Atomistic simulations of the sliding friction of graphene flakes

Using a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experiments probing the slipperiness of graphene flakes made slide against a graphite surface. Compared to previous theoretical models, where the flake was assumed to be geometrically perfect and rigid, while the substrate is represented by a static periodic potential, our fully-atomistic model includes quantum mechanics with the chemistry of bond breaking and bond formation, and the flexibility of the flake. These realistic features, include in particular the crucial role of the flake rotation in determining the static friction, in qualitative agreement with experimental observations.     

Spin polarization in carbon nanostructures with disclinations

We performed ab initio calculations, using density functional theory, to study spin polarization in carbon nanostructures with disclinations. The results indicate that compounds with positive and negative Gaussian curvature may exhibit a net magnetic moment in the ground state. Additionally, we can conclude that, carbon compounds that display an odd number of pentagons and heptagons, present polarization in the ground state.     

Gas adsorption on a single walled carbon nanotube-model simulation

We simulate the conduction variation of a gas-adsorbed carbon nanotube by a hybridization model, which has been previously used to simulate the gas adsorption on a nanographite ribbon. Two energy parameters, hybridization interaction and orbital energy level, are employed to simulate and distinguish the adsorbed gases. Two mechanisms, carrier localization and charge distribution, coexist in the gas adsorption process and provide a qualitative explanation for the current increase or decrease in gas adsorption experiments for the carbon nanotube.     

Symmetry constraints on phonon dispersion in graphene

We calculate the phonon dispersion for graphene with interactions between the first, second, and third nearest neighbors in the framework of the Born-von Karman model taking into account the constraints imposed by the lattice symmetry. Analytical expressions give the nonzero sound velocity for the out-of-plane (bending) mode. The dispersion of four in-plane modes is determined by coupled equations. Values of the force constants are found in fitting with frequencies at critical points and elastic constants measured on graphite.     

Effects of peak current density on the mechanical properties of nanocrystalline Ni-Co alloys produced by pulse electrodeposition

Cobalt content, grain size, microhardness and tensile strength of nanocrystalline Ni-Co deposits produced from a solution containing saccharin and cobalt sulfate at constant electrodeposition conditions (pulse on-time T_on at 1 ms and pulse off-time T_off at 15 ms) but varying the peak current density J_p were investigated. It is found that an increase in J_p makes the deposit Co content lower, colony-like morphology more obvious, grain size smaller, and hardness and tensile strength higher. All of the facts are believed to result from the higher overpotential and nucleation rates caused by the J_p increase. But its further increase could lead to reduction in the hardness and tensile strength. Peak current densities in the range of 100-120 A dm⁻² are recommended for the preparation of nanostructured Ni-Co alloy deposits with grain sizes in the range of 15-20 nm, containing 7-8% Co, possessing hardness of 590-600 kg mm⁻² and tensile strength of 1180-1200 MPa—significantly higher than the strength of pure nickel deposit which is produced by the similar method and gets similar grain size.     

Recent advances in superconductivity of covalent superconductors

A summary and critical analysis are given of recent results on superconductivity in diamond, Si, SiC, III–V and IV–VI semiconductors, as well as graphite and fullerites intercalated with alkali metals. Superconductivity in III–V semiconductors is questioned, and proposals are outlined for further enhancement of the transition temperature in diamond and intercalated graphite.     

Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H₂, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. Received: 16 October 2000 / Accepted: 15 November 2000 / Published online: 9 February 2001     

High quality CVD diamond: a Raman scattering and photoluminescence study

High quality synthetic diamonds were grown on single-crystal silicon by microwave plasma enhanced chemical vapour deposition (CVD). A careful optimisation of both the experimental setup and the growth parameters was necessary before that the achievement of the best results was made possible. The films were deposited using a CH₄-H₂ gas mixture at methane concentrations variable in the range 0.6-2.2%, while the substrate temperature was fixed at 750 ^°C. Raman spectroscopy and photoluminescence (PL) were utilised to monitor the quality of the deposited films and to study the spatial distribution of defects, respectively. Micro-Raman analysis shows that linewidths of the diamond peak lower than 2.4 cm^-1 can be easily measured at the growth surface, indicating that the crystalline quality of individual grains is comparable to that of the best natural diamonds. The excellent phase purity of the diamond microcrystals at the growth surface is witnessed by the complete absence of any non-diamond carbon feature and by a very weak luminescence background in the 1.6-2.4 eV spectral range. A worsening of the quality of the diamond particles is found moving from the growth surface towards the film-substrate interface. A photoluminescence feature at about 1.68 eV, commonly associated to Si impurities, is distinctly observed as the exciting laser beam is focused close to the interface. A progressive degradation of the global quality of the films is found with increasing methane concentration in the gas mixture, as witnessed by an increased PL background in the films grown at higher methane concentrations. Received 24 November 2000     

Electroluminescence from Multilayered Diamond/CeF3/SiO2 Films

We report a thin film electroluminescent device with a three-layer structure （diamond/CeF3/SiO2 films）, which has a luminance of 1.5 cd/m^2 at dc voltage 215 V. The electroluminescence spectrum at room temperature shows that the main peaks locate at 527 and 593nm, which are attributed to isolated emission centers of Ce^3＋ ions.     

Hydrogen storage using physisorption – materials demands

A survey is presented of the storage capacities of a large number of different adsorbents for hydrogen at 77 K and 1 bar. Results are evaluated to examine the feasibility and perspectives of transportable and reversible storage systems based on physisorption of hydrogen on adsorbents. It is concluded that microporousadsorbents, e.g. zeolites and activated carbons, display appreciable sorption capacities. Based on their micropore volume (∼1 ml/g) carbon-based sorbents display the largest adsorption, viz. 238 ml (STP)/g, at the prevailing conditions. Optimization of sorbent and adsorption conditions is expected to lead to adsorption of ∼560 ml (STP)/g, close to targets set for mobile applications. Received: 9 January 2001 / Accepted: 27 January 2001 / Published online: 23 March 2001