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diffraction and synchrotron radiation, respectively, and vibrational spectro-
scopic techniques including infrared and Raman scattering. This study was
undertaken in an attempt to satisfy the demand for such ''larger'' carbyne
single crystals.
Successful growth of carbyne crystals would move the
different structural models proposed for carbyne from inspired speculations
to undeniable reality.
Several theories have been advanced to explain the co-existence of
conjugated triple bonds (polyyne-type isomer) and cumulated double bonds
(cumulene-type isomer) in close proximity without the expected explosive
collapse of the linear structure towards graphene sheets. To stabilize the
sp-hybridization obviously ''spacers'' are required to keep the parallel
neighboring chains (''pencil-in-box'' model of the carbyne structure) apart
beyond the van der Waals radius of the carbon atom. Such spacers were
found to be metal atoms such as potassium [3], iron [4], copper [5] or
rhenium [6], bulky alkyl or aryl groups [7], end-capping CO molecules [8] or
alkali metal fluorides [9,10]. Recently, strongly linear 1-D ''carbon nano-
wires'' consisting of long carbon chains of more than 100 atoms were
produced and inserted into, and hence stabilized by, a multiwalled carbon
nanotube (MWNT) [11].
Nevertheless, the inability of researchers to synthesize carbyne single
crystals large enough to conduct structural analyses has hampered the
general acceptance of carbyne as the expected sp-hybridized carbon
allotrope. Why carbyne stubbornly resists forming crystals of suitable size
and purity is a matter of contention at present. It may be that the kinks
postulated as a precondition of termination of the otherwise infinite carbon
chain [12] introduce statistical short range order in the
direction
parallel to the chains. Consequently, the idea of a paracrystalline nature of
carbyne was advanced [13], which is thought to prevent the formation of
a long range-ordered 3-D structure. This conjecture is consistent with the
fact that, so far, only more or less 2-D thin films of carbyne, produced by
either chemical dehydrohalogenation of halogenated polymers [14,15] or
condensation of carbon vapor obtained from laser ablated [16] or sputtered
[17] graphite surfaces, has led to a reasonably well-ordered nano-to-
microcrystalline carbyne structure. It is in this context that the contribution
by Onuma et al. [17,18] take on specific significance. The hot tungsten
filament used by these authors acts as a mean to crack longer carbon chains
and clusters, respectively, into much smaller C
n
units that are thought to
reassemble easily in the linear carbyne structure at the substrate surface.
This techniques has been applied in the present study, with modifications
introduced to provide a polycrystalline diamond template to stabilize
epitaxially the growing carbyne structures [2,19]. This epitaxially mediated
h
00.1
i
Credit should be given here to Dr. Arthur Whittaker, 20753 Exhibit Court, Woodland Hills,
CA 91367, USA who provided the senior author (R.B.H) with the idea of epitaxial growth of
crystalline carbyne on diamond [2].
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