A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS/MoS, deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS or MoS monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε(ω) by solving the Bethe-Salpeter equation with G W, we concluded that the three PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.

Single photon emission (SPE) by a solid-state source requires presence of a distinct two-level quantum system, usually provided by point defects. Here we note that a number of qualities offered by novel, two-dimensional materials, their all-surface openness and optical transparence, tighter quantum confinement, and reduced charge screeningare advantageous for achieving an ideal SPE. On the basis of first-principles calculations and point-group symmetry analysis, a strategy is proposed to design paramagnetic defect complex with reduced symmetry, meeting all the requirements for SPE: its electronic states are well isolated from the host material bands, belong to a majority spin eigenstate, and can be controllably excited by polarized light. The defect complex is thermodynamically stable and appears feasible for experimental realization to serve as an SPE-source, essential for quantum computing, with Re Mo V S in MoS 2 as one of the most practical candidates.

Despite being only a few atoms thick, single-layer two-dimensional (2D) materials display strong electron-photon interactions that could be utilized in efficient light modulators on extreme subwavelength scales. In various applications involving light modulation and manipulation, materials with strong optical response at different wavelengths are required. Using qualitative analytical modeling and first-principles calculations, we determine the theoretical limit of the maximum optical response such as absorbance ( A) and reflectance ( R) in 2D materials and also conduct a computational survey to seek out those with best A and R in various frequency ranges, from mid-infrared to deep-ultraviolet. We find that 2D boron has broadband reflectance R > 99% for >100 layers, surpassing conventional thin films of bulk metals such as silver. Moreover, we identify 2D monolayer semiconductors with maximum response, for which we obtain quantitative estimates by calculating quasiparticle energies and accounting for excitonic effects by solving the Bethe-Salpeter equation. We found several monolayer semiconductors with absorbances ≳30% in different optical ranges, which are more than half of the maximum possible value, A = 1/2, for a freestanding 2D material. Our study predicts 2D materials which can potentially be used in ultrathin reflectors and absorbers for optoelectronic application in various frequency ranges.

Two-dimensional single-layer boron (borophene) has emerged as a new material with several intriguing properties. Recently, the β polymorph of borophene was grown on Ag(111), and observed to host Dirac fermions. Similar to graphene, β borophene can be described as atom-vacancy pseudoalloy on a closed-packed triangular lattice; however, unlike graphene, the origin of its Dirac fermions  is yet unclear. Here, using first-principles calculations, we probe the origin of Dirac fermions in freestanding and Ag(111)-supported β borophene. The freestanding β sheet hosts two Dirac cones and a topologically nontrivial Dirac nodal line with interesting Dirac-like edge states. On Ag(111), the Dirac cones develop a gap, whereas the topologically protected nodal line remains intact, and its position in the Brillouin zone matches that of the Dirac-like electronic states seen in the experiment. The presence of nontrivial topological states near the Fermi level in borophene makes its electronic properties important for both fundamental and applied research.

While pressure-induced changes in the electronic, magnetic, and optical properties of Cu-Cl hybrid perovskites have been studied intensively, the correlation between these properties and pressure-induced structural changes is still vaguely understood. Here, by first-principles calculations on a model system (EDBE)[CuCl4] (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) (a Cu-Cl hybrid perovskite), we correlate the evolution of a series of exciting physical properties with pressure while resolving some of the long-standing debates on the fundamental electronic nature of this important class of material. The material shows two structural phase transitions and an anisotropy in compressibility with increasing pressure. After a critical pressure of 17 GPa, the structure becomes highly symmetric, thereby suppressing the Jahn-Teller distortions. At zero pressure, mapping the optical transitions with the Laporte selection rules, lower and higher energy excitations are found to be of Mott-Hubbard (MH) and charge transfer (CT) type, respectively, signifying the material to be a Mott insulator. The material shows a red shift in the charge transfer band edge with increasing pressure and temperature, demonstrating the piezochromism and the thermochromism, respectively. Piezochromism originates from the changes in mixing of Cl-Cu p-d states, while thermochromism is due to broadening of conduction band states, thereby showing different electronic and structural evolution with pressure and temperature. Furthermore, the magnetic ordering in the material was found to be stable up to higher pressures, making pressure a tool to tune the electronic property without perturbing the magnetic property.