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

Abstract

Ultra long linear carbon chains of more than 6000 carbon atoms have recently been synthesized within doublewalled carbon nanotubes, and they show a promising new 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 for the first time 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. arXiv:1705.02259v2 [cond-mat.mtrl-sci]

Bulk synthesis of linear carbon chains confined inside single‐wall carbon nanotubes by vacuum discharge

“…Inner nanotubes in DWCNTs and MWCNTs are suggested to serve as templates in the formation of LCC . Very recently, effects of the inner tube diameter on vibration and electronic properties of encapsulated LLCs are studied carefully . In the present study, by transmission electron microscopy (TEM) and Raman spectroscopy, we found the presence of a bulk amount of LCCs in CNT films after an electric discharge of the films that were used as a cathode of field emission (FE).…”

Extraction of Linear Carbon Chains Unravels the Role of the Carbon Nanotube Host

“…It is has been reported that the interaction between the outer and inner tubes could shift the RBM of inner tubes by 30 cm −1 at the maximum. 41,42 In our case, the frequency-shifts of the extracted (6,4) and (6,5) chirality tubes compared to those inside DWCNTs are both within 1 cm −1 (Fig.1c and Figs.5c and 5d), suggesting that the effect of the outer tubes is comparable to the one from the surrounding IO/surfactants as well as the LCC in total. This effect resembles that of bundling regarding energy space increments and broadening of the van Hove singularities of the bundled tubes, explaining the shifts in the RBM frequencies.…”

“…Furthermore, the Raman spectra shown in Fig.1c illustrate the difference between the two separated parts and confirm the successful extraction again. From these spectra (6,4) and (6,5) tube chiralities can be clearly identified in the low-frequency radial breathing mode (RBM) region for the top layer, [33][34][35] whereas much more signal from both the inner and outer tubes of DWCNTs can be seen in the bottom region sample. Note that the peak labelled IO conresponding to the signal arising from the iodixanol density gradient medium (see supplementary Fig.S1).…”

“…Note that the peak labelled IO conresponding to the signal arising from the iodixanol density gradient medium (see supplementary Fig.S1). The strong RBMs of (6,4) and (6,5) tubes is not only due to their enrichment in the sample (also see the absorption), but also because the tubes were excited by 568 nm laser whose energy is close to the nanotubes' E 22 resonance condition. 33,34 In addition, the narrower G + -band, more pronounced G − -band, and the shifted 2D-band (inset of Fig.…”

“…It is well understood that the longer the LCC, the lower the frequency, and the smaller electronic band gap. 4,5,37 Therefore, the 568 nm and 584 nm lasers are better for the detection of short and long chains, respectively. In addition, from the upper insets of Fig.4, the resonance behaviour has also been observed for (6,5) and (6,4) tubes.…”