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.

We present here the minimal tight-binding model for a single layer of transition metal dichalcogenides (TMDCs) ${MX}_2(M$, metal; $X$, chalcogen) which illuminates the physics and captures band nesting, massive Dirac fermions, and valley Landé and Zeeman magnetic field effects. TMDCs share the hexagonal lattice with graphene but their electronic bands require much more complex atomic orbitals. Using symmetry arguments, a minimal basis consisting of three metal $d$ orbitals and three chalcogen dimer $p$ orbitals is constructed. The tunneling matrix elements between nearest-neighbor metal and chalcogen orbitals are explicitly derived at $K,-K$, and $\mathrm{\Gamma }$ points of the Brillouin zone. The nearest-neighbor tunneling matrix elements connect specific metal and sulfur orbitals yielding an effective $6\times 6$ Hamiltonian giving correct composition of metal and chalcogen orbitals but not the direct gap at $K$ points. The direct gap at $K$, correct masses, and conduction band minima at $Q$ points responsible for band nesting are obtained by inclusion of next-neighbor Mo-Mo tunneling. The parameters of the next-nearest-neighbor model are successfully fitted to ${MX}_2(M=\text{Mo}$; $X=\text{S})$ density functional ab initio calculations of the highest valence and lowest conduction band dispersion along $K-\mathrm{\Gamma }$ line in the Brillouin zone. The effective two-band massive Dirac Hamiltonian for ${\mathrm{MoS}}_2$, Landé $g$ factors, and valley Zeeman splitting are obtained.

Quantum confinement and manipulation of charge carriers are critical for achieving devices practical for quantum technologies. The interplay between electron spin and valley, as well as the possibility to address their quantum states electrically and optically, makes two-dimensional (2D) transition metal dichalcogenides an emerging platform for the development of quantum devices. In this work, we fabricate devices based on heterostructures of layered 2D materials, in which we realize gate-controlled tungsten diselenide (${\text{WSe}}_2$ ) hole quantum dots. We discuss the observed mesoscopic transport features related to the emergence of quantum dots in the ${\text{WSe}}_2$ device channel, and we compare them to a theoretical model.

Atomically thin semiconductors from the transition metal dichalcogenide family are materials in which the optical response is dominated by strongly bound excitonic complexes. Here, we present a theory of excitons in two-dimensional semiconductors using a tight-binding model of the electronic structure. In the first part, we review extensive literature on 2D van der Waals materials, with particular focus on their optical response from both experimental and theoretical points of view. In the second part, we discuss our ab initio calculations of the electronic structure of MoS₂, representative of a wide class of materials, and review our minimal tight-binding model, which reproduces low-energy physics around the Fermi level and, at the same time, allows for the understanding of their electronic structure. Next, we describe how electron-hole pair excitations from the mean-field-level ground state are constructed. The electron–electron interactions mix the electron-hole pair excitations, resulting in excitonic wave functions and energies obtained by solving the Bethe–Salpeter equation. This is enabled by the efficient computation of the Coulomb matrix elements optimized for two-dimensional crystals. Next, we discuss non-local screening in various geometries usually used in experiments. We conclude with a discussion of the fine structure and excited excitonic spectra. In particular, we discuss the effect of band nesting on the exciton fine structure; Coulomb interactions; and the topology of the wave functions, screening and dielectric environment. Finally, we follow by adding another layer and discuss excitons in heterostructures built from two-dimensional semiconductors.

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.

We explore a two-qubit system defined on valley isospins of two electrons confined in a gate-defined double quantum dot created within a ${\mathrm{Mo}\mathrm{S}}_2$ monolayer flake. We show how to initialize, control, interact, and read out such valley qubits by only electrical means using voltages applied to the local planar gates, which are layered on the top of the flake. By demonstrating the two-qubit exchange or readout via the Pauli blockade, we prove that valley qubits in the transition-metal-dichalcogenide semiconductor family fulfill the universality criteria and represent a scalable quantum-computing platform. Our numerical experiments are based on the tight-binding model for a ${\mathrm{Mo}\mathrm{S}}_2$ monolayer, which gives single-electron eigenstates that are then used to construct a basis of Slater determinants for the two-electron configuration space. We express screened electron-electron interactions in this basis by calculating the Coulomb matrix elements using localized Slater-type orbitals. Then we solve the time-dependent Schrödinger equation and obtain an exact time evolution of the two-electron system. During the evolution we simultaneously solve the Poison equation, finding the confinement potential controlled via voltages applied to the gates.