Search results for "Direct and indirect band gaps"
showing 10 items of 48 documents
Surface band-gap narrowing in quantized electron accumulation layers.
2010
An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.
High-pressure structural, lattice dynamics, and electronic properties of beryllium aluminate studied from first-principles theory
2021
Abstract The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic prop…
Dipoles in 4,12,4-graphyne
2021
Abstract In present work, B-N pairs as dipole source were introduced into 4,12,4-graphyne. According to the density functional theory (DFT) simulations, the electronic configurations of the doped 4,12,4-graphyne systems were significantly modified owing to the built-in electric fields caused by the B-N dipoles. Different B-N concentrations and arrangements can alter the electronic structure of 4,12,4-graphyne. Consequently, an obvious in-plane piezoelectricity can also be induced. Moreover, the direct band gap can be delicately modulated from 150 meV to 660 meV at PBE level. The B-N dipoles can also greatly enhance the light absorption instead of shifting the absorption region. According to…
Charge transfer and tunable minority band gap at the Fermi energy of a quaternaryCo2(MnxTi1−x)GeHeusler alloy
2010
We investigate the distribution of element-specific magnetic moments and changes in the spin-resolved unoccupied density of states in a series of half-metallic ${\text{Co}}_{2}({\text{Mn}}_{x}{\text{Ti}}_{1\ensuremath{-}x})\text{Ge}$ Heusler alloys using x-ray magnetic circular dichroism. The Co and Mn magnetic moments are oriented parallel while a small Ti moment shows antiparallel to the mean magnetization. The element-specific magnetic moments remain almost independent on the composition. Therefore, a replacement of Ti by Mn results in an increase in magnetization. The increase in magnetization with increasing $x$ follows the Slater-Pauling rule. The Fermi level decreases with respect to…
Spectral and kinetic characteristics of pyroelectric luminescence in LiGaO2
2019
Abstract Pyroelectric luminescence was observed in noncentrosymmetrical crystal LiGaO2 with the direct band gap around 6 eV. For the first time spectral and kinetic characteristics of pyroelectric luminescence were obtained. The temporal structure of the PEL signal was determined as a sequence of pulses with duration not longer than several nanoseconds. This allowed proposing of the luminescence mechanism: in vacuum conditions in LiGaO2 crystal pyroelectric luminescence occurs inside the sample due to radiative recombination of electrons with the positively charged intrinsic luminescence centres.
Two-Dimensional Antimony Oxide
2020
Two-dimensional (2D) antimony, so-called antimonene, can form antimonene oxide when exposed to air. We present different types of single- and few-layer antimony oxide structures, based on density functional theory (DFT) calculations. Depending on stoichiometry and bonding type, these novel 2D layers have different structural stability and electronic properties, ranging from topological insulators to semiconductors with direct and indirect band gaps between 2.0 and 4.9 eV. We discuss their vibrational properties and Raman spectra for experimental identification of the predicted structures.
Exciton-Phonon Coupling in the Ultraviolet Absorption and Emission Spectra of Bulk Hexagonal Boron Nitride
2018
We present an \textit{ab initio} method to calculate phonon-assisted absorption and emission spectra in the presence of strong excitonic effects. We apply the method to bulk hexagonal BN which has an indirect band gap and is known for its strong luminescence in the UV range. We first analyse the excitons at the wave vector $\overline{q}$ of the indirect gap. The coupling of these excitons with the various phonon modes at $\overline{q}$ is expressed in terms of a product of the mean square displacement of the atoms and the second derivative of the optical response function with respect to atomic displacement along the phonon eigenvectors. The derivatives are calculated numerically with a fin…
Photo-induced cubic-to-hexagonal polytype transition in silicon nanowires
2019
Transformation of the crystalline lattice in silicon nanowires from cubic diamond (cub-Si) to hexagonal diamond (hex-Si) was observed under laser irradiation at intensities above 10 kW cm−2 (wavelength of 473 nm) by appearance of an additional peak in their Raman spectra in the range from 490 to 505 cm−1. Formation of the hex-Si phase in SiNWs is favoured by strong mechanical stresses caused by inhomogeneous photo-induced heating, which results in a singlet–doublet splitting of the Raman peaks for LO and TO phonons at about 517 and 510 cm−1, respectively. The estimated values of the photo-induced mechanical stresses and temperatures required for the polytype transformation in SiNWs correspo…
First-principles LCAO study of the low and room temperature phases of CdPS$_3$
2020
A.K. is grateful to the Latvian Council of Science project no. lzp-2018/2-0353 for financial support. Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.
Room temperature electrodeposition of photoactive Cd(OH)2 nanowires
2009
Cd(OH)2 nanowires (NWs) were successfully prepared by room temperature electrogeneration of base using Cd(NO3)2 aqueous electrolyte and Anodic Alumina Membrane (AAM) as template. Cd(OH)2 films have been also deposited on tin-doped indium oxide (ITO) for comparison. SEM analysis shows high quality deposits made of closely packed nanowires (NWs) into AAM and uniform flake-like surface on ITO. XRD analysis reveals that Cd(OH)2 films on ITO are polycrystalline, while the nanowires grow along the preferential directions [100] and [110]. Photoelectrochemical measurements show that Cd(OH)2 NWs are photoactive materials with indirect and direct band gap of 2.15 and 2.75 eV, respectively. Keywords: …