碳化物价电子结构及其界面电子密度分析

运用固体与分子经验电子理论计算了六种过渡金属碳化物(MC)的价电子结构及其部分低指数晶面的电子密度。计算结果表明：MC最强键为最近邻M—C键，相邻(111)面间以最强键结合，滑移困难。通过对NiAl和MC的不同晶面的共价电子密度的分析发现：(1lO)Nial(100)MC在一级近似范围内基本上保持连续；采取适当的制备工艺，使复合材料中存在尽可能多的(110)Nial／(100)MC，有可能使这种复合材料具有更优异的力学性能。     

电子的谷自由度

电子在晶格周期性势场影响下的运动遵循布洛赫定理. 布洛赫电子除了具有电荷和自旋两个内禀自由度外, 还有其他内禀自由度. 能带色散曲线上的某些极值点作为谷自由度, 具有独特的电子结构和运动规律. 本文从布洛赫电子的谷自由度出发, 简单介绍传统半导体的谷电子性质研究现状, 并重点介绍新型二维材料体系, 如石墨烯、硅烯、硫族化合物等材料中谷相关的物理特性. 有效利用谷自由度的新奇输运特性, 将其作为信息的载体可以制作出新颖的纳米光电子器件, 并有望造就下一代纳电子器件的新领域, 即谷电子学(valleytronics).     

Charge density wave states in phase-engineered monolayer VTe2

Charge density wave (CDW) strongly affects the electronic properties of two-dimensional (2D) materials and can be tuned by phase engineering. Among 2D transitional metal dichalcogenides (TMDs), VTe$_{2}$ was predicted to require small energy for its phase transition and shows unexpected CDW states in its T-phase. However, the CDW state of H-VTe$_{2}$ has been barely reported. Here, we investigate the CDW states in monolayer (ML) H-VTe$_{2}$, induced by phase-engineering from T-phase VTe$_{2}$. The phase transition between T- and H-VTe$_{2}$ is revealed with x-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) measurements. For H-VTe$_{2}$, scanning tunneling microscope (STM) and low-energy electron diffraction (LEED) results show a robust $2\sqrt 3 \times 2\sqrt 3 $ CDW superlattice with a transition temperature above 450 K. Our findings provide a promising way for manipulating the CDWs in 2D materials and show great potential in its application of nanoelectronics.     

Role of buffer layer in electronic structures of iron phthalocyanine molecules on Au(111)

We investigate the electronic structures of one and two monolayer iron phthalocyanine (FePc) molecules on Au(111) surfaces. The first monolayer FePc is lying flat on the Au(111) substrate, and the second monolayer FePc is tilted at～15° relative to the substrate plane along the nearest neighbour [101ˉ] direction with a lobe downward to the central hole of the unit cell in the first layer. The structural information obtained by first-principles calculations is in agreement with the experiment results. Furthermore, it is demonstrated that the electronic structures of FePc molecules in one-monolayer FePc/Au(111) system are perturbed significantly, while the electronic structures of FePc molecules in the second monolayer in two-monolayer FePc/Au(111) system remain almost unchanged due to the screening of the buffer layer on Au(111).     

Effect of strain on geometric and electronic structures of graphene on Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations.Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately.Considering the strain induced by the lattice mismatch,we optimize the atomic structures and investigate the electronic properties of the graphene.Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N × N superlattice and the substrate.The strain does not induce significant changes in electronic structure.Furthermore,the results calculated in the local density approximation(LDA) are compared with those obtained in the generalized gradient approximation(GGA),showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.     

无序化对有序铁铝金属间化合物磁性的影响

在平衡态时，化学计量比成份Fe3Al具有DO3超结构，并表现出铁磁性，平均每个Fe原子的磁矩是1．7μB，而具有B2超结构的FeAl却是无磁的。利用快速凝固、冷加工处理、溅射和机械合金化等非平衡加工技术可以使DO3-Fe3A1和B2-FeAl等有序铁铝金属间化合物相无序化。本文利用固体与分子经验电子理论（EET）和Jaccarino—Walker模型对铁铝金属间化合物的磁性进行了计算，研究了无序化对Fe-Al有序相磁性的影响。结果表明：完全无序的Fe15Al25和Fe50Al50，每个Fe原子的平均磁矩分别是2．01μB、1．41μB，铁铝金属间化合物的磁性可以通过无序化提高。     

Band engineering of double-wall Mo-based hybrid nanotubes

Hybrid transition-metal dichalcogenides(TMDs) with different chalcogens on each side(X-TM-Y) have attracted attention because of their unique properties. Nanotubes based on hybrid TMD materials have advantages in flexibility over conventional TMD nanotubes. Here we predict the wide band gap tunability of hybrid TMD double-wall nanotubes(DWNTs) from metal to semiconductor. Using density-function theory(DFT) with HSE06 hybrid functional, we find that the electronic property of X-Mo-Y DWNTs(X = O and S, inside a tube; Y = S and Se, outside a tube) depends both on electronegativity difference and diameter difference. If there is no difference in electron negativity between inner atoms(X) of outer tube and outer atoms(Y) of inner tube, the band gap of DWNTs is the same as that of the inner one. If there is a significant electronegativity difference, the electronic property of the DWNTs ranges from metallic to semiconducting, depending on the diameter differences. Our results provide alternative ways for the band gap engineering of TMD nanotubes.     

Growth behaviors and emission properties of Co-deposited MAPbI3 ultrathin films on MoS2

Hybrid organic-inorganic perovskite thin films have attracted much attention in optoelectronic and information fields because of their intriguing properties. Due to quantum confinement effects, ultrathin films in nm scale usually show special properties. Here, we report on the growth of methylammonium lead iodide (MAPbI₃) ultrathin films via co-deposition of PbI₂ and CH₃NH₃I (MAI) on chemical-vapor-deposition-grown monolayer MoS₂ as well as the corresponding photoluminescence (PL) properties at different growing stages. Atomic force microscopy and scanning electron microscopy measurements reveal the MoS₂ tuned growth of MAPbI₃ in a Stranski-Krastanov mode. PL and Kelvin probe force microscopy results confirm that MAPbI₃/MoS₂ heterostructures have a type-II energy level alignment at the interface. Temperaturedependent PL measurements on layered MAPbI₃ (at the initial stage) and on MAPbI₃ crystals in averaged size of 500 nm (at the later stage) show rather different temperature dependence as well as the phase transitions from tetragonal to orthorhombic at 120 and 150 K, respectively. Our findings are useful in fabricating MAPbI₃/transition-metal dichalcogenide based innovative devices for wider optoelectronic applications.