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Towards Ligand-Customized Quantum Dots

Left, a cadmium selenide with ligand molecule of trioctylphosphine oxide (TOPO). Right, the agreement between model and measured spectra establishes predictive capability for theoretical approach.
A team of researchers headed by Foundry User Eric Schwegler and Tony van Buuren of Lawrence Livermore National Laboratory has shed light on a long-standing question about passivating quantum dot surfaces with organic molecules known as ligands. Pairing theoretical modeling with X-ray absorption spectroscopy, they revealed how ligands modify optical and electronic properties of quantum dots, insights highly relevant to designing nanocrystal-based solar cells.

Passivating ligands help stabilize nanocrystals, but they can also change nanocrystal properties, influencing everything from photon absorption to magnetic behavior. Although these effects are central to the performance of nanocrystal quantum dots in device applications, scientists have so far struggled to disentangle the influence of ligands from other factors.

Van Buuren’s team repeatedly characterized x-ray absorption spectra of cadmium-selenide quantum dots at Berkeley Lab’s Advanced Light Source, exchanging surface ligands between spectra for comparison. Working with the Molecular Foundry’s David Prendergast, Schwegler’s team employed first-principles calculations to pinpoint how the different ligands modify electronic structure of surface cadmium atoms.

Their results establish a predictive capability that can be extended to other quantum dot systems and pave the way toward engineering quantum dots with specialized ligands for photovoltaics and related technologies.

 
“Ligand-Mediated Modification of the Electronic Structure of CdSe Quantum Dots,” J. R. I. Lee, H. D. Whitley, R. W. Meulenberg, A. Wolcott, J. Z. Zhang, D. Prendergast, D. D. Lovingood, G. F. Strouse, T. Ogitsu, E. Schwegler, L. J. Terminello, and T. van Buuren, Nano Letters 12, 2763 (2012). DOI: 10.1021/nl300886h