2D Excitation Emission Matrices Show a Ligand Coverage Effect on Lead Sulfide Quantum Dot Dynamical Lineshapes

Quantum light applications of quantum dots require narrow spectral linewidths. To achieve this, we need to eliminate size dispersion broadening and synthetically control single-dot linewidths. Synthesis of quantum dots has advanced to a stage that the ensemble spectroscopic lineshapes traditionally used to estimate the size dispersion of a synthesis are affected as much by homogeneous single-dot linewidths as by size dispersion. This implies that one can no longer use the ensemble lowest-exciton linewidth to compare the size-dispersion in modern, high-quality synthesis. Since single-dot emission spectra differ from each other even for quantum dots with the same bandgap, quantum dots do not have a “homogeneous” lineshape. The average single-dot lineshape for dots with the same bandgap is called dynamical lineshape. In 2013, Cui et al. used photon correlation Fourier spectroscopy to measure the symmetrized dynamical lineshape and showed that the dynamical linewidth changes by 30% upon shelling the CdSe QDs.[1] Furthermore, in 2021, Ryu et al. used femtosecond 2D Fourier transform electronic spectroscopy to extract dynamical lineshapes for 4.3 nm diameter lead sulfide quantum dots (PbS QDs) and observed that changing the ligand coverage changed the dynamical linewidth by more than the size dispersion broadened the ensemble linewidth.[2] In the same study, they also discovered that these quantum dots obey single-molecule generalized Einstein relations between their absorption and emission spectra.
The above techniques for measuring the dynamical lineshape require specialized instruments and experimental expertise. Here, we describe efforts to develop a much easier, faster, and more accessible procedure for separating the dynamical lineshape from bandgap dispersion using spectra from commercially available absorption and fluorescence spectrometers. Our procedure is based on acquiring incoherent 2D Excitation Emission Maps (EEM), which is the spontaneous emission analogue of the stimulated emission signal in the fully relaxed 2DFT spectra. The 2D EEM eliminates complications from ground state bleach and excited state absorption signals in 2DFT spectra. We measured 2D EEM spectra for the oleate capped 3.6 nm PbS QDs and applied various analysis procedures to obtain several estimates of the dynamical linewidth that are consistent with each other. The obtained dynamical linewidth for the PbS QDs is in close agreement with Bawendi’s photon-correlation Fourier spectroscopy measurement on dots with a similar bandgap.[3] The 2D EEM method has three advantages: it is experimentally easier (it takes a few minutes to an hour to record the full 2D-EEM spectra); it gives a smaller error bar on the linewidth; and it need not assume a symmetric dynamical lineshape. We applied this technique to measure the effect of ligand coverage on the dynamical linewidth for these PbS QDs and observed that decreasing the ligand coverage increases the dynamical linewidth. In this talk, I will be discussing these results and progress towards developing a robust procedure that can be used to extract asymmetric dynamical lineshapes from 2D-EEM spectra.

Acknowledgement: This work was supported by the National Science Foundation under Grant No. DMR-2019444. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

[1] “Direct probe of spectral inhomogeneity reveals synthetic tunability of single-nanocrystal spectral linewidths” Jian Cui et al., Nature Chemistry 5, 602-606 (2013).
[2] “Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra" Jisu Ryu et al., Science Advances 7, eabf4741 (2021).
[3] “PbS Nanocrystal Emission Is Governed by Multiple Emissive States” Justin R. Caram et al., Nano Letters 16, 6070-6077 (2016).