Stephan Steinhauer

Formation Mechanism of Fe Nanocubes by Magnetron Sputtering Inert Gas Condensation

In this work, we study the formation mechanisms of iron nanoparticles (Fe NPs) grown by magnetron sputtering inert gas condensation and emphasize the decisive kinetics effects that give rise specifically to cubic morphologies. Our experimental results, as well as computer simulations carried out by two different methods, indicate that the cubic shape of Fe NPs is explained by basic differences in the kinetic growth modes of {100} and {110} surfaces rather than surface formation energetics. Both our experimental and theoretical investigations show that the final shape is defined by the combination of the condensation temperature and the rate of atomic deposition onto the growing nanocluster. We, thus, construct a comprehensive deposition rate-temperature diagram of Fe NP shapes and develop an analytical model that predicts the temporal evolution of these properties. Combining the shape diagram and the analytical model, morphological control of Fe NPs during formation is feasible; as such, our method proposes a roadmap for experimentalists to engineer NPs of desired shapes for targeted applications.

Single CuO nanowires decorated with size-selected Pd nanoparticles for CO sensing in humid atmosphere

We report on conductometric gas sensors based on single CuO nanowires and compare the carbon monoxide (CO) sensing properties of pristine as well as Pd nanoparticle decorated devices in humid atmosphere. Magnetron sputter inert gas aggregation combined with a quadrupole mass filter for cluster size selection was used for single-step Pd nanoparticle deposition in the soft landing regime. Uniformly dispersed, crystalline Pd nanoparticles with size-selected diameters around 5 nm were deposited on single CuO nanowire devices in a four point configuration. During gas sensing experiments in humid synthetic air, significantly enhanced CO response for CuO nanowires decorated with Pd nanoparticles was observed, which validates that magnetron sputter gas aggregation is very well suited for the realization of nanoparticle-functionalized sensors with improved performance.

Nanoparticle design by gas-phase synthesis

Abstract Gas-phase synthesis characterizes a class of bottom-up methods for producing multifunctional nanoparticles (NPs) from individual atoms or molecules. This review aims to summarize recent achievements using this approach, and compare its potential to other physical or chemical NP fabrication techniques. More specifically, emphasis is given to magnetron-sputter gas-phase condensation, since it allows for flexible growth of complex, sophisticated NPs, owing to the fast kinetics and non-equilibrium processes it entails. Nanoparticle synthesis is decomposed into four stages, i.e. aggregation, shell-coating, mass-filtration, and deposition. We present the formation of NPs of various functionalities for different applications, such as magnetic, plasmonic, catalytic and, gas-sensing, emphasizing on the primary dependence of each type on a different stage of the fabrication process, and their resultant physical and chemical properties. Graphical Abstract

Low-frequency noise characterization of single CuO nanowire gas sensor devices

Low-frequency noise properties of single CuO nanowire devices were investigated under gas sensor operation conditions in dry and humid synthetic air at 350 °C. A 1/f noise spectrum was found with the normalized power spectral density of current fluctuations typically a factor of 2 higher for humid compared to dry atmosphere. A core-shell nanowire model is proposed to treat the noise as parallel combination of gas-independent bulk and gas-dependent surface noise components. The observed increase in 1/f noise in the presence of water vapor is explained in terms of Hooges mobility fluctuation model, where the increased surface noise component is attributed to carrier scattering at potential fluctuations due to hydroxyl groups at the nanowire surface.Low-frequency noise properties of single CuO nanowire devices were investigated under gas sensor operation conditions in dry and humid synthetic air at 350 °C. A 1/f noise spectrum was found with the normalized power spectral density of current fluctuations typically a factor of 2 higher for humid compared to dry atmosphere. A core-shell nanowire model is proposed to treat the noise as parallel combination of gas-independent bulk and gas-dependent surface noise components. The observed increase in 1/f noise in the presence of water vapor is explained in terms of Hooges mobility fluctuation model, where the increased surface noise component is attributed to carrier scattering at potential fluctuations due to hydroxyl groups at the nanowire surface.

Nanoscale Heterogeneity of Multilayered Si Anodes with Embedded Nanoparticle Scaffolds for Li-Ion Batteries

Abstract A new approach on the synthesis of Si anodes for Li‐ion batteries is reported, combining advantages of both nanoparticulated and continuous Si films. A multilayered configuration prototype is proposed, comprising amorphous Si arranged in nanostructured, mechanically heterogeneous films, interspersed with Ta nanoparticle scaffolds. Particular structural features such as increased surface roughness, nanogranularity, and porosity are dictated by the nanoparticle scaffolds, boosting the lithiation process due to fast Li diffusion and low electrode polarization. Consequently, a remarkable charge/discharge speed is reached with the proposed anode, in the order of minutes (1200 mAh g−1 at 10 C). Moreover, nanomechanical heterogeneity self‐limits the capacity at intermediate charge/discharge rates; as a consequence, exceptional cycleability is observed at 0.5 C, with 100% retention over 200 cycles with 700 mAh g−1. Higher capacity can be obtained when the first cycles are performed at 0.2 C, due to the formation of microislands, which facilitate the swelling of the active Si. This study indicates a method to tune the mechanical, morphological, and electrochemical properties of Si electrodes via engineering nanoparticle scaffolds, paving the way for a novel design of nanostructured Si electrodes for high‐performance energy storage devices.

Bimetallic nanoparticles for optimizing CMOS integrated SnO 2 gas sensor devices

We present gas sensor devices based on ultrathin SnO2 films, which are integrated on CMOS fabricated micro-hotplate (μhp) chips. Bimetallic nanoparticles (NPs) such as PdAu, PtAu, and PdPt have been synthesized for optimizing the sensing performance of these sensors. We demonstrate that functionalization of nanocrystalline SnO2 gas sensing films with PdAu-NPs leads to a strongly improved sensitivity to the toxic gas carbon monoxide (CO) while the cross sensitivity to humidity is almost completely suppressed. We conclude that specific functionalization of CMOS integrated SnO2 thin film gas sensors with different types of NPs is a powerful strategy towards sensor arrays capable for distinguishing several target gases. Such CMOS integrated arrays are highly promising candidates for realizing smart multi-parameter sensing devices for the consumer market.

Modeling and Analysis of Spray Pyrolysis Deposited SnO2 Films for Gas Sensors

Metal oxide materials such as tin oxide (SnO2) show powerful gas sensing capabilities. Recently, the deposition of a thin tin oxide film at the backend of a CMOS processing sequence has enabled the manufacture of modern gas sensors. Among several potential deposition methods for SnO2, spray pyrolysis deposition has proven itself to be relatively easy to use and cost effective while providing excellent surface coverage on step structures and etched holes. A model for spray pyrolysis deposition using a pressure atomizer is presented and implemented in a Level Set framework. A simulation of tin oxide deposition is performed on a typical gas sensor geometry and the resulting structure is imported into a finite element tool in order to analyze the electrical characteristics and thermo-mechanical stress present in the grown layer after processing. The deposition is performed at 400 °C and the subsequent cooling to room temperatures causes a stress to develop at the material interfaces due to variations in the coefficient of thermal expansion between the different materials.

Probing electron beam effects with chemoresistive nanosensors during in situ environmental transmission electron microscopy

We report in situ and ex situ fabrication approaches to construct p-type (CuO) and n-type (SnO2) metal oxide nanowire devices for operation inside an environmental transmission electron microscope (TEM). By taking advantage of their chemoresistive properties, the nanowire devices were employed as sensitive probes for detecting reactive species induced by the interactions of high-energy electrons with surrounding gas molecules, in particular, for the case of O2 gas pressures up to 20 mbar. In order to rationalize our experimental findings, a computational model based on the particle-in-cell method was implemented to calculate the spatial distributions of scattered electrons and ionized oxygen species in the environmental TEM. Our approach enables the a priori identification and qualitative measurement of undesirable beam effects, paving the way for future developments related to their mitigation.