Metal deposition onto biomolecular layers on silicon surfaces: a study of interface formation using Raman spectroscopy S.D. Silaghi * , R. Scholz, G. Salvan, Yu J. Suzuki, M. Friedrich, T.U. Kampen, D.R.T. Zahn Institut fu ¨r Physik, Technische Universitat Chemnitz, Reichenhainer Street 70/615, TU Chemnitz, 09107 Chemnitz, Germany Available online 2 July 2004 Abstract In this work the interface formed between the DNA base cytosine and hydrogen passivated Si(1 1 1) substrates is investigated by spectroscopic methods and density functional calculations of optimized adsorbate geometries. The cytosine was thermally evaporated by organic molecular beam deposition onto flat and vicinal H-passivated Si(1 1 1) substrates under ultra-high vacuum (UHV) conditions. In order to take advantage of surface-enhanced Raman scattering at rough metal surfaces, silver was evaporated onto the biomolecular adsorbate. Polarisation-dependent Raman measurements reveal that the cytosine molecules align along the steps on vicinal H–Si(1 1 1) surfaces. The orientation of the molecular plane of the cytosine molecule deduced from the SER spectra can be well reproduced with density functional calculations of the optimized geometries of an adsorbed cytosine molecule at a step edge of H-passivated Si(1 1 1) slab. As the binding energy of cytosine at the substrate step is as large as 0.41 eV, one can conclude that the coverage with silver does not affect the preferential adsorption geometry of the cytosine molecule. # 2004 Elsevier B.V. All rights reserved. PACS: 78.30 Jw; 78.66 Qn Keywords: Cytosine; Silver; Silicon; SERS; DFT 1. Introduction In the last years, the long-range charge transport in DNA was investigated for applications in nano- electronic technologies [1]. Electrical transport mea- surements on micrometer-long DNA ‘‘ropes’’ [2] and on DNA molecular films [3] indicated that DNA has a metallic conductivity. On the other hand, the electrical transport measurements performed on poly(guanine)- poly(cytosine), a double-stranded DNA polymer, showed semiconducting behaviour with a large band gap [4]. Charge migration through DNA takes place via the overlap of the p orbitals in adjacent base pairs in a single strand. It was shown that irregular base-pair sequences lead to localisation of charge carriers and reduce the transfer rate of electrons. Single DNA bases may also become key molecules in the field of mole- cular nano-technology. For instance, the temperature dependence of the electrical admittance and permit- tivity of thin cytosine layers showed that these films exhibit electrical ordering in the temperature range Applied Surface Science 234 (2004) 113–119 * Corresponding author. Tel.: þ49 371 531 3137; fax: þ49 371 531 3060. E-mail address: simona-dorina.silaghi@physik.tu-chemnitz.de (S.D. Silaghi). 0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.154