Heiko Reith

A Tunable Strain Sensor Using Nanogranular Metals

This paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)3]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals.

Catalytic purification of directly written nanostructured Pt microelectrodes.

In the majority of cases, nanostructures prepared by focused electron beam induced deposition employing an organometallic precursor contain predominantly carbon-based ligand dissociation products. This is unfortunate with regard to using this high-resolution direct-write approach for the preparation of nanostructures for various fields, such as mesoscopic physics, micromagnetism, metaoptical phenomena in the visible spectral range, or others. Following early attempts of postprocessing Pt-based structures prepared by focused electron beam induced deposition at several hundred degrees Celsius in a reactive gas atmosphere, recent work has focused on developing in situ purification processes by using a stationary O2 flux in combination with electron irradiation to oxidize the carbonaceous component of the deposits. Here we show that this purification process is driven by the catalytic activity of Pt and in fact does not rely on the parallel electron irradiation process to function, if the O2 exposure is done in a pulsed fashion. We suggest a multistep cleaning mechanism which results in pure, nanoporous Pt. By suitably chosen beam parameters, high-resolution Pt dot and line structures with dimensions below 10 nm can thus be conveniently obtained. In temperature-dependent resistance measurements, we find the typical metallic behavior of Pt. In low-temperature magnetoresistance measurements, we see clear evidence for weak antilocalization effects and deduce a dephasing length of 234 nm at 1.2 K. We consider this to be a promising starting point for developing this approach into a versatile preparation technique for Pt-based mesoscopic structures, in particular since the purification process can be run in parallel on different deposits. We furthermore anticipate that our results will spur further research on purification approaches for nanostructures prepared by focused electron beam induced deposition containing a catalytically active metal species such as Pd-, Fe-, or Co-based deposits.

Berry phase and band structure analysis of the Weyl semimetal NbP

Weyl semimetals are often considered the 3D-analogon of graphene or topological insulators. The evaluation of quantum oscillations in these systems remains challenging because there are often multiple conduction bands. We observe de Haas-van Alphen oscillations with several frequencies in a single crystal of the Weyl semimetal niobium phosphide. For each fundamental crystal axis, we can fit the raw data to a superposition of sinusoidal functions, which enables us to calculate the characteristic parameters of all individual bulk conduction bands using Fourier transform with an analysis of the temperature and magnetic field-dependent oscillation amplitude decay. Our experimental results indicate that the band structure consists of Dirac bands with low cyclotron mass, a non-trivial Berry phase and parabolic bands with a higher effective mass and trivial Berry phase.

Influence of irradiation-induced disorder on the Peierls transition in TTF-TCNQ microdomains

The combined influence of electron irradiation-induced defects, substrate-induced strain and finite size effects on the electronic transport properties of individual micron-sized thin film growth domains of the organic charge transfer compound tetrathiafulvalene– tetracyanoquinodimethane (TTF–TCNQ) have been studied. The TTF–TCNQ domains have been isolated and electrically contacted by focused ion beam etching and focused ion and electron-beam-induced deposition, respectively. This allowed us to measure the temperature-dependent resistivity and the current–voltage characteristics of individual domains. The dependence of the resistivity on temperature follows a variable-range hopping behaviour which shows a crossover of the exponents as the Peierls transition is approached. The low temperature behaviour is analysed within the segmented rod model of Fogler, Teber and Shklovskii which was developed for charge-ordered quasi one-dimensional electron crystals (Fogler et al 2004 Phys. Rev. B 69 035413). The effect of substrate-induced biaxial strain on the Peierls transition temperature is discussed with regard to its interplay with the defect-induced changes.

Polyethenetetrathiolate or polytetrathiooxalate? Improved synthesis, a comparative analysis of a prominent thermoelectric polymer and implications to the charge transport mechanism

1,1,2,2-Ethenetetrathiolate (ett4−) coordination polymers, such as poly[Kx(Ni-ett)], have been known for decades for their excellent thermoelectric properties. However in reality, ett4− is neither a “true” comonomer which participates in the polymerization, nor represents a “true” repeat unit of the target polymer. Indeed, poly[K2(Ni-ett)], which is formally the product of Ni-induced polymerization of ett4−, has a poor conductivity and needs to be oxidized to show attractive thermoelectric characteristics. The polymerization and oxidation processes are poorly controllable which causes irreproducibility of the polymer properties. To improve the synthesis reproducibility, we studied polymerization of potassium tetrathiooxalate (K2tto), the convenient synthesis of which was developed in our recent work. Because K2tto is the “true monomer”, and not its precursor, a high quality product is reproducibly formed simply by mixing K2tto with NiCl2 at room temperature. The procedure does not require additional components (bases), or special conditions (prolonged heating), which are usually needed for the preparation of this polymer from the monomer precursor 1,3,4,6-tetrathiapentalene-2,5-dione (TPD). Furthermore, as tto2− is formally the product of two-electron oxidation of ett4−, the poorly controllable oxidation process is avoided and poly[Ni-tto] almost free from K is directly formed upon the complexation of Ni2+ and tto2−. Thus-obtained poly[Ni-tto] possesses conductivity in the range of 27–47 S cm−1 and a Seebeck coefficient in the range of −38 to −55 μV K−1, which are superior thermoelectric properties compared to poly[Kx(Ni-ett)] samples obtained by the previously reported methods. Redox and structural properties of poly[Ni-tto] were compared with those of poly[Kx(Ni-ett)] obtained by the reported methods. Furthermore, DFT calculations were performed to shed more light on generally promising properties of this class of materials. Particularly, possible packing models have been predicted for polymers, and the molecular dynamics simulations have been used to simulate the molecular arrangements under ambient conditions.