Gunther Richter

Ultrahigh Strength Single Crystalline Nanowhiskers Grown by Physical Vapor Deposition

The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focused-ion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.

Reversible cyclic deformation mechanism of gold nanowires by twinning-detwinning transition evidenced from in situ TEM

Mechanical response of metal nanowires has recently attracted a lot of interest due to their ultra-high strengths and unique deformation behaviours. Atomistic simulations have predicted that face-centered cubic metal nanowires deform in different modes depending on the orientation between wire axis and loading direction. Here we report, by combination of in situ transmission electron microscopy and molecular dynamic simulation, the conditions under which particular deformation mechanisms take place during the uniaxial loading of [110]-oriented Au nanowires. Furthermore, by performing cyclic uniaxial loading, we show reversible plastic deformation by twinning and consecutive detwinning in tension and compression, respectively. Molecular dynamics simulations rationalize the observed behaviours in terms of the orientation-dependent resolved shear stress on the leading and trailing partial dislocations, their potential nucleation sites and energy barriers. This reversible twinning-detwinning process accommodates large strains that can be beneficially utilized in applications requiring high ductility in addition to ultra-high strength.

Measuring surface dislocation nucleation in defect-scarce nanostructures

Linear defects in crystalline materials, known as dislocations, are central to the understanding of plastic deformation and mechanical strength, as well as control of performance in a variety of electronic and photonic materials. Despite nearly a century of research on dislocation structure and interactions, measurements of the energetics and kinetics of dislocation nucleation have not been possible, as synthesizing and testing pristine crystals absent of defects has been prohibitively challenging. Here, we report experiments that directly measure the surface dislocation nucleation strengths in high-quality 〈110〉 Pd nanowhiskers subjected to uniaxial tension. We find that, whereas nucleation strengths are weakly size- and strain-rate-dependent, a strong temperature dependence is uncovered, corroborating predictions that nucleation is assisted by thermal fluctuations. We measure atomic-scale activation volumes, which explain both the ultrahigh athermal strength as well as the temperature-dependent scatter, evident in our experiments and well captured by a thermal activation model.

Epitaxy of Pd thin films on (100) SrTiO3: A three-step growth process

Control of the orientation of thin Pd films on (100) SrTiO3 surfaces was obtained by changing the growth temperature. In particular, a three-step growth method was applied to deposit thin single-crystal Pd films on single-crystal (100) SrTiO3 surfaces. This was realized by first growing epitaxial Pd seeds at elevated temperatures. Subsequently, the seeds were overgrown at room temperature by polycrystalline Pd which fully covered the substrate at a low thickness. Annealing of these films promoted growth of the epitaxial seeds, resulting in single-crystal Pd films: (100) SrTiO3∥(100) Pd, [010] SrTiO3∥[010] Pd. The three-step growth method turned out to be a useful method to overcome surface roughening and the creation of crystalline imperfections in thin Pd films. This is essential for the growth of thin metallic epitaxial buffer layers. The microstructure of the films was analyzed by reflection high-energy electron diffraction, scanning electron microscopy, x-ray diffraction, and transmission electron mic...

Recoverable plasticity in penta-twinned metallic nanowires governed by dislocation nucleation and retraction

There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside scanning and transmission electron microscopes show that penta-twinned silver nanowires undergo stress relaxation on loading and complete plastic strain recovery on unloading, while the same experiments on single-crystalline silver nanowires do not exhibit such a behaviour. Molecular dynamics simulations reveal that the observed behaviour in penta-twinned nanowires originates from the surface nucleation, propagation and retraction of partial dislocations. More specifically, vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery.

Amorphous versus crystalline state for ultrathin Al2O3 overgrowths on Al substrates

The thermodynamic and kinetic background of the stability of ultrathin (<3nm) amorphous Al2O3 overgrowths on Al{111}, Al{100}, and Al{110} substrates was investigated by thermal oxidation of the bare substrates in pure oxygen gas for oxidation times up to 6000s in the temperature range of T=350–650K. The microstructural evolutions of the developing oxide films were analyzed by angle-resolved x-ray photoelectron spectroscopy, low energy electron diffraction, and high-resolution transmission electron microscopy. For sufficiently small thicknesses, stable amorphous Al2O3 films form on all substrates. The critical thickness values beyond which a crystalline state for the Al2O3 film is thermodynamically preferred can be reliably calculated provided that a layer-by-layer mode of oxide-film growth occurs. With increasing temperature, a transition from a layer by layer to an island-by-layer type of oxide growth mode occurs and, consequently (tensile), growth strain in a crystalline Al2O3 overgrowth can be more re...

Insight into the atomic-scale mechanism of liquid metal embrittlement

A method has been developed to investigate material embrittlement on the atomistic level. By ion implantation the embrittling agent is introduced into a small volume enclosing a segment of a grain boundary of a thin specimen prepared for transmission electron microscopy (TEM). After allowing the implanted ions to segregate at the grain boundary, the position of the ions is determined by atomic-resolution TEM. In the present study this was applied to the Al(Ga) system. Close correspondence with previous ab initio calculations is found and a model for the initial stages of embrittlement is proposed.

Large-area fabrication of TiN nanoantenna arrays for refractory plasmonics in the mid-infrared by femtosecond direct laser writing and interference lithography [Invited]

Robust plasmonic nanoantennas at mid-infrared wavelengths are essential components for a variety of nanophotonic applications ranging from thermography to energy conversion. Titanium nitride (TiN) is a promising candidate for such cases due to its high thermal stability and metallic character. Here, we employ direct laser writing as well as interference lithography to fabricate large-area nanoantenna arrays of TiN on sapphire and silicon substrates. Our lithographic tools allow for fast and homogeneous preparation of nanoantenna geometries on a polymer layer, which is then selectively transferred to TiN by subsequent argon ion beam etching followed by a chemical wet etching process. The antennas are protected by an additional Al2O3 layer which allows for high-temperature annealing in argon flow without loss of the plasmonic properties. Tailoring of the TiN antenna geometry enables precise tuning of the plasmon resonances from the near to the mid-infrared spectral range. Due to the advantageous properties of TiN combined with our versatile large-area and low-cost fabrication process, such refractory nanoantennas will enable a multitude of high-temperature plasmonic applications such as thermophotovoltaics in the future.

Low-temperature growth of silicon nanotubes and nanowires on amorphous substrates

Silicon one-dimensional (Si 1D) materials are of particular relevance due to their prospect as versatile building materials for nanoelectronic devices. We report the growth of Si 1D structures from quasi-hexagonally ordered gold (Au) nanoparticle (NP) arrays on borosilicate glass (BSG) and SiOx/Si substrates. Using hydrogen instead of oxygen plasma during NP preparation enhances the catalytic activity of AuNPs (diameters of 10-20 nm), enabling Si 1D growth at temperatures as low as 320 degrees C. On BSG, Si nanowires (SiNWs) are identified and reasonable vertical alignment is achieved at 420 degrees C. On SiOx/Si, only Si nanotubes (SiNTs) are obtained right up to 420 degrees C. A mixture of SiNTs and SiNWs is observed at 450 degrees C and only SiNWs grow at 480 degrees C.

Surface dislocation nucleation controlled deformation of Au nanowires

We investigate deformation in high quality Au nanowires under both tension and bending using in-situ transmission electron microscopy. Defect evolution is investigated during: (1) tensile deformation of 〈110〉 oriented, initially defect-free, single crystal nanowires with cross-sectional widths between 30 and 300 nm, (2) bending deformation of the same wires, and (3) tensile deformation of wires containing coherent twin boundaries along their lengths. We observe the formation of twins and stacking faults in the single crystal wires under tension, and storage of full dislocations after bending of single crystal wires and after tension of twinned wires. The stress state dependence of the deformation morphology and the formation of stacking faults and twins are not features of bulk Au, where deformation is controlled by dislocation interactions. Instead, we attribute the deformation morphologies to the surface nucleation of either leading or trailing partial dislocations, depending on the Schmid factors, whic...