Nanotube Content in Arc Generated Carbon Powder

 A novel experimental technique for the separation of nanotubes from other unwanted carbon species in arc generated carbon soot is described. A conjugated polymer was used to bind to nanotubes in solution. The resultant hybrid was soluble, whereas extraneous carbon material formed a sediment at the bottom of the sample bottle. The process was monitored using electron paramagnetic resonance (EPR) spectroscopy showing that 63% of nanotubes were kept in solution and 98.1% of impurities were rejected. This non-destructive purification allowed the calculation of the nanotube content in the carbon soot using EPR and thermogravimetric analysis (TGA). The measurement of nanotube content gave a purity value of 34% for the soot used in this study; this is compatible with estimates from electron microscopic determinations.     

Recent Developments in Conjugated Polymer Based Plastic Solar Cells

Summary.  Recent developments on photovoltaic elements based on solid state composites of conjugated, semiconducting polymers mixed with buckminsterfullerene are reviewed. The photoinduced charge transfer from donor-type semiconducting conjugated polymers onto acceptor-type conjugated polymers or acceptor molecules such as buckminsterfullerene is reversible, ultrafast (within 100 fs) with a quantum efficiency approaching unity, and the charge separated state is metastable (up to ms at 80 K). This phenomenon of photoinduced electron transfer leads to a number of potentially interesting applications which include, among others, sensitization of the photoconductivity, reverse saturable absorption (optical limiting), and photovoltaic phenomena. Recent studies on the realization of photovoltaic elements with 3% power conversion efficiency are reported. Received December 19, 2000. Accepted December 22, 2000     

EXAFS Investigations on Nanocomposites Composed of Surface-Modified Zirconium and Zirconium/Titanium Mixed Metal Oxo Clusters and Organic Polymers

Summary.  The surface-modified oxometallate clusters Zr₆(OH)₄O₄(OMc)₁₂, Ti₄Zr₄O₆(OBu)₄ (OMc)₁₆, and Ti₂Zr₄O₄(OBu)₂(OMc)₁₄ (OMc = methacrylate) as well as their nanocomposites with polystyrene, poly(methacrylic acid) and poly(methyl methacrylate) were investigated by EXAFS. Studies on the nanocomposites revealed that the structure of the cluster core is retained in the hybrid materials. Received October 23, 2001. Accepted November 12, 2001     

Excitonic Bands in the Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units

Summary.  The optical absorption, photoluminescence, and photoconductivity spectra of some compounds of the formulas [R(CH₂) _n NH₃] _x M _y X _z , [R(CH₂) _n NH(CH₃)₂] _x M _y X _z , [R(CH₂) _n S(CH₃)₂] _x M _y X _z , [R(CH₂) _n SC(NH₂)₂] _x M _y X _z , and [R(CH₂) _n SeC(NH₂)₂] _x M _y X _z (R = organic residue; M = Bi(III), Pb(II), Sn(II), Cu(I), Ag(I) etc; X = I, Br, Cl; n, x, y, z = 0, 1, 2, 3, …) are briefly reviewed, and some new results are reported. The position, intensity, and shape of the excitonic bands depend on the dimensionality and size of the inorganic network as well as on the nature of the M, X, R, and onium moieties. Received June 23, 2000. Accepted August 1, 2000     

A Phenyl α-Nitronyl Nitroxide with a Forced Chiral Conformation

Summary.  A new phenyl α-nitronyl nitroxide bearing a hydroxyl group at position 2 of the aromatic ring and a chiral methyl lactate substituent at position 5 has been synthesized with the aim of combining the magnetic properties of this kind of radical with the optical properties endowed by the chiral group. The optically active compound forms intramolecular hydrogen bonds between the OH group and one of the NO groups and shows a large torsion angle between the two rings when compared with similar radicals with no substituent in this position. Therefore, the optical properties are distinct. The optical and magnetic properties of the new radical in both solution and solid state are presented. Received June 23, 2000. Accepted (revised) September 18, 2000     

Influence of Al2O3 Nanoparticles on the Thermal Stability of Ultra-Fine Grained Copper Prepared by High Pressure Torsion

Summary.  Ultra-fine grained (UFG) Cu (grain size 80 nm) containing 0.5 wt.% Al₂O₃ nanoparticles (size 20 nm) was prepared by high pressure torsion (HPT). Positron lifetime spectroscopy was employed to characterize the microstructure of this material, especially with respect to types and concentration of lattice defects. The evolution of microstructure with increasing temperature was studied by positron lifetime spectroscopy and X-ray diffraction measurements. The thermal stability of the Cu + 0.5 wt.% Al₂O₃ nanocomposite was compared with that of pure UFG Cu prepared by the same technique. The processes taking place during thermal recovery of the initial nanoscale structure in both studied materials are described. Received October 5, 2001. Accepted (revised) December 20, 2001     

Surface Enhanced Raman Spectroscopy on Carbon Filaments

 A method for analysis of carbon-containing thin films by using surface enhanced Raman spectroscopy (SERS) is described. Thin films of boron nitride or silicon carbide which are deposited on carbon filaments were coated additionally with silver nanoparticles. A very thin plasma polymer film was deposited on the silver particles to give a better long time stability. Using these layers, very intensive carbon band were detected.     

Electronic Structure and Size of TiO2 Nanoparticles of Controlled Size Prepared by Aerosol Methods

Summary.  A complete characterization of nanostructures has to deal both with electronic structure and dimensions. Here we present the characterization of TiO₂ nanoparticles of controlled size prepared by aerosol methods. The electronic structure of these nanoparticles was probed by X-ray absorption spectroscopy (XAS), the particle size by atomic force microscopy (AFM). XAS spectra show that the particles crystallize in the anatase phase upon heating at 500°C, whereas further annealing at 700°C give crystallites of 70% anatase and 30% rutile phases. Raising the temperature to 900°C results in a complete transformation of the particles to rutile. AFM images reveal that the mean size of the anatase particles formed upon heating at 500°C is 30 nm, whereas for the rutile particles formed upon annealing at 900°C 90 nm were found. The results obtained by these techniques agree with XRD data. Received October 5, 2001. Accepted (revised) December 6, 2001     

Microstructure and Defect Characterization of Nanostructured Ni<Subscript>3</Subscript>Al

Summary.  Nanostructured Ni₃Al was produced by the inert gas condensation and in situ compaction technique and characterized by means of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and density measurements. The defect structure was investigated using positron annihilation lifetime spectroscopy (PALS). It is shown that in some samples besides the cubic also the martensitic phase can be present. The defect structure can be divided into three major components: vacancy-like defects in the grain boundaries and nano-voids with a size of 1 nm as seen with PALS, and large pores with sizes up to 8 nm as seen with HRTEM. Furthermore, it is shown that an increasing compaction temperature leads to significantly smaller nano-voids. Received October 5, 2001. Accepted (revised) November 12, 2001     

Luminescence in Organic Silicons Prepared from Organic Precursors in Plasma Discharges

Summary.  The photoluminescence of plasma-prepared polysilanes during the change from linear 1D Si chains to an amorphous 3D Si network was studied. The excitonic absorption band with a maximum at 353 nm in 1D Si experiences a blue shift and broadening upon introduction of branching and networking defects. With the gradual transition from 1D to 3D structure, an extensive redistribution of oscillator intensity along the absorption edge, accompanied by a decrease of the resolution of the σ-σ^* band, was observed. In the short wavelength region of the excitation spectra there is an enormous increase of excitonic emission at 328 nm. This effect is tentatively attributed to the excitation of the phenyl group or to the phenyl-silicon bond as confirmed by effusion spectra of the phenyl species. Received July 10, 2000. Accepted (revised) September 8, 2000     

Novel Functional Fullerene Materials: Fullerenes as Energy Acceptors

Summary.  Fullerene derivatives exhibit absorption throughout the UV/Vis region up to 750 nm. This feature is important for easy excitation. Once excited, fullerenes may become both electron and energy acceptors. The distinction between these two pathways depends strongly on the partner involved (donor) and on the conditions employed (solvent, etc.). Received June 23, 2000. Accepted July 7, 2000     

Inorganic Clusters in Organic Polymers and the Use of Polyfunctional Inorganic Compounds as Polymerization Initiators

Summary.   Silicon oxide or metal oxide clusters or small particles with polymerizable organic groups covalently bonded to their surface can be copolymerized with organic monomers by various polymerization techniques. Whereas the preparation and properties of the polymers reinforced by R ₈Si₈O₁₂ have already been well investigated, analogous materials with incorporated transition metal oxide clusters are only beginning to show their potential as an interesting new class of inorganic-organic hybrid polymers. In the second part of the article, approaches are reviewed in which the inorganic building block serves as an initiator for polymerization reactions. This results in materials in which the organic polymer is grafted from an inorganic core. Most work has been done with surface-modified silica particles. Free radical polymerizations and atom transfer radical polymerizations with macroinitiators are summarized. The latter method results in polymeric particles in which an inorganic core is surrounded by an organic polymer shell. A new approach is the use of polyfunctional inorganic molecules or molecular clusters as initiators. Received July 28, 2000. Accepted August 7, 2000     

A Study of PbTiO3 Crystallization in Pure and Composite Nanopowders Prepared by the Sol-Gel Technique

Summary.  In this investigation the crystallization of PbTiO₃ upon annealing of pure nanopowders and PbTiO₃–SiO₂ (1:1 v/v) nanocomposite powders prepared by the sol-gel technique was studied. Using X-ray diffraction phase analysis, the start of PbTiO₃ crystallization in pure PbTiO₃ powders was detected at 400°C. Distinct crystallization of PbTiO₃ in PbTiO₃–SiO₂ nanocomposites starts at 700°C, whereas SiO₂ remains amorphous. There are indications that an interface interaction between the PbTiO₃ and the SiO₂ phase plays an important role in hindering the crystallization of PbTiO₃. The particle size (size of coherently scattering regions) was estimated from the broadening of the X-ray diffraction line profiles. The average size of PbTiO₃ nanocrystallites increases with temperature and time of annealing, the influence of temperature being more significant than that of the annealing time. Differential scanning calorimetry confirmed the results of the X-ray diffraction with respect to the start of the crystallization. Laser beam scattering and scanning electron microscopy provided the statistical distribution of the grain size and the morphology of the powder grains, showing that each grain of the powders contains several nanocrystallites (coherently scattering regions). Received October 4, 2001. Accepted (revised) December 14, 2001     

Rotational Energy Barrier of the Polarized Carbon–Carbon Double Bond in Quinophthalone

Summary.  A dynamic NMR effect is observed in the ¹³C NMR spectra of anhydrous quinophthalone (quinoline yellow) and its monohydrate in the vicinity of 47°C and 0°C, respectively, and is attributed to a restricted rotation around the polarized carbon–carbon double bond. The free energy of activation for this process in anhydrous quinophthalone and the monohydrate is 65±2 and 55±2 kJ · mol⁻¹, respectively, in CDCl₃. Received September 25, 2001. Accepted (revised) November 14, 2001     

Porphyrin-Based Bola-Amphiphiles as Structure-Directing Agents for the Synthesis of Mesostructured Silica

Summary.  Amphiphilic porphyrins with a bola-type arrangement of trimethylammonium headgroups can be used to generate M41S-type silica mesostructures with a lamellar topology and alternating organic–inorganic layers. Within the organic layers, the porphyrin entities are aggregated as shown by UV-Vis spectroscopy. The use of a bola amphiphile as structure-directing agent extends the possibilities of surfactant-controlled assembly of inorganic structures. In addition, the direct functionalization of the organic part of organic–inorganic hybrid nanostructures is possible due to the combination of the amphiphilic function with that of a chromophore. Corresponding author. E-mail: peter.behrens@mbox.acb.uni-hannover.de Received February 22, 2002; accepted May 8, 2002     

Magnetic Nanoparticles and Biosciences

Summary.  Magnetic nanoparticles represent an interesting material both present in various living organisms and usable for a variety of bioapplications. This review paper will summarize the information about biogenic magnetic nanoparticles, the ways to synthesize biocompatible magnetic nano- particles and complexes containing them, and the applications of magnetic nanoparticles in various areas of biosciences and biotechnologies. Received October 4, 2001. Accepted November 19, 2001     

Magnetic Properties of Ball-Milled Nanocrystalline Alloys Fe78B13Si9

Summary.  Magnetic properties of nanocrystalline Fe₇₈B₁₃Si₉ alloys are studied for three series prepared by ball milling starting from amorphous ribbons, crystallized ribbons, and elemental powders. Temperature variation of static magnetization results in strong ferromagnetic interaction which is weakly dependent on the initial material. Magnetic hysteresis loops show that saturation magnetization, magnetic remanence, and coercive field increase with frequency for both series of ribbon samples, whereas they decrease for alloys prepared from elemental powders. Power losses raise faster for the alloys prepared from elemental powders than for the two other alloys. Received October 5, 2001. Accepted (revised) November 12, 2001     

Stable Monopyrrole Atropisomers

Summary.  Malonic ester derivatives of ethyl and methyl 3,5-dimethyl-4-(1′-iodoneopentyl)-1H-pyrrole-2-carboxylate exhibit restricted rotation about the pyrrole C(4)–C(1′) bond due to the bulky 1′-tert-butyl and malonic ester groups and the ortho effect at C(4) of the sterically crowded 3,5-dimethylpyrrole. The malonates belong to a rare class of atropisomers with restricted rotation about an sp³–sp² C–C bond, and they undergo diastereomeric separation by TLC and crystallization: the diastereomers are stable in solution at room temperature. A crystal of one of the diastereomers, suitable for X-ray crystallography, gave the relative configuration of the chiral axis and stereogenic center at C(1′). Dynamic NMR studies of the purified diastereomers provide kinetic and thermodynamic parameters associated with the atropisomerism: ΔG ^‡ = 132–134 kJ/mol (∼32 kcal/mol) at 383 K in C₂D₂Cl₄ solvent. Corresponding author. E-mail: lightner@scs.unr.edu Received July 1, 2002; accepted July 8, 2002     

Characterization of Nanocomposite Coatings in the System Ti–B–N by Analytical Electron Microscopy and X-Ray Photoelectron Spectroscopy

Summary.  Superhard nanocomposite coatings of different composition in the quasi-binary system TiN–TiB₂ were deposited onto stainless steel sheets by means of unbalanced DC magnetron co-sputtering using segmented TiN/TiB₂ targets. The chemistry and microstructure of a TiB_0.6N_0.7 coating was investigated using X-ray and electron diffraction, photoelectron spectroscopy, energy-filtering transmission electron microscopy, and electron energy-loss spectrometry. High resolution elemental mapping of the elements Ti, B, N, and O with energy-filtering TEM reveals a homogeneous distribution on the nanometer scale. X-Ray and electron diffraction exhibit only TiN crystallites of nanometer size, but no information on the boron-rich phase. The near-edge fine structures of the BK and NK ionization edges in the EELS spectra of the Ti–B–N coatings were used to derive information on the phases by comparing the edges with those of reference compounds. It was found that the TiN nanocrystals occur together with TiO _x particles; the grains are embedded in a strongly disordered or quasi-amorphous matrix consisting mainly of TiB₂ particles and, near the steel substrate, also boron oxide (B₂O₃). Received October 4, 2001. Accepted (revised) January 10, 2002     

The Role of Interfaces in Photovoltaic Devices

Summary.  Organic opto-electronic devices comprise one or more organic layers and the electrodes. The interfaces between these very different components play a crucial role to the performance of the devices. In donor–acceptor composites for photovoltaics, the electronic processes occurring at the interface will benefit from a particular interface morphology on the 10–100 nanometer level; this is demonstrated for composites of oligophenylenevinylene and C₆₀. Phase separation on such a scale may be achieved naturally in diblock copolymers. The synthesis of an OPV–C₆₀ diblock copolymer is described. Received June 23, 2000. Accepted (revised) July 18, 2000