Molecular dynamics simulation of microcrystalline Si deposition processes by silane plasmas

Molecular dynamics (MD) simulations have been performed to examine enhanced surface diffusion of Si adatoms during silane(SiH₄)-based plasma enhanced chemical vapor deposition (PECVD) processes. Such high surface diffusion, if it actually takes place, has been known to account for the growth of microcrystalline silicon(μc-Si) in the PECVD process. Focused in the present study is the motion of a silicon (Si) adatom on a hydrogenated Si surface and the surface diffusion coefficient of Si adatoms on the fully hydrogenated (111) Si surface at 600 K was evaluated from MD simulations. The obtained diffusion coefficient is much larger than those of typical clean Si surfaces known in the literature. The interatomic potential functions for Si-H systems used for the simulations, which we have developed for this study based on ab initio calculations of the interatomic energies, are also presented.     

Ab initio supercell calculations of the (0001) α-Cr2O3 surface with a partially or totally Al-substituted external layer

Ab initio supercell calculations employing the periodic Hartree-Fock formalism are presented of the (0001) α-Cr₂O₃ surface with a partially or totally Al-substituted external layer. In the simulations a fraction of the Cr atoms at the surface of the chromia slab are replaced by Al atoms, and the Al surface coverage is varied between zero (pure chromia) and 100% (Al-terminated chromia). The surface Al atoms are found to relax inwards considerably, with the magnitude of the relaxation decreasing with increasing Al surface coverage. The calculations also reveal that the surface energy of the slab decreases with increasing Al coverage. Finally, the electronic properties at the surface of the Al-substituted (0001) α-Cr₂O₃ slabs are investigated. Here the calculations show that the substitution of Cr by Al gives rise to an increase in the covalency of the AlO bonds compared to slabs of pure alumina. In contrast, the influence of the surface Al atoms on the electrostatic potential in the (0001) plane of metal ions is relatively small. These findings support the utilisation of α-chromia substrates for the templated growth of α-alumina, which is consistent with recent experiments.     

Transport current densities of MgB2 wire, cable and continually transposed conductor

Multi-core MgB₂ wire, cable and continually transposed conductor (CTC) have been assembled from a single-core in situ made Ti/Cu sheathed composite. It was shown that the filament’s current densities J_c of composed wire, cable and CTC are comparable, but the engineering current densities J_e and the window current densities J_w are much different. MgB₂ cable shows apparently lower sensitivity to bending strain than monolithic wire. Instead of the highest J_w for CTC, it offer also high surface to volume ratio, which is important parameter for efficient cooling and thermal stability. The measurement of the resistance to tensile strain has shown the best performance for monolithic wire and the lowest irreversible strain for CTC.     

A review on the stability and surface modification of layered transition-metal oxide cathodes

An ever-increasing market for electric vehicles (EVs), electronic devices and others has brought tremendous attention on the need for high energy density batteries with reliable electrochemical performances. However, even the successfully commercialized lithium (Li)-ion batteries still face significant challenges with respect to cost and safety issues when they are used in EVs. From a cathode material point of view, layered transition-metal (TM) oxides, represented by LiMO₂ (M = Ni, Mn, Co, Al, etc.) and Li-/Mn-rich xLi₂MnO₃·(1–x)LiMO₂, have been considered as promising candidates because of their high theoretical capacity, high operating voltage, and low manufacturing cost. However, layered TM oxides still have not reached their full potential for EV applications due to their intrinsic stability issues during electrochemical processes. To address these problems, a variety of surface modification strategies have been pursued in the literature. Herein, we summarize the recent progresses on the enhanced stability of layered TM oxides cathode materials by different surface modification techniques, analyze the manufacturing process and cost of the surface modification methods, and finally propose future research directions in this area.     

Characterizing individual SnO2 nanobelt field-effect transistors and their intrinsic responses to hydrogen and ambient gases

The intrinsic electrical properties of individual single-crystalline tin dioxide nanobelts, synthesized via catalyst-free physical vapor deposition, were studied and correlated to the surface oxygen deficiency with the presence of various ambient gases, especially hydrogen. Four-terminal field-effect transistor (FET) devices based on individual SnO₂ nanobelts were fabricated with SiO₂/Si as back gate and RuO₂/Au as contacts. Four-probe I–V measurements verify channel-limited transistor characteristics and ensure that the hydrogen gas sensing reflect electrical modification of the nanobelt channel. The demonstrated results of the intrinsic SnO₂ nanobelt based hydrogen sensor operating at room temperature provide useful information on the synthesis of room temperature chemo-resistive gas sensors with good sensitivity and stability. To evaluate the impact of surface gas composition on the electrical properties of SnO₂ nanobelts, their temperature-dependent resistivity (ρ), effective carrier mobility (μ_eff) and effective carrier concentration (n_e) were determined under different oxygen concentrations.     

Surface energy modification of SiOxCyHz film using PECVD by controlling the plasma processes for OMCTS (Si4O4C8H24) precursor

SiO_x films produced from octamethylycyclodisiloxane (Si₄O₄C₈H₂₄, OMCTS) with oxygen carrier gas have a low contact angle. The surface energy of the SiO_x films can be changed by controlling the plasma process. SiO_xC_yH_z films were deposited on polycarbonate substrates by plasma enhanced chemical vapor deposition using OMCTS without oxygen carrier gas. The input power in the radio frequency plasma was changed to optimize the surface energy of the resulting SiO_xC_yH_z film. The plasma diagnostics, surface energy and surface morphology were characterized by optical emission spectrometry, contact angle measurements and atomic force microscopy, respectively. The chemical properties of the coatings were examined by Fourier transform infrared spectroscopy. The surface energy of the SiO_xC_yH_z films produced using a room temperature plasma process could be controlled by employing the appropriate intensity of excited neutrals, ionized atoms, molecules and energy (input rf power and bias), as well as the suitable dissociation of OMCTS.     

Multiresolution atomistic simulations of dynamic fracture in nanostructured ceramics and glasses

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic fracture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO₂) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO₂ is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si₃N₄). Structural correlations in n-Si₃N₄ reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si₃N₄ is found to be six times larger than that of crystalline -Si₃N₄. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si₃N₄ are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si₃N₄ nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K=0.4 MPa m^1/2, we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K=0.5 MPa m^1/2, water molecules decompose to oxidize Si or break Si-Si bonds.     

Surface nanostructures created by cluster-surface impact

This paper presents a review of the production of surface nanostructures from the controlled deposition of size-selected clusters on graphite. At high enough impact energies, the clusters can be implanted into the graphite surface to create open ‘well’ structures. Scanning tunnelling microscopy (STM) measurements, coupled with molecular dynamics (MD) simulations, for Au₇⁺, Ag₇⁺ and Si₇⁺ clusters exhibit scaling relations which reveal that the implantation depth scales linearly with the momentum of the clusters in all cases. At lower impact energies, the clusters can be pinned on the graphite surface when the impact energy exceeds a critical (threshold) value, thus allowing the fabrication of monodispersed cluster arrays which are stable at room temperature (and above). One application of such cluster arrays is to bind protein molecules; atomic force microscopy (AFM) measurements in buffer solution demonstrate dispersed arrays of both chaperonin and horseradish peroxidase molecules on graphite substrates decorated with size-selected Au_N⁺ clusters.     

Interaction diagrams for carbon nanotubes under combined shortening-twisting

A theoretical investigation on the strength and stiffness of carbon nanotubes (CNTs) under combined shortening and twisting strains is presented. CNTs with similar length-to-diameter aspect ratios, L/D, but different atomic structures (zig-zag, armchair and chiral) have been selected. Molecular dynamics (MD) simulations have been performed to study the critical buckling behaviour and the pre-critical and post-critical stiffness of CNTs under combined shortening-twisting conditions. The main results are presented in the form of interaction diagrams between the critical strain and the critical angle of twist per unit of length. An interaction equation is proposed and validated by comparison with the MD results. If shortening is more dominant than twisting, the strain energy at the onset of buckling drops considerably with the increase of the twisting-shortening rate. If twisting is more influential than shortening, the energy at the onset of buckling decreases very slowly with the twisting-shortening rate. We also found an interaction factor of 1.5 for CNTs under combined shortening-twisting, which is much lower than the value 2.0 commonly adopted for circular tubes at macro-scale. We conclude that CNTs are much more sensitive to buckling under shortening-twisting interaction than macro-scale tubes.     

Tensile behavior of glass/epoxy laminates at varying strain rates and temperatures

In the present work tensile tests at different strain rates and temperatures were performed in glass fiber reinforced polymer (GFRP). It is observed that such kind of composite presents an elasto–viscoplastic behavior – the rate dependency only occurs for loading levels above a given elasticity limit. Strain rate strongly affects the ultimate tensile strength (σ_u) and the modulus of elasticity is almost insensitive to it while temperature only influences the modulus. Analytical expressions to predict the modulus of elasticity and (σ_u) as a function of the temperature and strain rate are proposed and compared with experimental data showing a reasonable agreement.     

In situ X-ray studies of metal organic chemical vapor deposition of PbZrxTi1 − xO3

In situ synchrotron X-ray scattering and fluorescence techniques were used to simultaneously observe the evolution of the strain and composition of a growing crystal surface in real time. Control of the X-ray incidence angle allows us to obtain high surface sensitivity. We studied metal organic chemical vapor deposition (MOCVD) of epitaxial PbZr_xTi_1 − xO₃ (PZT) onto SrTiO₃ (001) substrates under various growth conditions. We observe a strong increase in Zr incorporation as strain relaxation occurs, consistent with the effect of compositional strain on the thermodynamic driving force for growth.     

Approaches for evaluating Young's and shear moduli in terms of a single SAW velocity via the SAM technique

In ultrasonic material investigations Young's modulus, E, and shear modulus, G, conventionally expressed in terms of the velocities of longitudinal (V_L,) and transverse (V_T) modes, are usually difficult to determine from a single measurement, in particular for scanning acoustic microscopy. Therefore, using Viktorov formula and physically acceptable approximations, we derive simple expressions for E and G in terms of the velocity, V_i, of just one propagating mode (including Rayleigh mode V_R). It was found that (E, G) = (α_i, β_i)ρV_i². The validity of these expressions is successfully tested, analytically and graphically, for a great number of materials representing a large spectrum of densities, 1000 kg/m³ < ρ < 22 000 kg/m³, and characterized by a wide interval of velocities, 1400 m/s < V_L < 12 000 m/s.     

Elastic–plastic material response of fatigue crack surface profiles due to overload interactions

The deformation caused by single and periodic overloads on the crack surface profile is studied using finite element fatigue crack closure simulations in a material with linear kinematic hardening. Differential surface profiles (difference of crack surface displacements before and after overloads), Δu_y, are found useful in understanding the role and the interaction between overloads. Three parameters, ΔK_OL/ΔK, ΔK and R, are found necessary to characterize deformation response of a single overload on the crack surface profile. The simulation procedure and results are discussed based on experimental and numerical studies reported in literature on overload interactions.The deformation occurred on the crack surface due to an applied single overload (hump) inhibits reversed plastic deformations by acting like a spring. Therefore, a second single overload leads to a larger deformation response even if this second overload is applied outside the overload plastic zone of the first single overload. This second deformation response is found equivalent to the response of a single overload with a higher K_min value.     

Material characterisation and constitutive modelling of a tungsten-sintered alloy for a wide range of strain rates

The behaviour of a tungsten-sintered alloy has been investigated using a combination of tension tests, modified Taylor-impact tests and planar-plate-impact (PPI) tests using the VISAR technique. A logarithmic yield stress–strain rate dependency as it is predicted by the original Johnson–Cook (JC) strength model covering a strain rate range of 10 orders of magnitude has been measured. With the PPI tests the Hugoniot elastic limit and the spall strength, as well as the U_s–u_p relation have been determined. Model parameters for the JC strength model and an equation of state have been determined from the experimental results. The validation of the material model has been performed by numerical simulations of the modified Taylor-impact tests where an enhanced model validation has been done by comparing the measured and calculated VISAR signals while this technique is normally used for PPI tests only.     

Generalised plane strain problem for a circular inclusion in anisotropic elasticity

This paper is concerned with a generalised plane deformation problem in the linear theory of anisotropic elasticity. As is well known, the generalised plane deformation is the deformation of a body of infinite length bounded by a cylindrical surface, when all the stress and strain components exist but they are functions of two co-ordinates x₁, and x₂ only. It may be shown that if u₃ = 0, it is impossible to satisfy all the three equations of equilibrium of anisotropic elastic body. One has to choose u₃ as a non-zero function of x₁, x₂ for satisfying equations of equilibrium. In isotropic elasticity, u₃ = 0, makes the third equation of equilibrium identically equal to zero.The problem in this paper concerns an elastic circular cylindrical inclusion embedded in a matrix of different anisotropic material. The matrix and the inclusion are perfectly bonded at the interface. Each of the two materials possesses anisotropy of a general form with all the 21 elastic constants. The matrix is subjected to a uniform stress at infinity. The equations of elasticity theory demand that the rotation component ω₃ must also be prescribed at infinity. The complex variable technique is used and exact analytical expressions are derived for the elastic field in both the regions.     

Molecular dynamics simulations for nitridation of organic polymer surfaces due to hydrogen-nitrogen ion beam injections

Interaction of organic polymer surfaces with energetic reactive ions during etching processes by hydrogen-nitrogen plasmas has been investigated microscopically with the use of classical molecular dynamics (MD) simulations. Especially examined in the present study are interactions of atomic-nitrogen, molecular-nitrogen, or ammonia beams with a polyparaphenylene (PPP) surface and the resulting surface modification. It has been observed in the simulations that, when reactive atomic-nitrogen (N) beams are injected into a PPP surface, a carbon nitride layer with carbon-nitrogen bonds of higher bond orders (i.e., bonds containing π bonds) tend to be formed and it also acts as a source of carbon-nitride clusters for sputtered species. This observation is consistent with the fact that excessive supply of nitrogen to a carbon nitride film makes the film structurally weak as nitrogen atoms tend to break up carbon chains. On the other hands, when ammonia (NH₃) beams are injected, carbon-nitrogen single bonds (i.e., σ bonds only) are more likely to be formed since hydrogen atoms would efficiently terminate π bonds should they be formed.     

A study of α- and β-phase poly(9,9-dioctylfluorene) by electroabsorption spectroscopy

The excited-state structure of β-phase poly(9,9-dioctylfluorene) (F8) is investigated by means of optical absorption and electroabsorption spectroscopies in order to reveal the effect of β-phase formation on the excited-state structure in F8. The excited-state structure of β-phase F8 consists of six essential states, where two new B_u states are formed on the four essential states of α-phase F8. The two additional B_u states result from the energy splitting of the B_u states of α-phase F8, caused by intermolecular interaction in β-phase F8.     

Effects of size and surface on the elasticity of silicon nanoplates: Molecular dynamics and semi-continuum approaches

In this paper, the size effects on the elastic behavior of single crystal silicon nanoplates terminated by {100} surfaces is studied by means of molecular dynamics (MD) using a modified embedded atom method. The results indicate that the {100} surfaces undergo 2 × 1-type reconstruction, which significantly influences the mechanical properties of nanoplates. The simulations are carried out at room temperature and structural relaxation is performed. The effective Young's modulus, in extensional mode, is determined for different thicknesses. The surface energy, surface stress and surface elasticity of layers near the surfaces (non-bulk layers) are obtained. These surface properties are used as inputs for a recently developed two-dimensional plane-stress semi-continuum framework. The framework can be seen as the link between the surface effects calculated by atomistic simulations and the overall elastic behavior. The surface properties of nanoplates of a few layers are shown to deviate from thicker plates, suggesting a size dependence of surface parameters and, especially, surface energy. For thicknesses below 3 nm, there is a difference between the effective Young's modulus, calculated by the semi-continuum approach and that calculated directly by MD. The difference is due to the size dependence of surface parameters.     

Nitrogen ion induced nitridation of Si(111) surface: Energy and fluence dependence

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N₂⁺ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N₂⁺ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N₂⁺ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of Si_xN_y on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N₂⁺ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.