Water adsorption on and desorption from crystalline copolymers of vinylidene fluoride with trifluoroethylene

Water adsorption and absorption on crystalline polyvinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE, 70:30), was examined by thermal desorption spectroscopy. Two distinctly different water adsorption sites are identified: one adsorbed species that resembles ice and another species that interacts more strongly with the polymer thin film. The existence of the latter species is consistent with X-ray diffraction studies of water absorbed into the bulk of copolymers of polyvinylidene fluoride with trifluoroethylene crystalline thin films. There are strong steric effects observed in the angle-resolved thermal desorption that may be a result of the large polymer thin film surface dipoles.     

Chemisorbed halogen adlayers on Fe(110): Electronic band structure and surface geometry

A series of overlayer structures, starting with a c(3 × 1) geometry are formed during the dissociative chemisorption of chlorine, bromine, and iodine on Fe(110). These adlayers provide an opportunity to systematically examine the corresponding changes in electronic band structure as a function of coverage and adsorbate. At coverage levels greater than low-energy electron diffraction patterns obtained from the halogen-covered Fe(110) surface evolve continuously with coverage. The electronic band structure was examined as a function of coverage and adsorbate species via angle-resolved ultraviolet photoelectron spectroscopy. It is concluded that the electronic band structure measurements support an incommensurate structural model for halogen overlayer geometry on Fe(110) in which an overlayer lattice, that is in general incommensurate with the substrate, rotates and compresses with increases in coverage. An alternative interpretation of the diffraction results based on antiphase domain boundaries within the overlayer is not consistent with the photoemission data. The incommensurate model emphasizes adatom-adatom interactions over adatom-substrate interactions while the reverse is true for the antiphase boundary model.     

The n-type Gd-doped HfO<Subscript>2</Subscript> to silicon heterojunction diode

Gd-doped HfO₂ films were deposited on p-type silicon substrates in a reducing atmosphere. Photoemission measurements indicate the n-type character of Gd-doped HfO₂ due to overcompensation with oxygen vacancies. The Gd 4f photoexcitation peak at 5.5 eV below the valence band maximum is identified using both resonant photoemission and first-principles calculations of the f hole. The rectifying (diode-like) properties of Gd-doped HfO₂ to silicon heterojunctions are demonstrated. PACS 79.60.-i; 68.55.Ln; 29.40.Wk; 81.05.Je     

Surface charging at the (1 0 0) surface of Cu doped and undoped Li2B4O7

We have compared the photovoltaic charging of the (1 0 0) surface termination for Cu doped and undoped Li₂B₄O₇. While the surface charging at the (1 0 0) surface of Li₂B₄O₇ is significantly greater than observed at (1 1 0) surface, the Cu doping plays a role in reducing the surface photovoltage effects. With Cu doping of Li₂B₄O₇, the surface photovoltaic charging is much diminished at the (1 0 0) surface. The density of states observed with combined photoemission and inverse photoemission remains similar to that observed for the undoped material, except in the vicinity of the conduction band edge.     

The energetics of mercury adsorption on Cu(100)

We have investigated the adsorption of mercury overlayers on Cu(100) by atom beam scattering, low energy electron diffraction and angle resolved photoemission. From our data we have calculated the isosteric heats in the adsorbed Hg layer on Cu(100) and compared these with results obtained for mercury on Fe(100), W(100) and Ni(100). We observe changes in the isosteric heat of adsorption that can be associated with the ordering of a c(2 × 2) Hg overlayer phase and the transition from a c(2 × 2) overlayer to a c(4 × 4) overlayer. The isosteric heat of adsorption is 0.50 ± 0.07 eV/atom (48 ± 7 kJ/mol) at zero coverage and reaches a maximum of 0.73 ± 0.04 eV/atom (70 ± 4 kJ/mol). From a combination of ABS and LEED, the structures of the two equilibrium ordered phases of Hg on Cu(100) have been identified, as well as the structures of several non-equilibrium phases.     

When are thin films of metals metallic?

There is an increasing body of experimental information suggesting that very thin films of materials, normally considered to be metals, exhibit behavior characteristic of a nonmetal. In almost all cases, there is a nonmetal-to-metal transition as a function of film density or thickness, frequently accompanied by a structural transition. Amazingly, this behavior seems to occur for metal films on metal substrates, as well as for metals on semiconductors. The identification of this phenomena and the subsequent explanation has been slow in developing, due to the inability to directly measure the conductivity of a submonolayer film. This paper will discuss the evidence accumulated from variety of spectroscopic experimental techniques for three systems: a Mott-Hubbard transition, a Peierls-like distortion, and a Wilson transition.     

Resonant photoemission at the 3s threshold of Ni

A resonant enhancement of valence band photoemission features in Ni near the 3s threshold is presented. The emission behavior with photon energy of the Ni-3d band is characteristic of a Fano-type resonance. In addition to the main 3d-band emission and 6eV binding energy satellite, a weak satellite is observed at 7.2eV below the Fermi level with photon energies in the vicinity of the 3s threshold.     

Halogen adsorption on Fe(100): I. The adsorption of Cl2 studied by AES,LEED, work function change and thermal desorption

The initial sticking probability of chlorine on Fe(100) at room temperature is calculated to be 0.13, and there is evidence to suggest that the chlorine adsorbs into a short lived mobile precursor state above the surface. The work function change, Δφ, is proportional to coverage and reaches a maximum value of 1.43 eV at saturation. At this coverage a c(2 × 4) LEED pattern is formed. On heating, chlorine is lost from the surface, but the mechanism is such that no detectable loss is incurred at a constant elevated temperature. The c(2 × 4) pattern is shown to be a coincidence structure formed from a $\text{(}\text{1}\text{2}\text{3}\text{?1}\text{2}\text{3}\text{)}$ net of chlorine atoms on the Fe(100) substrate. This structure is a special case of the more general $\text{(}\text{1}\text{2}\text{tan}\text{α}\text{?1}\text{2}\text{tan}\text{α}\text{)}$ structure formed at lower concentrations of chlorine. The c(2 × 4) is formed when α = 56.31°, which gives the chlorine atoms a hard sphere diameter of 0.345 nm and a concentration of 0.75 atoms per four-fold site.