Tunable Ambipolar Polarization-Sensitive Photodetectors Based on

Tunable Ambipolar Polarization-Sensitive Photodetectors Based on High-Anisotropy ReSe2 Nanosheets

Enze Zhang,†,⊗ Peng Wang,‡,⊗ Zhe Li,† Haifeng Wang,§,∥ Chaoyu Song,† Ce Huang,† Zhi-Gang Chen,⊥ Lei Yang,⊥ Kaitai Zhang,† Shiheng Lu,† Weiyi Wang,† Shanshan Liu,† Hehai Fang,‡ Xiaohao Zhou,‡ Hugen Yan,† Jin Zou,⊥,& Xiangang Wan,§,∥ Peng Zhou,*,# Weida Hu,*,‡ and Faxian Xiu*,† †

State Key Laboratory of Surface Physics and Department of Physics, Collaborative Innovation Center of Advanced Microstructures and #State Key Laboratory of ASIC and System, Department of Microelectronics, Fudan University, Shanghai 200433, China ‡ National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China § National Laboratory of Solid State Microstructures, School of Physics and ∥Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ⊥ Materials Engineering and &Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia S Supporting Information *

ABSTRACT: Atomically thin 2D-layered transition-metal dichalcogenides have been studied extensively in recent years because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe2 is an emerging material that exhibits a stable distorted 1T phase and strong in-plane anisotropy due to its reduced crystal symmetry. Here, the anisotropic nature of ReSe2 is revealed by Raman spectroscopy under linearly polarized excitations in which different vibration modes exhibit pronounced periodic variations in intensity. Utilizing high-quality ReSe2 nanosheets, top-gate ReSe2 field-effect transistors were built that show an excellent on/off current ratio exceeding 107 and a well-developed current saturation in the current−voltage characteristics at room temperature. Importantly, the successful synthesis of ReSe2 directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 500 times and the hole mobility over 100 times at low temperatures. Strikingly, corroborating with our density-functional calculations, the ReSe2-based photodetectors exhibit a polarizationsensitive photoresponsivity due to the intrinsic linear dichroism originated from high in-plane optical anisotropy. With a back-gate voltage, the linear dichroism photodetection can be unambiguously tuned both in the electron and hole regime. The appealing physical properties demonstrated in this study clearly identify ReSe2 as a highly anisotropic 2D material for exotic electronic and optoelectronic applications. KEYWORDS: ReSe2, ambipolar, anisotropy, linear dichroism photodetection ince the discovery of graphene,1 atomically thin 2D materials have been widely studied owing to their striking physical properties and promising applications in spintronics,2 electronics,3 and optoelectronics.4,5 Among them, the most studied systems include transition-metal chalcogenides (TMDs)6−8 and hexagonal boron nitride (hBN).9,10 Recently, 2D materials with strong in-plane anisotropy such as black phosphorus have received surging research interest due to their anisotropic electrical, optical, and phonon properties.11−13 Monolayer black phosphorus has been

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© 2016 American Chemical Society

shown to have highly anisotropic and robust excitons with a very large binding energy.14 Experiments also reveal that black phosphorus exhibits very different thermal conductivity along its crystal “zigzag” and “armchair” direction.15,16 Moreover, its photodetectors using ionic-gel gated vertical p−n junctions display a broadband and fascinating liner dichroism photoReceived: June 23, 2016 Accepted: July 29, 2016 Published: July 29, 2016 8067

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Figure 1. Characterizations of ReSe2 nanosheets grown by CVD. (a) (Top) Top view of the crystal structure of ReSe2. Red dashed line, a-axis and b-axis of ReSe2 crystal. (Bottom) side view of monolayer ReSe2. (b) Optical images of ReSe2 nanosheets with different thicknesses and shapes: (i−iii) samples grown on SiO2, (iv) samples grown on hBN. Scale bar, 5 μm. (c) TEM image of ReSe2 nanosheets. Scale bar: 500 nm. (d) High-resolution TEM image. Scale bar: 1 nm. Inset: corresponding electron diffraction pattern. (e) Raman spectra of the ReSe2 on hBN and SiO2 substrates. There are small shifts (less than 2 cm−1) of the peak position to the negative direction as the thickness increases. (f) Polar plots of the extracted angular-dependent Raman intensity of peak 238 cm−1 under “only incident”, “parallel”, and “perpendicular” configurations. Inset: schematic measurement configuration of the Raman measurements. The “only incident” situation is where only the incident light polarization direction changes (PI). β is the angle between the polarization direction of the incident light and scattering light, β = 0° and 90° correspond to parallel (PI∥PS) and perpendicular(PI⊥PS) configuration, respectively. Red solid line, a-axis and b-axis of ReSe2 crystal.

detection capability.17 However, discovery of such anisotropic members in various 2D families is still in its early stage, and the exploration of material candidates and applications based on the anisotropy nature of layered materials has certainly rendered practical significance that is widely recognized and anticipated.6,15,18

Compared to other hexagonal TMDs such as MoS219 and WS220 which are isotropic in crystals, ReX2 (X denotes Se or S) has a stable distorted 1T phase in which the underlying 1D arrangement of Re chains lead to a strong in-plane anisotropy.18,21 In addition, the structural distortion gives rise to much weaker interlayer coupling and a lack of interlayer registry that makes its bulk material behave electronically and 8068

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ACS Nano vibrationally as decoupled monolayers.22 Layer-dependent Raman spectroscopy based on ReS222−24 has shown nearly no dependence of Raman vibration modes on the number of layers due to the weak interlayer coupling. Few-layer ReS2 has been incorporated into digital inverters,25 field-effect transistors (FETs),26 and photodetectors.27,28 Although the ReS2 photodetector shows very high external quantum efficiency, its relatively slow photoresponse hinders its applications in highspeed photodetection. ReSe2, as its twin sister, has been predicted to exhibit dramatic spatial-anisotropy optical response.29 The 1D cluster of Re chains in the ReSe2 crystal makes it optically biaxial, creating numerous possibilities for conceptually anisotropic optoelectronic and nanomechanical devices.30−32 Previously, absorption studies have shown that bulk ReSe2 exhibits in-plane anisotropic indirect bandgaps ranging from 1.17 to 1.20 eV for electric fields parallel and perpendicular to the direction of the Re chains, respectively.33 Recently, few-layer ReSe2 was demonstrated to exhibit optical anisotropy in its van der Waals plane,34 and it can be used for FETs35,36 and photodetectors,37,38 but yet, the limited device performance and inaccessibility of polarization-sensitive photodetection make it impossible to fully realize attributes of the high anisotropy in this material. Here, we report the successful synthesis of ReSe2 nanosheets onto SiO2 and hBN substrates via chemical vapor deposition (CVD). By performing angular-dependent Raman spectroscopy on ReSe2 nanosheets, the strong in-plane anisotropy nature of ReSe2 is unveiled. With the measurement polarization configuration being parallel or perpendicular, the intensity of different vibration modes in ReSe2 exhibits a pronounced periodic variation (90° or 180°). Top-gate FET devices based on few-layer ReSe2 show an excellent on/off current ratio exceeding 107 and a stable current saturation behavior. Moreover, back-gate FET devices based on ReSe2 directly synthesized on hBN substrates have been successfully fabricated. Due to an atomically flat surface without dangling bonds and charge impurities on hBN, the field-effect mobility of the ReSe2 devices has been greatly enhanced over 500 times for electrons and over 100 times for holes at low temperatures. Furthermore, the ReSe2-based photodetectors possess a reasonable high-speed photoresponse time down to 2 ms, based on which the polarization-dependent photocurrent mapping can be acquired. The devices also allow for a gatetunable polarization-sensitive photoresponsivity both in the electron and hole regime due to the high in-plane optical anisotropy.

a transmission electron microscopy (TEM) image and shows typical ReSe2 nanosheets. Figure 1d and its inset are the corresponding high-resolution TEM image and selected area electron diffraction pattern that verify the single-crystalline characteristics. Figure 1e displays the unpolarized Raman spectra of ReSe2 grown on SiO2 and hBN substrates with different thicknesses. A small shift of the peak position is witnessed as the sample thickness increases (for details, see Figure S1). However, due to the decoupled lattice vibrations between the adjacent layers, all the shifts are less than 2 cm−1, which is similar to that in ReS2.23 Also, the anisotropy of ReSe2 can be studied by angular-dependent Raman spectroscopy, where the sample can be rotated with respect to the incident polarized laser beam. α in the Figure 1f inset is defined as the angle between the crystal edge and the incident light. A polarizer is placed in front of the spectrometer to select the scattered light polarized parallel (β = 0° in Figure 1f inset) or perpendicular (β = 90°) to the incident light. Figure 1f displays the extracted Raman intensity of peak 238 cm−1 of a 7 nm thick ReSe2 nanosheet at varying angles α (see Figure S3j−l for another sample with a thickness of 5 nm). Three measurement configurations show periodic intensity changes with a strong angular dependence, illustrating direct evidence of an anisotropic crystal23 (Figure 1f and Supporting Information, Figures S2 and S3). The situation where only the incident light polarization changes (PI in Figure 1f) yields a two-lobed shape, while the parallel (P I ∥PS ) and perpendicular (P I⊥PS ) configurations exhibit a four-lobed shape. Here PI and PS represent the polarization of the incident and scattered light, respectively. To understand the anisotropic Raman behavior of ReSe2, we consider the Raman tensor R which corresponds to the light polarized in the layer plane. With the incident light perpendicular to the sample layer plane, the Raman tensor of ReSe2 can be written as34

⎡a d⎤ R=⎢ ⎥ ⎣d b ⎦

(1)

where a, b, and d are constants for a specific mode and can be obtained by fitting to the experimental data. For a given active Raman mode, the observed scattered light intensity is denoted by the equation23 I ∝ |eiRes|2, where ei and es are the polarization vectors of the incident and scattered light, respectively. After performing the transformation to the Raman tensor, we can derive equations to fit the Raman intensities under these three circumstances (Supporting Information, section 2 and Figure S2). As shown in Figure 1f, the equations fit well to the experimental results. The intensity of these Raman modes reaches a local maximum or minimum value as α varies, which is consistent with the scattering intensity calculated by the Raman tensors. It should be noted that all of the Raman peak positions do not change as the polarization of the incident light varies in all three configurations (Figure S2). In addition, different Raman modes have different polarization directions at their maximal peak intensity due to different dominating phonon vibration directions (Figure S3).40 All of the above results further demonstrate the optical in-plane anisotropic nature of ReSe2, and importantly, by carefully analyzing the angle-dependent intensity change of different Raman peaks, the crystal direction of as-grown samples can be successfully verified: our CVDsynthesized samples always crystallize with well-defined edges

RESULTS AND DISCUSSION Few-layer ReSe2 nanosheets were synthesized onto hBN/SiO2/ Si or SiO2/Si substrates in a CVD tube furnace using ReO3 and selenium powder as the source materials (see the Methods). Typically, ReSe2 crystallizes in a distorted 1T structure and the cluster of Re4 units forms a 1D chain inside each monolayer (purple bonds in Figure 1a). There are two principle crystal axes corresponding to the shortest (b-axis) and second-shortest axis (a-axis) in the basal plane (red dash arrows in Figure 1a). The angle between a-axis and b-axis of the ReSe2 crystal is either 61.09° or 118.91°, and the b-axis corresponds to the direction in which the 1D atomic chain forms.39 Figure 1b depicts the optical images of as-grown ReSe2 nanosheets on hBN/SiO2/Si or on SiO2/Si substrates. Different thicknesses can be verified through optical contrast and precisely determined by atomic force microscopy (AFM). Figure 1c is 8069

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Figure 2. Top-gate field-effect transistors based on few-layer ReSe2. (a) Schematic structure of ReSe2 top-gate FET. (b) Small-range output characteristics (IDS − VDS) of the device under different VTG, Inset, a SEM picture and IDS − VDS of the device under different VBG. The thickness of ReSe2 nanosheet is 3 nm. Scale bar, 4 μm. (c) IDS − VDS curves in a larger voltage range showing saturation behavior under various VTG. (d) Room-temperature transfer curves (IDS − VTG) of the ReSe2 top-gate FET. An on/off current ratio of 107 at VDS = 3 V is achieved. Inset, IDS − VBG of the device obtained at VDS = 50 mV.

along the b-axis or a-axis (see the Supporting Information, section 3, for details).30 We first studied the anisotropic conductivity of the ReSe2 by angular-dependent field-effect mobility measurements. As anticipated and also confirmed by our calculations, the maximum field-effect mobility of two-terminal back-gate devices occurs when the electric field is parallel to b-axis while the minimum one develops along the a-axis (Figure S4 and Table S1). In seeing this, we intentionally made the sourcedrain contacts oriented along b-axis of the ReSe2 crystal when fabricating top-gate FETs in order to achieve the maximum mobility value. Figure 2a shows a schematic device structure of the top-gate FET based on a 3 nm thick ReSe2 nanosheet on SiO2. A 30 nm thick Al2O3 layer was deposited by atomic layer deposition (ALD) serving as a high-κ gate dielectric material. A scanning electron microscopy (SEM) picture of the device is displayed in the inset of Figure 2b. The electrical conduction of the ReSe2 FET was probed by current−voltage (I−V) measurements under the modulation of top-gate voltage (VTG) or back-gate voltage (VBG) at room temperature. As shown in Figure 2b, the source−drain current IDS varies linearly with VDS under different VTG or VBG (the inset of Figure 2b), indicating a well-developed contact.41 As VDS increases, a welldefined current saturation behavior can be observed under different VTG (Figure 2c). The saturation behavior occurs in the top-gate ReSe2 devices because of the pinch-off effect of the conducting channel at the drain side under high VDS.42 Such a

well-developed current saturation is crucial for potential applications in devices that are required to be operated in the saturation region such as radiofrequency devices or thin film transistors in organic light-emitting diode displays.11 The transfer curves of the device (IDS − VTG) can be obtained by sweeping VTG while keeping the back gate floating (Figure 2d). At VD = 3 V, a maximal on/off current ratio of more than 107 is obtained, which is significantly larger than the required 104 for CMOS logic devices.26 Note that we also display the back-gate IDS − VBG transfer curves in the inset of Figure 2d. Meanwhile, a subthreshold swing of 850 mV per decade is extracted. This value is of the same order of magnitude as the reported MoS2 FET devices.41 An optimized subthreshold swing could be further obtained by using gate dielectric with a larger dielectric constant like HfO2.43 Using the equation42 μ = (dIDS/dVTG)[L /(WCiVDS)]

(5)

the field-effect mobility of the top-gate device can be extracted to be 10 cm2 V−1 s−1, where L = 2 μm and W = 3 μm are the channel length and width, respectively. Ci is the capacitance per unit area between the top gate and the ReSe2 channel (Ci = ε0εr/d; εr = 6.5; d = 30 nm). It should be noted that the scattering of the carrier mobility could also be affected by the substrates in addition to the anisotropy of the crystal as we will discuss below. Intentionally designing the electrode’s orientation presents one way to maximize the device performance while another 8070

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Figure 3. Temperature-dependent transport properties of four-terminal few-layer ReSe2 FETs on SiO2 and hBN substrates. (a) Schematic structure of ReSe2 back-gate FET on SiO2 substrate. (b) Temperature-dependent sheet conductance versus back-gate voltage showing an evident ambipolar behavior of the device on SiO2 (285 nm) substrate. The channel is 6 nm thick. Inset: optical image of the FET device. Scale bar, 5 μm. (c) Extracted four-terminal field-effect electron and hole mobilities versus temperature for the device on SiO2 substrate. (d) Schematic structure of device on hBN/SiO2 substrate. (e) Temperature-dependent sheet conductance versus back-gate voltage of the device on hBN/SiO2 (7 nm/285 nm) substrate with channel thickness of 7 nm. Inset: optical image of the sample before and after the electrode fabrication. Scale bar, 4 μm. (f) Extracted four-terminal field-effect electron and hole mobilities versus temperature for the device on hBN/ SiO2 substrate.

channel was initially switched from the “on” to the “off” state, and then it was turned on again at the negative gate voltages, exhibiting a clearly ambipolar behavior, which suggests different Fermi levels of our CVD samples compared to previously exfoliated ones from bulk crystals.35 The corresponding temperature-dependent electron and hole mobilities of the devices can be extracted using the field-effect mobility formula, as summarized in Figure 3c,f. Compared with the device directly on SiO2, a drastic increase of mobility of both electrons and holes has been found when hBN is used as the substrate. Similar phenomena were also reported in exfoliated monolayer MoS2 encapsulated in a high-κ HfO2 dielectric environment.44 Remarkably, the electron and hole mobilities of these devices show very different behavior at low temperatures. The electron and hole mobilities of the SiO2-based device decrease monotonically as the temperature decreases, while the mobilities of those devices on hBN remain nearly unchanged. Consequently, the direct synthesis of ReSe2 on hBN has significantly improved the electron motility over 500 (3) times and hole mobility over 100 (60) times at low (room) temperature. It should be noted that the decrease of the mobility in the SiO2-based device is attributed to the scattering from the charged impurities.44,46 Therefore, the lack of the charged impurity scattering on the hBN surface is accounted for the enhancement of mobility at low temperatures. It should be noted that the demonstration of TMD field-effect transistors that are directly synthesized onto hBN substrates and the effective enhancement of the mobility suggest a general route of

approach is through substrate engineering.44 Owing to its ultraflat surface without dangling bonds and charged traps, hBN has been proven to be an excellent gate-insulating substrate for graphene10 and black phosphorus.45 Here, we aim to probe the intrinsic transport properties of ReSe2 by building back-gate FET devices based on the samples directly synthesized onto hBN substrates. As a comparison, two types of four-terminal back-gate FETs were fabricated onto different substrates. For the samples grown on SiO2/Si, e-beam lithography (EBL) and metal deposition were performed to build the electrical contacts. For the samples grown on hBN/SiO2/Si, the thickness of hBN was examined by AFM before the fabrication of metal contacts, as shown in the inset of Figure 3e. Parts a and d of Figure 3 display a schematic device structure of two kinds of devices; correspondingly, the optical images are provided in the insets of Figure 3b,e. The back-gate voltage was applied on the degenerately doped silicon to tune the Fermi level of ReSe2 with SiO2 (285 nm) or hBN/SiO2 (7/285 nm). Both hBN and SiO2 serve as the gate dielectric. For these types of devices, the conductance was measured in the four-probe configuration to avoid complications from the electrical contacts. Typically, conductance was defined as44 G = IDS/ (V1 − V2), where IDS is the source−drain current and V1 − V2 is the measured voltage drop between the middle two voltage probes. Parts b and 3 of Figure 3 display the logarithmic plots of temperature-dependent conductance as a function of VBG of the devices built on SiO2 and hBN substrates, respectively. When the gate voltage was swept from positive to negative, the 8071

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Figure 4. Polarization-sensitive photodetectors based on ReSe2 nanosheets. (a) Schematic structure of ReSe2 photodetectors. The laser was set to pass through a linear polarizer, and then a half-wave plate was used to change the polarization direction, θ here stands for angle between the laser polarization direction and b-axis of ReSe2. (b) IDS − VDS of the photodetector with incident laser power PIN changes from 0 to 20 μW and the corresponding power density ranging from 0 to 0.25 μW/μm2. Inset: small range of IDS − VDS with different PIN. (c) Time-resolved photoresponse of the device under different VDS and at PIN = 25 μW, corresponding to a power density of 0.32 μW/μm2. (d) Photoresponse time of the device. (e) SEM image of the ReSe2 photodetector; the source-drain electrodes are along b-axis of the ReSe2 crystal. θ is the same angle as shown in (a). Scale bar, 5 μm. (f) Polarization-dependent photocurrent mapping of the device, showing prominent linear dichroic photodetection. The thickness of the ReSe2 channel is 12 nm.

carriers as the light intensity increases.50 An oscilloscope was used to measure the time-resolved photoresponse with the laser being switched on and off. The device exhibits a stable and repeatable response to the laser illumination with different VDS (Figure 4c). The response speed of the device to the laser excitation is determined by the rise in photocurrent upon turning on the laser light and its decay after removal of the incident light (Figure 4d). The device shows a reasonably fast response with a typical τrise = τdecay = 2 ms, which is faster than most TMD-based photodetectors.50−52 The fast photoresponse of ReSe2 photodetectors allows us to perform spatially polarization-sensitive photoresponse mapping of the device. As shown in Figure 4a, a polarizer and a half-wave plate are used to change the angle θ between the polarization direction of incident laser (633 nm) and the b-axis of the sample. A fixed dc voltage (VDS = 1 V) was applied between the source and drain electrodes. Figure 4f displays the large-area photocurrent images for the whole channel under different polarization directions of the incident laser (Figure 4e). We note that the photocurrent signals are originated from the ReSe2 channel rather than the electrodes, eliminating the influence of Schottky barriers.53 The photocurrent reaches its

using hBN as the growth substrate for the optimized device performance. Probing the absorption difference of the light being polarized parallel or perpendicular to an orientation axis is defined as linear dichroism.47 Usually, it requires an anisotropy of the configuration and orientation of material or device structure, where they have either an intrinsically anisotropic crystal structure or extrinsically anisotropic device patterns.48 The Raman spectroscopy of ReSe2 has demonstrated a high in-plane crystal anisotropy which offers a great opportunity to exploit it in linear-polarization-sensitive photodetectors.29 As schematically displayed in Figure 4a, the unpolarized photodetection behavior of the device was probed by a focused laser beam of 633 nm with an illumination power ranging from 1 to 20 μW. The IDS − VDS curves of the device with and without the laser illumination show a systematic quasilinear behavior, indicating well-developed contacts (Figure 4b).49 Compared to the dark current background (the black curve in Figure 4b), IDS shows a significant increase of several orders of magnitude as the device is illuminated. Furthermore, the photocurrent (IPH = Iilluminated − Idark) also shows a strong dependence on the laser power originated from the increased number of photon-generated 8072

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Figure 5. Influence of the back-gate voltage on the linear dichroic photodetection characteristics. (a)Transfer curves (IDS − VBG) of the device under different PIN showing an ambipolar gate tunability. Inset: extracted photoresponsivity under different VBG. (b) Photocurrent mapping of the device with the channel thickness of 12 nm under various VBG at θ = 90° and 0°, where θ = 90° shows a larger increase as the increase of | VBG| than θ = 0°.

maximum value when the incident light is polarized along the b crystal axis (0° polarization) and reaches its minimum at the perpendicular setup (90° polarization). To exclude the possible geometric edge effect17 in the metal−ReSe2 interface, we also fabricated several control samples with semicircle electrodes (Supporting Information, sections 10−12). As expected, the intrinsic polarization-dependent photoresponse indeed comes from the ReSe2 channel itself.48 In addition, we tried to change the wavelength of the incident laser to 520 nm, and it produced similar results (Supporting Information, section 12). These phenomena unambiguously demonstrate that the incident laser with different polarization directions encounters a varying absorption when it travels across the ReSe2 channel, reflecting the intrinsically anisotropic nature of the ReSe2 crystals. Compared to carbon nanotubes54 and CdSe nanowires55 which require a sophisticated nanofabrication process, ReSe2 photodetectors utilize its intrinsic crystal anisotropic properties

and show a much more robust linear dichroism photodetection behavior. To further enhance the polarization-dependent photoresponse of the ReSe2 photodetectors, a back-gate voltage was applied to tune the Fermi level of the ReSe2 channel. Figure 5a displays the ambipolar photoresponse of ReSe2 under several illumination powers, where the channel current is increased for all values of VBG. Moreover, the photocurrent IPH increases as |VBG| increases; this is because the increased |VBG| can tune the Fermi level of ReSe2 closer to the conduction band (or valence band), making it easier for the carriers to overcome the barrier between the channel and electrodes. As a result, the extracted photoresponsivity, defined as the ratio of photocurrent to the incident laser power56 R = IPH/PIN, also shows an ambipolar relation with VBG (the inset of Figure 5a). Correspondingly, the photoresponsivity was improved by 30 times at VBG = −40 V and 300 times at VBG = 40 V. In view of this, we then tried to utilize the back-gate to enhance the linear 8073

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Figure 6. (a) Density functional theory calculated electronic band structures of bulk ReSe2, indicating an indirect bandgap of 1.27 eV. (b) Theoretical calculation of optical absorption spectra of bulk ReSe2, showing an anisotropic in-plane behavior as the polarization direction of incident light varies. Inset: calculated optical absorption spectra of monolayer ReSe2, exhibiting consistent behavior with bulk ReSe2. (c) Experimental polarization-dependent transmission spectra of ReSe2; the incident light is linearly polarized in directions ranging from parallel to perpendicular to the b crystal axis (γ in the inset changes from 0° to 90°). Inset: experimental transmission spectra in a wider range with unpolarized light. The dips in the spectra are located at ∼1.32 and ∼1.45 eV, respectively.

The photocurrent difference near the left and right electrodes in Figure 5b at VBG = 30 V also suggests the development of asymmetric barrier heights11 ( inset of Figure S16d) at such a VBG. To better understand the linear dichroism photodetection behavior of ReSe2, we first performed ab initio calculations on the electronic structure of ReSe2. The calculated band structure for bulk ReSe2 is shown in Figure 6a (monolayer, see Figure S17), where an indirect bandgap of 1.27 eV, defined from conduction band minimum at the Γ point and the valence band maximum located close to the Γ point, is revealed (monolayer, 1.24 eV). In Figure 6b, the optical absorption spectra of bulk ReSe2 were obtained by computing the dielectric function63 (monolayer, see the inset of Figure 6b). As the incident light polarization direction varies along different in-plane directions with respect to b crystal axis, both the bulk and monolayer ReSe2 demonstrate an evident change of absorption rate within the incident photon energy of 1.7−2.4 eV, indicative of a linear dichroism behavior.12,29 Remarkably, our theoretical prediction agrees well with the polarization-dependent transmission measurements carried out on CVD-grown ReSe2 nanosheets (see the Methods for more details). In the regime where the photon energy is larger than 1.32 eV (Figure 6c), the infrared transmission spectrum with incident light polarization parallel to the b-axis (γ = 0°) of the ReSe2 crystal shows a more prominent dip than the transmission of light polarized perpendicular to the b-axis (γ = 90°). It can be seen that there is a noticeable evolution of the transmission spectra when the polarization angle γ changes from 0° to 90°. For instance, the transmission ratio at 1.86 eV changes from 0.66 to 0.98 as γ changes from 0° to 90°. This suggests that more photons have been absorbed with γ = 0° than γ = 90°.17 It should be noted that these results are also consistent with the previously reported polarization-dependent absorption experiments performed on ReSe2 bulk material.33 Figure 6c inset shows the transmission spectra obtained for unpolarized light, in which the curve has dips for photon energies at 1.32 and 1.45 eV, indicating a rather complex optical response.

dichroic photodetection of the ReSe2 photodetectors. Figure 5b shows the photomapping of the device under illumination of 90° and 0° polarization at different VBG. It can be seen from the mapping that there is a pronounced increase of 0° illumination photocurrent as |VBG| increases while the 90° illumination photocurrent is not significantly changed as |VBG| varies. Consequently, the ratio of the photoresponsivities between the two polarization directions of the incident light is enhanced at large |VBG|. It is indispensable to reveal the main photocurrent generation mechanism in ReSe2 photodetectors under various VBG. As for layered-material based photodetectors, there are mainly three kinds of photocurrent generation mechanisms: photoconductive effect, photovoltaic effect, and photothermoelectric effect.57 Here, we employed a VDS = 1 V during the photocurrent mapping measurements. Since there are no opposite directions of current flow and zero photocurrent crossover17 along the channel under VDS ≥ 0.5 V (Supporting Information, section 14), the device works beyond the photothermoelectric regime. Typically for the photoconductive effect, the photocurrent arises from the increase of the photogenerated carriers and a main signature is the linear increase of photocurrent with the incident layer power,58 that is IPH ∝ PIN. While for the photovoltaic effect, the photocurrent arises from the shift of the threshold voltage resulting in the increase of VDS, and generally, the increase of photocurrent with PIN follows a relation59,60 IPH ∝ PηIN, η < 1. From the power dependence of the photocurrent under various VBG, we can capture the dominant mechanism through the power law IPH ∝ PηIN (Figure S16b,c). Figure S16d shows the relation of η as a function of VBG where η ranges from ∼1 (photoconduction) in the hole regime to