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We investigate the deposition of SiO x films by dual ion beam sputtering system with emphasis on secondary ion beam irradiation effect. We observe the intense photoluminescence (PL) from SiO x samples with apparent oxygen contents, x app , in the narrow range of 1.1≤ x app ≤1.4, after post-deposition annealing at 1100 °C. This indicates the formation of Si nanocrystals as evidenced by cross-sectional transmission electron microscope (TEM) image. Ar ion beam irradiation changes PL peak positions in ion-beam exposed region. This effect is utilized for achieving the area-selective formation of Si nanocrytals by inserting a shadow mask in secondary ion beam during deposition.

We investigate the charge retention characteristics in a metal–insulator–semiconductor capacitor containing Ge nanocrystals (~3 nm in diameter) using capacitance–voltage measurements with various gate bias sweep rates. The metal–insulator–semiconductor capacitor is fabricated by pulsed laser deposition and rapid thermal oxidation. The capacitance–voltage curve shows the strong hysteresis indicating charging/discharging of carriers during the gate bias sweeping. The stored charge densities, evaluated from the flat band voltage shifts, show the non-dispersive carrier relaxation, which is a characteristic property for nanocrystals arranged in a layer. In addition, we observe an interesting capacitance ledge during forward bias sweeping. From the trap distribution determination by the Terman method, the capacitance ledge is due to an energetically localized trap at 0.33 eV + Ev. The energetic position of the trap is in agreement with the ground-state hole energy level in the Ge nanocrystals. This suggests the possibility that the trap level is related to the fact that the stored charges initially tunnel out at the ground-state hole energy level.

An amorphous Si layer prepared by ion-beam-assisted electron beam deposition (IBAED) method is oxidized by a rapid thermal oxidation technique. We observe a nanocrystal band located at about 4 nm from the Si/SiOx interface by cross-sectional transmission electron microscope observation. The metal–oxide–semiconductor (MOS) structure employing the oxide layer with the nanocrystal band exhibits a large capacitance–voltage hysteresis indicative of trapping of electrons/holes. In contrast, a relatively small capacitance–voltage hysteresis is found for the MOS diode prepared by conventional electron beam deposition (EBD) without ion-beam assistance. Such a marked difference shows that the ion-beam irradiation plays an important role in the formation of nuclei, which would grow to nanocrystals during subsequent rapid thermal oxidation process. Interestingly, the MOS prepared by IBAED shows a characteristic capacitance transient behavior, indicative of non-dispersive carrier relaxation. In addition, the charge retention times shows a bias dependence and a maximum of 72 s near the mid-gap voltage. Such a bias-dependent retention time is interpreted in terms of the tunneling of trapped charges in nanocrystals through empty interface states. © 2001 Elsevier Science B.V. All rights reserved.

First-principles pseudopotential calculations were carried out to investigate atomic and electronic structures of small GaAs clusters containing even numbers of atoms. The most stable atomic structure of the GaAs cluster significantly depends on its size and composition. The structure becomes substantially distorted as the cluster size increases. The binding energy per atom increases as the size of cluster increases. As-rich clusters have larger binding energies than Ga-rich ones. A distribution of small GaAs clusters obtained in laser vaporization experiments is well explained via inequalities between binding energies. Ionization energies and electron affinities of the stoichiometric Ga n As n ( n =2–6) clusters were evaluated.

The energetics and structure of vacancies in the second layer of GaAs(110) were examined using the ab initio total energy calculation method. Structural changes due to the presence of the vacancies are substantial. The changes are different in shape, depending on the type of vacancy and charged states. Both the Ga vacancy and the As vacancy in the second layer of the surface favor negatively charged states.

and SB steps induces a strong buckling along the rebonded dimer row in the lower terrace. On the upper terrace of the SB step, the kink induces a local p(2×2) structure directed toward the center of the step. The strains of second-layer atoms supporting the dimers are responsible for the reconstruction near the monatomic steps on the Si(001) surface.

We study the mechanism of the As-mediated epitaxial growths of Ge and Si on Si(100) surfaces through first-principles pseudopotential calculations. On an As-monolayer-covered Si(100) surface, individual Ge or Si adatoms are found to incorporate rapidly into subsurface As sites with minimum surface diffusion. The segregation of As is initiated by the substitutional adsorptions of individual Ge or Si adatoms. Because of the rapid adatom incorporation, adatom mobility is drastically reduced, compared with the growth without surfactants, resulting in a high density of two-dimensional islands. The inclusion of gradient corrections to the LDA energies does not change our results.

The structure of a clean Si(100) and a Ni-contaminated si(100) was investigated using scanning tunneling microscopy. The clean Si (100) shows the 2 × 1 reconstruction with a surface dimer vacancy density less than 2%. The major defects on the clean surface are a single dimer vacancy and the C defect. A small amount of Ni on the surface drastically changes the surface structure and produces 2 × n reconstructions. The formation of vacancy clusters is favored. A rebonded SB step is preferred on the clean Si(100) while a nonrebonded SB step with a split-off dimer is mainly observed on the Ni-contaminated Si(100) and in the vicinity of dimer vacancies of the lower terrace on the clean Si(100).

We investigate the atomic and electronic structure of Li-adsorbed Si(100) surfaces through first-principles pseudopotential calculations. We find that Li adatoms interact mainly with the dangling-bond orbitals of Si dimers, however, no hydrogenlike directional bonds exist between the Li and substrate Si atoms, based on calculated scanning tunneling microscopy images. The analysis of charge densities demonstrates a large charge transfer from the Li adatom to a dangling-bond orbital of a Si dimer, which is responsible for a large decrease of work function at submonolayer coverages. The Li atoms form linear chains either by occupying the interdimer bridge sites along the dimer row direction or the dimer bridge and cave sites perpendicular to the dimer row. As Li coverage (\ensuremath{\Theta}) increases, repulsive interactions between the adatoms in the chains are also found to increase. Thus, at 1-ML Li coverage, the stable adsorptions sites are changed to either the cave and pedestal sites or the valley-bridge and pedestal sites, with all the Si dimers symmetric, resulting in a $2\ifmmode\times\else\texttimes\fi{}1$ structure. From the calculated formation energies, we find a $1\ifmmode\times\else\texttimes\fi{}1$ phase to be thermodynamically stable at $\ensuremath{\Theta}=2,$ in contrast to other alkali-metal adsorptions.

We have investigated Sb overlayer structures on Si(111) with low energy electron diffraction and scanning tunneling microscope (STM). The Sb/Si(111)-53×53 structure was constructed via the desorption of Sb from the saturated 2×1 surface at elevated temperature (700–750 °C). Its atomic structure and formation process were extensively studied with STM images, considering the structural stability. This Sb structure could be described in terms of the site selective replacement of outermost Si atoms with Sb atoms in the dimer-adatom-stacking fault 5×5 structure, which is generated by both the saturation of dangling bonds and strain due to the large lattice mismatch.

The atomic structure at monatomic-height ${\mathit{S}}_{\mathit{B}}$ steps on clean Si(001) and a Ni-contaminated Si(001) was investigated using high-resolution scanning tunneling microscopy at room temperature. Rebonded ${\mathit{S}}_{\mathit{B}}$ steps are dominant on clean Si(001). Nonrebonded ${\mathit{S}}_{\mathit{B}}$ steps with split-off dimers are favored on the Ni-contaminated Si(001) and in the vicinity of dimer vacancies on clean Si(001). The nonrebonded step with the split-off dimer is generated by the strain due to dimer vacancies. A buckled dimer was observed in the vicinity of a kink of ${\mathit{S}}_{\mathit{B}}$ step edges on clean Si(001). \textcopyright{} 1996 The American Physical Society.

Temperature-dependent evolution of surface corrugation and the interface dislocation in In 0.15 Ga 0.85 As epilayer on GaAs(100) substrate grown by chemical beam epitaxy using unprecracked monoethylarsine have been investigated by atomic force microscope (AFM) and transmission electron microscopy (TEM). AFM images showed that the line direction of surface ridge changes from [011] to [011] with increasing temperature. However, TEM micrographs showed that dislocation networks are formed along both [011] and [011] directions at the interface. These results indicate that growth kinetics on the terrace and at surface steps generated by the dislocations play an important role in determining the direction of surface corrugation. We suggest that the temperature-dependent change of surface corrugation is caused by an anisotropic surface diffusion on the terrace and different sticking probability of adsorbates on the surface steps which were produced by interface misfit dislocation along the two orthogonal surface directions.

First-principles pseudopotential calculations were carried out to evaluate barrier heights during the process of fragmentation of the perfect C60 into the perfect C58 and the C2. Three important paths were taken into account: (i) the perfect C60 → a defect C58 + C2 → the perfect C58 + C2 (removal of single-bonded two carbon atoms), (ii) the perfect C60 → a defect C58 + C2 → a defect C58 + C2 → the perfect C58 + C2 (removal of double-bonded two carbon atoms), and (iii) the perfect C60 → a defect C60 → the perfect C58 + C2. The third process is energetically favored.

We investigate the effect of Ar-ion-beam irradiation during the deposition of SiOx films by dual-ion-beam deposition system. Ion-beam irradiation effectively increases the oxygen content, x, in SiOx films indicative of the preferential sputtering of Si phase as compared to SiO2 phase in SiOx films. We observe the intense photoluminescence from nonirradiated sample after postdeposition annealing at 1100°C indicating the formation of Si nanocrystals as shown by a cross-sectional transmission electron microscope. However, the increased oxygen content in ion-beam-irradiated sample results in small optical volume of small Si nanocrystals not sufficient for yielding appreciable photoluminescence intensity after postdeposition annealing. The property is utilized for achieving the area-selective formation of Si nanocrytals by inserting a shadow mask in assist ion beam during deposition.

We observe a completely different growth regime of silicon-rich oxide (SRO) layers by rapid thermal chemical vapor deposition for the formation of luminescent nanocrystals. The growth regime is characterized by low [N2O]/[SiH4] ratios ( 700 °C). High-resolution cross-sectional transmission electron microscopy (XTEM) shows the bimodal distribution of large polycrystals and nanocrystals after post-deposition annealing. The luminescence is attributed to the nanocrystals. Fourier transform infrared spectroscopy in conjunction with XTEM and energy-dispersive x-ray studies show the phase separation and bonding reconfiguration in as-deposited SRO layers. The effectively increased oxygen content in the oxide matrix by phase separation and bonding reconfiguration reduces the diffusion coefficient of Si in the matrix, resulting in the formation of nanocrystals during post-deposition annealing.

The rapid-thermal-annealing effect on lateral charge loss in metal–oxide–semiconductor capacitors with Ge nanocrystals is investigated by means of capacitance-voltage (C–V) and capacitance decay measurements. The C–V curves show the hysteresis indicating the charge storage effect in Ge nanocrystals. The hysteresis width shows strong annealing temperature dependence and shows the maximum at 700 °C meaning the maximum nanocrystal density. Capacitance decay experiment at flat-band voltage shows that the decay is dominated by two decay mechanisms. The initial fast discharge is more significant for samples annealed at lower temperatures. The cross-sectional transmission electron microscopic observations show the quasi-continuous Ge layer with Ge nanocrystals and Ge-rich amorphous regions for samples annealed at lower temperatures. Therefore, the fast discharging is attributed to lateral charge loss of insufficiently localized nanocrystals. On the other hand, the slow discharge is attributed to tunneling out of...

We report results of ab initio calculations on structural properties of 20-atom carbon clusters including the ring, the corrannulene and the dodecahedron. The corrannulene and the ring are energetically less stable than the dodecahedron for both the neutral and the singly-charged states. The clusters have well-defined ionization potentials and electron affinities. The structural transformation of these clusters can occur through a series of “concerted exchange” and bondbreaking (or bond formation). The barrier heights for the transformation and the structures at the saddle points are investigated at the initial stage of transformation from the corrannulene to the other isomers. The barrier heights even at the initial stage are found to be substantial. The results explain the abundance of the ring structure in experiments.

Scanning tunneling microscope (STM) images of natural MoS2 revealed protrusions with a diameter of 30–40 A and ring-type structures with two different diameters of 40 and 80 A on the surface. The STM images were taken varying the tip bias voltage from −0.04 to −1.1 eV. Ring-type structures existed in the whole range of tip bias voltages used, but the ring intensity which was subtracted by the background drastically decreased at tip bias voltages beyond −0.9 eV. Atomic force microscope (AFM) images of the same MoS2 sample did not show such structures. In order to understand the origin of the ring-type structures observed on the natural MoS2 surface, the impurities in the sample were analyzed by using glow discharge mass spectrometry (GDMS). The concentrations of the major impurities such as V and Ti were comparable to the average number density of ring structures on the sample. STM and AFM images suggest that the ring-type structures appear as an electronic effect due to the sinuous band structure caused by two kinds of band bending near the impurities included in the MoS2 sample.

We describe the results of extensive quantum molecular dynamics calculations of the properties of fullerenes and microtubules. The topics to be discussed include: (i) stability of C 60 isomers and barriers to isomerization; (ii) reactivity of C 60 and C 58 with C 2 and C 3 , and its implications on the formation and growth of fullerenes; and (iii) atomic and electronic structure and doping of semiconducting microtubules. We also discuss the structures, stabilities and atomic transformations of large multishell fullerenes and offer an explanation for the formation of spheroidal “onions” under high fluence electron irradiation conditions. The last results, which involved calculations for up to ∼ 15 000 atoms, were obtained using classical three-body potentials.

The energetics and reactions of fullerenes have been investigated by ab initio molecular dynamics. ${\mathrm{C}}_{60}$ and other fullerenes of similar sizes are reactive towards ${\mathrm{C}}_{2}$, which can insert into the fullerene cage without an activation barrier. The computed stability and reactivity patterns suggest that the increase of abundance of ${\mathrm{C}}_{60}$ and other locally stable fullerenes upon high-temperature annealing is due to atom and fragment exchange with other carbon clusters.

We describe the results of extensiveab initio molecular dynamics calculations of the properties of fullerenes and microtubules. Our finite temperature quantum MD simulations for solid C60 are in excellent agreement with NMR, photoemission and neutron scattering data. The C60 isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C60, but the transformation between these two structures is hindered by a 5.4 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C20 isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C60. The corannulene structure is a natural precursor for the formation of C60. Simulations of reactions show that C2 can insert into C58, perfect C60, and defect C60 fullerenes without an activation barrier, while C3 attaches only to their surfaces. Evaporative fragmentation of carbon clusters during annealing is unlikely, but atom and fragment exchange during collision favor "locally" most stable structures, such as C60. These results may explain the large increase in the abundance of C60 and C70 when carbon clusters are annealed at high density. We have also carried out calculations for paradigmatic microtubules, both reflection-symmetric and chiral. We find that the optimized geometries of the tubules are close to the ideal ones. It is possible to fabricate tubules with direct band gaps away from the Γ point by exploiting the similarities between the projected band structure of graphite and that of the tubule. The semiconducting tubules can be doped n- and p-type by substitutional N and B, respectively.

Amorphous silicon films are deposited by ion-beam-assisted electron beam deposition and subsequently oxidized by a rapid thermal oxidation process. The oxidized film contains a large density of nanocrystals specifically localized at a certain depth from the Si/SiOx interface, whereas no evidence of nanocrystals is found for oxidized films deposited without ion beam assistance. Such a marked contrast resulted from the enhancement of nucleation rate by ion beam irradiation. The metal-oxide-semiconductor structure utilizing the film shows an ultralarge capacitance‐voltage hysteresis whose width is over 20 V. In addition capacitance‐time measurement shows a characteristic capacitance transient indicating nondispersive carrier relaxation. The retention time shows a dependence on applied bias and the maximum time of ;70 s is obtained near midgap voltage. The retention time dependence on applied bias and large capacitance‐voltage hysteresis are attributed to direct tunneling of trapped charges in the deep traps of nanocrystals to the interface states. © 2001 American Institute of Physics. @DOI: 10.1063/1.1337618#

Owing to negative electron affinity (NEA), diamond has emerged as one of the most important materials for a low threshold cold cathode in vacuum microelectronics and in flat-panel display technology [1]. N-type doping of diamond is an important problem to be solved for effective emission of electrons. Nitrogen is one of common impurities in both natural and synthetic diamond. A recent report showed the possibility of nitrogen for n-type doping in the diamond matrix [2]. Substitutional nitrogen produces a deep-lying donor level below the conduction band minimum. The activation energy is roughly 1.7 eV. Therefore, the level is too deep to be ionized at room temperature. A dramatic improvement of field emission characteristics has been reported for a heavily nitrogen-doped diamond [3]. Urea [(NH3)2CO], N2 and melamine [C3H6N6] have been utilized as nitrogen doping sources [4–6]. Specifically, Okano et al. achieved the lowest turn-on field of ∼0.5 V/μm with nitrogen-doped diamond films by employing urea as a doping source [7]. However, a significant degradation of diamond quality has been observed during the nitrogen incorporation [5, 8]. In particular, the nitrogen incorporation substantially affects the surface morphology and thereby results in a spiked surface morphology with very low growth rate. This result means that there is a trade-off between surface morphology and doping density. The trade-off has been a technical hurdle in achieving heavy nitrogen doping while maintaining a reasonable surface morphology. In this paper, we introduce a modulated growth technique to overcome the hurdle. A home-built hot filament chemical vapor deposition (HFCVD) system was used for deposition of diamond films. Twisted W wires (0.45 mm diameter) were used for a filament. Prior to deposition, the filament was carburized at least for 7 h while monitoring the resistivity of W wires. The filament temperature was measured by an optical pyrometer. The separation between the substrate (n-type Si(111), 5× 5 mm2) and filament was 6.5 mm. The substrate surface was scratched with 1 μm diamond suspension. After dipping into a diluted

The results of recent ab initio molecular dynamics studies of C and Al clusters are presented. The simulations have shown that C 60 molecular structure is well preserved in the solid and that the individual C 60 molecules start to rotate at relatively low temperatures. Our results are in very good agreement with NMR, photoemission, and neutron scattering data. At high temperatures C 60 undergoes large amplitude soccerball-rugbyball oscillations, but the cage structure is still preserved. The C 60 isomer containing two pairs of adjacent pentagons has a binding energy only 1.6 eV smaller than that of perfect C 60, but high temperatures and long annealing times are required for the transformation between these two structures. Its activation energy is 5.4 eV. We have also studied the various isomers of C 20, since it could form the smallest possible fullerene. At T=0 , the lowest energy isomer is indeed a dodecohedral structure. However, high temperatures favor the corannulene structure, which is a perfect precursor for the formation of C 60. These results are consistent with the experimental data, since high temperatures are needed for efficient formation of C 60. The atomic and electronic structure and doping properties of semiconducting microtubules have also been investigated. The distortions from the ideal geometries are small in microtubules. Substitutional N and B are effective shallow donors and acceptors, respectively. For Al clusters, we focused on Al 13 and Al 55, which can assume perfect icosahedral and cubic structures. However, the distortions from these ideal structures are substantial. For Al 55, several inequivalent but nearly energetically degenerate structures are found. It is shown that this is due to the short range of the screened interatomic interactions.

First-principles pseudopotential calculations were performed to investigate the atomic structure, structural transformation, and reactivity of ${\mathrm{Ga}}_{13}$ clusters. ${\mathrm{Ga}}_{13}$ energetically favors a distorted decahedron. The decahedron is more stable than a relaxed icosahedron and a relaxed cuboctahedron by 0.22 and 0.67 eV, respectively. Structural transformations from the cuboctahedron and the icosahedron to the lowest-energy structure need to overcome barrier heights of less than 0.55 and 0.05 eV, respectively. Small activation energies are responsible for the flexibility and floppiness of ${\mathrm{Ga}}_{13}$ clusters. Reactions of the decahedral ${\mathrm{Ga}}_{13}$ cluster with Ga and As atoms induce structural changes of the substrate and produce cohesive-energy gains of 3.65 and 4.71 eV, respectively. Migrations of the Ga and As adatoms on the surface of the ${\mathrm{Ga}}_{13}$ cluster need to go over activation barriers with heights of less than 0.30 and 0.34 eV, respectively.

The energetics and structures of metal clusters are studied by the Car-Parrinello method with Al chosen as a paradigm. For the 13- and 55-atom clusters which can assume perfect icosahedral and cubic structures, the distortions from these ideal structures are substantial. For the 55-atom cluster, several inequivalent but energetically nearly degenerate structures are found. The origin of this near-degeneracy is traced to the short range of the effective interatomic interactions, which provides an ab initio justification for their parametrization in terms of short-range potentials.

We present a convenient implementation of constant-thermodynamic-speed simulated annealing for molecular dynamics calculations. Sample results are shown for finding the ground states of 13-Al and 55-Al clusters by classical molecular dynamics and by ab initio quantum molecular dynamics using the Car-Parrinello formalism.

Results of ab initio molecular-dynamics simulations of the properties of solid C 60 are reported. The soccer-ball structure of molecular C 60 is well preserved in the solid. The T=0 structure consists of alternating single and double bonds, but this distinction becomes blurred at relatively low temperatures. The intermolecular interactions are so weak that the individual C 60 can rotate at relatively low temperatures. At high temperatures vibrations cause large distortions, but the cage structure is still preserved. The present results are in excellent agreement with photoemission and NMR data

The energetics of transitions between icosahedral and bulk-like structures is studied theoretically via the Car—Parrinello (CP) method. Total energy calculations show that the energy differences between icosahedral and cuboctahedral structures for 13-, 19-, and 55-atom clusters are surprisingly small, indicating that the transition between icosahedral and bulk structures may occur very early in Al clusters. Bulk embedded atom potentials are found useful for estimates of atomic relaxations for a given structure, but they do not reproduce well the energy differences between different structures. Ionization potentials and electron affinities were calculated within the CP formalism and found to differ by 0.4–1.0 eV from the experimental results.

Ab initio total-energy calculations have been performed for the energetics and diffusion of surface vacancies on GaAs(110). The energies required to evaporate a Ga atom, an As atom, and a GaAs dimer are 4.9, 5.4, and 6.5 eV, respectively, in the neutral state. After evaporations, relaxations of the surface are substantial. The vacancies favor negatively charged states. The diffusion of the vacancies was examined by calculating the potential-barrier heights.

We investigate the energetics, diffusion, and reaction paths of Si and Ge adatoms on As-covered Si(100) surfaces through first-principles pseudopotential calculations. We find that at a monolayer coverage, individual Ge or Si adatoms incorporate rapidly into subsurface As sites with minimum surface diffusion. Thus, the segregation of As to the surface is initiated by single adatom exchange. Due to rapid adatom exchanges, the adatom mobility on surface is drastically reduced, compared to the growth mode without surfactants, resulting in a high density of two-dimensional islands. Generalized gradient corrections to the LDA energies do not alter our results.

Car–Parrinello calculations show that the transformation between C60 isomers involves barriers in excess of 5 eV, although the energetic difference between the buckyball and the next lowest isomer is only 1.6 eV. C60 isomerization thus requires high temperatures and long annealing times.

Young-Jo Ko,1 Jae-Yel Yi,2 Seong-Ju Park,3,* El-Hang Lee,3 and K. J. Chang1 1Department of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-338, Korea 2Korea Research Institute of Standards and Science, P.O. Box 102, Yusung, Taejon 305-600, Korea 3Research Department, Electronics and Telecommunications Research Institute, P.O. Box 106, Yusung, Taejon 305-600, Korea (Received 28 August 1995)

Dimer-vacancy defects on clean Si(001)-2\ifmmode\times\else\texttimes\fi{}1 and Ni-contaminated Si(001)-2\ifmmode\times\else\texttimes\fi{}n surfaces are investigated by scanning tunneling microscopy (STM). The clean Si(001) surface shows the 2\ifmmode\times\else\texttimes\fi{}1 reconstruction irrespective of cooling rates faster than 150 \ifmmode^\circ\else\textdegree\fi{}C/sec. On the Si(001)-2\ifmmode\times\else\texttimes\fi{}1 surface with a surface dimer-vacancy density of 1.7%, the most abundant dimer-vacancy defect is a randomly distributed one dimer vacancy (1-DV) of the Wang-Arias-Joannopoulos model. Appreciable amounts of (1+2)-DV and 2-DV are observed. The ordered defects on the Si(001)-2\ifmmode\times\else\texttimes\fi{}n surface are mainly composed of (1+2)-DV and 2-DV. The real-space STM images reveal that the dimer adjacent to the unrebonded side of 2-DV is depressed by more than 0.5 \AA{}, representing the highly asymmetric characteristics. A small amount of Ni contamination on Si(001) drastically increases the dimer-vacancy density from below 2% to above 20%.

Atomic structures of Ga and As atoms on GaAs(110) were examined employing a first-principles pseudopotential method. Both Ga and As atoms reside in the center of a triangle consisting of a surface Ga and two surface As atoms in the single-atom chemisorbed state. Adsorption energies for Ga and As atoms are 3.1 and 3.5 eV, respectively. Energy barrier heights of Ga and As atoms for the migration along the path through the interstitial channel were found to be 0.6 and 1.0 eV, respectively. Simulations on the deposition of two atoms reveal that a pair formation is stable against separate single-atom chemisorptions.

We examine the atomic and electronic structure of Na-adsorbed Si(100)-p(2\ifmmode\times\else\texttimes\fi{}2) surfaces for various Na coverages (FTHETA) through first-principles pseudopotential calculations. At FTHETA=1/4, we find that the 4\ifmmode\times\else\texttimes\fi{}1 structure with linear Na chains adsorbed on the valley bridge sites is energetically most stable, while substrate Si dimers are rearranged by buckling towards the Na chains. At FTHETA=1/2, the adsorption site of Na is the valley bridge site; however, the 2\ifmmode\times\else\texttimes\fi{}2 structure is found to be more stable than the 2\ifmmode\times\else\texttimes\fi{}1 structure. From the calculated formation energies, we suggest that the saturation coverage is one monolayer with the Na atoms occupying the pedestal and valley bridge sites, exhibiting a 2\ifmmode\times\else\texttimes\fi{}1 reconstruction. The coverage dependences of surface geometry and work function are discussed.

The atomic and electronic structures of microtubules of graphitic carbon have been investigated by ab initio molecular dynamics. Fully optimized atomic structures of two microtubules, a reflection symmetric and a chiral, are found to differ little from their ideal, graphite-derived geometries. Both are semiconductors with direct band gaps at the \ensuremath{\Gamma} point and along the \ensuremath{\Delta} direction, respectively. It is shown by direct calculations in large supercells that substitutional N and B dope microtubules n and p type.

Publisher Summary This chapter discusses quantum molecular dynamics (QMD) studies of the structure and dynamics of metal clusters. What is present in the chapter focuses on metal clusters with Al chosen as a paradigm. The structures of the clusters will play an important role in the assembly process and influence the properties of the final material. The chapter discusses the structures of larger Al clusters, looking at the icosahedral to fee phase transition via the first-principles Car-Parrinello (quantum molecular dynamics) approach. The results of such calculations can be used for the development of empirical potentials that would be suitable for classical molecular dynamics simulations of much larger clusters, cluster fusion, and the assembly of cluster materials. The chapter describes Car-Parrinello calculations that are focused on (1) investigating the transition between icosahedral and bulk (fee) cluster structures in Al, as a function of cluster size, and (2) on developing a database of first-principles results for determining an embedded atom potential for clusters.

Atomic structures of Al on GaAs(110) are studied by ab initio molecular dynamics for coverages of 1/8 to 1 monolayer (ML). A single chemisorbed Al atom resides at the center of a triangle of one Ga and two As atoms. Al dimers have very long bond lengths and bind due to substrate-mediated interactions. Epitaxial growth of 1 ML of Al is less stable than the formation of islands. Preformed clusters bond strongly to the substrate, which shows that the absence of Fermi-level pinning in samples grown by cluster deposition is due to suppression of reactivity rather than lack of interactions

Recent quantum molecular dynamics studies of Al and carbon clusters are described. For Al, we focused on the 13- and 55-atom clusters, which can assume perfect icosahedral and cubic structures. However, the distortions from these ideal structures are substantial. For the 55-atom cluster, several inequivalent but nearly energetically degenerate structures are found, due to the short range of the screened interatomic interactions. For solid C60, it is found that the soccerball structure is well-preserved in the solid. The intermolecular interactions are so weak that the individual C60 can rotate at relatively low temperatures. At high temperatures vibrations cause large distortions, but the cage structure is still preserved. The C60 isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C60, but the transformation between these two structures is hindered by a 5.5 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C20 isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C60. The corannulene structure is a natural precursor for the formation of C60. These results are consistent with the experimental findings that high temperatures are necessary for the formation of substantial quantities of C60. A formulation and the first applications of a new, real space quantum molecular dynamics method, particularly suitable for cluster calculations, are also described.

We report the first structure optimization and quantum molecular dynamics simulations for solid C60. The symmetry-unconstrained optimization results in nearly ideal buckminsterfullerene structure. This is due to the closed shell nature of C60 and the weakness of the intermolecular interactions in the solid. The lack of substantial intermolecular interactions allows for rotations at relatively low temperatures and these rotations are observed during simulations. Although the zero temperature structure consists of alternating single and double bonds, this distinction becomes blurred at moderate temperatures. However, computer generated movies show that the cage structure is preserved even at high temperatures, despite vibrational motion with substantial amplitude.

The energetics and structures of metallic clusters containing up to 55 atoms are studied by the Car-Parrinello (quantum-molecular dynamics) method with Al chosen as a paradigm. It is found that the structural energy differences between small cluster (icosahedral) and bulk (fee) structures are surprisingly small, indicating that the icosahedron-fee structural transition can occur very early in Al clusters. Simulated annealing studies for 13- and 55-atom clusters, where both perfectly symmetric icosahedral and cuboctahedral (fee) structures exist, show that the distortions from the ideal structures are substantial. For the 55-atom cluster several inequivalent but energetically nearly degenerate structures are found, which is likely to lead to floppiness at finite temperatures as well as a low melting point. Although the structure factors for the annealed structures are nearly identical, they are very different from those of either of the ideal structures. The computed IPs and EAs differ by less than 0.1 eV for weakly annealed cuboctahedral and icosahedral clusters, making the structural identification on the basis of IP and EA measurements very difficult. The quantum-mechanical results are also used to develop a classical potential for long-time molecular dynamics simulations of large clusters.

The methods for first-principles calculation of the structure and dynamics of clusters have now progressed to a point where clusters containing ca. 50 non-transition-metal atoms can be studied. As a paradigm, we studied the energetics of structural transformations in 13- and 55-atom Al clusters, which can assume both perfect icosahedral and cuboctahedral structures. Using the Car–Parrinello (quantum molecular dynamics) formalism, we found Al13 has a unique structure, a slightly distorted icosahedron, but Al55 has several inequivalent but energetically nearly degenerate structures. The degeneracy in Al55 is due to the short range of the effective interatomic interactions in a metallic cluster and should lead to floppiness at finite temperatures. A new accurate procedure for calculating ionization potentials (Ei) and electron affinities (Eea) within the Car–Parrinello formalism was developed and applied to Al clusters. Unfortunately, it appears that at least for some clusters, most notably for Al55, the Ei and Eea are very similar for different structural models of this cluster. A formulation and the first tests of a new multigrid-based method for real space electronic structure calculations are briefly described. This method should make possible calculations similar to the above for clusters containing transition metal and/or first-row atoms in the fairly near future.

Green's function is used in order to derive the analytical solution for the change in impedance due to a presence of the flaws in a conductor. This solution is applied to a cylindrical flaw and a spherical flaw whose radii are much smaller than the radius of the test coil. For both cases, the change in impedance is obtained within Born's limit.

The impedance of a single-turn coil which surrounds a conducting cylinder with a flaw is calculated employing Green's function technique. The Born approximation is used in order to know how the impedance change due to the presence of a flaw depends on the conductivity and size of a flaw.

The eddy current tests of ferromagnetic materials are theoretically analyzed employing a Globus model for the magnetization process, and the impedance change of a coil due to the presence of flaws contained in a magnetic conductor is derived employing Green’s function technique. The results show that for an excitation field which is lower than the critical field, the impedance change is maximum when the residual magnetization (or the dc magnetic field applied for the magnetic saturation of the conductor) is absent but for a large field the impedance change becomes maximum when the appropriate dc magnetic field in which the differential permeability reaches its maximum value is applied. For small surface flaws, the volume inhomogeneity mainly contributes to the reactance change of the coil whereas the discontinuity in magnetic permeability contributes to both real and imaginary parts of the impedance change. The dependence of the impedance change on flaw shape is shown explicitly in the derived formula.