Abstract
Almost all experiments and future applications of transition metal dichalcogenide monolayers rely on a substrate for mechanical stability, which can significantly modify the optical spectra of the monolayer. Doping from the substrate might lead to the domination of the spectra by trions. Here we show by ab initio many-body theory that the negative trion (A−) splits into three excitations, with both inter- and intra-valley character, while the positive counterpart (A+) consists of only one inter-valley excitation. Furthermore, the substrate enhances the screening, which renormalizes both band gap and exciton as well as the trion-binding energies. We verify that these two effects do not perfectly cancel each other, but lead to red-shifts of the excitation energies for three different substrates ranging from a wide-bandgap semiconductor up to a metal. Our results explain recently found experimental splittings of the lowest trion line as well as excitation red-shifts on substrates.
Highlights
Almost all experiments and future applications of transition metal dichalcogenide monolayers rely on a substrate for mechanical stability, which can significantly modify the optical spectra of the monolayer
transition metal dichalcogenides (TMDCs) monolayers often exhibit a second kind of excited state next to excitons: trions[12,13,14,15,16,17,18,19]
The first point that is striking in both trion spectra is the huge variety of excitations compared to only two excitons below 2.4 eV
Summary
Almost all experiments and future applications of transition metal dichalcogenide monolayers rely on a substrate for mechanical stability, which can significantly modify the optical spectra of the monolayer. Semiconducting transition metal dichalcogenides (TMDCs) have attracted much attention as candidates for future optoelectronic applications due to their rich physical properties: the indirect-to-direct band-gap transition from bulk to monolayer[1, 2], selective valley- and spin-excitations[3,4,5,6,7], and high on–off ratios as field-effect transistors[8]. The goal of this work is to address these experimental findings, i.e., to understand the interplay between a TMDC monolayer and a substrate, including the formation and properties of both negative and positive trions and the role of the additional substrate screening quantitatively[25] For this we apply the framework of ab initio many-body perturbation theory (MBPT, see Supplementary Notes 1 and 2)[26, 27] and extend it to include trions (Supplementary Note 3). We extend the previous findings and verify that these two large effects do not exactly cancel each other, instead they result in small, but robust shifts of both the excitonic and trionic transition energies to the red (lower energies) for TMDC monolayers
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