Characterization of Self-Assembled Organic Films Using Differential Charging in X-ray Photoelectron Spectroscopy Manish Dubey, † Irina Gouzman, ‡ Steven L. Bernasek,* ,† and Jeffrey Schwartz* ,† Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, and Space EnVironmental Section, Soreq NRC, YaVne 81800, Israel ReceiVed December 20, 2005. In Final Form: March 3, 2006 Differential charging is often regarded as a problem in X-ray photoelectron spectroscopic studies, especially for insulating or partially conducting samples. Application of a positive bias can reduce the effect of differential charging by attracting stray electrons from the system, thereby compensating for the electron loss. On the other hand, differential charging effect can be enhanced by the application of a negative bias to the sample during spectrum acquisition. The successful use of the differential charging technique to distinguish between multi- and monolayer organophosphonate films on oxide-covered silicon has been reported. A detailed description of this technique is now presented which shows how differential charging can be used as an important tool for the characterization of self-assembled films deposited on various surfaces. The dependence of this technique on the conductivity of the substrate has been investigated by studying the spectral behavior of the deposited films of phosphonic acid on conducting, semiconducting, and insulating samples (stainless steel, silicon, and glass). Application of either positive or negative dc electrical bias affects the carbon core-level (C1s) line shape and intensity, which is dependent on the atom’s physical location above the surface. Introduction Self-assembled monolayers (SAMs) are widely used to modify conductor and semiconductor surfaces and have applications in fields such as electronic devices or biological sensors; 1 under- standing the growth and structure of these SAMs is key to their successful implementation in these devices. In this context, the uniformity of SAMs is of great importance and the presence of multilayer islands is undesirable. However, distinguishing SAMs from ultrathin multilayers can be a challenge for most conventional surface characterization techniques. The T-BAG method (tether- ing by aggregation and growth) has been shown to be simple and reliable to grow SAMs of alkylphosphonic acids on oxide surfaces. 2 SAM films are by definition very thin, and therefore, investigation of structure and bonding in SAMs requires characterization techniques that are sensitive to only a few atomic layers. XPS analysis of organic SAM films has been widely used to provide information on film structure, composition, and bonding. 3,4 However, sample charging is often a problem when using XPS to analyze insulating or only partially conducting materials such as these SAM films due to the incomplete neutralization of the photoemitted electrons. 5-10 The low-energy electron flood-gun technique has been used successfully for the neutralization of these excess electrons. 11-13 An alternative approach to this problem is to use an external bias, as first reported by Dickinson et al. 14 However, there is a fundamental difference between the two methods. For the electron flood-gun technique, excess electrons are supplied by an external source located above the sample, and thus results are independent of the conductivity of the sample. Moreover, over-neutralization leading to an excess of negative charge on the surface is also possible, especially for the samples with poor conductivity. In the latter case, the flow of electrons is from the ground to the sample and is, therefore, dependent on the conductivity of the sample. Since the early development of surface analytical techniques, charging effects have been widely observed and reported to be problematic. 15,16 However, surface charging has also been shown to yield chemical, physical, and structural information. 12,13,17-23 The increasing use of charging effects to provide sample information is reflected in extensive work by Suzer et al. 24-29 * To whom correspondence should be addressed. † Princeton University. ‡ Space Environmental Section, Soreq NRC. (1) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73 (1-3), 1-56. (2) Hanson, E. L.; Schwartz, J.; Nickel, B.; Koch, N.; Danisman, M. F. J. Am. Chem. Soc. 2003, 125 (51), 16074-16080. (3) Gouzman, I.; Dubey, M.; Carolus, M. D.; Schwartz, J.; Bernasek, S. L. Surf. Sci. 2006, 600 (4), 773-781. (4) Popat, K. C.; Swan, E. E. L.; Desai, T. A. 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Interface Anal. 2004, 36 (7), 619-623. 4649 Langmuir 2006, 22, 4649-4653 10.1021/la053445f CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006