High-Resolution TOF-SIMS Study of Varying Chain Length Self-Assembled Monolayer Surfaces Kurt V. Wolf, † David A. Cole, ‡ and Steven L. Bernasek* ,† Department of Chemistry, Princeton University, Princeton, New Jersey 08544, and Evans East, East Windsor, New Jersey 08520 A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) has been used to investigate chain length effects in hydrocarbon self-assembled mono- layer (SAM) surfaces on gold substrates. A wide range of n-alkanethiols was used to make homogeneous SAM surfaces, which included both odd and even hydrocarbon chain length thiols. Variations in coverage, extent of oxidation, and high-mass cluster formation as a function of hydrocarbon chain length of the alkanethiol SAM surfaces were investigated. Long-short chain length ef- fects were observed for the relative coverage of the SAM surfaces, which directly influences the extent of oxidation for the thin films. The formation of gold-sulfur and gold- adsorbate cluster ions was also observed, since the mass range of the TOF-SIMS made it possible to monitor all of the cluster ions that were formed following the high-energy ion/ surface interactions. The technology of time-of-flight secondary ion mass spectrom- etry (TOF-SIMS) has improved dramatically in the past decade. Along with other methods such as X-ray photoelectron spectros- copy (XPS), Auger electron spectroscopy (AES), and infrared spectroscopy (IR), TOF-SIMS has become an indispensable tool for surface analysis. 1 In this paper, the use of a commercially available TOF-SIMS instrument to study self-assembled monolayer (SAM) surfaces is described. The SIMS ionization method when coupled with a time-of-flight mass analyzer allows for high mass resolution. This specific technique is able to detect low-mass fragments as well as ions with mass-to-charge ratios of thousands of atomic mass units with parallel detection of all ions for a given polarity. This process also has the ability to image surfaces, providing spatially resolved molecular information about the surface. 2 The detailed molecular information that can be obtained from a mass spectrum makes static SIMS suitable for the analysis of organic surfaces when compared to dynamic SIMS. 3 Often when metallic substrates are used, cluster ions are detected in the secondary ion mass spectrum. Uniquely, when an organic thin film is deposited on a metallic surface, a combination of metal- metal, organic, and metal-organic cluster ions is formed. 4,5 Studying cluster formation using a well-characterized system such as SAM surfaces can increase our knowledge of high-energy ion/ surface collisions. The purpose of this study was to analyze the structure and composition of alkanethiol self-assembled monolayer surfaces as a function of chain length using the TOF-SIMS technique. A wide range of unmixed alkanethiols, CH 3 (CH 2 ) n SH, were used to form the thin films. Alkanethiols and disulfides self-assemble on Au (111) surfaces to form well-ordered monolayers, which are bound to the surface through a sulfur-gold bond. 6-10 In the standing- up phase, the backbone of the SAM surface, which is composed of the methylene groups of the alkyl chains, is oriented in an all- trans conformation with an overall chain tilt angle of ∼30° from the surface normal, as determined by ellipsometry and IR spectroscopy. 11,12 The TOF-SIMS technique can provide detailed information about the chemical composition of self-assembled monolayer systems. 13 This method makes use of the fact that positively and negatively charged ions are emitted from the surface upon ion bombardment and are readily detected by the time domain mass analyzer. The bombardment of the surface with the primary ion beam leads to the desorption of intact adsorbate ions as well as characteristic fragment ions. 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Chem. 2002, 74, 5009-5016 10.1021/ac020275s CCC: $22.00 © 2002 American Chemical Society Analytical Chemistry, Vol. 74, No. 19, October 1, 2002 5009 Published on Web 08/24/2002