Increasing the quantum yield of upconverting NaYF${}_4$ nanocrystals

Nanoparticles consisting of sodium yttrium fluoride (NaYF${}_4$) gain a lot of attention due to their countless possible applications. For example, by adding ytterbium and erbium to the host lattice, low energy infrared light can be converted to high energy visible light. Additionally, NaYF${}_4$ can be prepared in nanometer size and therefore can be used inside cells or living organisms because the infrared excitation light can easily pass the tissue. At the beginning of this thesis, even core/shell particles had much lower quantum yields and short luminescence decay times than the bulk reference material. In this thesis the synthesis method for preparing upconverting NaYF${}_4$ nanoparticles has been modified by using different starting materials and observing the impact of these changes on the particle properties. It could be shown that using NaHF${}_2$ leads to really narrow size distributed particles, but with the disadvantage of not being able to grow a shell on these particles. In the second part of the work presented here, the optimization was aimed at improving the optical properties of the particles. By using dried solvents, anhydrous rare earth acetates and NH${}_4$F it was possible for the first time to synthesize core/shell particles (<50 nm) with a quantum yield close to the bulk reference material. But also for very small core/shell particles (≤15 nm) the quantum yield could be raised to match the bulk material within a factor of 3-4. By measuring the energy level decay times, the biggest energy loss could be assigned to the Yb${}^{3+}$ emission at 940 nm and that was successfully prohibited by adding a shell.