3 research outputs found
Molecule-Adsorbed Topological Insulator and Metal Surfaces: A Comparative First-Principles Study
We
compare electronic structure characteristics of three different
kinds of benzene-adsorbed (111) surfaces: that of Bi<sub>2</sub>Te<sub>3</sub>, a prototypical topological insulator, that of Au, a prototypical
inert metal, and that of Pt, a prototypical catalytic metal. Using
first-principles calculations based on dispersion-corrected density
functional theory, we show that benzene is chemisorbed on Pt, but
physisorbed on Au and Bi<sub>2</sub>Te<sub>3</sub>. The adsorption
on Bi<sub>2</sub>Te<sub>3</sub> is particularly weak, consistent with
a minimal perturbation of the electronic structure at the surface
of the topological insulator, revealed by a detailed analysis of the
interaction of the molecular orbitals with the topological surface
states
Rashba Effect in a Single Colloidal CsPbBr<sub>3</sub> Perovskite Nanocrystal Detected by Magneto-Optical Measurements
This study depicts the influence
of the Rashba effect on the band-edge exciton processes in all-inorganic
CsPbBr<sub>3</sub> perovskite single colloidal nanocrystal (NC). The
study is based on magneto-optical measurements carried out at cryogenic
temperatures under various magnetic field strengths in which discrete
excitonic transitions were detected by linearly and circularly polarized
measurements. Interestingly, the experiments show a nonlinear energy
splitting between polarized transitions versus magnetic field strength,
indicating a crossover between a Rashba effect (at the lowest fields)
to a Zeeman effect at fields above 4 T. We postulate that the Rashba
effect emanates from a lattice distortion induced by the Cs<sup>+</sup> motion degree of freedom or due to a surface effect in nanoscale
NCs. The unusual magneto-optical properties shown here underscore
the importance of the Rashba effect in the implementation of such
perovskite materials in various optical and spin-based devices
High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide
Cesium lead iodide
possesses an excellent combination of band gap and absorption coefficient
for photovoltaic applications in its perovskite phase. However, this
is not its equilibrium structure under ambient conditions. In air,
at ambient temperature it rapidly transforms to a nonfunctional, so-called
yellow phase. Here we show that chloride doping, particularly at levels
near the solubility limit for chloride in a cesium lead iodide host,
provides a new approach to stabilizing the functional perovskite phase.
In order to achieve high doping levels, we first co-deposit colloidal
nanocrystals of pure cesium lead chloride and cesium lead iodide,
thereby ensuring nanometer-scale mixing even at compositions that
potentially exceed the bulk miscibility of the two phases. The resulting
nanocrystal solid is subsequently fused into a polycrystalline thin
film by chemically induced, room-temperature sintering. Spectroscopy
and X-ray diffraction indicate that the chloride is further dispersed
during sintering and a polycrystalline mixed phase is formed. Using
density functional theory (DFT) methods in conjunction with nudged
elastic band techniques, low-energy pathways for interstitial chlorine
diffusion into a majority-iodide lattice were identified, consistent
with the facile diffusion and fast halide exchange reactions observed.
By comparison to DFT-calculated values (with the PBE exchange-correlation
functional), the relative change in band gap and the lattice contraction
are shown to be consistent with a Cl/I ratio of a few percent in the
mixed phase. At these incorporation levels, the half-life of the functional
perovskite phase in a humid atmosphere increases by more than an order
of magnitude