Fine Structure Control and Multi-luminescence Property of Cadmium-free Quantum Dots with Rare Earth

Abstract

Colloidal semiconductor quantum dots (QDs) are capable of changing the typical emission and absorption wavelengths because of the band gap widening effect of nano-sized particles. So far, these properties have created many applications for nanocrystals in light-emitting diodes, lasers, photovoltaic devices, advanced displays and biomedical complexes. Recently, alternative materials that do not contain toxic cadmium are strongly desired. Although the photoluminescence quantum yield and emission band width of colloidal InP QDs have been greatly improved in recent years, the materials have not yet suitable for replacing widely used CdSe-based colloidal QDs. The ZnSe can emit overall region visible light by adding Mn or Cu source into the ZnSe quantum dot. Further, Rare earth (RE) materials possessing special 4f are recognized as excellent candidates for luminescence centers of the doped Ⅱ-VI nanocrystals due to their many optical advantages, such as sharp fluorescent emissions via intra of 4f-5d transition, large stokes shift, no photobleaching and long luminescent lifetime and unique luminescent of Europium etc. Among the various quantum dot materials, the ZnSe/ZnS has thermal stable as wide band gap (2.7 eV and 3.5 eV, respectively). In addition, core/shell of Type I systems passivate the surface of the core with the goal to develop their optical properties. Because the band gap of the shell material is larger than that of the core one, and both electrons and holes are confined in the core. In the dissertation, new multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) QDs introduce by controlling the fine-structure of QDs with rare earth (RE). Therefore, optical and structure property of QDs with RE was investigated from surface to inner-surface structure. The core/shell structural QDs have luminescence intensity three times that of ZnSe QDs due to the passivation effect of the Shell compared to core. The broad spectrum was confirmed at 450–550 nm emission of Eu2+ (4F65D1→4F7) and Eu3+ sharp peaks in the red at 579, 592, 615, 651, and 700 nm due to the electronic transitions of 5D0→7Fn (n = 0, 1, 2, 3, 4). While the core/shell system rapidly decreased Eu2+ emission but only the Eu3+ peaks depending on leisurely decreased with increased reaction time compared to core. Microscopic analyses show that the core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. The lattice constant in the central portion of the core/shell QDs are around d111 = 3.13Å, which is between the outside and inside ring patterns (d111 = 3.27 and 3.07 Å, respectively). This shows the effective over-capping of shell onto the core QDs. The core/shell structure have including oxygen less than core that Eu2O3 and EuO bonding the over-coated ZnS surface on the Eu3+-doped ZnSe core.