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The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface. Application to the MoS2/WS2 heterostructure yields good agreement with experiments, indicating near complete charge transfer within a timescale of 100 fs.

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Combining high resolution scanning tunneling microscopy and first principles calculations, we identified the major native defects, in particular the Se vacancies and Se interstitial defects, that are responsible for the bulk conduction and nanoscale potential fluctuations in single crystals of archetypal topological insulator Bi_{2}Se_{3}. Here it is established that the defect concentrations in Bi_{2}Se_{3} are far above the thermodynamic limit, and that the growth kinetics dominate the observed defect concentrations. Furthermore, through careful control of the synthesis, our tunneling spectroscopy suggests that our best samples are approaching the intrinsic limit with the Fermi level inside the band gap without introducing extrinsic dopants.

Native defects in pnictogen chalcogenides are currently a great barrier toward the realization of the exotic properties of this class of topological insulators. Previous first-principles results of low-energy defects in Bi 2 Te 3 and Sb 2 Te 3 are in qualitative agreement with experiments. However, for Bi 2 Se 3 the calculated low-energy defects are antisites, opposed to Se vacancy (V Se) as observed experimentally. We find that the inclusion of spin-orbit interaction drastically shift the band-edge energies of the bulk states with respect to defect transition energies. It turns Bi antisite (Bi Se) from an acceptor to a donor and makes V Se more stable than Bi Se. This brings the calculated results for native defects in pnictogen chalcogenides into agreement with experiments.