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.

Three-Legged 2,2'-Bipyridine Monomer at the Air/Water Interface: Monolayer Structure and Reactions with Ni(II) Ions from the Subphase.

The behavior of compound 2 [1,3,5-tri(2,2'-bipyridin-5-yl)benzene] with three bipyridine units arranged in a star geometry is investigated in the presence and absence of Ni(ClO). Its properties at the air-water interface as well as after transfer onto a solid substrate are studied by several techniques including Brewster angle microscopy, X-ray reflectivity, neutron reflectivity, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and atomic force microscopy combined with optical microscopy. It is found that compound 2 within the monolayers formed stays almost vertical at the interface and that at high Ni/2 (Ni/2 = 4000, 20'000) ratios two of the three bipyridine units of 2 are complexed, resulting in supramolecular sheets that are likely composed of arrays of linear metal-organic complexation polymers.

Synthesis of a Two-Dimensional Covalent Organic Monolayer through Dynamic Imine Chemistry at the Air/Water Interface.

A two-dimensional covalent organic monolayer was synthesized from simple aromatic triamine and dialdehyde building blocks by dynamic imine chemistry at the air/water interface (Langmuir-Blodgett method). The obtained monolayer was characterized by optical microscopy, scanning electron microscopy, and atomic force microscopy, which unambiguously confirmed the formation of a large (millimeter range), unimolecularly thin aromatic polyimine sheet. The imine-linked chemical structure of the obtained monolayer was characterized by tip-enhanced Raman spectroscopy, and the peak assignment was supported by spectra simulated by density functional theory.