Well separated trion and neutral excitons on superacid treated MoS2 monolayers

Abstract

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs the photoluminescence (PL) spectra even at low temperature consists typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here we show that in superacid treated MoS2 MLs the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but with similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emission. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.Introduction.-Transition metal dichalcogenide (TMD) monolayers (ML) such as MoS 2 , MoSe 2 , WS 2 and WSe 2 are a new class of two-dimensional semiconductors with a direct bandgap in the visible region of the spectrum [1-3] and very unique properties. Strong spin orbit coupling combined with a crystal lattice that has no inversion symmetry allows for optical manipulation of the spin and valley degree of freedom in these materials [4]. In addition to their potential for unconventional, atomically thin and flexible electronics or valleytronics, they also are ideal candidates for optoelectronic and spin-optronic applications. For example, solar cells

Synergetic photoluminescence enhancement of monolayer MoS2via surface plasmon resonance and defect repair

“…Note that various PL enhancement factors due to the TFSI treatment ranging from 60 to 190 have been reported, which depends on the quality of the MoS 2 monolayers. 20,34,46 Nevertheless, our result indicates that the defects in MoS 2 played an important role in the PL process. Even without the LSPR enhancement from Au NPs, the defects passivation in MoS 2 monolayers could have a notable enhancement effect.…”

Temperature dependence of the double‐resonance Raman bands in monolayer MoS2

“…Figure a shows the fitting of the PL spectra at T = 173 K using four Voigt components. The most intense feature, named T A , is associated with the A trion . The strong intensity of T A is due to the excess of charge (possible optical doping) induced by the relatively high laser power used in our experiments .…”

“…The strong intensity of T A is due to the excess of charge (possible optical doping) induced by the relatively high laser power used in our experiments . The two smaller bands, better observed in Figure a, are named X A and X B and associated with the A and B excitons, respectively. The positions of T A , X A , and X B as a function of temperature are shown in Figure b and were fitted according to the equation: where E X ( T = 0) is the extrapolated band gap at 0 K, a is a fitting parameter, and Θ is an average phonon energy .…”

“…The most intense feature, named T A , is associated with the A trion. [59][60][61][62][63][64] The strong intensity of T A is due to the excess of charge (possible optical doping) induced by the relatively high laser power used in our experiments. [64] The two smaller bands, better observed in Figure 3a, are named X A and X B and associated with the A and B excitons, [59][60][61][62][63] respectively.…”

Repairing atomic vacancies in single-layer MoSe2 field-effect transistor and its defect dynamics

“…23,24,29 For the SLM FET, such defects lead to various carrier mobilities, 19,24,[30][31][32][33] and defect repairing is the focus of concern during these years. 17,28,34,35 Here we report that a simple drop of chemical solution may repair the defect-rich SLM FET, by which the carrier mobility increases from 0.1 to around 30 cm 2 /Vs. The aberration-corrected high resolution transmission electron microscopy (HRTEM), Raman scattering, and density functional theory (DFT) calculations reveal the defect dynamics for the Selenium (Se) vacancy repair, which suggests the localization source of the transport carriers.…”