The formation of carbon nanofibers and thin films from the catalytic decomposition of ethylene by palladium

It has been demonstrated that palladium can be an exceptional catalyst toward the deposition of solid carbon from ethylene in two distinct forms: nanofibers and thin films. Four forms of palladium were tested: sputtered film, foil, sub-micron powder, and nanopowder. The deposition of carbon can be achieved by a very simple method. In this method ethylene and oxygen or hydrogen are flowed through a single-zone, horizontal tube furnace at atmospheric pressure and temperatures typically from 550-700°C. The addition of a secondary gas such as oxygen or hydrogen is vital in driving the deposition. Although both gases improve deposition, the manner in which they do differs. Ethylene-oxygen mixtures are preferred at lower temperatures (i.e. 550°C) than ethylene-hydrogen mixtures (i.e. 700°C). Pd sub-micron was the most prolific form of palladium at producing solid carbon in a combustion environment, whereas nanopowder was in ethylene-hydrogen mixtures. Palladium, of any form, did not catalyze appreciable carbon deposition at any temperature in ethylene alone. These findings suggest that radical species may be imperative to inciting carbon deposition. Independent of the previous finding, it is suggested different mechanisms of growth exist for fibers and thin films. This difference in mechanism is attributed to carbon acting to self-catalyze further deposition. The resulting carbon deposition rate and morphology were found to be a function of temperature, position in the reactor, duration of the reaction, gaseous environment, and form of palladium. These factors were all interconnected, and had to be considered collectively to predict the efficacy of the reaction toward solid carbon production. Crystallinity was found to increase with temperature, and ethylene-hydrogen mixtures produced more crystalline structures than were formed in a combustion environment, however the carbon produced under any conditions tested here was never fully graphitic, and instead was turbostratic or nearly amorphous. Based on the findings of the general catalysis study, the promise of application for the carbon nanofibers was anticipated and demonstrated through the formation of fibrous carbon foams. These foams can be generated using a small quantity of palladium (\u3c5% carbon output by mass), and both the macro- and microscale properties will define the overall properties, and therefore the projected use. These fibrous carbon foams can be combined with other materials to form composites which can be integrated during the formation of the foam. Because the foam process does not require high temperatures, a variety of materials with low melting temperatures can be safely incorporated. Also discussed is the potential of carbon nanofibers as an improved method of polymer reinforcement by tailoring morphology through reaction parameters

Electromagnetic energy transfer and switching in nanoparticle chain arrays below the diffraction limit

Electromagnetic energy transfer in plasmon wires consisting of chains of closely spaced metal nanoparticles can occur below the diffraction limit by means of coupled plasmon modes. Coherent propagation with group velocities that exceed 0.1 c is possible in straight wires and around sharp corners (bending radius much less than wavelength of visible light). Energy transmission through chain networks is possible at high efficiencies and is a strong function of the frequency and polarization direction of the plasmon mode. Although these structures exhibit transmission losses due to heating of about 3 dB/500 nm, they have optical functionality that cannot be obtained in other ways at a length scale ≪1 μm

Electromagnetic energy transport along arrays of closely spaced metal rods as an analogue to plasmonic devices

The transport of electromagnetic energy along structures consisting of arrays of closely spaced metal rods (spacing = 0.2 cm) was investigated in the microwave regime at 8.0 GHz (lambda= 3.7 cm). The dispersion relation shows that information transport occurs at a group velocity of 0.6c. The electromagnetic energy is highly confined to the arrays (90% within a distance of 0.05lambda from the array). The propagation loss in a straight array is 3 dB/8 cm. Routing of energy around 90° corners is possible with a power loss of 3–4 dB. Analogies to plasmon wires consisting of arrays of nm-size metal clusters are discussed

Models for quantitative charge imaging by atomic force microscopy

Two models are presented for quantitative charge imaging with an atomic-force microscope. The first is appropriate for noncontact mode and the second for intermittent contact (tapping) mode imaging. Different forms for the contact force are used to demonstrate that quantitative charge imaging is possible without precise knowledge of the contact interaction. From the models, estimates of the best charge sensitivity of an unbiased standard atomic-force microscope cantilever are found to be on the order of a few electrons

Kinetics governing phase separation of nanostructured Sn_xGe_(1–x) alloys

We have studied the dynamic phenomenon of Sn_xGe_(1–x)/Ge phase separation during deposition by molecular beam epitaxy on Ge(001) substrates. Phase separation leads to the formation of direct band gap semiconductor nanowire arrays embedded in Ge oriented along the [001] growth direction. The effect of strain and composition on the periodicity were decoupled by growth on Ge(001) and partially relaxed Si_yGe_(1–y)/Ge(001) virtual substrates. The experimental results are compared with three linear instability models of strained film growth and find good agreement with only one of the models for phase separation during dynamic growth

Localized charge injection in SiO_2 films containing silicon nanocrystals

An atomic-force microscope (AFM) is used to locally inject, detect, and quantify the amount and location of charge in SiO2 films containing Si nanocrystals (size ~2–6 nm). By comparison with control samples, charge trapping is shown to be due to nanocrystals and not ion-implantation-induced defects in samples containing ion-beam-synthesized Si nanocrystals. Using an electrostatic model and AFM images of charge we have estimated the amount of charge injected in a typical experiment to be a few hundred electrons and the discharge rate to be ~35±15 e/min

Carbon Fiber Foam Composites and Methods for Making the Same

Exemplary embodiments provide methods and apparatus of forming fibrous carbon foams (FCFs). In one embodiment, FCFs can be formed by flowing a fuel rich gas mixture over a catalytic material and components to be encapsulated in a mold to form composite carbon fibers, each composite carbon fiber having a carbon phase grown to encapsulate the component in situ. The composite carbon fibers can be intertwined with one another to form FCFs having a geometry according to the mold

Wafer bonding and layer transfer processes for 4-junction high efficiency solar cells

A four-junction cell design consisting of InGaAs, InGeAsP, GaAs, and Ga0.5In0.5P subcells could reach 1 x AMO efficiencies of 35.4%. but relies on the integration of non-lattice-matched materials. Wafer bonding and layer transfer processes show promise in the fabrication of InP/Si epitaxial templates for growth of the bottom InGaAs and InGaAsP subcells on a Si support substrate. Subsequent wafer bonding and layer transfer of a thin Ge layer onto the lower subcell stack can serve as an epitaxial template for GaAs and Ga0.5In0.5P subcelis. Present results indicate that optically active III/V compound semiconductors can be grown on both Ge/Si and InP/Si heterostructures. Current-voltage electrical characterization of the interfaces of these structures indicates that both InP/Si and Ge/Si interfaces have specific resistances lower than 0.1 Ωcm^2 for heavily doped wafer bonded interfaces, enabling back surface power extraction from the finished cell structure