Air-Stable In-Plane Anisotropic GeSe2 for Highly Polarization

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Air-Stable In-Plane Anisotropic GeSe2 for Highly Polarization-Sensitive Photodetection in Short Wave Region Yusi Yang, Shun-Chang Liu, Wei Yang, Zongbao Li, Yang Wang, Xia Wang, Shishu Zhang, Yun Zhang, Mingsheng Long, Gengmin Zhang, Ding-Jiang Xue, Jin-Song Hu, and Li-Jun Wan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01234 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Journal of the American Chemical Society

Air-Stable In-Plane Anisotropic GeSe2 for Highly PolarizationSensitive Photodetection in Short Wave Region Yusi Yang,†,‡,⊥ Shun-Chang Liu,†,§,⊥ Wei Yang,‡ Zongbao Li,# Yang Wang, Xia Wang,# Shishu Zhang,‡ Yun Zhang,† Mingsheng Long, Gengmin Zhang,‡ Ding-Jiang Xue,†,§,* Jin-Song Hu,†,§,* and Li-Jun Wan†,§ ∥

∥

†

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Beijing National Research Center for Molecular Sciences, CAS Research/Education Center for Excellence in Molecule Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China

#

School of Material and Chemical Engineering, Tongren University, Tongren 554300, China

∥

State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

§

School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 10049, China

KEYWORDS: germanium diselenide; anisotropy; linear dichroic polarization; photodetector; 2D materials ABSTRACT: In-plane anisotropic layered materials such as black phosphorus (BP) have emerged as an important class of two-dimensional (2D) materials that bring a new dimension to the properties of 2D materials, hence providing a wide range of opportunities for developing conceptually new device applications. However, all of recently reported anisotropic 2D materials are relatively narrow-bandgap semiconductors (< 2 eV), and there has been no report about this type of materials with wide bandgap, restricting the relevant applications such as polarization-sensitive photodetection in short wave region. Here we present a new member of the family, germanium diselenide (GeSe2) with a wide bandgap of 2.74 eV, and systematically investigate the in-plane anisotropic structural, vibrational, electrical, and optical properties from theory to experiment. Photodetectors based on GeSe2 exhibit a highly polarization-sensitive photoresponse in short wave region due to the optical absorption anisotropy induced by in-plane anisotropy in crystal structure. Furthermore, exfoliated GeSe2 flakes show an outstanding stability in ambient air which originates from the high activation energy of oxygen chemisorption on GeSe2 (2.12 eV) through our theoretical calculations, about three times higher than that of BP (0.71 eV). Such unique in-plane anisotropy and wide bandgap, together with high air stability, make GeSe2 a promising candidate for future 2D optoelectronic applications in short wave region.

1.

INTRODUCTION

As a new member of two-dimensional (2D) materials, black phosphorus (BP) has attracted significant attention due to its intriguing in-plane anisotropic properties stemmed from the low symmetry of puckered in-plane lattice structure and promise for novel devices.1-7 Inspired by the fascinating properties of BP, many other anisotropic 2D materials with complementary properties to BP, such as some transition metal dichalcogenides (ReS2 and ReSe2),8-10 and group IV monochalcogenides (Ge and Sn monochalcogenides),11-17 have recently emerged. Compared with conventional in-plane isotropic 2D materials such as graphene and MoS2,5 the emergence of these unique anisotropic 2D materials greatly enriches the properties of 2D materials and provides a new degree of freedom to design and modulate electronic and

optoelectronic devices,18,19 having been utilized in integrated digital inverters,10 crystal orientation-induced synaptic devices for neuromorphic diodes,20,21 applications,22 polarization-sensitive photodetectors and so on.23,24 Among these unique applications mentioned above, the application of anisotropic 2D materials in polarizationsensitive photodetection has been considered as an important research field in recent years, being applied to various fields ranging from communication to military applications.25,26 In contrast to conventional linear dichroic photodetectors based on extrinsic geometric effects such as one-dimensional nanostructure, which usually require complicated process to pattern the devices and align channel materials,27,28 the emerging anisotropic 2D materials are intrinsically sensitive to the linear

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polarization of incident light due to their anisotropic nature of crystal structure.23,25 The most typical example is that photodetectors using the well-known BP can distinguish polarized light along different directions over a broad bandwidth.25 Other examples include recent demonstrations of GeSe,12 an isostructural analogue of BP, and ReS2,23 fully displaying the superiority of anisotropic 2D materials for linear dichroism photodetection. But so far, this application has mainly focused on anisotropic 2D materials with relatively narrow bandgap (< 2 eV), and the wide-bandgap 2D semiconductors with in-plane anisotropy are rarely explored. Although narrow-bandgap semiconductors can be directly used to perform short wave photodetection, the costly and complicated low aperture optical system is usually indispensable in order to adjust the photodetecting system to the desired spectral range.29,30 Instead, the wide-bandgap detectors can easily realize short wave detection without extra optical accessories. Consequently, it is highly desirable to explore new anisotropic 2D materials with wide bandgap suitable for short-wavelength polarized photodetection. Germanium diselenide (GeSe2), a typical binary IV−VI chalcogenide with wide bandgap, crystallizes in three phases, including orthorhombic α-GeSe2, monoclinic βGeSe2 and hexagonal γ-GeSe2.31,32 All of the three phases have a layered structure, in which α- and β-GeSe2 are inplane anisotropic due to the low lattice symmetry, while γ-GeSe2 exhibits in-plane isotropic behavior.32 In particular, β-GeSe2 is the most stable form with the lowest energy among the three phases, thereby offering the advantage of easy preparation.32 Meanwhile, β-GeSe2 is an intrinsically p-type semiconductor exhibiting a wide bandgap of 2.7 eV,33,34 and displays a high absorption coefficient of ~ 104 cm-1.35 Additionally, β-GeSe2 shows great stability in ambient environment and is comprised of earth-abundant and environmentally friendly Ge and Se elements.36-38 All these features make β-GeSe2 a new member of anisotropic 2D materials and promising candidate for short-wave photodetection. Until now, however, there is only one study reporting the anisotropic behavior of GeSe2 in terms of its angle-resolved polarized Raman scattering.33 The in-plane anisotropic properties including structural, electrical, optical anisotropies and its relevant applications such as polarization sensitive photodetection have rarely been studied. Here, we introduce β-GeSe2 as a new member of inplane anisotropic 2D materials with a distinct wide bandgap of 2.74 eV from conventional members of this family explored to date. The in-plane anisotropic properties including structural, vibrational, electrical, and optical anisotropies were systematically investigated from theory to experiment. Guided by the anisotropic absorption, a polarization-sensitive photodetector in short wave region was then demonstrated, the first observed linear dichroic photodetection based on GeSe2. Finally, we found that ultrathin GeSe2 displayed great stability in ambient environment, a significant advantage for its practical application. The presented results establish wide-bandgap GeSe2 as a new member of

anisotropic 2D materials, greatly complementing the recently reported anisotropic 2D analogues with narrow bandgaps. 2.

RESULTS AND DISCUSSION

2.1. Material Characterizations. GeSe2 thin flakes were mechanically exfoliated from bulk crystal onto SiO2/Si (300 nm for SiO2) wafer using scotch tape technique due to the weak interlayer bonding. Atomic force microscopy (AFM) was used to characterize the thickness of the flakes. As shown in Figure 1a, the thickness of few-layer GeSe2 was determined as 3.5 nm, corresponding to 6 layers. Raman spectroscopy was carried out to confirm the crystal structure of GeSe2. It can be clearly seen from Figure 1b that the Raman spectra of GeSe2 flakes showed a prominent peak at 210 cm-1, corresponding to the typical Ag mode of β-phase GeSe2.39 Notably, no Raman peaks at 200 cm-1,34 assigned to the Raman vibration mode of α-GeSe2, was observed proving no α-GeSe2 in our exfoliated sample. Raman spectra on GeSe2 thin flakes with different thickness (7.0 nm, 17.5 nm and 24.9 nm) showed that the relative intensity of Raman peak at 210 cm-1 was enhanced with increasing thickness due to the increase of scattering centers in thicker areas (Figure 1b and Figure S1), in good agreement with the previous reports.40 Meanwhile, with the increasing thickness to 17.5 nm and 24.9 nm, there were two emerging Raman peaks at 96 cm-1 and 116 cm-1, also corresponding to the vibrational modes of β-GeSe2.34

Figure 1. (a) AFM image of exfoliated GeSe2 flake transferred onto SiO2/Si substrate. (b)Raman spectra of GeSe2 flakes with different thicknesses, determined by AFM profiles. (c) SAED pattern of a single GeSe2 flake. (d) HRTEM image of a single GeSe2 flake.

High-resolution transmission electron microscopy (HRTEM) and elected-area electron diffraction (SAED) were used to characterize the crystalline quality of exfoliated GeSe2 flakes. The SAED pattern taken from a single GeSe2 flake in Figure 1c exhibited sharp diffraction spots, indicating that the flake was a single crystal with high crystalline quality. Furthermore, the diffraction spots can be also well indexed to monoclinic β-GeSe2.41 Figure


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Journal of the American Chemical Society 1d was the HRTEM image of GeSe2 flake, in which the lattice fringe of (100) was clearly observed and measured to be 0.7 nm, consistent with the monoclinic crystal structure of β-GeSe2.42 We further applied energy dispersive X-ray spectroscopy (EDS) mapping to probe the spatial distribution of different compositional elements. As shown in Figure S3, the Ge and Se elements exhibited no apparent separation or aggregation and were distributed uniformly within the flake. Moreover, the Ge:Se atomic ratio determined through EDS was very close to 1:2, in good agreement with the stoichiometry of GeSe2 (Figure S3b). In brief, the above material characterizations gathered from AFM, Raman, HRTEM, SAED and EDS confirmed the crystal structure of β-GeSe2 and high crystalline quality of GeSe2 flakes.

Figure 2. Crystal structure of monoclinic GeSe2 from (a) side and (b) top view. (c) Schematic of three-dimensional Brillouin zone of GeSe2. (d) Calculated band structure of bulk GeSe2. Insets in (d) illustrate the side-view structure of GeSe2 showing the stacking of tetrahedrons along y axis (left) and x axis (right), respectively.

2.2. Structural Anisotropy. We now focus on the inplane crystal structure anisotropy of GeSe2 induced by low lattice symmetry. As shown in Figure 2a and 2b, GeSe2 has a monoclinic crystal structure with the P21/c space group and crystallizes in double layers. The two in-plane directions are defined as x and y, and the out-of-plane direction is defined as z. The unit cell dimensions of bulk GeSe2 are a = 7.016 Å (x), b = 16.796 Å (y), and c = 11.831 Å (z), with the interlayer distance being 5.916 Å. Atoms within the layers are covalently bonded, while van der Waals forces along the z-axis separate the layers.43 The basic building blocks of GeSe2 are GeSe4 tetrahedra, which are mutually connected via corner-sharing forming (GeSe4)n chains along the x-axis and via edge-sharing Ge2Se8 double tetrahedra along the y-axis (Figure 2d insets),44 thus forming a layered structure with two inequivalent directions within the in-plane lattice. The different connection modes along the x-axis and y-axis

lead to low in-plane lattice symmetry, creating an anisotropic band structure. We then applied theoretical band calculations to study the band dispersion anisotropy of GeSe2. As shown in Figure 2d, the bands along the Γ-X and Γ-Y directions had different evolving slopes. The valence band along the ky direction was a relatively flat band and gave a larger hole effective mass of ~1.562 m0, compared to that along the kx direction with a hole effective mass of ~0.755 m0. It thus directly reflected the electronic anisotropy along the Γ-X and Γ-Y directions while approximately implied that the mobility ratio of 𝜇" /𝜇$ might be estimated to about 2.1. Therefore, GeSe2 offers an ideal material for studying 2D materials with low symmetry. 2.3. Vibrational Anisotropy. To investigate the vibrational anisotropy of GeSe2, angle-resolved polarized Raman spectroscopy (ARPRS) was carried out, which was an ideal method to elucidate the anisotropic light-matter interaction in low symmetry materials.7,45 The polarized Raman spectra were recorded in a backscattering geometry using a 633 nm laser with a fixed linear polarization. The scattered light polarization was taken along the direction parallel to the incident laser polarization through inserting a linear polarizer in front of the detection system. We chose the range of Raman shifts between 160 and 240 cm-1, since the strongest Ag mode located at 210 cm-1 occur in this frequency range, making it convenient to analyze the angle dependence. Figure 3b showed the contour color map of Raman spectra at different rotation angles in steps of 30 o during measurement. It was clear that the intensities of the Raman mode varied periodically with the sample rotated from 0o to 180 o, which was maximized at 0o and 180 o while minimized at 90 o (Figure S4). These observations revealed the close dependence between incident light polarization and crystalline orientation.

Figure 3. (a) Optical image of GeSe2 flake with four pairs o electrodes spaced at 45 apart. (b) Contour color map of the angle-resolved polarized Raman spectra of GeSe2 flake. (c) -1 Polar plot of the Raman peak intensity of Ag mode at 210 cm as a function of rotation angle. (d) Polar plot of the angleresolved normalized conductance.



Figure 4. (a) Absorption spectrum of GeSe2 flake. (b) Tauc plot for GeSe2 flake. (c) Calculated absorptance along the x axis and y axis. (d) 3D view of polarization-resolved absorption spectra of GeSe2 flake. (e) Schematic structure of the GeSe2 photodetector. (f) Polar plot of the normalized angle-resolved photocurrent as a function of the polarization angle.

To quantitatively understand the periodical changes in Raman intensities, we applied the classical Raman selection rules, where the Raman scattering intensity (I) is proportional to 𝑒& ∙ 𝑅 ∙ 𝑒) * , in which 𝑒& and 𝑒) are the unit vectors of incident and scattered light, respectively, and R is the Raman tensor.12,17 After defining θ as the angle of incident polarization relative to y axis, the incident light polarization can be expressed as 𝑒& = (cos 𝜃 , 0, sin 𝜃) , while the 𝑒) = (cos 𝜃 , 0, sin 𝜃) and (− sin 𝜃 , 0, cos 𝜃) under parallel or cross polarization configurations, respectively. For the monoclinic phase (P21/c space group), the Raman tensor R of Ag mode can be described as 𝑎 𝑑 0 R(𝐴9 ) = 𝑑 𝑏 0 0 0 𝑐 where a, b, c, d are constants of the Raman tensor.33 Then, the angular dependence of Raman intensity of Ag mode can be further written as 𝐼 ∝ (a 𝑐𝑜𝑠 * 𝜃 + 𝑐 𝑠𝑖𝑛* 𝜃)* under the parallel configuration, showing a 180o variation period. As shown in Figure 3c, the fitted curve based on the above Raman intensity equation matched well with the experimental data. Notably, Zhai et al. have recently reported that Ag mode at 210 cm-1 reached a maximum intensity when the polarization of incident laser was parallel to the y axis of GeSe2 through combined HRTEM and ARPRS measurements on the same GeSe2 flake.33 Combined with this conclusion, we can directly determine the crystal orientation of the flake through our ARPRS measurement, which indicated that the y axis of GeSe2 was along 0o reference direction as depicted by a yellow dashed line in Figure 3a. Briefly, the ARPRS measurement confirms the vibrational anisotropy of GeSe2, and most importantly, it can be used as a simple and nondestructive method to directly identify the

crystalline orientation of GeSe2 flakes, which is benefit to the further study of anisotropic properties of GeSe2. 2.4. Electrical Anisotropy. After identifying the crystal orientation of GeSe2 flake, we applied angle-resolved conductance (ARC) measurement on the same GeSe2 flake characterized in ARPRS to experimentally verify the electrical anisotropy of GeSe2. An exemplary device was shown in Figure 3a. Four pairs 500 nm wide Ti/Au (5 nm/90 nm) electrodes were fabricated on a single GeSe2 flake with a step of 45 o, while the length of channel was 3 μm. Through AFM measurement, the thickness of GeSe2 flake was 58.9 nm (Figure S5). The θ was set as the angle between the measured electrodes and the direction of y axis. The current-voltage (I - V) curves were measured by applying an electric field between each pair of diagonally positioned electrodes separated at 180o apart. It was clear that the device showed good ohmic contact between GeSe2 and electrodes, as evidenced by the nearly linear I V curves shown in Figure S5. On the basis of the I - V data analysis, conductance along four directions can be obtained and then processed to normalized conductance for the convenience of comparing anisotropic electrical property. The angle-resolved normalized conductance was plotted in Figure 3d in polar coordinate, displaying strong anisotropic angular-dependent electrical conductance. The low-field conductivity in an anisotropic material at a certain θ can be expressed as 𝜎H = 𝜎$ 𝑐𝑜𝑠 * 𝜃 + 𝜎" 𝑠𝑖𝑛* 𝜃, where 𝜎" and 𝜎$ are the normalized conductance along the x axis and y axis directions, respectively; θ means the angle relative to y axis direction (0o reference direction), along which both the electric field is applied and the conductance is measured.46 Experiment data was fitted with the above equation and the result was shown in Figure 3d. The calculated maximum conductance ratio


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Figure 5. AFM (a, b) and spatial Raman mappings (c, d) images of exfoliated GeSe2: (a, c) fresh-made; (b, d) stored in ambient air after 30 days. Physisorption and chemisorption processes of an O2 molecule on monolayer (e) BP and (f) GeSe2. was 1.58, slightly smaller than the ratio of effective mass (2.1) along the x axis and y axis directions from our theoretical calculations, probably due to existence of fringing current along the conducting path counteracting against anisotropic transport, thus ultimately underestimating the anisotropic conductance. It should be noted that this ratio was comparable to that of many other reported anisotropic 2D materials such as BP and SnS.5,16 Subsequently, we can directly identify the specific crystal orientation of the GeSe2 flake through ARC measurement, where GeSe2 exhibited its maximum conductance along the x axis direction while minimum conductance along the y axis direction. To conclude, the ARC measurement clearly reveals the large in-plane electrical anisotropy of GeSe2. 2.5. Optical Anisotropy and Linear Dichroism Photodetection. Before studying the optical anisotropy of GeSe2, we first investigated the basic optical properties through unpolarized light absorption spectroscopy on mechanically cleaved GeSe2 flake on a quartz substrate using the microscopic spectrophotometer. The thickness of GeSe2 flake was first characterized as 27.3 nm by AFM measurement (Figure S6). As shown in Figure 4a, the absorbance began to sharply decline at the wavelength of approximately 400 nm, and gradually dropped to almost zero at wavelength shorter than 500 nm. Furthermore, the bandgap of GeSe2 flake was calculated as 2.74 eV through the equation α(𝛼ℎ𝑣)* = 𝐴(ℎ𝑣 − 𝐸9 ) (Figure 4b), consistent with previously reported values for bulk GeSe2.34,39 We also predicted the optical absorption spectra of bulk GeSe2 through calculating the dielectric

function. Two absorption spectra were shown in Figure 4c for light incident along the z axis and linearly polarized in x and y axes, respectively, demonstrating an obvious linear dichroism. As evidenced by our theoretical band calculations, the anisotropic electronic structure of GeSe2 can induce the anisotropy of dielectric function, which leads to variations in real and imaginary parts of the complex refractive index as a function of the crystalline direction, thus resulting in the optical anisotropy of GeSe2. In order to experimentally verify the optical anisotropy of GeSe2, we then carried out polarizationdependent absorption measurement on the same GeSe2 flake. As shown in Figure 4d, anisotropy of absorption was observed by changing the polarization angle from 0o to 180o at steps of 30o, indicative of a linear dichroism behavior. This optical absorption anisotropy offered us a great opportunity to further exploit GeSe2 in linearpolarization-sensitive photodetector. As schematically shown in Figure 4e, two-terminal photodetector were fabricated on a single GeSe2 thin flake. The comparison between dark current and photocurrent under a 450 nm unpolarized incident light indicated an obvious photoresponse (Figure S7a). The cyclability of photodetector was further measured under the same illumination conditions with a bias of 10 V, demonstrating a good stability (Figure S7b). This obvious photoresponse of GeSe2 photodetector prompted us to study the polarization sensitive detection of the device. As shown in Figure 4e, the polarization of the incident light (450 nm) was controlled through a polarizer and half-wave plate. A



fixed bias of 10 V was applied between the source and drain electrodes. The angle-resolved normalized photocurrent as a function of the polarization angle was plotted in Figure 4f in polar coordinate. It was clear that the photocurrent changed dramatically through rotating the polarization of light, exhibiting a high dichroic ratio of 3.4 in the device, comparable to that of many other reported anisotropic 2D materials such as BP and ReS2 in other wavelengths (Table S1).23,25 Notably, short-wave photodetectors have been a hot topic recently due to their wide commercial and military applications.29 For instance, pulsed blue and green laser (450 nm~530 nm) has important applications in underwater detection and communication. The wide bandgap of GeSe2 (2.74 eV) makes it an ideal material for this band photodetection. In particular, compared with other reported polarizationsensitive photodetectors based on anisotropic 2D materials, which usually have narrow bandgaps lower than 2 eV such as BP and ReS2,23,24 our wide-bandgap GeSe2 can be directly used in the photodetection without any costly and complicated low aperture optical accessories, demonstrating the unique potential of GeSe2 for short-wave photodetection. In short, the above results fully demonstrated the polarization-dependent anisotropic absorption of GeSe2 and the resulting linear dichroic photodetection in the short wave region. 2.6. Stability Investigation. Finally, we estimated the stability of GeSe2 in ambient conditions, which was very crucial for the practical application. Although BP has raised much attention due to its attractive electronic and optoelectronic properties, its air instability remains to be solved, hampering its development for practical devices.47,48 In contrast, GeSe2 thin flakes exhibited excellent long-term air stability through our systematic characterization of AFM and Raman spectroscopy. As shown in AFM images of Figure 5a and b, the morphology of GeSe2 thin flake displayed no change after 30 days aging in air. Similar results were also observed when comparing the spatial Raman mapping images of fresh exfoliated GeSe2 and aging sample. Moreover, we can observe that the I - V curve of the aging GeSe2 device exhibited slight difference after 30-day air exposure compared with the fresh device, demonstrating excellent air stability (Figure S8). To further reveal the mechanism of this excellent air-stability, we performed DFT calculations to study the oxidation behaviors of monolayer GeSe2 while comparing with that of BP. As shown in Figure 5e and f, the physisorption and chemisorption processes of an O2 molecule on monolayer GeSe2 and BP were simulated, respectively. The binding energy (𝐸N&OP ), representing the interaction between O2 and a monolayer, can be described as 𝐸N&OP = 𝐸QRQST − 𝐸URORTS$VW − 𝐸XY , where 𝐸QRQST refers to the energy of monolayer 2D material absorbing an individual O2 molecule; 𝐸URORTS$VW and 𝐸XY mean the energies of pristine monolayer and an individual O2 molecule, respectively. The black line segment of the left part means the energy level of the initial state, while black line segments of the middle and right parts represent that of the transition

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state and final state, with each corresponding atomic structures beside the energy level. From the above equation, we could conclude that 𝐸N&OP below zero implied the exothermic absorption of O2 molecule. At the initial stage of the absorption, physisorption process can be observed, and the corresponding binding energy between an O2 molecule and monolayer were -0.61 eV and -0.15 eV for GeSe2 and BP system, respectively. As the O2 molecule got closer to the monolayer, the O2 molecule underwent a transition from physisorption to chemisorption by dissociating O2 molecule into two O atoms. The dissociative reaction of O2 on GeSe2 and BP were exothermic releasing 0.4 eV and 4.17 eV, indicating GeSe2 and BP could be oxidized in air. However, from the reaction barrier calculation, we can see that there existed an activation energy for the transition from physisorption to chemisorption. The activation energy for BP was only 0.71 eV, consistent with previously reported value through theoretical calculation,49 while 2.12 eV for GeSe2, about three times higher than that of BP, probably due to the highest valence state of Ge in GeSe2. The above calculated results suggested that the oxidation of GeSe2 would not happen at room temperature in ambient air while BP was more vulnerable to oxidation. Therefore, our detailed experimental and theoretical investigation indicated the excellent air stability of GeSe2, demonstrating its great potential for further studies and practical applications. 3.

CONCLUSIONS

In summary, we have introduced a new member of inplane anisotropic 2D materials, GeSe2 with a wide bandgap of 2.74 eV, providing a powerful complement to conventional members of this family with narrow bandgaps such as BP (0.3 eV), GeSe (1.14 eV) and ReS2 (1.5 eV). GeSe2 exhibited strong in-plane anisotropic behaviors in structural, vibrational, electrical, and optical properties. Polarization-sensitive photodetectors based on GeSe2 displayed competitive performance with a high dichroic ratio of 3.4. More importantly, GeSe2 showed excellent long-term stability under ambient conditions. All these results make GeSe2 a very promising 2D material candidate for future optoelectronic applications in short wave region.

ASSOCIATED CONTENT Supporting Information. Experimental section and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ORCID Ding-Jiang Xue: 0000-0002-7599-0008 Jin-Song Hu: 0000-0002-6268-0959

Author Contributions


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These authors contributed equally.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21573249, 61671022), the National Key Project on Basic Research (2015CB932302), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100) and the Youth Innovation Promotion Association CAS (2017050). The authors thank Prof. Jin Zhang for the assistance of material characterization and useful discussion.

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