Effects of crystal phase mixing on the electrical properties of InAs nanowires

We report a systematic study of the relationship between crystal quality and electrical properties of InAs nanowires grown by MOVPE and MBE, with crystal structure varying from wurtzite to zinc blende. We find that mixtures of these phases can exhibit up to 2 orders of magnitude higher resistivity than single-phase nanowires, with a temperature-activated transport mechanism. However, it is also found that defects in the form of stacking faults and twin planes do not significantly affect the resistivity. These findings are important for nanowire-based devices, where uncontrolled formation of particular polytype mixtures may lead to unacceptable device variability.     

Unit cell structure of crystal polytypes in InAs and InSb nanowires

The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.     

Wurtzite-zincblende superlattices in InAs nanowires using a supply interruption method

Crystal phase control in single III-V semiconductor nanowires has emerged recently as an important challenge and possible complement to conventional bandgap engineering in single material systems. Here we investigate a supply interruption method for precise crystal phase control in single nanowires. The nanowires are grown by metalorganic vapor phase epitaxy using gold particles as seeds and are analyzed by transmission electron microscopy. It is observed that wurtzite segments with controlled length and position can be inserted on demand into a pure InAs zincblende nanowire. The interface between wurtzite and zincblende segments can be made atomically sharp and the segments can be made only a few bilayers in thickness. The growth mechanisms, applicability and limitations of the technique are presented and discussed.     

Optimization and Fabrication of a Thick Printed Thermoelectric Device

Thermoelectric power generation technology aims to convert thermal energy into electricity. Micromodule design optimization depends directly on the thermal environment. For low thermal energy input, optimized thermoelectric devices require 100 μm to 500 μm element thickness. These dimensions currently present a challenge for standard mass-production manufacturing techniques. In this paper, a unique printing technology for micromodule fabrication is presented. This technology is compared with a traditional bulk thermoelectric manufacturing process to highlight the advantages of the printing process to obtain scalable thermoelectric devices. Initial thermoelectric materials have been integrated in inks and then deposited by a spray technology onto a polymer substrate. A complete micromodule for application on nonplanar surfaces is also presented.     

Sintering of TCP-TiO2 biocomposites: influence of secondary phases

TCP-TiO2 ceramic biocomposites with various alpha-to-beta TCP ratios can be prepared by quenching the alpha phase. The presence of dopants (Ca, P, or Na) leads to the precipitation of secondary phases, which decreases the densification of titania. In the system TCP-TiO2, there is a eutectic with a composition of 63 wt% TCP-37 wt% TiO2 at a temperature of 1380 degrees C.     

Supercurrent and multiple Andreev reflections in an InSb nanowire Josephson junction

Epitaxially grown, high quality semiconductor InSb nanowires are emerging material systems for the development of high performance nanoelectronics and quantum information processing and communication devices and for the studies of new physical phenomena in solid state systems. Here, we report on measurements of a superconductor-normal conductor-superconductor junction device fabricated from an InSb nanowire with aluminum-based superconducting contacts. The measurements show a proximity-induced supercurrent flowing through the InSb nanowire segment with a critical current tunable by a gate in the current bias configuration and multiple Andreev reflection characteristics in the voltage bias configuration. The temperature dependence and the magnetic field dependence of the critical current and the multiple Andreev reflection characteristics of the junction are also studied. Furthermore, we extract the excess current from the measurements and study its temperature and magnetic field dependences. The successful observation of the superconductivity in the InSb nanowire-based Josephson junction device indicates that InSb nanowires provide an excellent material system for creating and observing novel physical phenomena such as Majorana fermions in solid-state systems.     

Electron microscopy of frozen biological objects: a study using cryosectioning and cryosubstitution

Freezing of bulk biological objects was investigated by X-ray cryodiffraction. Freezing at atmospheric pressure of most microscopic biological samples gives rise to large hexagonal crystals and leads to poor structural preservation of these specimens. High-pressure freezing induces the formation of different ices (hexagonal, cubic and a high-pressure form) consisting of crystals having sizes smaller than those formed at atmospheric pressure. With both freezing methods, a cryoprotectant has to be added to the biological object to avoid the formation of ice crystals. However, special cases can be encountered: some biological objects contain large amounts of natural cryoprotectant or have a low water content. In these cases, vitrification can be achieved, especially using high-pressure freezing. Cryo-sectioning can be performed on vitrified samples, and the sections studied by electron cryomicroscopy. Images and electron diffraction patterns having a resolution better than 2 and 0.2 nm, respectively, can be obtained with such sections. Because samples containing crystalline ices cannot be cryosectioned, their structure has to be studied using cryosubstitution and resin embedding. We show that bacteria, yeast, and ciliate and marine worm elytrum have cellular compartments with an organization that has not been described by classical techniques relying on chemical fixation of the tissues. A high-pressure artefact affecting the Paramecium trichocysts is described. Such artefacts are not general; for example, we show that 70% of high-pressure frozen yeast cells survive successive high-pressure freezing and thawing steps.     

Changes in cerebrospinal fluid monoamine metabolites, tryptophan, and gamma-aminobutyric acid during the 1st year of life in normal infants. comparison with victims of sudden infant death syndrome

Cerebrospinal fluid (CSF) levels of 3-methoxy-4-hydroxyphenylglycol, 5-hydroxyindoleacetic acid, homovanillic acid, tryptophan, and gamma-aminobutyric acid were measured using high-performance liquid chromatography in 102 infants during the 1st year of life (preterm and term neonates included). CSF levels are expressed versus corrected age (postnatal days - preterm days) which reflects the stage of maturity of the central nervous system. These results are compared to those obtained in CSF of 53 victims of sudden infant death syndrome (SIDS). All components were significantly higher in SIDS than in the age-matched control group. This increase does not seem to be an artefact related to death. Indeed, under the same conditions concerning postmortem time interval before CSF sampling and analysis, the levels are not significantly higher in infants who died from a known pathology than in living infants. Moreover, in living infants as regards a pathology such as asphyxia or hypoventilation in comparison with SIDS, similar profiles are observed in some neurotransmitters or metabolites. Other studies are necessary to explore further neurotransmission systems in SIDS.     

Matrix-implanted laser desorption/ionization mass spectrometry

The implantation of low-velocity massive gold clusters is shown to be a method of choice for homogeneous incorporation of a metallic matrix into the near-surface region of a solid biopolymer for subsequent laser desorption/ionization (LDI) MS analysis. Matrix implanted (MI)LDI spectra from cluster-implanted pure test peptide or tissue exhibit molecular ion peaks similar to those observed by matrix-assisted LDI. Moreover, the ion emission is very reproducible from any spot on the surface of these test samples. MILDI promises to be a powerful technique for mass spectrometric analysis of native biological samples as demonstrated by the first results on rat brain tissues.     

High yield of self-catalyzed GaAs nanowire arrays grown on silicon via gallium droplet positioning

We report and detail a method to achieve growth of vertical self-catalyzed GaAs nanowires directly on Si(111) with a near-perfect vertical yield, using electron-beam-defined arrays of holes in a dielectric layer and molecular beam epitaxy. In our conditions, GaAs nanowires are grown along a vapor-liquid-solid mechanism, using in?situ self-forming Ga droplets. The focus of this paper is to understand the role of the substrate preparation and of the pre-growth conditioning. Without changing temperature or the V/III ratio, the yield of vertical nanowires is increased incrementally up to 95%. The possibility to achieve very dense arrays, with center-to-center inter-wire distances less than 100?nm, is demonstrated.