Linear carbon - the explosive member of the nanocarbon family

(Nanowerk Spotlight) The combination of sp3, sp2, and sp hybridized atoms can give rise to a large number of carbon allotropic forms and phases, starting from carbon crystals based on all sp3 (diamond) and sp2 (graphite, fullerene) are well known and characterized. In addition there are innumerable transitional forms of carbon where sp2 and sp3 hybridization bonds coexist in the same solid such as in amorphous carbon, carbon black, soot, cokes, glassy carbon, etc. Solids based on sp hybridization, although subject of intense experimental efforts, seem to be the most elusive of the different carbon families.
Such one-dimensional (1D) structures - "real" carbon nanowires - are linear chains of carbon atoms linked by alternating single and triple bonds (polyynes) or only double bonds (polycumulene). They are considered the building blocks for the elusive "carbyne": an ideal crystal constituted by carbon atoms with sp hybridization only. In solid and stable form this would represent a new carbon allotrope whose existence was a matter of great debate in the 1980s.
With the immense interest in carbon nanomaterials, sp carbon nanostructures have become objects of renewed interest in recent years since they are considered precursors in the formation of fullerenes and carbon nanotubes; moreover they are interesting in astrophysics since they are considered constituents of interstellar dust. 1D carbon nanowires are expected to show interesting optical, electrical and mechanical properties. Some techniques already permit the synthesis of linear carbon chains in solution. However, their extremely high reactivity against oxygen - they can literally explode - and a strong tendency to interchain crosslinking makes synthesis of pure carbyne solids a major challenge.
Researchers in Italy have now presented a simple method to obtain a solid system where polyynes in a silver nanoparticle assembly display long-term stability at ambient conditions.
"Sp carbon chains display high non-linear optical properties" Dr. Carlo Casari explains to Nanowerk. "In order to develop an optical device a solid and stable material is required. Stabilization of sp chains by silver nanoparticles addresses these issues (and it represents also an easy to handle system) opening the possibility to develop an optical device for instance in the form of a thin film."
Casari is a research professor in the Micro and Nanostructured Materials Lab (Prof. C.E. Bottani) at the Politecnico di Milano in Italy. He is first author of a recent paper in Applied Physics Letters describing the capability to obtain a solid system containing stable linear carbon structures by simply drying a solution of sp carbon chains mixed with silver colloids ("Stabilization of linear carbon structures in a solid silver nanoparticle assembly"). This work is in collaboration with the Vibrational Spectroscopy and Molecular Dynamics Lab (prof. G. Zerbi) at the Politecnico di Milano and with prof. F. Cataldo (Soc. Lupi Chemical Research, Rome).
Casari notes that linear carbon chains in solution are stable thanks to the solution itself that maintains the chains isolated and prevents cross-linking reactions. "These structures reorganize to the sp2 carbon (amorphous carbon) when heated, when UV irradiated and when dried from the solvent" he says. "For our research we used silver nanoparticles to stabilize the chains even when completely dried. The result is a solid nanostructured assembly of silver nanoparticles with sp carbon structures which show long-term stability even in ambient conditions."
For their experiments, the Italian scientists produced hydrogen terminated linear carbon chains with polyyne structure by electric arc discharge between two graphite electrodes submerged in methanol. Then they added 40-60 nm-sized silver colloids, dispersed in water, to the polyyne solution and then deposited this on glass or silicon substrates.
SEM image of the silver nanoparticle solid assembly deposited on a silicon substrate. (Image: Politecnico di Milano)
In order to test the stability of linear structures in the silver assembly, they left this sample in air, just protecting it from the environmental dust and light. After about 3 months the SERS (Surface Enhanced Raman Scattering) spectrum clearly shows that polyyne chains are still present and this evidence suggests an enhanced stability in the solid state sample.
"Our observations confirm that 1D carbon structures are stabilized by termination with silver nanoparticles and that a solid silver nanoparticle assembly containing stable sp carbon structures can be obtained" says Casari.
Metals are already known as stabilizing agents for the sp phase. Therefore it is not surprising that silver may form complexes with linear carbon chains.
"More surprising instead is the stability enhancement observed in our system and this effect could be ascribed to the same interaction mechanism between an sp carbon chain and a silver nanoparticle which enables the SERS chemical effect" explains Casari. "In other words, a bonding with silver may help the sp chain to stabilize its electronic configuration, thus preventing a reorganization pointing to a more stable sp2 configuration."
From a technological point of view, such a solid system containing stable linear carbon structures could be a good candidate for optical and electronic applications owing to the electronic transport properties and to the nonlinear optical properties theoretically predicted and recently experimentally detected.
There are several hurdles still to overcome. One of the major problems is that the concentration of sp chains in the solution is low and as a consequence the concentration of linear chains in the solid system is low. Casari also points out that stability still remains a general problem to deal with, in addition to the synthesis of longer chains (>20 carbon atoms).
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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