Atomistic simulation of the growth of defect-free carbon nanotubes

Evolution Mechanism of Solid-Phase Catalysts During Catalytic Growth of Single-Walled Carbon Nanotubes

Using solid nanoparticles (NPs) as catalysts is the most effective method to achieve catalytic growth of single-walled carbon nanotubes (SWCNTs) with ultrapure chirality. Until now, SWCNTs with a suitable chirality purity have not been prepared in experiments. That is, the evolution of solid NPs during the catalytic growth of SWCNTs is in contradiction with the original concept of a changeless structure. Hence, in this work, the evolution mechanism of solid cobalt NPs during the nucleation process of SWCNTs is analyzed through molecular dynamics. Similar to the experimental observations, the results show that a drastic structural fluctuation of the NPs occurs during the nucleation of SWCNTs. This structural fluctuation is caused by the fact that the elastic strain energy and surface energy of the NPs can be tuned when a carbon gradient exists between the subsurface and interior of the NP. Furthermore, such a carbon gradient can be reduced by changing the carbon feeding rate. This work not only reveals the evolution mechanism of solid catalysts during the nucleation of SWCNTs but also provides prospects for realizing solid catalysts with a changeless structure by tuning the experimental parameters.

Dynamics of growing carbon nanotube interfaces probed by machine learning-enabled molecular simulations

Carbon nanotubes (CNTs), hollow cylinders of carbon, hold great promise for advanced technologies, provided their structure remains uniform throughout their length. Their growth takes place at high temperatures across a tube-catalyst interface. Structural defects formed during growth alter CNT properties. These defects are believed to form and heal at the tube-catalyst interface but an understanding of these mechanisms at the atomic-level is lacking. Here we present DeepCNT-22, a machine learning force field (MLFF) to drive molecular dynamics simulations through which we unveil the mechanisms of CNT formation, from nucleation to growth including defect formation and healing. We find the tube-catalyst interface to be highly dynamic, with large fluctuations in the chiral structure of the CNT-edge. This does not support continuous spiral growth as a general mechanism, instead, at these growth conditions, the growing tube edge exhibits significant configurational entropy. We demonstrate that defects form stochastically at the tube-catalyst interface, but under low growth rates and high temperatures, these heal before becoming incorporated in the tube wall, allowing CNTs to grow defect-free to seemingly unlimited lengths. These insights, not readily available through experiments, demonstrate the remarkable power of MLFF-driven simulations and fill long-standing gaps in our understanding of CNT growth mechanisms.

Carbon nanowires made by the insertion-and-fusion method toward carbon-hydrogen nanoelectronics

Carbon nanowires (CNWs), long linear carbon chains encapsulated inside carbon nanotubes, exhibit sp hybridization characteristics as one of one-dimensional nanocarbon materials. The research interests on CNWs are accelerated by the successful experimental syntheses from the multi-walled to double-walled until single-walled CNWs recently but the formation mechanisms and structure-property relationships of CNWs remain poorly understood. In this work, we studied the insertion-and-fusion formation process of CNWs at an atomistic level using ReaxFF reactive molecular dynamics (MD) and density functional theory (DFT) calculations with particular focus on the hydrogen (H) adatom effects on the configurations and properties of carbon chains. The constrained MD shows that short carbon chains can be inserted and fused into long carbon chains inside the CNTs due to the van der Waals interactions with little energy barriers. We found that the end-capped H atoms of carbon chains may still remain as adatoms on the fused chains without C-H bond breaking and could transfer along the carbon chains via thermal activation. Moreover, the H adatoms were found to have critical effects on the distribution of bond length alternation as well as the energy level gaps and magnetic moments depending on the varied positions of H adatoms on the carbon chains. The results of ReaxFF MD simulations were validated by the DFT calculations and ab initio MD simulations. The diameter effect of the CNTs on the binding energies suggest that multiple CNTs with a range of appropriate diameters can be used to stabilize the carbon chains. Different from the terminal H of carbon nanomaterials, this work demonstrated that the H adatoms could be used to tune the electronic and magnetic properties of carbon-based electronic devices, opening up the door toward rich carbon-hydrogen nanoelectronics.

Is the Carbon Nanotube-Catalyst Interface Clean during Growth?

Revealing a "true" picture of the carbon nanotube (CNT) growth front at the catalyst surface is critical to understanding the mechanism of CNT growth. If the CNT-catalyst interface is clean or messy, which will greatly affect the mechanism of controlled CNT growth, has never been properly solved either experimentally or theoretically. Here, this issue by ab initial calculation-based kinetic analysis and classical molecular dynamic (MD) simulations is revisited. It is found that the appearance of carbon chains at the CNT-catalyst interfaces or the "messy" interfaces in MD simulations is a consequence of the very short simulation time, and a "clean" CNT-catalyst interface will emerge if the simulation time is close to that in real experiments. This study reveals that, during real CNT experimental growth, a "clean" CNT-catalyst interface with zigzag, armchair, and/or kink sites dominates the growth kinetics, and therefore, the controllable CNT growth by tuning the CNT-catalyst interface is feasible.

Catalytic Growth of Ultralong Graphene Nanoribbons on Insulating Substrates

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 µm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of ≈1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. This study provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.

The promoter role of sulfur in carbon nanotube growth

We investigate the effect of sulfur on the interaction of iron catalyst nanoparticles and carbon nanotubes (CNTs), typically present in a floating catalyst chemical vapor deposition (FCCVD) process. As a reference, the interaction of graphene with the Fe fcc(111) surface is used. In both systems we performed a systematic density functional theory (DFT) study on the interaction with different sulfur contents. We found that the presence of sulfur changes the nature and strength of interaction between graphene and the iron surface from strong chemisorption to weak physisorption. Furthermore, sulfur significantly reduces the CNT-iron binding, indicating a beneficial effect on the CNT growth and its promoter role. We believe that these results induce further experimental studies and optimization of the CNT synthesis process.

Why Carbon Nanotubes Grow

Despite three decades of intense research efforts, the most fundamental question "why do carbon nanotubes grow?" remains unanswered. In fact, carbon nanotubes (CNTs) should not grow since the encapsulation of a catalyst with graphitic carbon is energetically more favorable than CNT growth in every aspect. Here, we answer this question using a theoretical model based on extensive first-principles and molecular dynamics calculations. We reveal a historically overlooked yet fundamental aspect of the CNT-catalyst interface, viz., that the interfacial energy of the CNT-catalyst edge is contact angle-dependent. The contact angle increases via graphitic cap lift-off, drastically decreasing the interfacial formation energy by up to 6-9 eV/nm, overcoming van der Waals cap-catalyst adhesion, and driving CNT growth. Mapping this remarkable and simple interplay allows us to understand, for the first time, why CNTs grow.

Dynamic State and Active Structure of Ni-Co Catalyst in Carbon Nanofiber Growth Revealed by in Situ Transmission Electron Microscopy

Alloy catalysts often show superior effectiveness in the growth of carbon nanotubes/nanofibers (CNTs/CNFs) as compared to monometallic catalysts. However, due to the lack of an understanding of the active state and active structure, the origin of the superior performance of alloy catalysts is unknown. In this work, we report an in situ transmission electron microscopy (TEM) study of the CNF growth enabled by one of the most active known alloy catalysts, i.e., Ni-Co, providing insights into the active state and the interaction between Ni and Co in the working catalyst. We reveal that the functioning catalyst is highly dynamic, undergoing constant reshaping and periodic elongation/contraction. Atomic-scale imaging combined with in situ electron energy-loss spectroscopy further identifies the active structure as a Ni-Co metallic alloy (face-centered cubic, FCC). Aided by the molecular dynamics simulation and density functional theory calculations, we rationalize the dynamic behavior of the catalyst and the growth mechanism of CNFs and provide insight into the origin of the superior performance of the Ni-Co alloy catalyst.

Nanoscale Patterning of Carbon Nanotubes: Techniques, Applications, and Future

Carbon nanotube (CNT) devices and electronics are achieving maturity and directly competing or surpassing devices that use conventional materials. CNTs have demonstrated ballistic conduction, minimal scaling effects, high current capacity, low power requirements, and excellent optical/photonic properties; making them the ideal candidate for a new material to replace conventional materials in next-generation electronic and photonic systems. CNTs also demonstrate high stability and flexibility, allowing them to be used in flexible, printable, and/or biocompatible electronics. However, a major challenge to fully commercialize these devices is the scalable placement of CNTs into desired micro/nanopatterns and architectures to translate the superior properties of CNTs into macroscale devices. Precise and high throughput patterning becomes increasingly difficult at nanoscale resolution, but it is essential to fully realize the benefits of CNTs. The relatively long, high aspect ratio structures of CNTs must be preserved to maintain their functionalities, consequently making them more difficult to pattern than conventional materials like metals and polymers. This review comprehensively explores the recent development of innovative CNT patterning techniques with nanoscale lateral resolution. Each technique is critically analyzed and applications for the nanoscale-resolution approaches are demonstrated. Promising techniques and the challenges ahead for future devices and applications are discussed.

Contact-Induced Phase Separation of Alloy Catalyst to Promote Carbon Nanotube Growth

In this Letter, using density functional theory based molecular dynamics simulations, we report that contact to a carbon nanotube (CNT) induces phase separation in an alloy catalyst, which promotes CNT growth. During growth of a CNT, the growth front tends to preferentially bond to the more active metal atom in the alloy catalyst, thus triggering a phase separation of the alloy catalyst particle. The accumulation of the active metal stabilizes the open end of the CNT, attracts carbon precursors to rapidly diffuse to the growth front, and avoids catalyst poisoning by preventing the encapsulation of the catalyst. This study resolves a long-term mystery surrounding the higher efficiency of alloy catalysts in CNT growth as compared to a pure metal catalyst and thereby paves the way to a more rational catalyst design for controlled CNT growth.

Molecular Dynamics of Chirality Definable Growth of Single-Walled Carbon Nanotubes

In order to achieve the chirality-specific growth of single-walled carbon nanotubes (SWCNTs), it is crucial to understand the growth mechanism. Even though many molecular dynamics (MD) simulations have been employed to analyze the SWCNT growth mechanism, it has been difficult to discuss the chirality determining kinetics because of the defects remaining on the SWCNTs grown in simulations. In this study, we demonstrate MD simulations of defect-free SWCNTs, that is, chirality definable SWCNTs, under the optimized carbon supply rate and temperature. The chiralities of the SWCNTs were assigned as (14,1), (15,2), and (9,0), indicating the preference of near-zigzag and pure-zigzag SWCNTs. The SWCNTs contained at least one complete row of defect-free walls consisting of only hexagons. The near-zigzag SWCNTs grew via a kink-running process, in which bond formation between a carbon atom at a kink and a neighboring carbon chain led to formation of a hexagon with a new kink at the SWCNT edge. Defects including pentagons and heptagons were sometimes formed but effectively healed into hexagons on metal surfaces. The pure-zigzag SWCNTs grew by the kink-running and the hexagon nucleation processes. In addition, chirality change events along SWCNTs with incorporation of pentagon-heptagon pair defects were observed in the MD simulations. Here, pentagons and heptagons were frequently observed as adjacent pairs, resulting in ( n, m) chirality changes by (±1,0), (0,±1), (1,-1), or (-1,1).

How a Solid Catalyst Determines the Chirality of the Single-Wall Carbon Nanotube Grown on It

Although the growth of single-wall carbon nanotubes (SWCNTs) with a chirality selectivity up to 90% has been successfully achieved using solid catalysts ( Yang , F. Nature , 2014 , 510 , 522 ; Zhang , S. ; Nature , 2017 , 543 , 234 , etc.), the underlying mechanism that governs the chirality selection is far from clear. Here we propose a mechanism to understand how a solid catalyst particle determines the structure of the SWCNT grown on it. The mechanism has to satisfy three criteria: (i) thermodynamic selection of SWCNTs that possess a structural symmetry the same as that of the catalyst surface; (ii) kinetic elimination of the achiral SWCNTs with extremely low growth rates; (iii) rough control over the catalyst particle size leads to SWCNTs with only one or a few dominant chiralities. Besides the deep understanding on the mechanisms of experimentally synthesized (12, 6) and (8, 4) SWCNTs, the preference growth of other SWCNTs of the (2 n, n) family, such as the (10, 5) or (6, 3) SWCNTs, by using catalyst surface with a 5- or 3-fold symmetry is predicted. Such a simple three-criteria mechanism deepens our understanding of the selective growth of SWCNTs and provides a guideline for catalyst design for controlled SWCNT synthesis.

Modified potential for atomistic simulation of the growth of carbon materials from binary alloy catalysts

A new hybrid bond order potential has been developed and implemented to describe carbon–bimetallic alloy interactions, involved in the catalytic growth of low dimensional carbon materials on the surface of binary alloy catalysts.

Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.

Root-growth of boron nitride nanotubes: experiments and ab initio simulations

We have synthesized boron nitride nanotubes (BNNTs) in an arc in the presence of boron and nitrogen species. We find that BNNTs are often attached to large nanoparticles, suggesting that root-growth is a likely mechanism for their formation. Moreover, the tube-end nanoparticles are composed of boron, without transition metals, indicating that transition metals are not necessary for the arc synthesis of BNNTs. To gain further insight into this process we have studied key mechanisms for root growth of BNNTs on the surface of a liquid boron droplet by ab initio molecular dynamics simulations. We find that nitrogen atoms reside predominantly on the droplet surface where they organize to form boron nitride islands below 2400 K. To minimize contact with the liquid particle underneath, the islands assume non-planar configurations that are likely precursors for the thermal nucleation of cap structures. Once formed, the caps are stable and can easily incorporate nitrogen and boron atoms at their base, resulting in further growth. Our simulations support the root-growth mechanism of BNNTs and provide comprehensive evidence of the active role played by liquid boron.

The kinetics of chirality assignment in catalytic single-walled carbon nanotube growth and the routes towards selective growth

Depending on its specific structure, or so-called chirality, a single-walled carbon nanotube (SWCNT) can be either a conductor or a semiconductor. This feature ensures great potential for building ∼1 nm sized electronics if chirality-selected SWCNTs could be achieved. However, due to the limited understanding of the growth mechanism of SWCNTs, reliable methods for chirality-selected SWCNTs are still pending. Here we present a theoretical model on the chirality assignment and control of SWCNTs during the catalytic growth. This study reveals that the chirality of a SWCNT is determined by the kinetic incorporation of pentagons, especially the last (6^th) one, during the nucleation stage. Our analysis showed that the chirality of a SWCNT is randomly assigned on a liquid or liquid-like catalyst surface, and two routes of synthesizing chirality-selected SWCNTs, which are verified by recent experimental achievements, are demonstrated. They are (i) by using high melting point crystalline catalysts, such as Ta, W, Re, Os, or their alloys, and (ii) by frequently changing the chirality of SWCNTs during their growth. This study paves the way for achieving chirality-selective SWCNT growth for high performance SWCNT based electronics.

Controlled Synthesis of Ultralong Carbon Nanotubes with Perfect Structures and Extraordinary Properties

Carbon nanotubes (CNTs) have drawn intensive research interest in the past 25 years due to their excellent properties and wide applications. Ultralong CNTs refers to the horizontally aligned CNT arrays which are usually grown on flat substrates, parallel with each other with large intertube distances. They usually have perfect structures, excellent properties, and lengths up to centimeters, even decimeters. Ultralong CNTs are promising candidates as building blocks for transparent displays, nanoelectronics, superstrong tethers, aeronautics and aerospace materials, etc. The controlled synthesis of ultralong CNTs with perfect structures is the key to fully exploit the extraordinary properties of CNTs. CNTs are typical one-dimensional single-crystal nanomaterials. It has always been a great challenge how to grow macroscale single-crystals with no defects. Thus, the synthesis of ultralong CNTs with no defect is of significant importance from both fundamental and industrial aspects. In this Account, we focus on our progress on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. A deep understanding of the CNT growth mechanism is the first step for the controlled synthesis of ultralong CNTs with high quality. We first introduce the growth mechanism for ultralong CNTs and the main factor affecting their structures. We then discuss the strategies to control the defects in the as-grown ultralong CNTs. With these approaches, ultralong high-quality CNTs with different structures can be obtained. By completely eliminating the factors which may induce defects in the CNT walls, ultralong CNTs with perfect structures can be obtained. Their chiral indices keep unchanged for several centimeters long along the axial direction of the CNTs. The defect-free structures render the ultralong CNTs with excellent electrical, mechanical and thermal properties. The as-grown ultralong CNTs exhibit superhigh mechanical strength (>100 GPa) and their breaking strain (>17.5%) reach the theoretical limits. They also show excellent electrical and thermal properties. In addition, centimeters long CNTs showed macroscale interwall superlubricious properties due to their defect-free structures. Ultralong, defect-free CNTs with controlled structures are highly desirable for many high-end applications. We hope that this Account will shed light on the controlled synthesis of ultralong CNTs with perfect structures and excellent properties. Moreover, the growth mechanism and controlled synthesis of ultralong CNTs with perfect structures also offers a good model for other one-dimensional nanomaterials.

Horizontally aligned carbon nanotube arrays: growth mechanism, controlled synthesis, characterization, properties and applications

This review summarizes the growth mechanism, controlled synthesis, characterization, properties and applications of horizontally aligned carbon nanotube arrays.

A smart temperature and magnetic-responsive gating carbon nanotube membrane for ion and protein transportation

Carbon nanotube (CNT) nanoporous membranes based on pre-aligned CNTs have superior nano-transportation properties in biological science. Herein, we report a smart temperature- and temperature-magnetic-responsive CNT nanoporous membrane (CNM) by grafting thermal-sensitive poly(N-isopropylacrylamide) (PNIPAM) and Fe₃O₄ nanoparticles (Fe₃O₄-NPs) on the open ends of pre-aligned CNTs with a diameter around 15 nm via surface-initiated atom transfer radical polymerization (SI-ATRP) method. The inner cavity of the modified CNTs in the membrane is designed to be the only path for ion and protein transportation, and its effective diameter with a variation from ~5.7 nm to ~12.4 nm can be reversible tuned by temperature and magnetic field. The PNIPAM modified CNM (PNIPAM-CNM) and PNIPAM magnetic nanoparticles modified CNM (PNIPAM-MAG-CNM) exhibit excellent temperature- or temperature-magnetic-responsive gating property to separate proteins of different sizes. The PNIPAM-CNMs and PNIPAM-MAG-CNMs have potential applications in making artificial cells, biosensors, bioseparation and purification filters.

Large scale atomistic simulation of single-layer graphene growth on Ni(111) surface: molecular dynamics simulation based on a new generation of carbon-metal potential

To explore the mechanism of graphene chemical vapor deposition (CVD) growth on a catalyst surface, a molecular dynamics (MD) simulation of carbon atom self-assembly on a Ni(111) surface based on a well-designed empirical reactive bond order potential was performed. We simulated single layer graphene with recorded size (up to 300 atoms per super-cell) and reasonably good quality by MD trajectories up to 15 ns. Detailed processes of graphene CVD growth, such as carbon atom dissolution and precipitation, formation of carbon chains of various lengths, polygons and small graphene domains were observed during the initial process of the MD simulation. The atomistic processes of typical defect healing, such as the transformation from a pentagon into a hexagon and from a pentagon-heptagon pair (5|7) to two adjacent hexagons (6|6), were revealed as well. The study also showed that higher temperature and longer annealing time are essential to form high quality graphene layers, which is in agreement with experimental reports and previous theoretical results.

ZnO quantum dots decorated on optimized carbon nanotube intramolecular junctions exhibit superior field emission properties

Sensitive optical and superior field emission properties can be attained through the use of ZnO selective quantum well heterostructures grown on carbon nanotubes to fabricate carbon-zinc-oxide (CZO) nanotubes.