Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe2.

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition-metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe2, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe2, in contrast to the 2H materials, the out-of-plane transition metal dz2 and chalcogen pz orbitals do not contribute significantly to the top of the valence band, which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe2 using scanning tunneling spectroscopy and explore the implications on the gap following surface doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe2. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place among two-dimensional crystals.

Note: Simple leak sealing technique for ultra-high vacuum cryostat by using freezable liquid.

Here we introduce a simple, low-cost, contamination-free, and highly reliable technique for sealing an ultra-high vacuum (UHV) cryostat by using cryogenically freezable liquid. We demonstrate it by sealing an UHV cryostat with dry leaks in the high vacuum range; ethanol was utilized to fill and block the leakage pathways through the subsequent in situ solidification by LN2. The seal is reversible and can be maintained as long as the cryostat is kept at cryogenic temperature.

Time-resolved energy transduction in a quantum capacitor.

The capability to deposit charge and energy quantum-by-quantum into a specific atomic site could lead to many previously unidentified applications. Here we report on the quantum capacitor formed by a strongly localized field possessing such capability. We investigated the charging dynamics of such a capacitor by using the unique scanning tunneling microscopy that combines nanosecond temporal and subangstrom spatial resolutions, and by using Si(001) as the electrode as well as the detector for excitations produced by the charging transitions. We show that sudden switching of a localized field induces a transiently empty quantum dot at the surface and that the dot acts as a tunable excitation source with subangstrom site selectivity. The timescale in the deexcitation of the dot suggests the formation of long-lived, excited states. Our study illustrates that a quantum capacitor has serious implications not only for the bottom-up nanotechnology but also for future switching devices.

Error analysis of the residence time of bistable Poisson states obtained by periodic measurements.

We performed error analysis on the periodic measurement schemes to obtain the residence time of bistable Poisson states. Experimental data were obtained by periodical level-sensitive samplings of oxygen-induced states on Si(111)-7 x 7 that stochastically switches between two metastable states. Simulated data sequences were created by the Monte Carlo numerical method. The residence times were extracted from the experimental and simulation data sequences by averaging and exponential-fitting methods. The averaging method yields the residence time via the summation of the detected temporal width of each state weighed by the normalized frequency of the state and the exponential fitting via fitting a single exponential function to the frequency histogram of the data. It is found that the averaging method produces consistently more accurate results with no arbitrariness, when compared to the exponential fitting method. For further understanding, data modeling using the first-order approximation was performed; the enhanced accuracy in the averaging method is due to the mutual cancellation of errors associated with detection of zero-width states and long-tail states. We investigated a multi-interval detection scheme as well. Similar analysis shows that the dual-interval scheme produces larger error compared to the single interval one, and has narrower optimum region.

Trapping-mediated chemisorption of ethylene on Si(001)-c(4 x 2).

Adsorption of ethylene molecules on Si(001)-c(4 x 2) was studied using scanning tunneling microscopy at low temperatures. Ethylene molecules trapped at the surface at 50 K were imaged only after decay to chemisorption, each bonding to a Si dimer. Atomic-scale observations of temperature-dependent kinetics show that the decay exhibited Arrhenius behavior with the reaction barrier of 128 meV in clear evidence of the trapping-mediated chemisorption, however, with an anomalously small preexponential factor of 300 Hz. Such a small prefactor is attributed to the entropic bottleneck at the transition state caused by the free-molecule-like trap state.