Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide

Abstract

Two-dimensional (2D) layered materials, transition metal dichalcogenides and black phosphorus, have attracted much interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to their remarkable optical properties and prospects for new devices. Here, we report the anisotropic optical properties of layered 2D monochalcogenide of germanium sulfide (GeS). Three Raman scattering peaks corresponding to the B3g, A 1 g , and A 2 g modes with strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at around 1.66 eV that originates from the direct optical transition in GeS at room temperature. Moreover, determination of the polarization dependent characteristics of the PL and absorption reveals an anisotropic optical transition near the band edge of GeS, which is also supported by the density functional theory calculations. This anisotropic layered GeS presents the opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties.

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p–n Diodes

“…The R drastically increases with decreasing P , as shown in the inset of Figure c. The nonlinearity is attributed to the presence of defect states either in the GeS channel and/or at the interface between GeS and the metal electrodes consistent with the previous reports on the 2D materials based photodetectors . The obtained R reaches 3.5 × 10 4 A/W, which is much higher than those reported for 2D material HJ photodetectors such as ReSe 2 /MoS 2 , MoS 2 /WS 2 , MoS 2 /BP, MoS 2 /WSe 2 , and SnS/WS 2 .…”

“…Raman peaks are observed at 83.4, 152.5, and 190.4 cm −1 ; these peaks are assigned to the , B 3g , and phonon modes, respectively . The A g and B 3g phonons are shear modes in which adjacent layers move parallel to one another in the armchair and zigzag directions, respectively . Figure d shows the Raman scattering spectrum of MoS 2 .…”

“…Recently, group‐IV monochalcogenides MX (M = Ge, Sn; X = S, Se) have emerged as a new group of 2D layered materials and are attracting increasing attention because of their intriguing properties, which include high carrier mobility, high thermoelectric efficiency, and anisotropic optical response . GeSe is a layered p‐type semiconductor with a distorted rocksalt orthorhombic structure.…”

The Optical Properties and Plasmonics of Anisotropic 2D Materials

“…Thus, the most critical value for determining the hyperbolic regime is not the effective mass ratio, but the anisotropy of the intraband and interband couplings along the two principle axes. In layered anisotropic semiconductor materials, such as BP, BP‐analog materials (e.g., SnSe, SnS, GeSe, and GeS), the 1T phase of TMDCs (e.g., ReSe 2 ), and the trichalcogenides (e.g., TiS 3 ), strong anisotropic in‐plane electronic properties have been demonstrated via anisotropic optical absorptions, polarization‐dependent PL, and anisotropic conductivities, while the low carrier densities (low intensity of intraband transitions) and relatively large bandgap (from the mid‐infrared to the visible spectrum) are against forming strong coupling between the intraband and interband excitations unless the plasmon frequencies are tuned to the vicinity of interband excitations. In such cases, the wave vector for plasmons would be extremely large, which is impractical in experiments.…”

Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe2 and MoSe2

“…They belong to the transition metal dichalcogenide (TMD) family reported by Wilson and Yoffe 1 but, unlike more wellknown TMDs, their structure is highly anisotropic in the layer plane and this is reflected in all their physical properties. [2][3][4] The class of van der Waals layered semiconductors with in-plane anisotropy is a topic of great current interest and, besides ReS 2 and ReSe 2 , it includes black phosphorus, [5][6][7] GeS, 8 transition metal trichalcogenides 9 and Sb 2 Se 3 . 10 Proposed applications of ReX 2 (X = S, Se) include plasmonic materials, 11 polarization-sensitive photodetectors with high sensitivity, [12][13][14][15] inverters, 16 catalytic devices, 17,18 and few-layer field effect 19,20 or heterojunction 21 transistor structures.…”