Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes

Abstract

We studied the exciton energy transfer in pairs of semiconducting nanotubes using high-resolution optical microscopy and spectroscopy on the nanoscale. Photoluminescence from large band gap nanotubes within bundles is observed with spatially varying intensities due to distancedependent internanotube transfer. The range of efficient energy transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation responsible for low photoluminescence quantum yield.Single-walled carbon nanotubes (SWNTs) feature unique electronic properties, making them ideal candidates for ultrahigh density devices in electronics, photonics, and optoelectronics.1-3 At nanoscale distances, energy transfer from large to small band gap nanotubes is expected to occur, facilitating novel architectures including crossbars and 3D arrays but also imposing design restrictions. Photoluminescence (PL) in semiconducting nanotubes results from exciton recombination [4][5][6] and is found to be quenched in bundles. 7-10Very recently, resonant exciton energy transfer between semiconducting nanotubes has been observed for SWNTs in micelles suspensions for the first time and was explained by near-field coupling corresponding to fluorescence resonance energy transfer (FRET) well-known for molecular systems.11 In ref 12, spectroscopic signatures of internanotube transfer were observed, and it was suggested that efficient coupling results from carrier migration requiring direct physical contact. In ensemble measurements, however, the identification of donor and acceptor spectral signatures is complicated by overlapping contributions from different nanotube species, including phonon-assisted absorption and possible emission from lower lying defect-associated states. [13][14][15] Up to now, the range of exciton transfer and its efficiencies are unknown.We used tip-enhanced near-field optical microscopy (TEN-OM [16][17][18] ) as a tool to visualize energy transfer in pairs of semiconducting nanotubes forming bundles and crossings on a nanometer length scale. Near-field PL and topography images of a single nanotube bundle reveal the presence of two semiconducting nanotubes with different chiralities having an internanotube spacing ranging from 1 to 4 nm. Photoluminescence from large band gap nanotubes was observed with unexpectedly high intensities although varying spatially along the nanotubes due to distance-dependent internanotube energy transfer. Efficient transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation and can be explained in terms of electromagnetic near-field coupling. From the experimental data, we estimate transfer efficiencies and time scales.The setup used for near-field optical microscopy and spectroscopy is based on an inverted confocal optical microscope that is combined with a scan head for shearforce detection providing tip-sample distance control. [16][17][18] The signal is detected either by a combination of a spectrograph and a cooled charged coupl...

Photoluminescence and Förster Resonance Energy Transfer in Elemental Bundles of Single-Walled Carbon Nanotubes

“…At the individual nanotube level, near-field imaging studies show clear evidence of exciton transfer between two adjacent SWNTs. 5 In all of these studies, the PL intensity of the higher band gap SWNT was reportedly transferred to the lower band gap SWNT, giving rise to an extra feature in the PLE map.…”

“…16,19 Upon bundling, similar red shifts have also been observed by Rayleigh scattering (E 33 and E 44 transitions) 20 and by near-field spectroscopy. 5 Although the magnitude of the shifts is comparable, the different experimental conditions (e.g., smaller or larger nanotube diameter, and differing dielectric environments) make direct comparisons difficult.…”

“…Recently, a few studies have addressed the luminescence from SWNT bundles and revealed a signature of energy transfer to SWNTs with the lowest energy. [3][4][5][6] Förster resonance energy transfer (FRET) is an important research topic with direct applicability to biological systems 7 and potential impact for the design of light emitting or harvesting devices. 8 In FRET, 9 electronic excitations in a fluorescent donor (D) molecule transfer to an acceptor (A) molecule, which shows fluorescence at lower energy (see ref 10 for a general review).…”

Exciton transfer and propagation in carbon nanotubes studied by near‐field optical microscopy

“…PL of semiconducting nanotubes arises from excitons with Bohr-radii of 2-5 nm that decay on picosecond time-scales [2][3][4][5][6]. Observation of PL energies and dynamics from single nanotubes provides access to the properties of excitonic states in SWNTs with specific chirality (n,m) [7][8][9]. Compared to diffraction limited conventional confocal microscopy, near-field microscopy offers high spatial resolution optical information resolving relevant length scales, such as the exciton diffusional range [10,11].…”

Effectiveness of sorting single-walled carbon nanotubes by diameter using polyfluorene derivatives

“…This interaction is one of the most elusive effects in carbon nanotubes photophysics, in that the investigation of these effects is strongly limited by the quality and control of the nanotubes separation [29]. Up to now, very few studies on energy transfer in SWCNT aggregates based on steadystate measurements such as high-resolution optical microscopy and photoluminescence excitation spectroscopy have been reported [30][31][32][33][34][35]. It is known, however, that precise information of the energy transfer can be obtained only with time-resolved spectroscopy [36].…”