Simulation of sputtering and desorption of gold nanoclusters under bombardment with 180-eV/atom Au400 nanoclusters using the classical molecular dynamics method

Computer simulation of the interaction of an Au₄₀₀ nanocluster (the total energy E = 72 keV) with free spherical Au _N nanoclusters (6 and 12 nm in diameter) and Au₆₀₅₁ clusters deposited on the (111) surface of an Al substrate is performed by means of the classical molecular dynamics method. The distributions of the absorbed energy (ε) converted to one atom of the bombarded nanocluster and the sputtering yield are analyzed. It has been ascertained that the most probable values are either the small (ε ? ε_max = E/N) or the maximum possible (ε ～ ε_max) values of absorbed energy. The total sputtering yield and the absorbed energy decrease with increasing impact parameter. It has been demonstrated that, with a probability of ～10%, a direct impact can lead to ejection of the entire bombarded nanocluster from the substrate. This event occurs in the case where an incident cluster initiates the secondary emission of target-cluster atoms mainly in the direction of the substrate. As a result, the nonsputtered part of the target cluster acquires the momentum in the opposite direction. This recoil effect can be regarded as one of the possible mechanisms by which nanoclusters deposited on substrate surfaces desorb under ion and cluster bombardment.     

Study of gold nanocluster sputtering under 38-keV Au ion bombardment by a classical molecular dynamics method

The computer simulation of the interaction of 38-keV Au₁ ions with isolated spherical Au _N nanoclusters of diameters 2.6 and 18 nm is performed in the framework of the classical molecular dynamics (MD) method. The distribution of the absorbed energy ε per one atom of the irradiated cluster and the sputtering yields are analyzed for different ratios of the nanocluster diameter D to the average projective range R _p of the bombarding ion. It is established that the small values of the absorbed energy (ε ? ε_max = E/N) are most probable for D < R _p, and either small (ε ? ε_max) or the maximum possible (ε ～ ε_max) values are mainly realized for D ≤ R _p. It is shown that the total sputtering yield depends weakly on the impact parameter. It is demonstrated for the first time that the irradiated cluster, as a whole, can be ejected by direct impact with a probability of approximately 6–13%. Such events are realized in the cases where the bombarding ion causes secondary cluster-atom emission in the dominant direction to a substrate, with the result that an unsputtered cluster fraction acquires momentum in the opposite direction. This recoil effect can be one of the mechanisms for desorption of nanoclusters deposited on the surface under ion (or cluster) bombardment.     

Stability of gold cages (Au<Subscript>16</Subscript> and Au<Subscript>17</Subscript>) at finite temperature

We have employed ab initio molecular dynamics to investigate the stability of the smallest gold cages, namely Au₁₆ and Au₁₇, at finite temperatures. First, we obtain the ground state structure along with at least 50 distinct isomers for both the clusters. This is followed by the finite temperature simulations of these clusters. Each cluster is maintained at 12 different temperatures for a time period of at least 150 ps. Thus, the total simulation time is of the order of 2.4 ns for each cluster. We observe that the cages are stable at least up to 850 K. Although both clusters melt around the same temperature, i.e. around 900 K, Au₁₇ shows a peak in the heat capacity curve in contrast to the broad peak seen for Au₁₆.      

Self-assembly of Mo<Subscript> 6</Subscript>S<Subscript> 8</Subscript> clusters on the Au(111) surface

The preferred adsorption sites and the propensity for a self-organised growth of the molybdenum sulfide cluster Mo₆S₈ on the Au(111) surface are investigated by density-functional band-structure calculations with pseudopotentials and a plane wave basis set. The quasi-cubic cluster preferentially adsorbs via a face and remains structurally intact. It experiences a strong, mostly non-ionic attraction to the surface at several quasi-isoenergetic adsorption positions. A scan of the potential energy surface exhibits only small barriers between adjacent strong adsorption sites. Hence, the cluster may move in a potential well with degenerate local energy minima at room temperature. The analysis of the electronic structure reveals a negligible electron transfer and S-Au hybridised states, which indicate that the cluster-surface interaction is dominated by S-Au bonds, with minor contributions from the Mo atom in the surface vicinity. All results indicate that Mo₆S₈ clusters on the Au(111) surface can undergo a template-mediated self-assembly to an ordered inorganic monolayer, which is still redox active and may be employed as surface-active agent in the integration of noble metal and ionic or biological components within nano-devices. Therefore, a classical potential model was developed on the basis of the DFT data, which allows to study larger cluster assemblies on the Au(111).     

Classical molecular dynamic simulation of (111) Si and Al surface sputtering under bombardment by polyatomic clusters

The self-sputtering processes of (111) Si and Al surfaces under bombardment by Si _N and Al _N ions and clusters (N = 1−60) with the same energy per particle-projectile atom (1 keV/atom) are studied in this paper. The nonlinear effects produced in the target during the development stage of an atomic-collision cascade and during the postcascade stage are analyzed, and a correlation between these effects and secondary emission characteristics is found. The study has been carried out in the framework of classical molecular dynamics. As a result, a number of features of (111) Si and Al surface sputtering and erosion have been revealed. Thus, it has been established that the sputtering yield increases nonadditively as the size N of the implanted cluster increases at N > 10, which is related to the appearance of nonlinear cascades and the postcascade heat spike, and is accompanied by microcrater formation. It is shown that the implantation of clusters into the Si target leads to the formation of amorphous regions.     

Simulation of doping and primary radiation damage to the SiC(111) surface under bombardment by Si<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> atomic and cluster ions (<Emphasis Type="Italic">N</Emphasis> = 1, 5, and 60) using classical molecular dynamics

The features of the cascade of atomic collisions, the spatial distribution of dopes, and primary radiation damage in a near-surface region of cubic silicon carbide under bombardment by Si _N ions and clusters (N = 1, 5, and 60) in the case of the same energy per one atom of the particle-projectile (200 and 1000 eV/atom) are studied in this paper. The study is carried out using classical molecular dynamics. As a result, several features of the low-energy implantation of polyatomic clusters in SiC(111) are revealed, namely, a relatively weak effect of the size of the implanted cluster on the distribution of ranges of incorporated atoms, a low degree of nonlinear effects at the cascade and postcascade stages, and formation of amorphous regions in the target during cluster implantation.     

Deposition of Ni<Subscript> 13</Subscript> and Cu<Subscript> 13</Subscript> clusters on Ni(111) and Cu(111) surfaces

The soft deposition of Ni₁₃ and Cu₁₃ clusters on Ni(111) and Cu(111) surfaces is studied by means of constant-energy molecular-dynamics simulations. The atomic interactions are described by the Embedded Atom Method. It is shown that the shape of the nickel clusters deposited on Cu(111) surfaces remains rather intact, while the copper clusters impacting on Ni(111) surfaces collapse forming double and triple layered products. Furthermore, it is found that for an impact energy of 0.5 eV/atom the structures of all investigated clusters show the lowest similarity to the original structures, except for the case of nickel clusters deposited on a Cu(111) surface. Finally, it is demonstrated that when cluster and substrate are of different materials, it is possible to control whether the deposition results in largely intact clusters on the substrate or in a spreading of the clusters. This separation into hard and soft clusters can be related to the relative cohesive energy of the crystalline materials.     

Longitudinal dynamics of <Superscript>197</Superscript>Au<Superscript>32+</Superscript> and <Superscript>197</Superscript>Au<Superscript>79+</Superscript> ions in NICA injection chain

The main objective of the NICA project developed at the Joint Institute for Nuclear Research (JINR) is to conduct experimental studies with colliding heavy ion beams in an energy range of 1–4.5 GeV/nucleonucleon with luminosity on the level of 1 × 10²⁷ cm⁻² s⁻¹. In this paper the operation regime of the collider injection chain providing the bunch with experimentally desirable parameters at the output of the Nuclotron is considered for gold ions as an example.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Modification of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface by high-energy bismuth ions

This paper reports on an atomic-force microscopy study of the surface of α-Al₂O₃ single crystals irradiated by Bi ions with energies of 710, 557, 269, and 151 MeV. The shape of the radiation defects produced by single ions was established to depend on the ionization energy loss. The threshold ionization density above which the surface topography is observed to change lies in the 27–35 keV/nm interval. Possible mechanisms of defect formation in the thermal-spike model, namely, a phase transition and the creation of thermoelastic stresses in the high-energy ion track, are considered.     

Equilibrium state of C<Subscript>60</Subscript>, C<Subscript>70</Subscript>, and C<Subscript>72</Subscript> nanoclusters and local defects of the molecular skeleton

The stability of C₆₀ and C₇₀ fullerenes and C₆₀ and C₇₂ nanotubes devoid of 2–12 atoms of the cluster skeleton was theoretically studied. It was established that C_n molecules with an even number of atoms remain stable, which was confirmed by experimental studies of monomolecular decay of clusters with the number of atoms n≥30. The change in the internuclear distances and in the ionization potential of nanoclusters was determined depending on the number of eliminated atoms. Such defects were shown to decrease the ionization potential of nanoclusters by 0.5–0.8 eV. The electron spectrum was calculated within the Harrison semiempirical tight-binding model in the Goodwin modification. A new parametrization of interatomic matrix elements of the Hamiltonian and atomic terms for carbon nanoclusters was suggested.     

Composition and properties of nanoscale Si structures formed on the CoSi<Subscript>2</Subscript>/Si(111) surface by Ar<Superscript>+</Superscript> ion bombardment

The variations in the composition and structure of CoSi₂/Si(111) surface layers under Ar⁺ ion bombardment with subsequent annealing has been studied. It has been demonstrated that nanocluster phases enriched with Si atoms form on the CoSi₂ surface at low doses D ≤ 10¹⁵ cm^–2, and a pure Si nanofilm forms at high doses.     

A molecular dynamics study on melting point and specific heat of Ni<Subscript>3</Subscript>Al alloy

Using the Embedding Atom Method (EAM) for highly undercooled Ni₃Al alloy, the melting point and the specific heat were studied by a molecular dynamics simulation. The simulation of melting point was carried out by means of the sandwich method and the NVE ensemble method, and the results show a good agreement, whereas are larger than the experimental value of 1663 K. This difference is attributed to the influence of surface melting on experimental results, which causes the smaller measurements compared with the thermodynamic melting point. The simulated specific heat of Ni₃Al alloy weakly and linearly increases with the increase of undercooling in the temperature range from 800 K to 2000 K. Supported by the National Natural Science Foundation of China (Grant No. 50395101)     

Photoactivity passivation of TiO<Subscript>2</Subscript> nanoparticles using molecular layer deposited (MLD) polymer films

Pigment-grade anatase TiO₂ particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100–160 °C. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO₂ particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al₂O₃ film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO₂ particles.     

Evidence for oxygen abstraction from NO<Subscript>2</Subscript> upon thermal scattering from an Al(111) surface

Evidence is presented for the existence of a non-adiabatic reaction channel, namely the abstraction of an oxygen atom from a nitrogen dioxide molecule upon scattering from an aluminium(111) surface. This reaction channel was studied by exposing the sample to an NO₂ molecular beam and subsequently analysing the scattered flux using REMPI spectroscopy. In these experiments, a considerable amount of NO emitted from the surface was detected. The emitted NO has a rotational temperature of ca. 260 K that increases only slowly with surface temperature. In summary, these results provide first evidence for the abstraction reaction NO_2(g)NO_(g)+O_(a) upon NO₂/Al(111) scattering, which may arise when two electrons are rapidly transferred to the incoming molecule while it is unfavourably oriented for concurrent adsorption of both fragments. PACS 82.65.+r; 68.43.-h     

Lattice dynamics of BiFeO<Subscript>3</Subscript> under hydrostatic pressure

The vibrational frequencies of the BiFeO₃ crystal lattice in the cubic phase (Pm3m) and the rhombohedral paraelectric phase (R3c) are calculated in terms of the ab initio model of an ionic crystal with the inclusion of the dipole and quadrupole polarizabilities. In the ferroelectric phase with the symmetry R3c, the calculated spontaneous polarization of 136 μC cm^?2 agrees well with the experimental data. The dependences of the unit cell volume, the elastic modulus, and the vibrational frequencies on the pressure are calculated. It is found that the frequency of an unstable ferroelectric mode in both the cubic (Pm3m) and rhombohedral (R3c) phases are almost independent of the applied pressure, in contrast to classical ferroelectrics with a perovskite structure, where the ferroelectric instability is very sensitive to a variation in the pressure.     

Study of the formation and unimolecular fragmentation of Si<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>O<Stack><Subscript><Emphasis Type="Italic">m</Emphasis></Subscript><Superscript>+</Superscript></Stack> clusters under ion bombardment

The dependences of the emission and fragmentation of clusters sputtered by Xe⁺ ions from the surface of Si _n O _m ⁺ on the oxygen pressure near the bombarded surface are studied using secondary ion mass spectrometry. It is shown that the process of Si _n O _m ⁺ cluster formation under ion bombardment can be described within the framework of the mechanism of combinatorial synthesis by taking into account the mutual reversibility of the reactions of formation and unimolecular decay.     

The structure and properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al coatings deposited by microarc oxidation on graphite substrates

Protective α-Al₂O₃ coatings on the surface of a graphite article have been obtained by method of electric-arc metallization with aluminum and microarc oxidation (anodic spark process). Investigation of the obtained coating by scanning electron microscopy (SEM), X-ray diffraction (XRD), and proton elastic recoil detection analysis (ERDA) showed good quality of the Al and α-Al₂O₃ coatings on graphite. The proposed technology can be used for obtaining protective coatings in low-accessible sites of graphite articles.     

Luminescence of Cr<Superscript>3+</Superscript> impurity ions in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> nanocrystals and clusters embedded in lithium germanate glasses

The properties of Cr³⁺-doped Li₂Ge₇O₁₅ (LGO) lithium germanate nanocrystals produced in lithium germanate glasses under isothermal heating were studied. The samples were characterized by x-ray diffraction and small-angle scattering, as well as by transmission electron microscopy. The luminescence spectra of the impurity chromium measured in lithium germanate glasses containing LGO crystals revealed transitions in Cr³⁺ ions residing in the glass phase and in LGO crystallites starting from extremely small clusters. This provided the possibility of following the process of crystallization of the lithium germanate glass from Cr³⁺ luminescence spectra. The effects observed in the Cr³⁺ luminescence spectrum revealed a ferroelectric phase transition in LGO nanocrystals embedded in the glass.