Timur Halicioglu

Stress calculations for carbon nanotubes

Abstract Atomic stresses were calculated for carbon nanotubes under strain conditions. Graphitic tubules with radii ranging from approximately 2 to 11Aand two different tubule structures with varying atomic orientations were included in the calculations. Elongations and contractions were applied in the axial direction and atomic stress values were calculated for infinitely long tubules. The calculations were carried out using Brenners function which was developed for carbon species. Results indicate that the stress is tensile in the radial direction while it is compressive in the tangential direction. Variations in stress values in the direction of the cylindrical axis were investigated as a function of applied strain. Furthermore, using the stress-strain curve (calculated based on atomic considerations), the values of Youngs modulus and Poissons ratio for nanotubules were also estimated.

Structures of microclusters: An atomistic approach with three-body interactions

Abstract The structure and stability of small clusters (with 8, 9 and 13 particles) were investigated by minimizing the total energy which comprised two-body and three-body interactions. Variation in the energetically preferred geometry of the cluster is analyzed parametrically as a function of the intensity of the three-body interactions. The results are consistent with those of other atomistic calculations for rare gas atoms and with some of the ab initio calculations for metallic clusters.

Mechanism for the self-diffusion of Au and Ir adatoms on Pt(110) surface

Abstract A molecular dynamics computer technique was used to simulate the diffusion of a Au and and Ir adatom on the Pt(110) surface. Details of the exchange mechanism associated with cross-channel diffusion were observed. For the Au adatom at the low temperature, the expected channel diffusion occurred, whereas at the higher temperature a temporary exchange of the Pt wall atom by the Au adatom was seen. In the Ir case at the low temperature no exchange was observed, although there was a partial displacement of the wall atom by the adatom. At the higher temperature, the exchange mechanism was observed. The results are consistent with experimental observations and also indicate the importance of the stability of the channel wall atoms (via their thermal motion) on the occurrence of the exchange mechanism.

Effect of three‐body interactions on the structure of small clusters

Minimum energy configurations of microclusters (up to six atoms) have been calculated using two‐ and three‐body interactions. Structural changes were parametrically analyzed as a function of the intensity of three‐body forces. The results are qualitative in nature; they indicate, however, that three‐body interactions play an important role in the equilibrium structure of microclusters. The effect of the intensity of the three‐body interactions on the structure of small clusters is not manifested in a continuous manner. Rather, changes in the energetically most stable structure occur abruptly. The results are in qualitative agreement with experimental observations as well as other calculations.

A calculation of the diffusion energies for adatoms on surfaces of F.C.C. metals

Abstract The activation energies for diffusion were determined for gold, platinum and iridium adatoms on (110) and (311) Pt surfaces and were found to be in good agreement with the measurements reported by Bassett and Webber. The Lennard-Jones pair potentials were used to model the interatomic forces, and relaxation of the substrate atoms in near proximity to the adatom was considered in detail. The present calculations clarify the mechanism of the observed two-dimensional diffusion of platinum and iridium atoms on a (110) Pt surface. The results are compared with those obtained using Morse potential functions and different relaxation techniques.

An atomistic calculation of two-dimensional diffusion of a Pt adatom on a Pt(110) surface

Results of an atomistic calculation supporting two-dimensional diffusion of a Pt atom on a Pt(110) surface are presented. The calculations for diffusion energies indicate that interchannel diffusion by the replacement of a channel-wall atom by the adatom is energetically favorable.

Fluorination of Si(001)-2×1 surface near step edges: A mechanism for surface defect induced etching

Fluorination of a dimerized Si(001)-2×1 surface near single-layer high step edges has been studied with molecular statics and dynamics simulations using the Stillinger–Weber potential for Si–Si, Si–F, and F–F interactions. Binding energies for up to three F atoms on Si atoms at the step edges have been investigated for fully relaxed systems. We find that F-bonded Si atoms relax away from the surface with increasing fluorination which lowers the desorption/etching energies of SiFx(x=1,3) species. Room temperature molecular dynamics simulations then show direct etching of SiF3 from Si atoms bonded to step edges. The calculations predict an anisotropy in the step edge induced etching of the Si(001)-2×1 surface. This is in qualitative agreement with an experimental observation on the same surface though using Cl instead of F atoms.

Energetics for bonding and detachment steps in etching of Si by Cl

Simulation calculations were carried out to investigate the energetics of adsorption and desorption processes taking place during etching of the Si(100) surface by Cl atoms. Clean and fully chlorinated surfaces with step edges were taken into consideration. Attachment energies of Cl atoms at step edge sites and subsequent detachment (etching) energies of SiClx species were calculated. Configurational characteristics of Cl bonded regions were also investigated. Results provide a systematic description of Cl adsorption and etching processes.

Analysis of the cross-channel diffusion mechanism

Abstract A molecular dynamics simulation was used to determine the effect of temperature and adatom size and energy parameters on the occurrence of the exchange mechanism in cross-channel diffusion on the Pt( 110) surface. Results showed that the exchange mechanism is highly dependent on temperature, occurring readily at sufficiently high temperatures, and is dependent upon adatom size in the case of a weakly interacting adatom, but, within the limits of the study, is not so in the case of a strongly interacting adatom.

Cluster adsorption on amorphous and crystalline surfaces - A molecular dynamics study of model Pt on Cu and model Pd on Pt

Molecular dynamics was used to study the structure, dispersion and short‐time behavior of ten‐atom clusters adsorbed onto amorphous and crystalline substrates, in which the cluster atoms differed from the substrate atoms. Two adatom–substrate model systems were chosen; one, in which the interaction energy between adatom pairs was greater than that between substrate pairs, and the other, in which the reverse was true. At relatively low temperature ranges, increased dispersion of cluster atoms occurred: (a) on the amorphous substrate as compared to the FCC(100) surface, (b) with increasing reduced temperature, and (c) with adatom–substrate interaction energy stronger than adatom–adatom interaction. Two dimensional clusters (rafts) on the FCC(100) surface displayed migration of edge atoms only, indicating a mechanism for the cluster rotation and shape changes found in experimental studies.