arXiv subjects

Lorenz Kienle

Publications and source records attributed to Lorenz Kienle.

17 recordsLinked to original sources

arXiv2026

Laser-generated CuPdAgPtAu High-Entropy Alloy Nanoparticles -- Thermal Segregation Threshold and Elemental Segregation

High-entropy alloy nanoparticles synthesized via laser ablation in liquid are promising for catalysis due to their ability to form simple solid solutions despite chemical complexity. In this study, noble metal HEA NPs (CuPdAgPtAu) are produced from equimolar and Cu- or Ag-enriched bulk targets. Advanced electron microscopy, XRD, and atomistic simulations are used for structural and compositional analysis. Equimolar targets and NPs exhibit a single fcc phase. In contrast, Cu- or Ag-enriched targets show phase segregation into two fcc phases, which is not observed in the synthesized NPs. Simulations predict segregation tendencies, including Ag surface enrichment and Pt core enrichment due to surface energy differences. However, experimentally, individual NPs remain compositionally homogeneous. Thermal stability studies reveal that phase segregation can be induced post-synthesis. Upon heating, Cu-Ag segregation occurs, forming a second fcc phase similar to bulk targets. These findings demonstrate that rapid quenching during laser ablation suppresses thermodynamically driven segregation and stabilizes metastable solid solutions under kinetic control. Subsequent slow heating overcomes kinetic barriers, enabling equilibrium phase formation at higher temperatures. The thermal stability of these NPs and their tunable composition, including Cu enrichment beyond equilibrium limits, make them promising for high-temperature catalytic applications while reducing noble metal usage.

cond-mat.mtrl-sci↗

arXiv2025

Effect of Ti$_2$Pd(Ni) on the Transformation Behavior in Sputtered Ti-rich TiNiPd Shape Memory Alloys

TiNiPd based shape memory alloys (SMAs) share similar microstructural features as TiNiCu-based SMAs known for their exceptional resistance to functional fatigue due to their high crystallographic compatibility, nanometer sized grains and coherent precipitates, making them an ideal system to further explore the critical factors influencing cyclic stability. In this study, we investigate the effect of heat treatments (500 °C, 600 °C, 700 °C and 800 °C) on the cyclic stability and microstructure of free-standing, magnetron-sputtered Ti$_{53.6}$Ni$_{35.2}$Pd$_{11.2}$ films. All heat treatments promote the formation of Ti$_2$Pd(Ni) precipitates and result in a similar grain size (~1-4 $μ$m). Lower heat treatment temperatures improve the cyclic stability of the stress induced transformation while reducing transformation temperatures and latent heat. Temperature dependent X-ray diffraction reveals a complex microstructure for the martensite phase with Ti$_2$Pd(Ni), Ti$_2$Ni(Pd), TiNiPd(B2), B19/B19$'$ and R-phase. The thermal phase transition changes from a distinct 1st order to a 2nd order like transition, accompanied by increasing amount of remanent austenite and R-phase, with nearly no change for the sample heat treated at 500 °C. In situ stress dependent X-ray diffraction demonstrates a significant difference between the temperature and stress induced phase transformation for this heat treatment. The observed semi crystalline microstructure, featuring nano domains of Ti$_2$Pd(Ni) precipitates in the sample heat-treated at 500 °C, leads to a mixture of long range martensitic and strain glass transition. This study highlights the impact of heat treatment and microstructure on the phase transformation behavior and functional fatigue in Ti-rich TiNiPd alloys.

cond-mat.mtrl-sci↗

arXiv2025

Interfacial Polarization Switching in Al0.92Sc0.08N/GaN Heterostructures Grown by Sputter Epitaxy

The integration of ferroelectric nitride thin films such as Al1-xScxN onto GaN templates could enable enhanced functionality in novel high-power transistors and memory devices. This requires a detailed understanding of the ferroelectric domain structures and their impact on the electric properties. In this contribution, the sputter epitaxy of highly coherent Al0.92Sc0.08N thin films grown on GaN approaching lattice-matching conditions is demonstrated. Scanning transmission electron microscopy investigations reveal the formation of polar domains and the mechanism of domain propagation upon ferroelectric switching. Atomic resolution imaging suggests that polarization inversion is initiated by an interfacial switching process in which already the first atomic layer of Al1-xScxN changes its polarization from the as-grown M- to N-polarity. An atomically sharp planar polarization discontinuity is identified at the Al0.92Sc0.08N/GaN interface and described by atomic modeling and chemical structure analysis using electron energy loss spectroscopy, considering local lattice spacings. Moreover, residual domains with M-polarity are identified at the top Pt electrode interface. These insights on the location and the atomic structure of ferroelectric inversion domains in sputter deposited Al1-xScxN/GaN heterostructures will support the development of future non-volatile memory devices and novel HEMT structures based on ferroelectric nitride thin films via interface engineering.

cond-mat.mtrl-sci↗

arXiv2024

Improved Leakage Currents and Polarity Control through Oxygen Incorporation in Ferroelectric Al0.73Sc0.27N Thin Films

This article examines systematic oxygen (O)-incorporation to reduce total leakage currents in sputtered wurtzite-type ferroelectric Al0.73Sc0.27N thin films, along with its impact on the material structure and the polarity of the as-grown films. The O in the bulk Al0.73Sc0.27N was introduced through an external gas source during the reactive sputter process. In comparison to samples without doping, O-doped films showed almost a fourfold reduction of the leakage current near the coercive field. In addition, doping resulted in the reduction of the steady-state leakage currents by roughly one order of magnitude sub-coercive fields. Microstructure analysis using X-ray diffraction 1and scanning transmission electron microscopy (STEM) revealed no significant structural degradation of the bulk Al0.73Sc0.27N. In case of the maximum O-doped film, the c-axis out-of-plane texture increased by only 20% from 1.8° and chemical mapping revealed a uniform distribution of oxygen incorporation into the bulk. Our results further demonstrate the ability to control the as-deposited polarity of Al0.73Sc0.27N via the O-concentration, changing from nitrogen- to metal-polar orientation. Thus, this article presents a promising approach to mitigate the leakage current in wurtzite-type Al0.73Sc0.27N without incurring any significant structural degradation of the bulk thin film quality, thereby making ferroelectric nitrides more suitable for microelectronic applications.

physics.app-ph↗

arXiv2023

Demonstration and STEM Analysis of Ferroelectric Switching in MOCVD-Grown Single Crystalline Al$_{0.85}$Sc$_{0.15}$N

Wurtzite-type Al$_{1-x}$Sc$_x$N solid solutions grown by metal organic chemical vapour deposition are for the first time confirmed to be ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive field of 5.5 MV/cm at a measurement frequency of 1.5 kHz. Single crystal quality and homogeneous chemical composition of the film was confirmed by X-ray diffraction spectroscopic methods such as time of flight secondary ion mass spectrometry. Annular bright field scanning transmission electron microscopy served to proof the ferroelectric polarization inversion on unit cell level. The single crystal quality further allowed to image the large-scale domain pattern of a wurtzite-type ferroelectric for the first time, revealing a predominantly cone-like domain shape along the c-axis of the material. As in previous work, this again implies the presence of strong polarization discontinuities along this crystallographic axis, which could be suitable for current transport. The domains are separated by narrow domain walls, for which an upper thickness limit of 3 nm was deduced, but which could potentially be atomically sharp. We are confident that these results will advance the commencing integration of wurtzite-type ferroelectrics to GaN as well as generally III-N based heterostructures and devices.

physics.app-ph↗

arXiv2023

Decorative coating or corrosion product? TEM and SEM Study on a Late Neolithic Axe to find origins of silver metal on the surface

Here we report on the analysis of a metallic Late Neolithic copper axe by means of Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The axe was found at Eskilstorp, south-west Scania during archaeological excavations in autumn of 2015. It showed a hint of silver en-richment on the surface which is unusual for Late Neolithic axes from Pile type hoard. To identify the origin of the silver content, we extracted a thin lamella of the axe interior using Focused Ion Beam (FIB) technology to reach the internal structure to keep the process extremely minimally invasive. The results revealed the presence of porous external layer of copper oxide enriched with silver particles. It is shown with the corrosion test performed on copper-silver experimental replica that the silver en-richment is attributed to the selective dissolution of copper metal as a resut of corrosion process on the axe. The corrosion experiment is performed in the presence of an organic electrolyte such as Hemic Acid (HA) which replicates soil and natural water environment. In the scope of this research work, we found a strong evidence that the silver on the surface of the axe was not decrative layer and instead remained on the surface due to its more noble character while copper leached out into the soil due to corrosion.

physics.chem-ph↗

arXiv2023

Functional light diffusers based on hybrid CsPbBr$_3$/SiO$_2$ aero-framework structures for laser light illumination and conversion

The new generation of laser-based solid-state lighting (SSL) white light sources requires new material systems capable of withstanding, diffusing and converting high intensity laser light. State-of-the-art systems use a blue light emitting diode (LED) or laser diode (LD) in combination with color conversion materials, such as yellow emitting Ce-doped phosphors or red and green emitting quantum dots (QD), to produce white light. However, for laser-based high-brightness illumination in particular, thermal management is a major challenge, and in addition, a light diffuser is required to diffuse the highly focused laser beam. Here, we present a hybrid material system that simultaneously enables efficient, uniform light distribution and color conversion of a blue LD, while ensuring good thermal management even at high laser powers of up to 5W. A highly open porous (> 99%) framework structure of hollow SiO$_2$ microtubes is utilized as an efficient light diffuser that can drastically reduce speckle contrast. By further functionalizing the microtubes with halide perovskite QDs (SiO$_2$@CsPbBr$_3$ as model system) color conversion from UV to visible light is achieved. Under laser illumination, the open porous structure prevents heat accumulation and thermal quenching of the QDs. By depositing an ultrathin (~ 5.5 nm) film of poly(ethylene glycol dimethyl acrylate) (pEGDMA) via initiated chemical vapor deposition (iCVD), the luminescent stability of the QDs against moisture is enhanced. The demonstrated hybrid material system paves the way for the design of advanced and functional laser light diffusers and converters that can meet the challenges associated with laser-based SSL applications.

physics.optics↗

arXiv2023

Hybrid aeromaterials for enhanced and rapid volumetric photothermal response

Conversion of light into heat is essential for a broad range of technologies such as solar thermal heating, catalysis and desalination. Three-dimensional (3D) carbon nanomaterial-based aerogels have shown to hold great promise as photothermal transducer materials. However, till now, their light-to-heat conversion is limited by surface-near absorption, resulting in a strong heat localization only at the illuminated surface region, while most of the aerogel volume remains unused. We present an innovative fabrication concept for highly porous (>99.9%) photothermal hybrid aeromaterials, that enable an ultra-rapid and volumetric photothermal response with an enhancement by a factor of around 2.5 compared to the pristine variant. The hybrid aeromaterial is based on strongly light-scattering framework structures composed of interconnected hollow silicon dioxide (SiO${_2}$) microtubes, which are functionalized with extremely low amounts (in order of a few $μ$g cm${^-}$${^3}$) of reduced graphene oxide (rGO) nanosheets, acting as photothermal agents. Tailoring the density of rGO within the framework structure enables us to control both, light scattering and light absorption, and thus the volumetric photothermal response. We further show that by rapid and repeatable gas activation these transducer materials expand the field of photothermal applications, like untethered light-powered and -controlled microfluidic pumps and soft pneumatic actuators.

physics.app-ph↗

arXiv2023

In-Grain Ferroelectric Switching in Sub-5 nm Thin AlScN Films at 1 V

Analog switching in ferroelectric devices promises neuromorphic computing with highest energy efficiency, if limited device scalability can be overcome. To contribute to a solution, we report on the ferroelectric switching characteristics of sub-5 nm thin Al$_{0.74}$Sc$_{0.26}$N films grown on Pt/Ti/SiO2/Si and epitaxial Pt/GaN/sapphire templates by sputter-deposition. In this context, we focus on the following major achievements compared to previously available wurtzite-type ferroelectrics: 1) Record low switching voltages down to 1 V are achieved, which is in a range that can be supplied by standard on-chip voltage sources. 2) Compared to the previously investigated deposition of thinnest Al$_{1-x}$Sc$_x$N films on epitaxial templates, a significantly larger coercive field to breakdown field ratio is observed for Al$_{0.74}$Sc$_{0.26}$N films grown on silicon substrates, the technologically most relevant substrate-type. 3) The formation of true ferroelectric domains in wurtzite-type materials is for the first time demonstrated on the atomic scale by scanning transmission electron microscopy investigations of a sub-5 nm thin partially switched film. The direct observation of inversion domain boundaries within single nm-sized grains supports the theory of a gradual domain-wall motion limited switching process in wurtzite-type ferroelectrics. Ultimately, this should enable the analog switching necessary for mimicking neuromorphic concepts also in highly scaled devices.

cond-mat.mtrl-sci↗

arXiv2022

Strain-invariant, highly water stable all-organic soft conductors based on ultralight multi-layered foam-like framework structures

Soft and flexible conductors are essential in the development of soft robots, wearable electronics, as well as electronic tissue and implants. However, conventional soft conductors are inherently characterized by a large change in conductance upon mechanical deformation or under alternating environmental conditions, e.g., humidity, drastically limiting their application potential and performance. Here, we demonstrate a novel concept for the development of strain-invariant, fatigue resistant and highly water stable soft conductor. By combining different thin film technologies in a three-dimensional fashion, we develop nano- and micro-engineered, multi-layered (< 50 nm), ultra-lightweight (< 15 mg/cm$^3$) foam-like composite framework structures based on PEDOT:PSS and PTFE. The all-organic composite framework structures are characterized by conductivities of up to 184 S/m, remaining strain-invariant between 80 % compressive and 25 % tensile strain. We further show, that the multi-layered composites are characterized by properties that surpass that of framework structures based on the individual materials. Both, the initial electrical and mechanical properties of the composite framework structures are retained during long-term cycling, even after 2000 cycles at 50 % compression. Furthermore, the PTFE functionalization renders the framework structure highly hydrophobic, resulting in stable electrical properties, even when immersed in water for up to 30 days. The here presented concept overcomes the previous limitations of strain-invariant soft conductors and demonstrates for the first time a versatile approach for the development of innovative multi-scaled and multi-layered functional materials, for applications in soft electronics, energy storage and conversion, sensing, catalysis, water and air purification, as well as biomedicine.

physics.app-ph↗

arXiv2022

Automated Classification of Nanoparticles with Various Ultrastructures and Sizes

Accurately measuring the size, morphology, and structure of nanoparticles is very important, because they are strongly dependent on their properties for many applications. In this paper, we present a deep-learning based method for nanoparticle measurement and classification trained from a small data set of scanning transmission electron microscopy images. Our approach is comprised of two stages: localization, i.e., detection of nanoparticles, and classification, i.e., categorization of their ultrastructure. For each stage, we optimize the segmentation and classification by analysis of the different state-of-the-art neural networks. We show how the generation of synthetic images, either using image processing or using various image generation neural networks, can be used to improve the results in both stages. Finally, the application of the algorithm to bimetallic nanoparticles demonstrates the automated data collection of size distributions including classification of complex ultrastructures. The developed method can be easily transferred to other material systems and nanoparticle structures.

cond-mat.mtrl-sci↗

arXiv2022

Control of magnetoelastic coupling in Ni/Fe multilayers using He$^+$ ion irradiation

This study reports the effects of post-growth He$^+$ irradiation on the magneto-elastic properties of a $Ni$ /$Fe$ multi-layered stack. The progressive intermixing caused by He$^+$ irradiation at the interfaces of the multilayer allows us to tune the saturation magnetostriction value with increasing He$^+$ fluences, and even to induce a reversal of the sign of the magnetostrictive effect. Additionally, the critical fluence at which the absolute value of the magnetostriction is dramatically reduced is identified. Therefore insensitivity to strain of the magnetic stack is nearly reached, as required for many applications. All the above mentioned effects are attributed to the combination of the negative saturation magnetostriction of sputtered Ni, Fe layers and the positive magnetostriction of the Ni$_{x}$Fe$_{1-x}$ alloy at the intermixed interfaces, whose contribution is gradually increased with irradiation. Importantly the irradiation does not alter the layers polycrystalline structure, confirming that post-growth He$^+$ ion irradiation is an excellent tool to tune the magneto-elastic properties of magnetic samples. A new class of spintronic devices can be envisioned with a material treatment able to arbitrarily change the magnetostriction with ion-induced "magnetic patterning".

cond-mat.mtrl-sci↗

arXiv2022

Ultrathin AlScN for low-voltage driven ferroelectric-based devices

Thickness scaling of ferroelectricity in AlScN is a determining factor for its potential application in neuromorphic computing and memory devices. In this letter, we report on ultrathin (10 nm) Al0.72Sc0.28N films that are ferroelectrically switchable at room temperature. All-epitaxial Al0.72Sc0.28N/Pt heterostructures are grown by magnetron sputtering onto GaN/sapphire substrates followed by an in situ Pt capping approach to avoid oxidation of the Al0.72Sc0.28N film surface. Structural characterization by X-ray diffraction and transmission electron microscopy reveals the established epitaxy. The thus obtained high-quality interfaces in combination with the in situ capping is expected to facilitate ferroelectric switching of AlScN in the ultrathin regime. The analysis of the relative permittivity and coercive field dependence on the Al0.72Sc0.28N film thicknesses in the range of 100 nm down to 10 nm indicates only moderate scaling effects, suggesting that the critical thickness for ferroelectricity is not yet approached. Furthermore, the deposited layer stack demonstrates the possibility of including ultrathin ferroelectric AlScN into all-epitaxial GaN-based devices using sputter deposition techniques. Thus, our work highlights the integration and scaling potential of all-epitaxial ultrathin AlScN offering high storage density paired with low voltage operation desired for state of the art ferroelectric memory devices.

physics.app-ph↗

arXiv2021

On the exceptional temperature stability of ferroelectric AlScN thin films

Through its dependence on low symmetry crystal phases, ferroelectricity is inherently a property tied to the lower temperature ranges of the phase diagram for a given material. This paper presents conclusive evidence that in the case of ferroelectric AlScN, low temperature has to be seen as a purely relative term, since its ferroelectric-to-paraelectric transition temperature is confirmed to surpass 1100°C and thus the transition temperature of virtually any other thin film ferroelectric. We arrived at this conclusion through investigating the structural stability of 0.4 - 2 $μ$m thick Al$_{0.73}$Sc$_{0.27}$N films grown on Mo bottom electrodes via in situ high-temperature X-ray diffraction and permittivity measurements. Our studies reveal the wurtzite-type structure of Al$_{0.73}$Sc$_{0.27}$N is conserved during the entire 1100°C annealing cycle, apparent through a constant c over a lattice parameter ratio. In situ permittivity measurements performed up to 1000°C strongly support this conclusion and include what could be the onset of a diverging permittivity only at the very upper end of the measurement interval. Our in situ measurements are well-supported by ex situ (scanning) transmission electron microscopy and polarization and capacity hysteresis measurements. These results confirm the structural stability on the sub-$μ$m scale next to the stability of the inscribed polarization during the complete 1100°C annealing treatment. Thus, AlScN is the first readily available thin film ferroelectric with a temperature stability that surpasses virtually all thermal budgets occurring in microtechnology, be it during fabrication or the lifetime of a device - even in harshest environments.

cond-mat.mtrl-sci↗

arXiv2021

Multidimensional thermally-induced transformation of nest-structured complex Au-Fe nanoalloys towards equilibrium

Bimetallic nanoparticles are often superior candidates for a wide range of technological and biomedical applications, thanks to their enhanced catalytic, optical, and magnetic properties, which are often better than their monometallic counterparts. Most of their properties strongly depend on their chemical composition, crystallographic structure, and phase distribution. However, little is known of how their crystal structure, on the nanoscale, transforms over time at elevated temperatures, even though this knowledge is highly relevant in case nanoparticles are used in, e.g., high-temperature catalysis. Au-Fe is a promising bimetallic system where the low-cost and magnetic Fe is combined with catalytically active and plasmonic Au. Here, we report on the in situ temporal evolution of the crystalline ordering in Au-Fe nanoparticles, obtained from a modern laser ablation in liquids synthesis. Our in-depth analysis, complemented by dedicated atomistic simulations, includes a detailed structural characterization by X-ray diffraction and transmission electron microscopy as well as atom probe tomography to reveal elemental distributions down to a single atom resolution. We show that the Au-Fe nanoparticles initially exhibit highly complex internal nested nanostructures with a wide range of compositions, phase distributions, and size-depended microstrains. The elevated temperature induces a diffusion-controlled recrystallization and phase merging, resulting in the formation of a single face-centered-cubic ultrastructure in contact with a body-centered cubic phase, which demonstrates the metastability of these structures. Uncovering these unique nanostructures with nested features could be highly attractive from a fundamental viewpoint as they could give further insights into the nanoparticle formation mechanism under non-equilibrium conditions.

cond-mat.mtrl-sci↗

arXiv2018

AlScN: A III-V semiconductor based ferroelectric

Ferroelectric switching is unambigiously demonstrated for the first time in a III-V semiconductor based material: AlScN -- A discovery which could help to satisfy the urgent demand for thin film ferroelectrics with high performance and good technological compatibility with generic semiconductor technology which arises from a multitude of memory, micro/nano-actuator and emerging applications based on controlling electrical polarization. The appearance of ferroelectricity in AlScN can be related to the continuous distortion of the original wurtzite-type crystal structure towards a layered-hexagonal structure with increasing Sc content and tensile strain, which is expected to be extendable to other III-nitride based solid solutions. Coercive fields which are systematically adjustable by more than 3 MV/cm, high remnant polarizations in excess of 100 μC/cm$^2$ which constitute the first experimental estimate of the previously inaccessible spontaneous polarization in a III-nitride based material, an almost ideally square-like hysteresis resulting in excellent piezoelectric linearity over a wide strain interval from -0.3% to +0.4% as well as a paraelectric transition temperature in excess of 600°C are confirmed. This intriguing combination of properties is to our knowledge as of now unprecedented in the field of polycrystalline ferroelectric thin films and promises to significantly advance the commencing integration of ferroelectric functionality to micro- and nanotechnology, while at the same time providing substantial insight to one of the central open questions of the III-nitride semiconductors - that of their actual spontaneous polarization.

cond-mat.mtrl-sci↗

arXiv2017

In depth nano spectroscopic analysis on homogeneously switching double barrier memristive devices

Memristors based on a double barrier design have been analysed by various nano spectroscopic methods to unveil details about its microstructure and conduction mechanism. The device consists of an AlOx tunnel barrier and a NbOy/Au Schottky barrier sandwiched between Nb bottom electrode and Au top electrode. As it was anticipated that the local chemical composition of the tunnel barrier, i.e. oxidation state of the metals as well as concentration and distribution of oxygen ions, have a major influence on electronic conduction, these factors were carefully analysed. A combined approach was chosen in order to reliably investigate electronic states of Nb and O by electron energy-loss spectroscopy as well as map elements whose transition edges exhibit a different energy range by energy-dispersive X-ray spectroscopy like Au and Al. The results conclusively demonstrate significant oxidation of the bottom electrode as well as a small oxygen vacancy concentration in the Al oxide tunnel barrier. Possible scenarios to explain this unexpected additional oxide layer are discussed and kinetic Monte Carlo simulations were applied in order to identify its influence on conduction mechanisms in the device. In light of the strong deviations between observed and originally sought layout, this study highlights the robustness in terms of structural deviations of the double barrier memristor device.

cond-mat.mtrl-sci↗