Citation
Nagel, John Frederick (1997) I. The Use of Spherosiloxanes as Molecular Building Blocks for Material and Thin Films. II. A Method of Using SC-Cut Quartz Oscillators for Chemical Sensing. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wkz6-jt81. https://resolver.caltech.edu/CaltechTHESIS:03212025-215725447
Abstract
The ability to prepare a priori ordered materials of a desired structure is a long standing goal in materials research. Recently, there has been great interest in the use of well-defined molecular precursors that can, in principle, be combined in a regular way without degradation to produce ordered materials. This process has been dubbed "lego-chemistry." This work examines the use of spherosiloxanes, a family of polyhedral silicate cage molecules, as molecular building blocks for the synthesis of bulk microporous materials and microporous thin films.
The process for synthesizing and functionalizing the spherosiloxanes with a variety of reactive functional ligands is presented. The reactive molecules are characterized using a suite of analytical and spectroscopic techniques. In order to achieve some degree of control over the condensation process, a binary reaction mechanism utilizing nonhydrolytic reaction conditions is proposed for the production of bulk materials. Experimental results from the non-hydrolytic condensation of spherosiloxanes are presented. The effects of adding various catalysts to the reaction mixture are described. A multinuclear solution NMR study was performed on monomeric silicate analogues of the spherosiloxanes in order to elucidate the non-hydrolytic reaction pathways. It is found that under the conditions of interest ligand exchange predominates over condensation.
A reaction scheme and growth mechanism for the production of a microporous thin film from spherosiloxane precursors is presented and discussed. It is speculated that such an ordered film could function as a framework for the production of a molecular electronic device. The first important steps in this scheme, namely the condensation behavior of spherosiloxanes on the Si (100) - (2x1) reconstructed surface, are explored through a variety of surface characterization techniques. In particular, low temperature STM is used to record the nature of the species condensed on the surface. An image of a single isolated (CH3O)8Si8O12 is recorded and its position and orientation are discussed in the context of the reaction scheme. A HREELS/TPD study of the spherosiloxanes on the same surface is performed and the results discussed.
A new technique for chemical sensing is proposed and is based on the recently developed SC-cut quartz oscillator used in conjunction with the analytical technique known as thermal programmed desorption (TPD). This technique is developed in order to address the need to develop new sensing techniques that are sensitive, selective and cost efficient, for application to the rising threats of terrorism and pollution present today. The technique utilizes two vibrational modes of the crystal, one very sensitive and one relatively insensitive to changes in temperature, that are monitored throughout the experiment. The two mode sensing allows in principle for the simultaneous monitoring of the mass and temperature of the sensor.
A system for evaluating the performance of this sensor is designed and built, and includes an automated data acquisition and control system. The sensor is tested using a variety of chemically selective coatings and analytes. The structure and morphology of the coating used is shown to have a significant effect on the oscillation behavior of the crystal used. TPD experiments are performed with the sensor in order to evaluate its characteristics. In nearly all cases, it is impossible to discern any desorption signal during the TPD experiment. A dual mode analysis that is used to deconvolute the temperature and mass responses of the sensor fails to distinguish any desorption signals except from a very heavily loaded sample, i.e., water on a polyethyleneimine coated crystal where bulk absorption occurs. A TPD experiment is performed on a hydrated, PEI coated crystal using a Cahn microbalance, and a relationship between the mass loading and frequency change is determined for this system. These values are used to calculate the frequency response for a typical coating where only surface adsorption occurs, and the result is below the minimum detection level of the system. Based on these data, it is determined that the SC-cut sensor fails to show the sensitivity necessary for application as a practical sensor device.
Item Type: | Thesis (Dissertation (Ph.D.)) |
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Subject Keywords: | (Chemical Engineering and Chemistry) |
Degree Grantor: | California Institute of Technology |
Division: | Chemistry and Chemical Engineering |
Major Option: | Chemical Engineering |
Minor Option: | Chemistry |
Awards: | Constantin G. Economou Memorial Prize, 1994. |
Thesis Availability: | Public (worldwide access) |
Research Advisor(s): |
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Thesis Committee: |
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Defense Date: | 19 November 1996 |
Record Number: | CaltechTHESIS:03212025-215725447 |
Persistent URL: | https://resolver.caltech.edu/CaltechTHESIS:03212025-215725447 |
DOI: | 10.7907/wkz6-jt81 |
Default Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. |
ID Code: | 17086 |
Collection: | CaltechTHESIS |
Deposited By: | Benjamin Perez |
Deposited On: | 25 Mar 2025 18:10 |
Last Modified: | 25 Mar 2025 18:16 |
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