Abstract
ABSTRACT Multicolor upconversion luminescence of Er3+ was successfully enhanced by optimizing the interface in dye-coordinated nanoparticles with a core/shell structure. Red and green upconversion emissions of Er3+ were obtained at the interface of oxide nanoparticles via the intramolecular energy transfer from the coordinating squaraine dye with high light-absorption ability, which was more efficient than emissions through the energy transfer from metal ions such as Yb3+. Additionally, CaF2 nanoparticles as a core material minimized the energy loss with nonradiative downward relaxations in Er3+, resulting in the observation of unusual blue upconversion emissions from the upper energy level of Er3+ by nonlaser excitation using a continuous-wave (CW) Xe lamp at an excitation power of 1.2 mW/cm2.
Highlights
The upconversion phenomenon, which can convert two or more low-energy photons into one high-energy photon, has great potential for effective light energy use
The upconversion emission of Ln ions occurs through the metal-to-metal multiple energy transfer from near-infrared (IR) light absorbers (Yb3+) to visible light emitters (Tm3+, Er3+, Ho3+, Nd3+, etc.), in which the emission color strongly depends on the dopant concentration [5], dispersing solvent [6], core/shell structure [7], and host material [8]
We successfully developed a novel upconversion emission system composed of core/ shell structured Tm/Yb oxide nanoparticles coordinated with indigo dyes [17]
Summary
The upconversion phenomenon, which can convert two or more low-energy photons into one high-energy photon, has great potential for effective light energy use. Lndoped upconversion nanoparticles have some limitations to obtaining the desired emission colors, especially blue emission, since their forbidden transitions of 4f-4f configurations have a low light-absorption coefficient (ɛ = 1–100 dm3mol−1cm−1) [16] and nonradiative downward relaxations often take place with the upward transitions In this investigation, an active interface composed of dye-coordinated Ln nanoparticles with a core/shell structure has been developed to control multicolor upconversion luminescence of Er3+ (Figure 1(a)), where the emissions of Er3+ can be enhanced mainly by two efficient energy migration pathways at the interface: (1) the intramolecular energy transfer from coordinating dyes and (2) the nonradiative downward relaxation on core nanoparticles. CaF2 nanoparticles as the core can suppress the vibration deactivation pathway in the upconversion process due to their low phonon energy, high refractive index, optical transparency, and high chemical and thermal stabilities [19,20,21,22]
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