1. Chemical Self-Doping of Organic Nanoribbons for High Conductivity and Potential Application as Chemiresistive Sensor

Na Wu, Chen Wang, Benjamin R. Bunes, Yaqiong Zhang, Paul M. Slattum, Xiaomei Yang, and Ling Zang

ACS Appl. Mater. Interfaces, 8 (2016) 12360-12368. 

Intrinsically low electrical conductivity of organic semiconductors hinders their further development into practical electronic devices. Herein, we report on an efficient chemical self-doping to increase the conductivity through one-dimensional stacking arrangement of electron donor–acceptor (D–A) molecules. The D–A molecule employed was a 1-methylpiperidine-substituted perylene tetracarboxylic diimide (MP-PTCDI), of which the methylpiperidine moiety is a strong electron donor, and can form a charge transfer complex with PTCDI (acting as the acceptor), generating anionic radical of PTCDI as evidenced in molecular solutions. Upon self-assembling into nanoribbons through columnar π–π stacking, the intermolecular charge transfer interaction between methylpiperidine and PTCDI would be enhanced, and the electrons generated are delocalized along the π–π stacking of PTCDIs, leading to enhancement in conductivity. The conductive fiber materials thus produced can potentially be used as chemiresistive sensor for vapor detection of electron deficient chemicals such as hydrogen peroxide, taking advantage of the large surface area of nanofibers. As a major component of improvised explosives, hydrogen peroxide remains a critical signature chemical for public safety screening and monitoring.

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2. Interfacial Donor-acceptor Nanofibril Composites for Selective Alkane Vapor Detection

Chen Wang, Benjamin R. Bunes, Miao Xu, Na Wu, Xiaomei Yang, Dustin E. Gross, Ling Zang

ACS Sensors,  1 (2016) 552-559.

The detection of alkane vapors has strong implications for safety, health, and the environment. Alkanes are notoriously difficult to detect because of their chemical inertness at room temperature. Herein, we introduce a tunable photoinduced charge transfer strategy to selectively detect alkane vapors under ambient condition. A unique donor–acceptor nanofibril composite comprising a compatible interface was fabricated, which is preferential for alkane adsorption. Then the enhanced adsorption disrupts the charge transfer across the interface and decreases the photocurrent, enabling the design of alkane gas sensor. We demonstrate a critical relationship between the tunable donor–acceptor interface and alkane response. The composite sensor is able to provide specific distinction between different alkanes based on their kinetics of the response profiles, and outstanding general selectivity against the common polar solvents. The work described herein may provide a basis for a new type of sensing material for detecting inert chemicals at room temperature.

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3. Self–Assembly of Perylene Imide Molecules into 1D Nanostructures: Methods, Morphologies and Applications

Shuai Chen, Paul Slattum, Chuanyi Wang, Ling Zang

Chem. Rev., 115 (2015) 11967-11998.  

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4. Interfacial Donor−Acceptor Engineering of Nanofiber Materials To Achieve Photoconductivity and Applications

Ling Zang

Accounts of Chemical Research, 48 (2015) 2705-2714.

Self-assembly of π-conjugated molecules often leads to formation of well-defined nanofibril structures dominated by the columnar π–π stacking between the molecular planes. These nanofibril materials have drawn increasing interest in the research frontiers of nanomaterials and nanotechnology, as the nanofibers demonstrate one-dimensionally enhanced exciton and charge diffusion along the long axis, and present great potential for varying optoelectronic applications, such as sensors, optics, photovoltaics, and photocatalysis. However, poor electrical conductivity remains a technical drawback for these nanomaterials. To address this problem, we have developed a series of nanofiber structures modified with different donor–acceptor (D–A) interfaces that are tunable for maximizing the photoinduced charge separation, thus leading to increase in the electrical conductivity. The D–A interface can be constructed with covalent linker or noncovalent interaction (e.g., hydrophobic interdigitation between alkyl chains). The noncovalent method is generally more flexible for molecular design and solution processing, making it more adaptable to be applied to other fibril nanomaterials such as carbon nanotubes. In this Account, we will discuss our recent discoveries in these research fields, aiming to provide deep insight into the enabling photoconductivity of nanofibril materials, and the dependence on interface structure.

The photoconductivity generated with the nanofibril material is proportional to the charge carriers density, which in turn is determined by the kinetics balance of the three competitive charge transfer processes: (1) the photoinduced electron transfer from D to A (also referred to as exciton dissociation), generating majority charge carrier located in the nanofiber; (2) the back electron transfer; and (3) the charge delocalization along the nanofiber mediated by the π–π stacking interaction. The relative rates of these charge transfer processes can be tuned by the molecular structure and nanoscale interface engineering. As a result, maximal photoconductivity can be achieved for different D–A nanofibril composites. The photoconductive nanomaterials thus obtained demonstrate unique features and functions when employed in photochemiresistor sensors, photovoltaics and photocatalysis, all taking advantages of the large, open interface of nanofibril structure. Upon deposition onto a substrate, the intertwined nanofibers form networks with porosity in nanometer scale. The porous structure enables three-dimensional diffusion of molecules (analytes in sensor or reactants in catalysis), facilitating the interfacial chemical interactions. For carbon nanotubes, the completely exposed π-conjugation facilitates the surface modification through π–π stacking in conjunction with D–A interaction. Depending on the electronic energy levels of D and A parts, appropriate band alignment can be achieved, thus producing an electric field across the interface. Presence of such an electric field enhances the charge separation, which may lead to design of new type of photovoltaic system using carbon nanotube composite.

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5. Organic Optoelectronic Materials for Trace Explosive Sensing.

Chengyi Zhang, Ling Zang

Imaging Science and Photochemistry, invited review, 30 (2012) 161-174.

Detection of trace explosives is of great concern for homeland security, battlefield protection, and industrial and environmental safety control. Fluorescence quenching based sensing has proven to be one of the most promising approaches for trace explosives detection. This review is focused on examples reported by Porf. Ling Zang's group and Prof. William Trogler's group related to fluorescence quenching sensors based on one-dimensional small molecular nanomaterials, fluorescent coordination polymers (or metal-organic frameworks) and polysiloles. It is also introduced the use of organic semiconductors, such as phthalocyanine thin films and perylene diimide nanowires, to construct electronic sensors for trace explosive sensing.

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6. Ambient Photodoping of p-Type Organic Nanofibers: Highly Efficient Photoswitching and Electrical Vapor Sensing of Amines

Yanke Che, Xiaomei Yang, Zengxing Zhang, Jianmin Zuo, Jeffrey S Moore, and Ling Zang

Chem. Commun. 46 (2010) 4127-4129.

P-type organic nanofibers were fabricated from a reducing tetracyclic macromolecule. High photoconductivity was achieved for these nanofibers though a simple photodoping process under ambient conditions via photoinduced electron transfer from nanofiber to the surface adsorbed oxygen. The high photoconductivity obtained for the nanofibers, together with theirintrinsic high surface area and porosity when deposited on a substrate, enables the application in electrical vapor sensing of organic amines.

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7. Ultrathin N-type Organic Nanobelts with High Photoconductivity and Application in Optoelectronic Vapor Sensing of Explosives

Yanke Che, Xiaomei Yang, Guilin Liu, Chun Yu, Hongwei Ji, Jianmin Zuo, Jincai Zhao, and Ling Zang

J. Am. Chem. Soc. 132 (2010) 5743-5750.

Well-defined ultrathin nanoribbons have been fabricated from an amphiphilic electron donor-acceptor (D-A) supramolecule comprising perylene tetracarboxylic diimide (PTCDI) as the backbone scaffold to enforce the one-dimensional intermolecular assembly via the strong π-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.

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Ling Zang, Yanke Che, Jeffrey S. Moore

Accounts of Chemical Research, a special issue on nanoscience, 41 (2008) 1596-1608.

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 p-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 to 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 to control these factors to achieve the desired 1D nanomaterials, e.g., 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 to correlate the inherent optoelectronic properties of the nanomaterials 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 p-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.

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

Yanke Che, Xiaomei Yang, Stephen Loser, and Ling Zang

Nano Lett., 8 (2008) 2219-2223.

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 mesh-like, highly porous film, which enables expedient diffusion of gaseous analyte molecules within the film matrix, leading to milliseconds response for the vapor sensing.

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