Chemical Mapping of Nanodefects within 2D Covalent Monolayers by Tip-Enhanced Raman Spectroscopy

Nanoscale defects in monolayers (MLs) of two-dimensional (2D) materials, such as graphene, transition-metal dichalcogenides, and 2D polymers, can alter their physical, mechanical, optoelectronic, and chemical properties. However, detailed information about nanodefects within 2D covalent monolayers is difficult to obtain because it requires highly selective and sensitive techniques that can provide chemical information at the nanoscale. Here, we report a 2D imine-linked ML prepared from two custom-designed building blocks by dynamic imine chemistry at the air/water interface, in which an acetylenic moiety in one of the blocks was used as a spectroscopic reporter for nanodefects. Combined with density functional theory calculations that take into account surface selection rules, tip-enhanced Raman spectroscopy (TERS) imaging provides information on the chemical bonds, molecular orientation, as well as nanodefects in the resulting ML. Additionally, TERS imaging visualizes the topography and integrity of the ML at Au(111) terrace edges, suggesting possible ductility of the ML. Furthermore, edge-induced molecular tilting and a stronger signal enhancement were observed at the terrace edges, from which a spatial resolution around 8 nm could be deduced. The present work can be used to study covalent 2D materials at the nanoscale, which are expected to be of use when engineering their properties for specific device applications.