MD study on high-energy reactive cluster impact on diamond (111) and (100) surfaces

Molecular dynamics (MD) simulations of single argon, CO₂ and O₂ cluster impacts on diamond (100) and (111) surfaces are performed in order to investigate the surface erosion process. The transient crater on the (100) surface seems rather unpherical and skew compared to the typical hemispherical crater appeared on the (111) surface due to the orientation-dependent hardness. Argon cluster impacts on the diamond (100) surface resulted in a slightly higher erosion rate than on the (111) surface while it is lowered on the (111) surface for CO₂ cluster impacts. The difference in the susceptibility to the physical erosion appears in the rim or the crater.     

Possibilities of C 1s XPS and N(E) C KVV Auger spectroscopy for identification of inherent peculiarities of diamond growth

The interaction of C-atoms and CH_n-radicals with uncleaned and argon cleaned silicon substrate and with diamond surface after H-treatment have been studied in situ by XPS and Auger spectroscopy. It was found the formation of a new chemical surface state of carbon atoms in the case of carbon atoms and radicals interaction with cleaned silicon. The same chemical state was revealed on the H-treated diamond surface. Graphite-like structure of carbon atoms was observed on the surface of unlearned silicon and H-treated diamond after interaction with carbon atoms and radicals. N(E) C KVV Auger spectrum for the new chemical state of carbon atoms significantly differs from typical spectra for sp²- and sp³-bonded carbon materials. The high energy part of this spectrum was interpreted under the hypothesis of sp³-bonded carbon atoms but with shifted fermi level position.     

Nanofriction studies of reactively or non-reactively cluster-eroded single crystal diamond

Friction properties of cluster-eroded surfaces of synthetic single crystal diamond (Monodite) are compared after erosion with high-speed CO₂ cluster beams as well as with corresponding Ar cluster beams, the cluster impact kinetic energy being 100 keV in both cases. The respective friction values are determined by atomic force microscope measurements. Using CO₂ clusters, the reactive accelerated cluster erosion (RACE) of the single crystal diamond substrates leads to more than seven times higher friction values than those observed after erosion with non-reactive accelerated Ar clusters. Molecular dynamics calculations reveal related differences in the simulations of respective single cluster impacts already at 2 ps after impact.     

Simulations of C60 bombardment of Si, SiC, diamond and graphite

Molecular dynamics simulations of the 20-keV C₆₀ bombardment at normal incidence of Si, SiC, diamond and graphite targets were performed. The unique feature of these targets is that strong covalent bonds can be formed between carbon atoms from the C₆₀ projectile and atoms in the solid material. The mesoscale energy deposition footprint (MEDF) model is used to gain physical insight into how the sputtering yields depend on the substrate characteristics. A large proportion of the carbon atoms from the C₆₀ projectile are implanted into the lattice structure of the target. The sputtering yield from SiC is ∼twice that from either diamond or Si and this can be explained by both the region of the energized cylindrical tract created by the impact and the number density. On graphite, the yield of sputtered atoms is negligible because the open lattice allows the cluster to deposit its energy deep within the solid. The simulations suggest that build up of carbon with a graphite-like structure would reduce any sputtering from a solid with C₆₀⁺ bombardment.     

Spectroscopy of calcium deposited on large argon clusters

Spectroscopic experiments have been performed, providing emission and excitation spectra of calcium atoms trapped on argon clusters of average size 2 000. The two experimental spectra fall in the vicinity of the calcium resonance line ¹P ₁ → ¹S₀ at 422.9 nm. The excitation spectrum consists in two bands located on each side of the resonance line of the free calcium. In addition, Monte Carlo calculations, coupled to Diatomics-In-Molecule potentials are employed to simulate the absorption spectrum of a single calcium atom in the environment of a large argon cluster of average size 300. The theoretical absorption spectrum confirms the existence of two bands, and shows that these bands are characteristic of a calcium atom located at the surface of the argon cluster and correspond to the excited 4p orbital of calcium either perpendicular or parallel to the cluster surface. The precise comparison between the shape of the absorption spectrum and that of the fluorescence excitation spectrum shows different intensity ratios. This could suggest the existence of a non adiabatic energy transfer that quenches partly the fluorescence of trapped calcium. Another explanation, although less likely, could be a substantial dependence of the calcium oscillator strength according to the alignment of the calcium excited orbital with respect to the cluster surface. The emission spectrum always shows a band in the red of the resonance line which is assigned to the emission of calcium remaining trapped on the cluster. When exciting the blue band of the excitation spectrum, the emission spectrum shows a second, weak, component that is assigned to calcium atoms ejected from the argon clusters, indicating a competition between ejection and solvation. Received 7 May 2002 Published online 1st October 2002 RID="a" ID="a"e-mail: jmm@drecam.saclay.cea.fr RID="b" ID="b"URA 2453 du CNRS RID="c" ID="c"UMR 5626 du CNRS     

Effects of CO2 activation on porous structures of coconut shell-based activated carbons

In this paper, textural characterization of an activated carbon derived from carbonized coconut shell char obtained at carbonization temperature of 600 °C for 2 h by CO₂ activation was investigated. The effects of activation temperature, activation time and flow rate of CO₂ on the BET surface area, total volume, micropore volume and yield of activated carbons prepared were evaluated systematically. The results showed that: (i) enhancing activation temperature was favorable to the formation of pores, widening of pores and an increase in mesopores; (ii) increasing activation time was favorable to the formation of micropores and mesopores, and longer activation time would result in collapsing of pores; (iii) increasing flow rate of CO₂ was favorable to the reactions of all active sites and formation of pores, further increasing flow rate of CO₂ would lead carbon to burn out and was unfavorable to the formation of pores. The degree of surface roughness of activated carbon prepared was measured by the fractal dimension which was calculated by FHH (Frenkel-Halsey-Hill) theory. The fractal dimensions of activated carbons prepared were greater than 2.6, indicating the activated carbon samples prepared had very irregular structures, and agreed well with those of average micropore size.     

The magnetic property of carbon-doped TiO2

Despite carbon and TiO₂ are nonmagnetic, we detected ferromagnetism at room temperature over samples of carbon-doped TiO₂. The materials were prepared by standard solid-state reaction and sintered either in an argon or nitrogen atmosphere. According to Raman results, the samples sintered in nitrogen showed lower D-bond (disordered) and G-bond (graphitic) concentration, plausibly a result of nitrogen incorporation into the carbon-doped TiO₂ materials. All the samples are ferromagnetic at room temperature. With increase of carbon concentration, there is decline of magnetic moment per carbon (in carbide form) due to antiferromagnetic interaction among the carbon atoms. Compared to the sample sintered in argon, the one sintered in nitrogen is lower in magnetic moment due to partial replacement of carbon atoms by nitrogen atoms. We found that the electrons-mediated mechanism is more suitable than the holes-mediated one for the explanation of ferromagnetism of the carbon-doped TiO₂ materials.     

Hyperthermal cluster-surface scattering

Using molecular-dynamics simulation, we study the processes occurring after impact of clusters on a rigid wall. Comparing the impact of model clusters consisting of 13 atoms, or of 13 diatomic molecules with varied bond strength, the systematics in the results of the collision process are investigated. Four regimes of impact-induced cluster fragmentation are identified: intact reflection, shattering into large fragments, complete fragmentation, and molecule dissociation. The effect of the number of degrees of freedom activated in the collision on the translational and internal energies of the reflected fragments is discussed in detail. As a rule, with increasing number of degrees of freedom which can be activated in the collision, the translational energy sinks. On the other hand, for weak intramolecular bonding, intramolecular vibrations are easily excited at small impact energies, reducing the resulting translational energy. The presence of even a very weak attractive well epsilon_w at the surface has a major influence on the sticking behavior of the clusters — and hence also on the absolute reflected energies — even at impact energies E₀ ≫ epsilon_w.     

Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO₂) adsorbent. In general, CO₂ adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO₂ adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N₂ adsorption/desorption at −196 °C. CO₂ capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO₂ adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800 °C) considerably enhanced the CO₂ uptake at higher temperatures.     

Friction and energy dissipation mechanisms in adsorbed molecules and molecularly thin films

This review provides an overview of recent advances that have been achieved in understanding the basic physics of friction and energy dissipation in molecularly thin adsorbed films and the associated impact on friction at microscopic and macroscopic length scales. Topics covered include a historical overview of the fundamental understanding of macroscopic friction, theoretical treatments of phononic and electronic energy dissipation mechanisms in thin films, and current experimental methods capable of probing such phenomena. Measurements performed on adsorbates sliding in unconfined geometries with the quartz crystal microbalance technique receive particular attention. The final sections review the experimental literature of how measurements of sliding friction in thin films reveal energy dissipation mechanisms and how the results can be linked to film-spreading behavior, lubrication, film phase transitions, superconductivity-dependent friction, and microelectromechanical systems applications. Materials systems reported on include adsorbed films comprised of helium, neon, argon, krypton, xenon, water, oxygen, nitrogen, carbon monoxide, ethane, ethanol, trifluoroethanol, methanol, cyclohexane, ethylene, pentanol, toluene, tricresylphosphate, t-butylphenyl phosphate, benzene, and iodobenzene. Substrates reported on include silver, gold, aluminum, copper, nickel, lead, silicon, graphite, graphene, fullerenes, C60, diamond, carbon, diamond-like carbon, and YBa₂Cu₃O₇, and self-assembled monolayers consisting of tethered polymeric molecules.     

Heat transfer between tungsten surface and glow discharge plasmas in argon and CO<Subscript>2</Subscript>

A hot-filament method is used to study the heat transfer between tungsten surface and hollow-cathode glow discharge plasmas in argon and CO₂. The dependence of the electric power supplied to a tungsten wire on the discharge current is determined for argon and carbon dioxide in the temperature range between 1000 and 1700 K. A difference in heat transfer at the tungsten wire surface is found between experiments on argon and carbon dioxide. The difference is attributed to heterogeneous recombination in CO₂ plasma.     

CO2 gas resonance absorption at CO2 laser wavelength in?multiple laser coating

Multiple laser beams demonstrate many advantages as energy sources in diamond synthesis. In a reported amazingly-fast multiple laser coating technique, CO₂ gas is claimed as the sole precursor or secondary precursor for forming a diamond or diamond-like carbon, which remains poorly understood. The absorption coefficient changes under the irradiation of multiple lasers are one of the keys to resolve the mysteries of multiple laser beam coating processes. This study investigates the optical absorption in CO₂ gas at the CO₂ laser wavelength. The resonance absorption process is modeled as an inverse process of the lasing transitions of CO₂ lasers. The well-established CO₂ vibrational-rotational energy structures are used as the basis for the calculations with the Boltzmann distribution for equilibrium states and the three-temperature model for non-equilibrium states. Based on the population distribution, our predictions of the CO₂ absorption coefficient changes as a function of temperature are in agreement with the published data.     

Experimental and theoretical studies of interactions between Si<Subscript>7</Subscript> clusters

The possibility of using magic Si₇ clusters to form a cluster material was studied experimentally and theoretically. In experiments Si₇ clusters were deposited on carbon surfaces, and the electronic structure and chemical properties of the deposited clusters were measured using X-ray photoelectron spectroscopy (XPS). A non bulk-like electronic structure of Si₇ was found in the Si 2p core level spectra. Si₇ is suggested to form a more stable structure than the non-magic Si₈ cluster and Si atoms upon deposition on carbon surfaces. Theoretically it was possible to study the interaction between the clusters without the effect of a surface. Density functional theory (DFT) calculations of potential curves of two free Si₇ clusters approaching each other in various orientations hint at the formation of cluster materials rather than the fusion of clusters forming bulk-like structures.     

Ab initio modeling of the electronic and spin properties of the [NV]<Superscript>−</Superscript> centers in diamond nanocrystals

The electronic and spin properties of different nanocrystals of carbon are studied. The properties of these cluster systems are modeled in terms of the ab initio (Hartree-Fock) and semiempirical (PM3, AM1) quantum-chemical methods. The calculations are performed for different carbon nanocluster systems: defect-free and with [NV]^? centers, hydrogen passivated (C₃₈H₄₂, C₇₁H₈₄, C₈₆H₇₈), and with a free (unpassivated) surface (C₃₈, C₇₁, C₈₆). The spin properties of unhydrated nanoclusters were studied for the first time. The structure of all the clusters under study was optimized using the total energy minimization principle. It is shown that, in the case of hydrated carbon nanocrystals passivated by hydrogen atoms, diamond-like clusters are formed. The atomic structure of an unpassivated nanocrystal depends on the number of atoms in the cluster, as well as on its initial geometrical parameters. In some cases, clusters with a fullerene-like surface are formed. In hydrogenpassivated diamond nanocrystals with [NV]^? centers, the spin density is localized at the nuclei of C atoms nearest to the center vacancies. For the unpassivated counterparts, the spin density is localized at the nuclei of C atoms forming the surface of the corresponding nanocrystal.     

A new study on the catalytic mechanism of Fe-based alloys in diamond formation by Mössbauer spectroscopy

M?ssbauer spectroscopy has been used to systemically study the catalytic mechanism of Fe-based alloys in diamond formation at high temperature–high pressure (HTHP) for the first time. M?ssbauer spectra reveal the magnetic state of the 3d electrons of a Fe atom in the Fe-based alloy catalyst during diamond formation at HTHP. During carburization at lower temperatures than that required for diamond formation and diamond formation in the diamond-stability region using Fe-based alloys as a catalyst, both the quadrupole splitting QS and the isomer shift IS change from negative to positive, especially reaching a state in which they are zero. It was indicated that the state of the 3d-shell electrons of the iron atom changes greatly during carburization and diamond formation and that the incomplete 3d sub-bands of Fe atoms in the catalyst alloys could be filled up in proper order by electrons of interstitial carbon atoms. During diamond formation, the unpaired 3d-shell electrons of an iron atom in the Fe-based alloy absorb and interact with 2P_z electrons of the carbon atoms. There exist a Fe–C bonding and an electron charge transfer stage. The 2P_z electrons of the carbon atoms could be dragged into the metal atoms in the catalyst alloy and would make a transition of triangular (sp²π) hybridization of valence electrons to tetrahedral (sp³) hybridization of valence electrons (a transition of sp²π bonds of graphite to sp³ bonds of diamond), resulting in a transition of graphite structure to diamond. Although the conclusion of this study is strictly applicable only to Fe-based alloy catalysts, it could be considered more general because of the chemical similarities between the transition elements used as solvent catalysts for diamond synthesis. Received: 2 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Thermally Evaporated Ag–Au Bimetallic Catalysts for Efficient Electrochemical CO2 Reduction

CO₂ reduction reaction (CO₂RR) has indispensable significance for carbon recycling and renewable energy production. As typical electrochemical catalysts, Au and Ag show relatively high reaction activity and selectivity in CO₂RR. In this study, a series of Ag–Au bimetallic catalysts are designed and synthesized through the thermal evaporation method for efficient yet massive production of electrochemical catalysts. The Ag–Au catalysts show significantly enhanced activity and selectivity in CO₂RR, which is mainly attributed to the increased grain boundaries with well-dispersed single Ag atoms. After the optimization, Au20Ag10 exhibits the best performance with a CO Faraday efficiency of 89% at −0.9 V (vs the reversible hydrogen electrode) with good stability.     

Quantum-chemical modeling of structural,electronic, and spin characteristics of NV centers in nanostructured diamond: Surface effect

The effect of the surface of diamond on atomic, electronic, and spin properties of diamond nanocrystals containing single nitrogen-vacancy defects ([NV]⁻ centers) is studied. The surface was modeled with clusters C₃₃H₃₀[NV]⁻, C₆₆H₇₂[NV]⁻, which were constructed based on bulk clusters C₃₃H₃₆[NV]⁻ and C₆₉H₈₄[NV]⁻, respectively. In all cases, clusters in the triplet state S = 1 are considered with the cluster charge being −1. The geometric structure of clusters is optimized using the principle of minimization of the total energy of the system; then, the electronic and spin characteristics of clusters are calculated by the density functional theory. The isotropic and anisotropic hyperfine interaction constants of the electron spin of the NV center with the nuclear spin of the nitrogen atom and ¹³C atoms located at different sites in the cluster are calculated. It is found that, in contrast to bulk clusters with [NV]-centers in which the spin density is mainly localized at the three carbon atoms that are the nearest neighbors of the vacancy of the center, upon arrangement of the NV center in the immediate proximity to the surface, the spin density is redistributed such that it is mainly localized at the three carbon atoms that are the nearest neighbors of the nitrogen atom of the center and at C atoms that form the first atomic layer of the (111) surface of the nanocrystal.     

Diamond coating deposition by synergy of thermal and laser methods—A problem revisited

Diamond coatings were deposited by synergy of the hot filament CVD method and the pulse TEA CO₂ laser, in spectroactive and spectroinactive diamond precursor atmospheres. Resulting diamond coatings are interpreted relying on evidence of scanning electron microscopy as well as microRaman spectroscopy. Thermal synergy component (hot filament) possesses an activating agent for diamond deposition, and contributes significantly to quality and extent of diamond deposition. Laser synergy component comprises a solid surface modification as well as the spectroactive gaseous atmosphere modification. Surface modification consists in changes of the diamond coating being deposited and, at the same time, in changes of the substrate surface structure. Laser modification of the spectroactive diamond precursor atmosphere means specific consumption of the precursor, which enables to skip the deposition on a defined substrate location. The resulting process of diamond coating elimination from certain, desired locations using the CO₂ laser might contribute to tailoring diamond coatings for particular applications. Additionally, the substrate laser modification could be optimized by choice of a proper spectroactive precursor concentration, or by a laser radiation multiple pass through an absorbing medium.     

Cold Ar 3 + generated by ionization of argon clusters in large helium clusters: Temperature dependence of the photofragmentation

Ionized argon clusters were generated by electron impact ionization of neutral argon clusters embedded in large neutral helium clusters. Photofragmentation spectroscopy of Ar ₃ ⁺ and Ar ₃ ⁺ He produced in this way demonstrates the strong influence of vibrational excitation on the photodissociation dynamics, and indicates the low internal energy of the latter cluster.