Number Of Adatoms Research Articles

Insight into the graphitization mechanism of the interface between iron and diamond: A DFT study

It is an important topic in the manufacturing research field to understand the mechanisms for the wear of diamond surfaces. The density functional theory (DFT) methods have served as useful tools for studying chemical reaction mechanisms in processing. In this work, we studied the changes in the diamond structure during the interface contact between the iron and diamond. The iron-diamond contact area is simplified to different coverage rates of Fe atom, corresponding to different numbers of adatom. With the different numbers of the Fe adatoms, the interlayer CC bonds have different changes. Below the Fe adatom region, the interlayer CC bonds are strengthened, inhibiting the graphitization. It is noteworthy that the CC bonds became significantly weakened or even broken at the boundary of the interface contact. Moreover, the interface sliding simulation verifies that the graphitization preferentially occurs at the edge of the interface between iron and diamond. The actual cause of graphitization is the synergistic effect of the charge transfer of the FeC bonds and the asymmetric contraction and stretching on the diamond surface. Our work provides a theoretical basis for suppressing excessive diamond tool wear and polishing the diamond surfaces.

Connection of Kinetic Monte Carlo Model for Surfaces to One-Step Flow Theory in 1+1 Dimensions

The Burton--Cabrera--Frank (BCF) theory of step flow has been recognized as a valuable tool for describing nanoscale evolution of crystal surfaces. We formally derive a single-step BCF-type model from an atomistic, kinetic Monte Carlo (KMC) model of a crystal surface in 1+1 dimensions without external material deposition. Through an averaging procedure, consistent with Boltzmann statistics, we introduce definitions of the continuous quantities of the BCF theory, i.e., the step position and density of adsorbed atoms (adatoms), as expectation values taken over the discrete probabilities of the KMC model. The equations of our BCF-type model describe the time evolution of these expectation values. A central idea of our approach is to exploit the observation that the number of adatoms on a surface is typically small at experimentally relevant temperatures. Accordingly, we (i) show that our BCF-type theory arises from a KMC model in which only one adatom is allowed to move, and (ii) characterize corrections to ...

Coverage temperature phase diagram of LEED spot and arced streak intensities for Li on Cu(0 0 1) surface using lattice gas model

It has been established that the arced streaks connecting four spots observed in LEED for a Li system adsorbed on a Cu(0 0 1) surface originate from the Bragg reflection from parallel adatomic lines on a c(2 × 2) lattice site. For example, one streak at about k y = π/ a originates from the parallel atomic lines including two atoms separated at a distance of d y = 2 a, which is the second-neighbor distance in a c(2 × 2) lattice. The c(2 × 2) structure sites consist of two sublattices with y = 2 na and y = (2 n + 1) a. Here, the difference in the number of adatoms on the two sublattices is the cause of the intensity of the midpoint of the streak, where the differences depend on the coverage of adatoms, Θ. In this study, using a lattice gas model on the substrate lattice with Monte Carlo simulation, we obtain the coverage and temperature dependence of intensities of the spots for the c(2 × 2) structure and the streaks. We found that the intensity of the streaks increase and decrease within the coverage range 0 < Θ < 0.5. That of the spots increases monotonically in this coverage range. These theoretical findings are similar to the experimental results. On the other hand, as temperature is increased, the intensity of the streaks increases and becomes saturated. We found a similar phenomenon using analytical calculation by statistical mechanics. In addition, the intensity of the spots decreased with the second-order transition.

Arsenic dependence on the morphology of ultrathin GaAs layers on In0.53Ga0.47As∕InP(001)

Thin GaAs films grown on lattice-matched In0.53Ga0.47As∕InP(001) have been imaged by in vacuo scanning tunneling microscopy. We observe that the morphological evolution of these strained films depends on the deposition and diffusion of group III adatoms and the incorporation of As from the vapor, in addition to Asaro–Tiller–Grinfeld instabilities. We compare step edge densities to a model for the Ga adatom density and find that the absolute magnitude of the group V and group III fluxes has a strong effect on the number of adatoms on the surface and, thus, the morphology of the films.

On absorption on polycrystalline substrates

The effect of a polycrystalline substrate on the charge of adatoms and on the work function is considered in terms of the Anderson-Newns model. Polycrystallinity is characterized by deviation δϕi of the work function of the ith face from mean value 〈ϕi〉. It is found that polycrystallinity leads to an increase in the occupation numbers of adatoms as compared to absorption on a monocrystalline surface. This leads to a decrease in the value of charge in the case of electropositive adsorption and to an increase in its value in the case of electronegative adsorption. It is shown that the polycrystallinity effect must be manifested most strongly in the case of adsorption of gases at metals and semiconductors.

Dislocation Injection, Reconstruction, and Atomic Transport on {001} Au Terraces

High-resolution electron microscopy investigations of Au films show that adatoms on (100) surfaces insert into the underlying terrace to form surface dislocations. This injection readily occurs when the number of adatoms on a terrace is approximately 20 atoms or less. The surface dislocation glides along the terrace, but is repelled from the edges. The dislocation escapes by squeezing out in the dislocation line direction (not gliding out the terrace edge). Atomistic simulations confirm the dislocation stability, easy glide along the terrace and trapping at the terrace edge. These results have profound implications for film growth.

An “inverse” growth of Ag(1 1 1) on Cu(0 0 1) obtained by superficial segregation

We report the existence of layering transitions induced by superficial segregation in Cu(Ag)(0 0 1). Moreover, Monte Carlo (MC) simulations in conjunction with a N-body potential show that these chemical transitions are coupled with structural ones. Thus, the first three surface planes transform successively from almost pure Cu planes with a (1×1) square structure to almost pure Ag planes with a pseudo-hexagonal c(10×2) superstructure. We find a large hysteresis effect for the transitions affecting the surface underlayers, partly due to the difficulty to re-incorporate a given number of adatoms during the reverse transition c(10×2)→(1×1). A matching of the MC results on a lattice gas model allows us to discuss the relative influence of the chemical effect and of the structural one on the layering transitions.

Stability and diffusion of surface clusters

Using kinetic Monte Carlo simulations and a bond-counting ansatz, thermal stability and diffusion of an adatom island on a crystal surface are studied. At low temperatures, the diffusion constant $D$ is found to decrease for a wide range of island sizes like $D \propto N^{-\alpha}$, where $\alpha$ is close to one, $N$ being the number of adatoms in the cluster. By heating up the surface, the system undergoes a phase transition above which the island disappears. Characteristics of that transition are discussed.

Self-organized network structure appearing in the B/Si(111)-( &lt; ![IGNORE[$\sqrt{3}$]]× &lt; ![IGNORE[$\sqrt{3}$]])R30° phase formation process studied by scanning tunneling microscopy

× )R30° structural formation by HBO2 molecular irradiation onto a Si(111)-7×7 surface at ∼750 °C, B atoms interacted with each other to form chains from the beginning, and a ( × )R30° structure consisting of Si and B atoms was formed. The observed result is completely different from the ( × )R30° structural processes induced by other group III atoms; in general, when metals such as Al are deposited on a Si(111)-7×7 surface, large 7×7 and ( × )R30° structure domains exist separately. With an increase in the amount of the B component to about half the number of adatoms, B atoms formed a network with the same ( × )R30° phase, resulting in the formation of an ordered ( × )R30° structure. Off-phase disordered areas remaining on the surface were limited to the areas occupied by Si atoms. The obtained results indicate that the interaction caused by B atoms plays an essential role in the formation process of the ordered ( × )R30° structure of this surface.

Two step simulation of low energy ion beam mixing at different temperatures

Two step simulation of low energy ion beam mixing based on combined use of the Molecular Dynamics technique and mixing equations have been performed for 100 eV Ar bombardment of a Cu (100) crystal at temperatures of 0 K, 300 K and 500 K. The influence of temperature results in an increased sputtering yield at 300 K and 500 K as compared to 0 K. The change of the sputtering yield from 300 K to 500 K is not significant. Production of stable vacancies, which occur in the vicinity of the sputtered surface, and the number of ad-atoms at the surface increase with temperature. Contrary to this the number of stable interstitials in the depth of the crystal decreases with temperature. The general mixing effect, which has been estimated as the mean square displacement of atoms in the collision cascade increases with ambient temperature, but the mixing distortion of an initial pseudo-marker in the 17th atom layer during sputter removal decreases with temperature, due to the increase of the sputtering yield with temperature and due to a reduction of the effective mixing range at 500 K as compared to 0 K and 300 K.

Ion-beam induced generation of Cu adatoms on Cu(100)

Low-energy ion scattering was used to study on-beam induced adatom generation during irradiation of a Cu(100) surface with 6 keV Ne ions at a sample temperature of 60 K. It was found that the number of adatoms produced per incoming ion decreases from an average of 3.5 to a saturation level of 1.8 after prolonged irradiation. Two mechanisms are believed to contribute to the adatom generation: ion-beam induced atom-replacement sequences and diffusion of self-interstitial atoms to the surface. From the adatom production as a function of irradiation dose the relative contributions of the two mechanisms were determined.

Surface diffusion length under kinetic growth conditions.

We obtain, using both a direct numerical simulation and a kinetic scaling argument, the atomistic surface diffusion length under kinetic growth conditions such as molecular beam epitaxy. The diffusion length shows strong crossover effects arising from two different physical mechanisms: The number of adatoms (two or three) in the stable islands which define incorporation, and the ratio of the atomistic deposition rate (high or low) to the atomistic hopping rate at the growth front. We give detailed results for the functional dependence of the diffusion length on the growth rate.

Energetics and Structures of Pt, Pd, and Ni Adatom Clusters on the Pt (001) Surface

ABSTRACTUsing the embedded atom method (EAM), we have calculated the stable configurations of Pt, Pd, and Ni clusters, containing up to nine adatoms, on the Pt (001) surface. For Pt, we predict that the stable configurations are linear chains oriented along the &lt;110&gt; directions for three and five adatoms, and close-packed islands otherwise. These results, for clusters containing up to seven adatoms, were reported previously in reference 4. For Pd, the results are the same except that the stable configuration predicted for five adatoms is a close-packed island. For Ni, we predict that linear chains are the stable configurations for all numbers of adatoms. In determining these stable configurations, we allowed substrate relaxations. To assess the importance of substrate relaxations, we also performed calculations on an unrelaxed substrate. We found that substrate relaxations are important with regard to the stable configurations, and that allowing relaxation lowers the energies of chains with respect to islands.

Structure of platinum adatom clusters on Pt(100): Experimental observations and embedded-atom-method calculations.

The structure of Pt adatom clusters on Pt(100) is observed to oscillate between chain- and island-type configurations as the number of adatoms is increased from three through six. Embedded-atom-method calculations are found to predict these results and show that lattice relaxations are a critical factor in causing this unusual series of structural transformations.

Coverage dependence of Schottky barrier formation

When the metal coverage at the surface of a semiconductor is sufficiently small so that there are isolated metal adatoms and therefore localized states, there is a distinction between the energy (0,+) to remove an electron from a neutral adatom and the energy (−,0) to add an electron to a neutral adatom. The acceptor level (−,0) is higher in energy than the donor level (0,+) by an energy U*, just as in the free atom, but here U* is modified by the presence of the surface. For these isolated metal adatoms, the donor level (0,+) frequently lies in the semiconductor gap with the acceptor level (−,0) coming in the conduction band. This circumstance leads to an asymmetry between the low-coverage band bending for n-type semiconductors versus that for p-type. As the number of adatoms is increased, the (0,+) and (−,0) levels shift relative to the semiconductor bands at the surface. These shifts can be understood in terms of the polar bonds between the metal adatoms and the semiconductor surface atoms. At sufficiently large metal coverages the wavefunctions of adjacent metal adatoms overlap and the discrete levels broaden into bands. Eventually the donor and acceptor bands overlap and the asymmetry between n-type and p-type band bending disappears. This picture is modified if the metal adatoms are clustered rather than uniformly distributed.

Direct observation of overlayer structures on W(110)

Observations in the field ion microscope have resolved the individual atoms in two-dimensional overlayers of platinum, palladium, iridium and silicon on W(110). All these layers, but most notably silicon, have quite an open arrangement of atoms. With iridium, transformations in the structure of the layer occurs as the number of adatoms is changed.

Direct observation of overlayer structures on W(110)

Observations in the field ion microscope have resolved the individual atoms in two-dimensional overlayers of platinum, palladium, iridium and silicon on W(110). All these layers, but most notably silicon, have quite an open arrangement of atoms. With iridium, transformations in the structure of the layer occurs as the number of adatoms is changed.

Thermal disorder of adatoms in a one-dimensional model

As a model for disordered adatoms, the author uses a lattice gas in thermal equilibrium, with short-range repulsions between atoms. Fraunhofer diffraction from this model gives a calculated diffraction pattern, whose dependence on temperature is comparable to that found by Germer, May, and Szostak in their experiment with O on Ni(110). Also, the calculation is used to discuss the splitting (of the 1 2 -order diffracted beam) which is caused by a difference between the numbers of adatoms and of vacant sites.