Journal ofMaterials Chemistry C

PAPER

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Band gap engine

Department of Chemistry, Visva-Bharati

E-mail: pranab.sarkar@visva-bharati.ac.in

Cite this: DOI: 10.1039/c4tc01735h

Received 5th August 2014Accepted 3rd September 2014

DOI: 10.1039/c4tc01735h

www.rsc.org/MaterialsC

This journal is © The Royal Society of

ering of graphene–CdTe quantumdot hybrid nanostructures

Biplab Rajbanshi, Sunandan Sarkar and Pranab Sarkar*

We report the results of our theoretical studies on the electronic structure of graphene–CdTe quantum dot

(QD) hybrid nanostructures. We put emphasis on the possibility of engineering the electronic energy levels

of hybrid systems either through the variation of the size of the CdTeQD or by controlling the H coverage of

graphene. We also extend our study to see the effect of Se doping on the electronic energy levels of

graphene–CdTeQD hybrid nanostructures. It is well known that for solar cell applications the composite

system should have a type-II band alignment that hinders the recombination of charge carriers, thereby

improving the photovoltaic performance. By analyzing the electronic energy levels of the composite

systems we have shown that one can engineer the band gap of the system by controlling either the size

of the QD or percentage of the hydrogen atoms on graphene to achieve the type-II band alignment.

Based on the relative positions of the frontier energy levels we offer qualitative understanding of the

dynamics of electron transfer from QDs to the graphene and also the dynamics of recombination of the

charge carriers.

Introduction

Due to their unique size- and shape-dependent properties,semiconductor nanoparticles have become the focus of intenseresearch for the last couple of decades. The primary focus hasbeen the exploitation of their size- and shape-dependent prop-erties for application in diverse elds such as optoelectronicsand photovoltaic devices etc. Although the development of newoptoelectronic devices made of semiconductor nanoparticles isnow a reality, the use of individual semiconductor nano-particles in designing solar cells is rather limited. The utility ofsemiconductor quantum dots in the designing of photovoltaicdevices primarily depends on the ability of charge transfer froma photoexcited quantum dot (QD) before the trapping andrecombination of the charge carriers.1 Most of the studies onindividual QDs reveal that these events take place on an ultra-fast time scale, so that before the carrier gets transferred to asuitable acceptor the recombination takes place, therebyseverely limiting the use of these materials in solar cell appli-cations. To overcome this problem inorganic–organic hybridnanostructures with suitable donor–acceptor systems havereceived serious attention because these can slow down therecombination process and thus serve as ideal building blocksfor the design of solar cells.2–9 In the past few years severalexperimental groups have synthesized and studied differentinorganic–organic nanocomposites with a goal towards under-standing the ultra-fast charge transfer dynamics, and their

University, Santiniketan-731235, India.

Chemistry 2014

studies suggest that the charge separation is a prerequisite ofefficient photovoltaic devices and is greatly improved incomposite materials compared to that of an individual semi-conductor.10 In this context, the composite systems made ofgroup II–VI QDs (such as CdS, CdSe and CdTe) and carbonnanostructures such as carbon nanotubes (CNTs), fullerenesand graphene have been investigated by several groups.3,4,7,8,11–23

These materials, having versatile donor–acceptor properties,may be potential alternatives in fabricating photo-activemolecular devices. In this work, we would like to explore theelectronic structure of one such type of nanocomposite system,namely the graphene–CdTeQD (GR–CdTeQD) system. Thechoice of CdTe as the inorganic component of the hybrid systemstems from the fact that in the last few years several authorshave demonstrated that thiol-capped CdTeQDs can serve as anideal model system for building various functional nano-structures and devices. Their applicability ranges from biolog-ical labeling to third-generation solar cells.24–29 Graphene, byvirtue of its extraordinary properties such as higher drivelocities, may be a good alternative material to CNT andfullerene as an organic component to be integrated with inor-ganic quantum dots for designing novel nanohybrids withinteresting properties.30,31

The different experimental studies on QD–graphene nano-composites have established their potential application inphotovoltaic cells and it has been understood that graphene canextract a photo-excited electron from QDs very efficiently whichcan also delay the process of recombination of chargecarriers.19–23,32–34 Using cyclic voltammetry and spectroscopy,Markad et al. have studied the interaction between quantum

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dots of CdTe and reduced graphene oxide.23 In a very recentarticle Kaniyankandy et al.20 have reported the synthesis ofwater soluble CdTe decorated graphene (G–CdTe). Using ultra-fast transient absorption spectroscopy and time-resolvedemission spectroscopy these authors have indicated the effi-cient ultra-fast charge separation on G–CdTe nanohybrids.20

This novel nanohybrid can provide a new and promisingdirection toward developing high-performance light-harvestingdevices for next-generation solar cells. Debgupta et al.21 haverecently reported the synthesis of a graphene–CdSe QDcomposite and have shown a 2–3 fold increase in photocurrentin a graphene–QD composite.

Although there are number of experimental studies devotedto the synthesis and understanding of the ultra-fast chargetransfer dynamics of QD–graphene hybrid systems, theoreticalstudies addressing the electronic structure of QD–graphenenanohybrids are still missing. However, the basic under-standing of the charge transfer dynamics of such hybridsystems, which essentially depends on the position of thefrontier energy levels of its components, is therefore of crucialimportance for their effective exploitation in designing photo-voltaic devices. In view of that we herein performed the elec-tronic structure calculation of GR–CdTeQD nanohybrids. Byanalyzing the electronic energy levels we seek to have qualitativeunderstanding of the dynamics of charge carriers in both elec-tron transfer rate and recombination rate of charge carriers. Weemploy here the self-consistent charge density-functional tight-binding (SCC-DFTB) method for our electronic structurecalculation of the GR–CdTeQD hybrid nanostructures. In a veryrecent article, Gao et al.40 beautifully demonstrated the bandgap tunability of partially hydrogenated graphene by controllingthe extent of hydrogenation. This study motivates us to invokethe possibility of designing solar cells via the formation ofhydrogenated graphene–CdTeQD (HG–CdTeQD) hybrid nano-structures. We therefore extend our study to explore the elec-tronic structure of HG–CdTeQD hybrid nanostructures. Wehave also investigated the effect on electronic structure of thisnanocomposite by doping Se into the CdTeQDs in differentpercentages.

Details of computation

In the present work we have employed the self-consistent chargedensity-functional tight-binding (SCC-DFTB) method to studythe electronic structure of GR–CdTe nanohybrids as a functionboth of the size of the CdTeQDs and of the extent of hydroge-nation in the graphene sheets. The SCC-DFTB method,35–38

which is a compromise between reasonable accuracy andcomputational efficiency, is one of the most promisingapproaches in addressing systems composed of large numbersof atoms. The SCC-DFTB method is a parametrized density-functional method and has been described in detail else-where.35–38 In one of our previous studies41 we have establishedthe accuracy of our SCC-DFTB parameter set for Cd-chalco-genides and their interaction with several different systems bycalculating several physical parameters (viz. the latticeconstants, cohesive energy and band gap) and the electronic

J. Mater. Chem. C

structure of materials of various dimensions. The close agree-ment between our parametrized SCC-DFTB and ab initio resultsstrengthens the belief that the derived parameter set is reliablein studying the Cd–chalcogenide nanostructures. For the detailsof the parametrization the readers are referred to our earlierpaper.41

Tomodel our desired systems, we have initially chosen a unitcell of graphene with two carbon atoms. We then considered a16 � 16 supercell of graphene sheet in the XY plane whichcontains 512 carbon atoms. In the middle portion of thatsupercell, one carbon atom has been functionalized by a–COOH group. For balancing the charge of the graphenesheet, one –H group is attached to the neighboring carbonatom to where the –COOH group is attached. On the other hand,we have chosen four different thiol-capped CdnTen quantumdots (n ¼ 17, 34, 65, 98) of zinc-blende structure. The radiusof the QDs are 0.91, 1.08, 1.28 and 1.45 nm, respectively. Allthe surface Cd atoms of the bare CdTe clusters are passivatedby thiol groups (–SH) except one, which is functionalizedwith a long chain thiol molecule, 3-aminopropane-1-thiol(–S–CH2–CH2–CH2–NH2). From the concept of bio-conjugation,we then attached the functionalized CdTeQDs to the –COOH onthe graphene sheet through amide linkage i.e., the GR–CdTehybrid material (GR–CONH–CH2CH2CH2–S–CdTe) is formed byelimination of one molecule of H2O. In this context we wouldlike to mention that Chaban et al. in a recent study on CdSeQD–fullerene nanocomposites have shown that this type of covalentlinking can increase the photo-conversion efficiency by 2–3orders of magnitude as compared to non-interacting compositesystems.42 To avoid the interaction of the QDs with their peri-odic replicas on innite graphene sheet we have consideredonly one QD per graphene sheet. Again to avoid spuriousinteractions among consecutive periodic replicas along the zdirection, the calculations have been performed with a suitablevacuum region of 100 A along that direction. Geometry opti-mizations have been performed with the conjugated gradientalgorithm, until all forces became smaller than 0.0001 eV A�1.We have used a (4� 4� 1) Monkhorst–Pack grid for all periodiccalculations. In the optimized structure of the hybrid systems,the carbon atoms of GR that are attached to ligands exhibitstructural distortion because of a change in hybridization fromsp2 to sp3. We employed the Slater type orbital (STOs) basis setsand Perdew–Burke–Ernzerhof (PBE)43 exchange correlationenergy functional in this study. All calculations have been per-formed with the DFTB+ program.39 The optimized structures(both top and side views) of one representative GR–CdTeorganic–inorganic hybrid nano-composite [GR–(CdTe)65] areshown in Fig. 1.

Results and discussionGraphene–CdTeQD nanocomposites

To understand the contribution of electronic energy levels ofgraphene and QDs in the nanocomposites we have in Fig. 2shown the band structure of graphene (GR), GR–COOH and GR–CdTeQD with QDs of four different sizes. The band structure ofgraphene and GR–COOH clearly indicates that the chemical

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Optimized structures [both top (upper) and side (lower) views]of one representative GR–CdTeQD [GR–Cd65Te65 QD] nanohybridsystem.

Fig. 3 Density of states (DOS) of GR–CdTeQD nanohybrids with QDsof four different size: (a) Cd17Te17, (b) Cd34Te34, (c) Cd65Te65 and (d)Cd98Te98. The zero energy is set at the Fermi level.

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functionalization opens the band gap of the graphene sheet.From the band structure plot of the different GR–CdTeQDcomposites, it is also clear that the band gap region of thenanocomposites is dominated by the graphene sheet, while thecontributions of CdTeQDs are in the depth of both the valenceand conduction band regions. But it may be noticed that as thesize of the QDs increases both the HOMO and LUMO of QDsapproach toward the Fermi level. This nding gets support fromthe density of states (DOS) plot of GR–CdTeQD nano-composites. In Fig. 3, the DOS of four GR–CdTeQD nano-composite systems with CdTeQDs of different size are shown.

Fig. 2 Band structure of (a) graphene (GR), (b) GR–COOH, and GR–CdCd65Te65 and (f) Cd98Te98. The zero energy is set at the Fermi level.

This journal is © The Royal Society of Chemistry 2014

We have also shown the PDOS of individual QDs and graphenein the same gure. The DOS gure reveals one interestingfeature that the difference between the lowest unoccupied

TeQDs with QDs of four different size: (c) Cd17Te17, (d) Cd34Te34, (e)

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Fig. 4 Optimized structures of both top (left) and side (right) views of one representative 50%HG–CdTeQD with a QD of radius 1.28 nm.

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molecular orbital (LUMO) of the QD and conduction band (CB)of the graphene, which is key for dictating the dynamics ofelectron transfer, decreases with an increasing size of the QDs.

Fig. 5 Density of states (DOS) of HG–CdTeQD nanohybrids with differesize (radius): 12.5% HG with 0.91, 1.08 and 1.28 nm CdTeQDs (a–c); 25% Hand 1.28 nm CdTeQDs (g–i) and 62.5% HG with 0.91, 1.08 and 1.28 nm

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As the driving force for the electron transfer is the difference inenergy between the donor LUMO and the acceptor CB, onecould expect a faster electron transfer rate for a smaller CdTeQD

nt percentages of hydrogen, each coupled with three QDs of differentG with 0.91, 1.08 and 1.28 nm CdTeQDs (d–f); 50% HG with 0.91, 1.08CdTeQDs (j–l). The zero energy is set at the Fermi level.

This journal is © The Royal Society of Chemistry 2014

Table 1 The variation of band gap values of HG–CdTeQD nanocomposites with the percentage of hydrogen and size of the QDs. The boldfigures indicate type-II band alignments

Percentage ofH on graphene

Band gap of partiallyhydrogenated graphene (eV)

Radius of CdTeQD(nm)

Band gap ofQD (eV)

Band gap of the HG–CdTeQDhybrid system (eV)

12.5% 1.06 0.91 2.74 1.041.08 2.50 1.021.28 2.06 0.99

25% 1.23 0.91 2.74 1.201.08 2.50 1.181.28 2.06 0.96

50% 1.29 0.91 2.74 0.951.08 2.50 0.921.28 2.06 0.62

62.5% 3.53 0.91 2.74 1.401.08 2.50 1.621.28 2.06 1.01

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as compared to larger QDs when they are coupled with thegraphene. However, it should be mentioned that faster electrontransfer rates with smaller sized QDs do not necessarily meanthat these systems will show better photovoltaic efficienciessince these depend on other factors also, such as the rate of holetransfer and the rate of recombination of charge carriers. Thisfast transfer of electrons from the QD to the graphene thereforeresults in the uorescence quenching of the composite systemas compared to an isolated CdTeQD that has already been foundin experimental studies on this type of system.20,21

The band gap values of all nanocomposites are small ascompared to isolated CdTeQDs. From the band structure plot ofthe composite system it is clear that the graphene states are

Fig. 6 VBM and CBM densities of (a) 25% HG with 0.91 nm CdTeQD, (b)The red surfaces represent the densities of VBM whereas the blue surfac

This journal is © The Royal Society of Chemistry 2014

introduced in between the HOMO and LUMO states of the QD.Although the UV absorption shows a red-shi when thecomposite system is formed, it should be noticed that the quasi-particle band gap is increased for the CdTeQD23 due to a chargetransfer interaction. Our theoretical result reports a blue-shi ofthe quasi-particle band gap of 0.03 eV for the smallest QD of0.91 nm radius. The estimated HOMO and LUMO positions forthe CdTeQD in the composite system are reported to be at�5.98and �3.21 eV, respectively, while the Fermi energy of grapheneis estimated to be �4.63 eV. These results are in excellentagreement with the earlier report on similar sized QDs.20

However, the absorption spectra of the GR–CdTeQD nano-composite will show a clear red shi as compared to an isolated

25% HG with 1.28 nm CdTeQD and (c) 50% HG with 0.91 nm CdTeQD.es represent the CBM.

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Fig. 7 Density of states (DOS) of 50%HG–CdTeQDs with differentpercentages of doped Se: (a) 0%, (b) 25%, (c) 50%, (d) 75% and (e) 100%.The zero energy is set at the Fermi level.

Table 2 The variation in band gap and ECBM � ELUMO of the dopedhybrid system

Doped hybrid systemBand gap of hybridsystem (eV)

ECBM �ELUMO (eV)

50%HG–CdTe 0.62 0.8050%HG–CdTe0.75Se0.25 0.72 0.7050%HG–CdTe0.5Se0.5 0.79 0.6350%HG–CdTe0.25Se0.75 0.92 0.5050%HG–CdSe 0.99 0.43

Fig. 8 The variation in band gap of (CdTe1�xSex) nanocomposites withdiffering composition of the QD.

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CdTeQD and this observation is in good agreement with theexperimental studies of Markad et al.23 As the electron transferfrom the QD to the graphene is faster for smaller QDs comparedto larger ones, the uorescence quenching rate will decreasewith increasing size of the QDs. This qualitative picture of theuorescence quenching rate has recently been observed byKundu et al. in their experimental studies on CdTeQD–gra-phene oxide composite systems.22 In this regard it should be

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pointed out that the surface states of the QD also have a greatrole in uorescence quenching. It has been observed that theluminescence decay kinetics may be governed by both intrinsicand surface states.20 In our case, upon passivation these statesare removed and hence only the intrinsic valence and conduc-tion bands of the CdTeQD are involved during the chargetransfer processes.

The decrease in the band gap of the GR–CdTeQD nano-composite compared to the isolated CdTeQD conrms a stronginteraction of the CdTeQD with the graphene in the compositesystem. This decrease in band gap of the GR–CdTeQD nano-hybrids is in agreement with the experimental red shi in theUV-vis spectra of this particular system observed by Markadet al.23 We have also calculated the band gap of the mixture ofCdTeQD and the graphene without covalent linking betweenthem and found that the band gaps are the same as that ofeither isolated graphene or the isolated QD. So, the electronicstructure of GR–CdTeQD nanocomposites is different from themechanical mixture of the two components as it is found indifferent experimental studies.23,42

Partially hydrogenated graphene–CdTeQD (HG–CdTeQD)hybrid nanocomposites

Graphene, by virtue of its unique linear electronic dispersionnear the K-points of the Brillouin zone, can in principle act as anefficient relay material for injected electrons. However, thegapless feature of graphene limits its applications in electronicsand photonics.31 Thus, a tunable band gap in the intermediateenergy range (e.g., 1–3 eV) is highly desirable. Many theoret-ical44–50 as well as experimental24,40,51–58 studies demonstratedthat chemical functionalization of graphene can efficientlytransform the hybridization state of carbon atoms from sp2 intosp3 and thus open a large band gap up to several electron volts.The zero band gap of graphene and the large band gap ofgraphane46 (5.4 eV) suggest that it is possible to tailor the bandgap across a wide range by controlling the hydrogen coverage.Gao et al.40 reported a series of patterned partially hydrogenatedgraphene. Keeping in mind their report, we choose fourpartially hydrogenated graphene (HG) with different

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percentages of hydrogen of 12.5%, 25%, 50% and 62%. Thetunability trend of the band gap of these partially hydrogenatedgraphenes agrees well with the results reported by Gao et al.40

We have now considered the HG–CdTeQD hybrid nano-composites and would like to investigate the effect of both theextent of hydrogen passivation on graphene and also the size ofthe CdTeQD on the electronic structure of the nanocomposite.For each HG with a particular percentage of hydrogen we haveconsidered three composite systems with three QDs of differentsize (radius 0.91 nm, 1.08 nm, 1.28 nm). The optimized struc-tures (both top and side view) of one representative HG–CdTeQD organic–inorganic hybrid nanocomposite, which is50% HG–CdTeQD with a QD of radius 1.28 nm, are shown inFig. 4. The DOS and PDOS of few nanocomposites along withthe individual QDs and hydrogenated graphenes are plotted inFig. 5. The DOS gure clearly reveals the interesting feature thatfor HG–CdTeQD with a lower percentage of hydrogen (12.5% or25%) again the band gap region is dominated by graphenefor smaller QDs (of radius 0.91 nm or 1.08 nm) as in case ofGR–CdTeQD nanocomposite systems. Thus, both the VBMand CBM of the composite system are localized on a singlecomponent, namely graphene. So, we can conclude that theHG–CdTeQD with a lower percentage of hydrogen and smallerQDs shows type I band alignment. But if we increase the size ofthe QDs (to radius 1.28 nm) keeping the percentage of hydrogenon graphene xed, the VBM of the HG-CdTeQD hybrid system isdominated by the valance band maximum (VBM) of HG, whilethe CBM of the HG–CdTeQD hybrid system is controlled by theLUMO of the CdTeQD. This situation arises due the appearanceof the LUMO of the CdTeQD at a lower energy than theconduction band minimum (CBM) of GR. As a result, the bandgap of the HG–CdTeQD hybrid system decreases from the bandgap of HG. Alternatively, we can say that the HG–CdTeQDhybrid system with a lower percentage of hydrogenated gra-phene and larger QDs shows type-II band alignment. Hence,the variation of size (from smaller to larger) of QDs in theQD–graphene nanocomposite becomes one possible pathwayfor making a transition from type-I to type-II band alignment.On the other hand, HG–CdTeQDs with a larger percentage ofhydrogen (50% or more), due to the large band gap of HG,shows type-II alignment with all three QDs as the LUMO of theQDs appears at a lower energy than the CBM of HG, whereasthe VBM of these HG–CdTeQDs is dominated by the VBM ofHG as before. This results in the decrease in band gap of theHG–CdTeQDs with a larger percentage of hydrogen than thecorresponding isolated HG.

The band gap variations from HG of the HG–CdTeQD hybridsystem are tabulated in Table 1. In this table we display thepercentage of hydrogen on graphene, the size (radius) of the QDattached to that partially hydrogenated graphene, the bandgap of that HG, the band gap of the QD and the band gap of theHG–CdTeQD hybrid system. From the table it can be found thatHG–CdTeQDs with 12.5% and 25% hydrogen on graphene bothwith the QDs of radius 0.91 nm and 1.08 nm show band gapsclose to that of the corresponding HG. But for the larger QD ofradius 1.28 nm the band gap of the HG–CdTeQD decreases fromthat of HG, whereas for HG–CdTeQDs with higher percentages

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of hydrogen on graphene (50% and 62.5%) the band gapdecreases from the corresponding HG for all three QDs.

The transition from type-I to type-II band alignment canclearly be understood from VBM and CBM densities as given inFig. 6. From the analysis of these results we propose that forCdTeQD–graphene nanocomposites, there are two strategies forthe transition from type-I to type-II band alignment: one caneither change the size of the QD for a given percentage ofhydrogenated graphene (#50%) and/or one can change thepercentage of hydrogenation on graphene for a xed size of theQD in the nanocomposite. To demonstrate such a situation wehave plotted VBM and CBM densities of three HG–CdTeQDnanocomposites such as (a) 25% hydrogenated graphene withQD of radius 0.91 nm, (b) 25% hydrogenated graphene with QDof radius 1.28 nm and (c) 50% hydrogenated graphene with QDof radius 0.91 nm respectively in Fig. 6. From Fig. 6(a) it can benoticed that both the VBM and CBM are residing on thehydrogenated graphene sheet, indicating that it is a type-Isystem. But, if we increase the size of the QD from radius 0.91nm to radius 1.28 nm, keeping the percentage of hydrogenationon graphene xed (i.e. the same HG), then the VBM density isseen to be delocalized over the HG whereas CBM density isshied over the QD as shown in Fig. 6(b). Thus, these systemsrepresent the type-II band alignment. Hence, the desiredstrategy for transitioning from type-I to type-II band alignmentcan be achieved just by changing the size of the QDs. Alterna-tively, from Fig. 6(c), if we increase the percentage of hydroge-nation on graphene (from 25% to 50%), keeping the size of theQD as in (a) (i.e. the QD with radius 0.91 nm), then the VBMdensity also remains delocalized over the HG and the CBM isdelocalized over the QD. So, again this is a transition to a type-IIsystem from a type-I system. And this is being achieved throughthe second desired strategy by changing the percentage ofhydrogenation on the graphene sheet.

The CdTeQD–graphene nanocomposite with type-II bandalignment may be a suitable donor–acceptor system to be usedin solar cells. The type-II band alignment results in a localiza-tion of the charge carriers in different components of thecomposite system, thus the charge recombination dynamicsbecome slower. This delayed charge recombination increasesthe photovoltaic performance for this nanocomposite. Ourqualitative understanding of delayed charge recombination isin good agreement with the experimental results of Kaniyan-kandy et al.20 The bleach recovery kinetic study by these authorsindicates that the recombination dynamics of CdTeQD–gra-phene nanocomposites are much slower compared to those ofthe isolated CdTeQD.

Doped HG–CdTeQD hybrid nanocomposites

We are now interested in investigating what the effect isof doping into the QDs on the electronic properties of theseHG–CdTeQD hybrid systems. In view of this, we doped theCdTeQD of radius 1.28 nm with Se in four different percentages(25%, 50%, 75% and 100%). In the next step, we attached all thedoped QDs on a 50% hydrogenated graphene sheet in a similarway to that described earlier. In Fig. 7, the DOS of ve doped

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HG–CdTeQD nanocomposites with different percentages of Seas well as PDOS of individual doped QDs and hydrogenatedgraphene are shown. It is worth pointing out that 0% dopingsimply means a pure CdTeQD whereas 100% doping implies apure CdSeQD. The DOS gures reveal two interesting features.Firstly, as the percentage of Se doping increases the band gap ofHG–CdTeQD nanocomposites increases. The 0% doped HG–CdTeQD i.e. pure HG–CdTeQD shows the lowest band gapwhereas 100% doped i.e. HG–CdSeQD shows the highest bandgap. With an increase in the percentage of doping, the LUMO ofthe QD is shied towards the higher energy, keeping the posi-tion of the HOMO the same, which results in the increase in theband gap. Secondly, as the band gap increases i.e. the LUMO ofthe QD is shied towards higher energy and thus the energydifference between LUMO of the QD and the CB of GR decreasescontinuously. Thus, we could expect a faster electron transferrate for undoped CdTeQD than for doped CdTeQD andCdSeQD. In other words if one starts with CdSeQD as the initialQD and then dope Te into it then the band gap would decreasewith an increasing doping percentage and hence the electrontransfer rate should increase with the doping.

Finally, to get a quantitative picture, the values of the bandgap of doped HG–CdTeQD composites and the energy differ-ence between the CBM of graphene and the LUMO of the QD[ECBM � ELUMO] are shown in Table 2. The values also suggest anincrease in band gap and decrease in ECBM � ELUMO withincreasing percentage of Se doping into the CdTeQD. Thevariation in the band gaps of (CdTe1�xSex) nanocomposites withrespective composition of the QD is shown in Fig. 8, whichshows a linear relationship between the composition of the QDand the band gap.

Conclusion

We have investigated the electronic structures of graphene–CdTeQD and partially hydrogenated graphene–CdTeQD nano-hybrid systems. We also extend our study towards under-standing the effect of doping on the electronic structure of thesepartially hydrogenated graphene–CdTeQD nanohybrid systems.The band gap region is dominated by graphene for GR–CdTeQDcomposites, while the contributions of CdTe QDs to the DOS arein the depth of both the valence and conduction band regions.The HOMO and LUMO of the QDs continuously move towardsthe Fermi level as the size of the QDs increases. This means thatthe energy difference between the LUMO of the QDs and the CBof graphene continuously decreases as the size of the QDsincrease. This observation reveals a faster electron transfer ratefor smaller CdTeQDs as compared to larger ones attached to thegraphene. When the percentage of hydrogen passivation on thegraphene sheet increases then the band gap also increases in atunable manner. The electronic structures of the HG–CdTeQDsshow type-I to type-II transition depending on either the size ofthe CdTeQD or the extent of hydrogenation on graphene. Thus,the HG–CdTeQDs of lower percentages of hydrogenated gra-phene, when coupled with larger QDs, show type-II alignmentbut they show type-I alignment when coupled with the smallerQDs. On the other hand, if the percentage of hydrogen

J. Mater. Chem. C

passivation on graphene increases, the type-II alignment is alsoseen for smaller QDs. Hence, by two strategies, changing the QDsize as well as changing the percentage of hydrogenation ongraphene, the transition from type-I to type-II can be achieved.The band gap of doped GR–CdTe1�xSex changes linearly withthe composition of the QD. At the same time, the energydifference between the LUMO of the QD and the CB of HGdecreases continuously as the percentage of Se dopingincreases. The HG–CdTeQDs with pure CdTeQDs show fasterelectron transfer rate than that of the fully doped QD i.e.CdSeQD. Alternatively, we can say that if CdSeQDs were taken asinitial QDs and then doped by Te in different percentage, onewould nd a decrease in band gap with increasing electrontransfer rate, as the percentage of doping increases, when theyare coupled with HG. We hope that our study reveals somequalitative understanding of the electronic energy levels of GR–CdTeQDs and will encourage experimentalists towards furtherexploration of these nanomaterials in designing nanodevices.

Acknowledgements

This paper is dedicated to Prof. Kankan Bhattacharyya, IACS,Jadavpur on the happy occasion of his reaching sixty. One of theauthors (B. R.) is grateful to UGC, New Delhi for awarding theRajiv Gandhi National Fellowship. The nancial support fromCSIR [01(2744)/13/EMR-II] and UGC (SAP), New Delhi throughresearch grants is gratefully acknowledged. The authors wouldlike to thank Dr Anup Pramanik for many fruitful discussions.

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