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Research was conducted to examine the interactions of SiH radicals with crystalline and amorphous silicon surfaces based on an atomic-scale simulation study. Focus was on elucidating the key parameters that determine the nature of the radical-surface interactions. Results indicate that the surface atomic coordination and the surface hydrogen concentration largely determine both the reaction mechanism and the reactivity of the radical with the surface.

We report a systematic analysis on the effects of hydrogenation on the mechanical behavior of irradiated single-layer graphene sheets, including irradiation-induced amorphous graphene, based on molecular-dynamics simulations of uniaxial tensile straining tests and using an experimentally validated model of electron-irradiated graphene. We find that hydrogenation has a significant effect on the tensile strength of the irradiated sheets only if it changes the hybridization of the hydrogenated carbon atoms to sp3, causing a reduction in the strength of irradiation-induced amorphous graphene by ∼10GPa. Hydrogenation also causes a substantial decrease in the failure strain of the defective sheets, regardless of the hybridization of the hydrogenated carbon atoms, and in their fracture toughness, which decreases with increasing hydrogenation for a given irradiation dose. We characterize in detail the fracture mechanisms of the hydrogenated irradiated graphene sheets and elucidate the role of hydrogen and the extent of hydrogenation in the deformation and fracture processes. Our study sets the stage for designing hydrogenation and other chemical functionalization strategies toward tailoring the properties of defect-engineered ductile graphene. [ABSTRACT FROM AUTHOR]

We report the results of a systematic atomic-scale analysis of the reactions of small mobile helium clusters (Hen, 4≤n≤7) near low-Miller-index tungsten (W) surfaces, aiming at a fundamental understanding of the near-surface dynamics of helium-carrying species in plasma-exposed tungsten. These small mobile helium clusters are attracted to the surface and migrate to the surface by Fickian diffusion and drift due to the thermodynamic driving force for surface segregation. As the clusters migrate toward the surface, trap mutation (TM) and cluster dissociation reactions are activated at rates higher than in the bulk. TM produces W adatoms and immobile complexes of helium clusters surrounding W vacancies located within the lattice planes at a short distance from the surface. These reactions are identified and characterized in detail based on the analysis of a large number of molecular-dynamics trajectories for each such mobile cluster near W(100), W(110), and W(111) surfaces. TM is found to be the dominant cluster reaction for all cluster and surface combinations, except for the He4 and He5 clusters near W(100) where cluster partial dissociation following TM dominates. We find that there exists a critical cluster size, n=4 near W(100) and W(111) and n=5 near W(110), beyond which the formation of multiple W adatoms and vacancies in the TM reactions is observed. The identified cluster reactions are responsible for important structural, morphological, and compositional features in the plasma-exposed tungsten, including surface adatom populations, near-surface immobile helium-vacancy complexes, and retained helium content, which are expected to influence the amount of hydrogen re-cycling and tritium retention in fusion tokamaks. [ABSTRACT FROM AUTHOR]

Numerical simulations of the surface morphological evolution of uniaxially stressed elastic crystalline solids have demonstrated that in addition to Asaro-Tiller/Grinfeld (surface cracking) instabilities, long-wavelength perturbations from the planar surface morphology can trigger a tip-splitting instability that causes formation of a pattern of secondary ripples, which cannot be explained by linear stability theory. In this study, we develop a weakly nonlinear stability theory, which can explain the occurrence of such secondary rippling instabilities and predict the number of secondary ripples that form on the surface as a function of perturbation wavelength. The theory shows that this type of surface pattern formation arises entirely due to the competition between surface energy and elastic strain energy, regardless of surface diffusional anisotropy or the action of externally applied fields. The origin of secondary rippling is explained through nonlinear terms included in the analysis which generate sub-harmonic ripples in the surface morphology with wave numbers that are multiples of the original surface perturbation wave number. Based on the weakly nonlinear theory, we have developed simple analytical expressions that predict the critical wavelength for the onset of secondary rippling, the increase in the number of secondary ripples with increasing perturbation wavelength, and how the onset of the secondary rippling instability and the rippled surface pattern are affected by surface diffusional anisotropy and the action of an applied electric field. The conclusions of the theory are validated by systematic comparisons with results of self-consistent dynamical simulations of surface morphological evolution. [ABSTRACT FROM AUTHOR]

We study the mechanical behavior and fracture of graphene nanomeshes (GNMs) consisting of hexagonal lattices of unpassivated circular pores based on molecular-dynamics simulations of uniaxial tensile deformation tests. We analyze the GNMs' mechanical response as a function of their porosity for porosities up to 80%. We find that the fracture strain exhibits a minimum at a porosity ~15%, which marks the onset of a transition in the mechanical behavior of the nanomeshes; beyond this critical porosity, the GNM ductility increases and the toughness remains practically constant with increasing porosity. The mechanism of crack initiation and propagation is characterized in both cases of mechanical response. [ABSTRACT FROM AUTHOR]

We analyze the surface morphological stability of bulk conducting face-centered cubic (fcc) crystalline solids in uniaxial tension under the simultaneous action of an electric field and a temperature gradient. The analysis is based on self-consistent dynamical simulations, in conjunction with linear stability theory, according to a well validated fully nonlinear surface mass transport model that accounts for surface electromigration and thermomigration induced by the externally applied fields, surface diffusional anisotropy, and the Arrhenius temperature dependence of surface diffusivity. Our simulation results validate the findings of linear stability theory and establish that the electric field and the thermal gradient, if properly directed, can work synergistically to stabilize the planar surface morphology against the Asaro-Tiller/Grinfeld (ATG) instability when the strength of the resulting effective external field is higher than a critical level. We also show that the temperature dependence of the surface diffusivity does not change the criticality criterion for surface stabilization but only affects the rate of growth or decay of the surface morphological perturbation from its planar state. Furthermore, we establish that, in fcc crystals, the morphological response of h111ioriented surfaces is superior to that of differently oriented surfaces. In case of failure due to ATG instability, the super-exponential growth of the surface perturbation amplitude exhibits a logarithmic singularity as the time to failure is approached. Our study provides an effective practical solution to inhibit the surface cracking of crystalline conducting solids based on the optimal combination of the simultaneous action of externally applied electric fields and thermal gradients. [ABSTRACT FROM AUTHOR]

We report results of atomistic computations for the interactions of small mobile helium clusters (Hen) with free surfaces and grain boundaries (GBs) in tungsten toward development of continuum drift-diffusion-reaction models for the dynamics of mobile helium clusters in plasma-exposed tungsten. Molecular-statics (MS) simulations based on reliable many-body interatomic potentials are carried out for Hen (1 ⩽ n ⩽ 7) clusters near sinks to obtain the potential energy profiles of the Hen clusters as a function of the clusters' center-of-mass distance from a sink. Sinks investigated include surfaces, GBs, and regions in the vicinity of junctions where GBs intersect free surfaces. Elastic interaction potentials based on elastic inclusion theory provide an excellent description of the MS results for the cluster-sink interactions. The key parameter in the elastic models is the sink segregation strength, which is found to increase with increasing cluster size. Such cluster-sink interactions are responsible for the migration of small helium clusters by drift and for helium segregation on surfaces and grain boundaries in tungsten. Such helium segregation on sinks is observed in large-scale molecular-dynamics simulations of helium aggregation in model polycrystalline tungsten at 933K upon helium implantation. [ABSTRACT FROM AUTHOR]

We have developed a coarse-grained description of the phase behavior of the isolated 38-atom Lennard-Jones cluster (LJ38). The model captures both the solid-solid polymorphic transitions at low temperatures and the complex cluster breakup and melting transitions at higher temperatures. For this coarse model development, we employ the manifold learning technique of diffusion mapping. The outcome of the diffusion mapping analysis over a broad temperature range indicates that two order parameters are sufficient to describe the cluster's phase behavior; we have chosen two such appropriate order parameters that are metrics of condensation and overall crystallinity. In this well-justified coarse-variable space, we calculate the cluster's free energy landscape (FEL) as a function of temperature, employing Monte Carlo umbrella sampling. These FELs are used to quantify the phase behavior and onsets of phase transitions of the LJ38 cluster. [ABSTRACT FROM AUTHOR]

Based on a continuous-domain model, capable of accessing the spatiotemporal scales relevant to fuzz formation on the surface of plasma-facing component (PFC) tungsten, we report self-consistent simulation results that elucidate the effects of elastic softening and helium (He) accumulation kinetics on the surface morphological response of PFC tungsten. The model accounts for the softening of the elastic moduli in the near-surface region of PFC tungsten, including both thermal softening at high temperature and softening due to He accumulation upon He implantation. The dependence of the elastic moduli on the He content follows an exponential scaling relation predicted by molecular-dynamics simulations, while the He content in the near-surface region of PFC tungsten evolves according to a first-order saturation kinetics, consistent with experimental and simulation results reported in the literature. We establish that this elastic softening accelerates both nanotendril growth on the PFC surface and the onset of fuzz formation. We also explore the role of the rate of He accumulation to a saturation level in the near-surface region of irradiated tungsten in the onset of fuzz formation. For PFC tungsten surfaces such as W(110) where, under typical irradiation conditions, the characteristic time scale for stress-driven surface diffusion is comparable to the characteristic time scale for He accumulation, we find that accelerating the rate of He accumulation accelerates the growth rate of nanotendrils emanating from the surface. Additionally, we present a systematic parametric study of the PFC surface morphological response to explore its dependence on the He accumulation kinetics that is controlled by the irradiation conditions for low-energy implantation. Finally, we introduce an incubation time for nanotendril growth on the PFC surface, a concept equivalent to that of incubation fluence discussed in the literature, to predict and explain the minimum exposure time required to observe fuzz formation on PFC tungsten surfaces. [ABSTRACT FROM AUTHOR]

Two of the simulations discussed in a prior article (Hammond et al 2019 Nucl. Fusion 59 066035) were affected by a simulation glitch. We repeated the affected calculations and discuss them here. The overall conclusions are essentially unchanged, though the details are different. In particular, observations that we referred to as 'concerted bursting' were caused primarily by non-physical heating and cooling applied by the thermostat after most atoms' velocities were deleted (for reasons that are not known for certain). The phenomenon of one bubble bursting and causing another nearby bubble to burst does exist, though its effects are much less spectacular in the absence of non-physical driving forces. The observation of an interconnected network of sub-surface cavities formed by burst bubbles is real, and the observation of holes on the surface 1–2 nm in diameter is also confirmed. [ABSTRACT FROM AUTHOR]

We report results of a systematic computational study of the electromigration-driven complex surface dynamics of voids in mechanically stressed thin films of face-centered cubic metals with <100>-oriented film planes. The films are subjected to an external electric field simultaneously with biaxial mechanical loading, which can be either purely compressive, ranging from purely isotropic to strongly anisotropic including uniaxial, or a mixed type of loading with both tensile and compressive stress components in the applied stress tensor. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological evolution following a well validated, two-dimensional, and fully nonlinear model. We find that depending on the electromechanical conditions, void size, and surface diffusional anisotropy, two types of asymptotic states can be stabilized in the void surface dynamical response, namely, morphologically steady or time-periodic traveling voids, and film failure can be caused by void tip extension. The loading mode as well as the loading anisotropy are found to be the significant factors in determining the void morphological stability domains and can be tailored to stabilize steady or time-periodic states and to increase the film's resistance to failure. Under a mixed (tensile + compressive) loading mode, we find that it is impossible to stabilize steady states in the void morphological response and that the stress levels that the film can sustain prior to failure are much lower than those under purely tensile or purely compressive biaxial loading. [ABSTRACT FROM AUTHOR]

Based on first-principles density functional theory calculations, we report a novel class of carbon nanostructures consisting of superlattice arrangements of caged fullerene configurations of various sizes embedded within planes of twisted bilayer graphene. Formation of these structures is the outcome of interlayer C-C bonding between pairs of graphene planes chemically modified with certain patterns of chemisorbed hydrogen and rotated with respect to each other by angles around 30°. A specific subclass of these nanostructures preserves the main features of the electronic structure of pristine single-layer graphene. Our study proposes possible functionalization strategies to systematically tailor the electronic properties of bilayer graphene. [ABSTRACT FROM AUTHOR]

We analyze the surface morphological stability of a coherently strained thin film that has been grown epitaxially on a compliant substrate of finite thickness and is subjected simultaneously to an external electric field, which drives surface electromigration. The compliant substrate has the ability to accommodate elastically some of the misfit strain that is developed in the epitaxial film due to the lattice mismatch between the film and substrate materials. We develop a three-dimensional model for the surface morphological evolution of the thin film and conduct a linear stability analysis for the morphological stability of the heteroepitaxial film's planar state; of particular importance for the analysis is the elastostatic boundary-value problem for the heteroepitaxial film/substrate system. The analysis shows that surface electromigration due to a properly applied and sufficiently strong electric field can inhibit Stranski-Krastanow-type instabilities. Furthermore, we determine the critical electric-field strength as a function of material properties and heteroepitaxial system parameters, as well as the optimal direction of the electric field for the most efficient stabilization of the surface morphology. We find that using a compliant substrate reduces the critical strength of the externally applied electric field required for planar film surface stabilization by approximately two orders of magnitude compared to that needed to stabilize the planar surface of the same thin film when grown epitaxially on a practically infinite substrate. This critical electric-field strength also is found to be substantially lower than that required for planar film surface stabilization for the same film grown on an elastic substrate that is clamped to a holder and has thickness equal to the compliant substrate thickness. This critical strength requirement can be reduced further by decreasing the ratio of the film's shear modulus with that of the substrate, and it can be minimized for an optimal value of the compliant substrate thickness. We conclude that surface electromigration can be used to control the onset of island formation on epitaxial film surfaces and that the required electric-field strength for such control can be optimized by efficient use of substrate engineering techniques. [ABSTRACT FROM AUTHOR]

We report the findings of a systematic computational study that addresses the effects of surface segregation on the atomic distribution at equilibrium of constituent group-III atoms in freestanding ternary semiconductor InxGa1-xAs nanocrystals. Our analysis is based on density functional theory calculations in conjunction with Monte Carlo simulations of the freestanding nanocrystals using a DFT-re-parameterized valence force field description of interatomic interactions. We have determined the equilibrium concentration profiles as a function of nanocrystal size (d), composition (x), and temperature (T). The ranges of d, x, and T are explored and demonstrate surface segregation and phase separation that leads to different extents of alloying in the nanocrystal core and in the near-surface regions. We find that formation of core/shell-like quantum dots characterized by an In-deficient core and an In-rich shell with a diffuse interface is favored at equilibrium. The analysis elucidates the relationship between the constituent species distribution in the nanocrystal and the parameters that can be tuned experimentally to design synthesis routes for tailoring the properties of ternary quantum dots. [ABSTRACT FROM AUTHOR]

We report results of first-principles density functional theory (DFT) calculations for the electronic structure of ZnSe1-xTex ternary quantum dots (TQDs) and the effects of composition and compositional distribution on the electron density distribution, electronic density of states, and band gap. We analyze the electronic structure of five types of nanocrystal configurations, namely, pristine ZnSe and ZnTe quantum dots, as well as ZnSe/ZnTe core/shell, ZnTe/ZnSe reverse core/shell, and randomly alloyed ZnSe1-xTex TQDs. We find that the band gaps for ZnSe/ZnTe core/shell TQDs are nonlinearly dependent on the number of Te atoms in the shell, whereas presence of Te in the core of alloyed ZnSe1-xTex TQDs modifies the electronic energy levels abruptly and significantly in the limits of x → 0 and x → 1. Our results imply that distribution of Te atoms in the TQD in the form of a ZnSe/ZnTe core/shell configuration allows for optimum tunability of the band gap and wave function confinement in TQDs. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of colloidal crystallization in a 32-particle system at a fixed value of interparticle depletion attraction that produces coexisting fluid and solid phases. Free energy landscapes (FELs) and diffusivity landscapes (DLs) are obtained as coefficients of 1D Smoluchowski equations using as order parameters either the radius of gyration or the average crystallinity. FELs and DLs are estimated by fitting the Smoluchowski equations to Brownian dynamics (BD) simulations using either linear fits to locally initiated trajectories or global fits to unbiased trajectories using Bayesian inference. The resulting FELs are compared to Monte Carlo Umbrella Sampling results. The accuracy of the FELs and DLs for modeling colloidal crystallization dynamics is evaluated by comparing mean first-passage times from BD simulations with analytical predictions using the FEL and DL models. While the 1D models accurately capture dynamics near the free energy minimum fluid and crystal configurations, predictions near the transition region are not quantitatively accurate. A preliminary investigation of ensemble averaged 2D order parameter trajectories suggests that 2D models are required to capture crystallization dynamics in the transition region. [ABSTRACT FROM AUTHOR]

We present a systematic computational analysis of the complex, electromechanically driven surface dynamics of voids in thin films of face-centered cubic metals for <100>-oriented film planes characterized by four-fold symmetry of surface diffusional anisotropy. The voids are located at an edge of the metallic thin film, and the film is subjected simultaneously to an external electric field and an anisotropic biaxial tensile stress. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological response according to a well-validated, two-dimensional, and fully nonlinear model. We examine thoroughly the effects of the anisotropic mechanical loading on the morphological evolution of the electromigration-driven void surface and the resulting asymptotic states of the surface morphological response. We have found supercritical Hopf-bifurcation transitions from stable steady to stable time-periodic states. For such films and over the range of electromechanical conditions examined, the only possible stable asymptotic states are either time-periodic states characterized by a single period of oscillation or steady states without any change in the void shape. We have determined the stability domain boundaries of the various asymptotic states and their dependence on the anisotropy of the applied stress tensor. The loading anisotropy has significant effects on the stability domain boundaries, but it does not introduce any more complex void dynamics in <100>-oriented films than that under isotropic mechanical loading. [ABSTRACT FROM AUTHOR]

We report a systematic analysis of the surface morphological stability of mechanically stressed electrically conductive crystalline solids that are disturbed from their planar surface state according to long-wavelength shape perturbations under the simultaneous action and in the absence of an applied electric field. The analysis is based on self-consistent dynamical simulations according to a fully nonlinear model of driven surface morphological evolution in conjunction with linear stability theory. We find that for perturbations with a longer-than-critical wavelength a morphological instability is triggered that leads to the formation of a pattern of secondary ripples on the surface morphology. Special emphasis is placed on the effects of the amplitude and the wavelength of the initial perturbation on the surface morphological evolution beyond the onset of the rippling instability. The analysis establishes the relationship between this secondary rippling instability and surface cracking instabilities and provides a detailed characterization of the rippled surface morphologies. The effects of surface diffusional anisotropy on this rippling phenomenon also are examined. We demonstrate that this secondary rippling is a general long-wavelength surface morphological instability phenomenon and is not due to the action of the applied electric field. Furthermore, we show that a sufficiently strong electric field can inhibit both this rippling instability and surface cracking instabilities. [ABSTRACT FROM AUTHOR]

A systematic computational study is presented of the effects of atomic hydrogen chemisorption on the structure and morphology of graphene layers and single-walled carbon nanotubes (SWCNTs). The study is based on a combination of classical molecular-dynamics (MD) and Monte Carlo simulations of structural and compositional relaxation of the hydrogenated surfaces, employing hydrogen distributions consistent with experimental observations and first-principles calculations. Results are reported for the strains induced on the graphene and the SWCNTs, as a result of sp2-sp3 bonding transitions due to atomic H chemisorption, and their dependence on the H surface coverage, Θ, over the entire range 0≤Θ≤1 and on nanotube diameters and chiralities. Detailed structural analysis of the relaxed hydrogenated surfaces demonstrates a tendency for clustering of hydrogenated and of nonhydrogenated sites; this leads to surface morphologies characterized by ripples, which consist of hills that form due to clustering of hydrogenated sites and are surrounded by valleys appearing through the formation of long chains of nonhydrogenated sites. These features introduce surface roughness that depends on the degree of hydrogenation and reaches its maximum levels at intermediate levels of H coverage. [ABSTRACT FROM AUTHOR]

We report results for the surface morphological stability of a thin film that lies on a finite-thickness substrate and is subjected simultaneously to an external electric field. The film is grown epitaxially on the substrate and may undergo a Stranski-Krastanow instability due to its lattice mismatch with the substrate material. We develop a model for the surface morphological evolution of the thin film and conduct a linear stability analysis to examine the morphological stability of the epitaxial film's planar surface state. Our analysis shows that surface electromigration due to a properly applied and sufficiently strong electric field can inhibit Stranski-Krastanow-type instabilities, which can be used to control the onset of island formation on the film surface. We find that using a finite-thickness substrate can have the beneficial effect of reducing the critical strength of the electric field required to stabilize the planar surface morphology of the epitaxial film with respect to the field strength required in the case of an infinitely thick substrate. We determine the critical electric-field strength as a function of material properties and heteroepitaxial system parameters, as well as the field's optimal direction for the most efficient stabilization of the surface morphology. Detailed results are presented for the effects of the finite-thickness substrate on the stabilization of the film's surface over a range of mechanical properties of the heteroepitaxial system's constituents. [ABSTRACT FROM AUTHOR]

Results are reported of a comprehensive analysis of the surface morphological response of an electrically conducting crystalline solid under the simultaneous action of an electric field and mechanical stress. The analysis is based on self-consistent dynamical simulations according to a fully nonlinear model of driven surface morphological evolution in conjunction with linear stability theory. The possibility is examined of current-induced stabilization of the surface morphology due to surface electromigration in stressed conductors that, in the absence of the electric current, would undergo cracklike surface instabilities. Current-induced surface stabilization of stressed solids is demonstrated and the corresponding stability domain boundaries are determined over a range of electromechanical conditions. Systematic comparisons are carried out of the numerical simulation results with the predictions of the linear stability theory and are used to establish the range of validity of the linear stability theory. [ABSTRACT FROM AUTHOR]

A comprehensive analysis is presented of the effects of surface diffusional anisotropy on the morphological stability of planar surfaces of stressed crystalline solids under the simultaneous action of an electric field. The analysis is based on self-consistent dynamical simulations of driven surface morphological evolution in conjunction with linear stability theory. Results are reported of a systematic study of the effects on the surface morphological response of varying surface diffusional anisotropy parameters; these include the direction of the applied electric field with respect to fast surface diffusion directions, the surface crystallographic orientation, and the strength of the anisotropy that increases with decreasing temperature. For face-centered cubic crystalline solids under stress, we find that there exists a preferred direction for the applied electric field that optimizes their surface morphological response. In addition, we find that for these solids under given surface electromigration conditions, the morphological response of their (111)-oriented surfaces is superior to that of their (100)-oriented and (110)-oriented surfaces. Furthermore, we demonstrate that increasing the strength of the surface diffusional anisotropy in these materials has beneficial effects on their surface morphological stability under the simultaneous action of an electric field. Our predictions can provide guidance for experimental studies of surface morphological response to the combined action of electric fields and mechanical stresses, as well as for tailoring operating conditions to improve the mechanical reliability of thin films in service. [ABSTRACT FROM AUTHOR]