Niklas Wolff

Localized Synthesis of Iron Oxide Nanowires and Fabrication of High Performance Nanosensors Based on a Single Fe2O3 Nanowire

A composed morphology of iron oxide microstructures covered with very thin nanowires (NWs) with diameter of 15-50 nm has been presented. By oxidizing metallic Fe microparticles at 255 °C for 12 and 24 h, dense iron oxide NW networks bridging prepatterned Au/Cr pads are obtained. X-ray photoelectron spectroscopy studies reveal formation of α-Fe2 O3 and Fe3 O4 on the surface and it is confirmed by detailed high-resolution transmission electron microscopy and selected area electron diffraction (SAED) investigations that NWs are single phase α-Fe2 O3 and some domains of single phase Fe3 O4 . Localized synthesis of such nano- and microparticles directly on sensor platform/structure at 255 °C for 24 h and reoxidation at 650 °C for 0.2-2 h, yield in highly performance and reliable detection of acetone vapor with fast response and recovery times. First nanosensors on a single α-Fe2 O3 nanowire are fabricated and studied showing excellent performances and an increase in acetone response by decrease of their diameter was developed. The facile technological approach enables this nanomaterial as candidate for a range of applications in the field of nanoelectronics such as nanosensors and biomedicine devices, especially for breath analysis in the treatment of diabetes patients.

CuV2S4: A High Rate Capacity and Stable Anode Material for Sodium Ion Batteries

The ternary compound CuV2S4 exhibits an excellent performance as anode material for sodium ion batteries with a high reversible capacity of 580 mAh g-1 at 0.7 A g-1 after 300 cycles. A Coulombic efficiency of ≈99% is achieved after the third cycle. Increase of the C-rate leads to a drop of the capacity, but a full recovery is observed after switching back to the initial C-rate. In the early stages of Na uptake first Cu+ is reduced and expelled from the electrode as nanocrystalline metallic Cu. An increase of the Na content leads to a full conversion of the material with nanocrystalline Cu particles and elemental V embedded in a Na2S matrix. The formation of Na2S is evidenced by 23Na MAS NMR spectra and X-ray powder diffraction. During the charge process the nanocrystalline Cu particles are retained, but no crystalline materials are formed. At later stages of cycling the reaction mechanism changes which is accompanied by the formation of copper(I) sulfide. The presence of nanocrystalline metallic Cu and/or Cu2S improves the electrical conductivity, leading to superior cycling and rate capability.

Identifying and overcoming the interface originating c-axis instability in highly Sc enhanced AlN for piezoelectric micro-electromechanical systems

Enhancing the piezoelectric activity of AlN by partially substituting Al with Sc to form Al1–xScxN is a promising approach to improve the performance of piezoelectric micro-electromechanical systems. Here, we present evidence of an instability in the morphology of Al1–xScxN, which originates at, or close to, the substrate/Al1–xScxN interface and becomes more pronounced as the Sc content is increased. Based on Transmission electron microscopy, piezoresponse force microscopy, X-ray diffraction, and SEM analysis, it is identified to be the incipient formation of (100) oriented grains. Approaches to successfully reestablish exclusive c-axis orientation up to x = 0.43 are revealed, with electrode pre-treatment and cathode-substrate distance found to exert significant influence. This allows us to present first measurements of the transversal thin film piezoelectric coefficient e31,f and dielectric loss tangent tan δ beyond x = 0.3.

Phase formation and stability in TiOx and ZrOx thin films: Extremely sub-stoichiometric functional oxides for electrical and TCO applications

Abstract Most functional materials are thermodynamic equilibrium phases representing minima in the thermodynamic phase space. However, it is expected that many metastable phases with highly interesting properties also exist. Here, we report on a systematic approach to prepare thin-films of such non-equilibrium phases based on the gas phase deposition methods sputtering and pulsed laser deposition (PLD). Our synthetic strategy is to deposit a “precursor phase” which is amorphous or already a crystalline non-equilibrium phase. Subsequent heat treatment leads to the nucleation of crystalline phases which again may be metastable or stable compounds. In the present paper we focus on the binary systems Ti–O and Zr–O, both systems being widely applied and technologically relevant. Highly oxygen-deficient titanium oxide (TiO1.6) and zirconium oxide (ZrO) films prepared by pulsed laser deposition at room temperature are optically absorbing and possess electronic conductivities in the range of 10 S/cm. Both materials are metastable in respect to both composition and structure. For TiO1.6 we find an amorphous matrix with embedded grains of cubic titanium monoxide (γ-TiO) directly after deposition. Upon annealing nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid-state disproportionation whereas the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S/cm) at about 450 °C. Congruently, room temperature deposited ZrO films with an average composition of Zr:O= 1:1 contain small ZrO nanocrystals within an amorphous matrix. Heat treatment again leads to an internal disproportionation reaction whereas small crystals of Zr2O and ZrO2 precipitate at temperatures as low as 75 °C. Increasing the temperature then results in the crystallization of metastable tetragonal ZrO2 at about 400 °C. Sputter deposition allows a subtler control of the oxygen partial pressure. Slightly non-stoichiometric TiO2−x films form a degenerate semiconductor with room temperature conductivities as high as 170 S/cm. Moreover, controlling both, the doping level and the vacancy concentration of these films allows to control the phase formation and the transition temperature between the rutile and anatase TiO2 polymorphs. Niobium doping of sputter deposited TiO2 can lead to films with very high electrical conductivities while maintaining a high optical transmittance demonstrating the potential of the material as an alternative transparent conducting oxide (TCO) with extraordinary properties.

Functional NiTi grids for in situ straining in the TEM

In situ measurements are a pivotal extension of conventional transmission electron microscopy (TEM). By means of the shape memory alloy NiTi thin film Functional Grids were produced for in situ straining as alternative or at least complement of expensive commercial holders. Due to the martensite-austenite transition temperature straining effects can be observed by use of customary heating holders in the range of 50 to 100°C. The grids can be produced in diversified designs to fit for different strain situations. Micro tensile tests were performed and compared with finite element simulations to estimate the applied forces on the sample and to predict the functionality of different grid designs. As a first example of this Functional Grid technology, we demonstrate the impact of applying a strain to a network of ZnO tetrapods.