Self-Assembled Host-Guest Thin Films for Functional Interfaces
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The functionalization of surfaces has received attention because the process allows the design and tailoring of substrate surfaces with a new or improved function. â Host-guestâ thin film complexes are composed of â hostâ molecules attached the substrate surface, either through physisorption or covalent bonds, with cavities for the inclusion of desired â guestâ molecules for the functionalization of the surface. Two methods for fabricating functional â host-guestâ thin films were investigated: Langmuir-Blodgett (LB) deposition and self-assembly monolayer (SAM). Langmuir films were created at the air-water interface using octadecanesulfonic acid (C18S) as the amphiphilic â hostâ molecules separated by hydrophilic guanidinium (G) spacer molecules, which created a cavity allowing the inclusion of desired â guestâ molecules. Surface pressure-area isotherms of the (G)C18S, with and without guests, are characterized by the lift-off molecular areas and are use to determine the proper deposition surface pressure. â Host-guestâ Langmuir films are deposited onto silicon substrates using the LB deposition technique. The LB films were then subjected to stability testing using different solvents over increasing periods of time. Grazing-angle incidence X-ray diffraction (GIXD), specular X-ray reflectivity (XRR) and transfer ratio measurements were used to characterize the crystallinity, film thickness, overall film stability and film coverage. The GIXD data revealed that the crystallinity of the deposited film varies with the â guestâ molecules and can be disrupted by the functional group on the â guestâ molecule through hydrogen bonding. After modeling the XRR data using StochFit, it was discovered that the more polar solvent, tetrahydrofuran (THF), removed the film completely while the nonpolar solvent, hexane, compacted the thin film and increased the electron density. With transfer ratios around 0.95 to 1.05, the deposited films were homogenous. The second method used was self-assembly monolayers, which differs from Langmuir films in that they are created by a spontaneous chemical synthesis from immersing a substrate into a solution containing an active surfactant. Octadecyltrichlorosilane (OTS) was used initially as a molecule to study the self-assembled monolayer procedure. To study a â host-guestâ self-assembled monolayer system, a compound is being synthesized from 9-bromoanthracene. This compound would already contain the cavity necessary for the inclusion of â guestâ molecules. The solution that contained OTS was composed of a 4:1 mixture of anhydrous octadecane: chloroform. Silicon substrates with a deposited oxide layer were hydroxylated for the surfactant binding chemical reaction to occur. The OTS SAMs were exposed to the same stability tests as the LB films. Surface contact angle measurements were taken of the OTS SAMs before and after the stability tests. The contact angle prior to the stability tests was 110Â° (Â±2Â°). The contact angle after immersion in THF was 101Â° (Â±2Â°) while the contact angle resulting from immersion in hexane was 105Â° (Â±2Â°). From the contact angle measurements, the degradation of the OTS SAMs was less extensive than that of the (G)C18S LB films.
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