In recent years, surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. In the Ullmann-like coupling reaction, halogenated precursor molecules undergo thermally activated dissociation of aromatic carbon–halogen bonds resulting in the formation of organometallic or covalent networks. As a result, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogenous 2D system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution.<br/><br/>Here, the tandem measurement method of scanning tunneling microscopy (STM) tip-enhanced Raman spectroscopy (TERS) was used to interrogate the Ullmann-like coupling reaction of two types of precursor molecules on a Ag(100) substrate. With STM imaging, 3,6-dibromo-phenanthrequinone (DBPQ) molecules were found to self-assemble through hydrogen and halogen interactions, resulting in well-ordered molecular islands which steered the eventual formation of organometallic networks [1]. Expanding on this work to include spectroscopic imaging, STM-TERS was used to investigate the formation of 2D networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br<sub>4</sub>TPP) molecules [2]. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirmed the covalent nature of the 2D networks and suggested that the apparent disorder arises from molecular flexibility. These works demonstrate the ability to provide new insight into low-dimensional structures with atomic-scale topographic imaging and spectroscopy with the same spatial resolution.

Studies of low-dimensional nanostructures formed by on-surface reactions of molecular precursors with scanning tunneling microscopy and tip-enhanced Raman spectroscopy

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