RESUMEN
An atomic layer deposition reactor has been constructed with quantitative, precision dose control for studying precursor adsorption characteristics and to relate dose quantity and exposure dynamics to fluid flow in both the viscous and molecular flow regimes. A fixed volume of gas, held at a controlled temperature and measured pressure, is dosed into the reaction chamber by computer-controlled pneumatic valves. Dual in situ quartz crystal microbalances provide parallel mass measurement onto two differently coated substrates, which allows adsorption coverage and relative sticking coefficients to be determined. Gas composition in the reaction chamber was analyzed in situ by a quadrupole mass spectrometer. Absolute reactant exposure is unambiguously calculated from the impingement flux, and is related to dose, surface area, and growth rates. A range of control over the dose amount is demonstrated and consequences for film growth control are demonstrated and proposed.
RESUMEN
A new class of nonlithographically prepared surface enhanced Raman spectroscopy (SERS) substrates based on metalized, nanostructured poly(p-xylylene) films has been developed and optimized for surface plasmon response with a view to applications of SERS detection of microbial pathogens, specifically, bacteria and viruses. The main emphasis has been on achieving high spot to spot, sample to sample reproducibility of the SERS signals while maintaining useful enhancement factors. The use of these surfaces, metalized with either Ag or Au, provides a noninvasive and nondestructive method for spectral fingerprint analyses of both bacteria and viruses. Examples are given for the detection of bacteria (E. coli and B. cereus) and viruses (respiratory syncytial virus and Coxsackievirus). Our method is able to distinguish Gram positive from Gram negative bacterial strains as well as enveloped and nonenveloped viruses. The results demonstrate the development of a new class of SERS substrates which can provide rapid, selective identification of infectious agents without amplification of cultures.
RESUMEN
In situ time-of-flight secondary ion mass spectrometry, infrared spectroscopy, and X-ray photoelectron spectroscopy measurements have been used to characterize the interfacial chemistry that occurs upon physical vapor deposition of Ti and Ca atoms onto a -OCH(3) terminated alkanethiolate self-assembled monolayer (SAM) on Au{111}. While the final result for both metals is near-exhaustive degradation of the methoxy terminal group and partial degradation of the alkyl chains to inorganic products such as carbides, hydrides, and oxides, the reaction mechanisms differ significantly. Titanium reacts in parallel with the -OCH(3) and -CH(2)- units, extensively degrading the latter until a metallic overlayer forms preventing further degradation. At this point, there is a cessation of the Ti-SAM reactions. In contrast, Ca is initially consumed by the -OCH(3) terminal group via a reaction mechanism involving two -OCH(3) groups; subsequent depositions lead to alkyl chain degradation, but at a rate slower than that for Ti deposition. These results demonstrate the subtle differences in chemistry that can arise in the vapor deposition of reactive metals, and have important implications for the behavior of electrical interfaces in organic and molecular devices made with Ti or Ca top contacts.
RESUMEN
We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.
RESUMEN
We have examined p-tert-butylcalix[4]arenetetrathiolate (BCAT) monolayers for their potential use as molecular recognition elements for in situ aqueous chemical sensors. Spectroscopic and wetting studies of BCAT monolayers on Au{111} reveal that the calixarene molecules exist in monolayers, preferentially oriented with their phenyl rings parallel to the surface normal axis. Using quartz crystal microbalance (QCM) sensors with gold-coated electrodes, the chemical specificity of monolayers and thin films to a variety of aromatic and aliphatic analytes in aqueous solution was examined. The response of BCAT sensors was compared to the responses of p-tert-butylcalix[4]arene (BCA)- and decanethiolate (DT)-coated QCM electrodes. BCAT is very selective for alkylbenzenes, much more so than either its spray-coated thin-film analogue, BCA, or the highly ordered DT monolayer. From these measurements, the factors behind molecular differentiation in each film are explored. Drawing upon these findings, the roles of cavitation and film order in molecular recognition for calixarene films are discussed.
RESUMEN
This paper provides background on the basic chemical processes which can occur during the exposure of polymers to typical end-use environments such as air, sunlight, water vapor, and various atmospheric pollutant gases.
