Molecular dynamics computer simulations of 5 keV C60 bombardment of benzene crystal

Coarse-grained molecular dynamics computer simulations have been used to investigate the damage of a benzene crystal induced by 5 keV C₆₀ projectile bombardment. The sputtering yield, mass distributions and the depth distributions of ejected organic molecules are analyzed. The temporal evolution of the system reveals that impinging C₆₀ cluster leads to creation of almost hemispherical crater. Most of the molecules damaged by the projectile impact are ejected into the vacuum during cluster irradiation. This “cleaning up” effect may explain why secondary ion mass spectrometry (SIMS) analysis of some organic samples with cluster projectiles can produce significantly less accumulated damage compared to analysis performed with atomic ion beams.     

Organic mass spectrometry with low-energy projectiles

Molecular dynamics computer simulations have been employed to elucidate mechanisms responsible for uplifting of a monolayer of benzene and polystyrene molecules adsorbed on Ag{1 1 1} by low-energy atomic and cluster Ar projectiles. The sputtering yield and mass distributions of ejected particles are analyzed depending on the type and the kinetic energy of a projectile. It is shown that the relative contribution of intact molecules can be greatly enhanced if the kinetic energy of atomic projectile is reduced below 60 eV. At these energies, however, the efficiency of desorption is low and the ejection process is limited only to loosely bound molecules. Much better results can be obtained for cluster projectiles containing hundreds of Ar atoms with the incident energy of a few eV per atom. The impact of such particles leads to a gentle and very efficient removal of intact organic molecules originally adsorbed at the surface.     

Desorption of organic overlayers by Ga and C60 bombardment

This paper reviews our recent work on computer simulations of Ga and C₆₀ bombardment of thin organic overlayers deposited on metal substrate. A multilayer of benzene, a monolayer of PS4 on Ag{1 1 1} and a self-assembled monolayer of octanethiol molecules on Au{1 1 1} were irradiated with 15 keV monoatomic (Ga) and polyatomic (C₆₀) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C₆₀, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C₆₀ and Ga projectiles. Our results indicate that the chemical analysis of thin organic films performed by detection of sputtered neutrals will not benefit from the use of C₆₀ projectiles.     

Applications of fullerene beams in analysis of thin layers

Molecular dynamics computer simulations have been employed to model ejection of particles from Ag{1 1 1} metal substrate and thin benzene overlayer bombarded by fullerene cluster projectiles. The sputtering yields are analyzed depending on the size (from C₂₀ up to C₅₄₀) and the kinetic energy (5-20 keV) of a projectile. It has been found that for clean metal substrate bombarded by 15 keV projectiles the maximum ejection is stimulated by the impact of the C₆₀ cluster. However, the size of the cluster projectile maximizing the yield depends on the kinetic energy of the cluster, shifting towards larger clusters as the impact energy increases. For a thin benzene overlayer, the yield increases monotonically with the size of the cluster within investigated range of fullerene projectiles and kinetic energies.     

Evaporation processes of water molecules from graphene edge: DFT and MD study

The evaporation processes of water molecules adsorbed in the edge region of graphene have been investigated by means of direct MO–MD method. A large system composed of 29 water molecules and a graphene sheet (C₉₆H₂₄) was used as a model system. The edge carbon atom of graphene was terminated by hydrogen atom. The geometry optimization showed that the water molecules interact with the hydrogen atoms in the edge region of graphene. At low temperature (300 K), the water molecules were dissociated as water clusters from the graphene. On the other hand, in addition to the dissociation of water clusters, the isolated water molecule was also found as dissociation product at high temperature (500 K). The mechanism of water evaporation was discussed on the basis of theoretical results.     

The mechanism of TiO2 deposition by direct current magnetron reactive sputtering

Thin films are generally thought of as being the product of a reaction between the surface and atoms and/or molecules in the gas phase. However, a relatively new theory, the theory of charged clusters (TCC), suggests that charged clusters nucleate in the gas phase and become the growth unit for a thin film. The aim of this study was to determine whether or not TiO₂ thin film deposition by DC reactive sputtering occurs via this mechanism. TiO₂ was deposited on unheated transmission electron microscopy grids to observe TiO₂ clusters, as well as glass and silicon substrates to observe the resulting thin films. The results showed that TiO₂ clusters were indeed produced in the chamber of a direct current reactive sputtering system. Furthermore, these clusters were observed as close as 50 mm away from the target. Clusters 3 nm and <2 nm in diameter were found 250 mm and 50 mm away from the target, respectively The cluster size was found to have a direct effect on the film deposited. Smaller clusters produced a facetted crystalline anatase film whereas larger clusters produced an amorphous film.     

High-temperature sputtering of bimetallic clusters by low-energy argon clusters

Molecular dynamic simulation of sputtering of bimetallic bipartite clusters bombarded by Ar₁ and Ar₁₃ particles is performed. The bombarded targets are 390-atom clusters consisting of two parts with equal number of atoms of the monocomponent fractions, Cu–Au and Ni–Al. The energy distributions of atoms in clusters and distributions of sputtered atoms with respect to the time of emission from clusters are obtained; these distributions testify to the increasing contribution of thermal evaporation of atoms into the sputtering yield of the metallic cluster with increasing size and energy of bombarding particles.     

The influence of surface structures on sputtering: Angular distribution and yield from faceted surfaces

Solid surfaces subject to energetic particle bombardment generally develop characteristic structures, which may significantly change the total and differential sputtering yield. The change is due to two competing effects, a yield-increase by an enhanced effective projectile incidence angle, and a yield-reduction by recapture of obliquely ejected particles. Both effects have been included in calculations of the sputtering yields from faceted surfaces in the regime where the plane surface yield follows a cos^−v dependence on the incidence angle. Except at very low energies, the total sputtering yield is always increased by faceting. The angular distribution function is mainly influenced at large polar angles and may significantly deviate from that of the corresponding flat surface. This has important consequences both in comparisons between experimental and theoretical distribution functions as well as in applications such as thin film production, plasma contamination, secondary ion mass spectrometry etc. EURAT0M Association.     

Sputtering of ice films by the bombardment of Ar ions

The process of sputtering ice films covered on Au(1 1 1) surface at normal incidence by Ar ions with an initial energy of E₀ = 300 eV and 700 eV has been investigated by the molecular dynamic simulation. The mass spectrum and kinetic energy distributions have been calculated. Our simulations clearly show that the mass spectrum contains peaks of the water molecules, water clusters and Au atoms. These results are of interest for the study of mass spectrometry in films and surfaces.     

Molecular dynamics simulations of CF3 etching of SiC

Molecular dynamics simulations were performed to investigate CF₃ continuously bombarding SiC surfaces with energies of 100, 150 and 200 eV at normal incidence and room temperature. The simulated results show that the etching rates of Si and C atoms increase linearly with the incident energy. The etch rate of Si atoms is much more than that of C atoms. A carbon-rich surface layer is observed which is in good agreement with experiments. Under bombarding by CF₃, an F-containing reaction layer is formed through which Si and C atoms are removed. In reaction layer, SiF and CF species are dominant. The formation mechanisms of ejected products are discussed. In etching products, SiF₃ is dominant. It is found that etching of C atoms in SiC is controlled by physical sputtering, while etching of Si atoms in SiC is controlled by chemical sputtering.     

Synthesis and characterization of titanium and zirconium oxynitride coatings

Thin films of zirconium oxynitride (ZrNO) and titanium oxynitride (TiNO) have been deposited onto Si(100) substrates at room temperature by radiofrequency magnetron sputtering in an argon-oxygen-nitrogen atmosphere. Single oxynitride layers have been stacked to obtain a multilayer structure. The film structure has been determined by X-ray diffraction while compositional analysis has been performed by X-photoelectrons spectroscopy. Structural analysis has shown that TiNO can be represented as a cubic structure where oxygen atoms replace nitrogen ones while ZrNO can be described as a cubic ZrO₂ where nitrogen atoms replace oxygen ones. Besides the main peak, the multilayer films show satellite peaks, proving the formation of the stacked structure. The final films stoichiometry has been explained by a growth model. It establishes that in TiNO films the nitrogen vacancies filling by reactive reactions with oxygen atoms is favourite while for ZrNO films the oxygen vacancies filling by energetic nitrogen atoms is more likely to happen. The different behaviour between TiNO and ZrNO is further confirmed during the multilayer growth.     

ESR and X-ray Investigations of Deuterium Atoms and Molecules in Impurity-Helium Solids

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. We measured the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D₂-He solids. These measurements show that D atoms are mainly stabilized in D₂ clusters. Using an x-ray method we found the size of D₂ clusters to be ～90Å in diameter and the densities of D₂ molecules in the samples to be of order 2.5?10²¹ cm^?3 . The highest average concentration of D atoms achieved in D-D₂-He solids was ～1.5?10¹⁸ cm^?3 . The local concentrations of D atoms within D₂ clusters is found to be large (～2?10¹⁹ cm^?3).     

Low temperature diffusion of Au into Si in the Si(substrate)-Au(film) system

When the clean surface of a Si crystal is covered with an evaporated Au layer and heated in an oxidizing atmosphere at 100°–300°C, i.e. below the Si-Au eutectic point (370°C), Si atoms are ejected from the Si-Au interface and migrate through the Au film to its surface to appear as a SiO₂ layer. For an understanding of this low temperature ejection, an AES study of the Si-Au interface region has been undertaken. It is proposed that the Si at the interface is metallic and forms a metallic bonding with Au, the melting point of the interface being comparable with that of Si-Au eutectic. The metallic Si was identified from Si (LVV) Auger spectra of “vapor-quenched” metastable Si-Ag, Si-Au and Si-Cu alloys. The low temperature ejection is possible, therefore, from this metallic interface. To support this statement, low temperature ejection of Au atoms from the interface into the bulk of Si was also studied by photoconductivity measurements of the above specimen. The analysis of the photoresponse spectra of the ejected Au atoms inside the Si crystal suggests that the atoms diffuse interstitially with a diffusion coefficient of about 10^-9 cm² sec^-1 at 340°C.     

Self-assembly and photocatalysis of mesoporous TiO2 nanocrystal clusters

Mesoporous nanocrystal clusters of anatase TiO₂ with large surface area and enhanced photocatalytic activity have been successfully synthesized. The synthesis involves the self-assembly of hydrophobic TiO₂ nanocrystals into submicron clusters, coating of these clusters with a silica layer, thermal treatment to remove organic ligands and improve the crystallinity of the clusters, and finally removing silica to expose the mesoporous catalysts. With the help of the silica coating, the clusters not only maintain their small grain size but also keep their mesoporous structure after calcination at high temperatures (with BET surface area as high as 277 m²/g). The etching of SiO₂ also results in the clusters having high dispersity in water. We have been able to identify the optimal calcination temperature to produce TiO₂ nanocrystal clusters that possess both high crystallinity and large surface area, and therefore show excellent catalytic efficiency in the decomposition of organic molecules under illumination by UV light. Convenient doping with nitrogen converts these nanocrystal clusters into active photocatalysts in both visible light and natural sunlight. The strategy of forming well-defined mesoporous clusters using nanocrystals promises a versatile and useful method for designing photocatalysts with enhanced activity and stability.     

The effect of sputtering deposition rate on the agglomeration and crystallization of Si clusters

Si/SiO₂ films were fabricated by alternative sputtering silicon and SiO₂ targets following by rapid-thermal-annealing (RTA) and furnace-annealing (FA) treatment. After RTA treatment, the extent of crystallization and the density of Si clusters increase with sputtering rate, while the size distributions of clusters obtained at low and high sputtering rates are more uniform than that at the mid-sputtering rate. Initial clusters and structure defects in the as-deposited films combine with atomic diffusion during RTA to determine the size distribution and density of Si clusters. During the FA process, excessive Si atoms can precipitate by either adhering to the existent Si clusters or congregating into new clusters.     

The synthesis of Nb3Ge by RF sputtering

The conditions necessary for the formation of Nb ₃ Ge by low-pressure rf sputtering with a superconducting transition temperatureT _c >21 K have been investigated. Samples have been deposited onto cooled substrates so that the film is first amorphous and then is crystallized by a subsequent annealing, and onto hot substrates, in which case the film is crystalline upon deposition. The highestT _c samples were obtained for a substrate temperature of 735±25° C. The optimum substrate temperature is the same as the optimum annealing temperature for crystallizing films which were first deposited onto cooled substrates. Special conditions are necessary for the formation of single-phase A15 samples of Nb ₃ Ge with an optimumT _c . We have utilized the collisions that the sputtered atoms undergo with the sputtering gas molecules to thermalize the sputtered atoms. We report here on sputtering in both krypton and in argon.     

Modelling interaction cross sections for intermediate and low energy ions

When charged particles slow in tissue they undergo electron capture and loss processes that can have profound effects on subsequent interaction cross sections. Although a large amount of data exists for the interaction of bare charged particles with atoms and molecules, few experiments have been reported for these 'dressed' particles. Projectile electrons contribute to an impact-parameter-dependent screening of the projectile charge that precludes straightforward scaling of energy loss cross sections from those of bare charged particles. The objective of this work is to develop an analytical model for the energy-loss-dependent effects of screening on differential ionisation cross sections that can be used in track structure calculations for high LET ions. As a first step a model of differential ionisation cross sections for bare ions has been combined with a simple screening model to explore cross sections for intermediate and low energy dressed ions in collisions with atomic and molecular gas targets. The model is described briefly and preliminary results compared to measured ejected electron energy spectra.     

Angular dependence of sputtering effects by ethanol cluster ion irradiation on solid surfaces

In order to clarify the interactions of ethanol cluster ions with solid surfaces such as Si(100), SiO₂ and Au surfaces, sputtering effects were investigated by changing the acceleration voltage and incident angle (θ) of the cluster ions. The sputtered depth at a normal incidence of θ = 0° increased with an increase of the acceleration voltage. The sputtering ratio of Si to SiO₂ was approximately 10, which suggested that chemical reactions between Si and ethanol produced silicon hydride as a dominant etching material. Furthermore, the sputtered depth of Si surfaces by ethanol cluster ion irradiation had a maximum value at an incident angle between 10° and 60°, and the angle corresponding to the maximum peak increased with an increase of the acceleration voltage. On the other hand, for the physical sputtering of Au surfaces by ethanol cluster ion irradiation, the sputtered depth decreased with the increase of the incident angle, and the change was in accordance with cos θ.With regards to the angular distribution of sputtered particles, Si surface atoms were ejected by ethanol cluster ion irradiation according to a cosine law distribution. This indicated that the Si surfaces were chemically sputtered by ethanol cluster ion irradiation. On the other hand, for the case of Au surfaces, the ejection of the sputtered particles changed to the under-cosine law. This was ascribed to the lateral sputtering effect of ethanol cluster ion irradiation.     

Depth profiling of fullerene-containing structures by time-of-flight secondary ion mass spectrometry

A new variant of depth profiling for thin-film fullerene-containing organic structures by the method of time-of-flight (TOF) secondary ion mass spectrometry (SIMS) on a TOF.SIMS-5 setup is described. The dependence of the yield of C₆₀ molecular ions on the energy of sputtering ions has been revealed and studied. At an energy of sputtering Cs⁺ ions below 1 keV, the intensity of C₆₀ molecular ions is sufficiently high to make possible both elemental and molecular depth profiling of multicomponent (multilayer) thin-film structures. Promising applications of TOF-SIMS depth profiling for obtaining more detailed information on the real molecular composition of functional organic materials are shown.     

The synthesis of the Nb3(Al-Ge) system by sputtering

The variation of the superconducting transition temperatureT _chas been studied in the Nb₃Al _1–x Ge _x system for0x1 for samples produced by rf sputtering onto heated substrates. The effect that different sputtering gas pressures have upon the rate of energy loss of the sputtered atoms due to collisions with neutral sputtering gas atoms is considered. Also considered is how thermalization can be achieved in the fewest number of collisions by matching the mass of the sputtering gas atoms to that of the sputtered atoms. For the case of Nb ₃ (Al-Ge) we show that it is advantageous to use a mixture of sputtering gases so that the light Al atoms can be thermalized as well as the heavier Nb and Ge atoms. It is also thus shown that the same sputtering conditions that are optimal for forming high-T _cNb₃Ge onto heated substrates are not optimal for forming high-T _cNb ₃ Al.Supported by CUNY FRAP and National Science Foundation DMR 74-18138.