Diameter control of single-walled carbon nanotube forests from 1. 3–3. 0 nm by arc plasma deposition

ABSTRACT We present a method to both precisely and continuously control the average diameter of single-walled carbon nanotubes in a forest ranging from 1.3 to 3.0 nm with ~1 Å resolution.

The diameter control of the forest was achieved through tuning of the catalyst state (size, density and composition) using arc plasma deposition of nanoparticles. This 1.7 nm control range

and 1 Å precision exceed the highest reports to date. SIMILAR CONTENT BEING VIEWED BY OTHERS ONE STEP FABRICATION OF ALIGNED CARBON NANOTUBES USING GAS RECTIFIER Article Open access 25

January 2022 DETERMINISTIC TRANSFER OF OPTICAL-QUALITY CARBON NANOTUBES FOR ATOMICALLY DEFINED TECHNOLOGY Article Open access 25 May 2021 COALESCENCE OF CARBON NANOTUBES WHILE PRESERVING THE

CHIRAL ANGLES Article Open access 05 February 2025 INTRODUCTION For single-walled carbon nanotubes (SWCNTs), the diameter represents a critical structural aspect as it determines many

electronic and chemical properties. For instance, diameter determines the metallicity due to the inverse proportionality with the band gap1. Similarly, thermal properties are also dictated

by the diameter as curvature is related with Umklapp scattering2. Finally, diameter determines the radius of curvature of the tube which affects surface functionalization3. Therefore,

diameter represents an important structural parameter of SWCNTs; however, precise control of the SWCNT diameter over a wide range has yet to be demonstrated. Previous results for the SWCNT

diameter control can be grouped into two techniques: (1) floating catalyst method, where the SWCNTs are grown from catalysts flowing through a reactor tube and (2) substrate growth, where

the SWCNTs are grown from catalysts deposited onto a substrate. From our survey of previous reports, the floating catalyst method has demonstrated the widest range of SWCNT diameter control

with the highest precision, particularly in the smaller diameter range (~<1 to ~2.1 nm)4,5,6. For example, Saito _et al._ demonstrated control from 1.2–1.8 nm with ~0.15 nm resolution by

utilizing the differing activation temperatures of carbon sources4. Tian _et al._ controlled the diameter from 1.2–1.9 nm with a resolution of ~0.2 nm by controlling the CO2 concentration5.

The floating catalyst method affords this level of diameter control as the catalysts are not fixed in a single plane (_i.e._ the substrate) and therefore possess increased degrees of freedom

to lessen the probability of aggregation. For substrate growth, one basic approach has demonstrated the widest range of diameter control: increase catalyst spacing to minimize unwanted

particle aggregation7,8,9,10,11. In this manner, Lu _et al._ reported diameter control of SWCNTs from 0.6 to 2.1 nm by selectively activating either the smaller or larger nanoparticles7.

Durrer _et al._ used centrifugation method to separate the nanoparticles prior to deposition to control the diameter with a narrow distribution8. Song _et al._ achieved a narrow distribution

of 0.1 nm with diameter control from 0.8 to 1.4 nm by using a sandwiched catalyst system9. Chen _et al._ controlled the diameter of CNTs from 0.9–1.8 nm by controlling the size of SiO2

nanoparticles10. In each of these cases, the sparse distribution of nanoparticles (_i.e._ large catalyst spacing) was critical in reducing catalyst aggregation which resulted in entangled

mats of SWCNTs. When SWCNTs are efficiently grown from catalysts on a substrate, they self-organize into a vertically aligned assembly, called a “forest”12,13,14,15. However, the methods

mentioned above do not grow in sufficient density to create a forest structure which is unique in several aspects. First, the forest represents a high efficiency and pure synthesis of CNTs

from substrate-bound catalysts. Second, the forest structure cannot be formed at any other time than at the synthesis stage. Third and most importantly, the forest structure provides a

unique aligned structure for specialized processing, such as drawing CNT sheets and yarns16,17,18 or aligned pre-impregnated composites19. Further, as the properties of a CNT depend on its

structure, such as diameter, height, wall number, _etc_20,21, several reports have demonstrated that the properties of a forest also depend on the forest assembly structure, such as

alignment16,18,22,23. Therefore, the ability to control the structure of the forest, in general, triggers advances in the application of CNTs, such as yarning16,18, prepregs for composite

reinforcement19, viscoelastic materials23 and thermal management materials24, etc. For SWCNT forests, sufficient catalyst density is required to create a condition where the

non-self-supporting SWCNTs can self-organize into a vertically aligned ensemble through a “crowding effect”25. Arc plasma deposition (APD) is a physical nanoparticle deposition method where

nanoparticles are generated in pulsed arc plasma from a target cathode at high voltages and allowed to deposit onto a substrate. This method possesses significant advantages over the

conventional thin film catalyst systems commonly used to grow SWCNT forests. First, unlike thin film catalyst systems where the average nanoparticle size is determined by the film thickness,

the size of nanoparticles can be easily controlled by adjusting the arcing voltage. Second, as nanoparticles are generated through an arc pulse, the number (_i.e._ density) of nanoparticles

of a given size can be easily controlled by adjusting the pulse number. These two points allow for the independent control of nanoparticle size and density not allowable by the thin film

catalyst system. Third, as the composition of the nanoparticles has been shown to agree (within 3%) with the target composition26, APD allows for simple compositional changes to the

catalyst. In previous reports, APD has been used to grow SWCNT, double-walled CNT and multiwalled CNT (MWCNT) forests with diameters of 1.5, 4.0 and 5.5 nm, respectively, demonstrating that

APD can make catalyst nanoparticles of different sizes27. In this paper, we have explored this opportunity to control the catalyst nanoparticle size and density, by which we realized the

precise and continuous control of the average diameter of SWCNT forests from 1.3 to 3.0 nm with a resolution of ~1 Å. The control of catalyst size, density and composition, which was carried

out by tuning the APD power setting and target composition, was found to be critical in achieving wide range of diameter control to reduce catalyst aggregation. Finally, we found that the

SWCNT forest height decreased with the average diameter, indicating that the synthesis of tall SWCNT forests with small diameter is inherently difficult. RESULTS By the APD method, we were

able to control the average diameter of SWCNT forests over a wide range (2.0 to 3.0 nm) with a precision of ~1 Å using Fe catalysts and grown by thermal chemical vapor deposition (CVD). This

size control was achieved by the following strategy: At each APD power setting, which produced a specific size of nanoparticles, the pulse number was adjusted to achieve the sufficient

nanoparticle density to allow the SWCNTs to self-assemble into a forest (Fig. 1a, see the detailed information in Supplementary Table S1 online). We should note that care was taken not to

exceed this level too greatly as catalyst nanoparticle aggregation increased, which was particularly more prominent at the small diameter regime. We demonstrated the wide average diameter

control of SWCNT forests from 2.0 to 3.0 nm with ~1 Å precision as shown by a series of Fourier transform infrared (FTIR) spectra (Fig. 1b). In short, the average diameter (_d_ (nm)) was

determined from the peak position of the S11 absorbance band and converted to diameter by the relation between binding energy and diameter (_d_ = 0.77/_E_, where _E_ (eV) is the peak

position of S11 absorbance peak)28. Measurement reproducibility was confirmed by calculating the variance (~0.01 nm) of several measurements for a single sample. Measurement reproducibility

due to processing was also confirmed by testing two other samplings taken from different areas of a single forest and found that the difference was ~0.01 nm. In fact, the standard deviation

from the three locations was found only ~0.03 nm. These results demonstrated the high reproducibility and accuracy of this FTIR measurement. This method was applied to the family of SWCNT

forests and the family of FTIR spectra showed a clear and gradual upward shift in the S11 band associated with decreased diameter. To further support the FTIR measurements, diameter

characterization by transmission electron microscopy (TEM) for ten members of the family of the SWCNT forests spanning the control range were performed and the results agreed exceptionally

well with FTIR results (Fig. 1c). Importantly, TEM observation also confirmed the 100% SWCNT selectivity as multi-shelled CNTs would not be detectable in the FTIR method. As mentioned above,

synthesizing SWCNT forests requires sufficient catalyst density so that SWCNTs can self-organize into a forest structure through a “crowding effect”25. This requirement of high catalyst

density represents a fundamental obstacle for the SWCNT diameter control when synthesizing SWCNT forests with the conventional thin film catalyst, where the catalyst size and catalyst

density are linked29. The APD method overcomes the limitation of the thin film catalyst system to simultaneously control the catalyst size and the sufficient catalyst density, which is

particularly difficult at the smaller diameter regime. To demonstrate this point, four varieties of deposited nanoparticles showing size control were observed by atomic force microscopy

(AFM) and these images showed the independent control of the particle size and density by the power setting (voltage) and discharge number (pulse number), respectively (Fig. 2). This aspect

was critical to provide the required particle density for any particle size to drive the “crowding” of the individual SWCNTs into a self-assembled unit. Through this approach, we could

control the diameter range from 2.0 to 3.0 nm using the Fe catalyst. While the Fe catalyst offered a wide range of diameter control (2.0–3.0 nm), we found that the diameter range could be

further extended by using alloyed Fe-Ni-Cr catalysts. This marks an additional advantage of APD where catalyst composition can be easily adjusted by the choice of target. We deposited

nanoparticles of various sizes and densities for several Fe-Ni-Cr alloys to examine their limiting range (see Supplementary Table S1 online). Fe and Ni were chosen because of the high

catalytic activity and carbon solubility for CNT synthesis. The major obstacle in controlling the diameter of CNTs, particularly for small diameter SWCNTs, is the migration of the catalyst

material resulting in unwanted nanoparticle aggregation. To prevent or minimize this effect, Cr was chosen because the previous reports showed that it could suppress the catalyst migration

by acting as spacer30 or dispersant31 between active catalysts or by reacting with the Al2O3 buffer layer to bind the catalyst nanoparticles32. After synthesis, the diameters were

characterized and plotted as a function of each alloy in a ternary plot with colors representing the resulting SWCNT average diameters for forests, as shown in Fig. 3a. This ternary plot

showed a strong dependence of the minimum diameter on the catalyst composition. Specifically, we observed that the increased Cr content resulted in the smaller diameter forests (as small as

1.3 nm) while the increased Ni content resulted in the larger diameter forests. It should be noted that we could not grow forest from 100% Ni catalysts at our growth conditions. To clarify

the effect of the respective Ni and Cr content, we plotted the average SWCNT diameter as a function of Ni and Cr content shown in Figs. 3b and 3c, respectively. These plots showed the

dependence of the lower achievable diameter range on the Ni and Cr content. Generally speaking, higher levels of Ni led to larger diameters, while higher levels of Cr led to smaller

diameters. For practical purposes, we divided the entire diameter range into two regimes. In one regime, we used the Fe-Ni-Cr alloy catalyst to control the diameter in the small region

(1.3–2.0 nm). In the second regime, we used the pure Fe catalyst for the large diameter SWCNTs (2.0–3.0 nm), as shown in Fig. 1b. Our results showed that tuning the catalyst condition (size,

density and composition) by APD was an effective method for controlling the average diameter of the SWCNTs in a forest where Cr was indispensable for extending the control range into the

small diameter regime. DISCUSSION The results presented, herein, represent the widest range of average diameter control with the highest precision not only for SWCNT forests, but also for

SWCNTs in general (Fig. 4). This level of diameter control in terms of range and precision rivals the highest reported levels and highlights our ability to control the structure of the SWCNT

forests. From a literature survey, the floating catalyst method has demonstrated the widest and most precise control of SWCNT diameter (~<1 to ~2.1 nm)4,5,6. In addition, previous

approaches, which used the sparse distribution of catalysts on substrates, have demonstrated a coarse control over a substantial diameter range7,8,9,10. Recently, the diameter control of

SWCNT forests has also showed remarkable progress33,34,35,36,37. The mean diameter of SWCNT forests, spanning 1.2–2.5 nm, has been reported by changing both the relative and absolute amounts

of Co and Mo in a binary catalyst system34. Further, the average diameter of SWCNTs was shown to reduce from 2.1 to 0.7 nm by the addition of acetonitrile to an ethanol carbon feedstock35.

However, neither of these methods showed a fine diameter control. More recently, a report demonstrated the continuous and wide diameter range control of 1.9–3.2 nm of SWCNT forests by

adjusting the catalyst formation temperature and the amount of hydrogen exposure for a thin film catalyst system to control Ostwald ripening and subsurface diffusion of the catalysts36.

However, it should be noted that the APD method presented here possesses several advantages in both the results and approach. First, as the nanoparticles are generated independent of the CVD

process, a large amount of hydrogen gas is not required. This is of particular importance when considering production costs and safety for mass production. Second, by using the flow rate

control and the thin film catalyst, a strong dependence of the forest density and diameter was observed, while the APD method does not exhibit such a strong density dependence. This opens

the opportunity to tune the forest structure for specific properties, such as higher porosity (lower density) or higher mechanical stiffness (higher density). Finally, our results

demonstrate a wider control range of 1.3–3.0 nm, which is the widest level of control for the SWCNT average diameter in general. It is noteworthy to indicate that the forests produced in

this study possess a broad diameter distribution. Sharp diameter distributions are not common in CVD methods, particularly in forests and/or on substrates. While post-processing separation

techniques have shown extremely sharp diameter distributions and high resolutions38,39,40,41, the disadvantage is the need for mono-dispersed SWCNTs and the low yield. Our method of the

direct synthetic control shows much higher yield and wider diameter range despite the wider diameter distributions. The widest range of diameter control of SWCNT forests was achieved by

using Fe and Fe-Ni-Cr alloy catalysts, with Cr playing an important role in extending the control to smaller diameters. To further understand the effect of Cr, migration studies of the

deposited catalysts were performed by AFM before and after identical thermal reduction processes for Fe and Fe-Ni-Cr alloy (SUS329J4L) catalyst nanoparticles. For Fe, the size of

nanoparticles exhibited obvious increase after thermal annealing due to the Ostwald ripening (see Supplementary Fig. S1 online). In contrast, for Fe-Ni-Cr alloy, the catalyst nanoparticles

showed a much smaller change in size and density, indicating that the presence of Cr suppressed the nanoparticle aggregation. The actual mechanism behind this remains uncertain, but a

literature survey showed two different reports believed that Cr could act as spacer30 or dispersant31 between active catalysts. In a separate report, the Cr was thought to bind with the

Al2O3 buffer layer to resist the migration32. It is our belief that the level of Cr in our study was insufficient to act as spacers; therefore, we attribute the suppression in aggregation to

the binding to the Al2O3 buffer layer. As a result, by using alloyed Fe-Ni-Cr catalysts with high-concentration of Cr, we were able to further extend the average diameter of SWCNT forests

to small region. The ability to precisely control the average diameter of SWCNT forests provides the opportunity to examine the dependence of the growth properties on the diameter. We

plotted the height (achievable height in a fixed time) of our SWCNT forests as a function of the average diameter in Fig. 5, which showed that the height decreased with decreased diameter.

Specifically, for the larger diameter range, ~2.8–3.0 nm, the forest was highest, ~300–500 μm, under these growth conditions, as shown in the upper right inset scanning electron microscopy

(SEM) image of Fig. 5; however, for the medium diameter range, ~2.2–2.6 nm, the forest height dropped to about half (~100–200 μm). Finally, the forest height for ~1.3–1.6 nm fell to several

tens of μm. These results may explain the absence of long length and small diameter SWCNT forests. Our results are completely the opposite trend reported by Patole _et al._ where smaller

particles, due to their smaller surface area and volume, would grow CNTs at faster growth rates (height per growth time)42. We suggest the most plausible reason is that the effect of gas

diffusion becomes an increasingly greater effect as the diameter is reduced. As stated previously, one requirement for SWCNT forest growth is a critical catalyst density to create sufficient

crowding among the SWCNTs so as to form the forest and this critical density increases with decreased diameter25. By calculating the inter-tube spacing from the linear mass density (mass

per unit length) of the average diameter and bulk mass density43, the inter-tube spacing for the 1.3-nm-diameter forests was ~1/3 of that for the 3.0-nm-diameter forests (_i.e._ ~5.4 nm

versus ~13.6 nm, see Supplementary Table S1 online). A recent report on CVD synthesis of CNTs concluded that gas diffusion occurred in the Knudsen diffusion regime and the effective Knudsen

diffusivity was proportional to the channel radius (_i.e._ the inter-tube spacing in this work) and the porosity of the forest and inversely proportional to the tortuosity factor, at a fixed

temperature44. As stated, the channel radius was ~1/3 for the smaller diameter CNTs, but the calculated porosity of these two forests was nearly the same (~97%). We estimated that the

effective Knudsen diffusivity for the small diameter (1.3 nm) forests was at least 1/3 lower than that of the large diameter (3.0 nm) forests. We expect this value (1/3) to be an upper limit

as the tortuosity factor of a forest of small diameter and flexible SWCNTs is expected to be higher than one of larger diameter and stiffer SWCNTs. Therefore, we conclude that difference in

gas diffusion is one potential cause of the observed decreased height of forests with diameter. This explains the apparent difference between our results and that of Patole _et al._, where

the inter-tube spacing was significantly larger and diffusion was not a major factor42. Another reason for the height decrease with diameter could be based on recent findings that the mass

of the individual SWCNT is determined by the individual catalyst volume and consequently the forest yield is determined by the total catalyst volume36,45. This implies that larger diameter

SWCNTs will provide larger length whereas smaller diameter SWCNTs will provide shorter length. Finally, due to the addition of low carbon solubility element, Cr, into the Fe catalyst

nanoparticles, we would expect an overall decrease in carbon solubility. This decreased carbon solubility may explain not only the diminishing height of forests even after the optimization

of CVD conditions but also the slower growth rates. Therefore, our results suggest that the synthesis of tall SWCNT forests with small diameter is inherently difficult. To summarize, this

work demonstrated the precise and continuous control of SWCNT average diameter in a forest ranging from 1.3 to 3.0 nm with ~1 Å resolution by using APD of catalyst nanoparticles. Through

APD, the catalyst condition (size, density and composition) was tuned and shown as an effective method for controlling the SWCNTs in a forest where Cr led to smaller diameters and Ni led to

larger diameters. Our results represent the widest range of the average diameter control with the highest precision for SWCNTs in general. Investigation of the growth properties showed a

direct relationship between the achievable height and the diameter suggesting the fundamental difficulty in synthesizing tall and small diameter SWCNT forests. METHODS CATALYST DEPOSITION

Catalyst nanoparticles were deposited by APD in a high vacuum chamber (~4 × 10−4 Pa) onto silicon substrates with a 40 nm thick sputtered Al2O3 buffer layer. Deposition parameters included

the voltage and pulse number (see Supplementary Table S1 online). Voltage, determining arc current, was increased to increase nanoparticle size and ranged from 80–180 V at a fixed

capacitance of 720 μF. The density of deposited nanoparticles was determined by the number of arc pulses, which ranged from 100 to 170. SYNTHESIS SWCNT forests were synthesized in a 1″ tube

furnace by thermal CVD using helium as the carrier gas and 1% ethylene as the carbon source at 1000 sccm total flow. Catalyst aggregation was minimized by reducing the accumulated heat

exposure to the catalyst nanoparticles. Specifically, the 1 × 1 cm silicon substrate was inserted into a preheated CVD system (750°C) to reduce the metallic nanoparticles (90% hydrogen in

helium) for 1 min, immediately followed by a growth phase for 6 min. In our previous reports we added water to increase the height of forests14. However, in this study, we have not added

water as it increased the average diameter of SWCNTs. Therefore, the CVD conditions for the data presented herein were fixed for simplicity. CHARACTERIZATION The catalyst nanoparticles were

observed by AFM (BRUKER Dimension FastScan). The forest structures were characterized by SEM (Hitachi S-4800). The average diameter of SWCNTs was analyzed by FTIR spectroscopy (Nicolet

6700). Diameter evaluation by FTIR is based on the conversion of the peak of the S11 absorption band to diameter by the relation between SWCNT diameter and binding energy (_i.e._ Kataura

plot)28. In this method, a sample of SWCNTs was removed from the forest and dispersed in organic solvent, such as ethanol, dimethylformamide, etc. and then dropped onto a stainless steel

mesh and dried. Transmission FTIR spectroscopy was performed and the location of the S11 absorption band (_E_ (eV)) was converted to diameter (_d_ (nm)) by the following relation, _d_ =

0.77/_E_, which was derived simply from the Kataura plot. It should be noted that while the shape of the FTIR absorption peak is dependent on the SWCNT dispersion, the peak position is

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volume. Materials 6, 2633–2641 (2013). Article  ADS  CAS  PubMed  PubMed Central  Google Scholar  Download references ACKNOWLEDGEMENTS Support by Technology Research Association for Single

Wall Carbon Nanotubes (TASC) is acknowledged. AUTHOR INFORMATION Author notes * Chen Guohai and Seki Yasuaki contributed equally to this work. AUTHORS AND AFFILIATIONS * Nanotube Research

Center, National Institute of Advanced Industrial Science and Technology (AIST) and Technology Research Association for Single Wall Carbon Nanotubes (TASC), Central 5, 1-1-1 Higashi,

Tsukuba, Ibaraki, 305-8565, Japan Guohai Chen, Yasuaki Seki, Hiroe Kimura, Shunsuke Sakurai, Motoo Yumura, Kenji Hata & Don N. Futaba * Department of Pure and Applied Sciences, Tsukuba

University, Tsukuba, Ibaraki, 305-8573, Japan Hiroe Kimura & Kenji Hata * Japan Science and Technology Agency (JST), Kawaguchi, Saitama, 332-0012, Japan Kenji Hata Authors * Guohai Chen

View author publications You can also search for this author inPubMed Google Scholar * Yasuaki Seki View author publications You can also search for this author inPubMed Google Scholar *

Hiroe Kimura View author publications You can also search for this author inPubMed Google Scholar * Shunsuke Sakurai View author publications You can also search for this author inPubMed 

Google Scholar * Motoo Yumura View author publications You can also search for this author inPubMed Google Scholar * Kenji Hata View author publications You can also search for this author

inPubMed Google Scholar * Don N. Futaba View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS G.C. and Y.S. designed and conducted the

experiments. H.K. and S.S. helped with the experiment equipment. K.H. and D.N.F. supervised the overall research. G.C., Y.S. and D.N.F. drafted the paper. M.Y. guided this work. All authors

read and commented on the manuscript. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. ELECTRONIC SUPPLEMENTARY MATERIAL SUPPLEMENTARY

INFORMATION Supplementary Table S1 and Figure S1 RIGHTS AND PERMISSIONS This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy

of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Chen, G., Seki, Y., Kimura, H. _et al._ Diameter

control of single-walled carbon nanotube forests from 1.3–3.0 nm by arc plasma deposition. _Sci Rep_ 4, 3804 (2014). https://doi.org/10.1038/srep03804 Download citation * Received: 30 April

2013 * Accepted: 23 December 2013 * Published: 22 January 2014 * DOI: https://doi.org/10.1038/srep03804 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this

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