Sticking Of Atoms Research Articles

A multi-timescale model predicts the spherical-to-cubic morphology crossover of magnetron-sputtered niobium nanoparticles

Sputter-based cluster beam deposition offers a solvent- and effluent-free physical method to synthesize nanoparticles (NPs) with tailored characteristics. Despite the broad capabilities to control the NP architecture via multiple parameters such as magnetron power, gas pressure and flow, type of gas, etc., the technique is still far from being at hand because of insufficient understanding of the processes of NP nucleation, growth, and transport. Here, we synthesize Nb NPs of spherical (diameter 32 nm) and cubic (side length 19 nm) shape, depending on pressure, and perform a detailed structural characterization. A known analytical model of the NP growth and thermalization was modified to account for ion bombardment and non-ideal atomic sticking in the plasma. The model was enforced with ad hoc multi-timescale molecular dynamic simulations to unveil the peculiarities of the growth of Nb NPs and explain the difference in their morphology.

Revealing the mechanisms for inactive rolling and wear behaviour on chemical mechanical planarization

The paper examines the influences of abrasive conditions, silica thickness, and motion parameters on the polishing process of a silicon substrate covered by a thin silica film. The silica film minimizes the direct collision of the abrasive on the silicon substrate. During the polishing process, the adhesion phenomenon plays the main role in atomic removal. Notably, the abrasive sliding motion can generate an inactive rolling motion. The inactive rolling motion can be used to explain the stick-slip motion by showing the system of periodic high-strain marks. In rolling, the deformation behaviors of the workpiece rely on the correspondence between the rolling and the moving speeds. In the fixed diamond abrasive (FDA) cases, the atomic sticking rate is lower but the workpiece deformation is higher than the loose diamond abrasive (LDA) cases. Besides, the FDA cannot create the inactive rolling motion and the stick-slip motion as the LDA. In general, the paper points out the existence of the inactive rolling motion and its role in the atomic removal process through the stick-slip motion phenomenon at the nanoscale, the advantage of this type of motion in a conventional chemical mechanical polishing (CMP) process, and the padding role of the silica layer.

Angular dependence of nanofriction of mono- and few-layer MoSe2

The frictional properties of two-dimensional (2D) materials are strongly dependent on the crystallographic orientation and number of layers. Although friction anisotropy caused by crystallographic orientations has been reported for various 2D materials, the flexural deformations and different defects complicate the insight into the mechanism of the in-plane friction anisotropy of these materials. Here, the anisotropic friction behavior between an atomic force microscopy tip and monolayer (ML) and few-layer (FL) MoSe2 flakes grown by CVD was performed by nanofriction measurements at different crystallographic directions, applied loads (FA), and tip scanning velocities. Our results reveal that the angular dependence of the friction forces is highly anisotropic for both types of MoSe2 flakes, and the anisotropy decreases with FA. Importantly, the anisotropies demonstrate opposite angular dependence for ML and FL MoSe2 flakes. As confirmed by the friction scanning velocity dependences, this difference seems to originate from different friction mechanisms for ML and FL MoSe2 flakes. The friction of FL flakes was predominantly influenced by atomic stick–slip motion. In contrast, ML MoSe2 is characterized by a coexistence of deformation-induced and atomic stick–slip motion. The experimental results presented here extend the understanding of the tribological properties of dry lubricants operating at the nanoscale.

Effects of He3 impurities on the mass decoupling of He4 films

We carried out quartz crystal microbalance experiments of a 5 MHz AT-cut crystal for superfluid 4He films on Grafoil (exfoliated graphite) with a small amount of 3He up to 0.40 atoms/nm2. We found that the mass decoupling from oscillating substrate is considerable sensitive even in a small amount of 3He doping. In a 4He film of 29.3 atoms/nm2, we observed a small drop in resonance frequency at T3 of ~0.4 K for a small amplitude, which is attributed to sticking of 3He atoms on the 4He solid atomic layer. For a large amplitude, the 4He solid layer shows a reentrant mass decoupling at TR close to T3. This decoupling can be explained by the suppression of the superfluid counterflow due to the adsorption of 3He atoms on edge dislocations. As the 4He areal density increases, TR shifts to the lower temperature, and vanishes around a 4He film of 39.0 atoms/nm2.

Gas-grain model of carbon fractionation in dense molecular clouds

ABSTRACT Carbon containing molecules in cold molecular clouds show various levels of isotopic fractionation through multiple observations. To understand such effects, we have developed a new gas-grain chemical model with updated 13C fractionation reactions (also including the corresponding reactions for 15 N, 18O, and 34S). For chemical ages typical of dense clouds, our nominal model leads to two 13C reservoirs: CO and the species that derive from CO, mainly s-CO and s-CH3OH, as well as C3 in the gas phase. The nominal model leads to strong enrichment in C3, c-C3H2, and C2H in contradiction with observations. When C3 reacts with oxygen atoms, the global agreement between the various observations and the simulations is rather good showing variable 13C fractionation levels that are specific to each species. Alternatively, hydrogen atom reactions lead to notable relative 13C fractionation effects for the two non-equivalent isotopologues of C2H, c-C3H2, and C2S. As there are several important fractionation reactions, some carbon bearing species are enriched in 13C, particularly CO, depleting atomic 13C in the gas phase. This induces a 13C depletion in CH4 formed on grain surfaces, an effect that is not observed in the CH4 in the Solar system, in particular on Titan. This seems to indicate a transformation of matter between the collapse of the molecular clouds, leading to the formation of the protostellar disc, and the formation of the planets. Or it means that the atomic carbon sticking to the grains reacts with the species already on the grains giving very little CH4.

Thickness-dependent frictional behavior of topological insulator Bi2Se3 nanoplates

Two-dimensional Bi2Se3 TIs were recently found to be the most promising room-temperature topological insulators for its relatively large bulk gap, but its surface frictional response is little investigated. Here, we prepared single-crystalline Bi2Se3 nanoplates with a lateral dimension up to ~ 1 μm and a thickness of less than 200 nm via a simple polyol method, and the molecular structure and morphology were characterized in detail using different methods. The micro-frictional behavior of Bi2Se3 nanoplates with different thickness is inventively investigated with AFM technique. The atomic stick–slip friction stemming from periodic crystal lattice, and the larger friction force of thinner nanoplates is attributed to the larger adhesion force and enhanced energy dissipation. This work has, for the first time, built the link of the behavior of topological protected surface and mechanical friction behavior of Bi2Se3.

Interpreting the dynamic equilibrium during evaporation in a cesium environment.

The cesium ovens for the prototype source of the ITER neutral beam injectors are currently tested in the CAesium Test Stand (CATS) facility, with a background pressure of 10-6 mbar. Different diagnostics are here installed: two Langmuir-Taylor detectors allow us to determine the Cs vapour evaporation rate from the oven and the Cs density at different positions in the vacuum chamber; and laser absorption spectroscopy is used to measure the density integrated over a line of sight and a quartz crystal microbalance to detect the cesium mass deposited in time over a surface. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS with the first oven tested in order to gain information about the Cs sticking coefficient at the walls. The model hence includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces, and provides the Cs atom density at any location in the volume. By this model, we simulate the Cs evaporation and the equilibrium density, comparing the modeled results with the experimental data. As a result, a sticking coefficient of 2% is obtained.

Tagging fullerene ions with helium in a cryogenic quadrupole trap

Helium tagging of charged C60q+ ions (q = 1–3) has been used for measuring electronic and infrared gas phase spectra, allowing astronomers to improve the identification of fullerenes in space. Here, we present a detailed study of the attachment of He to cold mass selected fullerene ions. Experiments were performed in the temperature variable radio frequency (rf) ion trap ISORI at high He densities and a few K. For all three charge states, the ternary rate coefficients for forming He-C60q+ are below 10−31 cm6s–1 and all three show temperature dependences, proportional to exp(-T/T0) with T0.< 1.5 K. While most of our knowledge on the interaction of C60+ with He atoms comes from ab initio calculations, from gas phase ion mobility experiments, and from fullerene ions ejected from He-droplets, the sequential addition of He to C60q+ in ter- or maybe bimolecular collisions provides complementary information. Our experimental results are tentatively explained with difficulties to form and stabilize the short-lived He-fullerene complexes while relaxation of hot fullerenes in collisions with cold He seems to be rather efficient. Severe restrictions are due to the fact that thermalized C60q+ ions behave like fast rotating rigid spheres. Despite the low efficiency in attaching the first He, measurements reveal a fast subsequent growth as soon as a few He atoms stick to the fullerene. In the case of triply charged ions, almost all primaries can be quickly converted into the particular structure He32-C603+. IR excitation of this thermalized complex with one 1329 cm-1 photon provides interesting insight into the decay kinetics.

The roughening kinetics of hydrogenated graphene

The roughness is a common property of all growing surfaces – however, the way the roughness of a growing surface changes with time and space is uniquely related to the underlying growth process, i.e. to how the atoms stick to the surface during the first stage of nucleation. This concept allows getting insights on the nucleation process of a growing surface by measuring two scaling exponents, α and β, known as roughness and growth exponents, respectively. In this work, we studied hydrogenation of graphene using the roughening kinetics. The coverage of graphene will depend on how the H ions stick on the surface, giving rise to a unique roughness evolution in time and space. We measured a roughness exponent of ~0.5 (derived from a Fourier index of ~3), and a growth exponent of ~0.3. The values of the growth and roughness exponents are close to those reported for clustered carbon, suggesting a roughening mechanism by clustering, in good agreement with the theory. We also compared our coverage data with a different model, used to describe the dynamics of graphene coverage, during chemical vapour deposition. Our data are in agreement with a nucleation-dominated growth, further confirming that hydrogenation is happening by clustering.

Microscopic modeling of gas-surface scattering. I. A combined molecular dynamics-rate equation approach

A combination of first principle molecular dynamics (MD) simulations with a rate equation model (MD-RE approach) is presented to study the trapping and the scattering of rare gas atoms from metal surfaces. The temporal evolution of the atom fractions that are either adsorbed or scattered into the continuum is investigated in detail. We demonstrate that for this description one has to consider trapped, quasi-trapped and scattering states, and present an energetic definition of these states. The rate equations contain the transition probabilities between the states. We demonstrate how these rate equations can be derived from kinetic theory. Moreover, we present a rigorous way to determine the transition probabilities from a microscopic analysis of the particle trajectories generated by MD simulations. Once the system reaches quasi-equilibrium, the rates converge to stationary values, and the subsequent thermal adsorption/desorption dynamics is completely described by the rate equations without the need to perform further time-consuming MD simulations. As a proof of concept of our approach, MD simulations for argon atoms interacting with a platinum (111) surface are presented. A detailed deterministic trajectory analysis is performed, and the transition rates are constructed. The dependence of the rates on the incidence conditions and the lattice temperature is analyzed. Based on this example, we analyze the time scale of the gas-surface system to approach the quasi-stationary state. The MD-RE model has great relevance for the plasma-surface modeling as it makes an extension of accurate simulations to long, experimentally relevant time scales possible. Its application to the computation of atomic sticking probabilities is given in the second part (paper II).

Line Scan Reconstruction: A Viable Approach for Tracking Atomic Stick–Slip Events and True Tip Position in Atomic Force Microscopy

Since its first report on graphite, atomic stick–slip friction has been widely observed on various crystalline surfaces in atomic force microscopy (AFM) experiments. Theoretically, one can extract the key intrinsic interface parameters, e.g., functional shape of the atomic interaction potential energy, thermal hopping frequency and effective damping coefficient, from statistics of the atomic stick–slip data. However, this approach is seriously impeded by the lack of a reliable method to extract the physically meaningful statistical data. In this work, we present an easy and robust approach to reconstruct the atomic stick–slip process such that individual slip events with arbitrary slip distance can be automatically identified, while the slip forces and true tip position can be obtained simultaneously. By implementing the new scheme for both one-dimensional and two-dimensional simulated data and real AFM experiments, we clearly demonstrate that this new method can reliably resist noise interference while accurately preserving the essential thermodynamic information. The new technique enables the physical process of atomic friction to be unveiled with far more details and provides a new paradigm for examining the statistics of atomic processes in sliding interfaces.

Energy corrugation in atomic-scale friction on graphite revisited by molecular dynamics simulations

Although atomic stick–slip friction has been extensively studied since its first demonstration on graphite, the physical understanding of this dissipation-dominated phenomenon is still very limited. In this work, we perform molecular dynamics (MD) simulations to study the frictional behavior of a diamond tip sliding over a graphite surface. In contrast to the common wisdom, our MD results suggest that the energy barrier associated lateral sliding (known as energy corrugation) comes not only from interaction between the tip and the top layer of graphite but also from interactions among the deformed atomic layers of graphite. Due to the competition of these two subentries, friction on graphite can be tuned by controlling the relative adhesion of different interfaces. For relatively low tip-graphite adhesion, friction behaves normally and increases with increasing normal load. However, for relatively high tip-graphite adhesion, friction increases unusually with decreasing normal load leading to an effectively negative coefficient of friction, which is consistent with the recent experimental observations on chemically modified graphite. Our results provide a new insight into the physical origins of energy corrugation in atomic scale friction.

Electron-hole pair excitation determines the mechanism of hydrogen atom adsorption.

How much translational energy atoms and molecules lose in collisions at surfaces determines whether they adsorb or scatter. The fact that hydrogen (H) atoms stick to metal surfaces poses a basic question. Momentum and energy conservation demands that the light H atom cannot efficiently transfer its energy to the heavier atoms of the solid in a binary collision. How then do H atoms efficiently stick to metal surfaces? We show through experiments that H-atom collisions at an insulating surface (an adsorbed xenon layer on a gold single-crystal surface) are indeed nearly elastic, following the predictions of energy and momentum conservation. In contrast, H-atom collisions with the bare gold surface exhibit a large loss of translational energy that can be reproduced by an atomic-level simulation describing electron-hole pair excitation.

Friction force microscopy at a regularly stepped Au(665) electrode: Anisotropy effects

Abstract Using friction force microscopy, friction was determined for the AFM-tip scanning parallel and vertically to the monoatomic steps of Au(665) electrode for different coverages of Cu in sulfuric acid. When the tip was scanning parallel to the steps, the results were similar to those obtained before for a Au(111) surface: a higher coverage of Cu leads to an increased friction. However, differently from Au(111), no transitions in the friction coefficient were observed with increasing load. Atomic stick slip was observed both for the Au surface and the 3 × 3 honeycomb Cu adlayer with a Cu coverage of 2/3. When the tip was scanning perpendicular to the steps, friction did not depend much on coverage; astonishingly, atomic stick slip was also observed.

Ionization of short-lived isotopes in a hot cavity – Numerical simulations

An extended Monte Carlo method based numerical model of the hot cavity ion source is presented. Not only the radioactive decay and delays due to the sticking of atoms to hot walls are taken into account, but also (i) delays due to the diffusion of nuclides, (ii) contributions from electron impact ionization, (iii) calculations of ion release curves and (iv) the case of hemispherical cavities are implemented. The code enables calculations of ionization efficiency for a broader range of parameters including ionizer material, size, geometry and temperature; extraction voltage; timescales characterizing radioactive decay, particle sticking, out-diffusion; and electron impact ionization cross-sections. The dependences of ion source efficiency on decay half-life, sticking time and diffusion timescale are shown and discussed. Two different schemes of radioactive nuclide generation are introduced and compared. Influence of ionizer length and extraction voltage is extensively studied. The importance of the electron ionization for short-lived isotope ion production is for the first time demonstrated. Two new analytical models of ionization in the case short-lived nuclides are introduced and compared to simulation results. Good agreement of experimental and simulated data (efficiency vs. temperature) is shown. New features enabling simulations of ion release curves are demonstrated.

Molecular Basis of Crystal Morphology-Dependent Adhesion Behavior of Mefenamic Acid During Tableting

The molecular basis of crystal surface adhesion leading to sticking was investigated by exploring the correlation of crystal adhesion to oxidized iron coated atomic force microscope (AFM) tips and bulk powder sticking behavior during tableting of two morphologically different crystals of a model drug, mefenamic acid (MA), to differences in their surface functional group orientation and energy. MA was recrystallized into two morphologies (plates and needles) of the same crystalline form. Crystal adhesion to oxidized iron coated AFM tips and bulk powder sticking to tablet punches was assessed using a direct compression formulation. Surface functional group orientation and energies on crystal faces were modeled using Accelrys Material Studio software. Needle-shaped morphology showed higher sticking tendency than plates despite similar particle size. This correlated with higher crystal surface adhesion of needle-shaped morphology to oxidized iron coated AFM probe tips, and greater surface energy and exposure of polar functional groups. Higher surface exposure of polar functional groups correlates with higher tendency to stick to metal surfaces and AFM tips, indicating involvement of specific polar interactions in the adhesion behavior. In addition, an AFM method is identified to prospectively assess the risk of sticking during the early stages of drug development.

Friction force microscopy: a simple technique for identifying graphene on rough substrates and mapping the orientation of graphene grains on copper

At a single atom thick, it is challenging to distinguish graphene from its substrate using conventional techniques. In this paper we show that friction force microscopy (FFM) is a simple and quick technique for identifying graphene on a range of samples, from growth substrates to rough insulators. We show that FFM is particularly effective for characterizing graphene grown on copper where it can correlate the graphene growth to the three-dimensional surface topography. Atomic lattice stick–slip friction is readily resolved and enables the crystallographic orientation of the graphene to be mapped nondestructively, reproducibly and at high resolution. We expect FFM to be similarly effective for studying graphene growth on other metal/locally crystalline substrates, including SiC, and for studying growth of other two-dimensional materials such as molybdenum disulfide and hexagonal boron nitride.

Nanocrystal Growth Observed

Two research groups have independently monitored the real-time dynamics of nano­crystal growth via electron microscopy, observing particles form and then attach to one another in solution (Science, DOI: 10.1126/science.1219185 and 10.1126/science.1219643). Such particle-level knowledge of nanocrystal growth should help scientists better control the synthesis and properties of nanomaterials. Scientists have been kicking around two theories of how nanocrystals grow, says Haimei Zheng, leader of one of the groups to make the measurements. According to the classic theory of crystal growth, nano­particles form when individual atoms stick to high-energy crystal faces, she says. A second theory posits that small nanoparticles initially form, approach one another, align their crystal faces, and finally attach to create larger nanoparticles. “It’s been conjectured for some time that some nanostructures come about during synthesis through the process of oriented attachment,” which is the second theory, says James J...

Oxygen Adsorption on β-Quartz Model Surfaces: Some Insights from Density Functional Theory Calculations and Semiclassical Time-Dependent Dynamics

The O/β-quartz interaction is described by combining our time-dependent semiclassical approach to atom-molecule/surface scattering with first-principles electronic structure calculations at the DFT (PBE0) level of accuracy. In particular, the O, O(2) interaction potentials with an on-top Si atom and its nearest O atom both localized over three different silica clusters have been calculated as a function of the oxygen-silica approaching distance. The calculated DFT potential energy surface has been used in semiclassical trajectory calculations to investigate the sticking and inelastic reflection of oxygen atoms from a model β-quartz surface. The collisional mechanism, including the role played by the phonon dynamics, is brought to light and accurate sticking probabilities are calculated at five impact energies in the range [0.05-0.8] eV and T(S) = 1000 K. The different catalytic response of β-quartz and β-cristobobalite to the atomic oxygen flux is also discussed and highlighted.

When sticking influences H2formation

Aims. Interstellar dust grains, because of their catalytic properties, are crucial to the formation of H2, the most abundant molecule in the Universe. The formation of molecular hydrogen strongly depends on the ability of H atoms to stick on dust grains. In this study we determine the sticking coefficient of H atoms chemisorbed on graphitic surfaces, and estimate its impact on the forma- tion of H2. Methods. The sticking probability of H atoms chemisorbed onto graphitic surfaces is obtained using a mixed classical-quantum dynamics method. In this, the H atom is treated quantum- mechanically and the vibrational modes of the surface are treated classically. The implications of sticking for the formation of H2 are addressed by using Kinetic Monte Carlo simulations that follow how atoms stick, move and associate with each other on dust surfaces of different temper- ature. Results. In our model, molecular hydrogen forms very efficiently for dust temperatures lower than 15 K through the involvement of physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures lower than 2000 K, H2 formation differs strongly if the H atoms com- ing from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by DFT calculations to become chemisorbed. The product of sticking times efficiency can be increased by many orders of magnitude when realistic barriers are considered. If graphite phonons are taken into account in the dynamics calculations, then H atoms stick better on the surface at high energies, but the overall H2 formation efficiency is only slightly affected. Our results suggest that H2 formation can proceed efficiently in photon dominated regions, X-ray dominated regions, hot cores and in the early Universe when the first dust is available.