Inorganic semiconductor nanocrystals, or quantum dots, are widely used in commercial displays and imaging technology. They could be used in additional optoelectronic applications if their charge-transport properties could be improved without dimming their intense luminescence. A new study reports a way to do that by treating the crystals with metal salts to customize the quantum dots’ surface chemistry (Nat. Commun. 2023, DOI: 10.1038/s41467-022-35702-7).
Quantum dots are typically capped with organic ligands containing long hydrocarbon chains. The ligands control crystal growth during synthesis and stabilize the particles in colloidal suspensions. They also form electrically insulating barriers that impede charge flow between particles, a property needed for electronic applications.
Replacing the organic ligands with inorganic ones can boost charge transport, but that approach dramatically dims the particles. For example, the photoluminescence quantum yield, a measure of emission intensity, of a set of red-, green-, and blue-emitting crystals composed of cadmium selenide and zinc sulfide—capped with organic ligands—reaches 97%, 84%, and 82%, respectively. Switching to common inorganic ligands such as halides and chalcogenides causes the yields of that set to fall to less than 20% for red and green emitters and below 5% for blue emitters.
To bypass that shortcoming, a team led by Yuanyuan Wang of Nanjing University and Dmitri V. Talapin of the University of Chicago treated organically capped quantum dots with salts of cadmium, zinc, and other cations, and nitrate, tetrafluoroborate, and triflate anions. Those salts differ from the halides and others used previously for ligand exchange, the team explains, in that the cations passivate Lewis basic sites on the crystal surfaces, and the anions do not bind tightly to the particles. The new treatment preserves the luminescence intensity with almost no decrease in red and green emitters and just a 10% decrease for blue ones relative to quantum dots capped with organic ligands. The team used the particles to create high-resolution patterns via optical lithography and inkjet printing, a key step toward electronic applications.