Free-standing, interconnected metallic nanowire networks with densities as low as 40 mg/cm3 have been achieved over centimeter-scale areas, using electrodeposition into polycarbonate membranes that have been ion-tracked at multiple angles. Networks of interconnected magnetic nanowires further provide an exciting platform to explore 3-dimensional nanomagnetism, where their structure, topology, and frustration may be used as additional degrees of freedom to tailor the materials properties. New magnetization reversal mechanisms in cobalt networks are captured by the first-order reversal curve method, which demonstrate the evolution from strong demagnetizing dipolar interactions to intersection-mediated domain wall pinning and propagation, and eventually to shape-anisotropy dominated magnetization reversal. These findings open up new possibilities for 3-dimensional integrated magnetic devices for memory, complex computation, and neuromorphics.

Because of their outstanding mechanical properties, carbon nanotubes (CNTs) are attractive 1-D nanoscale building blocks for lightweight composites that could potentially outperform naturally occurring materials. A still unsolved challenge for fully exploiting CNT superior mechanical properties for reinforcement of macroscopic systems is control of the interfacial interaction across multiple length scales to favor load transfer. Toward overcoming this challenge, we investigate here the effect of high-energy He+ ion irradiation on the elastic modulus and tensile strength of thin films of randomly oriented double-walled nanotube (DWCNT) mats and DWCNT-epoxy composites. We correlate irradiation-induced reinforcement with the formation of cross-links at different hierarchical levels of the DWCNT network. Our measurements reveal a rapidly increasing reinforcement at low ion doses, attributed to intra-bundle CNT cross-links, followed by a slow mechanical reinforcement at higher doses, associated to inter-bundle cross-links. At the highest ion irradiation dose of this study, the strength and elastic modulus of DWCNT-epoxy composites approach similar to 900 MPa and 25 GPa, respectively, which is among the best mechanical performances reported for randomly-oriented CNT mats. We also develop a model accounting for radiation-induced hierarchical reinforcement of DWCNT films. The model is in good agreement with the experimental results across the entire dose range. (C) 2015 Elsevier Ltd. All rights reserved.

We study the propagation of high-power laser beams in micro-structured carbon foams by monitoring the x-ray output from deliberately introduced Cu content. In particular, we characterize this phenomenon measuring absolute time-resolved x-ray yields, time-resolved x-ray imaging, and x-ray spectroscopy. New experimental results for C–Cu foams show a faster heat front velocity than simulation that assumed homogeneous plasma. We suggest the foam micro-structure may explain this trend.

High energy light-ion (3.8MeV He) bombardment is used to introduce lattice defects in a 3-dimensional (3D) interconnected network of graphene aerogels (GAs). When these materials are used as anodes for lithium ion batteries, we observe improved percentage reversible capacity and cycle stability compared to those without ion-beam treatment. Furthermore, all ion-beam treated 3D graphene samples exhibit substantially higher Coulombic efficiencies, suggesting at beneficial role of vacancy-type defects in stabilizing solid-electrolyte interphases. Although 3D graphene exhibits initial reversible capacities that are 2–3 times higher than that of graphite (∼372mAh/g), fast capacity fading is observed but becomes more stable after ion-beam treatment. Our experimental results demonstrate that ion-beam treatment is an effective route to tune and produce good-performance graphene electrodes, and that vacancy-type defects help to promote reversible lithium storage capacity in graphene. We further observe that 3D GAs irradiated to the highest dose studied (1016cm−2) fail rapidly upon electrochemical cycling, likely caused by the excessive ion-beam damage and graphene restacking. Raman I(D)/I(D′) signature is considered linked to defect type in graphene and thus is proposed, for the first time, as an indicator of the reversible capacity for GAs.

Atomic hydrogen exists ubiquitously in graphene materials made by chemical methods. Yet determining the effect of hydrogen on the electrochemical performance of graphene remains a significant challenge. Here we report the experimental observations of high rate capacity in hydrogen-treated 3-dimensional (3D) graphene nanofoam electrodes for lithium ion batteries. Structural and electronic characterization suggests that defect sites and hydrogen play synergistic roles in disrupting sp2 graphene to facilitate fast lithium transport and reversible surface binding, as evidenced by the fast charge-transfer kinetics and increased capacitive contribution in hydrogen-treated 3D graphene. In concert with experiments, multiscale calculations reveal that defect complexes in graphene are prerequisite for low-temperature hydrogenation, and that the hydrogenation of defective or functionalized sites at strained domain boundaries plays a beneficial role in improving rate capacity by opening gaps to facilitate easier Li penetration. Additional reversible capacity is provided by enhanced lithium binding near hydrogen-terminated edge sites. These findings provide qualitative insights in helping the design of graphene-based materials for high-power electrodes.

Controlled synthesis of porous metal-carbon nanocomposites is challenging. Here, we describe the fabrication of porous Cu-C nanocomposites by water-ice templating in an aqueous CuSO4 solution with a suspension of graphene oxide and carbon nanotubes. Upon the crystallization of water, the salt and carbon constituents segregate at grain boundaries of water crystallites. Freeze drying (vacuum desiccation) is used to remove water, creating pores. Final Cu-C composites are obtained by thermally-induced reduction of graphene oxide and decomposition of CuSO4. Pore walls in resultant Cu-C foams consist of a random network of reduced graphene oxide sheets and carbon nanotubes decorated with nearly spherical Cu nanoparticles with average diameters tunable in the range of similar to 30-80 nm. With this approach, we demonstrate nanoporous Cu-C composites with an ultralow C matrix density of >= 5 mg cm(-3) and controllable Cu loading in the range of similar to 0-65 wt%. This versatile method could be extended to fabricate other nanoporous metal-carbon composites geared for specific applications. As an example, we demonstrate Ag-C nanocomposites by using AgNO3 instead of CuSO4.

Low-density foams irradiated by a 20 kJ laser at the Omega laser facility (Laboratory for Laser Energetics, Rochester, NY, USA) are shown to convert more than 5% of the laser energy into 4.6 to 6.0 keV x rays. This record efficiency with foam targets is due to novel fabrication techniques based on atomic-layer-deposition of Ti atoms on an aerogel scaffold. A Ti concentration of 33 at. % was obtained in a foam with a total density of 5 mg/cm3. The dynamics of the ionization front through these foams were investigated at the 1 kJ laser of the Gekko XII facility (Institute for Laser Engineering, Osaka, Japan). Hydrodynamic simulations can reproduce the average electron temperature but fail to predict accurately the heat front velocity in the foam. This discrepancy is shown to be unrelated to the possible water adsorbed in the foam but could be attributed to effects of the foam micro-structure.

Realization of macroscale three‐dimensional isotropic carbons that retain the exceptional electrical and mechanical properties of graphene sheets remains a challenge. Here, a method for fabricating graphene‐derived carbons (GDCs) with isotropic properties approaching those of individual graphene sheets is reported. This synthesis scheme relies on direct cross‐linking of graphene sheets via the functional groups in graphene oxide to maximize electronic transport and mechanical reinforcement between sheets and the partial restacking of the sheets to increase the material density to about 1 g cm‐3. These GDCs exhibit properties 3–6 orders of magnitude higher than previously reported 3D graphene assemblies.

Current synthesis methods of nanoporous Cu-C composites offer limited control of the material composition, structure, and properties, particularly for large Cu loadings of greater than or similar to 20 wt%. Here, we describe two related approaches to realize novel nanoporous Cu-C composites based on the templating of recently developed carbon-nanotube aerogels (CNT-CAs). Our first approach involves the trapping of Cu nanoparticles while CNT-CAs undergo gelation. This method yields nanofoams with relatively high densities of greater than or similar to 65 mg cm(-3) for Cu loadings of greater than or similar to 10 wt%. Our second approach overcomes this limitation by filling the pores of undoped CNT-CA monoliths with an aqueous solution of CuSO4 followed by (i) freeze-drying to remove water and (ii) thermal decomposition of CuSO4. With this approach, we demonstrate Cu-C composites with a C matrix density of similar to 25 mg cm(-3) and Cu loadings of up to 70 wt %. These versatile methods could be extended to fabricate other nanoporous metal-carbon composite materials geared for specific applications.

We report the near through mid-infrared (MIR) optical absorption spectra, over the range 0.05–1.3 eV, of monocrystalline silicon layers hyperdoped with chalcogen atoms synthesized by ion implantation followed by pulsed laser melting. A broad mid-infrared optical absorption band emerges, peaking near 0.5 eV for sulfur and selenium and 0.3 eV for tellurium hyperdoped samples. Its strength and width increase with impurity concentration. Its strength decreases markedly with subsequent thermal annealing. The emergence of a broad MIR absorption band is consistent with the formation of an impurity band from isolated deep donor levels as the concentration of chalcogen atoms in metastable local configurations increases.

We study the effect of 2 MeV Xe ion bombardment at room temperature on the microstructure and mechanical properties of cross-linked carbon-nanotube-based nanoporous carbons. Irradiation causes a gradual densification and a decrease in the surface roughness of monoliths, an increase in Young’s modulus and failure stress, a decrease in the failure strain, and smoothening of nanoligament surfaces. Our results demonstrate a potential of heavy-ion bombardment for a controlled modification of nanoporous carbons.

We investigate the possibility of creating an intermediate band semiconductor by supersaturating Si with a range of transition metals (Au, Co, Cr, Cu, Fe, Pd, Pt, W, and Zn) using ion implantation followed by pulsed laser melting (PLM). Structural characterization shows evidence of either surface segregation or cellular breakdown in all transition metals investigated, preventing the formation of high supersaturations. However, concentration-depth profiling reveals that regions of Si supersaturated with Au and Zn are formed below the regions of cellular breakdown. Fits to the concentration-depth profile are used to estimate the diffusive speeds, vD, of Au and Zn, and put lower bounds on vD of the other metals ranging from 102 to 104 m/s. Knowledge of vD is used to tailor the irradiation conditions and synthesize single-crystal Si supersaturated with 1019 Au/cm3 without cellular breakdown. Values of vD are compared to those for other elements in Si. Two independent thermophysical properties, the solute diffusivity at the melting temperature, Ds(Tm), and the equilibrium partition coefficient, ke, are shown to simultaneously affect vD. We demonstrate a correlation between vD and the ratio Ds(Tm)/ke0.67, which is exhibited for Group III, IV, and V solutes but not for the transition metals investigated. Nevertheless, comparison with experimental results suggests that Ds(Tm)/ke0.67 might serve as a metric for evaluating the potential to supersaturate Si with transition metals by PLM.

Radiation-generated point defects in solids often experience dynamic annealing-diffusion and interaction processes after the thermalization of collision cascades. The length scale of dynamic annealing can be described in terms of the characteristic defect diffusion length (Ld). Here, we propose to measure Ld by a pulsed beam method. Our approach is based on the observation of enhanced defect production when, for individual ion pulses, the average separation between adjacent damage regions is smaller than Ld. We obtain a value for Ld of ~30 nm for float-zone Si crystals bombarded at room temperature with 500 keV Ar ions.

Room-temperature heavy-ion bombardment of polar (0001) ZnO leads to the formation of intermediate peak and step features in damage–depth profiles measured by ion channeling. Here, we show that these anomalous disorder effects are strongly suppressed for crystals with (112¯0) and (101¯0) non-polar surface terminations. Possible defect interaction scenarios responsible for the enhanced radiation tolerance of non-polar-terminated ZnO are discussed.