Photoluminescence Properties of Mn2+-Doped CdS/ZnS Core/Shell Nanocrystals with Different Core Sizes and Shell Thicknesses

The spectral line width of doped semiconductor nanocrystals (NCs) is a critical parameter governing their optical performance, yet isolating the contributions of static structural disorder and dynamic electron–phonon interactions remains a fundamental challenge. In this study, we systematically synthesized CdS:Mn/ZnS core/shell NCs with precisely controlled core-to-shell ratios to elucidate the mechanisms governing the Mn2+ photoluminescence (PL) spectral width. Using a time-gated photon-counting technique, we successfully isolated the long-lived Mn2+ PL and analyzed its temperature and shell-thickness dependencies. Low-temperature (15 K) measurements revealed that the static inhomogeneous broadening, dictated by the local coordination environment of the Mn2+ ions at the core/shell interface, is intrinsically independent of the core/shell ratio. In contrast, at room temperature, significant spectral broadening was observed in NCs with thinner shells. This phenomenon is attributed to the strong coupling between the highly localized Mn 3d electrons and broadly distributed surface phonon modes. Crucially, we demonstrate that a robust inorganic ZnS shell acts not only as a surface passivant but also as a physical shield that effectively decouples the Mn2+ ions from these surface phonons, thereby suppressing the line broadening. These findings provide a comprehensive physical chemistry perspective on the interplay between dopant localization and surface phonon coupling, offering critical design principles for achieving a high spectral purity in doped nanomaterials.