Low-Powered Photodetector Based on Two-Dimensional InS 0.3 Se 0.7 /WS 2 Heterostructure

: Photodetectors based on two-dimensional (2D) materials have great potential applications in the ﬁeld of new energy, such as fuel cells, solar cells, and other ﬁelds. Van der Waals (vdW) heterojunction photodiodes are expected to be one of the promising applications of two-dimensional materials due to the photoelectric properties without consideration of lattice mismatch. High-efﬁciency photoelectric sensors based on two-dimensional materials have great signiﬁcance to reducing the energy consumption of devices. Here, we build a complex vdW heterostructure by combining InS 0.3 Se 0.7 with another suitable 2D material WS 2 . Few-layer graphite was used as electrodes to enhance the optoelectronic performance of indium monochalcogenides. Evident photocurrent is observed in the InS 0.3 Se 0.7 /WS 2 vdW heterostructure device arising from the formed p–n junction at the interface. The uniformity and photoresponse of the InS 0.3 Se 0.7 /WS 2 vdW heterostructure has been further investigated by the photocurrent mapping. It shows that the entire photovoltaic current was originated from the InS 0.3 Se 0.7 /WS 2 vdW heterojunction by scanning photocurrent microscope images. Furthermore, the response speed is enhanced at small bias voltage. The transient photoresponse can be well reproduced in almost 100 cycles, indicating the good repeatable optoelectronic performance. Our study indicates that the as-prepared InS 0.3 Se 0.7 /WS 2 vdW heterostructures are attractive building blocks for photodetectors application. Our ﬁndings will open up a new way to further develop high-performance, low-power, and energy-efﬁcient photodetectors based on indium monochalcogenides.


Introduction
In recent years, the development of advanced semiconductors based on two-dimensional (2D) materials is expected to solve the energy problem, which is caused by the huge growth of the number of sensors bringing by high integration and smaller size of electronic devices [1][2][3][4]. 2D semiconductor devices with low power consumption and high performance can be widely used in new energy fields, such as photocatalysis, fuel cells, solar cells, and others [3,[5][6][7][8][9][10][11]. Photodetectors are expected to be one of the promising applications of materials due to the photoelectric properties produced in single-or multilayer 2D materials. Especially, the low power photodetector with good visible light response, high stability, and fast photoresponse is very crucial for low-power applications, such as future internet of things, wearable electronics, and flexible electronics [12][13][14][15][16].
The complex heterostructures composed of different 2D layered materials open up a new platform for electronics and optoelectronics because of these novel and extraordinary properties, such as tunable bandgaps, passivated surfaces, and high mobility [12][13][14]. 2D materials usually possess different carrier polarities; when combining with different 2D materials to form the vdW heterostructures, the heterostructures can generate a pronounced photovoltaic effect [10,17,18]. In the past decade, various photodetectors based on 2D materials have been proven to have superior performance [19][20][21][22]. More recently, indium monochalcogenides InX (InS, InSe, or InS x Se 1-x ) have attracted much attention due to their excellent stability, high carrier mobility, and moderate band gap, showing promising applications in storage devices, optical sensors, thermoelectric devices, and low-cost and environmentally friendly solar cells [23][24][25][26][27][28][29][30]. Previous research on InS x Se 1−x alloys shows that the bandgap shifts from~1.27 to~1.42 eV as S composition increases from 0 to 0.3 without changing crystal structure [31]. Interestingly, when InSe is fabricated from singlelayer to multilayer with thickness larger than seven layers, its band structure changes from indirect gap to direct gap [23,28]. However, the photoelectric properties are significantly hindered by the little absorption of light induced by indirect band gap. On the other hand, tungsten disulfide (WS 2 ) has inspired great efforts in optoelectronics because of their suitable and direct band gap [32][33][34][35][36][37]. Therefore, WS 2 can act as one active absorber to construct many kinds of heterojunctions. Typically, the semiconductor heterojunctions possess three kinds of band arrangement, i.e., Type I-straddling, Type II-staggered, and Type III-broken-gap [20]. Type II possesses alternating band gap alignment between conduction bands and valence bands of the heterojunctions [20,37,38], and thus brings the decrease of the band gap and significantly increases carrier recombination rate because of the built-in electric field, leading to a wider detection wavelength range and faster response time. When the InSe layer and WS 2 layer are stacked together, a Type-II band alignment will be formed [21,32,33]. This construction of vdW heterostructures can enhance the optoelectronic performance.
In this work, the photoelectric properties of a type II vdW heterostructure based on InS 0.3 Se 0.7 /WS 2 were investigated. Graphene was used as electrodes by placing on the top of mechanically exfoliated few-layers InS 0.3 Se 0.7 flakes and a single-layer WS 2 flake. Graphene-on-semiconductor has been proven to be a quality heterojunction with efficient photoelectric conversion. Based on this hybrid heterostructure, we analyzed the performance of a photodetector. It can be responsive to the light, and the photoresponse region coincides with the overlapping area of InS 0.3 Se 0.7 layers and WS 2 single layer, which means all photovoltaic currents were originated from the InS 0.3 Se 0.7 /WS 2 heterojunction. The photocurrent waveform with a chopping frequency of 5 Hz indicated a good repeatable optoelectronic performance.

Results and Discussion
From the Raman spectra of the InS 0.3 Se 0.7 flakes, as shown in Figure S1 in Supporting Information, four typical Raman active modes at about 41 cm −1 , 119 cm −1 , 180 cm −1 , and 220 cm −1 , respectively, are observed in both areas, corresponding to the E, A 1 1g , E 1 2g , and A 2 1g modes, which is also consistent with the previous reports [31]. The scanning electron microscopy (SEM) image ( Figure S2a) of the as-synthesized InS 0.3 Se 0.7 crystals are of layered structures, energy-dispersive X-ray spectroscopy (EDS) ( Figure S2b-e) shows the elements of In, S, and Se evenly spread and the atomic ratio of In, S, and Se elements is estimated to be about 1:0.3:0.7. The photoluminescence spectra (PL) at different temperatures are also shown in Figure S3, indicating that the PL peak gradually shifts to a long wavelength for both WS 2 flakes and InS 0.3 Se 0.7 flakes with increasing temperature. Figure 1a shows the optical image of the InS 0.3 Se 0.7 /WS 2 vdW heterostructure. The n-type monolayer of WS 2 flakes was mechanically exfoliated from bulk crystal onto the SiO 2 substrates (marked by the green line in Figure 1a). The p-type InS 0.3 Se 0.7 flakes with the thickness of several tens of nanometers (marked by the orange line in Figure 1a) were mechanically exfoliated onto the PDMS-based gel supplier and then were transferred on the selected areas of n-type monolayer WS 2 flakes using a micromanipulator and digital camera to form the vdW p-n junction [38,39]. Afterwards, two pieces of few-layer graphite were transferred on InS 0.3 Se 0.7 flake and WS 2 flake, respectively, using the same processes, as marked by the black line in Figure 1a [40]. The Cr/Au electrodes were patterned on the few-layer graphite using the standard electron-beam lithography process in order to increase the response speed of the InS 0.3 Se 0.7 and WS 2 with high photoresponsivity, by using the transparent electrodes to modify the difference work function between the electrodes and the InS 0.3 Se 0.7 and WS 2 [22]. For example, for InSe/graphene, it can increase about 40 times faster than that of InSe/metal device [25,29]. This design of crystal/graphene electrical contacts could be important for high-performance optoelectronic devices.  When the laser is illuminated on the p-n junction, the photogenerated electron-hole pairs can be effectively separated: electrons and holes reside preferably in the InS 0.3 Se 0.7 and WS 2 layers, respectively [32,33], and finally result in the observation of open-circuit voltage (V oc ) and short-circuit current (I sc ). To explore the optoelectronic properties of the as-prepared InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices, an argon green laser at 532 nm with different power density was adopted to irradiate the device. Figure 2a shows the I-V characteristics of the InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices under the dark condition and different laser power densities (P = 1.5, 2.5, and 3 mW). As can be seen, the magnitude of the V oc and I sc increases gradually with increasing laser power, as indicated by blue triangles and red triangles in Figure 2b. Such increment of the V oc and I sc is believed to originate from the increased number of photo-generated carriers under laser irradiation with increased power. Under 15.3 mW/cm 2 illumination with the conditions of V ds = −0.6 V, V g = 0 V, λ =532 nm, the values of responsivity, detectivity were 0.25 A/W, 1.8 × 10 10 Jones, respectively. To further study the optoelectronic properties of the InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices, the photocurrent mapping is measured at zero bias potential and room temperature. The 2D plot of Figure 3 shows the intensity of photocurrent as a function of X and Y axis. By comparing with the optical image in Figure 3, it can be seen that the photocurrent is mainly generated at the overlap areas of the InS 0.3 Se 0.7 /WS 2 vdW heterostructure, as indicated by the blue dashed line. It is worth mentioning that no measurable photocurrent was observed in non-overlapping regions, with only the InS 0.3 Se 0.7 layers or single-layer WS 2 , demonstrating that the two graphene electrodes do enhance the photoresponse by contributing efficient carriers. This phenomenon further demonstrates that the measured photocurrent in our InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices is originated from the p-n junction formed at the interface, in which the photo-generated electron-hole pairs are effectively separated by the built-in field of the p-n junction. As shown in Figure S4, the depletion zone (as indicated by the green area) is formed at the interface of the InS 0.3 Se 0.7 flakes and WS 2 flakes. When the light is irradiated on the InS 0.3 Se 0.7 /WS 2 heterostructures, the photo-induced electron-hole will be separated at the interface. Electrons are driven to WS 2 side, while holes are driven to InS 0.3 Se 0.7 side. Finally, the electrons and holes were collected by the drain and the source, respectively, resulting in negative I sc and positive V os , as shown in Figure 2. In addition, the intensity of the photocurrent in the overlap area is almost the same, indicating the good uniformity of the as-prepared InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices. Therefore, the whole photovoltaic current was originated from the heterojunction. This can eliminate the contingency of photoresponse and further determine the position of the photocurrent.
The transient photoresponse characteristics were evaluated employing a 532 nm constant wave laser with optical power of 3 mW, and a frequency-programmable chopper. The photocurrent waveform with a chopping frequency of 5 Hz is presented in Figure 4. When the laser is irradiated, the resolved transient photoresponse cycles is clearly observed. The transient photoresponse can be well reproduced in almost 100 cycles, indicating the good repeatable optoelectronic performance of the InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices.

Conclusions
The optoelectronic properties of InS 0.3 Se 0.7 /WS 2 vdW heterostructure devices was systemically investigated. The optoelectronic properties originate from the p-n junction formed at the InS 0.3 Se 0.7 /WS 2 vdW heterostructure. Furthermore, the response speed is enhanced at a small bias voltage. The uniformity and the repeatable photoresponse features at the interface make the InS 0.3 Se 0.7 /WS 2 vdW heterostructure device a promising candidate for the potential photodetector devices application. Our study indicates that the InS 0.3 Se 0.7 /WS 2 vdW heterostructures are covered by graphene as electrodes are attractive building blocks in low-power photodetectors application, which could play an important role in the sustainable development of energy.