PVA-assisted metal transfer for vertical WSe₂ photodiode with asymmetric van der Waals contacts

1 Introduction

Recently, vertical electronic and optoelectronic devices based on 2D materials have received considerable attention, in which the source electrode, semiconductor channel material, and drain electrode are vertically stacked together [1–6], and the transport direction of charge carriers is parallel to the direction of electric field generated by the gate electrode. In this case, the carriers are collected vertically rather than laterally, and hence the channel length is determined by the thickness of the semiconductor, which can be easily down-scaled to sub-10 nm without involving sophisticated lithography process. Therefore, the vertical configuration is particularly suitable for the fabrication of ultra-short-channel FETs. Compared with the laterally contacted 2D devices, the vertical geometry can offer great advantages, such as higher current density, faster switch speed, and superior short-channel control under a low operating voltage [4, 7]. In addition, in view of the unique charge transport direction, the vertical geometry also affords exceptional mechanical flexibility [2, 3, 8]. Diverse vertical devices have been reported based on graphene and other 2D semiconductors, including light-emitting diodes [9, 10], Schottky diodes [11, 12], tunneling transistors [1, 13], [14], [15], and photovoltaic cells [8, 16, 17], demonstrating great potential of vertical design in the development of future electronic and optoelectronic devices.

For the vertical geometry devices, due to the source and drain electrode fully cover the whole channel, the role of metal contacts becomes very important, and the properties of metal–semiconductor interfaces can even dominate the device performance, including maximum current, on/off ratio, carrier mobility, polarity, and so on [18–24]. Therefore, high-quality metal–semiconductor contact is crucial to the performance of vertical geometry devices. However, the atomically thin structure of 2D materials leads to their crystal lattice to be easily damaged during metal deposition processes, such as electron-beam (or thermal) evaporation and sputtering, which produce considerable defects and surface states at the metal–semiconductor interface, leading to strong Fermi-level pinning effect and/or the formation of the leakage current paths underneath the metal contact region [19–25]. With decreasing channel length, the leakage current gradually increases and ultimately leads to the device failure. Therefore, the fabrication of vertical devices with short-channel length using 2D semiconductors is still a serious challenge.

In this work, we report a simple and efficient metal transfer technique that used to fabricate high-performance vertical WSe₂ devices. Predeposited metal electrodes are peeled off with PDMS/PVA stamp and then transferred on top of WSe₂, so that a pristine channel and a clean metal–semiconductor interface can be obtained, which enabled the tunability of p-type and n-type behaviors of WSe₂ transistors by using metals with different work functions. With this method, we fabricate vertical WSe₂ FETs with a 18-nm-thick channel and a vertical WSe₂ photodiode with graphene and Pt as asymmetric vdW contacts. The WSe₂ photodiode exhibit a high rectification ratio (>10⁵) and excellent self-powered photodetection performance, demonstrating an photo-to-dark current ratio up to 1.5 × 10⁵, photoresponsivity of 0.28 A W⁻¹, and detectivity of 1.35 × 10¹⁰ Jones, together with fast response speed of 24 μs, which are proved to be one of the best performance for vertical photodiodes. This work validates the importance of metal–semiconductor contact and provides a new solution for the construction of high-performance short-channel vertical photodiode.

2 Experimental section

2.1 The preparation, release, and transfer process of patterned metal electrodes

A series of 50-nm-thick metal electrodes with different work functions, including Pt (ϕ_m ∼ 5.5 eV), Au (ϕ_m ∼ 5.1 eV), In (ϕ_m ∼ 4.12 eV), and Al (ϕ_m ∼ 4.06 eV) (ZhongNuo Advanced Material [Beijing] Technology Co., Ltd), were firstly deposited on a sacrificial SiO₂/Si chips following standard photolithography, thermal evaporation (sputtering for Pt), and lift-off procedures. The poly(vinyl alcohol) power (PVA, Sigma-Aldrich) with a molecular weight of 9000∼10,000 was dissolved into deionized water with a weight ratio of 10 %. The PVA aqueous solution was then dropped on a CD disk and baked at 70 °C for 1 h to form a thin film. Afterward, the PVA was cut into small pieces and brought into conformal contact with the PDMS stamp to form the transfer holder (Figure 1a(i)), which was then attached to a glass slide and clamped onto the arm of 3D transfer platform (Shanghai Onway technology Co., Ltd).

Figure 1:

The fabrication processes of WSe₂ transistors using PVA-assisted metal transfer technique. (a) Schematic illustration of the PVA-assisted metal transfer technique. (b) Photographs and optical micrograph (OM) images of metal electrode transfer process.

The release and transfer process for metal electrode is schematically illustrated in Figure 1a. A Si/SiO₂ chip containing metal electrodes was placed onto the stage of the transfer platform, and the desired metal electrode was centered in the field of view. By monitoring the process with a microscope, the PDMS/PVA stamp can be precisely aligned and contacted with the underlying target metal electrodes, as illustrated in Figure 1a(ii). After that, the stage was heated to 90 °C for 2 min to fully melt the PVA film, which could not only ensure PVA adheres firmly to the underlying metal electrodes but also weaken the adhesion between the PDMS and PVA film. Once the PVA layer has fully melted, the arm of the transfer platform is lifted up slowly, making the PDMS stamp peeled off from the PVA, which adhered to the target substrate (Figure 1a(iii)). After cooling down to room temperature, the PVA film was solidified and the PVA film carrying the metal electrodes can be picked up from the substrate with a tweezer as shown in Figure 1a(iv). The stronger interaction between PVA/metal electrodes compared to that between metal electrode/substrate ensures the reliable peel-off of the metal electrode. The obtained PVA/metal can be reattached to PDMS stamp to form a new stack of glass slide/PDMS/PVA/metal electrode.

The glass slide/PDMS/PVA/metal electrode was mounted on the transfer platform and lowered slowly until it contact with the target WSe₂ flake under an optical microscope (Figure 1a(v)). For the drop-down of metal electrode, the substrate is heated to 90 °C to make the PVA fully melted, giving rise to the strong contact between PVA and substrate together with the favored vdW adhesion of WSe₂/metal electrode, and then the glass slide is raised slowly with the PVA carrying metal electrodes left on the substrate (Figure 1a(vi)). Finally, the PVA layer was removed by being dissolved in deionized water, and the metal electrodes were successfully transferred to the target WSe₂ (Figure 1a(vii) and (viii)). The above pick-up and drop-off steps can also be used for constructing WSe₂/graphene heterojunction.

3 Results and discussion

The key to the successful transfer of metal electrode is the use of the bilayer film (PDMS/PVA) as a stamp. As reported in previous work, the PMMA [22, 26] and PPC [27] could also be used as the transfer mediator to transfer 2D materials; however, it usually cause polymer residuals and sample damage [28]. In addition, some volatile toxic solvents such as acetone and toluene are needed to remove the PMMA, which are harmful to human health. PVA is a small-molecule, water-soluble, and environment-friendly mediator, which can maintain the integrity and intrinsic properties of the samples after being removed. In addition, the PVA could enable selective transfer of the target sample, rather than the entire materials that prepared with CVD or mechanical exfoliation method.

The schematic illustration of the PVA-assisted metal transfer technique is shown in Figure 1a, and the corresponding photographs and OM images of the transfer process are shown in Figure 1b. With this method, contact electrodes can be laminated directly on the 2D materials without thermal damage. Therefore, a pristine channel and a clean vdW metal/semiconductor interface can be obtained. Due to PVA could be easily dissolved in hot water, the fabricated patterns of metal contacts can be well preserved, and there are no residues on the surface of the transferred metal electrodes and channel materials, as demonstrated in Figure 1b. Therefore, this method has the advantages of simplicity, efficient, and high accuracy. With this method, metal electrodes with different patterns can be easily transferred onto SiO₂/Si substrates (Figure S1).

In order to compare the optoelectronic performance between the devices with evaporated and transferred metal contacts, we fabricated two back-gated WSe₂ field-effect transistors (FETs) with the similar thickness. One with the transferred Au electrodes as source and drain contacts, and another with thermally evaporated Au contacts. The OM images of both devices are shown in the inset of Figure 2a–d, and their thickness was estimated to be 5.1 and 4.3 nm, respectively, according to the AFM images shown in Figure S2. Figure 2d presents the I_d–V_d curves of the WSe₂ FETs with transferred Au electrodes under dark and 405-nm laser irradiation. The nonlinear I_d–V_d curves indicate the existence of Schottky barrier at WSe₂/Au interface, which is consistent with previous experimental results and theoretical simulation [19, 29], [30], [31]. By contrast, for device with evaporated Au contacts, the chemical interaction between Au atoms and WSe₂ perturbs the electrical properties of WSe₂. As explained in previous report [19], the surface of WSe₂ was metalized under the Au electrode, which leads to an ohmic contact between Au and WSe₂. Therefore, a linear I_d–V_d curve was observed (Figure 2a). Compared with the device with transferred electrode, the one with evaporated Au has a higher photocurrent at the same laser intensity due to the lower contact resistance (Figure 2b–e). Furthermore, the photocurrent for the device with transferred Au electrode gradually increases with the increases of laser power owing to the enhancement of photoexcited carriers. While for the device with evaporated Au electrodes, the photocurrent saturates rapidly with increase of laser power owing to the trap filling.

Figure 2:

The I_d–V_d curves of the WSe₂ FETs with (a) evaporated and (d) transferred Au electrodes measured under dark and 405 nm laser irradiation with different intensities. (b) and (e) Are the corresponding time-resolved photoresponse of both devices at V_d = 1 V. (c) and (f) Are the rise and decay time of both devices acquired under pulsed illumination at V_d = 1 V.

Figure 2b–e shows the transient photoresponses of both devices under consecutive on/off light modulation under 405 nm laser illumination with V_d = 1 V. It is evident that the photocurrent of both devices increases with laser intensity, and they can be switched between on and off states with excellent reproducibility and stability. Benefiting from the ohmic contacts, the photodetector with evaporated Au contact has a higher photocurrent of about 4.2 nA at 21.7 mW cm⁻² and V_d = 1 V. However, the relative large dark current and low I_light/I_dark ratio (8.7) makes it unattractive for practical applications. Fortunately, the I_light/I_dark ratio (642) was greatly improved by the introduction of Schottky contacts through transferring metal electrode, and the dark current was reduced from 475 pA for evaporated electrode to 1.8 pA for transferred contacts at V_d = 1 V. The response time τ, which is defined as the time for the photocurrent to rise/fall from 10/90 % to 90/10 % of the peak value, is affected by many factors, including the metal–semiconductor contact, trap, and defect states in the semiconductors. As shown in the transient photoresponse curve in Figure 2c and f, the photodetector with evaporated Au contact exhibits the rise/decay time of 41/925 ms. While for the vdW contacted device, the response time is significantly reduced to 26.2/2.4 ms. The reduced response time can be attributed to the Schottky contact that resulted in built-in fields at the Au/WSe₂ interface, which are favorable for accelerating the separation and transport of photogenerated carriers, thus shortening the response times.

In order to investigate the influence of the work function of the contact metal, different WSe₂ FETs were fabricated with Pt (ϕ_m ∼ 5.5 eV), Au (ϕ_m ∼ 5.1 eV), In (ϕ_m ∼ 4.1 eV), and Al (ϕ_m ∼ 4.05 eV) [18, 19, 31], [32], [33], [34] contacts using the same transfer method. Figure S3 shows the transfer curves of these back-gated WSe₂ FETs. It was found that the charge transport properties of WSe₂ FETs can be tailored by changing the work function of the transferred contact metals. It was changed from p-type (Pt) to p-type dominant ambipolar (Au) and finally to n-type dominant ambipolar (In and Al), as the work function of the metals decreasing. This result is accordance with previous reports and can be explained by the band diagram of metal–semiconductor [18, 19, 31], [32], [33], [34]. Pristine multilayer WSe₂ is an ambipolar semiconductor with Fermi level (E_F) close to the middle of the band gap, which can be tuned either toward conduction or valence band by contacting with different metals. According to the Schottky–Mott rule, n-type/p-type charge transport behavior would be dominant in WSe₂ FETs contacting metals with low/high work function due to the low electron/hole Schottky barrier height. In comparison, previous reports have demonstrated that the WSe₂ FETs with evaporated metals exhibited similar n-type unipolar characteristics regardless of work function owing to the strong Fermi-level pinning (FLP) [18, 32]. In a word, the PVA-assisted metal transfer method could overcome the FLP effect and achieve a clean and vdW WSe₂–metal interface.

To demonstrate the significance of the metal/semiconductor contact properties to the performance of the vertical devices, vertical WSe₂ FETs were fabricated with Au as metal contacts. The bottom Au electrode was prepared with conventional thermal evaporation technique, while the top Au contact was transferred on top of the WSe₂ flake. For comparison, another Au electrode is also deposited on the same WSe₂ flake using thermal evaporation method (Figure S4a and b). As shown in Figure S4d, the WSe₂ FET contacted with evaporated Au electrodes displays a linear I_d−V_d curve, with the dark current as high as 0.86 μA at V_d = 1 V, while the photocurrent under 405 nm illumination (25.7 mW cm⁻²) is too weak to be distinguished. Therefore, there is no photoresponse was observed under a series of periodical light stimulation via 405 nm laser (Figure S4f). For the vdW contacted vertical WSe₂ device, the nonlinear I_d–V_d curves confirm the existence of Schottky barrier at WSe₂ and transferred Au interface, which leads to the dramatically reduced dark current of 1.53 nA under 1 V bias and enhanced photoresponse, as shown in Figure S4b. The photocurrent increases with laser intensity and the on/off ratio reaches 308 under 405 nm laser illumination (Figure S4e). This result confirms that the transferred contacts could minimize the damage of metal electrode to the 2D channel materials and reduce the dark current of vertical devices.

Vertical short-channel WSe₂ FETs was fabricated to demonstrate the potential of the PVA-assisted metal transfer method. The OM image of the device was shown in the inset of Figure 3a, and its thickness measured with AFM is 18.1 nm (Figure 3b and c). The I–V curves in logarithmic scale under dark condition and 405 nm laser irradiation are shown in Figure 3d. When the WSe₂ device is biased with a low voltage (V_d < 0.65 V), the dark current remains at a low level (<1 nA) due to the back-to-back Schottky barrier. Hence, the illumination of 405 nm laser generate an obvious photocurrent, thereby achieving an on/off current ratio of nearly 10 under a light intensity of 14.9 mW cm⁻² at V_d = 0.5 V, as shown in Figure 3e. When the reverse bias voltage reaches a certain value (0.7 V in Figure 3d), the dark current increases rapidly and almost coincides with photocurrent, indicating the breakdown of the Schottky junction.

Figure 3:

The (a) OM and (b) AFM image of the vertical short-channel WSe₂ FETs. (c) The height profile along the white line in panel b indicates the thickness of WSe₂ flake was ∼18.1 nm. (d) I_d–V_d curves of the vertical WSe₂ FETs under dark and 405 nm laser irradiation with different intensities. (e) The photoswitching characteristics of the WSe₂ device under 405 nm laser irradiation at V_d = 0.5 V. (f) The transfer curve of the vertical WSe₂ FETs under dark condition at V_d = 1 V.

To evaluate the gate modulation ability of the vertical WSe₂ device, the transfer characteristic was measured. Figure 3f shows the double-sweep transfer curve of the vertical short-channel WSe₂ device at V_d = 1 V. Although bottom Au electrode may partially shields the vertical electric field effect to the upper WSe₂ channel, the transfer curve measurement revealed that the device still retain gate coupling effect with on/off ratio 5.6 × 10², which may be caused by the large area of the WSe₂ flakes that some part of the WSe₂ is in direct contact with the SiO₂/Si substrate. In addition, the vertical WSe₂ transistor has a hysteresis window of 60 V under a gate voltage of ±60 V, which indicates the potential of the device for memory applications, and previous studies have demonstrated that the water and oxygen molecules trapped between SiO₂/Si substrate and WSe₂ channel plays the dominant role in the hysteresis of the transfer curves [35, 36].

In order to further improve the photoresponse performance of the vertical WSe₂ device, we fabricated a vertical Pt/WSe₂/graphene/Au photodiode. As shown in Figure 4a, the device is constructed by vertically sandwiching multilayer WSe₂ between the top graphene and the bottom Pt electrode. Figure 4b displays the OM of the vertical heterostructure device. The effective area of WSe₂ between the graphene and Pt is calculated to be about 3159 μm². The Raman spectrum of the vertical WSe₂/graphene heterojunction exhibits five main peaks (Figure 4c). The peaks located at 245.87, 254.89, and 305.25 cm⁻¹ correspond to the E_2g¹, A_1g, and B_2g¹ modes of multilayer WSe₂, respectively [12]. While the peaks appeared at 1579.26 and 2714.81 cm⁻¹ can be ascribed to the G and 2D phonon modes of graphene, respectively [37]. There is no shift for the characteristic Raman peaks of both WSe₂ and graphene in the heterojunction compared to the spectrum of individual crystal, suggesting good quality of each component after transfer and device fabrication process. The AFM image of the heterojunction device is shown in Figure S5, and the thickness of WSe₂ and graphene are 58.9 and 16.7 nm, respectively.

Figure 4:

Device structure and characterization of vertical WSe₂ photodiode. (a) Schematic and (b) OM image of the vertical Pt/WSe₂/graphene/Au FETs. (c) Raman spectra of individual WSe₂, graphene, and the WSe₂/graphene heterojunction with 532 nm laser excitation. (d) I_d–V_d curve of the vertical Pt/WSe₂/graphene/Au in logarithmic and liner scale under dark condition. (e) Energy band diagram of the vertically stacked Pt/WSe₂/graphene heterostructures.

Figure 4d shows the I_d–V_d curve of the vertical device at a bias voltage range of V_d = ±1 V measured in dark. The bottom Pt electrode is defined as drain electrode while the Au on graphene as source electrode. The vertical device exhibits strong rectification behavior with a large forward current over 7.1 μA at V_d = 1 V and a low reverse current below 18.4 pA at V_d = −1 V, and thus a very high current rectification ratio of 3.9 × 10⁵ at |V_d| = 1 V was obtained, which is superior to most vdWs 2D heterostructures reported in recent literatures [11, 12, 38, 39]. The I–V curve can be fitted with the Shockley diode equation: I = I_s(e^qV/nKT−1), where I_s is the reverse saturation current, n is the ideality factor, q is the elementary charge, K is the Boltzmann constant, and T is the absolute temperature. The ideality factor that determined from the slope of linear region of the I–V curve in Figure 4d is 1.52. This value is slightly higher than ideal value (n = 1), but smaller than many reported Schottky photodetectors, indicating good junction interface. The energy band diagram of the Pt/WSe₂/graphene heterojunction is illustrated in Figure 4e. According to previous reports, the bandgap and electron affinity of multilayer WSe₂ are approximately 1.3 and 4.2 eV [12, 34, 40], respectively, and the work function of graphene and Pt are 4.6 and 5.5 eV, respectively [34, 40]. The work function of the bottom Pt electrode matches well with the valence band edge of WSe₂, which provides an ohmic contact to WSe₂ (Figure S6). Graphene, on the other hand, forms a vdW Schottky barrier after contacting WSe₂, as demonstrated by previous experimental results and theoretical simulation [8, 40], [41], [42]. The difference in work function of Pt and graphene leads to a built-in potential in the device, which generates an excellent rectification ratio.

Subsequently, the optoelectronic properties of the vertical Pt/WSe₂/graphene/Au photodiode were investigated to examine its potential as a photodetector. The I_d–V_d curves on a logarithmic scale under dark and 405 nm laser illumination were shown in Figure 5a, with the corresponding linear-scale curves plotted in Figure S7. Apparently, the I_d–V_d curves are upshifted under laser illumination, exhibiting a distinct photovoltaic response, which demonstrates the device has a self-powered detection feature. When the device is illuminated with 405 nm laser, electron–hole pairs are excited in both graphene and WSe₂. The V_OC and I_SC were extracted and plotted as a function of the power density in Figure 5b, and the maximum V_OC of 0.37 V and J_SC of 331 nA were generated under a light intensity of 30.6 mW cm⁻². Therefore, the vertical Pt/WSe₂/graphene/Au device shows remarkable photovoltaic behavior under the 405 nm illumination. In addition, the I_SC followed a logarithmic dependence on the light intensity, which can be fitted by the equation: I_SC ∝P ^θ , where θ is a power law exponent that related to the recombination processes of the photogenerated carriers [43], and P is the light intensity. By fitting the experimental data, θ is deduced to be 1.28. For the ideal case, the value of θ is equal to 1, which implies all the photoexcited electron–hole pairs contributed to the photocurrent without any trapping and recombination [44]. Practically, the value of θ is usually lower than 1, and I_ph usually exhibits a sublinear dependence on light intensity due to some unwanted recombination. However, in this work, an unusual superlinear light intensity dependence of photocurrent was observed (θ > 1), and it features increased photoresponsivity with power density. Similar superlinear behavior has been recently observed in a series of 2D semiconductors and vdW heterojunctions, such as graphene [45], Ta₂NiSe₅ [46], graphene/h-BN [47], and WS₂/MoS₂ heterojunctions [48]. The typical origin of superlinearity can be attributed to the photothermionic (PTI) emission. The photon energy absorbed by graphene could generate a thermalized hot carrier distribution, and those carriers with an energy larger than the Schottky barrier at the WSe₂/graphene interface can be injected into WSe₂ [49], which could significantly extend the light response range and enhance the photoresponsivity.

Figure 5:

The optoelectronic properties of vertical WSe₂ photodiode. (a) I_d–V_d curves of the vertical Pt/WSe₂/graphene/Au device under dark and 405 nm laser illumination with different intensities. (b) Power density–dependent short-circuit current (I_SC) and open-circuit voltage (V_OC). (c) The V_d-dependent output electrical power (P_el) under 405 nm light illumination. (d) The photoswitching characteristics of the device at zero bias under 405 nm laser illumination. (e) Power-dependent responsivity of the device at zero bias. (f) The I_d–V_d curves of the vertical Pt/WSe₂/graphene/Au device under dark and 808 nm laser illumination with different intensities.

The output electrical power P_el, defined as P_el = I_d × V_d, was plotted in Figure 5c. The maximum P_el could reach 44.7 nW at a bias voltage of 0.21 V when light intensity is 30.6 mW cm⁻², corresponding to a power conversion efficiency of 4.6 %, which is comparable to or higher than that of most vdW heterojunction solar cells [8, 16, 17]. To investigate the photoswitching behavior of the vertical Pt/WSe₂/graphene/Au device under light illumination, Figure 5d exhibits the multicycle I–t curves of the device under 405 nm laser illuminations with various intensities at zero bias. It is evident that the device could be switched between on and off states with excellent reproducibility and stability. In particular, the drain current increases rapidly from about 2.3 pA at dark condition to 0.33 μA, thereby achieving an ultrahigh on/off current ratio of ∼1.4 × 10⁵ under a light intensity of 30.6 mW cm⁻², which signifies outstanding photoresponse of the device to the visible light.

To further evaluate the optoelectronic performance of the vertical Pt/WSe₂/graphene/Au device as a photodetector, the responsivity (R) is calculated by the following equation:

where I_light, I_dark, P, and S are the photocurrent, dark current, incident power intensity, and effective area of the device, respectively.

Based on the above formulas, the calculated R of the Pt/WSe₂/graphene/Au device under various power densities at zero bias is shown in Figure 5e. A decent R of 0.28 A W⁻¹ was achieved at a power density of 30.6 mW cm⁻¹. Simultaneously, the responsivity increases slowly with increasing incident power, which further confirms the superlinear behavior. In addition to responsivity, the specific detectivity (D*) is another significant parameter that reflects the ability of a photodetector to detect weak signal, which can be calculated by the equation:

where NEP is the noise equivalent power, S_device is the device area, and S_id is the noise power spectral density of the detector. To accurately characterize D*, the noise power spectral density is measured in dark condition and shown in Figure S8, which displays a typical 1/f power density in the low frequency range. At small measurement bandwidth of Δf = 1, the D* is calculated to be 1.35 × 10¹⁰ Jones.

Next, the photoresponse performance of this device upon excitation by a 532 and 808 nm laser was investigated. Figures S9 and 5f exhibit the corresponding dark and photo-induced I–V curves of the WSe₂ photodiode on a logarithmic scale. Because WSe₂ multilayers have a bandgap of 1.3 eV, the WSe₂ photodiode can respond to NIR light even though the response is weaker than that to visible light. The device displays an obvious photovoltaic behavior with a maximum V_OC of 0.25(0.3) V and I_SC of 18(71) nA to 808(532) nm laser. The time-resolved photoresponse at zero bias under different light intensities is shown in Figures S10 and S11. The device shows excellent photocurrent switching behaviors and negligible changes under periodic 532 and 808 nm laser illuminations, and the current on/off ratio decreases from 2.7 × 10³ for 532 nm laser to 4.0 × 10² for 808 nm laser, demonstrating the device has a self-powered photodetection feature from visible to NIR light.

The photoresponse speed of the device to 532 nm laser at zero bias voltage was recorded with an oscilloscope. The rise/decay time defined as the time taken for photocurrent increasing/decreasing from 10/90 % to 90/10 % of the maximum value was calculated to be 25.8/24.2 µs (Figure 6a), indicating the fast separation of the photogenerated electron–hole pairs under the internal electric field. For practical applications, the long-term working stability is another key parameter for a high-performance photodetector. The vertical Pt/WSe₂/graphene/Au device exhibits excellent long-term stability by storing it in an airtight container. The photocurrent exhibits no noticeable deviation and the on/off ratio decrease slightly after about 2 weeks (Figure 6b). To study the photosensitive region of the device, scanning photocurrent measurement was carried out with a confocal optical microscope. The focused 532 nm laser spot was scanned over the sample, and the resultant photocurrent at zero bias was recorded as a function of laser spot position. The photocurrent map presented in Figure 6d indicates that the photocurrent is mainly generated in the region where the three main layers (Pt/WSe₂/graphene) are superimposed, which could be attributed to the efficient separation and transportation of photogenerated electron–hole pairs by built-in electric field. There is no current generated in the area covered by the Au electrode as it blocks the entrance of incident light into the WSe₂ layer. No distinct photocurrent generation away from the Pt/WSe₂/graphene overlapping region, which may be due to the photocarriers generated in the WSe₂/graphene far away from the Pt electrode cannot be effectively collected.

Figure 6:

The temporal response, stability, and photocurrent mapping of vertical WSe₂ photodiode. (a) Rise and decay time of the Pt/WSe₂/graphene/Au device measured at V_d = 0 V under the illumination of 520 nm laser. (b) The photoresponse of the device was monitored before and after stored in an airtight container for 2 weeks. (c) The OM image of the vertical Pt/WSe₂/graphene/Au device. The top Au and bottom Pt electrode are outlined by blue and black dotted lines, respectively. (d) Scanning photocurrent mapping at zero bias with an illumination wavelength of 520 nm.

4 Conclusions

In summary, we reported an effective PVA-assisted metal transfer technique that enables predeposited metal electrode to be reliably peeled off and transferred onto target 2D materials. Compared to traditional evaporated contacts, this transfer printing method could reduce Fermi-level pinning and enables the tunability of p-type and n-type WSe₂ transistors by using metals with different work functions. With this technique, a vertical short-channel WSe₂ FETs and a vertical WSe₂ photodiode with asymmetric graphene and Pt vdW contacts were fabricated. The WSe₂ photodiode exhibited superior current rectification exceeding 10⁵ and excellent self-powered photoresponse characteristics, including an photo-to-dark current ratio up to 1.5 × 10⁵, photoresponsivity of 0.28 A W⁻¹, and detectivity of 1.35 × 10¹⁰ Jones, together with fast response speed of 24 μs, which are among the highest values ever reported for vertical photodiodes. This work validates the importance of metal–semiconductor contact and provides a new solution for the construction of high-performance short-channel vertical photodiode.

Supporting Information

The Au electrodes with different patterns that before and after transfer with PDMS/PVA stamp; AFM analysis of the WSe₂ FETs with evaporated and transferred Au electrodes; the transfer curves of WSe₂ FETs with different metal contacts; the optoelectronic properties of the vertical WSe₂ FETs prepared with evaporated and transferred metal methods on the same WSe₂ flakes; AFM analysis of the Pt/WSe₂/graphene/Au FETs; I–V curves of the two terminal Pt–WSe₂–Pt device under dark and 405 nm laser illumination; I_d–V_d curves of the vertical Pt/WSe₂/graphene/Au device under dark and 405 nm laser illumination; I_d–V_d curves of the vertical Pt/WSe₂/graphene/Au device under dark and 532 nm laser illumination; the photoswitching characteristics of the device at zero bias under 532 and 808 nm laser irradiation.