Interlayer Bound Wannier Excitons in Germanium Sulfide.

Sara Postorino,Maurizia Palummo,Laurent O Lee Cheong Lem,Saskia Fiedler,Jianbo Sun,Luca Camilli

doi:10.3390/ma13163568

Abstract

We report a cathodoluminescence (CL) study of layered germanium sulfide (GeS) where we observe a sharp emission peak from flakes covered with a thin hexagonal boron nitride film. GeS is a material that has recently attracted considerable interest due to its emission in the visible region and its strong anisotropy. The measured CL peak is at ~1.69 eV for samples ranging in thickness from 97 nm to 45 nm, where quantum-confinement effects can be excluded. By performing ab initio ground- and excited-state simulations for the bulk compound, we show that the measured optical peak can be unambiguously explained by radiative recombination of the first free bright bound exciton, which is due to a mixing of direct transitions near the -point of the Brillouin Zone and it is associated to a very large optical anisotropy. The analysis of the corresponding excitonic wave function shows a Wannier–Mott interlayer character, being spread not only in-plane but also out-of-plane.

Highlights

Over the past two decades, a wide range of layered/2D materials has been under extensive experimental and theoretical scrutiny because of their stability, interesting chemical and physical properties, the possibility to be exfoliated or grown in ultra-thin films, and the potentiality to realize flexible electronic and opto-electronic devices [1,2,3,4,5,6,7,8,9,10]
We investigate the optical properties of hBN-coated germanium monosulfide (GeS) by using cathodoluminescence (CL) measurements and ab initio calculations based on density function theory (DFT) and Many-body Perturbation Theory (MBPT)
We show that this peak is attributed to the radiative recombination of intrinsic direct bound excitons near Γ point in the Brillouin Zone (BZ) of GeS

Summary

Introduction

Over the past two decades, a wide range of layered/2D materials has been under extensive experimental and theoretical scrutiny because of their stability, interesting chemical and physical properties, the possibility to be exfoliated or grown in ultra-thin films, and the potentiality to realize flexible electronic and opto-electronic devices [1,2,3,4,5,6,7,8,9,10]. The family of group-IV monochalcogenides recently started to raise interest both at experimental and theoretical level Owing to their large in-plane anisotropy, they offer one more dimension to manipulate the physical properties when compared with isotropic layered/2D materials, making them attractive to realize novel angle-dependent electronic, opto-electronic [11], and tunable spintronic devices [12]. Our study illustrates the optical emission characteristics of GeS, which is instructive for the exploration of its applications in diverse fields

Experimental and Theoretical Methods

Results and Discussion

Conclusions