As defects frequently govern the properties of crystalline solids, the precise microscopic knowledge of defect atomic structure is of fundamental importance. We report a new class of point defects in single-layer transition metal dichalcogenides that can be created through 60° rotations of metal–chalcogen bonds in the trigonal prismatic lattice, with the simplest among them being a three-fold symmetric trefoil-like defect. The defects, which are inherently related to the crystal symmetry of transition metal dichalcogenides, can expand through sequential bond rotations, as evident from in situ scanning transmission electron microscopy experiments, and eventually form larger linear defects consisting of aligned 8–5–5–8 membered rings. First-principles calculations provide insights into the evolution of rotational defects and show that they give rise to p-type doping and local magnetic moments, but weakly affect mechanical characteristics of transition metal dichalcogenides. Thus, controllable introduction of rotational defects can be used to engineer the properties of these materials.

Raman scattering and photoluminescence (PL) emission are used to investigate a single layer of tungsten disulfide (WS${}_2$) obtained by exfoliating $n$-type bulk crystals. Direct gap emission with both neutral and charged exciton recombination is observed in the low temperature PL spectra. The ratio between the trion and exciton emission can be tuned simply by varying the excitation power. Moreover, the intensity of the trion emission can be independently tuned using additional subband gap laser excitation.

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.