Molecular Heterostructure and Aggregate Nanostructure Materials

Low-dimension organic/inorganic hybrid nanostructures can be used to produce new classes of organic/inorganic solid materials with properties that are not observed in either the individual nanosize components or the larger bulk materials. The properties of these hybrid materials arise from the sum of the contributions of the individual components as well as from processes that occur at the interfaces between the components. Completely novel or improved physical and chemical properties can result from strong interactions between the organic and inorganic units, thereby synergistic performance ("1 + 1 > 2" or "more than the sum of its parts") can be realized. We developed the combined self-assembly and templating technique to construct various nanostructured arrays of organic and inorganic semiconductors. The combination of hybrid aggregate nanostructures displays distinct optical and electrical properties compared with their individual components. Such hybrid structures show promise for applications in electronics, optics, photovoltaic cells, and biology.

 

Carbon-based Materials (Graphyne and Fullerene)

Naturally existing carbon allotropes are only in two forms, diamond and graphite, which composed of extended networks of sp3- and sp2-hybridized carbon atoms, respectively. Theoretically, carbon allotropes can be constructed by changing the periodic binding motif within the networks of sp3-, sp2- and sp-hybridized carbon atoms. Graphyne, consisting of layers containing sp and sp² carbons, was proposed as one of the synthetically approachablecarbon allotropes. This planar layer material, containing both hexagonal rings and acetylenic linkages, was named as graphyne in connection with graphite and acetylenic composition. Graphdiyne contains two diacetylenic linkages between a repeating pattern of carbon hexagons. Graphynefamilieshaveattracted great attention of many structural, theoretical, and synthetic scientists due to their promising electronic, optical, and mechanical properties.

High-yield direct preparation of graphdiyne had so far remained a challenge until we developed an in situ cross-coupling reaction on copper foil to fabricate a large-area ordered crystal film of graphdiyne from hexaethynylbenzene. The copper foil is not only the catalyst for the cross-coupling reaction but is also the substrate which provides a large flat space making the polymerization directional when growing the graphdiyne film. This key factor in the polymerization is the flat surface of copper, which drives the reaction towards forming a diyne-polymer. We also prepared graphdiyne nanotube (GDNT) arrays through an anodic aluminum oxide template catalyzed by Cu foil. The morphology-dependent field emission properties of graphdiyne arrays were measured and display high performance field emission properties. A novel aggregate structure of graphdiyne nanowires (GDNWs) can also been constructed which exhibits a very high quality defect-free surface by the VLS growth process using ZnO nanorod arrays on a silicon slice as a substrate. The GDNWs produced are excellent semiconductors with a conductivity of 1.9 × 10³ S m^-1 and a mobility of 7.1 ×10² cm² V^-1 s^-1. The results have confirmed that GDNW is indeed a promising and key novel material in electronic and photoelectric fields for both fundamental and practical applications.

 

Large-Area, Ordered Molecular Aggregate Nanostructures and Principle Devices

We have designed and synthesized functional conjugated organic molecules with structural features that favor assembly into aggregate nanostructures via weak intermolecular interactions. These large-area ordered molecular aggregate nanostructures are based on a variety of simpler structures such as fullerenes, perylenes, anthracenes, porphyrins, polydiacetylenes, and their derivatives. We have developed new methods to construct these larger structures including organic vapor-solid phase reaction, natural growth, association via self-polymerization and self-organization, and a combination of self-assembly and electrochemical growth. These methods are both facile and reliable, allowing us to produce ordered and aligned aggregate nanostructures, such as large-area arrays of nanowires, nanorods, and nanotubes. In addition, we can synthesize nanoscale materials with controlled properties. Large-area ordered aggregate nanostructures exhibit interesting electrical, optical, and optoelectronic properties. We also describe the preparation of large-area aggregate nanostructures of charge transfer (CT) complexes using an organic solid-phase reaction technique. By this process, we can finely control the morphologies and sizes of the organic nanostructures on wires, tubes, and rods. Through field emission studies, we demonstrate that the films made from arrays of CT complexes are a new kind of cathode materials, and we systematically investigate the effects of size and morphology on electrical properties.