Probing negatively charged and neutral excitons in MoS2/hBN and hBN/MoS2/hBN van der Waals heterostructures.

High-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer transition metal dichalcogenides enable observations of subtle optical and spin-valley properties whose identification was beyond the reach of structures exfoliated directly on standard SiO2/Si substrates. Here, we describe different van der Waals heterostructures based on uncapped single-layer MoS2 stacked onto hBN layers of different thicknesses and hBN-encapsulated monolayers. Depending on the doping level, they reveal the fine structure of excitonic complexes, i.e. neutral and charged excitons. In the emission spectra of a particular MoS2/hBN heterostructure without an hBN cap we resolve two trion peaks, T1 and T2, energetically split by about 10 meV, resembling the pair of singlet and triplet trion peaks (T S and T T ) in tungsten-based materials. The existence of these trion features suggests that monolayer MoS2 has a dark excitonic ground state, despite having a 'bright' single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of -2.47 for hBN-encapsulated structures, which reveals a nearly neutral doping regime. In the uncapped MoS2 structures, the excitonic g-factor varies from -1.15 to -1.39 depending on the thickness of the bottom hBN layer and decreases as a function of rising temperature.

Probing of free and localized excitons and trions in atomically thin WSe2, WS2, MoSe2 and MoS2 in photoluminescence and reflectivity experiments.

We report on detailed temperature dependent (T = 7-295 K) optical spectroscopy studies of WSe2, WS2, MoSe2 and MoS2 monolayers exfoliated onto the same SiO2/Si substrate. In the high energy region of absorption type (reflectivity contrast-RC) and emission (photo-luminescence-PL) spectra of all the monolayers resonances related to the neutral and charged excitons (X and T) are detected in the entire measured temperature range. The optical amplitudes of excitons and trions strongly depend on the temperature and two dimensional carrier gas (2DCG) concentration. In the low energy PL spectra of WSe2 and WS2 we detect a group of lines (L) which dominates the spectra at low temperatures but rapidly quenches with the increase in the temperature. Interestingly, in the same energy range of the RC spectra recorded for WS2, we observe an additional line (L 0), which behaves in the same way as the L lines in the PL spectra. The optical amplitude of L 0 and T resonances in the RC spectra strongly increases with the growth of the 2DCG concentration. On the base of these observations we identify the L 0 resonance in the RC spectra as arising from the fine structure of the trion. We also propose that the line interpreted previously in PL spectra of WSe2 and WS2 as related to the biexciton emission is a superposition of the biexciton, trion and localized exciton emission. We find that with the temperature increase from 7-295 K the total PL intensity decreases moderately in WSe2 and WS2, strongly in MoS2 and dramatically in MoSe2.

Exciton binding energy and hydrogenic Rydberg series in layered ReS2.

Unlike monolayers of transition metal dichalcogenides such as MoS2, which possess high in-plane symmetry, layered ReS2 exhibits reduced in-plane crystal symmetry with a distorted 1 T structure. This unique symmetry leads to anisotropic optical properties, very promising for light polarization devices. Here, we report on low temperature polarization-resolved emission and absorption measurements of excitons in ReS2 from bulk to monolayer. In photoluminescence and reflectivity contrast spectra we distinguish two strongly polarized excitons X1 and X2 with dipole vectors along different crystal directions, which persist from bulk down to monolayer. Basing on the PL and RC spectra of bulk crystals we determine the energy of the ground and first four excited states of both excitons, which follow the usual hydrogenic Rydberg series of energy levels of 3D excitonic states (En = Ry*/n2). From the numerical fit we estimate that the energy gap is direct and equal to 1671.7 meV and binding energy of X1 and X2 is equal to 117.5 and 86.6 meV, respectively. In magneto-PL spectra of bulk ReS2 up to B = 10 T, the energy shift of all the states is below 2 meV. On reducing the crystal thickness from bulk to monolayer the ground state experience a strong blue shift.

Room temperature multi-phonon upconversion photoluminescence in monolayer semiconductor WS2.

Photon upconversion is an anti-Stokes process in which an absorption of a photon leads to a reemission of a photon at an energy higher than the excitation energy. The upconversion photoemission has been already demonstrated in rare earth atoms in glasses, semiconductor quantum wells, nanobelts, carbon nanotubes and atomically thin semiconductors. Here, we demonstrate a room temperature upconversion photoluminescence process in a monolayer semiconductor WS2, with energy gain up to 150 meV. We attribute this process to transitions involving trions and many phonons and free exciton complexes. These results are very promising for energy harvesting, laser refrigeration and optoelectronics at the nanoscale.