Narrow Chirality Distribution Research Articles

High‐Temperature Growth of Chirality‐Enriched, Highly Crystalline Carbon Nanotubes for Efficient Single‐Chirality Separation

AbstractThere has been significant interest in producing single‐walled carbon nanotubes (SWNTs) with specific chirality to ensure consistent and predictable performance. However, chirality‐selective growth has traditionally required low reaction temperatures, causing SWNTs with low graphitization degrees. In this work, a magnesia supported cobalt catalyst (Co/MgO) is introduced for chemical vapor deposition growth of SWNTs with a narrow chirality distribution even at a high temperature of 900 °C, which promotes enhanced graphitization of SWNT walls. The achievement is attributed to the strong metal‐support interactions that prevent catalyst particle sintering under harsh reaction conditions. The resulting SWNTs provide fundamental raw materials for high‐efficiency single‐chirality separation. Because of the relatively large chiral differences between the major species and their narrow chirality distribution, the study can readily isolate six distinct (n, m) SWNT types including (6, 5), (8, 3), (9, 2), (7, 5), (7, 6), and (8, 4), with purities exceeding 91% by gel chromatography. Compared with commercially HiPco‐ and CoMoCAT SG65i‐ SWNTs, the separation efficiency and yield are greatly improved. This work demonstrates the growth of high‐quality SWNTs with a narrow chirality distribution, paving the way toward their efficient single‐chirality separation.

An efficient approach toward production of near-zigzag single-chirality carbon nanotubes.

Synthesizing single-walled carbon nanotubes (SWCNTs) with a narrow chirality distribution is essential for obtaining pure chirality materials through postgrowth sorting techniques. Using carbon monoxide chemical vapor deposition, we devise a ruthenium (Ru) catalyst supported by silica for the bulk production of SWCNTs containing only a few (n, m) species. The result is attributed to the limited carbon dissociation on the supported Ru clusters, favoring the growth of only small-diameter SWCNTs at comparable growth rates. The resulting materials expedite high-purity single chirality separation using gel chromatography, leading to unprecedented yields of 3.5% for (9, 1) and 5.2% for (9, 2) nanotubes, which surpass those separated from HiPco SWCNTs by two orders of magnitude. This work sheds light on the large-quantity synthesis of SWCNTs with enriched species beyond near-armchair ones for their high-yield separation.

Growth of Single-Walled Carbon Nanotubes from Solid Supported Heterogeneous Catalysts: Achievements and Challenges

Single-walled carbon nanotubes (SWNTs) have gained worldwide attention because of their exceptional electronic, optical, and mechanical properties, which are contingent on their structures denoted by chiral indices (n, m). To fully harness the extraordinary properties and advance SWNT-based technology, large scale synthesis of SWNTs with a narrow chirality distribution or even a specific (n, m) is highly demanded. Among various SWNT preparation techniques, chemical vapor deposition growth from solid supported heterogeneous catalysts has emerged as a promising approach in terms of large yield and (n, m) structure control. In this review, we provide an overview of the progress in developing heterogeneous catalysts for bulk synthesis of SWNTs. Firstly, the advantages and disadvantages of using solid supported catalysts for chemical vapor deposition growth of SWNTs are discussed. Secondly, the key components in supported catalysts, including the metallic component and the nature of support material, as well as their respective roles in affecting the catalyst performances and SWNT chirality distribution, are highlighted. Finally, the current challenges and future directions in the field of SWNT synthesis from solid supported catalysts are proposed.

Solid supported ruthenium catalyst for growing single-walled carbon nanotubes with narrow chirality distribution

The past two decades witnessed the thriving of non-conventional nanoparticles in catalytic synthesis of single-walled carbon nanotubes (SWNTs). However, most of the SWNTs are limited to growing on a flat surface which usually suffers low output, thus making the evaluation of SWNT chirality distribution challenging. In this work, we report a solid magnesia (MgO) supported ruthenium (Ru) catalyst for chemical vapor deposition (CVD) growth of SWNTs with sufficient quantity for optical characterizations. Both MgO supported atomically dispersed Ru catalyst and Ru nanoparticles pre-deposited onto porous MgO are active for catalytic synthesis of small diameter SWNTs. Particularly, enriched (6, 5) SWNTs are achieved at temperatures higher than 800 °C by CO CVD, which is partially attributed to the high activity of Ru clusters. Besides, density functional theory (DFT) calculations reveal that the charge transfer from the MgO support to the Ru clusters promotes the carbon precursor absorption and dissociation, accounting for the activation of Ru clusters. This work not only demonstrates the application of atomically dispersed catalyst for CVD growth of SWNTs, but also paves the way for the chirality-selective growth of SWNTs from non-magnetic nanoparticles.

Laser switching characteristics of enriched (7,5) single-walled carbon nanotubes at 640 nm

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter or chirality distribution have great potential in promoting carbon nanophotonics and nanoelectronics. In this paper, SWCNTs with dominant (7,5) species, which are fabricated by chemical vapor deposition method from a magnesia-supported Co catalyst, are applied for the nonlinear optical measurements. The SWCNTs exhibit efficient saturable absorption properties at a wavelength of 640 nm in terms of a modulation depth of 27.7% and a saturable intensity of 5.7 GW/cm2. Such a 640 nm absorption is correlated with the S22 absorbance bands of the semiconducting SWCNTs. Consequently, by employing the SWCNTs as a saturable absorber (SA), stable Q-switched pulses with the shortest pulse width of 200 ns and maximum pulse repetition rate of 238 kHz are achieved at 640 nm. Our findings indicate that subnanometer semiconducting SWCNT could act as an excellent optical switcher device at 640 nm, which may help to explore potential applications in visible nonlinear optics.

Synthesis of (9,8) single-walled carbon nanotubes on CoSO4/SiO2 catalysts: The effect of Co mass loadings

Selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality distribution at high production yield is critical to realize their applications in electronics and medicine. However, there is a significant trade-off between chirality selectivity and SWCNT yield. In this study, a series of CoSO4/SiO2 catalysts containing from 1 to 9 wt% of Co were prepared to systematically study how Co mass loadings affect the chirality selectivity and carbon production yield. SWCNTs were analyzed by spectroscopies and microscopes, while catalysts at three stages of SWCNT synthesis were studied by multiple characterization tools. Our results show that all CoSO4/SiO2 catalysts can grow (9,8) tubes. Specifically, the catalyst containing 3 wt% Co has the highest yield of (9,8) tubes, which grow from metallic Co particles with a size of around 1.2 nm. These Co particles are in-situ produced by reducing small Co9S8 particles of 2 nm in size, while unreduced Co9S8 particles help to stabilize metallic Co particles. High Co mass loadings lead to the formation of large Co and Co9S8 particles, which favor the formation of graphitic carbon and multi-walled carbon nanotubes. These findings provide useful insights to guide the development of novel catalysts for the chiral selective production of SWCNTs.

VQS (Vapor-Quasiliquid-Solid, Vapor-Quasisolid-Solid) Mechanism for Realizing Narrow Distributions of Chirality and Diameters of Single-Walled Carbon Nanotubes (SWCNTs).

Diameter-selective and chiral-selective single-walled carbon nanotubes (SWCNTs), all on a single wafer, have vast technological promise. An investigation has been carried out to study the role of the physicochemical state of the nanoparticle surface, and of the growth parameters and growth mechanism, on the relatively-low-temperature control of SWCNT diameter and chirality. For this, amorphicity, porosity, coarsening, high energy states, etc. resulting from controlled surface disturbance, disorder and/or restructuring of nanoparticle have been considered. Possible relationship between growth rate, chiral angle, diameter, and monodispersity of SWCNTs has been explored. The influence of the nanoparticle's peripheral hill on the diameter selectivity has been examined. Surface segregation and formation of shell have been described. The impact of charge redistribution, creation of contour of charges, and dipole moment in shell in creating chiral-selective SWCNTs has been discussed. The diffusion of the carbon species through the nanopores of the shell via Knudsen dynamics has been elucidated. The role of growth parameters in the growth process has been studied. The fundamental understanding of chiral selectivity and diameter selectivity of SWCNTs has been tried. Possible strategies for realizing narrow diameter and chirality distributions have been presented. The optimized chemical state of nanoparticle surface and the degree of ease and spontaneity, with which SWCNTs are grown on this surface, have been found to govern the chiral-selectivity, diameter-selectivity and high growth rates of SWCNTs. Good agreements of the calculated results with available experiments attest to the validity of the present study.

Improvement in growth yield of single-walled carbon nanotubes with narrow chirality distribution by pulse plasma CVD

A pulse plasma chemical vapor deposition (CVD) technique was developed for improving the growth yield of single-walled carbon nanotubes (SWNTs) with a narrow chirality distribution. The growth yield of the SWNTs could be improved by repetitive short duration pulse plasma CVD, while maintaining the initial narrow chirality distribution. Detailed growth dynamics is discussed based on a systematic investigation by changing the pulse parameters. The growth of SWNTs with a narrow chirality distribution could be controlled by the difference in the nucleation time required using catalysts comprising relatively small or large particles as the key factor. The nucleation can be controlled by adjusting the pulse on/off time ratio and the total processing time.

Direct Synthesis of Colorful Single-Walled Carbon Nanotube Thin Films.

In floating catalyst chemical vapor deposition (FC-CVD), tuning chirality distribution and obtaining narrow chirality distribution of single-walled carbon nanotubes (SWCNTs) is challenging. Herein, by introducing various amount of CO2 in FC-CVD using CO as a carbon source, we have succeeded in directly synthesizing SWCNT films with tunable chirality distribution as well as tunable colors. In particular, with 0.25 and 0.37 volume percent of CO2, the SWCNT films display green and brown colors, respectively. We ascribed various colors to suitable diameter and narrow chirality distribution of SWCNTs. Additionally, by optimizing reactor temperature, we achieved much narrower (n,m) distribution clustered around (11,9) with extremely narrow diameter range (>98% between 1.2 and 1.5 nm). We propose that CO2 may affect CO disproportionation and nucleation modes of SWCNTs, resulting in SWCNTs’ various diameter ranges. Our work could provide a new route for high-yield and direct synthesis of SWCNTs with narrow chirality distribution and offer potential applications in electronics, such as touch sensors or transistors.

High temperature growth of single-walled carbon nanotubes with a narrow chirality distribution by tip-growth mode

An MgO supported Co catalyst prepared by atomic layer deposition was applied for the growth of single-walled carbon nanotubes (SWNTs). Different from other catalysts, which generally nucleate SWNTs by base-growth mode, the Co-MgO catalyst uniquely generates SWNTs by tip-growth mode, indicating a weak metal-support interaction. During chemical vapor deposition process, the lift-off of Co nanoparticles from support inhibits the aggregation of Co nanoparticles and favors the synthesis of small-diameter SWNTs with a narrow chirality distribution, even at a high reaction temperature. Particularly, product grown at 800 °C maintains a preferential synthesis of small-diameter (6, 5) SWNTs. The enrichment of (6, 5) SWNT arises from the combination of SWNT-catalyst interface thermodynamics with growth kinetic. The growth dynamics of SWNTs was further investigated by in situ Raman spectroscopy, which revealed that SWNTs with smaller diameters appeared prior to the growth of large-diameter counterparts.

High-yield and high-throughput single-chirality enantiomer separation of single-wall carbon nanotubes

Single crystals of single-wall carbon nanotubes are desired for precise analysis of the physical properties of carbon nanotubes, necessitating preparation of single-chirality enantiomers in large quantities. In this work, by investigating the effects of surfactant concentration and temperature on the enantiomer separation, we achieved large-scale single-chirality enantiomer separation for the first time using triple surfactant stepwise elution chromatography with a naturally produced dextran-based gel as the column medium. Through one-round programmed separation, milligram-scale of (11,–5) (equivalent to (5,6)) and (6,5) enantiomers were separated from CoMoCAT carbon nanotube soot. The separation yields estimated from the optical absorbance at 280 nm were 5.6% (11,−5) and 2.6% (6,5) per load of all SWCNTs, which correspond 28% and 13% per load of total (11,–5) and (6,5) enantiomers, respectively. This high yield is mainly attributed to the narrow chirality distribution and high concentrations of (11,–5) and (6,5) in CoMoCAT and the high-resolution selectivity of the triple surfactant system for two enantiomers. The high throughput is sufficient to prepare high-purity (11,–5) and (6,5) bucky papers.

Anchoring effect of Ni2+ in stabilizing reduced metallic particles for growing single-walled carbon nanotubes

The suitability of the NiMgO catalyst as a catalyst in chiral-selective growth of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition has been assessed. It reveals that catalyst calcination temperature plays an important role in affecting the catalyst performances. Using CO as the carbon precursor and a chemical vapor deposition reaction temperature of 600 °C, NiMgO pre-calcined at 600 °C demonstrates the best performances in catalyzing the growth of SWNTs with predominant (6, 5) species. Systematic characterizations on catalysts calcinated at different temperatures indicate that Ni2+ ions diffuse towards the interior of MgO matrix upon annealing. DFT-based calculations reveal that the binding energy between Ni2+ and adjacent Ni(0) is larger than that between Mg2+ and Ni (0), while Ni2+ situated deep inside MgO has weak interactions with surface Ni atoms. This work highlights the importance of subsurface Ni2+ in anchoring reduced surface Ni atom, which inhibits the aggregation of Ni particles and therefore, facilitates the growth of SWNTs with a narrow chirality distribution.

High‐Efficiency Separation of (6,5) Carbon Nanotubes by Stepwise Elution Gel Chromatography

We adopted stepwise elution gel chromatography method for CoMoCAT single‐walled carbon nanotubes that is well known as narrow chirality distribution. High‐efficiency separation of (6,5) single‐chirality is realized. Because of high throughput of this method, even a lab scale apparatus can separate 0.43 milligram of (6,5) per one‐round separation procedure (4.5 h) with a separation yield of 24% with respect to the loaded (6,5) nanotubes. This method can also separate high‐purity metallic and rest of semiconducting carbon nanotubes simultaneously. Because of the high‐scalability of column chromatography, this high‐efficiency method is suitable for the large scale production of single chirality single‐walled carbon nanotubes in an industrial scale.

Selective Growth of Subnanometer Diameter Single-Walled Carbon Nanotube Arrays in Hydrogen-Free CVD.

Small diameter single-walled carbon nanotube (SWNT) arrays with larger bandgap are more desirable as near-infrared optical absorbers for the fabrication of high performance photovoltaic and photodetector devices. We report herein a rational approach to selective growth of well-aligned subnanometer diameter (∼84% between 0.75 and 0.95 nm) SWNT arrays with a density of 0.3-0.5 tubes/μm on quartz surfaces using solid Mo2C catalysts for short-time growth by low carbon feeding in hydrogen-free CVD. These subnanometer diameter SWNTs have a narrow chirality distribution (the ratio of (8,4), (8,5) and (7,6) is higher than 73%). During nanotube growth, only small size Mo nanoparticles are carbonized into stable Mo2C for catalyzing the growth of SWNTs through low carbon feeding rate over short time in the hydrogen-free environment, whereas larger catalysts are inactive due to underfeeding. Meanwhile, solid Mo2C catalysts are effective in reducing the chirality distributions of the as-grown SWNTs. Additionally, combining an annealing process after loading catalyst on the sapphire substrates, the average density is increased to ∼15 tubes/μm while maintaining small diameter and narrow chirality distribution. Our results offer more choices for structurally controlled growth of aligned-SWNTs, with potential applications in nanoelectronics.

Chiral-selective growth of single-walled carbon nanotubes on Fe-based catalysts using CO as carbon source

MgO supported monometallic Fe (FeMgO) and bimetallic FeMn (FeMnMgO) catalysts were developed for growing carbon nanotubes using CO as carbon source. Characterizations on the FeMgO catalyst revealed that Fe cations were well-dispersed in the porous MgO support, forming a solid solution. Since most Fe cations in the solid solution are difficult to reduce, FeMgO can only catalyze the growth of single-walled carbon nanotubes (SWCNTs) at temperatures of 700 °C and above. While Fe cations in FeMnMgO could be reduced at lower temperatures with the assistance of Mn, catalyzing the subsequent growth of SWCNTs. Compared with most SWCNTs grown on components like Co or Ni at ambient reaction pressure, the Fe-catalyzed SWCNTs demonstrate rather narrow chirality distributions. Particularly, preferential (6, 5) tube growth on FeMnMgO was achieved at 600 °C. The narrow SWCNT chirality distribution could be inherently related to the high carbon solubility of Fe nanoparticles, favoring the nucleation of SWCNTs by a perpendicular mode. The present studies not only offer essential insights into SWCNT growth mechanisms, but can guide the design of novel catalysts for chirality-controlled growth of SWCNTs.

Single-walled carbon nanotube synthesis using Pt catalysts under low ethanol pressure via cold-wall chemical vapor deposition in high vacuum

We study single-walled carbon nanotube (SWCNT) growth from Pt catalysts by a cold-wall chemical vapor deposition (CVD) method in a high vacuum using a nozzle injector for the ethanol gas supply. By optimizing the ethanol pressure, we could grow SWCNTs between 330 and 700 °C, and the optimal ethanol pressure to obtain the highest SWCNT yield was reduced as the growth temperature decreased. Using transmission electron microscopy, Raman spectroscopy and photoluminescence measurements, we determined the diameters and chirality distribution of the SWCNTs and showed that the grown SWCNTs have a narrow chirality distribution and that the diameters of most SWCNTs were below 1.0 nm. In addition, based on the size distribution and the chemical states of Pt catalyst particles, we proposed a growth model of the SWCNTs.

Some possible rules governing the syntheses and characteristics of nanotubes, particularly carbon nanotubes

Universally true rules constitute the very foundation of modern science and engineering. They are, in fact, the backbones of modern science and engineering. Nanotubes are promising materials, and nanotube science and engineering do not yet, to our knowledge, have these rules. Attempts have been made to explore if nanotube syntheses and characteristics follow any rule. Simple theoretical calculations were performed. Results of these calculations suggest that there may indeed be well-defined rules for nanotubes. The theoretical predictions are widely supported by available experiments. They indicate that the proposed rules may have broad appeal. They may have implication on exploring mechanisms most useful for growths of vertically aligned nanotubes of narrow chirality distributions.

Enrichment of large-diameter semiconducting SWCNTs by polyfluorene extraction for high network density thin film transistors

A systematic study on the use of 9,9-dialkylfluorene homopolymers (PFs) for large-diameter semiconducting (sc-) single-walled carbon nanotube (SWCNT) enrichment is the focus of this report. The enrichment is based on a simple three-step extraction process: (1) dispersion of as-produced SWCNTs in a PF solution; (2) centrifugation at a low speed to separate the enriched sc-tubes; (3) filtration to collect the enriched sc-SWCNTs and remove excess polymer. The effect of the extraction conditions on the purity and yield including molecular weight and alkyl side-chain length of the polymers, SWCNT concentration, and polymer/SWCNT ratio have been examined. It was observed that PFs with alkyl chain lengths of C10, C12, C14, and C18, all have an excellent capability to enrich laser-ablation sc-SWCNTs when their molecular weight is larger than ∼10 000 Da. More detailed studies were therefore carried out with the C12 polymer, poly(9,9-di-n-dodecylfluorene), PFDD. It was found that a high polymer/SWCNT ratio leads to an enhanced yield but a reduced sc-purity. A ratio of 0.5-1.0 gives an excellent sc-purity and a yield of 5-10% in a single extraction as assessed by UV-vis-NIR absorption spectra. The yield can also be promoted by multiple extractions while maintaining high sc-purity. Mechanistic experiments involving time-lapse dispersion studies reveal that m-SWCNTs have a lower propensity to be dispersed, yielding a sc-SWCNT enriched material in the supernatant. Dispersion stability studies with partially enriched sc-SWCNT material further reveal that m-SWCNTs : PFDD complexes will re-aggregate faster than sc-SWCNTs : PFDD complexes, providing further sc-SWCNT enrichment. This result confirms that the enrichment was due to the much tighter bundles in raw materials and the more rapid bundling in dispersion of the m-SWCNTs. The sc-purity is also confirmed by Raman spectroscopy and photoluminescence excitation (PLE) mapping. The latter shows that the enriched sc-SWCNT sample has a narrow chirality and diameter distribution dominated by the (10,9) species with d = 1.29 nm. The enriched sc-SWCNTs allow a simple drop-casting method to form a dense nanotube network on SiO2/Si substrates, leading to thin film transistors (TFTs) with an average mobility of 27 cm(2) V(-1) s(-1) and an average on/off current ratio of 1.8 × 10(6) when considering all 25 devices having 25 μm channel length prepared on a single chip. The results presented herein demonstrate how an easily scalable technique provides large-diameter sc-SWCNTs with high purity, further enabling the best TFT performance reported to date for conjugated polymer enriched sc-SWCNTs.

Selective synthesis of single-walled carbon nanotubes on Fe–MgO catalyst by chemical vapor deposition of methane

Abstract The selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality and diameter distribution by methane decomposition over Fe–MgO catalyst is reported. The catalyst was examined by nitrogen physisorption, X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. High resolution electron microscopy, Raman and optical absorption spectroscopy, temperature programmed oxidation, energy dispersive X-ray spectroscopy and nitrogen physisorption were used to probe reaction selectivity, SWCNT chirality and diameter distribution, carbon yield and effectiveness of purification protocols. The yield of carbon increased with an increase in temperature, although SWCNTs selectivity decreased above the optimum synthesis temperature. Results established a clear link between the degree of dispersion of iron oxide species inside the MgO lattice and the catalyst selectivity for SWCNT growth.

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.