Nucleation and Growth Retardation of PbS Quantum Dots (QDs) by Nd-O Clusters in Silicate Glasses

Abstract

Semiconductor quantum dots (QDs) have been intensely studied as promising candidates for near-infrared lasers, light-emitting diodes (LEDs), and fiber-optic amplifiers because of their size-dependent optical and electronic properties. In particular, lead chalcogenide (PbS) QDs exhibit a strong quantum confinement effect over a wide size range because of their large Bohr radius (rB). To date, PbS QDs have been embedded in various matrices, including colloidal solutions, zeolites, polymers, and glasses. The colloidal precipitation method has been most frequently used because it permits the fabrication of QDs exhibiting high emission efficiencies. On the other hand, glass matrices offer good mechanical and chemical stability, and as such, glasses with embedded QDs show potential for application in solid-state optical devices. It is important to precisely control the size and spatial distribution of QDs in glasses to realize the desired optical properties. To fabricate robust optical devices, simple thermal treatment is not adequate, because as even slight fluctuations in temperature often lead to an uncontrolled QD size distribution. Using rare-earth (RE) ion clusters as nucleating sites is considered as a suitable alternative strategy for controlling the growth of QDs in glasses. For example, Shim et al. incorporated Er2O3 into silicate glasses and observed a blue shift in photoluminescence (PL) from λ = 1817 to λ = 1546 nm upon increasing the Er2O3 content from 0.1 to 0.4 mol% under equivalent heat treatment conditions. No significant change in the Er3+:4I13/2 fluorescence lifetime was observed before (7.0±0.1 ms) or after (7.0±0.0 ms) heat treatment. In addition, Kim et al. doped Tm2O3, La2O3, and Ho2O3 to control the precipitation of PbS QDs in glasses, and observed a similar blue shift in the peak wavelengths of the absorption and PL bands with increasing RE concentration. Therefore, the purpose of this study is to use RE ions as size controller of PbS QDs in glass matrix and to understand the relation between RE ions and precipitation of PbS QDs. First, Nd2O3-doped silicate glasses was made by conventional melt-quenching method. The glasses thus obtained were annealed at 400°C for 1 h to eliminate the residual thermal stress. PbS QDs were precipitated by heat-treatment at 500oC for various duration. The formation of PbS QDs was confirmed through the observation of XRD patterns characteristic of PbS crystals. The diameters of PbS QDs decreased from ~4.4 nm to 3.7 nm as the Nd3+ content was increased from 0.0 mol% to 0.6 mol%. It means, the absorption and photoluminescence peaks shifted to shorter wavelengths with increasing Nd3+ concentrations. Second, local environment of Nd3+ ions was investigated by using extended x-ray absorption fine structure (EXAFS) and atom probe tomography (APT) measurements. EXAFS results revealed that the majority of Nd3+ ions were surrounded by ~7 numbers of O2- ions at a distance of ~2.5 Å after phase correction both before and after the precipitation of PbS QDs in Nd2O3 containing silicate glasses. In addition, Nd-O clusters in glass matrix were analyzed by APT and the 3-dimensional elemental distribution map shows the several clustered area with the average volume of 1.18 nm3. Approximately, 80 Nd-O clusters were evaluated by maximum separation method which is famous clustering analysis method and the compositional analysis shows 20 times higher concentration of Nd atoms in clusters rather than glass matrix. Moreover, the most important thing is Pb atoms are preferentially located near Nd atoms. Finally, the effect of Nd2O3 on the formation of PbS QDs was investigated by EXAFS and TEM analysis. In the growth kinetics, QD diameters (D) increased slower than predicted from the normal growth process, with a time dependence of D ≈ t0.270–0.286 likely due to the low concentrations of Pb2+ and S2− ions. Electron dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) analysis have revealed that the concentration of RE ions, such as Nd3+, is considerably higher inside the PbS QDs than in the glass matrix. At the conclusion, we clearly understood the role of Nd2O3 addition on the formation of PbS QDs and we found the precipitation of QDs was retarded due to the presence of Nd2O3 in the glasses, as the large NdOx polyhedra blocked the diffusion of Pb2+ and S2− ions. Therefore, we believe that Nd3+ ions were primarily located inside PbS QDs in the form of Nd-O clusters, with the PbS QDs being built on top of these clusters.