Origins of Linewidth Broadening in Highly-Confined CsPbBr₃ Nanorods

Colloidal semiconductor nanocrystals, or quantum dots (QDs), are an exciting playground for photophysics, exhibiting atom-like behaviour while retaining advantages of molecular crystal size tunability and solution-phase synthesis.¹ Recently, colloidal QDs have been explored as two-level systems for single photon sources.² In particular, weakly-confined lead halide perovskite nanocrystals (LHPs) exhibit superior properties to conventional II-VI and III-V materials with a facile synthesis and minimal engineering.³ However, these particles have high biexciton quantum yields at low temperature and emit from multiple fine structure states, both of which are sources of noise affecting coherence.⁴ These can be mitigated by moving to the highly confined regime, where there are heightened size and morphology effects.<br/>Highly confined CsPbBr₃ nanorods are a particularly interesting system to study, as the transition dipole aligns with the long axis of the material, resulting in strongly polarized, directional emission from a single fine structure state.⁵ However, these particles are more unstable than their large counterparts due to a high surface area to volume ratio. For this reason, LHP nanorods have not been studied to the same degree as nanocubes and nanoplatelets. Our group has synthesized, for the first time, highly confined, stable LHP nanorods that exhibit complete antibunching, but whose linewidths are extremely broad on the single particle level. In this work, we examine the relevant contributing factors to the spectral linewidth and find that high-lying (35 meV ± 6 meV) longitudinal optical phonons are the dominant mechanism of vibrational broadening in these materials. Taking into account a large fine structure (15 meV) and structural inhomogeneities, we can begin to understand the relevant photophysics of these materials, with the goal of eventually controlling exciton-phonon coupling and fine structure towards single photon emitter applications. Our results give insight into future synthetic handles to develop stable, narrow, and bright emitters.<br/><br/>References<br/>1. Murray, C. B., Norris, D. J., & Bawendi, M. G. (1993). <i>Journal of the American Chemical Society</i>, <i>115</i>(19), 8706–8715.<br/>2. Kuhlmann, A. v, et al. (2015). <i>Nature Communications</i>, <i>6</i>(1), 8204.; Senellart, P., Solomon, G., & White, A. (2017). <i>Nature Nanotechnology</i>, 12 (11), 1026–1039.<br/>3. Krieg, F., et al. (2018). <i>ACS Energy Lett. 20</i>, 55.<br/>4. Utzat, H., et al. (2017) <i>Nano Letters</i>, <i>17</i>(11), 6838–6846.; Kaplan, A.E.K., et al. (2023) Nat. Photon. 17, 775–780.<br/>5. Hou, L., Tamarat, P., & Lounis, B. (2021). <i>Nanomaterials</i>, <i>11</i>(4) 1053.; Sercel, P. C., et al. (2019) <i>Nano Letters</i>, <i>19</i>(6), 4068–4077.<br/>6. Hua Zhu, et al. Nano Letters 2022 22 (20), 8355-8362