Aidong Peng

Construction and optoelectronic properties of organic one-dimensional nanostructures.

In the last 10 years, nanomaterials based on small organic molecules have attracted increasing attention. Such materials have unique optical and electronic properties, which could lead to new applications in nanoscale devices. Zero-dimensional (0D) organic nanoparticles with amorphous structures have been widely studied; however, the systematic investigation of crystalline one-dimensional (1D) organic nanostructures has only emerged in recent years. Researchers have used inorganic 1D nanomaterials, such as wires, tubes, and belts, as building blocks in optoelectronic nanodevices. We expect that their organic counterparts will also play an important role in this field. Because organic nanomaterials are composed of molecular units with weaker intermolecular interactions, they allow for higher structural tunability, reactivity, and processability. In addition, organic materials usually possess higher luminescence efficiency and can be grown on almost any solid substrate. In this Account, we describe recent progress in our group toward the construction of organic 1D nanomaterials and studies of their unique optical and electronic properties. First, we introduce the techniques for synthesizing 1D organic nanostructures. Because this strategy is both facile and reliable, liquid phase synthesis is most commonly used. More importantly, this method allows researchers to produce composite materials, including core/sheath and uniformly doped structures, which allow to investigate the interactions between different components in the nanomaterials, including fluorescent resonance energy transfer and photoinduced electron transfer. Physical vapor deposition allows for the synthesis of organic 1D nanomaterials with high crystallinity. Nanomaterials produced with this method offer improved charge transport properties and better optoelectronic performance in areas including multicolor emission, tunable emission, optical waveguide, and lasing. Although inorganic nanomaterials have developed rapidly, our findings highlight the importance of organic compounds as components of novel 1D nanomaterials.

Chemically responsive luminescent switching in transparent flexible self-supporting [EuW10O36]9−-agarose nanocomposite thin films

Europium-containing polyoxometalates (Eu-POMs) are widely used for the fabrication of hybrid inorganic-organic luminescent materials. A few efforts have been devoted to develop active Eu-POM-based luminescent sensors and switches. In this study, highly transparent flexible self-supporting decatungsteuropate(EuW10)-agarose thin films were successfully fabricated by a facile hydrogel casting technique. It was identified that strong interactions between agarose and EuW10 by hydrogen bonds at the hydroxyl sites and densely-packed 3D network structure of agarose in the gel state account for the homogenous distribution of EuW10 and good mechanical properties of the nanocomposite films. More importantly, the obtained thin films displayed strong red emission of Eu(III) ion, and the luminescence of these thin films was sensitive to the acid and base gases. When the thin films were exposed to HCl gas, their luminescence was sharply decreased, while the luminescence was recovered upon subsequently exposing the films to NH3 gas. Such process could be repeated many times and a portable switch based on these thin films was proposed.

Transparent and flexible phosphomolybdate–agarose composite thin films with visible-light photochromism

Polyoxometalates (POMs) are promising photochromic materials, but flexible POM-based thin films sensitive to visible-light have not been reported so far. In this study, highly transparent flexible self-supporting phosphomolybdate(PMo12)–agarose thin films were successfully fabricated by the combination of hydrogel chemistry and a facile casting technique. It was identified that strong interaction between polyhydroxyl site of agarose and PMo12 by hydrogen bonds together with densely-packed 3D network structure in the gel state avoided the phase separation efficiently and resulted in the homogenous distribution of PMo12 as well as the good mechanical properties of the nanocomposite films. More importantly, the obtained thin films exhibited excellent visible-light photochromic performance. When exposed to blue light irradiation through a TEM copper grid as photomask, the image recorded on the film displayed good resolution at 2 μm scale. Therefore, the developed PMo12–agarose photochromic thin films are promising for applications in portable high-resolution displays and high-density memory devices.

Net-like Assembly of Au Nanoparticles as a Highly Active Substrate for Surface-Enhanced Raman and Infrared Spectroscopy

Anodic aluminum oxide (AAO) templates were employed to filtrate and assemble Au nanoparticles by the pressure difference method. It was found that the colloidal Au nanoparticles can be uniformly arranged as nanonet assembly on the AAO surface. The net-assembled Au nanoparticles are clean and closely packed with nanochains. Taking fullerene C60/C70 as probe molecules, high-quality surface-enhanced Raman scattering (SERS) spectra were observed. The net-assembled Au nanoparticles even synchronously support the observation of surface-enhanced infrared absorption (SEIRA) spectra of the fullerene C60/C70. These results indicate that the AAO template filtrated with net-assembled Au nanoparticles is a highly active substrate for surface-enhanced spectroscopy.

Distinct nanostructures from isomeric molecules of bis(iminopyrrole) benzenes: effects of molecular structures on nanostructural morphologies

The effects of molecular structures on nanostructural morphologies have been studied through the preparation of nanospheres, square nanowires, and nanocubes from three isomeric molecules of bis(iminopyrrole)benzene.

An application of AAO template: orderly assembled organic molecules for surface-enhanced Raman scattering

High-density ordered arrays of core–shell nano-pillars of Ag–perylene were fabricated using an anodic aluminium oxide (AAO) template which was first embedded with the perylene molecules, followed by an electrochemical deposition of Ag. The surface-enhanced Raman scattering (SERS) spectrum obtained from this system showed well-resolved Raman peaks with good signal-to-noise ratios and little fluorescence background. This is in sharp contrast to the SERS of the individual Ag–perylene nanorods removed from the same AAO template, and the SERS of perylene molecules adsorbed on Ag colloidal nanoparticles. In the latter two cases, the SERS spectra consisted of broad and not-so-well-resolved Raman peaks with a strong fluorescence background. It is believed that the orderly assembly of the perylene molecules on the inner walls of the pores of the AAO template along the Ag nano-pillars led to fluorescence quenching. The high-density ordered arrays of Ag nano-pillars brought forth a surface plasmon resonance for the SERS effect. The present AAO template system offers a new substrate for studying SERS of highly fluorescing molecules.

The fabrication of TiO2 nanorods from TiO2 nanoparticles by organic protection assisted template method

We report here the fabrication of TiO(2) nanorods from TiO(2) nanoparticles by using the organic protection assisted template method. After the deposition of perylene nanotubes in the pores of the porous alumina membranes, implantation of TiO(2) nanoparticles resulted in the formation of TiO(2)/perylene composite nanorods. The diameters and lengths of the nanorods correspond well to the diameter of the pores of the membrane and the thickness of the template used. After removing the perylene protection layer by calcination, the TiO(2) nanorods obtained have an anatase structure, the same as that of the original TiO(2) nanoparticles. It is supposed that the organic layers protected the TiO(2) rods from damage during removal of the template by alkaline etching. Such an organic layer protection method presents a new approach to fabricating one-dimensional nanostructures from the corresponding nanoparticles by an AAO template.

Photoinduced electron transfer in coaggregates of dicyanonaphthalene and pyrazoline.

The photophysical properties of mixed coaggregates containing 1,4-dicyanonaphthalene (DCN) and 1,3,5-triphenyl-2-pyrazoline (TPP) have been studied. The absorption spectra of mixed coaggregates indicate that a charge-transfer complex is not formed in the ground state between DCN and TPP. The fluorescence of TPP in the mixed coaggregates is quenched by DCN, accompanied with a broad and structureless emission at about 560 nm from an exciplex between DCN and TPP. The color of the emission from mixed coaggregates is tunable by changing the DCN content. The excited-state properties of the TPP-DCN molecule pair are investigated theoretically with a quantum chemistry method. The theoretical results have also confirmed that the broad emission at about 560 nm in the mixed coaggregates originates from the exciplex rather than from the charge-transfer complex.

Core-shell nanopillars of fullerene C60/C70 loading with colloidal Au nanoparticles: a Raman scattering investigation.

High-density ordered core-sheath nanopillars of fullerene C60/C70 loading with colloidal Au nanoparticles were fabricated with a template method. The anodic aluminum oxide (AAO) template was first imbedded with the fullerene C60/C70 molecules and then followed by a pressure-difference approach for Au colloid. High-quality surface-enhanced Raman scattering (SERS) spectra of fullerene C60/C70 were obtained. The spectra show intense SERS signals with a fluorescence-free background, even with a 514 nm excitation at which the normal Raman of fullerene C60/C70 present poor signal-to-noise. The assembly of the fullerene C60/C70 on the inner walls of the AAO pores along the Au nanopillars lead to fluorescence quenching; meanwhile, the high-density and ordered arrays of Au nanopillars contribute to surface plasmon resonance for the SERS effect.

Wide bandgap small molecular acceptors for low energy loss organic solar cells

Non-fullerene organic solar cells (OSCs) have attracted great attention due to their advantages including tunable light absorption and low cost fabrication. Many important strategies have been used to achieve high performing OSCs including increasing the charge transport mobility and reducing the energy loss (Eloss). In this contribution, two wide bandgap small molecular acceptors (IDTzCR and IDTCR) were designed and synthesized for OSCs. Through replacing the thiophene moieties with thiazole ones, charge transport mobility was increased due to introducing S⋯N noncovalent conformational locks, resulting in a significant enhancement of photovoltaic performances. Furthermore, IDTCR based OSCs afforded a record low Eloss value for “narrow bandgap donor:wide bandgap acceptor” systems due to the small LUMO/LUMO energy offset. This contribution showed a novel method to achieve excellent wide bandgap acceptors for OSCs and sheds lights on understanding the relationship between the materials properties and device performances.