Origin of Photoluminescence and XAFS Study of (ZnS)1-x(AgInS2)x Nanocrystals.

Donor-Acceptor transition was previously suggested as a mechanism for luminescence in (ZnS)1-x(AgInS2)x nanocrystals. Here we show the participation of delocalized valence/conduction band in the luminescence. Two emission pathways are observed: Path-1 involves transition between a delocalized state and a localized state exhibiting higher energy and shorter lifetime (∼25 ns) and Path-2 (donor-acceptor) involves two localized defect states exhibiting lower emission energy and longer lifetime (>185 ns). Surprisingly, Path-1 dominates (82% for x = 0.33) for nanocrystals with lower x, in sharp difference with prior assignment. Luminescence peak blue shifts systematically by 0.57 eV with decreasing x because of this large contribution from Path-1. X-ray absorption fine structure (XAFS) study of (ZnS)1-x(AgInS2)x nanocrystals shows larger AgS4 tetrahedra compared with InS4 tetrahedra with Ag-S and In-S bond lengths 2.52 and 2.45 Å respectively, whereas Zn-S bond length is 2.33 Å along with the absence of second nearest-neighbor Zn-S-metal correlation.