Clean Graphite Surface Research Articles

Adsorption and migration behavior of molybdenum atom on graphite (0001) surface

Molybdenum atom adsorption and migration behavior on clean graphite (0001) surface have been investigated systematically by using first-principles calculations based on the density functional theory. The Mo atom was not adsorpted at the bridge site of graphite (0001) surface because Mo atom moves towards to the hollow site after geometry optimization. Mo atom has been predicted to adsorb preferentially at the hollow site of graphite (0001) surface because of the larger adsorption energy and the smaller work function. It is found from electronicproperties that Mo adsorption on graphite (0001) surface belongs to chemisorptions, which is attributed to the electrons of Mo 4d states transfer to C atoms of graphite (0001) surface. The calculated adsorptive rate suggests that the Mo atom adsorption at the hollow site of graphite (0001) surface is more stable with the increasing of temperature. The optimum energy migration paths for Mo atom diffusion from the hollow site to the nearby hollow site is hollow 1 site → bridge site → hollow 2 site by using LST/QST transition state seeking method.

FIRST PRINCIPLES DENSITY FUNCTIONAL INVESTIGATION OF SUPPORTED TUNGSTEN CLUSTER (Wn; n = 1 TO 6) ON ANCHORED GRAPHITE (0001) SURFACE

Size-selected Wn clusters can be deposited firmly on a graphite (0001) surface using a novel technique, where the positive ions (of the same metal atom species) embedded on the graphite surface by ion implantation, act as anchors. The size selected metal clusters can then soft land on this anchored surface m [Hayakawa et al., 2009]. We have carried out a systematic theoretical study of the adsorption of Wn (n = 1-6) clusters on anchored graphite (0001) surface, using state-of-art spin-polarized density functional approach. In our first-principles calculations, the graphite (0001) surface has been suitably modeled as a slab separated by large vacuum layers. Wn clusters bond on clean graphite (0001) surface with a rather weak Van-der-Waals interaction. However, on the anchored graphite (0001) surface, the Wn clusters get absorbed at the defect site with a much larger adsorption energy. We report here the results of our first-principles investigation of this supported Wn cluster system, along with their reactivity trend as a function of the cluster size (n).

Dynamics of the Oxygen Molecules Scattered from the Graphite (0001) Surface and Comparison with Experimental Data

A quasiclassical trajectory dynamics study of molecular oxygen colliding over a free of defects and clean graphite (0001) surface has been performed with a recently published density functional theory based flexible periodic London–Eyring–Polanyi–Sato potential energy surface (PES). Although the PES was mainly constructed for describing accurately the recombination of atomic oxygen over an O-preadsorbed surface, here we show that this PES is also reliable to study the scattering of O2 over a graphite surface. Thus, several initial conditions have been explored: collision energies (0.2 eV ≤ Ecol ≤ 1.2 eV), incident angles (θv = 0, 45°), surface temperatures (100 K ≤ Tsurf ≤ 900 K), and some rovibrational O2 levels (v = 0, 1, 2 and j = 1, 17, 25). The calculated polar scattering angular distributions are in good agreement with the experimental ones in a wide range of explored conditions. Moreover, the comparison with hyperthermal experimental data, which was unclear in a previous work, has been finally clar...

Model for thermal desorption of hydrogen atoms from a graphite surface based on kinetic Monte Carlo simulations

Kinetic Monte Carlo simulations are carried out to describe the mechanism which is responsible for the occurrence of two peaks in the thermal recombinative desorption of molecular ${\text{H}}_{2}$ (or ${\text{D}}_{2}$) from clean graphite surfaces, after chemisorption of H (or D) atoms. At sufficiently low hydrogen exposure to prevent formation of clusters on the surface, the main peak is shown to be due to paradimers, while the second, less intense peak is due to the loss of orthodimers and results of a competition between diffusion and evaporation of monomers and paradimers.

Photodesorption of NO from graphite(0 0 0 1) surface mediated by silver clusters

The influence of the size and shape of nanometer-scale silver particles on the photodesorption of NO from a graphite(0 0 0 1) surface has been investigated. The initial photodesorption yield from a silver precovered surface increases upto 20 times compared to the clean graphite surface under irradiation with 355 nm photons. The yield fluctuates with cluster size and shows atleast one pronounced maximum. The observation is attributed to increased optical absorption due to particle plasmon excitations. Possible mechanisms for dissipation of the plasmon energy leading to NO desorption are discussed. The mechanism behind the photodesorption of NO from clean graphite(0 0 0 1) is also investigated and believed to be a substrate-mediated process.

Atomic scale friction between clean graphite surfaces

We investigate atomic scale friction between clean graphite surfaces by using molecular dynamics. The simulation reproduces atomic scale stick-slip motion and a low frictional coefficient, both of which are observed in experiments using frictional force microscope. It is made clear that the microscopic origin of low frictional coefficients of graphite lies on the honeycomb structure of each layer, not only on the weak interlayer interaction as believed so far.

Photophysics and photochemistry of ice films on graphite

AbstractWe consider model systems consisting of submonolayer‐ to several monolayers‐thick ice films grown at ultrahigh vacuum (UHV) and low (from ∼15 K) temperatures on an atomically clean graphite (0001) surface. They are characterized experimentally with temperature‐programmed desorption‐isothermal desorption spectroscopy (TPD‐ITD) and high‐resolution electron energy loss spectroscopy (HREELS). We provide examples of how ice structure and chemical properties are altered by incoming photons. The phenomena of “photon annealing” of pure amorphous films and their structure‐selective laser ablation are discussed. In the case of water ice doped with alkali metals and coadsorbed with NO, we report on the observations of photoreactions taking place on the graphite/ice interface. Depending on the composition of the coadsorbed ice layer, irradiation leads to formation of H2, CH4, CO, NH3, and CO2. More specifically, we address the mechanisms of water photo‐dissociation at surfaces and interfaces with emphasis on the role of (i) perturbation of water molecules by coadsorbates and/or substrate; (ii) the light‐initiated electron (charge) transfer reactions leading to water splitting; and (iii) confinement of the precursor system and reaction products. In all examples the attention is on the operating mechanism for the transformations. The role of photo‐excited charge carriers in the graphite substrate is underlined. Increased understanding of photo‐stimulated reactions and processes involving water is needed to allow these processes to be effectively controlled and fully utilized.

The interaction of molecular oxygen with active sites of graphite: a theoretical study

The adsorption of molecular oxygen at defective edge sites of zigzag and armchair graphite surfaces has been investigated by adopting cluster models in conjunction with density functional theory. Several different types of chemisorbed O 2 species are identified. It was found that the defect edge sites exhibit the significant catalytic role toward the adsorption and activation of molecular oxygen. The O 2 molecule is not only able to strongly bind to these edge sites, but the O–O bond strength is obviously weakened. Moreover, the calculated adsorption energy for O 2 adsorbed on the clean graphite basal surface is fairly consistent with the weak interaction nature of O 2 with the surface observed in the experiment, indicating one-layer cluster model is an effective way to study O 2 adsorption on graphite surface in terms of accuracy and computational cost, which is in agreement with previous experience. Whereas, we note that the local detailed arrangement of edge carbon atoms can play an important effect on the adsorption of O 2 on defect surfaces.

Photophysics and photochemistry of water ice films on graphite

Our model system consists of water ice films with submonolayer to several monolayer thickness grown at ultrahigh vacuum (UHV) and low (from ∼25 K) temperatures on an atomically clean graphite (0001) surface. We characterize them experimentally with temperature programmed desorption‐isothermal desorption spectroscopy (TPD‐ITD) and high‐resolution electron energy loss spectroscopy (HREELS) and provide examples of how ice structure and chemical properties are altered by incoming photons in broad wavelength and photon flux ranges (1.6 × 1016–5.5 × 1024 photons cm−2 s−1 for λ = 220–700 nm). The phenomena of “photon annealing” of pure amorphous films and their structure‐selective laser ablation are discussed. In the case of water ice doped with alkali metals and coadsorbed with NO, we report on the observations of photoreactions taking place on the graphite‐ice interface. Depending on the composition, irradiation leads to formation of H2, CH4, CO, NH3, and CO2. In all examples the attention is on the operating mechanism for the transformations. The role of photo‐excited charge carriers in the graphite substrate is underlined.

Films of silicon nanoparticles grown by gas aggregation

The growth of nanometer-sized spherical particles was observed after condensation of pure silicon vapor on a clean graphite surface. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy were performed on these silicon nanoparticles. The particles are arranged densely packed in perfectly flat layers and exhibit a very narrow size distribution. The tunneling current plotted versus the voltage between the STM tip and the sample (I–V curves) suggests an energy gap of these films of silicon particles of several eV. However, no ultraviolet photoluminescence could be observed in the visible. The particles can be swept from the surface by the scanning tip without reduction of the resolution of the STM. Thus, the coupling of the particles to the graphite can be expected to be weak.

Adsorption of hydrogen and deuterium atoms on the (0001) graphite surface

Adsorption of H and D on HOPG surfaces was studied with thermal desorption (TDS), electronic (ELS), and high-resolution electron-energy-loss (HREELS) spectroscopies. After admission of H (D) from thermal (2000 K) atom sources to clean graphite surfaces TD spectra revealed recombinative molecular H2 (D2) desorption in a main peak around 445 K (490 K) and a minor peak at 560 K (580 K). After admission of higher fluences the main peak shifts to 460 K (500 K) and develops a shoulder at 500 K (540 K). The saturation coverages were calculated as 0.4±0.2 for H and D and initial sticking coefficients of 0.4±0.2 were obtained. Through leading edge analysis of the TD spectra desorption activation energies for H and D were determined as 0.6 and 0.95 eV, respectively. EL spectra suggest a 16% loss of the sp2 character of the surface carbon 2sp electrons upon D adsorption. HREEL spectra of H (D) graphite covered surfaces reveal in addition to two graphite-intrinsic optical phonon losses vibrational features at 1210 and 2650 cm−1 (and 640 and 1950 cm−1). These frequencies are in excellent agreement with those obtained from a recently published H (D)/graphite potential energy surface. A theoretical description of the desorption process through calculated H+H/graphite potential surfaces reveals the desorption mechanism and desorption activation energies which are in good agreement with the measured data.

Dissociation and desorption of ferrocene on graphite by low energy electron impact

Ferrocene is an iron-containing organometallic molecule with the potential to be used for fabrication of metallic structures on the nanometre scale. We have used high resolution electron energy loss spectroscopy and scanning tunnelling microscopy to study the influence of low energy electron impact on ferrocene physisorbed on graphite. At electron energies below 10 eV we find that the ferrocene molecules are readily desorbed, leaving a clean graphite surface behind. For energies of 20–100 eV we find evidence of partial fragmentation of the ferrocene. At higher energies, exceeding 200 eV, there is complete disintegration of the molecules, leaving iron-containing nanoclusters in a stable film on the surface. Comparisons are drawn with photon-, thermal- and very high electron-energy induced dissociation and desorption.

Multistage gas–surface interaction model for the direct simulation Monte Carlo method

A multistage (MS) gas–surface interaction model for a monatomic/diatomic gas molecule interacting with a solid surface is presented, based on the analysis of molecular dynamics (MD) simulations and a model equation derived from the classical theory of an ellipsoid hitting a hard cube. This model is developed for use with the direct simulation Monte Carlo (DSMC) method and belongs to the thermal scattering regime. The molecular dynamics method is used for the molecular-level understanding of the scattering of O2, N2, and Ar from a graphite surface. The basic idea of the model is to separate the collision into three stages. At stages 1 and 2, the energy and scattering direction are determined by the model equation. At stage 3, according to the translational energy, the molecule is determined to scatter, re-enter or be trapped by the surface. Re-entering molecules return to stage 1. The model parameters are determined from our MD database. Experiments are also performed by scattering a supersonic O2 molecular beam from a clean graphite surface in an ultrahigh vacuum chamber. The in-plane scattering distribution, out-of-plane scattering distribution, and in-plane velocity distribution of the model show good agreement with those of molecular beam experiments. A model equation was included in the MS model to maintain thermal equilibrium between a gas and a surface at the same temperature when applied to DSMC simulations and the results are also shown. The high accuracy of the model clearly shows that such multiple-scale analysis can lead to the development of realistic models of the gas–surface interaction.

Molecular Dynamics Study of Gas-Surface Interaction. Construction of a Gas-Surface Scattering Model.

The Multistage model for diatomic molecules scattering from a solid surface is presented, which is based on a large body of data obtained from molecular dynamics simulations and molecular beam experiments. The molecular dynamics method is used for the numerical analysis of the scattering of an oxygen gas molecule from a clean graphite surface. The angular and velocity distributions agree well with experimental results, which were shown in our previous report. The scattering direction, translational energy and rotational energy of the gas molecule after each collision are saved as the data base. The basic idea of the model is to separate the collision into different stages. At each stage, the energy loss, the scattering direction and the trapping probability of the gas molecule are determined. The angular and velocity distribution of the model agrees with those of molecular beam experiments.

Molecular Beam Study of Gas-Surface Interaction.(1st Report, In-plane and Out-of-plane Scattering of Gas Molecule).

The experiment of a gas molecule scattering from a clean graphite surface is performed using the molecular beam technique. The molecular beam experiment is performed in the ultra high vacuum molecular beam scattering system. The supersonic molecular beam is produced in a three stage differentially pumped vacuum system by expansion of gas through a nozzle. In the second stage of the beam line, a rotating disc chopper is mounted to do time of-flight measurements. The base pressure in the main chamber where the graphite is placed is 3.7×10-7Pa (2.8×10-9 Torr). The graphite sample is mounted on a six-axis manipulator. The scattered molecule is detected by a rotatable quadrupole mass filter detector. By using the manipulator and the detector, the in-plane and out-of-plane scattering of a gas molecule can be measured.

Adsorption of potassium and oxygen on graphite: A theoretical study

We have performed electronic structure calculations of the interaction of potassium and oxygen with graphite (GR), individually and as coadsorbates. We use up to three graphite planes to represent the graphite surface, but we show that the main physics is correctly described by a single graphite layer. At low coverage the potassium–graphite bond is largely ionic, and the variation of the K–GR bond energy with the lateral position of the K atom in the graphite unit cell is very small. We study the interaction between atomic oxygen and graphite. We find that O binds strongest at the bridge site, but the barrier for diffusion is rather small. The frequency for the perpendicular O–graphite vibrational mode is remarkably low (53 meV), reflecting the relative slow variation of the O–graphite interaction energy with the separation z between the O atom and the graphite surface. We consider the adsorption of O2 on a clean graphite surface and on a graphite surface with a low concentration of potassium. On the clean surface the O2–graphite interaction is found to be repulsive (the weak attractive van der Waals interaction is not included in our theoretical method), in accordance with the extremely low sticking coefficient observed for O2 on clean graphite. When potassium is adsorbed on the graphite surface, O2 chemisorbs at the potassium sites which is consistent with the large sticking coefficient observed for O2 on a potassium covered surface. The energy barrier towards dissociation of O2 on the clean graphite surface is estimated to be similar to that of gas phase O2. For O2 on K/graphite we find that O2 chemisorbs “side-on” K, and that the barrier for dissociation is much smaller than in the gas phase or on the clean graphite surface.

Molecular Dynamic Study of Gas-Surface Interaction. 3rd Report. The Construction of a Gas-Surface Scattering Model.

The Multi-Stage model for diatomic molecules scattering from solid surfaces is presented, which is based on the large body of data obtained from molecular dynamics calculations and molecular beam experiments. The molecular dynamics method is used for the numerical analysis on the scattering of an oxygen gas molecule from a clean graphite surface. The angular and velocity distribution agrees well with experimental results, which was shown in our previous report. The scattering direction, translational energy and rotational energy of the gas molecule after each collision is saved for the data base. The basic idea of the model is to separate the collision into different stages. At each stage, the energy loss, the scattering direction and the trapping probability of the gas molecule will be determined. The angular and velocity distribution of the model agrees with those of molecular beam experiments.

Molecular Dynamics Study of Gas-Surface Interaction. 2nd Report. Analysis on the Energy Distribution after the Collision and the Vibration of the Potential Energy Surface.

The scattering of an oxygen gas molecule from a clean graphite surface is simulated using the molecular dynamics method. According to our previous report, the numerical simulation has shown quantitative agreement with experimental results, indicating that the method can simulate the trajectory of a scattering gas molecule with high accuracy. Here, the translational and rotational energy distribution after the first collision is analyzed. The translational energy distribution is fitted to a gaussian distribution. The parameters obtained by the fitting process obey the model equation that was obtained by assuming that the gas-surface collision is an ellipse hitting a hard-cube. The rotational energy distribution is fitted to the Boltzmann distribution and the non-Boltzmann distribution. The parameters for rotation also obey the model equation. Also, the vibration of the potential energy surface was analyzed and it was found that the amplitude can be defined as a function of surface temperature.

Measurement of Adsorbed Amount of Oxygen on Clean (0001) Graphite Surfaces by Using the Knudsen Flow Capillary Method.

The amounts of adsorbed oxygen on clean (0001) surface of natural graphite at 23°C has been determined by using the Knudsen flow capillary method. Graphite samples were the flattened (0001) single crystals (135 pieces, total weight: 0.146g, total geometrical surface area: 22.2cm2 ) occur in South Africa and the scaly single crystals (36, 326 pieces, total weight 0.35g, total geometrical surface area 219cm2) occur in Guatemala. The results obtained indicate that the saturated quantities of adsorbates are 1.7×1015 (molecules/cm2) for the South Africa-graphite and 9×1014 (molecules/cm2) for the Guatemala-graphite, respectively. These data suggest that oxygen molecules are adsorbed on the basal plane of graphite surface.

Molecular Dynamics Study of Gas-Surface Interaction. 1st Report. Numerical Simulation of O2/Graphite.

The scattering of an oxygen gas molecule from a graphite surface is simulated using the molecular dynamics method. The numerical simulation is compared with experimental results obtained by scattering a supersonic molecular beam of O2 from a clean graphite surface in a UHV chamber. The angular distributions of scattered O2 of both results show good agreement. The translational and rotational energy loss of the gas molecule at the first collision is also analyzed. The rotational energy loss is most affected by the initial rotational energy, and the translational energy loss is most affected by the initial normal translational energy. Both energy losses are also affected by energy transfer. Comparison with a monatomic gas molecule shows that the core size and the potential well are important factors for the scattering distribution and the energy loss of the gas molecule.