MgO、SnO_2、TiO_2表面氧空位性质对气体光学传感特性的影响

光学气敏传感器是当今研究领域的一个热门方向.文章采用密度泛函理论(DFT)体系下广义梯度近似(GGA)第一性原理平面波超软赝势方法,分析和计算了光学气敏材料岩盐型MgO、金红石型SnO_2和锐钛矿型TiO_2表面氧空位的特性.以CO作为吸附分子进行微观机理研究,研究不同氧化物表面吸附气体分子的机理.对氧化物表面的几何结构、吸附能、态密度、差分电荷密度、电荷布居、电荷转移、光学性质等进行分析.研究发现:含有氧空位缺陷的MgO(001)、SnO_2(110)和TiO_2(101)能稳定的吸附CO分子,吸附后造成了材料光学性质的变化,可作为光学气敏传感材料.分析发现:氧空位氧化能力的大小是光学性质改变的核心原因.表面吸附CO分子后,发现SnO_2(110)表面对分子的吸附能最大,分子与表面的吸附距离最短.通过差分电荷密度和电荷布居数发现,表面与CO分子间存在电荷转移,其转移电子数目大小为:SnO_2(110)TiO_2(101)MgO(001),由此得出不同氧化物表面氧化性的大小为:SnO_2(110)TiO_2(101)MgO(001);通过对比吸收谱和反射谱发现:吸附气体分子后SnO_2(110)表面的光学性质变化最为明显,是一种较好的光学气敏传感材料.     

三氧化钨表面氢吸附机理的第一性原理研究

采用基于密度泛函理论的第一性原理平面波超软赝势方法,在广义梯度近似下,研究了立方WO_3,WO_3(001)表面结构及其氢吸附机理.计算结果表明立方晶体WO_3理论带隙宽度为0.587 eV.WO_3(001)表面有WO终止(001)表面和O终止(001)表面两种结构,表面结构优化后W—O键长和W—O—W键角改变,从而实现表面弛豫;WO终止(001)表面和O终止(001)表面分别呈现n型半导体特征和p型半导体特征.分别计算了H原子吸附在WO终止(001)表面和O终止(001)表面的H—O_(2c)—H,H—O_(2c)…H—O_(2c),H—O_(1c)—H和H—O_(1c)…H—O_(1c)四种吸附构型,其中H—O_(1c)—H吸附构型的吸附能最小,H—O键最短,H失去电子数最多,分别为-3.684 eV,0.0968 nm和0.55e,此吸附构型最稳定.分析其吸附前后的态密度,带隙从吸附前的0.624 eV增加到1.004 eV,价带宽度基本不变.H的1s轨道电子与O的2p,2s轨道电子相互作用,在-8和-20 eV附近各形成了一个较强的孤立电子峰,两个H原子分别与一个O_(1c)原子形成化学键,最终吸附反应生成了一个H_2O分子,同时产生了一个表面氧空位.     

双层石墨烯吸附碱金属原子的第一性原理研究

基于密度泛函的第一性原理方法,研究了Li、Na、K和Rb碱金属原子吸附在双层石墨烯(BLG)表面的吸附能、迁移行为、电子性能.研究发现,Li和Na原子在BLG表面吸附易形成团簇,K和Rb原子能够分散吸附.碱金属原子在BLG表面的扩散能垒随原子半径的增加而减小.碱金属原子吸附使电子部分转移至BLG,使体系Fermi能级贯穿导带,表现出金属性.电荷密度差和电荷转移的分析表明,Li、Na、K和Rb与BLG表面以离子键结合.     

双层石墨烯吸附碱金属原子的第一性原理研究

基于密度泛函的第一性原理方法,研究了Li、Na、K和Rb碱金属原子吸附在双层石墨烯(BLG)表面的吸附能、迁移行为、电子性能. 研究发现,Li和Na原子在BLG表面吸附易形成团簇,K和Rb原子能够分散吸附. 碱金属原子在BLG表面的扩散能垒随原子半径的增加而减小. 碱金属原子吸附使电子部分转移至BLG,使体系Fermi能级贯穿导带,表现出金属性. 电荷密度差和电荷转移的分析表明,Li、Na、K和Rb与BLG表面以离子键结合.     

碱金属在磷烯表面吸附和迁移行为的第一性原理研究

利用基于密度泛函理论的平面波赝势方法研究了碱金属(Li、Na、K)-磷烯体系,分析了体系的吸附性质、电学性质和迁移行为.结果表明:碱金属在磷烯表面的最稳定吸附位都是H位.吸附过程中电荷由碱金属原子转移到磷烯,碱金属-磷烯体系表现出一定的离子性.碱金属-磷烯体系的吸附能从大到小为ΔE_(Li-磷烯)ΔE_(K-磷烯)ΔE_(Na-磷烯).Li→Na→K,随着原子序数的递增,体系的离子性逐渐增强;碱金属原子越来越容易在磷烯表面迁移.     

碱金属在GaAs(110)表面上的吸附

本文讨论Li,Na,K,Cs在GaAs(110)表面上的吸附,考虑理想表面和弛豫表面两种情况。计算采用集团模型,用电荷自洽的ExtendedHucheltheory(缩写为EHT)方法进行。结果表明,吸附后表面原子趋向于理想位置,碱金属原子位于垂直于表面沿[001]方向横跨表面Ga原子的对称平面上。碱金属吸附后的费密能级在价带顶以上约0.7eV处,是由表面Ga原子与碱金属原子间的相互作用决定的。而在价带中碱金属原子主要与表面As原子成键。 关键词：     

碱金属在石墨烯表面吸附,迁移行为的第一性原理研究

利用基于密度泛函理论的第一性原理方法计算了3种碱金属(Li,Na,K)在石墨烯表面的吸附性质,迁移行为和电学性质.结果表明:3种碱金属在石墨烯表面的最稳定吸附位都是H位;吸附过程中电荷由碱金属原子向石墨烯片层转移.Li→Na→K,吸附能先减小再增大,吸附的强弱顺序为Li-石墨烯体系K-石墨烯体系Na-石墨烯体系;体系的离子性逐渐增强;碱金属原子在石墨烯表面的迁移激活能逐渐降低,迁移行为更容易实现.     

第一性原理研究氧空位对Ag在MgO(001)面吸附的影响

采用基于密度泛函理论的第一性原理研究方法,应用Vienna Ab-initio Simulation Package (VASP),计算了氧空位对Ag原子在MgO(001)面吸附的影响．通过成键过程中电荷密度的变化以及电荷转移的讨论,从原子尺度上分析了MgO(001)面空位点Fs和Fs+对其吸附、聚集与成核属性的影响以及吸附的能量属性.结果表明,相对清洁的MgO表面而言,Ag原子吸附在O空位时,能够更牢固地与MgO表面结合,并吸引更多的Ag原子聚集在一起,形成一个个独立的Ag原子岛.     

N_2在Co掺杂Ru(001)表面吸附的DFT研究

采用密度泛函理论与周期性平板模型相结合的方法,对N_2在Ru(001)表面top、fcc、hcp、bridge四个吸附位和Ru-Co(001)表面Ru-top、Co-top、Ru(Ru)Ru-bridge、Co(Co)Co-bridge、Ru(Co)Co-bridge、Ru(Ru)Co-bridge、Ru_2Co-hcp、RuCo_2-hcp、Ru_2Co-fcc、RuCo_2-fcc十个吸附位的14种吸附模型进行了构型优化、能量计算,得到了N_2较有利的吸附位;并对清洁表面进行能带分析,对最佳吸附位进行总态密度分析.结果表明:掺杂Co后,Ru催化剂的能带变宽,催化活性增强;N_2在Ru(001)表面的最稳定吸附位top的吸附能是-88.94 kJ·mol~(-1),在Ru-Co(001)表面的最稳定吸附位Ru-top的吸附能是-95.71 kJ·mol~(-1),而且N_2与金属表面成键,属于化学吸附.     

FeOx上AsH3吸附净化的密度泛函研究

基于密度泛函理论研究了AsH₃和O₂分子在α-Fe₂O₃(001)表面和FeO(100)表面的吸附及共吸附性质.结果表明：AsH₃和O₂分子在α-Fe₂O₃(001)表面最稳定的吸附构型都是Hollow吸附位点. AsH₃分子在FeO(100)表面最稳定的吸附位点为Top O吸附位点. O₂分子在FeO(100)表面最稳定的吸附位点为Hollow吸附位点. O₂分子在α-Fe₂O₃(001)和FeO(100)表面吸附后均被活化从而促进AsH₃分子的催化氧化. AsH₃分子在α-Fe₂O₃(001)表面最小的吸附能为-0.7991 eV,在FeO(100)表面最小的吸附能为-0.9117 eV.吸附值数据表明AsH     

Interaction of alkali metals with the Fe3O4(1 1 1) Surface

The adsorption and interaction of alkali metals (Li, Na, K, Rb and Cs) with the Fe_tet1-terminated Fe₃O₄(1 1 1) surface have been computed at the level of density functional theory. At low coverage, adsorption of alkali metals on site-1 (O_a-O_c-O_d) is energetically more favorable than on site-2 (O_a-O_c-O_d). Li has the strongest adsorption energy, followed by K, Rb, Cs and Na. The computed net charges show that the alkali metals can donate electrons to surface Fe and O atoms in the order of Li < Na < K ≈ Rb ≈ Cs. It is also noted that increasing the coverage does not significantly improve the promoting effect of alkali metals. In addition, alkali metals can move facilely on the Fe₃O₄(1 1 1) surface.     

Theoretical study of ZnO adsorption and bonding on Al_2O_3(0001) surface

Sapphire (α-Al2O3) and silicon (Si) are widely applied as the substrates of the highquality ZnO thin films prepared by pulse laser deposition (PLD) and molecule beamepitaxy (MBE) technology. The adhesion, diffusivity, and bonding of the particles on thesubstrates play a significant role in the forming and initial growing of nucleation for filmgrowth, and directly influence the quality of the entire thin films[1]. No sufficient studiesand experiments are available on the surface atomic str…     

Effect of alkali metals and superalkali species on electronic properties of graphdiyne with open hexagonal edges

In the present work, we study the new two dimensional materials with tunable bandgap and high chemical reactivity via locating the alkali metals and superalkalis (Li, Na, K, Li3O, Na3O, and K3O) on the graphdiyne (GDY) sheets with open hexagonal edges. Our density functional theory (DFT) calculations reveal that the bandgap energy of GDY with a different open zigzag or armchair edges (ZGDY and AGDY) is decreased by interacting with alkali metals and superalkali species. We found that the alkali metals and superalkalis lowered the bandgap energy of AGDY and ZGDY by more than 170% and converted them into the semimetals. Our results also show that the M3O/ZGDY systems show higher chemical reactivity as compared to the other studied systems. Among the studied M(M3O)/ZGDY(AGDY) systems, the maximum binding energy, maximum means static polarisability, minimum bandgap energy, and minimum chemical hardness are related to the K3O/ZGDY complex. Consequently, the K3O/ZGDY is a promising chemically reactive material for applications in various fields such as gas detectors, catalysts in the oxygen reduction reactions, and drug delivery application.     

The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.     

Light Absorption of GaN Nanowire Array Cathode with Alkali Metal Nanoparticle Modified on the Surface

Since the adsorption of alkali metals is necessary for the negative electron affinity (NEA) of the photocathode, light absorption models of GaN nanowire (NW) arrays with alkali metal (Li, Na, K, and Cs) nanoparticles (NPs) modified on the NW surface based on the finite difference time domain (FDTD) method are constructed. The absorption spectra of hemispherical, spherical alkali metal NPs adsorbed on the outer surface of the NW, and spherical alkali metal embedded on the inner surface and center of the NW are studied. When the ratio of NW diameter to period (D/P) is greater than 0.5, the adsorption of alkali metal NPs cannot improve the absorption of GaN NW arrays. Alkali metal decoration can cause the absorption gain of NW arrays and optical loss of NPs, so the diameter and spacing of alkali metal NPs need to be balanced. When Li NPs are embedded in NW, plasmons can enhance the generation of electron-hole pairs, making GaN NWA obtain higher optical absorption and quantum efficiency. Therefore, the method of Li and Cs NPs embedded in GaN NW can provide a reference for the process NEA design, which will contribute to the development of the ultraviolet photocathode with high absorption characteristics.     

Mechanism of oxygen adsorption on surfaces of γ-TiAl

We studied the adsorption behavior of oxygen on low index surfaces of γ-TiAl via first principles to investigate the mechanism that drives the adsorption behavior. The (100) surface is the most stable surface energetically followed by the (111), (110) and (001) surfaces. A study of the adsorption of a single oxygen atom on surfaces of TiAl showed that the O atom prefers the Ti-rich environment that has a high potential of generating TiO₂. Competition between O-Al bonding and O-Ti bonding was observed in the O adsorbed surface regions. However, the O-Ti interaction dominates the adsorption behavior in all considered systems except when O is adsorbed on an Al-terminated (001) surface as the O–Al bond is stronger than O–Ti bond. A linear relationship between adsorption energy and integration of orbital overlaps between the O atom and the metals is obtained, which indicates that the electronic structure controls the adsorption behavior of an O atom on a γ-TiAl surface — an opportunity to improve the oxidation resistance of γ-TiAl based alloys.     

Tuning the optical properties of phosphorene by adsorption of alkali metals and halogens

Optical properties of phosphorene are tuned by adsorption of alkali metals (Li and Na) and halogens (Br and Cl). It has been found that on increasing the size of alkali metals and halogen adsorbed phosphorene layer the absorption coefficient reduces and shifts towards visible region. The refractive index in alkali metal adsorbed phosphorene increases with size of phosphorene layer. For halogen adsorbed structure it decreases with increase in size of phosphorene layer. Optical absorption is observed to depend on both dielectric constant and refractive index. Since adsorption of alkali and halogen materials modifies the refractive index of phosphorene, absorption is seen to reduce in all cases where refractive index increases due to adsorption even when the dielectric constant was high.     

TiO2分子在GaN(0001)表面吸附的理论研究

采用基于密度泛函理论的平面波超软赝势法,计算了TiO₂分子在GaN(0001)表面的吸附成键过程、吸附能量和吸附位置. 计算结果表明不同初始位置的TiO₂分子吸附后,Ti在fcc或hcp位置,两个O原子分别与表面两个Ga原子成键,Ga-O化学键表现出共价键特征,化学结合能达到7.932-7.943eV,O-O连线与GaN[1120]方向平行,与实验观测(100)[001] TiO₂//(0001)[1120]GaN一致. 通过动力学过程计算分析,TiO₂分子吸附过程经历了物理吸附、化学吸附与稳定态形成的过程,稳定吸附结构和优化结果一致. 关键词： GaN(0001)表面 2分子')" href="#">TiO₂分子 密度泛函理论 吸附     

Alkali-induced enhancement of surface electronic polarizibility

From results of ab initio electronic structure calculations based on density functional theory for a set of prototype systems, we find alkali adsorbates to cause a dramatic enhancement of the electronic polarizability of the metal surface extending it several angstroms into the vacuum. This phenomenon is traceable to an unusual feature induced in the surface potential on alkali adsorption. The effect appears to be general, as we find it to be present on metals as varied as Pd and Cu, and helps explain the observed substantial decrease in the vibrational frequency of molecules when coadsorbed with alkalis on metal surfaces. Specifically, for two dissimilar molecules CO and O(2), we trace the softening of the frequencies of their stretching mode when coadsorbed with K on Pd(111) to the enhanced electronic polarizability.     

Lattice relaxation at alkali metal surfaces

The surface relaxation of the first interlayer spacing for both the (110) and (100) faces of alkali metals has been calculated selfconsistently using the density functional formalism and a new density matrix method which obviates the use of wavefunctions. For the (110) surfaces of all alkali metals we find no or only very small relaxation. The (100) surfaces show bigger relaxation effects and these vary from 3.5% inward for Li to 1% outward for Cs. These findings are consistent with general trends found in LEED or Ion scattering studies. Force constants for the movement of the last atomic layer are also determined.