EspLORE: carbon atomic wires and the future of carbon nanomaterials

EspLORE: carbon atomic wires and the future of carbon nanomaterials

The ERC EspLORE project aims at addressing the potential of carbon atomic wires to develop novel functional coatings for advanced technology applications.

Carbon is cheap and abundant, and carbon-based materials, in many different forms from graphitic carbon, carbon black, synthetic diamond and carbon fibre composites (CFCs), play a leading role in a wide number of technology applications. In the last 30 years, the advent of fullerenes, nanotubes and graphene has pushed further the potential of carbon, opening it up to nanotechnology applications.

Research efforts on graphene, supported by European initiatives such as the Graphene Flagship, are now looking at transferring its outstanding properties from science to industry and, in some cases, commercial products containing nanotubes or graphene are already on the market. All these achievements have nurtured the interest in looking for novel carbon allotropes including 1-dimensional carbon nanostructures such as carbyne.1

What is carbyne?

Carbon atom shows a peculiar versatility in producing a zoology of different structures, a chief example being represented by the wide variety of molecules of organic chemistry. In fact, the family of alkanes, alkenes and alkynes groups molecules containing carbon atoms with sp3, sp2 and sp hybridisation, respectively. While all the possible hybridisations of the carbon atom occur in carbon-based molecules, the same does not happen for pure carbon forms in the solid state. In fact, besides graphite and diamond, the allotropes of carbon based on sp2 and sp3 hybridisation, respectively, scientists are looking for the ‘lacking’ allotrope based on sp hybridisation.

Such an allotrope consists of a solid made by linear monoatomic chains, i.e. carbynes (see Fig. 1). As graphene is the ultimate 2D carbon system (one atom thick), carbyne represents the ultimate 1D structure made of carbon (one atom in diameter) with the ideal form of an infinite linear chain of covalently bonded carbon atoms.

Carbyne can assume two distinct possible structural configurations: a chain with alternated single-triple bonds (polyyne) and a chain with a sequence of all double bonds (cumulene). Theoretical analysis indicates cumulene as a metal and polyyne as a semiconductor/insulator.2 The present difficulties in controlling the chirality of carbon nanotubes and in opening a gap in graphene make sp-carbon structures appealing systems for potential applications.

Outstanding properties predicted by theory

In the last few years the properties of carbyne attracted the interest of theoreticians and recent calculations have outlined outstanding properties potentially able to outperform any other existing material, in particular:
1) Ultra-high mechanical properties with the prediction of the highest mechanical properties (i.e. Young modulus) ever predicted
2) Super high thermal conductivity outperforming diamond and graphene
3) Large electron mobility comparable to graphene
4) Very high optical absorption
5) Huge effective surface tripling graphene

While carbyne is the ideal infinite chain, realistic systems have a well-defined length (i.e. number of sp-carbon atoms) and terminations. In such carbon atomic wires, optical and electronic properties show a strong relation with the structure, thus opening the possibility to tune the properties by controlling the length (i.e. n° of carbon atoms) and the chain terminations. As an example, by controlling the type of termination and the wire length, the properties can be modulated from metal-like to semiconductor and insulator and correspondingly the optical gap from the visible to the UV.3 Hence, the control of structure can open new perspectives for exploiting these systems as functional building blocks of novel nanostructured materials.

EspLORE for emerging carbon nanomaterials

The present knowledge of carbyne and carbon atomic wires and the fundamental research conducted so far naturally open a number of opportunities and challenges that need to be addressed for a realistic implementation of carbon atomic wires as a new class of materials for engineering applications.

Hence the main aim of the project EspLORE (‘Extending the science perspectives of linear wires of carbon atoms from fundamental research to emerging materials’) is to exploit the potential of carbon wires to fabricate emerging materials for advanced applications.
EspLORE is a five-year project (2017-2022) funded by the European Research Council (ERC) under the Consolidator Grant scheme (ERC-CoG Grant N° 724610). It is based on both experimental and theoretical research with a multidisciplinary approach merging different competencies from physics and chemistry to material science and engineering and nanotechnology. The project wants to contribute to the foundations of the engineering of carbon wire-based materials and their realistic implementation in advanced technological applications. These materials, able to provide complementary properties to graphene, will synergistically contribute to open new perspectives for an innovative ‘all-carbon’ approach to present and future challenges in many fields of engineering and technology.

Recent strategies allow for fabrication of stable carbon atomic wires

The interest in carbyne has seen ups and downs in the last 50 years. One of the major concerns has been the chemical stability and carbyne is still considered elusive or even impossible by the wider scientific community. However, in the last few years research has shown the capability to produce stable carbon atomic wires. The main strategy is based on the use of suitable terminations able to provide stability against cross-linking reactions leading to graphitic carbon.

At present, there are many different examples of carbon atomic wires with long stability in ambient conditions and even at moderate temperatures. Different fabrication techniques are presently available, both chemical and physical, and ranging from film deposition in vacuum to liquid-based.4,5 The challenge is the scale up of the fabrication and the proper control of the wire structure to engineer the overall properties of the material for the targeted applications.

Towards novel 2D carbon materials

The wire terminations play a key role in both imparting stability and in tuning the electronic properties from metal-like to semiconductor-like.3 By using sp2 carbon as a termination, it is possible synthesise sp-sp2 carbon atomic wires up to the model graphene-wire-graphene system (see Fig. 2).

In addition, the exploitation of carbon atomic wires allows to build interesting 2D materials by a combination of sp and sp2 hybridised carbon atoms (see Fig. 2). Such structures are known as ‘graphyne’ or ‘graphdiyne’, depending on the length of the linear connections. The simplest one is made of linear chains connected in hexagonal fashion in a sort of super-graphene in which the porosity can be modulated by the length of the linear connections.

Graphynes and graphdiynes were first proposed in 1987, but only recently theoretical calculations have outlined peculiar properties such as the high electron mobility and the existence of multiple Dirac’s cones. Theoretical and experimental research is now looking at this class of materials for applications ranging from nanoelectronics to photovoltaics and Li-ion batteries and to water purification. Other completely novel nanostructures can be foreseen by considering the rolling up of graphyne and graphdiyne in forming nanotubes or closed cages in a sort of fullerene made of sp-carbon wires.

Within EspLORE, we are focusing on all the relevant aspects for the development of novel sp-carbon based materials with activities already having been established and which include quantum-chemical calculations of 1D and 2D sp-carbon based systems, the fabrication of size-selected wires and the atomic-scale investigation by state of the art microscopy techniques, the development of nanocomposite materials and the assessment of functional properties for applications in optoelectronics, energy conversion devices and energy storage.

References
1 C.S. Casari, A Milani ‘Carbyne. ‘From the elusive allotrope to stable carbon atom wires’. MRS Communications 8(2) 2, 207-219 (2018)
2 C.S. Casari, M. Tommasini, R.R. Tykwinski, A. Milani. ‘Carbon-atom wires: 1-D systems with tunable properties’. Nanoscale 8, 4414 (2016)
3 A. Milani, M. Tommasini, V. Barbieri, A. Lucotti, V. Russo, F. Cataldo, C.S. Casari ‘Semiconductor-to-Metal Transition in Carbon-Atom Wires Driven by sp2 Conjugated End Groups’. J. Phys. Chem. C, 121 (19), 10562–10570 (2017)
4 F. Cataldo, O. Ursini, A. Milani, C. S. Casari ‘One-pot synthesis and characterization of polyynes end-capped by biphenyl groups (α,ω-biphenylpolyynes)’. Carbon 126, 232-240 (2018)
5 C.S. Casari, C.S. Giannuzzi, V. Russo. ‘Carbon-atom wires produced by nanosecond pulsed laser deposition in a background gas’. Carbon 104, 190-195 (2016)

Carlo S. Casari
Associate Professor in the physics
of matter
Department of Energy
Politecnico di Milano
+39 02 2399 6331
carlo.casari@polimi.it
www.esplore.polimi.it

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