Normal and surface-enhanced Raman spectroscopy of nitroazobenzene submonolayers and multilayers on carbon and silver surfaces

Functionalization of Cerium Oxide Nanoparticles to Influence Hydrophobic Properties

Electroless functionalization of cerium oxide nanoparticles (NPs) based on the grafting of aryl groups from the reduction of diazonium salts is presented as a useful and facile method for enhancing the properties of the NPs. For this study, 4-methyl-, 4-ethyl-, and 4- n-butyl-benzene diazonium salts were used as model molecules to demonstrate the ability to change the hydrophobic properties of the cerium oxide (CeO₂) NPs. The grafting reaction was investigated under two reducing environments: the addition of a chemical reducing agent and the use of cerium oxide's native reducing property. Spectroscopic evidence for the successful attachment of aryl groups to the CeO₂ NPs was given by infrared and ¹³C SS-NMR, which clearly detect characteristic aryl C-C peaks and the alkyl chains. X-ray diffraction results confirmed that the NPs underlying the crystal structure was unaffected by the grafting process. Thermal gravimetric analysis of the NPs suggested that this method enables the formation of multilayers at the surface, as well as an increase in the hydrophobic character. Hydrophobic properties of the resultant NPs further examined with a water contact angle test on pressed pellets revealed increase in hydrophobicity with increasing alkyl chain length. This research opens up new possibilities for controlling the surface chemical composition of CeO₂ NPs as well as other NPs using procedures operated in aqueous environments at room temperature.

Charge transfer process at the Ag/MPH/TiO2interface by SERS: alignment of the Fermi level

A nanoscale metal–molecule–semiconductor assembly (Ag/4-mercaptophenol/TiO₂) has been fabricated over Au nanoparticle (NP) films as a model to study the interfacial charge transfer (CT) effects involved in Ag/MPH/TiO₂.

Electron-beam evaporated silicon as a top contact for molecular electronic device fabrication

This paper discusses the electronic properties of molecular devices made using covalently bonded molecular layers on carbon surfaces with evaporated silicon top contacts. The Cu "top contact" of previously reported carbon/molecule/Cu devices was replaced with e-beam deposited Si in order to avoid Cu oxidation or electromigration, and provide further insight into electron transport mechanisms. The fabrication and characterization of the devices is detailed, including a spectroscopic assessment of the molecular layer integrity after top contact deposition. The electronic, optical, and structural properties of the evaporated Si films are assessed in order to determine the optical gap, work function, and film structure, and show that the electron beam evaporated Si films are amorphous and have suitable conductivity for molecular junction fabrication. The electronic characteristics of Si top contact molecular junctions made using different molecular layer structures and thicknesses are used to evaluate electron transport in these devices. Finally, carbon/molecule/silicon devices are compared to analogous carbon/molecule/metal junctions and the possible factors that control the conductance of molecular devices with differing contact materials are discussed.

Analytical chemistry in molecular electronics

This review discusses the analytical characterization of molecular electronic devices and structures relevant thereto. In particular, we outline the methods for probing molecular junctions, which contain an ensemble of molecules between two contacts. We discuss the analytical methods that aid in the fabrication and characterization of molecular junctions, beginning with the confirmation of the placement of a molecular layer on a conductive or semiconductive substrate. We emphasize methods that provide information about the molecular layer in the junction and outline techniques to ensure molecular layer integrity after the complete fabrication of a device. In addition, we discuss the analytical information derived during the actual device operation.

Optical interference effects in the design of substrates for surface-enhanced Raman spectroscopy

This paper presents results showing that the design of substrates used for surface-enhanced Raman spectroscopy (SERS) can impact the apparent enhancement factors (EFs) obtained due to optical interference effects that are distinct from SERS, providing additional enhancement of the Raman intensity. Thus, a combination of SERS and a substrate designed to maximize interference-based enhancement is demonstrated to give additional Raman intensity above that observed for SERS alone. The system explored is 4-nitroazobenzene (NAB) and biphenyl (BP) chemisorbed on a nanostructured silver film obtained by vacuum deposition of Ag on thermally oxidized silicon wafers. The enhancing silver layer is partially transparent, enabling a standing wave to form as a result of the combination of the incident light and light reflected from the underlying Si substrate (i.e., light that passes through the Ag and the intervening dielectric layer of SiO(x)). The Raman intensity is measured as a function of the thickness of the thermal oxide layer in the range from approximately 150 to approximately 400 nm, and despite a lack of morphological variation in the silver films, there is a strong dependence of the Raman intensity on the oxide thickness. The Raman signal for the optimal SiO(x) interlayer thickness is 38 times higher than the intensity obtained when the Ag particles are deposited directly onto Si (with native oxide). To account for the trends observed in the Raman intensity versus thickness data, calculations of the relative mean square electric field (MSEF) at the surface of the SiO(x) are carried out. These calculations are also used to further optimize the experimental setup.

Ultraviolet-visible spectroelectrochemistry of chemisorbed molecular layers on optically transparent carbon electrodes

Pyrolysis of diluted commercial photoresist spun onto quartz slides yields optically transparent graphitic films. Transparent carbon electrodes approximately 6 nm thick can be reproducibly prepared, with a maximum absorbance in the ultraviolet-visible (UV-vis) range of 0.25 at 270 nm. These electrodes are sufficiently conductive for electrochemistry, enabling modification of the surface via diazonium ion reduction and spectroelectrochemistry. Good quality ultraviolet-visible absorption spectra of covalently bonded molecular layers of nitroazobenzene, nitrobiphenyl, and azobenzene, with thicknesses of 1.4-4 nm, were obtained after subtracting the spectrum of the unmodified substrate. The spectra of all three molecules immobilized on the carbon surface showed red shifts of the absorption maxima relative to a solution of free molecules, indicating substantial electronic interactions between chemisorbed molecules and the Pi system of the substrate and/or intermolecular coupling. Spectroelectrochemical measurements show that reduction of free and chemisorbed molecules produce new absorption features in the 500-800 nm range; these spectral changes are partially reversible upon repeated potential cycling. Finally, density functional calculations correlate the new bands to the formation of anion radical or "methide" species that have more extensive electron delocalization than the parent molecules. The results from this work are useful for linking structural transformations in molecular layers "buried" under conductive top contacts in a type of molecular junction to changes in the electronic properties of the junction.