Nature of the chemical bond of hydrogen and oxygen atoms with PT(100) surface: Quantum chemical calculation and disappearance potential spectra

Electronic structures of clean, hydrogen covered, and oxygen covered Pt(100)-(1×1) surface have been calculated. Both absorbates form surface subzones localized below the metal conduction band but overlapping partially with. Furthermore, the local density of states (LDOS) reveals a peak of the resonant state on the absorbed atom which is narrower for hydrogen than for oxygen. The comparison of LDOS on absorbed and Pt atoms shows that subzone surface states are responsible for the covalent component of the chemical bond between absorbed and platinum atoms, while resonant states make the ionic contribution. The obtained LDOS were used to calculate disappearance potential spectra. Theoretical spectra are well consistent with experimental ones.     

Calculation of Electronic Structure of Anatase TiO2 Doped with Transition Metal V,Cr,Fe and Cu Atoms by the Linearized Augmented Plane Wave Method

The electronic density of states and band structures of doped and un-doped anatase TiO2 were studied by the Linearized Augmented Plane Wave method based on the density functional theory. The calculation shows that the band structures of TiO2 crystals doped with transition metal atoms become narrower. Interesting, an excursion towards high energy level with increasing atomic number in the same element period could be observed after doping with transition metal atoms.     

Tailoring of Electronic Structure and Thermoelectric Properties of a Topological Crystalline Insulator by Chemical Doping

Topological crystalline insulators (TCIs) are a new quantum state of matter in which linearly dispersed metallic surface states are protected by crystal mirror symmetry. Owing to its vanishingly small bulk band gap, a TCI like Pb_0.6Sn_0.4Te has poor thermoelectric properties. Breaking of crystal symmetry can widen the band gap of TCI. While breaking of mirror symmetry in a TCI has been mostly explored by various physical perturbation techniques, chemical doping, which may also alter the electronic structure of TCI by perturbing the local mirror symmetry, has not yet been explored. Herein, we demonstrate that Na doping in Pb_0.6Sn_0.4Te locally breaks the crystal symmetry and opens up a bulk electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. Na doping in Pb_0.6Sn_0.4Te increases p‐type carrier concentration and suppresses the bipolar conduction (by widening the band gap), which collectively gives rise to a promising zT of 1 at 856 K for Pb_0.58Sn_0.40Na_0.02Te. Breaking of crystal symmetry by chemical doping widens the bulk band gap in TCI, which uncovers a route to improve TCI for thermoelectric applications.     

Interfacial charge-transfer absorption: semiclassical treatment

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features but has not been experimentally observed. However, Gerischer characterized photocurrents arising from such absorption between adsorbed metal atoms and semiconductor conduction bands. Interfacial charge-transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here we provide a framework for modeling and predicting IFCTA spectra. The key feature of optical charge transfer to or from a band of electronic levels (taken to have a constant density of states and electronic coupling element) is that the absorption probability reaches half intensity at lambda + DeltaG(theta), where lambda and DeltaG(theta) are the reorganization energy and free-energy gap for the optical charge transfer, attains >90% intensity at lambda + DeltaG(theta) + 0.9 square root[4lambdak(B)T], and remains essentially constant until the top (bottom) level of the band is attained. However, when the electronic coupling and transition moment are assumed to be independent of photon energy (Mulliken-Hush model), a peaked, highly asymmetric absorption profile is predicted. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that absorption is not detectable, whereas for semiconductors there may be intense features involving coupling to surface states.     

Infrared spectra of two sexithiophenes in neutral and doped states: a theoretical and spectroscopic study

The FT-infrared spectra of two sexithiophenes having their end ,′-positions substituted by n-hexyl or -thiohexyl groups, in neutral and doped states, are studied with the main aim of deriving information about the π-electrons delocalization and about the electronic structure of the charged defects created upon doping with iodine. The analysis of the experimental data is aided by Density Functional Theory calculations. The modifications in the electronic structure of the sexithiophene backbone induced by the n-thiohexyl encapsulation are discussed from the point of view of single molecule interactions in thiol-terminated π-conjugated oligomers bound to metallic or cluster electrodes.     

EuB6的能带结构与成键及其光学常数的计算

采用密度泛函方法研究了EuB6的电子结构, 得到高精度能带结构和态密度分布信息; 分析成键情况, 从原子间轨道相互作用的角度说明EuB6能带结构的特征及其半金属性质. 计算出EuB6的介电函数、反射率、光电导率和能量损失函数谱等, 与实验结果一致, 表明了能带计算获得的电子结构信息的可靠性.     

Correlation induced insulator to metal transition: a systematic density functional study on highly doped n-type trans-polyacetylene

Since its discovery in 1977, a number of quantum chemical calculations have been attempted to simulate the metallic state of highly doped trans-polyacetylene. These simulations have focused on the possible closure of the band gap at high doping level due to a charge-induced elimination of Peierls distortion; however, conclusive demonstration of a metallic state has not been achieved. The present study presents density functional theory calculations of the band structure of highly doped trans-polyacetylene with explicit inclusion of the metal atoms in a one-dimensional periodic structure. The results indicate (i) small lattice dimerization, i.e., remnant of Peierls distortion exists even in the heavily doped trans-polyacetylene sample, (ii) charge induced closure of the Peierls gap is not a necessary condition to arrive at a metallic state in such systems, and (iii) electronic correlation, as described at the density functional theory level, with a charge induced small Peierls distortion is sufficient to achieve metallic state of highly doped n-type trans-polyacetylene even in one dimension. Furthermore, comparison of functionals that include differing degrees of electron correlation suggest that correlation promotes formation of the metallic state.     

A first-principles DFT study of UN bulk and (001) surface: comparative LCAO and PW calculations

LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribution, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low temperatures. All calculations indicate mixed metallic-covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and electronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches.     

石墨炔的化学修饰及功能化

石墨炔特殊的电子结构和孔洞结构使其在信息技术、电子、能源、催化以及光电等领域具有潜在、重要的应用前景。近几年石墨炔的基础和应用研究已取得了重要成果,并迅速成为了碳材料研究中的新领域。石墨炔中炔键单元的高活性为其化学修饰与掺杂提供了良好的平台。在这篇综述中,我们将重点介绍石墨炔的非金属杂原子掺杂、金属原子修饰以及表面改性,并深入探讨掺杂与衍生化对石墨炔材料的电子性质的影响及其对光电化学催化性能的协同增强。     

Complete basis set, Gaussian, and hybrid density functional theory evaluation of the proton affinities of water and ammonia

An ab inito computation of reorganization energy for the electron transfer (ET) reactions between metal–benzene and metal ion–benzene complexes is presented. The geometry optimization of the metal–benzene complexes was performed. The metal atoms (or metal ions)– benzene molecule separation distances computed using an ab initio method were found to agree with earlier reported results. Values of reorganization energies using George-Griffith Marcus (GGM) method (the contribution from only diagonal elements of force constant matrix) and Hessian matrix method (including the contribution from both diagonal and off-diagonal elements) were computed. Results of reorganization energy show that the GGM method gives much lower values compared to those obtained using the Hessian method, suggesting that the coupling interactions between different vibrational modes are important to the inner-sphere reorganization energy for the ET reactions in gaseous phase.     

The high-pressure electronic structure of the [Ni(ptdt)2] organic molecular conductor

The electronic structure of the single component molecular crystal [Ni(ptdt)(2)] (ptdt = propylenedithiotetrathiafulvalenedithiolate) is determined at ambient and high pressure using density functional theory. The electronic structure of this crystal is found to be of the "crossing bands" type with respect to the dispersion of the HOMO and LUMO, resulting in a small, non-zero density of states at the Fermi energy at ambient pressure, indicating that this crystal is a "poor quality" metal, and is consistent with the crystal's resistivity exhibiting a semiconductor-like temperature dependence. The ambient pressure band structure is found to be predominantly one-dimensional, reflecting enhanced intermolecular interactions along the [100] stacking direction. Our calculations indicate that the band structure becomes two-dimensional at high pressures and reveals the role of shortened intermolecular contacts in this phenomenon. The integrity of the molecular structure is found to be maintained up to at least 22 GPa. The electronic structure is found to exhibit a crossing bands nature up to 22 GPa, where enhanced intermolecular interactions increase the Brillouin zone centre HOMO-LUMO gap from 0.05 eV at ambient pressure to 0.15 eV at 22 GPa; this enhanced HOMO-LUMO interaction ensures that enhancement of a metallic state in this crystal cannot be simply achieved through the application of pressure, but rather requires some rearrangement of the molecular packing. Enhanced HOMO-LUMO interactions result in a small density of states at the Fermi energy for the high pressure window 19.8-22 GPa, and our calculations show that there is no change in the nature of the electronic structure at the Fermi energy for these pressures. We correspondingly find no evidence of an electronic semiconducting-metal insulator transition for these pressures, contrary to recent experimental evidence [Cui et al., J. Am. Chem. Soc. 131, 6358 (2009)].     

Effect of Nitrogen Substitution in V2O3 on the Metal–Insulator Transition

The effect of N‐doping on the paramagnetic–antiferromagnetic transition associated with the metal–insulator (M–I) transition of V₂O₃ at 150 K has been studied in bulk samples as well as in nanosheets. The magnetic transition temperature of V₂O₃ is lowered to ～120 K in the N‐doped samples. Electrical resistivity data also indicate a similar lowering of the M–I transition temperature. First‐principles DFT calculations reveal that anionic (N) substitution and the accompanying oxygen vacancies reduce the energy of the high‐temperature metallic corundum phase relative to the monoclinic one leading to the observed reduction in Nèel temperature. In the electronic structure of N‐substituted V₂O₃, a sub‐band of 2p states of trivalent anion (N) associated with its strong bond with the vanadium cation appears at the top of the band of O(2p) states, the 3d‐states of V being slightly higher in energy. Its band gap is thus due to crystal field splitting of the degenerate d‐orbitals of vanadium and superexchange interaction, which reduces notably (ΔE_g=?0.4 eV) due to their hybridization with the 2p states of nitrogen. A weak magnetic moment arises in the monoclinic phase of N‐substituted V₂O₃ with O‐vacancies, with a moment of ?1 μ_B/N localized on vanadium atoms in the vicinity of oxygen vacancies.     

Influence of Se and Zn substitution on the electronic transport on a CdTe nanotube-based molecular device: a first-principles study

The electronic transport property of pure cadmium telluride (CdTe) nanotube, selenium-substituted and zinc-substituted CdTe nanotube-based molecular device are investigated with density functional theory. The electronic transport property of CdTe nanotube is studied in terms of device density of states (DOS), electron density, transmission spectrum, and transmission pathways. The substitution of selenium and zinc atoms along the left electrode and bias voltage has the impact in the DOS. The electron density is found to be more at cadmium site in the left electrode. The transmission spectrum provides the insight into the transmission of electrons from valence band to conduction band across CdTe nanotube. The transmission pathway provides the visualization of electron transmission along CdTe nanotube. The results of present work provide a clear vision to tailor CdTe nanostructures with enhanced electronic property with substitution impurity for optoelectronic devices and photovoltaic cells.     

Inverse photoemission from metal surfaces

Ultraviolet inverse photoemission spectroscopy (IPES) is a technique for exploring unoccupied electronic states, particularly in the energy range between the Fermi level and vacuum level, a range inaccessible in ordinary photoemission. Theories of inverse photoemission and its special instrumentation requirements are outlined. IPES measurements on clean metal surfaces have revealed an abundance of new Shockley surface states and the Rydberg series of image states converging on the vacuum level. Empty surface states of d-like character have also been seen. The systematics of the occurrence of surface states (including image states) associated with s,p bulk band gaps are well described by a simple adaptation of multiple-reflection theory. This model is propounded and its implications discussed with regard to effective masses, surface corrugation, and determination of the surface barrier potential. IPES measurements on adsorbate-covered metal surfaces have revealed antibonding levels (O on Ni surfaces being the prime example) and valuable information on empty molecular levels (the 2π state in adsorbed CO and NO being the prime example). A straightforward Blyholder interpretation is modified by considerations of electronic relaxation and screening in the emission process. We compare and contrast the role of these effects in photoemission and inverse photoemission. Polarization selection rules and molecular shape resonances are also discussed.     

A DFT study on structural stability and electronic property of VIIIB transition metal-doped carbon nanocaps

Carbon nanocap (CNC) was selected for the systems doped with VIIIB transition metal (TM) atoms. The geometrical structures and electronic properties of TM-doped CNCs were calculated using the density functional theory method. It was found that TM atoms can interact with CNC to form TM–CNC complexes, which corresponded with the large partial charge transfer. All of molecular orbitals of TM–CNC complexes were localized in vicinity doping site. The density of states of these TM-doped CNCs were exhibited mostly metallic or narrow–gap semiconductor.     

LMTO Band Structure Calculations of ThCr2Si2-Type Transition Metal Compounds

The electronic structures of ThCr₂Si₂-type compounds were studied by means of self-consistent LMTO band structure calculations. Different bonding interactions in SrRh₂P₂are analyzed and their dependence on the electron count are discussed in terms of the formal substitution of elements. The overall bonding situation can be characterized as an interplay between covalent, metallic, and ionic interactions, although metal–metal bonding plays an important role. Particularly, the evolution of the interlayer bonding between the nonmetal atoms by changing the transition metal is examined in more detail. It turns out that the shortening of the interlayer bonds by filling thedshell of the transition metal is due to an increasing occupation of nonmetal bonding states which are pushed up to the vicinity of the Fermi level by antibonding metal–nonmetal interactions. The band structures of superconducting LuNi₂B₂C and nonsuperconducting SrRh₂P₂are compared and their similarities are pointed out. A van Hove singularity, generated by metal–metal interaction, coincides with the Fermi level in LuNi₂B₂C and lies about 0.2eV higher in SrRh₂P₂. By doping, it should be possible to induce superconductivity in SrRh₂P₂and related compounds.     

Comparative first-principles study of structural and optical properties of alkali metal azides

A comparative first-principles study of the structural and optical properties of the alkali metal azides has been performed with density functional theory within the generalized gradient approximation. The crystal structures of the alkali azides compare well with experimental data. Their ionic character is manifested by the closeness of their internitrogen distances to the calculated N-N bond length for the free azide ion. An analysis of electronic structure, charge transfer, and bond order shows that the alkali azides are all wide-gap insulators and ionic compounds. The energy band and density of states for lithium azide and alpha-sodium azide are very similar, while these for potassium azide, alpha-rubidium azide, and alpha-cesium azide are alike, but some modifications are observed with the increment of alkali metals' electropositivity. These changes are closely related to the differences of the crystal structures. The general shapes of the real and imaginary parts of the dielectric function, adsorption coefficient, and electron energy-loss spectra are quite similar. The peaks originate from the electron transitions from the alkali metal s and p states to the conduction band. Our calculated optical properties for the alkali azides are found to be in good agreement with available experimental data. The absorption spectra of the alkali azides show a number of absorption peaks, which are believed to be associated with different exciton states, in the fundamental absorption region. In general, the electron energy-loss spectra have two plasma frequencies.     

含空位和杂质缺陷的LiFePO4电子结构的第一性原理计算

采用基于密度泛函理论的第一性原理研究了含空位和杂质缺陷的LiFePO_4电子结构,通过能带、态密度、布居分布分析,阐明缺陷及阴离子掺杂对材料电化学性能的影响,为LiFePO_4的结构设计和实验研究提供理论基础。结果表明,Li、Fe和O空位型缺陷对LiFePO_4的带型变化影响较小,禁带中无新的导带,禁带宽度有一定程度缩小,有利于电子的传导,但总能量上升,造成结构的不稳定性,在实际高温制备过程中,可能产生少量杂相,影响LiFePO_4正极材料的电化学性能;P空位缺陷对LiFePO_4的带型影响同样较小,但在禁带中产生了两条新的导带,禁带宽度明显变窄,有利于电子的传导,虽然总能量上升,造成结构的不稳定性,但在实际高温制备过程中,可能产生微量有利于电化学性能的杂相;F掺杂LiFePO_4的带型出现了明显的变化,半导体类型由p型转变为n型,极大地促进了电子的导电性,总能量下降,结构稳定,对LiFePO_4正极材料的电化学性能有正面的影响。     

直流反应磁控溅射制备的Mo掺杂TiO2薄膜的光电特性

通过直流反应磁控溅射制备了不同Mo掺杂量的Mo-TiO2薄膜.用原子力显微镜(AFM)、X射线衍射(XRD)仪、X射线光电子能谱(XPS)仪、紫外-可见(UV-Vis)分光光度计详细研究了Mo掺杂量对薄膜表面形貌、晶体结构、元素价态及吸收带边的影响.用瞬态光电流和循环伏安法考察了不同Mo含量ITO/Mo-TiO2电极的光电特性.结果表明:在TiO2薄膜中掺入的Mo以Mo6+和Mo5+两种价态存在;随着Mo掺杂量的增加,Mo-TiO2薄膜的晶粒尺寸逐渐减小,晶格畸变增大,吸收阈值显著红移;薄膜的禁带宽度先减小后增大,在Mo掺杂量为2.7%(n(Mo)/n(Ti))时禁带宽度最小;Mo掺杂量为0.9%的样品在氙灯下的光生电流最大,且随着所加阳极偏压的提高光生电流并未呈现出饱和的趋势.此后随着掺杂量的提高,薄膜的光生电流开始下降,当Mo掺杂量达到3.6%时,薄膜的光电流小于未掺杂的样品;说明适当浓度的Mo掺杂能够提高Mo-TiO2薄膜光电性能,光生电流最大可达未掺杂的2.4倍.     

Electronic properties of a pure and sodium-doped C(70) single layer adsorbed on Al polycrystalline surface

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C(70) layer adsorbed on polycrystalline Al (1 ML-C(70)/Al) (ML-monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C(70) molecules tend to orient themselves with the C(5v) axis perpendicular to the surface in analogy to what observed for 1 ML-C(70)/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C(70) molecules takes place, as underlined by the gradual increasing intensity of the C(70) LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step.