Filamentary growth of metals: Microstructure and properties of (nano-) whiskers

One dimensional nanostructures have the prospect to change the properties of materials used in contemporary devices. Physical properties change with dimension and size. Ceramics, semiconductor and carbon materials are easily synthesized as one dimensional structures with typical diameters of several nanometers and length-diameter ratios of 1000:1. However, only the metals as one of the oldest are difficult to fabricate in similar geometries. In contrast, micrometer diameter, millimeter length macroscopic metallic nanowires were grown and reported decades ago via the reduction of metal halides, based on a process described already in 1574. Recently we developed a process to grow perfect defect and flaw free nanostructures with diameters of several ten nanometers, attached on substrates. The initiator mediated filamentary crystal growth process is based on the physical vapour deposition technique. Metals with face centered (Cu, Ag, Au, Pd, Ni, Co) crystal structure were synthesized successfully with the new technique. Typical diameters of the nanowhiskers are 100 nm and lengths of up to 200 μm are observed, giving aspect ratios of up to 2000:1. Traditional theories attribute the growth of whiskers with the presence of a screw-dislocation. However, studies by transmission electron microscopy did disprove this growth mechanism. An alternative growth mechanism will be discussed. The formation of the nanowhiskers is controlled by diffusion processes of adatoms on the substrate surface and in the emerging interface under the growing nanostructures. By changing substrate material or sequencing different materials during growth allow for the formation of alloy nanowhiskers. This will be described exemplarily by Metal-Si and binary metal alloys (Au-Ni and Ag-Ni). Please click Additional Files below to see the full abstract

Fabrication of Hollow AlAu2 Nanoparticles by Solid State Dewetting and Oxidation of Al on Sapphire Substrate

The Al-Au binary diffusion couple is a classic example of the system exhibiting Kirkendall voiding during interdiffusion. We demonstrate that this effect, which is a major reason for failures of the wire bonds in microelectronics, can be utilized for producing hollow AlAu2 nanoparticles attached to sapphire substrate. To this end, we produced the core-shell Al-Au nanoparticles by performing a solid state dewetting treatment of Al thin film deposited on sapphire substrate, followed by the deposition of thin Au layer on the top of dewetted sample. Annealing of the core-shell nanoparticles in air resulted in outdiffusion of Al from the particles, formation of pores, and growth of the AlAu2 intermetallic phase in the particles. We demonstrated that the driving force for hollowing is the oxidation reaction of the Al atoms at the Au-sapphire interface, leading to the homoepitaxial growth of newly formed alumina at the interface. We developed a kinetic model of hollowing controlled by diffusion of oxygen through the Au thin film, and estimated the solubility of oxygen in solid Au. Our work demonstrates that the core-shell nanoparticles attached to the substrate can be hollowed by the Kirkendall effect in the thin film spatially separated from the particles.Comment: 27 pages, 8 figure

Characterization of the CRISPR-Cas subtype I-B proteins Cas6b and Cas8b of Methanococcus maripaludis C5

The CRISPR-Cas system is an adaptive immune system found in archaea and bacteria to defend themselves against mobile genetic elements (e.g. phages). The system employs base complementarity of small RNA species (crRNAs) to target the foreign nucleic acids for degradation. The hallmark of the system is the CRISPR array or locus, which is composed of repetitive DNA sequences (repeats) that are interspersed by unique sequences (spacers). Spacer sequences can be derived from earlier encounters with viruses and, as part of the crRNAs, confer the base complementarity during a reoccurring attack. During the ongoing battle between prokaryotes and viruses diverse CRISPR-Cas systems evolved into three main types that are further subdivided. This thesis shows the first characterization of a subtype I-B CRISPR-Cas system. RNA-Seq data proved the in vivo activity of this CRISPR-Cas system in Methanococcus maripaludis C5. The data further revealed that the crRNAs are always composed of a complete spacer sequence flanked by an 8 nt 5' repeat tag and a 2 nt 3' repeat tag. Eigth cas genes were identified for M. maripaludis. Two Cas proteins, Cas8b and an annotated hypothetical protein were characterized in more detail. The hypothetical protein was shown to be the endoribonuclease responsible for the single-turnover catalysis of precursor crRNA into mature crRNA and was termed Cas6b. The reaction performed by Cas6b yields the 8 nt 5' terminal tag of the mature crRNAs. Despite sharing only low sequence identity of 11 %, the two Cas6 proteins of M. maripaludis and Pyrococcus furiosus could be well aligned using a structural model of Cas6b and the crystal structure of P. furiosus Cas6. Cas6b mutant analysis was used to determine four amino acid residues (lysine 30, histidine 38, histidine 40 and tyrosine 47) that comprise the catalytic site of Cas6b. The RNA binding properties of Cas6b were determined and showed a dimerization upon binding to a non-cleavable substrate. Further analyses including RNA crosslinking experiments followed by mass spectrometry identified a methionine residue (M185) that tightly coordinated to a uridine (U15) of the repeat sequence. Cas6b activity assays employing differently structured repeat variants of M. maripaludis and a 37 nt repeat sequence of Clostridium thermocellum could show, that the processing reaction performed by Cas6b does not recognize a secondary structure of the substrate. In addition to the verification to the in vivo activity of the CRISPR-Cas system, the RNA-Seq data also revealed a varying abundance pattern of crRNAs. To assess the crRNA abundance a experimental procedure was designed, which was aimed to analyse the influence of spacer sequences on a) the processing by Cas6 and b) the stability of crRNAs. With the help of this global approach influences of the spacer length and spacer sequence on the crRNA maturation and in vitro stability were recognized. In this context, future experiments will also determine further possible influences on crRNA abundance including i) crRNA loading into the Cas protein interference complex (Cascade) and ii) possible regulatory effects in terms of crRNA utilization dependent regulation. The characterization of the subtype-specific protein Cas8b revealed a splitting of the recombinant protein into two defined fragments. The exact point of cleavage was determined by Edman sequencing and provides evidence for a proteolytic cleavage of the full-length protein (either autocatalytically or by a protease). Other CRISPR-Cas subtypes were reported to contain two proteins serving as small and big subunit of the interference complex Cascade. For subtype I-B on the other hand Cas8b was found to be the only equivalent to these two proteins and it was proposed that the identified cleavage generates the large and small Cascade subunit. A biochemical analysis of Cas8b with respect to its putative roles during CRISPR-Cas immunity showed an unspecific binding to nucleic acids while no nucleolytic cleavage was observed. Possible functions of Cas8b are discussed and future studies will focus on the analysis of the protein functions in the context of a complete Cascade

IS THE LOCAL MINIMUM IN THE FORCE TIME HISTORY IN COUNTERMOVEMENT JUMPS RELATED TO JUMP PERFORMANCE?

The purpose of this study was to investigate the relationship between joint kinematics and joint kinetics of the lower extremity to the decrease in the force time curve in counter movement jumps. Thirty-five sports students performed maximum voluntary countermovement jumps. Two force plates and a 10 camera optoelectronic system as well as a full-body model were used to measure joint kinematics and kinetics. Twelve subjects showed a characteristic double peak in force-time-history in the concentric phase. This pattern was not directly related to jumping height. However the time for the total movement cycle as well as for the concentric phase of the jump was longer. In some sports it is necessary the produce maximum impulse in the concentric phase in a short period of time. In those cases a different movement strategy might be beneficial

The role of Si impurities in the transient dopant segregation and precipitation in yttrium-doped alumina

Y-doped alumina was sintered at 1500 degrees C for 10 h under ultra-clean experimental conditions without experiencing any abnormal grain growth. The yttrium was fairly homogeneously distributed at the grain boundaries, with a mean value of (Gamma) over bar (Y) = 5.5 at nm(-2). The Y-Al-O precipitates in the clean, Y2O3-doped alumina specimen were the YAP (YAlO3) phase, whereas only the YAG (Y3Al5O12) phase was present in the Y2O3-doped alumina samples contaminated with SiO2. The excess concentrations of Y and Si atoms at the grain boundaries that, at the same time, provoke the formation of structurally complex YAG precipitates and abnormal grain growth were both estimated to be at 4-5 at nm(-2). The compositions of the triple point pocket phases found in the region of the exaggeratedly grown alumina grains indicate the presence of alumino-silicate bulk liquids at the sintering temperature

Suitability study for real-time depth map generation using stereo matchers in OpenCV and Python

Stereo imaging provides an easy and cost-effective method to measure 3D surfaces, especially due to the availability of extensive free program libraries like OpenCV. An extension of the application to the field of forestry was aimed at here in the context of a project to capture the elevation profile of forest roads by means of stereo imaging. For this purpose, an analysis of the methods contained in OpenCV for the successful generation of depth maps was carried out. The program sections comprised the reading of the image stream, the image correction on the basis of calibrations carried out in advance as well as the generation of the disparity maps by the stereo matchers. These are then converted back into depth maps and stored in suitable memory formats. A data set of the image size 1280x864 pixels consisting of 30 stereo image pairs was used. The aim was to design an evaluation program which allows the processing of the described steps within one second for 30 image pairs. With a sequential processing of all steps under the used test system and the usage of a local stereo matcher a processing time of 4.37 s was determined. Steps to reduce the processing time included parallelizing the image preparation of the two frames of the image pair. Further reduction in total processing time was achieved by processing multiple image pairs simultaneously and using storage formats without compression. A total processing time of 0.8 s could be achieved by outsourcing the stereo matching to the graphics card. However, the tested method did not achieve the desired resolutions in depth as well as in the image plane. This was made possible by using semi-global matchers, which are up to 10 times slower but significantly more accurate, and which were therefore used for further investigations of the forest path profile

In situ ultrafine force measurement with nanowire based cantilevers in SEM

In nanomechanics the measurement of ultrafine forces becomes increasingly important for unravelling subtle details of elastic and plastic deformation processes. In particular, achieving high force resolution in combination with in situ imaging is a major challenge which is becoming exceedingly difficult with conventional methods. In this work, we introduce a novel systematic method to measure ultrafine forces using well-defined nanowires as cantilever beams in situ in the Scanning Electron Microscope (SEM). Forces can be measured variably in the range from micro-newtons (mN) down to femto-newtons (fN), depending on the chosen reference nanowire. The reference wires are picked with a manipulator tip without the use of FIB (see Figure 1 a). Please click Additional Files below to see the full abstract

Lattice Anharmonicity in Defect-Free Pd Nanowhiskers

We have investigated anharmonic behavior of Pd by applying systematic nanoscale tensile testing to near defect-free nanowhiskers offering a large range of elastic strain. We measured size-dependent deviations from bulk elastic behavior in nanowhiskers with diameters as small as ∼30  nm. In addition to size-dependent variations in Young’s modulus in the small strain limit, we measured nonlinear elasticity at strains above ∼1%. Both phenomena are attributed to higher-order elasticity in the bulklike core upon being biased from its equilibrium configuration due to the role of surface stresses in small volumes. Quantification of the size-dependent second- and third-order elastic moduli allows for calculation of intrinsic material nonlinearity parameters, e.g., δ. Comparison of the size-independent values of δ in our nanowhiskers with studies on bulk fcc metals lends further insight into the role of length scales on both elastic and plastic mechanical behavior

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 a thorough understanding of dislocation structure and their 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. In this talk, experiments that directly measure the surface dislocation nucleation strengths in high qualityPd nanowhiskers subjected to uniaxial tension will be presented. 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. Modeling of the probabilistic nature of surface dislocation nucleation suggests activation energies consistent with surface self-diffusion as the rate-limiting step needed to promote displacive activity. In this context, approaches allowing for modification of the surface chemistry and structure of metallic nanostructures to either inhibit or enhance surface diffusion will be discussed