表面非金属掺杂对锐钛矿相TiO2(101) 面吸附NH3特性的影响

采用基于密度泛函理论(DFT-D)体系下的第一性原理平面波超软赝势方法,研究了被不同非金属（B、C、N、F）掺杂的TiO2(101)表面吸附NH3的特性与作用机理。研究发现：被非金属掺杂后的表面对NH3的吸附效果要优于未掺杂表面。不同元素掺杂对比发现：C掺杂后的表面吸附能最大,稳定后吸附距离最小,为最稳定吸附结构。通过Mulliken电荷分布和分态密度的分析,得到了不同吸附条件下NH3在TiO2掺杂表面的催化氧化还原作用机理,并发现各模型吸附能的不同是由于掺杂（X）位原子与NH3分子的相互作用强弱不同所造成。掺杂原子在费米面附近的电子态密度贡献越强,掺杂原子与NH3分子电荷转移的净值越小,吸附距离越小,吸附能越大,吸附更稳定。     

表面非金属掺杂对锐钛矿相TiO_2(101)面吸附NH_3特性的影响

采用基于密度泛函理论(DFT-D)体系下的第一性原理平面波超软赝势方法,研究了被不同非金属(B、C、N、F)掺杂的TiO_2(101)表面吸附NH_3的特性与作用机理.研究发现:被非金属掺杂后的表面对NH_3的吸附效果要优于未掺杂表面.不同元素掺杂对比发现:C掺杂后的表面吸附能最大,稳定后吸附距离最小,为最稳定吸附结构.通过Mulliken电荷分布和分态密度的分析,得到了不同吸附条件下NH_3在TiO_2掺杂表面的催化氧化还原作用机理,并发现各模型吸附能的不同是由于掺杂(X)位原子与NH_3分子的相互作用强弱不同所造成.掺杂原子在费米面附近的电子态密度贡献越强,掺杂原子与NH_3分子电荷转移的净值越小,吸附距离越小,吸附能越大,吸附更稳定.     

H2O分子在锐钛矿型TiO2(101)表面的吸附特性与微观机理

近年来,随着着光纤技术和光集成技术的发展,光学气敏湿度传感器被广泛应用,在气敏传感材料领域中,由于TiO2具有灵敏度高、响应时间快等优点而受到广泛关注。本文采用基于密度泛函理论(DFT)的平面波超软赝势方法,模拟计算H2O分子在锐钛矿型TiO2(101)无氧空位和有氧空位表面的吸附行为,对吸附能、吸附距离、吸附前后表面电子态密度以及光学性质分别进行分析,结果表明：H2O分子在无氧空位锐钛矿型TiO2(101)表面不容易被吸附,在含有氧空位缺陷的表面容易被吸附;稳定吸附后,H2O分子平面垂直于TiO2表面;负电中心（O端）距空位越近,吸附越稳定,且氧空位浓度越高,吸附效果越明显;通过电子态密度分析发现,H2O分子吸附于含氧空位的表面后,由于H2O分子中O原子的2p孤对电子掺入,在费米能级附近出现新峰值,提高了材料在可见光低能区域的跃迁几率,明显改善了对可见光的吸收系数和反射率,光学气敏传感特性显著。     

H_2O分子在锐钛矿型TiO_2(101)表面的吸附特性与微观机理

本文采用基于密度泛函理论(DFT)的平面波超软赝势方法,模拟计算H2O分子在锐钛矿型TiO2(101)无氧空位和有氧空位表面的吸附行为,对吸附能、吸附距离、吸附前后表面电子态密度以及光学性质分别进行分析,结果表明:H2O分子在无氧空位锐钛矿型TiO2(101)表面不容易被吸附,在含有氧空位缺陷的表面容易被吸附;稳定吸附后,H2O分子平面垂直于TiO2表面;负电中心(O端)距空位越近,吸附越稳定,且氧空位浓度越高,吸附效果越明显;通过电子态密度分析发现,H2O分子吸附于含氧空位的表面后,由于H2O分子中O原子的2p孤对电子掺入,新峰值在费米能级附近出现,提高了材料在可见光低能区域的跃迁几率,明显改善了对可见光的吸收系数和反射率,光学气敏传感特性显著.     

金红石相TiO_2(110)面对NH_3吸附的微观机制和光学气敏特性研究

研究光学气敏材料吸附气体后的光学气敏特性是气敏传感研究的热点问题。研究了金红石相TiO2(110)面吸附NH3的微观机制和光学气敏特性。结果表明,NH3分子容易被含氧空位缺陷的金红石相TiO2(110)面所吸附,且提高表面氧空位的比例,有利于NH3稳定吸附。表面吸附NH3分子以负电荷中心向下为主,当氧空位比例为33%时,吸附能为1.7313eV。表面吸附NH3为化学吸附,NH3的H原子被还原而N原子被氧化。含氧空位的表面吸附NH3在可见光1.5~3.1eV范围内,表面氧空位比例越大,其对可见光吸收和反射能力越强,光学气敏性越灵敏。     

非金属元素掺杂锐钛矿TiO_2(101)表面对提升NH_3光学气敏传感特性的影响

本文采用第一性原理平面波超软赝势的方法,模拟计算了含氧空位锐钛矿TiO_2(101)表面单掺杂非金属C元素、N元素、F元素以及双掺杂C-N元素、C-F元素、N-F元素后表面的氧化还原能力,分析对NH_3分子吸附的微观机理,研究杂质掺入对光学传感特性的影响.结果表明:非金属元素是比较容易掺入到锐钛矿TiO_2(101)表面,掺杂表面对NH_3分子吸附较未掺杂的表面要好,表面吸附NH_3分子后,吸附距离都出现缩短,C-N元素掺杂后吸附距离最小且吸附能最大;通过Mulliken电荷布居分布分析,C掺杂提升了表面的氧化性,N元素对表面的氧化性提升不明显,而F掺杂降低了表面的氧化性;通过态密度分析可知,C掺杂在禁带中产生了受主能级,而N掺杂提高了价带顶的电子态密度,F掺杂在导带底产生了施主能级;通过光学性质的分析可知:C掺杂提升了材料对低能可见光的响应,使材料对570 nm～760 nm范围内的可见光吸收提高了大约3.5倍;而C-N双掺杂体系,使材料对400 nm～570 nm范围内的可见光吸收提高了大约3倍.总的来说,单掺杂C元素以及双掺杂C-N元素都能明显的提高材料的光学气敏传感特性.     

过渡金属Cu、Cr掺杂TiO2表面氧化性气体NO2光学气敏传感特性

采用密度泛函理论（DFT）体系广义梯度近似（GGA）第一性原理平面波超软赝势方法,分析锐钛矿型TiO₂（101）表面吸附NO₂分子光学气敏传感的微观机理.结果表明：Cu和Cr原子易于掺入TiO₂（101）表面,掺杂表面能稳定地吸附NO₂分子且吸附后光学性质发生显著变化.表面吸附NO₂分子后,Cu掺杂TiO₂（101）表面对分子的吸附能最大,吸附后结构更稳定,分子与表面的距离最短.通过分析差分电荷密度和电荷布居数发现,NO₂分子与基底表面间发生电荷转移,转移电子数目：Cu掺杂表面 > Cr掺杂表面 > 无掺杂表面.对比吸收光谱和反射光谱发现,在Cu掺杂表面吸附分子后,光学性质变化最明显,说明表面与吸附分子间氧化还原能力是决定光学气敏传感性能的核心因素.在过渡金属中,Cu与Cr都有4s价电子结构,其4s电子降低了材料表面氧空位的氧化性,增加了其还原性.对于氧化性气体,可以提升表面与分子的氧化还原作用,而Cu的4s电子更加活泼,从而光学气敏传感特性更加明显.因此,Cu掺杂的TiO₂对氧化性气体是一种较好的光学气敏传感材料.     

非金属元素掺杂锐钛矿TiO_2(101)表面对提升NH_3光学气敏传感特性的影响

本文采用第一性原理平面波超软赝势的方法,模拟计算了含氧空位锐钛矿TiO_2(101)表面单掺杂非金属C元素、N元素、F元素以及双掺杂C-N元素、C-F元素、N-F元素后表面的氧化还原能力,分析对NH_3分子吸附的微观机理,研究杂质掺入对光学传感特性的影响.结果表明:非金属元素是比较容易掺入到锐钛矿TiO_2(101)表面,掺杂表面对NH_3分子吸附较未掺杂的表面要好,表面吸附NH_3分子后,吸附距离都出现缩短,C-N元素掺杂后吸附距离最小且吸附能最大;通过Mulliken电荷布居分布分析,C掺杂提升了表面的氧化性,N元素对表面的氧化性提升不明显,而F掺杂降低了表面的氧化性;通过态密度分析可知,C掺杂在禁带中产生了受主能级,而N掺杂提高了价带顶的电子态密度,F掺杂在导带底产生了施主能级;通过光学性质的分析可知:C掺杂提升了材料对低能可见光的响应,使材料对570 nm～760 nm范围内的可见光吸收提高了大约3.5倍;而C-N双掺杂体系,使材料对400 nm～570 nm范围内的可见光吸收提高了大约3倍.总的来说,单掺杂C元素以及双掺杂C-N元素都能明显的提高材料的光学气敏传感特性.     

非金属元素掺杂锐钛矿TiO_2(101)表面对提升NH_3光学气敏传感特性的影响

本文采用第一性原理平面波超软赝势的方法,模拟计算了含氧空位锐钛矿TiO_2(101)表面单掺杂非金属C元素、N元素、F元素以及双掺杂C-N元素、C-F元素、N-F元素后表面的氧化还原能力,分析对NH_3分子吸附的微观机理,研究杂质掺入对光学传感特性的影响.结果表明:非金属元素是比较容易掺入到锐钛矿TiO_2(101)表面,掺杂表面对NH_3分子吸附较未掺杂的表面要好,表面吸附NH_3分子后,吸附距离都出现缩短,C-N元素掺杂后吸附距离最小且吸附能最大;通过Mulliken电荷布居分布分析,C掺杂提升了表面的氧化性,N元素对表面的氧化性提升不明显,而F掺杂降低了表面的氧化性;通过态密度分析可知,C掺杂在禁带中产生了受主能级,而N掺杂提高了价带顶的电子态密度,F掺杂在导带底产生了施主能级;通过光学性质的分析可知:C掺杂提升了材料对低能可见光的响应,使材料对570 nm～760 nm范围内的可见光吸收提高了大约3.5倍;而C-N双掺杂体系,使材料对400 nm～570 nm范围内的可见光吸收提高了大约3倍.总的来说,单掺杂C元素以及双掺杂C-N元素都能明显的提高材料的光学气敏传感特性.     

过渡金属Pd掺杂AsP结构及其吸附甲醛和一氧化碳的性能研究

基于第一性原理方法,研究了单层本征磷砷AsP和过渡金属钯（Pd）掺杂磷砷AsP的结构,并对比研究了本征和掺杂后的AsP吸附甲醛（HCHO）和一氧化碳（CO）气体分子的稳定性、能带结构、态密度以及电荷差分密度。研究结果表明：经Pd掺杂后AsP由半导体转变为导体;本征AsP吸附一氧化碳最稳定的位置为P-As键顶上,吸附甲醛最稳定的位置为P原子顶上;本征吸附时气体分子与基底之间的距离在3 Å左右,气体分子与基底之间未形成化学键。过渡金属Pd原子掺杂AsP后形成两种结构,分别为Pd原子替换超胞结构中的As原子或P原子。两种掺杂结构分别吸附一氧化碳或甲醛气体分子时,除了Pd原子替换AsP中的As原子形成的结构吸附甲醛的吸附能未明显增加外,其余掺杂结构吸附一氧化碳或甲醛的吸附能和电荷转移较本征吸附时均显著增强,吸附CO分子时,C原子与Pd原子之间形成了化学键。特别是,Pd原子替换AsP中的P原子形成的结构对一氧化碳和甲醛气体分子的吸附性能明显强于Pd原子替换AsP中的As原子所形成的结构。     

Surface energy modification by electron beam

We observe a pronounced variation of wettability properties in solid state materials induced by a low-energy electron beam. The phenomenon occurs in several stages characterized by various mechanisms. We show that for low electron doses the irradiation leads to decrease in the wetting of a dielectric surface due to induced surface electric potential. The higher electron charge leads to formation of a chemical monolayer on material’s surface. It has been found that the electron irradiation strongly modifies the surface free energy of SiO₂ by decreasing its total surface free energy value, almost twice. However, electron-induced variations of dispersive and polar components of the surface free energy are quite different and depend of incident electron charge.     

Unique edge structure and stability of fabricated dimer islands on Si(001)

The edge structure and stability of monolayer-high islands fabricated on Si(001) surfaces by scanning tunneling microscopy have been analyzed theoretically. In contrast to the edges of similar islands grown by depositing Si, the properties of edges of fabricated islands are determined by the length of the trench of dimers that are removed to create the island. We demonstrate the possibility of controlling the edge structure, and thus the stability, through a selective process of atom removal.     

Investigation of surface properties of liquid transition metals: Surface tension and surface entropy

In the present study, surface properties namely surface tension and surface entropy of liquid transition metals have been reported. The surface entropy of liquid Fe, Co and Ni metals has been investigated using the expression derived by Gosh et al. [R.C. Gosh, A.Z. Ziauddin Ahmed, G.M. Bhuiyan, Eur. Phys. J. B 56 (2007) 177]. To describe interionic interaction the pseudopotential approach has been used and radial distribution functions have been determined from the solution of Ornstein-Zernike integral equation. The calculated values of surface tension and surface entropy agree well with experiment. The present study shows that the expression derived by Gosh et al. leads to a good estimation for the surface entropy.     

On the surface trapping parameters of polytetrafluoroethylene block

Surface flashover phenomena under high electric field are closely related to the surface characteristics of a solid insulating material between energized electrodes. Based on measuring the surface potential decaying curve of polytetrafluoroethylene (PTFE) block charged by a needle-plane corona discharge, its surface trapping parameters are calculated with the isothermal current theory, and the correlative curve between the surface trap density and its energy level is obtained. The maximum density of electron traps and hole traps in the surface layer of PTFE presents a similar value of ∼2.7 × 10¹⁷ eV⁻¹ m⁻³, and the energy level of its electron and hole traps is of about 0.85-1.0 eV and 0.80-0.90 eV, respectively. Via the X-ray photoelectron spectroscopy (XPS) technique, the F, C, K and O elements are detected on the surface of PTFE samples, and F shows a remarkable atom proportion of ∼73.3%, quite different from the intrinsic distribution corresponding to its chemical formula. The electron traps are attributed to quantities of F atoms existing on the surface of PTFE due to its molecular chain with C atoms surrounded by F atoms spirally. It is considered that the distortions of chemical and electronic structure on solid surface are responsible for the flashover phenomena occurring at a low applied voltage.     

Comments on ‘‘Effect of the surface free energy on the behaviour of surface and guided waves”, by V. Vlasie Belloncle,M. Rousseau,Ultrasonics, 45 (2006) 188–195

In this short communication, it is demonstrated that the main results obtained by the authors of the commented paper, ‘‘Effect of the surface free energy on the behaviour of surface and guided waves”, by V. Vlasie Belloncle, M. Rousseau Ultrasonics, 45 (2006) 188–195, have been well-established long before publication of this paper. Therefore, the claim to novelty asserted by the authors is incorrect.     

Surface diffusivity of thin polymer films measured by a curvature driven flow and Rouse dynamics

Nanodefects induced by nanoindentation on thin polystyrene (PS) films spin cast on silicon (Si) relax upon annealing at 110 °C. The relaxation process for low molecular weight PS is interpreted in terms of a curvature driven flow which leads to the measurement of a diffusion coefficient. The latter is compared with the expected Rouse predictions using (i) bulk and (ii) surface glass transition temperature data, found in the literature. Deviations from the Rouse predictions are observed when is used for the analysis of the data. On the contrary, excellent agreement with the Rouse model is reported when is used.     

Surface plasmon polariton scattering by small ellipsoid particles

Scattering of surface plasmon polaritons (SPP’s) by small ellipsoid particles placed near a dielectric–metal interface is theoretically considered. Using the Green’s function formalism and the dipole approximation, we consider the differential and total scattering cross-sections associated with the SPP-to-SPP scattering as well as with the SPP scattering into waves propagating away from the interface, analyzing the influence of system parameters. As an example, scattering cross-sections of differently shaped gold spheroid particles placed near an air–gold interface are evaluated at the light wavelength of 800 nm. It is shown that the differential and total cross-sections depend strongly upon the particle-to-surface distance, the ratio between the major and minor axes and their orientation with respect to both the interface and the direction of SPP incidence. Implications of the obtained results to the design of SPP micro-optical components are also discussed.     

Model for degradation of band gap photo-luminescence in GaAs

Summary Degradation of near band gap photo-luminescence emission in GaAs with time of exposure to low power, c.w. laser excitation at room temperature is quantitatively described by a model based on defect reactions that are promoted by trapping and recombination of excess carriers at nonradiative recombination sites. The proposed model accurately describes the observed degradation rate, its power and temperature dependence, as well as the absence of degradation at a surface with shallow ion implantation. Summer student from the Materials Sciences Department at Massachusetts Institute of Technology, Cambridge, Mass., USA.