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Solid solutions of nanophase cerium oxides have been prepared and the relationship between their bulk crystal structure and surface characteristics has been studied at room temperature with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS, Addresses: Palmqvist AEC, Royal Inst Technol KTH, Dept Chem Mat, S-10044 Stockholm, Sweden. Royal Inst Technol KTH, Dept Chem Mat, S-10044 Stockholm, Sweden. Uppsala Univ, Dept Phys, S-75121 Uppsala, Sweden.

The formation of self-assembled chemisorbed layers of cystamine, cysteamine, and 4-aminothiophenol on gold has been studied by XPS and voltammetry. These compounds, often used in the preparation of biosensors and modified electrodes, are shown to yield surface coverages of approximately 80% of that of a octadecanethiol monolayer within 5 min in millimolar aqueous and ethanolic solutions. The results of the XPS experiments reveal that a shoulder on the S 2p(3/2) peak (situated at 162.1 eV) develops at 161.3 eV upon increasing the adsorption time from minutes to 1 week and that the initial rate of formation of the shoulder is higher for cystamine than for cysteamine. This shoulder is believed to be due to the presence of a sulfur species with a higher coordination number with respect to gold. Increased adsorption times also give rise to increased amounts of oxidized carbon and sulfur in the films. The oxidation of the sulfur in the thiols results in a detachment of the molecules from the gold surface, as indicated by XPS experiments with different takeoff angles. The main N 1s peak for cystamine is shifted toward higher binding energies for increasing adsorption times while two prominent nitrogen peaks are generally seen for cysteamine. For cysteamine, increasing adsorption times result in an increase of the main nitrogen component at the higher binding energy, yielding an apparent shift in the nitrogen peak with time similar to that seen for cystamine. Possible explanations for these experimental findings are discussed. Cystamine, cysteamine, and 4-aminothiophenol films on gold are shown to be irreversibly oxidized in the gold oxide formation region. On the basis of evaluation of the oxidation charge, surface coverages of approximately 1 x 10(-9) mol/cm(2) were obtained for adsorption times between 5 min and 1 week., Addresses: Wirde M, Univ Uppsala, Dept Phys, Box 530, SE-75121 Uppsala, Sweden. Univ Uppsala, Dept Phys, SE-75121 Uppsala, Sweden. Univ Uppsala, Dept Analyt Chem, SE-75121 Uppsala, Sweden.

Tricyclohexylphosphine (TCHP) adsorbates on rhodium, prepared both from solution and by sublimation in UHV are studied by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRAS), and temperature-programmed desorption (TP, Addresses: Uvdal K, Linkoping Univ, Appl Phys Lab, Dept Phys & Measurement Technol, S-58183 Linkoping, Sweden. Linkoping Univ, Appl Phys Lab, Dept Phys & Measurement Technol, S-58183 Linkoping, Sweden. Swedish Univ Agr Sci, Dept Chem, S-75007 Uppsala, Swe

The adsorption of 3-(2-thioethyl)pyrrole (3-TEP) and 1-(2-thioethyl)pyrrole (1-TEP) on gold has been studied in ultrahigh vacuum with infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption mass, Addresses: Liedberg B, Linkoping Univ, Dept Phys & Measurement Technol, Appl Phys Lab, S-58183 Linkoping, Sweden. Linkoping Univ, Dept Phys & Measurement Technol, Appl Phys Lab, S-58183 Linkoping, Sweden. Univ Uppsala, Dept Phys, S-75121 Uppsala, Sweden.

Molecular gradients of omega-substituted alkanethiols HS(CH2)(15)X (X:CH3, CH2OH, CO2CH3, CH2C drop N) were prepared on sputtered gold films by using a novel cross-diffusion methodology. Ellipsometry, contact angle measurements, infrared spectroscopy, and, Addresses: Liedberg B, LINKOPING UNIV, APPL PHYS LAB, S-58183 LINKOPING, SWEDEN. ACAD SINICA, INST CHEM, TAIPEI 115, TAIWAN.

Self-assembled films of octadecanethiols on polycrystalline Au have been studied with monochromated X-ray photoelectron spectroscopy (XPS). The focus has been on the S-Au interface. It is demonstrated that ionizing radiation causes breaking of S-Au bonds, Addresses: Wirde M, UNIV UPPSALA, DEPT PHYS, BOX 530, S-75121 UPPSALA, SWEDEN. PENN STATE COLL, DEPT CHEM & MAT SCI, UNIVERSITY PK, PA 16802.

alpha-Functionalized terthiophenes containing disulfide (-S-T-3-H)(2) and alkanethiol (HS-(CH2)(11)-T-3-H) anchoring groups have been synthesized for direct immobilization onto gold. Monolayer structures of these compounds are prepared by spontaneous asse, Addresses: Liedberg B, LINKOPING UNIV, APPL PHYS LAB, S-58183 LINKOPING, SWEDEN. UNIV UPPSALA, DEPT PHYS, S-75121 UPPSALA, SWEDEN. UNIV ULM, INST ORGAN CHEM 2, D-89081 ULM, GERMANY. UNIV TUBINGEN, INST PHYS & THEORET CHEM, D-72076 TUBINGEN, GERMANY.

Mixed self-assembled monolayers provide an attractive model system for investigating the role of different molecules in biological membranes. This paper describes the preparation and characterization of a novel type of mixed monolayer assemblies composed, Addresses: LINKOPING UNIV, APPL PHYS LAB, S-58183 LINKOPING, SWEDEN. FREE UNIV BRUSSELS, INST PHARM, B-1050 BRUSSELS, BELGIUM. UNIV UPPSALA, DEPT PHYS, S-75121 UPPSALA, SWEDEN.

Mixed self-assembled monolayers provide an attractive model system for investigating the role of different molecules in biological membranes. This paper describes the preparation and characterization of a novel type of mixed monolayer assemblies composed of thiol analogues of cholesterol and fatty acid. The mixed: monolayers are prepared by coadsorbing 11-mercaptoundecanoic acid (MUA) and thiocholesterol(cholest-5-ene-3 beta-thiol, TC) from solution directly onto evaporated gold surfaces. The influence of TC on the molecular composition and conformation in the mixed monolayer is analyzed by using a combination of infrared reflection-absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), ellipsometry, contact angle measurement, and cyclic voltammetry. The results indicate that the TC molecules maintain their conformation in the mixed monolayers, whereas the MUA molecules display a significantly more disordered conformation as compared to the MUA molecules in the pure monolayer. Cyclic voltammetry shows that the mixed monolayers are more densely packed and less permeable than the pure TC and MUA monolayers. The kinetics of the coadsorption of TC and MUA from ethanol indicates that adsorption of TC initially is strongly preferred over MUA but that MUA dominates over TC at long coadsorption times. This is because there is a larger energy gain per unit area in forming monolayers with MUA, Further, it is also seen that the number of molecules per unit area changes with the molecular composition, as a consequence of the different sizes of TC and MUA. We present herein a method for calculating the mole fraction of TC on the gold surface, (chi TC), which accounts for this variation. As a consequence of the dissimilar size and shape of the two molecules, the wetting properties of the mixed monolayer are found to be mainly governed by the fractional area of TC, rather than by the molecular composition of TC, (chi TC).

alpha-Functionalized terthiophenes containing disulfide (-S-T-3-H)(2) and alkanethiol (HS-(CH2)(11)-T-3-H) anchoring groups have been synthesized for direct immobilization onto gold. Monolayer structures of these compounds are prepared by spontaneous assembly from ethanol solutions on evaporated gold substrates and thoroughly characterized by ellipsometry, contact angle goniometry, infrared and X-ray photoelectron spectroscopy, and cyclic voltammetry. The two molecules coordinate to the gold substrate exclusively via the anchoring groups upon formation of gold-thiolate bonds. The kinetics of monolayer formation vary dramatically for the two compounds. The alkanethiol analogue assembles rapidly, within a few minutes, and forms a densely packed and highly organized monolayer, with the alkyl chains in an almost perfect all-trans conformation and the C-alpha-C-alpha axis of the alpha-T-3 units tilted about 14 degrees away from the surface normal. The assembly process is much slower for the disulfide, but an organized monolayer with an average alpha-T-3 chain tilt of about 33 degrees will eventually form when the assembly is allowed to equilibrate with a solution containing the disulfide for at least 1 day. Moreover, the two monolayer assemblies also display a remarkably different electrochemical, behavior. The heterogeneous electron-transfer rate at the disulfide-covered gold substrate is almost indistinguishable from that at bare gold, suggesting that the assembly contains a large number of easily accessible defects. An alternative mechanism for explaining the large electron-transfer rate involving electronic coupling via the conjugated pi-system of the alpha-T-3 units is also proposed. The electrochemical response is significantly reduced for the HS-(CH2)(11)-T-3-H assembly, but another type of defects, the so-called shallow defects originating from sparsely populated areas on the electrode surface, can be identified.