Surface-dominated conduction in a 6 nm thick Bi2Se3 thin film.

We report a direct observation of surface dominated conduction in an intrinsic Bi(2)Se(3) thin film with a thickness of six quintuple layers grown on lattice-matched CdS (0001) substrates by molecular beam epitaxy. Shubnikov-de Haas oscillations from the topological surface states suggest that the Fermi level falls inside the bulk band gap and is 53 ± 5 meV above the Dirac point, which is in agreement with 70 ± 20 meV obtained from scanning tunneling spectroscopies. Our results demonstrate a great potential of producing genuine topological insulator devices using Dirac Fermions of the surface states, when the film thickness is pushed to nanometer range.

Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition.

Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are promising semiconductors for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. Here, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS2 single crystals. By illuminating WS2 with off-resonance circularly polarized light and measuring the resulting spatially resolved circularly polarized emission (Pcirc), we find significantly large circular polarization (Pcirc up to 60% and 45% for α- and ß-domains, respectively) already at 300 K, which increases to nearly 90% in the α-domains at 80 K. Studies of spatially resolved photoluminescence (PL) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Kelvin-probe force microscopy, and conductive atomic force microscopy reveal direct correlation among the PL intensity, defect densities, and chemical potential, with the α-domains showing lower defect densities and a smaller work function by 0.13 eV than the ß-domains. This work function difference indicates the occurrence of type-two band alignments between the α- and ß-domains. We adapt a classical model to explain how electronically active defects may serve as nonradiative recombination centers and find good agreement between experiments and the model. Scanning tunneling microscopic/spectroscopic (STM/STS) studies provide further evidence for tungsten vacancies (WVs) being the primary defects responsible for the suppressed PL and circular polarization in WS2. These results therefore suggest a pathway to control the opto-valleytronic properties of TMDCs by means of defect engineering.