Yanke Che

One-Dimensional Self-Assembly of Planar π-Conjugated Molecules: Adaptable Building Blocks for Organic Nanodevices

In general, fabrication of well-defined organic nanowires or nanobelts with controllable size and morphology is not as advanced as for their inorganic counterparts. Whereas inorganic nanowires are widely exploited in optoelectronic nanodevices, there remains considerable untapped potential in the one-dimensional (1D) organic materials. This Account describes our recent progress and discoveries in the field of 1D self-assembly of planar pi-conjugated molecules and their application in various nanodevices including the optical and electrical sensors. The Account is aimed at providing new insights into how to combine elements of molecular design and engineering with materials fabrication to achieve properties and functions that are desirable for nanoscale optoelectronic applications. The goal of our research program is to advance the knowledge and develop a deeper understanding in the frontier area of 1D organic nanomaterials, for which several basic questions will be addressed: (1) How can one control and optimize the molecular arrangement by modifying the molecular structure? (2) What processing factors affect self-assembly and the final morphology of the fabricated nanomaterials; how can these factors be controlled to achieve the desired 1D nanomaterials, for example, nanowires or nanobelts? (3) How do the optoelectronic properties (e.g., emission, exciton migration, and charge transport) of the assembled materials depend on the molecular arrangement and the intermolecular interactions? (4) How can the inherent optoelectronic properties of the nanomaterials be correlated with applications in sensing, switching, and other types of optoelectronic devices? The results presented demonstrate the feasibility of controlling the morphology and molecular organization of 1D organic nanomaterials. Two types of molecules have been employed to explore the 1D self-assembly and the application in optoelectronic sensing: one is perylene tetracarboxylic diimide (PTCDI, n-type) and the other is arylene ethynylene macrocycle (AEM, p-type). The materials described in this project are uniquely multifunctional, combining the properties of nanoporosity, efficient exciton migration and charge transport, and strong interfacial interaction with the guest (target) molecules. We see this combination as enabling a range of important technological applications that demand tightly coupled interaction between matter, photons, and charge. Such applications may include optical sensing, electrical sensing, and polarized emission. Particularly, the well-defined nanowires fabricated in this study represent unique systems for investigating the dimensional confinement of the optoelectronic properties of organic semiconductors, such as linearly polarized emission, dimensionally confined exciton migration, and optimal pi-electronic coupling (favorable for charge transport). Combination of these properties will make the 1D self-assembly ideal for many orientation-sensitive applications, such as polarized light-emitting diodes and flat panel displays.

Expedient Vapor Probing of Organic Amines Using Fluorescent Nanofibers Fabricated from an n-Type Organic Semiconductor

A new type of fluorescence sensory material with high sensitivity, selectivity, and photostability has been developed for vapor probing of organic amines. The sensory material is primarily based on well-defined nanofibers fabricated from an n-type organic semiconductor molecule, N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide. Upon deposition onto a substrate, the entangled nanofibers form a meshlike, highly porous film, which enables expedient diffusion of gaseous analyte molecules within the film matrix, leading to milliseconds response for the vapor sensing.

Ultrathin n-Type Organic Nanoribbons with High Photoconductivity and Application in Optoelectronic Vapor Sensing of Explosives

Well-defined ultrathin nanoribbons have been fabricated from an amphiphilic electron donor-acceptor (D-A) supramolecule comprising perylene tetracarboxylic diimide as the backbone scaffold to enforce the one-dimensional intermolecular assembly via strong pi-stacking. These nanoribbons demonstrated high photoconductivity upon illumination with white light. The high photoconductivity thus obtained is likely due to the optimal molecular design that enables a good kinetic balance between the two competitive processes, the intramolecular charge recombination (between D and A) and the intermolecular charge transport along the nanoribbon. The photoconduction response has also proven to be prompt and reproducible with the light turning on and off. The photogenerated electrons within the nanoribbon can be efficiently trapped by the adsorbed oxygen molecules or other oxidizing species, leading to depletion of the charge carriers (and thus the electrical conductivity) of the nanoribbon, as typically observed for n-type semiconductor materials as applied in chemiresistors. Combination of this sensitive modulation of conductivity with the unique features intrinsic to the nanoribbon morphology (large surface area and continuous nanoporosity when deposited on a substrate to form a fibril film) enables efficient vapor sensing of nitro-based explosives.

Diffusion-controlled detection of trinitrotoluene: Interior nanoporous structure and low highest occupied molecular orbital level of building blocks enhance selectivity and sensitivity

Development of simple, cost-effective, and sensitive fluorescence-based sensors for explosives implies broad applications in homeland security, military operations, and environmental and industrial safety control. However, the reported fluorescence sensory materials (e.g., polymers) usually respond to a class of analytes (e.g., nitroaromatics), rather than a single specific target. Hence, the selective detection of trace amounts of trinitrotoluene (TNT) still remains a big challenge for fluorescence-based sensors. Here we report the selective detection of TNT vapor using the nanoporous fibers fabricated by self-assembly of carbazole-based macrocyclic molecules. The nanoporosity allows for time-dependent diffusion of TNT molecules inside the material, resulting in further fluorescence quenching of the material after removal from the TNT vapor source. Under the same testing conditions, other common nitroaromatic explosives and oxidizing reagents did not demonstrate this postexposure fluorescence quenching; rather, a recovery of fluorescence was observed. The postexposure fluorescence quenching as well as the sensitivity is further enhanced by lowering the highest occupied molecular orbital (HOMO) level of the nanofiber building blocks. This in turn reduces the affinity for oxygen, thus allocating more interaction sites for TNT. Our results present a simple and novel way to achieve detection selectivity for TNT by creating nanoporosity and tuning molecular electronic structure, which when combined may be applied to other fluorescence sensor materials for selective detection of vapor analytes.

Enhanced fluorescence sensing of amine vapor based on ultrathin nanofibers

The fluorescence sensing of amine vapor was largely enhanced upon using ultrathin nanofibers, which were fabricated from N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide by a new self-assembly approach.

Organic nanofibrils based on linear carbazole trimer for explosive sensing

Organic fluorescent nanofibrils were fabricated from a linear carbazole trimer and employed for expedient detection of nitroaromatic explosives (DNT and TNT) and highly volatile nitroaliphatic explosives (nitromethane).

Reversible Dispersion and Release of Carbon Nanotubes Using Foldable Oligomers

Foldamers are synthetic and designable oligomers that adopt a conformationally ordered state in selected solvents. We found that oligo(m-phenylene ethynylene)s, which are single-stranded foldamers, can be made to reversibly disperse and release single-walled carbon nanotubes (SWCNTs) simply by changing the solvent, consistent with a change from an unfolded state to a folded state. Using absorption spectroscopy, atomic force microscopy, Raman spectroscopy, and electrical measurements, we observed that the foldamer-dispersed SWCNTs are individually well-dispersed and have a strong interfacial interaction with the foldamers. In contrast, the released SWCNTs appeared to be free of foldamers. Under illumination, transistors based on the foldamer-dispersed SWCNTs demonstrated significant photoresponse, apparently due to photoinduced charge transfer between the foldamers and SWCNTs. The reported nanocomposites may open an alternative way of developing optoelectronic devices or sensors based on carbon nanotubes.

Interfacial engineering of organic nanofibril heterojunctions into highly photoconductive materials.

Photoconductive organic materials have gained increasing interest in various optoelectronics, such as sensors, photodetectors, and photovoltaics. However, the availability of such materials is very limited due to their intrinsic low charge carrier density and mobility. Here, we present a simple approach based on nanofibril heterojunction to achieve high photoconductivity with fast photoresponse, that is, interfacial engineering of electron donor (D) coating onto acceptor (A) nanofibers via optimization of hydrophobic interaction between long alkyl side-chains. Such nanofibril heterojunctions possess two prominent features that are critical for efficient photocurrent generation: the nanofibers both create a large D/A interface for increased charge separation and act as long-range transport pathways for photogenerated charge carriers toward the electrodes, and the alkyl groups employed not only enable effective surface adsorption of D molecules on the nanofibers for effective electron-transfer communication, but also spatially separate the photogenerated charge carriers to prevent their recombination. The reported approach represents a simple, adaptable method that allows for the development and optimization of photoconductive organic materials.

Simple pyrolysis of cobalt alginate fibres into Co3O4/C nano/microstructures for a high-performance lithium ion battery anode

Cobalt tetroxide (Co3O4) has attracted much attention as a promising anode material for rechargeable lithium-ion batteries (LIBs) owing to its high theoretical capacity (890 mA h g−1). However, its poor electronic conductivity and weak ability to accommodate large volume changes during a repeated charging–discharging process, which results in the poor cycling performance, have hindered the practical application of Co3O4. In this article, Co3O4/C fibres were prepared by simple pyrolysis of wetspun cobalt alginate fibres. The composites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). It was found that the resulting material possesses a unique hierarchical nano/microstructure in which Co3O4 nanoparticles (NPs) are capsulated in a micro-sized carbonaceous fibrous matrix. This nano/microstructure can combine the merits of the Co3O4 nanostructure and the carbonaceous microfibre matrix, and thus exhibits a high reversible capacity of 780 mA h g−1 at 89 mA g−1 after 100 cycles as well as excellent cycling stability and rate performance when used as an anode material. This finding could open up a new direction in sustainable use of natural seaweed resources as new energy storage materials.

Ambient photodoping of p-type organic nanofibers: highly efficient photoswitching and electrical vapor sensing of amines.

High photoconductivity of p-type nanofibers fabricated from a reducing tetracyclic macromolecule was achieved though a simple photodoping process under ambient conditions, which, together with the intrinsic high surface area and porosity of the nanofibers when deposited on a substrate, enables application in electrical vapor sensing of organic amines.