Electronic and vibrational properties of sp carbon atomic wires : effects of boundary constraints

The role of carbon-based devices in post-silicon nanoelectronics is gaining a rapidly increasing importance, due to the availability of carbon nanosystems, such as fullerenes, nanotubes, graphenes. These can be considered as potential building blocks of complex architectures with novel electronic and magnetic properties. Carbon nanowires consisting of sp-hybridized linear carbon chains are potentially another strategic component to integrate these carbon nanosystems in complex architectures, as it is routinely done in silicon nanoelectronics. Linear sp-carbon chains, characterized by double bonds (cumulene) or alternating single and triple bonds (polyyne), have been object of theoretical studies as ideal molecular conductors, and as building blocks of low dimensional solids with unusual properties. Despite their potential properties, the role of sp nanowires in the carbon-based electronics arena is still generally perceived as somehow marginal: polyynes and cumulenes have been considered for several decades as exotic allotropic forms due to their high reactivity and their tendency to undergo cross-linking reaction to form sp2 carbon. Recently, we demonstrated that large carbon clusters where sp and sp2 hybridization coexist can be formed in the gas phase [1] and deposited on a substrate to form a nanostructured film where sp chains are stable at room temperature [2, 3]. Here we report on first-principles DFT calculations combined with experimental Raman spectra on cluster-assembled sp-sp2 carbon films [4]. Ab-initio total energy and phonon calculations were performed on a selected range of model structures sampling significantly the infinite variety of three-dimensional arrangements of sp-chains bridging graphitic fragments in different hybridization states (Figure 1). Theoretical results suggest that sp-carbon chains are stabilized by sp2 or sp3 terminations (in particular by bonding to the edges of graphitic nanofragments) and allow us to interpret the nontrivial features and decay of the experimental Raman spectra. Moreover, the data for sp2-terminated chains point towards a rich phenomenology driven by even/odd alternation effects and by the effects of torsional strain. This unexplored effect promises many exciting applications since it allows one to modify the conductive states near the Fermi level, suggesting the possibility to control the nanowire conductance, and to switch on and off the on-chain pi-electron magnetism, purely by twisting its sp2 termination. Carbon atomic chains bridging graphene nanogaps, recently proposed as an explanation of the conductance switching in two-terminals graphene devices [5, 6], could hence originate an unexpected variety of geometrically-driven physical effects with many potential applications. Recently such systems have also been experimentally realized and observed by HR-TEM [7], thus opening the way to a completely new class of all-carbon devices. [1] M. Bogana et al., New J. Phys. 2005, 7, 81. [2] L. Ravagnan et al., Phys. Rev. Lett. 2002, 89, 285506. [3] L. Ravagnan et al., Phys. Rev. Lett. 2007, 98, 216103. [4] L. Ravagnan et al., Phys. Rev. Lett. in print, arXiv:0902.2573v4. [5] B. Standley et al., Nano Lett. 2008, 8, 3345. [6] Y. Li, A. Sinitskii, et al., Nat. Mater. 2008, 7, 966. [7] C. Jin et al., Phys. Rev. Lett. 2009, 102, 205501.