Investigation of the Structure of Alkyne Self-Assembled Monolayers on Gold and the Development of Matrix-Enhanced Secondary Ion Mass Spectrometry Methods

Abstract

Characterization of thin films is critical to the understanding of many technological and biological processes. The focus of this dissertation is to develop methods to characterize very low concentration species present on surfaces. A reflection adsorption infrared spectroscopy: RAIRS) system was constructed and tested. The instrument comprises an Fourier transform infrared: FTIR) spectrometer, an optical pathway and a vacuum chamber. The RAIRS system is designed to investigate in situ the interaction of vapor-deposited metals and gases, such as CVD precursors, with organic thin films, and so a vacuum chamber is required. To accommodate the vacuum chamber, an external IR optical pathway was designed and assembled because there was not enough room in the internal optical pathway of the FTIR spectrometer. The synthesis and characterization of terminal alkyne monolayers: TAMs) adsorbed on gold was investigated by RAIRS, single wavelength ellipsometry, time-of-flight secondary ion mass spectrometry: TOF SIMS) and x-ray photoelectron spectroscopy: XPS). TAMs have the potential to transform surface functionalization for many technological applications because they have increased temperature stability and electrical conductance. However, the data suggest that TAMs are not well-ordered and can be oxidized, which may limit their application. For TAMs with less than 11 methylene units in the backbone, the adsorbed layer is highly disordered, oxidized and has a multilayer structure. Longer chain length TAMs form disordered monolayers on gold. As the methylene chain length increases, the conformational order of the TAMs increases with the alkynes in an upright conformation and bound to the surface via a Au-C¡ÕC- bond. The use of room temperature ionic liquids: ILs) as matrices in TOF SIMS was examined to further characterize biological thin films. The data indicate that the secondary ion intensities of lipids, steroids, peptides, proteins and proteins are significantly enhanced using IL matrices. Secondary ion enhancements of at least an order of magnitude are typically observed. Limits of detection are also greatly improved. For example, the limits of detection of 1,2-dipalmityl-sn-glycero-phosphocholine: DPPC) and 1,2-dipalmityl-sn-glycero-phosphoethanolamine: DPPE) were at least two orders of magnitude better. The data also show that ILs are suitable matrices for imaging SIMS. The IL matrices did not cause changes to the sample surface; no ¡°hot spots¡± were observed. The mechanism of the secondary ion intensity enhancements using IL matrices was then investigated to optimize use in characterization. Only protic ILs, which are formed by the transfer of a proton from a Br©ªnsted acid to base, were observed to increase analyte signals. The matrix enhancement mechanism therefore involves the transfer of protons from, or to, the analyte, to, or from the matrix. The magnitude of the analyte signal enhancements is dependent on the chemistry of the matrix cation, anion and analyte. The pKa of the matrix acid and base do not appear to have a strong effect on the ion-intensity enhancements. The results also indicate that the chemical identity of the matrix anion has a stronger effect on analyte signal enhancements than the matrix cation