Interaction of hydrogen molecules on Ni-doped single-walled carbon nanotube

Adsorption of hydrogen molecules on an Ni-doped (8,0) single-walled carbon nanotube (SWNT) is investigated by using first-principles density functional calculations. The result shows that a single Ni atom adsorbed on the bridge site of the tube could cannot dissociate the H₂, however it can chemisorb three H₂ at most, with the average binding energy per H₂ suitable for the hydrogen storage at the room temperature. More H₂ would physisorb around an Ni atom weakly. As for the SWNT with an Ni dimer adsorbed, we find that when the H₂ approaches the Ni--Ni bond, it dissociates without overcoming any barrier and makes bonds with Ni atom.     

Defect-controlled transport properties of metallic atoms along carbon nanotube surfaces

The diffusion mechanism of indium atoms along multiwalled carbon nanotubes is studied by means of photoemission spectromicroscopy and density functional theory calculations. The unusually high activation temperature for diffusion (approximately 700 K), the complex C 1s and In 3d5/2 spectra, and the calculated adsorption energies and diffusion barriers suggest that the indium transport is controlled by the concentration of defects in the C network and proceeds via hopping of indium adatoms between C vacancies.     

Thermodynamics and kinetics of adsorption of atoms and molecules by carbon nanotubes

Thermodynamics and kinetics of adsorption of atoms and molecules by carbon nanotubes are investigated. Expressions are derived that can be used to perform calculations taking into account various external factors and interactions and, therefore, analyze both physical and chemical adsorption processes and determine its thermodynamic parameters. A mathematical model is developed that makes it possible to find parameters of desorption kinetics separately for molecules adsorbed at different locations and reduce possible systematic errors in analysis of experimental results. Theoretical models are validated by analyzing physical and chemical adsorption of hydrogen and physical adsorption of oxygen.     

Quantum scattering of fast atoms and molecules on surfaces

We present evidence for the diffraction of light keV atoms and molecules grazingly scattered on LiF(001) and NaCl(001) surfaces. At such energies, the de Broglie wavelength is 2 orders of magnitude smaller that the mean thermal atomic displacement in the crystal. Thus, no coherent scattering was expected and interaction of keV atoms with surfaces is routinely treated with classical mechanics. We show here that well-defined diffraction patterns can be observed indicating that, for grazing scattering, the pertinent wavelength is that associated with the slow motion perpendicular to the surface. The experimental data are well reproduced by an ab initio calculation.     

Pattern formation on carbon nanotube surfaces

Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250 degrees C. The effect of solvents on fluorine binding and surface diffusion is explored.     

Nonequilibrium desorption of atoms and molecules from surfaces

A model, based on concepts from the theory of atom-surface accommodation coefficients, for nonequilibrium desorption of atoms and molecules from surfaces is proposed. We contend that the nonequilibrium effects should be expected to be general properties of not only polyatomic molecule-surface systems, but of all desorbing systems. The model predicts large deviations from the equilibrium dependences, on desorption angle, of (a) the desorbing atomic flux, (b) the average energy of desorbing atoms and (c) the desorbing atomic speed ratio. The deviations are larger when the accommodation coefficient is smaller.     

Effects of electromigration on copper atoms in carbon nanotube channels

The effects of electromigration on copper in carbon nanotube (CNT) channels are investigated using molecular dynamics simulations. The study shows that the potential energy of copper and the resistive forces on copper are dependent on the shape of the CNT junction, and the increase in bias voltages magnifies these effects. Bias voltages affect the density of copper in the downstream CNT. The velocity of copper in the downstream CNT is relatively lower than that in the upstream CNT when the biased voltage is high.     

Interaction of antihydrogen with ordinary atoms and solid surfaces

The characteristic features of cold atom-antiatom collisions and antiatom-surface interactions are discussed and illustrated by the results for hydrogen-antihydrogen scattering and for quantum reflection of ultracold antihydrogen from a metallic surface. We discuss in some detail the case of spin-exchange in ultracold $\bar{H}-H$ collisions, exposing the interplay of Coulombic, strong and dispersive forces, and demonstrating the sensitivity of the spin-exchange cross sections to hypothetical violations of Charge-Parity-Time (CPT) symmetry.     

Quasi-elastic lineshapes for atoms and simple molecules undergoing jump rotation on surfaces

We apply an analytic model for discrete jump rotation of a species moving around a circle, to quasi-elastic scattering from single crystal surfaces. Results for several molecule-like geometries are presented and the subsequent form of the intermediate scattering function is identified. These functions give clear signatures, enabling different forms of jump rotation to be distinguished. A simple data reduction is discussed and we show how the true ISF will appear within this simplification.     

Structurization of hydrogen molecules during carbon nanotube filling

Certain forms of ordering of hydrogen molecules have been discovered during their alternate inflow into a carbon nanotube and outflow from it, and also during their further withdrawal, confinement, and accumulation in the nanotube with a lid in the form of half a fullerene molecule at one of its ends. It is shown that hydrogen forms a peculiar chain of molecules that extends along the nanotube axis. In this case, the molecules also turn out to be oriented along the axis. Estimates of the nanotube filling and the increase in the nanotube lengths and diameters are given.     

Threshold resonances in the diffraction of atoms and molecules by surfaces

We examine the problem of threshold resonances in the diffraction of atomic and molecular beams by solid surfaces. We show that the divergence in the slope of the intensity as a function of incident polar angle appears generally in all diffraction peaks. However, for interactions with realistically soft repulsive parts we find that the resonances are weakened to the point where they will be difficult to observe experimentally.     

Adsorption-desorption kinetics of carbon monoxide on rhodium polycrystalline surfaces

Adsorption isotherms, the absolute adsorption rate, and the absolute desorption rate of carbon monoxide on rhodium polycrystalline surfaces were measured in the same manner as in our previous work on palladium. The absolute adsorption rate was proportional to pressure, independent of temperature and was a slightly non-linear function of the coverage of CO. The absolute desorption rate was proportional to the coverage and was an increasing function of temperature and pressure. These results are similar to those of palladium.     

Reactions of excited atoms and molecules with atoms and molecules

Penning electron distributions arising from the ionization of Na and K by He (1s 2s ^1,3 S)-metastables in thermal collisions, as well as the absolute cross section for Penning ionization of Na by He (2³ S) and relative cross sections for ionization of Na and K by He(2¹ S) and He(2³ S) are measured. It is shown that under fairly general conditions the well depth ε* of the interaction potential between the metastable and the target particle can be obtained directly from the measured electron distributions. ε*-values are reported for the moleules He(1s 2s ^1,3 S)-Na(² S), K(² S) (^2,2Ω), and for He(1s 2s ^1,3 S)-Hg(¹ S)(^1,3Ω). These latter values are obtained from previously published measurements and are to be considered preliminary. Further, additional evidence is given, that Penning ionization with metastables is an electron exchange process.     

Reactions of excited atoms and molecules with atoms and molecules

From a He-beam excited by electron impact we eliminated the He(2¹ S) component to better than 0.5% by irradiating light from a He discharge. The quenching process ishv(2¹ P→2¹ S)+He(2¹ S)→He(2¹ P)→He(1¹ S +hv) (2¹ P 1)¹ S. By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He (2¹ S) and He(2³ S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He-beam, 320 and 90 °K. It is found that the cross section ratio of associative — to Penning ionization increases considerably as the temperature is decreased for both, He(2¹ S) and He(2³ S), the effect being much more pronounced for He(2¹ S). The results of this work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.     

Single-file transport of water molecules through a carbon nanotube

Recent molecular dynamics simulations of water transport through the interior channel of a carbon nanotube in contact with an aqueous reservoir showed that conduction occurred in bursts with collective water motion. A continuous-time random-walk model is used to describe concerted transport through channels densely filled with molecules in a single-file arrangement, as also found in zeolites, as well as ion channels and aquaporins in biological membranes. Theoretical predictions for different collective properties of the single-file transport agree with the simulation results.     

Reactions of excited atoms and molecules with atoms and molecules

Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH⁺, RH ₂ ⁺ , and H ₂ ⁺ by collisions of excited rare gas atoms R^* with H₂. With HD as the target molecule the isotope effect for the production of RD⁺ and RH⁺ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.     

Detection and quantized conductance of neutral atoms near a charged carbon nanotube

We describe a novel single atom detector that uses the high electric field surrounding a charged single-walled carbon nanotube to attract and subsequently field-ionize neutral atoms. A theoretical study of the field-ionization tunneling rates for atomic trajectories in the attractive potential near a nanowire shows that a broadly applicable, high spatial resolution, low-power, neutral-atom detector with nearly 100% efficiency is realizable with present-day technology. Calculations also show that the system can provide the first opportunity to study quantized conductance phenomena when detecting cold neutral atoms with mean velocities less than 15 m/s.     

Si表面间水平碳纳米管束的分子动力学模拟研究

本文采用分子动力学模拟方法研究了Si表面间单壁水平碳纳米管束SWCNT (10,10)的变形和摩擦特性.系统在弛豫平衡后,首先对碳纳米管束施加压力至碳纳米管或Si表面结构破坏.之后在无压力和高压力两种情况下使上表面沿水平方向做剪切运动以研究碳纳米管束的摩擦特性.结果表明,由于碳纳米管的柔韧性,碳纳米管束在加载过程中出现明显变形,但直至3.8 GPa高压下并无结构破坏.系统无压力时SWCNT (10,10)在原地轻微随机滚动,压力为3.8 GPa时,碳纳米管束出现了整体的轻微滑动,同时伴随无规律的轻微滚动, 关键词： 碳纳米管束 摩擦 分子动力学模拟     

Distribution and thermophysical properties of n-heptacosane molecules confined in carbon nanotube

Molecular dynamics (MD) simulations were performed to provide an insight about the molecule distribution and thermophysical properties of n-heptacosane confined in the (25, 25) single-walled carbon nanotube (CNT). The results show that an orderly distribution of n-heptacosane molecules along the CNT inner wall is clearly observed. Meanwhile, n-heptacosane confined in CNT exhibits an increased self-diffusion coefficient, a decreased melting point and an enhanced thermal conductivity, compared to the bulk. The simulations reveal that MD is an effective and convenient method to understand the variation characteristics of alkane-based phase change materials confined in CNT on molecular and atomic scale.