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ABSTRACT: We have studied intersubband decay of E22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n,m) can be mapped onto a connectivity grid with curves of constant (n - m) and (2n + m). Moreover, the global behavior of E22 line widths is best characterized by a strong increase with energy irrespective of their (n-m)mod(3) = +/-1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E22 line widths are dominated by phonon assisted coupling to higher momentum states of the E11 and E12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1.
Nano Letters 02/2008; 8(1):87-91. · 13.20 Impact Factor
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ABSTRACT: Low-energy, dark excitonic states have recently been predicted to lie below the first bright (E11) exciton in semiconducting single-walled carbon nanotubes [Phys. Rev. Lett. 93, 157402 (2004)10.1103/PhysRevLett.93.157402]. Decay into such deep excitonic states is implicated as a mechanism which reduces photoluminescence quantum yields. In this study we report the first direct observation of deep excitons in SWNTs. Photoluminescence (PL) microscopy of suspended semiconducting single-walled carbon nanotubes (SWNTs) reveals weak emission satellites redshifted by approximately 38-45 and approximately 100-130 meV relative to the main E11 PL emission peaks. Similar satellites, redshifted by 95-145 meV depending on nanotube species, were also found in PL measurements of ensembles of SWNTs in water-surfactant dispersions. The relative intensities of these deep exciton emission features depend on the nanotube surroundings.
Physical Review Letters 01/2008; 99(23):237402. · 7.37 Impact Factor
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ABSTRACT: Aqueous suspensions of length selected single-walled carbon nanotubes were studied by atomic force microscopy (AFM) in order to probe the influence of sonication on nanotube scission. The maximum of the tube length distribution, lM, initially exhibits a power law dependence on the sonication time, t - roughly as lM approximately t(-0.5). This and the limiting behavior observed at longer times can be rationalized to first order in terms of a continuum model deriving from polymer physics. In this picture, the strain force associated with cavitation scales with the square of the nanotube length. Scission stops when the strain force falls below the critical value for nanotube disruption.
The Journal of Physical Chemistry B 04/2007; 111(8):1932-7. · 3.70 Impact Factor
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ABSTRACT: Photoluminescence (PL) laser microscopy was applied to determine optical transition energies E-11 and E-22 of individual semiconducting single-walled carbon nanotubes (SWNTs) suspended on top of carbon nanotube "forests," grown by chemical vapor deposition (CVD) on silicon substrates. A uniform increase of E-11 and E-22 energies by 40-55 and 24-48 meV, respectively, was found for 19 different (n,m) nanotube species suspended in air or a vacuum-relative to SWNTs in a reference water-surfactant dispersion. CVD-grown SWNTs embedded in paraffin oil and 1-methylnaphthalene show nearly the same PL peak positions as SWNTs in aqueous dispersion, indicating similar dielectric screening of excitons in SWNTs in these media.
Physical Review B 02/2007; 75(7). · 3.69 Impact Factor
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ABSTRACT: Resonance Raman spectroscopy/microscopy was used to study individualized single-walled carbon nanotubes (SWNTs) both in aqueous suspensions as well as after spin-coating onto Si/SiO2 surfaces. Four different SWNT materials containing nanotubes with diameters ranging from 0.7 to 1.6 nm were used. Comparison with Raman data obtained for suspensions shows that the surface does not dramatically affect the electronic properties of the deposited tubes. Raman features observed for deposited SWNTs are similar to what was measured for nanotubes directly fabricated on surfaces using chemical vapor deposition (CVD) methods. In particular, individual semiconducting tubes could be distinguished from metallic tubes by their different G-mode line shapes. It could also be shown that the high-power, short-time sonication used to generate individualized SWNT suspensions does not induce defects in great quantities. However, (additional) defects can be generated by laser irradiation of deposited SWNTs in air, thus giving rise to an increase of the D-mode intensity for even quite low power densities (approximately 10(4) W/cm2).
The Journal of Physical Chemistry B 07/2005; 109(21):10567-73. · 3.70 Impact Factor
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ABSTRACT: Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.
Nano Letters 04/2005; 5(3):511-4. · 13.20 Impact Factor
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ABSTRACT: Photoluminescence spectra of water−surfactant dispersions of semiconducting single-walled carbon nanotubes (SWNTs) show large shifts of interband transition energies upon freezing and cooling the dispersions to 16 K. This is accompanied by an increase of the emission intensities up to 10 times in the presence of poly(vinylpyrrolidone). The shifts develop mainly in the temperature interval of 100−240 K and are reversible by cycling the temperature. Two groups of nanotubes classified by the value of (n-m) mod 3, where n,m are structure indices, demonstrate opposite shifts, the largest ones from nanotubes with small chiral angles. The experimental data agree well with calculations of Yang et al. [Phys. Rev. B 1999, 60, 13874] for SWNTs under axial compression and indicate that large stresses of up to 5 GPa are generated in individual nanotubes by thermal contraction of the ice matrix.
10/2004;
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ABSTRACT: We have applied the FTIR-luminescence/FT-Raman technique to map the near-infrared photoluminescence (PL) of water–surfactant dispersions of single-walled carbon nanotubes (SWNTs) in broad excitation (250–1500 nm) and emission (800–1700 nm) ranges. The excitation wavelength was scanned by using the monochromatized light of standard xenon and tungsten halogen lamps. The PL maps are presented for SWNTs with a mean diameter of ~1.3 nm prepared by the pulsed laser vaporization method. When dispersed by powerful ultrasonic agitation and separated by ultracentrifugation, these nanotubes show structured absorption bands and a PL quantum yield as high as ~10-3. This indicates a large fraction of individual nanotubes in the dispersion. Electronic interband transition energies of nanotubes derived from the PL data correspond reasonably to the energies calculated in the modified tight-binding model of Ding et al.
New Journal of Physics 10/2003; 5(1):140. · 4.18 Impact Factor
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ABSTRACT: We report the resonant Raman spectra of individualized =debundled single-walled carbon nanotubes SWNTs having diameters d 0.8– 1.3 nm subject to compression up to 10.5 GPa. Both SWNTs in water-surfactant dispersions as well as SWNTs deposited onto glass microfibers and compressed with methanol-ethanol were studied. In the low pressure regime 1 GPa, linear and reversible up-shifts of the Raman bands associated with the radial breathing vibrational mode RBM and tangential G mode were observed. The pressure derivative of the RBM frequency increases with increasing d and, unexpectedly, is larger for metallic than for semiconducting tubes. Above 1 – 2 GPa, RBM bands of additional SWNT species appear. This is due to pressure-induced shifts into resonance and broadening of the corresponding optical transitions. The latter could be quantified by photoluminescence PL measurements of the corresponding semiconducting tubes that are also reported here. Disappearance of the RBM and G bands contributed by nanotubes with d 0.8– 0.9 and 1.2– 1.3 nm at 10 and 4 GPa, respectively, is tentatively assigned to extensive radial deformation of the nanotubes, in accordance with theoretical predictions. After the application of several GPa pressure, a signifi-cant loss of Raman signals and a relative increase of the defect-induced D band are found, in particular for metallic SWNTs. We attribute these and other irreversible effects in the Raman spectra to defects generated in nanotubes under compression most likely via chemical processes. Comparable structural deterioration of semiconducting nanotubes is evidenced by strong irreversible changes in their PL spectra.
