Organic Sulfenyl Chlorides as Precursors for the Modification of Gold Surfaces
Self-assembled monolayers (SAMs) of organosulfur precursors on gold have been extensively used since they offer a wide range of technological applications such as corrosion inhibition, lubrication, adhesion promotion/inhibition, nanofabrication, chemical and biosensors, catalysis, and molecular electronics. Furthermore, the electronic and optical properties of aromatic SAMs make them a potential candidate for molecular electronics. However, these practical applications are limited by the short-range ordering, low packing density, irreproducibility, and inferior quality of SAMs, which are more critical for aromatic SAMs. Therefore, the discovery of alternative precursors is essential. This thesis reports for the first time, the use of organic sulfenyl chlorides as precursors for the modification of gold surfaces. These precursors may help to overcome some practical limitations of the traditional organosulfur precursors. The modification is done in a non-aqueous medium. Characterization of the modified surfaces is performed by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and scanning tunnelling microscopy (STM). Through the use of 4-nitrophenyl sulfenyl chloride, evidence for the formation of well-ordered aromatic SAMs formation on gold is provided. XPS data shows that the modification involves the scission of the S-Cl bond. PM-IRRAS studies further indicate that the adsorbed molecules are nearly vertically oriented on the surface. Both short and long-range well-ordered aromatic SAMs (a 4 x √3 rectangular and √3 x √3 hexagonal unit cells) are obtained from the STM images using two different modification conditions. This molecular density is usually only observed for aliphatic SAMs using the traditional precursors. Along with the main hexagonal lattice, the reversible distinct superstructures including hexagons, partial hexagons, parallelograms, and zigzags resulting from specific arrangements of adsorbed molecules provide submolecular details. This is the first direct experimental example, where the STM has shown its effectiveness to provide physical structure information of standing-up aromatic SAMs at room temperature. This work also provides some insight into a heavily debated issue regarding the origin of the various features and contrasts obtained in STM images of SAMs. The use of 2-nitrophenyl sulfenyl chloride and 2,4-dinitrophenyl sulfenyl chloride for the formation of aromatic SAMs on Au provides some insight regarding the modification extent and the effect of a nitro substituent (at ortho position ) on the quality of nitrophenyl thiolate SAMs on gold. XPS, PM-IRRAS, electrochemistry and STM provide evidence for the formation of less ordered, low density and less stable SAMs that may decompose to sulfur at longer modification times. The efficient deposition of sulfur on gold is observed using a series of substituted methane sulfenyl chlorides (triphenylmethane sulfenyl chloride, trichloromethane sulfenyl chloride and chlorocarbonyl sulfenyl chloride). The XPS, STM and electrochemical data show the formation of high density sulfur phases. These include rhombus, rectangular, and zig-zag sulfur structures. A mechanism is suggested involving the cleavage of the S-Cl bond and the ejection of the molecular backbone. This study also suggests that substituted methane sulfenyl chlorides do not form long-range ordered SAMs.