半导体性单壁碳纳米管与其石墨衬底表面相互作用的第一性原理计算

利用基于密度泛函理论的计算方法,研究了处于石墨（HOPG）表面的半导体性单壁碳纳米管,发现碳纳米管下面的石墨受碳管的作用向下凹陷,而纳米管本身虽然保持其形状不变,但它的电子态受石墨衬底影响,造成导带下移,禁带宽度明显减小. 关键词： 单壁碳纳米管 密度泛函理论 局域密度近似方法     

氟阴离子掺杂对NaMgH3储氢性能影响的第一性原理研究

化学氢化物NaMgH₃由于具有较高的氢质量和体积密度而被认为是颇具开发潜力的贮氢材料之一.文中采用基于密度泛函理论（DFT）的平面波赝势（PW-PP）方法,初步探讨了氟阴离子影响NaMgH₃放氢性能的机理.计算结果表明,氟取代氢化物中的部分氢导致了氢化物形成焓及反应焓的降低,改善了体系的热力学性能,有利于氢化物的放氢. 关键词： 储氢材料 氟掺杂 形成焓 反应焓     

Mg,O共掺杂实现p型AlN的第一性原理研究

采用密度泛函理论下的第一性原理平面波超软赝势方法研究了纤锌矿本征AlN,Mg单掺杂AlN和Mg,O共掺杂AlN体系的晶格参数、能带结构、电子态密度、差分电荷密度及电子布居数.计算结果显示:在Mg,O共掺杂AlN体系中,激活施主O原子的引入能使受主能级降低,形成浅受主掺杂.同时,体系的非局域化特征显著,受主能带变宽.因而提高了Mg原子的受主掺杂浓度和系统的稳定性.Mg,O共掺杂更有利于制备p型AlN.     

硼掺杂富勒烯储氢-从物理吸附到化学吸附的转变

本文采用密度泛函理论(DFT)中的局域密度近似(LDA)方法对硼(B)掺杂富勒烯(C_(35)B)储氢问题在前人的基础上做了进一步研究,结果表明被C_(35)B吸附的氢分子很容易解离,经历从物理吸附到化学吸附的转变,并且发现解离产物C_(34)BHCH有分子内氢转移反应发生,这时B原子仍能与氢分子有很强的相互作用,最终导致B位置以及与B最邻近的三个C原子上都有氢原子吸附.并利用过渡态理论从热力学上分析了这种反应的发生趋势.     

5d过渡金属原子掺杂氮化硼纳米管的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法,研究了当氮化硼纳米管（BNNT）中的B原子和N原子被5d过渡金属原子（Lu,Hf,Ta,W,Re,Os,Ir,Pt,Au,Hg）取代时BNNT的几何结构、电子结构和磁性性质. 作为对比,给出了理想BNNT,B缺陷体系（V_B）和N缺陷体系（V_N）的相应结果. 研究发现：5d原子取代B（B_5d）时体系的局域对称性接近于C_3v,但是取代N（N_5d）时体系的局域对称性偏离C_3v对称性较大;利用相同的5d原子进行掺杂时,B_5d的成键能比N_5d的成键能大;对于B_5d或者N_5d,其成键能基本上随着5d原子的原子序数的增大而降低;掺杂体系中出现了明显的杂质能级,给出了态密度等结果;不同掺杂情况的磁矩不同,取代B 时体系的总磁矩呈现出较强的规律性. 利用对称性和分子轨道理论解释了5d原子取代B时杂质能级的产生和磁性的变化规律. 关键词： 第一性原理计算 5d过渡金属原子 氮化硼纳米管 密度泛函理论     

Mn掺杂ZnS(110)表面的电子结构和磁性

采用基于密度泛函理论的第-性原理方法,研究Mn掺杂ZnS(110)表面的电子结构和磁性.计算分析不同掺杂组态的几何参数、形成能、磁矩、电子态密度以及电荷密度.结果表明:单个Mn原子掺杂,替位于表面第二层的Zn原子时体系形成能最低,说明该层是最稳定的掺杂位置.对于两个Mn原子的掺杂,当Mn与Mn之间呈反铁磁耦合时体系最稳定.体系的总磁矩和自由Mn原子的磁矩差别很小,但是Mn原子的局域磁矩却依赖于Mn原子的3d态和近邻S原子的3p态的杂化作用,即受周围S原子环境的变化影响较大.此外,分析电荷密度图得出Mn原子替换Zn原子后与S原子形成了更强的共价键.     

Be, O共掺杂实现p型AlN的第一性原理研究

基于密度泛函理论框架下的第一性原理平面波超软赝势方法,研究了掺杂和非掺杂AlN体系的晶格参数、 能带结构、总体态密度、分波态密度、差分电荷分布及电荷集居数.计算结果表明: Be掺杂AlN晶体能够在能隙中形成深受主能级,空穴载流子局域于价带顶, 而引入了激活施主O原子的Be, O共掺杂方法,能使受主能带变宽、非局域化特征明显. 同时,受主能级向低能方向移动,形成了浅受主能级, 从而提高了Be原子的掺杂浓度和系统的稳定性. Be, O共掺杂更有利于获得p型AlN.     

Mg，Cu 掺杂CdS电子结构的第一性原理研究

采用基于密度泛函理论（DFT）的第一性原理的平面波超软赝势方法,对闪锌矿结构CdS晶体及CdS:M（M=Mg, Cu）的几何结构、能带结构、电子态密度、集聚数和电荷密度分布进行了研究.对掺杂后体系的几何结构进行了优化计算,发现Mg和Cu原子掺入CdS后晶格常量均减少,晶格发生畸变.在此基础上研究了掺杂对体系电子结构的影响.结果表明,Mg,Cu掺入CdS都能提供较多空穴态,形成p型电导,并且Cu较Mg是更好的p型掺杂剂. 关键词： 密度泛函理论 电子结构 p型掺杂     

Ga掺杂δ-Pu的密度泛函理论计算

δ-Pu为Pu的高温相,掺杂少量的Ga即可使其在室温下稳定存在.本文采用密度泛函理论方法,对不同掺杂量体系进行晶体结构和电子结构计算,主要包括体系的晶格常数、密度、形成能、态密度、电荷密度和Mulliken布居分析.结果表明：在研究范围内,Ga掺杂后,体系晶格常数降低,密度增大,6.25%（原子百分比,下同）掺杂量体系的稳定性高于3.125%和12.5%掺杂量的体系;Ga掺杂使得Pu周围体系电子的局域性增强,成键能力增强,揭示了Ga稳定δ-Pu的电子机制.Ga和Pu之间为金属键,发生的作用主要由Pu的7s、6p、6d和Ga的4s、4p轨道电子贡献,但这种成键作用相对较弱,使得掺杂体系可以保持原有的力学性能和机械加工性能.Ga对δ-Pu的稳定作用主要在于改善Pu原子的成键性能,而不是与Pu原子直接成键.     

Cr掺杂BaTiO_3几何结构、电子结构和磁性的第一性原理研究

采用基于密度泛函的第一性原理平面波赝势方法以及广义梯度近似+U(GGA+U)方法,计算了钙钛矿结构四方相BaTiO_3晶体结构、电子结构以及Cr掺杂对体系磁性的影响.研究表明,Cr离子的掺杂会使BaTiO_3的晶格参数和晶胞体积减小,使结构逐渐从四方相的BaTiO_3向立方相的BaCrO3演化;能带和态密度的计算结果显示掺杂剧烈影响体系的电子结构,体系基态逐渐由绝缘态变为半金属态.同时,掺杂调控了体系的磁性.Cr离子在掺杂后为+4价,具有2μB的局域磁矩,可使体系具有磁性.结果预测了Cr掺杂对BaTiO_3改性的有效性,拓宽了BaTiO_3在自旋电子器件方面的应用.     

Photoconductive detection of tetrahedrally coordinated hydrogen in ZnO

In this Letter we apply an innovative experimental approach, which allows us to improve the sensitivity of detecting local vibrational modes (LVMs) even in highly absorbing spectral regions. This photoconductive technique allowed us to confirm a recent suggestion of a new multicenter bond for hydrogen in ZnO [A. Janotti and C. G. Van de Walle, Nature Mater. 6, 44 (2007)]. The two LVMs of the hydrogen substituting oxygen in ZnO are identified at 742 and 792 cm(-1). The modes belong to a nondegenerated A(1) and a twofold degenerated E representations of the C(3v) point group. The tetrahedral coordination of the hydrogen atom is the result of a newly detected multicenter bond for defects in solids.     

Hydrogen bonds and van der waals forces in ice at ambient and high pressures

The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated.     

Ab initio simulation of the spin transition during chemisorption: H/Al(111)

Electronically nonadiabatic effects during the chemisorption of hydrogen atoms on an Al(111) surface are simulated ab initio using time-dependent density-functional theory for the electrons in combination with Ehrenfest dynamics for the nuclei. Strongly nonadiabatic effects close to the spin transition of the H atom are identified, and the dissipated energy as well as the electron-hole pair excitation spectra are calculated. The recent Newns-Anderson-model approach by Mizielinski et al. is confirmed. The simulations illustrate the physical processes that contribute to internal exoelectron emission.     

Hydrogenation mechanism in lanthanum-activated magnesium films

With density-functional theory, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and La-doped Mg(0001) surfaces are studied. Calculation results show that the energy barrier obtained for hydrogen dissociation on the La-doped Mg(0001) surface is smaller due to back-donated bonding between molecular H₂ and doped La atom. The obtained diffusion barriers (0.8–0.22 eV) imply a fast motion of atomic H on La-doped Mg(0001) surface.     

Electrophilicity Indices for Peroxyl Radicals

In this work, electrophilicity indices are calculated for 50 peroxyl radicals with density-functional theory (B3LYP hybrid functional and the 6-311+G(d,p) basis set). There is a relationship between general and local electrophilicity indices of peroxyl radicals and Taft inductive constants (σ*) of the substituents at the–OO^? group.     

Inference and Learning in a Latent Variable Model for Beta Distributed Interval Data

Latent Variable Models (LVMs) are well established tools to accomplish a range of different data processing tasks. Applications exploit the ability of LVMs to identify latent data structure in order to improve data (e.g., through denoising) or to estimate the relation between latent causes and measurements in medical data. In the latter case, LVMs in the form of noisy-OR Bayes nets represent the standard approach to relate binary latents (which represent diseases) to binary observables (which represent symptoms). Bayes nets with binary representation for symptoms may be perceived as a coarse approximation, however. In practice, real disease symptoms can range from absent over mild and intermediate to very severe. Therefore, using diseases/symptoms relations as motivation, we here ask how standard noisy-OR Bayes nets can be generalized to incorporate continuous observables, e.g., variables that model symptom severity in an interval from healthy to pathological. This transition from binary to interval data poses a number of challenges including a transition from a Bernoulli to a Beta distribution to model symptom statistics. While noisy-OR-like approaches are constrained to model how causes determine the observables’ mean values, the use of Beta distributions additionally provides (and also requires) that the causes determine the observables’ variances. To meet the challenges emerging when generalizing from Bernoulli to Beta distributed observables, we investigate a novel LVM that uses a maximum non-linearity to model how the latents determine means and variances of the observables. Given the model and the goal of likelihood maximization, we then leverage recent theoretical results to derive an Expectation Maximization (EM) algorithm for the suggested LVM. We further show how variational EM can be used to efficiently scale the approach to large networks. Experimental results finally illustrate the efficacy of the proposed model using both synthetic and real data sets. Importantly, we show that the model produces reliable results in estimating causes using proofs of concepts and first tests based on real medical data and on images.     

Application of Lanczos-based time-dependent density-functional theory approach to semiconductor nanoparticle quantum dots

We present a density-functional theory study of Si nanoparticle quantum dots, focusing on determination of their optical properties. To calculate the absorption spectra of our quantum dot models we use a recently-developed method based on the application of Lanczos algorithms to linear-response time-dependent density-functional theory (LR-TDDFT). Quantum dot models are obtained by cutting Si atoms from the bulk crystal lattice and adding appropriate H terminating atoms to the surface; these structures are relaxed using density-functional theory in the plane-wave pseudopotential supercell approach, and then absorption spectra are calculated. We verify that with increasing size of the nanoparticle, the optical gap/onset of absorption steadily moves to lower energies. The paper represents an important demonstration of this new methodology on a class of systems that are the focus of significant current research in nanoscience.     

Self-passivating edge reconstructions of graphene

Planar reconstruction patterns at the zigzag and armchair edges of graphene were investigated with density-functional theory. It was unexpectedly found that the zigzag edge is metastable and a planar reconstruction spontaneously takes place at room temperature. The reconstruction changes electronic structure and self-passivates the edge with respect to adsorption of atomic hydrogen from a molecular atmosphere.     

Density-functional study of methanol adsorption on the Al(100) surface

The interaction of methanol (CH₃OH) molecules with the Al(100) surface is investigated in the framework of the density-functional theory. Numerical parameters, such as the decrease in the kinetic energy, number of special points used for integration over the Brillouin zone, broadening of the filling function, and number of atomic metal layers, are determined. The density-functional theory calculations with these parameters offer reliable data on the total energy and structural optimization of the system under consideration. The geometric characteristics of a methanol molecule in both isolated and adsorbed states on the metal surface are calculated. It is shown that the electron pseudodensity at the metal surface changes in the presence of an adsorbed molecule. The influence of the orientation of the methanol molecule (with respect to the metal surface and nearest neighbor methanol molecules) on the total energy of the system is analyzed. It is found that the total energy of the system is affected by the metal deformation and the interaction of adsorbed molecules with each other.