Charge Transfer and Vibrational Structure of sp-Hybridized Carbon Atomic Wires Probed by Surface Enhanced Raman Spectroscopy

Abstract

One-dimensional carbon atomic wires displaying sp hybridization have an appealing electronic and vibrational structure which profoundly affects their optical and transport properties. Here we investigated charge transfer in alternating triple–single bond carbon atomic wires (polyynes) terminated by phenyl rings and its effects on the structure of the system. The occurrence of a charge transfer between carbon wires and metal nanoparticles (both in liquids and supported on surfaces) is evidenced by Raman and surface enhanced Raman scattering (SERS) as a softening of the vibrational stretching modes. This is interpreted, with the support of density functional theory (DFT) calculations of the Raman modes, as a modification of the bond length alternation of carbon atoms in the wire. As a consequence of the charge transfer, carbon wires rearrange their structure toward a more equalized geometry which corresponds to a tendency toward a cumulenic structure (i.e., all double bonds). These observations open potential perspectives for developing carbon-based atomic devices with tunable electronic properties.

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

“…For comparison, the phonon curves obtained by removing all long-range interactions ( f n terms in eqn (1) chain conformation effects on the Raman spectrum have also been studied both experimentally and theoretically. 75,76,79,88 These studies show that first-principles calculations can be helpful to elucidate the connections among structural, vibrational, and electronic properties of sp-carbon chains. Yang et al 80 and Tommasini et al 73 have analysed the relationship between the BLA of hydrogen-endcapped polyynes and their Raman response, by means of DFT calculations.…”

“…[9][10][11]77 CAWs of increasing length show a decreasing BLA as a result of increasing π-conjugation. 54,73,[78][79][80][81] However, the BLA = 0 limit (cumulene) is not reached due to Peierls distortion. In this framework, the alternate ( polyyne-like) structure found in most of the synthesized sp-carbon wires 56 has been usually interpreted as the result of Peierls distortion, as also reported for other classes of π-conjugated materials.…”

“…However, for fixed endgroups, BLA decreases with the chain length until it reaches an asymptotic lower limit, as generally expected by the increasing π-conjugation. 54,61,73,[78][79][80][81]84 Fig. 8 Correlation between the LO phonon wavenumber (ECC mode) and bond length alternation (BLA) for different values of the electronic band gap, as obtained on the basis of Hückel theory applied to the linear infinite carbon chain.…”

“…120 Although the correct Raman intensity behaviour as a function of chain length is not described very accurately by DFT methods, by summing up the contributions from each chain length (weighted by their abundance in the sample), one can obtain a reasonable representation of the experimental spectra in hydrogen-endcapped polyynes 86 and phenyl-endcapped polyynes. 79 This approach allows for interpretation of the main Raman features of the spectra of CAW samples containing a distribution of chain lengths.…”

“…For example, phenyl-endcapped polyynes (Ph-C n -Ph) up to n = 12 are stable under ambient conditions even in a solid film when the solvent is completely removed. 79 The Raman spectrum of samples of phenyl-endcapped polyynes with a distribution of chain lengths is characterized by the well-resolved peaks in the 2050-2250 cm −1 region. As depicted in Fig.…”