Aqueous phase near-infrared emitters: water transfer of colloidal 2D PbS, PbSe and PbTe nanoplatelets

Colloidal two-dimensional (2D) lead chalcogenide PbX (X = S, Se, Te) nanoplatelets (NPLs) are strongly confined narrow band gap semiconductors with tuneable efficent photoluminescence (PL) in the near-infrared (NIR). They hold high potential for the use as classical and quantum emitters in fiber-based photonics that operate at telecommunication wavelengths. Up to now, the insolubility of 2D PbX NCs in water and other polar solvents has been a challenge that complicates their post-synthesic processing, e.g. into future functional nanocomposites. Here, we describe a phase transfer protocol from hexane to water using 11-mercaptoundecanoic acid (MUA), which yields aqueous phase 2D PbS, PbSe, and PbTe NPLs with preserved shape, crystallinity and NIR PL (e.g. PbS: 724 nm, PbSe: 1023 nm and PbTe: 1184 nm). Water-soluble 2D PbSe shows efficient emission (up to 13% PL quantum yield at 1023 nm), thereby retaining 65% of the initial quantum yield and making it highly interesting as an aqueous NIR light source. By using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR), we follow the phase transfer on a molecular level and find two binding motifs of MUA to the 2D PbX surfaces: X-type bound thiolate and L-type bound thiol. Our results shine new light on mercaptocarboxylic acid based nanomaterial phase transfers and represent a crucial step for incorporating NIR-emissive 2D PbX into (fiber) optics.