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Title: Bright and dark excitons in semiconductor carbon nanotubes

Abstract

We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description ofmore » excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.« less

Authors:
 [1]
  1. Los Alamos National Laboratory
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
960964
Report Number(s):
LA-UR-08-06809; LA-UR-08-6809
Journal ID: ISSN 1463-9076; TRN: US1002679
DOE Contract Number:  
AC52-06NA25396
Resource Type:
Journal Article
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Name: Journal of Chemical Physics; Journal ID: ISSN 1463-9076
Country of Publication:
United States
Language:
English
Subject:
74; CALCULATION METHODS; CARBON; DENSITY; DENSITY FUNCTIONAL METHOD; EFFICIENCY; ELECTRONIC STRUCTURE; ENERGY; EXCITATION; EXCITONS; HAMILTONIANS; HYBRIDIZATION; HYPERFINE STRUCTURE; LUMINESCENCE; MATRICES; NANOTUBES; RELAXATION; SEMICONDUCTOR MATERIALS

Citation Formats

Tretiak, Sergei. Bright and dark excitons in semiconductor carbon nanotubes. United States: N. p., 2008. Web.
Tretiak, Sergei. Bright and dark excitons in semiconductor carbon nanotubes. United States.
Tretiak, Sergei. Tue . "Bright and dark excitons in semiconductor carbon nanotubes". United States. https://www.osti.gov/servlets/purl/960964.
@article{osti_960964,
title = {Bright and dark excitons in semiconductor carbon nanotubes},
author = {Tretiak, Sergei},
abstractNote = {We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.},
doi = {},
url = {https://www.osti.gov/biblio/960964}, journal = {Journal of Chemical Physics},
issn = {1463-9076},
number = ,
volume = ,
place = {United States},
year = {2008},
month = {1}
}