Ligand enhanced upconversion of near-infrared photons with nanocrystal light absorbers

摘要

We designed and synthesized a tetracene derivative 4-(tetracen-5-yl)benzoic acid (>CPT) as a transmitter ligand used in PbS/PbSe nanocrystal (NC) sensitized upconversion of near infrared (NIR) photons. Under optimal conditions, comparing >CPT functionalized NCs with unfunctionalized NCs as sensitizers, the upconversion quantum yield (QY) was enhanced 81 times for 2.9 nm PbS NCs from 0.021% to 1.7%, and 11 times for 2.5 nm PbSe NCs from 0.20% to 2.1%. The surface density of >CPT controls the solubility of functionalized NCs and the upconversion QY. By increasing the concentration of >CPT in the ligand exchange solution, the number of >CPT ligand per NC increases. The upconversion QY is maximized at a transmitter density of 1.2 nm^–2 for 2.9 nm PbS, and 0.32 nm^–2 for 2.5 nm PbSe. Additional transmitter ligands inhibit photon upconversion due to triplet–triplet annihilation (TTA) between two neighboring >CPT molecules on the NC surface. 2.1% is the highest reported QY for TTA-based photon upconversion in the NIR with the use of earth-abundant materials.