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In our group we investigate phenomena emerging in novel solid state materials with dimensions in the nanometer range. Such materials are generated by colloidal synthesis, chemical vapor deposition, and electron-beam lithography. We characterize them by means of electrical transport measurements, spectroscopical methods, and electron microscopy. Of special interest are the electrical properties of inorganic nanoparticles, carbon nanotubes, and composites.  NEWS 03/07/2012 We published a new paper in Advanced Materials. Recently, we found that a larger friction coefficient is present when the tip of an AFM is sliding perpendicular to the axis of a carbon nanotube, as compared with sliding along the tube axis (Nature Mater. 8 (2009) 876). This behavior is explained by a deformation, like a lateral swaying (or a "hindered rolling") of the tube during the transverse sliding, which produces additional friction dissipation. This soft deformation mode is absent, or partially absent, when the tip slides along the nanotube axis, thus, for the longitudinal sliding the friction force arises mostly only from sliding the hard nano-contact between the tip and the tube. The ratio between transverse and longitudinal friction per unit area is called friction anisotropy. Here, we show how structural defects, surface chemistry and possibly chirality can couple the transverse and longitudinal sliding, modulating nanotubes frictional properties. A simple analytical model has been developed to compute the amount of coupling, between the transverse and longitudinal sliding, the "intrinsic" hard contact sliding shear strength, and the soft "hindered rolling" shear strength. This model captures very well the observed experimental behavior indicating for all the nanotubes a common supralinear decrease of the coupling with increasing ratio between transverse and longitudinal friction. In the experiments, the amount of structural defects in the CNTs is controlled by using different growth methods, whereas the surface chemistry is controlled with after-growth chemical functionalization. Reference: Hsiang-Chih Chiu et al., Adv. Mater. (2012) accepted. 12/05/2011 We published a new paper in the Journal of Physical Chemistry C. We report on the fabrication of carbon nanotube arrays with spatial dimensions that are suitable for application in polymer-based solar cells. We demonstrate that nanotubes can be grown on ITO covered glass at temperatures below 600 °C by static pressure chemical vapor deposition (CVD). Using short growth times, we were able to obtain nanotube arrays with a rather uniform length limited to about 200 nm. With a plasma-enhanced CVD process, we were also able to produce wall-like carbon nanostructures (multilayered graphene sheets) with controllable height on ITO. Both types of carbon nanostructures were investigated in test solar cells, in order to explore their suitability for application in organic photovoltaics. Reference: Holger Borchert et al., J. Phys. Chem. C 116 (2012) 412. 11/11/2011 We published a new paper in Nanoscale. In this work we study the charge transfer in individual double-walled carbon nanotubes highly covered with uniform ZnO nanoparticles. The composite material was used as conductive channel in a field-effect transistor device and the electrical photo-response was analysed under various conditions. By means of the transfer characteristics we could elucidate the mechanism of charge transfer from non-covalently attached semiconducting nanoparticles to carbon nanotubes. The role of positive charges remaining on the nanoparticles is discussed in terms of a gating effect. Reference: Alina Chanaewa et al., Nanoscale 4 (2012) 251. 10/07/2011 We published a new paper in ACS Nano. We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe–PbSe samples fabricated from colloidal core–shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core–shell material combinations for future thermoelectric materials of large electric conductivities in combination with an increased thermopower by energy filtering. Reference: Marcus Scheele et al., ACS Nano 5 (2011) 8541. 09/01/2011 Mirjam Volkmann joined our group as diploma student. She will work on the synthesis of hybrids materials composed of inorganic nanoparticles and nanotubes. 08/01/2011 Thomas Bielewicz joined our group as diploma student. He will work on the synthesis of sheet-like colloidal nanostructures. 12/22/2010 We published a new paper in ACS Nano. We report about the electrical transport through monolayers of monodisperse cobalt−platinum nanoparticles. Upon annealing we observe an increase of conductivity over more than 4 orders of magnitude. A first attempt of explanation of this unanticipated effect, a nanoparticle displacement, could not be confirmed for annealing temperatures below 400 °C. A second approach, a carbonization of the ligands, however, could be confirmed by Raman spectroscopy. The simple thermal treatment allows tuning essential properties of electronic devices based on nanoparticles by the manipulation of the interparticle coupling, namely the electrical conductivity, the Coulomb blockade characteristic, and the activation energy of the system. Reference: Yuxue Cai et al., ACS Nano 5 (2011) 67. 10/26/2010 We published a new paper in Journal of Materials Chemistry. We report the growth of an unstable shell-like gold structure around dihexagonal pyramidal CdSe nanocrystals in organic solution and the structural transformation to spherical domains by two means: (i) electron beam irradiation (in situ) and (ii) addition of a strong reducing agent during synthesis. By varying the conditions of gold deposition, such as ligands present or the geometry of the CdSe nanocrystals, we were able to tune the gold domain size between 1.4 nm and 3.9 nm and gain important information on the role of surface chemistry in heteronanoparticle synthesis and seed reactivity, both of which are crucial points regarding the chemical design of new materials for photocatalysis and optoelectronic applications. Reference: Michaela Meyns et al., J. Mater. Chem. 20 (2010) 10602. 09/01/2010 Sedat Dogan joined our group as PhD student. He will work on the electrical characterization of two-dimensional colloidal nanostructures. 07/30/2010 We published a new paper in Science. Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials. For this concept, oriented attachment of nanocrystal building blocks, a self-assembly of particles into larger single-crystalline objects, is one of the most promising approaches in nanotechnology. We report here the two-dimensional oriented attachment of lead sulfide (PbS) nanocrystals into ultrathin single-crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by cosolvents, which alter nucleation and growth rates during the primary nanocrystal formation, and is finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photodetector device without further treatment. More information in English and German. Reference: Constanze Schliehe et al., Science 329 (2010) 550. 06/25/2010 We published a new paper in ACS Nano. We report a solution-processed, ligand-supported synthesis of 15−20 nm thick Sb(2−x)BixTe3 nanoplatelets. After complete ligand removal by a facile NH3-based etching procedure, the platelets are spark plasma sintered to a p-type nanostructured bulk material with preserved crystal grain sizes. Due to this nanostructure, the total thermal conductivity is reduced by 60% in combination with a reduction in electric conductivity of as low as 20% as compared to the bulk material demonstrating the feasibility of the phonon-glass electron-crystal concept. An enhancement in the dimensionless thermoelectric figure of merit of up to 15% over state-of-the-art bulk materials is achieved, meanwhile, shifting the maximum to significantly higher temperatures. Reference: Marcus Scheele et al., ACS Nano 4 (2010) 4283. 04/01/2010 Michaela Meyns joined our group as PhD student. She will work on combined semiconductor-metal nanoparticles. 03/01/2010 We published a new paper in ACS Nano. The formation of monodisperse, tunable sized, alloyed nanoparticles of Ni, Co, or Fe with Pt and pure Pt nanoparticles attached to carbon nanotubes has been investigated. Following homogeneous nucleation, nanoparticles attach directly to non-functionalized singlewall and multiwall carbon nanotubes during nanoparticle synthesis as a function of ligand nature and the nanoparticle work function. Raman spectroscopy reveals that the sp2 hybridization of the carbon lattice is not modified by the attachment. In order to better understand the interaction between the directly attached nanoparticles and the non-functionalized carbon nanotubes we employed first-principles calculations on model systems of small Pt clusters and both zig-zag and armchair singlewall carbon nanotubes. The detailed comprehension of such systems is of major importance since they find applications in catalysis and energy storage. Reference: Beate Ritz et al., ACS Nano 4 (2010) 2438. 11/18/2009 Timm Reumann joined our group as diploma student. He will work on theoretical aspects of organic molecules adsorbed on carbon nanotubes. 10/07/2009 We published a new paper in Advanced Functional Materials. A novel synthesis for near monodisperse, sub-10 nm Bi2Te3 nanoparticles is reported. A new reduction route to bismuth nanoparticles is described, which are then applied as starting materials in the formation of rhombohedral Bi2Te3 nanoparticles. After ligand removal by a novel hydrazine hydrate etching procedure, the nanoparticle powder is spark plasma sintered to a pellet with preserved crystal grain sizes. Unlike previous works on the properties of Bi2Te3 nanoparticles, the full thermoelectric characterization of such sintered pellets shows a highly reduced thermal conductivity and the same electric conductivity as bulk n-type Bi2Te3. Reference: Marcus Scheele et al., Adv. Func. Mater. 19 (2009) 3476. 09/27/2009 Beatriz H. Juarez and Christian Klinke received the German Nanotech Prize 2009 (Nanowissenschaftspreis) at the Schlosshotel Cecilienhof in Potsdam. The prize is offered by the Arbeitsgemeinschaft der Nanotechnologie-Kompetenzzentren Deutschlands (AGeNT-D) and the Federal Ministry of Education and Research (BMBF). 09/21/2009 Francisco Martinez joined our group for three months as exchange PhD student from the Jose Dobado group in Granada, Spain. He will work on theoretical aspects of combined nanoparticle-nanotube systems. 09/13/2009 We published a new paper in Nature Materials. We present a combined theoretical and experimental study of the frictional forces encountered by a nano-size tip sliding on top of a supported multiwall carbon nanotube along a direction parallel or transverse to the nanotube axis. Surprisingly, we find a higher friction coefficient in the transverse direction compared to the parallel direction. This behavior is explained by a simulation showing that transverse friction elicits a soft "hindered rolling" of the tube and a frictional dissipation that is absent, or partially absent for chiral nanotubes, when the tip slides parallel to the nanotube axis. Reference: Marcel Lucas et al., Nature Mater. 8 (2009) 876. 09/04/2009 Michaela Meyns joined our group as diploma student. She will work on combined semiconductor-metal nanoparticles. 07/03/2009 Successful participation in the science initiative called "Landesexzellenzinitiative". We are part of the cluster project Nano spintronics and a graduate school C1 chemistry. |