We employ multibillion time step embedded-atom molecular dynamics simulations to investigate the homoepitaxial growth of Pt(111) from hyperthermal Pt atoms (EPt=0.2–50eV) using deposition fluxes approaching experimental conditions. Calculated antiphase diffraction intensity oscillations, based on adatom coverages as a function of time, reveal a transition from a three-dimensional multilayer growth mode with EPt<20eV to a layer-by-layer growth with EPt≥20eV. We isolate the effects of irradiation-induced processes and thermally activated mass transport during deposition in order to identify the mechanisms responsible for promoting layer-by-layer growth. Direct evidence is provided to show that the observed transition in growth modes is primarily due to irradiation-induced processes which occur during the 10ps following the arrival of each hyperthermal atom. The kinetic pathways leading to the transition involve both enhanced intralayer and interlayer adatom transport, direct incorporation of energetic atoms into clusters, and cluster disruption leading to increased terrace supersaturation.
Embedded-atom molecular dynamics simulations were used to investigate the effects of low-energy (5–50 eV) normally-incident self-ion irradiation of two-dimensional compact Pt3, Pt7, Pt19, and Pt37 clusters on Pt(111). We follow atomistic pathways leading to bombardment-induced intra- and interlayer mass transport. The results can be described in terms of three impact energy regimes. With E ≤ 20 eV, we observe an increase in 2D island dimensions and negligible residual point defect formation. As the impact energy is raised above 20 eV, we observe an increase in irradiation-induced lateral mass transport, a decrease in island size, and the activation of interlayer processes. For E ≥ 35 eV, this trend continues, but point defects, in the form of surface vacancies, are also formed. The results illustrate the richness of the dynamical interaction mechanisms occurring among incident energetic species, target clusters, and substrate atoms, leading to island preservation, reconfiguration, disruption and/or residual point defects formation. We discuss the significance of these results in terms of thin film growth.
Embedded-atom^{ }molecular dynamics simulations are used to investigate the effects of^{ }low-energy self-ion irradiation of Pt adatoms on Pt(111). Here, we^{ }concentrate on self-bombardment dynamics, i.e., isolating and monitoring the atomic^{ }processes, induced by normally incident Pt atoms with energies E^{ }ranging from 5 to 50 eV, that can affect intra-^{ }and interlayer mass transport.. We find that adatom scattering, surface^{ }channeling, and dimer formation occur at all energies. Atomic intermixing^{ }events involving incident and terrace atoms are observed at energies^{ }15 eV, while the collateral formation of residual surface vacancies is^{ }observed only with E>40 eV. The overall effect of low-energy self-ion^{ }irradiation is to enhance lateral adatom and terrace atom migration. ©2005 American Institute of Physics
The^{ }effects of substitutional additives on the properties and phase stability^{ }of - and -alumina (Al_{2}O_{3}), are investigated by density functional^{ }theory total energy calculations. The dopants explored are 5 at. %^{ }of Cr, Mo, Co, and As substituting for Al, respectively,^{ }N and S substituting for O, in the and^{ } lattices. Overall, the results show that it is possible^{ }to shift, and even reverse, the relative stability between -^{ }and -alumina by substitutional additives. The alumina bulk moduli are,^{ }in general, only slightly affected by the dopants but density^{ }of states profiles reveal additional peaks in the alumina band^{ }gaps. We also show that phase separations into pure oxides^{ }are energetically favored over doped alumina formation, and we present^{ }results on a number of previously unstudied binary oxides.
In this work we report the investigation of defect generation in Mo/W superlattices (SL). The study has been carried out using molecular dynamics (MD) and the embedded atom (EAM) potential. Mechanisms for the generation of observed defect patterns are proposed.
We report here the results of a Molecular Dynamics-Embedded Atom Method-investigation of the pathways generating point defects in Mo/W superlattices during bombardment with energetic (50 to 200 eV) Ar and Kr neutrals. Energy accommodation coefficients are computed for the different structures and are found to be roughly independent of the incident energy, and substantially higher for structures with Mo on top. Several different types of defects are shown, and two general processes generating those are discussed. Trapping of the incoming noble gas was observed for the case of Kr impinging on structures with Mo as the top monolayer; this is interpreted as an effect of the small mass difference between the Mo and the Kr atoms. An increase in atomic mass of the gas translates into a more disparate behaviour of the studied structures. The energy exchange with the surface layer dictates the behaviour of the superlattice; this is accentuated when bombarding with the heavier gas, Kr. (C) 1998 Elsevier Science S.A.
An investigation of defect generation at the interface during growth of epitaxial (100) oriented Mo/W superlattices by ion-assisted deposition has been carried out using molecular-dynamics simulations. The influence of the impact parameter within the irreducible bcc unit cell [001] surface and the incident ion energy on the energy accommodation, the dynamics of energy transfer, and energy dissipation are discussed. A detailed model of the generation of point defects is presented and the influence of materials upon the type and the number of defects as well as the energy accommodation of the superlattice is revealed. It is shown that the behavior of the superlattice as a whole is largely dominated by the material in the surface monolayer.
Embedded-atom molecular dynamics simulations were used to follow the diffusion dynamics of compact Pt clusters with up to 19 atoms on Pt (111) surfaces. The results reveal a novel cluster diffusion mechanism, involving successive shear translations of adjacent subcluster regions, which give rise to reptation, a snake-like gliding motion. We show that for compact clusters with 4 to 6 atoms, this mechanism competes energetically with that of island diffusion through concerted motion. However, as the cluster size increases from > 7 to ? 20 atoms, reptation becomes the energetically favored diffusion mechanism. The concerted shear motion of subcluster regions, leading to reptation, is also shown to play a significant role in dendritic-to-compact morphological transitions of Pt island.
Embedded-atom molecular-dynamics simulations were used to follow the diffusion dynamics of compact platinum clusters with up to 19 atoms on Pt(lll). The results reveal a cluster diffusion mechanism on (111) face-centered-cubic (fcc) surfaces involving successive shear translations of adjacent subcluster regions giving rise to reptation, a snake-like gliding motion. We show that for compact clusters with <7 atoms, this mechanism competes energetically with that of island diffusion through concerted motion. However, for cluster sizes of between 8 and similar or equal to 20 atoms, reptation becomes energetically favorable, especially for elongated clusters. Reptation is also shown to be an important migration mechanism for fractal (randomly ramified) and dendritic (symmetrically branched) islands. (C) 1999 Elsevier Science B.V. All rights reserved.
We use molecular dynamics simulations to follow the dynamics of small two-dimensional Pt clusters on Pt(111) at 1000 K. While close-packed Pt-7 heptamers are extremely stable structures, the addition of a single cluster vacancy or an on-top adatom immediately results in intracluster bond breaking, reconfigurations, rotations, the introduction of stacking faults, and greatly enhanced cluster diffusion rates. Mapping center-of-mass motion for total simulation times >145 ns revealed increases in cluster velocities by more than an order of magnitude with cluster migration occurring primarily by concerted motion and a novel diffusion mechanism involving double shearing of dimers/trimers. Contrary to some previous reports, edge-atom diffusion plays only a minor role. (C) 1998 American Institute of Physics.
Improved toughness is one of the central goals in the development of wear-resistant coatings. Previous studies of toughness in transition metal nitride alloys have addressed the effects of chemical composition in these compounds. Herein, we use density functional theory to study the effects of various metal sublattice configurations, ranging from fully ordered to fully disordered, on the mechanical properties of VM2N and TiM2N (M2 = W, Mo) ternary alloys. Results show that all alloys display high incompressibility, indicating strong M-N bonds. Disordered atomic arrangements yield lower values of bulk moduli and C_{11} elastic constants, as well as higher values of C_{44} elastic constants, compared to ordered structures. We attribute the low C_{44} values of ordered structures to the formation of fully-bonding states perpendicular to the applied stress. We find that the ductility of these compounds is primarily an effect of the increased valence electron concentration induced upon alloying.
We use classical molecular dynamics and the modified embedded atom method to determine residence times and descent pathways of Ti and N adatoms on square, single-atom-high, TiN islands on TiN(001). Simulations are carried out at 1000 K, which is within the optimal range for TiN(001) epitaxial growth. Results show that the frequency of descent events, and overall adatom residence times, depend strongly on both the TiN(001) diffusion barrier for each species as well as the adatom island-edge location immediately prior to descent. Ti adatoms, with a low diffusion barrier, rapidly move toward the island periphery, via funneling, where they diffuse along upper island edges. The primary descent mechanism for Ti adatoms is via push-out/exchange with Ti island-edge atoms, a process in which the adatom replaces an island edge atom by moving down while pushing the edge atom out onto the terrace to occupy an epitaxial position along the island edge. Double push-out events are also observed for Ti adatoms descending at N corner positions. N adatoms, with a considerably higher diffusion barrier on TiN(001), require much longer times to reach island edges and, consequently, have significantly longer residence times. N adatoms are found to descend onto the terrace by direct hopping over island edges and corner atoms, as well as by concerted push-out/exchange with N atoms adjacent to Ti corners. For both adspecies, we also observe several complex adatom/island interactions, before and after descent onto the terrace, including two instances of Ti islandatom ascent onto the island surface.
It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiN_{x} complexes (x = 1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiN_{x} (x = 1–3) admolecules on 8 × 8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000 K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN_{2} admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN_{2} migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN_{2} trimers descend exclusively by hopping. In contrast, TiN_{3} admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes.
Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200K are carried out using incident flux ratios N/Ti -1, 2, and 4. The films are analyzed as a function of composition, island size distribution, island edge orientation, and vacancy formation. Results show that N/Ti-1 films are globally understoichiometric with dispersed Ti-rich surface regions which serve as traps to nucleate 111-oriented islands, leading to local epitaxial breakdown. Films grown with N/Ti=2 are approximately stoichiometric and the growth mode is closer to layer-by-layer, while N/Ti-4 films are stoichiometric with N-rich surfaces. As N/Ti is increased from 1 to 4, island edges are increasingly polar, i. e., 110-oriented, and N-terminated to accommodate the excess N flux, some of which is lost by reflection of incident N atoms. N vacancies are produced in the surface layer during film deposition with N/Ti-1 due to the formation and subsequent desorption of N-2 molecules composed of a N adatom and a N surface atom, as well as itinerant Ti adatoms pulling up N surface atoms. The N vacancy concentration is significantly reduced as N/Ti is increased to 2; with N/Ti-4, Ti vacancies dominate. Overall, our results show that an insufficient N/Ti ratio leads to surface roughening via nucleation of small dispersed 111 islands, whereas high N/Ti ratios result in surface roughening due to more rapid upper-layer nucleation and mound formation. The growth mode of N/Ti-2 films, which have smoother surfaces, is closer to layer-by-layer. (C) 2016 American Vacuum Society.
The authors report the growth and mechanical properties of epitaxial B1 NaCl-structure V_{1-x}W_{x}N/MgO(001) thin films with 0 ≤ x ≤ 0.60. The Gibbs free energy of mixing, calculated using density functional theory (DFT), reveals that cubic V_{1-x}W_{x}N solid solutions with 0 ≤ x ≤ 0.7 are stable against spinodal decomposition and separation into the equilibrium cubic-VN and hexagonal-WN binary phases. The authors show experimentally that alloying VN with WN leads to a monotonic increase in relaxed lattice parameters, enhanced nanoindentation hardnesses, and reduced elastic moduli. Calculated V_{1-x}W_{x}N lattice parameters and elastic moduli (obtained from calculated C_{11}, C_{12}, and C_{44} elastic constants) are in good agreement with experimental results. The observed increase in alloy hardness, with a corresponding decrease in the elastic modulus at higher x values, combined with DFT-calculated decreases in shear to bulk moduli ratios, and increased Cauchy pressures (C_{12}–C_{44}) with increasing x reveal a trend toward increased toughness.
Using a combination of experiments and density functional theory (DFT), we demonstrate the first example of vacancy-induced toughening, in this case for epitaxial pseudobinary NaCl-structure substoichiometric V_{0.5}Mo_{0.5}N_{x} alloys, with N concentrations 0.55 ≤ x ≤ 1.03, grown by reactive magnetron sputter deposition. The nanoindentation hardness H(x) increases with increasing vacancy concentration from 17 GPa with x = 1.03 to 26 GPa with x = 0.55, while the elastic modulus E(x) remains essentially constant at 370 GPa. Scanning electron micrographs of indented regions show ductile plastic flow giving rise to material pile-up, rather than cracks as commonly observed for hard, but brittle, transition-metal nitrides. The increase in alloy hardness with an elastic modulus which remains constant with decreasing x, combined with the observed material pile-up around nanoindents, DFT-calculated decrease in shear to bulk moduli ratios, and increased Cauchy pressures (C_{12}-C_{44}), reveals a trend toward vacancy-induced toughening. Moreover, DFT crystal orbital overlap population analyses are consistent with the above results.
Hardness is an essential property for a wide range of applications. However, hardness alone, typically accompanied by brittleness, is not sufficient to prevent failure in ceramic films exposed to high stresses. Using VN as a model system, we demonstrate with experiment and density functional theory (DFT) that refractory VMoN alloys exhibit not only enhanced hardness, but dramatically increased ductility. V0.5Mo0.5N hardness is 25% higher than that of VN. In addition, while nanoindented VN, as well as TiN reference samples, suffer from severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Moreover, the wear resistance of V0.5Mo0.5N is considerably higher than that of VN. DFT results show that tuning the occupancy of d-t_{2g} metallic bonding states in VMoN facilitates dislocation glide, and hence enhances toughness, via the formation of stronger metal/metal bonds along the slip direction and weaker metal/N bonds across the slip plane.
A study was performed on quantum design and synthesis of a boron-oxygen-yttrium (BOY) phase. The calculations predicted that the BOY phase was 0.36 eV/atom more stable than crystalline BO0.17. The results showed that films with Y/B ratios ranging from 0.10 to 0.32, as determined via elastic recoil detection analysis, were grown over wide range of temperatures (300-600°C) and found to withstand 1000°C.
The effect of chemical composition on the elastic and electrical properties is studied for the BOxYz system with 0.27less than or equal toxless than or equal to1.14 and 0.36less than or equal tozless than or equal to0.08. We use ab initio calculations to obtain the elastic constants and density of states for BO1.5 and the BOY phase (yttrium substituting for oxygen in the boron suboxide structure). For decreasing x values, the elastic modulus is predicted to increase from 11 to 340 GPa, while electronic structure calculations suggest a shift in electrical properties from insulating to metallic. Thin films in the B-O-Y system are grown by reactive rf magnetron sputtering. As x decreases from 1.14 to 0.27, the elastic modulus increases from 12 to 282 GPa, which is a factor of 24, while resistivity decreases from 7.6+/-0.4 to (3.8+/-0.1)x10(-2) Omegam. The observed shifts in elasticity and resistivity are shown to be induced by the associated changes in chemical bonding from van der Waals type in BO1.5 to icosahedral type in the BOY phase.
Elastic modulus of amorphous boron suboxide thin films was studied by theoretical and experimental methods. It was shown that the increase of x in the a-BOx films from 0.08 to 0.18 decreased the magnitude of the elastic modulus from 273 to 231 GPa. The decrease of the elastic modulus with an increasing amount of O was correlated to the presence of the long B-O bonds with ionic contribution and the reduction of the film density.
Molecular dynamics simulations were used to follow the dynamics of the motion of hexagonal Pt heptamers on Pt(111). Close packed Pt-7 clusters on fee sites were found to be very stable structures with reconfiguration or translation events occurring only rarely over simulation times >30 ns at 1000 K. The adsorption of a single adatom on the cluster surface, however, induced rapid intracluster bond breaking, reconfiguration, the introduction of stacking faults, and greatly enhanced cluster diffusion rates. Cluster migration occurred primarily through sequences of individual atom and concerted dimer jumps, but concerted cluster motion was also observed. The adatoms eventually descended to the terrace, predominantly through push-out/exchange reactions with cluster atoms at B edges.
We use embedded-atom molecular dynamics simulations to follow the dynamics of adatoms, vacancies, and adatom/vacancy pairs on two-dimensional hexagonal Pt19 clusters on Pt(1 1 1) surfaces at 1000 K. All configurations are found to be quite stable and have essentially the same migration mobilities as compact hexagonal clusters. However, the presence of a single vacancy dramatically decreases the lifetime of an adatom on the cluster by a factor of three. This occurs primarily through an enhancement of the rate of push-out/exchange reactions at the outer cluster edge resulting from vacancy-induced softening of edge atom bonds. Overall, adatoms in the presence of vacancies descend to the terrace via vacancy filling 10% of the time, and through reactions with outer cluster edges the remaining 90%. Direct vacancy filling mechanisms are analogous to, and have similar activation energies with, those at outer cluster edges: adatom hopping over descending steps and push-out/exchange reactions. © 2003 Elsevier B.V. All rights reserved.
Cluster migration is known to be an important process during film growth at elevated temperatures, but relatively little quantitative data is available. We have used molecular dynamics simulations to follow the dynamics of small two-dimensional Pt clusters on Pt(lll) at 1000 K. While close-packed Pt-7 heptamers are extremely stable structures, the addition of a single-cluster vacancy or an on-top adatom immediately results in intracluster bond breaking, reconfigurations, rotations, the introduction of stacking faults, and greatly enhanced cluster-diffusion rates. Mapping center-of-mass motion for total simulation times > 145 ns revealed increases in cluster velocities by more than an order of magnitude with cluster migration occurring primarily by concerted motion and a novel diffusion mechanism involving double shearing of dimers/trimers. Contrary to some previous reports, edge-atom diffusion plays only a minor role. (C) 1998 Elsevier Science S.A.
A combinatorial method was employed to grow TiAlN-WNx films by DC sputtering as well as by High Power Pulsed Magnetron Sputtering (HPPMS) where the W concentration was varied between 10-52 at.% and 7-54 at.%, respectively. Experiments were paired with ab initio calculations to investigate the correlation between composition, structure, and mechanical properties. During all depositions the time averaged power was kept constant. As the W concentration was increased, the lattice parameter of cubic TiAlN-WNx films first increased and then decreased for W concentrations above approximate to 29 at.% (DCMS) and approximate to 27 at.% (HPPMS) as the N concentration decreased. Calculations helped to attribute the increase to the substitution of Ti and Al by W and the decrease to the presence of N vacancies. Youngs modulus and hardness were around 385-400 GPa and 29-31 GPa for DCMS and 430-480 GPa and 34-38 GPa for HPPMS, respectively, showing no significant trend as the W concentration was increased, whereas calculations showed a continuous decrease in Youngs modulus from 440 to 325 GPa as the W concentration was increased from 0 to 37.5 at.%. The presence of N vacancies was shown to increase the calculated Youngs modulus. Hence, the relatively constant values measured may be understood based on N vacancy formation as the W concentration was increased. HPPMS-deposited films exceed DCMS films in Youngs modulus and hardness, which may be a consequence of the larger degree of ionization in the HPPMS plasma. It is reasonable to assume that especially the ionized film forming species may contribute towards film densification and N vacancy formation.
Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl)(1-x)MxN thin films in the B1 structure, with 0.06 andlt;= x andlt;= 0.75 obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti0.5Al0.5 N. For (TiAl)(1-x)WxN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t(2g) metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses.
We use classical molecular dynamics and the modified embedded atom method formalism to investigate the dynamics of atomic-scale transport on a low-index model compound surface, TiN(001). Our simulations, totaling 0.25 mu s for each case study, follow the pathways and migration kinetics of Ti and N adatoms, as well as TiNx complexes with x = 1-3, which are known to contribute to the growth of TiN thin films by reactive deposition from Ti, N-2, and N precursors. The simulations are carried out at 1000 K, within the optimal range for TiN(001) epitaxial growth. We find Ti adatoms to be the highest-mobility species on TiN(001), with the primary migration path involving jumps of one nearest-neighbor distance d(NN) between adjacent fourfold hollow sites along in-plane andlt; 100 andgt; channels. Long jumps, 2d(NN), are also observed, but at much lower frequency. N adatoms, which exhibit significantly lower migration rates than Ti, diffuse along in-plane andlt; 110 andgt; directions and, when they intersect other N atoms, associatively form N-2 molecules, which desorb at kinetic rates. As expected, TiN and TiN3 complexes migrate at even lower rates with complex diffusion pathways involving rotations, translations, and rototranslations. TiN2 trimers, however, are shown to have surprisingly high diffusion rates, above that of N adatoms and almost half that of Ti adatoms. TiN3 motion is dominated by in-place rotation with negligible diffusion.
Ab initio molecular dynamics simulations based on density functional theory show that N adatoms are chemisorbed in threefold sites close to a N surface atom and between the two diagonally opposed neighboring Ti surface atoms on TiN(001). The most probable N adatom reaction pathway, even in the presence of nearby N adatoms, is for the N adatom and N surface atom pair to first undergo several exchange reactions and then desorb as a N_{2} molecule, resulting in a surface anion vacancy, with an activation barrier E_{des} of 1.37 eV and an attempt frequency A_{des} = 3.4 × 10^{13} s^{− 1}. E_{des} is essentially equal to the N adatom surface diffusion barrier, E_{s} = 1.39 eV, while A_{s} is only three to four times larger than A_{des}, indicating that isolated N adatoms migrate for only short distances prior to N_{2} desorption. The probability of N_{2} desorption via recombination of N adatoms on TiN(001) is much lower due to repulsive adatom/adatom interactions at separations less than ~ 3 Å which rapidly increase to ~ 2 eV at a separation of 1.5 Å. We obtain good qualitative and quantitative agreement with the above results using the modified embedded atom method potential to perform classical molecular dynamics simulations.
Ab initio and classical molecular dynamics (AIMD and CMD) simulations reveal that Ti adatoms on TiN(001) surfaces migrate between neighboring fourfold hollow sites primarily along in-plane less than100greater than channels. less than100greater than and less than110greater than single jumps, as well as less than100greater than double jump rates, obtained directly from MD runs as a function of temperature, are used to determine diffusion activation energies Ea, and attempt frequencies A, for the three preferred Ti adatom migration pathways on TiN(001). From transition rates Aexp[-Ea / (k(B)T)], we determine adatom surface distribution probabilities as a function of time, which are used to calculate adatom diffusion coefficients D(T). AIMD and CMD predictions are consistent and complementary. Using CMD, we investigate the effect on the adatom jump rate of varying the phonon wavelength degrees of freedom by progressively increasing the supercell size. We find that long-wavelength phonons significantly contribute to increasing adatom mobilities at temperatures less than= 600 K, but not at higher temperatures. Finally, by directly tracking the Ti adatom mean-square displacement during CMD runs, we find that Ti adatom jumps are highly correlated on TiN(001), an effect that yields lower D-s values (D-s(corr)) than those estimated from uncorrelated transition probabilities. The temperature-dependent diffusion coefficient is D-s(corr) (T) = (4.5 x 10(-4) Cm-2 s(-1)) exp[-0.55 eV / (k(B)T)].
Toughness, besides hardness, is one of the most important properties of wear-resistant coatings. We use ab initio density-functional theory calculations to investigate the mechanical properties of ternary metal nitrides TixM1-xN, with M=Mo and W, for x=0.5. Results show that Mo and W alloying significantly enhances the toughness of TiN. The electronic mechanism responsible for this improvement, as revealed by electronic structure calculations, stems from the changes in charge density induced by the additional transition-metal atom. This leads to the formation of a layered electronic arrangement, characterized by strong, respectively, weak, directional bonding, which enables a selective response to strain, respectively, shear, deformations of the structures and yields up to 60% decrease in C-44 values.
We use density functional theory calculations to explore the effects of alloying cubic TiN and VN with transition metals M = Nb, Ta, Mo, W in 50% concentrations. The obtained ternaries are predicted to become supertough as they are shown to be harder and significantly more ductile compared to the reference binaries. The primary electronic mechanism of this supertoughening effect is shown in a comprehensive electronic structure analysis of these compounds to be the increased valence electron concentration intrinsic to these ternaries. Our investigations reveal the complex nature of chemical bonding in these compounds, which ultimately explains the observed selective response to stress. The findings presented in this paper thus offer a design route for the synthesis of supertough transition metal nitride alloys via valence electron concentration tuning.
We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5, as estimated by stress strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [1111-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along < 110 > directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of < 111 >-ordered Tio.5W0.5N with respect to mixing of cubic-BI TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5MocoN epitaxial layers. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (N-ad) on VN(001) favor epitaxial atop-V positions and diffuse among them by transiting through 4-fold hollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V -amp;gt; FFH -amp;gt; atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (N-surf) atom. Frequent N-ad/N-surf pair exchange reactions lead to N-2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane (110) jumps and act as efficient catalysts for the dissociative chemisorption of incident N-2 molecules. During exposure of VN(001) to incident atomic N gas atoms, N-ad/N-ad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N-2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with N-ad or N-surf atoms. Similarities and differences between: N/VN(001) vs. previous N/TiN(001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning.
We use molecular dynamics (MD) based on the modified embedded atom method (MEAM) to determine diffusion coefficients and migration pathways for Ti and N adatoms (Ti-ad and N-ad) on TiN(111). The reliability of the classical model-potential is verified by comparison with density functional theory (DFT) results at 0 K. MD simulations carried out at temperatures between 600 and 1800 K show that both Ti-ad and N-ad favor fcc surface sites and migrate among them by passing through metastable hcp positions. We find that N-ad species are considerably more mobile than Ti-ad on TiN(111); contrary to our previous results on TiN(001). In addition, we show that lattice vibrations at finite temperatures strongly modify the potential energy landscape and result in smaller adatom migration energies, E-a = 1.03 for Ti-ad and 0.61 eV for N-ad, compared to 0 K values E-aOK = 1.55 (Ti-ad) and 0.79 eV (N-ad). We also demonstrate that the inclusion of dipole corrections, neglected in previous DFT calculations, is necessary in order to obtain the correct formation energies for polar surfaces such as TiN(111). (C) 2016 Elsevier B.V. All rights reserved.
We carried out a combined experimental and theoretical study of grain boundaries in polycrystalline diamond, aimed at achieving the conditions in which grain boundaries are equilibrated. Raman spectra of compacted at high-pressure and high-temperature diamond powders allow us to identify signals from sp(2)-bonded atoms, in addition to a strong peak at 1332 cm(-1), corresponding to sp(3)-bonded carbon. To verify our interpretation of the experiment, Sigma 5 (001) twist grain boundaries of polycrystalline diamond were studied by means of molecular dynamics simulations using the technique proposed by von Alfthan et al. [Phys. Rev. Lett. 96, 055505 (2006)]. We find that grain-boundary (GB) configurations, from which one atom is removed, have significantly lower energy compared to those obtained with conventional techniques. These calculated GBs are highly ordered, a few monolayers thick, in agreement with experimental observations, and are primarily sp(2) bonded. This paper underlines the importance of varying the number of atoms within GBs in molecular dynamics simulations to correctly predict the GB ground-state structure.
It is of high fundamental and practical importance to be able to control the formation and stability of the different crystalline phases of alumina (Al_{2}O_{3}). In this study, we have used density functional theory methods to investigate the changes induced in the thermodynamically stable α phase and the metastable θ phase as one eighth of the Al atoms are substituted for different additives (Sc, W, Mo, Cr, Cu, Si, and B). The calculations predict that the additives strongly affect the relative stability between the two phases. Most tested additives are shown to shift the relative stability towards, and in some cases completely stabilize, the θ phase, while Cu doping is predicted to increase the relative stability of the α phase. The reasons for these effects are discussed, as are possible implications on the growth and use of doped aluminas in practical applications. In addition, the effects of the additives on bulk moduli and densities of states have been investigated.
The interactions of Al, O, and O_{2} with different α- Al_{2}O_{3} (0001) surfaces have been studied using ab initio density functional theory methods. All three surface terminations obtainable by cleaving the bulk structure [single Al-layer (AlO), double Al-layer (AlAl), and O terminations] have been considered, as well as a completely hydrogenated O-terminated surface. Adsorbed Al shows strong ioniclike interaction with the AlO - and O-terminated surfaces, and several metastable adsorption sites are identified on the O-terminated surface. On the completely hydrogenated surface, however, Al adsorption in the bulk position is found to be unstable or very weak for the studied configurations of surface H atoms. Atomic O is found to interact strongly with the AlAl -terminated surface, where also O_{2} dissociative adsorption without any appreciable barrier is observed. In contrast, O adsorption on the AlO -terminated surface is metastable relative to molecular O_{2}. On the O-terminated surface, we find the creation of O surface vacancies to be plausible, especially upon exposure to atomic O at elevated temperatures. The results are mainly discussed in the context of alumina thin film growth and provide insight into phenomena related to, e.g., preferred adsorption sites and effects of hydrogen on the growth.
As one of the technologically most important ceramic materials, alumina (Al2O3) thin film growth has been studied extensively in the past. However, the mechanisms behind the formation of different phases and microstructures are still poorly understood, especially for physically vapor deposited films. An increased atomic scale understanding of alumina surface processes would thus be an important step towards a more complete understanding and control of the deposition process. In the present work, density functional theory based methods were used to study the adsorption of Al, O, AlO, and O2 on different terminations of alpha-alumina (0001) surfaces. The results show the existence of several metastable adsorption sites on the O-terminated surface and provide a possible explanation for the well-known difficulties in growing -Ñ-alumina at lower temperatures. Moreover, we demonstrate that Al adsorption in bulk positions is unstable, or considerably weaker, for completely hydrogenated surfaces, indicating that hydrogen stemming from residues in vacuum systems, might hinder the growth of crystalline alpha-alumina. Furthermore, nudged elastic band investigations of dynamic energy barriers for different surface diffusion processes show that Al diffusion, on the Al-terminated (0001) surface, requires only ~0.7 eV. This value is considerably lower than what is generally expected for the low temperature synthesis of alpha-alumina phase. These results add significantly to understanding the effects of several important factors on alumina growth, and their implication, on optimizing deposition processes for the synthesis of alumina films with desired properties, will be discussed.
Investigations of activation energy barriers for Al surface hopping on alpha-Al2O3 (0 0 0 1) surfaces have been carried out by means of first-principles density functional theory calculations and the nudged elastic band method. Results show that surface diffusion on the (most stable) Al-terminated surface is relatively fast with an energy barrier of 0.75 eV, whereas Al hopping on the O-terminated surface is slower, with barriers for jumps from the two metastable positions existing on this surface to the stable site of 0.31 and 0.99 eV. Based on this study and on the literature, the governing mechanisms during low-temperature alpha-alumina thin film growth are summarized and discussed. Our results support suggestions made in some previous experimental studies, pointing out that limited surface diffusivity is not the main obstacle for alpha-alumina growth at low-to-moderate temperatures, and that other effects should primarily be considered when designing novel processes for low-temperature alpha-alumina deposition.
We describe a theoretical analysis of the structures of self-organizing nanoparticles formed by Pt and Ru-Pt on carbon support. The calculations provide insights into the nature of these metal particle systems-ones of current interest for use as the electrocatalytic materials of direct oxidation fuel cells- and clarify complex behaviors noted in earlier experimental studies. With clusters deposited via metalloorganic Pt or PtRu5 complexes, previous experiments [Nashner et al. J. Am. Chem. Soc. 1997, 119, 7760, Nashner et al. J. Am. Chem. Soc. 1998, 120, 8093, Frenkel et al. J. Phys. Chem. B 2001, 105, 12689] showed that the Pt and Pt-Ru based clusters are formed with fcc(111)-stacked cuboctahedral geometry and essentially bulklike metal-metal bond lengths, even for the smallest (few atom) nanoparticles for which the average coordination number is much smaller than that in the bulk, and that Pt in bimetallic [PtRu5] clusters segregates to the ambient surface of the supported nanoparticles. We explain these observations and characterize the cluster structures and bond length distributions using density functional theory calculations with graphite as a model for the support. The present study reveals the origin of the observed metal-metal bond length disorder, distinctively different for each system, and demonstrates the profound consequences that result from the cluster/carbon-support interactions and their key role in the structure and electronic properties of supported metallic nanoparticles. © 2006 American Chemical Society.