Bacterially synthesized tellurium nanostructures for broadband ultrafast nonlinear optical applications.

Elementary tellurium is currently of great interest as an element with potential promise in nano-technology applications because of the recent discovery regarding its three two-dimensional phases and the existence of Weyl nodes around its Femi level. Here, we report on the unique nano-photonic properties of elemental tellurium particles [Te(0)], as harvest from a culture of a tellurium-oxyanion respiring bacteria. The bacterially-formed nano-crystals prove effective in the photonic applications tested compared to the chemically-formed nano-materials, suggesting a unique and environmentally friendly route of synthesis. Nonlinear optical measurements of this material reveal the strong saturable absorption and nonlinear optical extinctions induced by Mie scattering over broad temporal and wavelength ranges. In both cases, Te-nanoparticles exhibit superior optical nonlinearity compared to graphene. We demonstrate that biological tellurium can be used for a variety of photonic applications which include their proof-of-concept for employment as ultrafast mode-lockers and all-optical switches.

Percolating conductive networks in multiwall carbon nanotube-filled polymeric nanocomposites: towards scalable high-conductivity applications of disordered systems.

Disordered polymeric composite systems ordinarily exhibit poor bulk electronic transport properties, restricting their use to low-conductivity applications. In this work, highly electroconductive multi-walled carbon nanotube (MWCNT)/polyurethane (PU) nanocomposites were assembled via an aqueous solvent-blending method. Low percolation thresholds of 0.001 wt% and 0.093 wt% were obtained using pristine MWCNTs (P-MWCNTs) and mildly oxidized MWCNTs (O-MWCNTs), respectively. Corresponding critical values of dimensionality of 2.067 ± 0.094 and 2.304 ± 0.114 were calculated for P-MWCNT/PU and O-MWCNT/PU composites, respectively, strongly suggesting the formation of three-dimensional percolating conductive networks permeating the PU host matrix above the percolation threshold. Saturated direct current conductivities as high as 839 ± 72 S cm-1 were measured for O-MWCNT/PU composites at a filler-loading of 30.9 wt%. MWCNT/PU composite surfaces functionalized with superhydrophobic perfluoroalkyl moieties via chemical vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane exhibited sessile contact angles as high as 154° without adversely affecting electroconductivity.

Formation of tellurium nanocrystals during anaerobic growth of bacteria that use Te oxyanions as respiratory electron acceptors.

Certain toxic elements support the metabolism of diverse prokaryotes by serving as respiratory electron acceptors for growth. Here, we demonstrate that two anaerobes previously shown to be capable of respiring oxyanions of selenium also achieve growth by reduction of either tellurate [Te(VI)] or tellurite [Te(IV)] to elemental tellurium [Te(0)]. This reduction achieves a sizeable stable-Te-isotopic fractionation (isotopic enrichment factor [epsilon] = -0.4 to -1.0 per ml per atomic mass unit) and results in the formation of unique crystalline Te(0) nanoarchitectures as end products. The Te(0) crystals occur internally within but mainly externally from the cells, and each microorganism forms a distinctly different structure. Those formed by Bacillus selenitireducens initially are nanorods ( approximately 10-nm diameter by 200-nm length), which cluster together, forming larger ( approximately 1,000-nm) rosettes composed of numerous individual shards ( approximately 100-nm width by 1,000-nm length). In contrast, Sulfurospirillum barnesii forms extremely small, irregularly shaped nanospheres (diameter < 50 nm) that coalesce into larger composite aggregates. Energy-dispersive X-ray spectroscopy and selected area electron diffraction indicate that both biominerals are composed entirely of Te and are crystalline, while Raman spectroscopy confirms that they are in the elemental state. These Te biominerals have specific spectral signatures (UV-visible light, Raman) that also provide clues to their internal structures. The use of microorganisms to generate Te nanomaterials may be an alternative for bench-scale syntheses. Additionally, they may also generate products with unique properties unattainable by conventional physical/chemical methods.

Poly(L-lactide) (PLLA)/multiwalled carbon nanotube (MWCNT) composite: characterization and biocompatibility evaluation.

Much effort has been directed at the fabrication of carbon nanotubes (CNTs)/polymer composites and the characterization of their physical properties. Among them, composites comprising CNTs and the biocompatible polymers are of special interest due to their potential for specific biomedical applications. we report the preparation of the MWCNT/poly(L-lactide) composite and the corresponding spectroscopic (Raman) and the microscopic (SEM, TEM) characterization. The electronic transport, thermal properties, and biocompatibility of this composite have also been investigated. The Raman spectroscopic analysis suggests the interaction between PLLA and MWCNT occurs mainly through the hydrophobic C-CH3 functional groups. The DC conductivity of the composite increases as the MWCNT loading is increased. Such behavior can be described by a percolation mechanism in which a percolation threshold at about 14 wt % MWCNT loading is observed with the maximum end conductivity of 0.1 S x cm(-1). The DSC study of the PLLA/MWCNT composite reveals that the MWCNTs in the composite have the effect of inducing crystallization and plasticizing the polymer matrix. The results from the cell culture test suggest that the presence of MWCNT in the composite inhibits the growth of the fibroblast cells.

Doping properties of polydithienylmethine: a study on the correlation between polymer chain length, spectroscopy, and transport.

(1S)-(+)-10-Camphorsulfonic acid-doped polydithienylmethine was prepared through an acid-catalyzed condensation reaction of alpha,alpha'-di-2-thienyl-(2,2'-bithiophene)-5,5'-dimethanol and was characterized by 1H NMR spectroscopy and size exclusion chromatography (SEC). The electronic and vibrational properties of the resulting polymer thin films vary with the loadings of the (1S)-(+)-10-camphorsulfonic acid. The dark conductivity and drift mobility, which is significantly high, of the polymer thin films were enhanced with increasing doping levels and reached maximum values of 8.0x10(-5) S.cm-1 and 3.5x10(-2) cm2.V-1.s-1, respectively, at a 7 mol % dopant loading. Higher doping levels (>7 mol %) result in nonuniform polymer thin films with degraded optical quality due to the formation of nanocrystalite and thus a decrease in conductivity and drift mobility was observed. The doped polydithienylmethine thin film also exhibited a photoconductivity response with an excitation at 457 nm and the highest photoconductivity (2x10(-4) S.cm-1) was again observed at the 7 mol % doping level. Spectroscopic investigation suggests that the enhanced transport properties can be attributed to polaronic species present. The electronic and vibrational changes which relate to such doping were characterized by electronic absorption spectroscopy, Raman spectroscopy, and FTIR spectroscopy. The changes in transport values can be directly related to the changes we see in our spectroscopic investigations.

Aligned carbon nanotube-polymer hybrid architectures for diverse flexible electronic applications.

We present the fabrication and electrical characterization of a flexible hybrid composite structure using aligned multiwall carbon nanotube arrays in a poly(dimethylsiloxane) (PDMS) matrix. Using lithographically patterned nanotube arrays, one can make these structures at any length scale from submicrometer levels to bulk quantities. The PDMS matrix undergoes excellent conformal filling within the dense nanotube network, giving rise to extremely flexible conducting structures with unique electromechanical properties. We demonstrate its robustness against high stress conditions, under which the composite is found to retain its conducting nature. We also demonstrate that these structures can be utilized directly as flexible field-emission devices. Our devices show some of the best field-enhancement factors and turn-on electric fields reported so far.

Meso-structure formation for enhanced organic photovoltaic cells.

[reaction: see text] Formation of a controlled fullerene mesophase within an organic host system has enabled us to create high-power conversion efficiency photovoltaics. This mesophase is formed using thermal gradients that provide a fluidic mobility of the fullerenes allowing for greater dispersion of nanocrystalline 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) within regioregular poly(3-hexylthiophene) (P3HT). From this reorganization of the component materials in the matrix the overall efficiency of the system jumps dramatically from the roughly 2.4% to 5.2%.

Single-walled carbon nanotube purification, pelletization, and surfactant-assisted dispersion: a combined TEM and resonant micro-raman spectroscopy study.

Resonant Raman spectroscopy and transmission electron microscopy were used to characterize the structural changes of three single-walled carbon nanotube samples processed with purification, pelletization, and surfactant-assisted dispersion. A two-stage purification process selectively removes metallic tubes as well as small-diameter ones, enriching large-diameter semiconducting tubes. Pelletizing reduces the intertube distance but greatly increases the intensity ratio of the D band to the G band. Single-walled nanotube (SWNT) bundle size decreases during ultrasonication dispersion aided by a surfactant. SWNT bundles composed of large-diameter tubes are prone to debundling.

Structural and spectral features of selenium nanospheres produced by Se-respiring bacteria.

Certain anaerobic bacteria respire toxic selenium oxyanions and in doing so produce extracellular accumulations of elemental selenium [Se(0)]. We examined three physiologically and phylogenetically diverse species of selenate- and selenite-respiring bacteria, Sulfurospirillum barnesii, Bacillus selenitireducens, and Selenihalanaerobacter shriftii, for the occurrence of this phenomenon. When grown with selenium oxyanions as the electron acceptor, all of these organisms formed extracellular granules consisting of stable, uniform nanospheres (diameter, approximately 300 nm) of Se(0) having monoclinic crystalline structures. Intracellular packets of Se(0) were also noted. The number of intracellular Se(0) packets could be reduced by first growing cells with nitrate as the electron acceptor and then adding selenite ions to washed suspensions of the nitrate-grown cells. This resulted in the formation of primarily extracellular Se nanospheres. After harvesting and cleansing of cellular debris, we observed large differences in the optical properties (UV-visible absorption and Raman spectra) of purified extracellular nanospheres produced in this manner by the three different bacterial species. The spectral properties in turn differed substantially from those of amorphous Se(0) formed by chemical oxidation of H(2)Se and of black, vitreous Se(0) formed chemically by reduction of selenite with ascorbate. The microbial synthesis of Se(0) nanospheres results in unique, complex, compacted nanostructural arrangements of Se atoms. These arrangements probably reflect a diversity of enzymes involved in the dissimilatory reduction that are subtly different in different microbes. Remarkably, these conditions cannot be achieved by current methods of chemical synthesis.