Interface properties of metal/cytosine/Si(1 1 1):H heterostructures studied by means of SERS and DFT

In this work the interface formation between cytosine and hydrogen-passivated Si(1 1 1) substrates, the growth of cytosine layers as well as the interface between a metal (In or Ag) and the cytosine layers are studied. Cytosine was thermally evaporated by organic molecular beam deposition (OMBD) onto H-passivated Si(1 1 1) substrates under ultra-high vacuum conditions. Metal deposition on monolayers (∼0.4 nm) of cytosine leads to an enhancement of the Raman signal via the surface-enhanced Raman scattering (SERS) effect. The interaction with metals is found to be very different due to the large difference in the ionisation potential of Ag and In (IPAg = 7.58 eV, IPIn = 5.78 eV). The signal enhancement arises mainly from contributions due to molecule-metal charge transfer. Density functional theory calculations were employed for modelling the interaction of metal atoms with cytosine. Computational approaches were carried out on silver-cytosine and indium-cytosine complexes using the B3LYP density functional with the LANL2DZ basis set