Single-walled Carbon Nanotube Forests Research Articles

Addressing the Trade-Off between Crystallinity and Yield in Single-Walled Carbon Nanotube Forest Synthesis Using Machine Learning.

Synthetic trade-offs exist in the synthesis of single-walled carbon nanotube (SWCNT) forests, as growing certain desired properties can often come at the expense of other desirable characteristics such as the case of crystallinity and growth efficiency. Simultaneously achieving mutually exclusive properties in the growth of SWCNT forests is a significant accomplishment, as it requires overcoming these trade-offs and balancing competing mechanisms. To address this, we trained a machine-learning regression model with a set of 585 "real" experimental synthesis data, which were taken using an automatic synthesis reactor. Subsequently, 16000 exploratory "virtual" experiments were performed by our trained model to examine potential routes toward addressing the current crystallinity-height trade-off limitation, and suggestions on growth conditions were predicted. Importantly, additional validation using "real" experimental syntheses showed good agreement with the predictions as well as a 48% increase in growth efficiency while maintaining the high crystallinity (G/D-ratio). This highlighted the effectiveness and accuracy of the predictive capability of our machine-learning model, which achieved improved results in less than 50 validation tests. Furthermore, the trained model revealed the surprising importance of the nature of the carbon feedstock, particularly the reactivity and concentration, as a route for overcoming the trade-off between the SWCNT crystallinity and growth efficiency. These results of the high-efficiency synthesis of highly crystalline SWCNT forests represent a significant advance in overcoming synthetic trade-off barriers for complex multivariable systems.

Dynamic analysis of multiple-nanobeam-systems acted upon by multiple moving nanoparticles accounting for nonlocality, lag, and lateral inertia

This paper deals with nonlocal transverse vibrations of vertically aligned single-layered membranes from single-walled carbon nanotubes (SWCNTs) for transporting of nanoparticles. The consisting nanotubes are modeled via classical and shear deformable beams, while the moving nanoparticles is assumed as rigid-in contact-particles with dissimilar velocities accounting for their weight force, inertia, and lags. The nonlocal equations of motion are derived by considering the intertube van der Waals (vdW) forces and then solved for elasto-dynamic fields methodically. For two decorations of the nanosystem, the effects of inertia, shear deformation, nonlocality, velocities and lags of moving nanoparticles on the dynamic response of the nanosystem are explained in some detail. Among various scenarios of loadings, lags, and velocities of moving nanoparticles, the most critical case is introduced and discussed. By increasing the slenderness ratio of nanotubes, the role of shear deformation diminishes and the results of various models approach each other. Further, the influence of vdW forces on the dynamic deflections reduces and the discrepancies between the results of two understudy nanosystem’s patterns would reduce. The newly developed models in the present study can be employed for modeling of more complex nanosystems like as vertically aligned forests of SWCNTs for delivery of nanoparticles with different lags and velocities.

Thermal properties of single-walled carbon nanotube forests with various volume fractions

Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data.

Improving the synthetic efficiency of single-wall carbon nanotube forests using a gas-analysis-designed mixed carbon feedstock

A high efficiency, mixed gas carbon feedstock was designed by gas analysis of ethylene-based single-wall carbon nanotube (SWCNT) chemical vapor deposition. Gas analysis identified acetylene and 1,3-butadiene as the primary contributors to CNT growth based on the observed correlation between the decrease in respective concentrations and the CNT growth rate. In addition, a semi-empirical chemical kinetics growth model was developed which further verified that the experimentally observed decrease of acetylene and 1,3-butadiene were due to their contribution to the synthesis reaction. Using these results, we designed a mixed carbon feedstock of acetylene and 1,3-butadiene at a ratio of 2:5 which efficiently grew high purity and selectivity SWCNTs at a high growth efficiency, four-times that of pure ethylene and two-times that of pure acetylene. The combination of acetylene and 1,3-butadiene proved synergetic due to the inherent differences in reactivity and hydrogen content through “carbon scavenging” by the hydrogen byproducts. As a result, unlike single-source carbon feedstocks, the total carbon feedstock concentration could be increased, which resulted in increasing yields without saturating the catalysts. Our results demonstrate the complementary roles of carbon feedstocks in developing high efficiency synthesis, which is important for the advancement of the synthesis of long and clean CNTs.

Large Area, High-Density SWCNT Forests with Remarkably Invariant Structural Properties over a Wide Range of Synthesis Conditions

Driven by the exceptional thermal, electrical, optical, mechanical and fluidic properties of single-walled carbon nanotubes (SWCNT), commercial use of vertically aligned “forests” are on the horizon for applications1 ranging from optical absorbers and thermal interfaces to selective membranes and advanced fabrics.2,3 Large-area SWCNT forests with high densities and small diameters are especially important for many applications, yet they are conspicuously absent from the literature due to synthesis challenges.Here, we demonstrate synthesis of high-density (> 1012 CNTs/cm2), small-diameter (~ 2 nm) SWCNT forests with structural characteristics that remain uniform over large areas (up to 6-in. diameter)4 and, surprisingly, largely unchanged over a wide range of synthesis conditions. We find also that our growth regime enables a record-high carbon conversion efficiency and a stable mass kinetics, both of which are invariant with catalyst type (Fe vs Fe/Mo mixes). To identify the limiting steps in the SWCNT growth kinetics and the factors contributing to the process robustness, we determine the dependence of the reaction rate and carbon conversion efficiency on broad ranges of synthesis parameters. Effects of catalyst area (from 1 × 1 cm2 to 6-in wafers), growth pressure (10-750 mbar), gas composition (30-fold acetylene partial pressure variation), and flowrates are presented.Our demonstration of such exceptionally robust and reproducible synthesis of high-density forests over large areas provides valuable insights for future scale-up efforts of SWCNT forest production that are necessary for widespread adoption of advanced CNT technologies. 1. R. Rao et al., ACS Nano., 12 (2018) 11756.2. N. Bui, E. R. Meshot, S. Kim, J. Peña, P. W. Gibson, K. J. Wu, F. Fornasiero, Adv. Mater., 28 (2016) 5871.3. Y. Li, C. Chen, E. R. Meshot, S. F. Buchsbaum, M. Herbert, R. Zhu, O. Kulikov, B McDonald, N. Bui, M. L. Jue, S. Park, C. Valdez, S. Hok, C. J. Doona, K. J. Wu, T. M. Swager, F. Fornasiero, submitted (2019)4. E. R. Meshot, S. Park, S. F. Buchsbaum, M. L. Jue, T. R. Kuykendall, E. Schaible, L. B. Bayu Aji, S. Kucheyev, K. J. Wu, F. Fornasiero, submitted (2019).This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-797268

Shape-dependent infrared reflectance properties of CNT forest metamaterial arrays.

In this work, shape-dependent mid-infrared properties of novel split ring resonator (SRR) metamaterials composed of single-walled carbon nanotube (CNT) forest are investigated. The introduction of the gap and dip shape to the closed ring geometry reduced the total reflectance by 15%, due to the generation of circular currents and LC resonances in SRRs. The increase of the SRR height reduced the total IR reflectance by 25%. Unique one-dimensional anisotropic electric and photonic properties of CNTs, combined with an artificial refractive index induced in SRR circuits, will stimulate the development of new optoelectronics applications.

High-yield growth kinetics and spatial mapping of single-walled carbon nanotube forests at wafer scale

Emerging commercial applications of vertically aligned, single-walled carbon nanotube (SWCNT) “forests” require synthesis that minimizes nanotube diameter while maximizing number density across substrate areas exceeding centimeter scale. To address this need, we synthesized SWCNT forests on full silicon wafers with notable reproducibility and uniformity, and co-optimized growth for small diameters and high densities across large areas to access new territory in this 3D parameter space. We mapped the spatial uniformity of key structural features using Raman microscopy, synchrotron X-ray scattering, and Rutherford backscattering spectrometry. Low C2H2 flux over sub-nm Fe/Mo catalysts produced small-diameter SWCNTs (2.1 nm) at high number densities (2.26 × 1012 cm−2) on wafers up to 6 in. Although removing Mo resulted in larger SWCNT diameters and lower densities (<0.7 × 1012 cm−2), mass conversion rates from C2H2 to SWCNT product were high and remarkably invariant for catalyst compositions and densities (i.e., 47.7% or 1.30 × 106% g-catalyst−1 on 4-in. wafers). These carbon conversion efficiencies far exceed typical benchtop reactors and are on par with the best reported literature values. Our detailed elucidation of correlations among structural characteristics within this resource-efficient process is expected to guide future scale-up efforts of SWCNT forest growth beyond wafer scale.

Gd-Enhanced Growth of Multi-Millimeter-Tall Forests of Single-Wall Carbon Nanotubes.

Multi-millimeter-tall vertically aligned single-wall carbon nanotube (VA-SWCNT) forests were grown using Fe/Gd/Al2Ox catalyst with high initial growth rate of ∼2 μm s-1 and long catalyst lifetime of ∼70 min at 800 °C. The addition of Gd with a nominal thickness of 0.3 nm drastically prolonged the catalyst lifetime. The analysis of the VA-SWCNT forests by a transmission electron microscope showed that the average diameter of the SWCNTs grown with Gd is constant from the top to the bottom of the forests, while it monotonically increased without Gd. This indicated that Gd suppresses the structure change of the Fe nanoparticles in the lateral direction during the CNT growth. By X-ray photoelectron spectroscopy, it was found that the longer catalyst lifetime with Gd stems from the suppression of the interaction between Fe and C resulting in the smaller structure change of the Fe nanoparticles.

Correlation between the Carbon Nanotube Growth Rate and Byproducts in Antenna-Type Remote Plasma Chemical Vapor Deposition Observed by Vacuum Ultraviolet Absorption Spectroscopy.

For sp2 or sp3 carbon material growth, it is important to investigate the precursors or intermediates just before growth. In this study, the density of ethylene (C2 H4 ) outside the plasma discharge space and just before reaching the carbon nanotube (CNT) growth region is investigated by vacuum ultraviolet absorption spectroscopy for plasma discharge in an antenna-type remote plasma chemical vapor deposition with a CH4 /H2 system, with which the growth of very long (≈0.5 cm) CNT forests is achieved. Single-wall CNT forests have the potential for application as electrodes in battery cells, vertical wiring for high current applications, and thermal interface materials. It is observed that the plasma discharge decomposes the CH4 source gas and forms C2 Hx species, which reversibly reform to C2 H4 in the plasma-off state. In addition, the density of the formed C2 H4 has a strong correlation to the CNT growth rate. Therefore, the C2 H4 density is a good indicator of the density of C2 Hx species for CNT growth in the CH4 /H2 plasma system.

Synthesis and Metrology for Large-Area Applications of Single-Wall Carbon Nanotube Forests

Commercial applications of vertically aligned single-wall carbon nanotube (SWCNT) “forests” are on the horizon, with the emergence of their use in nanocomposite membranes,1 electronic interconnects, thermal interface materials, among other promising demonstrations. To realize these potential applications, we sought to not only design reproducible wafer-scale synthesis of high-density SWCNT forests but also to establish advanced metrology to enable quantitative, large-area mapping of several key forest characteristics. This detailed information will unlock better understanding of interdependent correlations among the characteristics and how these correlations inform SWCNT forest growth mechanisms toward enhanced synthetic control. We performed low-pressure chemical vapor deposition (CVD) on 4-in. Si wafers in an AIXTRON® Black Magic cold-wall furnace. The combination of a low flux of carbon precursors with sub-nm Fe/Mo catalyst films on alumina-coated Si wafers maintained small-diameter SWCNTs (mean as small at 1.87 nm) with number densities up to 2.2 x1012 cm-2. Although removing Mo resulted in categorically larger CNT diameters and lower densities (< 0.9 x1012 cm-2), the mass conversion rate from C2H2 to SWCNT product was essentially constant (~1.2 x106 % g catalyst-1) for all catalyst compositions and CNT densities. These results corroborate the notion that hydrocarbon dissociation may be thermally driven without assistance from the catalyst under these conditions, and that the role of Mo was primarily a physical one in creating smaller, denser SWCNTs. Furthermore, we leveraged a suite of advanced characterization techniques to generate area maps across a 4-in wafer to visualize and evaluate the uniformity of various forest properties, including Raman microscopy (forest height, graphitization level, diameters), synchrotron X-ray scattering (mean diameter),2 and Rutherford backscattering spectroscopy (forest density). The uniformity and coupled correlations among different structural characteristics (e.g., diameter-density, rate-density) within a single wafer were consistent with those observed for multiple wafers grown by sequential runs (up to 5 in a day). This validates the reproducibility of our process and suggests that these correlations may be more universal phenomena. Lastly, our developments in synthesis represent to our knowledge best-in-class co-optimization of small-diameter, high-density, and large-area SWCNT forest growth. We will briefly discuss growth benchmarks in context of membranes, electrochemical devices, and antireflective surfaces as examples. Bui, N.; Meshot, E. R.; Kim, S.; Peña, J.; Gibson, P. W.; Wu, K. J.; Fornasiero, F. Advanced Materials 2016, 28 (28), 5871-5877.Meshot, E.; Zwissler, D. W.; Bui, N.; Kuykendall, T. R.; Wang, C.; Hexemer, A.; Wu, K. J. J.; Fornasiero, F. ACS Nano 2017, 11 (6), 5405-5416.

Synthesis Procedure of Highly Densely Packed Carbon Nanotube Forests on TiN.

The goal of this research was to obtain high-density single-walled carbon nanotube forests (SWNTs) on conductive substrates for different applications, including field emission. For this, dip-coating was chosen as the catalyst deposition method, to subsequently grow SWNTs by Alcohol Catalytic Chemical Vapor Deposition (AC-CVD). Si (100) was chosen as the substrate, which was then coated with a TiN thin film. By sputtering with Ar, it was possible to generate alternating TiN and Si lanes, with a different wettability and, therefore, a different affinity for the catalysts. As a result, the Mo-Co catalyst was mainly deposited on TiN and not on sputtered-Si, which allowed the selective growth of SWNT forests on the TiN conductive surfaces. These as-synthesized SWNTs were used for field emission measurements in a high vacuum chamber.

Efficient growth of vertically-aligned single-walled carbon nanotubes combining two unfavorable synthesis conditions

We demonstrate an unexpected recipe for the efficient synthesis of vertically-aligned single-walled carbon nanotubes (SWCNTs). The growth of high-density arrays is achieved by combining two unfavorable conditions: low temperature and mono-metallic Co catalyst, neither of which was previously found to be effective for the synthesis of SWCNT forest. As a result of this new recipe, the as-synthesized SWCNTs have a narrower diameter distribution than those obtained in previous studies, with 85% of the SWCNTs between 1.6 and 2.4 nm. Meanwhile, we utilize our recently established transmission electron microscopy (TEM) technique to characterize this monometallic Co catalyst directly on SiO2. Nanoparticles in a 600 °C reaction are confirmed to be smaller, denser and more uniform than those formed at high temperatures. Our technique further allows to image catalyst at high temperatures, by which Ostwald ripening is visualized at a single particle level. In addition, in situ electron diffraction and imaging confirm that monometallic Co catalyst remains as a solid phase even at 800 °C. The discovery of new growth recipe increases the compatibility of synthesizing SWCNT onto other substrates. The new TEM technique provides experimental evidence for some long-standing hypotheses of catalytic particles and is capable of guiding the design of new catalysts.

Millimetre-scale growth of single-wall carbon nanotube forests using an aluminium nitride catalyst underlayer

ABSTRACTWe have demonstrated the high yield (∼900 μm) and highly single-wall selective (&gt;95%) growth of carbon nanotube (CNT) forest using aluminium nitride (AlN) as a catalyst underlayer. Such high efficiency and single-wall selectivity have not been previously reported using this underlayer system. Evaluation with transmission electron microscopy showed that the average diameter of the grown carbon nanotubes was ∼3.0 nm, which is similar to those grown on alumina underlayers. In addition, characterization of the catalyst/underlayer system using atomic force microscopy and X-ray photoelectron spectroscopy suggests that neither Ostwald ripening along the surface nor catalyst subsurface diffusion into the AlN underlayer are severely occurring at the growth temperature, leading to the creation of the stable and dense small nanoparticle array to achieve an efficient growth of single-wall CNTs.

Modulation of carbon nanotube yield and type through the collective effects of initially deposited catalyst amount and MgO underlayer annealing temperature

ABSTRACTRecently, the millimetre-scale, highly efficient growth of single-wall carbon nanotube (SWCNT) forests from iron (Fe) catalysts has been reported through the annealing of the magnesia (MgO) underlayer. Here, we report the modulation of the CNT yield (height) and average number of CNT walls for a Fe/MgO catalyst system through the collective effects of initial Fe amount and MgO annealing temperature. Our results revealed the existence of a well-defined region for high yield SWCNT forest growth in the domain of deposited Fe thickness and MgO annealing temperature. Through topographic examinations of the catalyst surface using atomic force microscopy, we confirmed that our results stem from the collective effects of increased amounts of surface-bound Fe through the amount of deposition and suppression of Fe subsurface diffusion, together govern the amount of surface-bound catalyst. The combination of these mechanisms determined the final nanoparticle size, density, and stability and could explain the three distinctly defined regions: low yield SWCNT growth, high yield SWCNT growth, and high yield multiwall CNT growth. Furthermore, we explained the observed borders between these three regions.

Designing Single-Wall Carbon Nanotube Forest Growth for Nanofluidic Applications

Ultrafast fluid transport through the core of single-wall carbon nanotube (SWNT) channels promises to advance membrane applications, from efficient water purification and low-cost separation of high-value components, to next-generation protective garments. For the last application, we recently demonstrated cm2-area, vertically-aligned, SWNT-polymer composite membranes that combined both high breathability and protection in a single functional material, thus breaking the typically elusive trade-off between the two.1 As we seek to further enhance the performance of these membranes, we target vertically-aligned SWNT “forest” growth toward: a) minimizing SWNT diameter to maximize protection via size exclusion; b) maximizing SWNT number density to maximize breathability; and c) scaling up SWNT growth area to eventually incorporate into garments. We perform low-pressure chemical vapor deposition (CVD) in an AIXTRON® Black Magic cold-wall furnace, featuring a wafer-scale, local heater stage and gas showerhead. The combination of a low flux of carbon precursors with sub-nm Fe/Mo catalyst films on alumina-coated Si wafers maintains small-diameter SWNTs. High-resolution TEM and X-ray scattering2 confirm that our forests contain >99% SWNTs below 3.5 nm diameter (on wafers up to 4 inches). Consistent with literature reports, 6 at% Mo optimally preserves small, densely packed particles, templating growth of forests with high number densities up to 1.4x1012 cm-2. In an effort to control the forest thickness and uniformity across large substrate areas, we investigate the area-dependent growth kinetics and temporal density decay. Despite exhibiting a growth rate that is initially uniform across the substrate, growth at the center self-terminates first, while edge growth continues at a sublinear rate for tens of minutes. Understanding these varying reaction lifetimes that arise on a single substrate is key to engineering large-area SWNT membranes with highly uniform transport characteristics. To scale SWNT forest production and overcome non-uniformities in wafer-scale growth, we seek to simplify the catalyst film formulation. While wafer-scale growth from Fe films of equal thickness in the absence of Mo is markedly more robust over multiple sequential batches, the SWNT diameter distributions have larger means and tails, which are detrimental to the rejection properties of our membranes. Tuning the Fe thickness down renders smaller SWNTs, but the range of densities grown from these Fe-only films is 0.2-0.6x1012 cm-2, regardless of thicknesses studied (0.35-0.65 nm). This challenge of accessing a parameter space that co-optimizes growth yield of small, monodisperse diameter, high-density SWNT forests highlights a deficiency in conventional catalyst design and suggests a need for improved designs that could unlock a range of SWNT applications. This work is supported by the Defense Threat Reduction Agency (DTRA) D[MS]2 project under Contract No. BA12PHM123 and was performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Bui, N.; Meshot, E. R.; Kim, S.; Peña, J.; Gibson, P. W.; Wu, K. J.; Fornasiero, F., Ultrabreathable and Protective Membranes with Sub-5 nm Carbon Nanotube Pores. Advanced Materials 2016, 28 (28), 5871-5877.Meshot, E.; Zwissler, D. W.; Bui, N.; Kuykendall, T. R.; Wang, C.; Hexemer, A.; Wu, K. J. J.; Fornasiero, F. Quantifying the Hierarchical Order in Self-Aligned Carbon Nanotubes from Atomic to Micrometer Scale. ACS Nano 2017, 11 (6), 5405-5416.

Application of nonlocal higher-order beam theory to transverse wave analysis of magnetically affected forests of single-walled carbon nanotubes

Recently, it has been proved that the mechanical properties of arrays of single-walled carbon nanotubes (SWCNTs) are enhanced along the direction of the applied magnetic field, however, no study has been performed to explain characteristics of waves within magnetically affected forests of SWCNTs with three-dimensional configuration. Using nonlocal higher-order beam theory, characteristics of traveling transverse waves in vertically aligned jungles of SWCNTs in the presence of a longitudinal magnetic field are going to be explored carefully. Both nonlocal discrete and continuous models are established and the capabilities of the continuous model in capturing the characteristics of waves by the discrete model are declared. The newly established continuous model would be very useful in examining vibrations of magnetically affected nanosystems with high population in which the discrete model suffers from huge amount of computational effort and labor costs. Roles of magnetic field strength, small-scale parameter, radius of SWCNTs, intertube distance, longitudinal wavenumber, and number of constitutive SWCNTs of the nanosystem on dispersion curves, phase velocities, and group velocities are investigated. The influences of nonlocality and magnetic field strength on these characteristics of elastic waves are also highlighted.

One millimeter per minute growth rates for single wall carbon nanotube forests enabled by porous metal substrates.

We report an exceptionally high-efficiency synthesis of long single-wall carbon nanotube (SWCNT) forests using porous substrates (metal meshes) in place of nonporous flat substrates. This study examined the dependence of the growth efficiency on various mesh structures, including wire diameter, aperture size, and total surface area. We demonstrated that the synthesis of SWCNT forests is highly dependent on the initial porosity as well as maintaining the open pores throughout the duration of the growth. Our results show that carbon nanotubes (CNTs) can be grown on all surfaces of the mesh in high efficiency with the optimum growth efficiency observed for a mesh porosity of ∼30%. Based on these results, we demonstrated the high efficiency synthesis of SWCNT forests (height: >3.47 mm, average growth rate: 301 μm min−1, and yield: 12.7 mg cm−2 in 10 min growth time). Furthermore, we showed that the initial growth rates exceeded 1 millimeter per minute (1000 μm min−1). Our results further indicate that metal meshes represent a viable alternative to nonporous flat substrates for the efficient synthesis of tall and high yielding SWCNTs.

Limitation in growth temperature for water-assisted single wall carbon nanotube forest synthesis

ABSTRACTIn this study, we examined the limitations in growth temperatures for single-walled carbon nanotube (SWNT) forest synthesis using a water (H2O) growth enhancer as highlighted by a dramatic decrease in growth efficiency at high temperatures. Comparative synthesis using a carbon monoxide (CO) growth enhancer demonstrated a wider temperature window for SWNT forest synthesis. We found that SWNT forests taller than 100 μm could not been synthesized above 850 °C by using H2O, but by using CO, tall (&gt;800 μm) forests could be synthesized at growth temperatures exceeding 900 °C. The dependency of H2O concentration showed that H2O, itself, was the cause for the reduced growth efficiency observed at higher temperatures. In contrast, increased CO concentrations did not result in any drop in the carbon nanotube (CNT) yield across the entire temperature range. While a severe drop of CNT yield above 950 °C was observed when CO was used, the origin was found to stem from catalyst particle aggregation enhanced by diffusion on the surface rather than CO itself. Therefore, the ability to synthesize at higher growth temperatures is advantageous when using more stable carbon feedstocks, such as methane.

FIB Secondary Etching Method for Fabrication of Fine CNT Forest Metamaterials

Anisotropic materials, like carbon nanotubes (CNTs), are the perfect substitutes to overcome the limitations of conventional metamaterials; however, the successful fabrication of CNT forest metamaterial structures is still very challenging. In this study, a new method utilizing a focused ion beam (FIB) with additional secondary etching is presented, which can obtain uniform and fine patterning of CNT forest nanostructures for metamaterials and ranging in sizes from hundreds of nanometers to several micrometers. The influence of the FIB processing parameters on the morphology of the catalyst surface and the growth of the CNT forest was investigated, including the removal of redeposited material, decreasing the average surface roughness (from 0.45 to 0.15 nm), and a decrease in the thickness of the Fe catalyst. The results showed that the combination of FIB patterning and secondary etching enabled the growth of highly aligned, high-density CNT forest metamaterials. The improvement in the quality of single-walled CNTs (SWNTs), defined by the very high G/D peak ratio intensity of 10.47, demonstrated successful fine patterning of CNT forest for the first time. With a FIB patterning depth of 10 nm and a secondary etching of 0.5 nm, a minimum size of 150 nm of CNT forest metamaterials was achieved. The development of the FIB secondary etching method enabled for the first time, the fabrication of SWNT forest metamaterials for the optical and infrared regime, for future applications, e.g., in superlenses, antennas, or thermal metamaterials.

The double-edged effects of annealing MgO underlayers on the efficient synthesis of single-wall carbon nanotube forests.

Recently, the millimetre-scale, highly efficient synthesis of single-wall carbon nanotube (SWCNT) forests from Fe catalysts has been reported through the annealing of the magnesia (MgO) underlayer. Here, we report the double-edged effects of underlayer annealing on the efficiency and structure of the SWCNT forest synthesis through a temperature-dependent examination. Our results showed that the efficiency of the SWCNT forests sharply increased with increased underlayer annealing temperatures from 600 °C up to 900 °C due to a temperature-dependent structural modification, characterized by increased grain size and reduced defects, of the MgO underlayer. Beyond this temperature, the SWCNT fraction also decreased as a result of further structural modification of the MgO underlayer. This exemplifies the double-edged effects of annealing. Specifically, for underlayer annealing below 600 °C, the catalyst subsurface diffusion was found to limit the growth efficiency, and for excessively high underlayer annealing temperatures (>900 °C), catalyst coalescence/ripening led to the formation of double-wall carbon nanotubes. As a result, three distinct regions of synthesis were observed: (i) a "low yield" region below a threshold temperature (∼600 °C); (ii) an "increased yield" region from 600 to 900 °C, and (iii) a "saturation" region above 900 °C. The efficient SWCNT forest synthesis could only occur within a specific annealing temperature window as a result of this double-edged effects of underlayer annealing.