Curing Effects on Interfacial Adhesion between Recycled Carbon Fiber and Epoxy Resin Heated by Microwave Irradiation.

The interfacial adhesion of recycled carbon fiber (CF) reinforced epoxy composite heated by microwave (MW) irradiation were investigated by changing the curing state of the epoxy resin. The recycled CF was recovered from the composite, which was prepared by vacuum-assisted resin transfer molding, by thermal degradation at 500 or 600 °C. Thermogravimetric analysis showed that the heating at 600 °C caused rough damage to the CF surface, whereas recycled CF recovered at 500 °C have few defects. The interfacial shear strength (IFSS) between recycled CF and epoxy resin was measured by a single-fiber fragmentation test. The test specimen was heated by MW after mixing the epoxy resin with a curing agent or pre-curing, in order to investigate the curing effects on the matrix resin. The IFSSs of the MW-irradiated samples were significantly varied by the curing state of the epoxy resin and the surface condition of recycled CF, resulting that they were 99.5 to 131.7% of oven heated samples Furthermore, rheological measurements showed that the viscosity and shrinking behaviors of epoxy resin were affected based on the curing state of epoxy resin before MW irradiation.

Determination of the stacking order of curved few-layered graphene systems.

We report a facile method to efficiently visualize the atomic carbon network of curved few-layered graphitic systems including folded bi-layer graphene, nanoribbon edges and multi-walled carbon nanotubes (straight and bent), via the processing of aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) images. This technique is also able to atomically resolve the structure of overlapping graphene layers with different orientations, thus enabling us to determine the stacking order of multiple graphene layers. To the best of our knowledge, we are the first to identify the stacking order of a misoriented 4-layer closed-edge graphene and a metal-semiconductor double-walled carbon nanotube junction.

Chirality-dependent transport in double-walled carbon nanotube assemblies: the role of inner tubes.

A fundamental understanding of the electrical properties of carbon nanotubes is vital when fabricating high-performance polymeric composites as well as transparent conductive films. Herein, the chirality-dependent transport mechanisms in peapod- and chemical vapor deposition-grown double-walled carbon nanotubes (DWNTs) films are discussed by identifying the chiralities of the inner and the outer tubes using fast Fourier transform image processing, as well as optical studies (e.g., Raman, UV, and photoluminescence spectroscopies). The observed conduction mechanisms are strongly dependent on the total fraction of the metallic inner and outer tubes within the DWNT samples. Furthermore, the contribution of the inner tubes to the electronic transport properties of DWNT films is confirmed by photochemically deactivating the outer tubes in both types of DWNT samples.

Photocatalysis-induced selective decoration of semiconducting single walled carbon nanotubes: hole-doping effect.

We have examined the time-dependent effect of the titanium oxide photocatalysis on N-methyl-2-pyrrolidone individually dispersed single walled carbon nanotube (SWNT) suspensions. From optical spectroscopic studies, we found a selective decoration of the semiconducting tubes. Such selectivity is attributed to the preferential attack of the photogenerated active species on the hole-doped semiconducting SWNTs.

Optical spectroscopic studies of thermally coalesced single-walled carbon nanotubes.

Changes in the optical properties of thermally coalesced single-walled carbon nanotubes (SWCNTs) caused by heat treatment between 1300 and 2800 degrees C in argon, have been monitored using optical absorption and photoluminescence spectroscopy. For SWNTs heat treated at 1900 degrees C, we found a complete disapperance of small-diameter tubes (<1.0 nm) as well as the appearance of enlarged, defective tubes. The decreased sp2/sp3 ratio for tubes heat treated below 2100 degrees C suggests that adjacent small-diameter SWNTs transform into energetically stable larger-diameter SWNTs by the presence of structural non sp2 defects.

Sensitive G-band Raman features for the electrical conductivity of multi-walled carbon nanotubes.

We have studied the structural parameters of catalytically grown highly disordered multi-walled carbon nanotubes that were heat treated at temperatures between 1200 degrees C and 2600 degrees C in an argon atmosphere. Rather than the interlayer spacing or the R value (the intensity of the D band divided by the intensity of the G band), we found that the half width at half maximum intensity of the G band was the most sensitive parameter that is correlated with the altered electrical conductivity of an individual carbon nanotube that had been heat treated at high temperatures. This is because one-dimensional nanocarbons exhibit a preference for two-dimensional structural development along the length of the tube due to the limited mobility of carbon atoms along the circumferential direction. Tubes heat treated at 2200 degrees C exhibited both a high electrical conductivity and an absence of lithium-ion intercalation, and thus are the best conductive filler for the active materials of lithium-ion batteries for long-term stability.

Raman and fluorescence spectroscopic studies of a DNA-dispersed double-walled carbon nanotube solution.

We performed resonant Raman/fluorescence spectroscopic studies on double-walled carbon nanotubes (DWNTs) that were dispersed in an aqueous single stranded DNA solution. The luminescence signals from the inner tubes of DWNTs are intensified in the isolated state of each individual DWNT. The completely depressed radial breathing modes (RBMs) associated with the outer tubes (whether semiconducting or metallic) via the mechanical wrapping and the strong charge transfer between DNA and the outer tubes support our interpretation that the bright luminescence and sharp absorption spectra come from only the inner tubes, and not from isolated SWNTs. The circumferentially wrapped DNA on the outer tubes of individually isolated DWNTs in an aqueous solution gives rise to strong charge transfer to the semiconducting and metallic outer tubes as well as to generating physical strain in the outer tubes.

Properties of one-dimensional molybdenum nanowires in a confined environment.

The atomistic mechanism for the self-assembly of molybdenum into one-dimensional metallic nanowires in a confined environment such as a carbon nanotube is investigated using quantum mechanical calculations. We find that Mo does not organize into linear chains but rather prefers to form four atom per unit cell nanowires that consist of a subunit of a Mo body-centered cubic crystal. Our model explains the 0.3 nm separation between features measured by high-resolution transmission electron microscopy and why the nanotube diameter must be in the 0.70-1.0 nm range to accommodate the smallest stable one-dimensional wire. We also computed the electronic band structure of the Mo wires inside a nanotube and found significant hybridization with the nanotube states, thereby explaining the experimentally observed quenching of fluorescence and the damping of the radial breathing modes as well as an increased resistance to oxidation.

Correlation between in Situ Raman scattering and electrical conductance for an individual double-walled carbon nanotube.

In situ Raman scattering is performed on an individual semiconducting double-walled carbon nanotube (DWNT) in a field-effect transistor (FET) geometry, while the transfer characteristics of the DWNT-FET are measured. Through studying the Raman spectra with response to forward and backward gate voltage (V(gs)) sweeping, respectively, we observe hysteresis loops in the curves of G(-) peak frequency and the intensity ratio of G(-) to G(+) (I(G(-))/I(G(+))) as a function of V(gs). These loops correlate very well with the hysteretic transfer characteristics of the device. The clear correlations suggest that G(-) peak line width and I(G(-))/I(G(+)) increase with the carrier concentration in the DWNT induced by V(gs). In addition, unique G(-) peak line width variations with V(gs) can be attributed to interband electron transitions between the energy bands of two concentric shells of the DWNT excited by G phonons.

Transparent and conductive polyethylene oxide film by the introduction of individualized single-walled carbon nanotubes.

It is demonstrated that an optically transparent and electrically conductive polyethylene oxide (PEO) film is fabricated by the introduction of individualized single-walled carbon nanotubes (SWNTs). The incorporated SWNTs in the PEO film sustain their intrinsic electronic and optical properties and, in addition, the intrinsic properties of the polymer matrix are retained. The individualized SWNTs with smaller diameter provide high transmittance as well as good electrical conductivity in PEO films.

Selective optical property modification of double-walled carbon nanotubes by fluorination.

We found that by fluorination of double-walled carbon nanotubes (DWNTs), it is possible to suppress only the Raman radial breathing mode and absorption peaks from the outer (large diameter) tubes of DWNTs. In contrast, Raman signals from the inner shells showed no difference from the pristine DWNTs. The stability of the inner shells of fluorinated DWNTs was also confirmed from the photoluminescence (PL) map and the optical absorption spectra, which only showed the signals from the inner shells of DWNTs, with no distinct change in the optical properties of the inner tubes after fluorination. Our results indicate that once fluorinated, there exists only a weak, if not none, interaction between the inner tube and the outer fluorinated tube, proving that fluorination can be used to suppress the optical properties of carbon nanotubes without interfering the properties of inner tubes. The present finding can be important in electronic and sensor applications, keeping the inner tube from having unwanted contact with other substances that may distract from the inner tube's own characteristics.

Synthesis and isolation of molybdenum atomic wires.

One of the main challenges in nanoscience and nanotechnology consists in the production and isolation of metallic atomic-scale nanowires (Benzryadin, C. N.; Lau, A.; Tinkham, M. Q. Nature 2000, 404, 971-974; Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120-2123; Nilius, N.; Wallis, T. M.; Ho, W. Science 2002, 297, 1853-1856.). Here we report a unique and controllable way of isolating individual atomic molybdenum (Mo) chains by their encapsulation inside double-walled carbon nanotubes, exhibiting inner diameters ranging from 0.6 to 0.8 nm. We have found that these individual atomic chains form spontaneously within the hollow core of tubes in the absence of any reducing agent. We believe that these atomic-scale nanowires could now be studied without suffering oxidation, so that their physical and chemical properties are elucidated.