Coherent Exciton Delocalization in Strongly Coupled Quantum Dot Arrays Ryan W. Crisp, †,‡ Joel N. Schrauben, † Matthew C. Beard, † Joseph M. Luther, † and Justin C. Johnson* ,† † National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, Colorado 80401, United States ‡ Department of Physics, Colorado School of Mines, Golden, Colorado 80401, United States * S Supporting Information ABSTRACT: Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical tran- sitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron−hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter. KEYWORDS: Quantum dot, nonlinear, ultrafast, exciton, spin, delocalization E lectronic excitations in isolated quantum dots (QDs) are constrained by the physical size of the nanostructure and thus display a manifold of exciton states derived from confinement of the band structure of the material by an infinite spherical potential of well-defined diameter. 1,2 Reduc- tion in interparticle distance and the type or amount of material between QDs in a film can incite wave function expansion beyond the QD boundary, thus perturbing the isolated QD exciton state manifold. This perturbation in excited state electronic structure, which is inevitable in almost all practical schemes of solar photoconversion involving colloidal QDs, is bound to influence photophysical properties that have been studied in detail only for isolated nanostructures. For example, multiexciton processes like multiple exciton generation (MEG) 3 and Auger decay 4 depend on electron−hole Coulomb interactions that scale inversely with the effective size of the exciton. Moreover, the rate of thermal equilibration of carriers after photoexcitation in hot carrier extraction schemes may depend on the effective exciton size. 5 It is therefore crucial to measure quantitatively the exciton size after photoexcitation, which is the initial condition for all subsequent dynamics. The extent of the excited state wave function perturbation induced by close QD coupling is not trivial to isolate experimentally, as many material properties also exhibit continuous variation in behavior as the bulk limit is approached. During the past several years, researchers have reported on a large number of successful ligand exchange and surface manipulation treatments for creating electronically coupled nanocrystal arrays. 6 Short-chain organic ligands, 7 inorganic salts, 8 and metal chalcogenide complex (MCC) ligands 9 have increased charge carrier mobilities in close-packed films fabricated from surface-modified colloidal QDs compared to those retaining the native organic ligands from synthesis. Photophysically, these treatments produce a broadened and red-shifted, yet still distinct, lowest exciton absorption feature. Charge carrier mobilities in such films, determined by time- resolved microwave conductivity (TRMC) measurements or measuring the gate response of the array in a field-effect transistor (FET), have exceeded 10 cm 2 V −1 s −1 for PbSe QDs 10 and CdSe QDs. 11 Even in the absence of long-range order, transport was found to occur without thermal activation, implying a change from a charge hopping transport mechanism to a regime that is more coherent or band-like. While true band transport (i.e., fully delocalized states) across a macroscopic film seems unlikely in such samples, 12 coherent expansion of excited states over shorter ranges remains possible and could lead to the observed temperature dependence. 13,14 Neither TRMC nor FET measurements report directly on exciton delocalization since each is sensitive only to free charges, but these interesting observations may have at their origin the presence of a delocalized exciton state that is less sensitive to typical activation barriers to percolation and more efficiently moves charge over long distances. In this Letter we investigate the ultrafast transition from localized and quantum-confined excitons created by photo- absorption in coupled CdSe QD films to delocalized excitons extending over multiple QDs. We find that the MCC ligand In 2 Te 3 produces the largest effective exciton size without thermal annealing, about 2.2 times the known QD size. Mild thermal annealing of pyridine-capped QDs also produces a significantly larger exciton size, but the onset of necking or sintering at higher temperatures eventually destroys the Received: July 23, 2013 Revised: August 29, 2013 Published: September 16, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 4862 dx.doi.org/10.1021/nl402725m | Nano Lett. 2013, 13, 4862−4869