High-performance H(2)S detection by redox reactions in semiconducting carbon nanotube-based devices.

Here we report the highly effective detection of hydrogen sulfide (H2S) gas by redox reactions based on single-walled carbon nanotubes (SWCNTs) functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a catalyst and we also discuss the important role of water vapor in the electrical conductivity of SWCNTs during the sensing of H2S molecules. To explore the H2S sensing mechanism, we investigate the adsorption properties of H2S on carbon nanotubes (CNTs) and the effects of the TEMPO functionalization using first-principles density functional theory (DFT) and we summarize current changes of devices resulting from the redox reactions in the presence of H2S. The semiconducting-SWCNT (s-SWCNT) device functionalized with TEMPO shows a very high sensitivity of 420% at 60% humidity, which is 17 times higher than a bare s-SWCNT device under dry conditions. Our results offer promising prospects for personal safety and real-time monitoring of H2S gases with the highest sensitivity and low power consumption and potentially at a low cost.

One-dimensional self-assembly of a water soluble perylene diimide molecule by pH triggered hydrogelation.

A water soluble perylene diimide molecule has been fabricated into nanofibers via a pH triggered hydrogelation route. The one-dimensional self-assembly is dominated by the intermolecular π-π stacking interactions in concert with the hydrogen bonding between the carboxylic acid side chains. The anisotropic electronic and optical properties observed for the nanofibers are consistent with the one-dimensional intermolecular π-π arrangement.

Ultrafine nanofibers fabricated from an arylene-ethynylene macrocyclic molecule using surface assisted self-assembly.

Large area uniform nanofibers have been fabricated from a hexameric arylene-ethynylene macrocycle (1) through in situ self-assembly on a glass substrate during solvent evaporation. The fibril morphology is controlled by the solvophilic core of 1, in conjunction with the interfacial interactions between the side chains of 1 and the substrate.

Modulating the performance of carbon nanotube field-effect transistors via Rose Bengal molecular doping.

A simple, reliable, and large scale ambient environment doping method for carbon nanotubes is a highly desirable approach for modulating the performance of nanotube based electronics. One of the major challenges is doping carbon nanotubes to simultaneously offer a large shift in threshold voltage and an improved subthreshold swing. In this paper, we report on modulating the performance of carbon nanotube field-effect transistors (CNTFETs) by rationally selecting doping molecules. We demonstrated that Rose Bengal sodium salt (RB-Na) molecular doping can effectively shift the threshold voltage (ΔVth) of CNTFETs up to ∼6 V, decrease the subthreshold swing down to 130 mV/decade, and increase the effective field-effect mobility to 5 cm2 V(-1) s(-1). It is also shown that CNTFETs doped with Rose Bengal lactone (RBL) show a smaller variation in ΔVth (∼2 V) and subthreshold swing than those doped by RB-Na, which can be attributed to the difference in their molecular structures. The observed right-shift of the threshold voltage is attributed to the positive charge doping of the nanotube conduction channel from Rose Bengal molecules. The resultant lowering of the subthreshold swing is due to the reduced Schottky barrier at the CNT/metal/molecule interface. This room temperature chemical doping approach provides an efficient, simple, and cost-effective method to fabricate highly reliable and high-performance nanotube transistors for future nanotube based electronics.

Investigating photoinduced charge transfer in carbon nanotube-perylene-quantum dot hybrid nanocomposites.

In this study, we investigate photophysical and photoinduced current responses of a nanocomposite which consists of multiwalled carbon nanotubes (CNTs), thiol derivative perylene compound (ETPTCDI), and cadmium selenide quantum dots (QDs). These QDs as well as the ETPTCDI harvest photons and transfer their excited electrons or holes to CNTs to complete the circuit. Both QDs and ETPTCDI contribute charges to the carbon nanotubes, which increased the overall photon harvest efficiency of the nanocomposite. Herein, we investigate through a series of photophysical photoluminescence quenching studies the charge transfer between donors (QDs and ETPTCDI) and acceptor (CNTs). The incorporation of ETPTCDI into the nanocomposite significantly increases the adhesion between QDs and CNTs through bonding between QDs and thiol groups on ETPTCDI and π-π interactions between ETPTCDI and CNTs. Thus, ETPTCDI acted as a molecular linker between QDs and CNTs. Furthermore, a significant increase (>5 times) in the Stern-Volmer constant, K(sv), for QD emission after addition of ETPTCDI-tagged CNTs clearly indicates a large enhancement in the adhesion between CNTs and QDs. The nanocomposite shows a ∼2-4-fold increase in the photoconductivity when exposed to AM1.5 solar-simulated light. The damage to the nanocomposite from the intensity of the solar-simulated light is also investigated. The proposed nanocomposite has the potential for photovoltaic applications such as being the active component in a hybrid bulk heterojunction solar cell.

Linearly polarized emission of an organic semiconductor nanobelt.

Linearly polarized emission has been observed for the nanobelts fabricated from a perylene diimide molecule through both solution-based and surface-supported self-assembling. The measurement of polarized emission was performed over single nanobelts with use of a near-field scanning optical microscope (NSOM) adapted with emission polarization (by putting a planar polarizer before the detector). Rotating the emission polarizer (from 0 degrees to 180 degrees) changed the emission intensity in a way depending on the relative angle between the long axis of the belt and the polarizer with a minimum of intensity detected at ca. 78 degrees, which is indicative of the tilted stacking of molecules along the belt direction.

Effect of side-chain substituents on self-assembly of perylene diimide molecules: morphology control.

Effect of side-chain substitutions on the morphology of self-assembly of perylene diimide molecules has been studied with two derivatives modified with distinctly different side-chains, N,N'-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (DD-PTCDI) and N,N'-di(nonyldecyl)-perylene-3,4,9,10-tetracarboxylic diimide (ND-PTCDI). Due to the different side-chain interference, the self-assembly of the two molecules results in totally different morphologies in aggregate: one-dimensional (1D) nanobelt vs zero-dimensional (0D) nanoparticle. The size, shape, and topography of the self-assemblies were extensively characterized by a variety of microscopies including SEM, TEM, AFM, and fluorescence microscopy. The distinct morphologies of self-assembly have been obtained from both the solution-based processing and surface-supported solvent-vapor annealing. The nanobelts of DD-PTCDI fabricated in solution can feasibly be transferred to both polar (e.g., glass) and nonpolar (e.g., carbon) surfaces, implying the high stability of the molecular assembly (due to the strong pi-pi stacking). The side-chain-dependent molecular interaction was comparatively investigated using various spectrometries including UV-vis absorption, fluorescence, X-ray diffraction, and differential scanning calorimetry. Compared to the emission of ND-PTCDI aggregate, the emission of DD-PTCDI aggregate was significantly red-shifted (ca. 30 nm) and the emission quantum yield decreased about three times, primarily due to the more favorable molecular stacking for DD-PTCID. Moreover, the aggregate of DD-PTCDI shows a pronounced absorption band at the longer wavelength, whereas the absorption of ND-PTCDI aggregate is not significant in the same wavelength region. These optical spectral observations are reminiscent of the previous theoretical investigation on the side-chain-modulated electronic properties of PTCDI assembly.

Nanofibril self-assembly of an arylene ethynylene macrocycle.

Nanofibril structures have been fabricated from an arylene ethynylene macrocycle (AEM), which consists of a square frame corner-joined by four carbazole moieties. The fabrication was performed through a gelating process by cooling a warm, homogeneous solution in cyclohexane at high temperature (e.g., 100 degrees C) to room temperature. During the gelation, the molecules become organized, with optimal pi-pi stacking in cooperation with the side-chain association. The favorable pi-pi stacking facilitates the 1D growth of molecular assembly.

Surface-assisted one-dimensional self-assembly of a perylene based semiconductor molecule.

Chloroform-vapor annealing of thin films of propoxyethyl perylene tetracarboxylic diimide (PE-PTCDI, an n-type semiconductor) deposited on glass or mica leads to formation of well-defined one-dimensional self-assemblies (e.g. nanobelts), which show optically uniaxial properties as demonstrated by the linearly polarized emission.

Nanobelt self-assembly from an organic n-type semiconductor: propoxyethyl-PTCDI.

Nanobelt structures have been fabricated for an n-type semiconductor molecule, N,N'-di(propoxyethyl)perylene-3,4,9,10-tetracarboxylic diimide (PTCDI). The short alkyloxy side chain not only affords effective pi-pi stacking in polar solvents for self-assembling but also provides sufficient solubility in nonpolar solvents for solution processing. As revealed by both AFM and electron microscopies, the nanobelts have an approximately rectangular cross section, with a typical thickness of about 100 nm and a width in the range of 300-500 nm. The length of the nanobelts ranges from 10 to a few tens of micrometers. The highly organized molecular packing (uniaxial crystalline phase) has been deduced from the measurement of electron diffraction and polarized microscopy imaging. The detected optical axis is consistent with the one-dimensional stacking of the molecules.