Vertical 1T’‐WTe2/WS2 Schottky‐Barrier Phototransistor with Polarity‐Switching Behavior

In recent years, 2D reconfigurable phototransistors (RPTs) have been applied in broadband convolutional processing, retinomorphic hardware devices, and non‐volatile memorizers. However, there has been a lack of investigation into all‐2D Schottky junctions used in RPT with polarity control behavior. Herein, a vertically stacked multilayered WS2/WTe2 Schottky RPT is reported. The semimetal characteristics of 1T’‐WTe2 is designed to form a built‐in electric field of 69 meV across the heterojunction and WS2 exhibits gate‐tunable characteristics. Therefore, reconfigurable rectifying behavior and self‐driven bidirectional photo response can be achieved. The phototransistor possesses a gate‐tunable rectification ratio ranging from 10−2 to 105, and the corresponding logic half‐wave rectifier shows excellent switchable rectifying states. Under 635 nm illumination, the responsivity can be adjusted from −1325 to 430 mA W−1 with reversed signs. Meanwhile, the maximum power conversion efficiency is 2.84%, and the specific detectivity is 1.47 × 1012 Jones. The device shows both negative and positive responsivity with linear gate dependence within a voltage window of 10 V. Impressively, nonvolatile photovoltaic performance can be demonstrated by reversing short‐circuit current and open‐circuit voltage by applying and releasing pulsed gate voltage. Finally, reconfigurable polarization behavior, single‐pixel imaging, and the optical logic circuit are applicable to the heterostructure.


Introduction
During the post-Moore era, high integration levels, low power consumption, and small feature sizes are three key factors for the future of very large-scale integrated circuits with scaling DOI: 10.1002/aelm.202300672 and enhanced functionality.2D layered materials can meet the aforementioned demand due to their atomic thickness and tunable electrical and optical properties. [1]Among them, 2D transition metal dichalcogenides (TMDs) composed of MX 2 (where M represents a transition metal and X belongs to the chalcogen group in VIA, such as S, Se, and Te) exhibit thickness-dependent bandgap, strong light-matter interactions, atomically thin dangling-bond-free surfaces, and effective electrostatic control due to the quantum confinement limit. [2]Furthermore, 2D TMD family possesses distinctive physical properties and can be divided into insulators (such as HfS 2 ), [3] semiconductors (including WS 2 , MoS 2 , WSe 2 , MoSe 2 ), [4,5] semimetals (such as 1T' or T d -MoTe 2 and 1T' or T d -WTe 2 ), [6,7] metals and even superconductors (like NbSe 2 , TaS 2 ). [8,9]or 2D type-II Weyl semimetals, MoTe 2 and WTe 2 with a distorted structure display small interaction energy due to the longer distance between metal atoms, which makes them possessing extremely high mobility, high conductivity, temperaturedependent band gap, and out-of-plane polarization.They can be used in low-energy photodetection, field-effect transistors, and so on. [10,11]However, the gapless Dirac structure of the 2D semimetal allows for a broadband response spectrum regardless of its monolayer structure; Nevertheless, it results in a high dark current under biased. [12]Fortunately, 2D van der Waals (vdW) heterostructure obtained by artificial stacking or epitaxial growth is an efficient strategy for improving electrical and optoelectronic performance, without considering the lattice matching constraint. [13]Most importantly, various reconfigurable phenomena based on 2D vdW heterostructures can be discovered such as anti-ambipolar behavior, [14] dynamic polarity conduction, [15] gate-modulated rectifying behavior, [16] nonvolatile storage and memory, [17] bidirectional photoresponse, [18] etc.As a 2D reconfigurable phototransistor (RPT), the sign of the photocurrent can be modulated by bias, gate voltage (V g ), and wavelength.This leads to reversible band bending and transitions between band alignment types such as BP/MoS 2 , [19] SnSe 2 /MoTe 2 , [20] and PdSe 2 -MoTe 2 heterostructures. [21]They can be utilized in logic circuits, neuromorphic circuits, convolution processing, artificial synapse. [22]For example, Ghosh et al. fabricated a WSe 2 /SnSe 2 vdW diode with bidirectional photoresponse behavior.A high negative responsivity (R) of up to −2 × 10 4 A W −1 and a fast response speed of 1 μs can be achieved.The reversible transition between type-II and type-III band alignment for positive and negative photocurrent can be realized through the specific bias and V g . [23]n recent years, all-2D Schottky barrier junction (SBJ), which is a 2D semimetal or metal/semiconductor junction benefiting from the strategy of 2D vdW heterostructure, has emerged as a promising research topic.[26] On the one hand, such as 1T'-PtTe 2 /MoTe 2 , [27] 1T'-NbTe 2 /InSe, [28] and 1T'-WTe 2 /MoTe 2 , [29] can greatly suppress dark current and have tunable built-in electric fields across the SBJ through gate modulation, resulting in significant optoelectronic performance.Second, in the case of multilayered graphene/MoS 2 /T d -WTe 2 , the carrier concentration or polarity in the overlapped channel can be regulated via external bias and V g , allowing for the manipulation of reversed built-in electric field and tunable Schottky barrier height (SBH) for application in nonvolatile memorizers. [30]At last, the contact resistance can be reduced to an ultralow value by designing the Fermi level difference between the semimetal and semiconductor to approach the Schottky-Mott limit.It can result in ultrahigh mobility and high on-current density in field-effect transistors such as 1T'-WTe 2 /monolayer MoS 2 . [31]In addition, the bidirectional photoresponse, which depends on the wavelength and semimetal thickness, can be observed in T d -WTe 2 /MoS 2 .This is attributed to the thickness-dependent band structure and the anomalous response of T d -WTe 2 . [32]Above all, the 2D SBJ architecture is a potential candidate for applications in multiple optical sensing, computing capabilities, intelligent vision sensors, and logic optoelectronics.Specifically, one heterostructure can accommodate multiple devices.However, besides exploring bipolar semiconductor based heterostructures, it is necessary to investigate the polarity switching properties, bidirectional optical response, and other multifunctional phenomena of semi-MoTe 2 /MoS 2 and semi-WTe 2 /WS 2 heterojunctions based on all 2D semimetal/unipolar TMD SBJ.
In this paper, we develop an all-2D SBJ RPT using a 1T'-WTe 2 /WS 2 vdW heterostructure with symmetric electrodes through mechanical exfoliation and dry-transfer method.The Fermi level of WS 2 can be significantly tuned upward or downward through the V g benefiting from the self-gating effect of 1T'-WTe 2 , which results in a reversed built-in electric field at WTe 2 /WS 2 interface.Remarkably, the device shows reconfigurable rectification behavior with a rectification ratio ranging from 10 −2 to 10 5 and a large window of 10 7 that can be switched depending on V g .Meanwhile, the unipolar transistor can be reconfigured as either an n-type or p-type conductor by applying a negative or positive bias, allowing the fabricated device to function as a logic half-wave rectifier in integrated circuits.In addition, under forward or reverse V g , the diode can deliver highperformance reconfigurable photovoltaic response ranging from 450 to 900 nm with an optimal wavelength at ≈630 nm.The Schottky junction at the WTe 2 /WS 2 or Cr/WS 2 interface can be revealed by spatially resolved photocurrent mappings.Most importantly, it exhibits a linear gate-dependent responsivity from the positive to negative direction without a hysteresis loop on a h-BN substrate.Therefore, the photodiode can function as a lowpower logic optoelectronic device and enable in-sensor broadband convolution processing.Moreover, the sign of photovoltaic parameters can be switched by applying or releasing pulsed V g .The excellent nonvolatile self-driven photo response is also confirmed.For further applications, our photodiode reveals reconfigurable polarization-sensitive response and high-resolution single-pixel imaging.Above all, our fabricated WS 2 /WTe 2 RPF is believed to have great potential in broadband convolutional neural networks, frequent-doubling diodes, non-volatile memorizers, and polarization-sensitive imaging systems.

Results and Discussion
Figure 1a shows the 3D schematic diagram of a phototransistor based on a monolayered 1T'-WTe 2 /2H-WS 2 heterostructure.The global back gate is formed by a heavily doped Si substrate with a 300 nm SiO 2 layer.The Drain (D) and Source (S) terminals are connected to the WTe 2 and WS 2 sides, respectively.This clearly shows that 1T'-WTe 2 possesses an orthorhombic structure and shows a distorted octahedral structure between W atoms and Te atom layers. [33]WS 2 exhibits a hexagonal atom-packed structure, with the W metal atom sandwiched between two planes of S atoms. [34]First, multilayered WS 2 flakes are mechanically exfoliated on 3 M scotch tape and transferred onto the SiO 2 /Si substrate using the polydimethylsiloxane (PDMS) method. [35]Second, multilayered 1T'-WTe 2 is also exfoliated onto the PDMS stamp and artificially stacked to form a vertical SBJ via a 3D micro-zone transfer platform (see in Experimental Section).Finally, Figure S1 (Supporting Information) shows the corresponding optical image of device I consist of 10 nm Cr/50 nm Au electrodes.E1-E2 and E3-E4 pairs are used to measure the electrical characteristics of individual WS 2 and WTe 2 channels to confirm their contact quality and doping type.The reconfigurable performance of the SBJ can be measured by testing E2-E3.In Figure 1b, AFM results confirm that the thicknesses of as-prepared WS 2 and WTe 2 were measured along the pink line to be ≈32 nm and 57 nm with smooth and clean surfaces, respectively.In general, KPFM can be used to determine the surface potential difference (SPD) between the WTe 2 and WS 2 vdW interface, as well as the Fermi level of individual WS 2 and WTe 2 .As a result, Figure 1c displays the SPD image and the extracted potential profile.The averaged SPD of WTe 2 and WS 2 can be measured and calculated using the following equations: [36] eSPD WTe 2 = W tip − W WTe 2 (1) Where e is the electron charge, W tip , W WTe 2 and W WS 2 represent the work functions of KPFM tip, WTe 2 and WS 2, respectively.Therefore, after contact, the built-in electric field points from WS 2 to the WTe 2 side.Moreover, the Fermi level difference (ΔE f ) between WTe 2 and WS 2 can be obtained to be ≈69 meV using the following equation: [37] ΔE Benefiting from the small ΔE f value and the excellent gate modulation of semiconducting WS 2 , our designed 2D SBJ phototransistor with polarity control can be achieved by applying negative or positive V g .In addition, based on previous studies, the conduction band minimum (CBM), valence band maximum (VBM) and band gap (E g ) values of multilayer WS 2 can be theoretically determined to be approximately 4.1 eV, 5.07 eV, and 0.97 eV using the generalized gradient approximation of Perdew-Burke-Ernzerhof. [34,38] W WS 2 is likely located beyond the middle Fermi level of the band structure, which can be confirmed by its n-type doping characteristics later.Therefore, Figure 1d shows the energy band alignment between Cr, WS 2 , and WTe 2 before contact.
Raman spectrum is often used to show the phonon vibrations in individual 2D materials and interlayer coupling effects in vdW heterostructure.For multilayered WS 2 , two strong Raman peaks at 352 cm −1 and 420 cm −1 can be observed, representing the in-plane E 1 2g mode and out-of-plane A 1g mode, respectively. [39]hile for multilayered 1T'-WTe 2 , five Raman peaks at 77 cm −1 , 108 cm −1 , 130 cm −1 , 160 cm −1 , and 208 cm −1 are consistent with the previous report. [40]In the overlapped region, the reduced intensities of all Raman peaks (called "Raman softening") with negligible Raman shift indicate enhanced interlayer coupling and high crystalline quality at the WTe 2 /WS 2 interface, which leads to the effective charge transfer across the junction. [41,42]In addition, the emission peak of multilayered WS 2 and the charge transfer process can be demonstrated by the photoluminescence (PL) spectra.As shown in Figure 1f, except for the A and B peaks of WS 2 , a symmetric and broad PL peak located at ≈875 nm can be depicted, indicating an indirect band gap transition of WS 2 for I peak at ≈1.42 eV. [43]In the overlapping region, PL quenching with the quenching factor of 92.13% at the same peak is achieved, which can confirm the interlayer charge transfer process at the 1T'-WTe 2 /WS 2 Schottky barrier interface. [44]Above all, a vertical semimetal-WTe 2 /semiconducting WS 2 SBJ with high quality and efficient charge transfer is fabricated successfully.
The electrical properties of the WTe 2 /WS 2 phototransistor will be discussed.First, Figure 2a exhibits the transfer curves of WS 2 and WTe 2 at V ds = 1 V. Intuitively, WS 2 can be fully depleted via electrostatic modulation and shows asymmetric ambipolar behavior, which indicates that the Fermi level can be tuned upward and downward via positive or negative V g and enables the reconfigurable behavior in the heterostructure. [37]In particular, our exfoliated WS 2 displays ambipolar behavior with a typical n-type conducting at V g = 0 V, which is consistent with the previously reported n-type doping. [46]The corresponding hole and electron current on/off ratios are calculated to be 2.8 × 10 5 and 1.79 × 10 6 , respectively.The field-effect mobility of WS 2 for holes and electrons can be calculated by the following formula. [45] = dI ds dV g L WC i V ds (4)   Where dI ds /dV g can be extracted from the maximum slope of the transfer curve at a linear scale.L and W are the length and width of the channel.C i represents the gate capacitance per unit area according to the equation of C i =  0  r /d, where  0 and  r are the dielectric constants of the vacuum and SiO 2 , respectively, and d is the thickness of the SiO 2 .As a result, the hole and electron field-effect mobility of WS 2 at V ds = 1 V are calculated to be 15.38 cm 2 V −1 s −1 and 11.64 cm 2 V −1 s −1 , respectively.By contrast, WTe 2 channel shows high conductance and cannot be modulated by V g , which is consistent with the semi-metallic characteristics of multilayered 1T'-WTe 2 . [29,30]In addition, the Fermi level of WTe 2 is also confirmed to be pinned at the VBM at room temperature regardless of V g . [29]As shown in Figure S2 (Supporting Information), the I ds -V ds curves for individual WS 2 and WTe 2 show linear and symmetric relationship between I ds and V ds , revealing the Ohmic contact between WS 2 or WTe 2 and Cr/Au electrode. [47]We employ the transmission line method to compute the direct contact resistance of the device, the direct contact resistance between WTe 2 and Cr can be determined as 53 Ω μm.Meanwhile, the direct contact resistance between WS 2 and Cr can be calculated to be 2.6 MΩ μm, as illustrated in Figure S3 (Supporting Information).Meanwhile, we also explain Ohmic contact behavior using a band diagram, as shown in Figure S4 (Supporting Information).It can be also indicated that the rectification behavior with and without gate modulation is probably originated from the WTe 2 /WS 2 interface. [48]Benefiting from the ambipolar polarity of WS 2 and the semi-metallic property of WTe 2 , the transfer characteristics of the SBJ will be diversified and controlled by the V ds polarity.As shown in Figure 2b, the 1T'-WTe 2 /WS 2 channel shows asymmetric ambipolar characteristics and different doping types of majority carriers, which are determined by the V ds polarity.When V ds = −2 V, the SBJ shows weak p-type conducting as V g sweeps from -60 to 50 V and the maximum on-state current at the order of 10 −8 A at V g = -60 V. Nevertheless, the SBJ represents significant n-type doping as V g sweeps from -60 V to 50 V at V ds = 2 V and the maximum on-state current of 10 −5 A at V g = 50 V.Thus, our fabricated SBJ shows dynamic polarity control.To confirm the consistency, Figure 2c shows the 3D schematic output curves of the WTe 2 /WS 2 channel as scanning V g values from -60 to 60 V.[51] When V g > 0 V, a non-linearity and positive rectifying characteristic with a high current rectification ratio (I 1V /I −1V ) of about 10 3 is observed owing to the obvious large band offset at the WTe 2 /WS 2 junction.Conversely, When V g < 0 V, a negative rectifying behavior with a rectification ratio of ≈5 × 10 −2 can be achieved because of the reversed band offset at the junction.Above all, the rectification direction of the WTe 2 /WS 2 channel can be changed by applying the V g polarity to WS 2 , which is different from many other 2D conventional Schottky junction diodes. [52,53]The bias is swept between V ds = ±2 V at various V g to investigate the rectifying features of the WTe 2 /WS 2 diode.Figure 2d shows the current rectification ratio (I 2V /I −2V ) as a function of V gs extracted from the transfer curves in Figure 2b, which varies in a wide range from 10 −2 to 10 5 as the V g increases from -40 to 40 V.In particular, the transition point of the rectification direction occurs at V g = -25 V, which represents the reversed point for the built-in electric field.Moreover, the reconfigurable performance dependent on the thickness of WS 2 and WTe 2 are also illustrated in Figures S5 and S6 (Supporting Information).
As the thickness increases, the reconfigurable rectifying behavior and bidirectional photocurrent behavior under V g becomes more pronounced.To verify the polarity control tuned by bias and reversible rectification properties modulated by V g , the energy band alignments with charge transport process as a function of V g are exhibited in Figure 2e.When a negative V g of -60 V is applied, the E f position of WS 2 moves to near the VBM and lies below the E f of WTe 2 .After contact, the electrons in WTe 2 will diffuse to WS 2 , resulting in a built-in electric field pointing from WTe 2 to WS 2 .The band edge of WS 2 will bend downward near the interface.Thus, the minority electrons in WS 2 are accelerated to WTe 2 without a SBH, while holes in WTe 2 are prohibited.At negative V ds , the current at backward condition becomes conductive by overcoming the built-in electric field.While the current is forbidden at positive V ds by strengthening the built-in electric build.Thus, a backward rectification behavior can be achieved.When a large positive V g of 60 V is applied, the E f position of WS 2 will move near to the CBM and lies on top of W WTe 2 , leading to the built-in electric field from WS 2 to WTe 2 .The band edge of WS 2 bends upward near the interface.The majority of holes in WS 2 can easily drift to WTe 2 , while the electrons in WTe 2 hardly move across the large SBH.At positive V ds , the current at forward becomes conductive by overcoming the built-in electric field.Thus, a forward rectification behavior can be achieved.
In a word, when a positive/negative V ds is applied to WTe 2 (D) terminal, the external electric field will overcome/strengthen the built-in electric field.It can be verified that our device can switch the polarity via modulating V g and enable a large range of rectification ratio up to 10 7 , which can simplify the integrated level in large-scale circuit and broadens the application in half-wave rectifiers and reconfigurable photodetectors.
A half-wave rectifier is a circuit element that can use the conduction characteristics of a diode to rectify the current, which can convert alternating current into direct current.When a sinusoidal signal is input, the output can get the positive or negative portions, and the polarity of the half-wave rectifier can be switched because of the gate-modulation polarity switching in our designed WTe 2 /WS 2 diode in Figure 3a. [15]Figure 3b shows the schematic diagram of the polarity switchable logic half-wave rectifier.Figure 3c shows the output V out curve of a rectifier with an input sinusoidal signal at a peak-to-peak voltage (the difference between the maximum positive input voltage and the maximum negative input voltage) of 4 V at the frequency of 16 Hz. [15]With V g of 0 V, +20 V, +40 V, and +60 V, the device exhibits conventional p-n junction characteristics, allowing the positive voltages to pass through and the negative voltages to be filtered steadily.By contrast, while V g = -20 V, -40 V, and -60 V, the device undergoes polarity reversal, allowing negative voltages to pass through and positive voltages to be filtered out.This unique polarity reversal property in such a SBJ diode allows for a circuit layout design of complicated rectifiers. [54]he reconfigurable photoresponse properties will be also investigated as follows.Figure S7a-c   photodiode in the wavelength range from 450 to 900 nm at the light power density of 30 mW cm −2 and V ds = 0 V. Similarly, the optimal wavelength at ≈630 nm can be obtained with and without gate-modulation, indicating that WS 2 dominates the light absorption instead of WTe 2 . [55]Nevertheless, a higher illuminated I ds at V g = 60 V is demonstrated, which can predict the larger built-in electric field across the Schottky barrier interface for the acceleration of photo-generated carriers.The corresponding time-resolved curves with and without gate modulation are also shown in Figure S7d,e (Supporting Information).It can be noticed that the device possessed the highest photocurrent at the wavelength of 635 nm and a slightly wider response range including 808 nm under V g = ± 60 V.The photovoltaic performance of the WS 2 /WTe 2 phototransistor will be investigated under 635 nm illumination with varied light power densities at V g = ± 60 V. First, Figure 4a and c show the I ds -V ds curves under 635 nm irradiation ranging from 0.0064 to 11.59 mW cm −2 .When V g = 60 V, negative short-circuit current (I sc ) and positive open-circuit voltage (V oc ) can be obtained.By contrast, positive I sc and negative V oc are achieved under V g = -60 V.It means that our selfdriven phototransistor can deliver multilevel I sc states and exhibit sign reversibility under visible light irradiation.Meanwhile, the corresponding time-resolved switching on-off curves as a function of light power density (P) are displayed in Figure 4b,d.A highest negative I sc of 4 × 10 −8 A and a positive V oc of 159 mV at V g = 60 V are observed.While a highest positive I sc of 6.15 × 10 −9 A and a negative V oc of 1.2 mV at V g = -60 V are extracted.When P = 11.9 mW cm −2 , the maximum I light /I dark ratios reach to 2.74 × 10 2 and 3.53 × 10 3 at V g = -60 V and 60 V, respectively.To figure out the photocurrent distribution and accurately evaluate the level of our fabricated phototransistor, the scanning photocurrent microscopy (SPCM) at V g = 0 V, 60 V, and -60 V without bias under 638 nm are displayed in Figure 4e-g, respectively.When V g = 0 V, a highest negative photocurrent with a magnitude of 10 −8 A is generated at the edge of the WTe 2 /WS 2 contact, suggesting that the built-in electric field in the depletion region originates from the WTe 2 /WS 2 Schottky junction.The photocurrent can be obtained from the generation and separation process of photo-generated carriers driven by the built-in electric field of 69 meV at zero bias without V g . [56]In this condition, the sign of I sc turns to negative direction because of the hysteresis loop in the transfer curve and the presence of trapped holes at the SiO 2 /Si interface after applying a V g of 60 V.When V g = 60 V, the negative photocurrent is increased to the order of 5 × 10 −7 A and uniformly exists in the fully overlapped region between WTe 2 and WS 2 .The reason is that the strength and the width of the built-in electric field at the Schottky interface are significantly enhanced and broadened.Thus, more and more photogenerated carriers can be separated in a short time toward the opposite electrode, leading to a larger negative I sc value.In particular, the photocurrent area is obviously expanded to the individual WS 2 region because of the lower doping concentration of WS 2 compared to 1T'-WTe 2 . [57]The electrode contacts at WTe 2 /Cr (D) and WS 2 /Cr (S) show no photoresponse at V ds = 0 V, V g = 0 V, and 60 V. Nevertheless, when V g = -60 V, both positive and negative photocurrent are observed in the WS 2 /Cr interface as well as within the WS 2 channel between Cr/Au and WTe 2 .In general, the positive photocurrent direction is determined as flowing from WTe 2 /Cr (D) to WS 2 /Cr (S) electrodes.The built-in electric field at the WS 2 /Cr interface is larger than that at the WTe 2 /WS 2 interface, leading to the obvious photocurrent distribution at the WS 2 /Cr and WS 2 region.The above charge transport mechanism is displayed through energy band alignment under illumination in Figure S8 (Supporting Information).Above all, the photocurrent generation is mainly contributed by the Schottky barrier region of WTe 2 /WS 2 interface at V g = 0 V and 60 V.While the photocurrent distribution is turned to another Schottky barrier junction region of Cr/WS 2 interface at V g = -60 V. Importantly, the rise time ( r ) and decay time ( d ) represent the time range between 10% and 90% of the on-state photocurrent can be used to evaluate the response speed of the photodetector.As shown in Figure S9a (Supporting Information), a stable photo-switching curve was achieved at the frequency of 11 Hz under 620 nm illumination.Meanwhile, the photovoltaic  r and  d of 5.3 ms and 5.4 ms are obtained at V g = 0 V in Figure S9b (Supporting Information).Moreover, in Figure S9c (Supporting Information), the response speed is mainly accelerated to 1.7 ms/5.9 ms, and the corresponding bandwidth (ΔB) is approximately 588 Hz at V g = -60 V because of the effective built-in electric field across the Cr/WS 2 interface.Nevertheless, the response speed is slightly prolonged to 13 ms/12.3ms and ΔB is ≈77 Hz at V g = 60 V in Figure S9d (Supporting Information), which is probably attributed to the increased Auger recombination rate across the WTe 2 /WS 2 interface at higher light power density.In general, R represents the sensitivity of the photodetectors to incident light and is defined as the photocurrent generated per unit power of incident light on the effective area.EQE means the ratio of the number of absorbed charges to the number of effective photogenerated carriers stimulated by the incident photons.D* reflects its capacity to detect weak light signals and represents the signal-to-noise ratio per unit of radiated power of the photodetector in unit bandwidth and unit area.The above parameters serve as a crucial metric to assess the level of our fabricated reconfig-urable phototransistor, which can be calculated by the following formulas. [58]= I ph ∕P in S (5) Where S is the effective illumination area determined by the photocurrent mapping results at V g = ± 60 V, P in is the incident light power density, and I ph is the net photocurrent.Moreover, q is the electronic charge, h is Planck's constant, c is the velocity of light, and  is the incident wavelength.NEP, A, and i n are the photodetector's noise equivalent power, effective photo-active area, and noise current of the photodetector, respectively.To compare the photovoltaic performance of WTe 2 /WS 2 phototransistor at different photocurrent signs, Figure 4h depicts I ph and R as a function of P at V g = -60 V and 60 V. Herein, the parameter sign is omitted to enhance evaluation accuracy.Remarkably, I ph monotonously increases with the increasing P because of the larger number of photo-generated electron-hole pairs being separated by the built-in electric field under stronger P. According to the power law formula of I ph ∝ P  in , [58] the fitting index of  are extracted to be 0.95 (< 1) at V g = 60 V and 0.82 (< 1) at V g = -60 V after fitting the experimental data.The clear sub-linear power-dependent behavior of the photocurrent is obtained under the gate-modulation photovoltaic effect.A nearly ideal power exponent ( = 1) of 0.95 at V g = 60 V illustrates a relatively lower Auger recombination of photo-generated carriers driven by the large built-in electric field in the WTe 2 /WS 2 junction. [59]When V g = 60 V, R monotonously decreases from 1325 to 667 mA W −1 with the increment of P. The maximum R value of 1325 mA W −1 under 635 nm illumination is superior to many other WTe 2 -based Schottky diodes such as MoTe 2 /WTe 2 , [29] Gr/MoS 2 /WTe 2 [30] and WTe 2 /MoS 2 heterostructures. [32]While V g = -60 V, R also slightly decreases from 644 to 410 mA W −1 with the increment of P. As known, the photovoltaic R reaches its maximum at weak P because of the neglectable non-radiative recombination originating from the small number of filled trap states.However, most of the trap states of the minority carrier are gradually occupied, leading to the increased recombination probability and the decreased R. [60] Considering the sign of photocurrent, the self-driven R can be changed from -1325 to 410 mA W −1 by gate modulation.Meanwhile, in Figure 4i, all EQE show a decreasing trend from 259.28% to 130.47% at V g = 60 V and from 126.08% to 80.22% at V g = -60 V as the P increases.The EQE values beyond 100% can be ascribed to the internal photo-gain effect in the WTe 2 /WS 2 heterostructure at V g = 60 V. [61] Meanwhile, the optoelectrical performance with varying heterojunction regions is depicted in Figure S10 (Supporting Information).When the thicknesses for WS 2 and WTe 2 are fixed, an enlarged heterojunction region leads to enhanced light absorption and photocurrent generation.In addition, the maximum output electrical power (P el max ) of 2.62 nW under P = 11.59 mW cm −2 and V g = 60 V in Figure S11a (Supporting Information).In general, fill factor (FF) and photoelectric conversion efficiency (PCE) are two key parameters to evaluate photovoltaic performance.They can be calculated by the following formula. [62] = P el max I sc V oc (9)   PCE = P el P in (10)   Where P el is the output electrical power and P in represents the effective incident electrical power.Figure S11b (Supporting Information) shows the maximum values of FF and PCE at V g = 60 V are 0.44 and 2.84%.It can be confirmed that the gatemodulated photovoltaic parameters are enhanced at V g = 60 V because of the larger built-in electric field at WTe 2 /WS 2 interface.
To evaluate the photovoltaic sensitivity of the WTe 2 /WS 2 -based phototransistor, Figure S11c (Supporting Information) exhibits the current noise power density (S n ) spectrum from 1 to 10 5 Hz at V g = 0 V and ± 60 V without bias.When V g = 0 V, the frequencydependent noise (1/f, f is the frequency of the laser) shows a linear decreasing trend and dominates the noise power distribution up to 300 Hz, which can be attributed to the fluctuations in carriers being trapped and de-trapped by the interface defects and the local electronic states originating from the disordered geometry in 1T'-WTe 2 . [60,63]When the frequency is above 300 Hz, the S n shows a frequency-independent phenomenon and is determined by the white noise at ≈1 × 10 −14 A Hz −1/2 including Johnson noise (also known as thermal noise) and shot noise. [60]evertheless, all the S n show a linear decreasing tendency from 1 to 10 5 Hz by the control of at V g = ± 60 V and V ds = 0 V.The gate-modulated n values at V ds = 0 V can be extracted by the corresponding ΔB.Thus, the NEP values at V ds = 0 V are 0.36 pW Hz −1/2 at V g = 60 V and 0.28 pW Hz −1/2 at V g = -60 V, which indicates that our fabricated Schottky barrier phototransistor can detect the weakest signal of 0.28 pW from noise under gate-modulated photovoltaic mode.Furthermore, when V g = 60 V, D* slightly decreases from 1.47 × 10 12 Jones to 7.87 × 10 11 Jones.While at V g = -60 V, D* decreases from 2.08 × 10 12 Jones to 1.72 × 10 11 Jones.
Our phototransistor and other state-of-the-art Schottky photodetectors are shown in Table S1 (Supporting Information).
It can be shown that our WTe 2 /WS 2 heterostructure showed high-performance photovoltaic behavior under gate-modulation than those photodiodes under visible region.As known, broadband convolution processing in photovoltaic imaging sensors requires a linear light power density dependence and gate dependence for the photocurrent.Figure S11d (Supporting Information) displays the gate-tunable positive and negative photo response with a large hysteresis loop at P = 5.87 mW cm −2 under 635 nm illumination.The hysteresis loop under 635 nm illumination can be prohibited by incorporating a h-BN layer between WTe 2 /WS 2 and SiO 2 layer in Figure S12 (Supporting Information), indicating that the charge trapping states at the smooth h-BN layer can be suppressed. [64]As knows, the sub-linear relationships between P and photocurrent ( = 0.95 at V g = 60 V and  = 0.82 at V g = -60 V) are achieved, ensuring the collection of high-resolution image information.In Figure 5a, the calculated positive and negative R can be linearly changed from 100 to -400 mA W −1 by the applied V g scanning from -60 to 60 V.Moreover, a linear window can be maintained for a V g of 10 V. To investigate the nonvolatile polarity-switching behavior in WTe 2 /WS 2 phototransistor, a transient pulsed V g of ± 60 V is applied to figure out the reversible sign of the photocurrent at V ds = 0 V.As shown in Figure 5b, when a pulsed V g of 60 V at the frequency of 0.2 Hz is applied in 5 s, a negative I sc with stable and fast photo response can be remarkably obtained due to the built-in electric field of WTe 2 /WS 2 pointing from WS 2 to WTe 2 side.While the pulsed V g is removed, the sign of I sc can be turned to a positive direction and called to be Pos-diode, the photocurrent distribution is probably determined by the Cr/WS 2 region.Similarly, when a pulsed V g of -60 V at the same frequency is exerted, a positive I sc photo response curve can be observed because of the dominated built-in electric field of Cr/WS 2 pointing from Cr to WS 2 side compared to the WTe 2 /WS 2 interface under illumination.While the sign of I sc can be entirely reversed to negative direction and known as Neg-diode after erasing the pulsed V g in Figure 5c.The corresponding optoelectrical performance can be enhanced because of the effective charge transport across the WTe 2 /WS 2 region.Moreover, Figure 5d displays the time-resolved photo response curves under 635 nm illumination at various light power densities for Pos-diode and Neg-diode, respectively.When P increases from 0.0064 to 11.59 mW cm −2 , the Pos-diode, and Neg-diode can respond to 635 nm wavelength with stability.Figure 5e exhibits the corresponding I ph and R as a function of P for the Pos-diode and Neg-diode, respectively.In particular, the highest  of 0.96 can be achieved for the Pos-diode, which is ascribed to the fewer recombination centers existing in the heterojunction.Meanwhile, the calculated R monotonously decreases from 150.2 to 56.6 mA W −1 with the increase of P for Pos-diode.Though a lower  of 0.71 is obtained for the Negdiode, a maximum photovoltaic R value of 1278 mA W −1 can be achieved because of the larger generation of photo-carriers and increased intralayer recombination probability.This R value is comparable to that at V ds = 0 V and V g = 60 V.The maximum EQE values for Pos-diode and Neg-diode are 29.39% and 250%, respectively.At last, to absolutely evaluate the sensitivity of the self-driven photodetector without V g , D* should be calculated by the following formula. [58] Where S n is extracted to be ≈1 × 10 −14 A Hz −1/2 at f = 300 Hz from Figure S11c (Supporting Information).As a result, D* slightly decreases from 4.65 × 10 10 Jones to 1.75 × 10 10 Jones for the Pos-diode.While for Neg-diode, the corresponding D* decreases from 3.96 × 10 11 to 9.03 × 10 10 Jones.Above all, our fabricated WTe 2 /WS 2 based phototransistor possesses a nonvolatile bidirectional photo response behavior after programming and erasing the V g of ± 60 V, which can be used in high-frequency nonvolatile optoelectronic memorizers with high photoresponse performance.The logical outputs including the programmed state of "0" and the erased state of "1" can be defined by the illuminated I ds with negative and positive directions, respectively.It can simulate two types of neuromorphic vision functions of imaging pre-processing and classifier for the human retina. [65]The corresponding logic optoelectronic device under illumination will also be confirmed later.
In general, polarization-sensitive photodetection is used to extract the polarization information of light, which will enable a higher contrast degree in detection and imaging in complicated environments.Owing to the in-plane anisotropic structure of semi-metallic WTe 2 , it can be used in broadband polarized photodetection. [10,11]Nevertheless, the polarized-sensitive photodetection of 1T'-WTe 2 based Schottky phototransistor is rarely reported.Figure 6a displays the schematic diagram of the polarization photocurrent measurement setup.The circularly polarized light at the wavelength of 635 nm can be translated into linear polarized light with the help of a polarizer.When the polarizer is kept at 0°, the a-axis direction of WTe 2 is vertical to the linear polarized laser.Meanwhile, a half-wave plate between the polarizer and the device is rotated in a clockwise direction from 0°to 180°(equal to 0°to 360°) and the P is maintained at 3 mW cm −2 regardless of the polarization angle.To investigate the relationship between the gate-modulated photovoltaic performance and photocurrent anisotropic ratio, Figure 6b shows the switching onoff curves during the rotation angle under V g = 0 V, -60 V, and 60 V. respectively.It is well known that the light absorption efficiency of the heterostructure under 635 nm illumination is dominated by the bottom WS 2 layer by broadband spectrum.Indistinctively, the illuminated I ds changes periodically at V g = 0 V and -60 V when the polarized angle changes from 0°to 360°.While the illuminated I ds shows a weak periodical change at V g = 60 V. Intuitively, the relationship between angle-resolved photocurrent and the polarization angle can be demonstrated in the polar plot in Figure 6c.The fitting curves under various V g are derived from the experimental data and the following formula. [66]ds () = I max cos 2 ( + ) + I min sin 2 ( + ) (12)   Where  is the rotation angle between the polarization direction of the 635 nm laser and the a-axis direction of semi-metallic WTe 2 . represents the phase position. Imax and I min are the maximum and the minimum of the measured photocurrent, respectively.All the photocurrent with and without gate-modulation can reach their maximum at  = 90°while are prohibited at  = 0°and 180°, which is consistent with the previous report. Terefore, the polarization characteristics in heterojunction devices are mainly determined by WTe 2 .As known, the photocurrent anisotropic ratio can be expressed by . Therefore, the photovoltaic values at V g = 0 V, -60 V, and 60 V are calculated to be 2.76, 3.66, and 1.84, respectively.In particular, the highest photocurrent anisotropic ratio at V g = -60 V is probably due to the competition between the Cr/WS 2 and WTe 2 /WS 2 junctions.Though the polarization detection originated from the orthorhombic crystal structure of 1T'-WTe 2 regardless of the isotropic characteristic of WS 2 , [55] the domination of the Cr/WS 2 Schottky barrier and the reversed built-in electric field of WTe 2 /WS 2 interface can both exert a positive effect on the final polarization ratio.The maximum photocurrent direction is parallel to the a-axis direction of semi-metal WTe 2 . [10]Above all, the reconfigurable polarization photodetection of our fabricated WTe 2 /WS 2 phototransistor under V g control can apply several modes such as polarimetric sensor or not dependent on the specific application circumstances. [67]oreover, the imaging capability of a self-driven photodetector to collect optical signals and acquire high-resolution image information is of great importance in remote sensing detection.A light-emitting diode (LED) of 620 nm with a spot diameter of 27 μm at light power of 800 μW is implemented by a commercial single-pixel modulus (see in Measurement part).First, a color image of "HS" should be processed in grayscale and imported into the compact data acquisition (cDAQ) by a computer.160 × 300 pixels of the image of "HS" can be read by the cDAQ and converted into voltage values.Second, the LED of 620 nm can output various light power densities determined by the voltage value in each pixel.At last, the device is illuminated by the LED of 620 nm, the actual illuminated I ds at negative direction can be acquired by a software-programmed computer at V ds = 0 V and V g = 0 V, achieving a high-resolution and uniform distribution image of "HS" in Figure 6d.
Benefiting from the polarity switching in the photovoltaic mode under gate modulation in our fabricated WTe 2 /WS 2 heterojunction, a logic optoelectrical component can be designed to output negative and positive states of the electric signal by stimulating continuous V g of 60 V and -60 V under 635 nm illumi-nation at a fixed P in Figure 6e.As shown in Figure 6f, the reconfigurable device exhibits negative I sc at V g = 60 V while a positive I sc is obtained at V g = -60 V, enabling a wide range of applications in retinomorphic hardware device, in-memory optical sensing, broadband convolution processing, and optoelectronic logic devices with light-actuated sensing ability. [21,30,68,69]In order to verify the reproducibility of our designed structure, the other two devices with similar reversed rectification behavior and bidirectional photoresponse are shown in Figure S13 (Supporting Information).To verify that reconfiguration is independent of charge capture at the SiO 2 /2D materials interface, we fabricated WTe 2 /WS 2 and h-BN/WS 2 /WTe 2 with similar thickness and performed repeated tests after one week, demonstrating both reliability and controllability in Figure S14 (Supporting Information).To verify that the consistency of the time constant across different tests, the response speed of four different devices was evaluated, and the results are presented in Figure S15 (Supporting Information).It can be observed that the time constant remains stable primarily at the millisecond level.

Conclusion
In summary, a reconfigurable Schottky phototransistor based on WTe 2 /WS 2 vertical heterojunction with multi-functions is demonstrated.Through dry-transfer methods, our fabricated heterostructure shows strong layer-by-layer coupling effect and efficient charge transport process by Raman and PL analysis.As a result, the reconfigurable field-effect behavior including p-type and n-type conducting by the signs of bias and reversed rectification behavior modulated by V g inputs are achieved, ascribing to the reversed built-in electric field at WTe 2 /WS 2 interface.The rectification ratio ranges from 10 −2 to 10 5 at a crossover point of V g = -25 V.It can act as a logic half-wave rectifier showing excellent switchable rectifying states.Under illumination, WTe 2 /WS 2 phototransistor exhibits excellent photovoltaic performance at V g = 60 V with a maximum negative R of -1325 mA W −1 and D* of 1.47 × 10 12 Jones because of the strongest built-in electric field at WTe 2 /WS 2 interface.The photovoltaic behavior can change to positive response determined by the Cr/WS 2 Schottky barrier at V g = -60 V by SPCM and band alignment analysis.Moreover, a linear gate-dependent photoresponsivity in a V g window of 10 V can be achieved, allowing for tuning the kernels for several types of broadband convolution processing.Most importantly, a stable, pronounced, and nonvolatile photo response along negative and positive directions can be controlled by applying or erasing V g inputs.After releasing V g , the maximum self-driven R changes from 150.2 to -1278 mA W −1 .In addition, owing to the in-plane anisotropic structure of 1T'-WTe 2 , a gate-tunable polarization behavior of the I max I min ratio ranging from 1.84 to 3.66 with polarity switching can be observed, ascribing to the less domination of isotropic WS 2 at V g = -60 V. Finally, a clear single-pixel imaging of "HS" can be acquired, and the fast and stable negative/positive conductance states can be outputted by stimulating V g of ± 60 V. Our fabricated single Schottky junction based on WS 2 /WTe 2 with reconfigurable multi-functions can be considered into the applications in in-sensor broadband convolutional processing, retinomorphic hardware, and nonvolatile memorizers.

Experimental Section
Device Fabrication: All these different photodiodes were based on p + -Si wafers with a thickness of 300 nm SiO 2 or BN layers.h-BN flake was exfoliated using the micromechanical exfoliation technique.WS 2 and WTe 2 nanosheets were stripped from the bulk single crystal (2D Semiconductor Co., Ltd.bought from Shanghai Onway Technology Co., Ltd.) using 3 M scotch tape and were exfoliated in the same way.BN can be transferred by PDMS stamp to a silicon wafer as a substrate.WS 2 and WTe 2 were then transferred to the substrate in turn by the dry transfer method to form a WS 2 /WTe 2 heterostructure via a 3D transfer stage (Shanghai Onway Technology Co., Ltd.).The electron beam evaporation method was used to achieve electrodes.10 nm Cr/50 nm Au electrodes were prepared on WS 2 /WTe 2 Van der Waals heterojunction by using a positive photoresist of AR-P 5350 and a developing solution of AR 300-26 (ALLRESIST GmbH Company bought from Taizhou SUNANO New Energy Co., Ltd), maskless UV lithography system (TuoTuo Technology (Suzhou) Co., Ltd.) and electron beam evaporation process.Then, by annealing at 150 °C for 0.4 h under an argon environment, the contact quality between the electrode WS 2 /WTe 2 vdW heterojunction was improved and the contact barrier was reduced, and the photodiodes with WS 2 /WTe 2 heterojunction were prepared.
Characterization and Measurement: The morphology and size were characterized with the help of an optical microscope (ECLIPSE LV150N, Nikon).The thickness and surface potential were measured by scanning probe microscope equipped with AFM and KPFM (Dimension FastScan from Bruker Co., Ltd.).The Raman and PL spectra were performed by confocal microscope with a laser wavelength of 532 nm (Nost Technology Co., Ltd.).
The spectral response of heterojunctions from 400 to 1500 was studied by a variable-wavelength photocurrent instrument (MStarter ABS DUV-NIR Microscopic Absorption Spectroscopy System, Nanjing Metatest Optoelectronics Co., Ltd.) under a tungsten light source with a power of 30 mW cm −2 .All photoelectric characteristics of the device were performed through a three-probe station equipped with a Keithley 2636B SourceMeter.The photoresponse performances of the device were conducted under illumination at 405, 635 nm, whose power density was measured by a power meter (PM400, Thorlabs). of an optical-fiber laser.High-resolution time response curves were tested by a digital oscilloscope (Siglent SDS5104X) combined with a semiconductor analyzer (Fs-Pro, Primarius) (supplied by Guangdong Avit Technology Co., Ltd.).The polarization sensitivity was carried out by adding a Glan-Taylor prism to the incident light path.The single-point imaging was conducted by a photocurrent imaging instrument (Mstarter 200 High Precision Photocurrent Scanning Test Microscope) (supplied by Nanjing Maita Optoelectronic Technology Co., Ltd.).

Figure 1 .
Figure 1.a) 3D Schematic diagram of the 1T'-WTe 2 /2H-WS 2 heterojunction phototransistor.b) Atomic Force Microscopy (AFM) image of WS 2 and WTe 2 , respectively.Inset: The corresponding height profile along the pink line.c) Kelvin Probe Force Microscopy (KPFM) image and Surface potential difference profile of the heterojunction.d) Band diagram of WTe 2 and WS 2 before contact.e) Raman spectra of individual WTe 2 , WS 2 , and the overlapped region of WTe 2 /WS 2 .f) The photoluminescence (PL) spectra comparison between WS 2 and the heterojunction formed by WTe 2 /WS 2 .

Figure 2 .
Figure 2. a) Transfer curve of WS 2 at V ds = 1 V. Inset: Transfer curve of WTe 2 at V ds = 1 V; b) Transfer curve of the WS 2 /WTe 2 heterostructure at V ds = 2 V and V ds = −2 V; c) Output curves of WS 2 /WTe 2 at various values of V g ; d) Rectification ratio as a function of semi-logarithmic scale for different values of V g .Inset shows the schematic circuit diagram for the WS 2 /WTe 2 phototransistor.The green solid line (blue dashed line) diode in a forward direction only functions while V g is larger or smaller than −25 V. e) Energy band alignment diagram for the WS 2 /WTe 2 heterostructure after contact in dark dependent on V g .

Figure 3 .
Figure 3. a) Output curves of another WS 2 /WTe 2 phototransistor at V g = 60 V and V g = -60 V, also showing the reversible rectification behavior.b) Schematic illustration of the polarity-switchable logic half-wave rectifier.c) Rectifier output V out with a harmonic input signal V in (amplitude of 2 V at the frequency of 16 Hz) under varying V g values from -60 to 60 V.
(Supporting Information) show the wavelength-dependent I ds of the WS 2 /WTe 2

Figure 4 .
Figure 4.I ds -V ds curves of the WS 2 /WTe 2 photodiode under 635 nm laser illumination with various light power densities at a) V g = 60 V c) V g = -60 V. Time-resolved photo response curves under varied light power densities at b) V g = 60 V d) V g = -60 V under 635 nm illumination.e-g) Photocurrent mappings at V g = 0 V, 60 V, and -60 V under 638 nm illumination.The black, blue, and brown dashed lines illustrate the outlines of WS 2 , WTe 2 , and Cr/Au electrodes, respectively.The circular spot diameter is 1.5 μm and the incident light power is 50 μW;h) R and I ph as a function of light power density at V g = 60 V and -60 V under 635 nm illumination.i) D* and EQE as a function of light power density at V g = 60 V and -60 V under 635 nm illumination.

Figure 5 .
Figure 5. a) Extracted responsivity varies for 635 nm with V g in the range from -60 to 60 V. b) Positive and negative photo response at a wavelength of 635 nm after applying and releasing b) a transient pulsed V g of 60 V and c) a transient pulsed V g of -60 V. d) Time-resolved photo response curves of the phototransistor after releasing V g under varied light power densities at V ds = 0 V under 635 nm illumination.e) R and I ph as a function of light power density at V ds = 0 V under 635 nm illumination.f) D* and EQE as a function of light power density at V ds = 0 V under 635 nm illumination.

Figure 6 .
Figure 6.a) Schematic diagram of the polarized measured system.b) Polarized angle-dependent time-resolved I ds curves at V ds = 0 V under V g = 0 V, 60 V, and -60 V. c) Polarization sensitive photocurrent as a function of the polarization angle in the polar coordinates under linear-polarization laser of 635 nm at V ds = 0 V under different gate voltage.d) Single-pixel imaging of the photodiode at V ds = 0 V and V g = 0 V. e) Schematic diagram of the logic optoelectrical switch.e) Switching on-off behavior of the WTe 2 /WS 2 phototransistor by manipulating V g under continuous light illumination.