Optical and Surface Structural Properties of Mn2+-Doped ZnSe Nanoparticles

Mn 2+ -doped ZnSe nanoparticles were synthesized from molecular cluster precursors. Four ZnSe nanoparticle samples, one with low Mn 2+ concentration (A), one with an intermediate Mn 2+ concentration (B), one with a high Mn 2+ concentration (C), and one with no Mn 2+, were prepared and characterized using UV−vis, luminescence, electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) techniques. The sample with no Mn 2+ had a sharp ZnSe band edge emission peak and a quantum yield of ∼2%. The samples with Mn 2+ had a significant decrease in band edge emission. Sample A had no Mn 2+ 4 T 1 → 6 A 1 emission but showed some ZnSe band edge emission and trap state emission. Sample B had Mn 2+ 4 T 1 → 6 A 1 emission and a further reduction in ZnSe band edge emission and trap state emission. Sample C showed an increase in the Mn 2+ 4 T 1 → 6 A 1 emission, a dramatic increase in trap state emission, and essentially no ZnSe band edge emission. The overall emission from all four samples was quenched with time. To better understand these observations, XAFS and ESR data were taken to characterize the local structural and chemical environment of the Mn 2+ ions. The XAFS data indicated that there was a reduction in the Zn and Mn first neighbor Se coordination from the bulk value but a lack of a reduction in the Se first neighbor coordination. This suggests that the core of the nanoparticles resembles that of bulk ZnSe, and the surface of the particle has a higher concentration of metal atoms. We propose that the surface Mn 2+ possessed an octahedral geometry due to significant OH - /O 2- coordination and the interior Mn 2+ occupied the Zn 2+ tetrahedral site. The overall low Mn 2+ emission quantum yield (>0.1%) is primarily due to the presence of Mn 2+ on the particle surface, and the decrease in Mn 2+ emission overtime is attributed to the quenching of the luminescence by OH - /O 2- coordinated to the surface metal ions. In sample C, which had the highest Mn 2+ concentration, the surface Mn 2+ enhanced the disorder of the nanoparticle surface structure, resulting in an increase in trap state emission.