The Luminescence Dynamics In Lanthanide Doped Upconversion Core/Shell Nanoparticles

Lanthanide doped upconversion namomaterials(Ln UCNs) have the ability of converting near-infrared excitations into visible emissions, and therefore is extremely potential in the applications in solid laser, three-dimensional display, photovoltaic industry, and especially biomedicine. Due to the unique excitability by near-infrared excitation, Ln UCNs have a considerable depth of exciting penetration and a magnificent signal-to-noise ratio in biological tissue, which make Ln UCNs attractive in the applications in bio-imaging, bio-detection, and photodynamic therapy. These advantages are, however, shrouded by the low luminescence efficiency.The low luminescence efficiency of Ln UCNs is radically attribute to the luminescence mechanism of upconversion. Due to the complex electron configuration and energy level structures of rare earth ions, the spontaneous downconversion emissions and non-radiative transitions of the intermediate states is the most energy loss pathway in upconversion fluorescence. In the last decade, much effort has been made in improving upconversion efficiency. But the lacking of an unambiguous comprehension in upconverison luminescence dynamics process limited the understanding of the new approaches, such as “active shell”, of enhancing upconversion luminescence. As such, the radical energy loss pathway in upconversion fluorescence has not been discovered and controlled. Therefore, a deeper comprehension of the upconverison dynamics is essential.In this thesis, the luminescence dynamics in core/shell upconversion nanoparticles is established. By the study of the influence on the optical properties by the parameter such as doping concentration and excitation power density, we determined the real role of active-shell in enhancing the luminescence of lanthanides doped nanomaterials, and the optimal sensitizer doping concentration in the shell of core/active-shell nanostructure. And also, we fingered out the different origin of red emission in Na YF4:Yb3+,Er3+ core nanoparticles and Na YF4:Yb3+,Er3+ @Na YF4 core/inert-shell nanoparticles. The works are listed as follows:(1) We studied the luminescence dynamics in the new core/active-shell lanthanides doped nanomaterials. By the theoretical analysis and experimental verification, we revealed that the essence of active-shell strategy, actually just increased the absorption efficiency of the materials, the quantum yield was not improved.(2) We studied the luminescence dynamics in the new core/active-shell lanthanides doped nanomaterials and discussed the origin of concentration quenching effect in lanthanides doped materials. By the theoretical analysis and experimental verification, we figured out, in the core/active-shell nanoparticles, the optimal doping concentration of sensitizers in the shell should be lower than that in the core.(3) We studied the luminescence dynamics in Na YF4:Yb3+,Er3+ upconversion nanosystem, and discussed the origins of red emission of Er3+ under 980 nm excitation. By studying the different dependences of red emissions on the excitation power density in different nanostructure system and different doping concentration, we determined the origin of red emission in core nanoparticles come from the population by non-radiative relaxation and cross-relaxation, and the origin of red emission in core/active-shell nanoparticles only come from cross-relaxation, the non-radiative is negligible due to the shielding effect by the shell on the interaction between Er3+and surface defects.It was well known that upconversion luminescence of rare earth ions in nanomaterials has a irreplaceable and greater advantages, compared with conditional luminescence materials. Once realized the breakthrough of the both of science and technology, the multifunctional nanoplatform will be used in a great deal of fields, including in new laser, novel radiation source, optical sensor from middle to near infrared spectral region, luminescence bioprobe etc.