Determination of Interface Atomic Structure and Its Impact on Spin Transport Using Z-Contrast Microscopy and Density-Functional Theory

We combine Z-contrast scanning transmission electron microscopy with density-functional-theory calculations to determine the atomic structure of the Fe/AlGaAs interface in spin-polarized light-emitting diodes. A 44% increase in spin-injection efficiency occurs after a low-temperature anneal, which produces an ordered, coherent interface consisting of a single atomic plane of alternating Fe and As atoms. First-principles transport calculations indicate that the increase in spin-injection efficiency is due to the abruptness and coherency of the annealed interface.Comment: 16 pages (including cover), 4 figure

Data-driven simulation and characterisation of gold nanoparticle melting

The simulation and analysis of the thermal stability of nanoparticles, a stepping stone towards their application in technological devices, require fast and accurate force fields, in conjunction with effective characterisation methods. In this work, we develop efficient, transferable, and interpretable machine learning force fields for gold nanoparticles based on data gathered from Density Functional Theory calculations. We use them to investigate the thermodynamic stability of gold nanoparticles of different sizes (1 to 6 nm), containing up to 6266 atoms, concerning a solid-liquid phase change through molecular dynamics simulations. We predict nanoparticle melting temperatures in good agreement with available experimental data. Furthermore, we characterize the solid-liquid phase change mechanism employing an unsupervised learning scheme to categorize local atomic environments. We thus provide a data-driven definition of liquid atomic arrangements in the inner and surface regions of a nanoparticle and employ it to show that melting initiates at the outer layers

Biaxial strain tuning of exciton energy and polarization in monolayer WS2

We perform micro-photoluminescence and Raman experiments to examine the impact of biaxial tensile strain on the optical properties of WS2 monolayers. A strong shift on the order of -130 meV per % of strain is observed in the neutral exciton emission at room temperature. Under near-resonant excitation we measure a monotonic decrease in the circular polarization degree under applied strain. We experimentally separate the effect of the strain-induced energy detuning and evaluate the pure effect coming from biaxial strain. The analysis shows that the suppression of the circular polarization degree under biaxial strain is related to an interplay of energy and polarization relaxation channels as well as to variations in the exciton oscillator strength affecting the long-range exchange interaction.Comment: 29 pages, 11 figure

Spatially selective reversible charge carrier density tuning in WS_2 monolayers via photochlorination

Chlorine-doped tungsten disulfide monolayer (1L-WS_2) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal's electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron spectroscopy (XPS) are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS_2 with micrometric resolution. This method can also be extended for the controllable doping of other TMD monolayers

Electrical Spin Pumping of Quantum Dots at Room Temperature

We report electrical control of the spin polarization of InAs/GaAs self-assembled quantum dots (QDs) at room temperature. This is achieved by electrical injection of spin-polarized electrons from an Fe Schottky contact. The circular polarization of the QD electroluminescence shows that a 5% electron spin polarization is obtained in the InAs QDs at 300 K, which is remarkably insensitive to temperature. This is attributed to suppression of the spin relaxation mechanisms in the QDs due to reduced dimensionality. These results demonstrate that practical regimes of spin-based operation are clearly attainable in solid state semiconductor devices.Comment: 4 figures, accepted by Appl. Phys. Let

Reduction Of Spin Injection Efficiency by Interface Spin Scattering

We report the first experimental demonstration that interface microstructure limits diffusive electrical spin injection efficiency across heteroepitaxial interfaces. A theoretical treatment shows that the suppression of spin injection due to interface defects follows directly from the contribution of the defect potential to the spin-orbit interaction, resulting in enhanced spin-flip scattering. An inverse correlation between spin-polarized electron injection efficiency and interface defect density is demonstrated for ZnMnSe/AlGaAs-GaAs spin-LEDs with spin injection efficiencies of 0 to 85%.Comment: 13 pages, 5 figures; submitted to PR

Understanding the potential in vitro modes of action of bis(β‐diketonato) oxovanadium(IV) complexes

To understand the potential in vitro modes of action of bis(β-diketonato) oxovanadium(IV) complexes, nine compounds of varying functionality have been screened using a range of biological techniques. The antiproliferative activity against a range of cancerous and normal cell lines has been determined, and show these complexes are particularly sensitive against the lung carcinoma cell line, A549. Annexin V (apoptosis) and Caspase-3/7 assays were studied to confirm these complexes induce programmed cell death. While gel electrophoresis was used to determine DNA cleavage activity and production of reactive oxygen species (ROS), the Comet assay was used to determine induced genomic DNA damage. Additionally, Förster resonance energy transfer (FRET)-based DNA melting and fluorescent intercalation displacement assays have been used to determine the interaction of the complexes with double strand (DS) DNA and to establish preferential DNA base-pair binding (AT versus GC)