Nonlinear Absorption and Refraction of Highly Monodisperse and Luminescent ZnTe Quantum Dots and Their Self-Assembled Nanostructures: Implications for Optoelectronic Devices

Zinc telluride (ZnTe) quantum dots (QDs) were synthesized by a unique supersaturation-controlled aqueous route. For a given pH, increasing the degree of initial supersaturation led to a decrease in the average diameter (d avg) of the QDs and increased monodispersity. Three samples of ZnTe QDs having average sizes of 0. 8, 1. 7, and 2. 2 nm were synthesized (hence named ZnTe₀. 8, ZnTe₁. 7, and ZnTe₂. 2). Nonlinear absorption (NLA) and nonlinear refraction (NLR) of these colloidal ZnTe QDs of different sizes were investigated by the Z-scan technique using a continuous He-Ne laser (632. 8 nm, 15 mW). Isotropic assembly of ZnTe₂. 2 leads to the formation of nanoballs (hence named ZnTeNB). The NLA profile of smaller QDs, ZnTe₁. 7 and ZnTe₀. 8, was found to follow a three-photon absorption (3PA) model, while relatively bigger QDs, ZnTe₂. 2, followed a two-photon absorption (2PA) model. On moving from ZnTe₀. 8 to ZnTe₁. 7, the three-photon absorption coefficient (γ) decreases by 26% (3. 00 × 10-4 → 2. 21 × 10-4 cm3/MW2). The two-photon absorption coefficient (β) for ZnTe₂. 2 is 0. 3 cm/MW. For a 63% decrease in average diameter (2. 2 → 0. 8 nm), the refractive index (n 2) increases by 45% (2. 48 × 10-2 → 3. 6 × 10-2 cm2/MW). Overall, the NLR coefficient shows a decreasing trend with size. Upon isotropic self-assembly, ZnTeNB, there is a significant increase in the NLR coefficient by 40% (2. 48 × 10-2 → 3. 48 × 10-2 cm2/MW) and a simultaneous decrease in the NLA coefficient by 45% (0. 3 → 0. 166 cm/MW). The figure of merit was also determined for all of the samples, and it was found that ZnTe₂. 2 and ZnTe₀. 8 were best suited for all-optical device applications. Further, the self-assembled nanostructures are promising for making optical waveguides for supercontinuum generation (SCG).