Electronic band gaps of confined linear carbon chains ranging from polyyne to carbyne

Lei Shi, Philip Rohringer, Marius Wanko, Angel Rubio, Sören Waßerroth, Stephanie Reich, Sofie Cambré, Wim Wenseleers, Paola Ayala, and Thomas Pichler

Phys. Rev. Materials 1, 075601 – Published 12 December 2017

Abstract

Ultralong linear carbon chains of more than 6000 carbon atoms have recently been synthesized within double-walled carbon nanotubes (DWCNTs), and they show a promising route to one-atom-wide semiconductors with a direct band gap. Theoretical studies predicted that this band gap can be tuned by the length of the chains, the end groups, and their interactions with the environment. However, different density functionals lead to very different values of the band gap of infinitely long carbyne. In this work, we applied resonant Raman excitation spectroscopy with more than 50 laser wavelengths to determine the band gap of long carbon chains encapsulated inside DWCNTs. The experimentally determined band gaps ranging from 2.253 to 1.848 eV follow a linear relation with Raman frequency. This lower bound is the smallest band gap of linear carbon chains observed so far. The comparison with experimental data obtained for short chains in gas phase or in solution demonstrates the effect of the DWCNT encapsulation, leading to an essential downshift of the band gap. This is explained by the interaction between the carbon chain and the host tube, which greatly modifies the chain's bond-length alternation.

Received 15 March 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.1.075601

Physics Subject Headings (PhySH)

Band gap Semiconductors

1-dimensional spin chains Carbon-based materials Nanotubes

High-resolution transmission electron microscopy Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Lei Shi¹, Philip Rohringer^1,2, Marius Wanko³, Angel Rubio^3,4, Sören Waßerroth⁵, Stephanie Reich⁵, Sofie Cambré², Wim Wenseleers², Paola Ayala¹, and Thomas Pichler^1,*

¹Faculty of Physics, University of Vienna, 1090 Wien, Austria
²Experimental Condensed Matter Physics Laboratory, University of Antwerp, B-2610 Antwerp, Belgium
³Nano-Bio Spectroscopy Group and ETSF, Dpto. Material Physics, Universidad del País Vasco, 20018 San Sebastián, Spain
⁴Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
⁵Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

^*thomas.pichler@univie.ac.at

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Vol. 1, Iss. 7 — December 2017

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Images

Figure 1
An experimental HRTEM image (a), a simulated HRTEM image (b), and a molecular model (c) of a LLCC@DWCNT. The line profiles for the experimental (d) and the simulated (e) LLCC@DWCNT at the corresponding marked positions are shown in blue (d) and red (e).
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Figure 2
(a) Raman spectra of pristine DWCNTs (blue line) and LLCC@DWCNTs (red line) measured with a 568 nm laser excitation at room temperature. The LLCC-band region is highlighted by a box. (b) Raman spectra of the same samples conducted with a 590 nm laser at 38 K. The colored dashed lines indicate the blueshift of 4.2 and $3.8 {cm}^{- 1}$ for the RBM peaks of (6,4) and (6,5) tubes, respectively, after encapsulation of the LLCCs. The black numbers are the frequencies of all observed Raman bands of the LLCCs.
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Figure 3
(a) Resonance Raman mapping of LLCC@DWCNTs. The LLCC band is highlighted by a red box. The three arrows point out three LLCC-band peaks with different frequencies. The evolution of the Raman spectra of LLCC@DWCNTs excited by lasers with a wavelength 564–610 nm (b) and 650–680 nm (c). The resonance Raman spectra for the LLCC peaks at 1793, 1802, 1832, 1842, as well as the sum of 1850 and $1856 {cm}^{- 1}$ were highlighted by magenta, dark yellow, olive, red, and blue lines, respectively.
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Figure 4
(a) Resonance Raman excitation profiles for six LLCC-band peaks. The dashed lines are a fit to the experimental data using Eq. (1). (b) Band gap of the LCCs as a function of Raman frequency of the LCCs. The orange crosses are our theoretical prediction by ab initio calculations on the free chains in vacuum, the olive triangles represent LCCs terminated by bulky end groups in toluene (Raman frequencies) or hexane (band gap) [4, 5], and the blue squares are our work on LCCs inside DWCNTs. The linear lines are the fittings of the data points.
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Figure 5
The band gap as a function of the inverse number of carbon atoms by ab initio calculations (our work: orange crosses), predicted by Mostaani et al. for carbyne (green cross at $1 / N = 0$ ) [13], measured in gas phase (solid circles) [36] or dissolved in a solvent (solid triangles and solid stars represent LCCs terminated by different chemical ending groups) [5, 15] by absorption spectroscopy, and LCCs inside SWCNTs (open squares [18, 35])/DWCNTs (our work: the light blue shadow) by resonance Raman spectroscopy. The data for SWCNTs do not represent band-gap measurements but excited dark states [18, 35]. Inset: the enlarged part of the band gap of the confined chains. The dashed line is obtained by shifting the linear fit of the LCCs in solution, with the smallest observed band gap as the anchor point. The horizontal red lines represent the measured band gaps, and the blue shaded area indicates the possible length range for our data.
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