Introduction

Photodetectors that convert the light information into electrical information have been widely utilized in photography1, optical communication2, and analytical instruments3. Currently, the silicon-based photodetector is the workhorse behind the commercial image sensors embedded in mobile phones and digital cameras4,5,6. However, the application of silicon photodiode in the infrared and ultraviolet spectrum is limited due to its low absorption coefficient or high dark current7,8. To utilize the full light spectrum targeting emerging application fields such as biological imaging9, environment monitoring10, and flexible photodetectors11, emerging semiconductor materials and device structures are being assessed on their suitability for photodetector development12,13. The advent of two-dimensional (2D) materials has provided a tremendous amount of candidates covering almost all the interesting light spectrum for photodetectors exploration14. During the last decade, various photodetectors based on the 2D materials have been demonstrated with superior properties15,16,17,18. Early demonstrations used a back-gated field-effect transistor (FET) structure and the operational principle was mainly based on the photoconductive effect. The photodiodes are not readily obtained as reliable doping methods like ion-implantation are not suitable for 2D materials. Instead, local electrostatic gating by using a split gate has been proposed to realize a WSe2 PN junction, and a pronounced photovoltaic (PV) effect has been obtained19. Alternatively, as different 2D materials exhibit different carrier polarity, combining different 2D materials could form the heterostructures with pronounced PV effect20. The main advantages of the PV effect-based photodetectors, including low dark current and small operating voltage, would enable a low-power or even self-driven photodetectors for applications in internet of things (IoT) and wearable electronics21,22. 2D material-based self-driven photodetectors have received intense research efforts recently23, and these photodetectors have exhibited superior performances in terms of responsivity, response time, and detectivity. However, in terms of low dark current and linearity, as critically commented in recent works24,25, the 2D photodetectors are still much inferior to that of the commercial photodetectors.

In this paper, we designed an asymmetrical metal–semiconductor–metal (MSM) photodetector by using graphene and Au as the contacting electrodes to the WSe2 flake. To avoid the notorious Fermi-level pinning effect, a dry-transfer method is adopted to construct the van der Waals contacts. An ultrahigh photocurrent-to-dark current ratio up to 108 is obtained at a zero bias. The self-driven photodetector also exhibits a gate-tunable PV effect thanks to the gate-tunable Fermi level of the 2D WSe2 and graphene. The fabrication of the proposed device structure does not involve any external chemical doping or complex split gate structure, ensuring a stable and low-cost process for self-driven photodetectors.

Results and discussion

Device operational principle and fabrication process

Traditionally, the MSM structures are adopted for photodetectors due to the low dark current and also a simple planar fabrication process7. Figure 1a shows the band alignment of a traditional MSM photodetector with Au as the contact electrodes to WSe2. The left panel shows the band diagram of Au and WSe2 before contact. The valence band maximum (VBM) and conduction band minimum (CBM) is −5.18 and −4.03 eV, respectively26, and the Fermi level of WSe2 is located at the middle between the CBM and VBM, as confirmed by the ambipolar transfer curves of the WSe2 FET. The work function of the Au and Gr is about 5.1 and 4.56 eV, respectively27,28. As the MSM photodetector can be modeled as two MS junctions connected back to back, the total current density under light illumination flows through the MSM photodetector is given by

$$J = J_{\mathrm L} + J_{\mathrm R} = \left[ {J_{{\mathrm{SL}}}({\mathrm e}^{qV_{\mathrm{L}}/{\upeta}kT} - {\mathrm{1}}) + {\mathrm{q}}GW_{\mathrm{L}}} \right] - \left[ {J_{{\mathrm{SR}}}({\mathrm{e}}^{ - qV_{\mathrm{R}}/{\upeta}kT} - {\mathrm{1}}) + qGW_{\mathrm{R}}} \right],$$
(1)

where JL (JR) and JSL (JSR) are the current density and the reverse saturation current density of the MS junction at the left (right) side, respectively. VL and VR are the voltage applied to the left and right MS junction whose sum is determined by external bias voltage between the two electrodes Vds = VL + VR. η is the ideality factor of the MS junction, and k is the Boltzmann constant. JSL (JSR) is dependent on the Richardson constant A* and the Schottky barrier height ΦBL (ΦBR) as given by7

$$J_{{\mathrm{SL}}} = A^ \ast T^2{\mathrm{e}}^{ - q{\it{{\Phi}}}_{{\mathrm{BL}}}/kT}.$$
(2)
Fig. 1: Operational principle and the fabrication process of the photodetector.
figure 1

Band alignment of the Metal–WSe2–Metal structures with a Au as the two contact electrodes and b Au and multilayer graphene as the two contact electrodes, respectively, where the asymmetrical Schottky barriers at the two sides enabling a non-zero short-circuit current under light illumination. c Three-dimensional schematic of the process flow to fabricate the graphene–WSe2–Au photodetector.

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The thermionic emission theory predicts that the A* is solely dependent on the semiconductor side, which is WSe2 in this study. Thus for a symmetrical Au–WSe2–Au photodetector, ΦBL is equal to ΦBR, resulting in an equal reverse saturation current at the two MS junctions. The second term in the expression of JL (JR) is contributed from the photogenerated current, and G and WL (WR) denotes the carrier generation rate and the depletion width at the left (right) MS junction. The depletion width WL (WR) is related to the external voltage bias:

$$W_{\mathrm{L}} = \sqrt {2\varepsilon _{\mathrm{s}}(V_{{\mathrm{BL}}} + V_{\mathrm{L}})/qN_{\mathrm{d}}},$$
(3)
$$W_{\mathrm{R}} = \sqrt {2\varepsilon _{\mathrm{s}}(V_{{\mathrm{BR}}} - V_{\mathrm{R}})/qN_{\mathrm{d}}},$$
(4)

where εs and Nd are the dielectric constant and carrier concentration of WSe2, respectively. VBL (VBR) is the built-in potential of the MS junction at the left (right) side, which can be expressed as VBL = ΦBL − Ψn, where Ψn is the energy difference between the CBM and the Fermi level.

When the device is short circuited, both VL and VR are zero, and the total current is contributed by the difference in the second term, which gives J = qG(WL − WR). As shown in the right panel of Fig. 1a, for the symmetrical Au–WSe2–Au MSM photodetector, WL is equal to WR at zero external bias voltage, which results in a zero short-circuit photocurrent (Isc). In that case, a non-zero photocurrent can be induced by applying an external voltage which is mainly applied to the reverse-biased MS junction and causes a difference between WL and WR. Note that the semiconductor WSe2 is the same throughout this study, and Ψn is the same for both the two MS junctions. Thus a large Isc can be expected by forming two asymmetrical MS junctions with different barrier height. Figure 1b shows the energy band diagram of the asymmetrical graphene–WSe2–Au MSM photodetector, which schematically illustrates that a large difference between WL and WR can be obtained even without any external bias, thanks to the intrinsic difference between ΦBL and ΦBR.

As the key to obtaining the self-driven photodetector is to ensure a large difference of barrier height for the two constituent MS junctions, one may choose two electrodes with different work functions to construct the asymmetrical MSM photodetectors. However, traditional metal deposition methods, including sputtering and evaporation, induce defects and Fermi level pinning at the interface27, which makes it challenging to form asymmetrical MSM photodetectors with observable photovoltaic effects just by choosing two different metal materials. To address this problem, we adopted a PDMS-assisted dry-transfer method, as shown in Fig. 1c. The Au–WSe2 MS junction is formed first by transferring a WSe2 flake onto the pre-patterned Au electrodes, and the other constituent graphene–WSe2 MS junction is formed by transferring a multilayer graphene flake onto the WSe2 flake. In this way, both of the electrodes form a van der Waals contact with the WSe2 flake, and the barrier height difference is highly dependent on the work function difference of the two electrode materials.

Electrical characteristics of the photodetector

Figure 2a illustrates the optical microscope image of a fabricated MSM photodetector with asymmetrical contact electrodes. The red dashed lines show the contact edges of the Au–WSe2 and graphene–WSe2 MS junctions. The effective area of WSe2 in between the two electrodes is about 507.1 μm2, as calculated from the optical image. The Raman spectrum measured on the graphene–WSe2 contact area is presented in Fig. 2b. Both of the G (1580.5 cm−1) and G′ (2726.6 cm−1) peaks of graphene are observed, and the relatively weaker peaks at 247.9 and 256.6 cm−1 are corresponding to the E2g and A1g modes of multilayer WSe2, respectively. Note that a much higher Raman laser power is applied to obtain the WSe2 characteristics due to the graphene coated on top of the WSe2. The Raman spectra obtained from the graphene and WSe2 flakes near the overlapped region are presented in Supplementary Fig. 1a, b, which show the same peak positions but different intensities, especially for the WSe2 flake. Raman peak shift is usually used as an indicator to access the strain effect or the quenching effect29,30,31. The experimental results suggest the lack of observable strain effect or quenching effects in the heterostructures. The low Raman intensity of WSe2 is probably due to the relatively thick multilayer graphene on top of the WSe2, which absorbs a lot of incoming light and reflected light from the WSe2. The thicknesses of the WSe2 and graphene flakes are measured by atomic force microscopy (AFM) method. Figure 2c shows the AFM image of the device highlighting the graphene–WSe2 contact region, and the height profile along the red dashed line is presented in Fig. 2d, from which the graphene and WSe2 thicknesses are extracted as 41.9 nm and 54.2 nm, respectively. Considering the high absorption coefficient of the 2D material, the relatively thick WSe2 flake could ensure a nearly total light absorption for optimal photodetector development.

Fig. 2: Device fabrication and basic electrical characteristics.
figure 2

a Optical picture of one fabricated graphene–WSe2–Au structure by the dry-transfer process. The red dashed lines indicate the boundary of the van der Waals contacts. Scale bar 30 μm. b Raman spectrum obtained on the multilayer graphene–WSe2 contact region. For the lower Raman shift range, the laser power is increased to five times to identify the WSe2 characteristics. The excitation wavelength is 532 nm. c Atomic force microscope image of the fabricated graphene–WSe2–Au structure. d Height profile of the graphene–WSe2 heterostructure along the red dashed line in c. e Transfer curves and f output curves of the fabricated graphene–WSe2–Au structure.

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Figure 2e shows the transfer curves of the fabricated device. The gate voltage is applied to the silicon back gate and the thermally grown 100 nm SiO2 is used as the gate dielectric. The graphene–WSe2–Au structure exhibits ambipolar transport properties. Note that the Vds bias voltage is applied to the graphene electrode, and the Au electrode is set as the ground. When a small Vds is applied (Vds = 1 V), the device shows only hole transport, and electron transport is suppressed due to the large Schottky barrier between the Au and WSe2. This feature is further identified by the output curves shown in Fig. 2f, which shows clearly non-linear asymmetrical Ids–Vds characteristics. The output curves imply that a large contact resistance is present due to the reverse-biased MS junction. In terms of normal FETs or photodetectors based on the photoconductive mechanism, a low-resistance Ohmic contact is desired. While for MSM photodetectors, a large Schottky barrier and thus a high-resistance contact is desirable to achieve a low dark current. This advantage will be elaborated more in the following sections when we address the photoresponse of the graphene–WSe2–Au photodetector.

Self-driven photoresponse characteristics

Figure 3a shows the current–voltage (I–V) curves of the graphene–WSe2–Au MSM photodetector under the illumination of a 650 nm laser diode. During the photoresponse measurement, the photodetector is put on a probe station, and the laser light is illuminated on the whole device through a microscope mounted on top of the probe station. The light spot size is about 3 mm in diameter, and the full power of the 650 nm laser illuminated on the substrate is about 6.08 mW, which gives an equivalent power density of 0.86 mW mm−2. The total light power illuminated on the photodetector is about 507.1μm2 × 0.86 mW mm−2 = 436 nW. The I–V curves show the clear photovoltaic effect of the photodetector. The measured photocurrent is in the range of −8.51 to 7.68 μA, which is at least four orders magnitude higher than the dark current shown in Fig. 2e in the measured Vds range of −1 to 1 V. The self-driven characteristic is clearly identified by the large Isc in Fig. 3a. The Isc is about 3.29 μA under the zero gate bias condition, which corresponding to a responsivity of 7.55 A W−1. Another feature of the self-driven characteristic is the open-circuit voltage Voc, which is −0.24 V for the present photodetector. Figure 3b shows the I–V curves measured under varying light power density, and the dependence of Isc and Voc on the power density is summarized in Fig. 3c. It is noteworthy that the Voc is almost a constant of ~ −0.24 V over three orders of light power density, and the Isc exhibits a linear relationship with the light power density.

Fig. 3: Self-driven photoresponse of the photodetector.
figure 3

a Current–voltage (I–V) characteristics of the graphene–WSe2–Au photodetector under different gate voltage bias. A 650-nm laser with the full power of 0.86 mW mm−2 is used as the light source. b I–V characteristics of the graphene–WSe2–Au photodetector under varying light power densities and a zero gate voltage bias. c Dependence of Isc and open-circuit voltage (Voc) on the light power. d Dependence of the measured responsivity of the graphene–WSe2–Au photodetector on the light power density. e Dynamic Isc of the graphene–WSe2–Au photodetector under a zero gate voltage bias. f Fall time and rise time of the photodetector extracted from the dynamic voltage exerted to the serial resistor.

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Figure 3d shows the responsivity calculated according to the equation32:

$$R = \frac{{I_{{\mathrm{ph}}}}}{{P_{{\mathrm{in}}}}} = \frac{{I - I_{{\mathrm{dark}}}}}{{P_{{\mathrm{in}}}}},$$
(5)

where Iph is the photocurrent and is obtained by subtracting the dark current Idark from the total current measured under light illumination, and Pin is the power density of the light. The responsivity obtained without any external bias, that is Vds = 0 and Vg = 0, is almost invariant with the light power density in a dynamic range of 60 dB. The absolute value of the responsivity is about 4.38 A W−1 when the light power is in the range of 0.1–50% of the full light power, and it increases to 7.55 A W−1 when the light power increases to 100% of the full power. Even without any external bias, the responsivity is much larger than many 2D semiconductor based photodetectors operated with an external bias voltage19,32. Although the responsivity is not the highest among 2D materials based photodetectors, the high linearity over almost 60 dB is highly desirable for practical applications. In comparison, a typical silicon photodiode exhibits a responsivity <1 A W−1 in the visible light range, and features a linear relationship with the light power within a dynamic range of 40–60 dB6. We note that the responsivity of the graphene–WSe2–Au photodetector can be further increased by applying an external voltage bias Vds, which changes the depletion width according to Eqs. (3) and (4) and causes an even larger asymmetry between the two MS junctions. Supplementary Figure 2 shows the I–V curves obtained with Vds ranging from −3 to 3 V, from which the highest responsivity of ~25 A W−1 can be obtained.

Figure 3e shows the dynamic Isc measured under the 650 nm light illumination. The gate voltage bias is also zero for dynamic measurement. When the light is off, the photodetector has a low dark current ~10−14 A. As the photocurrent is around 10−6 A, the photocurrent-to-dark current ratio, which is denoted as the Iph/Idark ratio in this paper, is larger than 108. Based on the acquired information about the self-driven photodetector, including the responsivity R (7.55 A W−1), dark current Idark (10−14 A), device area A (507.1 μm2), the detectivity D* of the photodetector under zero bias can be estimated according to the equation32:

$$D^ \ast = \frac{{A^{{\mathrm{1/2}}}R}}{{\left( {{\mathrm{2}}qI_{{\mathrm{dark}}}} \right)^{1/2}}},$$
(6)

which is about 3.0 × 1012 Jones and among the highest reported value for 2D material-based photodetectors32. By connecting the MSM photodetector to an external resistor, and measuring the dynamic voltage of the resistor with an oscilloscope, a rise time and fall time of 332 and 554 μs are extracted from the dynamic voltage signal waveform, respectively (Fig. 3f). The response time of the present device is still longer than some reported values, such as 15 μs for the CNT-MoS2 heterojunction33, and 30 μs for the Au–MoS2–Au photodetector34, which could be attributed to the larger channel length (~30 μm) than these devices (3.5 μm in ref. 34). The response time could be further improved by optimizing the geometry of the devices (a shorter channel length), or eventually by adopting a vertical heterojunction that eliminates the lateral channel.

Scanning photocurrent measurement (SPCM) is widely applied to identify the sources of the photocurrent34,35,36. We have further performed SPCM on the same device. Supplementary Figure 3 shows SPCM image measured under a zero bias. The location of the highest photocurrent is at the Au–WSe2 junction, which is due to the large Schottky barrier height between the high work function Au and the ambipolar WSe2. As the depletion region extends to the WSe2 channel area, we could also observe the photocurrent in the WSe2 layer, although the magnitude is relatively smaller than that at the junction. The assumption that the Au–WSe2 junction and WSe2–Gr junction has a different barrier height can be verified from the distinct photocurrent observed at the two junction regions.

Gate-tunable characteristics of the self-driven photodetector

For a traditional MSM photodetector based on bulk semiconductor materials, a thick substrate is required to obtain a high absorbance of the incoming light, and the device is a two-terminal device. In the present study, as the 2D material has a much higher absorption, a thin film is enough to absorb the incoming light, which makes it possible to add the third terminal to control the Fermi level of the 2D semiconductor, as well as the Schottky barrier of the MS junctions. This effect is experimentally verified and reported in Fig. 4. It is noteworthy that the absolute value of Voc increases sharply from 0.02 to 0.25 V when the gate bias increases from −20 to 0 V, as shown in Fig. 4a. Referring to Fig. 2e, we note that the polarity of the carriers changes from the p-type to n-type when the gate bias increases from −20 to 0 V. For a p-type WSe2 whose Fermi level is closer to the Au electrode, a small Au–WSe2 barrier height is expected because of the high work function of Au. For the graphene–WSe2 MS junction, the Fermi levels of both WSe2 and graphene could be tuned by the applied gate bias; thus, the originally small barrier height does not change too much as the other MS junction. As a result, the difference of the barrier height of the two MS junctions is smaller for a negative gate voltage bias voltage. When the gate bias voltage is larger than −5 V, n-type transport is dominant and the Au–WSe2 exhibits a larger barrier height, which results in a larger asymmetry between the two MS junctions.

Fig. 4: Gate-tunable photoresponse.
figure 4

Dependence of the Voc and Isc on the gate voltage bias under 650 nm light illumination with a power density of a 0.86 μW mm−2, b 8.6 μW mm−2, c 0.086 mW mm−2, and d 0.86 mW mm−2.

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Figure 4c, d shows that as the light power increases, the Voc also increases for the negative gate bias region. For the largest power density of 0.86 mW mm−2 used in this study, the Voc is almost independent of the applied gate voltage bias, which indicates that the photogenerated carrier concentration dominates over the gate bias induced carrier concentration. In that case, the Fermi level, or more precisely the quasi-Fermi level of WSe2, is mainly controlled by photogenerated carriers, and the Schottky barriers difference and Voc is determined by the work function difference between graphene and Au electrodes. In accordance with the Voc, the Isc exhibits the same gate tunable and light power dependence characteristics.

We have also measured the transfer curves under illumination (650 nm, 100% intensity). Several phenomena could be observed from this measurement (Supplementary Fig. 4). Firstly, the photocurrent shows a constantly large value compared to the dark current. Secondly, for a relatively small Vds (1–3 V), the photocurrent is almost a constant over the strong p-type (−40 to −20 V) and n-type (10–40 V) regions, suggesting the electric field is strong enough to separate the photogenerated carriers. Thirdly, in the transition region from the p-type to the n-type, a local minimum and maximum photocurrent is observed, which is due to the rapid change of the Fermi level of the WSe2. Finally, we note that with a larger Vds and Vg, the increasing photocurrent is probably due to the tunneling current assisted by the strong electric field in the heterojunction.

Broadband photoresponse of the self-driven photodetector

The broadband photoresponse of the self-driven photodetector is investigated in the wavelength range from 405 to 780 nm. The same self-driven characteristics, as indicated by the Isc and Voc, are observed for all the light wavelengths, as shown in Fig. 5. For the 405 nm light which has the full power of 3.7 μW mm−2, an Isc of 38.65 nA is recorded and the corresponding responsivity of 20.6 A W−1 is obtained (Fig. 5a). Gate-tunable Voc and Isc follow the same trend as reported for the 650 nm light illumination (Supplementary Fig. 5) with a similar power density. The largest Voc of 0.295 V is obtained with a laser wavelength of 780 nm and a Pin of 1.9 μW mm−2 (Supplementary Fig. 6), while the highest zero-bias responsivity of 16.54 A W−1 is obtained under a Pin of 0.19 mW mm−2, as shown in Fig. 5c, d summarizes the dependence of Voc on the gate voltage bias and the power density of a 780 nm laser. In general, the Voc shows the same gate-tunable effect as that analyzed for the photodetector under 650 nm light illumination.

Fig. 5: Broadband photoresponse of the self-driven photodetector.
figure 5

I–V curves of the self-driven graphene–WSe2–Au photodetector under varying light wavelengths of a 405 nm, b 532 nm, and c 780 nm. d Dependence of the Voc on the gate voltage bias and light power density under 780 nm light illumination.

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Modeling and analysis of the self-driven photodetector

To get a deep understanding of the self-driven property of the graphene–WSe2–Au photodetector, we have performed numerical simulation of the photodetector based on the finite element analysis method. A 2D structure is constructed with both graphene–WSe2 and Au–WSe2 MS junction modeled as an ideal Schottky contact (Supplementary Fig. 7). From previous experimental results, we note that the highest Voc is obtained when the gate voltage is between −10 and 0 V, and the corresponding electric field-induced carrier concentration is below 2.16 × 1017 cm−3. Figure 6a shows the simulated I–V curves of three different MSM photodetectors under 650 nm light illumination. The only difference of the MSM photodetectors is the electrode configurations as shown in the legend in Fig. 6a. The work functions of the graphene and Au electrodes are chosen as 4.6 and 4.9 eV, respectively. While the three devices show almost the same saturated photocurrent at a large voltage bias, only the asymmetrical Gr–WSe2–Au structure exhibits the photovoltaic effect. The dark current of the three photodetectors, which is at least three orders of magnitude that the photocurrent (Supplementary Fig. 8), can be neglected when analyzing its contribution to the total current of the photodetector under light illumination with a large power density. As a comparison, we have also fabricated symmetrical Au–WSe2–Au photodetectors. The I–V curvers under illumination also shows symmetrical characteristics without any photovoltaic effect (Supplementary Fig. 9). As shown in Fig. 6b, the simulated Voc increases with increasing light power, and the largest absolute value of Voc is 0.3 V which is exactly the same as the barrier height difference between the two MS junctions. One origin of the relatively smaller Voc under a small Pin is the dark current contribution from the reversed-biased Gr–WSe2 junction, which is comparable to the photocurrent and effectively decreases the voltage bias needed to compensate the photocurrent difference from the two MS junctions. Figure 6c summarizes the dependence of Voc and Isc on the light power density. The high linearity of the Isc with light power can be obtained over at least five orders of magnitude of the light power, which is very beneficial for future practical applications.

Fig. 6: Theoretical analysis and performance comparison of the self-driven photodetector.
figure 6

a Simulated I–V curves of three MSM photodetectors with different electrode configurations. b Simulated I–V curves of the Gr–WSe2–Au photodetector under various light power densities. c Dependence of Voc and Isc on the light power density. d Comparison of the performances of self-driven photodetectors based on different materials and working mechanisms.

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Up to now, there are mainly three types of self-driven photodetectors based on 2D materials, including PN junctions built on the same material, PN junction by stacking two different semiconducting materials, and MSM photodetectors with asymmetrical device structures. In Fig. 6d, we compare the key parameters including responsivity and Iph/Idark of various self-driven photodetectors. These data points in Fig. 6d are extracted from recently reported self-driven photodetectors based on different materials and working mechanisms19,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53, and the detailed performance metrics are listed in Supplementary Table 1. In general, our device shows a relatively high responsivity, and the highest reported Iph/Idark ratio under a zero bias, which origins from the proposed asymmetrical device structure featuring both asymmetrical MS junctions for a high zero-bias photocurrent and an ultralow dark current.

In summary, an asymmetrical Gr–WSe2–Au MSM device structure is proposed to realize the self-driven photodetector. With both of the constituent MS junctions formed by the dry-transfer method, van der Waals contacts are obtained between WSe2 and the electrode materials, thus preventing the notorious Fermi-level pinning effect in 2D materials based photodetectors. As a result, a high responsivity of 7.55 A W−1 is obtained under zero external voltage bias. Combining the low dark current of ~10−14 A, a high detectivity of 3 × 1012 Jones and a high Iph/Idark of 108 is obtained for the present device. Furthermore, a numerical simulation is performed to explain the device physics of the proposed device structure, including the self-driven property and its dependence on the light power density. The photodetector also exhibits high linearity with the light power density, which is very important for its implementation with processing circuits for practical low-power applications.

Methods

Device fabrication

For device fabrication, a silicon wafer with a 100-nm-thick thermally grown SiO2 was chosen as the starting substrate. We prepared the Au electrodes on top of the SiO2/Si substrate by following the standard photolithography, Au/Ti (50 nm/10 nm) evaporation, and lift-off process flow. The WSe2 flake with proper size is transferred to the PDMS stamp, which is then attached to a glass slide and aligned to the pattered Au electrode. For the first transfer process, a part of the chosen WSe2 flake is coated on the Au electrode, while a large part of the WSe2 flake is transferred onto the SiO2/Si substrate. For the second transfer process, the graphene flake is aligned on top of the WSe2 flake which is attached to the SiO2/Si substrate. Part of the graphene flake is also contacted with another Au electrode for following electrical characterization.

Electrical and photoresponse characterizations

All the electrical and photoresponse measurements were conducted on a probe station connected to a semiconductor parameter analyzer Agilent 4156c. The laser is mounted on the microscope and the laser light is illuminated on the entire device through the lens of the microscope. The dynamic I–V measurements were conducted using the same equipment by using the time sampling mode of 4156C. While the dynamic I–V measurements could provide a high precision in measuring the ultralow dark current, the sampling speed is too low to extract the real response speed of the photodetector. By connecting the photodetector to an external resistor (20 MΩ), and measuring the dynamic voltage of the resistor by an oscilloscope (Wavesurfer 510 1 GHz Oscilloscope), both the rise time and the fall time can be recorded with a higher speed. The photodetector is illuminated with a red-light laser diode powered by an external signal generator (Rigol DG4102). No external voltage is applied to the photodetector during the dynamic I–V and voltage measurements.