We report on detailed temperature dependent ( T = 7–295 K) optical spectroscopy studies of WSe ₂ , WS ₂ , MoSe ₂ and MoS ₂ monolayers exfoliated onto the same SiO ₂ /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 WSe ₂ and WS ₂ 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 WS ₂ , 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 WSe ₂ and WS ₂ 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 WSe ₂ and WS ₂ , strongly in MoS ₂ and dramatically in MoSe ₂ .

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.

Unlike monolayers of transition metal dichalcogenides such as MoS_2,which possess high in-plane symmetry, layered ReS₂exhibits 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 ReS₂from bulk to monolayer. In photoluminescence and reflectivity contrast spectra we distinguish two strongly polarized excitons X₁and X₂with 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 (E_n = Ry^*/n²). From the numerical fit we estimate that the energy gap is direct and equal to 1671.7 meV and binding energy of X₁and X₂is equal to 117.5 and 86.6 meV, respectively. In magneto-PL spectra of bulk ReS₂up 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.

We have examined the influence of flake-substrate effects that affect one and few layers of MoS ₂ in terms of their electrical and optical properties. In the measurements, we used SiO ₂ /Si substrates with etched cavities and aluminum electrodes. Suspended areas are easily identifiable both on images depicting the topography and on the surface potential maps measured with the Kelvin probe force microscopy. Compared to the SiO ₂ /Si supported material, surface potential decrease is observable at the membrane. The surface potential value of the flakes located on the electrodes is the lowest. PL measurements prove that single MoS ₂ monolayer was obtained. Suspended regions are also correlated with maps obtained as a result of Raman spectroscopy.

Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe₂ monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe₂ transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with Λ- or K-valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials.

We present results of optical experiments and theoretical analysis on the high-quality single-layer MoS₂ which reveal the fine structure of charged excitons, i.e., trions. In the emission spectra we resolve and identify two trion peaks, T₁ and T₂, resembling the pair of singlet and triplet trion peaks (T_S and T_T) in tungsten-based materials. However, in polarization-dependent photoluminescence measurements we identify these peaks as intra- and inter-valley singlet trions due to the trion fine structure distinct from that already known in bright and dark 2D materials with large conduction-band splitting induced by the spin-orbit coupling. We show that the trion energy splitting in MoS₂ is a sensitive probe of inter- and intra-valley carrier interaction. With additional support from theory we claim that the existence of these singlet trions combined with an anomalous excitonic g-factor together suggest that monolayer MoS₂ has a dark excitonic ground state, despite having "bright" singleparticle arrangement of spin-polarized conduction bands.