Tenths Of meV Research Articles

Introducing a new atomic parameter of energy scale for wideband semiconductors and binary materials

The fundamental concept in the science of dislocations is the study of the optical properties of semiconductors. The purpose of this work is to investigate the dependence of the structural features of the low-temperature photo luminescence (LTPL) spectra with the lattice parameters of wideband semiconductors and binary materials. A decoding method of the LTPL spectra of the binary polytypic structures of SiC was used. As a result, we introduce here for the first time a new i-unit atomic parameter of energy scale for the LTPL spectra of binary polytypic nanostructures. The i-unit parameter equals 4.3 meV, i.e. which equals 1/2 of the distance between adjacent Si–C layers and gives the exact multiples of the basic spectral parameters to tenths of meV. The i-unit of energy scale allows the simplifying method of spectral analysis of fine structures in SiC polytypes. The proposed new atomic i-scale allows us to monitor the processes of phase transformations up to 0.0787 A. This allows observing the displacement in tetrahedron of silicon and carbon with dimensions 7.56 A. The introduction of a new energy parameter makes it possible to control the displacement and position of atoms with very high accuracy, which in turn is very important for understanding the nature of defects in semiconductors. It is shown that phase transformations in SiC are related to dislocation in the crystal and implemented by the deformation and diffusion mechanisms. Due to results the diagram of the interlayer scheme radiative recombination with resulting dislocation levels was introduced.

Influence of Co-doping on optical properties and traps localization of ZnO films obtained by spray pyrolysis

We report a systematic investigation on the structural and optical properties of Co-doped ZnO and Co3O4 thin films grown by spray pyrolysis using acetates as precursors in low molarity solutions. XRD and optical transmittance measurements do not show secondary phases. Optical measurements exhibit the Co2+ signatures related to tetrahedral coordination in absorption and emission. Temperature-dependent PL shows the influence of traps up to 200K when they are shielded by thermal energy. The qualitative potential fluctuations model allows the evaluation of traps depth of as tenths of meV and the influence of Co-doping in the neighborhood of optically active Co2+.

Design and characterization of an SRAM-based neutron detector for particle therapy

A neutron detector based on registration of radiation effects in Static Random Access Memories (SRAMs) has been developed at the University of Bergen for applications in particle therapy. Nine different SRAMs were tested and a 16 Mibit SRAM from Cypress was chosen for the final detector. The SRAMs were irradiated in beam lines at PTB Braunschweig, the Oslo Cyclotron Laboratory, The Svedberg Laboratory, The Institute for Energy Technology (IFE, Kjeller) and the CERN-EU high-energy reference field. The results from the measurements demonstrate the feasibility of using the selected SRAMs for neutron detection. The results indicate low or no sensitivity to thermal neutrons while the cross section for fast neutrons increases with neutron energy before reaching a more stable level at energies of several tenths of MeV.

Electronic structures, magnetism, and phonon spectra in the metallic cubic perovskite <span class="aps-inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi mathvariant="normal">BaOsO</mi></mrow><mn>3</mn></msub></math></span>

Using ab initio calculations, we have investigated a cubic perovskite BaOsO3 and a few related compounds that have been synthesized recently and formally have a metallic d^4 configuration. In BaOsO3, which shows obvious 3-dimensional fermiology, a nonmagnetism is induced by a large spin-orbit coupling (SOC), which is precisely equal to an exchange splitting ~0.4 eV of the $t_{2g}$ manifold. However, the inclusion of on-site Coulomb repulsion as small as $U^c$~1.2 eV, only 1/3 of the $t_{2g}$ bandwidth, leads to the emergence of a spin-ordered moment, indicating that this system is on the verge of magnetism. In contrast to BaOsO3, our calculations suggest that the ground state of an orthorhombic CaOsO3 is a magnetically ordered state due to the reduction of the strength of SOC (about a half of that of BaOsO3) driven by the structure distortion, although the magnetization energy is only a few tenths of meV. Furthermore, in the cubic BaOsO3 and BaRuO3, our full-phonon calculations show several unstable modes, requiring further research.

Conduction mechanisms in tungsten-polyoxometalate self-assembled molecular junctions

The dependence of the various possible physical mechanisms, governing electron flow in planar metal- molecular layer-metal junctions, upon voltage bias and temperature is investigated in this work. A molecular self-assembled monolayer containing inorganic tungsten-polyoxometalate molecules is studied. These molecules are interesting for molecular memory applications due to their redox properties. Low temperature (T < 150 K) conduction is attributed to indirect tunneling; the effective tunneling barrier decreases with the rise of temperature due to the variation of available electronic states in the metal electrodes. Upon further increase of temperature, hopping arises as conduction mechanism and actually dominates for temperatures higher than 200 K. The hopping activation energies found are several tenths of meV. The transition between conduction mechanisms depends also on the electrode distance.

Temperature dependent photoluminescence of photocatalytically active titania nanopowders

Temperature photoluminescence (PL) in the sub-ambient range has been developed as a tool for characterizing photocatalytic materials. The use of well-characterized TiO 2 nanoparticles with calibrated particle sizes allowed to face the temperature dependent PL results with the photocatalytic activity and several important physico-chemical parameters. In the relaxation of the photoexcited electron/hole pairs, the transfer towards surface sites is in competition with radiative and non-radiative recombinations. Temperature dependent PL appears thus to be a very sensitive technique to study the efficiency of the electron or the hole trapping at the surface of titania nanoparticles where they take part to the oxidation/reduction reactions. The PL shows a peak around 610 nm with a width of about 130 nm. The quenching of this emission is measured as a function of the temperature, from 9.9 up to 230 K. The kinetics of this phenomenon can be described by fitting the temperature dependent PL results with a double activation law while a simple activation model fails. It suggests that two ways are involved in the trapping process of the carriers, with activation energies in the range of a few meV and a few tenths of meV, respectively. The relationship between the PL parameters linked to the two different energetic processes associated to two energy barriers (activation energy and transition rate), the powder characteristics and the photoactivity is discussed.

Diamagnetism of quasicrystals due to Bragg's reflexions

We point out that in quasicrystals, one has to take into account a large but finite number of effective Bragg's reflexions out of the dense set. This leads to small gaps on the Fermi surface (some tenths of meV), much narrower than the usual Hume-Rothery one's (0.2-0.5 eV) invoked for explaining their stability. Small pockets of electrons and holes with very anisotropic effective masses are thus existing. The very low effective masses in some directions for electron pockets are responsible for the diamagnetism of quasicrystals. As these gaps are very sensitive to impurities, phasons, so is the diamagnetism.