Photoemission spectroscopy demonstrates the formation of a surface gold nitride upon irradiation of a $\mathrm{Au}(110)$ surface with $500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ nitrogen ions at room temperature. After irradiation two $\mathrm{N}1s$ peaks are observed at binding energies of $396.7\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $397.7\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ along with a broadening of the $\mathrm{Au}4{d}_{5∕2}$ line. Changes in valence-band spectra are also observed, including an additional density of states at $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ binding energy and new states at $\ensuremath{\sim}3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Annealing experiments indicate that the two $\mathrm{N}1s$ lines are associated with nitrogen compounds of differing thermal stability, possibly due to the formation of more than one nitride phase. To further investigate the properties of gold nitride we have undertaken ab initio pseudopotential calculations on the most likely nitride stoichiometry, ${\mathrm{Au}}_{3}\mathrm{N}$, and identified a novel triclinic crystal structure of a significantly lower energy than the anti-${\mathrm{ReO}}_{3}$ expected from a simple consideration of the periodic table, although the latter structure is also found to be stable. The triclinic structure is determined to be metallic, of importance to possible applications.
We present a detailed experimental study of the onset of rippling in highly crystalline carbon nanotubes. Modeling has shown that there should be a material constant, called the critical length, describing the dependence of the critical strain on the nanotube outer radius. Surprisingly, we have found very large variations, by a factor of three, in the critical length. We attribute this to a supporting effect from the inner walls in multiwalled concentric nanotubes. We provide an analytical expression for the maximum deflection prior to rippling, which is an important design consideration in nanoelectromechanical systems utilizing nanotubes.
A micromachined capacitive force sensor operating in the micro-Newton range has been calibrated using both dynamic and static methods. Both calibrations are non-destructive, accurate and traceable to Système International (SI) fundamental units. The dynamic calibration is a differential mass loading resonant method where the resonance frequency with and without an added mass is measured. This gives enough information to compute the spring constant. In this paper, we evaluate the resonant mass loading method for more complex MEMS devices. Analytical calculations and finite element analysis have been performed to investigate the dynamic properties of the sensor, e.g. modal interference. The frequency response was measured with the third harmonic method where the third harmonic of the current through the sensor was measured. To detect and analyse the resonance mode of the structure during excitation, a scanning laser Doppler vibrometer was used. Two designs of a capacitive nanoindenter force sensor with flexure-type springs have been evaluated using these methods. The quality of the resonant calibration method has been tested using static mass loading in combination with transmission electron microscopy imaging of the sensor displacement. This shows that the resonant method can be extended to calibrate more complex structures than plain cantilevers. Both calibration methods used are traceable to SI fundamental units as they are based on masses weighed on a calibrated scale. The masses used do not need to be fixed or glued in any way, making the calibration non-destructive.
Abstract Electrical characterization of nanostructures, such as nanotubes and wires, is a demanding task that is vital for future applications of nanomaterials. The nanostructures should ideally be analyzed in a free-standing state and also allow for other material characterizations to be made of the same individual nanostructures. Several methods have been used for electrical characterizations of carbon nanotubes in the past. The results are widely spread, both between different characterizations methods and within the same materials. This raises questions regarding the reliability of different methods and their accuracy, and there is a need for a measurement standard and classification scheme for carbon nanotube materials. Here we examine a two-probe method performed inside a transmission electron microscope in detail, addressing specifically the accuracy by which the electrical conductivity of individual carbon nanotubes can be determined. We show that two-probe methods can be very reliable using a suitable thermal cleaning method of the contact points. The linear resistance of the outermost nanotube wall can thus be accurately determined even for the highest crystallinity materials, where the linear resistance is only a few kΩ/μm. The method can thereby by used as a valuable tool for future classification schemes of various nanotube material classes.
Ortho-para conversion of ${\mathrm{H}}_{2}$ adsorbed at the step atoms of a Cu(510) surface proceeds with a short conversion time constant around 1 s as observed in electron-energy-loss measurements of rotational populations. We suggest that this rapid conversion is related to the special character of the adsorption state, which involves a short ${\mathrm{H}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{Cu}$ bond length of 1.8 \AA{}. On the flat Cu(100) surface, conversion is found to occur at active sites, most likely step atoms.
A capacitive force sensor for in situ TEM instrumentation is investigated. In order to prevent movement of the suspended plate in the capacitive sensor force feedback has been investigated, primarily using CV measurements. A manual feedback has successfully been implemented and an analytical model using a serial and a parallel capacitor is presented.
The properties of laser wakefield accelerated electrons in supersonic gas flows of hydrogen and helium are investigated. At identical backing pressure, we find that electron beams emerging from helium show large variations in their spectral and spatial distributions, whereas electron beams accelerated in hydrogen plasmas show a higher degree of reproducibility. In an experimental investigation of the relation between neutral gas density and backing pressure, it is found that the resulting number density for helium is $\ensuremath{\sim}30%$ higher than for hydrogen at the same backing pressure. The observed differences in electron beam properties between the two gases can thus be explained by differences in plasma electron density. This interpretation is verified by repeating the laser wakefield acceleration experiment using similar plasma electron densities for the two gases, which then yielded electron beams with similar properties.
