Sulfur and sulfur-oxide compounds as potential optically active defects on SWCNTs

TL;DR

This work uses density functional theory to identify sulfur- and sulfur-oxide-based covalent dopants on the (6,5) SWCNT as optically active defects. By evaluating multiple adsorbates ($S$, $SO$, $SO_2$, $SO_3$) and binding configurations, the authors show that $S$ and $SO$ strongly bind and introduce in-gap states that reduce the band gap by up to about $0.28$ eV, producing a red-shift compatible with experimental photoluminescence data. Larger $SO_x$ species bind weakly and have negligible electronic impact, making them unlikely dopants. The findings support sulfur-based defects as tunable emissive centers in semiconducting SWCNTs and provide a framework for assigning experimentally observed PL features to specific dopant configurations.

Abstract

Semiconducting single-walled carbon nanotubes (SWCNT) functionalized with covalent defects are a promising class of optoelectronic materials with strong, tunable photoluminescence and demonstrated single photon emission (SPE). Here, we investigate sulfur-oxide containing compounds as a new class of optically active dopants on (6,5) SWCNT. Experimentally, it has been found that when the SWCNT is exposed to sodium dithionite, the resulting compound displays a red-shifted and bright photoluminescence peak that is characteristic of doping with covalent defects. We perform density functional theory calculations on the possible adsorbed compounds that may be the source of doping (S, SO, SO2 and SO3). We predict that the two smallest molecules strongly bind to the SWCNT with binding energies of ~ 1.5-1.8 eV and 0.56 eV for S and SO, respectively, and introduce in-gap electronic states into the bandstructure of the tube consistent with the measured red-shift of (0.1-0.3) eV, consistent with measurements. In contrast, the larger compounds are found to be either unbound or weakly physisorbed with no appreciable impact on the electronic structure of the tube, indicating that they are unlikely to occur. Overall, our study suggests that sulfur-based compounds are promising new dopants for (6,5) SWCNT with tunable electronic properties.

Figures (3)

• Figure 1: a) The configuration of different sulfur compounds studied relative to the carbon atoms of (6,5) SWCNT (labeled R$_x$). b) Illustration of the two different orientations of the sulfur compounds relative to the SWCNT tube axis with a single sulfur atom shown as an example. The cylinder represents the (6,5) SWCNT.
• Figure 2: The predicted adsorption geometry of SO$_\text{x}$ derivatives on (6,5) SWCNT within PBE-D3. Relevant bond lengths are labeled.
• Figure 3: PBE-predicted bandstructure for pristine (6,5) SWCNT and (6,5) SWCNT with the mostly likely adsorbate structures for S, SO, SO$_\text{2}$ , and SO$_\text{3}$ . Occupied pristine-like valence bands and unoccupied conduction bands are shown in red and blue, respectively, while unoccupied states localized on the defect are in purple. The gray dash-dot line on each band structure represents the Fermi energy. All plots are shifted such that the top of the pristine-like valence band is at zero at $\mathbf{k} = 0$ (indicated by black dash line). The orbital charge density for the lowest energy unoccupied band is shown below the plots with an isosurface that captures $40\%$ of the density.