Low Temperature Emission Spectra of Individual Single-Walled Carbon Nanotubes: Multiplicity of Subspecies within Single-Species Nanotube Ensembles

Low-temperature photoluminescence (PL) studies of individual semiconducting single-walled carbon nanotubes reveal ultranarrow peaks (down to 0.25 meV linewidths) that exhibit blinking and spectral wandering. Multiple peaks appear within bands previously assigned to nanotubes of certain chiralities, indicating the existence of numerous subspecies within single-chirality specimens. The sharp PL features show two types of distinctly different shapes (symmetric versus asymmetric) and temperature dependences (weak versus strong), which we attribute to the presence of unintentionally doped nanotubes along with undoped species.

Figure 1

(a) Spectrally dispersed image of SWNTs recorded at 4 K. Bright horizontal line fragments represent the emission spectra of individual nanotubes. Vertical displacement ($y$) of these lines is determined by the position of the tubes along the entrance slit of the spectrometer. (b) Upper panel: a RT spectrum of the SWNT mixture in solution. Lower panel: a histogram showing the distribution of emission energies of spectral peaks recorded for 150 SWNTs prepared as dilute films [the data were collected from five images similar to that shown in panel (a)]. (c)–(e) These images are examples of three different “fluctuation” behaviors observed for individual nanotubes (see text for details). The false color represents the emission intensity and the 2D image shows the temporal evolution of the spectral position. The green traces at the bottom of each panel show the temporal behavior of the integrated PL intensity.

Figure 2

An expanded view of the RT PL band (a solution sample) corresponding to the $(8,3)$ nanotube structure (red circles) fit to a Gaussian (black line). This band is compared with narrow symmetric (blue) and asymmetric (green) lines of individual nanotubes that are observed in approximately the same spectral range. Left inset: An example of a narrow symmetric line of a single SWNT that has a 0.25 meV width (derived by taking into account the spectrometer spec­tral response function). Right inset: Distribution of peak widths derived from the analysis on spectra of 150 individual nanotubes.

Figure 3

Examples of symmetric (a) and asymmetric (b) PL spectra of two individual SWNTs taken at low ($8.5\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, dashed lines) and high ($1.5\mathrm{k}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$, solid lines) pump intensities; these data indicate that the contribution from high pump-intensity effects to observed spectral widths is negligible. The temperature dependence observed for symmetric (c) and asymmetric (d) PL spectra. While the symmetric peaks show weak temperature dependence, the asymmetric peaks are characterized by pronounced thermal broadening on their high-energy side. As expected for the FES effect, the high-energy side of the asymmetric spectra recorded at 50 and 60 K can be closely fit (red solid lines) assuming the Fermi distribution of carriers.