Polarization dependent optical absorption properties of single-walled carbon nanotubes and methodology for the evaluation of their morphology

Polarization dependence of the optical absorption properties of SWNTs is presented and investigated in detail for the energy range 0.5-6 eV. We found that the absorption peaks in the UV region at approximately 4.5 and 5.25 eV exhibit remarkable and different dependencies on the morphology of the SWNT film, or equivalently, on the incident light polarization relative to the SWNT axis. An analytical pathway to evaluate the physical degree of SWNT alignment for a vertically aligned SWNT film is developed with both transition dipoles parallel and perpendicular to the SWNT axis taken into account. This analytical procedure, coupled with polarized optical absorption measurements performed on the vertically aligned SWNT film grown on substrates, leads to the determination of the bare optical absorption cross-section of SWNTs for both parallel and perpendicular to SWNT axis. In the end, the proposed methodology for evaluating the SWNT film morphology is applied to investigate the transient change of the degree of alignment in the growth process of our vertically aligned SWNT films.     

Aligned millimeter-long carbon nanotube arrays grown on single crystal magnesia

Single crystal magnesium oxide (MgO) was found to be very beneficial to the growth of aligned carbon nanotube (CNT) arrays as long as 2.2 mm by chemical vapor deposition. Before growth, a thin film of catalyst (iron) was coated on the MgO by magnetron sputtering. Scanning electron microscopy was used to study the alignment and length, and transmission electron microscopy was used to exam the wall numbers, diameter, and graphitization. It was found that the number of walls as few as two can be controlled by the catalyst film thickness, whereas the length is a combined result of gas pressure, temperature, and time during growth. Water was found not to be a factor to the length of CNTs grown on MgO, but a significant factor when sapphire was used as the substrates.     

Textures of pyrolytic carbon formed in the chemical vapor infiltration of capillaries

Capillaries, 1.1 mm in diameter and 17.0 or 32.5 mm in length, were infiltrated at a temperature of 1100 °C and methane pressures from 5 to 30 kPa. Layer thickness and carbon texture were determined at cross-sections of 2, 16 and 32 mm from the open end of the capillaries using polarized light microscopy. Average deposition rates, determined from layer thickness and infiltration time, as a function of methane pressure indicate a rate increase up to a saturation adsorption at pressures between 10 and 15 kPa (range 1) and a strong rate increase above these pressures (range 2). This result implies carbon formations based on the growth mechanism in range 1 and the nucleation mechanism in range 2. The carbon texture shows a maximum in range 1 and a minimum in the transition from range 1 to range 2 followed by a clear increase in range 2. The maximum in range 1 corresponds to the particle-filler model describing formation of various textures of carbon by the ratio of aromatic species to C₂ species. Increasing texture degrees in range 2 suggest that the nucleation mechanism may lead to high textured carbon provided that the residence time for intramolecular rearrangments of polycyclic aromatic hydrocarbons is sufficient.     

Optimization of methane cold wall chemical vapor deposition for the production of single walled carbon nanotubes and devices

Carbon nanotubes are synthesized by cold wall chemical vapor deposition (CVD) using methane as the carbon source and iron thin film catalyst. The yield of thin nanotubes as determined by scanning electron microscopy (SEM) is strongly dependent on the precise CVD process and the preparation of the substrate. The effects of pressure (5–80 kPa), temperature (700–950 °C), substrate conditioning (air preheat) and metallization (Fe, Al, Mo) on thin nanotube yield are reported. High yields of thin nanotubes are obtained under optimum conditions. These thin nanotubes are candidates to be single walled carbon nanotubes (SWNTs) and Raman spectroscopy, photoluminescence spectroscopy and electrical transport provide evidence that, at least at optimum conditions, many, and perhaps all of the thin nanotubes are single walled. Single nanotube field effect transistors are fabricated and factors affecting device yield are reported. Optimum single nanotube device yield does not necessarily coincide with the optimum nanotube yield.     

Role of systemic T-cells and histopathological aspects after subcutaneous implantation of various carbon nanotubes in mice

We have evaluated the biological responses to four different types of carbon nanotubes (CNTs), by measuring CD4⁺ and CD8⁺ T-cells in peripheral blood, and by the histopathological study on tissues surrounding subcutaneously implanted CNTs for up to 3 months. All mice survived, and no large changes in their weights were observed within our experimental period. After 1 week, only single-walled carbon nanotubes (SWNTs) activated major histocompatibility complex (MHC) class I pathway of antigen-antibody response system (higher CD4⁺/CD8⁺ value), resulting in the appearance of an edematous aspect. After 2 weeks, significantly high values in CD4⁺ and CD4⁺/CD8⁺ without change in CD8⁺ signified an activated MHC class II for all samples. It is worth noting that the toxicological response of CNTs was absolutely lower than that of asbestos. As a result, we envisaged that our result (relatively low toxicity of CNTs) will spur the mass-production, as well widespread application of CNTs in the near future.     

Single- and multi-wall carbon nanotubes produced using the floating catalyst method: Synthesis, purification and hydrogen up-take

The floating catalyst (FC) method for synthesis of single- and multi-wall carbon nanotubes was optimized and scaled up to yield 6 g/h and 20 g/h of products, respectively. Different CNTs purification methods were compared. It was found that the procedure involving room temperature bromination is the most effective to purify the FC-CNTs. The hydrogen up-take capacities of the different products were measured using the quasi-equilibrium volumetric method. It was shown that, at room temperature and gas pressure up to 150 atm for both SWCNTs and MWCNTs, hydrogen up-take does not exceed 1.5 wt.% and is weakly dependent on the product purity.     

Considerable Enhancement of Field Emission of SnO2 Nanowires by Post-Annealing Process in Oxygen at High Temperature

The field emission properties of SnO₂ nanowires fabricated by chemical vapor deposition with metallic catalyst-assistance were investigated. For the as-fabricated SnO₂ nanowires, the turn-on and threshold field were 4.03 and 5.4 V/μm, respectively. Considerable enhancement of field emission of SnO₂ nanowires was obtained by a post-annealing process in oxygen at high temperature. When the SnO₂ nanowires were post-annealed at 1,000 °C in oxygen, the turn-on and threshold field were decreased to 3.77 and 4.4 V/μm, respectively, and the current density was increased to 6.58 from 0.3 mA/cm² at the same applied electric field of 5.0 V/μm.     

An ellipsometric analysis of CVD-diamond films at infrared wavelengths

The ellipsometric measurements of diamond films, which were deposited onto polished [100]-oriented silicon and rough alumina ceramic substrates by hot filament chemical vapor deposition (HFCVD) technique, have been performed over the spectral range from 3 to 12 μm. The parameters of the films, namely, film thickness and volume fraction for each constituent have been calculated from the ellipsometric data by the best fitting procedure using the optimized model. A two-layer stack with about 870-nm thick surface rough layer on top of diamond basis can be perfectly used to simulate the films on silicon substrates. However, the films on alumina substrates cannot be well described by the two-layer model. For the sake of good fit, a three-component interface layer, which consists of 64.13±4% alumina, 23.34±3% diamond and 12.53±1% void, must be appended to the model by Bruggeman effective medium approximation.     

Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation

We discuss growth of high-quality carbon nanotube (CNT) films on bare and microstructured silicon substrates by atmospheric pressure thermal chemical vapor deposition (CVD), from a Mo/Fe/Al₂O₃ catalyst film deposited by entirely electron beam evaporation. High-density films having a tangled morphology and a Raman G/D ratio of at least 20 are grown over a temperature range of 750-900 °C. H₂ is necessary for CNT growth from this catalyst in a CH₄ environment, and at 875 °C the highest yield is obtained from a mixture of 10%/90% H₂/CH₄. We demonstrate for the first time that physical deposition of the catalyst film enables growth of uniform and conformal CNT films on a variety of silicon microstructures, including vertical sidewalls fabricated by reactive ion etching and angled surfaces fabricated by anisotropic wet etching. Our results confirm that adding Mo to Fe promotes high-yield SWNT growth in H₂/CH₄; however, Mo/Fe/Al₂O₃ gives poor-quality multi-walled CNTs (MWNTs) in H₂/C₂H₄. An exceptional yield of vertically-aligned MWNTs grows from only Fe/Al₂O₃ in H₂/C₂H₄. These results emphasize the synergy between the catalyst and gas activity in determining the morphology, yield, and quality of CNTs grown by CVD, and enable direct growth of CNT films in micromachined systems for a variety of applications.     

Alpha particle transient response of a polycrystalline diamond detector

We report the transient response of room temperature pulses generated from alpha particles from a chemical vapor deposition (CVD) polycrystalline diamond detector. For transient signals dominated by electron transport only prompt pulse shapes were observed with an average rise time of 160 ns limited by the preamplifier rise time. For transient signals dominated by hole transport significant slow components were observed in the majority of pulses due to thermal emission of charge from shallow hole traps. These slow pulses were observed from the device when in its as-grown state, without any previous ‘priming’. Two separate slow components were identified from the hole pulses, with average rise times of 600 ns and 1–10 μs, respectively. These data suggest that room temperature electron transport in polycrystalline CVD diamond is prompt, with no evidence for thermal de-trapping. In contrast hole transport in our sample at room temperature contains a significant delayed component due to thermal emission of holes from at least two bands of shallow defects.     

UV Raman characteristics of nanocrystalline diamond films with different grain size

Nanocrystalline diamond films with different size were characterized by ultraviolet (UV) (244 nm) Raman spectroscopy. It was found that a diamond peak at 1333 cm⁻¹ was enhanced, while the D and G peak of graphite as well as photoluminescence was suppressed, compared with that measured by visible (514.5 nm) Raman. With decreasing the particle size from 120 to 28 nm, the diamond peak shifts from 1332.8 to 1329.6 cm⁻¹, the line width of the peak becomes broader, the intensity ratio of diamond and G peak decreases. The down shift and broadening of the diamond peak depending on the particle size by UV Raman measurements are consistent with the phonon confinement model.     

Template synthesis of microporous carbon for direct methanol fuel cell application

Ordered microporous carbon with a structure of amorphous carbon core and graphitic carbon shell was prepared using hydrogen-form zeolite Y as the template. Impregnation and chemical vapor deposition methods were employed to infiltrate carbon in the pores of the template. Physical adsorption of nitrogen, X-ray diffraction, thermogravimetric analysis, field-emission scanning electron microscope, and field-emission transmission electron microscope techniques were employed to study the structural and morphological properties of the samples. The electrochemical properties of Pt supported on the carbon samples were examined and compared with a commercial catalyst. It was observed that Pt catalyst supported on a carbon with a core/shell structure has a higher specific activity for room-temperature methanol oxidation than the commercial catalyst.     

Synthesis of multi-branched porous carbon nanofibers and their application in electrochemical double-layer capacitors

Multi-branched carbon nanofibers with a porous structure have been synthesized on a Cu catalyst doped with Li, Na, or K. The products were characterized by field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. Using this new type of nanofiber as polarized electrodes, an electrochemical double-layer capacitor with a specific capacitance of ca. 297 F/g was obtained using 6 M KOH as the electrolyte.     

Cu-Ni composition gradient for the catalytic synthesis of vertically aligned carbon nanofibers

The influence of catalyst alloy composition on the growth of vertically aligned carbon nanofibers was studied using Cu-Ni thin films. Metals were co-sputtered onto a substrate to form a thin film alloy with a wide compositional gradient, as determined by Auger analysis. Carbon nanofibers were then grown from the gradient catalyst film by plasma enhanced chemical vapor deposition. The alloy composition produced substantial differences in the resulting nanofibers, which varied from branched structures at 81%Ni-19%Cu to high aspect ratio nanocones at 80%Cu-20%Ni. Electron microscopy and spectroscopy techniques also revealed segregation of the initial alloy catalyst particles at certain concentrations.     

Urchin-like nano/micro hybrid anode materials for lithium ion battery

Inspired by a special biological structure in nature, a kind of urchin-like nano/micro hybrid design was proposed to modify conventional micrometer-sized electroactive materials for lithium ion battery (LIB). By catalytic chemical vapor deposition to in situ grow carbon nanofibers on the surface of natural graphite spheres, we fabricated the nano/micro hybrid composite with an urchin-like structure. Scanning electron microscopy (SEM), focused ion beam (FIB) workstation, transmission electron microscopy (TEM), and Raman spectroscopy were employed to characterize the composite. Electrochemical measurements indicated that the cyclability and rate capability of the composite as anode material for LIB were significantly improved. Furthermore, the design also demonstrated its effectiveness in other kinds of anode materials such as transition metal oxides.