Influence of Ag Nanoparticles on Growth Behavior and Optical Properties of Semiconductor Quantum Dots in Glasses

Abstract

Semiconductor quantum dots (QDs) doped glasses can have tunable absorption and photoluminescence (PL) wavelengths by adjusting the size of QDs because of quantum confinement effect. Group II-VI QDs doped glasses have a tunable band-gap in the visible wavelength region, which have been long used as sharp cutoff color filters. IV-VI doped glasses can absorb and emit light in the near-infrared wavelength region due to their narrow band-gap energies and strong quantum confinement, and they have potential applications as solid-state saturable absorbers for mode-locked lasers and as fiber-optic amplifiers in optical communication. Thermal treatment is the most common way to precipitate QDs in glasses, and the size of QDs can be tuned by tailoring the heat-treatment temperatures or durations. But, it is still very difficult to control the nucleation process and spatial distribution of QDs in glasses by this manner. Enhancing luminescence of QDs in glasses is another strong desire to push their applications forward. Ag nanoparticles (NPs) are well-known as nucleating agents for controlled crystallization of glasses. And recently, Ag NPs are used to enhance luminescence of QDs by keeping the distance between Ag NPs and QDs within tens of nanometers. Therefore, the purpose of this work is to use Ag NPs as nucleating agents to control the precipitation of PbS QDs in glasses and to enhance luminescence of CdS QDs in glasses by the induction of Ag NPs in glasses. First, melt-quenching method was used to prepare silicate glasses that contained PbS compounds and different contents of Ag+ ions (0 – 40 ppm). After thermal treatment, PbS QDs were formed and showed the tunable absorption and PL wavelengths (700 ≤ λ ≤ 1900 nm) in the near-infrared region. At the same heat-treatment conditions, the absorption coefficients and PL intensities of PbS QDs increased with the content of Ag+ ions. Ag nanoclusters formed by thermal treatment nucleated formation of PbS QDs in glasses. Second, l of Ag+ ions were incorporated into glasses using three methods to diffuse Ag+ ions: (1) dipping glasses in AgNO3 solutions or (2) in AgNO3 melts for Na+↔Ag+ ion-exchange, and (3) solid-state diffusion of Ag+ ions assisted by electric field. After subsequent thermal treatment, PbS QDs in Ag+-diffused regions photoluminesced at longer wavelengths than those in Ag+-free regions. This indicates that the sizes of PbS QDs thus formed in Ag+-diffused regions were larger than those in Ag+-free regions and the size difference was also confirmed from the transmission electron microscope (TEM) images. In addition, PbS QDs can preferentially precipitate in Ag+-diffused regions at low temperatures. The formation of Ag NPs at relatively low temperature was confirmed by X-ray photoelectron spectroscopy and absorption spectra. We hypothesized that these Ag NPs provided nucleation sites for formation of PbS QDs in glasses, and thus resulted in the larger PbS QDs in the Ag+-diffused glasses than in the Ag+-free glass and preferential precipitation of PbS QDs at low temperatures. These methods provide a promising approach to controlling the precipitation and spatial distribution of PbS QDs in glasses. Finally, enhancement and quenching of CdS quantum dot luminescence by induction of Ag NPs in glasses were investigated. Ag NPs were incorporated into glasses containing CdS QDs through Ag+ ion-exchange and subsequent thermal treatment. The formation of Ag NPs in the glasses was confirmed by TEM images and X-ray diffraction patterns, and their size was estimated to be ~5.0 nm. Confocal fluorescence microscopy was used to compare the changes in the luminescence intensities of CdS QDs. Luminescence intensity increased up to ~3 times when ion-exchange duration was 1 min due to the surface plasmons of Ag NPs, but was quenched when duration prolonged to 30 min. Increasing amount of Ag NPs upon ion-exchange decreased the average distance between CdS QDs and Ag NPs, and this decrease caused the quenching of PL due to the electron/energy transfer between the two.