Adsorbate doping of MoS2 and WSe2: the influence of Na and Co.

We have investigated the influence of metal adsorbates (sodium and cobalt) on the occupied and unoccupied electronic structure of MoS2(0 0 0 1) and WSe2(0 0 0 1), through a combination of both photoemission and inverse photoemission. The electronic structure is rigidly shifted in both the WSe2 and MoS2 systems, with either Na or Co adsorption, generally as predicted by accompanying density functional theory based calculations. Na adsorption is found to behave as an electron donor (n-type) in MoS2, while Co adsorption acts as an electron acceptor (p-type) in WSe2. The n-type transition metal dichalcogenide (MoS2) is easily doped more n-type with Na deposition while the p-type transition metal dichalcogenide (WSe2) is easily doped more p-type with Co deposition. The binding energy shifts have some correlation with the work function differences between the metallic adlayer and the transition metal dichalcogenide substrate.

The symmetry-resolved electronic structure of 2H-WSe2(0 0 0 1).

The orbital symmetry of the band structure of 2H-WSe2(0 0 0 1) has been investigated by means of angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT). The WSe2(0 0 0 1) experimental band structure is found, by ARPES, to be significantly different for states of even and odd reflection parities along both the [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] lines, in good agreement with results obtained from DFT. The light polarization dependence of the photoemission intensities from the top of the valence band for bulk WSe2(0 0 0 1) is explained by the dominance of W 5[Formula: see text] states around the [Formula: see text]-point and W 5d xy states around the [Formula: see text]-point, thus dominated, respectively, by states of even and odd symmetry, with respect to the [Formula: see text]-[Formula: see text] line. The splitting of the topmost valence band at [Formula: see text], due to spin-orbit coupling, is measured to be 0.49 ± 0.01 eV, in agreement with the 0.48 eV value from DFT, and prior measurements for the bulk single crystal WSe2(0 0 0 1), albeit slightly smaller than the 0.513 ± 0.01 eV observed for monolayer WSe2.

Low temperature growth of cobalt on Cr2O3(0 0 0 1).

The thickness and temperature dependence of in situ grown cobalt thin films on Cr2O3(0 0 0 1) single crystalline substrate has been studied by low energy electron microscopy (LEEM). The LEEM images indicate that growth of thin Co films (⩽5 monolayers) on chromia at 100 K tends to be continuous and flat with suppressed island growth compared to films grown on chromia at room temperature and above (to ~440 K). Low energy electron diffraction indicates that disorder builds and crystallinity of the cobalt thin film decreases with increased film thickness. Compared with cobalt thin films on Al2O3(0 0 0 1) single crystalline substrate, cobalt thin films on Cr2O3(0 0 0 1) show larger magnetic contrast in magnetic force microscopy indicating enhancement of perpendicular anisotropy induced by Cr2O3.

Spin-orbit coupling in the band structure of monolayer WSe2.

We used angle-resolved photoemission spectroscopy (ARPES) to map out the band structure of single-layer WSe2. The splitting of the top of the valence band because of spin-orbit coupling is 513 ± 10 meV, in general agreement with theoretical predictions and in the same range as that of bulk WSe2. Overall, our density functional theory (DFT) calculations of the band structure are in excellent agreement with the ARPES results. We have verified that the few discrepancies between theory and experiment are not due to the effect of strain. The differences between the DFT-calculated band structure using local density approximation (LDA) and that using the generalized gradient approximation (GGA), for single-layer WSe2, are caused mainly by differences in the respective charge densities.

The complete in-gap electronic structure of colloidal quantum dot solids and its correlation with electronic transport and photovoltaic performance.

The direct observation of the complete electronic band structure of a family of PbS CQD solids via photoelectron spectroscopy is reported. We investigate how materials processing strategies, such as the latest passivation methods that produce record-performance photovoltaics, achieve their performance advances. Halide passivated films show a drastic reduction in states in the midgap, contributing to a marked improvement in the device performance.

Graphene mediated domain formation in exchange coupled graphene/Co3O4(111)/Co(0001) trilayers.

Graphene grown directly on Co3O4(111)/Co(0001) by molecular beam epitaxy exhibits extrinsic p-type doping, as demonstrated by photoemission and conductivity measurements. Trilayer heterostructures of graphene/Co3O4(111)/Co(0001) reveal an unconventional magneto-optical Kerr hysteresis with vanishing remanence for temperatures up to 400 K. Magnetic force microscopy measurements demonstrate that the vanishing remanence is due to a complex domain state, indicating substrate-induced graphene spin polarization. The domain formation of the Co magnetization is in strong contrast to the magnetic behavior of Co in Co/Co3O4 bilayers. This suggests that the Co3O4 interlayer mediates the variable Co magnetization and induced graphene spin polarization, with possible retroaction of graphene on the Co film.