Electroluminescence from indirect band gap semiconductor ReS ₂

Abstract

It has been recently claimed that bulk crystals of transition metal dichalcogenide (TMD) ReS 2 are direct band gap semiconductors, which would make this material an ideal candidate, among all TMDs, for the realization of efficient opto-electronic devices. The situation is however unclear, because even more recently an indirect transition in the PL spectra of this material has been detected, whose energy is smaller than the supposed direct gap. To address this issue we exploit the properties of ionic liquid gated fieldeffect transistors (FETs) to investigate the gap structure of bulk ReS 2 . Using these devices, whose high quality is demonstrated by a record high electron FET mobility of 1,100 cm 2 /Vs at 4K, we can induce hole transport at the surface of the material and determine quantitatively the smallest band gap present in the material, irrespective of its direct or indirect nature. The value of the band gap is found to be 1.41 eV, smaller than the 1.5 eV direct optical transition but in good agreement with the energy of the indirect optical transition, providing an independent confirmation that bulk ReS 2 is an indirect band gap semiconductor. Nevertheless, contrary to the case of more commonly studied semiconducting TMDs (e.g., MoS 2 , WS 2 , etc.) in their bulk form, we also find that ReS 2 FETs fabricated on bulk crystals do exhibit electroluminescence when driven in the ambipolar injection regime, likely because the difference between direct and indirect gap is only 100 meV. We conclude that ReS 2 does deserve more in-depth investigations in relation to possible opto-electronic applications.

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

“…These values are in good agreement with those we obtain from DFPT as shown on Fig. 3, which are 367 and 417 cm −1 for the Etype modes (34,35) and A-type mode (36), respectively. The agreement is particularly good if we take into account the fact that our calculations for MoSe 2 systematically overestimate the phonon frequencies by about 5%, or~16 cm −1 in this frequency range.…”

“…[27][28][29][30] ReSe 2 is an indirect band gap semiconductor for all numbers of layers [31][32][33][34] with a band gap of 1.36 eV 34 whilst there is disagreement as to the nature of the band gap for bulk ReS 2 , with some groups reporting an indirect band gap of 1.41 eV and others claiming a direct band gap of 1.5 eV. [31][32][33][34][35][36] In this paper, we focus on the alloys of composition ReSe 2 -x S x . These alloys are of interest for two major reasons.…”

Electronic band structure of ReS2 by high-resolution angle-resolved photoemission spectroscopy

“…This behavior may account for some of the uncertainty in the literature regarding the direct or indirect gap nature of bulk ReS 2 , with other groups reporting electrical transport properties characteristic of indirect behavior in bulk samples [28,39] or PL peaks within this range close to the direct transition [7], while one group found a direct gap via electron energy loss spectroscopy of 1.42 eV at room temperature [22], consistent with the direct gap at 100 K of ∼1.5 eV that we find [40]. Unlike here, previous calculations have often not considered the full bulk BZ and have instead focused on 2D or quasi-2D simulations of ReS 2 , where calculations have predicted the gap to be direct at the 2D point [15] in the monolayer form.…”

Unraveling the Raman Enhancement Mechanism on 1T′‐Phase ReS2 Nanosheets

“…This indirect character was also confirmed by determining the bandgap of bulk ReS 2 using ionic liquid field-effect transistor method and k-space photoemission microscopy. [17] Therefore, it remains urgent to unveil the layer-number-dependent nature of the band structure as well as the related optical and electronic properties of ReS 2 materials. In terms of Raman enhancement on ReS 2 , the mechanism, charge transfer or energy transfer, is still under debate and needs to be clearly understood.…”