Reducing quantum dot inhomogeneity is critical for transport in quantum dot arrays and device applications. The size dispersion from transmission electron microscopy (TEM) is frequently used with empirical sizing curves to estimate inhomogeneity. Applying this approach to oleate-capped PbSe nanocrystals described in the literature and made in our laboratory generates a paradox: sizing curves systematically estimate bandgap dispersions larger than the total linewidth of absorption spectra. In this work, we use 2D electronic spectroscopy to resolve this discrepancy for a sample of PbSe nanocrystals with 1.10 eV bandgap. The combination of positive and negative signals in 2D spectra produces a nodal line. The nodal line slope allows a single parameter determination of excitonic 1S-1S absorption bandgap inhomogeneity for the entire ensemble (~10<sup>11</sup> nanocrystals). This approach only uses sample absorption and emission spectra plus the pulse spectrum used to record the 2D spectra and is insensitive to other unknown quantum dot parameters. The precisely determined bandgap inhomogeneity of ~85 meV allows extraction of the average single nanocrystal linewidth of ~80 meV. This better determination of the bandgap inhomogeneity by 2D spectroscopy has helped to resolve the TEM size dispersion vs. absorption linewidth paradox. An inhomogeneity of 80-90 meV due to nanocrystal size variation alone has been obtained from sizing curves after 3 key improvements were applied to TEM image analysis. The improvements consist of imaging nanocrystals with higher signal to noise ratio by ADF-STEM, analyzing the underlying size and shape distribution from major and minor image projections, and considering independent deviations in all three nanocrystal dimensions. The PbSe nanocrystal dimensions vary by ~1 atomic layer (0.3 nm).

Bandgap inhomogeneity from 2D spectra <i>vs. </i>size dispersion from TEM in PbSe quantum dots

253rd National Meeting (2017)

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Bandgap inhomogeneity from 2D spectra vs. size dispersion from TEM in PbSe quantum dots