Dynamics of Charged Excitons in Electronically and Morphologically Homogeneous Single-Walled Carbon Nanotubes
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Dynamics of charged excitons in electronically and morphologically homogeneous single-walled carbon nanotubes Yusong Baia, Jean-Hubert Oliviera, George Bullarda, Chaoren Liua, and Michael J. Theriena,1 aDepartment of Chemistry, French Family Science Center, Duke University, Durham, NC 27708-0346 Edited by José N. Onuchic, Rice University, Houston, TX, and approved December 8, 2017 (received for review July 22, 2017) The trion, a three-body charge-exciton bound state, offers unique SWNTs demonstrate a new lower-energy absorption, which opportunities to simultaneously manipulate charge, spin, and has been attributed to a direct ground-to-trion optical tran- + hυ + + excitation in one-dimensional single-walled carbon nanotubes sition (E 00 ! Tr 11, where Tr 11 denotes a low-lying elec- (SWNTs) at room temperature. Effective exploitation of trion tronically excited trion state) (5, 8); no experimental evidence, quasi-particles requires fundamental insight into their creation however, has confirmed the nature of the state produced by and decay dynamics. Such knowledge, however, remains elusive + SWNT E photon absorption. for SWNT trion states, due to the electronic and morphological 00 Here, we describe the transient absorptive and dynamical heterogeneity of commonly interrogated SWNT samples, and the properties of hole trions in length-sorted semiconducting (6,5) fact that transient spectroscopic signals uniquely associated with the trion state have not been identified. Here, we prepare length- SWNTs, wherein hole polaron densities are rigorously con- sorted SWNTs and precisely control charge-carrier-doping densi- trolled. Owing to the electronic and morphological homogeneity ties to determine trion dynamics using femtosecond pump–probe of these SWNT samples, we clearly identify a trion transient spectroscopy. Identification of the trion transient absorptive hall- absorptive hallmark, which in turn enables us to correlate dy- mark enables us to demonstrate that trions (i) derive from a pre- namical processes characteristic of bright excitons, hole polarons, cursor excitonic state, (ii) are produced via migration of excitons to and trions, and thus unambiguously unveil trion formation and CHEMISTRY stationary hole-polaron sites, and (iii) decay in a first-order man- decay dynamics. By comparing trion dynamics acquired from ner. Importantly, under appropriate carrier-doping densities, pumping in resonance with the E00 → E11 exciton transition, exciton-to-trion conversion in SWNTs can approach 100% at am- with those obtained from excitation of the previously assigned bient temperature. Our findings open up possibilities for exploit- “trion transition,” we ascertain charge-doped 1D SWNTs do ing trions in SWNT optoelectronics, ranging from photovoltaics + → + and photodetectors to spintronics. not possess a direct E 00 Tr 11 optical transition. Moreover, these dynamical studies demonstrate that under appropriate trion | exciton | single-walled carbon nanotube | dynamics | charge carrier-doping conditions, optical stimuli can result in near-unit conversion of excitons to trions, opening up possibilities for he trion, comprising an exciton and a charge (1), defines a SWNT-based optoelectronic devices that rely upon manipu- Tunique quasi-particle species by its hybrid nature: it simul- lating spin, energy, and charge. taneously carries excitation energy, net charge, and unpaired spin. Exploitation of trions in optoelectronics has been impeded Significance by their small binding energies (ΔETr) in conventional 3D (ΔETr ∼ 0.01–0.3 meV) (2) and 2D (ΔETr ∼ 1–5 meV) (3, 4) semicon- Formation of quasiparticles, such as excitons, polarons, and ductors, wherein trion observation is made possible under cryo- trions in semiconductors are the foundation for modern op- conditions (4). In sharp contrast, optical excitation of the semi- toelectronics. Unlike the widely investigated exciton and conducting single-walled carbon nanotube (SWNT) charged polaron, the trion, a three-body charge-exciton bound state, + ground state (E 00) gives rise to trions even at room temperature is less familiar due to its small binding energy in conventional (5–15), due to the drastically increased ΔETr (∼100 meV) in 1D inorganic semiconductors. Here, employing ultrafast spec- SWNTs that arises from reduced dielectric screening. Owing troscopy and rigorously controlled charge-doping levels, we characterize trion creation and decay in single-walled carbon to the substantial ΔETr, the tightly bound trion quasi-particles in SWNTs offer new opportunities to manipulate charge, spin, nanotubes (SWNTs), wherein trions are stable at room tem- and excitonic energy at room temperature. To fully un- perature. We show that SWNT trions derive exclusively from derstand and exploit the exceptional potential of SWNT trion a precursor exciton state, and importantly, that exciton-to- species, it is vitally important to attain fundamental insights trion conversion can approach unity under appropriate con- into the dynamics and mechanisms that characterize their ditions. Because trions simultaneously carry excitation en- creation and decay. ergy, charge, and spin, our findings may guide design of new Various transient optical methods have been used to indirectly SWNT-based optoelectronic devices, including photovoltaics, photodetectors, and spintronics. assess trion dynamics in SWNTs; such studies report trion for- mation and decay time constants that vary by many orders of Author contributions: M.J.T. designed research; Y.B., J.-H.O., and G.B. performed re- magnitude (11–14). Furthermore, corresponding mechanistic search; Y.B., J.-H.O., G.B., C.L., and M.J.T. analyzed data; and Y.B., J.-H.O., G.B., C.L., descriptions of trion generation and decay are ambiguous, due to and M.J.T. wrote the paper. the heterogeneity of the nanotube samples studied and the lack The authors declare no conflict of interest. of a clear trion spectral fingerprint. Indeed, recent experimental This article is a PNAS Direct Submission. and theoretical evidence for the suppression of exciton-free Published under the PNAS license. carrier scattering (16, 17) in SWNTs challenged the exciton- 1To whom correspondence should be addressed. Email: [email protected]. ∼ free hole scattering mechanism for ultrafast ( 50 fs) trion for- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. mation (15). Additionally, linear optical studies of charge-doped 1073/pnas.1712971115/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1712971115 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 Results and Discussion superstructures in D2O solvent (24). We emphasize that spec- S Acquiring Homogeneously Engineered SWNTs Samples. As a primary troscopic data demonstrate that the -PBN(b)-Ph5 polymer re- task for identifying trion dynamics, we acquired SWNTs having mains unoxidized by this procedure, as the semiconducting high uniformity of electronic structure (chirality) and length. polymer valence band energy is stabilized by over 400 meV rel- Dispersion of these SWNTs in the condensed phase by exploiting ative to that of the (6,5) SWNT (24). With length-sorted SWNTs a binaphthalene-based polyanionic semiconducting polymer in hand, and known nanotube concentrations in solution, the [S-PBN(b)-Ph ] that exfoliates, individualizes, and disperses stoichiometric oxidant enables precise control over hole density 5 h+ S SI Appendix SWNTs via a single-chain helically chiral wrapping mechanism, ([ ]) in -PBN(b)-Ph5-[(6,5) SWNTs] ( , Section 2). assures morphological homogeneity of these samples (Fig. 1A) Fig. 2A electronic absorption data that chronicle the oxidative S (18). These semiconducting polymer-SWNT superstructures titration of the -PBN(b)-Ph5-[(6,5) SWNT] superstructures → maintain a fixed polymer helical pitch length on the SWNT highlight the progressive diminution of E00 E11 transition surface (Fig. 1B). The robustness of the polymer-SWNT super- oscillator strength and the correlated rise of a heretofore structures in various aqueous and organic solvents enables unidentified lower-energy transition at ∼1,150 nm, with in- + − multiple rigorous separation procedures that permit isolation of creasing [h ] from 0 to 14.3 (100 nm) 1. The transition centered highly enriched (19) (purity > 90%), length-sorted (20) (700 ± at 1,150 nm has usually been ascribed to a trion optical transition + + 50 nm) (6,5) SWNTs: these S-PBN(b)-Ph -[(6,5) SWNTs] thus (E 00 → Tr 11) (5, 8); consistent with data described below, we 5 + + define uniquely engineered, consistent nanoscale carbon nano- denote this 1,150-nm transition as E 00 → E 11, an exciton tube superstructures (Fig. 1B and SI Appendix, Fig. S1) with transition dressed by the interactions with hole polarons. which to probe transient absorptive signatures and dynamics of trions. Fig. 1C provides benchmark transient absorption spectra Identification of SWNT Trion Transient Absorptive Signature. To for neutral S-PBN(b)-Ph5-[(6,5) SWNTs] in D2O solvent, fol- identify the trion transient absorptive signature, we acquired → broadband pump–probe transient data on hole-doped SWNTs in lowing E00 E11 (1,000 nm) optical excitation. Owing to the + high homogeneity of these (6,5) SWNT superstructures, tran- which [h ] was fixed and known; these studies unveiled a suspicious sient absorptive hallmarks for a broad range of quasi-particles transientabsorptionbandcenteredat1,190nm.Representative S are clearly visualized: these include a dominant E00 →