Many-particle induced band renormalization processes in few- and mono-layer MoS₂

Abstract

Band renormalization effects play a significant role for two-dimensional (2D) materials in designing a device structure and customizing their optoelectronic performance. However, the intrinsic physical mechanism about the influence of these effects cannot be revealed by general steady-state studies. Here, band renormalization effects in organic superacid treated monolayer MoS2, untreated monolayer MoS2 and few-layer MoS2 are quantitatively analyzed by using broadband femtosecond transient absorption spectroscopy. In comparison with the untreated monolayer, organic superacid treated monolayer MoS2 maintains a direct bandgap structure with two thirds of carriers populated at K valley, even when the initial exciton density is as high as 2.05 × 1014 cm−2 (under 400 nm excitations). While for untreated monolayer and few-layer MoS2, many-particle induced band renormalizations lead to a stronger imbalance for the carrier population between K and Q valleys in k space, and the former experiences a direct-to-indirect bandgap transition when the initial exciton density exceeds 5.0 × 1013 cm−2 (under 400 nm excitations). Those many-particle induced band renormalization processes further suggest a band-structure-controlling method in practical 2D devices.

Room Temperature Enhancement of Electronic Materials by Superacid Analogues

“…Studies on TFSA-treated flakes have reported either essentially no shift ,,− or a blue shift, , but all these studies used monolayers whereas our flakes were a few layers thick and so may behave differently. Studies on TFSA-treated CVD monolayers have observed blue shifts, ,,, consistent with our observations. One multiple treatment study on a monolayer flake showed an initial blue shift relative to the untreated case followed by red shifts with successive treatments .…”

Defect Engineering of Ultrathin WO3 Nanosheets: Implications for Nonlinear Optoelectronic Devices

“…We suspect the engineered oxygen vacancies of D-WO 3 introduced extra trap states within the forbidden gaps and lowered its band gap, as verified by the aforementioned XPS and ESR results in Figure d–f. Moreover, band renormalization, that is, the shrinkage of the band gap induced by the state filling by a large amount of electron–hole pairs, is another possible root. , Band modification, metal doping, and defect engineering are well-known factors that affect the band renormalization upon laser pulse excitation. − The TA spectroscopic techniques are powerful tools to identify band renormalization effects in 2D materials. Indeed, the negative signals in Figures c and S10 evidenced the occurrence of band renormalization that contributed to the saturable absorption behaviors of the metal oxides. , …”

Direct Observation of Room‐Temperature Intravalley Coherent Coupling Processes in Monolayer MoS2

“…Femtosecond Transient Absorption Setup: In the TA experiments, a mode-locked Ti: sapphire laser/amplifier system (Solstice, Spectra-Physics) was used. [40,41] The output laser pulse centered at 800 nm (repetition rate: 250 Hz) with 35 fs pulse width and 1.5 mJ pulse energy was split into two parts: the stronger beam through TOPAS to generate pump pulses of 670, 630, and 600 nm (with a relatively moderate initial exciton density of ≈5 × 10 12 cm −2 to obtain an acceptable signal-to-noise ratio); the other beam was penetrated through 5 mm thick sapphire, generating broadband (from 450 to 800 nm) white-light probe light. The circular (or linear) polarization of pump and probe lights are tuned by quarter (or half) wave plates.…”