Search results for "electronic band structure"
showing 10 items of 206 documents
High-spin bands in117,119I and118Xe
1984
Excited states in117I,118Xe and119I were populated in the reactions106Pd+16O and108, 110Cd+12C. The groundstate band in118Xe was observed to its (18+) member and the 11/2− bands in117I and119I to their (35/2−) and (43/2−) members, respectively. The structure of the bands in the I isotopes is interpreted as aπ h11/2 proton coupled to an even-even band structure in Xe, and the band crossings observed in118Xe and119I are interpreted in the Cranked Shell Model framework asABn ofv h11/2.
Influence of F centres on structural and electronic properties of AlN single-walled nanotubes
2007
We analyse the influence of uncharged N vacancies (neutral F centres), created either under conditions of AlN nanotube growth or by its soft irradiation, on the atomic and electronic structure. Periodic one-dimensional (1D) density functional theory (DFT) calculations on models of defective single-walled nanotubes (SW NTs) allow us to analyse how NT chirality and concentration of F centres change their properties compared to the corresponding defect-free nanotubes. We have simulated reconstruction around periodically repeated F centres on 1 nm AlN SW NTs with armchair- and zigzag-type chiralities. To achieve the limit of an isolated vacancy for both chiralities, we have considered different…
Theoretical determination of the geometric and electronic structures of oligorylenes and poli(peri‐naphthalene)
1992
We present a theoretical investigation of the electronic structure of oligorylenes (from perylene to heptarylene, including also the naphthalene molecule) and their corresponding polymer poly(peri‐naphthalene) (PPN) using the nonempirical valence effective (VEH) method. The geometry of the unit cell used to generate the polymer is extrapolated from the PM3‐optimized molecular geometries of the longest oligorylenes. That geometry shows some bond alternation along the perimeter carbon chains and a bond length of ≊1.46 Å is calculated for the peri bonds connecting the naphthalene units. The VEH one‐electron energy level distributions calculated for oligorylenes are used to interpret the experi…
Theoretical study of the effect of substituent and backbone conformation on the electronic properties of symmetrically substituted poly(di‐n‐alkylsil…
1994
We present the results of ab initio 3‐21G∗ geometry optimizations and valence effective Hamiltonian (VEH) band structure calculations aimed at determining the evolution of the geometric and electronic (ionization potential, electron affinities, and band gaps) properties of all‐trans poly(dimethylsilane), poly(diethylsilane), poly(di‐n‐propylsilane), and poly(di‐n‐butylsilane) when increasing the size of the alkyl group. In the latter polymer, we have also studied the 7/3 conformation, in order to analyze the effect of the backbone conformation on the geometric and electronic structure. The VEH ionization potentials of all‐trans poly(di‐n‐alkylsilanes) are almost equal, and as experimental p…
Tuning the band gap of PbCrO4 through high-pressure: Evidence of wide-to-narrow semiconductor transitions
2014
The electronic transport properties and optical properties of lead(II) chromate (PbCrO4) have been studied at high pressure by means of resistivity, Hall-effect, and optical-absorption measurements. Band-structure first-principle calculations have been also performed. We found that the low-pressure phase is a direct band-gap semiconductor (Eg = 2.3 eV) that shows a high resistivity. At 3.5 GPa, associated to a structural phase transition, a band-gap collapse takes place, becoming Eg = 1.8 eV. At the same pressure the resistivity suddenly decreases due to an increase of the carrier concentration. In the HP phase, PbCrO4 behaves as an n-type semiconductor, with a donor level probably associat…
Phase Behavior of TmVO4 under Hydrostatic Compression: An Experimental and Theoretical Study
2020
We present a structural and optical characterization of magnetoelastic zircon-type TmVO4 at ambient pressure and under high pressure. The properties under high pressure have been determined experimentally under hydrostatic conditions and theoretically using density functional theory. By powder X-ray diffraction we show that TmVO4 undergoes a first-order irreversible phase transition to a scheelite structure above 6 GPa. We have also determined (from powder and single-crystal X-ray diffraction) the bulk moduli of both phases and found that their compressibilities are anisotropic. The band gap of TmVO4 is found to be Eg = 3.7(2) eV. Under compression the band gap opens linearly, until it unde…
Electronic structure of CuAlO2 and CuScO2 delafossites under pressure
2007
The electronic structure of CuAlO 2 and CuScO 2 delafossites is investigated by means of optical absorption measurements under pressure and ab initio band structure calculations. Measurements are carried out on CuAlO 2 monocrystals and pulsed laser deposited CuAlO 2 and CuScO 2 thin films up to 20 GPa. CuAlO 2 is an indirect semiconductor that is stable in the pressure range explored here. The pressure coefficients of the indirect and direct gaps are found to be 15 meV/GPa and 2 meV/GPa respectively. CuScO 2 is a direct semiconductor and the pressure coefficient of the excitonic peak energy is -5.5 meV/GPa. Two reversible phase transitions are observed in CuScO 2 . At 13 GPa the delafossite…
Verwey-type transition in EuNiP
2006
High temperature 151Eu Mossbauer measurements provide proof for inhomogeneous mixed-valent behaviour in EuNiP. We observed that EuNiP undergoes a Verwey-type charge delocalisation transition when heated above 470 K prior to the structural γ-β phase transition at T ≈ 510 K. This finding confirms the results of photoemission spectroscopy in the isostructural compound EuPdP and of TB-LMTO-ASA band structure calculations. We discuss the role of a van Hove singularity associated with a high density of 4f states close to the Fermi energy in inhomogeneous mixed europium valency, and the microscopic mechanism of γ-β phase transition in compounds analogous to EuNiP.
Photoelectrochemical characterization of photocatalysts
2021
Abstract This chapter aims to provide an overview of the photoelectrochemical characterization of semiconducting photocatalysts and, in particular, present Photocurrent Spectroscopy (PCS) as a useful tool in determining the band structure of semiconducting/insulating materials. Some fundamentals on PCS will be provided looking at the experimental setup and underlying its advantages and disadvantages. Then, the photoelectrochemical behavior of a semiconductor/electrolyte junction under irradiation will be presented, also taking into account its crystalline or amorphous nature, highlighting how it is possible to get information on the energetics of the junction. This will be exploited to show…
Photoluminescence from silicon nanocrystals initiated by Auger recombination
2006
Abstract The mechanism of intense photoluminescence (PL) of silicon nanocrystals (nc-si), so interpreted as recombinative emission is reconsidered. Analysis of available theoretical and experimental data is presented to show that nc-si have an indirect band structure and, therefore, it is doubtful that electron–hole recombination is the only mechanism of intense emission. A model is proposed according to which a fraction of electrons reaches the second conduction sub-band by Auger recombination, a part of intense visible radiation being caused by direct electron transitions from the second conduction sub-band to the first one. Continuity equations are constructed in the first and the second…