We show that for metal/graphene/dielectric sandwich structures, charge doping in graphene depends on both the work functions of the metal and the dielectric. Using C-1s core level photoemission spectroscopy we determine the charge doping in graphene for one-sided metal contacts as well as for sandwich structures that are commonly used in graphene devices. The measured Fermi-level shifts are in good agreement with a model that predicts that the difference in charge doping for graphene on a metal compared to graphene sandwiched between a metal and dielectric is given by ΔEF ≈ 0.44 × √(Φmetal − Φdielectric)

No abstract

Integration of graphene with other materials by direct growth, i.e., not using mechanical transfer procedures, is investigated on the example of metal/graphene/dielectric heterostructures. Such structures may become useful in spintronics applications using graphene as a spin-filter. Here, we systematically discuss the optimization of synthesis procedures for every layer of the heterostructure and characterize the material by imaging and diffraction methods. 300 nm thick contiguous (111) Ni-films are grown by physical vapor deposition on YSZ(111) or Al2O3(0001) substrates. Subsequently, chemical vapor deposition growth of graphene in ultra-high vacuum (UHV) is compared to tube-furnace synthesis. Only under UHV conditions, monolayer graphene in registry with Ni(111) has been obtained. In the tube furnace, mono- and bilayer graphene is obtained at growth temperatures of ∼800 °C, while at 900 °C, non-uniform thick graphene multilayers are formed. Y2O3 films grown by reactive molecular beam epitaxy in UHV covers the graphene/Ni(111) surface uniformly. Annealing to 500 °C results in crystallization of the yttria with a (111) surface orientation.

The interactions of cyanide species with a copper (001) surface were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Adsorbed cyanide species (CN(a)) undergo recombinative desorption evolving molecular cyanogen (C(2)N(2)). As the adsorbed CN species charge upon adsorption, mutually repulsive dipolar interactions lead to a marked desorption energy reduction with increasing CN(a) coverages. Two new TPD analysis approaches were developed, which used only accurately discernible observables and which do not assume constant desorption energies, E(d), and pre-exponential values, ν. These two approaches demonstrated a linear variation of E(d) with instantaneous coverage. The first approach involved an analysis of the variations of desorption peak asymmetry with initial CN coverages. The second quantitative approach utilized only temperatures and intensities of TPD peaks, together with deduced surface coverages at the peak maxima, also as a function of initial surface coverages. Parameters derived from the latter approach were utilized as initial inputs for a comprehensive curve fit analysis technique. Excellent fits for all experimental desorption curves were produced in simulations. The curve fit analysis confirms that the activation energy of desorption of 170-180 kJ/mol at low coverage decreases by up to 14-15 kJ/mol at CN saturation.

Methane and molecular hydrogen desorption from a methyl and hydrogen exposed Cu(001) surface is investigated. Both gaseous products are observed nearly simultaneously within two temperature regimes separated by more than 100 K. The lower temperature desorption, at ∼325 K, is believed to result from two processes which compete for adsorbed atomic hydrogen: methyl reduction and associative hydrogen desorption. The higher-temperature competitive desorption is initiated after the onset of thermal decomposition of remaining methyl species, at ∼420 K. Kinetic simulations of the two presumed competing reactions are used to show observable and comparable methane and hydrogen evolution can occur in two temperature regimes, only with a precise balance of kinetic parameters, but fail to accurately reproduce the observed small differences in CH(4) and H(2) peak desorption temperatures. It is concluded that either the utilized desorption kinetics are inaccurate at low H((a)) coverages or rapid desorption, or the same reactions are not competitive at higher temperatures and an alternative active mechanism for product evolution must exist.