Tuning colloidal quantum dot band edge positions through solution-phase surface chemistry modification

Daniel M Kroupa,Nicholas P Brawand,Elisa M Miller,Giulia Galli,Arthur J Nozik,Márton Vörös,Alan Sellinger,Jing Gu,Brett W Mcnichols,Matthew C Beard

doi:10.1038/ncomms15257

Abstract

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells and light emitting diodes. Here we show that band edge positions of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned over 2.0 eV through surface chemistry modification. We achieved this remarkable control through the development of simple, robust and scalable solution-phase ligand exchange methods, which completely replace native ligands with functionalized cinnamate ligands, allowing for well-defined, highly tunable chemical systems. By combining experiments and ab initio simulations, we establish clear relationships between QD surface chemistry and the band edge positions of ligand/QD hybrid systems. We find that in addition to ligand dipole, inter-QD ligand shell inter-digitization contributes to the band edge shifts. We expect that our established relationships and principles can help guide future optimization of functional organic/inorganic hybrid nanostructures for diverse optoelectronic applications.

Highlights

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells and light emitting diodes
We explore a library of functionalized cinnamic acid molecules to systematically tune PbS quantum dots (QDs) surface chemistry, and find that thin films of fully ligand-exchanged QDs exhibit remarkable band edge shifts: the band edge position of QDs can be tuned over 2.0 eV, the largest value reported to date
R-CAH molecules are ideal ligands for solution-phase exchange: the optoelectronic properties of the R-CAHs are widely tunable through functionalization of the aromatic ring of the ligand motif; the vinyl linkage of R-CAH allows for electronic coupling of the dipole active portion of the ligand to the QD core; the R-CA À ligands impart long-term colloidal stability and prevents QD aggregation; the carboxylate surface coordination environment of native oleate (OA À )is conserved post-exchange; and the binding of R-CAHs induces broadband optical absorbance enhancement, which can be utilized to monitor the extent of exchange in situ[10,11,19]

Summary

Introduction

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells and light emitting diodes. The Bawendi[14,15], Luther[16] and Bent[17] groups found that surface chemistry modification using solid-state ligand exchange techniques can shift the ionization energy (IE) and work function (F) of lead sulfide (PbS) QD thin films, allowing for the engineering of more efficient QD solar cells All of these studies demonstrated that modifying the ligand/PbS QD interface produces quite distinct chemical systems, and some even suggested a link between QD band edge energy shifts and ligand dipole moment; due to the uncontrolled and ill-defined physicochemical nature of solid-state ligand exchanges, a clear and quantitative relationship has never been reported. We propose that inter-QD ligand shell inter-digitization likely present in close packed QD thin films must be accounted for, and we report quantitative comparisons between theory and experiment

Methods

Results

Conclusion