Suppressed Superconductivity of the Surface Conduction Layer in Bi2_2Sr2_2CaCu2_2O8+x_{8+x} Single Crystals Probed by {\it c}-Axis Tunneling Measurements

We fabricated small-size stacks on the surface of Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO-2212) single crystals with the bulk transition temperature $T_c$$\simeq$90 K, each containing a few intrinsic Josephson junctions. Below a critical temperature $T_c'$ ($\ll$ $T_c$), we have observed a weakened Josephson coupling between the CuO$_2$ superconducting double layer at the crystal surface and the adjacent one located deeper inside a stack. The quasiparticle branch in the $IV$ data of the weakened Josephson junction (WJJ) fits well to the tunneling characteristics of a d-wave superconductor($'$)/insulator/d-wave superconductor (D$'$ID) junction. Also, the tunneling resistance in the range $T_c'$$<$$T$$<$$T_c$ agrees well with the tunneling in a normal metal/insulator/d-wave superconductor (NID) junction. In spite of the suppressed superconductivity at the surface layer the symmetry of the order parameter appears to remain unaffected.Comment: 13 pages, 6 figure

Unusually low thermal conductivity of gallium nitride nanowires

We report measurements of thermal conductivity κ on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97 to 181 nm grown by thermal chemical vapor deposition. We observed unexpectedly small kappa values, in the range of 13–19 W/m K at 300 K, with very weak diameter dependence. We also observe unusual power law κ~Tn behavior with n=1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1–1 and 0.01–0.1 at. %, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low κ and the T1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path

Defects in GaN Nanowires

High resolution and cross-sectional transmission electron microscopy (HRTEM, XTEM) were used to characterize common defects in wurtzite GaN nanowires grown via the vapor-liquid-solid (VLS) mechanism. High resolution transmission electron microscopy showed that these nanowires contained numerous (001) stacking defects interspersed with cubic intergrowths. Using cross-sectional transmission electron microscopy, bicrystalline nanowires were discovered with two-fold rotational twin axes along their growth directions, and were concluded to grow along high index directions or vicinal to low index planes. A defect-mediated VLS growth model was used to account for the prevalence of these extended defects. Implications for nanowire growth kinetics and device behavior are discussed

Effect of the polar surface on GaN nanostructure growth and morphology

Wurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia. The resulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron microscopy and transmission electron microscopy revealed both thin smooth and thick corrugated nanowires. The growth axes of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (\u3c0001\u3e), while the corrugated nanowires and the large polyhedra grew parallel to \u3c0001\u3e. We propose a model to explain these morphology variations in terms of the Ga/N ratio and the different characteristic lengths of {0001} polar surfaces in the different nanostructures

Self-branching in GaN Nanowires Induced by a Novel Vapor-Liquid-Solid Mechanism

Nanowires have great potential as building blocks for nanoscale electrical and optoelectronic devices. The difficulty in achieving functional and hierarchical nanowire structures poses an obstacle to realization of practical applications. While post-growth techniques such as fluidic alignment might be one solution, self-assembled structures during growth such as branches are promising for functional nanowire junction formation. In this study, we report vapor-liquid-solid (VLS) self-branching of GaN nanowires during AuPd-catalyzed chemical vapor deposition (CVD). This is distinct from branches grown by sequential catalyst seeding or vapor-solid (VS) mode. We present evidence for a VLS growth mechanism of GaN nanowires different from the well-established VLS growth of elemental wires. Here, Ga solubility in AuPd catalyst is limitless as suggested by a hypothetical pseudo-binary phase diagram, and the direct reaction between NH3 vapor and Ga in the liquid catalyst induce the nucleation and growth. The self-branching can be explained in the context of the proposed VLS scheme and migration of Ga-enriched AuPd liquid on Ga-stabilized polar surface of mother nanowires. This work is supported by DOE Grant No. DE-FG02-98ER45701

Focused Ion Beam Platinum Nanopatterning for GaN Nanowires: Ohmic Contacts and Patterned Growth

Nanopatterned Pt by Ga+ focused ion beam (FIB) decomposition of an organometallic precursor forms low resistance ohmic contacts on 40–70 nm diameter GaN nanowires (NWs) grown by thermal reaction of Ga2O3 and NH3. With no intentional doping, the wires are presumed to be n-type. Thus, the linear I-V behavior is surprising since evaporated Pt usually forms Schottky barriers on n-GaN. Ohmic behavior was not obtained for 130–140 diameter wires, even with thicker Pt contacts. A second application of FIB Pt nanopatterning was demonstrated by position-selective growth of GaN NWs on Pt catalyst dots. NW locations and density are defined by the position, size, and thickness of the Pt deposit. Combining these techniques provides a versatile platform for nanostructure research and development

Applications of electron microscopy to the characterization of semiconductor nanowires

We review our current progress on semiconductor nanowires of β-Ga2O3, Si and GaN. These nanowires were grown using both vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. Using transmission electron microscopy (TEM) we studied their morphological, compositional and structural characteristics. Here we survey the general morphologies, growth directions and a variety of defect structures found in our samples. We also outline a method to determine the nanowire growth direction using TEM, and present an overview of device fabrication and assembly methods developed using these nanowires

Ultra‐high elastic strain energy storage in hybrid metal‐oxide infiltrated polymer nanocomposites

An understanding of the mechanical properties of materials at nanometer length scales, including a material’s ability to store and release elastic strain energy, is of great significance in the effective miniaturization of actuators, sensors and resonators for use in micro-/nano-electromechanical systems (MEMS/NEMS) as well as advanced development of artificial muscles for locomotion in soft robots. The measure of a material’s ability to store and release elastic strain energy, the modulus of resilience (R), is a crucial parameter in realizing such advanced mechanical actuation technologies. Typically, engineering a material system with a large R requires large increases in the material’s yield strength yet conservative increase in Young’s modulus, an engineering challenge as the two mechanical properties are strongly coupled; generally, strengthening methods results in considerable stiffening or increase in the Young’s modulus. Here, we present hybrid composite polymer nanopillars which achieve the highest specific R ever reported, by utilizing vapor-phase aluminum oxide infiltrations into lithographically patterned polymer resist SU-8. In-situ nanomechanical measurements reveal high, metallic-like yield strengths (~500 MPa) combined with a compliant, polymeric-like Young’s modulus (~7 GPa), a unique pairing never observed in known engineering materials. It is these exceptional elastic properties of our hybrid composite which allows for realization of R per density (Rs) values ~ 11200 J/kg, orders of magnitude greater than those in most engineering material systems. The high elastic energy storage/release capability of this material, as well as its compatibility with lithographic techniques, makes it an attractive candidate in the design of MEMS devices, which require an ultra-high elastic component for advanced actuation and sensor technologies. Furthermore, an opportunity for tunability of the elastic properties of the SU-8 polymeric material exists with this fabrication technique by varying the number of infiltration cycles or the organometallic precursor Please click Additional Files below to see the full abstract