The EPR, radioluminescence, and photoluminescence of cubic ZnS (cZnS) nanocrystals (NCs) with a narrow size distribution centered at 2 nm, doped with 0.1, 0.2, and $0.5\hspace{0.5em}\mathrm{at}.\%$ Mn${}^{2+}$ ions were investigated. Besides the main lines from substitutional Mn${}^{2+}$ ions localized in the core of the NCs next to a stacking defect, the EPR spectra exhibited two broader hyperfine sextets, attributed to the so-called Mn(II) and Mn(III) surface centers, which could be separated by adequate thermal treatments. The contribution to the photoluminescence from the Mn${}^{2+}$ ions at various sites was further determined from the analysis of the steady-state and time-resolved photoluminescence data from cZnS:Mn NCs subjected to thermal treatments and from cZnS:Mn single crystals. Thus, the main emission consisting of two intense overlapping bands peaking at 596 and 630 nm was attributed to the ${}^4{T}_1-{}^6{A}_1$ transition of the substitutional Mn${}^{2+}$ ions in the core of the cZnS nanocrystals and to residual aggregated Mn${}^{2+}$ ions, respectively, the last ones being responsible for a broad EPR line observed in the $X$-band spectrum. The Mn(II) and Mn(III) centers, consisting of Mn${}^{2+}$ ions in the oxidized and hydrolyzed surface layer of the NCs, respectively, are only indirectly involved in the energy transfer to the substitutional Mn${}^{2+}$ centers, very likely through pairs interaction.

• Received 27 May 2010

DOI:https://doi.org/10.1103/PhysRevB.83.045301