Visible Fluorescence Induced by the Metal Semiconductor Transition in Composites of Carbon Nanotubes with Noble Metal Nanoparticles

We show that single-walled carbon nanotube (SWNT) bundles emit visible fluorescence in the presence of noble metal nanoparticles and nanorods in the solid state. Conductivity measurements with metallic nanotubes, isolated from pristine SWNTs, show that they become semiconducting in the presence of the metal nanoparticles. Nanoparticle binding increases the defects in the nanotube structures which is evident in the Raman spectra. The metal-semiconductor transition removes the nonradiative decay channels of the excited states enabling visible fluorescence. Nanotube structures are imaged using this emission with resolution below the classical limits.

Figure 1

(A) Single spot Raman spectra of (a) Ag-SWNT, (b) Au-SWNT, (c) AuNR-SWNT, (d) pristine SWNTs, and (e) SWNTs treated with heated citrate solution. Traces (d) and (e) are shifted vertically for clarity. RBM is labeled and the $D$ and $G$ bands are indicated by asterisks (*). The visible emission maxima are marked with their respective wavelengths. Inset (i): the RBM region at a higher resolution. Inset (ii): the Raman spectra of a drop-cast film of AuNR, along with the Rayleigh to compare the intensities. (B) TEM image of the Au-SWNT. (C) Raman spectral images of AuNR-SWNT, based on the intensities of visible emission in the 595 to 675 nm window. The spots observed away from the nanotube structure are due to shorter nanotubes remaining in the centrifugate or due to the $z$-axis discrimination of confocal imaging. (D) Light intensity based transmission SNOM image of AuNR-SWNT. Inset (i): the three-dimensional view of (B), rotated suitably to show the increased emission. Inset (ii): corresponding topographic image. Raman and SNOM data are acquired with 514.5 nm excitation.

Figure 2

(a) Raman spectra of Ag-SWNT as a function of varying $C_{\mathrm{NP}}$ and $C_{\mathrm{SWNT}}$ (inset). (b) Fluorescence intensity variation from Au-SWNT as a function of helium pressure. The highest intensity trace is in the absence of helium at a vacuum of ${10}^{-2}\mathrm{torr}$. Inset: TEM of Au-SWNT at $C_{\mathrm{CTAB}}\sim {10}^{-4}\mathrm{M}$, showing a clear separation between nanoparticles and SWNTs.

Figure 3

(a) Raman spectra of pristine HiPco synthesized SWNTs (black trace) and extracted mSWNTs (red trace), in the RBM and $G$-band regions. Inset: Raman spectra of (i) extracted mSWNTs, (ii) Au-mSWNT, and (iii) pristine SWNTs in the $G$-band region. Note distinct change in FWHM with Au-mSWNT resembling pristine SWNTs. (b) Differential conductance of mSWNTs (red traces) and Au-mSWNT (black traces). Inset: Raman spectrum of Au-mSWNT, showing visible emission.