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Materials ExpressArticle

Copyright copy 2011 by American Scientific PublishersAll rights reservedPrinted in the United States of America

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Doping (10 0)-Semiconductor Nanotubes withNitrogen and Vacancy DefectsE Gracia-Espino1 F Loacutepez-Uriacuteas1 H Terrones2 and M Terrones3lowast

1Advanced Materials Department IPICYT Camino a la Presa San Joseacute 2055 Col Lomas 4a seccioacutenSan Luis Potosiacute SLP 78216 Meacutexico2Center for Nanophase Materials Sciences Oak Ridge National Laboratory One Bethel Valley RoadOak Ridge TN 37831-6367 USA3Department of Physics Department of Materials Science and Engineering amp Materials Research InstituteThe Pennsylvania State University University Park PA 16802-6300 USA and Research Center for ExoticNanocarbons (JST) Shinshu University Wakasato 4-17-1 Nagano 380-8553 Japan

The electronic properties of (10 0)-semiconductingsingle-walled carbon nanotubes (SWCNTs) con-taining structural defects such as vacancies (singleand di-vacancies) and pyridinic Nitrogen atoms areinvestigated using first-principles density functionaltheory The band structure electronic band gapformation energy structural relaxation and HOMO-LUMO wave functions were systematically calculatedusing various combinations of vacancies and nitro-gen concentrations It is found that depending onthe concentration and location of Nitrogen atomswith respect to the vacancy-sites semiconducting(10 0)-SWCNTs could become metallic After relax-ation di-vacancies with pyridine-like Nitrogen atomsundergo a reconstruction so as to form pentago-nal and octagonal rings in which Nitrogen behavesas a substitutional atom (not pyridinic) within thegraphitic lattice Interestingly some Nitrogen dopedconfigurations exhibit a p-type doping characteristicsThe possibility of having pndashn junctions in SWCNTsby doping with just one element as dopant is alsodiscussed

Keywords Carbon Nanotubes Doping NitrogenDefects First-Principles Self-Reconstruction SurfaceVacancies

lowastAuthor to whom correspondence should be addressedEmails mut11psuedu mtterronesshinshu-uacjp

1 INTRODUCTION

The study and control of defects on doped carbonnanotubes is of technological relevance because duringsynthesis different types of defects could result (inten-tionally or unintentionally) within the structures Nowa-days it has been found that the addition of differentelements such as nitrogen12 phosphorous34 or sulfur5

results in the chemical modification of carbon nanotubesas well as changes in their electronic and mechanicalproperties Depending on the foreign atom (dopant) it ispossible to observe different tubular morphologies Forexample bamboo-type morphologies could be generatedin the presence of nitrogen6 and Y -junctions could beproduced when S is present in the synthesis5 These andothers nanotube architectures have attracted the attentionof numerous researchers because of the possibility of tai-loring their band gap Therefore it is essential to under-stand the role and dynamics of the chemical dopants anddefects when embedded in the lattice of carbon nano-tubes During the first analysis and theoretical study ofN-doped carbon nanotubes (CNx-MWCNTs) reported byCzerw et al6 it was found that the nitrogen atom exhib-ited two types of chemical bonding (1) substitutionalwhen the nitrogen atom replaces a C atom in the hexago-nal carbon lattice (N bonded to three carbon atoms) and(2) pyridine-like in which a N atom is only bonded totwo carbon atoms in vacancy locations generated in thehexagonal tubular lattice The triple pyridine-like modelproposed by Czerw et al6 consisted of a vacancy embed-ded in the carbon nanotube surrounded by three nitrogenatoms with double coordination to the C atoms Using tight

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

binding calculations the authors determined that N-dopingwas responsible for the metallic behavior by the additionof electronic states at the Fermi level In addition theyreported experimental evidence indicating that N-dopingislands exhibited strong electron donor states near theFermi level and proposed that all N-doped nanotubesneeded to be metallic78 Generally speaking when dop-ing occurs within a carbon nanotube shifts in the Fermienergy are observed and the sites could enhance its chemi-cal activity910 The induced electronic states in the vicinityof the Fermi energy and therefore the transport propertiesof doped-nanotubes are sensitive not only to the concen-tration of nitrogen atoms but also to their distribution andnanotube chirality1112

The influence of a nitrogen-vacancy on the transportproperties of single-walled carbon nanotubes (SWCNTs)has been studied by Wei et al13 These authors demon-strated that a single vacancy having one or three N-atomsplaced on a metallic 44 and a semiconducting 80SWCNT generates a half-filled impurity band whichfavors transport of semiconducting SWCNTs but inhibitstransport for the metallic tubes Transport studies of sub-stitutional nitrogen doping zigzag and armchair nanotubeshave also been reported by Kaun et al14 They found that a70 zigzag semiconducting SWCNT doped with a sin-gle N impurity increases the current flow whereas for a1010 armchair metallic SWCNT a current reduction isobserved with substitutional N doping (elastic backscat-tering caused by the impurity) In addition the electronicand magnetic properties of graphite and carbon nano-tubes containing substitutional nitrogen or adsorbed N2

on their surface have been studied by Ma et al15 Theseauthors found that N adatoms form a bridge-like struc-ture on the nanotube surface Experimental and theoreti-cal calculations on the electrical transport characteristicsof N-doped SWCNTs (N-SWCNTs) have also been per-formed by Min et al16 They found that contrary to theexpectation that the nitrogen atoms may induce n-typetransport the electrical behavior through the N-SWCNTsis either ambipolar in vacuum or p-type in air Their first-principles electronic structure calculations considered adoped semiconducting 100 SWCNT and revealed thatthe nitrogen atom indeed favors the pyridine-like config-uration Along this line more recent first-principles den-sity functional calculations performed by Fujimoto et al17

who demonstrated that pyridine-type defects embeddedwithin a 100 N-SWCNT are energetically preferredwhen compared to substitutional nitrogen present in thenearby region of a vacancy These authors also foundthat pyridine-type defects promote localized states nearthe valence-band and acts as an acceptor Other stud-ies performed by Sumpter et al18 have demonstrated thepossibility of controlling the nanotube growth by addingnitrogen phosphorus and sulfur atoms in small concentra-tions Subsequent studies on the adsorption of metal atoms

or molecules on graphite19 graphene20 and nanotubes21

together with defects and nitrogen as a dopant have beenreported as well as the influence of the nitrogen atoms onthe radial breathing mode in carbon nanotubes22

In addition to the nitrogen doped cases discussed abovein this account we investigate novel ways of simultane-ously doping SWCNTs with N and vacancies For exam-ple one and two adjacent vacancies were introduced in asemiconducting 100 SWCNT and all possibilities wereexplored by replacing systematically the low-coordinatedcarbon atoms (C-atoms surrounded the vacancy) by nitro-gen atoms Our results have been carefully studied andwere compared with some of the existing published mate-rial related to nitrogen-doped SWCNTsThe density functional theory (DFT) in conjunction with

the local density approximation has been used to predictthe structure formation energy and the electronic proper-ties of these doped systems We found that the electronicproperties are very sensitive to the nitrogen concentrationits configuration and the number of vacancies introducedin the systems In addition the carbon nanotubes tend toavoid dangling bonds and its surface self reconstructs

2 COMPUTATIONAL DETAILS

Electronic calculations were performed using Den-sity Functional Theory2324 within the Local DensityApproximation (DFT-LDA) using the Ceperley-Alderparametrization25 as implemented in the SIESTA code26

The wave functions for the valence electrons were repre-sented by a linear combination of pseudo-atomic numer-ical orbitals using a double- polarized basis (DZP)27

while core electrons were represented by norm-conservingTroullierndashMartins pseudopotentials in the KleinmanndashBylander non-local form Refs [28 29] The real-spacegrid used for charge and potential integration is equivalentto a plane wave cut-off energy of 150 Ry The pseudo-potentials (pprsquos) were constructed from 4 5 valence elec-trons for the carbon and nitrogen atoms respectively(C2s22p2 and N2s22p3) The systems were constructedusing a supercell of 160 atoms taking 4 unit cells of the100 nanotubes (40 atoms per unit cell) Periodic bound-ary conditions were used and the inter-tube distance waskept to a minimum of 40 Aring in order to avoid lateral inter-actions Sampling of the 1D Brillouin zones were carriedout with 1times 1times 16 Monkhorst-Pack grids All nanotubeswere relaxed by conjugate gradient minimization until themaximum force was less than 004 eVAring The bindingenergy (Eb) is defined as Eb = EscminusNiEi) where ESC isthe energy of the super cell Ni is the number of atoms inthe super cell and Ei is the energy of the isolated atom Thestability of vacancies and nitrogen-doping sites within thecarbon nanotubes are now calculated using the formationenergy (Eform expression using the equation defined by

Eform = EbminusNCCminusNNN (1)

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

where Eb is the binding energy of the defective nanotube(doped or with vacancies) C is the cohesive energy of thecarbon atom embedded in a pristine 100-SWCNT N

is the cohesive energy of a Nitrogen atom calculated fromthe molecule N2 (gas phase) Nc and NN are the numbersof carbon and nitrogen atoms respectively Similar forma-tion energies were also applied in Ref [21] The differentstudied carbon nanotube systems are identified by the labelVpNkABCD where V refers to the vacancies and N to thenitrogen doping p and k refer to the number of vacanciesand nitrogen atoms respectively A B C and D correspondto the sites where the nitrogen atoms are placed

3 RESULTS AND DISCUSSION

Firstly we focus on the introduction of a single vacancy ina 100 semiconductor carbon nanotube and the differentways of accommodating one or two N atoms around thevacancy Figure 1 depicts all the non-equivalent geometriesof the 100 carbon nanotube generated by removing acarbon atom (creating one vacancy) and replacing the lowcoordination carbon atom by nitrogen atoms The unre-laxed system is shown in Figure 1(a) which correspondsto the starting geometry used in our calculations The A Band C labels in Figure 1(a) represent the type of site noticethat sites A and C are equivalent In order to dope carbonnanotubes we have removed one or more carbon atomslocated at sites A B and C and replaced them by nitro-gen atoms For an individual nitrogen atom we have twonon-equivalent configurations as shown in Figure 1(b) and(c) In this case the nitrogen atom was either located insite B (V1N1B configuration) or in site C (V1N1C config-uration) It should be noted that the configuration V1N1A

Fig 1 Molecular models depicting (a) The starting geometry of the single-walled carbon nanotube (SWCNT) of 100 chirality with one vacancy(b) and (c) relaxed structures when the carbon atom is replaced by a nitrogen atom in site B (V1N1B) and C (V1N1C) respectively The cases oftwo nitrogen atoms replacing the carbon atoms are shown in (d) V1N2AC and (e) V1N2BC configuration The case shown in (e) has two equivalentconfigurations All the relaxed structures have at least one pyridine-like site

is equivalent to the configuration V1N1C After structuralrelaxation significant structural changes were observedIn both cases one pentagonal ring appears (in front ofthe nitrogen atom) and the nitrogen atom remains witha pyridine-like structure It is noteworthy that a similarV1N1B structure was reported in Ref [17] For two nitro-gen atoms introduced in the vacancy we have two dif-ferent configurations (see Fig 1(d and e)) When thenitrogen atoms are located in sites A and C (V1N2ACconfiguration) the carbon nanotube preserves the originalmorphological structure with slight changes in the inter-atomic distances for atoms surrounding the vacancy (seeFig 1(d)) and the two nitrogen atoms remain doubly coor-dinated However if the two nitrogen atoms are incorpo-rated to the system and located in sites B and C (V1N2BCconfiguration which is equivalent to the configurationAB (V1N2AB) the formation of a pentagonal ring isagain observed with a triply-coordinated nitrogen atomand the other nitrogen atom remains doubly-coordinated(pyridine-like) This result is in agreement with previ-ous calculations17 Notice that this structure is similar tothat obtained for an individual nitrogen case shown inFigure 1(b) (V1N1B configuration) However the elec-tronic properties of these systems are completely differ-ent as discussed below We have also studied the triplepyridine-like nitrogen defect (V1N3ABC) substitutionalnitrogen doping (without vacancy) V0N1 and the effectof one single vacancy (without doping) V1N0 In generalwe observed that the introduction of one vacancy with(or without) pyridine-like nitrogen atoms makes it diffi-cult to have a self-reconstruction of the nanotube surface

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Kotakoski et al30 studied the role of vacancies in undopedcarbon nanotubes These authors used a nonorthogo-nal density-functional-theory-based tight-binding modeland removed 1ndash6 carbon atoms from the carbon nano-tubes They demonstrated that the formation energies ofsmall vacancy clusters from single vacancies are energet-ically more favorable in comparison to multi-vacanciesAnother recent study on multi-vacancies in undopedcarbon nanotubes demonstrated that the formation of pen-tagons eliminates dangling bonds thus lowering the for-mation energy31 Regarding nitrogen-doped nanotubes Yuet al32 reported (using ab initio density functional the-ory calculations) the effects of substitutional doping nitro-gen atoms on the structure and the electronic propertiesof zigzag carbon nanotubes They introduced two nitro-gen atoms in the nanotube lattice and demonstrated thatthe electronic properties depend on the sites that the twonitrogen atoms occupy in the hexagonal networkThe band structure and wave functions at the -point

for the top valence (HOMO the highest occupied molecu-lar orbital) and lowest conduction band (LUMO the low-est unoccupied molecular orbital) are shown in Figure 2(the corresponding relaxed structures could be seen inFig 1) Calculations on a 100 nanotube without defectsor dopants demonstrate that it exhibits a semiconductingbehavior with an electronic band gap (Eg) of 0754 eVin agreement with previous calculations1733 If a nitrogenatom is introduced in the nanotube lattice as depicted inFigures 1(b) and (c) (V1N1B and V1N1C configurations)the Fermi energy (which is located at zero) is shiftedtowards the valence band see Figures 2(a) and (b) thusshowing a metallic behavior with a partially filled bandThe wave functions for these structures are also shown inFigure 2 in both cases the LUMO wave function exhibitsstates mainly located in the pentagonal ring being themost localized for the V1N1B configuration whereas theHOMO wave function exhibits states along of the entirebody of the carbon nanotube in both structures Oppo-site results were obtained for two nitrogen atoms theV1N2AC configuration exhibits a small electronic bandgap (0064 eV) and the Fermi energy is shifted close tothe valence band showing a p-type doping (see Fig 2(c))The corresponding LUMO wave function exhibits highlylocalized states near the vacancy and nitrogen atoms (nondispersive band) In addition the HOMO wave functionexhibits localized states in the vicinity of the vacancydefect The V1N2BC system reveals Eg values of 0723 eVwhich is slightly reduced when compared to the pristine100 nanotube (see Fig 2(d)) Note that all systems pre-sented in Figure 2 exhibit significant changes around ofthe Fermi level however far away from the Fermi levelthe bands remain almost similar independently of defectsor doping concentrationThe case considering two vacancies and two nitrogen

atoms systems is shown in Figure 3 It is important to

Fig 2 Electronic band structure of the relaxed 100 single-walledcarbon nanotubes containing one vacancy and one or two nitrogen atomsIn all cases the Fermi level is set to zero The cases (a) V1N1B and (b)V1N1C correspond to one-nitrogen systems whose structures are shown inFigure 1(b) and (c) respectively these systems exhibit a metallic behav-ior and a p-type doping The cases shown in (c) and (d) correspond to theV1N2AC and V1N2BC configurations (two nitrogen atoms) respectivelyThe corresponding relaxed geometries can be seen in Figure 1(d) and(e) The V1N2AC system exhibits a small band gap and a p-type dopingwhereas the V1N2BC system shows a band gap similar to the pristinetube In all cases the wave functions are plotted at the gamma point withan isosurface value of plusmn005 Aringminus32

mention that the vacancies were generated by removingtwo adjacent carbon atoms (di-vacancy) along the tubeaxis (see Fig 3(a)) The initial configuration (Fig 3(a)not relaxed geometry) provides four options (site A BC or D) in which nitrogen atoms could replace the car-bon atoms After geometry relaxation it was observeda reduction in diameter of the tube around the vacancydefect For the relaxed structure with only two vacancies

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 3 Molecular models showing (a) The starting and (b) relaxedgeometries of the single-walled carbon nanotube (SWCNT) exhibiting the100 chirality with two vacancies The relaxed structures of the threenon-equivalent cases of accommodating two nitrogen atoms are shownin (c) V2N2AC (d) V2N2AD and (e) V2N2AB Note that for the caseshown in (c) the nitrogen atoms remain as a pyridinic type whereas for(d) and (e) the N-atoms adopt three nearest neighbors (substitutional-likebehavior)

V2N0 configuration (without nitrogen) the carbon nano-tube healed its surface by forming two pentagonal andone octagonal rings (5-8-5 defect) avoiding dangling bonds(see Fig 3(b)) The octagonal ring creates a saddle point inthe structure and in consequence local negative Gaussiancurvature is observed In this region the nanotube diame-ter is reduced Ab initio calculations performed by Amorimet al34 reported the role of di-vacancies on the transportproperties of carbon nanotubes They also demonstratedthat the 5-8-5 defects are energetically favorable for alarge range of nanotube diameters By introducing twonitrogen atoms there are three non-equivalent configura-tions (see Fig 3(c d and e)) When nitrogen atoms werelocated at site AC or equivalently BD (V2N2AC configura-tion) the resulting relaxed structure exhibited a pentagonalring which contains only carbon atoms The two nitrogenatoms remain within a pyridine-like configuration thusavoiding the closure of the local structure (see Fig 3(c))However if the two nitrogen atoms are located at sitesA and D (V2N2AD configuration) which is equivalent tothe V2N2BC configuration the resulting structure exhibitsa 5-8-5 defect (see Fig 3(d)) Similar results have beenobtained when nitrogen atoms are placed at sites A andB sites (V2N2AB or equivalently V2N2CD configuration)We have also studied the di-vacancy containing four nitro-gen atoms occupying sites A B C and D (see Fig 3(a)configuration V2N4ABCD) After geometry relaxation nosignificant changes in the structure of the carbon nanotubewere observed results are also in agreement with previouscalculations2135

Figure 4 depicts results on the band structure and theHOMO and LUMO wave function plots for the differ-ent systems are shown in Figure 3 The nanotube withtwo adjacent vacancies V2N0 (without nitrogen) exhibitsa small band gap (0012 eV) and its corresponding wavefunctions displays states distributed along the nanotube

Fig 4 Electronic band structure of the relaxed 100-SWCNT con-taining two vacancies In all cases the Fermi level is set to zero Theframe shown in (a) corresponds to the case without nitrogen where theFermi energy has been shifted to the valence band exhibiting a very smallband gap (0012 eV) Frames (b) (c) and (d) correspond to two nitrogensystem whose structures are shown in Figure 3(c d and e) respectivelyFor V2N2AC the structure exhibits a small band gap of 0106 eV anda p-type characteristic For the last two cases (V2N2AD and V2N2ABconfigurations) similar band structures and band gaps are obtained whencompared to the pristine carbon tubule In all cases the wave functionswere plotted at gamma point with an isosurface value of plusmn005 Aringminus32

waist (see Fig 4(a)) The cases concerning the introduc-tion of two nitrogen atoms in non-equivalent sites areshown in Figures 4(b)ndash(d) For the system exhibiting aV2N2AC configuration the Fermi energy (located at zero)has been shifted to the valence band and they exhibit asmall Eg equal to 0106 eV (Fig 4(b)) The wave func-tion of V2N2AC configuration exhibits localized states onthe atoms surrounding the nitrogen atoms and the pen-tagonal ring (see Fig 4(b)) However for the V2N2ADand V2N2AB configurations the carbon nanotube exhibits

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

a semiconductor behavior with an electronic band gapof 0719 and 0685 eV respectively Both configurationsexhibit wave functions with states distributed on the entirenanotube lattice (see Fig 4(c and d))In order to understand the corrugation effect and the

diameter variation due to the presence of vacancies andnitrogen doping we analyzed five representative cases (seeFig 5) The nanotube diameters experience a diameterreduction at the vacancy followed by a slight increase nearto the defect thus resulting in corrugation We have alsoobserved that the diameter reduction significantly dependsof the position and concentration of the dopant atomsIn addition we studied two more configurations for

two vacancies with one and three nitrogen atoms (V2N1Cand V2N3ABD) the different site types are depicted inFigure 3(a) The relaxed structures and the band struc-ture calculations are shown in Figure 6 The systemcorresponding to the V2N1C exhibits 5-8-5 defects (seeFig 6(a)) similar to nanotubes with only two vacanciesand without nitrogen dopants (see configuration V2N0 inFig 3(b)) also exhibits a p-type behavior see Figure 6(b)The relaxed structure corresponding to V2N3ABD exhibitsonly one pentagonal ring which contains a nitrogen atomthe other two nitrogen atoms remain as pyridine-like sites(see Fig 6(c)) and the corresponding band structure isshown in Figure 6(d) Here a p-type behavior was alsoobserved Both configurations (V2N1C and V2N3ABD)exhibit a diameter variation of sim1 Aring near the defectiveregionAs we have witnessed from our results various

pyridine-like N-doped 100-SWCNTs configurationsexhibit a p-type doping behavior (electron acceptor) con-trary to our expectations in which the nitrogen-dopantwithin carbon nanotubes only causes n-type doping behav-ior (electron donor) We have observed a p-type behav-ior in the systems corresponding to the V1N1B V1N1CV1N2AC V1N3ABC V2N1C V2N3ABD and V2N4ABCDconfigurations Our results are in agreement with pre-vious reports indicating that pyridine-like configurationsexhibit p-type doping73335 In addition experimental

Fig 5 Molecular models showing the diameter variations generated by vacancies and nitrogen doping in a 100 carbon nanotube Side viewsimages reveal the diameter variation along the nanotube axis for different representative cases (a) Two vacancies without nitrogen (V2N0) (b) V1N1C(c) V1N1B (d) V1N2AC and (f) V2N2AD The nitrogen atoms are indicated by the open cycles In (b) the diameter is slightly varied but for (c)(d) and (f) it is observed a difference of sim08 Aring between the larger and smaller diameters these values are similar to the V2N0 case These changesin diameter result in corrugated surfaces characteristics of the nitrogen-doped nanotubes It is important to mention that the diameter of a pristine(undoped and without vacancies) 100 carbon nanotubes is around 799 Aring

electrical-transport measurements on metallic nitrogen-doped SWNTs have shown both p- and n-type dopingcharacteristics1636

Figure 7(a) shows the formation energy (calculated fromEq (1)) of all studied configurations and Figure 7(b)depicts the electronic band gaps (Eg) of the correspondingconfigurations In Figure 7(a) the energies were referredto the pristine nanotube (undoped and without vacancies)In Figure 7(a) the ldquolowastrdquo symbol is placed next to the struc-ture with defects that to the best of our knowledge arenovel and have not been reported hitherto It is interestingto observe that generally the role of nitrogen doping inthe carbon nanotubes with one or two vacancies reducesthe formation energy In most cases the undoped con-figurations V2N0 and V1N0 exhibit higher energy whencompared to the doped cases In Figure 7(a) we observethat next to the pristine tube the most stable configura-tion is the V0N1 system (substitutional doping) followedby the nanotube with four-nitrogen atoms placed in a di-vacancy (V2N4ABCD) A similar trend was reported by Liet al35 using density functional calculations and DMOLpackage In addition the authors investigated such defectsas a function of the nanotube chirality demonstrating thatthese defects are more stable in armchair nanotubes InFigure 7(a) several configurations were observed in therange of 3 to 5 eV here some structures experience self-surface reconstruction favoring the formation of pentago-nal and octagonal membered rings In this energy rangeit is also observed that nanotubes doped with two nitro-gen atoms exhibit a maximal electronic band gap (seeconfigurations V2N2AD V2N2AB V1N2BC in Fig 7(b))Notice that the canonical configuration (three nitrogenatoms occupying the pyridine-like sites V1N3ABC) oneof the most studied cases in the literature is the less sta-ble doped system Table I compares the formation ener-gies of five different defects obtained in this study withenergies reported in the literature showing that in mostcases our formation energies are in agreement with pre-vious calculations However little is known about chemi-cal reactivity of the configurations shown in Figure 7(a)

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 6 (a) Relaxed geometry obtained by introducing two vacancies and one nitrogen atom (configuration V2N1C) into the 100 carbon nanotubeNote that the final structure exhibits a 5-8-5 defect the corresponding band structure is shown in (b) (c) Relaxed structure obtained by introducingtwo vacancies and three nitrogen atoms (configuration V2N3ABD) into the 100 carbon nanotube This structure exhibits a pentagonal ring defectand the corresponding band structure calculations are shown (d) In both cases the Fermi level shifted to the valence band thus exhibiting a p-typedoping behavior and localized states at the Fermi level The open circles indicate the position of the nitrogen atoms

and therefore additional theoretical and experimental stud-ies are needed in order to understand the capacity ofnitrogen-doped carbon nanotubes for adsorbing differentmolecules and atoms Along this direction Zhao et al21

studied the hydrogen adsorption on calcium dispersed in

Fig 7 (a) Formation energy (see Eq (1)) of the 100 single walled carbon nanotubes (SWCNTs) by considering different ways of introducingnitrogen atoms and vacancies (all structures were relaxed) The circles indicate the nitrogen atoms and all energies are referred to the pristine 100nanotube (undoped and without vacancies) The ldquolowastrdquo symbol corresponds to new N-doped structures that to the best of our knowledge have not beenreported hiterto The most stable doped nanotube corresponds to the substitutional nitrogen doping V0N1 configuration followed by the V2N4ABCDconfiguration Notice that the less stable configuration correspond to the one-vacancy nanotube without nitrogen (V1N0 The corresponding electronicband gaps (Eg) are shown in (b) Notice that the modified 100-SWCNTs exhibits a reduced Eg which become metallic in some cases

the V2N4ABCD structure The authors suggested that upto five H2 molecules could be bound per calcium atomFinally as we have mentioned above by a selective

introduction of nitrogen atoms and vacancies to a 100carbon nanotube it is possible to tune its electronic

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

where Eb is the binding energy of the defective nanotube(doped or with vacancies) C is the cohesive energy of thecarbon atom embedded in a pristine 100-SWCNT N

is the cohesive energy of a Nitrogen atom calculated fromthe molecule N2 (gas phase) Nc and NN are the numbersof carbon and nitrogen atoms respectively Similar forma-tion energies were also applied in Ref [21] The differentstudied carbon nanotube systems are identified by the labelVpNkABCD where V refers to the vacancies and N to thenitrogen doping p and k refer to the number of vacanciesand nitrogen atoms respectively A B C and D correspondto the sites where the nitrogen atoms are placed

3 RESULTS AND DISCUSSION

Firstly we focus on the introduction of a single vacancy ina 100 semiconductor carbon nanotube and the differentways of accommodating one or two N atoms around thevacancy Figure 1 depicts all the non-equivalent geometriesof the 100 carbon nanotube generated by removing acarbon atom (creating one vacancy) and replacing the lowcoordination carbon atom by nitrogen atoms The unre-laxed system is shown in Figure 1(a) which correspondsto the starting geometry used in our calculations The A Band C labels in Figure 1(a) represent the type of site noticethat sites A and C are equivalent In order to dope carbonnanotubes we have removed one or more carbon atomslocated at sites A B and C and replaced them by nitro-gen atoms For an individual nitrogen atom we have twonon-equivalent configurations as shown in Figure 1(b) and(c) In this case the nitrogen atom was either located insite B (V1N1B configuration) or in site C (V1N1C config-uration) It should be noted that the configuration V1N1A

Fig 1 Molecular models depicting (a) The starting geometry of the single-walled carbon nanotube (SWCNT) of 100 chirality with one vacancy(b) and (c) relaxed structures when the carbon atom is replaced by a nitrogen atom in site B (V1N1B) and C (V1N1C) respectively The cases oftwo nitrogen atoms replacing the carbon atoms are shown in (d) V1N2AC and (e) V1N2BC configuration The case shown in (e) has two equivalentconfigurations All the relaxed structures have at least one pyridine-like site

is equivalent to the configuration V1N1C After structuralrelaxation significant structural changes were observedIn both cases one pentagonal ring appears (in front ofthe nitrogen atom) and the nitrogen atom remains witha pyridine-like structure It is noteworthy that a similarV1N1B structure was reported in Ref [17] For two nitro-gen atoms introduced in the vacancy we have two dif-ferent configurations (see Fig 1(d and e)) When thenitrogen atoms are located in sites A and C (V1N2ACconfiguration) the carbon nanotube preserves the originalmorphological structure with slight changes in the inter-atomic distances for atoms surrounding the vacancy (seeFig 1(d)) and the two nitrogen atoms remain doubly coor-dinated However if the two nitrogen atoms are incorpo-rated to the system and located in sites B and C (V1N2BCconfiguration which is equivalent to the configurationAB (V1N2AB) the formation of a pentagonal ring isagain observed with a triply-coordinated nitrogen atomand the other nitrogen atom remains doubly-coordinated(pyridine-like) This result is in agreement with previ-ous calculations17 Notice that this structure is similar tothat obtained for an individual nitrogen case shown inFigure 1(b) (V1N1B configuration) However the elec-tronic properties of these systems are completely differ-ent as discussed below We have also studied the triplepyridine-like nitrogen defect (V1N3ABC) substitutionalnitrogen doping (without vacancy) V0N1 and the effectof one single vacancy (without doping) V1N0 In generalwe observed that the introduction of one vacancy with(or without) pyridine-like nitrogen atoms makes it diffi-cult to have a self-reconstruction of the nanotube surface

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Kotakoski et al30 studied the role of vacancies in undopedcarbon nanotubes These authors used a nonorthogo-nal density-functional-theory-based tight-binding modeland removed 1ndash6 carbon atoms from the carbon nano-tubes They demonstrated that the formation energies ofsmall vacancy clusters from single vacancies are energet-ically more favorable in comparison to multi-vacanciesAnother recent study on multi-vacancies in undopedcarbon nanotubes demonstrated that the formation of pen-tagons eliminates dangling bonds thus lowering the for-mation energy31 Regarding nitrogen-doped nanotubes Yuet al32 reported (using ab initio density functional the-ory calculations) the effects of substitutional doping nitro-gen atoms on the structure and the electronic propertiesof zigzag carbon nanotubes They introduced two nitro-gen atoms in the nanotube lattice and demonstrated thatthe electronic properties depend on the sites that the twonitrogen atoms occupy in the hexagonal networkThe band structure and wave functions at the -point

for the top valence (HOMO the highest occupied molecu-lar orbital) and lowest conduction band (LUMO the low-est unoccupied molecular orbital) are shown in Figure 2(the corresponding relaxed structures could be seen inFig 1) Calculations on a 100 nanotube without defectsor dopants demonstrate that it exhibits a semiconductingbehavior with an electronic band gap (Eg) of 0754 eVin agreement with previous calculations1733 If a nitrogenatom is introduced in the nanotube lattice as depicted inFigures 1(b) and (c) (V1N1B and V1N1C configurations)the Fermi energy (which is located at zero) is shiftedtowards the valence band see Figures 2(a) and (b) thusshowing a metallic behavior with a partially filled bandThe wave functions for these structures are also shown inFigure 2 in both cases the LUMO wave function exhibitsstates mainly located in the pentagonal ring being themost localized for the V1N1B configuration whereas theHOMO wave function exhibits states along of the entirebody of the carbon nanotube in both structures Oppo-site results were obtained for two nitrogen atoms theV1N2AC configuration exhibits a small electronic bandgap (0064 eV) and the Fermi energy is shifted close tothe valence band showing a p-type doping (see Fig 2(c))The corresponding LUMO wave function exhibits highlylocalized states near the vacancy and nitrogen atoms (nondispersive band) In addition the HOMO wave functionexhibits localized states in the vicinity of the vacancydefect The V1N2BC system reveals Eg values of 0723 eVwhich is slightly reduced when compared to the pristine100 nanotube (see Fig 2(d)) Note that all systems pre-sented in Figure 2 exhibit significant changes around ofthe Fermi level however far away from the Fermi levelthe bands remain almost similar independently of defectsor doping concentrationThe case considering two vacancies and two nitrogen

atoms systems is shown in Figure 3 It is important to

Fig 2 Electronic band structure of the relaxed 100 single-walledcarbon nanotubes containing one vacancy and one or two nitrogen atomsIn all cases the Fermi level is set to zero The cases (a) V1N1B and (b)V1N1C correspond to one-nitrogen systems whose structures are shown inFigure 1(b) and (c) respectively these systems exhibit a metallic behav-ior and a p-type doping The cases shown in (c) and (d) correspond to theV1N2AC and V1N2BC configurations (two nitrogen atoms) respectivelyThe corresponding relaxed geometries can be seen in Figure 1(d) and(e) The V1N2AC system exhibits a small band gap and a p-type dopingwhereas the V1N2BC system shows a band gap similar to the pristinetube In all cases the wave functions are plotted at the gamma point withan isosurface value of plusmn005 Aringminus32

mention that the vacancies were generated by removingtwo adjacent carbon atoms (di-vacancy) along the tubeaxis (see Fig 3(a)) The initial configuration (Fig 3(a)not relaxed geometry) provides four options (site A BC or D) in which nitrogen atoms could replace the car-bon atoms After geometry relaxation it was observeda reduction in diameter of the tube around the vacancydefect For the relaxed structure with only two vacancies

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 3 Molecular models showing (a) The starting and (b) relaxedgeometries of the single-walled carbon nanotube (SWCNT) exhibiting the100 chirality with two vacancies The relaxed structures of the threenon-equivalent cases of accommodating two nitrogen atoms are shownin (c) V2N2AC (d) V2N2AD and (e) V2N2AB Note that for the caseshown in (c) the nitrogen atoms remain as a pyridinic type whereas for(d) and (e) the N-atoms adopt three nearest neighbors (substitutional-likebehavior)

V2N0 configuration (without nitrogen) the carbon nano-tube healed its surface by forming two pentagonal andone octagonal rings (5-8-5 defect) avoiding dangling bonds(see Fig 3(b)) The octagonal ring creates a saddle point inthe structure and in consequence local negative Gaussiancurvature is observed In this region the nanotube diame-ter is reduced Ab initio calculations performed by Amorimet al34 reported the role of di-vacancies on the transportproperties of carbon nanotubes They also demonstratedthat the 5-8-5 defects are energetically favorable for alarge range of nanotube diameters By introducing twonitrogen atoms there are three non-equivalent configura-tions (see Fig 3(c d and e)) When nitrogen atoms werelocated at site AC or equivalently BD (V2N2AC configura-tion) the resulting relaxed structure exhibited a pentagonalring which contains only carbon atoms The two nitrogenatoms remain within a pyridine-like configuration thusavoiding the closure of the local structure (see Fig 3(c))However if the two nitrogen atoms are located at sitesA and D (V2N2AD configuration) which is equivalent tothe V2N2BC configuration the resulting structure exhibitsa 5-8-5 defect (see Fig 3(d)) Similar results have beenobtained when nitrogen atoms are placed at sites A andB sites (V2N2AB or equivalently V2N2CD configuration)We have also studied the di-vacancy containing four nitro-gen atoms occupying sites A B C and D (see Fig 3(a)configuration V2N4ABCD) After geometry relaxation nosignificant changes in the structure of the carbon nanotubewere observed results are also in agreement with previouscalculations2135

Figure 4 depicts results on the band structure and theHOMO and LUMO wave function plots for the differ-ent systems are shown in Figure 3 The nanotube withtwo adjacent vacancies V2N0 (without nitrogen) exhibitsa small band gap (0012 eV) and its corresponding wavefunctions displays states distributed along the nanotube

Fig 4 Electronic band structure of the relaxed 100-SWCNT con-taining two vacancies In all cases the Fermi level is set to zero Theframe shown in (a) corresponds to the case without nitrogen where theFermi energy has been shifted to the valence band exhibiting a very smallband gap (0012 eV) Frames (b) (c) and (d) correspond to two nitrogensystem whose structures are shown in Figure 3(c d and e) respectivelyFor V2N2AC the structure exhibits a small band gap of 0106 eV anda p-type characteristic For the last two cases (V2N2AD and V2N2ABconfigurations) similar band structures and band gaps are obtained whencompared to the pristine carbon tubule In all cases the wave functionswere plotted at gamma point with an isosurface value of plusmn005 Aringminus32

waist (see Fig 4(a)) The cases concerning the introduc-tion of two nitrogen atoms in non-equivalent sites areshown in Figures 4(b)ndash(d) For the system exhibiting aV2N2AC configuration the Fermi energy (located at zero)has been shifted to the valence band and they exhibit asmall Eg equal to 0106 eV (Fig 4(b)) The wave func-tion of V2N2AC configuration exhibits localized states onthe atoms surrounding the nitrogen atoms and the pen-tagonal ring (see Fig 4(b)) However for the V2N2ADand V2N2AB configurations the carbon nanotube exhibits

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Article

a semiconductor behavior with an electronic band gapof 0719 and 0685 eV respectively Both configurationsexhibit wave functions with states distributed on the entirenanotube lattice (see Fig 4(c and d))In order to understand the corrugation effect and the

diameter variation due to the presence of vacancies andnitrogen doping we analyzed five representative cases (seeFig 5) The nanotube diameters experience a diameterreduction at the vacancy followed by a slight increase nearto the defect thus resulting in corrugation We have alsoobserved that the diameter reduction significantly dependsof the position and concentration of the dopant atomsIn addition we studied two more configurations for

two vacancies with one and three nitrogen atoms (V2N1Cand V2N3ABD) the different site types are depicted inFigure 3(a) The relaxed structures and the band struc-ture calculations are shown in Figure 6 The systemcorresponding to the V2N1C exhibits 5-8-5 defects (seeFig 6(a)) similar to nanotubes with only two vacanciesand without nitrogen dopants (see configuration V2N0 inFig 3(b)) also exhibits a p-type behavior see Figure 6(b)The relaxed structure corresponding to V2N3ABD exhibitsonly one pentagonal ring which contains a nitrogen atomthe other two nitrogen atoms remain as pyridine-like sites(see Fig 6(c)) and the corresponding band structure isshown in Figure 6(d) Here a p-type behavior was alsoobserved Both configurations (V2N1C and V2N3ABD)exhibit a diameter variation of sim1 Aring near the defectiveregionAs we have witnessed from our results various

pyridine-like N-doped 100-SWCNTs configurationsexhibit a p-type doping behavior (electron acceptor) con-trary to our expectations in which the nitrogen-dopantwithin carbon nanotubes only causes n-type doping behav-ior (electron donor) We have observed a p-type behav-ior in the systems corresponding to the V1N1B V1N1CV1N2AC V1N3ABC V2N1C V2N3ABD and V2N4ABCDconfigurations Our results are in agreement with pre-vious reports indicating that pyridine-like configurationsexhibit p-type doping73335 In addition experimental

Fig 5 Molecular models showing the diameter variations generated by vacancies and nitrogen doping in a 100 carbon nanotube Side viewsimages reveal the diameter variation along the nanotube axis for different representative cases (a) Two vacancies without nitrogen (V2N0) (b) V1N1C(c) V1N1B (d) V1N2AC and (f) V2N2AD The nitrogen atoms are indicated by the open cycles In (b) the diameter is slightly varied but for (c)(d) and (f) it is observed a difference of sim08 Aring between the larger and smaller diameters these values are similar to the V2N0 case These changesin diameter result in corrugated surfaces characteristics of the nitrogen-doped nanotubes It is important to mention that the diameter of a pristine(undoped and without vacancies) 100 carbon nanotubes is around 799 Aring

electrical-transport measurements on metallic nitrogen-doped SWNTs have shown both p- and n-type dopingcharacteristics1636

Figure 7(a) shows the formation energy (calculated fromEq (1)) of all studied configurations and Figure 7(b)depicts the electronic band gaps (Eg) of the correspondingconfigurations In Figure 7(a) the energies were referredto the pristine nanotube (undoped and without vacancies)In Figure 7(a) the ldquolowastrdquo symbol is placed next to the struc-ture with defects that to the best of our knowledge arenovel and have not been reported hitherto It is interestingto observe that generally the role of nitrogen doping inthe carbon nanotubes with one or two vacancies reducesthe formation energy In most cases the undoped con-figurations V2N0 and V1N0 exhibit higher energy whencompared to the doped cases In Figure 7(a) we observethat next to the pristine tube the most stable configura-tion is the V0N1 system (substitutional doping) followedby the nanotube with four-nitrogen atoms placed in a di-vacancy (V2N4ABCD) A similar trend was reported by Liet al35 using density functional calculations and DMOLpackage In addition the authors investigated such defectsas a function of the nanotube chirality demonstrating thatthese defects are more stable in armchair nanotubes InFigure 7(a) several configurations were observed in therange of 3 to 5 eV here some structures experience self-surface reconstruction favoring the formation of pentago-nal and octagonal membered rings In this energy rangeit is also observed that nanotubes doped with two nitro-gen atoms exhibit a maximal electronic band gap (seeconfigurations V2N2AD V2N2AB V1N2BC in Fig 7(b))Notice that the canonical configuration (three nitrogenatoms occupying the pyridine-like sites V1N3ABC) oneof the most studied cases in the literature is the less sta-ble doped system Table I compares the formation ener-gies of five different defects obtained in this study withenergies reported in the literature showing that in mostcases our formation energies are in agreement with pre-vious calculations However little is known about chemi-cal reactivity of the configurations shown in Figure 7(a)

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 6 (a) Relaxed geometry obtained by introducing two vacancies and one nitrogen atom (configuration V2N1C) into the 100 carbon nanotubeNote that the final structure exhibits a 5-8-5 defect the corresponding band structure is shown in (b) (c) Relaxed structure obtained by introducingtwo vacancies and three nitrogen atoms (configuration V2N3ABD) into the 100 carbon nanotube This structure exhibits a pentagonal ring defectand the corresponding band structure calculations are shown (d) In both cases the Fermi level shifted to the valence band thus exhibiting a p-typedoping behavior and localized states at the Fermi level The open circles indicate the position of the nitrogen atoms

and therefore additional theoretical and experimental stud-ies are needed in order to understand the capacity ofnitrogen-doped carbon nanotubes for adsorbing differentmolecules and atoms Along this direction Zhao et al21

studied the hydrogen adsorption on calcium dispersed in

Fig 7 (a) Formation energy (see Eq (1)) of the 100 single walled carbon nanotubes (SWCNTs) by considering different ways of introducingnitrogen atoms and vacancies (all structures were relaxed) The circles indicate the nitrogen atoms and all energies are referred to the pristine 100nanotube (undoped and without vacancies) The ldquolowastrdquo symbol corresponds to new N-doped structures that to the best of our knowledge have not beenreported hiterto The most stable doped nanotube corresponds to the substitutional nitrogen doping V0N1 configuration followed by the V2N4ABCDconfiguration Notice that the less stable configuration correspond to the one-vacancy nanotube without nitrogen (V1N0 The corresponding electronicband gaps (Eg) are shown in (b) Notice that the modified 100-SWCNTs exhibits a reduced Eg which become metallic in some cases

the V2N4ABCD structure The authors suggested that upto five H2 molecules could be bound per calcium atomFinally as we have mentioned above by a selective

introduction of nitrogen atoms and vacancies to a 100carbon nanotube it is possible to tune its electronic

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

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Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Kotakoski et al30 studied the role of vacancies in undopedcarbon nanotubes These authors used a nonorthogo-nal density-functional-theory-based tight-binding modeland removed 1ndash6 carbon atoms from the carbon nano-tubes They demonstrated that the formation energies ofsmall vacancy clusters from single vacancies are energet-ically more favorable in comparison to multi-vacanciesAnother recent study on multi-vacancies in undopedcarbon nanotubes demonstrated that the formation of pen-tagons eliminates dangling bonds thus lowering the for-mation energy31 Regarding nitrogen-doped nanotubes Yuet al32 reported (using ab initio density functional the-ory calculations) the effects of substitutional doping nitro-gen atoms on the structure and the electronic propertiesof zigzag carbon nanotubes They introduced two nitro-gen atoms in the nanotube lattice and demonstrated thatthe electronic properties depend on the sites that the twonitrogen atoms occupy in the hexagonal networkThe band structure and wave functions at the -point

for the top valence (HOMO the highest occupied molecu-lar orbital) and lowest conduction band (LUMO the low-est unoccupied molecular orbital) are shown in Figure 2(the corresponding relaxed structures could be seen inFig 1) Calculations on a 100 nanotube without defectsor dopants demonstrate that it exhibits a semiconductingbehavior with an electronic band gap (Eg) of 0754 eVin agreement with previous calculations1733 If a nitrogenatom is introduced in the nanotube lattice as depicted inFigures 1(b) and (c) (V1N1B and V1N1C configurations)the Fermi energy (which is located at zero) is shiftedtowards the valence band see Figures 2(a) and (b) thusshowing a metallic behavior with a partially filled bandThe wave functions for these structures are also shown inFigure 2 in both cases the LUMO wave function exhibitsstates mainly located in the pentagonal ring being themost localized for the V1N1B configuration whereas theHOMO wave function exhibits states along of the entirebody of the carbon nanotube in both structures Oppo-site results were obtained for two nitrogen atoms theV1N2AC configuration exhibits a small electronic bandgap (0064 eV) and the Fermi energy is shifted close tothe valence band showing a p-type doping (see Fig 2(c))The corresponding LUMO wave function exhibits highlylocalized states near the vacancy and nitrogen atoms (nondispersive band) In addition the HOMO wave functionexhibits localized states in the vicinity of the vacancydefect The V1N2BC system reveals Eg values of 0723 eVwhich is slightly reduced when compared to the pristine100 nanotube (see Fig 2(d)) Note that all systems pre-sented in Figure 2 exhibit significant changes around ofthe Fermi level however far away from the Fermi levelthe bands remain almost similar independently of defectsor doping concentrationThe case considering two vacancies and two nitrogen

atoms systems is shown in Figure 3 It is important to

Fig 2 Electronic band structure of the relaxed 100 single-walledcarbon nanotubes containing one vacancy and one or two nitrogen atomsIn all cases the Fermi level is set to zero The cases (a) V1N1B and (b)V1N1C correspond to one-nitrogen systems whose structures are shown inFigure 1(b) and (c) respectively these systems exhibit a metallic behav-ior and a p-type doping The cases shown in (c) and (d) correspond to theV1N2AC and V1N2BC configurations (two nitrogen atoms) respectivelyThe corresponding relaxed geometries can be seen in Figure 1(d) and(e) The V1N2AC system exhibits a small band gap and a p-type dopingwhereas the V1N2BC system shows a band gap similar to the pristinetube In all cases the wave functions are plotted at the gamma point withan isosurface value of plusmn005 Aringminus32

mention that the vacancies were generated by removingtwo adjacent carbon atoms (di-vacancy) along the tubeaxis (see Fig 3(a)) The initial configuration (Fig 3(a)not relaxed geometry) provides four options (site A BC or D) in which nitrogen atoms could replace the car-bon atoms After geometry relaxation it was observeda reduction in diameter of the tube around the vacancydefect For the relaxed structure with only two vacancies

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 3 Molecular models showing (a) The starting and (b) relaxedgeometries of the single-walled carbon nanotube (SWCNT) exhibiting the100 chirality with two vacancies The relaxed structures of the threenon-equivalent cases of accommodating two nitrogen atoms are shownin (c) V2N2AC (d) V2N2AD and (e) V2N2AB Note that for the caseshown in (c) the nitrogen atoms remain as a pyridinic type whereas for(d) and (e) the N-atoms adopt three nearest neighbors (substitutional-likebehavior)

V2N0 configuration (without nitrogen) the carbon nano-tube healed its surface by forming two pentagonal andone octagonal rings (5-8-5 defect) avoiding dangling bonds(see Fig 3(b)) The octagonal ring creates a saddle point inthe structure and in consequence local negative Gaussiancurvature is observed In this region the nanotube diame-ter is reduced Ab initio calculations performed by Amorimet al34 reported the role of di-vacancies on the transportproperties of carbon nanotubes They also demonstratedthat the 5-8-5 defects are energetically favorable for alarge range of nanotube diameters By introducing twonitrogen atoms there are three non-equivalent configura-tions (see Fig 3(c d and e)) When nitrogen atoms werelocated at site AC or equivalently BD (V2N2AC configura-tion) the resulting relaxed structure exhibited a pentagonalring which contains only carbon atoms The two nitrogenatoms remain within a pyridine-like configuration thusavoiding the closure of the local structure (see Fig 3(c))However if the two nitrogen atoms are located at sitesA and D (V2N2AD configuration) which is equivalent tothe V2N2BC configuration the resulting structure exhibitsa 5-8-5 defect (see Fig 3(d)) Similar results have beenobtained when nitrogen atoms are placed at sites A andB sites (V2N2AB or equivalently V2N2CD configuration)We have also studied the di-vacancy containing four nitro-gen atoms occupying sites A B C and D (see Fig 3(a)configuration V2N4ABCD) After geometry relaxation nosignificant changes in the structure of the carbon nanotubewere observed results are also in agreement with previouscalculations2135

Figure 4 depicts results on the band structure and theHOMO and LUMO wave function plots for the differ-ent systems are shown in Figure 3 The nanotube withtwo adjacent vacancies V2N0 (without nitrogen) exhibitsa small band gap (0012 eV) and its corresponding wavefunctions displays states distributed along the nanotube

Fig 4 Electronic band structure of the relaxed 100-SWCNT con-taining two vacancies In all cases the Fermi level is set to zero Theframe shown in (a) corresponds to the case without nitrogen where theFermi energy has been shifted to the valence band exhibiting a very smallband gap (0012 eV) Frames (b) (c) and (d) correspond to two nitrogensystem whose structures are shown in Figure 3(c d and e) respectivelyFor V2N2AC the structure exhibits a small band gap of 0106 eV anda p-type characteristic For the last two cases (V2N2AD and V2N2ABconfigurations) similar band structures and band gaps are obtained whencompared to the pristine carbon tubule In all cases the wave functionswere plotted at gamma point with an isosurface value of plusmn005 Aringminus32

waist (see Fig 4(a)) The cases concerning the introduc-tion of two nitrogen atoms in non-equivalent sites areshown in Figures 4(b)ndash(d) For the system exhibiting aV2N2AC configuration the Fermi energy (located at zero)has been shifted to the valence band and they exhibit asmall Eg equal to 0106 eV (Fig 4(b)) The wave func-tion of V2N2AC configuration exhibits localized states onthe atoms surrounding the nitrogen atoms and the pen-tagonal ring (see Fig 4(b)) However for the V2N2ADand V2N2AB configurations the carbon nanotube exhibits

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

a semiconductor behavior with an electronic band gapof 0719 and 0685 eV respectively Both configurationsexhibit wave functions with states distributed on the entirenanotube lattice (see Fig 4(c and d))In order to understand the corrugation effect and the

diameter variation due to the presence of vacancies andnitrogen doping we analyzed five representative cases (seeFig 5) The nanotube diameters experience a diameterreduction at the vacancy followed by a slight increase nearto the defect thus resulting in corrugation We have alsoobserved that the diameter reduction significantly dependsof the position and concentration of the dopant atomsIn addition we studied two more configurations for

two vacancies with one and three nitrogen atoms (V2N1Cand V2N3ABD) the different site types are depicted inFigure 3(a) The relaxed structures and the band struc-ture calculations are shown in Figure 6 The systemcorresponding to the V2N1C exhibits 5-8-5 defects (seeFig 6(a)) similar to nanotubes with only two vacanciesand without nitrogen dopants (see configuration V2N0 inFig 3(b)) also exhibits a p-type behavior see Figure 6(b)The relaxed structure corresponding to V2N3ABD exhibitsonly one pentagonal ring which contains a nitrogen atomthe other two nitrogen atoms remain as pyridine-like sites(see Fig 6(c)) and the corresponding band structure isshown in Figure 6(d) Here a p-type behavior was alsoobserved Both configurations (V2N1C and V2N3ABD)exhibit a diameter variation of sim1 Aring near the defectiveregionAs we have witnessed from our results various

pyridine-like N-doped 100-SWCNTs configurationsexhibit a p-type doping behavior (electron acceptor) con-trary to our expectations in which the nitrogen-dopantwithin carbon nanotubes only causes n-type doping behav-ior (electron donor) We have observed a p-type behav-ior in the systems corresponding to the V1N1B V1N1CV1N2AC V1N3ABC V2N1C V2N3ABD and V2N4ABCDconfigurations Our results are in agreement with pre-vious reports indicating that pyridine-like configurationsexhibit p-type doping73335 In addition experimental

Fig 5 Molecular models showing the diameter variations generated by vacancies and nitrogen doping in a 100 carbon nanotube Side viewsimages reveal the diameter variation along the nanotube axis for different representative cases (a) Two vacancies without nitrogen (V2N0) (b) V1N1C(c) V1N1B (d) V1N2AC and (f) V2N2AD The nitrogen atoms are indicated by the open cycles In (b) the diameter is slightly varied but for (c)(d) and (f) it is observed a difference of sim08 Aring between the larger and smaller diameters these values are similar to the V2N0 case These changesin diameter result in corrugated surfaces characteristics of the nitrogen-doped nanotubes It is important to mention that the diameter of a pristine(undoped and without vacancies) 100 carbon nanotubes is around 799 Aring

electrical-transport measurements on metallic nitrogen-doped SWNTs have shown both p- and n-type dopingcharacteristics1636

Figure 7(a) shows the formation energy (calculated fromEq (1)) of all studied configurations and Figure 7(b)depicts the electronic band gaps (Eg) of the correspondingconfigurations In Figure 7(a) the energies were referredto the pristine nanotube (undoped and without vacancies)In Figure 7(a) the ldquolowastrdquo symbol is placed next to the struc-ture with defects that to the best of our knowledge arenovel and have not been reported hitherto It is interestingto observe that generally the role of nitrogen doping inthe carbon nanotubes with one or two vacancies reducesthe formation energy In most cases the undoped con-figurations V2N0 and V1N0 exhibit higher energy whencompared to the doped cases In Figure 7(a) we observethat next to the pristine tube the most stable configura-tion is the V0N1 system (substitutional doping) followedby the nanotube with four-nitrogen atoms placed in a di-vacancy (V2N4ABCD) A similar trend was reported by Liet al35 using density functional calculations and DMOLpackage In addition the authors investigated such defectsas a function of the nanotube chirality demonstrating thatthese defects are more stable in armchair nanotubes InFigure 7(a) several configurations were observed in therange of 3 to 5 eV here some structures experience self-surface reconstruction favoring the formation of pentago-nal and octagonal membered rings In this energy rangeit is also observed that nanotubes doped with two nitro-gen atoms exhibit a maximal electronic band gap (seeconfigurations V2N2AD V2N2AB V1N2BC in Fig 7(b))Notice that the canonical configuration (three nitrogenatoms occupying the pyridine-like sites V1N3ABC) oneof the most studied cases in the literature is the less sta-ble doped system Table I compares the formation ener-gies of five different defects obtained in this study withenergies reported in the literature showing that in mostcases our formation energies are in agreement with pre-vious calculations However little is known about chemi-cal reactivity of the configurations shown in Figure 7(a)

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 6 (a) Relaxed geometry obtained by introducing two vacancies and one nitrogen atom (configuration V2N1C) into the 100 carbon nanotubeNote that the final structure exhibits a 5-8-5 defect the corresponding band structure is shown in (b) (c) Relaxed structure obtained by introducingtwo vacancies and three nitrogen atoms (configuration V2N3ABD) into the 100 carbon nanotube This structure exhibits a pentagonal ring defectand the corresponding band structure calculations are shown (d) In both cases the Fermi level shifted to the valence band thus exhibiting a p-typedoping behavior and localized states at the Fermi level The open circles indicate the position of the nitrogen atoms

and therefore additional theoretical and experimental stud-ies are needed in order to understand the capacity ofnitrogen-doped carbon nanotubes for adsorbing differentmolecules and atoms Along this direction Zhao et al21

studied the hydrogen adsorption on calcium dispersed in

Fig 7 (a) Formation energy (see Eq (1)) of the 100 single walled carbon nanotubes (SWCNTs) by considering different ways of introducingnitrogen atoms and vacancies (all structures were relaxed) The circles indicate the nitrogen atoms and all energies are referred to the pristine 100nanotube (undoped and without vacancies) The ldquolowastrdquo symbol corresponds to new N-doped structures that to the best of our knowledge have not beenreported hiterto The most stable doped nanotube corresponds to the substitutional nitrogen doping V0N1 configuration followed by the V2N4ABCDconfiguration Notice that the less stable configuration correspond to the one-vacancy nanotube without nitrogen (V1N0 The corresponding electronicband gaps (Eg) are shown in (b) Notice that the modified 100-SWCNTs exhibits a reduced Eg which become metallic in some cases

the V2N4ABCD structure The authors suggested that upto five H2 molecules could be bound per calcium atomFinally as we have mentioned above by a selective

introduction of nitrogen atoms and vacancies to a 100carbon nanotube it is possible to tune its electronic

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 3 Molecular models showing (a) The starting and (b) relaxedgeometries of the single-walled carbon nanotube (SWCNT) exhibiting the100 chirality with two vacancies The relaxed structures of the threenon-equivalent cases of accommodating two nitrogen atoms are shownin (c) V2N2AC (d) V2N2AD and (e) V2N2AB Note that for the caseshown in (c) the nitrogen atoms remain as a pyridinic type whereas for(d) and (e) the N-atoms adopt three nearest neighbors (substitutional-likebehavior)

V2N0 configuration (without nitrogen) the carbon nano-tube healed its surface by forming two pentagonal andone octagonal rings (5-8-5 defect) avoiding dangling bonds(see Fig 3(b)) The octagonal ring creates a saddle point inthe structure and in consequence local negative Gaussiancurvature is observed In this region the nanotube diame-ter is reduced Ab initio calculations performed by Amorimet al34 reported the role of di-vacancies on the transportproperties of carbon nanotubes They also demonstratedthat the 5-8-5 defects are energetically favorable for alarge range of nanotube diameters By introducing twonitrogen atoms there are three non-equivalent configura-tions (see Fig 3(c d and e)) When nitrogen atoms werelocated at site AC or equivalently BD (V2N2AC configura-tion) the resulting relaxed structure exhibited a pentagonalring which contains only carbon atoms The two nitrogenatoms remain within a pyridine-like configuration thusavoiding the closure of the local structure (see Fig 3(c))However if the two nitrogen atoms are located at sitesA and D (V2N2AD configuration) which is equivalent tothe V2N2BC configuration the resulting structure exhibitsa 5-8-5 defect (see Fig 3(d)) Similar results have beenobtained when nitrogen atoms are placed at sites A andB sites (V2N2AB or equivalently V2N2CD configuration)We have also studied the di-vacancy containing four nitro-gen atoms occupying sites A B C and D (see Fig 3(a)configuration V2N4ABCD) After geometry relaxation nosignificant changes in the structure of the carbon nanotubewere observed results are also in agreement with previouscalculations2135

Figure 4 depicts results on the band structure and theHOMO and LUMO wave function plots for the differ-ent systems are shown in Figure 3 The nanotube withtwo adjacent vacancies V2N0 (without nitrogen) exhibitsa small band gap (0012 eV) and its corresponding wavefunctions displays states distributed along the nanotube

Fig 4 Electronic band structure of the relaxed 100-SWCNT con-taining two vacancies In all cases the Fermi level is set to zero Theframe shown in (a) corresponds to the case without nitrogen where theFermi energy has been shifted to the valence band exhibiting a very smallband gap (0012 eV) Frames (b) (c) and (d) correspond to two nitrogensystem whose structures are shown in Figure 3(c d and e) respectivelyFor V2N2AC the structure exhibits a small band gap of 0106 eV anda p-type characteristic For the last two cases (V2N2AD and V2N2ABconfigurations) similar band structures and band gaps are obtained whencompared to the pristine carbon tubule In all cases the wave functionswere plotted at gamma point with an isosurface value of plusmn005 Aringminus32

waist (see Fig 4(a)) The cases concerning the introduc-tion of two nitrogen atoms in non-equivalent sites areshown in Figures 4(b)ndash(d) For the system exhibiting aV2N2AC configuration the Fermi energy (located at zero)has been shifted to the valence band and they exhibit asmall Eg equal to 0106 eV (Fig 4(b)) The wave func-tion of V2N2AC configuration exhibits localized states onthe atoms surrounding the nitrogen atoms and the pen-tagonal ring (see Fig 4(b)) However for the V2N2ADand V2N2AB configurations the carbon nanotube exhibits

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

a semiconductor behavior with an electronic band gapof 0719 and 0685 eV respectively Both configurationsexhibit wave functions with states distributed on the entirenanotube lattice (see Fig 4(c and d))In order to understand the corrugation effect and the

diameter variation due to the presence of vacancies andnitrogen doping we analyzed five representative cases (seeFig 5) The nanotube diameters experience a diameterreduction at the vacancy followed by a slight increase nearto the defect thus resulting in corrugation We have alsoobserved that the diameter reduction significantly dependsof the position and concentration of the dopant atomsIn addition we studied two more configurations for

two vacancies with one and three nitrogen atoms (V2N1Cand V2N3ABD) the different site types are depicted inFigure 3(a) The relaxed structures and the band struc-ture calculations are shown in Figure 6 The systemcorresponding to the V2N1C exhibits 5-8-5 defects (seeFig 6(a)) similar to nanotubes with only two vacanciesand without nitrogen dopants (see configuration V2N0 inFig 3(b)) also exhibits a p-type behavior see Figure 6(b)The relaxed structure corresponding to V2N3ABD exhibitsonly one pentagonal ring which contains a nitrogen atomthe other two nitrogen atoms remain as pyridine-like sites(see Fig 6(c)) and the corresponding band structure isshown in Figure 6(d) Here a p-type behavior was alsoobserved Both configurations (V2N1C and V2N3ABD)exhibit a diameter variation of sim1 Aring near the defectiveregionAs we have witnessed from our results various

pyridine-like N-doped 100-SWCNTs configurationsexhibit a p-type doping behavior (electron acceptor) con-trary to our expectations in which the nitrogen-dopantwithin carbon nanotubes only causes n-type doping behav-ior (electron donor) We have observed a p-type behav-ior in the systems corresponding to the V1N1B V1N1CV1N2AC V1N3ABC V2N1C V2N3ABD and V2N4ABCDconfigurations Our results are in agreement with pre-vious reports indicating that pyridine-like configurationsexhibit p-type doping73335 In addition experimental

Fig 5 Molecular models showing the diameter variations generated by vacancies and nitrogen doping in a 100 carbon nanotube Side viewsimages reveal the diameter variation along the nanotube axis for different representative cases (a) Two vacancies without nitrogen (V2N0) (b) V1N1C(c) V1N1B (d) V1N2AC and (f) V2N2AD The nitrogen atoms are indicated by the open cycles In (b) the diameter is slightly varied but for (c)(d) and (f) it is observed a difference of sim08 Aring between the larger and smaller diameters these values are similar to the V2N0 case These changesin diameter result in corrugated surfaces characteristics of the nitrogen-doped nanotubes It is important to mention that the diameter of a pristine(undoped and without vacancies) 100 carbon nanotubes is around 799 Aring

electrical-transport measurements on metallic nitrogen-doped SWNTs have shown both p- and n-type dopingcharacteristics1636

Figure 7(a) shows the formation energy (calculated fromEq (1)) of all studied configurations and Figure 7(b)depicts the electronic band gaps (Eg) of the correspondingconfigurations In Figure 7(a) the energies were referredto the pristine nanotube (undoped and without vacancies)In Figure 7(a) the ldquolowastrdquo symbol is placed next to the struc-ture with defects that to the best of our knowledge arenovel and have not been reported hitherto It is interestingto observe that generally the role of nitrogen doping inthe carbon nanotubes with one or two vacancies reducesthe formation energy In most cases the undoped con-figurations V2N0 and V1N0 exhibit higher energy whencompared to the doped cases In Figure 7(a) we observethat next to the pristine tube the most stable configura-tion is the V0N1 system (substitutional doping) followedby the nanotube with four-nitrogen atoms placed in a di-vacancy (V2N4ABCD) A similar trend was reported by Liet al35 using density functional calculations and DMOLpackage In addition the authors investigated such defectsas a function of the nanotube chirality demonstrating thatthese defects are more stable in armchair nanotubes InFigure 7(a) several configurations were observed in therange of 3 to 5 eV here some structures experience self-surface reconstruction favoring the formation of pentago-nal and octagonal membered rings In this energy rangeit is also observed that nanotubes doped with two nitro-gen atoms exhibit a maximal electronic band gap (seeconfigurations V2N2AD V2N2AB V1N2BC in Fig 7(b))Notice that the canonical configuration (three nitrogenatoms occupying the pyridine-like sites V1N3ABC) oneof the most studied cases in the literature is the less sta-ble doped system Table I compares the formation ener-gies of five different defects obtained in this study withenergies reported in the literature showing that in mostcases our formation energies are in agreement with pre-vious calculations However little is known about chemi-cal reactivity of the configurations shown in Figure 7(a)

132 Mater Express Vol 1 2011

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 6 (a) Relaxed geometry obtained by introducing two vacancies and one nitrogen atom (configuration V2N1C) into the 100 carbon nanotubeNote that the final structure exhibits a 5-8-5 defect the corresponding band structure is shown in (b) (c) Relaxed structure obtained by introducingtwo vacancies and three nitrogen atoms (configuration V2N3ABD) into the 100 carbon nanotube This structure exhibits a pentagonal ring defectand the corresponding band structure calculations are shown (d) In both cases the Fermi level shifted to the valence band thus exhibiting a p-typedoping behavior and localized states at the Fermi level The open circles indicate the position of the nitrogen atoms

and therefore additional theoretical and experimental stud-ies are needed in order to understand the capacity ofnitrogen-doped carbon nanotubes for adsorbing differentmolecules and atoms Along this direction Zhao et al21

studied the hydrogen adsorption on calcium dispersed in

Fig 7 (a) Formation energy (see Eq (1)) of the 100 single walled carbon nanotubes (SWCNTs) by considering different ways of introducingnitrogen atoms and vacancies (all structures were relaxed) The circles indicate the nitrogen atoms and all energies are referred to the pristine 100nanotube (undoped and without vacancies) The ldquolowastrdquo symbol corresponds to new N-doped structures that to the best of our knowledge have not beenreported hiterto The most stable doped nanotube corresponds to the substitutional nitrogen doping V0N1 configuration followed by the V2N4ABCDconfiguration Notice that the less stable configuration correspond to the one-vacancy nanotube without nitrogen (V1N0 The corresponding electronicband gaps (Eg) are shown in (b) Notice that the modified 100-SWCNTs exhibits a reduced Eg which become metallic in some cases

the V2N4ABCD structure The authors suggested that upto five H2 molecules could be bound per calcium atomFinally as we have mentioned above by a selective

introduction of nitrogen atoms and vacancies to a 100carbon nanotube it is possible to tune its electronic

Mater Express Vol 1 2011 133

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

134 Mater Express Vol 1 2011

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

Mater Express Vol 1 2011 135

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IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

a semiconductor behavior with an electronic band gapof 0719 and 0685 eV respectively Both configurationsexhibit wave functions with states distributed on the entirenanotube lattice (see Fig 4(c and d))In order to understand the corrugation effect and the

diameter variation due to the presence of vacancies andnitrogen doping we analyzed five representative cases (seeFig 5) The nanotube diameters experience a diameterreduction at the vacancy followed by a slight increase nearto the defect thus resulting in corrugation We have alsoobserved that the diameter reduction significantly dependsof the position and concentration of the dopant atomsIn addition we studied two more configurations for

two vacancies with one and three nitrogen atoms (V2N1Cand V2N3ABD) the different site types are depicted inFigure 3(a) The relaxed structures and the band struc-ture calculations are shown in Figure 6 The systemcorresponding to the V2N1C exhibits 5-8-5 defects (seeFig 6(a)) similar to nanotubes with only two vacanciesand without nitrogen dopants (see configuration V2N0 inFig 3(b)) also exhibits a p-type behavior see Figure 6(b)The relaxed structure corresponding to V2N3ABD exhibitsonly one pentagonal ring which contains a nitrogen atomthe other two nitrogen atoms remain as pyridine-like sites(see Fig 6(c)) and the corresponding band structure isshown in Figure 6(d) Here a p-type behavior was alsoobserved Both configurations (V2N1C and V2N3ABD)exhibit a diameter variation of sim1 Aring near the defectiveregionAs we have witnessed from our results various

pyridine-like N-doped 100-SWCNTs configurationsexhibit a p-type doping behavior (electron acceptor) con-trary to our expectations in which the nitrogen-dopantwithin carbon nanotubes only causes n-type doping behav-ior (electron donor) We have observed a p-type behav-ior in the systems corresponding to the V1N1B V1N1CV1N2AC V1N3ABC V2N1C V2N3ABD and V2N4ABCDconfigurations Our results are in agreement with pre-vious reports indicating that pyridine-like configurationsexhibit p-type doping73335 In addition experimental

Fig 5 Molecular models showing the diameter variations generated by vacancies and nitrogen doping in a 100 carbon nanotube Side viewsimages reveal the diameter variation along the nanotube axis for different representative cases (a) Two vacancies without nitrogen (V2N0) (b) V1N1C(c) V1N1B (d) V1N2AC and (f) V2N2AD The nitrogen atoms are indicated by the open cycles In (b) the diameter is slightly varied but for (c)(d) and (f) it is observed a difference of sim08 Aring between the larger and smaller diameters these values are similar to the V2N0 case These changesin diameter result in corrugated surfaces characteristics of the nitrogen-doped nanotubes It is important to mention that the diameter of a pristine(undoped and without vacancies) 100 carbon nanotubes is around 799 Aring

electrical-transport measurements on metallic nitrogen-doped SWNTs have shown both p- and n-type dopingcharacteristics1636

Figure 7(a) shows the formation energy (calculated fromEq (1)) of all studied configurations and Figure 7(b)depicts the electronic band gaps (Eg) of the correspondingconfigurations In Figure 7(a) the energies were referredto the pristine nanotube (undoped and without vacancies)In Figure 7(a) the ldquolowastrdquo symbol is placed next to the struc-ture with defects that to the best of our knowledge arenovel and have not been reported hitherto It is interestingto observe that generally the role of nitrogen doping inthe carbon nanotubes with one or two vacancies reducesthe formation energy In most cases the undoped con-figurations V2N0 and V1N0 exhibit higher energy whencompared to the doped cases In Figure 7(a) we observethat next to the pristine tube the most stable configura-tion is the V0N1 system (substitutional doping) followedby the nanotube with four-nitrogen atoms placed in a di-vacancy (V2N4ABCD) A similar trend was reported by Liet al35 using density functional calculations and DMOLpackage In addition the authors investigated such defectsas a function of the nanotube chirality demonstrating thatthese defects are more stable in armchair nanotubes InFigure 7(a) several configurations were observed in therange of 3 to 5 eV here some structures experience self-surface reconstruction favoring the formation of pentago-nal and octagonal membered rings In this energy rangeit is also observed that nanotubes doped with two nitro-gen atoms exhibit a maximal electronic band gap (seeconfigurations V2N2AD V2N2AB V1N2BC in Fig 7(b))Notice that the canonical configuration (three nitrogenatoms occupying the pyridine-like sites V1N3ABC) oneof the most studied cases in the literature is the less sta-ble doped system Table I compares the formation ener-gies of five different defects obtained in this study withenergies reported in the literature showing that in mostcases our formation energies are in agreement with pre-vious calculations However little is known about chemi-cal reactivity of the configurations shown in Figure 7(a)

132 Mater Express Vol 1 2011

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 6 (a) Relaxed geometry obtained by introducing two vacancies and one nitrogen atom (configuration V2N1C) into the 100 carbon nanotubeNote that the final structure exhibits a 5-8-5 defect the corresponding band structure is shown in (b) (c) Relaxed structure obtained by introducingtwo vacancies and three nitrogen atoms (configuration V2N3ABD) into the 100 carbon nanotube This structure exhibits a pentagonal ring defectand the corresponding band structure calculations are shown (d) In both cases the Fermi level shifted to the valence band thus exhibiting a p-typedoping behavior and localized states at the Fermi level The open circles indicate the position of the nitrogen atoms

and therefore additional theoretical and experimental stud-ies are needed in order to understand the capacity ofnitrogen-doped carbon nanotubes for adsorbing differentmolecules and atoms Along this direction Zhao et al21

studied the hydrogen adsorption on calcium dispersed in

Fig 7 (a) Formation energy (see Eq (1)) of the 100 single walled carbon nanotubes (SWCNTs) by considering different ways of introducingnitrogen atoms and vacancies (all structures were relaxed) The circles indicate the nitrogen atoms and all energies are referred to the pristine 100nanotube (undoped and without vacancies) The ldquolowastrdquo symbol corresponds to new N-doped structures that to the best of our knowledge have not beenreported hiterto The most stable doped nanotube corresponds to the substitutional nitrogen doping V0N1 configuration followed by the V2N4ABCDconfiguration Notice that the less stable configuration correspond to the one-vacancy nanotube without nitrogen (V1N0 The corresponding electronicband gaps (Eg) are shown in (b) Notice that the modified 100-SWCNTs exhibits a reduced Eg which become metallic in some cases

the V2N4ABCD structure The authors suggested that upto five H2 molecules could be bound per calcium atomFinally as we have mentioned above by a selective

introduction of nitrogen atoms and vacancies to a 100carbon nanotube it is possible to tune its electronic

Mater Express Vol 1 2011 133

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IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

134 Mater Express Vol 1 2011

Delivered by Ingenta toUmea University Library

IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

Mater Express Vol 1 2011 135

Delivered by Ingenta toUmea University Library

IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

Fig 6 (a) Relaxed geometry obtained by introducing two vacancies and one nitrogen atom (configuration V2N1C) into the 100 carbon nanotubeNote that the final structure exhibits a 5-8-5 defect the corresponding band structure is shown in (b) (c) Relaxed structure obtained by introducingtwo vacancies and three nitrogen atoms (configuration V2N3ABD) into the 100 carbon nanotube This structure exhibits a pentagonal ring defectand the corresponding band structure calculations are shown (d) In both cases the Fermi level shifted to the valence band thus exhibiting a p-typedoping behavior and localized states at the Fermi level The open circles indicate the position of the nitrogen atoms

and therefore additional theoretical and experimental stud-ies are needed in order to understand the capacity ofnitrogen-doped carbon nanotubes for adsorbing differentmolecules and atoms Along this direction Zhao et al21

studied the hydrogen adsorption on calcium dispersed in

Fig 7 (a) Formation energy (see Eq (1)) of the 100 single walled carbon nanotubes (SWCNTs) by considering different ways of introducingnitrogen atoms and vacancies (all structures were relaxed) The circles indicate the nitrogen atoms and all energies are referred to the pristine 100nanotube (undoped and without vacancies) The ldquolowastrdquo symbol corresponds to new N-doped structures that to the best of our knowledge have not beenreported hiterto The most stable doped nanotube corresponds to the substitutional nitrogen doping V0N1 configuration followed by the V2N4ABCDconfiguration Notice that the less stable configuration correspond to the one-vacancy nanotube without nitrogen (V1N0 The corresponding electronicband gaps (Eg) are shown in (b) Notice that the modified 100-SWCNTs exhibits a reduced Eg which become metallic in some cases

the V2N4ABCD structure The authors suggested that upto five H2 molecules could be bound per calcium atomFinally as we have mentioned above by a selective

introduction of nitrogen atoms and vacancies to a 100carbon nanotube it is possible to tune its electronic

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Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

134 Mater Express Vol 1 2011

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IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

Mater Express Vol 1 2011 135

Delivered by Ingenta toUmea University Library

IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy Defects

Gracia-Espino et al

Article

Table I Comparison of the formation energy (Eform) of some defectsin the 100 semiconductor single walled carbon nanotube calculatedin our work (bold face values) and previous reported works from othergroups The different defects correspond to one vacancy (V1N0) diva-cancy (V2N0) substitutional nitrogen doping (V0N1) three nitrogenatoms in a pyridine-like island (V1N3ABC) and four nitrogen atoms anddouble vacancy (V2N4ABCD)

Type of defect Eform (eV)

V1N0 553 56237

V2N0 377 3934

V0N1 083 10435 09333 17810 4832

V1N3ABC 446 21617 6461040233 29935

V2N4ABCD 224 25835 28721

properties In this sense it is possible to build a pndashn junc-tion made of N-doped nanotubes Figure 8 depicts a pndashnjunction made entirely with one dopant within a 100carbon nanotube We have simultaneously introduced twodifferent types of defects that modify the electronic prop-erties of the 100 SWCNT First we added one nitrogenatom in a substitutional fashion (which creates a nega-tive doped semiconductor material) followed by the addi-tion of a V1N1B defect (which generates a positivelydoped material as is determined in Fig 2(a)) The result-ing geometry is shown in Figure 8(a) The band structure

Fig 8 (a) Relaxed geometry of the 100 carbon nanotube by simulta-neously introducing one nitrogen atom in a substitutional fashion (V0N1

and a V1N1B defect (such defects generate negatively and positivelydoped materials respectively the open circles illustrate the position ofthe nitrogen atoms) (b) Band structure and the corresponding HOMOand LUMO wave functions for the structure shown in (a) For the bandstructure the Fermi level is set to zero The tube exhibits semiconductorproperties with a band gap equal to 055 eV and formation energy of341 eV The wave functions are plotted at the gamma point (isosurfacevalue of plusmn005 Aringminus32) It is observed that the HOMO is located near theV1N1B defect whereas the LUMO is mainly situated at the substitutionalnitrogen atom

is depicted in Figure 8(b) in which the Fermi level is set tozero we observed that the resulting material shows semi-conductor properties with a band gap equal to 055 eVwhich is less than the band gap of the substitutional case(076 eV) The wave functions of the HOMO and LUMOare also shown in Figure 8(b) and it is observed thatthe HOMO is located near the V1N1B defect while theLUMO is mainly situated at the substitutional nitrogenatom This substitutional V1N1B defect exhibits a forma-tion energy of 341 eV which is higher than the formationenergy of the isolated substitutional nitrogen but lowerthan the isolated V1N1B case The formation energy andthe resulting electronic properties obtained for this specificcase (substititutional-V1N1B defect on the 100 carbonnanotube) open a theoretical and experimental challengeto tailor or improve the physico-chemical properties ofcarbon nanotubes by controlling and combining simulta-neously two or more different ways of nitrogen doping

4 CONCLUSIONS

The combined effect of nitrogen doping and vacancydefects were studied in semiconductor 100 SWCNTsusing first principle calculations For different cases therelative stability and the band structures were calculatedOur results demonstrated that the 100 semiconductornanotube could exhibit metallicity depending on the posi-tion of the nitrogen atoms along the nanotube structureWhen one vacancy and one nitrogen atom are introducedwithin the nanotube the surface remains open (vacancydoes not anneal out) and the bands cross the Fermi levelthus indicating metallicity It has also been observed thatone vacancy with two nitrogen atoms embedded symmet-rically exhibits a non dispersive conduction band whichresults in a LUMO-wave function with localized states inthe defective region In general when two vacancies wereintroduced the systems surface self-reconstructs thus pre-serving the semiconducting feature with a reduction of theelectronic band gap All energies associated with the dif-ferent systems are less stable than the pristine the 100nanotube However we found that pyridine-like dopingwith three double coordinated nitrogen atoms surroundinga vacancy (V1N3ABC) exhibits higher formation energieswhen compared to structures containing one two and fourpyridine-like nitrogen atoms

Acknowledgments The authors are grateful toK Goacutemez for technical assistance This work wassupported in part by CONACYT-Meacutexico grants 60218-F1 (FLU) and PhD Scholarship (EGE) MT thanksJST-Japan for funding the Research Center for ExoticNanoCarbons under the Japanese regional InnovationStrategy Program by the Excellence H Terrones acknowl-edges support as visiting professor at the CNMS atORNL

134 Mater Express Vol 1 2011

Delivered by Ingenta toUmea University Library

IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

Mater Express Vol 1 2011 135

Delivered by Ingenta toUmea University Library

IP 13023934175Thu 05 Jul 2012 070328

Materials ExpressDoping (10 0)-Semiconductor Nanotubes with Nitrogen and Vacancy DefectsGracia-Espino et al

Article

References and Notes

1 B G Sumpter V Meunier J-M Romo-Herrera E Cruz-SilvaD A Cullen H Terrones D J Smith and M Terrones ACS Nano1 369 (2007)

2 C P Ewels and M Glerup J Nanosci Nanotechnol 5 1345 (2005)3 E Cruz-Silva D A Cullen L Gu J M Romo-Herrera E Muntildeoz-

Sandoval F Loacutepez-Uriacuteas B G Sumpter V Meunier J C CharlierD J Smith H Terrones and M Terrones ACS Nano 2 441 (2008)

4 A Chen Q Y Shao and Z C Lin Sci China Ser G-Phys MechAstron 52 1139 (2009)

5 J M Romo-Herrera B G Sumpter D A Cullen H TerronesE Cruz-Silva D J Smith V Meunier and M Terrones AngewChem Int Ed 47 2948 (2008)

6 R Czerw M Terrones J C Charlier X Blase B FoleyR Kamalakaran N Grobert H Terrones D Tekleab P M AjayanW Blau M Ruumlhle and D L Carroll Nano Lett 1 457 (2001)

7 M Terrones A Jorio M Endo A M Rao Y A Kim T HayashiH Terrones J C Charlier G Dresselhaus and M S DresselhausMater Today 7 30 (2004)

8 M Terrones P M Ajayan F Banhart X Blase D L Carroll J CCharlier R Czerw B Foley N Grobert R Kamalakaran P Kohler-Redlich M Ruumlhle T Seeger and H Terrones Appl Phys A 74 355(2002)

9 K M Upadhyay J Appl Phys 105 024312 (2009)10 S H Lim R Li W Ji and J Lin Phys Rev B 76 195406 (2007)11 H S Kang and S Jeong Phys Rev B 70 233411 (2004)12 J Wei H F Hua H Zeng Z Zhou W Yang and P Peng Physica

E 40 462 (2008)13 J Wei H Hu Z Wang H Zeng Y Wei and J Jia Appl Phys

Lett 94 102108 (2009)14 C C Kaun B Larade H Mehrez J Taylor and H Guo Phys Rev

B 65 205416 (2002)15 Y Ma A S Foster A V Krasheninnikov and R M Nieminen

Phys Rev B 72 205416 (2005)16 Y S Min E J Bae U J Kim E H Lee N Park C S Hwang

W Park Appl Phys Lett 93 043113 (2008)

17 Y Fujimoto and S Saito Physica E 43 677 (2011)18 B G Sumpter J Huang V Meunier J M Romo-Herrera E Cruz-

Silva H Terrones and M Terrones Inter J Quantum Chem 109 97(2009)

19 J Akola and H Haumlkkinen Phys Rev B 74 165404 (2006)20 G Kim S H Jhi and N Park Appl Phys Lett 92 013106 (2008)21 J-X Zhao Y-H Ding X-G Wang Q-H Cai and X-Z Wang

Diamond amp Related Materials 20 36 (2011)22 I C Gerber P Puech A Gannouni and W Bacsa Phys Rev B

79 075423 (2009)23 P Hohenberg and W Kohn Phys Rev 136 B864 (1964)24 W Kohn and L J Sham Phys Rev 140 A1133 (1965)25 D M Ceperley and B J Alder Phys Rev Lett 45 566 (1980)26 J M Soler E Artacho J D Gale A Garciacutea J Junquera

P Ordejoacuten and D Saacutenchez-Portal J Phys Condens Matter14 2745 (2002)

27 J Junquera O Paz D Saacutenchez-Portal and E Artacho Phys RevB 64 235111 (2001)

28 N Troullier and J L Martins Phys Rev B 43 1993 (1991)29 L Kleinman and D M Bylander Phys Rev Lett 48 1425 (1982)30 J Kotakoski A V Krasheninnikov and K Nordlund Phys Rev B

74 245420 (2006)31 J E Padilla R G Amorim A R Rocha A J R da Silva and

A Fazzio Solid State Commun 151 482 (2011)32 S S Yu Q B Wen W T Zheng and Q Jiang Nanotechnology

18 165702 (2007)33 E Cruz-Silva F Loacutepez-Uriacuteas E Muntildeoz-Sandoval B G Sumpter

H Terrones J C Charlier V Meunier and M Terrones ACS Nano3 1913 (2009)

34 R G Amorim A Fazzio A Antonelli F D Novaes and A J Rda Silva Nano Lett 7 2459 (2007)

35 Y F Li Z Zhou and L B Wang J Chem Phys 129 104703(2008)

36 V Krstic G L J A Rikken P Bernier S Roth and M GlerupEurophys Lett 77 37001 (2007)

37 S Tang and Z Cao J Chem Phys 131 114706 (2009)

Received 28 February 2011 RevisedAccepted 1 May 2011

Mater Express Vol 1 2011 135