Temperature Dependant IrradiationInduced Desorption of Thiol SelfAssembled Monolayers

Temperature Dependant Irradiation-Induced Desorption of Thiol Self-Assembled Monolayers S. Wyczawska 1, M. Buck 2, P. Cyganik 3, P. Lievens 1, Z. Postawa 3, R. E. Silverans 1 1 Laboratorium voor Vaste-Stoffysica en Magnetisme, K. U. Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium 2 School of Chemisty, University of St Andrews, North Haugh, St Andrews, KY 16 9 ST, United Kingdom 3 Instytut Fizyki, Uniwersytet Jagiellonski, ul. Reymonta 4, PL 30 -059 Krakow 16, Poland Introduction Self-assembled monolayers (SAMs) of organic molecules chemisorbed on various surfaces are promising materials to be used as building elements in nanoscience and nanotechnology [1]. In order to optimize technological applications, it is important to fully understand both the formation and the controlled modification of such monolayers. Hereto we focus in this work on the influence of the exposure of organic surfaces to swift charged particles. This is not only of interest for technological applications in nanolithography but also to investigate the damage induced during characterization of these layers by standard spectroscopic techniques. Up to now, most of the fundamental studies of SAMs have been performed on monolayers consisting of alkanethiols [1]. Only recently, aromatic thiols have moved into focus due to their interesting electronic properties and potential application in future nano-electronics [2, 3]. Experimental results During the exposure of SAMs to a beam of charged particles, molecular fragments are emitted from the surface. We investigated the desorption process together with temperature dependence for phenylethyl mercaptan [C 6 H 5 CH 2 -SH, PEM] SAMs adsorbed on Au. The comparison of the mass spectra recorded in room and elevated temperature after photoionization of gas phase and desorbed molecules, allows differentiating between ion-induced. desorption and temperature induced cleavage of a bond. Experimental method ion detector laser pulse + + Dt ion pulse + The irradiation of a surface with swift charged particles (ions and electrons), induces the emission of secondary particles such as atoms, molecules and clusters. Although the desorbed particles are distributed over different charge and excited states, most of them are neutrals. To probe Ds the neutral particles we use nanosecond pulsed laser ionization in combination with time-of-flight mass spectrometry. The ionization energy of molecules is of the order of 710 e. V. We use a one-color two-step resonance enhanced multiphotoionization scheme (REMPI) to ionize the molecular fragments that contain a phenyl chromo-phore. REMPI has the advantage that lower photon fluences are needed to ionize the molecular fragments thus reducing photofragmentation. Experimental system The cloud of molecular fragments and substrate atoms desorbed from the sample surface by interaction with energetic Ar+-ions (15 ke. V, 600 ns pulse) is intersected by a linearly polarized beam of a pulsed tunable laser system, (l=259 nm; photon fluence 7 1017 photons/cm 2). The photo-ionized particles are extracted into a time-of-flight mass spectrometer and detected [4]. Latelly, we implemented a heating/cooling stage on the sample manipulator which allow us to change temperature in controlable way from 150 till 500 K. Desorption measurements were done in SPUTTERING temperature range (240 – 400 K). It is clearly visible that desorption of PEM fragments is highly sensitive to temperature. In low temperature (below 220 K) molecular signal of desorption vanishes totally (freezing surface). On the other hand, when temperature is too high molecules (or fragments) can very easily unbound and leave the surface [5, 6]. So far a lot of temperature measurements were done for simple alkanethiols [7], even though our measurements are compatible with predicted behavior. Our measurements shown that some molecules are more sensitive to GAS PHASE changing temperature than the others even some fragments are only visible in gas phase (94, 106, 115, 128 amu). Molecular sputtering to thermal gas phase ratio shown that fragment 91 is not really sensitive to temperature changes: in the room and elevated temperature ratio remains the same (Graph a). Additionally, ratio of main fragments in elevated temperature seems to be constant. Additionally we performed measurements circles: samples were heated till 393 K, kept in that conditions for at least 3 h and cooled naturally till room temperature. After this circle organic signal was still visible but pattern was changed (Graph b). a b References 1. J. C. Love, et al. , Chem. Rev. 105 (2005), 1103. 2. C. Olsen, P. A. Rowntree, J. Chem. Phys. 108 (1998) 3750. 3. M. Zharnikov, et al. , Langmuir 16 (2000) 2697. 4. E. Vandeweert, et al. , Appl. Phys. Lett. 82 (2003) 114. 5. Z. Postawa, et al. , Nucl. Instr. Meth. In Phys. Res. B 182 (2001) 148. 6. D. E. Riederer et al. , J. Am. Chem. Soc. 119 (1997) 8089. 7. F. Schreiber, Prog. Surf. Sci. 65 (2000) 151. Conclusions Our measurements show that we are able tune desorption profile but temperature changes. Because of this it may be possible selective sputtering. In addition, we may see phase transition. It is excellent starting point to implement that type of measurements to more complicated molecules. Nevertheless more probing are necessary.