Carbyne: A simple yet strong chain of carbon atoms

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Carbyne is a chain of carbon atoms: a truly one-dimensional material. Carbyne’s electronic properties have promising applications in nanoelectronics and photonics. However, preparing such materials is challenging. Researchers at the International Center for Polaritonics at Westlake University, led by Professor Alexey Kavokin, have studied monoatomic carbyne chains containing 8 to 24 carbon atoms, which were stabilised using gold nanoparticles and water at the Vladimir State University. These chains are molecular wires that can conduct electric current. Applying a voltage facilitates the deposition and ordering of one-dimensional crystals on a glass surface.

Carbon can exist in different forms called allotropes. Many of these carbon allotropes are semiconductors or semimetals, such as graphite, graphene, fullerenes and nanotubes. Semiconductors/semimetals have an electrical conductivity between that of a conductor (such as copper) and an insulator (such as glass), and their resistance to electric current falls as temperature increases. Semiconductors are used in electrical devices and carbon allotropes have promising applications in nanoelectronics – electronic components on the molecular scale.

One-dimensional carbon crystals

Certain semiconductors have been demonstrated to emit light and have been used in light-emitting diodes and lasers, for example. The known carbon allotropes, however, are poor light emitters. In theory, one-dimensional carbon crystals may be able to conduct electricity and emit light and so they are of great interest for applications in optoelectronics – the study of light-emitting devices. One-dimensional carbon crystals do exist in nature, but they are difficult to extract from natural sources without causing their degradation. The freestanding carbon chains needed to form such crystals, have also proven difficult to synthesise.

Nasky/Shutterstock.com

Lev Landau, who won the Nobel Prize in 1962 for his research into the physics of condensed matter, predicted these challenges. He demonstrated that a perfect, infinite, one-dimensional chain of atoms would be unstable at any temperature. Small fluctuations of the distances between the carbon atoms causes bending and folding of the chain, thus destroying the crystal symmetry. According to Landau’s theory, the maximum number of carbon atoms in a straight chain is six.

“Remarkably, carbon needles with metallic ends appeared to be sensitive to external electric fields.”

However, recent research efforts at the International Center for Polaritonics, led by Professor Alexey Kavokin, have produced a finite-size stable chain of carbon atoms, resulting in a revolutionary development in this field. In a project led by Dr Stella Kutrovskaya, Westlake University, China, Professor Pavlos Lagoudakis, Hybrid Photonics Laboratories, Skoltech, Russia and their colleagues from Vladimir State University, Russia, have successfully synthesised one-dimensional crystals with straight chains containing between 8 and 24 carbon atoms. Excitingly, the researchers demonstrate that these new materials have some interesting electronic properties that could offer a host of applications. The international researchers have recently published their findings in a series of papers.

Lev Landau predicted the challenges of extracting carbon crystals from nature.

Carbon chains

The carbon chains were obtained by mixing a colloidal suspension of shungite (a mineral form of carbon) with gold nanoparticles (cleaned with a laser) in distilled water. The role of the water was to reduce the strain forces responsible for bending the long carbon chains which were being formed. The gold nanoparticles attached to both ends of the carbon chains to stabilise them. Overall, the water and the gold permitted the creation of chains longer (8 to 24 carbons) than predicted by Landau’s theory, which does not take these stabilising factors into account.

The formation of the chains was driven by a laser and an electric field. The laser enabled the gold nanoparticles to supply the electric charge needed to chemically bond with the carbon chains. The electric field lowered the pH of the colloidal mixture, which reduced the zeta-potential (the electric charge at the interface between the water attached to the particles and the bulk water). This lowered the energy barriers and strengthened the Van der Waal’s interactions (hydrophobic forces) between assembling carbon atoms.

Fabricating the world’s thinnest wires

Monoatomic carbon chains (carbyne) are held together by either double or alternating single and triple bonds. They can be considered as ultra-thin, molecular wires, and are truly one-dimensional (unlike atom-thin sheets of graphene that have a top and a bottom, or hollow nanotubes). Electrons can be injected into these wires through gold nanoparticles serving as electric contacts. These carbon needles with metallic ends appeared to be sensitive to external electric fields, which made it possible to order them on a solid substrate such as glass. Alexey Kavokin explains, ‘The developed synthesis and deposition technology allows us to dream of integrated circuits made of atomically thin wires connecting nano-size transistors and diodes’.

Graphite and diamond are allotropes of carbon.

Based on recent theoretical work, two different allotropes were proposed for the carbon chains: cumulene, which has carbon atoms linked by double bonds (C=C=C=C=C=C), and polyyne or carbyne, which has alternate single and triple bonds between carbon atoms (C-C≡C-C≡C-C).

The Raman spectra of the deposited crystals were consistent with a polyyne structure with peaks at 1,050 and 2,150 cm-1 corresponding to single and triple bonds, respectively.

Electron diffraction data further corroborated the carbon chains having a polyyne form and also provided evidence for a one-dimensional crystal structure with kinks (step-like defects of the linear chain) due to the single bonds between carbon atoms.

In addition, direct visualisation of the carbyne wires attached to the gold nanoparticles was achieved using high resolution transmission electron microscopy.

Gold nanoparticles produced using a laser. Georgy Shafeev/Shutterstock.com
Distilled water, used to obtain the carbon chains. ggw/Shutterstock.com

Light emission

The electrons in carbon allotropes are arranged in molecular orbitals. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are of particular relevance. The absorption and the emission of light corresponds to electrons traversing the energy gap (the band gap) between the HOMO and the LUMO.

It has been established that the molecular orbitals associated with carbon-to-carbon triple bonds (known as sp hybrid orbitals) in carbon crystals have band gaps appropriate for light emission. In contrast, the carbon-to-carbon double bonds (known as sp2 hybrid orbitals) present in most carbon allotropes are unsuitable for light emission.

The research team carried out photoluminescence spectroscopy at various low temperatures and showed that the size of the band gap for the synthesised polyynes depends upon the chain length, i.e., the shorter the chain the greater the band gap. At 4 Kelvin, photoluminescence spectroscopy produced triplet peaks, composed of a sharp intense peak accompanied by two broad satellite peaks. The authors suggest that this is indicative of the presence of quasiparticles.

“Remarkably, carbon needles with metallic ends appeared to be sensitive to external electric fields.”

The concept of quasiparticles was developed by Lev Landau to describe emergent phenomena that occur when a solid behaves as if it contained different weakly interacting particles in a vacuum. For example, an electron travelling through a semiconductor has its motion perturbed by interactions with other electrons and with atomic nuclei.

The absorption and emission of light corresponds to electrons crossing the energy gap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). magnetix/Shutterstock.com

‘Exitons’ and ‘trions’

Professor Kavokin and Dr Kutrovskaya have detected two types of quasiparticles at the ends of the chains attached to the gold nanoparticles: ‘exitons’ and ‘trions’. An exiton consists of an electron and an electron hole (the absence of an electron from a position where one could exist) bound together by the electrostatic force, like the proton and the electron in a hydrogen atom. A negatively charged trion is composed of two electrons and an electron hole, and a positively charged trion is made of one electron and two electron holes.

Exitons are extremely important for applications in optoelectronics, laser applications, and quantum information processing. Professor Kavokin explains, ‘the discovery of excitons in monoatomic carbon chains paves the way to applications of carbon nanowires in world-smallest lasers or single-photon sources’. Trions have been previously studied in semiconductor quantum structures but they have only recently been detected in carbon crystals, so their detection in these chains is hugely significant.

New carbon-based nano-optoelectronics
The future looks promising. The fabrication of one-dimensional carbon chains may trigger an explosive development of carbon-based nano-optoelectronics. Considering the rapid development of quantum cryptography that requires compact and reliable single-photon sources, monoatomic carbon chains may well find other areas of application too.

An exiton is an electron and an electron hole bound together by electrostatic force.

Do you think that it will be possible to prepare even longer one-dimensional carbon chains?

Definitely, yes. Actually, we believe that these longer chains may be found in the samples we have grown already. We could not see them because their optical response is masked by stronger signals coming from a multitude of shorter chains. Currently, we are developing methods to single out individual chains of required lengths. Once this is done, chains longer than 32 atoms may be used for the realisation of nano-lasers emitting at sub-millimetre wavelengths, according to our estimations.

 

References

DOI
10.26904/RF-136-1450703609

Research Objectives

The International Center for Polaritonics is a world-leading research center combining the expertise and resources of three major research groups: the Quantum Optoelectronics Group headed by Dr Pavlos Savvidis, the Carbyne Group headed by Dr Stella Kutrovskaya, and the Theory Group headed by Prof Alexey Kavokin.

Funding

Westlake University, Project Number 041020100118, the Program 2018R01002 funded by Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang.

Collaborators

We acknowledge the team from Vladimir State University for the performed synthesis and deposition of LLCC. Raman spectra and absorbance were measured at the Center for Optical and Laser Materials Research, Research Park, St. Petersburg State University. We thank Prof Lagoudakis’ lab in Skoltech University for TRPL study at low temperatures.

Bio

Stella Kutrovskaya gained her PhD from the Moscow State University in 2012, and joined the faculty of Vladimir State University as an associate professor, and the Russian Quantum Center as a senior scientist. Since 2018 she has led the Carbyne Laboratory at the Westlake University, China, where she serves as a research professor.

Pavlos Lagoudakis obtained his PhD from the University of Southampton, UK, in 2003, and his chair professorship in 2008. Since 2015 he setup and heads the Hybrid Photonics Laboratories in Skoltech, Russia.

Alexey Kavokin received his PhD from the Ioffe Institute, Russia, in 1993 and his professorship from the Blaise Pascal University, France, in 1998. He became a chair professor at the University of Southampton, UK (2005) and the Westlake University (2018). He founded the Spin Optics Laboratory at the State University of St Petersburg (2011) and the Quantum Polaritonics Group at the Russian Quantum Center (2014).

Contact

Dr Stella Kutrovskaya
E: stella.kutrovskaya@westlake.edu.cn
T: +8618368165645

Prof Pavlos Lagoudakis
E: p.lagoudakis@skoltech.ru
T: +79856678864

Prof Alexey Kavokin
E: a.kavokin@westlake.edu.cn
T: +8613588434766

W: https://icp.westlake.edu.cn

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