Abstract
6-[4-(phenylazo)phenoxy]hexane-1-thiol self-assembled monolayers deposited on a gold surface form domain-like structures possessing a high degree of order with virtually all the molecules being identically oriented with respect to the surface plane. We show that, by using polarized near-field Raman spectroscopy, it is possible to derive the Raman scattering tensor of the ordered layer and consequently, the in-plane molecular orientation at the individual domain level. More generally, this study extends the application domain of the near-field Raman scattering selection rules from crystals to ordered organic structures.
Highlights
Raman spectroscopy is an extremely useful and widespread technique for material characterization [1,2]
Apart from offering distinctive spectral signatures allowing for the rapid identification of the chemical nature of the sample, even more detailed information on the molecular order can be retrieved by studying the signal dependence on the polarization of the two involved photons, as formally defined by the Raman scattering tensor [3]
Polarized Tip Enhanced Raman Spectroscopy (TERS) experiments were mainly conducted on smooth regions of the supporting metal film: Adsorption at an atomically flat surface is needed for the formation of the ordered thiol layer
Summary
Raman spectroscopy is an extremely useful and widespread technique for material characterization [1,2]. While reflection-absorption infrared spectroscopic and ellipsometric measurements confirmed a nearly perpendicular orientation of the azobenzene moieties [24,25,26], scanning probe microscopy studies revealed the presence of small domains having the azobenzene groups arranged in regular lattice structures The formation of such domains is possible due to the flexibility of the (sulfur terminated) alkyl chain linking the aromatic system to the surface (so-called bundle model) [27,28,29,30,31]. This makes azobenzene derivative SAMs interesting for polarized TERS studies to test the capability of Raman spectroscopy in the near-field optical regime while aiming at the determination of the crystalline domain orientation with sub-diffraction limited lateral resolution. Azobenzene-containing thiol on electrochemically roughened gold electrodes or on nanostructured substrates formed by electron beam lithography [32,33]
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