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Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
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Science of Advanced MaterialsVol 7 pp 789ndash793 2015(wwwaspbscomsam)
Piezoelectric Piezo-Phototronic and UV SensingProperties of Single Ultra Long NanobeltJawayria Mujtaba1 Umair Manzoor2lowast Sadaf Zia1 Muhammad Hafeez1 and Arshad Saleem Bhatti1
1Center for Micro and Nano Devices (CMND) Department of Physics COMSATS Institute of Information TechnologyIslamabad 44000 Pakistan2Alamoudi Water Chair King Saud University Riyadh 11451 Saudi Arabia
ABSTRACT
We present proof-of-concept of a potential new single ZnO nanobelt based sensor for converting piezoelectriceffect in to visible light Mechanical stress on ZnO ultra-long nanobelts was generated by bending at differentangles and systematic color change was observed IV curves suggested significant change in the electricalproperties with applied stress As-constructed single nanobelt UV sensor exhibit good sensitivity indicatingthat these structures can be a promising candidate for miniaturization of proposed device and UV irradiationmonitoring in many areas
KEYWORDS Sn Doping Nanostructures (ZnO) Piezoelectric Effect Optical and Electrical Properties
1 INTRODUCTIONZnO exhibits dual properties of semiconductor and piezo-electricity This amazing property of ZnO is because ofits crystal structure which has tetrahedrally bonded oxy-gen atoms and zinc atoms1 The coupling of electricaloptoelectronic and photochemical properties of ZnO givea unique combination for future applications in chemi-cal biology aerospace military and medical technologiesThe interest in this material is also fueled by its prospectsin optoelectronics applications2 ZnO nanostructures arevery promising candidates for piezoelectricity3 This sci-entific interest is mainly explained by the enhancement ofsome properties already present in the bulk material Asan example the piezoelectric coefficient d33 is higher inmono or bi-dimensional structures such as nanobelts thanin the bulk material4 Different research groups in theirindependent studies suggested change in electric proper-ties with applied stress in pure and doped ZnO5ndash9
Piezoelectricity is important for advance applicationsbut mechanical action however is not easy to be inter-faced directly with silicon technologies without innovativedesign and approaches The aim of this work is to providefor the first time to the best of our knowledge a com-prehensive experimental study on the change in color andelectrical properties of Sn-doped ZnO ultra-long nanobeltas a result of applied stress After an overview on themorphology structural and optical properties florescent
lowastAuthor to whom correspondence should be addressedEmail umanzoorksuedusaReceived 27 January 2014Accepted 19 March 2014
optical microscopy images IV characterization and UVsensing capability are reported
2 EXPERIMENTAL PROCEDUREZnO nanobelts were fabricated by simple vapor trans-port method Ball milled mixture of Zinc Oxide powder(999 Hayashi Pure Chemical Industries Osaka Japan)and Graphite in 11 weight ratio was used as catalystMilling and mixing of ZnO and C was done in planetryball mill (Fritsch Germany) for 25 hours at 200 rpm 02 gof SnO was added in 08 g of ZnO and C mixture (sourcematerial) for doping Alumina boat was loaded with sourcematerial and Si substrates were placed on top of the boatThe boat was then placed at the center of the tube furnaceinside a quartz tube (35 mm) The furnace temperaturewas set at 1030 C for 30 minutes and ramp rate was10 Cminute Ar gas mixed with a small amount of oxy-gen was used as a carrier gasScanning electron microscope (SEM Hitachi SU-1500)
was used for morphology and crystal structure of as-grownproducts were characterized by X-ray diffraction (XRDPANalytical X-pert Pro with Cu-K radiations) UV-VISspectroscope (Perkin Elmer Lambda 950) was used forspectroscopic studies and florescence images were takenusing optical microscope (Olympus BX61)2 mm steel wire was cut into 15 inch pieces and non-
conductive tape (sticky on both sides) was wrapped on thewire ZnO ultralong belts were shifted on a glass slide andthen wire with tape was gently pressed against the glassslide in such a way that both wire and the belts are parallelto each other The wire was then bent by applying force by
Sci Adv Mater 2015 Vol 7 No 4 1947-293520157789005 doi101166sam20151956 789
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt Mujtaba et alARTICLE
hand The same wire was bent at different angles to mea-sure optical and electrical properties at different angles
3 RESULTS AND DISCUSSIONWe used a mixture of ZnO and SnO powders in a weightratio of 41 as the source material to grow Sn-dopedZnO nanobelts It is known that SnO decompose intoSn and O2 at high temperature therefore no catalystwas used Figure 1(a) is SEM image of Sn-doped ZnONanobelts The image clearly suggests two distinct sizes ofNanobelts Majority had smaller dimensions with width of1238 mplusmn438 m and thickness 51912 nmplusmn206 nmLarger size belts are few in number but ultra-long with
Fig 1 (a) low magnification image of as-synthesized Sn-doped ZnONanobelts The image clearly suggests two distinct sizes of Nanobelts(b) SEM image of a complete nanobelt The inset SEM image is thecross sectional view conforming belt shape morphology (c) XRD datashows only ZnO peaks clearly indicting phase pure ZnO (d) SEM imageof ultralong ZnO nanobelt on a non-conducting tape
length more than 05 mm in most of the cases Onesuch as-synthesized belt is shown in Figure 1(b) Theinset clearly suggested belt shape rectangular morphologyFigure 1(c) is the XRD pattern clearly suggesting phasepure ZnO with high crystalinity EDX results are shownas inset in Figure 1(c) Zn O and Sn peaks are clearlyvisible suggesting that Sn is present in significant amountin the sample Figure 1(d) is SEM image showing ultra-long nanobelts isolated from the bulk by scratching fromthe substrate (using very sharp knife) and placing it on aglass piece with few drops of ethanol ZnO Nanobelts werethen sticked on non-conducting PVA tape wrapped arounda steel wire (2 mm) In short Figure 1 is a clear indi-cation that ultralong highly crystalline phase pure ZnOnanobelts can be synthesized and isolated on a noncon-ducting tapeFigure 2(a) shows the UV-Vis transmittance spectra
taken at room temperature The line is not very uniformindicating that light may be scattered but overall signal tonoise ratio is good The direct band emission (major peakin UV range) is wide It is expected that dopant atomsenter substitutionally into the zinc sites in the ZnO lat-tice so that they act as ionized donors (different oxidationstates)1 Therefore it is likely to be associated with oxy-gen deficiencies The donor electrons occupy the states atthe bottom of the conduction band (CB) and the bandgapwidensThe intensity of green emission (visible range) can be
enhanced by increasing the number of oxygen vacanciesand this was done by doping of Sn in ZnO nanowires2
Native or intrinsic defects (two minor peaks in the visiblerange) are due to imperfections in the crystal lattice thatinvolve the constituent elements Understanding the incor-poration and behavior of point defects in ZnO is essentialto its successful application in devices Defects are oftenelectrically active and introduce levels in the band gap ofthe semiconductor which involve transitions between dif-ferent charge states of the same defect3 and these transitionlevels are observable quantities that can be derived andestimatedFigure 2(b) shows the FTIR spectrum of Sn-doped
ZnO nanobelts Metal oxides generally give absorptionbands below 1000 cmminus1 arising from interatomic vibra-tions The bands around 418 cmminus1 and 572 cmminus1 corre-sponds to ZnndashO and SnndashO bonds respectively4 Wide bandat 3400 cmminus1 indicates the presence of water The bandsat 3400 cmminus1 and 1620 cmminus1 are due to adsorption ofhumidity5 One reasonable explanation and possibility ofthese peaks are because all the optical characterization wasperformed in ambient conditionsZnO is a Piezophototronic material The existence of
ldquopiezo-chargesrdquo at the interface introduce three possi-ble effects6 and one of the most important is the shiftin local electronic band structure due to the introducedlocal potential By carefully controlling the bending of the
790 Sci Adv Mater 7 789ndash793 2015
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
Fig 2 UV-Vis spectra (a) shows direct band emission in the UV range and 2 minor defect related peaks in the visible range (b) is the FTIR spectraof Sn-doped ZnO Nanobelts
nanobelt local applied pressure can be varied resulting in asystematic control of electronic band structure In our caseby carefully controlling the bending of nanobelt opticalemissions can be controlled in the visible range In thisreport ZnO was deliberately doped with SnO to observeoptical emissions in visible range When the nanobelt wasbended at different angles the local applied pressure wasdifferent subsequently effecting electronic band structurewhich result in different colors in the visible region7
Figures 3(i) and (ii) is the clear demonstration of thiseffect Selected part of the nanobelt (with significant colorchange upon bending) is shown Images (asimd) is bendingon one side and image (esimf) shows the same belt afterbending in the opposite direction Li and his group sug-gested a relationship between band gap and biaxial tensilestress8 as follows
Eg = 3171+0027xx
This again is an indication that bandgap of ZnO is relatedto the stress and changes with the applied stress The slightdifference in colors after bending in opposite directions(but similar angles) as shown in Figures 3(i)-(b) and (f) ismay be because the positive piezoelectric charges lowerthe energy band and the negative piezoelectric chargesraise the energy band in n-type semiconductors This effectcan also be related to crude experimental conditions butrepeated experiments show similar behavior and clearlyruled out the possibility of the experimental error It is alsointeresting to note that the results are reproducible with allSn-nanobelts
During the bending process the IndashV characteristicsof the belt were monitored Six typical bending curva-tures of the ZnO NW are shown in SEM images inFigures 3(a)ndash(f) and their corresponding IndashV curves arepresented in Figure 4 The symmetric shape of the IndashVcurves indicates good ohmic contacts at both ends ofthe belt The current drops significantly with increased
(i)
(ii)
Fig 3 (i) Optical micrograph florescent images (selected part) ofSn-doped ZnO Nanobelt after bending at different angles The colorchange is clearly visible with different bending angles (ii) Optical micro-graph full length florescent images of Sn-doped ZnO Nanobelt after bend-ing at different angles The color change is clearly visible with differentbending angles The schematics clearly suggesting the bending angles arealso shown against each bend
Sci Adv Mater 7 789ndash793 2015 791
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt Mujtaba et alARTICLE
Fig 4 IV curves of Sn-doped ZnO Nanobelt after bending at differ-ent angles The results clearly suggested increase in conductance withsubsequent bending
bending (Figs 4(bsime)) indicating decreased conductancewith increased strain The mechanism of the conductioncan be described by the following equation9
SnO2ZnO rarr SnprimeprimeZn+OxO+
12O2+2eprime
Sn+4 ions substituted Zn2+ ions in the lattice induce posi-tive charges in the material In order to maintain electricalneutrality two negative electrons are induced to compen-sate the excess positive charges When a small strain wasapplied on nanobelt by bending the steel wire the cur-rent increased further systematic increase was observedwith the subsequent increase in the bending angles Similarbehavior was observed when same nanobelt was bendedin the opposite direction In short we observed a bending-induced enhancement in conductance Bending can changethe band structure and this effect is also demonstrated inFigure 3 Bending induces outer tensile and inner compres-sive strains and thus the combination of this and its effectis very complex Han and co-workers discussed this issuein detail suggesting that tensile strain and shear strain tendto narrow the band gap since their states are much lowerthan the compressive states10 The free electron carrierswill be mainly distributed in the lower sub-bands and thusincreasing the conductivity The florescence microscopyresults (Fig 3) are in perfect agreement with each otherand the color change with subsequent bending was fromlower to higher wavelengths suggesting decrease in thebandgap However some of the research groups4 sug-gested bending-induced decrease in conductance (oppositeeffect)The low-conductance state can represent the off (ldquo0rdquo)
state and the high conductance the on (ldquo1rdquo) state There-fore modulation of the conductance in ZnO NWs viabending can be used as a nanoscale switch or otherelectromechanical device in nanoelectromechanical sys-tems (NEMS) However this is relatively a new fieldand more experiments and indepth theoretical calcu-lations are required to reveal the underlying complexmechanisms
Fig 5 UV sensing result of a single belt The first cycle was with lowUV intensity and intensity was doubled for next 4 cycles The resultsclearly suggest that Sn-doped ZnO Nanobelt can be used as an effectiveUV sensor
The photosensing properties were characterized at roomtemperature in air When Sn-doped ZnO nanobelt wasexposed to UV electrons transition takes place from thevalence band to the conduction band giving rise to a cur-rent flow and decrease in the resistance Figure 5 showsUV sensing results of a single belt In the 1st cycle UVintensity was low and it was doubled for the next 4 cyclesHowever overall sensitivity was low and the response timewas also slow Different researchers explained the decayprocess by different mechanisms For example Lupanet al reported that the decay process can be attributedto the recombination of electronndashhole pairs and the read-sorption of oxygen molecules on the surface11 He et alsuggested that the decay is mainly determined by the elec-tron free-carrier recombination and molecules adsorptionon the surface12 Cheng et al reported that when the UVillumination was removed the photocurrent fell from themaximum fast at the first stage which indicates the rapidloss of electrons in the conduction band via retrapping theelectrons Then the slow falling of the current to the ini-tial value can be attributed to the reabsorbed oxygen onthe ZnO which decreases its conductivity13 However thedecay mechanism needs further discussion
4 CONCLUSIONIn summary a possible concept that piezoelectric effect inSn-doped ZnO ultra-long nanobelts can be measured bychange in color was experimentally demonstrated Therewas a systematic change in color and electrical propertieswith change in bending angle One reasonable explanationis that because of the applied stress there is a modificationin the local band structure This change in the bandgap is
792 Sci Adv Mater 7 789ndash793 2015
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
responsible not only for the change in electrical propertiesbut also change in the color Sn was doped in ZnO to cre-ate defects which are responsible of emitting radiations inthe visible range under florescence microscope XRD andSEM confirmed phase pure ZnO and belt shape morphol-ogy respectively The as-constructed single nanobelt sensorexhibit good sensitivity towards UV irradiation indicat-ing that these sensors can be a promising candidate forminiaturization of device and UV irradiation monitoringin advance applications
Acknowledgment This project was supported byNSTIP strategic technologies program number (12-WAT-2451-02) in the Kingdom of Saudi Arabia
References and Notes1 U Ozgur Y I Alivov C Liu A Teke M A Reshchikov S Dogan
V Avrutin S-J Cho and H Morkoc J Appl Phys 98 1 (2005)
2 U Manzoor and D K Kim Scripta Mater 54 807 (2006)3 S Xu and Z Wang Nano Research 4 1013 (2011)4 X Xudong J Zhou H Jin J Liu N Xu and Z L Wang Nano
Lett 6 2768 (2006)5 K-H Kim B Kumar K Y Lee H-K Park J-H Lee H H Lee
H Jun D Lee and S-W Kim Nature 3 2017 (2012)6 B Kumar and S W Kim Nano Energy 1 342 (2012)7 M Y Choi D Choi M J Jin I Kim S H Kim J Y Choi S Y
Lee J M Kim and S W Kim Adv Mater 21 2185 (2009)8 A Onodera N Tamaki Y Kawamura T Sawada and H Yamashita
Jpn J Appl Phys 35 5160 (1996)9 J Wang W Chen and M Wang J Alloys Compd 449 44
(2008)10 B E Sernelius K F Berggren Z C Jin I Hamberg and C G
Granqvist Phys Rev 37 10244 (1988)11 S Zia M Amin U Manzoor and A S Bhatti Applied Physics A
Materials Science and Processing 115 275 (2013)12 Electronic Properties of Defects In Fundamentals of Semiconduc-
tors Springer Berlin Heidelberg (2001) pp 159ndash20213 F Gu S F Wang C F Song M K Luuml Y X Qi G J Zhou
D Xu and D R Yuan Chem Phys Lett 372 451 (2003)
Sci Adv Mater 7 789ndash793 2015 793
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt Mujtaba et alARTICLE
hand The same wire was bent at different angles to mea-sure optical and electrical properties at different angles
3 RESULTS AND DISCUSSIONWe used a mixture of ZnO and SnO powders in a weightratio of 41 as the source material to grow Sn-dopedZnO nanobelts It is known that SnO decompose intoSn and O2 at high temperature therefore no catalystwas used Figure 1(a) is SEM image of Sn-doped ZnONanobelts The image clearly suggests two distinct sizes ofNanobelts Majority had smaller dimensions with width of1238 mplusmn438 m and thickness 51912 nmplusmn206 nmLarger size belts are few in number but ultra-long with
Fig 1 (a) low magnification image of as-synthesized Sn-doped ZnONanobelts The image clearly suggests two distinct sizes of Nanobelts(b) SEM image of a complete nanobelt The inset SEM image is thecross sectional view conforming belt shape morphology (c) XRD datashows only ZnO peaks clearly indicting phase pure ZnO (d) SEM imageof ultralong ZnO nanobelt on a non-conducting tape
length more than 05 mm in most of the cases Onesuch as-synthesized belt is shown in Figure 1(b) Theinset clearly suggested belt shape rectangular morphologyFigure 1(c) is the XRD pattern clearly suggesting phasepure ZnO with high crystalinity EDX results are shownas inset in Figure 1(c) Zn O and Sn peaks are clearlyvisible suggesting that Sn is present in significant amountin the sample Figure 1(d) is SEM image showing ultra-long nanobelts isolated from the bulk by scratching fromthe substrate (using very sharp knife) and placing it on aglass piece with few drops of ethanol ZnO Nanobelts werethen sticked on non-conducting PVA tape wrapped arounda steel wire (2 mm) In short Figure 1 is a clear indi-cation that ultralong highly crystalline phase pure ZnOnanobelts can be synthesized and isolated on a noncon-ducting tapeFigure 2(a) shows the UV-Vis transmittance spectra
taken at room temperature The line is not very uniformindicating that light may be scattered but overall signal tonoise ratio is good The direct band emission (major peakin UV range) is wide It is expected that dopant atomsenter substitutionally into the zinc sites in the ZnO lat-tice so that they act as ionized donors (different oxidationstates)1 Therefore it is likely to be associated with oxy-gen deficiencies The donor electrons occupy the states atthe bottom of the conduction band (CB) and the bandgapwidensThe intensity of green emission (visible range) can be
enhanced by increasing the number of oxygen vacanciesand this was done by doping of Sn in ZnO nanowires2
Native or intrinsic defects (two minor peaks in the visiblerange) are due to imperfections in the crystal lattice thatinvolve the constituent elements Understanding the incor-poration and behavior of point defects in ZnO is essentialto its successful application in devices Defects are oftenelectrically active and introduce levels in the band gap ofthe semiconductor which involve transitions between dif-ferent charge states of the same defect3 and these transitionlevels are observable quantities that can be derived andestimatedFigure 2(b) shows the FTIR spectrum of Sn-doped
ZnO nanobelts Metal oxides generally give absorptionbands below 1000 cmminus1 arising from interatomic vibra-tions The bands around 418 cmminus1 and 572 cmminus1 corre-sponds to ZnndashO and SnndashO bonds respectively4 Wide bandat 3400 cmminus1 indicates the presence of water The bandsat 3400 cmminus1 and 1620 cmminus1 are due to adsorption ofhumidity5 One reasonable explanation and possibility ofthese peaks are because all the optical characterization wasperformed in ambient conditionsZnO is a Piezophototronic material The existence of
ldquopiezo-chargesrdquo at the interface introduce three possi-ble effects6 and one of the most important is the shiftin local electronic band structure due to the introducedlocal potential By carefully controlling the bending of the
790 Sci Adv Mater 7 789ndash793 2015
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
Fig 2 UV-Vis spectra (a) shows direct band emission in the UV range and 2 minor defect related peaks in the visible range (b) is the FTIR spectraof Sn-doped ZnO Nanobelts
nanobelt local applied pressure can be varied resulting in asystematic control of electronic band structure In our caseby carefully controlling the bending of nanobelt opticalemissions can be controlled in the visible range In thisreport ZnO was deliberately doped with SnO to observeoptical emissions in visible range When the nanobelt wasbended at different angles the local applied pressure wasdifferent subsequently effecting electronic band structurewhich result in different colors in the visible region7
Figures 3(i) and (ii) is the clear demonstration of thiseffect Selected part of the nanobelt (with significant colorchange upon bending) is shown Images (asimd) is bendingon one side and image (esimf) shows the same belt afterbending in the opposite direction Li and his group sug-gested a relationship between band gap and biaxial tensilestress8 as follows
Eg = 3171+0027xx
This again is an indication that bandgap of ZnO is relatedto the stress and changes with the applied stress The slightdifference in colors after bending in opposite directions(but similar angles) as shown in Figures 3(i)-(b) and (f) ismay be because the positive piezoelectric charges lowerthe energy band and the negative piezoelectric chargesraise the energy band in n-type semiconductors This effectcan also be related to crude experimental conditions butrepeated experiments show similar behavior and clearlyruled out the possibility of the experimental error It is alsointeresting to note that the results are reproducible with allSn-nanobelts
During the bending process the IndashV characteristicsof the belt were monitored Six typical bending curva-tures of the ZnO NW are shown in SEM images inFigures 3(a)ndash(f) and their corresponding IndashV curves arepresented in Figure 4 The symmetric shape of the IndashVcurves indicates good ohmic contacts at both ends ofthe belt The current drops significantly with increased
(i)
(ii)
Fig 3 (i) Optical micrograph florescent images (selected part) ofSn-doped ZnO Nanobelt after bending at different angles The colorchange is clearly visible with different bending angles (ii) Optical micro-graph full length florescent images of Sn-doped ZnO Nanobelt after bend-ing at different angles The color change is clearly visible with differentbending angles The schematics clearly suggesting the bending angles arealso shown against each bend
Sci Adv Mater 7 789ndash793 2015 791
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt Mujtaba et alARTICLE
Fig 4 IV curves of Sn-doped ZnO Nanobelt after bending at differ-ent angles The results clearly suggested increase in conductance withsubsequent bending
bending (Figs 4(bsime)) indicating decreased conductancewith increased strain The mechanism of the conductioncan be described by the following equation9
SnO2ZnO rarr SnprimeprimeZn+OxO+
12O2+2eprime
Sn+4 ions substituted Zn2+ ions in the lattice induce posi-tive charges in the material In order to maintain electricalneutrality two negative electrons are induced to compen-sate the excess positive charges When a small strain wasapplied on nanobelt by bending the steel wire the cur-rent increased further systematic increase was observedwith the subsequent increase in the bending angles Similarbehavior was observed when same nanobelt was bendedin the opposite direction In short we observed a bending-induced enhancement in conductance Bending can changethe band structure and this effect is also demonstrated inFigure 3 Bending induces outer tensile and inner compres-sive strains and thus the combination of this and its effectis very complex Han and co-workers discussed this issuein detail suggesting that tensile strain and shear strain tendto narrow the band gap since their states are much lowerthan the compressive states10 The free electron carrierswill be mainly distributed in the lower sub-bands and thusincreasing the conductivity The florescence microscopyresults (Fig 3) are in perfect agreement with each otherand the color change with subsequent bending was fromlower to higher wavelengths suggesting decrease in thebandgap However some of the research groups4 sug-gested bending-induced decrease in conductance (oppositeeffect)The low-conductance state can represent the off (ldquo0rdquo)
state and the high conductance the on (ldquo1rdquo) state There-fore modulation of the conductance in ZnO NWs viabending can be used as a nanoscale switch or otherelectromechanical device in nanoelectromechanical sys-tems (NEMS) However this is relatively a new fieldand more experiments and indepth theoretical calcu-lations are required to reveal the underlying complexmechanisms
Fig 5 UV sensing result of a single belt The first cycle was with lowUV intensity and intensity was doubled for next 4 cycles The resultsclearly suggest that Sn-doped ZnO Nanobelt can be used as an effectiveUV sensor
The photosensing properties were characterized at roomtemperature in air When Sn-doped ZnO nanobelt wasexposed to UV electrons transition takes place from thevalence band to the conduction band giving rise to a cur-rent flow and decrease in the resistance Figure 5 showsUV sensing results of a single belt In the 1st cycle UVintensity was low and it was doubled for the next 4 cyclesHowever overall sensitivity was low and the response timewas also slow Different researchers explained the decayprocess by different mechanisms For example Lupanet al reported that the decay process can be attributedto the recombination of electronndashhole pairs and the read-sorption of oxygen molecules on the surface11 He et alsuggested that the decay is mainly determined by the elec-tron free-carrier recombination and molecules adsorptionon the surface12 Cheng et al reported that when the UVillumination was removed the photocurrent fell from themaximum fast at the first stage which indicates the rapidloss of electrons in the conduction band via retrapping theelectrons Then the slow falling of the current to the ini-tial value can be attributed to the reabsorbed oxygen onthe ZnO which decreases its conductivity13 However thedecay mechanism needs further discussion
4 CONCLUSIONIn summary a possible concept that piezoelectric effect inSn-doped ZnO ultra-long nanobelts can be measured bychange in color was experimentally demonstrated Therewas a systematic change in color and electrical propertieswith change in bending angle One reasonable explanationis that because of the applied stress there is a modificationin the local band structure This change in the bandgap is
792 Sci Adv Mater 7 789ndash793 2015
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
responsible not only for the change in electrical propertiesbut also change in the color Sn was doped in ZnO to cre-ate defects which are responsible of emitting radiations inthe visible range under florescence microscope XRD andSEM confirmed phase pure ZnO and belt shape morphol-ogy respectively The as-constructed single nanobelt sensorexhibit good sensitivity towards UV irradiation indicat-ing that these sensors can be a promising candidate forminiaturization of device and UV irradiation monitoringin advance applications
Acknowledgment This project was supported byNSTIP strategic technologies program number (12-WAT-2451-02) in the Kingdom of Saudi Arabia
References and Notes1 U Ozgur Y I Alivov C Liu A Teke M A Reshchikov S Dogan
V Avrutin S-J Cho and H Morkoc J Appl Phys 98 1 (2005)
2 U Manzoor and D K Kim Scripta Mater 54 807 (2006)3 S Xu and Z Wang Nano Research 4 1013 (2011)4 X Xudong J Zhou H Jin J Liu N Xu and Z L Wang Nano
Lett 6 2768 (2006)5 K-H Kim B Kumar K Y Lee H-K Park J-H Lee H H Lee
H Jun D Lee and S-W Kim Nature 3 2017 (2012)6 B Kumar and S W Kim Nano Energy 1 342 (2012)7 M Y Choi D Choi M J Jin I Kim S H Kim J Y Choi S Y
Lee J M Kim and S W Kim Adv Mater 21 2185 (2009)8 A Onodera N Tamaki Y Kawamura T Sawada and H Yamashita
Jpn J Appl Phys 35 5160 (1996)9 J Wang W Chen and M Wang J Alloys Compd 449 44
(2008)10 B E Sernelius K F Berggren Z C Jin I Hamberg and C G
Granqvist Phys Rev 37 10244 (1988)11 S Zia M Amin U Manzoor and A S Bhatti Applied Physics A
Materials Science and Processing 115 275 (2013)12 Electronic Properties of Defects In Fundamentals of Semiconduc-
tors Springer Berlin Heidelberg (2001) pp 159ndash20213 F Gu S F Wang C F Song M K Luuml Y X Qi G J Zhou
D Xu and D R Yuan Chem Phys Lett 372 451 (2003)
Sci Adv Mater 7 789ndash793 2015 793
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
Fig 2 UV-Vis spectra (a) shows direct band emission in the UV range and 2 minor defect related peaks in the visible range (b) is the FTIR spectraof Sn-doped ZnO Nanobelts
nanobelt local applied pressure can be varied resulting in asystematic control of electronic band structure In our caseby carefully controlling the bending of nanobelt opticalemissions can be controlled in the visible range In thisreport ZnO was deliberately doped with SnO to observeoptical emissions in visible range When the nanobelt wasbended at different angles the local applied pressure wasdifferent subsequently effecting electronic band structurewhich result in different colors in the visible region7
Figures 3(i) and (ii) is the clear demonstration of thiseffect Selected part of the nanobelt (with significant colorchange upon bending) is shown Images (asimd) is bendingon one side and image (esimf) shows the same belt afterbending in the opposite direction Li and his group sug-gested a relationship between band gap and biaxial tensilestress8 as follows
Eg = 3171+0027xx
This again is an indication that bandgap of ZnO is relatedto the stress and changes with the applied stress The slightdifference in colors after bending in opposite directions(but similar angles) as shown in Figures 3(i)-(b) and (f) ismay be because the positive piezoelectric charges lowerthe energy band and the negative piezoelectric chargesraise the energy band in n-type semiconductors This effectcan also be related to crude experimental conditions butrepeated experiments show similar behavior and clearlyruled out the possibility of the experimental error It is alsointeresting to note that the results are reproducible with allSn-nanobelts
During the bending process the IndashV characteristicsof the belt were monitored Six typical bending curva-tures of the ZnO NW are shown in SEM images inFigures 3(a)ndash(f) and their corresponding IndashV curves arepresented in Figure 4 The symmetric shape of the IndashVcurves indicates good ohmic contacts at both ends ofthe belt The current drops significantly with increased
(i)
(ii)
Fig 3 (i) Optical micrograph florescent images (selected part) ofSn-doped ZnO Nanobelt after bending at different angles The colorchange is clearly visible with different bending angles (ii) Optical micro-graph full length florescent images of Sn-doped ZnO Nanobelt after bend-ing at different angles The color change is clearly visible with differentbending angles The schematics clearly suggesting the bending angles arealso shown against each bend
Sci Adv Mater 7 789ndash793 2015 791
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt Mujtaba et alARTICLE
Fig 4 IV curves of Sn-doped ZnO Nanobelt after bending at differ-ent angles The results clearly suggested increase in conductance withsubsequent bending
bending (Figs 4(bsime)) indicating decreased conductancewith increased strain The mechanism of the conductioncan be described by the following equation9
SnO2ZnO rarr SnprimeprimeZn+OxO+
12O2+2eprime
Sn+4 ions substituted Zn2+ ions in the lattice induce posi-tive charges in the material In order to maintain electricalneutrality two negative electrons are induced to compen-sate the excess positive charges When a small strain wasapplied on nanobelt by bending the steel wire the cur-rent increased further systematic increase was observedwith the subsequent increase in the bending angles Similarbehavior was observed when same nanobelt was bendedin the opposite direction In short we observed a bending-induced enhancement in conductance Bending can changethe band structure and this effect is also demonstrated inFigure 3 Bending induces outer tensile and inner compres-sive strains and thus the combination of this and its effectis very complex Han and co-workers discussed this issuein detail suggesting that tensile strain and shear strain tendto narrow the band gap since their states are much lowerthan the compressive states10 The free electron carrierswill be mainly distributed in the lower sub-bands and thusincreasing the conductivity The florescence microscopyresults (Fig 3) are in perfect agreement with each otherand the color change with subsequent bending was fromlower to higher wavelengths suggesting decrease in thebandgap However some of the research groups4 sug-gested bending-induced decrease in conductance (oppositeeffect)The low-conductance state can represent the off (ldquo0rdquo)
state and the high conductance the on (ldquo1rdquo) state There-fore modulation of the conductance in ZnO NWs viabending can be used as a nanoscale switch or otherelectromechanical device in nanoelectromechanical sys-tems (NEMS) However this is relatively a new fieldand more experiments and indepth theoretical calcu-lations are required to reveal the underlying complexmechanisms
Fig 5 UV sensing result of a single belt The first cycle was with lowUV intensity and intensity was doubled for next 4 cycles The resultsclearly suggest that Sn-doped ZnO Nanobelt can be used as an effectiveUV sensor
The photosensing properties were characterized at roomtemperature in air When Sn-doped ZnO nanobelt wasexposed to UV electrons transition takes place from thevalence band to the conduction band giving rise to a cur-rent flow and decrease in the resistance Figure 5 showsUV sensing results of a single belt In the 1st cycle UVintensity was low and it was doubled for the next 4 cyclesHowever overall sensitivity was low and the response timewas also slow Different researchers explained the decayprocess by different mechanisms For example Lupanet al reported that the decay process can be attributedto the recombination of electronndashhole pairs and the read-sorption of oxygen molecules on the surface11 He et alsuggested that the decay is mainly determined by the elec-tron free-carrier recombination and molecules adsorptionon the surface12 Cheng et al reported that when the UVillumination was removed the photocurrent fell from themaximum fast at the first stage which indicates the rapidloss of electrons in the conduction band via retrapping theelectrons Then the slow falling of the current to the ini-tial value can be attributed to the reabsorbed oxygen onthe ZnO which decreases its conductivity13 However thedecay mechanism needs further discussion
4 CONCLUSIONIn summary a possible concept that piezoelectric effect inSn-doped ZnO ultra-long nanobelts can be measured bychange in color was experimentally demonstrated Therewas a systematic change in color and electrical propertieswith change in bending angle One reasonable explanationis that because of the applied stress there is a modificationin the local band structure This change in the bandgap is
792 Sci Adv Mater 7 789ndash793 2015
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
responsible not only for the change in electrical propertiesbut also change in the color Sn was doped in ZnO to cre-ate defects which are responsible of emitting radiations inthe visible range under florescence microscope XRD andSEM confirmed phase pure ZnO and belt shape morphol-ogy respectively The as-constructed single nanobelt sensorexhibit good sensitivity towards UV irradiation indicat-ing that these sensors can be a promising candidate forminiaturization of device and UV irradiation monitoringin advance applications
Acknowledgment This project was supported byNSTIP strategic technologies program number (12-WAT-2451-02) in the Kingdom of Saudi Arabia
References and Notes1 U Ozgur Y I Alivov C Liu A Teke M A Reshchikov S Dogan
V Avrutin S-J Cho and H Morkoc J Appl Phys 98 1 (2005)
2 U Manzoor and D K Kim Scripta Mater 54 807 (2006)3 S Xu and Z Wang Nano Research 4 1013 (2011)4 X Xudong J Zhou H Jin J Liu N Xu and Z L Wang Nano
Lett 6 2768 (2006)5 K-H Kim B Kumar K Y Lee H-K Park J-H Lee H H Lee
H Jun D Lee and S-W Kim Nature 3 2017 (2012)6 B Kumar and S W Kim Nano Energy 1 342 (2012)7 M Y Choi D Choi M J Jin I Kim S H Kim J Y Choi S Y
Lee J M Kim and S W Kim Adv Mater 21 2185 (2009)8 A Onodera N Tamaki Y Kawamura T Sawada and H Yamashita
Jpn J Appl Phys 35 5160 (1996)9 J Wang W Chen and M Wang J Alloys Compd 449 44
(2008)10 B E Sernelius K F Berggren Z C Jin I Hamberg and C G
Granqvist Phys Rev 37 10244 (1988)11 S Zia M Amin U Manzoor and A S Bhatti Applied Physics A
Materials Science and Processing 115 275 (2013)12 Electronic Properties of Defects In Fundamentals of Semiconduc-
tors Springer Berlin Heidelberg (2001) pp 159ndash20213 F Gu S F Wang C F Song M K Luuml Y X Qi G J Zhou
D Xu and D R Yuan Chem Phys Lett 372 451 (2003)
Sci Adv Mater 7 789ndash793 2015 793
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt Mujtaba et alARTICLE
Fig 4 IV curves of Sn-doped ZnO Nanobelt after bending at differ-ent angles The results clearly suggested increase in conductance withsubsequent bending
bending (Figs 4(bsime)) indicating decreased conductancewith increased strain The mechanism of the conductioncan be described by the following equation9
SnO2ZnO rarr SnprimeprimeZn+OxO+
12O2+2eprime
Sn+4 ions substituted Zn2+ ions in the lattice induce posi-tive charges in the material In order to maintain electricalneutrality two negative electrons are induced to compen-sate the excess positive charges When a small strain wasapplied on nanobelt by bending the steel wire the cur-rent increased further systematic increase was observedwith the subsequent increase in the bending angles Similarbehavior was observed when same nanobelt was bendedin the opposite direction In short we observed a bending-induced enhancement in conductance Bending can changethe band structure and this effect is also demonstrated inFigure 3 Bending induces outer tensile and inner compres-sive strains and thus the combination of this and its effectis very complex Han and co-workers discussed this issuein detail suggesting that tensile strain and shear strain tendto narrow the band gap since their states are much lowerthan the compressive states10 The free electron carrierswill be mainly distributed in the lower sub-bands and thusincreasing the conductivity The florescence microscopyresults (Fig 3) are in perfect agreement with each otherand the color change with subsequent bending was fromlower to higher wavelengths suggesting decrease in thebandgap However some of the research groups4 sug-gested bending-induced decrease in conductance (oppositeeffect)The low-conductance state can represent the off (ldquo0rdquo)
state and the high conductance the on (ldquo1rdquo) state There-fore modulation of the conductance in ZnO NWs viabending can be used as a nanoscale switch or otherelectromechanical device in nanoelectromechanical sys-tems (NEMS) However this is relatively a new fieldand more experiments and indepth theoretical calcu-lations are required to reveal the underlying complexmechanisms
Fig 5 UV sensing result of a single belt The first cycle was with lowUV intensity and intensity was doubled for next 4 cycles The resultsclearly suggest that Sn-doped ZnO Nanobelt can be used as an effectiveUV sensor
The photosensing properties were characterized at roomtemperature in air When Sn-doped ZnO nanobelt wasexposed to UV electrons transition takes place from thevalence band to the conduction band giving rise to a cur-rent flow and decrease in the resistance Figure 5 showsUV sensing results of a single belt In the 1st cycle UVintensity was low and it was doubled for the next 4 cyclesHowever overall sensitivity was low and the response timewas also slow Different researchers explained the decayprocess by different mechanisms For example Lupanet al reported that the decay process can be attributedto the recombination of electronndashhole pairs and the read-sorption of oxygen molecules on the surface11 He et alsuggested that the decay is mainly determined by the elec-tron free-carrier recombination and molecules adsorptionon the surface12 Cheng et al reported that when the UVillumination was removed the photocurrent fell from themaximum fast at the first stage which indicates the rapidloss of electrons in the conduction band via retrapping theelectrons Then the slow falling of the current to the ini-tial value can be attributed to the reabsorbed oxygen onthe ZnO which decreases its conductivity13 However thedecay mechanism needs further discussion
4 CONCLUSIONIn summary a possible concept that piezoelectric effect inSn-doped ZnO ultra-long nanobelts can be measured bychange in color was experimentally demonstrated Therewas a systematic change in color and electrical propertieswith change in bending angle One reasonable explanationis that because of the applied stress there is a modificationin the local band structure This change in the bandgap is
792 Sci Adv Mater 7 789ndash793 2015
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
responsible not only for the change in electrical propertiesbut also change in the color Sn was doped in ZnO to cre-ate defects which are responsible of emitting radiations inthe visible range under florescence microscope XRD andSEM confirmed phase pure ZnO and belt shape morphol-ogy respectively The as-constructed single nanobelt sensorexhibit good sensitivity towards UV irradiation indicat-ing that these sensors can be a promising candidate forminiaturization of device and UV irradiation monitoringin advance applications
Acknowledgment This project was supported byNSTIP strategic technologies program number (12-WAT-2451-02) in the Kingdom of Saudi Arabia
References and Notes1 U Ozgur Y I Alivov C Liu A Teke M A Reshchikov S Dogan
V Avrutin S-J Cho and H Morkoc J Appl Phys 98 1 (2005)
2 U Manzoor and D K Kim Scripta Mater 54 807 (2006)3 S Xu and Z Wang Nano Research 4 1013 (2011)4 X Xudong J Zhou H Jin J Liu N Xu and Z L Wang Nano
Lett 6 2768 (2006)5 K-H Kim B Kumar K Y Lee H-K Park J-H Lee H H Lee
H Jun D Lee and S-W Kim Nature 3 2017 (2012)6 B Kumar and S W Kim Nano Energy 1 342 (2012)7 M Y Choi D Choi M J Jin I Kim S H Kim J Y Choi S Y
Lee J M Kim and S W Kim Adv Mater 21 2185 (2009)8 A Onodera N Tamaki Y Kawamura T Sawada and H Yamashita
Jpn J Appl Phys 35 5160 (1996)9 J Wang W Chen and M Wang J Alloys Compd 449 44
(2008)10 B E Sernelius K F Berggren Z C Jin I Hamberg and C G
Granqvist Phys Rev 37 10244 (1988)11 S Zia M Amin U Manzoor and A S Bhatti Applied Physics A
Materials Science and Processing 115 275 (2013)12 Electronic Properties of Defects In Fundamentals of Semiconduc-
tors Springer Berlin Heidelberg (2001) pp 159ndash20213 F Gu S F Wang C F Song M K Luuml Y X Qi G J Zhou
D Xu and D R Yuan Chem Phys Lett 372 451 (2003)
Sci Adv Mater 7 789ndash793 2015 793
Delivered by Publishing Technology to Umair ManzoorIP 21257215203 On Tue 17 Mar 2015 070804
Copyright American Scientific Publishers
Mujtaba et al Piezoelectric Piezo-Phototronic and UV Sensing Properties of Single Ultra Long Nanobelt
ARTICLE
responsible not only for the change in electrical propertiesbut also change in the color Sn was doped in ZnO to cre-ate defects which are responsible of emitting radiations inthe visible range under florescence microscope XRD andSEM confirmed phase pure ZnO and belt shape morphol-ogy respectively The as-constructed single nanobelt sensorexhibit good sensitivity towards UV irradiation indicat-ing that these sensors can be a promising candidate forminiaturization of device and UV irradiation monitoringin advance applications
Acknowledgment This project was supported byNSTIP strategic technologies program number (12-WAT-2451-02) in the Kingdom of Saudi Arabia
References and Notes1 U Ozgur Y I Alivov C Liu A Teke M A Reshchikov S Dogan
V Avrutin S-J Cho and H Morkoc J Appl Phys 98 1 (2005)
2 U Manzoor and D K Kim Scripta Mater 54 807 (2006)3 S Xu and Z Wang Nano Research 4 1013 (2011)4 X Xudong J Zhou H Jin J Liu N Xu and Z L Wang Nano
Lett 6 2768 (2006)5 K-H Kim B Kumar K Y Lee H-K Park J-H Lee H H Lee
H Jun D Lee and S-W Kim Nature 3 2017 (2012)6 B Kumar and S W Kim Nano Energy 1 342 (2012)7 M Y Choi D Choi M J Jin I Kim S H Kim J Y Choi S Y
Lee J M Kim and S W Kim Adv Mater 21 2185 (2009)8 A Onodera N Tamaki Y Kawamura T Sawada and H Yamashita
Jpn J Appl Phys 35 5160 (1996)9 J Wang W Chen and M Wang J Alloys Compd 449 44
(2008)10 B E Sernelius K F Berggren Z C Jin I Hamberg and C G
Granqvist Phys Rev 37 10244 (1988)11 S Zia M Amin U Manzoor and A S Bhatti Applied Physics A
Materials Science and Processing 115 275 (2013)12 Electronic Properties of Defects In Fundamentals of Semiconduc-
tors Springer Berlin Heidelberg (2001) pp 159ndash20213 F Gu S F Wang C F Song M K Luuml Y X Qi G J Zhou
D Xu and D R Yuan Chem Phys Lett 372 451 (2003)
Sci Adv Mater 7 789ndash793 2015 793
![Piezo‐Phototronic Effect on Selective Electron or …band 0.9 µm due to the indirect absorp-tion mechanism[12,13] and highly reflective surface across the electromagnetic spec-trum,[14]](https://img.pdfslide.net/doc/110x75/5fcb25c499bee5163e234ea6/piezoaphototronic-effect-on-selective-electron-or-band-09-m-due-to-the-indirect.jpg)
