Reduced absorption due to defect-localized interlayer excitons in transition metal dichalcogenide-graphene heterostructures

Associating the presence of atomic vacancies to excited-state transport phenomena in two dimensional semiconductors is of emerging interest, and demands detailed understanding of the involved exciton transitions. Here we study the effect of such defects on the electronic and optical properties of WS$_2$-graphene and MoS$_2$-graphene van der Waals heterobilayers by employing many-body perturbation theory. We find that the combination of chalcogen defects and graphene adsorption onto the transition metal dichalcogenide layer can radically alter the optical properties of the heterobilayer, due to a combination of dielectric screening, the impact of the missing chalcogen atoms in the intralayer and interlayer optical transitions, and the different nature of each layer. By analyzing the intrinsic radiative rates of the most stable subgap excitonic features, we find that while the presence of defects introduces low-lying optical transitions, resulting in excitons with larger oscillator strength, it also decreases the optical response associated to the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.Comment: 7 pages + 3 figures; Supporting Information (20 pages + 13 figures

Charge quenching at defect states in transition metal dichalcogenide-graphene van der Waals heterobilayers

We study the dynamical properties of point-like defects, represented by monoatomic chalcogen vacancies, in WS$_2$-graphene and MoS$_2$-graphene heterobilayers. Employing a multidisciplinary approach based on the combination of ab initio, model Hamiltonian and density matrix techniques, we propose a minimal interacting model that allows for the calculation of electronic transition times associated to population and depopulation of the vacancy by an additional electron. We obtain the "coarse-grained" semiclassical dynamics by means of a master equation approach and discuss the potential role of virtual charge fluctuations in the internal dynamics of impurity quasi-degenerate states. The interplay between the symmetry of the lattice and the spin degree of freedom through the spin-orbit interaction and its impact on charge quenching is studied in detail.Comment: 17 pages + 9 figures; Supplemental Material (10 pages + 4 figures

Defect-induced modification of low-lying excitons and valley selectivity in monolayer transition metal dichalcogenides

We study the effect of point-defect chalcogen vacancies on the optical properties of monolayer transition metal dichalcogenides using ab initio GW and Bethe-Salpeter equation calculations. We find that chalcogen vacancies introduce unoccupied in-gap states and occupied resonant defect states within the quasiparticle continuum of the valence band. These defect states give rise to a number of strongly-bound defect excitons and hybridize with excitons of the pristine system, reducing the valley-selective circular dichroism. Our results suggest a pathway to tune spin-valley polarization and other optical properties through defect engineering

Energy Level Alignment at Molecule-Metal Interfaces from an Optimally-Tuned Range-Separated Hybrid Functional

The alignment of the frontier orbital energies of an adsorbed molecule with the substrate Fermi level at metal-organic interfaces is a fundamental observable of significant practical importance in nanoscience and beyond. Typical density functional theory calculations, especially those using local and semi-local functionals, often underestimate level alignment leading to inaccurate electronic structure and charge transport properties. In this work, we develop a new fully self-consistent predictive scheme to accurately compute level alignment at certain classes of complex heterogeneous molecule-metal interfaces based on optimally-tuned range-separated hybrid functionals. Starting from a highly accurate description of the gas-phase electronic structure, our method by construction captures important nonlocal surface polarization effects via tuning of the long-range screened exchange in a range-separated hybrid in a non-empirical and system-specific manner. We implement this functional in a plane-wave code and apply it to several physisorbed and chemisorbed molecule-metal interface systems. Our results are in quantitative agreement with experiments, both the level alignment and work function changes. Our approach constitutes a new practical scheme for accurate and efficient calculations of the electronic structure of molecule-metal interfaces.Comment: 15 pages, 8 figure

Quantitative Prediction of Optical Absorption in Molecular Solids from an Optimally Tuned Screened Range-Separated Hybrid Functional

We show that fundamental gaps and optical spectra of molecular solids can be predicted quantitatively and nonempirically within the framework of time-dependent density functional theory (TDDFT) using the recently developed optimally tuned screened range-separated hybrid (OT-SRSH) functional approach. In this scheme, the electronic structure of the gas-phase molecule is determined by optimal tuning of the range-separation parameter in a range-separated hybrid functional. Screening and polarization in the solid state are taken into account by adding long-range dielectric screening to the functional form, with the modified functional used to perform self-consistent periodic boundary calculations for the crystalline solid. We provide a comprehensive benchmark for the accuracy of our approach by considering the X23 set of molecular solids and comparing results obtained from TDDFT with those obtained from many-body perturbation theory in the GW-BSE approximation. We additionally compare results obtained from dielectric screening computed within the random-phase approximation to those obtained from the computationally more efficient many-body dispersion approach and find that this influences the fundamental gap but has little effect on the optical spectra. Our approach is therefore robust and can be used for studies of molecular solids that are typically beyond the reach of computationally more intensive methods