Palladium was deposited gradually under ultrahigh vacuum onto a well-defined surface of (0001)-oriented n-type GaN, at room temperature. Each deposition step was followed by annealing. Physicochemical properties of the Pd adlayers were in situ investigated prior to and after annealing by the X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, low-energy electron diffraction, scanning tunneling microscopy and atomic force microscopy techniques. Annealing resulted in the formation of GaPd 2 and GaPd intermetallic compounds at 550°C and at 800°C. Even for thicker layers, the compounds were strongly dispersed, forming 3D nanostructures. The substrate uncovered by the compounds revealed Ga-rich GaN(0001)-(1 9 1) surface. Formation of Ga-Pd-N bonds or Pd nitrides was not detected at the surface. The Ga-Pd intermetallic compound surface engineered on the GaN(0001) substrate can be used as the strongly dispersed catalyst or a model catalyst.
Metal-lattice plasma is treated as a neutral two-component two-phase system of 2D surface and 3D bulk. Free electron density and bulk chemical potential are used as intensive parameters of the system with the phase boundary position determined in the crystalline lattice. A semiempirical expression for the electron screened electrostatic potential is constructed using the lattice-plasma polarization concept. It comprises an image term and three repulsion/attraction terms of second and fourth orders. The novel curve has two extremes and agrees with certain theoretical forms of potential. A practical formula for the electron work function of metals and a simplified schema of electronic structure at the metal/vacuum interface are proposed. This yields 10.44 eV for the Fermi energy of free electron gas; −5.817 eV for the Fermi energy level; 4.509 eV for the average work function of bcc tungsten. Selected data are also given for fcc Cu and hcp Re. For harmonic frequencies ∼ 10E16 per s of the self-excited metal-lattice plasma, energy gaps of 14.54 and 8.02 eV are found, which correspond to the bulk and surface plasmons, respectively. Further extension of this thermodynamics and metal-lattice theory based approach may contribute to a better understanding of theoretical models which are employed in chemical physics, catalysis and materials science of nanostructures.
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