Photocarrier relaxation pathway in two-dimensional semiconducting transition metal dichalcogenides

Abstract

Two-dimensional crystals of semiconducting transition metal dichalcogenides absorb a large fraction of incident photons in the visible frequencies despite being atomically thin. It has been suggested that the strong absorption is due to the parallel band or 'band nesting' effect and corresponding divergence in the joint density of states. Here, we use photoluminescence excitation spectroscopy to show that the band nesting in mono-and bilayer MX 2 (M ¼ Mo, W and X ¼ S, Se) results in excitation-dependent characteristic relaxation pathways of the photoexcited carriers. Our experimental and simulation results reveal that photoexcited electron-hole pairs in the nesting region spontaneously separate in k-space, relaxing towards immediate band extrema with opposite momentum. These effects imply that the loss of photocarriers due to direct exciton recombination is temporarily suppressed for excitation in resonance with band nesting. Our findings highlight the potential for efficient hot carrier collection using these materials as the absorbers in optoelectronic devices.

Gate‐Tunable Resonant Raman Spectroscopy of Bilayer MoS2

“…According to the photoluminescence excitation (PLE) spectroscopy of bilayer MoS 2 , [31] the emission of A exciton will be the strongest when the excitation is in resonance to B exciton instead of when in resonance to A exciton (electron doping will possibly cause the PLE profile to be not exactly the same with that in ref. [31], but we do not think it affects the statement here). Both 671.0 and 514.5 nm cannot cause the A exciton emission to dominate over the indirect gap emission, unlike the excitation of 612.2 or 633 nm.…”

Real-time monitoring of 2D semiconductor film growth with optical spectroscopy

“…This is a clear sign of the nucleation and the growth of the MoSe 2 thin film. Compared to the spectral line shape measured at room temperature [22,24,25], the peak C plotted in figure 2(b) is relatively broader. This observation can be attributed to the fact that the current spectra were recorded at an elevated substrate temperature of 250°C.…”

“…The two lowest energy absorption peaks, labeled as A and B, are attributed to the excitonic transitions occurring at the K (K′) point of the Brillouin zone. The pronounced absorption peak C at higher energies is associated to interband transitions occurring in the region near the Γ point of the Brillouin zone where the valance and conduction bands are nested [22,24,25]. These absorption features are characteristic of the 2D TMDs and depend sensitively on the number of layers, the dielectric environment, the defect density and the residual strain regarding the peak positions and spectral shape [5,23,26].…”

“…The absorption spectra of MX 2 (M = Mo, W and X = S, Se) exhibit characteristic peaks associated to the excitonic resonances and the interband electronic transitions [5,[22][23][24]. The two lowest energy absorption peaks, labeled as A and B, are attributed to the excitonic transitions occurring at the K (K′) point of the Brillouin zone.…”

“…These factors make the optical spectroscopy one of the most powerful method for the characterization of 2D-TMD materials. Indeed, deep understandings on the unique electronic and optical properties of atomically thin TMD films have been achieved with the help of optical spectroscopy [5,[22][23][24]. In this work, we have monitored, in real time and in situ, the evolution of the optical reflectance during the MBE growth of atomically thin MoSe 2 films using differential reflectance spectroscopy (DRS) [5,[22][23][24].…”