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

Mn2+-doped ZnSe nanoparticles were synthesized from molecular cluster precursors. Four ZnSe nanoparticle samples, one with low Mn2+ concentration (A), one with an intermediate Mn2+ concentration (B), one with a high Mn2+ concentration (C), and one with no Mn2+, were prepared and characterized using UV−vis, luminescence, electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) techniques. The sample with no Mn2+ had a sharp ZnSe band edge emission peak and a quantum yield of ∼2%. The samples with Mn2+ had a significant decrease in band edge emission. Sample A had no Mn2+ 4T1 → 6A1 emission but showed some ZnSe band edge emission and trap state emission. Sample B had Mn2+ 4T1 → 6A1 emission and a further reduction in ZnSe band edge emission and trap state emission. Sample C showed an increase in the Mn2+ 4T1 → 6A1 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 Mn2+ 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 Mn2+ possessed an octahedral geometry due to significant OH-/O2- coordination and the interior Mn2+ occupied the Zn2+ tetrahedral site. The overall low Mn2+ emission quantum yield (>0.1%) is primarily due to the presence of Mn2+ on the particle surface, and the decrease in Mn2+ emission overtime is attributed to the quenching of the luminescence by OH-/O2- coordinated to the surface metal ions. In sample C, which had the highest Mn2+ concentration, the surface Mn2+ enhanced the disorder of the nanoparticle surface structure, resulting in an increase in trap state emission.