Surface Matters: The Impact of Ligands on the Absorption Spectra of Multiexciton and Charged States in Cadmium Chalcogenide Nanocrystals
Katherine Shulenberger a b
a Brandeis University, South Street, 415, Waltham, United States
b University of Colorado Boulder, United States
Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)
#NANOFUN - Functional Nanomaterials: from optoelectronics to bio- and quantum applications
Torremolinos, Spain, 2023 October 16th - 20th
Organizers: Milena Arciniegas, Iwan Moreels and Gabriele Raino
Invited Speaker, Katherine Shulenberger, presentation 152
DOI: https://doi.org/10.29363/nanoge.matsus.2023.152
Publication date: 18th July 2023

The generation, identification, and utilization of excited states is of paramount importance for optimizing semiconductor nanocrystals for optoelectronic applications. Of particular interest is characterizing the number and type of excitations present in a nanocrystal to understand the efficiency of processes such as Auger recombination, carrier multiplication, or charge transfer. While charged (trion) and multiexciton states are functionally quite different from single excitons states with respect to their excited state lifetime, they are spectrally quite similar, resulting in challenges in conclusively identifying these states. Through power dependent transient absorption measurements and globally fitting the exciton and multiexciton component spectra, we directly identify both the recombination dynamics, spectral form, and relative intensities affiliated with each state in CdSe quantum dots. Through these measurements we illustrate that the biexciton transient absorption spectra is < 1.6 times as intense as the exciton spectrum at the band-edge across a range of CdSe quantum dot sizes. This is in contrast to the oft-assumed factor of 2 between the exciton and biexciton absorption due to the degeneracy of the state. We identify that differences in the spectral intensity between samples is most sensitive to the difference in peak energies, which is correlated with the surface termination rather than quantum dot size. Surface effects also have a large impact on the excited state properties of CdS nanocrystals. One such example is surface/ligand mediated photoreduction which we identify through both steady-state absorption and photoluminescence measurements [1]. Of particular importance is the ubiquitous nature of this ligand-mediated photoreduction, which can have an enormous impact on optical measurements and device functionality for a variety of applications of CdS nanocrystals. While problematic for spectroscopic measurements of exciton properties, these photogenerated charged states can be long-lived (minutes to hours), opening avenues for highly efficient charge transfer to enable photocatalytic applications.

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