Origin
of Photoluminescence and XAFS Study of (ZnS)<sub>1–<i>x</i></sub>(AgInS<sub>2</sub>)<sub><i>x</i></sub> Nanocrystals
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Abstract
Donor–Acceptor transition
was previously suggested as a
mechanism for luminescence in (ZnS)<sub>1–<i>x</i></sub>(AgInS<sub>2</sub>)<sub><i>x</i></sub> 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 <i>x</i> = 0.33) for nanocrystals with lower <i>x</i>, in sharp difference with prior assignment. Luminescence
peak blue shifts systematically by 0.57 eV with decreasing <i>x</i> because of this large contribution from Path-1. X-ray
absorption fine structure (XAFS) study of (ZnS)<sub>1–<i>x</i></sub>(AgInS<sub>2</sub>)<sub><i>x</i></sub> nanocrystals
shows larger AgS<sub>4</sub> tetrahedra compared with InS<sub>4</sub> 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