Formation of alkylsilane-based monolayers on gold

The formation of monolayers of alkylsilanes on a gold surface is characterized by X-ray photoelectron and reflection-absorption infrared spectroscopies. The reaction occurs through the activation of multiple Si-H bonds. Reactivity of the newly synthesized systems to oxygen and water is reported.     

Charging/discharging of Au (core)/silica (shell) nanoparticles as revealed by XPS

By recording XPS spectra while applying external voltage stress to the sample rod, we can control the extent of charging developed on core-shell-type gold nanoparticles deposited on a copper substrate, in both steady-state and time-resolved fashions. The charging manifests itself as a shift in the measured binding energy of the corresponding XPS peak. Whereas the bare gold nanoparticles exhibit no measurable binding energy shift in the Au 4f peaks, both the Au 4f and the Si 2p peaks exhibit significant and highly correlated (in time and magnitude) shifts in the case of gold (core)/silica (shell) nanoparticles. Using the shift in the Au 4f peaks, the capacitance of the 15-nm gold (core)/6-nm silica (shell) nanoparticle/nanocapacitor is estimated as 60 aF. It is further estimated that, in the fully charged situation, only 1 in 1000 silicon dioxide units in the shell carries a positive charge during our XPS analysis. Our simple method of controlling the charging, by application of an external voltage stress during XPS analysis, enables us to detect, locate, and quantify the charges developed on surface structures in a completely noncontact fashion.     

The compressibility of high purity Y bB(2)

The compressibility and phase stability of Y bB(2) are investigated under high pressure using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The bulk modules of high purity Y bB(2) is obtained as ～182?GPa using the Birch-Murnaghan equation of state. The patterns measured up to 20?GPa and the pressure dependence of normalized lattice parameters, a/a(0) and c/c(0), reveal that the compressibility of Y bB(2) is low and fairly isotropic, and this material can be classified as a hard material. X-ray photoemission studies demonstrate that Yb in Y bB(2) has a mostly trivalent valence state at room temperature. Moreover, sample preparation details provide a new insight into the high purity synthesis of Y bB(2) at ambient pressure and moderate temperatures. The presented structural and compressibility results are in agreement with the available theoretical and experimental data on binary rare-earth borides and can serve as a reliable reference for future studies.     

Characterization of Electroinitiated and Radiation Polymerized Poly(Butadiene Sulfone)

Polymerization of butadiene sulfone was accomplished by using two different methods. The first method involves electrochemical polymerization of the monomer using the constant potential electrolysis technique. Oxidation of the monomer under nitrogen atmosphere in an acetonitrile-tetrabutylammonium tetrafluoroborate system yields brown, tarry polymers. On the other hand, the second initiation method, which was radiation polymerization, yields white, powdery polymers. Structural analyses of the two polymers by various spectroscopic methods suggest a structure consisting of two different units in different proportions.     

X-ray-induced production of gold nanoparticles on a SiO(2)/Si system and in a poly(methyl methacrylate) matrix

Prolonged exposure to X-rays of HAuCl(4) deposited from an aqueous solution onto a SiO(2)/Si substrate or into a poly(methyl methacrylate) (PMMA) matrix induces reduction of the Au(3+) ions to Au(0) and subsequent nucleation to gold nanoclusters as recorded by X-ray photoelectron spectroscopy. The corresponding major oxidation product is determined as chlorine {HAuCl(4)(ads) + X-rays --> Au(ads) + (3/2)Cl(2)(ads) + HCl(ads)}, which is initially adsorbed onto the surface but eventually diffuses out of the system into the vacuum. The reduced gold atoms aggregate (three-dimensionally) into gold nanoclusters as evidenced by the variation in the binding energy during X-ray exposure, which starts as 1.3 eV but approaches a value that is 0.5 eV higher than that of the bulk gold. The disappearance of the oxidation product (Cl2p signal) and the growth of the nanoclusters (related to the measured binding energy difference between the Si2p of the oxide and Au4f of the reduced gold) exhibit first-order kinetics which is approximately 3 times slower than the reduction of Au(3+), indicating that both of the former processes are diffusion controlled. Similarly, gold ions incorporated into PMMA can also be reduced and aggregated to gold nanoclusters using 254 nm deep UV irradiation in air evidenced by UV-vis-NIR absorption spectrocopy.     

Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements

Angle-resolved X-ray photoelectron spectroscopy and contact-angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take-off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR-XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non-spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take-off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems.     

Dynamical XPS measurements for probing photoinduced voltage changes

Photoillumination with 405 nm laser causes shifts in XPS peaks of n-Si(100), and CdS. To distinguish between surface photovoltage (SPV), and charging, dynamical measurements are performed, while sample is subjected to square wave pulses of ± 10.00 V amplitude, and 10^?3–10⁵ Hz frequency. For n-Si, Si2p peaks are twinned at + 10.00 and ?10.00, yielding always 20.00 eV difference. Photoillumination shifts the twinned peaks to higher energies, but the difference is always 20.00 eV. However, for CdS, the measured binding difference of Cd3d peaks exhibits strong frequency dependence due to charging, which indicates that both fast SPV and slow charging effects are operative.     

Impedance-type measurements using XPS

An impedance type of measurement using X-ray photoelectron spectroscopy is applied for probing charging/discharging dynamics of a sample containing Rb deposited on a SiO₂/Si substrate containing an octadecyltrichlorosilane (OTS) bilayer coating. The OTS bilayer coatings have possible use as anti-relaxation wall coatings for alkali atom vapor cells in miniaturized instruments such as chip-scale atomic clocks, and/or magnetometers. The measurement consists of the application of bipolar square wave pulses of ±10.0 V amplitude to the sample with varying frequencies in the range of 10⁻² to 10² Hz while recording X-ray photoemission data. For a conducting sample this type of measurement twins all the photoemission peaks at −10.0 and +10.0 eV positions at all frequencies with exactly 20.0 eV difference between them. However, for samples amenable to charging, the difference between the twinned peaks is less than 20.0 eV, and gradually decreases at correspondingly lower frequencies. For the sample under consideration here at 0.01 Hz, the twinned O1s and Si2p peaks, representing the SiO₂ substrate, are separated by 18.2 eV, displaying a 1.8 eV charging shift. These positions differ from those of the C1s (18.0 eV) representing the OTS bilayer and the Rb3d peaks (18.1 eV). These results reveal that the Rb is electrically (perhaps also chemically) isolated from the OTS bilayer, which may be correlated with the improved performance of the OTS bilayers as anti-relaxation coatings in these alkali atom magnetometer cells.