LOW COVERAGE PHASE OF 1,4 BENZENEDIMETHANETHIOL ON Au(111) and Ag(111)

LEOLEONARDO SALAZAR ALARCON; MARIA LUZ MARTIARENA; OSCAR GRIZZI; ESTEBAN ALEJANDRO SANCHEZ

San Carlos de Bariloche

Congreso; 4º Congreso de la Asociación Argentina de Micrsocopia (SAMIC2016); 2016

SAMIC

Self-assembled monolayers (SAMs) of organothiols on metal surfaces have been investigated in detail due to their promising properties: corrosion inhibition, fabrication of sensors, new electronic devices, lithography [1]. In particular, aromatic thiols and dithiols (for example 1,4 benzenedimethanethiol (BDMT)) have been proposed for use in a number of applications and studied from both experimental and theoretical approaches [2]. Depending on the type of substrate, on its reactivity, it may be advantageous the use of a vacuum approach to form the SAM instead of its counterpart of solution growth [3]. In this case, a clean surface is exposed to the vapours of the corresponding thiol in high vacuum. During growth the system may undergo different phases prior to attaining the final SAM, and in some cases the system may grow continuously forming multilayers. The knowledge of all the phases formed during SAM preparation can help to optimize the preparation method in order to obtain films of better quality. Here we studied the growth of BDMT under ultra-high vacuum conditions on Au(111) and Ag(111), characterized in situ by different techniques including: low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), and ion scattering. Calculations within the functional density model (VASP code) were also performed to help the interpretation of the STM images. Ion scattering provided information on the formation of different phases as a function of BDMT exposure. In Fig. 1 we recognized two phases: for very low exposures the molecules are lying down, while for very high exposures a standing up configurations is formed on the surface. Surprisingly, it is seen that due to the higher reactivity of Ag, the initial phases formed are better defined and more ordered than on Au. The LEED pattern of Fig.2 shows a well-defined arrangement for BDMT/Ag while it is blurred for BDMT/ Au (not shown). The STM images acquired on BDMT/Ag (Fig.2) show that for very low exposures the surface covered with a single layer of BDMT molecules form a long range and very well ordered arrangement. A close inspection of the STM images shows that the molecules are tilted with respect to the surface. The molecular arrangement looks very complex, so a DFT calculation is very useful to provide extra information. The simulation was performed by putting the molecules in the lying down phase with the orientation of the molecules as shown in the STM images. If all the Ag atoms are left on the surface, the STM simulations did not agreed with experimental ones, showing BDMT parallel to the surface. The only way to reproduce the experimental molecular arrangement with tilted molecules was after including in the calculation Ag vacancies. The results of the DFT calculation with a comparison with the experimental result is shown in Fig. 3. In the presentation we emphasize the convenience of using spectroscopic, theoretical and STM techniques to achieve a full characterization of the surface.REFERENCES[1] Vericat C., Vela M. E., and R. C. Salvarezza, (2005) ?Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics? Phys. Chem. Chem. Phys. 7 (18): 3258-3268.[2] Nara J., Higai S., Morikawa Y., Ohno T. (2004) ?Density functional theory investigation of benzenethiol adsorption on Au(111)? J. Chem. Phys. 120 (14): 6705-6711. [3] Salazar Alarcón, L., Cristina, L.J., Shen, J., Jia, J., Esaulov, V.A., Sánchez, E.A., Grizzi, O., (2013) ?Growth of 1,4-benzenedimethanethiol films on Au, Ag, and Cu: Effect of surface temperature on the adsorption kinetics and on the single versus multilayer formation? J. Phys. Chem. C 117 (34): 17521-17530.ACKNOWLEDGEMENTSWe acknowledge support from U.N. Cuyo (06-C454) and CONICET (PIP 112-201101-00594).