A momentum space view of the surface chemical bond

Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum.     

Temperature dependence of sorptive deformation

A study has been made of the temperature dependence of deformation of CaA, CaX, and LaX zeolites when krypton or xenon is adsorbed, and also the deformation of activated carbon when dimethyl ether is adsorbed. It has been shown that the volume compressibility and the standard chemical potential of the adsorbate in a vacancy solution, within the interval of temperatures and pressures that were investigated, can be considered as linear functions of temperature. In this case, knowledge of the temperature dependence of sorptive deformation offers a means for calculating sorptive deformation curves for other temperatures.Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow. Institute of Petrochemical Synthesis, Russian Academy of Sciences. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 23–28, January, 1992.     

Fluctuations and phase separation in a quasi-one-dimensional system

Phase transitions in a quasi-one-dimensional surface system on a metal substrate are investigated as a function of temperature. Upon cooling the system shows a loss of long-range order, fluctuations, and a transition into an inhomogeneous ground state due to competition of local adsorbate-adsorbate interactions with an incommensurate charge density wave. This agrees with a general phase diagram for correlated systems and high-temperature superconductors. The model surface system allows direct imaging of the fluctuations and the glassy inhomogeneous ground state by scanning tunneling microscopy.     

Scanning tunneling microscopy imaging of NiO(100)(1 × 1) islands embedded in Ag(100)

The imaging of NiO(100)(1 × 1) islands embedded in Ag(100) by scanning tunneling microscopy is addressed. As a function of tunneling conditions and tip termination it is possible to resolve the NiO–vacuum interface, the second oxide layer as well as the NiO-substrate interface with atomic contrast. We find that for sub-monolayer coverages of NiO the oxide islands consist of an essentially defect-free surface layer at the vacuum interface with a number of NiO second layer patches incorporated into the Ag substrate underneath. The oxide layer is surrounded by a rim of a NiO bilayer of monoatomic width. A reduction of the density of states between a NiO monolayer and local NiO bilayer stackings is suggested to be responsible for the observed appearance of mosaic patches at the island surface.     

Diffusion versus sticking anisotropy: Anisotropic growth of organic molecular films

The molecular/crystal orientation and morphology of active molecular structures is a key determinant for the function of nanoscaled organic devices, yet little is known regarding the processes that govern thin film growth. Here, we show that either sticking or diffusion anisotropy can control the growth depending on preparation conditions. This is illustrated by an investigation into the growth of sexiphenyl (6P) on the anisotropic TiO₂(1 1 0)-(1 × 1) single crystal surface. The great differences in crystallite orientation and morphology observed are explained by the domination of the growth kinetics by either sticking or diffusion anisotropy depending on growth temperature.