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Defects Enable Dark Exciton Photoluminescence in Single-Walled Carbon Nanotubes Amanda R Amori, Jamie E. Rossi, Brian J. Landi, and Todd D. Krauss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10565 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018
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The Journal of Physical Chemistry
Defects Enable Dark Exciton Photoluminescence in Single-Walled Carbon Nanotubes Amanda R. Amori1, Jamie E. Rossi3,4, Brian J. Landi3,4, Todd D. Krauss1,2* 1
Department of Chemistry and 2The Institute of Optics, University of Rochester, Hutchison Hall, Box 270216, Rochester, New York 14627, United States 3
NanoPower Research Labs and the 4Department of Chemical Engineering, Rochester Institute of Technology, 1 Lomb Memorial Drive, Rochester, New York 14623, United States Corresponding Author E-mail:
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Abstract: Variable temperature photoluminescence excitation spectroscopy of three (n,m) species of single-walled carbon nanotubes revealed that at resonant S22 excitation, in addition to allowed excitonic optical transitions, several sidebands that should be forbidden based on selection rules were observed and appeared to have a strong temperature dependence. In particular, we found that a sideband located approximately 130 meV away from the bright S11 exciton peak relating to the K-momentum dark exciton state, called X1, decreased in intensity five-fold as the samples were cooled. Direct optical excitation of this dark state is nominally forbidden, thus calling into question how the state is populated, and why it is so prominent in the photoluminescence spectrum. Interestingly, the ratio of the integrated photoluminescence intensities of X1 to S11 scales with a Boltzmann factor unrelated to the phonon that is thought to be responsible for depopulating the K-momentum dark exciton state: an in-plane transverse optical phonon, A1’. Furthermore, photoluminescence spectra from individual (7,5) nanotubes show that only a small fraction exhibit the X1 feature, with varying oscillator strength, thus suggesting that intrinsic processes such as phonon scattering are not responsible for populating the dark state. Alternatively, we suggest that populating the K-momentum dark exciton state requires scattering from defects, which is consistent with the increased magnitude of the X1 feature for samples with increased sample purification and processing. Thus, the presence of an X1 peak in photoluminescence is an extremely sensitive spectroscopic indicator of defects on single-walled carbon nanotubes.
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The Journal of Physical Chemistry
Introduction Single-walled carbon nanotubes (SWCNTs) are quasi-one-dimensional nanomaterials whose optical properties arise from strongly bound excitons.1 Semiconducting SWCNTs have the potential to be integrated into numerous technologies including high performance field-effect transistors,2 nanoscale sensors,3 and electron conductive and proton permeable membranes.4 However, in order for the practical application of SWCNTs to be fully realized, a deep insight into their optical properties is important. While the optical properties of nanotubes (NTs) have been the subject of significant research interest in the last decade, even the most basic properties of NT photophysics are still not well understood. For example, the SWCNT cross section has been the subject of much debate5-7 as well as the SWCNT photoluminescence efficiency, or quantum yield.8-10 The transitions observed within SWCNT optical spectra are fundamentally derived from applying appropriate periodic boundary conditions for an electron confined to a cylinder to the energy band structure of graphene.11 The degeneracy of the K and K’ points of the graphene unit cell, in conjunction with the spins of charge carriers, produce the 16 possible excitonic states that exist in SWCNTs for each subband.12-16 Of these states, only the singlet-spin, odd-parity, and zero-angular-momentum state is optically allowed (bright). The remaining 15 states are optically forbidden (dark) and, thus, are much less understood. One exception is the singlet-spin, evenparity, zero-angular-momentum, optically dark state lying just below the bright exciton, which was predicted by theory,17 and was subsequently characterized by use of photoluminescence microscopy18 and magneto-optical spectroscopy.19-21
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Recently, two degenerate singlet-spin, odd-parity excitonic states with finite center-ofmass momenta have been discovered, termed K-momentum dark excitons (KDE).22-27 These excitons are nominally dark because they consist of having one charge carrier located at the K point and the other at the K’ point and thus, due to momentum conservation, these excitons cannot arise from direct optical transitions. Existence of the KDE state has been inferred from the presence of two bright phonon sidebands in the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of well-dispersed SWCNT samples of known chirality.22-27 Indeed, the vibronic nature of the sidebands was recently confirmed, including identification of the specific vibrational mode that most strongly couples to the KDE state: a zone-boundary, in-plane transverse optical (iTO), A1’, phonon, which is also the phonon that appears as the “D-band” mode in a SWCNT Raman spectrum.24, 28 The focus of recent experimental work has been to characterize the PL22-27 and PLE23-24 sidebands, X1 and X2 respectively, that are speculated to arise from phonon creation and annihilation processes from the KDE states. For example, X1 has been attributed to an indirect optical transition involving depopulation of the KDE and simultaneous emission of an A1’ (iTO) phonon.24 However, little work has directly addressed how the KDE state is populated in the first place. A single report has measured the temperature dependence of the X1 peak of a single (7,5) NT, but did not address how the temperature dependence relates to the population of the KDE state.26 While one may attribute the temperature dependent changes in X1 PL intensity to phonon creation, the large A1’ phonon energy relative to kBT means that the depopulation of the KDE state should not be temperature dependent. Likewise, the large A1’ phonon energy relative to kBT implies that a negligible phonon population is available to scatter an exciton from the S11 to the KDE state. Thus, the KDE state must be populated by some other mechanism.
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Herein, we describe the use of variable temperature PLE spectroscopy to investigate the origins of several sidebands featured in the PL spectra of chirality-enriched samples of SWCNTs. Specifically, three (n,m) chiral species were studied, (7,5), (6,5), and (10,5) by suspending the SWCNTs in a glass-forming organic solvent mixture in order to investigate the PL intensity over a wide range of temperatures in the solution phase, where effects from bundling and SWCNT aggregation are minimized. While all ensemble PL spectra showed a temperature dependent X1 peak, single molecule (SM) PL studies of individual (7,5) NTs at 295 K revealed that only a handful of NTs studied displayed X1 emission. Lack of an X1 contribution in the PL spectra from most individual NTs suggests that population of the KDE state cannot be phonon-related, as a phonon-related process should act similarly for every NT studied. Therefore, we postulate that temperature dependence of the X1 peak and the SM data are overall consistent with the KDE state being populated by a factor extrinsic to the NT: intervalley scattering from NT defects. By contrast, emission from the KDE state involves creation of an A1’ phonon for momentum conservation. Our data suggests that the X1 feature is a highly sensitive spectroscopic indicator for the presence of low numbers of defects in SWCNTs. Experimental Section SWCNTs were produced by the CoMoCAT method (SouthWest Nanotechnologies, Inc.) and also synthesized at the Rochester Institute of Technology via pulsed laser vaporization using a previously reported procedure.29 A Nd:YAG laser (1064 nm) was rastered over the surface of a graphite target (Alfa Aesar, Graphite Flake, median 7-10 µm, 99% metal basis) doped with 3% w/w of both Ni (Sigma-Aldrich,
