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Tuning the Energy Levels of ZnO/ZnS Core/Shell Nanowire to Design an Efficient Nanowire-Based Dye-Sensitized Solar Cell Supriya Saha, Sunandan Sarkar, Sougata Pal, and Pranab Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp402611j • Publication Date (Web): 12 Jul 2013 Downloaded from http://pubs.acs.org on July 17, 2013

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The Journal of Physical Chemistry

Tuning the Energy Levels of ZnO/ZnS Core/Shell Nanowires to Design an efficient Nanowire-based Dye-Sensitized Solar Cell Supriya Saha,† Sunandan Sarkar,† Sougata Pal,‡ and Pranab Sarkar∗,† Department of Chemistry, Visva-Bharati University, Santiniketan- 731235, India, and Department of Chemistry, University of Gourbanga, Malda E-mail: [email protected]

Abstract By using the self-consistent charge density-functional tight binding(SCC-DFTB) method we studied the electronic structure of ZnO/ZnS core/shell nanowire (NW) as a function of both core radius and shell thickness. By studying the band energy alignment, band structure, density of states and band edge wave functions we envisage the efficacy of this particular nano heterostructure in dye sensitized solar cell. The strong localization of valence band maximum (VBM) and conduction band minimum (CBM) in ZnS shell and ZnO core respectively, irrespective of core radius and shell thickness clearly indicates the spatial charge separation in this system. This spatial charge separation decreases the charge recombination rate thereby increasing the chance of better photovoltaic performance. We also investigated the electronic structure of anthraquinone(AQ) acid dye molecule- ZnO/ZnS nanowire composite system. We demonstrated that whether the composite system will form type-I or type-II band alignment that very much depends on the thickness of the ZnS shell and the nature of the functional group (electron withdrawing or electron donating) attached to AQ acid molecule. Keywords: ZnO/ZnS NW; Band energy alignment; NW-Dye interaction; Charge Separation ∗ To

whom correspondence should be addressed University ‡ University of Gourbanga † Visva-Bharati

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1 Introduction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Harnessing solar energy using photovoltaic technology is being widely recognized as an essential component of future global energy production. Dye sensitized solar cells (DSSCs) originally introduced by Gratzel et al. 1 are promising photovoltaic devices for low-cost, high-efficiency solarto-electric conversion. The configuration of a DSSC consists of a wide band gap semiconductor with a large surface area, a dye as sensitizers, and an electrolyte. Nanostructured TiO2 with a high surface to volume ratio is one of the most widely used wide band gap semiconductors (band gap, Eg = 3.2 eV) for this purpose. 2–9 Recent researches on ZnO semiconductor have shown that it has the potential to be an alternative material for improving the solar cell performance in dye-sensitized solar cells. 10–25 The emergence of ZnO as an alternative is due to having a band gap similar to TiO2 and also having a much higher electron mobility than that of TiO2 17,26 thus lowering the charge recombination. Apart from this, ZnO semiconductor can be fabricated to a variety of nanostructures and their morphology can be modified easily by using different synthetic methods. 27–31 Among different nanostructures, nanowire based DSSCs are the most attractive as they transport electrons, minimizing trapping/detrapping events thereby reducing the time of transport of electrons through the nanoparticle materials. 13,20,32–36 The first successful approach of designing nanowire dye-sensitized solar cell is due to Law et al. 18,19 These authors showed that nanowires have excellent electron transport capabilities because of internal electric fields in the direction of the c axis of wurtzite crystals and this causes the suppression of recombination for injected electrons from surrounding electrolytes. 13,18 TiO2 nanowires do not provide the advantage of electron transport because they agglomerate in a polycrystalline network. 20,37 Besides, ZnO nanowire because of its well-aligned nanostructure enables rapid electron transport with a large electron diffusion coefficient of 1.8 × 10−3 cm2 /s. 17 Despite its great potential, the cell efficiency of the ZnO nanowire based DSSCs is still lower than that of TiO2 nanoparticle-based ones. The low conversion efficiency of ZnO-based DSSCs is primarily due to the recombination of the electrons injected into the ZnO with either the dye or the redox electrolyte, thereby reducing the cell efficiency. 16,38 Another reason is chemical instability of ZnO in acid dye. The ZnO get dissolved by the -COOH 2


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group of the dye molecule forming a dye − Zn+ complex, which blocks the electron injection from 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dye to ZnO. 37,39 One of the most promising approach to improve DSSCs performance as shown by Law et al. 18 is to coat a thin layer on the ZnO nanowire surface to passivate the recombination sites and thereby decreasing the dark current and increasing the Voc (open-circuit voltage). The formation of core/shell nanowire also results a energy barrier between the core and the shell thereby reducing the recombination rates. In the recent literature there are extensive reports on the effect of coating the ZnS shell onto the ZnO nanowire to overcome the above limitations. 12,13,15,16,21,25 These authors have shown that ZnS layer is a viable option as a shell layer on ZnO nanowire for the application of solar cells. The oxygen vacancies of ZnO nanowires which were responsible for high recombination rate of electrons in DSSCs have been filled up by S atoms of the ZnS layer. The formation of a shell of ZnS nanowire over the core ZnO nanowire results a type II band alignment of heterostructures through the induction of charge separation at the interface of the two different materials. Thus the electrons and holes are localizing in different regions of the nanostructures and this reduces the chance of recombination of the charge carriers. The type II band alignment also form appropriate conduction band edge lines to facilitate electron transfer. Although in the past few years, there are extensive experimental studies 12–16,21,40–42 on synthesis of the ZnS shell over ZnO nanowire and its subsequent effect on the performance of solar cell efficiency, theoretical studies addressing the detail electronic structure of this heterostructure are scarce. 43,44 By using band-corrected pseudo-potential density-functional theory, Schrier et al. 43 studied optical absorption and carrier localization in nanowire-based ZnO/ZnS heterostructure. However, the detailed electronic structure viz. the variation of band gap, band edge wave functions, band alignment with the radius of the core and the thickness of the shell is still lacking. The recent experimental work of Yu et al. 16 on ZnO/ZnS hetero nanostructure showed that the variation of shell thickness and roughness of the ZnS shell has remarkable influence on the photovoltaic properties of ZnO-based DSSCs. Zhu et al. 45 in their study on CdSe/ZnS core/shell nanosystems have demonstrated that both the charge separation and recombination rates can be controlled by altering the shell thickness and the nature of the shell. So, extensive theoretical

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studies, in particular the evolution of the electronic structure as a function of core radius and shell 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

thickness are crucial importance for optimization of solar cell performance by controlling the core size or thickness of the shell. But theoretical studies of nanostructures composing large number of atoms are prohibitive because of their high computational demand. The self-consistent charge density-functional tight-binding (SCC-DFTB) method has been successfully applied to large-scale quantum-mechanical simulation, is a suitable candidate for the task. In the present work, we have employed the self-consistent charge density-functional tight-binding (SCC-DFTB) method 46–51 to investigate the electronic structure of ZnO/ZnS nanowire. By varying both the core radius and shell thickness, we made a systematic study to understand the dependence of band edge alignment of ZnO/ZnS core/shell heterostructures on the thickness of both the core and shell part. We found that the valence band maximum (VBM) for each core/shell system is located approximately at the same level of pure ZnO system but the conduction band minimum (CBM) differs depending on both core and shell thickness of the heterostructures. Tuning the energy levels of this heterostructure introducing ZnS shell onto the ZnO nanowire play a great role to perform a highly-efficient DSSCs using appropriate dye molecules. We also extend our study to investigate the electronic structure of adsorption of the dye (anthraquinone-2,3-dicarboxylic acid) molecules with various functional groups on ZnO/ZnS core/shell NW with a goal to have some qualitative understanding of the dynamics of electron injection and electron-hole recombination.

2 Computational Methodology and Model In the present work, we employed the self-consistent charge density-functional based tight-binding (SCC-DFTB) method to study the electronic structure of ZnO/ZnS core/shell heterostructure nanowires. The SCC-DFTB method being semi-empirical in nature can treat large number of atoms and has demonstrated a good compromise between accuracy and computational efficiency. This method has been described in detail elsewhere. 46–51 We have used Slater type orbitals (STOs) as basis sets and Perdew-Burke-Ernzerhof (PBE) 52 exchange correlation energy functional and our recently

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derived SCC-DFTB parameter set. 53 To test the accuracy of our SCC-DFTB calculation, we have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

compared the calculated structural and energetic properties viz. lattice constants, cohesive energies, band gap etc. of both bulk ZnO and ZnS with those of PP-PBE calculation and also with the experimental values. The details of this parametrization are given in our previous study. 53 ¯ facets infinite along the [0001] The nanowires with hexagonal cross section enclosed by [1010] direction were cut from the bulk (taking average lattice constant of ZnO and ZnS) wurtzite(wz) structure, keeping ZnS outside the NWs and ZnO in the core region and the surface atoms are passivated by H-atom fulfilling the co-ordinations of Zn and S atoms. The NWs are passivated with H-atoms to saturate the dangling bonds on the surface of the NWs. The optimized structural parameters are a=3.23 Å and c=5.24 Å for ZnO, and a=3.86 Å and c=6.30 Å for ZnS which are in good agreement with the experimental values. The choice of the wurtzite structure stems from the recent experimental observation of Chung et al. 13 These authors have shown that the synthesized nanowires had hexagonal wurtzite structures with preferential orientations along the c axis. The diameters of the NWs studied here are 24.85 Å, 31.95 Å, 39.05 Å, 46.15 Å and 53.25 Å, containing 240, 360, 504, 672 and 864 atoms per unit cell, respectively. All atomic coordinates as well as the lattice constants along the wire axis are fully relaxed by using periodic boundary conditions and suitably oriented supercells. The calculations have been performed with a suitable vacuum region of 100 Å surrounding the structures along the x and y directions to avoid spurious interactions among consecutive periodic replicas. We have used the conjugate gradient algorithm for geometry optimization until the force on each atom in all cases is reduced to less than 0.001 eV Å −1 . For the bulk and nanowire calculations, a (8 × 8 × 4) and a (1 × 1 × 4) Monkhorst-Pack k-point mesh were employed to yield converged results. We first consider four ZnO/ZnS core/shell NW heterostructures with fixed core radii and increasing shell thickness (FCIS) and four ZnO/ZnS core/shell NW heterostructures with increasing shell thickness and decreasing core radii (ISDC) gradually, keeping fixed core plus shell diameter. We denote these as (ZnO)3 /(ZnS)1, (ZnO)3 /(ZnS)2 , (ZnO)3 /(ZnS)3, (ZnO)3 /(ZnS)4 and (ZnO)5 /ZnS)1 , (ZnO)4 /(ZnS)2 , (ZnO)2 /(ZnS)4 , (ZnO)1 /(ZnS)5 , where the subscription indi-

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cate the number of atomic layers in the cross section in the ZnO core and ZnS shell regions, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

respectively.

3 Results and Discussion 3.1 Electronic properties of ZnO/ZnS core/shell heterostructure NWs We start our discussion on electronic properties of ZnO/ZnS core/shell heterostructure NWs by first mentioning the effect of -H passivation on the NW’s band gap values and also its effect on the stress of the NWs. The passivation has the effect of increasing the band gap values of the NWs and the effect is more pronounced for smaller NW as compared to larger one. For example the band gap of bare (ZnO)3 /(ZnS)2 core/shell NW is 1.83 eV and that of -H passivated NW is 2.20 eV while the band gap of bare (ZnO)3 /(ZnS)3 core/shell NW is 1.56 eV and that of -H passivated NW is 1.62 eV. The passivation has also effect in reducing the stress on the NWs. This is evident from the values of surface Zn-S bond lengths. The Zn-S bond lengths doe bare core/shell NWs varies from 2.18 Å to 2.5 Å as compared to bulk Zn-S bond length of 2.3 Å. However, the Zn-S bond lengths of -H passivated NWs remain close to the bulk value. Because of large lattice mismatch between bulk ZnO (c=5.24 Å) and ZnS(c=6.30 Å) we have first optimized the lattice parameter c with atomic position along the axial direction(c) for all cases by allowing all atoms to move without any symmetry constraints. Due to large lattice mismatch between the outer ZnS shell and inner ZnO core, a significant anisotropic strain is developed over the nanowire heterostructure both along and perpendicular to the NW axis. 54 The local volume of the ZnS shell is consequently compressed, though non-uniformly, due to the relaxation in the radial direction. The axial lattice constants of the ZnO/ZnS heterostructure NWs depend both on the inner core radius and outer shell thickness. For FCIS heterostructure NWs i.e. for (ZnO)3/(ZnS)1 , (ZnO)3 /(ZnS)2, (ZnO)3 /(ZnS)3 and (ZnO)3 /(ZnS)4 systems, the optimized lattice constants(c) are 5.70 Å, 5.80 Å, 5.90 Å and 6.00 Å respectively, 8.7%, 10.6%, 12.6% and 14.5% larger than bulk ZnO and 9.5%, 7.9%,6.3% and 4.7% smaller than that of bulk ZnS. For ISDC heterostructure NWs i.e. for (ZnO)5 /(ZnS)1, 6


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(ZnO)4 /(ZnS)2, (ZnO)2 /(ZnS)4 and (ZnO)1 /(ZnS)5 systems those are 5.60 Å, 5.70 Å, 6.10 Å 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and 6.20 Å respectively, 6.80%, 8.80%, 16.40% and 18.32% larger than bulk ZnO and 11.10%, 9.50%, 3.20% and 1.60% smaller than that of bulk ZnS. There is a large overall distortion of the ideal hexagonal arrangement as a result of increase in the inner Zn-S bonds along the radial direction, also the surface Zn-S bonds get reduced as compared to their respective bulk values. As it is noticed by Yang et al. 54 we also found that the local strain state would depend on the composition ratio, i.e., the core radius and the shell thickness. A higher content of ZnS would result in a larger lattice constant along z axis, imposing a larger strain on ZnO and a smaller strain on ZnS, and vice versa. It is obvious that the strain over the NWs will affect the electronic band structure i.e. the position of energy levels and thus on band gap. 43 This strain can be used to reduce the total system band gap and also to tune the band alignment. Depending on the geometry of the nanostructure, the strain effect can be varied, providing a degree of freedom for band alignment engineering along with band gap engineering. Beside the strain effect, quantum confinement effect also play a key role on this band alignment and band gap engineering. From the calculated band alignment of ZnO/ZnS superlattice we found that the ZnO/ZnS system has a type-II band alignment.

In Fig.1 and Fig.2, we represent the band alignment of both FCIS and ISDC systems in detail. The variation of band gap with core/shell thickness for both FCIS and ISDC systems are also shown in the inset. The band gap for FCIS system decreases with increasing shell thickness. From the band alignment it is clear that VBM of the composite system remains almost at the same position or close to the VBM of ZnO NW and the CBM of the composite system becomes more negative with increasing shell thickness. Thus keeping core radius fixed if we increase shell thickness, both the absorption and emission of the exciton band are red-shifted due to the increased leakage of exciton wave function into the ZnS shell. The red shift in the band gap for ZnO/ZnS core/shell NW with increasing shell thickness is in good agreement with experimental studies of this particular system. 12,14,21,25 For ISDC systems the VBM of the composite system slightly become more positive as compared to ZnO NW while the energies of CBM shifts to lower energies as compared to

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CBM of ZnO NW. In this case also red-shift occurs in the band gap as we go from (ZnO)5 /(ZnS)1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

to (ZnO)2 /(ZnS)4. In this particular case of ISDC system the size of the NW remain fixed so, it is not the quantum confinement effect rather the stress on the NW plays a crucial role in dictating the variation of the band gap. As the value of ’n’ increases from 0 to 3 for (ZnO)6−n /(ZnS)n core/shell NW, the stresses on the system increases because of the increase in heterogeneity. The increased stress causes the decrease in CB energies resulting decrease in the band gap. The further increase in ’n’ causes the reduction of stress on the system thus CB energy increases and accordingly band gap increases. Here, we should mention that ideally the minimum should have come at n=3. For n=6, the system will be pure ZnS and the band gap will be higher and be equal to that of pure ZnS NW. From both Fig.1 and Fig.2 it is clear that one can tailor the band alignment of ZnO/ZnS nanowire heterostructure by either varying the ZnS shell thickness or by varying both ZnO core diameter and ZnS shell thickness. The understanding of this energy level alignment plays a crucial role in choosing a perfect dye molecule as the difference between the conduction band energy levels of heterostructure NWs and the dye molecule, serves as a driving force for the inter particle electron transfer. Hence for a dye molecule whose LUMO energy remain above the CBM of the composite system, the charge injection rate increases with the increasing ZnS shell thickness of the core/shell NW.

In Fig.3 and Fig.4, we have shown the electronic band structures of FCIS and ISDC ZnO/ZnS core/shell NWs along with their density of states (DOSs). All the core/shell NWs have a direct band gap at the Γ point showing similar properties to their homo-structure components. For FCIS systems with increasing ZnS shell thickness, the conduction band(CB) states are mostly affected, getting more strongly dispersed along kz and distinct from the other conduction states at the Γ point. For ISDC system, where core diameter decreases and shell thickness gradually increases, both the CB and valence band (VB) states are affected. In these cases while CB states are dispersed along kz , VB states become distinct from the deeper occupied states. Due to dispersion of these CB and VB states, band gap decreases in core/shell system than their homo-structure components.

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The partial density of states (PDOS) analysis gives further insight into the surface states of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NWs and the individual orbital contributions. For both FCIS and ISDC core/shell NW systems, the p orbitals of S atoms which are on the shell have the dominant contributions to the VBM, while the major contribution to the CBM comes from Zn-s, O-s and O-p states, located in the core region.

To assess the extent of charge separation at the interface of the heterostructures, the wave functions corresponding to the VBM(upper row) and CBM (lower row) of the FCIS core/shell NWs for three different shell thickness at the Γ point are presented in Fig.5 [the first column is for (ZnO)3 /(ZnS)1 , 2nd column for (ZnO)3 /(ZnS)2 and 3rd column for (ZnO)3 /(ZnS)3 system]. The same for three different ISDC core/shell NW systems are shown in Fig.6 [the first column is for (ZnO)5 /(ZnS)1 , 2nd column for (ZnO)4 /(ZnS)2 and 3rd column for (ZnO)2 /(ZnS)4 system]. In case of individual NWs the wave functions for both the VBM and CBM are uniformly distributed throughout the whole NWs. However, as it is evident from both the figures(Fig.5 and Fig.6) we see that for ZnO/ZnS NW, the VBM is predominantly localized in the exterior ZnS layer while the CBM predominantly localized in the interior ZnO core. Thus we see that this particular hetero NW system form type-II band alignment irrespective of the shell thickness and also of the variation both the core radius and shell thickness of ZnO/ZnS core/shell nanowire. This kind of spatial localization of VBM and CBM is in good agreement with other studies on this system. 43 The interfacial stress on the NW increases as the number of layers of the shell material increases. From figure 5 where we have shown the VBM and CBM for three core/shell systems with different shell thickness it is clear that with increasing shell thickness or stress on the NW, the charge on the VBM is more localized at the interface. So the charge separation decreases with increasing shell thickness or stress and this is attributed to the decrease in the surface electron density with increasing shell thickness. We would like to mention here that our discussion on localization is based on the spatial distribution of the electrons in molecular or crystal Kohn-Sham (KS) orbitals. For a proper quantitative description of an electron localization one should have first idea of an indication for such localization which can be gained from the spatial distribution of KS orbitals. Our

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result agree qualitatively with the experimental results of Zhu et 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

al. 45

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and also with the theoretical

study of Yang et al. 54 The localization of the electron and the hole in core and the shell material suggests the possibility of charge separation of an exciton into an electron and hole in this particular nano-heterostructure which would results an envelope-function-induced dark exciton. This dark exciton can suppress electron-hole recombination, resulting in an enhancement of the carrier collection efficiency 43 making this NW system very useful for solar cell application.

Finally, as the self-diffusion of atoms at the interface of a core/shell NW is very common occurrence we studied the self-diffusion of atoms in one representative (ZnO)3/(ZnS)2 core/shell NW and its effect on the band structure and band alignment of this system. We have considered two different cases: (i) diffusion of one O and one S atom at the interface (ii) diffusion of two O atoms and two S atoms at the interface. We have calculated the band structures of the systems after diffusion and have shown in figure S1 along with the band structure of the original system. The figure clearly shows that the diffusion of atoms (either one or two atoms) has very little effect on the band structures of the core/shell NW and accordingly has no effect on the band alignments.

3.2 Interaction of ZnO/ZnS core/shell heterostructure with anthraquinone(AQ) acid dye molecules Having some understanding of the electronic structure of ZnO/ZnS core/shell nanowire in detail, we now study the electronic structure of interaction of a dye molecule and ZnO/ZnS nanowire. We have considered anthraquinone-2,3-dicarboxylic acid (AQ) dye molecule adsorbed on ZnO/ZnS NW. In one of our previous paper 53 we have tested the transferability of our SCC-DFTB parameter set for Zn and its interaction with -H, -C, -O, -N etc. by studying the bond lengths and bond angles of a set of model Zn compounds and also modelling the Zn interactions with specific organic functional groups. The good agreement of these structural parameters with either the available experimental values or ab initio results gives us the belief that we can apply the present SCC-DFTB 10


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parameter set to model the NW-Dye molecule interactions. The HOMO-LUMO gap value of an1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

thraquinone acid molecule is 2.08 eV and the details of the energy levels near the gap and the plot of HOMO, HOMO-1, LUMO and LUMO+1 densities of the molecule are shown in figure S2. The performance of DSSC depends very much on the relative position of the energy bands of semiconductors and dye molecules. This is because the electron injection efficiency that describes the probability of photo-generated electrons transferring from the dye molecule to the semiconductor heterostructures largely depends on the relative position of VBM and CBM of semiconductors and HOMO and LUMO of dye molecules. 55–58 So, the proper tuning of the energy levels is utmost important. One way to control the distribution of states in the gap as well as to adjust the ionization potential/electron affinity level of a semiconductor NW system is the functionalization of its NW surface via adsorption of suitable ligands, such as organic molecules and also with introducing a coating by different semiconductor materials. 59 In this section we present our theoretical results of the effect of the ZnS shell thickness on the electron injection efficiency, charge separation and recombination dynamics of ZnO/ZnS core/shell NW - anthraquinone-2,3-dicarboxylic acid composite system. Our objective is to see how the energy levels (CBM and VBM) of the dye adsorbed ZnO/ZnS nano heterosystems are modulated by either changing the thickness of ZnS shell of ZnO/ZnS core/shell NW and also with AQ-acid dye with different functional groups. The adsorption of AQ-acid on ZnS shell of CdSe/ZnS type-I core-shell QD was investigated earlier by Zhu et al. 45 These authors have shown that the recombination rates decreases exponentially with the shell thickness. In this study we have considered two representative core/shell NW viz. (ZnO)2 /(ZnS)1, (ZnO)2 /(ZnS)2 for the adsorption of anthraquinone-2,3-dicarboxylic acid. We considered two repeating units of the (ZnO)2 /(ZnS)1 , (ZnO)2 /(ZnS)2 nanowire along Z-direction as a unit cell, containing 216 and 384 atoms respectively, per unit cell. The starting geometries were prepared by putting the AQ acid dye molecules perpendicularly at 3Å above the ZnO/ZnS NW surface and then relaxing the geometry of the composite systems to their total energy minimum. We consider one dye-molecule per units to avoid the interaction of the molecules with their periodic replicas. We have used (1 × 1 × 4) Monkhorst-Pack grid and a periodic box of 100Å along x and y 11



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direction in our calculation to avoid the neighboring interactions. From the optimized structure we 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

found that AQ-dye molecules binds with the NW in the Zn-site through carboxylic group creating two Zn-O bonds, with two consecutive NW surface Zn atoms. Two H atoms of the two carboxylic groups are being released to corresponding neighboring O atoms of the NW and two oxygen atoms of the carboxylic groups(C=O group) leads to formation of two parallel H-bonds bonded with NW surface and generate a stable structure. Fig.7 represents the band alignment of two representative ZnO/ZnS core/shell hetero systems with fixed core diameter and increasing shell thickness along with few anthraquinone-2,3-dicarboxylic acid having different functional groups. We also present the band alignment of some composite systems(hetero structure NW s and AQ-molecules) in the same figure. Here we wish to mention that we have tested the validity of the SCC-DFTB method ¯ NWs and comparing those with by calculating the band structures of both ZnO and ZnS [1010] the PP-PBE calculations. The close agreement between the two gives us the belief that the method can well describe the band alignments of the systems we studied here. We would like to analyze in detail the energy level alignment of the composite systems (anthraquinone-2,3-dicarboxylic acid, 7-amino-anthraquinone-2,3-dicarboxylic acid and 7-nitro-anthraquinone-2,3-dicarboxylic acid adsorbed on both (ZnO)2 /(ZnS)1 and (ZnO)2 /(ZnS)2 core/shell hetero structure NWs) to see the effect of shell thicknesses as well as the effect of different functional groups. Pure ZnO and ZnS are wide band gap semiconductor and type-II band alignment of hetero structure can be formed through the ZnS coating on ZnO nanowires. We have seen in the previous section that the overall band gap of the ZnO/ZnS hetero systems are low as compared to each homo-structure systems. This type-II band alignment forms appropriate conduction band edge lines to facilitate electron transfer. From the figure 7(a) and (b) it is seen that the conduction band(CB) is most affected with the increase in ZnS shell thickness. This can be clearly understood from the DOSs plot of figure 3. The figure shows that with increasing shell thickness the CB states although primarily localized in the ZnO core extends to ZnS shell because of the increased strain and thus shifted to lower energies. This lowering of CB energy with increasing shell thickness is in good agreement with the study of Yang et al. 54 and Lo et al. 60 From the figures it is evident that the position of the conduction

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band edge very much depends on the shell thickness and also on the nature of the functional group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of the dye molecule, so the electron injection rate depends accordingly. When anthraquinone-2,3dicarboxylic acid is adsorbed on ZnS shell in ZnO/ZnS core/shell heterosystem then the band gap of this composite systems becomes lower as compared to each individual systems. Band gap of bare (ZnO)2 /(ZnS)1 and (ZnO)2 /(ZnS)2 heterostructures with two units are 2.39 eV and 1.99 eV respectively, and the HOMO-LUMO gap of AQ-acid is 2.08 eV. When AQ-acid molecule is adsorbed on these NW surfaces then the band gap of the composite systems are 2.07 eV and 1.87 eV respectively. Thus we conclude that core/shell NW-Dye composite system would show red shift in their absorption spectra with increasing shell thickness of ZnO/ZnS core/shell NW. This pronounced red shift with increasing shell thickness is in good agreement with recent experimental studies on these core/shell NW systems and other related systems. 12,13,15,25 Now, whether or not the adsorption of AQ-acid and their derivatives on ZnO/ZnS core/shell NW will form type-II band alignment depends very much on the shell thickness and also on the nature of the (electron withdrawing or electron donating) functional group attached to AQ-acid. When ZnS shell thickness is small, then the conduction band of the core/shell NWs remain above the LUMO level of AQmolecule [Fig.7(a)] but with increase of shell thickness the CBM of core/shell heterostructure goes to positive (on the NHE scale) and then the LUMO of the AQ-acid dye molecule remains above the heterosystems whereas HOMO levels remain almost at the same position of the ZnO/ZnS NW. When the shell thickness is small as in Fig.7(a), all systems except NH2 -AQ system show type II band alignment. But for ZnO/ZnS core/shell NW with larger ZnS shell thickness, all systems including −NH2 -AQ have type II band alignment. So, for NH2 -AQ-acid dye -ZnO/ZnS NW composite system a type-I to type-II band alignment crossover occurs with the increase of ZnS shell thickness. Again due to attachment of electron withdrawing group in AQ-acid dye such as NO2 AQ-acid dye, the CBM of dye molecule goes to more positive (on the NHE scale) and AQ acid derivative with electron withdrawing group always forms a type-II band alignment. So from this band alignment we conclude that the nature of band alignment (type-I or type-II) and the nature of electron injection (either from dye to NW or NW to dye) very much depends on the shell thick-

13



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ness along with the nature of absorbed dye molecules. From the several systems, studied here, we 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

suggest that the composite system made of NH2 -AQ-acid dye -(ZnO)2 /(ZnS)2 NW is the most suitable for use in NW-dye sensitized solar cell as it has the proper band alignment that facilitates electron injection (LUMO level of the dye molecule is above to that of CBM of ZnO/ZnS NW) and also suppress the electron hole recombination (electron and hole wave functions are on different parts of the composite system) As the thickness of the ZnS shell in ZnO/ZnS core/shell NW increases, the CBM of the NW shifts downward so the ∆G for electron transfer from dye to the NW (the energy difference between LUMO of dye and CBM of NW) increases and ∆G for recombination (the energy difference between CBM of NW and HOMO level of the dye molecule) decreases. The increased ∆G for electron transfer and decreased ∆G for recombination of electrons will be beneficial for charge transfer and suppression of charge recombination, respectively.

The electronic band structure and DOSs of the interface of the composite systems are shown in Fig.8 and Fig.9 respectively. In the DOSs figure, the solid red lines represent the ZnO NW, the dotted green lines for ZnS NW and solid blue lines identify the projection on AQ-dye-molecules respectively. In all cases, Fermi energy is set to zero energy level. From both the band structure and DOSs plot we see that some new states appears within the band gap region of ZnO/ZnS heterostructure NWs due to adsorption of AQ-dye molecules, thereby modifying the electronic structures of the ZnO/ZnS heterostructures. The adsorption of AQ-dye molecules on ZnO/ZnS core/shell NW introduces some new states near both the VB and CB edge and these states are responsible for the reduced band gap of the composite systems(ZnO/ZnS heterostructures + AQ-dye molecules). From the figures, it is also evident that for (ZnO)2/(ZnS)2 − AQ acid and (ZnO)2 /(ZnS)2 − NH2 AQ acid composite systems, the CBM comes from ZnO/ZnS core/shell NW, however, it appears in lower energy as compared to CBM of isolated ZnO/ZnS NW system. The major contribution to the VBM of these composite systems comes from the AQ-acid molecule. However, for (ZnO)2 /(ZnS)2 − NO2 AQ acid composite systems, the CBM comes from the AQ-acid molecule while the major contribution to the VBM comes from the ZnO/ZnS NW.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To understand the charge separation at the interface of the composite systems, the wave functions corresponding to the lowest unoccupied and highest occupied Kohn-Sham states (CBM and VBM) are presented in Fig.10. For AQ-acid molecules, with electron donating group forming a composite system then the VBM is distributed over the dye molecules and CBM are localized on the ZnO core region but when an electron withdrawing group such as −NO2 is attached with AQacid molecule then the VBM is distributed through out the outer ZnS shell while CBM is localized on the adsorbed AQ-molecules. When shell thickness is small then VBM is localized on a particular region of outer ZnS shell but with increase of shell thickness these states are delocalized over the surface region of the outer shell. One of the most important factor that determines the efficiency of photovoltaic cell is the rate of charge recombination. Following Marcus theory 61 we can write down the rate of charge recombination as follows: 2π |H(d)|2 λ + ∆G(d)2 √ exp − k(d) = 4λ kB T h¯ 4πλ kB T

(1)

where k(d) is the thickness(d) dependent recombination rate constant, H is the electronic coupling strength, λ is the total reorganization energy and ∆G is the driving force. From the equation it is clear that the charge recombination rate greatly depends on H which in turn depends on the overlap of VBM and CBM wave functions of the composite system. It is found in the previous section that the electron and hole wave functions are spatially separated from each other so there is negligible overlap for the ZnO/ZnS NW-AQ acid dye molecule composite system and this also depends on the thickness of the ZnS shell. 21,45,58 So, we can conclude at least qualitatively that for this particular composite system, the charge recombination rate is low and will found be suitable for application in dye sensitized solar cell.

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4 Conclusion 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In summary, we have investigated the structural and electronic properties of H-passivated ZnO/ZnS core/shell nanowires with different core radius and shell thickness using the self-consistent charge density-functional based tight-binding (SCC-DFTB) method. We have shown that due to large lattice mismatch, a large and highly anisotropic strain is induced over the ZnO/ZnS core/shell nanowire, leading to a large change in electronic structures. The possibility of band gap engineering is explored either by changing the shell thickness or by both core diameter and shell thickness. Keeping core radius fixed, with increase in shell thickness, band gap of the heterosystems decreases due to both a reduction in confinement and a large strain resulting from lattice mismatch. We also explored the electronic structure of interaction of ZnO/ZnS NW and the dye molecule with a probable ways to optimize the photovoltaic performance. The efficiency of photovoltaic efficiency relies primarily on two factors: rate of charge injection and also the rate of recombination of the charge carriers. The rate charge injection increases as the energy offset between the the LUMO of the dye molecule and the CBM of the NW increases and the rate of charge recombination decreases with radial charge separation i.e. through the formation of type-II band alignment. We have shown the way one can tune the band edge energy levels of the NW-dye composite system by varying the shell thickness of the core/shell NW to attain proper energy level alignment for better electron injection and less electron-hole recombination. Thus, we found a transition from type-I to type-II band alignment with increasing shell thickness for ZnO-ZnS NW-AQ-NH2 acid composite system while ZnO-ZnS NW-AQ-NO2 composite systems always exhibit type-II band alignment irrespective of the thickness of the shell. Finally, although both ZnO-ZnS NW-AQ-NH2 acid and ZnO-ZnS NW-AQ-NO2 composite systems exhibit type-II band alignment, the localization of the VBM and CBM are on different parts. For ZnO-ZnS NW-AQ-NH2 acid composite system, the VBM mainly localizes on the molecule while CBM is on the ZnO core but for ZnO-ZnS NW-AQNO2 composite system it is the ZnS shell, where the VBM is distributed and CBM is localized on the molecule. We hope that the results of this study have thrown some light on the possible ways to tune the band gap along with band alignment by controlling the core diameter and shell thickness 16


Page 17 of 35


to achieve efficient ZnO/ZnS semiconductor NW based dye sensitized solar cells. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Acknowledgement The first two authors would like to thank the CSIR, New Delhi for the award of senior research fellowships. The financial support from DST, Govt. of India[Ref. No. SR/NM/NS-47/2009] is gratefully acknowledged. Supporting Information Available The band structures of (ZnO)3 /(ZnS)2 core/shell NW before and after diffusion of atoms. The electronic energy levels near the gap and the densities of the orbitals close to the gap of AQ acid dye molecule. This information is available free of charge via the Internet at http://pubs.acs.org.

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The Journal of Physical Chemistry 3 2 1 0

Band gap (eV)

5 4

(ZnO)3/(ZnS)n

3

(ZnO)3/(ZnS)0

2

(ZnO)3/(ZnS)1

1

(ZnO)3/(ZnS)2 (ZnO)3/(ZnS)3

0 0

Energy(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2

3

4

(ZnO)3/(ZnS)4

n

-1

CBM

-1.72

-2 -3.07

-3

-3.45

-4

-4.25

-4.35

-5 -5.65

-5.87

-5.80

5.24

5.70 5.80 5.90 Lattice parameter (c in Å)

-5.88

-5.69

VBM

-6 -7

6.00

Figure 1: Band alignment of FCIS ZnO/ZnS heterostructure NW systems. The dotted line indicates the VBM and CBM energy levels of ZnO NW without ZnS shell. In the inset, the variation of band gap is shown as a function of ZnS layer.

25



1 0 -1

VBM-CBM gap(eV)

2 5

(ZnO)6-n/(ZnS)n

4

(ZnO)6/(ZnS)0

3

(ZnO)5/(ZnS)1

2

(ZnO)4/(ZnS)2

1

(ZnO)3/(ZnS)3 (ZnO)2/(ZnS)4

0 0

1

2

3

4

5

(ZnO)1/(ZnS)5

n

Energy(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-2

CBM

-2.33

-3

-3.38

-3.57

-4

-3.74 -4.25

-4.45

-5 -6 -7

-6.11

5.24

-5.87

5.60

-5.79

-5.88

5.70 5.90 Lattice parameter (c in Å)

-5.86

-5.96

6.10

6.20

VBM

Figure 2: Band alignment of ISDC ZnO/ZnS heterostructure NW systems. The dotted line indicates the VBM and CBM energy levels of ZnO NW without ZnS shell. Variation of band gap is shown in the inset.

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4 (a)

PDOS Zn(s) PDOS Zn(p)

E (eV)

PDOS Zn(d) PDOS S (s) PDOS S(p)

0

PDOS S(d) PDOS O(s) PDOS O(p)

-3 4

Γ

PDOS H(s)

Z

E (eV)

(b)

0

-3 4

Γ

Z

E (eV)

(c)

0

-3 4

Γ

Z

(d)

E (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-3

Γ

Z

PDOS (arb. units)

Figure 3: Electronic band structure along with PDOS of FCIS (a)(ZnO)3 /(ZnS)1, (b)(ZnO)3 /(ZnS)2 , (c)(ZnO)3 /(ZnS)3 and (d)(ZnO)3 /(ZnS)4 heterostructure NWs. The Fermi level is set to at zero energy level and is shown by dotted line.

27



4

PDOS Zn(s)

(a)

PDOS Zn(p)

E (eV)

PDOS Zn(d) PDOS S(s) PDOS S(p)

0

PDOS S(d) PDOS O(s) PDOS O(p)

-3 4

Γ

PDOS H(s)

Z

E (eV)

(b)

0

-3 4

Γ

Z

E (eV)

(c)

0

-3 4

Γ

Z

(d)

E (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

0

-3

Γ

Z

PDOS (arb. units)

Figure 4: Electronic band structure along with PDOS of ISDC (a)(ZnO)5 /(ZnS)1, (b)(ZnO)4 /(ZnS)2 , (c)(ZnO)2 /(ZnS)4 and (d)(ZnO)1 /(ZnS)5 heterostructure NWs. The Fermi level is set to at zero energy level and is shown by dotted line.

28


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5: Band-edge wave functions of the ZnO/ZnS FCIS heterostructure NWs. 1st row represent the valance band maximum(VBM)(red) and 2nd row for conduction band minimum(CBM)(blue). Grey, yellow, red, and white spheres represent Zn, S, O, and H atoms, respectively.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

Figure 6: Band-edge wave functions of the ZnO/ZnS ISDC heterostructure NWs. 1st row represent the valance band maximum(VBM)(red) and 2nd row for conduction band minimum(CBM)(blue). Grey, yellow, red, and white spheres represent Zn, S, O, and H atoms, respectively.

30


Page 31 of 35

-3.0

Band gap (eV)

-3.5

(a)

CBM

-3.68 -4.07

-4.0

-3.66

-3.93

-4.5

-4.64

-4.47

-5.0 -5.5 VBM

-6.0

-6.15 AQ

-6.5

-5.79

-6.01 Q

NW+A

-5.8

H -AQ NH 2-AQ NW+N 2

-6.62

-6.17

O 2-AQ

NW+N

NO 2-AQ

-7.0 -3.0

(b)

-3.5

-3.68

CBM

Band gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4.07

-4.0

-4.02

-4.06

-4.36

-4.5

-4.64

-5.0 -5.5 VBM

-6.0

-6.15

-6.5

AQ

-5.79

-5.94 Q

NW+A

-5.71

-AQ NH 2-AQ NW+NH 2

-6.62

-6.16

O 2-AQ

NW+N

NO 2-AQ

-7.0

Figure 7: VBM(red) and CBM(blue) energy alignment of NW-dye composite systems(ZnO/ZnS core/shell heterostructure NWs and AQ-molecules with different functional group). (a)(ZnO)2 /(ZnS)1+dye molecules and (b)(ZnO)2 /(ZnS)2+dye molecules. Dotted lines represent the VBM and CBM of corresponding ZnO/ZnS core/shell NWs without dye molecule.

31



Energy (eV)

[a]

[b]

3.0

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0

-0.5

-0.5

-1.0

-1.0

-1.5

-1.5

Γ

Z

Γ

[c]

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0

-0.5

-0.5

-1.0

-1.0

Γ

Z

[d]

3.0

-1.5

Page 32 of 35

-1.5

Z

Γ

Z

Figure 8: Electronic band structure for (ZnO)2 /(ZnS)2 core/shell NW and different functionalized AQ-dye composite systems.(a)(ZnO)2/(ZnS)2, (b)(ZnO)2 /(ZnS)2+AQ-acid, (C)(ZnO)2 /(ZnS)2 + NH2 -AQ-acid and (d)(ZnO)2 /(ZnS)2 + NO2 -AQ-acid. The Fermi level is set to at zero energy level.

32


Page 33 of 35

[a]

[b]

PDOS_ZnO-NW

DOS (arb. unit)

PDOS_ZnS-NW PDOS_AQ-Molecule

0 [c]

[d]

DOS (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5

Energy (eV)

Energy (eV)

Figure 9: Projected density contributions(PDOS) of ZnO(solid red lines) NWs, ZnS(dotted green lines)NWs and dye molecules (solid blue lines) in the ZnO/ZnS core/shell heterostructure NW and NW-dye composite systems. (a)(ZnO)2 /(ZnS)2, (b)(ZnO)2 /(ZnS)2+AQ-acid, (C)(ZnO)2 /(ZnS)2 + NH2 -AQ-acid and (d)(ZnO)2 /(ZnS)2 + NO2 -AQ-acid respectively. The Fermi energy is set to at zero and is shown by dotted line.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

Figure 10: Band-edge wave functions of the composite nanosystems ((ZnO)2 /(ZnS)2 coreshell NW + AQ-NH2 and AQ-NO2 dye molecules) . 1st row represent valance band maximum(VBM)(red) and 2nd row represent conduction band minimum(CBM)(blue).1st column for (ZnO)2 /(ZnS)2 + NH2 -AQ-acid dye and 2nd column for (ZnO)2 /(ZnS)2 + NO2 -AQ-acid dye respectively. Grey, yellow, red, blue and white spheres represent Zn, S, O, N and H atoms, respectively.

34


Page 35 of 35

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Table of Content

35


Shell Nanowires To

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