The lanthanide photoluminescence in the trivalent terbium (Tb 3+ ) incorporated zinc sulfide nanoparticles [Zn(Tb)S] has been reported with the nanoparticle size varying from 2.0 ± 0.3 to 14 ± 3 nm in diameter as a function of reaction temperature. In all the nanoparticles, the Tb 3+ luminescence has been sensitized by the nanoparticle acting as an optical antenna. The relative contribution of different excitation bands in sensitizing Tb 3+ luminescence in the Zn(Tb)S nanoparticles has been found to be dependent on the size of the nanoparticles. The observed Tb 3+ luminescence efficiency in the Zn(Tb)S nanoparticles has been rationalized by competing factors: (i) the sensitization efficiency that is guided by the relative energy level position of the Tb 3+ ground and excited states with respect to the valence and conduction bands of the ZnS and (ii) the extent of incorporation of Tb 3+ in the nanoparticles. Additionally, it has been argued that the spectral overlap between the nanoparticle (donor) emission and Tb 3+ (acceptor) absorption is not a prerequisite in determining the Tb 3+ emission in the Zn(Tb)S nanoparticles studied. ■ INTRODUCTIONDoped semiconductor nanoparticles are useful due to the possible modulation of chemical, optical, electrical, and magnetic properties of the materials. In the perspective of optically active doped semiconductor nanoparticles, trivalent lanthanide (Ln 3+ ) incorporated semiconductor nanoparticles find wide attraction. 1−6 The luminescence that originates from the intraconfigurational 4f−4f transitions in Ln 3+ is unique compared to that in the conventional luminophores and finds usage in various luminescence-based applications. 7−14 The luminescence of lanthanide cations exhibit sharp emission bands spanning the entire visible and near-infrared spectral region allowing multiplex assays; longer (microseconds to milliseconds) lifetime which makes time-gated measurements feasible; and resistance to photobleaching mechanisms, hence allowing longer experiment time thereby increasing the signal-to-noise ratio.In order to detect the Ln 3+ -based luminescence, two significant challenges need to be overcome. First, the molar extinction coefficient of these cations is at least 1000 times smaller than the corresponding values for the conventional organic fluorophores, allowing the direct excitation of the lanthanide cations to the higher lying excited state difficult. In addition to this, the Ln 3+ luminescence is quenched by the vibrational overtones of various chemical bonds present in the solvent and ligand molecules of the immediate environment. 15 Incorporating Ln 3+ in a semiconductor nanoparticle matrix offers a way to overcome these challenges. 1 In such a system, the nanoparticle matrix acts as an optical antenna and protective matrix simultaneously in order to realize the Ln 3+ luminescence. Specifically, the nanoparticles with high molar extinction coefficient absorb light and transfer the energy nonradiatively to the lanthanide center, and concomitantly an incorporation of ...

Lanthanide cations tune the infrared absorption characteristics of the capping ligands in Zn(Ln)S [Ln = Sm, Eu, Tb, Dy] nanoparticles.