EWGP--a work in progress....

The need for energy affects almost every aspect of modern society. Indeed, the advent of coal as a widely-used energy source is sometimes viewed as the spark that started the Industrial Revolution. However, we are about to enter a new era due to increased competition from developing nations for the world\u27s dwindling energy supplies and to the growing recognition that our current energy usage is unsustainable and is affecting the world\u27s climate. It is apparent that research on energy-related issues will become increasingly important in the coming years

Carbide-Derived Carbons with Tunable Porosity Optimized for Hydrogen Storage

Relevance: Improvements in gravimetric and volumetric capacity were realized by processes which increase pore volume, heat of adsorption and powder density. Volumetric capacity was more than doubled by rolling peels with PTFE binder and pellet pressing. Even larger gains may be achieved with bulk precursors. Approach: A suite of post-processing strategies were developed and optimized for specific precursors. Technical Accomplishments and Progress: Excess H2 adsorption over 4.3 wt.% and 0.034 kg/L was demonstrated in as-produced CDC having a moderate SSA and pore volume @ (77K, 55 atm). Max heat of H2 adsorption up to 11 kJ/mol (with average values ~ 8 kJ/mol) demonstrated. Proposed Future Research: Further science-based modification of CDC porosity, microstructure and chemistry for improved H2uptake

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

Metal-Insulator Transition in Doped Single-Wall Carbon Nanotubes

We find strong evidence for a metal-insulator (MI) transition in macroscopic single wall carbon nanotube conductors. This is revealed by systematic measurements of resistivity and transverse magnetoresistance (MR) in the ranges 1.9-300 K and 0-9 Tesla, as a function of p-type redox doping. Strongly H2SO4-doped samples exhibit small negative MR, and the resistivity is low and only weakly temperature dependent. Stepwise de-doping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with ρ(T) obeying power-law temperature dependence, ρ(T) ∝ T -β. In the insulating regime (high annealing temperatures) the ρ(T) behavior ranges from Mott-like 3-dimensional (3D) variable-range hopping (VRH), ρ(T) ∝ exp[(-T0/T)-1/4], to Coulomb-gap (CGVRH) behavior, ρ(T) ∝ exp[(-T0/T)-1/2]. Concurrently, MR(B) becomes positive for large B, exhibiting a minimum at magnetic field Bmin. The temperature dependence of Bmin can be characterized by Bmin(T) = Bc(1 - T/Tc) for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature Tc, MR(B) is positive for all B. The observed changes in transport properties are explained by the effect of doping on semiconducting SWNTs and tube-tube coupling

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

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