Minimally invasive characterization of covalent monolayer sheets using tip-enhanced Raman spectroscopy

Synthetic covalent monolayer sheets and their subclass, two-dimensional polymers are of particular interest in materials science because of their special dimensionality which renders them very different from any bulk matter. However, structural analysis of such entities is rather challenging, and there is a clear need for additional analytical methods. The present study shows how tip-enhanced Raman spectroscopy (TERS) can be performed on monomer monolayers and the covalent sheets prepared from them by [4 + 4]-cycloaddition to explore rather complex structural and mechanistic issues. TERS is a surface analytical method that combines the high lateral resolution of scanning probe microscopy (SPM) with a greatly enhanced Raman scattering intensity. The high spatial resolution (<60 nm) and the significantly improved sensitivity (contrast factor of >4000) compared to confocal Raman microscopy provides new insights into the formation of this new and exciting material, namely significant consumption of the reactive units (anthracenes) and exclusion of the alternative [4 + 2]-cycloaddition. Moreover, due to the high lateral resolution, it was possible to find a first spectroscopic hint for step growth as the dominant mechanism in the formation of these novel monolayer sheets. In addition, TERS was used to get first insights into the phase behavior of a comonomer mixture.