John V. Badding

Microstructured optical fibers as high-pressure microfluidic reactors

Deposition of semiconductors and metals from chemical precursors onto planar substrates is a well-developed science and technology for microelectronics. Optical fibers are an established platform for both communications technology and fundamental research in photonics. Here, we describe a hybrid technology that integrates key aspects of both engineering disciplines, demonstrating the fabrication of tubes, solid nanowires, coaxial heterojunctions, and longitudinally patterned structures composed of metals, single-crystal semiconductors, and polycrystalline elemental or compound semiconductors within microstructured silica optical fibers. Because the optical fibers are constructed and the functional materials are chemically deposited in distinct and independent steps, the full design flexibilities of both platforms can now be exploited simultaneously for fiber-integrated optoelectronic materials and devices.

High-pressure chemistry of hydrogen in metals: in situ study of iron hydride.

Optical observations and x-ray diffraction measurements of the reaction between iron and hydrogen at high pressure to form iron hydride are described. The reaction is associated with a sudden pressure-induced expansion at 3.5 gigapascals of iron samples immersed in fluid hydrogen. Synchrotron x-ray diffraction measurements carried out to 62 gigapascals demonstrate that iron hydride has a double hexagonal close-packed structure, a cell volume up to 17% larger than pure iron, and a stoichiometry close to FeH. These results greatly extend the pressure range over which the technologically important iron-hydrogen phase diagram has been characterized and have implications for problems ranging from hydrogen degradation and embrittlement of ferrous metals to the presence of hydrogen in Earths metallic core.

Benzene-derived carbon nanothreads

Low-dimensional carbon nanomaterials such as fullerenes, nanotubes, graphene and diamondoids have extraordinary physical and chemical properties. Compression-induced polymerization of aromatic molecules could provide a viable synthetic route to ordered carbon nanomaterials, but despite almost a century of study this approach has produced only amorphous products. Here we report recovery to ambient pressure of macroscopic quantities of a crystalline one- dimensional sp(3) carbon nanomaterial formed by high-pressure solid-state reaction of benzene. X-ray and neutron diffraction, Raman spectroscopy, solid-state NMR, transmission electron microscopy and first-principles calculations reveal close- packed bundles of subnanometre-diameter sp(3)-bonded carbon threads capped with hydrogen, crystalline in two dimensions and short-range ordered in the third. These nanothreads promise extraordinary properties such as strength and stiffness higher than that of sp(2) carbon nanotubes or conventional high-strength polymers. They may be the first member of a new class of ordered sp(3) nanomaterials synthesized by kinetic control of high-pressure solid-state reactions.

Thermal and Electrical Conductivity of Size‐Tuned Bismuth Telluride Nanoparticles

Quantum-confined semiconductors composed of heavy elements hold great promise as thermoelectric materials. An increase in the density of states near the Fermi level due to quantum confinement effects and an increased scattering of boundary phonons due to nanostructuring can lead to an increase in the dimensionless figure of merit, ZT, which is defined as s 2 STk � 1 . [1,2] Here, s is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and k is the thermal conductivity. To realize the highest ZTquantum-confined material possible from a conventional thermoelectric semiconductor material such as bismuth telluride, meeting several criteria are important. First, it is reasonable to expect that uniform, optimal size quantum domains will lead to the highest ZT at a given doping level. The electronic structure of the semiconductor, which determines the thermoelectric power, depends on the degree of quantum confinement and thus the domain size. Second, the interfacial electrical transport must be optimized. The presence of impurities between the domains/particles typically leads to barriers to transport that reduce the electrical conductivity. Third, while optimizing electrical transport between domains, it is important that the quantum architecture not be destroyed, or the phonon scattering will decrease and the thermal conductivity will increase. Fourth, it is well knownthatproperdopingiscriticaltotheperformanceofbulk

Transition element-like chemistry for potassium under pressure

At high pressure the alkali metals potassium, rubidium, and cesium transform to metals that have a d1 electron configuration, becoming transition metal-like. As a result, compounds were shown to form between potassium and the transition metal nickel. These results demonstrate that the chemical behavior of the alkali metals under pressure is very different from that under ambient conditions, where alkali metals and transition metals do not react because of large differences in size and electronic structure. They also have significant implications for the hypothesis that potassium is incorporated into Earths core.

Poly(phenylcarbyne): A Polymer Precursor to Diamond-Like Carbon.

The synthesis of poly(phenylcarbyne), one of a class of carbon-based random network polymers, is reported. The network backbone of this polymer is composed of tetrahedrally hybridized carbon atoms, each bearing one phenyl substituent and linking, by means of three carbon-carbon single bonds, into a three-dimensional random network of fused rings. This atomic-level carbon network backbone confers unusual properties on the polymer, including facile thermal decomposition, which yields diamond or diamond-like carbon phases at atmospheric pressure.

Deposition and characterization of germanium sulphide glass planar waveguides

Germanium sulphide glass thin films have been deposited on CaF2 and Schott N-PSK58 glass substrates directly by means of chemical vapor deposition (CVD). The deposition rate of germanium sulphide glass film by this CVD process is estimated about 12 microm/hr at 500oC. These films have been characterized by micro-Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Their transmission range extends from 0.5microm to 7microm measured by UV-VIS-NIR and FT-IR spectroscopy. The refractive index of germanium sulphide glass film measured by prism coupling technique was 2.093+/-0.008 and the waveguide loss measured at 632.8nm by He-Ne laser was 2.1+/-0.3 dB/cm.

Zinc Selenide Optical Fibers

Semiconductor waveguide fabrication for photonics applications is usually performed in a planar geometry. However, over the past decade a new field of semiconductor-based optical fiber devices has emerged. The drawing of soft chalcogenide semiconductor glasses together with low melting point metals allows for meters-long distributed photoconductive detectors, for example.[1,2] Crystalline unary semiconductors (e.g., Si, Ge) have been chemically deposited at high pressure into silica capillaries,[3,4] allowing the optical and electronic properties of these materials to be exploited for applications such as all-fiber optoelectronics.[5-7] In contrast to planar rib and ridge waveguides with rectilinear cross sections that generally give rise to polarization dependence, the cylindrical fiber waveguides have the advantage of a circular, polarization-independent cross section. Furthermore, the fiber pores, and thus the wires deposited in them, are exceptionally smooth[8] with extremely uniform diameter over their entire length. The high-pressure chemical vapor deposition (HPCVD) technique is simple, low cost, and flexible so that it can be modified to fill a range of capillaries with differing core dimensions, while high production rates can be obtained by parallel fabrication of multiple fibers in a single deposition. It can also be extended to fill the large number of micro- and nanoscale pores in microstructured optical fibers (MOFs), providing additional geometrical design flexibility to enhance the potential application base of the fiber devices.[9] Semiconductor fibers fabricated via HPCVD in silica pores also retain the inherent characteristics of silica fibers, including their robustness and compatibility with existing optical fiber infrastructure, thus presenting considerable advantages over fibers based on multicomponent soft glasses.

Low loss silicon fibers for photonics applications

Silicon fibers are fabricated using a high pressure chemical deposition technique to deposit the semiconductor material inside a silica capillary. The silicon is deposited in an amorphous state into pure silica capillaries and can be crystallized to polysilicon after the deposition via a high temperature anneal. Optical transmission measurements of various amorphous and polycrystalline core materials were performed in order to determine their linear losses. Incorporating silicon functionality inside the fiber geometry opens up new possibilities for the next generation of integrated silicon photonics devices.

Nonlinear transmission properties of hydrogenated amorphous silicon core optical fibers.

The nonlinear properties of a low loss hydrogenated amorphous silicon core fiber have been characterized for transmission of high power pulses at 1540 nm. Numerical modelling of the pulse propagation in the amorphous core material was used to establish the two-photon absorption, free-carrier absorption and the nonlinear refractive index, which were found to be larger than the values typical for crystalline silicon. Calculation of a nonlinear figure of merit demonstrates the potential for these hydrogenated amorphous silicon core fibers to be used in nonlinear silicon photonics applications.