Two-Dimensional Nonlayered CuInSe2 Nanosheets for High

Oct 9, 2018 - CuInSe2 has attracted great attention because of its superior optical properties, such as high optical absorption coefficient and direct...
0 downloads 0 Views 3MB Size
Download PDF
Letter Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

www.acsanm.org

Two-Dimensional Nonlayered CuInSe2 Nanosheets for HighPerformance Photodetectors He Liu,† Miaomiao Yu,† Fanglu Qin,† Wei Feng,*,† and PingAn Hu*,‡ †

Department of Chemistry and Chemical Engineering, College of Science, Northeast Forestry University, Harbin 150040, China Key Laboratory of Microsystem and Microstructure of Ministry of Education, Harbin Institute of Technology, Harbin 150080, China

‡

ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 193.93.195.16 on 10/13/18. For personal use only.

S Supporting Information *

ABSTRACT: CuInSe2 has attracted great attention because of its superior optical properties, such as high optical absorption coefficient and directband-gap structure (1.1 eV). In this letter, for the first time, highphotoresponse-performance two-dimensional (2D) nonlayered CuInSe2 nanosheets were demonstrated. 2D CuInSe2 nanosheets showed typical ptype semiconducting transport behavior with a high-field-effect mobility of 180 cm2 V−1 s−1. Photodetectors based on 2D CuInSe2 were fabricated and showed a high response to the UV−visible spectral range (254−700 nm). The photoresponsivity and detectivity of 2D CuInSe2 photodetectors were calculated as 1900 A W−1 and 7 × 1011 Jones under 700 nm light illumination, respectively. Our results open a pathway for 2D nonlayered CuInSe2 nanosheet applications in future photodetection and photovoltaic devices. KEYWORDS: nonlayered, CuInSe2, two-dimensional, photodetector, solid-state reaction, InSe

R

method.21 The CuInSe2 nanowires can be achieved by a simple solid-state reaction from a In2Se3 nanowire template.22This strategy was also suitable for synthesizing 2D CuInSe2 nanosheets.23 High-performance and broad-response photodetectors based on CuInSe2 nanocrystals have been demonstrated,24,25 indicating that the CuInSe2 nanostructure can be a promising candidate for optoelectronic and photovoltaic devices. Compared to other nanostructure semiconductors, 2D materials are more compatible with the current siliconbased complementary metal oxide semiconductor (CMOS) processing. However, the photodetection performance of 2D CuInSe2 nanosheets has never been explored. In this letter, for the first time, high-photoresponseperformance 2D nonlayered CuInSe2 nanosheets were demonstrated. 2D CuInSe2 field-effect transistors (FETs) exhibited a typical p-type semiconducting transport property with a high-field-effect mobility of 180 cm2 V−1 s−1. Photodetectors based on 2D CuInSe2 nanosheets were fabricated and showed a high photoresponse to the UV− visible spectral range (254−700 nm). The photoresponsivity and detectivity of 2D CuInSe2 photodetectors were 1900 A W−1 and 7 × 1011 Jones measured at 700 nm light illumination, respectively. Our results demonstrate that 2D CuInSe 2 nanosheets have great potential for application in highperformance photodetector and photovoltaic devices.

ecently, two-dimensional (2D) semiconductors have attracted vast attention because of their unique physical and chemical properties. 2D semiconductors have been investigated for application in next-generation nanoelectronic and optoelectronic devices, such as transistors,1 diodes,2 and photodetectors.3 2D materials show great potential as novel electronic and optoelectronic devices, but they have been mainly limited to layered materials, such as transition-metal dichalcogenides (MoS2 and WSe2),1,4 black phosphorus,5 and III and IV compounds (InSe6 and GaSe7). Although highquality and large-scale of 2D layered semiconductors can be successfully synthesized by chemical vapor deposition (CVD),8 liquid exfoliation,9 physical vapor deposition,10 and pulsedlaser deposition,11−15 it is challenging to synthesize 2D nonlayered semiconductors because it is difficult to break the chemical bond of nonlayered materials. 2D nonlayered materials could be synthesized by a colloidal template method.16 However, it is difficult to remove nonvolatile organic solvents and templates, and aggregation also makes application difficult in electronic and optoelectronic devices. Nonlayered 2D semiconductors also are successfully synthesized by van der Waals epitaxy on layered substrates.17−20 Copper−indium selenide (CuInSe2), an I−III−VI2 chalcopyrite semiconductor, is applied in optoelectronic and photovoltaic devices because of its high optical absorption coefficient and small direct-band-gap structure (1.1 eV).21 Recently, the synthesis and property explorations of CuInSe2 nanostructures have attracted increasing attention. CuInSe2 nanocrystals have been synthesized by a colloidal template © XXXX American Chemical Society

Received: September 1, 2018 Accepted: October 9, 2018 Published: October 9, 2018 A

DOI: 10.1021/acsanm.8b01527 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Nano Materials

Figure 1. Characterizations of 2D CuInSe2 nanosheets. (a) Raman spectra of pristine InSe (pink line) and as-synthesized CuInSe2 nanosheets (black line). (b) Optical image and corresponding Raman mapping of as-synthesized CuInSe2 nanosheets at the Raman peak of 211 cm−1. (c) Lowmagnification TEM image of CuInSe2 nanosheets. (d) SAED pattern of CuInSe2 nanosheets. (e) HRTEM image of CuInSe2 nanosheets.

Figure 2. Electronic characterization of 2D CuInSe2 FETs. (a) 3D schematic illustration of back-gated 2D CuInSe2 FETs. (b) Optical image of back-gated 2D CuInSe2 FETs. Device configuration (length/width): 20 μm/10 μm. (c) Transfer curve of 2D CuInSe2 FETs measured at Vds = 1 V. (d) Output curves of 2D CuInSe2 FETs.

The as-synthesized CuInSe2 and pristine InSe nanosheets are first identified by the optical image, as shown in parts b and c in Figure S1, respectively. The color contrasts of the products of CuInSe2 nanosheets are obviously different from those of the reactants of InSe nanosheets, suggesting that InSe nanosheets are transformed and this solid-state reaction can be briefly estimated via color contrast. The thickness of the thinnest 2D CuInSe2 is 7.5 nm, as shown in Figure S1d,e, which is determined by atomic force microscopy (AFM). In order to further confirm that the InSe nanosheets have been

transformed to CuInSe2 nanosheets, Raman characterization was performed. Figure 1a shows the Raman spectra of pristine InSe and as-synthesized CuInSe2, and they are totally different. In annealing samples, the Raman peaks of InSe are replaced by three new Raman peaks, which belong to the CuInSe2 crystal.26 Figure 1b is a Raman mapping (the peak of CuInSe2 at 211 cm−1), which is uniformly distributed on entire annealed nanosheets. Raman spectra and mapping clarify that InSe nanosheets were completely transformed into CuInSe2 nanosheets via a solid-state reaction. The crystalline structure B

DOI: 10.1021/acsanm.8b01527 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Nano Materials

Figure 3. Optoelectronic characterization of 2D CuInSe2 nanosheets. (a) I−V curves of 2D CuInSe2 photodetectors illuminated by various lights with P = 0.29 mW cm−2. (b) R and D* measured under various illumination lights with Vds = 0.5 V. (c) I−V curves of 2D CuInSe2 photodetectors illuminated by 700 nm light with various intensities. (d) R and D* measured under 700 nm light with various intensities and Vds = 0.5 V.

linear part of the transfer curve in Figure 2c using the following equation: μ = [L/(WCiVds)][dIds/dVg], where L = 20 μm (channel length), W = 10 μm (channel width), and Ci = ε0εr/d, in which ε0 is the vacuum permittivity (8.854 × 10−12 F m−1), εr = 3.9, and d = 300 nm. The field-effect mobility of the 2D CuInSe2 nanosheet is calculated as 180 cm2 V−1 s−1, which is comparable to that of p-type 2D WSe2.4 The current on/off ratio of 2D CuInSe2 FETs is calculated to be 1.2, which is extracted from the transfer curve. Figure 2d contains the output curves of 2D CuInSe2 FETs. Ids linearly depends on the source/drain bias voltage in a large voltage range, suggesting that electrode chromium and CuInSe2 nanosheets have good contact. The output current also decreases as Vg increases, further confirming p-type semiconducting transport behavior of the as-synthesized CuInSe2 nanosheet. To reveal the optoelectronic properties of 2D CuInSe2 photodetectors, the optical absorption of 2D CuInSe2 was measured and various monochromatic lights were vertically illuminated onto photodetectors (ultraviolet, 254 nm; visible, 490, 610, and 700 nm). Figure S4 is the absorption curve of 2D CuInSe2, which indicates that 2D CuInSe2 has a strong optical absorption from visible to ultraviolet light. The optoelectronic performance of 2D CuInSe2 photodetectors was investigated via various illumination lights and intensities. Under illumination, the 2D CuInSe2 photodetector exhibits a wide-response spectral range from ultraviolet (254 nm) to visible (700 nm) light, as shown in Figure 3a, which is consistent with the optical absorption result. The photocurrent, Iph (=Iillumination − Idark), linearly increases with increasing bias voltage Vds under different illumination lights because of decreasing carrier transit time and increasing carrier drift velocity under high Vds. Responsivity (R) is a key parameter in the evaluation of the photoresponse performance of a photodetector, which is defined by the following equation: R = Iph/PS. Here, Iph is the photocurrent, P is the illumination intensity, and S is the channel area of the photodetector (200 μm2). The Iph values

of 2D CuInSe2 was further characterized by transmission electron microscopy (TEM). The surface of the CuInSe2 nanosheet is clean, as shown in Figure 1c, demonstrating that excessive copper films are removed. Figure 1d corresponds to the selected-area electron diffraction (SAED) pattern of the CuInSe2 nanosheet, suggesting that as-synthesized 2D CuInSe2 nanosheets have good crystallinity. The good crystallinity of CuInSe2 nanosheets is further confirmed by the highresolution TEM (HRTEM) image (Figure 1e). The elemental compositions of CuInSe2 nanosheets were identified by energy-dispersive spectroscopy (EDS) in Figure S2. The corresponding result shows that the Cu/In/Se atomic ratio of CuInSe2 nanosheets is close to 1:1:2, which is consistent with its stoichiometric equation. All of the above characterization results clarify that as-synthesized CuInSe2 nanosheets possess good crystalline structure. Next, we are going to investigate the electronic properties of 2D CuInSe2 nanosheets. 2D CuInSe2 nanosheets were transferred to another silicon-substrate-covered 300 nm SiO2 by a poly(methyl methacrylate)-assisted method. Back-gated FETs were fabricated using a shadow mask, and source/drain electrodes were fabricated by metal thermal evaporation. Figure 2a is the 3D illustration of 2D CuInSe2 FETs with backgated structures, where chromium/gold are the drain and source, n+-doped silicon is the gate, and 300 nm SiO2 layer works is the dielectric. An optical image of 2D CuInSe2 FET is exhibited in Figure 2b; the length and width of the channel are 20 and 10 μm, respectively. The thickness of the channel is 20 nm, which is determined by AFM measurements, as shown in Figure S3. The surface of the CuInSe2 nanosheet is rough (find more discussion in the Supporting Information). The electronic characteristics of CuInSe2 FETs are measured under an ambient environment at room temperature. Figure 2c is a transfer curve of CuInSe2 FETs measured at Vds = 1 V. Ids decreases with Vg increasing in the transfer curve, indicating p-type semiconducting transport behavior of 2D CuInSe2. The field-effect mobility of CuInSe2 FETs is calculated from the C

DOI: 10.1021/acsanm.8b01527 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Nano Materials

Figure 4. Photoresponse stability and photoresponse speed of 2D CuInSe2 photodetectors. (a) Photoresponse stability of 2D CuInSe2 photodetectors measured at 700 nm light with Vds = 0.5 V and P = 0.29 mW cm−2. (b) Photoresponse speed of 2D CuInSe2 photodetectors.

lights (700, 610, 490, and 254 nm) after five cycles of switching from the “illumination” state to the “dark” state. The current rise and decay processes obviously involve two steps: fast and slow steps. To clearly understand rise and decay processes, the individual rise and decay curves were plotted and are shown in Figure 4b. 2D CuInSe2 photodetectors show a long response time, which is similar to that of the CVDgrown 2D MoS2 photodetector.28,29 This behavior is attributed to trap states in CuInSe2 nanosheets introduced by the solidstate reaction process. The response time for the rise process is the duration time values between the dark current and 63% (1 − 1/e ≈ 63%) of the peak illumination current, while the response time for the decay process is the duration time values between the peak illumination current and 63% (1 − 1/e ≈ 63%) of the peak illumination current.30 The calculated response times are 10.5 and 8.4 s for the rise and decay processes, respectively. The response time of the 2D CuInSe2 photodetector is larger than that of the 2D InSe photodetector, which is due to the existence of introduced defects in 2D CuInSe2 during the synthesis process as mentioned above. In conclusion, for the first time, the electronic and optoelectronic performances of 2D CuInSe2 were explored. FETs based on 2D CuInSe2 show typical p-type semiconducting transport behavior with a high-field-effect mobility of 180 cm2 V−1 s−1. 2D CuInSe2 photodetectors exhibit a wideresponse range from 254 to 700 nm. The R and D* values of 2D CuInSe2 photodetectors are as high as 1900 A W−1 and 7 × 1011 Jones illuminated by 700 nm light, respectively. Our results open a pathway for 2D nonlayered CuInSe2 nanosheet applications in future photodetection and photovoltaic devices.

are 10.6, 1.58, 1.41, and 1.25 μA for 254, 490, 610, and 700 nm measured at Vds = 0.5 V, respectively. Wavelength-dependent responsivities are black dates in Figure 3b. The calculated R values are 18295, 2731.1, 2162.7, and 1900 A W−1 for 254, 490, 610, and 700 nm, respectively. The results further demonstrate that 2D CuInSe2 photodetectors have a wide response range from ultraviolet (254 nm) to visible (700 nm) light. The R values of the 2D CuInSe2 photodetectors are larger than those of other 2D layered material-based photodetectors, such as multilayer GaSe7 and InSe.27 Detectivity (D*) is another important parameter in the estimation of the photoresponse property of a photodetector, and D* is calculated by the following equation: D* = RA1/2/ (2eId) 1/2, where R is the calculated responsivity, A is the channel area of the photodetector (200 μm2), e = 1.6 × 10−19 C (electronic charge), and Id is the corresponding dark current. The blue dates in Figure 3b are the calculated D* values of the 2D CuInSe2 photodetector illuminated under various wavelengths. For the whole response range, the values of D* are 1011−1012 Jones, which are comparable to those of the photodetector-based multilayer GaSe.7 Figure 3c shows Ids−Vds curves of 2D CuInSe2 photodetectors measured under 700 nm illumination light with various illumination intensities (0.29−0.71 mW cm−2). The generated Iph increases with increasing light intensity under the measured intensity range. Iph of 2D CuInSe2 photodetectors strongly depend on the illumination intensity. Figure S5 is the curve of Iph versus P at Vds = 0.5 V, which is extracted from the results in Figure 3c. Iph shows a linear dependence on P (Iph ∼ P0.75). The good linear behavior between Iph and P clarifies that generated Iph is mainly dependent on the quantity of photogenerated electrons. Figure 3d shows that both values of R and D* decrease with increasing P. This phenomenon is mainly due to defects in CuInSe2 nanosheets or traps at the interface between CuInSe2 nanosheets and SiO2 substrates. This phenomenon is common in a trap-dominated photodetector. With increasing P, the longest-lived traps are entirely occupied and the shorter-lived traps start to be mainly responsible for the carrier lifetime. Hence, photogenerated electrons and holes are easy to recombine, leading to the degraded R and D* values. To comprehensively evaluate the photodetection properties of a photodetector, the stability and response speed are investigated. Figures 4a and S6 are time-dependent photocurrent curves of the 2D CuInSe2 photodetector by various lights, which were measured under P = 0.29 mW cm−2 and Vds = 0.5 V. 2D CuInSe2 photodetectors show good photoresponse repeatability and stability to various illumination

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01527. Description of experimental methods, EDS of CuInSe2, AFM of CuInSe2 FETs, absorption curve of 2D CuInSe2 nanosheets, Iph versus illumination intensity of CuInSe2 photodetector, and stability of the photoswitching of CuInSe2 photodetectors (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Feng: 0000-0001-6902-0024 D

DOI: 10.1021/acsanm.8b01527 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Nano Materials

(16) Tan, C.; Zhang, H. Wet-Chemical Synthesis and Applications of Non-Layer Structured Two-Dimensional Nanomaterials. Nat. Commun. 2015, 6, 7873. (17) Wen, Y.; Wang, Q.; Yin, L.; Liu, Q.; Wang, F.; Wang, F.; Wang, Z.; Liu, K.; Xu, K.; Huang, Y.; Shifa, T. A.; Jiang, C.; Xiong, J.; He, J. Epitaxial 2D PbS Nanoplates Arrays with Highly Efficient Infrared Response. Adv. Mater. 2016, 28, 8051−8057. (18) Wen, Y.; Yin, L.; He, P.; Wang, Z.; Zhang, X.; Wang, Q.; Shifa, T. A.; Xu, K.; Wang, F.; Zhan, X.; Wang, F.; Jiang, C.; He, J. Integrated High-Performance Infrared Phototransistor Arrays Composed of Nonlayered PbS−MoS2 Heterostructures with Edge Contacts. Nano Lett. 2016, 16, 6437−6444. (19) Cheng, R.; Wen, Y.; Yin, L.; Wang, F.; Wang, F.; Liu, K.; Shifa, T. A.; Li, J.; Jiang, C.; Wang, Z.; He, J. Ultrathin Single-Crystalline CdTe Nanosheets Realized Via Van Der Waals Epitaxy. Adv. Mater. 2017, 29, 1703122. (20) Wang, F.; Wang, Z.; Shifa, T. A.; Wen, Y.; Wang, F.; Zhan, X.; Wang, Q.; Xu, K.; Huang, Y.; Yin, L.; Jiang, C.; He, J. TwoDimensional Non-Layered Materials: Synthesis, Properties and Applications. Adv. Funct. Mater. 2017, 27, 1603254. (21) Tang, J.; Hinds, S.; Kelley, S. O.; Sargent, E. H. Synthesis of Colloidal CuGaSe2, CuInSe2, and Cu(InGa)Se2 Nanoparticles. Chem. Mater. 2008, 20, 6906−6910. (22) Schoen, D. T.; Peng, H.; Cui, Y. Anisotropy of Chemical Transformation from In2Se3 to CuInSe2 Nanowires through Solid State Reaction. J. Am. Chem. Soc. 2009, 131, 7973−7975. (23) Tao, X.; Mafi, E.; Gu, Y. Synthesis and Ultrafast Carrier Dynamics of Single-Crystal Two-Dimensional CuInSe2 Nanosheets. J. Phys. Chem. Lett. 2014, 5, 2857−2862. (24) Jebakumar, D. S. I.; Chitara, B.; Krupanidhi, S. B. Solution-Cast Photoconductive Photodetectors Based on CuInSe2 Nanoparticles. J. Nanosci. Nanotechnol. 2017, 17, 1538−1542. (25) Guo, R.; Shen, T.; Tian, J. Broadband Hybrid Organic/CuInSe2 Quantum Dot Photodetectors. J. Mater. Chem. C 2018, 6, 2573− 2579. (26) Rincon, C.; Ramirez, F. Lattice Vibrations of CuInSe2 and CuGaSe2 by Raman Microspectrometry. J. Appl. Phys. 1992, 72, 4321−4324. (27) Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS Nano 2014, 8, 1263− 1272. (28) Zhang, W.; Huang, J.-K.; Chen, C.-H.; Chang, Y.-H.; Cheng, Y.-J.; Li, L.-J. High-Gain Phototransistors Based on a CVD MoS2 Monolayer. Adv. Mater. 2013, 25, 3456−3461. (29) Zheng, W.; Feng, W.; Zhang, X.; Chen, X.; Liu, G.; Qiu, Y.; Hasan, T.; Tan, P.; Hu, P. A. Anisotropic Growth of Nonlayered CdS on MoS2 Monolayer for Functional Vertical Heterostructures. Adv. Funct. Mater. 2016, 26, 2648−2654. (30) Jin, W.; Gao, Z.; Zhou, Y.; Yu, B.; Zhang, H.; Peng, H.; Liu, Z.; Dai, L. Novel Graphene-Oxide-Semiconductor Nanowire Phototransistors. J. Mater. Chem. C 2014, 2, 1592−1596.

PingAn Hu: 0000-0003-3499-2733 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51802038), China Postdoctoral Science Foundation (Grant 2018M630329), and Fundamental Research Funds for the Central Universities (Grant 2572018BC14).

■

REFERENCES

(1) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (2) Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Optoelectronic Devices Based on Electrically Tunable p−n Diodes in a Monolayer Dichalcogenide. Nat. Nanotechnol. 2014, 9, 262. (3) Wu, J. Y.; Chun, Y. T.; Li, S.; Zhang, T.; Wang, J.; Shrestha, P. K.; Chu, D. Broadband MoS2 Field-Effect Phototransistors: Ultrasensitive Visible-Light Photoresponse and Negative Infrared Photoresponse. Adv. Mater. 2018, 30, 1705880. (4) Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A. High-Performance Single Layered WSe2 p-FETs with Chemically Doped Contacts. Nano Lett. 2012, 12, 3788−3792. (5) Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9, 372. (6) Feng, W.; Zheng, W.; Cao, W.; Hu, P. Back Gated Multilayer InSe Transistors with Enhanced Carrier Mobilities Via the Suppression of Carrier Scattering from a Dielectric Interface. Adv. Mater. 2014, 26, 6587−6593. (7) Hu, P.; Wen, Z.; Wang, L.; Tan, P.; Xiao, K. Synthesis of FewLayer GaSe Nanosheets for High Performance Photodetectors. ACS Nano 2012, 6, 5988−5994. (8) Lee, Y. H.; Zhang, X. Q.; Zhang, W.; Chang, M. T.; Lin, C. T.; Chang, K. D.; Yu, Y. C.; Wang, J. T. W.; Chang, C. S.; Li, L. J.; Lin, T. W. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320−2325. (9) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. TwoDimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568−571. (10) Li, X.; Basile, L.; Huang, B.; Ma, C.; Lee, J.; Vlassiouk, I. V.; Puretzky, A. A.; Lin, M.-W.; Yoon, M.; Chi, M.; Idrobo, J. C.; Rouleau, C. M.; Sumpter, B. G.; Geohegan, D. B.; Xiao, K. Van Der Waals Epitaxial Growth of Two-Dimensional Single-Crystalline GaSe Domains on Graphene. ACS Nano 2015, 9, 8078−8088. (11) Yao, J.; Zheng, Z.; Yang, G. Promoting the Performance of Layered-Material Photodetectors by Alloy Engineering. ACS Appl. Mater. Interfaces 2016, 8, 12915−12924. (12) Yao, J.; Zheng, Z.; Yang, G. All-Layered 2D Optoelectronics: A High-Performance UV−Vis−NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes. Adv. Funct. Mater. 2017, 27, 1701823. (13) Zheng, Z.; Yao, J.; Yang, G. Centimeter-Scale Deposition of Mo0.5W0.5Se2 Alloy Film for High-Performance Photodetectors on Versatile Substrates. ACS Appl. Mater. Interfaces 2017, 9, 14920− 14928. (14) Yao, J.; Yang, G. Flexible and High-Performance All-2D Photodetector for Wearable Devices. Small 2018, 14, 1704524. (15) Yao, J.; Zheng, Z.; Yang, G. Layered Tin Monoselenide as Advanced Photothermal Conversion Materials for Efficient Solar Energy-Driven Water Evaporation. Nanoscale 2018, 10, 2876−2886. E

DOI: 10.1021/acsanm.8b01527 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX