Atomic scale volume and grain boundary diffusion elucidated by in situ STEM.

Diffusion is one of the most important phenomena studied in science ranging from physics to biology and, in abstract form, even in social sciences. In the field of materials science, diffusion in crystalline solids is of particular interest as it plays a pivotal role in materials synthesis, processing and applications. While this subject has been studied extensively for a long time there are still some fundamental knowledge gaps to be filled. In particular, atomic scale observations of thermally stimulated volume diffusion and its mechanisms are still lacking. In addition, the mechanisms and kinetics of diffusion along defects such as grain boundaries are not yet fully understood. In this work we show volume diffusion processes of tungsten atoms in a metal matrix on the atomic scale. Using in situ high resolution scanning transmission electron microscopy we are able to follow the random movement of single atoms within a lattice at elevated temperatures. The direct observation allows us to confirm random walk processes, quantify diffusion kinetics and distinctly separate diffusion in the volume from diffusion along defects. This work solidifies and refines our knowledge of the broadly essential mechanism of volume diffusion.

From metal nanowires to ultrathin crystalline ALD nanotubes: process development and mechanism revealed by in situ TEM heating experiments.

The creation of hollow nanomaterials based on metal oxides has become an important research topic, as they show potential in a broad range of technical applications. However, the controlled synthesis of long and at the same time thin nanotubes is still challenging. Here we present a universal approach to create ultrathin aluminum oxide nanotubes with a length/diameter ratio of >1200 and minimum wall thickness of ≤4 nm. We use a facile process based on defined heat treatment of specific core-shell nanowires. The metal nanowires act as a template, which is thermally removed during heat treatment until an empty tube is created. The core-shell nanowires are produced by Physical Vapour Deposition (PVD) with a subsequent coating via Atomic Layer Deposition (ALD). The custom-built PVD-ALD system enables a direct sample transfer without breaking the vacuum, which allows determining the effect of a native oxide layer on the metal-ALD bonding. In combination with correlative ex situ observations, in situ Transmission Electron Microscopy (TEM) heating experiments unravel the dynamical processes going on at small scales. Based on the microscopic analysis, the energetics of the core material is analyzed, giving insights about heat induced effects as well as the phase transition from the amorphous to the crystalline state.

Effects of biomechanical and biochemical stimuli on angio- and vasculogenesis in a complex microvasculature-on-chip.

The endothelium of blood vessels is a vital organ that reacts differently to subtle changes in stiffness and mechanical forces exerted on its environment (extracellular matrix (ECM)). Upon alteration of these biomechanical cues, endothelial cells initiate signaling pathways that govern vascular remodeling. The emerging organs-on-chip technologies allow the mimicking of complex microvasculature networks, identifying the combined or singular effects of these biomechanical or biochemical stimuli. Here, we present a microvasculature-on-chip model to investigate the singular effect of ECM stiffness and mechanical cyclic stretch on vascular development. Following two different approaches for vascular growth, the effect of ECM stiffness on sprouting angiogenesis and the effect of cyclic stretch on endothelial vasculogenesis are studied. Our results indicate that ECM hydrogel stiffness controls the size of the patterned vasculature and the density of sprouting angiogenesis. RNA sequencing shows that the cellular response to stretching is characterized by the upregulation of certain genes such as ANGPTL4+5, PDE1A, and PLEC.

Room Temperature Viscous Flow of Amorphous Silica Induced by Electron Beam Irradiation.

The increasing use of oxide glasses in high-tech applications illustrates the demand of novel engineering techniques on nano- and microscale. Due to the high viscosity of oxide glasses at room temperature, shaping operations are usually performed at temperatures close or beyond the point of glass transition Tg . Those treatments, however, are global and affect the whole component. It is known from the literature that electron irradiation facilitates the viscous flow of amorphous silica near room temperature for nanoscale components. At the micrometer scale, however, a comprehensive study on this topic is still pending. In the present study, electron irradiation inducing viscous flow at room temperature is observed using a micropillar compression approach and amorphous silica as a model system. A comparison to high temperature yielding up to a temperature of 1100 °C demonstrates that even moderate electron irradiation resembles the mechanical response of 600 °C and beyond. As an extreme case, a yield strength as low as 300 MPa is observed with a viscosity indicating that Tg has been passed. Those results show that electron irradiation-facilitated viscous flow is not limited to the nanoscale which offers great potential for local microengineering.

Microscale 3D Printing and Tuning of Cellulose Nanocrystals Reinforced Polymer Nanocomposites.

The increasing demand for functional materials and an efficient use of sustainable resources makes the search for new material systems an ever growing endeavor. With this respect, architected (meta-)materials attract considerable interest. Their fabrication at the micro- and nanoscale, however, remains a challenge, especially for composites with highly different phases and unmodified reinforcement fillers. This study demonstrates that it is possible to create a non-cytotoxic nanocomposite ink reinforced by a sustainable phase, cellulose nanocrystals (CNCs), to print and tune complex 3D architectures using two-photon polymerization, thus, advancing the state of knowledge toward the microscale. Micro-compression, high-res scanning electron microscopy, (polarised) Raman spectroscopy, and composite modeling are used to study the structure-property relationships. A 100% stiffness increase is observed already at 4.5 wt% CNC while reaching a high photo-polymerization degree of ≈80% for both neat polymers and CNC-composites. Polarized Raman and the Halpin-Tsai composite-model suggest a random CNC orientation within the polymer matrix. The microscale approach can be used to tune arbitrary small scale CNC-reinforced polymer-composites with comparable feature sizes. The new insights pave the way for future applications where the 3D printing of small structures is essential to improve performances of tissue-scaffolds, extend bio-electronics applications or tailor microscale energy-absorption devices.

Machine Learning-Based Characterization of the Nanostructure in a Combinatorial Co-Cr-Fe-Ni Compositionally Complex Alloy Film.

A novel artificial intelligence-assisted evaluation of the X-ray diffraction (XRD) peak profiles was elaborated for the characterization of the nanocrystallite microstructure in a combinatorial Co-Cr-Fe-Ni compositionally complex alloy (CCA) film. The layer was produced by a multiple beam sputtering physical vapor deposition (PVD) technique on a Si single crystal substrate with the diameter of about 10 cm. This new processing technique is able to produce combinatorial CCA films where the elemental concentrations vary in a wide range on the disk surface. The most important benefit of the combinatorial sample is that it can be used for the study of the correlation between the chemical composition and the microstructure on a single specimen. The microstructure can be characterized quickly in many points on the disk surface using synchrotron XRD. However, the evaluation of the diffraction patterns for the crystallite size and the density of lattice defects (e.g., dislocations and twin faults) using X-ray line profile analysis (XLPA) is not possible in a reasonable amount of time due to the large number (hundreds) of XRD patterns. In the present study, a machine learning-based X-ray line profile analysis (ML-XLPA) was developed and tested on the combinatorial Co-Cr-Fe-Ni film. The new method is able to produce maps of the characteristic parameters of the nanostructure (crystallite size, defect densities) on the disk surface very quickly. Since the novel technique was developed and tested only for face-centered cubic (FCC) structures, additional work is required for the extension of its applicability to other materials. Nevertheless, to the knowledge of the authors, this is the first ML-XLPA evaluation method in the literature, which can pave the way for further development of this methodology.

Thermally Stable Nanotwins: New Heights for Cu Mechanics.

Nanocrystalline and nanotwinned materials achieve exceptional strengths through small grain sizes. Due to large areas of crystal interfaces, they are highly susceptible to grain growth and creep deformation, even at ambient temperatures. Here, ultrahigh strength nanotwinned copper microstructures have been stabilized against high temperature exposure while largely retaining electrical conductivity. By incorporating less than 1 vol% insoluble tungsten nanoparticles by a novel hybrid deposition method, both the ease of formation and the high temperature stability of nanotwins are dramatically enhanced up to at least 400 °C. By avoiding grain coarsening, improved high temperature creep properties arise as the coherent twin boundaries are poor diffusion paths, while some size-based nanotwin strengthening is retained. Such microstructures hold promise for more robust microchip interconnects and stronger electric motor components.

Combinatorial Study of Phase Composition, Microstructure and Mechanical Behavior of Co-Cr-Fe-Ni Nanocrystalline Film Processed by Multiple-Beam-Sputtering Physical Vapor Deposition.

A combinatorial Co-Cr-Fe-Ni compositional complex alloy (CCA) thin film disk with a thickness of 1 µm and a diameter of 10 cm was processed by multiple-beam-sputtering physical vapor deposition (PVD) using four pure metal sources. The chemical composition of the four constituent elements varied between 4 and 64 at.% in the film, depending on the distance from the four PVD sources. The crystal structure, the crystallite size, the density of lattice defects (e.g., dislocations and twin faults) and the crystallographic texture were studied as a function of the chemical composition. It was found that in a wide range of elemental concentrations a face-centered cubic (fcc) structure with {111} crystallographic texture formed during PVD. Considering the equilibrium phase diagrams, it can be concluded that mostly the phase composition of the PVD layer is far from the equilibrium. Body-centered cubic (bcc) and hexagonal-close packed (hcp) structures formed only in the parts of the film close to Co-Fe and Co-Cr sources, respectively. A nanocrystalline microstructure with the grain size of 10-20 nm was developed in the whole layer, irrespective of the chemical composition. Transmission electron microscopy indicated a columnar growth of the film during PVD. The density of as-grown dislocations and twin faults was very high, as obtained by synchrotron X-ray diffraction peak profile analysis. The nanohardness and the elastic modulus were determined by indentation for the different chemical compositions on the combinatorial PVD film. This study is the continuation of a former research published recently in Nagy et al., Materials 14 (2021) 3357. In the previous work, only the fcc part of the sample was investigated. In the present paper, the study was extended to the bcc, hcp and multiphase regions.

Microstructure, Hardness, and Elastic Modulus of a Multibeam-Sputtered Nanocrystalline Co-Cr-Fe-Ni Compositional Complex Alloy Film.

A nanocrystalline Co-Cr-Ni-Fe compositional complex alloy (CCA) film with a thickness of about 1 micron was produced by a multiple-beam-sputtering physical vapor deposition (PVD) technique. The main advantage of this novel method is that it does not require alloy targets, but rather uses commercially pure metal sources. Another benefit of the application of this technique is that it produces compositional gradient samples on a disk surface with a wide range of elemental concentrations, enabling combinatorial analysis of CCA films. In this study, the variation of the phase composition, the microstructure (crystallite size and defect density), and the mechanical performance (hardness and elastic modulus) as a function of the chemical composition was studied in a combinatorial Co-Cr-Ni-Fe thin film sample that was produced on a surface of a disk with a diameter of about 10 cm. The spatial variation of the crystallite size and the density of lattice defects (e.g., dislocations and twin faults) were investigated by X-ray diffraction line profile analysis performed on the patterns taken by synchrotron radiation. The hardness and the elastic modulus were measured by the nanoindentation technique. It was found that a single-phase face-centered cubic (fcc) structure was formed for a wide range of chemical compositions. The microstructure was nanocrystalline with a crystallite size of 10-27 nm and contained a high lattice defect density. The hardness and the elastic modulus values measured for very different compositions were in the ranges of 8.4-11.8 and 182-239 GPa, respectively.

Mechanisms of Fluorine-Induced Separation of Mass Interference during TOF-SIMS Analysis.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of very few analytical techniques allowing sample chemical structure to be characterized in three-dimensional (3D) with nanometer resolution. Due to the excellent sensitivity in the order of ppm-ppb and capability of detecting all ionized elements and molecules, TOF-SIMS finds many applications for analyzing nanoparticle-containing systems and thin films used in microdevices for new energy applications, microelectronics, and biomedicine. However, one of the main drawbacks of this technique is potential mass interference between ions having the same or similar masses, which can lead to data misinterpretation. In this work, we present that this problem can be easily solved by delivering fluorine gas to a sample surface during TOF-SIMS analysis and we propose mechanisms driving this phenomenon. Our comprehensive studies, conducted on complex thin films made of highly mass-interfering elements, show that fluorine modifies the ionization process, leading to element-specific changes of ion yields (which can vary by several orders of magnitude), and affects the efficiency of metal hydride and oxide formation. In conjunction, these two effects can efficiently induce separation of mass interference, providing more representative TOF-SIMS data with respect to the sample composition and significant enhancement of chemical image resolution. Consequently, this can improve the chemical characterization of complex multilayers in nanoscale.

High Sensitivity of Fluorine Gas-Assisted FIB-TOF-SIMS for Chemical Characterization of Buried Sublayers in Thin Films.

In this work, we present the potential of high vacuum-compatible time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors, which can be integrated within focused ion beam (FIB) instruments for precise and fast chemical characterization of thin films buried deep under the sample surface. This is demonstrated on complex multilayer systems composed of alternating ceramic and metallic layers with thicknesses varying from several nanometers to hundreds of nanometers. The typical problems of the TOF-SIMS technique, that is, low secondary ion signals and mass interference between ions having similar masses, were solved using a novel approach of co-injecting fluorine gas during the sample surface sputtering. In the most extreme case of the Al/Al2O3/Al/Al2O3/.../Al sample, a <10 nm thick Al2O3 thin film buried under a 0.5 µm material was detected and spatially resolved using only 27Al+ signal distribution. This is an impressive achievement taking into account that Al and Al2O3 layers varied only by a small amount of oxygen content. Due to its high sensitivity, fluorine gas-assisted FIB-TOF-SIMS can be used for quality control of nano- and microdevices as well as for the failure analysis of fabrication processes. Therefore, it is expected to play an important role in the development of microelectronics and thin-film-based devices for energy applications.

Elemental Characterization of Al Nanoparticles Buried under a Cu Thin Film: TOF-SIMS vs STEM/EDX.

In this work, we present a comprehensive comparison of time-of-flight secondary ion mass spectrometry (TOF-SIMS) and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM/EDX), which are currently the most powerful elemental characterization techniques in the nano- and microscale. The potential and limitations of these methods are verified using a novel dedicated model sample consisting of Al nanoparticles buried under a 50 nm thick Cu thin film. The sample design based on the low concentration of nanoparticles allowed us to demonstrate the capability of TOF-SIMS to spatially resolve individual tens of nanometer large nanoparticles under ultrahigh vacuum (UHV) as well as high vacuum (HV) conditions. This is a remarkable achievement especially taking into account the very small quantities of the investigated Al content. Moreover, the imposed restriction on the Al nanoparticle location, i.e., only on the sample substrate, enabled us to prove that the measured Al signal represents the real distribution of Al nanoparticles and does not originate from the artifacts induced by the surface topology. The provided comparison of TOF-SIMS and STEM/EDX characteristics delivers guidelines for choosing the most optimal method for efficient characterization of nano-objects.

A set of empirical equations describing the observed colours of metal-anodic aluminium oxide-Al nanostructures.

Structural colours have received a lot of attention regarding the reproduction of the vivid colours found in nature. In this study, metal-anodic aluminium oxide (AAO)-Al nanostructures were deposited using a two-step anodization and sputtering process to produce self-ordered anodic aluminium oxide films and a metal layer (8 nm Cr and 25, 17.5 and 10 nm of Au), respectively. AAO films of different thickness were anodized and the Yxy values (Y is the luminance value, and x and y are the chromaticity values) were obtained via reflectance measurements. An empirical model based on the thickness and porosity of the nanostructures was determined, which describes a gamut of colours. The proposed mathematical model can be applied in different fields, such as wavelength absorbers, RGB (red, green, blue) display devices, as well as chemical or optical sensors.

Achieving micron-scale plasticity and theoretical strength in Silicon.

As the backbone material of the information age, silicon is extensively used as a functional semiconductor and structural material in microelectronics and microsystems. At ambient temperature, the brittleness of Si limits its mechanical application in devices. Here, we demonstrate that Si processed by modern lithography procedures exhibits an ultrahigh elastic strain limit, near ideal strength (shear strength ~4 GPa) and plastic deformation at the micron-scale, one order of magnitude larger than samples made using focused ion beams, due to superior surface quality. This extended elastic regime enables enhanced functional properties by allowing higher elastic strains to modify the band structure. Further, the micron-scale plasticity of Si allows the investigation of the intrinsic size effects and dislocation behavior in diamond-structured materials. This reveals a transition in deformation mechanisms from full to partial dislocations upon increasing specimen size at ambient temperature. This study demonstrates a surface engineering pathway for fabrication of more robust Si-based structures.

Fluorine Gas Coinjection as a Solution for Enhancing Spatial Resolution of Time-of-Flight Secondary Ion Mass Spectrometry and Separating Mass Interference.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors have been intensively developed in recent decades due to their unprecedented capability of representing a sample elemental composition in a three-dimensional space from nano- to submilliscale with high spatial resolution and mass resolution. A compact high-vacuum-compatible version of these detectors can be integrated into a focused ion beam (FIB) system which, assembled with scanning electron microscopy (SEM), is the most popular tool used in nanotechnology and material science. This gives a new opportunity for combining TOF-SIMS analysis with other instruments within the same analytical chamber. In this work we present the results of conducting elemental characterization of a dedicated model multilayer sample composed of 100 nm thick thin films of Cu, Zr, and ZrCuAg alloy in a fluorine gas atmosphere provided by an in situ gas injection system (GIS). In general, the secondary ion signals were significantly enhanced by up to 3 orders of magnitude, leading to much higher spatial resolution. The quality of elemental images and depth profiles was improved during a single measurement (which usually cannot be obtained at standard vacuum conditions) at a high beam energy of 20 keV. Moreover, fluorine assistance has enabled a mass interference between 107Ag+ and 91Zr16O+ ions to be separated. This remarkable finding has never been reported before and is expected to play an important role in the future evolution of TOF-SIMS analytical protocols, as currently the mass interference between ions remains one of the main drawbacks of the technique.

3D Imaging of Nanoparticles in an Inorganic Matrix Using TOF-SIMS Validated with STEM and EDX.

Imaging nano-objects in complex systems such as nanocomposites using time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a challenging task. Due to a very small amount of the material and a matrix effect, the number of generated secondary ions can be insufficient to represent a 3D elemental distribution despite being detected in a mass spectrum. Therefore, a model sample consisting of a ZrCuAg matrix with embedded Al nanoparticles is designed. A high mass difference between the light Al and heavy matrix components limits mass interference. The chemical structure measurements using a pulsed 60 keV Bi32+ beam or a continuous 30 keV Ga+ beam reveals distinct Al signal segregation. This can indicate a spatially resolved detection of single 10s of nanometer large particles and/or their agglomerates for the first time. However, TOF-SIMS images of 50 nm or smaller objects do not necessarily represent their exact size and shape but can rather be their convolutions with the primary ion beam shape. Therefore, the size of nanoparticles (25-64 nm) was measured using scanning transmission electron microscopy. Our studies prove the capability of TOF-SIMS to image chemical structure of nanohybrids which is expected to help building new functional materials and optimize their properties.

Application of a Gas-Injection System during the FIB-TOF-SIMS Analysis-Influence of Water Vapor and Fluorine Gas on Secondary Ion Signals and Sputtering Rates.

Combining a Gas-Injection System (GIS) with the Focused Ion Beam (FIB) has a broad scope of applications in sample preparation such as protective layer deposition, increasing material sputtering rates, and reducing FIB-related artifacts. On the other hand, injecting certain specific gases during a Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) analysis can significantly increase element ionization probability and, therefore, improve the quality of 3D representation of a sample elemental structure. In this work, for the first time, the potential of GIS for enhancing secondary ion signals acquired using a TOF detector incorporated into a commercial Ga+ FIB-SEM (Focused Ion Beam combined with Scanning Electron Microscope) instrument is presented. The depth profiles of pure metals (thin films of Cu, Zr, Ag, and W with the thickness in the order of 100 nm) were acquired under ambient vacuum conditions as well as under an exposure to water and fluorine gases. The influence of supplementary gases on the ion yields and sputtering rates was studied. Simulations were performed to assess the local gas pressure at the location of FIB-TOF-SIMS analysis. The highest enhancement of ionization probability was achieved in the case of the Cu thin film (10 times during water vapor coinjection and 510 times when using a fluorine gas). Regarding the sputtering rates, the response of Zr to the effect of the gases was the strongest. Compared to standard background pressure measurements, this thin film was milled around 6 times faster under exposure to water vapor and over 2 times faster when fluorine gas was supplied.

Dynamic Plasticity and Failure of Microscale Glass: Rate-Dependent Ductile-Brittle-Ductile Transition.

Glass has been recently envisioned as a stronger and more robust alternative to silicon in microelectromechanical system applications, including high-frequency resonators and switches. Identifying the dynamic mechanical properties of microscale glass is thus vital for understanding their ability to withstand shocks and vibrations in such demanding applications. However, despite nearly half a century of research, the micromechanical properties of glass and amorphous materials in general are primarily limited to quasi-static strain rates below ∼0.1/s. Here, we report the in situ high-strain-rate experiments of fused silica micropillars inside a scanning electron microscope at strain rates up to 1335/s. A remarkable ductile-brittle-ductile failure mode transition was observed at increasing strain rates from 0.0008 to 1335/s as the deformation flow transitions between homogeneous-serrated-homogeneous regimes. Detailed surface topography investigation of the tested micropillars revealed that at the intermediate strain rate (<∼6/s) serrated flow regime, the load drops are caused by the sequential propagation of individual shear bands. Further, analytical calculations and finite element simulations suggest that the atomistic mechanism responsible for the homogeneous stress-strain curves at very high strain rates (>∼64/s) can be attributed to the simultaneous nucleation of multiple shear bands along with dissipative deformation heating. This unique rate-dependent deformation behavior of the glass micropillars highlights the importance and need of extending such microscale high-strain-rate studies to other amorphous materials such as metallic glasses and amorphous metals and alloys. Such investigations can provide critical insights about the damage tolerance and crashworthiness of these materials for real-life applications.