The nanoparticle system we chose in such endeavor is “upconverting nanoparticles” (UCNPs). UCNPs are nanoparticles where lanthanide ions (Yb3+ and Er3+) are doped in the host material such as NaYF4 nanocrystals (~30 nm in diameter).
they have unique optical properties, that is, they absorb near-IR (980 nm/ 808 nm) photons and emit in the visible spectral range through a photophysical process called mult-photon "upconversion". Because of presence of long lifetime states in lanthanides, there is no need to use expensive femtosecond lasers providing strong laser pulses but tiny and cheap CW diode lasers are good enough for the excitation light source. Obviously, the detection of visible emission from UCNPs does not require any "IR-grade" optics and cameras to be integrated into the equipment. Most importantly, even wide-field imaging without scanning of the laser or sample is feasible, so that "high quality", "high-speed", "real-time", and "multi-photon" imaging can be achieved, which is unprecedented.
In recent year, the research area of lighting emitting diode, biological imaging, temperature/pH-sensor requires novel luminescent materials which have high quantum yield. Therefore, it is very important to synthesize luminescent materials and to find out unique photophysical phenomena. We synthesize materials such as Oxide / Halide – Perovskite, Layered perovskite, Carbon quantum dots, and semiconductor quantum dots using a synthesis method such as Hydrothermal synthesis, Solid-state reaction, Ligand-Assistant Re-precipitation, and Hot injection synthesis. We find new luminescence phenomena based on the concepts of ‘Electron’, ‘Spin’, ‘Photon’, ‘Phonon’ and make deep insights into spectroscopic behavior occurring in materials.
From the “Puzzle of Rare-Earth Spectra”, Studies on the 4f-4f transitions of the lanthanide ion have continued. Upconversion materials are composed of “HOST CRYSTALS” with “LANTHANIDE IONS”. Because of Laporte forbidden properties of lanthanide ion electric dipole transitions, the effect of host crystal local symmetry around emitter ions on upconversion luminescence intensity (quantum yield) is very important. Crystal field-induced Mixing of atomic configurations results in relaxation of parity forbidden selection rule of transitions. We are looking for hosts with various non-centrosymmetric sites and are studying the upconversion phenomenon through the process of doping lanthanides on such sites.