Valeriu Chirita

Toughness enhancement in hard ceramic thin films by alloy design

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 h ...

Reptation: a mechanism for cluster migration on (111) face-centered-cubic metal surfaces

Abstract Embedded-atom molecular-dynamics simulations were used to follow the diffusion dynamics of compact platinum clusters with up to 19 atoms on Pt(111). 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

Low-energy ion irradiation during film growth: Kinetic pathways leading to enhanced adatom migration rates

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. ...

Enhanced cluster mobilities on Pt(111) during film growth from the vapor phase

We use molecular dynamics simulations to follow the dynamics of small two-dimensional Pt clusters on Pt(111) at 1000 K. While close-packed Pt7 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.

Adatom-induced diffusion of two-dimensional close-packed Pt7 clusters on Pt(111)

Abstract Molecular dynamics simulations were used to follow the dynamics of the motion of hexagonal Pt heptamers on Pt(111). Closepacked Pt 7 clusters on fcc 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.

Large-scale molecular dynamics simulations of TiN/TiN(001) epitaxial film growth

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 fu ...

Low-temperature alpha-alumina thin film growth : ab initio studies of Al adatom surface migration

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 t ...

Elastic modulus of amorphous boron suboxide thin films studied by theoretical and experimental methods

Boron suboxide BOx thin films have been deposited on Si (100) by reactive rf magnetron sputtering of B powders in an Ar/O2 atmosphere. Elastic recoil detection analysis and x-ray diffraction were used to study the influence of the O incorporation on the film composition and structure and relate them to mechanical properties, which were evaluated by nanoindentation. As x in BOx was increased from 0.08 to 0.18, the elastic modulus of the x-ray amorphous films decreased from 273 to 231 GPa, by 15%. This can be understood using classical molecular dynamics (MD) with a Buckingham-like interaction potential: The increase in the O concentration and corresponding formation of B–O bonds, shown to be longer than the B–B bonds, resulted in larger ionic contributions as well as a density reduction. This increased ionicity was responsible for the observed decrease in elastic modulus. As even more O was incorporated (x>0.18), the H concentration increased, exceeding 0.3 at. %. This may cause the formation of boric acid...

Cluster diffusion and surface morphological transitions on Pt (111) via reptation and concerted motion

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 ...

Effects of incident N atom kinetic energy on TiN/TiN(001) film growth dynamics: A molecular dynamics investigation

Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K, a temperature within the optimal range for epitaxial TiN growth, with an incident N-to-Ti flux ratio of four, are carried out using incident N energies EN = 2 and 10 eV and incident Ti energy ETi = 2 eV. To further highlight the effect of EN, we grow a bilayer film with EN = 2 eV initially and then switch to EN = 10 eV. As-deposited layers are analyzed as a function of composition, island-size distribution, island-edge orientation, and vacancy formation. Results show that growth with EN = 2 eV results in films that are globally overstoichiometric with islands bounded by N-terminated polar 110 edges, whereas films grown with EN = 10 eV are flatter and closer to stoichiometric. However, EN = 10 eV layers exhibit local N deficiency leading to the formation of isolated 111-oriented islands. Films grown by changing the incident energy from 2 to 10 eV during growth are more compact than those grown entirel...