Semiconductor-to-Metal Transition in Carbon-Atom Wires Driven by sp² Conjugated End Groups

Abstract

Novel bis(biphenyl)-capped polyynes have been synthesized to investigate the modulation of the electronic and optical properties of sp-hybridized carbon-atom wires (CAWs) capped with π-conjugated sp 2 endgroups. Raman and Surface Enhanced Raman spectroscopy (SERS) investigation of these systems and Density Functional Theory (DFT) calculations reveal structural changes from polyyne-like with alternating single-triple bonds towards cumulene-like with more equalized bonds as a consequence of the charge transfer occurring when wires interact with metallic nanoparticles. While polyynes have semiconducting electronic properties, a more equalized system tends to a cumulene-like structure characterized by a nearly metallic behavior. The possibility to drive a semiconductor-to-metal transition has been investigated by systematic DFT calculations on a series of CAWs capped with different conjugated endgroups revealing that the modulation of the structural, electronic and vibrational properties of the sp-carbon chain towards the metallic wire cannot be simply obtained by using extended π-conjugated sp 2 carbon endgroups, but require a suitable chemical design of the endgroup and control of charge transfer. These results provide useful guidelines for the design of novel sp-sp 2 hybrid carbon nanosystems with tunable properties, where graphene-like and polyyne-like domains are closely interconnected. The capability to tune the final electronic or optical response of the material makes these hybrid sp-sp 2 systems appealing for a future all-carbon-based science and technology.KEYWORDS: sp-carbon, polyyne, cumulene, carbyne, Bond length alternation; Raman Spectroscopy; SERS; Density Functional Theory; electronic band gap Fullerenes, nanotubes and graphene represent a chief example of the versatility of carbon in producing nanostructures with different properties and dimensionality (zero, quasi-one and two, respectively).Such systems are mainly based on sp 2 hybridization, only one of the three different possibilities (sp, sp 2 and sp 3 ) available for carbon, two of which can also form stable allotropes occurring in nature (i.e., sp 2 and sp 3 for graphite and diamond, respectively). 1 The long quest for the still lacking "third carbon allotrope" based on sp-hybridization has led to the development of many interesting nanoscale/molecular systems in the form of carbon-atom wires (CAWs). [2][3][4][5] CAWs are linear systems with finite length trying to approach the ideal infinite wire (i.e., carbyne) representing the ultimate 1-D carbon system with intriguing properties, as evidenced by theoretical predictions [6][7][8] . The ideal carbyne can be in two different configurations: semiconducting wire with alternate single-triple bonds (i.e., polyyne) and metallic wire with all double bonds (i.e., cumulene). As finite systems, CAWs display structural electronic and optical properties strongly dependent on the length (i.e., number of carbon atoms) and the type of functional termination (i.e., endgroup) and are thus attract...

Size-selected polyynes synthesised by submerged arc discharge in water

“…In fact, after mixing Ag colloids with the filtered aqueous solution of polyynes after the synthesis, a shoulder at high wavelengths of the plasmonic band appears, as depicted in Figure 3. This allows to have SERS enhancement at 514.5 nm, in agreement with previous results [29,33,41,43].…”

One-pot synthesis and characterization of polyynes end-capped by biphenyl groups (α,ω-biphenylpolyynes)

“…Simply, the laser beam of the Raman spectrometer is placed on the external walls of the reaction flask. presented elsewhere [58].…”

Gas Phase Formation of the Interstellar Molecule Methyltriacetylene

“…Polyynes end‐capped with gold nanoclusters or ruthenium‐based organometallic fragments also define molecular wires, in which the transition from insulators via semiconductors to metals can be selectively tuned by adjusting the length of the carbon chain (Scheme ) . In deep space, since hydrocarbon molecules carrying linear carbon chain backbones constitute nearly one third of all detected interstellar organic molecules, the elucidation of their formation routes will unravel the most fundamental processes that drive the hitherto poorly understood exotic organic chemistry in low temperature interstellar environments.…”

“…[20][21] Polyynes end-capped with gold nanoclusters [24] or ruthenium-based organometallic fragments [25] also define molecular wires, [26] in which the transition from insulators via semiconductors to metals can be selectively tuned by adjusting the length of the carbon chain (Scheme 2). [25,[27][28][29] In deep space, since hydrocarbon molecules carrying linear carbon chain backbones constitute nearly one third of all detected interstellar organic molecules, [30] the elucidation of their formation routes will unravel the most fundamental processes that drive the hitherto poorly understood exotic organic chemistry in low temperature interstellar environments. Considering that hydrogen-deficient polyyne-type species may act as precursors to fullerenes (C 60 , C 70 ) [31][32] and even to carbonaceous nanoparticles (carbon-based interstellar grains), [22,[33][34][35][36][37][38][39][40][41] our combined experimental and computational study helps defining the molecular complexity of organic molecules, which can be synthesized in our galaxy, thus ultimately predicting where else in the universe the molecular precursors to fullerenes and carbonaceous nanoparticles can be formed.…”