Highest Occupied Valence Band Research Articles

Structural and electronic properties of amorphous bismuth calcium borate from first-principle calculations

We investigatethe electronic and structural properties of two new amorphous materials, Bi6Ca2O28B12 and Bi6Ca2 O28B11:Tm, using density functional theory with a short-range description similar to that for crystalline solids and by considering the Tm 4f states as core states. Our results show that the valence band is formed by broadband states with a strong presence of the O − 2p orbital states in both materials. The thulium-doped structure shows an increased band gap, indicating the strong presence of this rare earth in connection with the oxygen. Both materials exhibit a narrow band gap. Calculations related to the electronic transition of the orbitals between the highest occupied valence band and the lowest unoccupied conduction band and the density charge show a significant contribution of bismuth and oxygen in the formation of both materials. Our results present important information that can assist in the investigation of the technological applications of devices with a design focus for solid-state lasers.

Influence of Cr3+ doping on the enhanced dielectric and nonlinear optical features of pyroelectric Pb5Ge3O11 single crystals

It was shown a possibility to enhance the output laser stimulated second harmonic susceptibility for Pb5Ge3O11 single crystals in their ferroelectric phase using an external laser light and appropriate doping by Cr3+ ions. The improvement of the Nonlinear Optical effect is achieved due to local enhancement of the local charge density non-centrosymmetry (acentricity), changes of the effective excitation of the occupied states and additional photo-polarization of the 3d-localized Cr3+ states. The addition of Cr3+ ions leads to additional enhancement of hyperpolarizability. To clarify the origin of the observed effect, and to understand the origin of its interesting properties, the calculations of the band structure of Pb5Ge3O11 were performed using, for the first time, the first-principles DFT calculations. These reveal that Pb5Ge3O11 has an indirect energy band gap Eg equal to about 2.6 eV and that the highest occupied valence band is very flat and is separated from the remaining valence bands by a small energy gap equal to about 0.3–0.4 eV. This discovery could explain many unexplained puzzling experimental results and especially the different photo-activated effects found previously in Pb5Ge3O11. The possible origin of the DFT calculated band structure is discussed. The enhancement of the second order optical susceptibility was explained within a framework of additional charges of the particular dopants during the coherent laser illumination.

First-Principles Study on Structural and Electronic Properties of the Armchair GaN Nanoribbons

Calculations have been performed for the structures and electronic properties of GaN nanoribbons with armchair edge (AGaNNRs), using the first-principles projector-augmented wave (PAW) potential within density functional theory (DFT) framework. The lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) are always separated, representing a semiconductor character for the AGaNNRs. In addition, the majority and minority spin bands are fully superposition and therefore the AGaNNRs are non-magnetic. As the nanoribbons width increase, band gaps of AGaNNRs decrease monotonically and become close to their asymptotic limit of a single layer of GaN sheet.

The Structural and Electronic Properties of the Zigzag GaN Nanoribbons: A First-Principles Study

We have performed the first-principles calculations onto the structural and electronic properties of GaN nanoribbons with zigzag edge (ZGaNNRs). The results show that, the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) are always separated, representing a semiconductor character for the ZGaNNRs. In addition, the majority and minority spin bands are fully superposition and therefore the ZGaNNRs are non-magnetic. As the nanoribbons width increase, band gaps of ZGaNNRs decrease monotonically and become close to their asymptotic limit of a single layer of GaN sheet. It is found that the fewer coordination number will lead the most electrons to range in higher energy region of the occupancy state.

First-principles study on structural and electronic properties of AlNSix heterosheet

Under generalized gradient approximation (GGA), the structural and electronic properties of AlN and Si sheets, hydrogen terminated AlN and Si nanoribbons with hexagonal morphology and 2, 4, 6 zigzag chains across the ribbon width and the hexagonally bonded heterosheets AlNSix (x=2, 4, and 6) consisting of hexagonal networks of AlN (h-AlN) strips and silicene sheets with zigzag shaped borders have been investigated using the first-principles projector-augmented wave (PAW) formalism within the density function theory (DFT) framework. The AlN sheet is an indirect semiconductor with a band gap of 2.56eV, while the Si sheet has a metallic character since the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) meet at one k point from Γ to Z. In the semiconductor 6-ZAlNNR, for example, the states of LUCB and HOVB at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. In metallic 6-ZSiNR, a flat edge state is formed at the Fermi level EF near the zone boundary Z because its charges are localized at edge Si atoms. The hybridizations between the edge states of h-AlN strips and silicene sheets result in the appearance of border states in the zigzag borders of heterosheets AlNSix whose charges are localized at two atoms of the borders with either bonding or antibonding π character.

Structural and electronic properties of a single C chain doped zigzag AlN nanoribbon

The effects of a single C-chain on the structural and electronic properties of the H terminated zigzag AlN nanoribbons (ZAlNNRs) have been investigated systemically by using the first-principles. The results show that in perfect 7-ZAlNNR for example, the states of the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) at zone boundary Z are edge states their charges are localized at edge Al and N atoms, respectively. Introducing a single C-chain and changing its position lead to the LUCB and HOVB getting closer with each other. Similar to the edge states existing in perfect ZAlNNR, the flat dispersion border states also exist in a single C-chain decorated ZAlNNR, but their charges are localized at border C–N and C–Al for LUCB and HOVB, respectively. Furthermore, for N z -ZAlNNR-C( n) with ribbon width N z = 2, 3, 4, 5, 6, 7 and 10, only N z -ZAlNNR-C(1) has a direct band gap, while the other N z -ZAlNNR-C( n) has an indirect band gap. Variation of the band gap with C-chain position n shows that, for N z -ZAlNNR-C( n) of arbitrary width N z , the N z -ZAlNNR-C(1) and N z -ZAlNNR-C(2) have nearly identical the minimum band gap of 0.132 eV and nearly identical the maximum band gap of 1 eV, respectively, except the maximum band gap of 0.63 eV for 2-ZAlNNR-C(2) because it belongs to the group of C-chain substituting the right edge Al–N chain, the band gap of this group decreases linearly with increasing ribbon width N z . For 3-, 4-, 5-, 6-, 7- and 10-ZAlNNR-C( n), the band gap decreases successively for C-chain position n from 2 to 3, 4, 5, 6, 7 and 10, respectively.

First-principles study on the structural and electronic properties of AlNC x nanosheet

The structural and electronic properties of a hydrogen terminated hexagonally AlN nanoribbon with 6 zigzag Al–N chains across the ribbon width (6-ZAlNNR) and the hexagonally bonded hetero-sheets AlNC x ( x = 2 , 4 , 6 ) consisting of AlN and graphite strips with zigzag shaped borders have been investigated systemically by using the first-principles. The results show that in 6-ZAlNNR, the states of the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. Introducing the graphite strip C x and increasing its width lead to the LUCB and HOVB getting closer with each other especially in flat dispersion region around the zone boundary J y , thus decreasing in the energy gap of the hetero-sheets AlNC 2, AlNC 4 and AlNC 6 successively. Similar to the edge states existing in zigzag edged AlNNR, the flat dispersion border states also exist in the zigzag borders of hexagonally networked hetero-sheets AlNC x . Unlike the edge states whose charges are localized at one of the edge atoms, the border states are localized at two atoms of the borders with either bonding or antibonding character.

Electronic structure and chemical bonding properties of UO 2F 2 from first principles

Band theoretical results are presented on UO 2F 2 equi-oxyfluoride based on computation methods within the density functional theory. From pseudo-potential projector augmented wave PP–PAW calculations the equation of state is obtained with equilibrium properties in agreement with experiment. For isotropic volume change a bulk modulus of B 0 = 150 GPa indicates a relatively soft material but anisotropic compression along the c-axis leads to excessive hardness. Scalar-relativistic all-electron calculations detailing the band structure point to little dispersion of the bands with a large band gap of ∼3.2 eV. It is direct at Γ and occurs between highest occupied valence band (VB) [ U ( f z 3 ) – O ( p ) ] and lowest unoccupied conduction band [U(f x, y )]. Analyses of the density of states (PDOS) and chemical bonding properties show that the VB lower part is dominated by U(d)–F(p) bonding while the VB upper part comprises U(f)–O(p) strong covalent interactions. These results provide a band structure illustration of the complex uranyl cation UO 2 2+.