The electronic structure of pentacene revisited

Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5 eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50 meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(0 2 2). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(0 2 2) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25 eV) does indeed exist in 5A(0 2 2).     

Theoretical investigations of the effect of vacancies on the geometric and electronic structures of zinc sulfide

The effects of S-vacancy and Zn-vacancy on the geometric and electronic structures of zinc blende ZnS are investigated by the first-principles calculation of the plane wave ultrasoft pseudopotential method based on the density functional theory. The results demonstrate that both S-vacancy and Zn-vacancy decrease the cell volume and induce slight deformation of the perfect ZnS. Furthermore, this change of geometric structure caused by Zn-vacancy is more obvious than the one due to the S-vacancy. The formation energy of S-vacancy is higher than that of Zn-vacancy, indicating that Zn-vacancy is easier to form than S-vacancy in ZnS crystal. Electronic structure analysis shows that Zn-vacancy increases the band-gap of ZnS from 2.03 eV to 2.15 eV, while the S-vacancy has almost no effect on the band-gap of ZnS. Bond population analysis shows that Zn-vacancy increases covalence character of the Zn–S bonds around Zn-vacancy, while S-vacancy shows a relatively weak effect on the covalence character of Zn–S bonds.     

Anharmonic properties of rocksalt structure solids

An effort has been made for obtaining the anharmonic properties of rocksalt structure solids starting from primary physical parameters viz. nearest-neighbor distance and hardness parameter assuming long- and short-range potentials at elevated temperatures. The elastic energy density for a deformed crystal can be expanded as power series of strains for obtaining coefficients of quadratic, cubic and quartic terms which are known as the second-, third- and fourth-order elastic constants, respectively. When the values of the higher-order elastic constants are known for a crystal, many of the anharmonic properties of the crystal can be treated within the limit of the continuum approximation in a quantitative manner. In this study, higher-order elastic constants are computed up to their melting temperature for rocksalt structure solids, which are alkali cyanides, sodium and potassium halides. The first order pressure derivatives of second- and third-order elastic constants, the second-order pressure derivatives of second-order elastic constants and partial contractions are also evaluated at different temperatures for these substances. The results thus obtained are compared with experimental data and found in well agreement with present values.     

The CPA solution of the anomalous Lyapunov exponent for disordered binary chains

The Coherent Potential Approximation (CPA) self-consistent equation is calculated for a binary disordered chain introducing a simple transformation. The transformation reduces the CPA equation to a cubic polynomial whose complex roots are related to the Green function and their relation to the complex Lyapunov exponent is also established. This solution fruitfully captures essential aspects of the well-known anomalous scaling behaviors in a different and advantageous way. It is found that the anomalous behavior is strongly effected by the nature of these roots. A small disorder expansion is carried out for comparison with the previous weak disorder calculations. We found that the CPA reproduced the anomalous behavior of the exact calculations. Received 22 April 2002 Published online 1st October 2002 RID="a" ID="a"e-mail: avgin@bornova.ege.edu.tr     

Band gap structure of disordered chiral photonic crystals

Based on the non-symmetric transmission-line method, the band gap structure of disordered chiral photonic crystals (CPC) has been investigated. The influence of the chiral parameters, the disorder, the periods and the refractive index on the band gap structure has been discussed. It is found that the photonic band gap (PBG) in CPC is more obvious than that in the conventional photonic crystals, and the PBG can be increased by increasing the periods or the contrast of the refractive index of the two media. It is also found that the existence of disorder will influence the band edge of the PBG, and such influence will increase with the increment of periods or the contrast of the refractive index of the two media.     

Au@Sin: Growth behavior, stability and electronic structure

The configurations, stability, and electronic structure of AuSin (n=1-16) clusters have been investigated within the framework of the density functional theory at the B3PW91/LanL2DZ and PW91/DNP levels. The results show that the Au atom begins to occupy the interior site for cages as small as Si₁₁ and for Si₁₂ the Au atom completely falls into the interior site forming Au@Si₁₂ cage. A relatively large embedding energy and small HOMO-LUMO gap are also found for this Au@Si₁₂ structure indicating enhanced chemical activity and good electronic transfer properties. All these make Au@Si₁₂ attractive for cluster-assembled materials.     

Aging of the field-induced asymmetry in a disordered ferroelectric

The isothermal aging of the asymmetry induced in the disordered dielectric crystal (x=0.027) submitted to the biasing electric field E, is investigated. To this end, the response of the complex dielectric constant to infinitesimal field changes , applied to the sample after a variable aging delay, has been measured for different magnitudes of E and after different aging delays. Two different experimental procedures have been used: in both cases the response strongly depends on the time spent under field. For short aging delays, the response has a strong contribution proportional to δE and a weak quadratic contribution proportional to . As time elapses, the linear and the quadratic contributions age in opposite ways: the former decreases whereas the latter increases. This paradoxical behaviour is analyzed in the framework of a model which attributes aging and the related effects (rejuvenation, memory) to the evolution of polarization domain walls: the decrease of the linear contribution is related to the decrease of the total wall area, while the increase of the quadratic term is attributed to wall reconformations.     

Delocalization in disordered chains with random symmetrical impurities

We study in this paper, with the context of a tight-binding on-side model, the electronic properties of one-dimensional random lattices with correlated impurities. We show that, when symmetrical impurities are inserted in a host chain of site energy and a constant hopping interaction V, diffusion will occur even when is random. We provide analytic expressions for the transmittance and confirm the theoretical results by a great deal of numerical calculations. When = V, we find that the mean-square displacement (MSD) follows the law m ² ∝t ^β with β = 2.0 for = constant and β = 1.0 for = = random, respectively. Received 15 January 2001 and Received in final form 30 April 2001     

固态二氧化碳电子结构及性能的理论研究

采用基于赝势平面波理论的第一性原理计算方法,研究了9种结构的固态二氧化碳的结构和性质.经过结构的几何优化,得到α石英稳定结构晶格常数与他人的计算值基本一致.通过对平衡态能量的分析,我们得出β方石英(cristobalite)结构是能量最低的结构.这与文献的研究结果一致.对弹性性质的计算结果表明,除了超石英(stishovite)和立方黄铁矿(cubic-pyrite)结构之外,其他的结构都是弹性稳定的.利用基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法,预测了各个结构的本征硬度值.结果表明, 关键词： 第一性原理计算 固态二氧化碳 电子结构 硬度     

Theoretical determination of the K2 electronic structure

A theoretical study of the electronic structure of K₂⁺ is presented. Potential energy curves for the ground and various electronic excited states have been computed in the framework of a model potential method over a wide range of internuclear distances. Spectroscopic constants for the lowest short-range bound states have been determined and they are compared with available experimental and theoretical values. Long-range structures (wells and humps) have been also predicted, some of which being described from a long-range model.     

A study of electronic structure and lattice dynamics of CoSb3 skutterudite

The novel filled skutterudite materials have attracted much interest in recent years and experimental studies have revealed that electrical properties (electrical conductivity and Seebeck coefficient) in these materials are dominated by their electronic structure while the effective suppression of thermal conductivity is mainly determined by their lattice dynamics. To clarify the relationship between microstructure and properties in further, we report a systematic study of electronic structures and lattice dynamics of CoSb₃ in this paper using linearized augmented plane waves based on the density functional theory of first principles. By calculating band structure and partial density of states (PDOS), effects of electronic structures of CoSb₃ on electrical properties were investigated. Based on the calculated results of phonon dispersions and phonon density of states of CoSb₃, lattice dynamics of CoSb₃ (heat capacity, Debye temperature, mean free path and lattice thermal conductivity) are discussed in detail. The calculated results are excellently consistent with other work and experimental data.     

邻苯二甲酸酯类塑化剂电子结构特征的理论研究

采用量子化学密度泛函理论和极化连续介质模型在B3LYP/6-311++G**水平上对DEHP、MEHP、DBP、MBP和雌二醇在水溶液中的电子结构特征进行了理论研究，获得了上述化合物的几何构型、表观静电势、前线轨道等信息；采用电子密度拓扑分析方法分析获得了分子中的键鞍点电荷密度. 在此基础上，对分子电子结构与生物活性的关系进行了初步探讨. 结果表明，MBP、MEHP分子表观正、负电势程度比DBP、DEHP大，有利于与受体形成稳定的复合物；MBP、MEHP的ΔEgap分别比DBP、DEHP更小，反应活性更高；MBP、MEHP的分子偶极矩比DBP、DEHP相应提高了22%、34%，有利于与受体分子之间的氢键相互作用、偶极-偶极相互作用. 以上研究结果验证了MBP、MEHP毒性分别高于DBP、DEHP的实验结论. 研究还发现，邻苯二甲酸酯类塑化剂与雌二醇在电子结构特征上存在较大差异，该类化合物可能具有与雌二醇不同的受体或结合位点.     

邻苯二甲酸酯类塑化剂电子结构特征的理论研究

采用量子化学密度泛函理论和极化连续介质模型在B3LYP/6-311++G**水平上对DEHP、MEHP、DBP、MBP和雌二醇在水溶液中的电子结构特征进行了理论研究,获得了上述化合物的几何构型、表观静电势、前线轨道等信息;采用电子密度拓扑分析方法分析获得了分子中的键鞍点电荷密度. 在此基础上,对分子电子结构与生物活性的关系进行了初步探讨. 结果表明,MBP、MEHP分子表观正、负电势程度比DBP、DEHP大,有利于与受体形成稳定的复合物;MBP、MEHP的ΔEgap分别比DBP、DEHP更小,反应活性更高;MBP、MEHP的分子偶极矩比DBP、DEHP相应提高了22%、34%,有利于与受体分子之间的氢键相互作用、偶极-偶极相互作用. 以上研究结果验证了MBP、MEHP毒性分别高于DBP、DEHP的实验结论. 研究还发现,邻苯二甲酸酯类塑化剂与雌二醇在电子结构特征上存在较大差异,该类化合物可能具有与雌二醇不同的受体或结合位点.     

Theoretical prediction of the structural,elastic, electronic and thermodynamic properties of V3M (M = Si,Ge and Sn) compounds

Density functional theory (DFT), is used in our calculations to study the V₃M (M = Si, Ge and Sn) compounds, we are found that V₃Sn compound is mechanically unstable because of a negative C₄₄ = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(E_f) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).     

Theoretical calculations on the adhesion, stability, electronic structure, and bonding of Fe/WC interface

The adhesion, stability, electronic structure, and bonding of Fe/WC interfaces were studied using first-principles calculations. The preferred stacking sequence is HCP structure that Fe atoms continue the natural stacking sequence of the bulk WC. For two different interfaces with HCP stacking geometry (C-HCP and W-HCP), the work of adhesion of the optimized Fe/WC interfaces are 9.7 J m⁻² for C-HCP and 5.1 J m⁻² for W-HCP, respectively. The effects of the interface on the electronic structures of both the metal Fe and ceramic WC are mainly localized within the first and second layers of the interface. C-HCP interface has strong covalency and W-HCP interface is dominated by metallic bonds. The magnetic moments of Fe atoms at interface are decreased in both interfaces. Calculations of the interfacial energies provide theoretical evidence for the excellent wear behaviors of Fe/WC composites. Besides, the chemical bonding properties for the interfacial atoms are also discussed in this paper based on Milliken population method.     

Theoretical studies on the electronic structure and magnetic properties of Cu(2,5-dmpz)Cl2

First-principles calculations have been performed to study the electronic structure, the metallic and magnetic properties of Cu(2,5-dmpz)Cl₂. The calculations are based on the density functional theory (DFT) with the generalized gradient approximation (GGA) and the full-potential-linearized-augmented plane wave (FPLAPW) method. The total energy, magnetic moment, density of states (DOS) and electronic band structure are calculated. The results reveal that the compound has a stable semiconductive antiferromagnetic (AFM) ground state and a semiconductive ferromagnetic (FM) metastable state, which is in good agreement with the experimental results. Based on the spin distribution and the DOS, it is found that the spin magnetic moment is mainly from the Cu²⁺, and with relative small contribution from Cl, N atoms.     

Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

First-principle study on the electronic and optical properties of Mn-doped SnO2

Using the full-potential linearized augmented plane wave method (FP-LAPW), we have investigated the electronic and optical properties of Sn_1−xMn_xO₂ (x=0, 0.0625, 0.125, 0.1875, 0.25). The doped Mn results in reduction of the band gap, which can be attributed to a series of impurity bands at the bottom of the conduction band caused by the strong hybridization between Mn 3d and O 2p. The results also show that the Mn-doped systems tend to convert into p-type semiconductor with direct band gaps. With the increase of Mn concentration, both the imaginary part of dielectric function and the absorption spectrum show red-shift corresponding to the change of band gaps.     

FeVO_4电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理方法研究了三斜结构FeVO_4的结构,基态的能带结构、总态密度和分波态密度.将FeVO_4非共线的螺旋磁结构简化为六种不同的反铁磁结构,通过比较不同自旋构型的总能确定了基态磁结构.能带计算和总态密度结果均显示FeVO_4是能隙为2.19 e V的半导体,与实验结果相符.考虑Fe原子的在位库仑能,FeVO_4的能带结构和态密度都发生变化,说明FeVO_4晶体是一个典型的强关联电子体系.     

第一性原理研究3C-SiC/Mg复合材料界面的电子结构

基于密度泛函理论的第一性原理平面波赝势方法对4种3C-SiC(111)/Mg(0001)界面模型进行研究.界面间距和粘附功的计算表明,结构优化之后的界面模型只在z轴方向发生了移动,界面间距发生了不同程度的缩短;中心型模型的稳定性强于顶位型模型,C终端结构的稳定性强于Si终端结构,中心型C终端的界面模型具有最大的粘附功(2.5834 J/m~2)和最小的界面间距(1.7193?),是4种模型中最稳定的结构. Mulliken电荷、电荷密度分布、差分电荷密度和态密度的计算表明,中心性结构的Si终端和C终端模型界面处存在共价键、离子键和金属键.