Role of Catalyst Oxidation State in the Growth of Vertically Aligned Carbon Nanotubes

The impact of gas-phase pretreatment of supported iron-oxide catalyst utilized in aligned carbon nanotube (CNT) growth is studied to understand the correlation between the catalyst oxidation state and the growth characteristics of the aligned CNT forests. By varying the pretreatment conditions from a reducing to an oxidizing environment, notable changes are observed in both the collective CNT array growth behavior and the individual CNT characteristics. Although the greatest catalytic activity was observed following a full reduction to the zerovalent (metallic) Fe catalyst, growth is also observed from a catalyst composed of both Fe₂O₃ and Fe₃O₄ particles. XPS core-level analysis, following pretreatment of the catalyst, emphasizes the critical nature of the combined catalyst–underlayer interaction to achieve optimal catalyst activity during growth and hence the most efficient catalyst reduction process. Additionally, CNT diameters during growth were strongly affected by the pretreatment process. Overall, this work gives a collective picture of how the catalyst oxidation state affects the CNT growth based on the catalyst pretreatment environment and the nature of the catalyst–underlayer interactions. Such concepts are critical for the rational design of alternative catalyst–underlayer systems for efficient CNT synthetic processes

Room Temperature Fabrication of Dielectric Bragg Reflectors Composed of a CaF₂/ZnS Multilayered Coating

We describe the design, fabrication, and characterization of mechanically stable, reproducible, and highly reflecting distributed Bragg reflectors (DBR) composed of thermally evaporated thin films of calcium fluoride (CaF₂) and zinc sulfide (ZnS). CaF₂ and ZnS were chosen as the low and high refractive index components of the multilayer DBR structures, with <i>n</i> = 1.43 and <i>n</i> = 2.38 respectively, because neither material requires substrate heating during the deposition process in order to produce optical quality thin films. DBRs consisting of seven pairs of CaF₂ and ZnS layers, were fabricated with thicknesses of 96 and 58 nm, respectively, as characterized by high-resolution scanning electron microscopy (HR-SEM), and exhibited a center wavelength of λ_c = 550 nm and peak reflectance exceeding 99%. The layers showed good adhesion to each other and to the glass substrate, resulting in mechanically stable DBR coatings. Complete optical microcavities consisting of two such DBR coatings and a CaF₂ spacer layer between them could be fabricated in a single deposition run. Optically, these structures exhibited a resonator quality factor of <i>Q</i> > 160. When a CaF₂/ZnS DBR was grown, without heating the substrate during deposition, on top of a thin film containing the fluorescent dye Rhodamine 6G, the fluorescence intensity showed no degradation compared to an uncoated film, in contrast to a MgF₂/ZnS DBR coating grown with substrate heating which showed a 92% reduction in signal. The ability to fabricate optical quality CaF₂/ZnS DBRs without substrate heating, as introduced here, can therefore enable formation of low-loss high-reflectivity coatings on top of more delicate heat-sensitive materials such as organics and other nanostructured emitters, and hence facilitate the development of nanoemitter-based microcavity device applications

One-Step Synthesis of N‑Doped Graphene Quantum Dots from Chitosan as a Sole Precursor Using Chemical Vapor Deposition

We present a simple, environment-friendly, and fast synthesis of nitrogen-doped graphene quantum dots (N-GQDs) on copper foil by chemical vapor deposition using exclusively chitosan, a cheap and nontoxic biopolymer, as a carbon and nitrogen precursor. We characterized the synthesized N-doped graphene quantum dots using Raman spectroscopy, XPS, AFM, HRTEM, and HRSEM and found them to be in the range 10–15 nm in diameter and 2–5 nm-thick with 4.2% of maximum nitrogen content. The proposed growth mechanism process includes three key steps: (1) decomposition of chitosan into nitrogen-containing compounds, (2) adsorption of reactive species (HCN) on the copper surface, and (3) nucleation to form N-doped graphene quantum dots. The synthesized N-GQDs exhibit photoluminescence (PL) emission in the visible band region, thus making them suitable for applications in nano-optoelectronics

Synthesis of Carbon Nanotubes Networks Grown on Silicon Nanoparticles as Li-Ion Anodes

Using chemical vapor deposition, we grew carbon nanotubes (CNTs) on the surface of Si nanoparticles (NPs) that were coated with a thin iron shell. We studied the CNT growth mechanisms and analyzed the influence of (1) varying annealing times and (2) varying growth times. We show that an initial annealing is necessary to reduce the iron oxide shell and to start the formation of Fe NPs and their consequent coarsening. We characterize the evolution of the catalyst morphology and its influence of the morphology and structure of the CNTs grown. We studied this nanocomposite of Si NPs interconnected by CNTs grown on them as anode material for Li-ion batteries. Compared to the pristine Si NPs, the Si-CNT nanocomposite brings an increase of 40% in specific capacity after 100 cycles at 1800 mA/g_Si with a high stability and a very low capacity loss per cycle of 0.06%. The electrochemical performance demonstrates how efficient the CNT shell on the Si NP is to mitigate the usual failure mechanism of Si NPs. Thus, the in situ growth of CNTs on Si anode materials can be an efficient route toward the synthesis of more stable Si anode composites for a Li-ion battery

Nickel Overlayers Modify Precursor Gases To Pattern Forests of Carbon Nanotubes

We analyzed the effect of nickel overlayers positioned in close proximity (bridges) or in contact (stencils) with the catalytic layer on the growth of vertically aligned carbon nanotubes (VACNTs) using thermal chemical vapor deposition (CVD). We studied the physical–chemical mechanisms, namely, the interaction of the overlayer with the gases and with the catalyst. We demonstrate that nickel inhibits CNT growth by adsorbing carbon to form graphene and by interacting with the gas precursors, leading to their modification into species that do not nucleate and grow CNTs. We demonstrate that the effect of the nickel bridge extends to the length of its boundary layer. We tested overlayer patterns and showed that the patterns were replicated during CNT growth. This facile method is a valid alternative to pattern CNT forests without the need for complex lithography and lift-off of the catalyst in applications where lithographic precision is not required

High Rate of Hydrogen Incorporation in Vertically Aligned Carbon Nanotubes during Initial Stages of Growth Quantified by Elastic Recoil Detection

We quantified the amount of hydrogen in as-grown vertically aligned multiwall CNTs at different stages of growth using elastic recoil detection analysis (ERDA). We suggest that hydrogen is associated with atomic defects and/or amorphous carbon impurities formed at earlier deposition stages. We found that the highest amount of hydrogen (2.3 wt %) was incorporated during the initial growth stage (15–20 s). Our results show a decrease of hydrogen content with increasing deposition time and/or with decreasing growth rate, which points to dynamical self-annealing of hydrogen-saturated defects. Consequently, the decrease of hydrogen-related defects leads to a higher quality of MWCNTs, which can be easily detected by ERDA. This research provides new insight into the nanotube growth mechanism and provides a new characterization approach for quantifying hydrogen-saturated atomic defects in MWCNTs

Millimeter-Tall Carpets of Vertically Aligned Crystalline Carbon Nanotubes Synthesized on Copper Substrates for Electrical Applications

We synthesized millimeter-tall, dense carpets of crystalline CNTs on nonpolished copper substrates with a thin Al₂O₃ (below 10 nm) underlayer and Fe (1.2 nm) layer as a catalyst using chemical vapor deposition (CVD). Preheating of the hydrocarbon precursor gases and in-situ formation of controlled amounts of water vapor were critical process parameters. High-resolution microscopy showed that the CNTs were crystalline with lengths up to a millimeter. Electrical conduction between the CNTs and the copper substrate was demonstrated using multiple methods (probe station, electrodeposition, and hydrolysis of water). Through TEM characterizations of cross sections, we demonstrated that copper diffusion into the alumina layer during the thermal process was the key to explain the observed electrical conductivity. Additionally, the high electrical conductivity of a thermally processed sample compared to the insulating behavior of a pristine sample confirmed the mechanistic hypothesis. Adsorption isotherm measurements showed the mesoporous structure of the vertically aligned carbon nanotubes (VACNTs) with a surface area of 342 m²/g. Electrical conduction and high surface area of this nanostructure make it a promising platform to be functionalized for future battery electrodes