Effects of surface charges on phonon properties and thermal conductivity in GaN nanofilms

Surface charges can modify the elastic modulus of nanostructure, leading to the change of the phonon and thermal properties in semiconductor nanostructure. In this work, the influence of surface charges on the phonon properties and phonon thermal conductivity of GaN nanofilm are quantitatively investigated. In the framework of continuum mechanics,the modified elastic modulus can be derived for the nanofilm with surface charges. The elastic model is presented to analyze the phonon properties such as the phonon dispersion relation, phonon group velocity, density of states of phonons in nanofilm with the surface charges. The phonon thermal conductivity of nanofilm can be obtained by considering surface charges. The simulation results demonstrate that surface charges can significantly change the phonon properties and thermal conductivity in a GaN nanofilm. Positive surface charges reduce the phonon energy and phonon group velocity but increase the density of states of phonons. The surface charges can change the size and temperature dependence of phonon thermal conductivity of GaN nanofilm. Based on these theoretical results, one can adjust the phonon properties and temperature/size dependent thermal conductivity in GaN nanofilm by changing the surface charges.     

Spontaneous explosive crystallization and phase transformations in a selenium/copper bilayer nanofilm

Spontaneous explosive crystallization in a selenium/copper bilayer nanofilm has been experimentally studied. It has been shown that the formation of a microcrack network in the selenium film is a decisive factor for spontaneous explosive crystallization. The microcrack network is an efficient channel for the relaxation of the collected energy of elastic stresses in the selenium film. The relaxation of this energy accelerates crystallization and transfers it to the explosive regime. It has been found that crystalline phases appearing in the products of the reaction after spontaneous explosive crystallization depend on the ratio of the thicknesses of the copper and selenium films.     

Angle-resolved photoemission study of Ag nanofilms grown on fcc transition-metal (111) substrates

A comparative study of the electronic structures of Ag nanofilms on the pseudomorphic metastable fcc Fe(111) and bulk-like fcc Co(111) substrates has been carried out to investigate their quantized electronic structures. The photoemission spectra of both Ag nanofilms exhibit the fine structures derived from the quantized sates of Ag sp valence electron. The nanofilm-thickness dependences of the binding energy of the quantized states are reproduced by the calculated results based on the phase accumulation model. From the angle-resolved photoemission measurements, the effective masses of the quantized electronic states along the direction parallel to the nanofilm surface were directly determined. We discuss the electronic hybridization effect between quantized states in Ag nanofilm and 3d-derived electronic states in transition-metal substrates.     

基于飞秒瞬态反射/透射技术的纳米Au半透膜热效应研究

以飞秒激光放大器作为光源联合使用瞬态反射/透射实验技术研究了纳米Au半透明纳米薄膜中非平衡载能粒子的热传导过程. 在相同实验条件下, 发现该薄膜的瞬态透射和反射信号明显不同并且延迟时间在5.0–7.5 ps时瞬态透射信号的符号发生改变. 对纳米薄膜的透射和反射信号进行了对比分析, 分别使用双温模型和Crude近似进行数据模拟并拟合, 分析认为沿膜厚方向的温度梯度变化和界面热阻效应引起介电函数的变化不同, 从而引起了瞬态透射信号和反射信号的不同. 对于半透明金属纳米薄膜需要同时考虑其瞬态透射和反射影响才能得到准确的瞬态吸收结果. 随着抽运脉冲能量的增加, 可以看到上升时间约为1.0 ps, 电子-晶格弛豫时间增加.     

多晶碲化锌薄膜载能子超快动力学实验研究

利用双波长飞秒激光抽运-探测实验方法测量了掺氮多晶ZnTe薄膜在飞秒激光加热情况下载能子超快动力学过程. 采用包含电子弛豫过程和晶格加热过程的理论模型拟合实验数据, 二者符合得很好. 拟合得到10 ps以内影响掺氮多晶ZnTe薄膜表面超快反射率变化的三个弛豫过程的时间常数均为亚皮秒量级. 其中, 正振幅电子弛豫过程是由电子-光子相互作用引起的载流子扩散和带间载流子冷却过程, 负振幅电子弛豫过程是由缺陷造成的光激载能子的俘获效应引起的, 晶格加热过程主要通过电子-声子耦合过程进行的.     

B2-NiAl纳米薄膜厚度对其弹性性能影响的模拟研究

本文应用分子动力学(MD)技术和改进分析型嵌入原子模型(MAEAM)研究B2-NiAl纳米薄膜有关弹性性能的尺寸效应和表面效应. 首先计算块体B2-NiAl合金的弹性性能和该类薄膜的厚度尺寸对其表面能的影响, 发现块体B2-NiAl薄膜的弹性性能与已有的实验和理论计算结果接近, 而薄膜表面能仅与表面原子组分有关, 基本不受其厚度尺寸的影响. 在此基础上, 重点研究了纳米薄膜的弹性性能随其厚度尺寸变化关系, 发现所有纳米薄膜弹性性能都随其尺寸增加而呈指数变化, 并受表面原子组分调控. 并进一步分析尺寸影响纳米薄膜弹性性能的内在机理, 发现纳米薄膜的晶面间距偏离和表面是影响其弹性性能随尺寸变化的主要因素, 并与以前实验和理论研究结果相符合.     

Fabrication of triazinedithiol functional polymeric nanofilm by potentiostatic polymerization on aluminum surface

The functional polymeric nanofilm of 6-(N-allyl-1,1,2,2-tetrahydroperfluorodecyl)amino-1,3,5-triazine-2,4-dithiol monosodium (AF17N) was prepared on pure aluminum surface by potentiostatic polymerization at different potentials. The thickness and weight of polymeric nanofilm increased proportionally to electro-polymerization potential following linear equation. The chemical structure of nanofilm was characterized by Fourier transform-infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Adsorption peaks in FT-IR and C1s, N1s, S2p, F1s and Al2p peaks in XPS spectra indicated that the polymeric nanofilm was poly(6-(N-allyl-1,1,2,2-tetrahydroperfluorodecyl)amino-1,3,5-triazine-2,4-disulfide) (PAF17). The morphologies of polymeric nanofilm were also observed by atomic force microscopy (AFM). All the results showed that the optimal electro-polymerization potential and time were 8 V and 20 s, respectively. Uniform and compact nanofilm of PAF17 could be obtained under these conditions. It is expected that this technique will be applied in the preparation of lubricating, dielectric and hydrophobic surface on aluminum substrate.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

An energy approach to the modelling of particle removal by pulsed laser irradiation

An energy model to explain particle removal mechanism has been developed. This model is based on a detailed investigation of contact deformation of a particle on a solid surface, as well as particle motion during the process of substrate surface expansion under uniform laser irradiation. Calculation results show that small particles mainly gain kinetic energy during pulsed laser irradiation, whereas large particles mainly gain elastic deforming potential energy. The particle removal condition is derived from the viewpoint of energy. The relationship of particle removal efficiency with laser fluence and particle size is discussed. Theoretical results are compared with experimental results. Received: 30 July 1998 / Accepted: 14 December 1998 / Published online: 17 March 1999     

金属纳米薄膜在石墨基底表面的动力学演化

采用分子动力学方法研究了金属Au和Pt纳米薄膜在石墨(烯)基底表面的动力学演化过程,探讨了金属薄膜和石墨(烯)基底间的相互作用对金属纳米薄膜在固态基底表面的去湿以及脱附的动力学演化的影响.研究结果表明,在高温下,相同层数的Au和Pt纳米薄膜在单层石墨基底表面上存在不同的去湿现象,主要表现为厚度较小的Pt纳米薄膜在去湿过程中有纳米空洞形成,而同样厚度的Au薄膜在去湿过程中没有形成空洞.Au和Pt两种金属薄膜在高温下都去湿形成纳米液滴,这些液滴最终都以一定的速度脱离基底.在模拟的薄膜厚度范围内(0.2—2.3 nm),Au和Pt纳米液滴脱离基底的速度随厚度增加表现出不同的变化规律.Pt纳米液滴的脱离速度随薄膜初始厚度的增加先增加后减少,而Au脱离速度随厚度的增加先减少,达到一个临界厚度后脱离速度突然迅速增加.利用薄膜与基底间相互作用的不同导致去湿过程中的黏滞耗散不同,定性分析了这种变化规律的原因.此外,进一步研究还发现金属液滴的脱离时间与薄膜厚度和模拟温度的依赖关系,发现脱离时间随薄膜厚度的增加而增加,随模拟温度的升高而减小.这些研究结果可以为金属镀膜、浮选、表面清洁、器件表面去湿等工业生产过程提供理论指导.     

Transport and dynamics of optically excited electrons in metals

Time-resolved two-photon photoemission, based on the equal-pulse correlation technique, is used to measure the energy relaxation and the transport of the photoexcited carriers in thin Ag and Au films. The energy-dependent relaxation time shows a significant thickness dependence in the Ag film, whereas for Au a much smaller effect is observed. These experimental observations are compared with a theoretical model based on the Boltzmann equation, which includes secondary (Auger) electrons and transport. A good agreement between experimental and theoretical results is found for Au. However, in our calculations, we did not find any significant change in the thickness dependence in the case of Ag. In order to explain the strong effect in Ag, we discuss the possibility of surface excitations. Received: 15 May 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Au纳米薄膜的熔化机理及其结构演变的分子动力学模拟

利用分子动力学模拟详细研究了不同厚度的Au纳米薄膜的熔化机理和结构演变. 模拟结果表明所有Au纳米薄膜的熔化行为分为两个阶段,即表面预熔和均相熔化. 只有最外层原子出现了预熔化行为, 其他内层原子在均相熔化之前始终保持稳定的固态,这与零维的Au纳米团簇和一维的Au纳米线的预熔化行为是不同的. 同时Au纳米薄膜的熔化温度随着薄膜厚度的增加而升高. 在预熔化过程中,在原子水平上发现了所有的Au纳米薄膜的f100g晶面向f111g晶面转变的表面重建过程. 对于最薄的L2纳米薄膜,当温度低于500 K 时表面应力不能诱导这样的表面重建. 然而一维的Au纳米线在更低温度下就能够观察到了由表面应力诱导的表面重建过程. 这主要是因为Au纳米线具有更高的比表面积所导致的. 另外研究结果还表明当模拟温度达到某一特定值时,由双原子层组成的Au纳米薄膜能够分裂成一维的纳米线.     

A control volume derivation of the energy equation for LII modeling

This paper considers the unsteady energy equation for a particle undergoing processes relevant to laser-induced incandescence. The energy equation is derived using both an integral control volume formalism and a differential approach. Confusion in the previous literature over the form of the energy equation is traced to the evaluation of the energy flux terms to and from the particle surface. Terms such as the heat of sublimation or heat of combustion are shown to arise naturally in both the control volume and differential derivations. Problems associated with the confusion regarding the flux terms resulting in incorrect energy equations are also identified.     

Analysis of mechanically induced processes in the Langmuir film

The understanding of processes in the monolayer at the air-water interface induced by mechanical compression is important as a part of basic research of the system with reduced dimensionality as well as for the investigation of processes during the Langmuir-Blodgett deposition. The Maxwell displacement current technique provides a substantial contribution for the study of structural and electrical properties. Analysis based on imperfect gas approximation with semi-empirical intermolecular potentials is used. Detail theoretical study of molecular tilt in a continual lateral compression and dielectric relaxation phenomena (step-compression) is presented. Obtained results are confronted with standard surface pressure analysis and surface potential measurement.     

High-resolution nonlinear laser spectroscopy of exciton relaxation in GaAs quantum wells

This paper describes measurements of exciton relaxation in GaAs/AlGaAs quantum well structures based on high resolution nonlinear laser spectroscopy. The nonlinear optical measurements show that low energy excitons can be localized by monolayer disorder of the quantum well interface. We show that these excitons migrate between localization sites by phonon assisted migration, leading to spectral diffusion of the excitons. The frequency domain measurements give a direct measure of the quasi-equilibrium exciton spectral redistribution due to exciton energy relaxation, and the temperature dependence of the measured migration rates confirms recent theoretical predictions. The observed line shapes are interpreted based on solutions we obtain to modified Bloch equations which include the effects of spectral diffusion.     

悬空石墨烯对其表面金纳米膜形貌的影响

石墨烯与金属间的相互作用是石墨烯器件研究中的关键问题之一,其涉及石墨烯器件的电学接触、锂离子电池石墨烯电极、石墨烯金属光学等方面.本文重点研究了不同层数的悬空石墨烯表面金纳米膜退火前后的形貌演化过程,观测到两个重要的现象:1)排除基底影响后的悬空石墨烯层数可以通过金纳米膜的形貌特征进行确认,但其随层数的变化趋势与有基底支撑的石墨烯正好相反;2)退火处理后的悬空石墨烯上的金纳米膜形貌演化过程具有类似水滴在荷叶上的行为.对悬空石墨烯表面金属纳米膜在退火前后的形貌变化规律及其现象背后的物理机理进行了详细的讨论和理论解释.     

Elongation rate measurements of nanofilms by microbump method induced by laser pulse

A laser pulse-induced microbump method that aims to measure the elongation rate of nanofilms is proposed. The sample structure is designed as “substrate/active layer/nanofilm” and the laser pulse is used as energy source to heat the active layer and to create microbump. These cause the nanofilm to expand and elongate. The surface area and length change of nanofilm is calculated by measuring the deflections and diameters of the microbumps, as well as to obtain the elongation rate of the nanofilms. A series of microbumps with different deflections are obtained. The deflections are measured precisely by atomic force microscopy (AFM) by taking AgO_x and ZnS-SiO₂ as the active layer and nanofilm, respectively, and by controlling pulse laser parameters. The line elongation rate and plane elongation rate of ZnS-SiO₂ nanofilm are measured at thickness of only 10 nm. Results show that both the two elongation rates linearly increases with laser power from 3.2 to 5.2 mW. Plane elongation rate is a little higher than the line elongation rate at the same laser power. The rupture at 5.4 mW laser power corresponds to fracture strength of the film. The maximum line elongation rate and plane elongation rate are 13.241% and 19.766%, respectively. This method applies a reproducible and efficient method for its applications in the near future.     

Electrostatic Nature of Size Dependences of Adsorption Properties of Ytterbium Nanofilms Grown on the Surface of Silicon: the CO–Yb–Si(111) System

The adsorption of CO molecules onto ytterbium nanofilms with their thickness varying from 1 to 16 monolayers is studied. The dependences of the number of adsorbed CO molecules (adsorption isotherms) and the work function of ytterbium films on the dose of carbon monoxide are examined. It is demonstrated that both the number of adsorbed molecules and the work function depend (under equal conditions) on the nanofilm thickness; in other words, a size effect is revealed. It is found that this size effect is induced by the electrostatic interaction between the conduction electrons of ytterbium and the electrons localized on the nanofilm surface, which establish bonding between the surface and CO molecules. This interaction depends on the film thickness and limits the number of CO molecules that may be adsorbed onto the surface of a film with a given thickness.

     

Optimizating the seats surrounding the platform in a recital hall when considering source directivity

The influence of source directivity can be significant for solo and chamber music performances, particularly for arena type halls that have improved visual intimacy for the audience. Computer simulation was performed to analyze the possibility of optimizing the acoustical qualities of a 4000-m³ recital hall with a significant portion of seats surrounding the platform when considering source directivity. The effects of surface treatments, length-to-width proportion, seating arrangement and platform location were evaluated. Most acoustical parameters derived from a baritone singer source were categorized as one factor that can be best represented by 2-kHz band early sound strength G_E. Regardless of room proportion, front-back 2-kHz band energy difference derived from optimized models can be controlled within 4 dB, a value much smaller than the difference in direct component.     

Influence of surface scattering on the thermal properties of spatially confined GaN nanofilm

Gallium nitride(GaN), the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In this paper, we investigate the surface scattering effect on the thermal properties of GaN nanofilms. The contribution of surface scattering to phonon transport is involved in solving a Boltzmann transport equation(BTE). The confined phonon properties of GaN nanofilms are calculated based on the elastic model. The theoretical results show that the surface scattering effect can modify the cross-plane phonon thermal conductivity of GaN nanostructures completely, resulting in the significant change of size effect on the conductivity in GaN nanofilm. Compared with the quantum confinement effect, the surface scattering leads to the order-of-magnitude reduction of the cross-plane thermal conductivity in GaN nanofilm. This work could be helpful for controlling the thermal properties of Ga N nanostructures in nanoelectronic devices through surface engineering.