Size-dependent tuning of Mn²⁺ d emission in Mn²⁺-doped CdS nanocrystals: bulk vs surface

Abstract

We show that the characteristic Mn²⁺ d emission color from Mn²⁺-doped CdS nanocrystals can be tuned over as much as 40 nm, in contrast to what should be expected from such a nearly localized d−d transition. This is achieved surprisingly by a fine-tuning of the host particle diameter from 1.9 to 2.6 nm, thereby changing the overall emission color from red to yellow. Systematic experiments in conjunction with state-of-the-art ab initio calculations with full geometry optimization establish that Mn²⁺ ions residing at surface/subsurface regions have a distorted tetrahedral coordination resulting in a larger ligand field splitting. Consequently, these near-surface Mn²⁺ species exhibit a lower Mn²⁺ d emission energy, compared to those residing at the core of the nanocrystal with an undisturbed tetrahedral coordination. The origin of the tunability of the observed Mn²⁺ emission is the variation of emission contributions arising from Mn²⁺ doped at the core, subsurface, and surface of the host. Our findings provide a unique and easy method to identify the location of an emitting Mn²⁺ ion in the nanocrystal, which would be otherwise very difficult to decipher.