Christian Klinke

Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment

Manufacturing Nanomaterials The exploration of many materials at the nanoscale has revealed properties that only emerge when working at these small dimensions. For device manufacture, materials need to be deposited or assembled in specific patterns. Schliehe et al. (p. 550; see the cover) show the oriented attachment of lead sulfide nanocrystals into two-dimensional sheets. The packing is driven by the choice of solvents that influence the interactions between the nanocrystals. The nanocrystals have excellent photoconductive properties and were incorporated into a photodetector without any additional chemical processing. Self-assembled two-dimensional nanocrystals of lead sulfide have excellent photoconductive properties. 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.

Carbon nanotube films as electron field emitters

Carbon nanotubes have been recognized as one of the most promising electron field emitters currently available. We review the state of the art of current research on the electron field emission properties of carbon nanotube films and present recent results outlining their potential as field emitters as well as illustrating some current concerns in the research field.

Synthesis and Thermoelectric Characterization of Bi2Te3 Nanoparticles

Here, 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.

Self-aligned carbon nanotube transistors with charge transfer doping

This letter reports a charge transfer p-doping scheme which utilizes one-electron oxidizing molecules to obtain stable, unipolar carbon nanotube transistors with a self-aligned gate structure. This doping scheme allows one to improve carrier injection, tune the threshold voltage Vth, and enhance the device performance in both the “ON-” and “OFF-” transistor states. Specifically, the nanotube transistor is converted from ambipolar to unipolar, the device drive current is increased by 2–3 orders of magnitude, the device OFF current is suppressed and an excellent Ion∕Ioff ratio of 106 is obtained. The important role played by metal–nanotube contacts modification through charge transfer is demonstrated.

Radial elasticity of multiwalled carbon nanotubes.

We report an experimental and a theoretical study of the radial elasticity of multiwalled carbon nanotubes as a function of external radius. We use atomic force microscopy and apply small indentation amplitudes in order to stay in the linear elasticity regime. The number of layers for a given tube radius is inferred from transmission electron microscopy, revealing constant ratios of external to internal radii. This enables a comparison with molecular dynamics results, which also shed some light onto the applicability of Hertz theory in this context. Using this theory, we find a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30+/-10 GPa.

Charge Transfer Induced Polarity Switching in Carbon Nanotube Transistors

We probed the charge transfer interaction between the amine-containing molecules hydrazine, polyaniline, and aminobutyl phosphonic acid and carbon nanotube field effect transistors (CNTFETs). We successfully converted p-type CNTFETs to n-type and drastically improved the device performance in both the ON- and OFF-transistor states, utilizing hydrazine as dopant. We effectively switched the transistor polarity between p- and n- type by accessing different oxidation states of polyaniline. We also demonstrated the flexibility of modulating the threshold voltage (Vth) of a CNTFET by engineering various charge-accepting and -donating groups in the same molecule.

ZT enhancement in solution-grown Sb(2-x)BixTe3 nanoplatelets.

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.

Quantum Dot Attachment and Morphology Control by Carbon Nanotubes

Novel applications in nanotechnology rely on the design of tailored nanoarchitectures. For this purpose, carbon nanotubes and nanoparticles are intensively investigated. In this work, we study the influence of nonfunctionalized carbon nanotubes on the synthesis of CdSe nanoparticles by means of organometallic colloidal routes. This new synthesis methodology not only provides an effective path to attach nanoparticles noncovalently to carbon nanotubes but represents also a new way to control the shape of nanoparticles.

Self-aligned carbon nanotube transistors with novel chemical doping

We report an unconventional chemical p- and n- doping scheme utilizing novel materials and a charge transfer mechanism to obtain air-stable, self-aligned, unipolar carbon nanotube transistors. This scheme in addition to introducing the tunability of the threshold voltage V/sub th/, increases the drive current 2-3 orders of magnitude, transforms CNFET from ambipolar to unipolar, suppresses minority carrier injection and yields an excellent I/sub on//I/sub off/ ratio of 10/sup 6/.

Thermoelectric Properties of Lead Chalcogenide Core–Shell Nanostructures

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.