Magneto-Optical Properties of Bifunctional Nanoparticles

Research on the photoluminescence (PL) of lanthanide-doped bifunctional nanoparticles (NPs) has been one of the interesting topics in the past few years. Their magneto-optical (MO) properties in presence of external magnetic fields and the energy transfer mechanisms between different rare earth ions and different structures are still not uncovered completely, which blocked their applications in the fields of magnetic resonance imaging (MRI), aircraft guidance, optical detection of the magnetic fields, etc. Although there are many observations of PL in these bifunctional NPs, there are rarely reports focusing on the interaction between the optical and magnetic properties. In our work, we investigate systematically the optical, magnetic, and MO properties of lanthanide-doped bifunctional NPs with different structures and different dopants under different external magnetic fields and low temperatures. Moreover, we report on the magnetic shielding phenomenon at the nanoscale by employing a fluorescent probe of which the luminescent intensity is sensitive to the applied magnetic field and elucidate the MO interaction mechanisms based on the systematic research of the MO properties. The main goal of this Ph.D. thesis is to study the energy transfer mechanism in bifunctional NPs, control their MO properties and explore magnetic shielding at the nanoscale.

In Chapter 1 we introduce the background of the research. The upconversion properties and the mechanisms of energy transfer of PL in the studied NPs are discussed in the first part. Furthermore, their magnetic properties, MO properties and magnetic shielding characteristics are introduced in the second, third and fourth part, respectively.

In Chapter 2 the procedure for preparation of core@n-shell NPs is discussed and an overview of devices for characterization is given (XRD, TEM, SQUID, etc.). In particular, the high pulsed field and low-temperature facilities are introduced in detail

In Chapter 3 the principle mechanism of energy transformation in core@shell structures NPs (Na(Gd0.8Yb0.18Tm0.02)F4@NaLnF4 with Ln=lanthanide) is discussed. Different core@shell structures of this type are successfully synthesized for luminescence upconversion from 980 nm to 800 nm. We observe the phenomenon that the upconversion luminescence of the core@shell NPs (Na(Gd0.8Yb0.18Tm0.02)F4@NaGdF4) has a much stronger infrared emission centered at 800 nm than that of the core-only NPs (Na(Gd0.8Yb0.18Tm0.02)F4). By replacing the NaGdF4 shell with a NaYF4, the emission could further be enhanced. We suggest that the breaking of the crystal field symmetry plays a key role in this enhancement. The importance of this asymmetry is further revealed by investigating the energy transfer between the Er3+ ion in the shell and the Tm3+ ion in the core of the NP.

In Chapter 4 Dy3+ is chosen instead of Y3+ in the core, since Dy3+ exhibits the strongest magnetic moment among rare earth  elements due to the spin−orbital coupling and Na(Y0.8-xDyxYb0.18Er0.02)F4 (x=0, 5%, 10%, 15%, 20%, 25%, 30%) NPs are synthesized by the solvothermal method. It is found that both the optical properties and magnetization are highly dependent on the Dy3+ content. More specifically, the magnetization of Na(Y0.8-xDyxYb0.18Er0.02)F4 NPs depends non-monotonously on the Dy3+ concentration, which indicates that there might be an occurrence of magnetic coupling of Dy3+ ions and anti-ferromagnetic coupling of the magnetic dipoles of the magnetic domains of the Dy3+, Y3+, Yb3+, and Er3+ ions. Through this research, we conclude that the optimal content of Dy3+ ions for simultaneous control of PL emission and magnetization is about x =15%, and this is valuable for tuning the optical and magnetization properties of bifunctional NPs simultaneously.

In Chapter 5 we consider the influence of MO effects on PL in different structures and compositions under external magnetic fields at 6 K and 80 K. Through systematic research, we demonstrate that the PL of Na(Gd0.8Yb0.18Er0.02)F4 with shells can be tuned by temperature, external magnetic fields, and doping with different lanthanide ions based on the MO effect. Also, a nanoscale magnetic shielding phenomenon is observed and discussed. We expect that this route, based on magnetic field-sensitive fluorescent probes, can open a novel pathway for exploring magnetic shielding at the nanoscale.

Finally, we conclude our results with an outlook on energy transfer, MO effects in bifunctional NPs and magnetic shielding at the nanoscale.