Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

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

doi:10.1021/acsnano.6b02562

Willem Walravens, Jolien Dendooven + Show 7 more

Open Access

https://doi.org/10.1021/acsnano.6b02562

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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.

Highlights

Two dimensional superlattices of epitaxially connected quantum dots enable sizequantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport
We demonstrate that epitaxial quantum dot superlattices can be formed using additives such as Na2S or amines that render nanocrystal facets stoichiometric as chemical triggers
In the case of amine-triggering, we find that the extent of ligand displacement translates into superlattices with a different symmetry with the more basic, strong displacers yielding pseudo-hexagonal lattices and the less basic, mild displacers giving close to square lattices

Summary

Introduction

Two dimensional superlattices of epitaxially connected quantum dots enable sizequantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. We show that monolayers of oleate-capped PbSe QDs on ethylene glycol can be turned into epitaxial superlattices by addition of sodium sulfide or butylamine, two additives known for inducing a sulfide for oleate exchange or a lead oleate displacement, respectively.[27,48] Focusing on amine addition, we find that less basic additives such as pyridine or aniline markedly promote the formation of cubic superstructures, where long range order can be imposed by a selective stripping of the lead oleate ligands prior to the initial monolayer formation We link these observations to aniline and pyridine being weaker ligand displacing agents, as confirmed by density functional theory, and we underscore the general character of the approach developed here by demonstrating the formation of epitaxially connected PbSe superlattices, and of PbS epitaxial superlattices by similar methods

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