Laser Irradiation-Hindered Growth of Small-Diameter Single-Walled Carbon Nanotubes by Chemical Vapor Deposition

SWNTs are synthesized on a Co/MgO catalyst using “laser-disturbed” CVD with CO as the carbon source. Compared with SWNTs grown by thermal CVD without laser irradiation (normal CVD), SWNTs synthesized under laser irradiation demonstrate the suppression of small-diameter SWNT growth, as indicated by Raman spectroscopy. Such a phenomenon is also observed for other supported catalysts, such as Co/SiO2 and Fe/MgO. Controlled experiments were carried out to clarify the effects of lasers. On the one hand, laser irradiation increases the reaction temperature locally, favoring the growth of SWNTs at a set temperature as low as 350°C. On the other hand, laser irradiation inhibits the nucleation of small SWNT caps, leading to the growth of large-diameter SWNT species. This work opens a new avenue for growing SWNTs with controlled diameters.

Catalyst discovery through megalibraries of nanomaterials

The nanomaterial landscape is so vast that a high-throughput combinatorial approach is required to understand structure–function relationships. To address this challenge, an approach for the synthesis and screening of megalibraries of unique nanoscale features (>10,000,000) with tailorable location, size, and composition has been developed. Polymer pen lithography, a parallel lithographic technique, is combined with an ink spray-coating method to create pen arrays, where each pen has a different but deliberately chosen quantity and composition of ink. With this technique, gradients of Au-Cu bimetallic nanoparticles have been synthesized and then screened for activity by in situ Raman spectroscopy with respect to single-walled carbon nanotube (SWNT) growth. Au3Cu, a composition not previously known to catalyze SWNT growth, has been identified as the most active composition.

Chemical vapor deposition synthesis of near-zigzag single-walled carbon nanotubes with stable tube-catalyst interface

Chemical vapor deposition (CVD) growth is regarded as the most promising method for realizing structure-specific single-walled carbon nanotube (SWNT) growth. In the past 20 years, many efforts dedicated to chirality-selective SWNT growth using various strategies have been reported. However, normal CVD growth under constant conditions could not fully optimize the chirality because the randomly formed cap structure allows the nucleation of all types of SWNTs and the chirality of an SWNT is unlikely to be changed during the following elongation process. We report a new CVD process that allows temperature to be periodically changed to vary SWNT chirality multiple times during elongation to build up the energetically preferred SWNT-catalyst interface. With this strategy, SWNTs with small helix angles (less than 10°), which are predicted to have lower interfacial formation energy than others, are enriched up to ~72%. Kinetic analysis of the process suggests a multiple redistribution feature whereby a large chiral angle SWNT tends to reach the near-zigzag chirality step by step with a small chiral angle change at each step, and hence, we named this method “tandem plate CVD.” This method opens a door to synthesizing chirality-selective SWNTs by rational catalyst design.

CoPt/CeO2 catalysts for the growth of narrow diameter semiconducting single-walled carbon nanotubes

The selective growth of narrow diameter distributed semiconducting SWNTs using CoPt/CeO2 catalysts has been achieved by dispersing the catalysts onto substrates, followed by oxidation and sintering in a general CVD system for specific SWNT growth.

Support Vector Machine Classification of Single Walled Carbon Nanotube Growth Parameters

ABSTRACTSelective single-walled carbon nanotube (SWNT) growth is a challenging problem, limiting their use in a wide variety of applications. Significant degrees of freedom in these experiments may lead to synthesis of multi-walled carbon nanotubes (MWNTs), which are less preferred. Thus, a method for constraining the synthesis results to only SWNTs is desired. A machine learning based approach for selectively growing SWNTs using a laser-induced chemical vapor deposition growth system is introduced. This approach models the complex relationships between the associated synthesis parameters to predict SWNT growth. The parameters under consideration include argon, ethylene, hydrogen and carbon dioxide partial pressures, growth temperature, and water vapor concentration. The catalyst consists of 10 nm of alumina and 1 nm of nickel deposited onto 10 µm diameter silicon pillars with a height of 10 µm. Determination of SWNT growth is performed through in-situ Raman spectroscopy using a 532 nm excitation laser. A total of 121 experiments are used to train a SWNT vs. MWNT classifier with a resulting model accuracy of 94.21%. The classifier model is applied to a range of simulated inputs, and the subset of these inputs that meet a >90% probability of SWNT growth are investigated further. The simulated inputs consist of 531,201,645 unique growth parameter combinations spanning the entire parameter space. A reduced dataset of 449,117 growth parameter combinations define 90% probability of SWNT growth according to the model. Randomly selected input parameters from this reduced dataset were tested experimentally, resulting in SWNT growth for all performed experiments validating the classifier model. This approach maps input growth conditions to SWNT growth selectivity using a limited set of experimental data and allows for further investigation into SWNT growth rates and chiral dependencies.

Impact of Thermodiffusion on Carbon Nanotube Growth by Chemical Vapor Deposition

Thermal diffusion, the process by which a multicomponent mixture develops a concentration gradient when exposed to a temperature gradient, has been studied in order to understand if its inclusion is warranted in the modeling of single-wall carbon nanotubes (SWNTs) synthesis by thermal chemical vapor deposition (CVD). A fully coupled reactor-scale model employing conservation of mass, momentum, species, and energy equations with detailed gas phase and surface reaction mechanisms has been utilized to describe the evolution of hydrogen and hydrocarbon feed streams as they undergo transport, as well as homogeneous and heterogeneous chemical reaction within a CVD reactor. Steady state velocity, temperature, and concentration fields within the reactor volume are determined, as well as concentrations of adsorbed species and SWNT growth rates. The effect of thermodiffusion in differing reactor conditions has been investigated to understand the impact on SWNT growth. Thermal diffusion can have a significant impact on SWNT growth, and the first approximation of the thermal diffusion factor, based on the Chapman–Enskog molecular theory, is sufficient for modeling thermophoretic behavior within a CVD reactor. This effect can be facilitatory or inhibitory, based on the thermal and mass flux conditions. The results of this investigation are useful in order to optimize model and reactor designs to promote optimal SWNT deposition rates.

Growth of Single-Walled Carbon Nanotubes at Low Temperature and Low Pressure CVD Conditions

Controlling the detailed structures of single-walled carbon nanotubes (SWNTs) is imperative for realizing many SWNT applications, and understanding the SWNT growth mechanism is important to improve the growth techniques. In the present study, we performed SWNT growth by a catalytic chemical vapor deposition (CVD) method in wide temperature and pressure ranges, using a high-vacuum CVD chamber. We focused on low CVD gas pressure and low temperature conditions and investigated the SWNT growth mechanism. SWNTs were synthesized by using ethanol gas as the carbon source. As the catalyst, Co and Mo metal nano-particles deposited onto silicon substrates were used. SWNTs were grown via the reaction between ethanol gas and the catalytic metal nano-particles at high temperature. The ethanol gas pressure ranged from 10−3 Pa to 102 Pa, and the CVD temperature ranged from 400 to 900 °C. The yield of SWNTs was assumed to be proportional to the G-band intensity, which was measured by Raman scattering spectroscopy. SWNT samples were observed by scanning electron microscopy and transmission electron microscopy. An optimum CVD temperature existed for each ethanol gas pressure, and decreased with decreasing ethanol gas pressure. Moreover, SWNTs were grown even at 500 °C, when the ethanol gas pressure was low (less than 10−2 Pa). In this study, the minimum temperature and pressure at which SWNTs could be grown were 450 °C and 10−3 Pa. At low temperature and low CVD gas pressure, the activity of the catalyst and the growth rate of SWNTs were low, while the catalyst lifetime was long.

Kinetic Modeling of the SWNT Growth by CO Disproportionation on CoMo Catalysts

A kinetic model has been developed to describe the growth of single-walled carbon nanotubes (SWNT) in the CoMoCAT™ method, which is based on the disproportionation of CO on supported CoMo catalysts. The model attempts to capture mathematically the different stages involved in this method: (i) catalyst activation or in-situ creation of active sites, i.e., reduced Co clusters by transformation of CoMoOx precursor species, or oxidized sites; (ii) CO decomposition over active sites, which increases the surface fugacity of carbon until reaching a certain threshold; (iii) nucleation of ordered forms of carbon; (iv) C diffusion (both across the surface and into the metal particle); (v) SWNT growth; (vi) termination, by either deactivation of the catalyst active sites or by increase in the carbon concentration at the metal/SWNT interface, approaching that of the metal/gas interface and eliminating the driving force for diffusion. Previous investigations have only explained the growth termination by the former. Here, we emphasize the possible contribution of the later and propose a novel "hindrance factor" to quantify the effect of nanotube interaction with its surroundings on the growth termination. To test the kinetic model and obtain typical values of the physical parameters, experiments have been conducted on a CoMo/SiO2 catalyst in a laboratory flow reactor, in which the rate of carbon deposition was continuously evaluated by the direct measurement of the CO2 evolution as a function of time. The experimental data are fitted very well with model.