Influence of Cumulenic Chains on the Vibrational and Electronic Properties ofsp−sp2Amorphous Carbon

Abstract

We report the production and characterization of a form of amorphous carbon films with sp/sp 2 hybridization (atomic fraction of sp hybridized species ≥ 20%) where the predominant sp bonding appears to be (=C=C=) n cumulene. Vibrational and electronic properties have been studied by in situ Raman spectroscopy and electrical conductivity measurements. Cumulenic chains are substantially stable for temperatures lower than 250 K and they influence the electrical transport properties of the sp/sp 2 carbon through a self-doping mechanism by pinning the Fermi level closer to one of the mobility gap edges. Upon heating above 250 K the cumulenic species decay to form graphitic nanodomains embedded in the sp 2 amorphous matrix thus reducing the activation energy of the material. This is the first example of a pure carbon system where the sp hybridization influences bulk properties. Cumulenes are expected to tend to a metallic behavior, characterized by the homogeneous distribution of the π orbitals, as their length increases, while polyynes are expected to be semiconducing even for infinitely long chains, due to bond length alternation opening an energy gap at the edge of the Brilluoin zone [6,7].Sp chains can be stabilized by isolation in rare gas or inorganic matrices [8,5] or terminating the chain ends with suitable functional groups [6,9]. Calculations on infinite carbon chains indicate that polyynes are energetically more stable than cumulenes due to Peierls distortion [10], however for finite length chains the gain in energy is too low to be significant even a T = 0 K [11].Furthermore it has been shown that the electronic configuration of the chains is determined, regardless their length, by the group termination of the chain [6]. A conspicuous amount of experimental and theoretical results are available on the electronic structure and vibrational properties of isolated polyynes [12][13][14], whereas almost no experimental characterizations can be found on cumulenes.The possibility of stabilizing sp chains in cluster-assembled nanostructured carbon films Here we report on the production and characterization of amorphous sp/sp 2 carbon films where the dominant sp species are cumulenes. This is obtained by supersonic cluster beam deposition on a substrate kept at a temperature of 150 K. We show that cumulenes are stable up to a temperature of roughly 210 K and that they influence the electrical transport properties of the films acting as metallic doping species. By rising the film temperature to 325 K cumulenes undergo a reorganization inducing the formation of nanometric graphitic islands in the amorphous sp 2 matrix, while the amount of polyynes remains substantially constant in the investigated temperature range.Cluster-assembled films were grown in ultra high vacuum (~ 10 -9 mbar) by depositing on glass substrates a supersonic beam of carbon clusters produced by a pulsed microplasma cluster source as described in detail in refs. [19,20]; two parallel gold contacts were previously evaporated on the substrat...

Carbon-atom wires: 1-D systems with tunable properties

“…27,88 It has been observed that the cumulene fraction has a higher tendency to undergo transformation to sp 2 -carbon structures, confirming reduced stability toward oxygen exposure and high temperatures. 98,116,117 Different terminations that can be provided by the amorphous carbon network can produce cumulene or polyyne structures, respectively. 88 Short cumulenes can be synthesized in a similar way by controlling the termination-induced electronic arrangement.…”

“…The conductivity of disordered sp-sp 2 -carbon systems has been measured in situ as a function of the polyyne and cumulene content showing the contribution of sp-carbon to the electrical conductivity of the whole system. 98 Moving to single molecule transport, Wang et al firstly reported a measurement of the conduction of single CAWs by means of the STM-break junction method. 144 Such a method is based on using the STM metal tip and a metal substrate as the two contacts of a molecular junction.…”

“…The deposited material is usually composed of sp-sp 2 moieties where sp-wires are mixed in a mainly sp 2 -amorphous carbon matrix (sp-content up to 40% has been estimated in some cases). [98][99][100] Laser ablation (with both fs and ns pulses) of solid carbon targets or suspensions (e.g., fullerenes and nanodiamond) produces solutions of isolated wires in the form of polyynes with an even number of carbon atoms. [101][102][103] An easy-to-use and cost-effective technique for the synthesis of polyynes in solution is the arc discharge in liquids developed by F. Cataldo.…”

Mechanical properties of carbynes investigated byab initiototal-energy calculations

“…12,13,[27][28][29] The cumulene is anisotropic: one can make it bend either within the molecular plane, as sketched in the inset of Fig. 3, or perpendicularly to it, resulting in differently oriented double bonds 30 being perturbed.…”

“…9,10 In a pure-carbon environment carbynes are far more stable. [11][12][13][14][15] Effects of longitudinal straining [16][17][18][19] and lateral bending 20 were investigated very recently by ab initio methods. The outcome of those studies is that carbyne chains are at one time extremely stiff against longitudinal straining and very soft against a bending deformation, to the extent that even extreme bending only moderately affects their bonding properties.…”