As an alternative to organic ligands, ionically charged inorganic compounds have been explored as capping ligands for semiconducting QDs. These inorganic ligands, such as metal sulfide complexes, have been used to increase the electronic coupling between QDs, enhancing their performance in devices such as transistors and photodetectors. Additionally, these heavier inorganic ligands possess lower vibrational energies, which may help reduce nonradiative quenching of lanthanide excited states in UCNPs. However, it remains an open question whether the chemically-"soft" sulfide ligands-typically used to coordinate with the soft Lewis acid cations in QDs-are compatible with the hard lanthanide ions in UCNPs.