Modification Of Silicon Based Substrates With Cyclocondensation And Cycloaddition Reactions

The modification of several silicon based substrates through cycloaddition and cyclocondensation reactions and the surface adducts which they produce is examined. First a brief introduction is given in chapter 1 which elucidates the need to study the surface modification of silicon as it may help in further advancements in the areas of electronics, surface pacification, and sensors. Since the surface itself is a reactant, chapter 2 discusses the structure of the silicon surfaces examined in later chapters, which includes Si(100)-2 x 1, several hydrogen-terminated silicon surfaces (with either the Si(100) or Si(111) face), and silicon nanostructures grown in highly ordered pyrolytic graphite (HOPG) pits. These surfaces require specialized protocols to create. With the exception of the silicon nanostructures grown in HOPG pits, discussed in chapter 2, these protocols as well as the experimental and theoretical techniques used to examine the cycloaddition and cyclocondensation reactions with these surfaces are discussed in chapter 3. A detailed discussion about predicting N 1s binding energies of surface adducts is the subject of chapter 4. The silicon surface modification discussed here can be initially separated into two types of processes: cycloaddition reactions with Si(100)-2 x 1 or cyclocondensation reactions with hydrogen-terminated silicon. The goal of both reactions is to cleanly and controllably attach the reactant molecules to the surface. In chapter 5, it is determined that there are two competing cycloaddition reactions for azides with the Si(100)-2 x 1 surface: 1,2- and 1,3-cycloaddition. From an examination of the DFT predicted initial reaction barriers for the two competing reactions of phenyl- and benzylazide it is determined that 1,3-cycloaddition is preferred over 1,2-cycloaddition. However, these predictions indicate that in the case of benzylazide there is a significant percentage of 1,2-cycloaddition that should be observed at cryogenic temperatures. This is confirmed with infrared spectroscopy. Finally the last part of the reaction, N 2 elimination, is examined with TPD and found to occur above room temperature. In the next chapter, the cyclocondensation reaction of nitrobenzene (PhNO 2) with hydrogen-terminated silicon is examined. This reaction is a dehydrative cyclocondensation reaction that releases water, with the hydrogen-terminated silicon substrate supplying the hydrogen. This is a new class of reactions with this surface. Here infrared spectroscopy is used to confirm the loss of hydrogen from the surface by the loss of the Si-H stretches and the attachment of the compound to the surface by the growth of vibrational modes indicative of the compound. The reaction of nitrobenzene with silicon nanostructures in HOPG pits is the subject of chapter 7. Nitrobenzene is shown to react with these silicon nanostructures and not the HOPG or HOPG pits. Additionally, the distribution of products after this reaction is similar to the distribution of products after the cyclocondensation reaction of nitrobenzene with hydrogen-terminated silicon. The kinetics of the cyclocondensation reaction of nitrobenzene with hydrogen-terminated silicon is examined in chapter 8. Here the reaction is conducted inside an ultrahigh vacuum chamber with hydrogen-terminated surfaces created in situ. The kinetics are determined by monitoring the loss of the Si-H vibrations with increased exposure of the surface to nitrobenzene. While the cyclocondensation reactions presented in the previous chapters only involved the reaction of nitrobenzene with the surface, chapter 9 shows that the cyclocondensation reaction is a more general reaction. The reaction of two molecules, nitrohexane (C 6 H 13 NO 2) and nitrosobenzene (PhNO), with hydrogen-terminated silicon are examined using DFT predictions to construct reaction diagrams and infrared spectroscopy to confirm the attachment of the molecules to the surface and determine the rates of the reaction. (Abstract shortened by UMI.).