Brightness characterization of single core and core/shell Yb3+,Er3+-doped NaYF4 upconversion nanoparticles

Dijkstra, Remko R. M. (2012) Brightness characterization of single core and core/shell Yb3+,Er3+-doped NaYF4 upconversion nanoparticles.

Abstract:Part I: Upconversion Nanoparticles NaYF4:Yb3+, Er3+ upconversion nanoparticles are luminescent particles that are promising in a wide range of applications such as: biomarkers, solar-cells, displays and microbarcodes [1]. However, due to their small size, these particles are typically not very bright. Coating the particles with a uniform NaYF4 shell increases the brightness significantly. Interestingly, the brightness continues to increase even after applying very thick shells, but never reaches the brightness of the bulk counterpart. The reason for this is still unknown. The objective of this study is to characterize the brightness of core-only and core/shell particles with different shell thickness on a single particle scale. This study, being the first of its kind, aims for new insights on the reasoning behind the inability of core/shell particles to reach the brightness of the bulk counterpart. The results of the single particle characterization confirmed an increasing particle brightness with increasing shell thickness. A key observation is the broadening of the single-particle brightness distribution with increasing shell thickness. We attribute this broadening to the presence of dopants in the shell, which are incorporated into the shell during synthesis. This is contrary to the idea of a completely passive shell, which was always assumed. This new insight is important feedback for the materials group that synthesizes the particles. Alternative methods for creating dopant-free shells should be considered. Additional first experiments on the dependence of excitation power-vs-emission intensity showed a significant difference between core-only and core/shell particles, indicating that the shell does play a significant role in the particle brightness enhancement. Preliminary results on the particle emission spectrum showed an interesting additional peak at 700 nm in addition to the spectrum that is typically reported in literature. Part II: STED Setup Design The objective of the second part of the assignment was to design and realize an easy-touse and robust single molecule sensitive microscope setup with additional Stimulated Emission Depletion (STED) super-resolution capability. In this report, the design and initial characterization of this setup is presented. By imaging single quantum dots it is shown that the realized setup is single-emitter sensitive. Furthermore, we present that a diffraction limited resolution of ∼ 280 nm FWHM can be obtained in confocal-mode. For the STED functionality we chose to implement continuous-wave based STED, since this does not require tight laser pulse synchronization. A doughnut-shaped point-spread function (PSF) for the STED beam can be easily obtained in the realized setup. Optimization of the doughnut quality can be efficiently realized with the setup through polarization fine-tuning and spatial phase-adjustment of the STED beam. Furthermore, the realized setup allows easy initial alignment of the excitation PSF maximum with the intensity-null of the STED doughnut PSF to within an accuracy of ∼ 100 nm, which can be further improved by fine-tuning. These initial results demonstrate the possibility to obtain STED based super-resolution with the realized microscope setup.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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