Effects of acceptor–donor complexes on electronic structure properties in co-doped TiO2: A first-principles study

We theoretically investigate the doping effects induced by impurity complexes on the electronic structures of anatase TiO₂ based on the density functional theory. Mono-doping and co-doping effects are discussed separately. The results show that the impurity doping can make the band-edges shift. The induced defect levels in the band gaps by impurity doping reduce the band gap predominantly. The compensated acceptor–donor pairs in the co-doped TiO₂ will improve the photoelectrochemical activity. From the calculations, it is also found that (S+Zr)-co-doped TiO₂ has the ideal band gap and band edge, at the same time, the binding energy is higher than other systems, so (S+Zr)-co-doping in TiO₂ is more promise in photoelectrochemical experiments.     

Location of muonium and hydrogen in C60 fullerene and associated electronic structure and hyperfine properties

The unrestricted Hartree-Fock (UHF) procedure is used to investigate the locations, associated electronic structures and hyperfine interactions for muonium and hydrogen in C₆₀ fullerene. Our results indicate that from total energy considerations, in keeping with earlier investigations, the exohedral model has the lowest energy. However, the energies of the endohedral model involving the muonium (hydrogen) inside the fullerene and bonded to one of the carbon atoms, and of the muon at the center are found to be almost equal, contrary to earlier results. The hyperfine interaction constant for the endohedral site is in good agreement with that required to explain the lower observed muon spin-rotation (SR) frequency in the C₆₀-muonium system. The same appears to be the case for the exohedral model. However, there seems to be some uncertainty about the theoretical result in the latter case due to significant admixtures of higher spin states in the UHF wave-function. Additionally, in solid fullerene, the calculated location of the muonium for the exohedral model is such that it could be bonded to two fullerene molecules and therefore a muonium attached to a simple fullerene may not be representative of the exohedral state. This feature as well as the difficulty for the exohedral model of explaining the observed equality of the correlation times for relaxation effects associated with both SR and¹³C relaxation times in nuclear magnetic resonance (NMR) experiments suggests that the endohedral model for muonium cannot at present be ruled out as a viable model in favor of the exohedral model. Possible avenues for future investigations to resolve some of the problems for both exohedral and endohedral models are discussed. Results obtained for muonium at the center of fullerene are presented and compared to the features of the observed high frequency SR signal, and possible improvements in theory are discussed.     

Band engineering of silicon for light emission: First-principles approach to the effect of co-doping with nitrogen and fluorine

A Si-based light emitter has long been the final key component for electronic and photonic integrated circuits on Si, because Si has an indirect band gap. Atomistic and electronic structures and energy gains of formation of possible nitrogen (N) and fluorine (F) complexes in Si have been researched from first-principles, in order to engineer the band structure of Si for light emission. The calculated results show that the substitutional nitrogen N_S and bond center fluorine F_BC pair complex has large stabilization energy, and that the pair-complex-doped Si has direct band gap, which is reduced with respect to that of Si. These results lead to the possibilities of doping-based engineering of Si optical properties with introduction of deep-level impurity and charge compensation.     

A study of the electronic and structural properties of C28 and C16N12

Equilibrium geometry of the heterofullerene C₁₆N₁₂ is calculated at the Density Functional Theory level and further compared with that of fullerene C₂₈. Theoretical vibrational spectra as well as the infrared intensities and Raman activities are reported. In addition, the effect of N substitution on fullerene stability is investigated in comparison to exohedral complexes C₂₈Cl₄ and C₁₆N₁₂Cl₄. The results show that C₁₆N₁₂ is stabilized by strengthening the hexagon belts, while exohedral complexes stabilize the hexagon structure.     

Encapsulation of small fullerenes into nitrogenated holey nanotubes: a density functional theory study

Encapsulation of fullerene into nanotubes based on a C₂N sheet, known as nitrogenated holey graphene, was investigated using density functional theory. The structural and electronic properties of these carbon hybrid materials, consisting of nitrogenated holey nanotubes and a small C₂₀ fullerene, were studied. The formation energies showed that encapsulation of the fullerene into the nitrogenated holey nanotube is an exothermic process. To characterise the electronic properties, the electronic band structure and density of states of armchair and zigzag nitrogenated holey nanotubes were calculated. Filling these nanotubes with the C₂₀ fullerene resulted in a p-type semiconducting character. The energy band gap of the nitrogenated holey nanotubes decreased with fullerene encapsulation. The results are indicative of the possibility of band gap engineering by encapsulation of small fullerenes into nitrogenated holey nanotubes.     

Vibronic resonance in spectra of frozen solutions of the C60 fullerene derivatives

This paper reports on a study by site-selective laser spectroscopy of low-temperature broadband fluorescence spectra of N-methyl-2-(3,5-di-tert-butyl-4-hydroxyphenyl)-[C₆₀] fulleropyrrolidine (N-MBHPhFP fullerene) molecules embedded in the crystalline toluene matrix. It has been shown that monochromatic laser excitation in the region of vibrational satellites of the pure electronic absorption band initiates structurization of the emission band of a pure electronic transition, which suggests the existence of a set of impurity centers in the crystalline host matrix. An analysis has been made of the factors accounting for replacement of the wide, inhomogeneously broadened band by a series of narrow bands, with the most essential of them being the weakness of electron-phonon coupling in the impurity center, the absence of electronic excitation transfer between the centers, and the vibronic resonance in the excited state of the set of centers. Vibration frequencies of the N-MBHPhFP fullerene in the excited electronic state have been determined.     

DFT study on electronic structure and optical properties of N-doped,S-doped,and N/S co-doped SrTiO3

The electronic structures and optical properties of N-doped, S-doped and N/S co-doped SrTiO₃ have been investigated on the basis of density functional theory (DFT) calculations. Through band structure calculation, the top of the valence band is made up of the O 2p states for the pure SrTiO₃. When N and S atoms were introduced into SrTiO₃ lattice at O site, the electronic structure analysis shows that the doping of N and S atoms could substantially lower the band gap of SrTiO₃ by the presence of an impurity state of N 2p on the upper edge of the valence band and S 2p states hybrid with O 2p states, respectively. When the N/S co-doped, the energy gap has further narrowing compared with only N or S doped SrTiO₃. The calculations of optical properties also indicate a high photo response for visible light for N/S co-doped SrTiO₃. Besides, we find a new impurity state which separates from the O 2p states could improve the photocatalytic efficiency and we also propose a model for light electron-hole transportation which can explain the experiment results well. All these conclusions are in agreement with the recent experimental results.     

Bucky-corn: van der Waals composite of carbon nanotube coated by fullerenes

ABSTRACT

Can a C₆₀ layer cover a surface of single-wall carbon nanotube (SWCNT) forming an exohedral pure-carbon hybrid with only van der Waals interactions? The aim of the present paper is to address this question and to demonstrate that the fullerene shell layer in such a bucky-corn structure can be stable. Theoretical study of the structure, stability and electronic properties of bucky-corn hybrids is reported for the shell of C₆₀ and C₇₀ molecules on an individual SWCNT, C₆₀ dimers on an individual SWCNT as well as C₆₀ molecules on SWNT bundles. The geometry and total energies of the bucky-corn hybrids were calculated by the molecular dynamics method, while the density functional theory method was used to simulate the electronic band structures.     

Tuning the electronic properties of the fullerene C<Subscript>20</Subscript> cage via silicon impurities

The fullerene C₂₀ represents one of the most active classes of nanostructures, and they have been widely used as active materials for important applications. In this study, we investigate and discuss the tuning of the electronic properties of the fullerene C₂₀ cage via various consternations and locations of silicon atoms. All calculations are based on the density functional theory (DFT) at the B3LYP/3-21G level through the Gaussian 09W program package. The optimized structures, density of state (DOS) analysis, total energies, dipole moments, HOMO energies, Fermi level energies, LUMO energies, energy gaps, and the work functions were performed and discussed. Our results show that the electronic properties of C₂₀ cage do not only depend on the silicon impurity concentrations, but also depend on the geometrical pattern of silicon impurities in the C₂₀ cage. The tuning of the electronic properties leads to significant changes in the charge transport and the absorption spectra for C₂₀ cage via engineering the energy gap. So, we suggest that substitutional impurities are the best viable option for enhancement of desired electronic properties of C₂₀ cage for using these structures in nanoelectronics and solar cell applications.     

Modulation of electronic structure properties of C/B/Al-doped armchair GaN nanoribbons

ABSTRACT

The electronic structures of C/B/Al-doped armchair GaN nanoribbons (aGaNNRs) are systematically studied by using density functional theory. We find that the original aGaNNRs are direct band gap semiconductors and that the gaps monotonically decrease with increasing widths. Interestingly, the B- or Al-doped aGaNNRs are also direct-band gap semiconductors with a slightly larger gap than their undoped aGaNNRs, while the C-doped aGaNNRs display metallic characteristics with an impurity state across the Fermi level in band structures. The semiconducting or metallic behaviours of C/B/Al-doped aGaNNRs can be explained by the orbital coupling between the extrinsic atom and primary Ga, N in their partial density of states. Our results show a useful way to modulate the band gaps of aGaNNRs.

Using the density-functional theory, we performed a theoretical research to study the electronic structures of C/B/Al-doped armchair gallium nitride nanoribbons. The calculated band structures show that the perfect and original aGaNNRs are direct semiconductors regardless of ribbon widths, and gaps monotonically decrease with increasing the widths. The B/Al-doped aGaNNRs are semiconductors with a slightly larger gap, while metallic behavior presents in C-doped aGaNNRs with an impurity band across the EF. The results show a useful way to modulate the band gaps of aGaNNRs.     

Ab initio calculations of endo-and exohedral C60 fullerene complexes with Li+ ion and the endohedral C60 fullerene complex with Li2 dimer

The results of ab initio Hartree-Fock calculations of endo-and exohedral C₆₀ fullerene complexes with the Li⁺ ion and Li₂ dimer are presented. The coordination of the Li⁺ ion and the Li₂ dimer in the endohedral complexes and the coordination of Li⁺ ion in the exohedral complex of C₆₀ fullerene are determined by the geometry optimization using the 3–21G basis set. In the endohedral Li⁺C₆₀ complex, the Li⁺ ion is displaced from the center of the C₆₀ cage to the centers of carbon hexa-and pentagons by 0.12 nm. In the Li₂ dimer encapsulated inside the C₆₀ cage, the distance between the lithium atoms is 0.02 nm longer than that in the free molecule. The calculated total and partial one-electron densities of states of C₆₀ fullerene are in good agreement with the experimental photoelectron and X-ray emission spectra. Analysis of one-electron density of states of the endohedral Li⁺@C₆₀ complex indicates an ionic bonding between the Li atoms and the C₆₀ fullerene. In the Li⁺C₆₀ and Li⁺@C₆₀ complexes, there is a strong electrostatic interaction between the Li⁺ ion and the fullerene.     

四角晶相HfO2(001)表面原子和电子结构研究

采用基于第一性原理的密度泛函理论研究了四角晶相二氧化铪(t-HfO₂)体相及 其(001)表面的原子几何与电子结构.理论计算结果表明，t-HfO₂(001)表面不会 产生重构现象.与体相电子结构相比, t-HfO₂(001)表面态密度明显高于体相态 密度.其次，表面原子的态密度更靠近费米能级(E_F)，价带往低能量处移动，并 有表面态产生.计算结果表明了t-HfO₂表面禁带宽度明显低于体相的禁带宽度. t-HfO₂(001)的表面态产生以及表面禁带宽度减小是由于Hf原子与O原子的配位 数减少，表面原子周围的环境发生变化而引起的. 关键词： 密度泛函理论 2(001)')" href="#">t-HfO₂(001) 表面电子结构     

Oxygen vacancy in N-doped Cu2O crystals: A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory. The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure. In N anion-doped Cu2O, some N 2p states overlap and mix with the O 2p valence band, leading to a slight narrowing of band gap compared with the undoped Cu2O. However, it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

Local electronic structure around Mn and Ga in Mn-doped GaN films showing room temperature ferromagnetism

Local electronic structures around Ga and Mn in Mn-doped GaN film with T_c of 940 K are investigated by K X-ray absorption near edge structure (XANES) analysis. It was found that the shape of the Ga XANES spectrum is remarkably similar to that of the un-doped GaN film indicating that the local electronic structure around Ga is not disturbed with Mn doping. As for the Mn XANES spectra, obvious pre-edge peaks were observed: the fine structures in the pre-edges correspond with calculated Mn 3d partial density of states which predict impurity band formation with the Fermi energy stays in the spin-up band. These findings imply that Mn 3d levels stay within the gap with the Fermi energy stays in the spin-up band.     

第一性原理研究Fe掺杂SnO2材料的光电性质

采用基于第一性原理的线性缀加平面波(FP-LAPW)方法,研究Fe掺杂SnO₂材料电子结构和光学性质,包括电子态密度、能带结构、介电函数和其他一些光学图谱. 研究结果表明,掺Fe后材料均属于直接跃迁半导体,且呈现半金属性;随掺杂浓度增加,费米能级进入价带,带隙逐渐减小,Fe原子之间耦合作用增强;通过掺杂能够在一定程度上改变成键性质,使其具有金属键性质. 光学谱线(吸收谱、消光系数等)与介电函数虚部谱线相对应,均发生蓝移,各峰值与电子跃迁吸收有关,从理论上指出光学性质和电子结构的内在联系. 关键词： 能带结构 态密度 光学性质 介电函数     

The electronic and optical properties of Cu doped CdI2

Based on the density functional pseudopotential method, the electronic structures and the optical properties of CdI₂ doped with Cu are investigated in detail. The calculation results indicate that the defect of Cu(Cd) exists steadily with a certain solubility. For the Cu doped CdI₂, the new highly localized impurity bands induced by Cu 3d states lie just across the Fermi energy at the top of the valence band. The doping of Cu induces reduction of band gap of CdI₂; red shifts are revealed in both the imaginary part of dielectric function and the absorption spectra corresponding to the change in band gaps. Moreover, the study of the reflection spectrum and the loss function shows that the doped Cu is responsible for the increased reflection peak intensity and the red shift of the plasma resonant frequency of CdI₂.     

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x _thr, 1, (2) lead to a superconducting state for x _thr, 1 < x < x _thr, 2, and (3) decay as x > x _thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.     

Doping effects on metallic and semiconductor single-wall carbon nanotubes

In this paper we investigate nitrogen- and boron-doped zigzag and armchair single-wall carbon nanotubes (SWNTs) with theoretical models based on the density functional theory. We take into account nitrogen and boron doping for two isomers in which substitutive atoms are on opposite sides of the tube, but only in one isomer the impurity sites are symmetrical with respect to the diameter. The band structures show a strong hybridization with impurity orbitals that change the original band structure. Although the two isomers of armchair SWNT exhibit the same formation energy, their band structures are different. Indeed asymmetrical isomers are gapless and exhibit a crossing of valence and conduction bands at k?=?π/c, leading to metallic SWNTs. Band structures of symmetrical isomers, on the other hand, exhibit an energy gap of 0.4?eV between completely filled valence and empty conduction bands. We use density of charge in order to understand this difference. In zigzag SWNT an impurity band is introduced in the energy gap and for N doping this band is just partially occupied in such a way that the electronic behaviour is reversed from semiconductor to metallic. Whereas for a given isomer armchair SWNT shows similar behaviours of N- and B-doped structures, B-doped zigzag SWNTs present different band structure and occupation compared to the N-doped case.     

Theoretical study of magnetic ordering and electronic properties of AgxAl1−x N compounds

Ferromagnetic ordering of silver impurities in the AlN semiconductor is predicted by plane-wave ultrasoft pseudopotential and spin-polarized calculations based on density functional theory (DFT). It was found that an Ag impurity atom led to a ferromagnetic ground state in Ag_0.0625Al_0.9375N, with a net magnetic moment of 1.95 μ_B per supercell. The nitrogen neighbors at the basal plane in the AgN₄ tetrahedron are found to be the main contributors to the magnetization. This magnetic behavior is different from the ones previously reported on transition metal (TM) based dilute magnetic semiconductor (DMS), where the magnetic moment of the TM atom impurity is higher than those of the anions bonded to it. The calculated electronic structure band reveals that the Ag-doped AlN is p-type ferromagnetic semiconductor with a spin-polarized impurity band in the AlN band gap. In addition, the calculated density of states reveals that the ferromagnetic ground state originates from the strong hybridization between 4d-Ag and 2p-N states. This study shows that 4d transition metals such as silver may also be considered as candidates for ferromagnetic dopants in semiconductors.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.