Organic modification of a silicon(100)-2x1 surface through reversible coadsorption.

摘要

A major move toward miniaturization has been taking place over the last several years in the field of electronics. The advance of surface science techniques has led to new discoveries of surface chemistries that have found applications in the fields of microelectronics, biosensing, catalysis, and others. Developing such devices requires a thorough understanding of the chemistry that occurs at surfaces and how materials can be used to alter the electronic properties of a surface so that it may be used for desired applications.;This project has a two-fold purpose --- to study the patterning that occurs when multiple molecules are reacted with a semiconductor surface, namely silicon, and to characterize reactions and adsorption behavior of molecules that have not been previously observed on silicon. A chemisorbed system of two different molecules was successfully formed and characterized. Other molecules were studied for their reactive properties on silicon, as well as their possible use in multiple-molecule surface patterns.;A combined layer of ethylene and nitrobenzene was successfully formed on the Si(100) surface. Ethylene was adsorbed first on the surface and, due to the repulsive interactions between ethylene molecules, was expected to chemisorb to alternating surface sites at sufficiently low exposure. This arrangement was to allow nitrobenzene to react to the remaining surface sites and form an ordered combined monolayer. Once the amount of ethylene needed to produce a half monolayer of coverage on the surface was determined, nitrobenzene was introduced and confirmed to adsorb on the surface and coexist with ethylene. The surface was then heated above the desorption temperature of ethylene so that only nitrobenzene remained on the surface. All stages of the coadsorption were confirmed through the observation of relevant vibrational modes with multiple internal reflection infrared spectroscopy (MIR-FTIR). Information from theoretical methods, such as predicted vibrational frequencies and reaction energies, supported the formation of the partial and combined monolayers as feasible.;The adsorption behavior of 2,3-dimethyl-2-butene was investigated on the Si(100) surface. Evidence of physisorption at low temperatures was found. In IR spectra, there was a small absorption peak in the 1650 cm-1 -- 1700 cm-1 region; this peak corresponds to a stretching frequency predicted to exist for the C=C bond of 2,3-dimethyl-2-butene in a weakly-bound physisorbed precursor state. The lack of any IR peaks at room temperature indicated that chemisorption of 2,3-dimethyl-2-butene does not occur. DFT calculations supported this hypothesis by predicting a barrier to chemisorption that is significantly larger than the barrier to desorption from a physisorbed precursor. Significant dihedral angle seen in the cycloadduct structure was thought to result from steric hindrance from the methyl groups of the molecule. Opposed to the expected surface chemistry for unsaturated hydrocarbons, 2,3-dimethyl-2-butene was shown to be relatively chemically inert on the Si(100) surface.;The reaction chemistry of triethylenediamine on the Si(100) surface was investigated. Triethylenediamine was predicted to form a stable dative-bonded structure on the surface. It was found to adsorb intact on the surface at cryogenic temperatures, but submonolayer coverages desorbed at temperatures below room temperature. IR studies reveal that chemisorption occurs at room temperature, and heating to higher temperatures results in total dissociation of the compound, as evidenced by the loss of the nuC-H mode and the appearance of a nuSi-H mode around 500 K. AES studies show a trend of decreasing C/N ratio on the surface with increasing temperature. Two prominent desorption events, corresponding to molecular hydrogen and m/z = 26 fragments, were observed with TPD. The desorption of m/z = 26 could correspond to the release of ethylene from the dissociation of triethylenediamine. DFT was used to investigate possible dissociation mechanisms; a likely mechanism involved the dissociation of ethylene through the interaction with a neighboring dimer. XPS studies point to dissociation of triethylenediamine occurring at room temperature.