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Emanuele Marino1 Thomas Kodger2 Dolf Timmerman1 Antonio Capretti1 Tom Gregorkiewicz1 Peter Schall1

1, University of Amsterdam, Amsterdam, , Netherlands
2, Wageningen University & Research, Wageningen, , Netherlands

Semiconductor nanocrystals (QDs) are characterized by a discrete density of states that resembles that of atoms. When an atomic system condenses into a solid, the degeneracy of atomic levels is lifted in accordance to Pauli’s exclusion principle, resulting into the formation of bands. In analogy, when a QD dispersion condenses into a crystalline solid, a supercrystal, the formation of mini-bands may be expected [1]. To obtain a conductive supercrystal, polydispersity must be below 7% [2], self-assembly must be controlled, and the QDs surface must be passivated with short (conductive) ligands [3]. Perfecting the self-assembly of QDs into a supercrystal may enable the observation of mini-bands and consequently boost the performance of QD based devices such as light emitting diodes, transistors, and solar cells.
We evaporate an oil-in-water emulsion to produce three-dimensional QD supercrystals [4]. The crystallization of QDs within each emulsion droplet is followed in situ using synchrotron-based X-ray radiation; we find that under a wide range of conditions, large (ca. 300 nm) QD single crystals grow. We characterize the efficiency of light absorption and emission in these supercrystals as compared to that of dispersed QDs. Furthermore, after performing established ligand exchange protocols with short conductive ligands on supercrystals, we observe a substantial decrease in inter-dot distance and increase in volume fraction while retaining three-dimensional long range order. Finally, we measure the conductivity of a single supercrystal and study its dependence on the crystallinity. We believe these findings suggest new approaches towards QD supercrystal device fabrication exhibiting both conductivity and structural order to enhance performance.
[1] O.L. Lazarenkova and A.A. Balandin. "Miniband formation in a QD crystal." Journal of Applied Physics (2001).
[2] P.N. Pusey, “The effect of polydispersity on the crystallization of hard spherical colloids.” Journal de Physique (1987).
[3] M. Kovalenko et al., "Colloidal nanocrystals with molecular metal chalcogenide surface ligands." Science (2009).
[4] B. De Nijs et al., "Entropy-driven formation of large icosahedral colloidal clusters by spherical confinement." Nature materials (2015).

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