Density of Electronic States in the Conduction Band of Ultrathin Films of Naphthalenedicarboxylic Anhydride and Naphthalenetetracarboxylic Dianhydride on the Surface of Oxidized Silicon

The results of examination of the electronic structure of the conduction band of naphthalenedicarboxylic anhydride (NDCA) films in the process of their deposition on the surface of oxidized silicon are presented. These results were obtained using total current spectroscopy (TCS) in the energy range from 5 to 20 eV above the Fermi level. The energy position of the primary maxima of the density of unoccupied states (DOUS) of an NDCA film was determined based on the experimental TCS data and calculated data and compared with the position of the DOUS maxima of a naphthalenetetracarboxylic dianhydride (NTCDA) film. The theoretical analysis involved calculating the energies and the spatial distribution of orbitals of the molecules under study at the B3LYP/6-31G(d) DFT (density functional theory) level and correcting the obtained energies in accordance with the procedure that was proven effective in earlier studies of the conduction band of films of small conjugated organic molecules. It was found that the DOUS maxima of the NTCDA film in the studied energy interval from 5 to 20 eV above the Fermi level are shifted toward lower electron energies by 1–2 eV relative to the corresponding DOUS maxima of the NDCA film Subdivision of the Ufa Federal Research Centre of the.

     

Intrinsic electron accumulation at clean InN surfaces

The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.     

C掺杂TiO薄膜的制备及其第一性原理研究

TiO在微电子结构器件中有重要的应用前景. 本文以CO₂作反应气体, 采用直流反应磁控溅射方法成功制备出C掺杂TiO薄膜. 采用XRD, XPS和四探针电阻计对薄膜结构、成分和电阻率进行表征. 在实验结果的基础之上建立起TiO和C掺杂TiO的计算模型并采用第一性原理 方法计算其能带结构和电子态密度. 实验结果表明薄膜相结构为面心立方的岩盐结构, C取代O的阴离子掺杂为主要掺杂方式, 薄膜电阻率为52.2 μΩ·cm. 第一性原理计算结果表明, 费米能级穿过TiO的导带, TiO具有金属性导电的能带结构特征; C掺杂TiO 后, 其金属性导电的能带结构没有改变, 只是在费米面附近出现C 2p态提供的杂质能级, 杂质能级扩展了TiO的导带宽度并提高了费米面附近的电子能态密度, 从而导至TiO电导增加, 电阻率降低. 第一性原理计算结果与实验结果一致. 关键词： C掺杂TiO 直流反应磁控溅射 第一性原理 电子结构     

Band Structures of Ultrathin Bi(110) Films on Black Phosphorus Substrates Using Angle-Resolved Photoemission Spectroscopy

The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates.     

氧化石墨烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理方法,计算了氧化石墨烯纳米带的电荷密度、能带结构和分波态密度。结果表明,石墨烯纳米带被氧化后,转变为间接带隙半导体,带隙值为0.375 e V。电荷差分密度表明,从C原子和H原子到O原子之间有电荷的转移。分波态密度显示,在导带和价带中C-2s、2p,O-2p,H-1s电子态之间存在强烈的杂化效应。在费米能级附近,O-2p态电子局域效应的贡献明显,对于改善氧化石墨烯纳米带的半导体发光效应起到了主要作用。     

PbTe(001)表面原子几何结构和电子结构的第一性原理计算

用第一性原理的密度泛函理论计算了PbTe（001）表面的几何结构和电子结构.计算结果表明：PbTe（001）表面不发生重构，但表面几层原子表现出明显的振荡弛豫现象，其中第一、第二层间距减小4.5%，第二、第三层间距增加2.0%，并且表面层原子出现褶皱.表面带隙在X 点，带隙变宽，在基本带隙中不引入新的表面态，而导带底和价带顶附近等多处出现新的表 面共振态；弛豫后费米面处态密度很低，所以表面结构很稳定. 关键词： 密度泛函理论 表面几何结构 表面电子结构 PbTe     

Electronic structure and reactivity of ultra-thin Fe films on a Rh(100) surface

The electronic structure and reactivity of ultra-thin Fe films on a Rh(100) surface have been investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and thermal desorption spectroscopy (TDS). The dispersion of d-bands of a clean Rh surface is consistent with the projection of the bulk band structure. One monolayer of Fe film shows a systematic shift of d-bands toward large binding energy by 0.4 eV and a large reduction in the density of states just below the Fermi level, in particular, around the M point. In accordance with the decrease in the density of states at the Fermi level, the bonding energy of hydrogen is greatly reduced to 245 kJ/mol on a 1 ML (ML = monolayer) Fe film, although the sticking coefficient is still in the range of 0.1–0.3. The successive increase in activation energy for the desorption of hydrogen with the increase of Fe film thickness from 1 to 3 ML is associated with a recovery of the density of states at the Fermi level.     

二维应变作用下超导薄膜LiFeAs的磁性和电子性质

基于密度泛函理论的第一性原理计算,研究了二维应变作用下LiFeAs超导薄膜的磁性结构、电子能带和态密度变化,分析了应变对其超导电性的作用.结果显示,对体系施加1%—6%的二维平面张、压应变均不改变其基态条形反铁磁性结构,费米面附近的电子态密度主要来自于Fe-3d轨道电子以及少量的As-4p电子.研究发现,与无应变情形相比,当施加压应变时,体系中Fe离子的反平行的电子自旋局域磁矩减小,薄膜反铁磁性受到抑制,费米面上电子态密度增加,超导电性来自于以反铁磁超交换耦合作用为媒介的空穴型费米面和电子型费米面间嵌套的Cooper电子对.而在张应变作用时,局域反铁磁性增强,费米面上电子态密度减小,金属性减弱,特别是张应变时费米面上空穴型能带消失, Cooper电子对出现概率显著降低,将抑制超导相变.     

电致发光加速层二氧化硅的电子高场迁移率

根据非晶态半导体的能带理论，讨论了分层优化薄膜电致发光方案中非晶二氧化硅加速层中的电子在高电场中的输运行为.研究结果表明：在高电场下，由于电场的存在降低了陷阱之间的平均势垒高度.在费密能级附近处的杂质及缺陷定域态和导带尾定域态中，电子的输运主要表现为电场增强的热辅助式跳跃传导；而在导带扩展态中，电子的输运仍像晶态半导体那样表现为共有化运动.此外，以实验数据为基础，计算出了非晶二氧化硅中电子的迁移率、最小金属电导率、导带迁移率边界状态密度及费密能级处的状态密度. 关键词：     

In-gap density of states in hole- and electron-doped CuO2 planes probed by scanning tunneling spectroscopy

The low-energy electronic structure of a c-axis Sr_xA_yCuO₂ (A is alkaline earth cation, x+y≦1, hole- and electron-doped infinite layer) thin film, grown by laser-molecular-beam epitaxy on a SrTiO₃ (001) substrate, has been studied using ultrahigh-vacuum scanning tunneling microscopy/spectroscopy. Images have been obtained for co-deposited Sr_xA_yCuO₂ thin films, which show the surface consisting of flat terraces separated by steps that are unit cell high. Tunneling spectra of undoped Sr_0.3Ca_0.7CuO₂ indicate a wide band gap of 1.8 eV which is consistent with the charge transfer gap. Hole-doped Sr_0.85CuO₂ shows in-gap states appearing at both the valence and conduction band edges. In contrast, for the electron-doped Sr_0.9La_0.1CuO₂, in-gap states appear predominantly above the Fermi level, and the spectral shape becomes asymmetric around the Fermi level. When these two systems are compared, barrier-height measurements reveal that there is no apparent shift of the Fermi level measured from the vacuum level. This suggests that the framework of the rigid-band picture might break down implying a strongly correlated electron system. Received: 20 August 1998 / Accepted: 15 February 1999 / Published online: 5 May 1999     

Conductivity anisotropy and localization of charge carriers in TlInTe<Subscript>2</Subscript> single crystals

The electrical resistivity of TlInTe₂ chain-structure semiconductors in directions parallel and perpendicular to the chains is analyzed as a function of temperature. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range under investigation are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed band. In the low-temperature range, the conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity conduction is determined. The localization lengths and the density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures.     

Photoelectron spectroscopy measurements of the valence band structures of polymerized thin films of C60 and La0.1C60

The electronic valence band structures of polymerized thin films of C₆₀ and La_0.1C₆₀ have been studied by using ultra-violet photoelectron spectroscopy. Additionally, the films have been characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The valence band of the C₆₀ film shows major peaks at binding energies of 2.6, 7.2, 10.3, and 12.6 eV. In the case of the doped film, we observe (i) an additional peak with a binding energy of 13.7 eV, (ii) evidence for redistribution of the density of electronic states due to hybridization between the 5d orbitals of La and the C₆₀ cage, and (iii) significantly higher density of the electronic states near the Fermi energy. The valence band spectra of the doped film are in good agreement with recent results of the density functional theory that support strong hybridization between the d-valence orbitals of La and the C₆₀ cage.     

Effect of nitrogen incorporation and oxygen vacancy on electronic structure and the absence of a gap state in HfSiO films

The effect of nitrogen (N) incorporation into HfSiO on the electronic structure and band alignment of HfSiO films was investigated. N depth profile data obtained by medium energy ion scattering (MEIS) showed that the concentration of N or the bonding or electronic state of N in the film was stable when the film was annealed at 950 °C, while the oxygen (O) in HfSiON films was present in dissociated form, as evidenced by the unoccupied electronic state of O. The valence band offsets of the HfSiO films were strongly affected by N incorporation due to the presence of N in a 2p state. Moreover, a reduction in the conduction band offset of a HfSiO film was confirmed after the film was annealed in an atmosphere of N₂. The unoccupied state of the O vacancy is responsible for the change in the conduction band offset. The results of ab-initio calculations for the density of states (DOS) of HfSiO and HfSiON supercells were in agreement with the experimental results. The incorporation on N into HfSiO prevents the formation of a gap-state inside the band gap despite the fact that an O vacancy is generated in the film.     

The electronic structure of Ca-intercalated superconducting graphite CaC6

We have measured Ca-intercalated graphite superconductor CaC₆ (T_c = 11.2 K) by soft X-ray photoemission spectroscopy in order to understand the electronic structure. For the valence band, we observed several structures that correspond to those of calculated density of states with the partial density of states of Ca 3d at the Fermi level (E_F). We also observed core level spectra that are a very large asymmetric Ca 2p and asymmetric C 1s for CaC₆, suggesting the existence of conduction electrons derived from Ca 3d and a charge transfer from Ca to graphene layer. These results provide spectroscopic evidence for PDOS of Ca 3d at E_F. From a comparison of electronic structure of CaC₆ and other graphite intercalation compounds (GICs), we found the difference between CaC₆ and other superconducting GICs, which provides deeper understanding of the superconductivity of CaC₆.     

XPO4(X=Lu, Y)电子结构的第一性原理研究

基于密度泛函理论(DFT)第一性原理对LuPO₄和YPO₄两种磷酸盐晶体的电子结构进行计算模拟.结果表明:LuPO₄的带隙为5.639 eV,YPO₄的带隙为4.884 eV.通过对态密度的分析得知LuPO₄的导带主要贡献来自Lu的5d态电子,费米能级附近价带主要由Lu的4f态和O的2p态电子贡献. YPO₄的导带主要贡献来自Y的4d态电子,价带顶的主要贡献来自于O的2p态电子.通过电荷密度和布局分析得知了材料内部原子间的电荷转移情况,进而对原子间成键情况进行了分析与讨论.     

Thermally induced effects on structural and electrical properties of selenium-rich Cd-Se thin films

The effect of annealing in nitrogen atmosphere on structural and electrical properties of selenium rich CdSe (SR-CdSe) thin films deposited by thermal evaporation onto glass substrates were studied. X-ray diffraction (XRD) patterns showed that the as-prepared films were amorphous, whereas the annealed films were polycrystalline. Analyzing XRD patterns reveals the coexistence of both Se and CdSe crystalline phases which exhibits a hexagonal structure. The microstructure parameters (crystallite size, microstrain and dislocation density) were calculated for annealed films.Temperature dependence (300–500 K) of d.c. conductivity was studied for as-prepared and annealed thin films. The experimental results indicate that the electrical conduction taking place through thermally activated process. At higher temperatures, electrical conduction for as-prepared film is taking place in the extended states while localized states conduction in the band tails is most likely to take place for annealed films. Regarding the lower temperature range, conduction by hopping in the localized states near the Fermi level is found to be dominant. Thus, conductivity data in this range was analyzed using Mott's variable range hopping conduction, where Mott's parameters were calculated for SR-CdSe thin films.     

缺陷对单层MoS2电子结构的影响

为了研究缺陷对单层MoS2的电子结构, 本文基于密度泛函理论框架下的第一性原理, 采用数值基组的方法计算了MoS2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现：Mo位缺陷、S位缺陷的MoS2的能带结构中的价带顶与导带底都在Q点, 为直接带隙材料; 其中Mo位缺陷体的禁带区域都出现5条新能级, S位缺陷体的禁带区域出现了3条新能级; 缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言, Mo位缺陷体的费米能级处出现了峰值, 表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现, Mo缺陷周围存在着负电荷聚集的现象, S缺陷周围存在正电荷聚集的现象.     

缺陷对单层MoS_2电子结构的影响

为了研究缺陷对单层MoS_2的电子结构,本文基于密度泛函理论框架下的第一性原理,采用数值基组的方法计算了MoS_2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现:Mo位缺陷、S位缺陷的MoS_2的能带结构中的价带顶与导带底都在Q点,为直接带隙材料;其中Mo位缺陷体的禁带区域都出现5条新能级,S位缺陷体的禁带区域出现了3条新能级;缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言,Mo位缺陷体的费米能级处出现了峰值,表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现,Mo缺陷周围存在着负电荷聚集的现象,S缺陷周围存在正电荷聚集的现象.     

The electronic structure of liquid silver chalcogenides: a tight-binding approach

We present a computational study of the electronic structure of the stoichiometric liquid zero-gap semiconductors [Formula: see text], [Formula: see text] and [Formula: see text]. The geometry of the fluids is described by the primitive model of charged hard spheres; the electronic structure is modelled using a tight-binding Hamiltonian. The density of states is computed considering the Madelung potential fluctuations and the topological disorder characteristic of an ionic fluid. Only the introduction of nonzero tight-binding hopping matrix elements - equivalent to the formation of chemical bonds - induces a pseudogap between the chalcogenide conduction band and the silver valence band. The Fermi level can be located in a region of a small density of states; eigenstates at [Formula: see text] are likely to exhibit disorder-induced localization.     

硼烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理的方法计算H修饰边缘不同宽度硼稀纳米带的电荷密度、电子能带结构、总态密度和分波态密度。结果表明,硼烯纳米带的宽度大小影响着材料的导电性能,宽度5的硼烯纳米带是间接带隙简并半导体,带隙值为0.674 eV,而宽度7的硼烯纳米带却具有金属材料的性质。分波态密度表明,宽度5的硼烯纳米带的费米能级附近主要是由B-2s、2p电子态贡献,H-1s主要贡献于下价带且具有局域性,消除了材料边缘的不稳定性。宽度7的B-2p和H-1s电子态贡献的导带和价带处于主导地位,费米能级附近B-2p和H-1s电子态的杂化效应影响材料的整体发光性能。