Thiol Terminated 1,4-Benzenedimethanethiol Films Grown on Au, Ag and Cu Surfaces studied by Direct Recoiling Spectroscopy

L. SALAZAR ALARCÓN, L. CHEN, J. SHEN, J. JIA, V.A. ESAULOV, M.L. MARTIARENA, E. A. SÁNCHEZ, AND O. GRIZZI

Kyoto

Congreso; 25th International Congress on Atomic Collisions in Solids; 2012

Dithiol films deposited on metallic surfaces are promising systems for developing sensors. In most of the cases studied up to now, the dithiol films are grown by dipping an already prepared surface in a solution containing the appropriate dithiol [1]. This has been shown to work well for Au surfaces, however in more reactive surfaces a vacuum approach is desirable to obtain cleaner film-substrate interfaces. In the vacuum approach [2-5], at low exposures the dithiol molecule tends to adsorb with both S atoms bonded to the substrate. The question whether the molecule will stand up or not at higher exposures is open and the adsorption conditions to obtain this Self Assembled Monolayer (thiol exposed film) depend on the substrate-molecule interaction. Direct Recoil Spectroscopy (DRS) [5] has the appropriate surface and element sensitivity to test the thiol termination of the films, and the low damage imparted to the organic film allows carrying on detailed studies of the film stability with surface temperature. In this work we present a TOF-DRS study of 1,4-benzenedimethanethiol films grown in vacuum on Au, Ag and Cu surfaces. We discuss the thiol termination and compare the corresponding spectra to those for the same substrates under pure S exposures. DFT calculations for these systems are used to obtain the shape of the molecule in front of the surface, to study the energetic of its adsorption and to understand the main features of the TOF-DRS spectra. Different implementations of the Van der Waals correction within the VASP [6] code are tested to study how this term affect the system description. Studies performed at temperatures below room temperature suggest formation of a multilayer. This thicker film is identified from the multiple scattering of Ar or Kr projectiles at high incidence angles, i.e., involving collisions with both molecule and substrate. Thermal desorption by TOF-DRS allows the study of the stability of the thiol termination with temperature, determination of the temperature at which the multilayer desorbs, and the identification of the final molecule products remaining after annealing.