• Roo, Jonathan De
• Hens, Zeger
• Detavernier, Christophe
• Infante, Ivan
• Brinck, Stephanie Ten
• Walravens, Willem
• Dendooven, Jolien
• Drijvers, Emile
• Solano, Eduardo

Abstract

Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to restore nanocrystal stoichiometry can trigger the formation of epitaxial superlattices of PbSe and PbS quantum dots. More specifically, we show that both chalcogen-adding (sodium sulfide) and lead oleate displacing (amines) additives induce small area epitaxial superlattices of PbSe quantum dots. In the latter case, the amine basicity is a sensitive handle to tune the superlattice symmetry, with strong and weak bases yielding pseudohexagonal or quasi-square lattices, respectively. Through density functional theory calculations and in situ titrations monitored by nuclear magnetic resonance spectroscopy, we link this observation to the concomitantly different coordination enthalpy and ligand displacement potency of the amine. Next to that, an initial ∼10% reduction of the initial ligand density prior to monolayer formation and addition of a mild, lead oleate displacing chemical trigger such as aniline proved key to induce square superlattices with long-range, square micrometer order; an effect that is the more pronounced the larger the quantum dots. Because the approach applies to PbS quantum dots as well, we conclude that it offers a reproducible and rational method for the formation of highly ordered epitaxial quantum dot superlattices. KEYWORDS: nanomaterials, PbSe, self-assembly, quantum-dot solid, surface chemistry C olloidal nanocrystals made by highly precise synthesis methods such as hot injection have been widely used as building blocks of self-assembled nanocrystal superlattices. 1−5 Especially in the case of semiconductor nanocrystals or quantum dots (QDs), formation of highly involved binary or ternary superstructures has been demon-strated, 6−10 the symmetry of which could be rationalized using hard sphere crystallization theory. 10−13 Whereas this provides ample possibilities to combine different nanocrystals in a single ordered crystal, only a few studies have shown such an approach to result in metamaterials with new or enhanced properties. 14−16 For one thing, this is due to the use of nanocrystal building blocks capped by long, organic ligands, which inevitably leads to electrically insulating nanocrystal solids. Therefore, optoelectronic devices, such as transis-tors, 17−19 solar cells, 20−23 or photodetectors, 24−26 are based on disordered QD solids, where the interparticle distance is usually decreased by exchanging the long organic ligands with shorter organic or inorganic moieties. 27−31 Although this makes for QD devices with ever increasing performance, carrier mobilities remain well below 10 cm 2 V −1 s −1 and the approach leaves no room for any symmetry-induced collective effects.

Topics

• density
• impedance spectroscopy
• surface
• theory
• semiconductor
• Sodium
• density functional theory
• two-dimensional
• Nuclear Magnetic Resonance spectroscopy
• amine
• metamaterial
• crystallization
• quantum dot
• titration
• monolayer formation