Thiolate self-assembled monolayers studied with a tuneable infrared low temperature laser driven scanning tunnelling microscope

Abstract

This work describes the investigation of self-assembled monolayers (SAMs) with scanning tunnelling microscopy/spectroscopy (STM/S) and infrared laser-driven STM (LDSTM). As a tool STM is uniquely able to resolve atoms on a surface. Illuminating an STM with infrared radiation tuned to match modes in the SAM used as the sample provides a unique opportunity to investigate the combination of the well understood character of organic molecules with the atomic scale resolution of an STM. SAMs were prepared with octanethiol and dimethyl disulphide on Au(111) substrates. STM images and STS spectra of these samples recorded at 78K are presented. Typical surface features are observed in the octanethiolate monolayers. The results of STS experiments with an octanethiolate monolayer reveal correlations between surface features and conductivity at -1.0V. The differences between these STS data and equivalents from uncoated samples reveal the effect of the molecules upon the electronic surface states of the samples. Images of samples prepared with dimethyl disulphide show previously unseen low density structures and individual molecules scattered across the surface. Correlations are made between these low density structures and the reconstruction of the underlying gold surface. Comparisons with previously calculated models are used to identify these isolated molecules. STS data collected across a section of sample show how topography data can be used to categorise STS data. The response of the laser-STM interaction to the laser modulation is presented and compared with that of the STM alone. The modulation of the tunnelling current is found to be proportional to the equilibrium tunnelling current which is consistent with theoretical models. This response of the laser-STM interaction to the laser modulation frequency was found to differ between Au(111) substrates uncoated and coated with a methylthiolate SAM. Correlations were observed between the frequency response of the laser-STM interaction and the STS data.

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