Article pubs.acs.org/IC
Solvothermal Synthesis of Cu2SnS3 Quantum Dots and Their Application in Near-Infrared Photodetectors Sandra Dias,* Kishan Kumawat, Shinjini Biswas, and Salaru Babu Krupanidhi Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka 560012, India ABSTRACT: Cu 2SnS3 (CTS) quantum dots were synthesized by solvothermal technique with poly(vinylpyrrolidone) (PVP) as a surfactant. The structural and optical properties were studied using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV−vis spectroscopy. The electronic band gap was measured using cyclic voltammetry. The infrared photoresponse of the CTS quantum dotsincorporated device was measured under different illumination intensities of the infrared lamp, 1550 and 1064 nm lasers. The characteristics of the photodetector device, that is, responsivity, external quantum efficiency, and specific detectivity were calculated. This study proves the potential use of CTS quantum dots in infrared photodetectors.
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INTRODUCTION
Quantum dots (QDs) provide various features like band gap tunability and large surface area for efficient absorption of light leading to higher photocurrent.1−3 They can be synthesized using solution-based techniques leading to economic, large-area coatings. They can be incorporated onto flexible substrates that can fit onto any type of structures.4,5 Using solvothermal technique we can easily control the size, shape, and crystallinity of the quantum dots by varying the reaction temperature, reaction time, solvent type, surfactant type, and the type of precursors used.6 Infrared (IR) photodetectors are used for a variety of applications in night-vision cameras,7 biomedical imaging,8 optical fiber communication,9 environmental monitoring, and remote sensing.10 CdSe, CdTe, PbSe, CuInS2, and Cu(In,Ga)Se2 QDs have been intensively investigated and found to be best materials for solar cell and optical applications.11−15 However, because of the toxicity of Cd16 and Pb and scarcity of In, Ga,17 etc., these materials are not suitable for future requirement.18 So alternative materials are being researched to replace these materials. In this regard many nonconventional, earth abundant, and nontoxic nanocrystal materials like Cu3BiS3, Cu2ZnSnS4, Cu2CoSnS4 were investigated over the past few years.14,19,20 Cu2SnS3 (CTS), a p-type semiconductor, is a new emerging material for thin-film optoelectronic applications with a direct band gap in the range of 0.93−1.77 eV and a high absorption coefficient greater than 1 × 104 cm−1. It contains earth-abundant, nontoxic, and inexpensive elements.21 Because of the above properties CTS is used in photocatalysis, lightemitting diodes,22 nonlinear optics,23 and in photodetection.24 In this paper we synthesized CTS QDs by solvothermal method using poly(vinylpyrrolidone) (PVP) as the capping agent and studied the application of CTS QDs in infrared photodetection. © XXXX American Chemical Society
EXPERIMENTAL SECTION
Synthesis. A cationic solution of 0.3 mmol of SnCl2·2H2O and 0.62 mmol of CuCl2·2H2O was prepared in 20 mL of ethylene glycol followed by the addition of 1.6 M PVP (Mw = 2.5 g mol−1). A solution of 0.93 mmol of Na2S was prepared in 20 mL of ethylene glycol and was added dropwise to the cationic solution. The resultant brownishblack solution was transferred to a 50 mL autoclave and kept at 180 °C for 12 h. The product was obtained by centrifugation followed by washing with ethanol and then vacuum-dried. The reaction for this synthesis is given below. Here PVP is acting as a capping agent, preventing excessive growth and agglomeration, thus leading to the formation of QDs.
2CuCl 2· 2H 2O + SnCl 2· 2H 2O + 3Na 2S 180 ° C,12hrs
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Cu 2SnS3 + 6NaCl + 6H 2O
(1)
Characterization. The phase formation of the CTS QDs was confirmed from X-ray diffraction (XRD) using a PANalytical X’Pert PRO Diffractometer equipped with a Cu Kα source of 1.5405 Å wavelength. The morphology and interplanar spacing was determined using transmission electron microscopy (TEM; Jeol JEM-2100F) at an accelerating voltage of 200 kV. The absorbance spectra of the CTS QDs was measured using UV−vis−NIR spectro-photometer (PerkinElmer-Lambda 750). Raman spectroscopy was observed using Raman spectrometer (LabRAM HR) with 514 nm line of Ar+ laser. X-ray photoelectron spectroscopy (XPS) was taken using AXIS UltraDLD Xray photoelectron spectrometer with Al Kα X-ray source. Cyclic voltammetry measurements were taken by dropcasting the sample on glassy carbon working electrode. Pt was used as the counter electrode, and Ag/AgCl was used as the reference electrode. KOH (0.1 M) in deionized water was used as the electrolyte. A voltage scan rate of 0.1 V/s was used. Device Fabrication. Flexible substrates of In-doped SnO2 (ITO) coated poly(ethylene terephthalate) (PET; 1 × 1.5 cm2, surface resistivity 60 Ω/sq) were used. The substrates were precleaned by Received: November 28, 2016
A
DOI: 10.1021/acs.inorgchem.6b02832 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry sonicating in deionized water and isopropyl alcohol followed by flushing with nitrogen gas. The CTS QDs were dispersed in ethanol (23.3 mg/mL), and the solution was spin-coated at 500 rpm for 90 s. This was repeated for three coatings. The silver electrical contacts were deposited by thermal evaporation. The IR photoresponse was measured using an IR lamp (750 to 1100 nm), 1550 and 1064 nm IR lasers, and a Keithley source measure unit SMU 2400.
R exp =
RB =
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RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of the CTS QDs. The XRD data were taken with a step size of 0.026° and a scanning rate of
Rp =
∑M − P ∑ wmYo,2 m
(3)
∑ |I o ″ , k − Ic, k| ″ ∑ I o″,k ″ ∑ |Yo, m − Yc, m|
(4)
∑ Yo, m
(5)
GOF = χ 2 =
R wp R exp
=
∑ wm(Yo, m − Yc, m)2 M−P
(6)
where, Yo,m and Yc,m are the observed and calculated data, respectively, for data point m, M is the number of data points, P is the number of parameters, wm is the weight value for data point m, and I″o″,k and Ic,k are the observed and calculated intensities for the kth reflection. The values obtained from the Reitveld refinement are: lattice parameters: a = b = 5.45 Å and c = 10.6 Å, crystal volume V = 314.8 Å3, and interfacial angles α = β = γ = 90°. Figure 3a shows the TEM image of the CTS QDs of size ∼3 nm, which is less than the Bohr radius for CTS and hence can Figure 1. X-ray diffraction pattern of the CTS QDs.
0.2° min−1. The XRD pattern matched well with the JCPDS 089−4714 data, and the crystal structure was found to be tetragonal. Reitveld refinement was performed using the GSAS software as shown in Figure 2. The tetragonal crystal structure
Figure 3. (a) TEM image (b) SAED pattern (c) HRTEM of the CTS QDs (d) FFT of the HRTEM.
lead to quantum confinement.25 The selected area diffraction (SAED) pattern is shown in Figure 3b, using which, the interplanar spacings are indexed. Figure 3c shows the highresolution transmission electron microscope (HRTEM) image of the CTS QDs with the interplanar spacings marked. Figure 3d shows the fast Fourier transform (FFT) of the HRTEM with the interplanar distances. From the above figures, it was found that the planes correspond to that of tetragonal CTS. Figure 4 shows the formation mechanism of the CTS QDs. PVP has polyfunctional groups, that is, the vinyl groups are hydrophobic and the carbonyl groups are hydrophilic, which help in preventing the agglomeration of the QDs. Figure 5a,b shows the absorbance spectra and the extracted band gap of the CTS QDs from Tauc plot, respectively. The band gap of the CTS QDs was found to be 1.66 eV, which is higher than the generally reported band gap of 1.35 eV for CTS thin films. This is due to quantum confinement effect; that is, with decrease in the particle size, the band gap increases.
Figure 2. Reitveld refinement of the XRD pattern.
model of CTS was used. The fundamental parameters were used for the peak-shape profile. The background was refined using shifted Chebychev polynomial. The zero error, scale factors, and unit cell parameters were refined. The values of the fractional coordinates, occupancies, and the atomic displacement factors were used as such from the standard data. The fitting criteria as given by the eqs 2−6 were obtained as R weighted pattern (Rwp) = 2.95%, R expected (Rexp) = 1.58%, R Bragg (RB) = 5%, R pattern (Rp) = 2.22%, and goodness of fit (χ2) = 1.86. R wp =
∑ wm(Yo, m − Yc , m)2 ∑ wmYo, m
(2) B
DOI: 10.1021/acs.inorgchem.6b02832 Inorg. Chem. XXXX, XXX, XXX−XXX
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eV, respectively, corresponding to the +2 oxidation state of Cu.27 Sn 3d5/2 and Sn 3d3/2 have binding energies of 487 and 495.3 eV, respectively, corresponding to the +4 oxidation state of Sn.28 S 2p3/2 and S 2p1/2 have binding energies of 161.6 and 162.4 eV, respectively, corresponding to the −2 oxidation state of S.29 Cyclic voltammetry (CV) can be used to determine the band edges of semiconductors, that is, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Figure 7 shows the cyclic voltammetry Figure 4. Formation mechanism of the CTS QDs.
Figure 7. Cyclic voltammetry of the CTS QDs sample.
Figure 5. (a) Absorbance spectra and (b) energy band gap of the CTS QDs.
scan of the CTS QDs taken between −1.5 and 2 V. The HOMO (or ionization potential Ip) and LUMO (or electron affinity Ea) levels can be calculated using the eqs 7 and 8, where Eox is the onset oxidation potential and Ered is the onset reduction potential
The composition and oxidation state of the elements were determined using XPS. The survey spectra in Figure 6a shows
E HOMO = −Ip = −(Eox + 4.71) eV
(7)
E LUMO = −Ea = −(Ered + 4.71) eV
(8)
From the CV scan, the oxidation and reduction potentials were found to be 1.36 and −0.5 V, respectively. Using these values the HOMO and LUMO levels were found to be at −6.07 and −4.21 eV, respectively. The band gap of the CTS quantum dots was found to be 1.86 eV. This differs from the band gap calculated from optical measurements by 0.2 eV. Figure 8 shows the energy-level diagram for the ITO/CTS/ Ag device using the HOMO and LUMO levels for CTS determined using cyclic voltammetry. The schematic of the CTS QDs device, used for photodetection under the IR lamp and 1550 and 1064 nm lasers, is shown in Figure 9. The ITO acts as the transparent conducting electrode through which the IR light is incident on the CTS QDs, and Ag acts as the front electrode. The electrons and holes that are photogenerated are collected by the Ag and ITO contacts, respectively. The photoresponse was measured using the IR lamp over illumination intensities of 0.38 and 0.48 W/cm2 as shown in Figure 10. Figure 10a,b shows the time-dependent photoresponse over different ON-OFF cycles of the lamp. The photocurrent is found to be stable with time under the ONOFF cycles. The photocurrent was found to increase from 1.96 to 3.52 μA, as the illumination intensity increased from 0.38 to 0.48 W/cm2. This increment in the current value is due to the increase in the number of photogenerated electron hole pairs with increase in the light intensity. The photoresponse curves were fitted using exponential rise and decay eqs 9 and 10, and
Figure 6. XPS spectra of the CTS QDs. (a) Survey spectra, (b) Cu 2p, (c) Sn 3d, and (d) S 2p core-level spectra.
the presence of Cu, Sn, and S elements belonging to CTS along with C and O impurities. The core-level spectra of Cu 2p, Sn 3d, and S 2p are shown in Figure 6b−d. Cu 2p3/2 and Cu 2p1/2 have binding energies of 932.2 and 952.2 eV, respectively, corresponding to the +1 oxidation state of Cu.26 The satellite peaks of Cu 2p3/2 and Cu 2p1/2 were found at 944.2 and 963 C
DOI: 10.1021/acs.inorgchem.6b02832 Inorg. Chem. XXXX, XXX, XXX−XXX
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The rise and decay time constants were found to be 4.6 and 3.9 s for lamp illumination intensity of 0.38 W/cm2 and 5.2 and 5.5 s for illumination intensity of 0.48 W/cm2. The photoresponse was also measured for IR lasers of 1064 and 1550 nm wavelengths. The photoresponse using 1064 and 1550 nm lasers under different illumination intensities is shown in Figure 11a,b. With
Figure 8. Energy-level diagram of the CTS QDs as determined from cyclic voltammetry along with the ITO and Ag work function levels. Figure 11. (a, b) Photocurrent as a function of time for light on and off cycles at various illumination intensities of 1064 and 1550 nm lasers.
increase in the illumination intensity, the photocurrent was found to increase due to the increase in the generation of electron hole pairs with increased photon impingement. The photocurrent was tested for several ON-OFF cycles under 1064 nm laser (intensity 0.39 W/cm2) as shown in Figure 12a. The
Figure 9. CTS QDs photodetector device architecture.
Figure 12. (a) Photocurrent as a function of time for light on and off cycles at 0.39 W/cm2 illumination intensity of 1064 nm laser. (b) Exponential curve fits for determination of rise and decay constants, respectively.
photocurrent was found to be stable with time. The rise and decay curves as shown in Figure 12b were exponentially fitted using the equations
Figure 10. (a, b) Time-dependent photoresponse. (c, d) Rise and decay curve fits for IR lamp illumination intensities of 0.38 and 0.48 W/cm2.
I(t )decay = Idark + A exp[−t /τdecay ]
(11)
I(t )decay = Idark + A exp[−t /τ1] + B exp[−t /τ2]
(12)
The rise and decay time constants were found to be τ1 = 8.6 s, τ2 = 2.4 s and τ1 = 8.8 s, τ2 = 1.9 s. Similar ON-OFF cycles were tested under 1550 nm laser (intensity 0.76 W/cm2), and the photocurrent was found to be stable as shown in Figure 13a. The curves as shown in Figure 13b were exponentially fitted using the first-order exponential rise and decay equations. The rise and decay constants were found to be 4.1 and 3.8 s, respectively. The photodetector parameters such as responsivity, external quantum efficiency (EQE), and specific detectivity were
the rise and decay time constants were found as shown in Figure 10c,d. I(t )rise = Idark + A exp[t /τrise]
I(t )rise = Idark + A exp[t /τ1] + B exp[t /τ2]
(9) (10) D
DOI: 10.1021/acs.inorgchem.6b02832 Inorg. Chem. XXXX, XXX, XXX−XXX
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be more for the 1064 nm laser compared to the 1550 nm laser, even upon increased illumination. This is because the energy of the 1064 nm laser (1.17 eV) is much higher than that of 1550 nm laser (0.8 eV). Hence it is able to excite more electrons from the CTS material even at lower illumination intensities than the 1550 nm laser.
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CONCLUSIONS CTS QDs were synthesized by the solvothermal method. The structural and optical properties of the CTS QDs were determined. The CTS QDs were used in the IR detection using IR lamp, 1550 and 1064 nm lasers. The responsivity, EQE, and specific detectivity were determined for the CTS photodetector.
Figure 13. (a) Photocurrent as a function of time for light on and off cycles at 0.76 W/cm2 illumination intensity of 1550 nm laser. (b) Exponential curve fits for determination of rise and decay constants, respectively.
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calculated. The reponsivity Rλ of the device defined as the amount of photocurrent generated per unit illumination intensity, per unit area, is given by Rλ = Iλ/PλS, where Iλ is the photocurrent given by Iλ = Ilight − Idark, Pλ is the illumination intensity, and S is the effective area of illumination.30 The external quantum efficiency EQE defined as the number of electrons generated per incident photon is given by EQE = hcRλ/qλ, where h is Planck’s constant, c is the velocity of light, q is the electronic charge, and λ is the wavelength of the light source.31 The specific detectivity given by D* = Rλ/(2qId)1/2 is the ability of a photodetector to detect the weakest light signal, where Id is the dark current.32 The calculated values of responsivity, EQE, and specific detectivity are shown in Tables 1, 2, and 3, respectively, for the IR lamp and 1064 and 1550 nm
*E-mail:
[email protected]. ORCID
Sandra Dias: 0000-0003-1400-8399 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Mr. M. E. Panzi for the Reitveld analysis and Mr. D. Das for helping in the cyclic voltammetry measurements.
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responsivity (mA/W)
EQE %
specific detectivity (Jones)
0.38 0.48
5.92 7.66
0.92 1.19
1.53 × 1010 2.11 × 1010
Table 2. Calculated Values of Responsivity, EQE, and Specific Detectivity under 1064 nm Laser Illumination illumination intensity (W/cm2)
responsivity (mA/W)
EQE %
0.05 0.09 0.16 0.25 0.39
1.61 1.89 1.90 1.81 1.77
0.188 0.221 0.222 0.211 0.207
specific detectivity (Jones) 4.83 5.68 5.64 5.53 5.38
× × × × ×
109 109 109 109 109
Table 3. Calculated Values of Responsivity, EQE, and Specific Detectivity under 1550 nm Laser Illumination illumination intensity (W/cm2)
responsivity (mA/W)
EQE %
0.24 0.46 0.76 1.11
0.57 0.86 0.93 0.90
0.046 0.068 0.075 0.072
specific detectivity (Jones) 1.86 2.72 2.82 2.69
× × × ×
REFERENCES
(1) H. Sargent, E. Infrared Quantum Dots. Adv. Mater. 2005, 17, 515−522. (2) Jara, D. H.; Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. SizeDependent Photovoltaic Performance of CuInS2 Quantum DotSensitized Solar Cells. Chem. Mater. 2014, 26, 7221−7228. (3) Ko, D.-K.; Maurano, A.; Suh, S. K.; Kim, D.; Hwang, G. W.; Grossman, J. C.; Bulović, V.; Bawendi, M. G. Photovoltaic Performance of PbS Quantum Dots Treated with Metal Salts. ACS Nano 2016, 10, 3382−3388. (4) Chen, J.; Zhao, D.; Li, C.; Xu, F.; Lei, W.; Sun, L.; Nathan, A.; Sun, X. W. All Solution-processed Stable White Quantum Dot Lightemitting Diodes with Hybrid ZnO/TiO2 as Blue Emitters. Sci. Rep. 2014, 4, 4085. (5) Kramer, I. J.; Moreno-Bautista, G.; Minor, J. C.; Kopilovic, D.; Sargent, E. H. Colloidal quantum dot solar cells on curved and flexible substrates. Appl. Phys. Lett. 2014, 105, 163902. (6) Lai, J.; Niu, W.; Luque, R.; Xu, G. Solvothermal synthesis of metal nanocrystals and their applications. Nano Today 2015, 10, 240− 267. (7) Chang, R.-P.; Perng, D.-C. Near-infrared photodetector with CuIn1−x AlxSe2 thin film. Appl. Phys. Lett. 2011, 99, 081103. (8) Izzetoglu, M.; Bunce, S. C.; Izzetoglu, K.; Onaral, B.; Pourrezaei, A. K. Functional brain imaging using near-infrared technology. IEEE Eng. Med. Biol. Mag. 2007, 26, 38−46. (9) Jana, M. K.; Chithaiah, P.; Murali, B.; Krupanidhi, S. B.; Biswas, K.; Rao, C. N. R. Near infrared detectors based on HgSe and HgCdSe quantum dots generated at the liquid-liquid interface. J. Mater. Chem. C 2013, 1, 6184−6187. (10) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Ultrasensitive solutioncast quantum dot photodetectors. Nature 2006, 442, 180−183. (11) Landry, M. L.; Morrell, T. E.; Karagounis, T. K.; Hsia, C.-H.; Wang, C.-Y. Simple Syntheses of CdSe Quantum Dots. J. Chem. Educ. 2014, 91, 274−279. (12) Zhou, D.; Lin, M.; Chen, Z.; Sun, H.; Zhang, H.; Sun, H.; Yang, B. Simple Synthesis of Highly Luminescent Water-Soluble CdTe
Table 1. Calculated Values of Responsivity, EQE, and Specific Detectivity under IR Lamp Illumination illumination intensity (W/cm2)
AUTHOR INFORMATION
Corresponding Author
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laser illumination at different intensities. With increase in intensity of the light, the responsivity, EQE, and specific detectivity were found to increase. This follows due to the increase in the number of photogenerated electrons with increased illumination intensity. The photoresponse is found to E
DOI: 10.1021/acs.inorgchem.6b02832 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Quantum Dots with Controllable Surface Functionality. Chem. Mater. 2011, 23, 4857−4862. (13) Lipovskii, A.; Kolobkova, E.; Petrikov, V.; Kang, I.; Olkhovets, A.; Krauss, T.; Thomas, M.; Silcox, J.; Wise, F.; Shen, Q.; Kycia, S. Synthesis and characterization of PbSe quantum dots in phosphate glass. Appl. Phys. Lett. 1997, 71, 3406−3408. (14) Li, L.; Daou, T. J.; Texier, I.; Kim Chi, T. T.; Liem, N. Q.; Reiss, P. Highly Luminescent CuInS2/ZnS Core/Shell Nanocrystals: Cadmium-Free Quantum Dots for In Vivo Imaging. Chem. Mater. 2009, 21, 2422−2429. (15) Liao, Y.-K.; Brossard, M.; Hsieh, D.-H.; Lin, T.-N.; Charlton, M. D. B.; Cheng, S.-J.; Chen, C.-H.; Shen, J.-L.; Cheng, L.-T.; Hsieh, T.P.; Lai, F.-I.; Kuo, S.-Y.; Kuo, H.-C.; Savvidis, P. G.; Lagoudakis, P. G. Highly Efficient Flexible Hybrid Nanocrystal-Cu(In,Ga)Se2 (CIGS) Solar Cells. Adv. Energy Mater. 2015, 5.140128010.1002/ aenm.201401280 (16) Das, P.; Samantaray, S.; Rout, G. R. Studies on cadmium toxicity in plants: a review. Environ. Pollut. 1997, 98 (1), 29−36. (17) Phipps, G.; Mikolajczak, C.; Guckes, T. Indium and Gallium: long-term supply. Refocus 2008, 9, 56−59. (18) Riha, S. C.; Parkinson, B. A.; Prieto, A. L. Solution-Based Synthesis and Characterization of Cu2ZnSnS4 Nanocrystals. J. Am. Chem. Soc. 2009, 131, 12054−12055. (19) Zaberca, O.; Gillorin, A.; Durand, B.; Chane-Ching, J. Y. A general route to the synthesis of surfactant-free, solvent-dispersible ternary and quaternary chalcogenide nanocrystals. J. Mater. Chem. 2011, 21, 6483−6486. (20) Aihara, N.; Araki, H.; Takeuchi, A.; Jimbo, K.; Katagiri, H. Fabrication of Cu2SnS3 thin films by sulfurization of evaporated Cu-Sn precursors for solar cells. Phys. Status Solidi C 2013, 10, 1086−1092. (21) Li, B.; Xie, Y.; Huang, J.; Qian, Y. Synthesis, Characterization, and Properties of Nanocrystalline Cu2SnS3. J. Solid State Chem. 2000, 153, 170−173. (22) Liang, X.; Cai, Q.; Xiang, W.; Chen, Z.; Zhong, J.; Wang, Y.; Shao, M.; Li, Z. Preparation and Characterization of Flower-like Cu2SnS3 Nanostructures by Solvothermal Route. J. Mater. Sci. Technol. 2013, 29, 231−236. (23) Samanta, L. K. On some properties of I2-IV-VI3 compounds. Phys. Status Solidi A 1987, 100, K93−K97. (24) Chen, X.-a.; Wada, H.; Sato, A.; Mieno, M. Synthesis, Electrical Conductivity, and Crystal Structure of Cu4Sn7S16and Structure Refinement of Cu2SnS3. J. Solid State Chem. 1998, 139, 144−151. (25) Wang, J.-J.; Liu, P.; Ryan, K. M. A facile phosphine-free colloidal synthesis of Cu2SnS3 and Cu2ZnSnS4 nanorods with a controllable aspect ratio. Chem. Commun. 2015, 51, 13810−13813. (26) Liang, X.; Cai, Q.; Xiang, W.; Chen, Z.; Zhong, J.; Wang, Y.; Shao, M.; Li, Z. Preparation and Characterization of Flower-like Cu2SnS3 Nanostructures by Solvothermal Route. J. Mater. Sci. Technol. 2013, 29, 231−236. (27) Partain, L. D.; Schneider, R. A.; Donaghey, L. F.; McLeod, P. S. Surface chemistry of CuxS and CuxS/CdS determined from x-ray photoelectron spectroscopy. J. Appl. Phys. 1985, 57, 5056−5065. (28) Ettema, A. R. H. F.; Haas, C. An X-ray photoemission spectroscopy study of interlayer charge transfer in some misfit layer compounds. J. Phys.: Condens. Matter 1993, 5, 3817. (29) Nakai, I.; Sugitani, Y.; Nagashima, K.; Niwa, Y. X-ray photoelectron spectroscopic study of copper minerals. J. Inorg. Nucl. Chem. 1978, 40, 789−791. (30) Xie, X.; Kwok, S.-Y.; Lu, Z.; Liu, Y.; Cao, Y.; Luo, L.; Zapien, J. A.; Bello, I.; Lee, C.-S.; Lee, S.-T.; Zhang, W. Visible-NIR photodetectors based on CdTe nanoribbons. Nanoscale 2012, 4, 2914−2919. (31) Dias, S.; Krupanidhi, S. B. Solution processed Cu2SnS3 thin films for visible and infrared photodetector applications. AIP Adv. 2016, 6, 025217. (32) Zhang, H.; Zhang, X.; Liu, C.; Lee, S.-T.; Jie, J. HighResponsivity, High-Detectivity, Ultrafast Topological Insulator Bi2Se3/Silicon Heterostructure Broadband Photodetectors. ACS Nano 2016, 10, 5113−5122. F
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