The Optical Resonances in Carbon Nanotubes Arise from Excitons

Wang, Feng; Duković, Gordana; Brus, L. E.; Heinz, Tony F.

doi:10.1126/science.1110265

Cited by 1,181 publications

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“…All these phenomena are typical features of quasi-one-dimensional materials 20 such as conjugated organic 21 and organometallic 22 polymers, and mix-valence chains 23 . In particular, recent studies reveal many common electronic features observed in spectra of CNTs and conjugated polymers 7,9,13,24,25 . Figure 1 schematically shows the essential electronic states contributing to photophysics of these materials providing common comparison baseline.…”

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“…So far, extensive amount of work in nanotube science has been done on the excitonic properties of the singlet manifold only 4,5,6,7,8,9,10,11,12 . Triplet states in CNTs have not been detected experimentally yet.…”

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“…2 (top). Reference PPV compound is one of the most explored and understood luminescent polymer 27 , and (7,6) 31 . We use B3LYP and PBE1PBE hybrid functionals, containing 20% and 25% of the orbital exchange, respectively.…”

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Triplet State Absorption in Carbon Nanotubes: A TD−DFT Study

Tretiak

2007

Nano Lett.

116

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We predict properties of triplet excited states in single-walled carbon nanotubes (CNTs) using a time-dependent density-functional theory (TD-DFT). We show that the lowest triplet state energy in CNTs to be about 0.2-0.3 eV lower than the lowest singlet states. Like in π-conjugated polymers, the lowest CNT triplets are spatially localized. These states show strong optical absorption at about 0.5-0.6 eV to the higher lying delocalized triplet states. These results demonstrate striking similarity of the electronic features between CNTs and π-conjugated polymers and provide explicit guidelines for spectroscopic detection of CNT triplet states.

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Triplet State Absorption in Carbon Nanotubes: A TD−DFT Study

Tretiak

2007

Nano Lett.

116

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“…[1][2][3] Photoluminescence (PL) of nanotubes results from exciton recombination and occurs in the near-infrared spectral range with emission energies controlled mainly by the nanotube structure (n,m). [4][5][6][7][8] Since nanotubes consist of surface atoms only, the detected emission energy is very sensitive to the nanotube environment, making them promising candidates for sensing applications. 9 At present, the influence of the environment is described by its relative dielectric constant ε influencing exciton binding energies but also renormalizing the band gap through charge carrier screening.…”

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“…We found a strong red shift caused by DNA-wrapping of between 7 and 17 meV depending on the nanotube chirality. In the next step, TENOM was used to resolve PL variations along DNA-wrapped (6,5) and (6,4) nanotubes. Here two distinct emission bands are identified and assigned to emission from DNA-wrapped segments of the nanotube at E DNA and unwrapped segments at E 0 , distinguished by energy shifts of 18 meV for (6,5) and 30 meV for (6,4)-nanotubes, respectively.…”

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Visualizing the Local Optical Response of Semiconducting Carbon Nanotubes to DNA-Wrapping

et al. 2008

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We studied the local optical response of semiconducting single-walled carbon nanotubes to wrapping by DNA segments using high resolution tip-enhanced near-field microscopy. Photoluminescence (PL) near-field images of single nanotubes reveal large DNA-wrapping-induced red shifts of the exciton energy that are two times higher than indicated by spatially averaging confocal microscopy. Near-field PL spectra taken along nanotubes feature two distinct PL bands resulting from DNA-wrapped and unwrapped nanotube segments. The transition between the two energy levels occurs on a length scale smaller than our spatial resolution of about 15 nm.Semiconducting single-walled carbon nanotubes (SWNTs) as photoluminescent quasi-one-dimensional systems have attracted enormous scientific interest and have large potential for various applications in photonics and opto-and nanoelectronics. 1-3 Photoluminescence (PL) of nanotubes results from exciton recombination and occurs in the near-infrared spectral range with emission energies controlled mainly by the nanotube structure (n,m). [4][5][6][7][8] Since nanotubes consist of surface atoms only, the detected emission energy is very sensitive to the nanotube environment, making them promising candidates for sensing applications. 9 At present, the influence of the environment is described by its relative dielectric constant ε influencing exciton binding energies but also renormalizing the band gap through charge carrier screening. 10-14 As a result, the emission energy of nanotubes is modulated by the dielectric constant, which can be expected to be nonuniform along nanotubes, leading to nonuniform emission energies in single nanotube measurements. 8,15,16 The use of DNA for hybridization of carbon nanotube sidewalls has facilitated sorting nanotubes and building chemical sensors. 9,[17][18][19] Single-strand DNA-wrapping introduces DNA segments with finite length, while the details of the secondary DNA structure will be determined by a complex interplay between π-π stacking interactions between DNA bases and nanotube surface as well as electrostatic interactions of the phosphate backbone. [20][21][22] The effect of helical wrapping by the charged DNA backbone was modeled by applying a helical potential causing symmetry breaking of the nanotube electronic structure and small energetic shifts for semiconducting nanotubes (0.01 meV in water). 23,24 It is well-known that DNA-wrapping red-shifts the PL energy depending on the nanotube chirality by several tens of meV compared to the values reported for micelleencapsulated nanotubes in aqueous solution, 7,25,26 which can be attributed to an increasing ε. 14 The surface coverage with DNA segments of finite length is expected to result in a nonuniform dielectric environment along the nanotubes. 25 Limited by diffraction, the PL information collected in confocal microscopy contains the optical response from a nanotube length of about 300 nm, which is far too large to clarify details of individual DNA-nanotube interactions. Tipenhanced n...

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