Formation of Highly Ordered Self-Assembled Monolayers on Two-Dimensional Materials via Noncovalent Functionalization.

Functionalized two-dimensional materials (2DMs) are attracting much attention due to their promising applications in nanoscale devices. Producing continuous and homogeneous surface assemblies with a high degree of order has been challenging. In this work, we demonstrate that by noncovalently self-assembling molecular platforms on 2DMs, high-quality and highly ordered monolayers can be generated. The high degree of order and uniformity of the self-assembled monolayer layers were confirmed by a variety of analytic techniques including time-of-flight secondary ion mass spectrometry, scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. Furthermore, by selectively enhancing the molecular vibrations of the molecular platform, via a combination of graphene-enhanced Raman spectroscopy (GERS) and surface-enhanced Raman spectroscopy (SERS), we were able to determine the orientation of self-assembled molecular platforms with respect to the surface normal. The selective enhancement of the vibrational modes occurs by taking advantage of the distance dependence of the Raman enhancement either by the graphene surface (GERS) or the silver nanoparticules (SERS) that are located on top of the self-assembled monolayer.