Toward Confined Carbyne with Tailored Properties

Synthesis and properties of confined carbyne and beyond

Carbyne, a one-dimensional carbon allotrope characterized by sp1 hybridization, has attracted significant attention due to its unique structure and exceptional properties. In principle, carbyne is an infinite linear carbon chain, or a long linear carbon chain that its properties remain independent of its length. Despite being proposed a century ago, the existence of carbyne has been a subject of controversy, primarily because of its extreme instability and strong chemical reactivity. So far the longest end-capped polyyne and the carbon nanotube-confined linear carbon chain comprise up to 68 and 6000 carbon atoms, respectively. In this review, general synthesis methods for confined linear carbon chains, i.e., confined carbyne, are outlined, with a particular focus on the chronological development of routes towards carbyne. In addition, the structure and properties of the carbon chains unraveled by theoretical calculations and various Raman spectroscopy are discussed in detail. Finally, the current challenges in the synthesis and property-engineering of sp1-hybridized carbon but not limited to confined carbyne are addressed, offering insights into potential future directions for both fundamental research and applications.

Nitrogen Doping of Confined Carbyne

Low-dimensional carbon allotropes belong to the most revolutionary materials of the most recent decades. Confined carbyne, a linear chain of sp1-hybridized carbon encapsulated inside a small-diameter carbon nanotube host, is one extraordinary nanoengineering example. Inspired by these hybrid structures, we demonstrate the feasibility to synthesize nitrogen-doped confined carbyne by using azafullerenes (C59N) encapsulated in nanotubes ("peapods") as precursors for the growth of confined carbyne. Resonance Raman spectroscopy as a site selective local probe has served to identify the changes in the spectra of nitrogen-doped versus pristine carbon peapods and confined carbyne. We are able to disentangle frequency changes due to charge transfer from changes due to the difference in mass for both the nanotube and the carbyne, where different effects dominate. This study demonstrates a suitable pathway to achieve controlled doping of carbyne chains via the use of specifically doped precursors.

van der Waals one-dimensional atomic crystal heterostructure derived from carbon nanotubes

One-dimensional (1D) van der Waals (vdWs) heterojunctions, due to the dimensional reduction leading to 1D quantum confinement effects and interface effects of the heterojunctions, typically exhibit discrete energy levels and strong electron interactions, resulting in unique conductive and optical behaviors. Carbon nanotube (CNT)-derived 1D atomic crystal vdWs heterojunctions represent a new class of 1D vdWs heterojunctions. They leverage the excellent chemical stability, nanoscale cavities, and adjustable diameters provided by CNTs as templates, ensuring controlled synthesis and precise structural tuning. The 1D radial pathways can alter the photonic-electronic propagation characteristics. At the same time, their unique metal-semiconductor-like electronic structure creates conditions for constructing various types of heterojunctions. The CNTs and their encapsulated 1D materials can lead to synergistic enhancement in the fields of electronics, magnetism, and optics. Currently, research is concentrated on understanding the synthesis mechanisms, integration characteristics, and host-guest interactions, and the exploration of novel 1D atomic crystal vdWs heterojunctions derived from CNTs. This review is focused on the latest progress made in 1D vdWs heterojunctions using CNTs as growth templates, emphasizing the construction methods, selection criteria, and the unique properties and applications arising from these complex interfacial electronic or phonon interactions. We also propose several future directions for the development of CNT-derived 1D atomic crystal vdWs heterojunctions. This review aims to enhance the understanding of their synthesis mechanisms and fundamental properties, broaden the range of available materials, and explore new and broader applications.

Metallated carbon nanowires for potential quantum computing applications via substrate proximity

The realization of next-generation quantum computing devices is hindered by the formidable challenge of detecting and manipulating Majorana zero mode (MZM). In this study, we study if MZM exist in metallated carbyne nanowires. Through optimizations of distinct types of metallated carbyne, we have achieved an average magnetic moment surpassing 1μB for the cases of Mo, Tc, and Ru metallated carbyne. where their local moments exceed 2μB. The magnetism of the Ru atom displays periodic variations with increasing carbyne length. associated with a strong average spin-orbital coupling of ∼140meV. When the ferromagnetic Ru metallated carbyne, coupled with a superconducting Ru substrate, could trigger band inversions at the gamma (G) point and M point, where spin-orbital coupling triggers the transition between the band inversion and the Dirac gap. Our findings present an exciting opportunity to realize carbon-based materials capable of hosting MZM.

Systematic Optimization of the Synthesis of Confined Carbyne

Confined carbyne, an sp1-hybridized linear carbon chain inside a carbon nanotube, is a novel material with remarkable properties and potential applications. Among its currently successful synthesis methods, high temperature high vacuum annealing is prevalent. Further optimization could be achieved by tuning the synthesis pathway. Here, a systematic analysis of key synthesis parameters including precursor filling, annealing step sequences, and temperature conditions during high temperature vacuum processing is performed. A novel yield determination model that overcomes previous limitations related to the irregular resonance Raman behavior of carbyne is applied to evaluate bulk yield and realized growth potential. With this refined model, it is possible to make a quantitative assessment of bulk yield optimization potential in multi-step annealing processes. These results provide crucial insights into the fundamental formation mechanisms of confined carbyne, advancing our understanding of this promising hybrid nanomaterial system. It is therefore possible to establish improved protocols for maximizing confined carbyne yields through precise control of synthesis conditions.

A Universal Method to Transform Aromatic Hydrocarbon Molecules into Confined Carbyne inside Single-Walled Carbon Nanotubes

Carbyne, a sp1-hybridized allotrope of carbon, is a linear carbon chain with exceptional theoretically predicted properties that surpass those of sp2-hybridized graphene and carbon nanotubes (CNTs). However, the existence of carbyne has been debated due to its instability caused by Peierls distortion, which limits its practical development. The only successful synthesis of carbyne has been achieved inside CNTs, resulting in a form known as confined carbyne (CC). However, CC can only be synthesized inside multiwalled CNTs, limiting its property-tuning capabilities to the inner tubes of the CNTs. Here, we present a universal method for synthesizing CC inside single-walled CNTs (SWCNTs) with diameters of 0.9-1.3 nm. Aromatic hydrocarbon molecules are filled inside SWCNTs and subsequently transformed into CC under low-temperature annealing. A variety of aromatic hydrocarbon molecules are confirmed as effective precursors for the formation of CC, with Raman frequencies centered around 1861 cm-1. Enriched (6,5) and (7,6) SWCNTs with diameters less than 0.8 nm are less effective than the SWCNTs with diameters of 0.9-1.3 nm for CC formation. Furthermore, resonance Raman spectroscopy reveals that the optical band gap of the CC at 1861 cm-1 is 2.353 eV, which is consistent with the result obtained using a linear relationship between the Raman frequency and optical band gap. This approach provides a versatile route for synthesizing CC from various precursor molecules inside diverse templates, which is not limited to SWCNTs but could extend to any templates with appropriate size, including molecular sieves, zeolites, boron nitride nanotubes, and metal-organic frameworks.

Precursor-Driven Confined Synthesis of Highly Pure 5-Armchair Graphene Nanoribbons

Armchair graphene nanoribbons (AGNRs) known as semiconductors are holding promise for nanoelectronics applications and sparking increased research interest. Currently, synthesis of 5-AGNRs with a quasi-metallic gap has been achieved using perylene and its halogen-containing derivatives as precursors via on-surface synthesis on a metal substrate. However, challenges in controlling the polymerization and orientation between precursor molecules have led to side reactions and the formation of by-products, posing a significant issue in purity. Here a precision synthesis of confined 5-AGNRs using molecular-designed precursors without halogens is proposed to address these challenges. Perylene and its dimer quaterrylene are utilized for filling into single-walled carbon nanotubes (SWCNTs), following a precursor-driven transition into 5-AGNRs by heat-induced polymerization and cyclodehydrogenation. SWCNTs restrict the alignment of confined quaterrylene enabling their polymerization with a head-to-tail arrangement, which results in the formation of pure 5-AGNRs with three times higher yield than that of perylene, as the free rotation capability of perylene molecules inside SWCNTs lead to the formation of 5-AGNRs concomitant with by-products. This work provides a templated route for synthesizing desired GNRs based on molecular-designed precursors and confined polymerization, bringing advantages for their applications in electronics and optoelectronics.

Advanced 1D heterostructures based on nanotube templates and molecules

Recent advancements in materials science have shed light on the potential of exploring hierarchical assemblies of molecules on surfaces, driven by both fundamental and applicative challenges. This field encompasses diverse areas including molecular storage, drug delivery, catalysis, and nanoscale chemical reactions. In this context, the utilization of nanotube templates (NTs) has emerged as promising platforms for achieving advanced one-dimensional (1D) molecular assemblies. NTs offer cylindrical, crystalline structures with high aspect ratios, capable of hosting molecules both externally and internally (Mol@NT). Furthermore, NTs possess a wide array of available diameters, providing tunability for tailored assembly. This review underscores recent breakthroughs in the field of Mol@NT. The first part focuses on the diverse panorama of structural properties in Mol@NT synthesized in the last decade. The advances in understanding encapsulation, adsorption, and ordering mechanisms are detailed. In a second part, the review highlights the physical interactions and photophysics properties of Mol@NT obtained by the confinement of molecules and nanotubes in the van der Waals distance regime. The last part of the review describes potential applicative fields of these 1D heterostructures, providing specific examples in photovoltaics, luminescent materials, and bio-imaging. A conclusion gathers current challenges and perspectives of the field to foster discussion in related communities.

Unravelling the formation of carbyne nanocrystals from graphene nanoconstrictions through the hydrothermal treatment of agro-industrial waste molasses

The delicate synthesis of one-dimensional (1D) carbon nanostructures from two-dimensional (2D) graphene moiré layers holds tremendous interest in materials science owing to its unique physiochemical properties exhibited during the formation of hybrid configurations with sp-sp2 hybridization. However, the controlled synthesis of such hybrid sp-sp2 configurations remains highly challenging. Therefore, we employed a simple hydrothermal technique using agro-industrial waste as the carbon source to synthesize 1D carbyne nanocrystals from the nanoconstricted zones of 2D graphene moiré layers. By employing suite of characterization techniques, we delineated the mechanism of carbyne nanocrystal formation, wherein the origin of carbyne nanochains was deciphered from graphene intermediates due to the presence of a hydrothermally cut nanoconstriction regime engendered over well-oriented graphene moiré patterns. The autogenous hydrothermal pressurization of agro-industrial waste under controlled conditions led to the generation of epoxy-rich graphene intermediates, which concomitantly gave rise to carbyne nanocrystal formation in oriented moiré layers with nanogaps. The unique growth of carbyne nanocrystals over a few layers of holey graphene exhibits excellent paramagnetic properties, the predominant localization of electrons and interfacial polarization effects. Further, we extended the application of the as-synthesized carbyne product (Cp) for real-time electrochemical-based toxic metal (As3+) sensing in groundwater samples (from riverbanks), which depicted superior sensitivity (0.22 mA μM-1) even at extremely lower concentrations (0.0001 μM), corroborating the impedance spectroscopy analysis.

A Simulation of the Effect of External and Internal Parameters on the Synthesis of a Carbyne with More than 6000 Atoms for Emerging Continuously Tunable Energy Barriers in CNT-Based Transistors

Transistors made up of carbon nanotube CNT have demonstrated excellent current-voltage characteristics which outperform some high-grade silicon-based transistors. A continuously tunable energy barrier across semiconductor interfaces is desired to make the CNT-based transistors more robust. Despite that the direct band gap of the carbyne inside a CNT can be widely tuned by strain, the size of the carbyne cannot be controlled easily. The production of a monoatomic chain with more than 6000 carbon atoms is an enormous technological challenge. To predict the optimal chain length of a carbyne in different molecular environments, we have developed a Monte Carlo model in which a finite-length carbyne with a size of 4000-15,000 atoms is encapsulated by a CNT at finite temperatures. Our simulation shows that the stability of the carbyne@nanotube is strongly influenced by the nature and porosity of the CNT, the external pressure, the temperature, and the chain length. We have observed an initiation of the chain-breaking process in a compressed carbyne@nanotube. Our work provides much-needed input for optimizing the carbyne length to produce carbon chains much longer than 6000 atoms at ~300 K. Design rules are proposed for synthesizing ~1% strained carbyne@(6,5)CNT as a component in CNT-based transistors to tune the energy barriers continuously.

Anti-Stokes Raman Scattering of Single Carbyne Chains

We investigate the anti-Stokes Raman scattering of single carbyne chains confined inside double-walled carbon nanotubes. Individual chains are identified using tip-enhanced Raman scattering (TERS) and heated by resonant excitation with varying laser powers. We study the temperature dependence of carbyne's Raman spectrum and quantify the laser-induced heating based on the anti-Stokes/Stokes ratio. Due to its molecular size and its large Raman cross section, carbyne holds great promise for local temperature monitoring, with potential applications ranging from nanoelectronics to biology.

Isotopic Labelling of Confined Carbyne

Carbyne is a one-dimensional allotrope of carbon consisting of a linear chain of carbon atoms bonded to each other with exceptional strength. Its outstanding mechanical, optical, and electronic properties have been theoretically predicted, but its stability has only been achieved when grown encapsulated in the hollow core of carbon nanotubes. One of the advantages of this confinement is that its properties can be controlled by the chain's length and surrounding environment. We investigated an alternative way of gaining control of its properties is using isotope labelling as tuning mechanism. The optimized liquid precursor was first chosen among several options, which can greatly enhance the yield of the confined carbyne. Then isotopic labelled liquid precursor was encapsulated for further synthesis of isotopic labelled confined carbyne. This allowed us to obtain pioneering results on isotope engineered carbyne with around 11.9 % of ¹³ C-labelling using ¹³ C-methanol as precursor.