First‐principles identification of defect levels in Er‐doped GaN

Erbium (Er) doped GaN has been studied extensively for optoelectronic applications, yet its defect physics is still not well understood. In this work, we report a first‐principles hybrid density functional study of the structure, energetics, and thermodynamic transition levels of Er‐related defect complexes in GaN. We discover for the first time that Er_Ga–C_N–V_N, a defect complex of Er, a C impurity, and an N vacancy, and Er_Ga–O_N–V_N, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 eV and 0.46 eV below the conduction band, respectively. Together with Er_Ga–V_N, a complex of Er and an N vacancy which has recently been found to produce a donor level at 0.61 eV, these defect complexes provide explanation for the Er‐related defect levels observed in experiments. The role of these defects in optical excitation of the luminescent Er center is also discussed.     

氩原子基态波函数和能量的解析计算

以第一性原理和变分原理为基础,给出了氩原子基态波函数的一种解析表达式,计算了基态氩原子(含类氩离子)的能量,导出了所涉及的所有积分的解析表达式.对氩原子,所得到的能量理论值与实验值的相对误差为0.22%.     

N,Fe, La三掺杂锐钛矿型TiO2能带调节的理论与实验研究

采用基于密度泛函理论(DFT)的平面波超软赝势方法(PWPP), 利用Material studio 计算N, Fe, La三种元素掺杂引起的锐钛矿TiO₂晶体结构、能带结构和态密度变化. 并通过溶胶-凝胶法制得锐钛矿型本征TiO₂, N, Fe共掺杂TiO₂和N, Fe, La共掺杂TiO₂; 用X射线衍射和扫描电镜表征结构; 紫外-可见分光光度计检测TiO₂对甲基橙的降解效率变化. 计算结果表明, 由于N, Fe, La三掺杂TiO₂的晶格体积、键长等发生变化, 导致晶体对称性下降, 光生电子-空穴对有效分离, 同时在导带底和价带顶形成杂质能级, TiO₂禁带宽度由1.78 eV变为1.35 eV, 减小25%, 光吸收带边红移, 态密度数增加, 电子跃迁概率提升, 光催化能力增加. 实验结果表明: 离子掺杂使颗粒变小, 粒径大小: 本征TiO₂>N/Fe_TiO₂>N/Fe/La_TiO₂, 并测得N/Fe/La_TiO₂发光峰425 nm, 能隙减小, 光催化能力比N/Fe_TiO₂强, 增强原因是杂质能级和电子态数量增加引起.     

First passage percolation and escape strategies

Consider first passage percolation on with passage times given by i.i.d. random variables with common distribution F. Let be the time from u to v for a path π and the minimal time among all paths from u to v. We ask whether or not there exist points and a semi‐infinite path such that for all n. Necessary and sufficient conditions on F are given for this to occur. When the support of F is unbounded, we also obtain results on the number of edges with large passage time used by geodesics. © 2014 Wiley Periodicals, Inc. Random Struct. Alg., 47, 414–423, 2015     

Sulfur doping of AlN and AlGaN for improved n‐type conductivity

Achieving high levels of n‐type conductivity in AlN and high Al‐content nitride alloys is a long standing problem; significant decreases in conductivity are observed as the Al content is increased, a phenomenon that has been attributed to donors such as oxygen or silicon forming DX centers. We address this problem through a comprehensive first‐principles hybrid density functional study of potential n‐type dopants, identifying S_N and Se_N as two elements which are potential shallow donors because they do not undergo a DX transition. In particular, S_N is highly promising as an n‐type dopant because it also has a low formation energy and hence a high solubility. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Synthesis,spatial structure and spectral properties of pyrylo‐4 (thio) squaraines variously substituted in cyclobutene moiety

Synthetic routes have been developed to a number of (thio) squaraine dyes containing the residues of CH‐acids at the central cyclobutene ring. The electronic and spatial structure as well as the chemical conversions and optical behaviour of the compounds obtained have been studied both theoretically and by X‐ray diffraction analysis, ¹H NMR and electronic spectroscopy. As shown, the electronic nature and sterical characteristics of the central ring substituents give rise to some general conformational features and crystal packing regularities and also govern the spectral position of the first π–π^* absorption band. The structure–property relationships established in the study provide guidance for the purposeful design of deeply coloured (thio) squaraines. Copyright © 2015 John Wiley & Sons, Ltd.     

Large Hexagonal Bi‐ and Trilayer Graphene Single Crystals with Varied Interlayer Rotations

Bi‐ and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high‐quality large‐size bi‐ and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid‐like hexagonal bi‐ and trilayer graphene single‐crystal domains on Cu foils using chemical vapor deposition is reported. The as‐produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier‐transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first‐principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations.     

Origin of pressure-induced insulator-to-metal transition in the van der Waals compound FePS3 from first-principles calculations

Pressure-induced insulator-to-metal transition (IMT) has been studied in the van der Waals compound iron thiophosphate (FePS₃) using first-principles calculations within the periodic linear combination of atomic orbitals method with hybrid Hartree–Fock-DFT B3LYP functional. Our calculations reproduce correctly the IMT at ∼15 GPa, which is accompanied by a reduction of the unit cell volume and of the vdW gap. We found from the detailed analysis of the projected density of states that the 3p states of phosphorus atoms contribute significantly at the bottom of the conduction band. As a result, the collapse of the band gap occurs due to changes in the electronic structure of FePS₃ induced by relative displacements of phosphorus or sulfur atoms along the c-axis direction under pressure.     

Fe3O4(001)-B表面水煤气变换反应机理第一性原理研究

采用自旋极化的密度泛函理论方法系统地研究了Fe_3O_4(001)-B表面水煤气变换的反应机理,计算了整个反应历程。结果表明,对于Fe_3O_4(001)-B表面上的水煤气变换反应,氧化还原、联合和再生三种反应路径共存,但氧化还原和联合机理的有效能垒较低,因而更占优势。对于生成H_2的基元反应,其活性受表面H浓度和催化剂表面O缺陷浓度影响;较高的表面H浓度和O缺陷浓度均有利于H_2生成。这些结果有助于进一步认识铁氧催化剂上的水煤气变换反应机理。     

Ferromagnetism and structural deformation in monolayer alpha lead oxide induced by N and F doping: New insights from first principles

We study the structural, electronic, and magnetic properties of monolayer α-PbO_0.875A_0.125 (A = N, F), which are calculated using first principles. As a result, N doping induces local ferromagnetism centered at the N²⁻ site, originating from the spin-down N 2p valence states. On the other hand, F doping induces nonmagnetism and induces ab-plane deformation, where F⁻ receives one electron to its nearest-neighboring Pb^1.75+ ions. N doping redshifts the bandgap of the undoped system and transforms it to be indirect, while F doping blueshifts the bandgap through the Burstein-Moss effect. The hybridization of Pb 6p and O 2p orbitals is stronger near the A site than that of the crystal structure edge. Our result shows new insights, predicting possible experimental results for future functional device applications.