Article Cite This: J. Phys. Chem. C 2018, 122, 3599−3607
pubs.acs.org/JPCC
Defects Enable Dark Exciton Photoluminescence in Single-Walled Carbon Nanotubes Amanda R. Amori,† Jamie E. Rossi,§,∥ Brian J. Landi,§,∥ and Todd D. Krauss*,†,‡ †
Department of Chemistry and ‡The Institute of Optics, University of Rochester, Hutchison Hall, Box 270216, Rochester, New York 14627, United States § NanoPower Research Laboratories and the ∥Department of Chemical Engineering, Rochester Institute of Technology, 1 Lomb Memorial Drive, Rochester, New York 14623, United States S Supporting Information *
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 5-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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INTRODUCTION Single-walled carbon nanotubes (SWCNTs) are quasi-onedimensional 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 past decade, even the most basic properties of NT photophysics are still not well understood. For example, the SWCNT crosssection 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-angularmomentum state is optically allowed (bright). The remaining © 2018 American Chemical Society
15 states are optically forbidden (dark) and, thus, are much less understood. One exception is the singlet-spin, even-parity, zeroangular-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 Recently, two degenerate singlet-spin, odd-parity excitonic states with finite center-of-mass 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 Received: October 25, 2017 Revised: January 21, 2018 Published: January 24, 2018 3599
DOI: 10.1021/acs.jpcc.7b10565 J. Phys. Chem. C 2018, 122, 3599−3607
Article
The Journal of Physical Chemistry C 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. 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 glassforming 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 phononrelated, 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.
1 mg SWCNT:4 mg polymer:1 mL toluene by vortex mixing (VWR Scientific Mini Vortexer) for 1 min at 3200 rpm, probe sonicating (Branson 450 Sonifier) in an ice bath for 1 h at a power density of 125 W/cm2, centrifuging (Heraeus Biofuge Pico) for 1.5 h at 13000 rpm (16000g), and filtering using a 5 μm pore syringe filter (Millipore Millex-SV). All absorption spectra were collected on the as-produced samples in a 1 cm path length quartz microvolume cuvette (Starna Cells, Inc.) using a PerkinElmer Lambda 950 UV−visNIR spectrophotometer (Figure S1). All PL and PLE spectra were collected on the as-prepared SWCNT samples diluted 10:1 with an optically transparent glass former, 3-methylpentane (Acros Organics), in a thin-walled 300 MHz borosilicate NMR tube (Wilmad LabGlass) secured inside a double-walled quartz dewar. PL and PLE spectra were acquired with a 1 s integration time with excitation from a 75 W xenon lamp using a fluorometer system with an emission monochromator (750 nm blaze) and an excitation monochromator (1.25 μm blaze) in a right-angle geometry with respect to the sample chamber (Photon Technology International, Inc.). Near-infrared emission was detected using an InGaAs detector at ambient temperature, utilizing a chopper for increased S/N. PLE maps were obtained for each enriched sample using the same sample preparation and fluorometer system. For each map, a range of wavelengths were used to excite the sample and emission was monitored over a constant wavelength range with a 2 nm step and an integration time of 1 s (Table S2). All spectra were corrected for instrument response. Each PLE map was normalized to the peak of maximum intensity. PL spectra were deconvolved using IGOR Pro software version 7.02. Raman measurements were performed using a 660 nm cw laser (Cobalt Diode Lasers, Inc.) for excitation and a triple spectrograph (Princeton Instruments) coupled to a Si-CCD (Princeton Instruments) in a 180 deg backscattering geometry relative to the sample for emission collection through an aspheric lens. For single NT optical measurements, samples were spincoated onto a silica substrate (Esco Optics) with a 1% wt polystyrene coating. Measurements were performed at room temperature on a home-built scanning confocal microscopy setup. The NTs were excited near their S22 transition at 633 nm by a cw HeNe laser. The excitation power was kept at 10 kW/ cm2 at the objective (1/e2 beam diameter). A microscope objective (40×, extra-long working distance) was used to focus laser light and to collect emission from the samples. PL images and spectra were taken with a 1D InGaAs array mounted on a mirror and on a spectrograph, respectively (Princeton Instruments). Long-pass filters (633 and 850 nm) were used to block the laser beam and background emission from the substrates.
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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,
