M. Châtelet

On the mechanisms of precipitation of graphene on nickel thin films

Growth on transition metal substrates is becoming a method of choice to prepare large-area graphene foils. In the case of nickel, where carbon has a significant solubility, such a growth process includes at least two elementary steps: 1) carbon dissolution into the metal, and 2) graphene precipitation at the surface. Here, we dissolve calibrated amounts of carbon in nickel films, using carbon ion implantation, and annealing at 725°C or 900°C. We then use transmission electron microscopy to analyse the precipitation process in detail: the latter appears to imply carbon diffusion over large distances and at least two distinct microscopic mechanisms.

Evaporation of small fragments during the scattering of argon clusters at thermal kinetic energies from a graphite surface

We present a thermokinetic model together with new experimental results for the scattering of large argon clusters off a graphite surface. Both angular and time‐of‐flight distributions are shown for a large range of surface temperatures, incidence angles, and incident cluster sizes. A quantitative comparison between the proposed thermokinetic model and our measurements allows one to interpret most of the experimental results as due to thermal evaporation of very small fragments from their parent clusters gliding along the surface. The coefficient of tangential velocity conservation cF and the local temperature Tlocal of the evaporating fragments have been determined quantitatively. Although the investigated parameters were varied over a large range, Tlocal remains essentially constant around (140 ± 20) K. The coefficient cF turns out to be approximately (0.80 ± 0.05) independent of surface temperature and incident cluster size for all incidence angles larger than 40°. It increases, however, rapidly to 1.4...

Dynamic zone structure model for the surface scattering of large van der Waals clusters at thermal kinetic energies

A semiempirical dynamic zone structure model is presented to explain the behavior of the grazing exit angle component previously observed in experiment and simulation during the scattering of large van der Waals clusters from surfaces. The proposed model that is based on a simple energy balance and the Leidenfrost phenomenon does not only qualitatively reproduce the measured importance of the grazing exit angle component as a function of incident cluster size, incident velocity, angle of incidence, and surface temperature, but it also gives an order‐of‐magnitude estimate for the size of the large fragments scattered in this component and for the involved picosecond interaction times. Recent results obtained from both trajectory calculations and experiments show very good agreement with the predictions concluded from the proposed model.

Experimental evidence of enhanced diffuse monomer scattering in cluster-surface collisions. Arn on graphite

Abstract We report the first experimental observation of diffuse scattering of monomers exhibiting significant velocity thermalization in large argon clusters collisions with a pyrolytic surface. Angular distributions of the outcoming argon atoms show a diffuse component for which time-of-flight spectra are independent of incidence angle and incoming cluster size, while they vary significantly with surface temperature. The amplitude of the diffuse scattering component is enhanced by up to a factor of three for incident clusters over its value for incident monomers. This enhancement is discussed by using a simple model of atom trapping-desorption on flat surfaces.

Ultrahigh‐vacuum multichamber apparatus for molecule‐surface interaction studies

We present a complete apparatus for molecule‐surface interaction studies. Three UHV chambers are connected together allowing preparation, characterization, transfer, and experimental investigations with well‐defined surface samples under ultrahigh‐vacuum conditions. A chopped, supersonic molecular beam with well‐controlled profile enters the main UHV chamber where it is scattered by the sample under study. Detection systems have been designed to measure angular and time‐of‐flight distributions and rotational populations of beam particles scattered off the surface sample giving access to the energy exchange between the internal degrees of freedom of the scattered molecules and the surface. The characteristics of our apparatus are reported and experimental tests for nitrogen and argon molecular beams scattered off a graphite surface are shown.

Density control of electrodeposited Ni nanoparticles/nanowires inside porous anodic alumina templates by an exponential anodization voltage decrease.

Porous alumina templates have been fabricated by applying an exponential voltage decrease at the end of the anodization process. The time constant η of the exponential voltage function has been used to control the average thickness and the thickness distribution of the barrier layer at the bottom of the pores of the alumina structure. Depending on the η value, the thickness distribution of the barrier layer can be made very uniform or highly scattered, which allows us to subsequently fine tune the electrodeposition yield of nickel nanoparticles/nanowires at low voltage. As an illustration, the pore filling percentage with Ni has been varied, in a totally reproducible manner, between ∼3 and 100%. Combined with the ability to vary the pore diameter and repetition step over ∼2 orders of magnitude (by varying the anodization voltage and electrolyte type), the control of the pore filling percentage with metal particles/nanowires could bring novel approaches for the organization of nano-objects.

Normal to tangential velocity conversion in cluster‐surface collisions: ArN on graphite

Collisions between large neutral argon clusters and a pyrolytic graphite surface have been investigated at normal incidence by measuring angular distributions of density and angularly resolved time‐of‐flight distributions of scattered species (essentially monomers). These data have been taken for two surface temperatures (440 and 660 K) with cluster sizes N ranging from 400 to 8200 atoms per cluster, and at an incoming cluster energy of 72 meV/atom. A higher incident kinetic energy (115 meV/atom) has been investigated also for N=900 atoms per cluster. The experimental results have been analyzed by considering two components. First, an ‘‘ejection’’ contribution, dominant at large scattering angles, has been assigned to the evaporation of monomers from cluster material having acquired a flow velocity parallel to the surface. The average value of this flow velocity increases sharply with the incident cluster velocity in the investigated range. Second, a thermal contribution, dominant close to the surface nor...

Enrichment of binary van der Waals clusters surviving a surface collision

The dynamics of collisions between large van der Waals clusters and solid surfaces has been attracting increasing interest. Recent molecular dynamics simulations have shown that a novel chemistry, involving significant many-body effects, may take place within the cluster itself or with the surface, due to the extreme pressures and temperatures achieved in the impact region for initial cluster velocities of the order of 10 km s-1 [1].

Synthesis of conducting transparent few-layer graphene directly on glass at 450??C

Post-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq⁻¹. Finally the samples equipped with printed contacts appear to be efficient humidity sensors.

Experimental investigation of large nitrogen cluster scattering from graphite: Translational and rotational distributions of evaporated N2 molecules

We present experimental results on the translational and rotational energy transfers due to the collisions of large (N2)n clusters with a highly oriented pyrolytic graphite sample at 750 ms−1 incident velocity, for two surface temperatures Ts (440 and 580 K), for average cluster sizes n from 200 to 1000 monomers, and for incidence angles θi from 30° to 70°. Angular distributions of flux and angularly resolved time-of-flight profiles of scattered particles are measured with a quadrupole mass spectrometer while rotational state distributions are determined by resonantly enhanced multiphoton ionization. For all incidences but the largest (70°) the translational distributions of scattered molecules recorded at detection angles θd from 20° to 70° are compatible with a simple thermokinetic model assuming that monomer evaporation occurs at a well-defined temperature Tloc from the cluster gliding on the solid surface. Fits of the translational data yield Tloc values between 250 and 500 K, increasing with Ts and d...