Ligand-Free, Colloidal, and Luminescent Metal Sulfide Nanocrystals.

We report here a different kind of binary and ternary colloidal inorganic nanocrystals, where no capping ligand is used. The surfaces of these ligand-free nanocrystals were designed to exhibit negative charges and, therefore, electrostatically repel each other, forming a colloidal dispersion in a polar solvent. Undoped and Mn-doped ZnxCd1-xS nanocrystals were studied both in solution and close-packed films. While undoped samples exhibit poor luminescence because of surface defects, Mn-doped nanocrystals show strong luminescence both in solution (20% quantum efficiency) and the film. Strong Mn emission arises from the inner-core d electrons, which are less sensitive to nonradiative decay channels on the nanocrystal surface. Ligand-free AgInS2 nanocrystals exhibit donor-acceptor type luminescence. Our preliminary results suggest the possibility of electronic coupling between ligand-free nanocrystals in their film, and therefore, they are expected to be more suitable for electronic and optoelectronic applications compared to organic-capped nanocrystals.