Optical properties of atomically thin transition metal dichalcogenides: observations and puzzles

Abstract

Abstract:Recent results on the optical properties of monolayer and few layers of semiconducting transition metal dichalcogenides are reviewed. Experimental observations are presented and discussed in the frame of existing models, highlighting the limits of our understanding in this emerging field of research. We first introduce the representative band structure of these systems and their interband optical transitions. The effect of an external magnetic field is then considered to discuss Zeeman spectroscopy and optical pumping experiments, both revealing phenomena related to the valley degree of freedom. Finally, we discuss the observation of single photon emitters in different types of layered materials, including wide band gap hexagonal boron nitride. While going through these topics, we try to focus on open questions and on experimental observations, which do not yet have a clear explanation.

Dark excitations in monolayer transition metal dichalcogenides

“…Due to the inverted character of the conduction bands in MoX 2 , it is difficult to predict the energetic ordering of dark and bright excitations. In accordance with previous studies [38][39][40], we observe that the lowest exciton is dark in WX 2 (X = S/Se) and MoS 2 . However, in contrast to previous results, we also find the dark excitation slightly below the bright ones in MoSe 2 .…”

“…In the following, we extend the discussion to WS 2 , MoSe 2 , and MoS 2 [37]. As reported previously [22,38], the most significant qualitative difference between these materials with regards to low-lying excitations is the different alignment curvature of the spin-split conduction bands at K [ Fig. 3(a)], in particular, whether the conduction band minimum reveals the same spin as the highest valence band.…”

Excitonic effects in the optical properties of 2D materials:an equation of motion approach

“…whereψ(r) is the field operator (8) defined below and V (q) is the Fourier transform of the Keldysh potential given by equation (4). For simplicity, from here on we choose units such that v F = = e = 1; the usual units are reintroduced at the end of the calculations.…”

“…With the advent of two-dimensional materials [3], this topic regained interest since it became possible to study single-and few-layers of TMDC's [4,5]. Together with its two-dimensional nature, this new class of materials also has a hexagonal lattice structure as does graphene.…”

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe 2 , WSe 2 and WS 2 Monolayers

“…Both resonances are related to free neutral exciton states and are classified according to the valence sub-band involved in the optical transition [25][26][27][28][29][30][31][32]. The lower/higher energy excitons A/B are related to transitions involving a hole from the upper/lower valence band state, respectively.…”