Single-walled carbon nanotubes show stable emission and simple photoluminescence spectra with weak excitation sidebands at cryogenic temperatures

Low-temperature microphotoluminescence (PL) maps were measured over a broad range of emission and excitation wavelengths for various samples of individual single-walled carbon nanotubes (SWNTs). These included nanotubes produced by chemical vapor deposition which were probed either in direct contact with a sapphire substrate or suspended between supports (on top of a carbon nanotube “forest”). Whereas surfactant-coated SWNTs manifest PL intermittency and spectral diffusion, the surfactant-free samples show stable PL over minute time scales at temperatures down to $4\mathrm{K}$. This indicates that PL intermittency and spectral diffusion are not intrinsic properties of SWNTs, but can be caused by interactions with a surfactant coating. We also demonstrate that interactions between spatially close (presumably contacting) nanotubes may have contributed to the complexity of previously reported low-temperature PL spectra. Apart from such multipeak PL maps characterized by intense excitation sidebands, we also observed simple single-peak PL maps which can be attributed to only one SWNT within the laser excitation spot. In addition, we compare typical PL intensity (quantum efficiency) and structural assignment of PL peaks for the various types of samples studied.

Figure 1

SEM images of carbon nanotube samples produced by catalytic chemical vapor deposition techniques (see text for details). (a) Top view of an $\sim 20\mu \mathrm{m}$ high carbon nanotube forest on a $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_2$ substrate. Bare regions were not (pre)covered with sputtered iron catalyst film. (b) Nanotubes cleaved from the edge of a forest film. [(c) and (d)] Carbon nanotubes grown on sapphire partially covered with a thin alumina and/or Fe-Mo-catalyst film. The nanotubes and the film edge appear to be overexposed in the SEM image (c) due to the insulating nature of the substrate. (d) Many nanotubes were found grown over the catalyst film.

Figure 2

(Color online) (a) Low-temperature (at 7 and $8\mathrm{K}$) microphotoluminescence maps [emission intensity versus emission and excitation wavelengths shown as a three-dimensional (3D) plot] and (b) corresponding time evolution of the emission intensity and wavelength [shown as a two-dimensional (2D) contour plot] of separate SWNTs deposited on sapphire from a water and/or sodium cholate dispersion. Significant PLE sidebands are lettered with S. The maps (b) combine 60 sequential emission spectra each acquired during $10\mathrm{s}$ upon excitations at 805 and $786\mathrm{nm}$ for (9,8) and (9,7) nanotubes, respectively. The $(n,m)$ assignment of deposited surfactant-coated nanotubes follows that of Ref. 23 (see text for details).

Figure 3

(Color online) Low-temperature $(T=6\mathrm{K})$ microphotoluminescence maps (emission intensity versus emission and excitation wavelengths 3D plot) for separate CVD-grown (surfactant-free) SWNTs: (a) (8,7) nanotube suspended on the top surface of a carbon nanotube forest; (b) (9,4) nanotube grown over a fumed alumina and/or catalyst film. S denotes a weak PLE sideband. The inset maps (2D contour plots) show the time evolutions of the PL peaks. They are combined from $100∕40$ sequential spectra each acquired during $10\mathrm{s}$ excitation at $705∕710\mathrm{nm}$ for the $\left(\mathrm{8,7}\right)∕\left(\mathrm{9,4}\right)$ nanotubes, respectively. The $(n,m)$ assignment is according to Ref. 19 (see text for details).

Figure 4

(Color online) Further examples of simple, one-peak microphotoluminescence maps of separate CVD-grown nanotubes at low temperatures: (a) (10,2) nanotube suspended on top of a carbon nanotube forest, $T=6\mathrm{K}$; (b) (12,4) nanotube grown in a contact with a sapphire substrate, $T=4.8\mathrm{K}$; (c) PLE profiles at the emission wavelengths of 992 and $1320\mathrm{nm}$ for the (10,2) and (12,4) nanotubes, respectively. The PL peaks correspond to $E_{11}$ (emission) and $E_{22}$ (excitation) optical transitions and are accompanied by unusually weak PLE sidebands.

Figure 5

(Color online) Microphotoluminescence map of a region comprising several individual $(n,m)$ nanotubes detected on the top surface of a carbon nanotube forest at $T=6\mathrm{K}$. The main PL peaks ($E_{22}$ excitation, dark open circles) are assigned to air-suspended (13,2), (12,4), and (8,6) nanotubes. Dotted lines correspond to the Raman $G^{\prime }$ and $2G$ bands. Note that the PL assigned to the phonon-assisted $(E_{11}+2G)$ excitation of the (12,4) nanotube is brighter than that originating from the “main” $E_{22}$-excitation transition. White open ovals (S) indicate high-energy $(E_{22}+\Delta E)$ PLE sidebands of the (13,2) nanotube. White open circles (X) indicate unidentified PL features.