碳纳米管内金纳米线的结构与热稳定性

采用分子动力学模拟方法, 研究了填充在(8,8)单壁碳纳米管内的Au纳米线的结构和热稳定性. 研究表明, 经高温退火至室温, Au在碳纳米管内能生成多样而稳定的结构上明显区别于自由状态Au纳米线的壳层螺旋结构Au纳米线, 其螺旋结构会随着温度的变化而转变. 束缚在碳纳米管内的壳层螺旋结构Au纳米线有非常好的热稳定性, 稳定温度高于块体Au晶体的熔化温度. 关键词： 纳米线 碳纳米管 热稳定性 分子动力学模拟     

AuCu249合金团簇热稳定性的原子尺度计算研究

采用基于嵌入原子法的NVT正则系综分子动力学方法,在原子尺度上计算了包含249个原子的金属间化合物AuCu₂₄₉合金团簇由固态转变为熔体的结构演化过程. 根据对分布函数、原子密度分布函数和主要原子键对数目随温度的变化,发现在温度从低温升到高温的过程中,合金团簇内伴随着原子的连续位置交换,团簇呈现由外及里的分阶段结构转变. 同时还发现在团簇内原子堆积结构转变过程中,Au原子出现由团簇内层向外层运动的趋势,而Cu原子则有由外层向内层运动的趋势. 关键词： 合金团簇 分子动力学 计算机模拟 相变     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综（NVT）分子动力学,模拟了在10 K条件下,Au₉₅₉团簇负载于MgO（100）表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大. 关键词： 团簇 分子动力学 计算机模拟 表面     

熔融Cu55团簇在铜块体中凝固过程的分子动力学模拟

应用基于嵌入原子势函数的分子动力学方法,模拟了嵌入在具有面心立方结构同质块体中的熔融Cu₅₅团簇在不同急冷温度下微观结构的演变情况.通过计算熔融Cu₅₅团簇的均方位移和原子平均能量随时间步的变化,并应用键对分析技术,分析了急冷温度对熔融Cu₅₅团簇结构变化的影响.研究结果表明,由于受到块体结构的影响,在所研究的急冷温度范围内,熔融Cu₅₅团簇在凝固过程中形成了以面心立方结构为主的微观结构.结晶过程是原子不断交换其位置的过程,团簇原子位置的重排敏感于温度的变化.随着急冷温度的升高,原子的扩散范围增大.在100,300和500 K三个较低的温度下有利于形成稳定的面心立方结构,但当急冷到100 K时,团簇中的原子在没有找到其最佳位置之前就已经完成晶化.在急冷到500 K时,团簇中的原子在块体中扩散充分,与块体中的原子形成理想的面心立方结构.在700,900和1100 K三个较高的温度上,局域结构表现为随时间步波动性变化. 关键词： 团簇 分子动力学 计算机模拟 凝固     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综(NVT)分子动力学,模拟了在10 K条件下,Au959团簇负载于MgO(100)表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大.     

基于graphene条带的硅纳米结构

采用分子动力学模拟方法研究了graphene条带上生长硅纳米结构的过程,分析了不同温度下硅原子在graphene条带边沿生成的新型纳米结构.研究表明,随机分布的硅原子吸附到锯齿型graphene条带边沿在不同的温度T下可生成不同类型的硅纳米结构：300K≤T<2000K时形成无规则的团簇,2000K≤T≤2800K时形成单原子链结构,2800K<T<3900K时形成含缺陷的硅链结构,T≥3900K时硅原子逐渐替代条带边沿的碳原子直至graphene条带破坏.而硅原子吸附到扶手椅型graphene条带边沿在300K≤T<3000 K内仅能形成非链状的不定型的硅纳米结构. 关键词： graphene 硅 纳米结构 分子动力学模拟     

低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu₁₃团簇以不同接触条件与Cu（001）表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大. 关键词： 分子动力学 团簇 表面 计算机模拟     

熔融Cu55团簇在铜块体中凝固过程的分子动力学模拟

应用基于嵌入原子势函数的分子动力学方法,模拟了嵌入在具有面心立方结构同质块体中的熔融Cu55团簇在不同急冷温度下微观结构的演变情况.通过计算熔融Cu55团簇的均方位移和原子平均能量随时间步的变化,并应用键对分析技术,分析了急冷温度对熔融Cu55团簇结构变化的影响.研究结果表明,由于受到块体结构的影响,在所研究的急冷温度范围内,熔融Cu55团簇在凝固过程中形成了以面心立方结构为主的微观结构.结晶过程是原子不断交换其位置的过程,团簇原子位置的重排敏感于温度的变化.随着急冷温度的升高,原子的扩散范围增大.在100,300和500 K三个较低的温度下有利于形成稳定的面心立方结构,但当急冷到100 K时,团簇中的原子在没有找到其最佳位置之前就已经完成晶化.在急冷到500 K时,团簇中的原子在块体中扩散充分,与块体中的原子形成理想的面心立方结构.在700,900和1100 K三个较高的温度上,局域结构表现为随时间步波动性变化.     

熔融Cu55团簇在Cu(010)表面上凝固过程的分子动力学模拟

采用基于嵌入原子方法的分子动力学,模拟了熔融Cu₅₅团簇在Cu衬底（010）表面上以两个不同降温速率降温过程中结构的变化.模拟结果表明,降温速率对团簇结构的变化有很大影响.较快的降温速率使得降温过程中团簇原子具有较低的能量;较慢的降温速率有助于高温时位于衬底内的原子向衬底表面扩散,排列形成面心立方结构. 关键词： 团簇 凝固 分子动力学 表面     

冷却过程中Pd_(923)团簇的结构转变研究

本文利用分子动力学模拟方法,研究了钯团簇在不同冷却条件下形成晶体及非晶的过程.利用平均原子体积、双体分布函数、键对分析和键序参数方法研究了微观局域结构随温度的变化关系.研究发现:在50 K/ps冷却过程中,液态Pd923团簇在1000 K发生玻璃化转变,在100 K下形成非晶结构;而在0.1K/ps冷却过程中,液态Pd923团簇发生结晶,并最终形成六角密排(hcp)晶体结构.     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

Welding of gold nanowires with different joining procedures

Welding of single-crystal metallic nanowires is likely to have an important role in the bottom-up fabrication of nanodevices. The welding effects of free ends of two single-crystal gold nanowires (Au–Au) were demonstrated by Monte Carlo simulations in this paper. The quantum corrected Sutton–Chen type many-body potential was used to model the metal–metal interactions. Metallic nanowires were first placed closely with head-to-head, head-to-side and side-to-side joining procedures. Two ends were successfully welded together to form a continuous nanowire by annealing at different temperatures. The welding effects of the different joining procedure with different temperatures were compared. Structures of the welded specimens were characterized by the common neighbor analysis technique. Variable atomic mobility, freedom and contacting angles may result in different bonding strengths in the three different configurations. The results showed that the joint structure welded at low temperature were similar to the cold welding without fusion meanwhile the molten phase was presented in the joint when processing at the high temperatures.     

Vertical assembly of carbon nanotubes for via interconnects

The via interconnects are key components in ultra-large scale integrated circuits(ULSI).This paper deals with a new method to create single-walled carbon nanotubes(SWNTs) via interconnects using alternating dielectrophoresis(DEP).Carbon nanotubes are vertically assembled in the microscale via-holes successfully at room temperature under ambient condition.The electrical evaluation of the SWNT vias reveals that our DEP assembly technique is highly reliable and the success rate of assembly can be as high as 90%.We also propose and test possible approaches to reducing the contact resistance between CNT vias and metal electrodes.     

Investigation of photoplasma that is configured in an electrostatic extractor having high-temperature electrodes

In the present work we report the investigation of photoplasma in a high temperature electrostatic extractor. The extractor can operate at temperatures ~900 K, at electric field ~200 V/cm and in an environment where there is metal vapor and photon flux. It is used concurrently with another vapor generating furnace that is placed below it. It is observed that for operation over long duration (~5 h) barium atoms from the vapor-producing furnace coat the electrodes. They are re-evaporated from the hot electrodes thereby forming a collision-less vapor cell between them. Multi-step, multi-wavelength resonant ionization process using lasers produce photoplasma in the cell. Thermionic emission from barium coated hot electrodes provides the seed current flux for electron-impact ionization of the atoms in the cell. The ionized background plasma provides a steady state current between the electrodes. The transient photoplasma motion is superimposed on it. The photoions decay as the ions are collected. The Bohm current and current due to sheath boundary motion determines their evolution.     

Synthesis and characterization of SWNT-heavy alkali metal intercalation compounds,effect of host SWNTs materials

Singlewall carbon nanotubes (SWNTs) produced by electric-arc and laser ablation methods were characterized by X-ray diffraction before and after the reaction with alkali metals (M=K, Rb, and Cs). Reaction with annealed SWNTs gave MC₈ composition at saturation. The alkali metal lattice showed short range order incommensurate with graphene cylinders of SWNTs. X-ray diffractogram simulations have enabled the study of the influence of SWNTs structure on that of intercalation compounds. Chemically-purified bundles, constituted of open SWNTs, can be intercalated inside and between the tubes forming disordered structures. Annealed or pristine bundles were intercalated only between the tubes leading to short or long range ordered structure depending on host crystallinity and alkali metal (K, Rb or Cs). The expansion of the 2D SWNTs lattice after intercalation is comparable to graphite intercalation compounds. Some 2D arrangements of SWNTs and K atoms are proposed and discussed to reproduce XRD results. ¹³C NMR and ESR studies of annealed doped SWNTs emphasize the fact that the intercalation compounds of SWNTs are metallic.     

Curvature-induced condensation of lithium confined inside single-walled carbon nanotubes: First-principles calculations

We carry out first-principles calculations to explore the potential energy profiles of Li confined inside single-walled carbon nanotubes (SWNTs) and the subsequent condensation processes. We found that Li has high mobility around tube axis with the energy barrier less than 47 meV, whereas the diffusion barrier along radial direction is as higher as 380 meV. This characterizes the condensation of Li atoms when placed randomly into SWNTs, resulting in nanowires with single or multi-shelled morphologies depending on the diameter of SWNTs. The charge transfer from Li nanowires to SWNTs is significant, indicating stronger couplings between them.     

低温下单壁碳纳米管拉曼光谱温度效应研究

在单壁碳纳米管的低温拉曼光谱测量过程中,发现径向呼吸模(RBM)和正切拉伸模(GM)的拉曼频移在低温下的温度效应和在高温时的温度效应存在着很大的区别,在低温下拉曼光谱的频移和温度并不呈线性关系。而且,在温度为210K时,单壁碳纳米管内部的振动结构可能发生了变化。在低温下单壁碳纳米管拉曼峰的强度的变化是不可逆的。     

6H-SiC(0001)表面graphene逐层生长的分子动力学研究

利用经典分子动力学方法和模拟退火技术分析研究了6H-ＳiC（0001）表面graphene的逐层生长过程及其形貌结构特点．研究表明,经过高温蒸发表面硅原子后,6H-ＳiC（0001）表面的碳原子能够通过自组织过程生成稳定的局部单原子层graphene结构．这种过程类似于6H-ＳiC（0001）表面graphene的形成,其生长和结构形貌演化主要取决于退火温度和表面碳原子的覆盖程度． 研究发现,当退火温度高于1400K时,6H-ＳiC（0001）表面碳原子能形成局部的单原子层graphene结构．这一转变温 关键词： graphene 碳化硅 分子动力学     

热处理对MgO基片的表面形貌以及对CeO_2和Tl-2212薄膜生长的影响

研究了MgO基片在高温退火时表面形貌和表面结构的变化,以及它对CeO2缓冲层和Tl-2212超导薄膜生长的影响。原子力显微镜(AFM)研究表明,在流动氧环境中1100℃温度下退火,MgO的表面首先由未退火时的皱褶形貌,演化为光滑表面,随着退火时间的延长,表面形貌最终演化为具有光滑基底的独立生长峰结构。XRD测试表明,通过高温热处理可以大幅度提高MgO基片表面结晶的完整性。在1100℃温度下热处理8小时的MgO基片上可以生长出具有高度c轴取向的CeO2(001)缓冲层。然后在此缓冲层上制备了厚度为500nm的外延Tl-2212超导薄膜,其临界转变温度(Tc)达到108.6K,液氮温度下临界电流密度(Jc)为2.8mA/cm2,微波表面电阻Rs(77K,10GHz)约为360.9μΩ。     

Assembly of single-walled carbon nanotubes on patterns of Au nanoparticles

We report on the assembly of single-walled carbon nanotubes (SWNTs) and gold nanoparticles (NPs) hybrid structure without any surface modification of SWNTs on patterns of Au nanoparticles (NPs). Microscale Au NP patterns were created on composite self-assembled monolayer (SAM) templates of octadecanethiol (ODT) and octanedithiol (OD) through self-assembly of Au NPs via the thiol-Au chemical bond onto the OD region. On such templates, we observed extensive adhesion and strong affinity of SWNTs on the Au NPs and no SWNT on ODT. We also examined systematically the adhesion of SWNTs on ODT with varying coverage of vapour-deposited Au. We observed little SWNT attachment even when there are high-density of Au clusters on the ODT SAM. Extensive adhesion of SWNTs is observed only when the coverage of ODT by Au is almost complete. Dynamic contact angle measurements of dichlorobenzene on the ODT/Au substrates revealed a direct correlation between the surface wettability and the SWNT assembly on a molecular template.