DIRECT RECOIL SPECTROSCOPY OF ALKANETHIOL COVERED SURFACES

L. RODRÍGUEZ, J.E.GAYONE, E.A. SÁNCHEZ, O. GRIZZI, B. BLUM, R.C. SALVAREZZA

Schloss Hernstein, Austria

Workshop; 16th International Workshop on Inelastic Ion-Surface Collisions (IISC-16); 2006

The growth of thin organic layers on surfaces is a field of great interest due to its potential application in areas such as nanolithography, surface pasivation, molecular electronics. The technique of Direct Recoil Spectroscopy with Time of Flight analysis (TOF-DRS) is adequate to study these systems because it provides information that is complementary to that available from electron spectroscopies[2] and other techniques[3]. The main advantages of TOF-DRS are: 1) the high top layer sensitivity, 2) the low damage imparted to the film, and 3) the capability of detecting H. In this report we present a comparative study of the adsorption of alkanethiols (CH3 (CH2)n-1 SH, with n = 3, 6 and 12, Cn hereafter) on metals (Au(111), Ag(111)) and semiconductors (GaAs(110)). The substrate surfaces were cleaned by grazing sputtering and annealing and the films grown by exposure to the thiol vapours under UHV conditions. TOF-DRS was used to follow the adsorption ki netics in a broad range of exposures (from 10-2 to 105 Langmuirs). Recoiling atoms from both the molecule and the substrate were followed at a 45 deg scattering angle. For the metallic surfaces, the shape of the TOFDRS spectra and the dependence of the recoiling intensity versus dose (Fig.1) present characteristics that can be associated to adsorption in three steps: an initial one at surface defects (inset Fig.1), a slower growth mode that covers the whole surface, and a final, denser layer that can be associated with the self assembled layer. The data show a strong dependence in the sticking coefficient with the surface roughness and the length of the alkyl chain, being smaller for the case of C3. In comparison, adsorption of C6 on GaAs(110) shows a simpler, one step behaviour. Fig. 1, upper panel: recoil intensities for Ag, H and C vs C3 exposure. Inset: span of the lower range in linear scale [ref 4]. Arrows indicate major changes in sticking. The H recoil intensity during adsorption of C6 is also shown (full squares). Lower panel: width of H recoil peak vs exposure. Annealing of the films up to temperatures near 500 K resulted in a complete disappearance of the C and H recoil signals and the appearance of small S features. On Ag and Au, the amount of S left at the surface depends on the initial surface roughness. For GaAs it is higher than for both Au and Ag. After the adsorption / desorption cycles, the topography of the Ag substrate changes, giving rise to formation of many small terraces of atomic height. This effect is verified by STM/AFM images. The dependence of the H, C and substrate recoil intensities with sample  temperature is different for each substrate, and dependant on the length of the alkyl chain. Fig.2: Ag, H and C recoil intensity vs. sample temperature. The full line is a fit to the H recoil intensity with a first order desorption model. The derivative of the H recoil intensity (dotted line) is  included. For the metallic surfaces, the shape of the TOFDRS spectra and the dependence of the recoiling intensity versus dose (Fig.1) present characteristics that can be associated to adsorption in three steps: an initial one at surface defects (inset Fig.1), a slower growth mode that covers the whole surface, and a final, denser layer that can be associated with the self assembled layer. The data show a strong dependence in the sticking coefficient with the surface roughness and the length of the alkyl chain, being smaller for the case of C3. In comparison, adsorption of C6 on GaAs(110) shows a simpler, one step behaviour. Fig. 1, upper panel: recoil intensities for Ag, H and C vs C3 exposure. Inset: span of the lower range in linear scale [ref 4]. Arrows indicate major changes in sticking. The H recoil intensity during adsorption of C6 is also shown (full squares). Lower panel: width of H recoil peak vs exposure. Annealing of the films up to temperatures near 500 K resulted in a complete disappearance of the C and H recoil signals and the appearance of small S features. On Ag and Au, the amount of S left at the surface depends on the initial surface roughness. For GaAs it is higher than for both Au and Ag. After the adsorption / desorption cycles, the topography of the Ag substrate changes, giving rise to formation of many small terraces of atomic height. This effect is verified by STM/AFM images. The dependence of the H, C and substrate recoil intensities with sample  temperature is different for each substrate, and dependant on the length of the alkyl chain. Fig.2: Ag, H and C recoil intensity vs. sample temperature. The full line is a fit to the H recoil intensity with a first order desorption model. The derivative of the H recoil intensity (dotted line) is  included. REFERENCES [1] J. C. Love, L.A Estroff, J.K. Kriebel, R.G. Nuzzo and G.M. Whitesides Chem. Rev. 105 (2005) 1103. [2] S.S. Duewez J. Electron Spectroscopy and Related Phenomena 134 (2004) 97. [3] F. Schreiber, Progr. Surf. Sci. 65, 151 (2000). 151. [4] L.M. Rodríguez, J.E. Gayone, E.A. Sánchez, O. Grizzi, B. Blum and R.C. Salvarezza, J. Phys. Chem. B Lett. 110 (2006) 7095.