Two-dimensional (2D) materials have been of great importance in nature and in technology, owing to their astonishing properties, which are different from those of their bulk counterparts. As a structural analogue of graphene, 2D polymers (2DPs) are macromolecules as topologically planar, separable, monolayer (ML) sheets with covalent (strong) bonds and long-ranging internal periodicity, whose physical and chemical properties depend on their building blocks, linkages, and topographies. [4+4]-Photocycloaddition between neighboring anthracene pairs with a face-to-face stacked packing can afford the controlled lateral polymerization of suitable monomers both in single crystals and in Langmuir–Blodgett (LB) MLs on the air/water interface, resulting in the formation of 2DPs. However, it is still challenging to locally control [4+4]-photocycloaddition to form covalent 2D materials at the nanoscale and molecular level.
Tip-enhanced Raman spectroscopy (TERS) integrates nanoscale spatial resolution of scanning probe microscopy (SPM) with the chemical selectivity of Raman spectroscopy, and can simultaneously provide topographic and molecular information on samples in a label-free fashion. By means of field enhancement due to a combination of localized surface plasmon resonances and a lightning-rod effect at a metallic tip apex, TERS has shown single-molecule sensitivity and down to subnanometer spatial resolution for experiments carried out on special samples at cryogenic temperatures.
By means of TERS techniques, the fellow can
(a) manipulate and visualize photon–electron molecule interactions during plasmon-induced [4+4]-cycloaddition polymerization of anthracene-based monomers on Au(111) via TERS imaging in real-time and space;
(b) obtain a 2DP nanoribbon from certain locations in monomer 1 ML when sufficient hot carriers were generated;
(c) shed light on the nanolithography of 2DP monolayers by manipulating the plasmon-induced [4+4]-cycloaddition reaction.
The work carried out in this project can enhance innovation capacity in plasmon-induced chemical reactions (PICRs) and 2D organic monolayers:
(a) Catalysis mechanism: Understanding the interaction between incident light, hot carriers, and target molecules during PICRs at the nanoscale will help to recognize the reaction mechanisms and promote the chemical transformation of the plasmon-mediated photocatalysis, which in turn should provide insights into how to rationally design efficient plasmonic catalysts.
(b) Growth mechanism: The need to grow a 2D polymer in two directions rather than only in one is extremely important in planar polymerizations. Direct experimental evidence supported for a self-stimulating growth mechanism.
(c) Plasmon-induced nanolithography: Nanolithography can create nanoscale patterns on different media, e.g. on silicon wafers and molecular monolayers, used in various fields of technology from electronic to biomedical devices. The current plasmon-induced nanolithography is the potential to write new 2D patterns onto a molecular monolayer.
