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Thomas Nann

Publications and source records attributed to Thomas Nann.

17 recordsLinked to original sources
PubMed2006

Synthesis and spectroscopic investigations of Cu- and Pb-doped colloidal ZnS nanocrystals.

A novel organometallic synthesis method for the preparation of colloidal ZnS nanoparticles is presented. This method enables the synthesis of undoped ZnS nanocrystals as well as doping with Cu, Pb, or both. The particles can be covered with an undoped layer of ZnS, forming core/shell-type particles with the ZnS:Pb, ZnS:Cu, or ZnS:Cu,Pb cores. The particles were characterized via TEM, XRD, dynamic light scattering, and optical spectroscopy. We investigated the extrinsic surface defects and their coverage with an additional ZnS layer in detail by temperature-dependent luminescence and luminescence lifetime spectroscopy.

Colloids↗
PubMed2006

Silica encapsulation of hydrophobically ligated PbSe nanocrystals.

Spherical PbSe@SiO2 nanoparticles have been successfully synthesized within reverse micelles via metal alkoxide hydrolysis and condensation within a microemulsion system. These core-shell nanoparticles were characterized by transmission electron microscopy (TEM), NIR absorption spectroscopy, energy-dispersive X-ray analysis, and TEM electron diffractions. It shows that the obtained core-shell structures have a spherical shape with narrow size distribution (average size approximately 35 nm) and smooth surfaces. The size of the particles and the thickness of the shells can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) within the microemulsion.

Journal Article↗
PubMed2006

Rapid synthesis of high-quality InP nanocrystals.

A rapid new method for preparation of monodisperse InP-nanocrystals was developed. A highly reactive indium precursor and tris(trimethylsilyl)phosphine (TMS)3P was reacted within a weakly coordinating solvent in the presence of a supporting protic agent. The yielded InP-nanocrystals had a very narrow size distribution without any size selection process. The precursor and ligand effects were considered as critical factors in control of nucleation and crystal growth process. Different ligands were introduced to study the reaction mechanism. The new method not only yielded the "best" InP-nanocrystals so far, but also includes the potential for preparation within a continuous flow reactor, because the utilized ester is liquid at room temperature.

Journal Article↗
PubMed2006

One-pot synthesis of YF3@silica core/shell nanoparticles.

A new one-pot synthetic method for preparing core/shell YF3@SiO2 nanoparticles with different morphologies, from spherical to elongated structures ("pearl necklace"), is described; absorbance and photoluminescence spectroscopy reveals intrinsic but no extrinsic defects in the YF3.

Journal Article↗
PubMed2006

Shape control of II-VI semiconductor nanomaterials.

Anisotropic II-VI semiconductor nanocrystals and nanoparticles have become important building blocks for (potential) nanotechnological applications. Even though a wide variety of differently shaped nanoparticles of this class can be prepared, the underlying mechanisms are mostly not fully understood. This Review article provides a brief overview of the currently studied shape-evolution mechanisms and the most prominent synthesis methods for such particles, with an aim to provide a fundamental understanding on how different morphologies evolve, and to function as a tool to aid in the preparation of specific nanocrystals.

Anisotropy↗
PubMed2005

Determination of defect states in semiconductor nanocrystals by cyclic voltammetry.

Colloidal, monodisperse CdSe nanocrystals were homogeneously dispersed in an ionic liquid and investigated by means of cyclic voltammetry. Almost all known defect states in semiconductor nanocrystal were quantitatively measured with this nonoptical method (including nonradiative defect states). Variation of the illumination and temperature resulted in excitation of defect-trapped electrons into the conducting band. Thus, we succeeded for the first time to correlate defect states in nanocrystals with those in the corresponding bulk crystals.

Journal Article↗
PubMed2005

Synthesis and structural metastability of CdTe nanowires.

A new organometallic preparation method is described for CdTe nanowires with a high aspect ratio and a predominantly metastable wurtzite phase. The optical and morphological properties of the resulting nanowires were studied, as well as the influence of elevated temperatures on the crystallographic properties. A phase transition from wurtzite to sphalerite was observed at about 500 degrees C. The results show that the wurtzite phase is stabilized by the synthetic method and the surfactants.

Journal Article↗
PubMed2005

Phase-transfer of CdSe@ZnS quantum dots using amphiphilic hyperbranched polyethylenimine.

A new, straightforward method for the phase-transfer of CdSe@ZnS quantum dots from non-polar solvents into water and short-chained alcohols using amphiphilic hyperbranched polyethylenimine of different molar weights is suggested and the experimental procedure is discussed as well as the chemical properties of the resulting polymer-derivatised nanocrystals.

Journal Article↗
PubMed2004

Application of luminescent nanocrystals as labels for biological molecules.

Luminescent semiconductor nanocrystals, so called "quantum dots" (QD), have attracted increasing interest for bioanalytical labeling applications in recent years. This review describes the major optical and (bio)chemical features of this class of label, compared with organic dyes. Different conjugation methods are also discussed and the most important recent applications are presented. An overview over the current state-of-the-art is given, as also is an outlook on possibilities and limitations.

Biological Assay↗
PubMed2004

Charge transfer efficiency in hybrid bulk heterojunction composites.

On the basis of recently published electrochemical measurements, the charge transfer efficiency within CdSe nanocrystal/conducting polymer heterojunction composites was investigated by means of luminescence interaction strength. It was found that poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] and poly-9-vinylcarbazole luminescence was not totally quenched by nanocrystals, whereas poly-3-octylthiophene and polyvinylpyrrolidone was completely quenched. In case of poly-3-hexylthiophene, the nanocrystal luminescence was quenched. The results are in complete agreement with the electrochemical findings and thus, the CdSe nanocrystal/Polyvinylpyrrolidone composite should be a promising material for electroluminescent devices.

Journal Article↗
PubMed2004

Charge transfer mechanism in hybrid bulk heterojunction composites.

Charge transfer mechanisms of several conducting polymers and polymer/CdSe nanocrystal composites (hybrid bulk heterojunction composites) were investigated by means of cyclic voltammetry. Potential application of these composites in hybrid light-emitting diodes was discussed. It was found that charge transfer is observed in most of the composites, used so far, but was relatively slow or incomplete. The PVPy/CdSe nanocrystal composite showed promising results, and is favorable for use in electroluminescent devices.

Journal Article↗
PubMed2003

Hexagonal CdTe nanoparticles of various morphologies.

The present communication describes a new, organometallic synthesis method for CdTe nanocrystals using cadmium stearate as cadmium source; nanocrystals of different morphologies like spheres, rods and spikes were prepared and the metastable crystallographic hexagonal phase was stabilised in all samples.

Journal Article↗
PubMed2003

Visualizing the self-assembly of tubulin with luminescent nanorods.

We succeeded in labeling tubulin with luminescent CdSe nanorods. The labeled tubulin was still able to self-assemble into microtubules, demonstrating that the protein has remained functional. The comparison with rhodamine-labeled tubulin revealed that whereas the rhodamine bleached completely within the observed time frame, the nanorods did not show any bleaching, which makes it possible to follow the key events of self-assembly at high time resolution. Preparation and derivatization of nanorods are described. Water-soluble, protein-reactive group-containing particles were covalently coupled to tubulin. The conjugate was purified by one cycle of assembly and disassembly and used to induce the formation of fluorescent microtubules. These results suggest that the use of luminescent nanorods should allow continuous confocal monitoring of dynamic biological processes.

Biocompatible Materials↗
PubMed2002

Monodisperse CdSe nanorods at low temperatures.

A new synthetic method is presented that allows the preparation of highly monodisperse CdSe nanorods (so called quantum rods) at relatively low temperatures (160 degrees C). This method is characterized by a high aspect ratio of the particles and affords good reproducibility. The morphology of the resulting nanorods was examined by means of transmission electron microscopy (TEM) and the electro-optic properties by means of fluorescence spectroscopy. The conditions of the reaction of nanoparticle growth were examined by varying the concentration of the organometallic precursors, the growth temperature, and the growth time. The experimental findings correspond well with previously published semiempirical pseudo-potential calculations.

Journal Article↗
PubMed2002

Electrochemical metallization of self-assembled porphyrin monolayers.

Multifunctional sensor systems are becoming increasingly important in electroanalytical chemistry. Together with ongoing miniaturization there is a need for micro- and nanopatterning tools for thin electroactive layers (e.g. self-assembling monolayers). This paper documents a method for production of a micro-array of different metal-porphyrin monolayers with different sensor properties. A new method has been developed for the selective and local metallization of bare porphyrin monolayers by cathodic pulsing and sweeping. The metal-porphyrin monolayers obtained were characterized by cyclic voltammetry. It was shown that porphyrin monolayers can be metallized with manganese, iron, cobalt, and nickel by use of the new method. It is expected that all types of metal-porphyrin monolayers can be produced in the same manner.

Journal Article↗