A Fine Size Selection of Brightly Luminescent Water-Soluble Ag-In-S and Ag-In-S/ZnS Quantum Dots

A size-selected series of water-soluble luminescent Ag-In-S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by a precipitation technique. Up to 10-11 fractions of size-selected AIS (AIS/ZnS) QDs emitting in a broad color range from deep-red to bluish-green were isolated with the photoluminescence (PL) quantum yield reaching 47% for intermediate fractions. The size of the isolated AIS (AIS/ZnS) QDs varied from similar to 2 to similar to 3.5 nm at a roughly constant chemical composition of the particles throughout the fractions as shown by the X-ray photoelectron spectroscopy. The decrease of the mean AIS QD size in consecutive fractions was accompanied by an increase of the structural QD imperfection/disorder as deduced from a notable Urbach absorption tail below the fundamental absorption edge. The Urbach energy increased from 90-100 meV for the largest QDs up to 350 meV for the smallest QDs, indicating a broadening of the distribution of sub-bandgap states. Both the Urbach energy and the PL bandwidth of the size-selected AIS QDs increased with QD size reduction from 3-4 to similar to 2 nm, and a distinct correlation was observed between these parameters. A study of size-selected AIS and AIS/ZnS QDs by UV photoelectron spectroscopy on Au and FTO substrates revealed their valence band level E-VB at similar to 6.6 eV (on Au) and similar to 7 eV (on FTO) pinned to the Fermi level of conductive substrates resulting in a masking of any possible size-dependence of the valence band edge position.