Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
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Three-Dimensional Optical Anisotropy of Low-Symmetry Layered GeS Zongbao Li,†,∥ Yusi Yang,‡,∥ Xia Wang,† Wei Shi,† Ding-Jiang Xue,*,‡,§ and Jin-Song Hu‡,§ †
School of Material and Chemical Engineering, Tongren University, Tongren 554300, China Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100049, China Downloaded via GUILFORD COLG on July 17, 2019 at 13:28:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: In-plane anisotropic two-dimensional (2D) materials, especially black phosphorus and ReS2, have attracted significant interest recently as they can provide one more dimension to manipulate their physical properties when compared with isotropic 2D materials. As a representative anisotropic 2D material, germanium monosulfide (GeS) has emerged as a new research hot topic in this field because of its unique in-plane anisotropic physical properties. Despite the rapid growing progress in the study of GeS, many of their fundamental optical anisotropies are still absent. Here, we report the three-dimensional (3D) optical anisotropy of GeS from theory to experiment. The 3D optical anisotropic properties including extinction, refraction, absorption, and reflection were systematically investigated through density functional calculations. The anisotropic refraction and reflection of GeS were experimentally verified by polarization-resolved optical microscopy and azimuth-dependent reflectance difference microscopy, respectively. Finally, a GeS-based linear dichroic photodetector was demonstrated with a dichroic ratio of 1.45 because of its polarization sensitive absorption. Our results provide deep insights into the optical anisotropy of GeS, which is important for the further development of GeS-based optoelectronic and optical devices. KEYWORDS: germanium monosulfide, 2D materials, optical anisotropy, birefringence, photodetector
1. INTRODUCTION Two-dimensional (2D) layered materials have attracted wide attention for their potential application in optoelectronics and electronics because of their remarkable optical,1−5 mechanical,6−9 and electrical properties.10−13 In terms of their crystal structures, these materials can be divided into two categories: (i) one with a high in-plane symmetry, such as graphene14 and MoS2;15 (ii) the other with a low in-plane symmetry, including black phosphorus (BP)16−20 and ReS2.21,22 Therefore, besides the fundamental anisotropy between in-plane and out-of-plane directions, there is also a new type of in-plane anisotropy, demonstrating a three-dimensional (3D) anisotropy.23 For example, ReS2 with a low symmetry and very weak interlayer coupling24 shows different thermal conductivities along three directions: (70 ± 18) W m−1 K−1 (along the in-plane Rechains), (50 ± 13) W m−1 K−1 (transverse to the in-plane chains), and (0.55 ± 0.07) W m−1 K−1 (along the out-ofplane).22 This intriguing in-plane anisotropy could bring a new degree to the investigation of 2D materials and provide possibilities for novel optoelectronic and electronic applications that are otherwise impossible with in-plane isotropic 2D materials.25−28 As an important member of IV−VI monochalcogenides, germanium monosulfide (GeS) with a BP analogue-layered © 2019 American Chemical Society
structure possesses an in-plane anisotropic crystal structure originating from the unequal bond angles and strengths along the two in-plane crystal directions, armchair and zigzag.29−37 This low-symmetry structure of GeS can induce in-plane anisotropic physical properties, such as electrical,32 optical,31,33 and vibrational anisotropies,34 and their correlative application such as dichroism photodetection. Additionally, GeS has also demonstrated great potential application in solar cells because of the binary composition, proper band gap (∼1.6 eV),36 and high absorption coefficient (∼105 cm−1).38 Meanwhile, several high-performance photodetectors based on GeS flakes have been reported recently.26,36,37 For example, Matsuda et al. fabricated a GeS-based photodetector with high photoresponsivity (6.8 × 103 A W−1), specific detectivity (5.6 × 1014 jones), and broad spectral response (ranging from 300 to 800 nm).37 However, the current optical investigation of GeS is inadequate, which has only limited the in-plane anisotropic absorption and photoluminescence. The anisotropic optical properties including optical reflection, refraction, and the 3D optical anisotropy are still under exploration. Received: March 28, 2019 Accepted: June 18, 2019 Published: June 18, 2019 24247
DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Crystal structure of GeS from side view. (b) Calculated band structure of bulk GeS. (c) Calculated refractive index n and extinction coefficient k along the three directions. (d) Calculated real and imaginary parts of the dielectric function along the three directions. (e) Calculated absorbance along the three directions. (f) Calculated reflectivity along the three directions.
3. RESULTS AND DISCUSSION 3.1. Theoretical 3D Optical Anisotropy. As a representative layered group IV monochalcogenide, GeS adopted a distorted orthorhombic structure (space group 25−29 Pcmn-D16 as shown in Figure 1a. The two principal 2h) crystallographic axes represented the in-plane armchair (defined as x-axis) and zigzag (defined as y-axis) directions (Figure S1), while the vertical direction along the adjacent layers stacked by van der Waals (vdW) forces was defined as the z-axis.26 It was easy to see that the low lattice symmetry of both in-plane and out-of-plane directions resulted in an anisotropic band structure along the three directions with different evolving slopes, as shown in Figure 1b. Meanwhile, it could be seen that the band gap of GeS was indirect, but the energy difference between the indirect and direct band gaps was very small, which is only about 0.028 eV.37 Thus, GeS offers an ideal 2D vdW material for studying 3D anisotropic properties along the three crystallographic directions. In particular, the experimental typical thickness is of the order of 100 nm because of the strong vdW interaction.23 Consequently, as an important optoelectronic material used for photovoltaics and photodetectors, it is necessary to study the 3D optical anisotropy of GeS. To date, however, there are only a few reports about the in-plane anisotropic optical absorption of GeS, and other optical properties including refraction, reflection, and the 3D optical anisotropy of GeS have not yet been systematically investigated. On the basis of the above theoretical band structure calculations, we then studied the optical anisotropy of GeS. It is well known that the nature of light in an anisotropic medium is usually expressed by the complex refractive indices (N = n + ik) along the principal axes of the medium. Therefore, we first calculated the wavelength-dependent n and k along the three directions through using a scissors of 0.38 eV to get a good agreement between the calculated band gap and the experimental result of 1.6 eV,38 as shown in Figure 1c. The extracted birefringence (Δn) and linear dichroism (Δk) are plotted in Figure S2 (Supporting Information). The calculated results revealed that n and k exhibited large changing behavior
Here, the optical anisotropy of GeS along the in-plane and out-of-plane directions is systematically studied from theory to experiment. Specifically, we first applied density functional theory (DFT) calculations to study the 3D optical anisotropy of GeS, including the anisotropic optical extinction, refraction, absorption, and reflection. Then, the anisotropic optical refraction and reflection of GeS flakes were investigated using polarization-resolved optical microscopy (PROM) and azimuth-dependent reflectance difference microscopy (ADRDM). Finally, based on the in-plane anisotropic optical absorption, GeS photodetectors were fabricated to explore its utility in linear dichroism photodetection. Our theoretical and experimental results provide deep understanding of the optical anisotropy of GeS, promoting the development of GeS-based optoelectronic devices.
2. THEORETICAL AND EXPERIMENTAL METHODS 2.1. DFT Calculations. The crystal structure, band structure, and optical properties were calculated using DFT calculations in conjunction with generalized gradient approximation + Perdew− Burke−Ernzerhof39 as implemented in Vienna ab initio simulation package.40 The energy cutoff for the plane-wave basis was set as 600 eV; Monkhorst−Pack k-point was 8 × 8 × 8;41 maximum force criterion was 0.01 eV/Å, while uniform k-point meshes of 0.003/Å were adopted.42 Moreover, a corrected value of 0.38 eV for the calculated band gap was adopted for a good agreement with the experiment result of 1.60 eV. 2.2. Finite Element Method Calculations. The 3D polarization-resolved absorption spectra of GeS was calculated using COMSOL Multiphysics 5.1 code. The directed light into the GeS flake was set as the linearly incident polarized light with wavelength ranging from 350 to 800 nm along the three directions. The GeS flake was assumed to be a dielectric cube with sides a = b = c = 100 nm. The adopted refractive index n and the extinction coefficient k along the three directions were calculated by DFT. The mesh sizes of the region in GeS cube were set as 1 nm. 2.3. Polarization-Sensitive Photodetection Measurement. In this measurement, a half-wave plate was used to change the polarized direction of the incident laser when the GeS-based device was kept still. The photocurrents of GeS device was recorded by a Keithley 4200 system. 24248
DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a−c) Calculated absorption color maps of three different planes (y−z, x−z, and x−y) as a function of the polarized angle. (d−f) Polar plot of the absorption in different planes at the incident light of 557 nm.
as a function of the wavelength along the three directions, thus demonstrating the obvious 3D optical anisotropy of GeS. Furthermore, through the calculated n and k, the real (ε′) and imaginary (ε″) parts of the complex in the range of 300−900 nm along the three directions were then plotted in Figure 1d. Meanwhile, the calculated absorbance along the three axes displayed different absorption spectra (Figure 1e), directly exhibiting the linear dichroism of GeS. As shown in Figure 1f, the optical reflection of GeS also presented a similar polarization-dependent phenomenon. To further visually describe the linear dichroism of GeS, we displayed the polarization-resolved absorption spectra of GeS in three dimensions through a contour color map (Figure 2a− c). As schematically shown in Figure S3, the absorption of GeS with a100 nm thickness along one defined crystalline axis was obtained using the finite element method. It was obvious that the values of absorption presented a 3D anisotropic character in the range of 500−700 nm. The anisotropies in y−z and x−z planes exhibited a similar tendency while that of x−y plane reversed, indicating the different interaction between the x−y plane (strong covalent bonding) and y−z as well as x−z planes (weak vdW interaction). Furthermore, the absorbances of GeS plotted in polar coordination at the incident light wavelength of 557 nm are shown in Figure 2d−f. The two-lobed shapes indicate the large optical absorption anisotropy of GeS in three dimensions. Briefly, the calculated optical properties including extinction, refraction, absorption, and reflection clearly demonstrate the 3D optical anisotropy of GeS. 3.2. Anisotropic Absorption. We now focus on the optical anisotropy of GeS from the experiment. Our previously reported polarization-resolved absorption spectroscopy (PRAS) method43 was applied to investigate the anisotropic absorption experimentally (Figure 3a). Figure 3b displays the unpolarized absorption spectrum of a mechanically cleaved GeS flake with a thickness of 83.4 nm determined by atomic force microscopy (AFM) measurement (Figure S4). It was clear that the steep absorbance reduction started at a wavelength of about 500 nm and decreased to almost zero at about 800 nm. Furthermore, the band gap of the GeS flake was calculated as 1.59 eV by plotting (αhν)1/2 versus (hν), as
Figure 3. (a) Schematic diagram of the testing principle of PRAS. (b) Absorption spectrum of a GeS flake. The inset is the Tauc plot for the GeS flake. (c) Polarization-resolved absorption spectra with the spectral range 300−1000 nm. (d) Evolution of the absorbance plotted in the polar coordination at a wavelength of 633 nm.
shown in the Figure 3b inset, which is well consistent with previously reported values.38 Then, using the same GeS flake, PRAS measurement was carried out through rotating the direction of probe light’s polarization from 0° to 180° at a step of 30°. Figures 3c and S5 clearly display the anisotropic absorption of GeS, and the evolution of absorbance plotted in polar coordination at a wavelength of 633 nm is shown in Figure 3d, which presents a typical “8” shape, experimentally confirming the linear dichroism of GeS. Furthermore, the evolutions of the experimental and theoretical energy of the absorption edge are also plotted in polar coordination (Figure S6), presenting an in situ visualization of the anisotropic band gap of GeS with the two-lobed shapes. 3.3. Anisotropic Refraction. Before experimentally studying the anisotropic refraction of GeS, vibrational anisotropy of the layered GeS was first investigated using angle-resolved 24249
DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Optical image of a GeS flake for ARPRS and PROM measurements. (b,c) Polar plots of Raman peak intensities of A1g and A2g modes as a function of the rotation angle. (d) Transmitted polarization-resolved optical images of the GeS flake under crossed-polarized light illumination as a function of the rotation angle.
Figure 5. (a) Schematic diagram of the testing principle of ADRDM. (b) ADRDM results of a GeS flake in the region marked with a red spot in (c). (c) Optical image of GeS flakes transferred onto a SiO2/Si substrate by mechanical exfoliation. (d) ADRDM images of GeS flakes as a function of the rotation angle.
armchair or zigzag direction of the GeS flake, consistent with the recent report of Tan et al.36 and its counterpart GeSe reported by Wang et al.44 Thus, we can directly identify the two in-plane crystalline orientations of our tested GeS flake through the ARPRS measurement. In our experiment, the crystalline orientations of the GeS flake were along the x- and y- directions, as shown in Figure 4a. The PROM measurement13 was then applied to investigate the anisotropic refraction on the same GeS flake. We set the initial angle (0°) at where the x-direction of GeS was horizontal (polarization direction of the incident light was parallel to the crystal orientation) and recorded the corresponding optical images through a CCD camera when the sample was rotated at a step of 15°. Figure 4d shows the PROM images of the GeS flake as the sample was rotated, where the polarizer and analyzer were set orthogonal to each other (cross-polarization). When the linearly polarized light
polarized Raman spectroscopy (ARPRS). The details of the experiment is the same as our previous works.13,43 In our experiment, the GeS flake (Figure 4a) with a thickness of 165.5 nm (Figure S7) was rotated every 15° while keeping the polarization of the incident light and the scattered light under a parallel configuration. Figure S8a depicts the evolution of the Raman spectra at different angles, and three active Raman photon modes (A1g, A2g, and B3g) located at ∼240, 270, and 213 cm−1 were clearly observed with a clear intensity change, which agreed well with the previous results.36 Furthermore, the angular dependence of Raman intensities of both the A1g, and A2g modes can be further written as I ∝ (a cos2 θ + c sin2 θ)2 and exhibited a periodic variation of 180°, whereas the B3g mode showed a periodicity of 90°, as presented in Figures 4b,c and S8b. It should be further noted that the intensity of the A1g mode reached a local maximum or minimum value when the polarization direction of the incident light was parallel to the 24250
DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a) Schematic diagram of the GeS photodetector. (b) AFM image of the GeS photodetector. (c) Polar plot of photocurrents as a function of the polarization angle.
can provide us with a more direct comprehend of the anisotropic level in a tested sample by using the imaging function.5 As presented in Figure 5d, the anisotropy contrast in the in-situ visualization of the ARDRM images further revealed the variety by different color exhibition at different rotation angles, while signals from the isotropic SiO2/Si substrate were always zero (green). Therefore, the ADRDM result reveals the anisotropic reflection of GeS, forcefully verifying our theoretical results. 3.5. Linear-Polarization-Sensitive Photodetection. The linear dichroism of GeS discussed above offers us a great opportunity to probe it in a linear-polarization-sensitive photodetector. Figure 6a schematically presents the photodetector fabricated by a single GeS flake with a thickness of 28.7 nm (Figure 6b). A fixed dc voltage of 1 V was used between the drain and source electrodes. A half-wave plate was applied to control the linearly polarized direction of the incident light (633 nm). Figure 6c displays the angle-resolved photocurrent in the polar coordinate as a function of the direction of incident light. By rotating the polarization of the light while fixing the incident power and external voltage, the photocurrent changed dramatically, indicating the polarizationsensitive photodetection with a dichroic ratio of 1.45. Notably, compared to conventional linear dichroic photodetectors that usually required an anisotropy of extrinsic geometric effects such as an one-dimensional nanostructure, the GeS polarization-sensitive photodetector utilized its intrinsic crystal anisotropic properties directly without the sophisticated nanofabrication processes. Meanwhile, the device performance may be further improved through synthesizing high-quality GeS flakes while constructing a heterojunction by combining GeS with other 2D materials to form a type-II band alignment. Hence, the above results prove that GeS can be used as a potential material for linear dichroism photodetection.
went through the crystalline orientations, the polarization direction of the output light was unchanged, which would be still vertical to the analyzer’s polarization direction. Therefore, the analyzer could not detect the light, and the image was dark. Conversely, the transmitted light would be elliptically polarized, and thus the optical contrast can be observed. From the recorded images, it can be seen that the lowest brightness of the GeS sample appeared when the incident light’s polarized direction was parallel to the crystalline orientation at θ = 0° and 90°, whereas the highest brightness appeared at θ = 45° and 135°. This phenomenon was due to the birefringence of anisotropic crystals of GeS, which was also observed in BP18 and ReS2.22 Thus, the PROM measurement clearly demonstrates that the birefringence of GeS originates from its in-plane anisotropic crystal structure, in good agreement with our theoretical results. 3.4. Anisotropic Reflection. After the investigation of anisotropic refraction, we applied ADRDM measurements on GeS flakes to experimentally verify the in-plane anisotropic reflection. In our previous work, the ADRDM has been confirmed to be a direct way to characterize the anisotropic reflection of 2D anisotropic materials.13 The detection principle of the ADRDM is schematically illustrated in Figure 5a and is further defined as13 R − Rn ΔR =2 m = 2N R Rm + Rn
(1)
where ΔR is the normalized reflectance difference along two arbitrary in-plane directions (m and n) of the sample at a perpendicular incidence angle; Rm and Rn are the reflectance intensities along the m and n directions. Meanwhile, the dimensionless value N(θ) can be described as N (θ ) =
R zz − Rac cos 2 (θ − θ0) R zz + Rac
(2)
4. CONCLUSIONS In summary, we have investigated the 3D optical anisotropy of GeS from theory to experiment. DFT calculations demonstrated the anisotropic optical extinction, refraction, absorption, and reflection of GeS along the three axes. The in-plane anisotropies of vibration and absorption were experimentally confirmed by ARPRS and PRAS measurements. The polarized optical microscopy including PROM and ADRDM visually displayed the anisotropic refraction and reflection of GeS. Finally, the polarization-sensitive two-terminal photodetector was fabricated by the GeS flake and exhibited a dichroic ratio of 1.45. This theoretical and experimental investigation of 3D optical anisotropy of GeS will enable the further development of GeS-based optoelectronic and optical devices.
where θ is the rotated angle of the linearly incident polarized light and θ0 is the initial value; Rzz and Rac are the reflectance intensities along the zigzag and armchair crystalline axes of the sample, respectively. From the above equation, it can be readily acquired that N(θ) of anisotropic materials displays a periodical change as the angle of incident polarized light changes, offering a direct way to characterize the anisotropic property of 2D anisotropic materials. As shown in Figure 5b, the ADRDM result of a GeS flake is obtained with the incident polarization rotating from 0° to 180°. The thickness of the GeS flake was 172.1 nm through AFM measurement (Figure S9). It was clear that the N(θ) value reached a maximum at about 150° and a minimum at about 60°, matching well with eq 2. Furthermore, ADRDM 24251
DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
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ACS Applied Materials & Interfaces
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05543.
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Crystal structure of a GeS flake; calculated birefringence (Δn) and linear dichroism (Δk) for wavelengths ranging from 300−800 nm; schematic diagram of absorbance along one direction; AFM images and profiles of a GeS flake in testing of PRAS,; 3D view of polarizationresolved absorption spectra of a GeS flake; evolution of the energy of absorption edges in experiment and DFT calculation plotted in the polar coordination; AFM image and profile of a GeS flake in ARPRS and PROM measurements; angle-resolved polarized Raman spectra of a GeS flake under parallel configuration and polar plots of the Raman-peak intensities of the B3g mode; and AFM image and profile of a GeS flake in testing of ADRDM (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Ding-Jiang Xue: 0000-0002-7599-0008 Jin-Song Hu: 0000-0002-6268-0959 Author Contributions ∥
Z.L. and Y.Y. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21573249, 61671022, and 21875264), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB12020100), the Youth Innovation Promotion Association CAS (2017050), the Natural Science Foundation of Guizhou Province (20161150), and the Provincial Key Disciplines of Chemical Engineering and Technology in Guizhou Province (no. ZDXK20178).
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REFERENCES
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DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
Research Article
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DOI: 10.1021/acsami.9b05543 ACS Appl. Mater. Interfaces 2019, 11, 24247−24253
