X-ray Induced Damage of Self-Assembled Alkanethiols on Gold and Indium Phosphide D. Zerulla † and T. Chasse ´* ,‡ Wilhelm-Ostwald-Institut fu ¨ r Physikalische und Theoretische Chemie, Universita ¨ t Leipzig, Linne ´ str.2, D-04103 Leipzig, Germany Received March 11, 1998. In Final Form: March 15, 1999 We have performed an X-ray photoelectron spectroscopy (XPS) study in order to investigate thiolate monolayers on gold and indium phosphide. Evidence is presented that alkanethiols (chain length 10, 12, 16, and 18) also adsorb to form passivating monolayers on InP(110). An increasing degradation of the thiolate films has been observed on both types of substrates during extended exposure to X-rays. Several features in the spectral shapes of the core level spectra and their intensity evolution are discussed in terms of different types of layer damage. The chemical damage is indicated by the appearance of a new radiation- induced sulfur species. Spectroscopic evidence as obtained from both XPS and X-ray absorption near edge structure has been provided that indicates this species might be of disulfidic nature. Both sulfur and carbon disappear from the surface in about comparable amounts, clearly demonstrating significant structural damage of the thiolate films. The layer damage by the X-rays proceeds much faster and significantly stronger on gold films than on InP. This result is discussed in terms of structural defects and electron- induced effects. Evidence is provided for a significant contribution of backscattered and secondary electrons to the damage of the adsorbed organic layers. Patterning of alkanethiolate layers by an electron probe of a scanning Auger unit has been demonstrated. Introduction Alkanethiols are well-known to form self-assembling monolayers (SAMs) on substrates such as gold, silver, and copper. 1-6 Such highly ordered organic films are very promising initial or intermediate states in order to achieve well-characterized tailoring of surfaces. Well-defined chemical modification and control of surfaces and inter- faces are also essential for a variety of applications of semiconducting materials, as in photocatalysis and opto- electronic and photovoltaic devices. 7-9 Recently, alkane- thiolate films were also investigated on semiconductors because of their application potential for such purposes. But to date still much less is known about alkanethiolate films on surfaces of compound semiconductors compared to the extensively studied surfaces of gold, silver, and copper. Passivation of GaAs surfaces using adsorbed thiols has been reported by Lunt et al. 10 A study on alkanethiols on chemically prepared InP(100) surface has been pub- lished by Gu et al. 11 However, the chemical composition of thiol monolayer films regarding both the sulfur species being present and their bonding to the substrate is still not completely understood in the case of self-assembled films on metals such as gold. Transformation of the sulfur headgroup into a disulfidic species has been reported by Zubra ¨gel et al. and Nuzzo et al. 6 Evidence for radiation-induced damage of SAMs has been presented by Wirde et al. 12 and Ja ¨ ger et al. 13 in their photoemission studies, and the sulfur species generated by the X-rays has been attributed to disulfides in contrast to sulfidic species in the undisturbed monolayers. Contradicting to these references, other authors have proposed a disulfidic nature for the sulfur in the original SAMs already. 14 Thus, this subject is still discussed controversially. On the other hand, ionizing radiation (e.g., photons and electrons) has been used as a tool for structuring by taking advantage of the severe damage due to high doses of radiation, as has been demonstrated for several substrates including III-V semiconductors (e.g., GaAs) by Tiberio et al. and Lercel et al. 15-17 Here we report on an XPS study of alkanethiolate SAMs on the surfaces of gold films and indium phosphide single crystals. The investigations have focused on the influence of X-rays on the bonding and structure of alkanethiolate films. The paper is structured as follows. First, the results of the investigations of radiation-induced effects on thiolate layers on gold will be presented. Second, we will comple- ment these data by observation of SAMs on InP(110). 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