Anisotropic Optical and Vibrational Properties of GeS

Abstract

The optical response of bulk germanium sulfide (GeS) is investigated systematically using different polarization-resolved experimental techniques, such as photoluminescence (PL), reflectance contrast (RC), and Raman scattering (RS). It is shown that while the low-temperature (T = 5 K) optical band-gap absorption is governed by a single resonance related to the neutral exciton, the corresponding emission is dominated by the disorder/impurity- and/or phonon-assisted recombination processes. Both the RC and PL spectra are found to be linearly polarized along the armchair direction. The measured RS spectra over a broad range from 5 to 300 K consist of six Raman peaks identified with the help of Density Functional Theory (DFT) calculations: Ag1, Ag2, Ag3, Ag4, B1g1, and B1g2, which polarization properties are studied under four different excitation energies. We found that the polarization orientations of the Ag2 and Ag4 modes under specific excitation energy can be useful tools to determine the GeS crystallographic directions: armchair and zigzag.

Resonant exciton scattering reveals Raman forbidden phonon modes in layered GeS

“…2(b), the Raman active phonon modes A 2 g , B 2 1g and A 3 g are observed in the Stokes range from 1.853 to 1.830 eV. Considering the spectral positions of the Raman lines with respect to the laser excitation energy , the phonon energies correspond to our theoretical calculation and to previous reports [11,29]: 116 cm −1 14.4 meV (A 2 g ), 218 cm − 1 27.0 meV (B 2 1g ), and 243 cm − 1 30.1 meV (A 3 g ). They also exhibit distinct polarization properties.…”

“…Complementary photoluminescence and re ectance experiments determine the energy and polarization of the neutral exciton (X) at the -point of the Brillouin zone. In non-resonant Raman spectra, in agreement with previous reports [11,26], four Raman active modes A 2 g , A 3 g , A 4 g and B 2 1g are observed; however, when the excitation energy is tuned towards the X energy, 18 Raman peaks are resolved, among which 14 have not been reported previously in the backscattering geometry. The phonon modes with the symmetries B 1u 2 , B 2u 2 , B 3u 2 , and B 1u 3 , which are Raman forbidden (dark), are also observed in the optical spectra.…”

“…The laser light propagated along the axis of the GeS ake (laser light wave vector is parallel to ), while its linear polarization (electric eld vector ) was parallel to the axis. The PL exhibits ve signi cant lines originating from the exciton (X) and most probably localized (impurity or defect) states [11] denoted by L 1 , L 2 , L 3 , and L 4 in the low-energy part of the PL spectrum. By comparison, in the RC spectrum only a single resonance is observed, whose energy coincides with the maximum of the X PL line positioned at about eV.…”

“…In contrast to TMDCs exhibiting a strong emission only as monolayers, the orthorhombic germanium monosul de has strong emission, both in the bulk and fewlayer form [10,11]. Bulk and few-layer GeS possesses an excitonic energy gap located at the Γ-point of the Brillouin zone with energies of about 1.6 to 1.77 eV [10][11][12], while monolayer GeS is an indirect semiconductor with a band gap approaching 2.34 eV [13]. Due to unique physical properties such as strongly polarized absorption [14,15], emission [10], conductivity [16], and piezoelectricity [17] as well as a high ratio of external quantum e ciency and detectivity, GeS is very promising material for applications in novel opto-electronic devices [18,19].…”

A Theoretical Study on Adjustment of Negative Differential Resistance Effect in Monolayer GeS via Substitutional Doping

“…[15,16] Currently, investigations are underway to modulate the electronic properties of GeS and explore many novel applications. [13,[17][18][19][20][21][22] For example, Kong et al studied the quantum confinement effect on the electronic structure and transport properties of monolayer GeS nanoribbions, and they found that edge-reconstructed GeS nanoribbons are promising for optoelectronics and photocatalytics. [23] Mislav et al reported the transport properties of GeS nanoribbons and the ballistic device performance of sub-4 nm field-effect transistors.…”