Nano-Raman imaging of Cu-TCNQ clusters in TCNQ thin films by Scanning Near-Field Optical Microscopy

We have combined scanning near-field optical microscopy (SNOM) techniques with Raman scattering in order to measure the topography and the local optical properties of solid samples together with the chemical properties of molecular adsorbates, on subwavelength scales. The proposed experimental setup is simple, but very sensitive. In particular, we have focussed our attention on the optimization of the collection and detection of the scattered light, in order to circumvent the limited imaging capabilities of the SNOM due to the low cross-section of the Raman effect. The sample we will report on is a tetracyanoquinodimethane (TCNQ) thin film kept in contact with Cu powder in order to give rise to local complexes of Cu-TCNQ. A comparative investigation based on independent micro- and nano-Raman maps, allows us to chemically discriminate the aggregates and to assess the enhanced lateral resolution of the SNOM in the nanometre range.