Interfacial supramolecular assemblies with advanced functions.
Date of Issue2013
School of Materials Science and Engineering
Supramolecular assemblies have a key component with regards to supramolecular electronics, the study of its self-assembly behavior as well as functional properties will bring great impact in the sense of understanding and improving the performance of such devices from a fundamental point of view. We study supramolecular assemblies of hexa-peri-hexabenzocoronene (HBC) derivatives bearing different substituents, adsorbed on highly oriented pyrolytic graphite (HOPG) by using scanning tunneling microscopy at the solid-liquid interface. Two effects of intermolecular interaction were found to play a significant role in controlling the interfacial supramolecular assembly of these C3-symmetric HBC derivatives at the solid-liquid interface. One is hydrogen bonding interactions; the other is intermolecular dipole-dipole interactions. This work demonstrates how intermolecular interactions could enable fine control over the self-assembly of disk-like π-conjugated molecules. Furthermore, a host-guest system is established by utilizing the hydrogen bonding assisted honeycomb network established from HBC derivatives, and by incorporating coronene inside the porous structure, it offers potential applications such as use as molecular rectifiers in the future. Besides the bottom up approach of self-assembly of rigid molecules like HBC derivatives to establish a 2D π-conjugated structure, graphene has been viewed as the alternative material from the top-down point of view to possess 2D large π-conjugated area for promoting electron transport. By chemically modulating the monolayer graphene devices, positive photoreponse has been achieved, and with the help of conductive atomic force microscopy (C-AFM), the photoreponse performance could be correlated to the degree of functionalization at spatial distribution with nanoscale resolution. The mechanism was found to be desorption and adsorption of gas molecules in light and dark conditions; the positive photoresponse is due to the n-type doping behavior after functionalization. From the spatially distributed correlation between photocurrent and functionalization, it makes us realize that every functionalized pixel (~ nm2) offers a positive photoreponse, and the overall device performance is equivalent to the photoresponse of each individual pixel. This demonstrates that it is reliable to use the chemical method to generate a photoreponse from a 2D material like graphene. In order to explore further the relationship between the assembly structure behavior and its functional properties, π-conjugation interrupted frameworks (CIFs) have been studied and it was found that they are not only offering great three dimensionality, but are also sensitive to dopants and post-modification of their nanostructure based thin film with diazonium salt, which rendered their electrical properties highly tunable by up to two degrees of magnitude. Hence, it has been demonstrated that the π-conjugation-interrupted frameworks and their doped counterparts broaden the options for organic electronics. As a result of the H-shaped conformation and photosensitive properties of CIFs, we envisage their potential role as active components of memory devices and photodetectors in the future. In conclusion, this study offers valuable platforms by rational designing the self-assembly building blocks for the development and advancement in the field of supramolecular assemblies. It demonstrates the correlation between their structural behavior and resulted functional properties by means of scanning probe microscopes. HBC derivatives are designed for the bottom up approach of generating 2D π-conjugated area for molecular electronics; while graphene has been referred as the top-down alternative of producing such extended 2D structures and with chemical modulation, photoreponse of such devices are achieved with great repeatability. From 2D to 3D, a class of π-conjugation interrupted dendrimers exhibits tunable electrical properties by decorating small molecules. Finally, the perspectives on future directions and unsolved challenges are also addressed.