Strong Electronic Coupling in Two-Dimensional Assemblies of Colloidal PbSe Quantum Dots
K. J. Williams,1 W. A. Tisdale,2 K. S. Leschkies,2 G. Haugstad,3 D. J. Norris,2 E. S. Aydil,2 and X.-Y. Zhu1
1Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
2Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455
3Characterization Facility, University of Minnesota, Minneapolis, Minnesota 55455
Thin films of colloidal PbSe quantum dots can exhibit very high carrier mobilities when the surface ligands are removed or replaced by small molecules, such as hydrazine. Charge transport in such films is governed by the electronic exchange coupling energy (Beta) between quantum dots. Here we show that twodimensional quantum dot arrays assembled on a surface provide a powerful system for studying this electronic coupling. We combine optical spectroscopy with atomic force microscopy to examine the chemical, structural, and electronic changes that occur when a submonolayer of PbSe QDs is exposed to hydrazine. We find that this treatment leads to strong and tunable electronic coupling, with the value as large as 13 meV, which is 1 order of magnitude greater than that previously achieved in 3D QD solids with the same chemical treatment. We attribute this much enhanced electronic coupling to reduced geometric frustration in 2D films. The strongly coupled quantum dot assemblies serve as both charge and energy sinks. The existence of such coupling has serious implications for electronic devices, such as photovoltaic cells, that utilize quantum dots.