Probing the upper band gap of atomic rhenium disulfide layers

Abstract

Here, we investigate the ultrafast carrier dynamics and electronic states of exfoliated ReS2 films using time-resolved second harmonic generation (TSHG) microscopy and density functional theory (DFT) calculations. The second harmonic generation (SHG) of layers with various thicknesses is probed using a 1.19-eV beam. Up to ~13 nm, a gradual increment is observed, followed by a decrease caused by bulk interferometric light absorption. The addition of a pump pulse tuned to the exciton band gap (1.57 eV) creates a decay-to-rise TSHG profile as a function of the probe delay. The power and thickness dependencies indicate that the electron–hole recombination is mediated by defects and surfaces. The two photon absorptions of 2.38 eV in the excited state that are induced by pumping from 1.57 to 1.72 eV are restricted because these transitions highly correlate with the forbidden d–d intrasubshell orbital transitions. However, the combined usage of a frequency-doubled pump (2.38 eV) with wavelength-variant SHG probes (2.60–2.82 eV) allows us to vividly monitor the variations in TSHG profiles from decay-to-rise to rise-to-decay, which imply the existence of an additional electron absorption state (s-orbital) at an approximate distance of 5.05 eV from the highest occupied molecular orbital states. This observation was critically examined by considering the allowance of each electronic transition and a small upper band gap (~0.5 eV) using modified DFT calculations.

Stacking‐Order‐Driven Optical Properties and Carrier Dynamics in ReS2

“…[ 31,32 ] Similar inconsistencies also exist in the determination of indirect‐direct bandgap transition, [ 24 ] Raman vibrational modes and second harmonic generation (SHG) spectra. [ 26–30,34 ] These results suggest that some other intrinsic parameter that governs the electronic and optical properties of multilayer ReS 2 , such as stacking order, is not well understood. Even though some Raman studies indicated the existence of different stacking orders, [ 28–30 ] possible stacking order in ReS 2 has not been identified.…”

“…[16] The uniqueness of ReS2 lies in its in-plane anisotropic properties, which have been demonstrated as early as 2001 in bulk. [9] In 2D ReS2, properties observed are polarizationdependent excitons, [17,18] non-linear absorption, [19] electron transport and SHG emission, [20,21] etc. Compared with black phosphorous (BP), which also shows in-plane anisotropic properties, ReS2 is more stable in ambient environments, which makes it more suitable for optoelectronic devices.…”

“…[25,26] Similar inconsistencies also exist in the determination of indirect-direct bandgap transition, [18] Raman vibrational modes and second harmonic generation (SHG) spectra. [20][21][22][23][24]28] These results suggest that some other intrinsic parameters governing the electronic and optical properties of multilayer ReS2 are not well understood. In trilayer graphene, researchers found that the two most stable stacking orders of ABC and ABA display dramatically different behavior in electronic screening of external field, [29] Raman vibrations, [30] and even electron transport.…”

“…[46] A third example is the SHG spectra, as reported by Song et al where SHG signals of ReS2 can only be observed in even number layers, [20] while Dhakal et al showed that the SHG signal increases with thickness of ReS2 starting from monolayer, regardless of the number of layers. [21] All of these discrepancies are explained with ease if the factor of stacking order is considered. SHG is determined by the symmetry of crystal.…”

Dual‐Enhanced Doping in ReSe2 for Efficiently Photoenhanced Hydrogen Evolution Reaction

“…[ 13 ] In ReSe 2 NSs, hot electrons can be excited into the CB and nonradiatively transfer energy to an extra charge (Figure 4e). [ 41 ] With low doping concentrations (Figure 4f), doped Mo atoms introduce impurity states near the VB edge, and this leads to the absorption of additional lower energy photons via a two‐step process. [ 42 ] However, excessive Mo doping leads to a wider IB (Figure 4g), and this generates recombination centers in the band gap.…”