Adsorption of Organic Phosphines and Thiols on Metal Surfaces

Abstract: In this thesis investigations of the adsorption of small molecules on metal surfaces are presented. Several surface sensitive analytical methods are used in the studies.Surface modification of gold is a field in which an enormous amount of work has been published in recent years. A convenient and reliable method to prepare well-defined self assembled monolayers have been developed during the past ten years, namely the use of long chain alkanethiols on gold. For the platinum group metals no similar surface modification method is at hand. In this thesis the possibility to use tertiary phosphines to modify platinum, iridium and rhodium surfaces is analyzed.Tertiary phosphines belong to a class of molecules that has extremely high electron donating capacity and therefore are expected to form strong bonds with soft electron acceptors, such as noble and platinum group metals. Using infrared and X-ray photoelectron spectroscopy it is confirmed that this is the case. Temperature programmed desorption is used to determine the strength of the bonds.On the coinage metals, gold, silver and copper, stable layers of dimethylphenylphosphine is prepared by adsorption from dilute toluene solutions as well as from gaseous dimethylphenylphosphine in ultra high vacuum. Films of monolayer or submonolayer thickness are formed on all the coinage metals with both deposition methods, but the binding is stronger for the solution deposited than for the gas phase deposited films.For the platinum group metals there are two different types of chemisorption; direct binding of the phosphine to the metal surface and binding via an oxygen bridge with Me-O-P units. The latter mechanism gives stronger bonds and the former is only observed on films deposited on sputter cleaned metals in ultra high vacuum. The difference between these two binding mechanisms is easily detected by X-ray photoelectron spectroscopy.Monolayers of short chain alkane thiols terminated by pyrrole rings prepared on gold are also studied. The purpose of this project is to develop a molecular glue to promote the adhesion of the conjugated polymer polypyrrole on gold. Depositions are made by adsorption from dilute solutions, by exposure the saturated vapor at atmospheric pressure and from gas phase in ultra high vacuum. All three deposition methods give the same result: a chemisorbed monolayer is formed on the gold surface. One of the investigated molecules, 3-thioethylpyrrole, decomposed during adsorption, loosing a substantial fraction of the pyrrole rings whereas the other compound, 1-thioethylpyrrole, adsorbed intact, but decomposed at high temperature. The adsorption, decomposition and desorption of the molecules are studied with infrared reflection absorption spectroscopy, temperature programmed desorption and X-ray photoelectron spectroscopy.