Exciton-acoustic phonon coupling revealed by resonant excitation of single perovskite nanocrystals

ARTICLE https://doi.org/10.1038/s41467-021-22486-5 OPEN Exciton-acoustic phonon coupling revealed by resonant excitation of single perovskite nanocrystals 1234567890():,; Yan Lv1,5, Chunyang Yin1,5, Chunfeng Zhang 1, Xiaoyong Wang 1 ✉, Zhi-Gang Yu 2,3 ✉ & Min Xiao 1,4 ✉ Single perovskite nanocrystals have attracted great research attention very recently due to their potential quantum-information applications, which critically depend on the development of powerful optical techniques to resolve delicate exciton photophysics. Here we have realized resonant and near-resonant excitations of single perovskite CsPbI3 nanocrystals, with the scattered laser light contributing to only ~10% of the total collected signals. This allows us to estimate an ultranarrow photoluminescence excitation linewidth of ~11.32 µeV for the emission state of a single CsPbI3 nanocrystal, corresponding to an exciton dephasing time of ~116.29 ps. Meanwhile, size-quantized acoustic phonons can be resolved from a single CsPbI3 nanocrystal, whose coupling with the exciton is proposed to arise from the piezoelectric potential. The ability to collect resonance fluorescence from single CsPbI3 nanocrystals, with the subsequent revelation of exciton-acoustic phonon coupling, has marked a critical step towards their steady advancement into superior quantum-light sources. 1 National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. 2 ISP/Applied Sciences Laboratory, Washington State University, Spokane, WA 99210, USA. 3 Department of Physics and Astronomy, Washington State University, Pullman, WA 99164, USA. 4 Department of Physics, University of Arkansas, Fayetteville, AR 72701, USA. 5These authors contributed equally: Yan Lv, Chunyang Yin. ✉email: ; ; NATURE COMMUNICATIONS | (2021)12:2192 | https://doi.org/10.1038/s41467-021-22486-5 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-22486-5 S emiconductor perovskite nanocrystals (NCs) have attracted great research attention in the single-particle optical studies since their first successful synthesis in 2015 (ref. 1), with the subsequent observations of single-photon emission2,3, suppressed photoluminescence (PL) blinking and spectral diffusion4, and stable exciton fine structures5,6. By means of photon-correlation Fourier spectroscopy, the PL linewidth was measured to be ~17.0 μeV for the emission-state excitons of single CsPbBr3 NCs7, while the exciton dephasing time of the absorption state could be longer than ~10 ps in single CsPbI3 NCs based on the quantum interference measurement8. These coherent optical properties, which are rarely achievable in traditional colloidal NCs despite several decades of active pursuits, promise great potential of single perovskite NCs in quantum-information applications. In the aforementioned pioneering works7,8, a single perovskite NC was first excited into the absorption state with the coherent information being extracted next from the emission state, which would cause not only linewidth broadening to reduce the exciton coherence9,10 but also timing jitter in the photon generation and emission events11. To circumvent such undesired situations in the coherent optical studies of single perovskite NCs, it is imperative to realize resonant excitation of the emission state. This is analogous to the historical development of single epitaxial quantum dots (QDs), whose routine demonstrations of Mollow-triplet spectra12 and indistinguishable single photons13 are critically dependent on the ability to collect the resonance fluorescence14. Even under resonant excitation of a single epitaxial QD, it is still possible for the emission state to be disturbed by environmental spin and charge fluctuations15,16, as well as lattice vibrations of the acoustic-phonon modes16–18. Specifically, the exciton-acoustic phonon coupling could induce a broad sideband around the emission state, which fundamentally determines the upper limits that can be achieved for the lifetime of exciton coherence and the degree of photon indistinguishability19,20. The soft ionic lattice of semiconductor perovskites is featured with a strong anharmonicity21–23 to bring about low-energy and short-lived acoustic phonon modes23–27, with the accompanied low thermal conductivity28,29 and strong acoustic-optical phonon up-conversion30. These acoustic phonons can significantly influence the carrier transport and relaxation dynamics of semiconductor perovskites at the cryogenic temperatures. In contrast to the optical phonons that have been widely studied in the literature31,32, the acoustic phonons are yet to be experimentally detected in single perovskite NCs, let alone their possible influences on the exciton photophysical properties. By adopting an orthogonal polarization geometry for laser excitation and PL collection, we show here that resonance fluorescence can be collected from single perovskite CsPbI3 NCs at the cryogenic temperature, with the residual contribution of scattered laser light being as low as ~10%. This allows us to resolve an ultranarrow PL excitation linewidth of ~11.32 μeV for the emission state of a single CsPbI3 NC, corresponding to a dephasing time of ~116.29 ps for the band-edge excitons. We further demonstrate that a single CsPbI3 NC can be efficiently excited when the laser energy is tuned across hundreds of μeV both above and below its emission state, which unambiguously confirms the participation of continuous acoustic phonons in the exciton generation processes. Moreover, a size-quantized acoustic-phonon mode is revealed under both near-resonant and resonant excitations of single CsPbI3 NCs, whose energy changes from ~150 to ~180 μeV with increase of the exciton’s emission energy from ~1.70 to ~1.73 eV due to the reduction of NC size. Results Chemical synthesis and optical setup. The perovskite CsPbI3 NCs are synthesized according to a standard hot-injection method5 (see 2 Supplementary Methods in the Supplementary Information) with a cubic edge length of ~9.31 ± 0.68 nm (see the transmission electron microscopy image in Supplementary Fig. 1). Meanwhile, their longterm stability is obtained by the addition of tri-octylphosphine ligands in the post-synthesis treatment33. One drop of the diluted NC solution is spin-coated onto a fused silica substrate, which is then attached to the cold finger of a helium-free cryostat for the optical studies of single CsPbI3 NCs. The experimental setup is schematically shown in Supplementary Fig. 2 (see Supplementary Methods in the Supplementary Information), where a He-Ne laser (~1.96 eV) and a tunable diode laser (~1.687–1.739 eV) both operated at the continuous-wave mode are employed for the above-bandgap and resonant/near-resonant excitations, respectively. For the purpose of observing resonance fluorescence from a single CsPbI3 (...truncated)