Direct Synthesis of CdSe Quantum Dot Inks in Polar Solvents with Conductive Ligands

Colloidal quantum dots (QDs) have unlocked advancements in technologies like displays and light detectors, thanks to their tuneable photoelectronic properties and scalability. However, several device schemes based on electronically conducting QD layers, such as photodetectors and LEDs, require processing QDs as concentrated inks with short, conductive ligands. Currently, this is only achievable after replacing the insulating ligands used during their colloidal synthesis, a process that is both time-consuming and resource-intensive. Direct synthesis of QD inks with short ligands remains underdeveloped in non-aqueous medium, yet such straightforward synthesis route would immensely facilitate QD device integration. In this study, we focus on cadmium selenide (CdSe), a well-understood QD system with established syntheses and surface chemistry, to showcase a strategy for directly synthesising inks with conductive ligands. We report a one-step synthesis of inorganic-passivated 3.4 nm wurtzite CdSe QDs in polar aprotic solvents. Our findings reveal a non-classical nucleation pathway with 1.5 nm magic-size clusters as intermediates. We demonstrate that highly Lewis-basic solvents and weakly coordinating cadmium counteranions are essential for producing high-quality CdSe QDs, particularly with hexamethylphosphoramide as a solvent and tetrafluoroborate counteranions for cadmium. Surface characterisations, including solid-state NMR, Raman, and IR spectroscopy, and ζ-potential measurements, reveal a disordered QD surface rich in cadmium-oxygen bonds and phosphates, which quenches the photoluminescence.