Enhanced Photoresponse of SnSe-Nanocrystals-Decorated WS2

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Enhanced photoresponse of SnSe nanocrystals decorated WS monolayer phototransistor 2

Zhiyan Jia, Jianyong Xiang, Fusheng Wen, Ruilong Yang, Chunxue Hao, and Zhongyuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12137 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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ACS Applied Materials & Interfaces

Enhanced photoresponse of SnSe nanocrystals decorated WS2 monolayer phototransistor Zhiyan Jiaa, Jianyong Xianga,*, Fusheng Wena, Ruilong Yanga, Chunxue Haoa, Zhongyuan Liua,* a

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinghuangdao 066004, People’s Republic of China.

Abstract Single-layer WS2 has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic thickness limited adsorption and bandgap-limited spectral selectivity. Here we have carried out investigations on WS2 monolayer-based phototransistors with and without decoration of SnSe nanocrystals (NCs) for comparison. Compared to the solely WS2 monolayer, SnSe NCs decoration leads to not only huge enhancement of photoresponse in visible spectrum but also extension to near infrared. Under excitation of visible light in a vacuum, the responsivity at zero gate bias can be enhanced by more than 45 times to ~99 mA/W, and the response time is retained in millisecond level. Particularly, with extension of photoresponse to near infrared (1064 nm), a responsivity of 6.6 mA/W can be still achieved. The excellent photoresponse from visible to near infrared is considered to benefit from synergism of p-type SnSe NCs and n-type WS2 monolayer, or in other words, the formed p-n heterojunctions between p-type SnSe NCs and n-type WS2 monolayer.

Keywords: Photoresponsivity, monolayer WS2, SnSe nanocrystal, chemical vapour deposition,

single mode microwave-assisted hydrothermal technique *Corresponding authors. Email: [email protected] (J. Y. Xiang), [email protected] (Z. Y. Liu)

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1. Introduction Low-cost, high-performance photodetection has been involved in numerous fields, such as biomedical diagnostics, imaging, remote sensing, and environmental monitoring etc.. Recent advances in fabrication of nanoscale materials and devices have stimulated tremendous interest in two-dimensional (2D) layered materials, such as graphene and transition metal dichalcogenides (TMDCs), for photodetection applications1-13. TMDCs are a class of layered semiconducting materials with the indirect bandgap in the range of 1-2 eV, and their single and few layers can be obtained by exfoliation14-16 and other synthesis routes17-22. In single layer, TMDCs transition to direct bandgap semiconductors. Single- and few-layer TMDCs have shown rich unusual fundamental physical properties and promising applications in wide fields such as advanced digital electronics, optoelectronics, and sensors5, 22-25. For optoelectronic applications, in contrast to the widely studied no-bandgap graphene, single- and few-layer TMDCs shows a certain advantage because the existence of finite bandgap allows for tunable carrier transport via a back gate voltage5. Single- and few-layers of TMDCs and heterostructures based on different TMDCs, particularly MoS2 (WS2), have been investigated for potential photodetection applications6-13, 26-29. In spite of the attractive photoresponse performances, they have faced weak light adsorption due to atomically thin profiles and limited spectral range owing to finite band gap, the common barriers of 2D materials in optoelectronic devices5. The metal dichalcogenides of IV and VI group elements (MDCs), such as SnS(Se), PbS(Se), GeS(Se), etc., are one other famous family of layered semiconducting materials. The colloidal nanocrystals (NCs) of MDCs have been widely studied for potential optoelectronic applications30-32, because their narrow band gaps can be tuned across a broad spectral range through quantum confinement by controlling the NC size33-34. Recently, the fabricated hybrid photodetectors with a thin PbS quantum dots film coated on graphene and MoS2 monolayer have demonstrated extraordinary gain and responsivity and extension of spectral sensitivity to near infrared4, 9.


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Among TMDCs, besides the first representative MoS2, WS2 is probably another more interesting material. WS2 monolayer has a direct band gap of at least 1.95 eV. The WS2 monolayers have been found to show about 20 times higher photoluminescence (PL) emission efficiency than that in the natural MoS2 crystal35-36, implying a relatively high quantum efficiency in potential WS2 monolayer-based optoelectronic devices. Recent investigations have revealed the excellent photoresponse properties of single- and few-layer WS210-13, but their spectral sensitivity is limited to the visible spectrum owing to the finite band gap. Compared with PbS, SnSe possesses the advantages of low toxicity and relative abundance of constituent elements, and SnSe NCs also show tunable band gap from near-infrared to visible wavelengths37-39. Thereby, by using the Chemical Vapor Deposition (CVD)grown triangular n-type WS2 monolayers and the synthesized p-type SnSe NCs via single mode microwave-assisted hydrothermal technique (SMMHT), we have fabricated the WS2 monolayer–based phototransistors with and without the decoration of SnSe NCs for comparison. Scattered SnSe NCs on WS2 monolayer lead to the formation of separated p-n heterojunctions. Compared to the only WS2 monolayer phototransistors, the decorated WS2 monolayer with SnSe NCs has shown not only huge enhancement of photoresponse in the visible spectral range but also extension to near-infrared range.

2. Results and discussion Synthesis of SnSe NCs via single mode microwave-assisted hydrothermal technique (SMMHT), growth of WS2 monolayers by CVD and fabrication of SnSe NCs-decorated WS2 monolayer phototransistors are schematically illustrated in Fig. S1 (Supporting information). As-synthesized SnSe NCs were thoroughly washed and centrifuged with alcohol. The crystal structure and morphology of as-synthesized SnSe NCs were checked by using X-ray diffraction and (high-resolution) transmission electron microscopy. As shown in XRD pattern (Fig. 1a) and HRTEM image (Fig. 1c), assynthesized SnSe NCs are well crystallized in the orthorhombic structure (JCPDS No. 89-0233), but they are not uniform in sizes as revealed in the TEM image (Fig. 1b). In order to obtain SnSe NCs in small and uniform size, as-synthesized SnSe NCs (5 mg) were sonicated in alcohol (10 ml) for 300 mins, and after centrifugation to remove



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the large SnSe NCs, the supernatant was extracted for later use. As demonstrated in the TEM image of Fig. 1d, the SnSe NCs in supernatant are uniform in size with lateral dimensions of ~ 5 nm. In the HRTEM image of a small SnSe NC (Inset on bottom right corner in Fig. 1d), recognizable fringes indicate the retention of good crystallinity after sonication. WS2 monolayers were grown onto a Si/SiO2 (300 nm) substrate with CVD in a quartz tube, and their structure and morphology are consistently verified by using optical microscopy, atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopy. The distinct contrast of the optical image indicates a triangular shape of as-grown WS2 monolayers (see bottom inset of Fig. 1f), similar to those reported single layers of WS219, 40-41. By AFM measurement (Fig. S2 in supporting information), the typical height of WS2 monolayers is determined to be 0.83 nm, a little larger than the theoretically and experimentally determined values of bulk WS242, however, falls into the range of observed values (< ~1.0 nm)10, 19, 35, 41, 43. This deviation is a common phenomenon in the AFM-derived monolayer thickness of 2D materials44-45. The collected Raman spectrum from a triangular WS2 monolayer (Fig. 1f) is in agreement with previously reported ones of single- and few-layer WS212, 35, 46-47. Raman spectrum of bulk WS2 involves one first-order acoustic phonon mode of LA(M), two first-order optical phonon modes of in-plane E12g(Γ) and out-of-plane A1g(Γ), and additional multiphonon combinations of the first-order modes47-48. For few-layer WS2, the peak positions of LA(M), E12g(Γ) and A1g(Γ) modes in Raman spectrum show small thickness-dependent deviations from bulk ones46-48. In Fig.1e, the Raman peaks at 175 and 418.6 cm-1 are recognized to arise from LA(M) and A1g(Γ) modes, respectively; E12g(Γ) mode at ~356 cm-1 is overwhelmed with high-intensity 2LA(M) mode at 353 cm-1. By using excitation wavelength of 532 nm in Raman studies on single- and few-layer WS2, identification of a single-layer WS2 can be made by the induced much higher (double) intensity of 2LA(M) than that of A1g(Γ) for a single layer owing to a double-resonance Raman process47. The inset of Fig 1e shows the intensity mapping of A1g(Γ), which is quite uniform over the entire WS2. Bulk WS2 is an indirect semiconductor and shows almost no PL. Enhancement of the PL in intensity can be several orders of magnitude as the thickness of WS2


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approaches to that of a single layer36, which has a direct bandgap of ~ 2 eV. Fig. 1f gives a room-temperature PL spectrum of as-grown WS2 monolayer, showing a single characteristic peak at 630 nm (1.97 eV) with full width at half maximum of ~14.2 meV and intensity of ~32 relative to the intensity of the Raman mode E12g. The observable single PL peak is consistent with the theoretical prediction of only one direct electronic transition at the K point for a WS2 monolayer49. The PL maximum position at 1.97 eV lies in the previously observed range between 1.99 and 1.94 eV for WS2 monolayers13, 35-36, 48. Top inset of Fig. 1f gives the PL scan of the intensity at the energy of 1.97 eV over an as-prepared triangular WS2 monolayer. The uniform distribution of the PL intensity over a large area of the triangular WS2 indicates good quality of the crystal. The suppression of PL at the edges was also observed in CVDgrown monolayer WS250, and is ascribed to the structural defects51. The solely WS2 monolayer back-gated field-effect transistor (FET) were fabricated with the help of a laser direct writing device. Fig. 2a shows the optical image of an only WS2 monolayer transistor. The metal contacts are composed of Ti/Au (10/60 nm). SnSe NCs-decorated WS2 monolayer (WS2/SnSe) FET devices were prepared directly by drop casting of SnSe NCs on WS2 monolayer transistors. Raman measurements performed on the hybrid device confirmed the successful decoration of SnSe NCs on monolayer of WS2 (Fig. S4). As revealed in the AFM image (Fig. 2b), SnSe NCs are uniformly distributed on the WS2 monolayer, leading to the formation of uniformly separated p-n junctions between p-type SnSe NCs and n-type WS2 monolayer. A schematic in Fig. 2c illustrates the cross-sectional view of SnSe NCsdecorated WS2 monolayer transistor and the connected configuration of voltage sources for electrical characterization. All measurements were carried out with a dual sourcemeter at room temperature in a vacuum (20 Pa) before and after decoration of SnSe NCs for comparison. The output characteristic curves at gate voltages (Vg) of 0 and 20 V were measured for the only WS2 monolayer and WS2/SnSe hybrid devices under dark conditions (see Fig. S3 in Supporting information). The nice linear behavior of Ids vs Vds verifies the Ohmic contacts between WS2 monolayer and Ti/Au electrodes.



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Fig. 2d shows typical transport performances of the only WS2 monolayer device under dark condition and the WS2/SnSe hybrid device under dark condition and illumination of three wavelengths (457, 671, and 1064 nm). The field-effect electron mobility (µFE) is found to be 0.1 cm2 V-1 s-1 for the only WS2 monolayer under dark condition, but after decoration with SnSe NCs, it increases to 2.2 cm2 V-1 s-1. Compared to the only WS2 monolayer device, the WS2/SnSe hybrid one displays a shift of threshold voltage toward the positive gate voltage and increase of ON/OFF current ratio under dark condition. These observations should be associated with the formed p–n junctions between the SnSe NCs and WS2 monolayer. Electrons flow from n-type WS2 monolayer into p-type SnSe NCs until formation of built-in fields at the p–n-junctions and equilibrium Fermi levels. The decorated WS2 monolayer with SnSe NCs thereby becomes less n-type doped. As shown in Fig. 2d, the WS2/SnSe hybrid phototransistor displays the drift of threshold voltage to more negative gate voltage under illumination. Probably, the photoexcited electron-hole pairs upon illumination are separated at the interfaces between SnSe NCs and WS2 monolayer. The WS2/SnSe heterojunctions favor injection of photogenerated electrons into WS2 monolayer from SnSe NCs, while photoproduced holes are retained in the SnSe NCs9. It has been revealed that with decoration of SnSe NCs, the spectral sensitivity is extended to near infrared. Our discussion in the following will turn to the photoresponse at Vg=0 V of only WS2 monolayer and WS2/SnSe hybrid phototransistors for comparison. Laser lights of three different wavelengths (457, 671 and 1064 nm) were used in the measurements. All the laser beams have a spot waist of 1.2 mm in diameter, and cover the whole WS2 monolayer. Figs. 3a and b show the Ids vs Vds curves at Vg=0 V for only WS2 and WS2/SnSe devices under dark condition and light illumination of 457, 671 and 1064 nm at a fixed power density of 23 W/cm2, and Figs. 3c, d give the measured timedependent photoresponse at Vg=0 V and Vds=5 V by periodically switching on and off laser light. The photocurrent of Iph is the difference between the currents under light excitation and dark condition, namely Iph = Ilight – Idark. Under light illumination, the photocurrent Iph shows a linear increase with the rise of Vds. For the only WS2 device, the highest photocurrent is produced under 457 nm excitation, and at Vds=5 V, a


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large value of ~60 nA is obtained. With the increase of wavelength, the photoresponse decays rapidly. Under light illumination of 671 nm, only a photocurrent of ~8 nA is observed at Vds=5 V. Under the 1064 nm excitation, as shown in Fig. 3c, the only WS2 device displays no sensitivity. This rapid decay of photoresponse with the increase of wavelength can be attributed to the large bandgap of WS2 monolayer, similar to those observed in the other layered materials9, 12, 52-54

. In comparison to the only WS2 monolayer device, as shown in Figs. 3b, d, the

WS2/SnSe hybrid one exhibits not only great enhancement of photoresponse but also extension of spectral sensitivity to near infrared. Under the excitation of 457 nm, the photocurrent of Iph is enhanced to µA level, and a high photocurrent of ~1600 nA is obtained at Vds=5 V, more than 26 times larger than that of the only WS2 monolayer device. With the increase of wavelength, in spite of reduction in photocurrent, the enhancement of photoresponse owing to the decoration of SnSe NCs becomes more obvious. Under the 671 nm excitation, a photocurrent of ~260 nA is achieved at Vds=5 V, more than 32 times larger than that of the only WS2 monolayer device. With the increase of wavelength to 1064 nm, as shown in Fig. 3d, the SnSe NCs-decorated WS2 monolayer device still exhibits clear photoresponse, and the photocurrent of ~25 nA at Vds=5 V is more than three times larger than that of the only WS2 monolayer device under 671 nm excitation. Therefore, with decoration of SnSe NCs, operation of WS2 monolayer as high-quality photosensitive switch can be extended to near-infrared range. Investigations were also performed on the photoresponse speeds of the only WS2 and WS2/SnSe hybrid devices to incoming light excitation for comparison. Figs. 4a and b show time-resolved photocurrents of one period for the only WS2 and WS2/SnSe hybrid devices under the 457 nm excitation, which were measured by a lock-in amplifier equipped with an optical chopper. For the only WS2 monolayer device, the rise and fall times are determined to be 4.1 and 5.2 ms, respectively, comparable to those reported ones for WS2 monolayer12. For the WS2/SnSe hybrid device, the rise and fall times are found to be 8.2 and 8.4 ms, respectively. Though the response becomes a little slower owing to the slow charge transfer process between WS2 monolayer and SnSe NCs, it is still fast in comparison to those reported



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in other layered materials6, 9, 18, 55-56 (see Table S1 in Supporting Information for the comparison in detail.). The only WS2 monolayer and WS2/SnSe hybrid devices were also characterized for comparison by varying excitation power under illumination of 457, 671 and 1064 nm. Figs. 4c and d show the photocurrent (Iph) and spectral responsivity (R) and their dependences on power density Φ, respectively. Iph and R=Iph/Φ can be fitted to a power relations Iph ∝ Φα and R ∝ Φ α-1, respectively, where Φ is defined as Φin(Sdevice/Sspot) with Φ in, Sdevice and Sspot representing the total incoming light power, areas of device and laser spot, respectively, α is an exponent. As shown in Fig. 4c, the photocurrent of Iph increases with increasing power density of Φ at all the three wavelengths. The linear curves imply that the photocurrent is determined by the amount of photogenerated carriers. By fitting the data with the above-mentioned power law relation, the extracted α was found to increase from 0.45 to 0.69 when the wavelength of the incident light was decreased from 1064 to 457 nm, and it was changed slightly after decoration. These values of α are smaller than the ideal factor of unit which is related to the loss of photocurrent, indicating that both trap states and interactions between the photogenerated carriers are involved in the recombination kinetics of photocarriers.57 Compared to the only WS2 monolayer device, the decoration of SnSe NCs leads to huge enhancement of the photocurrent. Under 457 nm excitation at a power density 23 W/cm2, the photocurrent can be enhanced from ~60.8 nA to ~1.6 μA. Particularly, the spectral sensitivity of WS2 monolayer is extended to near infrared owing to decoration of SnSe NCs. Under near-infrared illumination of 1064 nm, the photocurrent becomes higher than or comparable to those of the only WS2 monolayer device under visible illumination. Fig. 4d shows the variations of R with power density for the only WS2 and WS2/SnSe hybrid devices under light illumination of 457, 671 and 1064 nm. Both the only WS2 and WS2/SnSe hybrid devices exhibit the decrease of R with increasing power density, a common observation in photodetectors based on layered materials, such as WS211, black phosphorus52, graphene58, MoS254, InSe53. This phenomenon is considered to be induced by a reduction in photogenerated carriers available for extraction under photon flux owing to Auger processes or the saturation of recombination/trap states


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that influence the lifetime of the generated carriers4, 57-58. The only WS2 monolayer device shows no spectral responsivity under near-infrared illumination of 1064 nm due to the fact that the energy of the laser (1.17 eV) is too lower to excite considerable electron-hole pairs in WS2. With the decoration of SnSe NCs, the WS2/SnSe hybrid device exhibits not only hugely enhanced responsivity under visible excitation but also extension of spectral responsivity to near infrared. At a low power density of 0.44 W/cm2, the WS2/SnSe hybrid device shows the responsivity up to 99.0 (31.8) mA/W under visible illumination of 457 (671) nm, more than 26 (45) times higher than that of the only WS2 monolayer device. Even under near-infrared excitation, in comparison to no spectral responsivity of the solely WS2 monolayer device, a high value of R=6.6 mA/W is achieved at power density of 0.44 W/cm2 due to decoration of SnSe NCs. For the enhanced responsivity and extension of spectral sensitivity to near infrared in WS2/SnSe hybrid phototransistor, contribution from the SnSe NCs channel can be ruled out. As demonstrated by the AFM image (Fig. 2b), the uniformly scattered SnSe NCs on the WS2 monolayer are completely separated from each other, and they are not able to form any additional channel besides the WS2 one. Since the energy of the incident near-infrared light is comparable to the band gap of SnSe NCs ( ~ 1.05-1.27 eV31, 59-60), considerable amount of carrier in SnSe NCs were photoexcited to its conduction band and then transferred into the conduction band of monolayer WS2, as illustrated by the schematic band diagram (Fig. S5 in the supporting Information), resulting in the extension of the response spectra. Here, SnSe NCs in smaller size and narrower distribution in size are essential for the hybrid devices facilitating fast and effective carrier transfer process. On one hand, it was believed that the photon-tocharge carrier generation efficiency increases with decreasing the size of the NCs61, whereas the increase of bandgap due to the quantum confinement effect associated with the shift in conduction band towards less negative potentials (decrease in electron affinity energy) increases the driving force and favors fast electron injection.62-64 Moreover, the uniformly distributed NCs structures on monolayer WS2 suppress lateral carrier diffusion, preventing trapping in nonradiative recombination centres. One the other hand, more p-n junctions can be formed with smaller isolated



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SnSe NCs, although the current don’t pass through the SnSe NCs, the photoexcited electrons in SnSe NCs can transfer into the WS2 under the energy gradient at the interfaces and make contribution to its conductivity. While NCs in larger size could overlap on the surface of monolayer WS2, leading to the formation of additional channels. It can be concluded that the synergism of WS2 monolayer and SnSe NCs, or in other words, the separated p-n heterojunctions between the decorated SnSe NCs and WS2 monolayer, can be considered as the origin for enhanced photoresponse under visible light illumination and extension of spectral sensitivity to near infrared in the fabricated WS2/SnSe hybrid phototransistor.

3. Conclusions We have carried out the investigations on photoresponse of WS2 monolayer based phototransistors with and without decoration of p-type SnSe NCs for comparison. In comparison to the devices based on only WS2 monolayer, the WS2/SnSe hybrid devices are found to show hugely enhanced photoresponse performance owing to the synergism of SnSe NCs and WS2 monolayer. Under illumination of visible light, the WS2/SnSe hybrid transistors exhibit the enhanced responsivity of ~99 mA/W at zero gate bias, more than 45 times higher than that of the devices without SnSe NCs decoration, while the response time is retained at milliseconds. Particularly, the SnSe NCs decoration leads to the extension of spectral selectivity to near infrared, and a responsivity of 6.6 mA/W is still achieved under 1064 nm excitation. These excellent results indicate the efficient electronic coupling between n-type WS2 monolayer and p-type SnSe NCs, which can be used for fabrication of novel hybrid optoelectronic devices with significantly enhanced performances.

4. Experimental 4.1 Materials Powder materials of WO3 (99.8%), Sulfur (99.99%), SeO2 (99.99%) and SnCl4·5H2O (99.99%) were purchased from Alfa Aesar. Si/SiO2 (300 nm) was ordered from the Silicon Quest International Inc (USA). The fabrication process is similar to that of electron beam lithography. The metal contacts of Ti (10 nm)/Au (60 nm) were


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deposited by evaporation of Ti (99.999% purity, Alfa Aesar) and Au (99.999% purity, Alfa Aesar) sources. 4.2 Preparation of SnSe NCs SnSe NCs were synthesized via single mode microwave-assisted hydrothermal technique (SMMHT), and powders of SeO2 (99.99%) and SnCl4·5H2O (99.99%) (Alfa Aesar) were used as the starting materials. SeO2 (0.1 mmol) and SnCl4·5H2O (0.1 mmol) were put into a reaction bottle, and then Oil amine (8 mL, Alfa Aesar) was injected as reaction solvent. The reaction was performed at 100 °C for 24 hours. Assynthesized SnSe powder was thoroughly washed and filtered with alcohol. Then, the as-synthesized SnSe powder (5 mg) was put into 10mL alcohol for 300 mins sonication at power of 30 W in an ultrasonic cell crusher noise isolating chamber (HN-1000Y, HANU, Shanghai). The sonication-produced solution was centrifuged at 2500 rpm for 1 hour, and the supernatant was extracted for later use. 4.3 Growth of WS2 monolayer WS2 monolayers were grown on a Si/SiO2 (300 nm) substrate (Silicon Quest International Inc, USA) in a Quartz tube. Before use, the Si/SiO2 substrates were firstly cleaned with acetone in an ultrasonic condition for 10 mins, and then with isopropyl alcohol. Finally, the substrates were blow dried by N2 gas. During the process of growth, sulfur powder (1 g, 99.99% purity, Alfa Aesar) was put on the upstream of quartz tube, WO3 powder (500 mg, 99.8% purity, Alfa Aesar) was put on the downstream. The sulfur and WO3 vapors were carried with an argon flow at 100 sccm controlled by a mass-flow controller to the Si/SiO2 substrates, on which the reaction occurs for 50 mins. 4.4 Fabrication of WS2 monolayer-based phototransistors The electrodes of WS2-based devices were patterned using a standard photolithography with the help of a laser direct writing apparatus (Microwriter Baby Plus, Durham Magneto Optics Ltd, UK), followed by thermal evaporation of Ti (10 nm)/Au (60 nm) and lift-off. The fabricated WS2 monolayer-based phototransistors were annealed at 350 °C for 2 h in a vacuum (7×10-7 mbar) to improve contacts. The



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SnSe NCs-decorated WS2 monolayer phototransistors were directly prepared from the WS2 monolayer phototransistors by simply drop-casting of dispersion of SnSe NCs in alcohol. 4.5 Characterization The monolayer WS2 and SnSe NCs were characterized by the Raman, PL, AFM, XRD, SEM and TEM. Both Raman and PL spectroscopy was carried out using a Horiba Jobin Yvon LabRAM HR-Evolution Raman microscope with a laser radiation of 532 nm and power of 10 µW. The surface morphology images of sample were obtained with an atomic force microscope (AFM, MultiMode 8, Veeco Instruments Inc, USA). The XRD patterns were collected on a SmartLab X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å, Rigaku, Japan). The morphology images were obtained by a scanning electron microscopy (FESEM S-4800, Hitachi, Japan). The TEM images of SnSe nanocrystals were obtained in a transmission electron microscopy operating at 200 kV (TEM, JEM-2010, JEOL, Japan). The electrical data was collected with a Keithley 2612B system. All measurements were performed in a vacuum of ~20 Pa at room temperature. Time-resolved photocurrents were measured by a lock-in amplifier (SR830, Stanford Research System, USA) equipped with an optical chopper (SR540, Stanford Research System, USA).

Acknowledgements We thank the National Natural Science Foundation of China (Grant No. 51271214, 51102206, 51421091, 51571172), National Science Fund for Distinguished Young Scholars (Grant No. 51025103), Program for New Century Excellent Talents in University (NCET-13-0993). Supporting Information. Brief statement of the schematic diagram of the sample synthesis, AFM characterization of the as-grown monolayer WS2, the current-voltage curves of the only WS2 and hybrid devices at low voltage with different gated voltage, Raman spectra of the hybrid devices, schematic energy level diagram of the hybrid device, and a performance comparison of our devices with those from other research groups.


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Figure 1. Characterizations of SnSe NCs and WS2 monolayers. XRD pattern (a), TEM image (b) and HRTEM image (c) of as-synthesized SnSe NCs by SMMHT. (d) TEM image of SnSe NCs in extracted supernatant after sonication and centrifugation. (e) Raman spectrum of triangular WS2 monolayer grown by CVD. Inset is the intensity mapping of A1g(Γ) mode. The scale bar indicates 5 µm. (f) Photoluminescence and PL mapping (top inset) of a triangular WS2 monolayer. Bottom inset is an image of optical microscopy. The scale bar indicates 5 µm. 88x109mm (300 x 300 DPI)



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Figure 2. (a) Optical image of a fabricated WS2 monolayer phototransistor. (b) AFM image of uniformly distributed SnSe NCs on WS2 monolayer. (c) Schematic cross-sectional view of SnSe NCs-decorated WS2 monolayer transistor and the connected configuration of voltage sources for electrical characterization. (d) Transport performances of the only WS2 monolayer device under dark condition (dash line) and the WS2/SnSe hybrid device under dark condition and illumination of three wavelengths (457, 671 and 1064 nm). Back-gate voltage (Vg) sweeping range: -60 to 60 V; Source-drain bias (Vds): 5 V; Laser power density: 23 W/cm2. 68x52mm (300 x 300 DPI)


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Figure 3. Ids vs Vds curves at Vg=0 V for the only WS2 monolayer (a) and WS2/SnSe hybrid (b) phototransistors under dark condition and illumination of 457, 671 and 1064 nm at power density of 23 W/cm2. Time-dependent photocurrents with periodically turning on and off laser beams for the only WS2 monolayer (c) and WS2/SnSe hybrid (d) phototransistors. 98x109mm (300 x 300 DPI)



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Figure 4. Time-resolved photocurrents to show the typical rise and fall times for the only WS2 monolayer (a) and WS2/SnSe hybrid (b) phototransistors under illumination of 457 nm (Power density of 23 W/cm2, Vg=0 V and Vds=5 V). Variations of Photocurrent (c) and responsivity (d) with power density for the only WS2 monolayer and WS2/SnSe hybrid phototransistors under illumination of 457 nm (Vg=0 V and Vds=5 V). 92x71mm (300 x 300 DPI)


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TOC 60x85mm (600 x 600 DPI)


Enhanced Photoresponse of SnSe-Nanocrystals-Decorated WS2

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