Coincident Site Epitaxy at the Junction of AuâCu2ZnSnS4

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Coincident Site Epitaxy at the Junction of Au-Cu2ZnSnS4 Hetero-nanostructures Biplab K. Patra, Arnab Shit, Amit K. Guria, Suresh Sarkar, Gyanaranjan Prusty, and Narayan Pradhan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504434q • Publication Date (Web): 12 Jan 2015 Downloaded from http://pubs.acs.org on January 19, 2015

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Chemistry of Materials

Coincident Site Epitaxy at the Junction of Au-Cu2ZnSnS4 Heteronanostructures Biplab K Patra,† Arnab Shit,¥ Amit K Guria,† Suresh Sarkar,† Gyanaranjan Prusty,† and Narayan Pradhan*† ¥

†

Department of Materials Science and Center for Advanced Materials, Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata, India 700032 KEYWORDS: Heterostructures, Epitaxial Junction, Au-Cu2ZnSnS4 (Au-CZTS), Photoresponse, Photocurrent ABSTRACT: Considering the chemistry of formation and physics at the interface, we report on the heterostructure of a promising new energy material, Au-Cu2ZnSnS4 (Au-CZTS), and investigate the impact of coupling on Au in improving the photo-stability as well as the photo-response behavior. We focus primarily on the fundamental issues involved in bringing together two dissimilar materials having different chemical and physical properties in a single building block where one is a multinary semiconductor nanomaterial and other is a plasmonic noble metal. The formation of heteroepitaxy at the junction of Au and CZTS has been investigated for two different phases of CZTS. Considering epitaxy formation along the {111} planes of Au, it is observed that the wurtzite and tetragonal phases of CZTS exhibit coincident site epitaxy with different periodic intervals. A detailed study of this epitaxy formation with Au in both phases of CZTS has been carried out and reported. Since Au-CZTS is a promising new material, we have further investigated its photo-current and photoresponse behavior and compared with pure CZTS. We believe these findings will help the community, providing guidelines for investigating new functional materials and their applications.

has been studied for Au-CdSe27,28 and Au-ZnSe;7 in both cases the large lattice mismatch is compensated by coincident lattice matching at periodical intervals. Preferential crystallographic alignment is essential for obtaining stability of these materials;27 hence, study of the epitaxial junction formation is critically important.

INTRODUCTION Metal-semiconductor hetero-nanostructures are emerging as new functional materials for efficient photogenerated carrier transport, which is typically required for various photo-induced applications.1-15 Fabrication protocols for different heterostructures, where two different materials having different physical and chemical properties are brought together in a single building block, can often elucidate fundamental chemistry and provide new physical insights at the interfacial junction.6,11,12,16-21 The coupled metal strongly influences the photo-generated carrier movement which in turn influences the absorption behavior as well as the carrier transport properties of the semiconductor.8,9,14,22 Literature reveals that a large number of such materials with different shapes/phases has been designed in solution, and several induced material properties have been studied in detail.3,4,7,12,15,16,18-20,23-31 Among these, binary semiconductor nanostructures coupled with Au have been widely reported.7-11,13,16,23,28,32-41 However, in spite of the number of such reports, detailed studies on epitaxy formation are limited. Typically, the lattice mismatch between Au and the semiconductor nanomaterial is high, and hence it is critically important to understand how such structures are formed. Recently, the formation mechanism of epitaxy at the hetero-junctions

Recent development in the field of energy materials, which is one of the most demanding fields of current research, reveal that multinary semiconductors with variable compositions are emerging materials with exciting new functional properties. To date, extensive efforts have been put forward for the coupling of Au with binary nanocrystals; but coupling of Au with quaternary semiconductors, studying the synergic effect of plasmon– exciton coupling and studying carrier movement processes remain important issues. CZTS is one of the next generation materials having absorption in the solar spectral window42,43,52-62 and provides potential opportunities for photovoltaic applications. Very recently, it has also been established that Au-CZTS can act as an efficient visible light photo-catalyst for hydrogen evolution from water, as well as for dye degradation.62,63 The core issue in designing Au-CZTS structures is the formation chemistry of the hetero-junction. A few

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very recent reports have appeared on the synthesis of AuCZTS62, 63 while our work was underway, but the details of the crystallographic phase orientation and the microscopic analysis at the interface of the metal-semiconductor hetero-junctions have not been investigated. CZTS nanostructures are primarily reported in two phases: wurtzite (WZ) and tetragonal (Tg). Coupling of facecentered cubic (fcc) Au(0) with these different phases would require different chemical as well as physical pathways. However, the latest understanding of heterostructure formation between fcc Au-CdSe (WZ) and fcc Au-zinc blende ZnSe suggests that the hetero-junctions can be formed by coincident site epitaxy.7,27,28 As CZTS is a multinary nanocrystal, the epitaxy formation with fcc Au(0) on two different phases of CZTS may involve different combinations of the lattice planes to minimize the interfacial energy, and can also introduce new materials properties which need to be investigated in depth.

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X-ray Diffraction (XRD). XRD of the purified sample was taken by a Bruker D8 Advance powder diffractometer, using Cu Kα (λ= 1.5406 Ǻ) as the incident radiation. Current-Voltage (I-V) and Photo-response Measurement. Current-voltage (I-V) characteristics under dark and various illumination conditions were recorded with a Keithley 2401 source meter. The photocurrent was measured by illuminating the samples with white light at 100 mW/cm2. A 150 W Newport-Stratfort Solar Simulator model 65005, equipped with AM 1.5 filters, acted as the source for illumination. Using the same light source we also measured the photo-response properties of these materials. Synthesis Methods. (a)Preparation of Wurtzite (WZ) CZTS nanostructures. Wurtzite CZTS nanocrystals were prepared by following a modified method from the prior literature.55

Considering all these issues, we report the formation of heteroepitaxy at the junction of the goldmultinary semiconductor heterostructure, Au-CZTS. CZTS with two different crystal phases has been synthesized and their epitaxy formation with Au(0) has been analyzed. In both cases, coincident site epitaxy at different combinations of the Au and CZTS facets has been distinguished as the key factor for the formation of the epitaxial junction. The detailed studies including the chemistry of formation, epitaxy at the junction, and the lattice matching were carried out systematically. In addition, the carrier transport properties following the photoresponse activation of these new nanostructures were also studied.

In a typical synthesis, copper (II) acetylacetonate (0.06 g, 0.25 mmol), zinc acetate (0.048 g, 0.25 mmol), tin (IV) chloride (0.088 g, 0.25 mmol), TOPO (1g, 2.58 mmol), were loaded with 4 ml of ODE in a three-neck round bottom flask and degassed under N2 flow at room temperature for ~20 min. Then the reaction mixture was heated to 260 °C, maintained for 5 min and cooled to 200 °C. At this temperature, a mixture of 1 ml DDT and 1 ml t-DDT was rapidly injected into the reaction flask. The reaction mixture was further annealed at that temperature for 1 hr. Finally the nanocrystals were purified with acetone as a nonsolvent and then dispersed in chloroform for further use.

EXPERIMENTAL SECTION

(b) Preparation of fcc Au-CZTS (WZ) Twin Heterostructures.

Chemicals. Tin (IV) chloride (98%), copper (II) acetylacetonate (>99.99%), tin (IV) acetate (>99.99%), zinc acetate (>99.99%), trioctylphosphine oxide (TOPO, 99%), 1-octadecene (ODE, 90% tech), 1-dodecanethiol (1-DDT, 98%), tert-dodecyl mercaptan (t-DDT), oleic acid (tech., 90%), oleylamine (tech., 70%), S powder (99.98%), gold (III) chloride hydrate (HAuCl4. H2O, 99.9%), poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and indium tin oxide (ITO) coated glass slides were purchased from Sigma Aldrich. All chemicals were used without further purification.

Preparation of Au stock solution: The stock solution of gold was prepared by dissolving HAuCl4 (0.068 g, 0.2 mmol) in 2 ml of oleylamine in a 10 ml vial at 70 °C under N2 atmosphere for 2 min. Then the solution was cooled to room temperature and used as the gold source for the preparation of Au-CZTS heterostructures. In a typical synthesis, copper (II) acetylacetonate (0.06 g, 0.25 mmol), zinc acetate (0.048 g, 0.25 mmol), tin (IV) chloride (0.088 g, 0.25 mmol) and TOPO (1g, 2.58 mmol) were loaded with 4 ml of ODE in a three-neck round bottom flask and degassed by purging with N2 at room temperature for ~20 min. The reaction mixture was then heated to 260 °C, maintained for 5 min and cooled to 200 °C. Next, a mixture of (1 ml DDT and 1 ml t-DDT) was prepared separately and rapidly injected into the reaction flask. Soon after the injection (5-20 sec), the stock solution of gold was swiftly injected and the mixture was further annealed for 60 min. The twin structures were formed and then purified after cooling the reaction to room temperature by using acetone as a nonsolvent and chloroform as the solvent.

Optical Measurements (UV-VIS). UV-Vis measurements were taken with an Agilent-8453 UV-Vis spectrophotometer. Transmission Electron Microscopy (TEM). TEM, and High Resolution TEM images were taken on a UHR-FEGTEM, JEOL, JEM 2100 F model using a 200 kV electron source. Specimens of the samples were prepared by dropping a purified nanocrystal solution in chloroform on a carbon coated copper grid, and the grid was then dried in air.


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(c) Preparation of Tetragonal CZTS Nanostructures. Tetragonal CZTS nanostructures were prepared by following a modified method from the prior literature.42

RESULTS AND DISCUSSION Synthesis protocol and the optical changes in the formation of Au-CZTS (WZ) and Au-CZTS (Tg) heterostructures.

Preparation of S stock Solution: Sulfur stock solution was prepared by dissolving 1 mmol of S powder in 2.5 ml of ODE in a three neck flask at 155 °C under N2 atmosphere. After 60 min, the color of the solution turned to light yellow. Then the reaction was stopped and this solution was used as the S source.

For fabrication of Au-CZTS heterostructures, we have adopted a typical synthesis protocol where the nucleation of CZTS occurs on the surface of pre-formed Au(0) particles. From literature, it is understood that epitaxy formation usually follows one of three different chemical processes: The first is the growth of one material on the other epitaxially,28 the second is the recrystallization of one pre-formed material on the other27 and the third is the thermal fusion of appropriate lattice planes after both materials have pre-formed.7,27 Here we have followed the first case where Au particles are synthesized separately and introduced into the reaction system containing the precursors of Cu, Sn and Zn for the heterostructure formation at the desired temperature (see experimental section)

For the CZTS synthesis, copper (II) acetate (0.045g, 0.25 mmol), zinc acetate (0.022g, 0.12 mmol), tin (IV) acetate (0.045g, 0.12 mmol), oleic acid (0.8 ml), oleylamine (1 ml), ODE (2ml) and S (2.5 ml from stock) were mixed in a three neck flask and degassed for 20 min under N2. The temperature of the reaction system was increased to 240 °C and the reaction mixture was annealed for 60 min. Then, the reaction was cooled down to room temperature and the obtained tetragonal phase CZTS nanocrystals were purified using acetone as a nonsolvent and chloroform as the solvent.

The heterostructure formation was monitored spectroscopically as well as microscopically. The presynthesized gold particles have a prominent plasmon absorption peak at 530 nm (Figure 1) which becomes broadened and red-shifted to 630 nm within a minute of its treatment in the reaction flask; this reflects the formation of the heterostructure. Similar behavior has also been observed in the heterostructures of Au with other semiconductors due to the delocalization of the Au plasmon over the semiconductor.7,9,10,13,17,35 However, a shift of ~100 nm is significant and suggests that efficient coupling of the Au plasmon with the exciton of CZTS is occurring here. On the other hand, when the Au particles are introduced at the beginning, core/shell structures of Au/CZTS are obtained (Figure S1). However, for the study of epitaxy, we preferred analyzing these twin-shaped Au-CZTS heterostructures because lattices of both the entities in these structures are distinctly visible under microscopic analysis.

(d) Preparation of Tetragonal Au-CZTS Twin Heterostructures. For Au-CZTS, in a typical synthesis, copper (II) acetate (0.045g, 0.25 mmol), zinc acetate (0.022g, 0.12 mmol), tin (IV) acetate (0.045g, 0.12 mmol), oleic acid (0.8ml), oleylamine (1ml) and ODE (2ml) were loaded in a three neck flask and degassed for 15 min by purging with N2. Then the reaction temperature was raised to 240 °C. Next, 2.5 ml of S stock solution and 2 ml of gold stock solution were mixed together in a vial and injected into the reaction flask. Soon after the injection, the temperature was lowered to 220 °C and the reaction mixture was annealed for 60 min. Then tetragonal Au-CZTS nanostructures were formed and purified using acetone as a nonsolvent and dispersed in toluene for further use. (e) Device Fabrication. For the device fabrication, ITO-coated glass slides were etched using Zn dust/HCl keeping the width of the ITO portion at 2 mm. Then the slide was washed several times using soap water/acetone/ethanol. Then a holeconducting polymeric layer (PEDOT:PSS) was spin coated at 2000 rpm for 20 s over the ITO. The film was annealed at 100 °C for 10 min. Next, purified CZTS nanoparticles dispersed in a chloroform/toluene mixture were spin coated over the hole-transporting layer at the same rotational speed and dried under an inert atmosphere at 100 °C for 10-20 min. Approximately five to six layers of CZTS nanoparticles were spun onto the device following the same procedure. Lastly, the film was heated to 160 °C for 45 min to remove excess ligands. At this point it was ready for the I-V characteristics and the photosensitivity of the material to be measured. The procedure for making the film was same for CZTS and Au-CZTS. In both the cases, the same concentration of the materials was used for device fabrication.

To confirm the crystal phase of Au-CZTS, we have carried out powder X-ray diffraction of the purified samples; the observed patterns are shown in Figure 2. In both cases, the peaks at two theta values of 38, 44.4 and 64.5 degree are reflected from fcc Au(0). In the first case(Fig 2a), the peak positions are matched with WZ CZTS55 and the second one (Fig 2b), for Tg CZTS (JCPDS # 26-0575). Microscopic analysis of Au-CZTS (WZ) and Au-CZTS (Tg) heterostructures. Figure 3 shows the Transmission Electron Microscopic (TEM), High Resolution TEM, Fast Fourier Transform (FFT) and simulated HRTEM images of Au-CZTS (WZ) heterostructures. Figure 3a presents a wide view of the TEM images showing the twin shapes of the heterostructures, where Au(0) is connected preferably on the



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side of CZTS. Figures 3b-3d show the HRTEM images of a single heterostructure. Figure 3d depicts a HRTEM image where we have analyzed the FFT (Figure 3f and 3g); the simulated HRTEM along the epitaxial direction is shown in Figure 3h. The selected area FFTs of Figure 3e are shown separately in Figure 3f and 3g focusing on Au-CZTS and on bare CZTS respectively. The dark contrast of the image is the Au(0) and the light contrast part is the CZTS. The circle drawn in the FFT pattern obtained from area 1 (CZTS) and shown in Figure 3f contains three planes (010), (100) and (1 0) with d-spacing = 0.338 nm. This indicates that the CZTS nanostructure is in the WZ phase and it is viewed along the WZ C-axis. On the other hand, the FFT pattern obtained from area 2 where both Au and CZTS are present reflects both Au as well as CZTS planes. Comparing the FFT with Figure 3f, it can be easily concluded that the d-spacing of 0.235 nm is the reflection of the fcc Au (111) plane. This clearly implies that the epitaxy is formed between the (100) plane of CZTS and the (111) of Au. This is different from the WZ CdSe-fcc Au case, where the epitaxy has been observed between the (001) plane of CdSe and the (111) plane of Au. For clarity, we further simulate the HRTEM from the FFT pattern. Considering only the epitaxy along {111} planes of Au(0), the created simulated HRTEM is shown in Figure 3h. Interestingly, coincident site epitaxy is observed here with 3×d(100) of CZTS nearly equal to 4×d(111) of Au. For more visibility, lines are marked on both sides of the planes.

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matching of 4×d(111)fcc Au 5×d(220)Tg CZTS. Details of the lattice mismatch in these planes and the commensurate misfit are shown in the supplementary information (Table S1). Prior literature reports that three unit cells of fcc Au(0) with {111} facets coincide with two unit cells of wurtzite (WZ) CdSe with {002} facets to form the fcc AuCdSe (WZ).27 This reduces the typical lattice mismatch from ~ 50% to only 3.5%. A schematic model of such matching is shown in Figure 5a. We can simply state that the sum of three d-spacings of fcc Au (111) nearly matches with the sum of two d-spacings of WZ CdSe (002). Similar behavior has also been found for fcc Au(0) with zincblende (ZB) ZnSe where the sum of four unit cells on the {111} facets of Au(0) matches closely with that of three unit cells of ZnSe {111}.7 The coincident lattice matching of this combination is shown schematically in Figure 5b. In summary: It has been found that WZ CZTS matches with 4×d(111) Au 3×d(100)CZTS, and that Tg CZTS, matches with the ratio 4d(111)Au 5d(220)CZTS. Representative schematic presentations of both types of lattice matching are shown in Figure 5. From all the above results on the epitaxy formation between the Au (111) facet with different facets of WZ and Tg CZTS, it is clear that the lattice mismatch at the interface is the key issue for the formation of heterostructures. Moreover, it is also clear that in spite of large lattice mismatch, the planes can approach each other and form epitaxy following the coincident lattice matching which is observed in periodical intervals between the two entities. Calculation of the energy of the facets is certainly important, but in this article, we mostly focus on the variable lattice matching possibilities in different phases of the same materials on fcc Au. A detailed density functional theory calculation is warranted as a topic for further research.

But the scenario of the epitaxy formation at the junction in Tg CZTS-fcc Au is different. Here, the (111) plane of Au(0) forms the epitaxy with a different plane of Tg CZTS than that of the WZ CZTS. Figure 4a and 4b show the typical TEM and HRTEM images of the Tg CZTS-Au heterostructures (also see Figure S2). Figure 4c also presents the HRTEM image of a single heterostructure which we have analyzed in detail in a later section. Further, to understand the detail of the phase of CZTS, the selected area FFT pattern is analyzed and depicted in Figure 4d from the area 1 of Figure 4c. The circle marked in the pattern in Figure 4d has a diameter of 10.273 nm-1, which reflects the planes of Tg CZTS with d-spacing of 0.195 nm. The 60 degree angle between each plane confirms the (1 0), (100) and (010) facets, and a typical atomic model corresponding to this atomic arrangements is shown in Figure 4e. This suggests that the Au-CZTS HRTEM image in Figure 4c is viewed along the [ 21] direction. Further, to understand the epitaxy along (111) of Au(0), the selected area FFT pattern is obtained from area 2 where both Au(0) and CZTS are present and shown in Figure 4f. The spots with distance 8.5 nm-1 marked here reflect the d-spacing of 0.235 nm which is from (111) planes of Au(0). Masking only the (111) of Au(0) and (220) of CZTS which remain in one line in the FFT pattern we have separated them out and show them in Figure 4g. These planes are further simulated and the obtained HRTEM is provided in Figure 4h. Here, lines are marked for clarity about the coincident lattice matching and from these results it is clear that the epitaxy is formed with

Chemistry of formation of the epitaxy of the heterostructures. In spite of the vast prior literature on the synthesis of heterostructured materials involving Au, information on preferential crystallographic planes between the two materials is limited. Hence, it is important to understand the formation chemistry of the heterostructures of Au-CZTS with their epitaxial junctions. The reaction pathways reveal that the formation of CZTS is mostly triggered on the surface of Au. When pre-formed CZTS and Au are mixed and annealed at temperatures and conditions similar to those used in our study, we do not observe the heterostructures (Figure S3); hence the S precursors along with Au are the key factors for initiation of the heterostructure formation. We assume that a thin atomic layer of gold sulfide is first formed which initiates the CZTS formation. In addition, the presence of Au does not alter the nucleation and growth of the respective phase here. However, the interesting thing is that the epitaxial growth


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Chemistry of Materials from the materials WZ CZTS and Au-CZTS (WZ) respectively. Interestingly, the current intensity was observed to gradually decrease for plain CZTS; this was confirmed by several sets of repeated experiments. On the other hand, the peak current level remains almost the same for AuCZTS materials even after 6 cycles. This corroborates the improved photo-stability of the multinary CZTS materials while coupled with Au, which can produce almost the same amount of photo current with continuous on/off cycles. Apart from the stability, a significant increase in the intensity of the current flow has been noticed; this further supports the enhancement of the efficiency of the photo-response of the coupled materials compared to pure CZTS. Similar differences in the photo-response properties have also been observed for Tg CZTS and AuCZTS (Tg) (See Figure S5 of supplementary information).

of CZTS on the surface of Au(0) has been initiated as per the coincident site epitaxy conditions required for the formation of heterostructures. The mechanism is not like that of Au-ZnSe or the warm like structure of Au-CdSe reported by Figuerola et al.27 where the Au particles are mixed with pre-synthesized semiconductor nanocrystals and then annealed. Here, the two materials orient with preferential facets and then the epitaxy commences. Since the prior existence of the Au particles seems to be a requirement in our case, we propose the growth of the semiconductor on the preferential facets of Au. Our synthesis process also suggests that high temperature is not necessary for formation of such structures; rather a moderate reaction temperature can also lead to high quality nanostructures with clear epitaxial hetero-junctions. A recent report on CZTS synthesis64 suggests that Cu2S nanostructures are formed first, and then other cations are diffused. Even though we did not derive the mechanistic process of CZTS, we still believe that such a process can also be possible even if the nanostructures nucleate on Au.

CZTS, being a multinary material, contains defects and the photo-generated charge carriers are normally trapped in these defect states. Literature reports suggest that for fast transfer or separation of these carriers, in some cases the traps are filled with proper dopants that enhance the efficiency of the photodetector.65 We have noticed similar behavior in the case of our heterostructures where the photo-generated charge carrier has been trapped by the noble metal. For understanding the details of this process, we analyze the possible fate of the plasmon and the semiconductor exciton on photo-excitation. It has been reported that the surface plasmon (SP) state of Au lies close to the conduction band of CZTS.62 As a consequence, the plasmon electrons couple with the semiconductor excitons and vice versa, and are delocalized.66 This idea is corroborated by the loss of the original Au plasmon absorption characteristics as shown in Figure 1. Under illumination, the excited electron of the semiconductor migrates through the gold SP states, resulting in high photocurrent enhancement as well as better photoresponse. A plausible band position alignment of the CZTS conduction band, the Au SP state and the Al state is shown in Figure S6 which also suggests that the electrons can migrate from the CZTS conduction band to the Au and then transfer via the Al electrode, resulting in efficient photocurrent generation.

Study of the carrier transport, improved stability and efficiency of the Au-CZTS photodetector. In this study, we mainly focused on the heteroepitaxy formation, but exploration of new properties of these new materials is also desirable. Hence, we have designed appropriate devices to study the photo-current and photo-response behavior of these heterostructures. CZTS is an active material for absorbing solar light and can generate the charge carriers, while Au is expected to improve carrier transport. We have observed better photo-stability and higher photocurrent of the coupled materials in comparison to simple CZTS. To fabricate devices, we deposited a holeconducting polymeric layer (PEDOT:PSS) on ITO-coated glass and then the samples of hetero-nanostructures were spin-coated on top of the polymeric layer. Around 5-6 sample layers were deposited on the glass substrate. Next, Al was deposited as an electrode for the current-voltage measurement. The same measurement has also been performed using plain CZTS for comparison. A schematic view of the device fabrication is shown in Figure 6a, and the scanning electron microscope (SEM) image of a portion of the device is presented in Figure S4 (see supplementary information).

CONCLUSIONS In conclusion, we have studied the formation of epitaxial hetero-junctions of fcc Au with two different phases of CZTS. Considering the lattice along the {111} facets of Au, epitaxy has been observed along (100) of WZ and (220) of Tg CZTS in spite of their large lattice mismatches. From the simulated HRTEM, we conclude that in both phases the hetero-junction has coincident site epitaxy, but with different ratios for the different phases. For fcc Au-CZTS (WZ), the lattice matches when 4×d(111) Au ≅ 3×d(100) CZTS, and for fcc Au-Tg CZTS, it matches when 4×d(111) Au ≅ 5×d(220) CZTS. This result provides convincing evidence that heteroepitaxy can occur between two different materials irrespective of their large lattice mismatch. Moreover, our study was conducted with a promising new

Figures 6b and 6c show the current versus voltage plots of WZ CZTS and Au-CZTS (WZ) nanostructures in the dark and under illumination. In both cases, the photocurrent gain was higher for Au-CZTS than for plain CZTS. From the plots, we calculated the photocurrent gain (Iphoton /Idark). In the case of CZTS at 1 volt bias, (Iphoton /Idark) is 1.5; the value for Au-CZTS is 4.2. Further, we have carried out the current versus time measurement by turning the polychromatic illuminated light (Intensity 100 mW/cm2, Xe lamp) ON and OFF at 20 sec intervals at constant bias. Figure 6c and 6d show the plots obtained



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material, Au-CZTS, which has several new functional properties required for today’s technological applications. We have also studied the photocurrent and photoresponse behavior of Au-CZTS, and compared it with only CZTS. The results are promising and the coupled heterostructures not only increase the photo-stability of the materials but show better photodetector device performances. Hence, the study provides new outcomes to strongly support the fundamental science of the formation of heteroepitaxy in two dissimilar materials, and also shows a pathway for designing the functional materials required for advanced technology.

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ASSOCIATED CONTENT Supporting Information Six supporting figures composed of additional TEM, SEM, Current-Voltage plots and also a schematic diagram with a supporting Table of Lattice Mismatch are available in the supporting information. This material is available free of charge at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGEMENTS We acknowledge the DST of India for funding this study.

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Figures and Captions:

Figure 1. Absorption spectra of pre-synthesized Au particles (< 10 nm), Au-CZTS (WZ) hetero-structure and pure CZTS particles. After CZTS formation on Au(0) particles, the sharp plasmon peak is broadened and red-shifted.

Figure 2. Powder X-ray diffraction pattern of (a) WZ CZTS and (b) Tg CZTS. The JCPDS reference number for Tg 55 CZTS is 26-0575; for WZ CZTS, the peaks are matched from data provided in the literature.


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Figure 3. (a) and (b) TEM images of twin-shaped Au-CZTS nanostructures at different resolutions, (c and d) HRTEM images of single Au-CZTS twins. (e) A typical HRTEM image where Au and CZTS areas are marked to achieve the FFT of the selected area. (f and g) FFT from area 1 and 2 of the HRTEM presented in (e), respectively. (h) Simulated HRTEM along the interface of (111) of Au(0) and (100) of CZTS.

Figure 4.(a) TEM image of Au-Tg CZTS twin structures. (b and c) HRTEM of single twin Au-CZTS structures; in (c) areas near Au(0) and on CZTS are marked. (d) FFT from area 1 and (e) is a schematic atomic model designed as per the FFT in (d). (f) FFT from the selected area of (2) in panel (c). (g) FFT masking only the spots along the junctions of Au (111) and CZTS (220) facets. (h) Simulated HRTEM as per the FFT in panel (g).



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Figure 5. Schematic presentation of the matching of the periodic distances of the planes of (a) WZ CZTS-fcc Au and (b) Tg CZTS-fcc Au hetero-structures. Details of the combination of facets are provided at the bottom of the figure. All the semiconductor facets are calculated in terms of binding to the {111} facets of fcc Au(0).

Figure 6.(a) Presents the schematic construction of a model device where the thickness of the PEDOT:PSS is ~80 nm and that of the sample is ~ 250 nm. (b) and (c) show the current versus voltage plots of WZ CZTS and Au-CZTS (WZ) nanostructures respectively. In both cases the measurements were taken in the dark and under white light illumination. (d) and (e) present the current versus time plots during ON and OFF cycles. While (d) shows a decrease of peak current with successive cycles of ON/OFF, (e) retains constant peak current intensity.


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TOC

(111) Au fcc 4xd

3xd Au 20 nm

CZTS

1 nm

(100) CZTS WZ


Coincident Site Epitaxy at the Junction of AuâCu2ZnSnS4

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