Optical properties and modulated luminescence of metal ion doped phosphors

Abstract

Metal ions (including rare-earth and transition metal ions) doped phosphors have draw much attention due to their applications in light-emitting displays, lasers, optoelectronic devices and biomedicine. Among them, rare-earth doped upconversion phosphors are unique and attractive for fundamental research and great potential applications. The ability to manipulate the spectral properties of phosphors is highly desirable for understanding physical processes of energy transition and widespread applications. It is of considerable interest to find new approaches to rationally modulate the luminescence of phosphors. To date, modification of luminescence in phosphors excited by a given excitation source can normally be achieved through a conventional chemical approach, i.e., changing the composition of host materials and/or doping ions, thus limiting our understanding of the detailed process of luminescence and applications. Until now, there are no reports of an established approach to modulate upconversion emission in an in-situ and real-time way. The unique crystal structure of ferroelectric materials provides us an opportunity to couple variables including electric field and temperature to crystal symmetry in a single compound. In this thesis, optical properties of rare-earth-doped ferroelectrics are investigated, and the influences of factors such as temperature and site substitution on the phosphors are observed. We also present a new approach to enhance and modulate upconversion emission through applying a relatively low bias voltage to the rare-earth-doped BaTiO₃ (BTO) thin films. We investigated the luminescence properties of rare-earth-doped perovskite type hosts. Upconversion PL of Er³⁺ -doped BTO with perovskite ABO₃ structure was studies in terms of Er³⁺ substitutions for Ba (A-) and Ti (B-site) with different Er³⁺ doping concentrations. Photoluminescence (PL) quenching with an increasing Er3+ doping concentration was investigated based on the structural change and energy transfer of cross-relaxation process in BTO: Er, i.e. ²H₁₁/₂ + ⁴I₁₅/₂ → ⁴I₉/₂ + 4I₁₃/₂. Temperature dependence of the PL in BTO: Er was revealed, which was associated with phase transitions of BTO host. The results imply that the emission from substituted Er³⁺ ions may be used as structural probes for the ferroelectric titanates. Significant tuning and enhancement of upconversion PL represent a great challenge. In contrast conventional chemical and plasmonic approaches, we firstly showed an enhancement and modulation of upconversion emission by applying low bias voltage to the rare-earth-doped BTO thin films on different substrates. The emission intensity at 523 nm of ITO/BTO:Yb/Er/SrRuO₃/SrTiO₃ heterostructure under the bias voltage at 10 V is almost 2.7 times as that of the unbiased one. Moreover, the PL intensity of the BTO:Yb/Er can be modulated with ac electric field. This work provides a real-time and dynamic way to control PL under an electric field. The phenomena of electroluminescence (EL) have attracted an ever-increasing demand for applications throughout the illumination and display industries. In this thesis, we reported on the fabrication and characteristic of strain-induced piezoelectric potential stimulated luminescence from ZnS:Mn film on Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ (PMN-PT) substrate. The light emission of the ZnS:Mn arises from the piezoelectric potential, resulting from the converse piezoelectric effect of PMN-PT substrate. In contrast to conventional electroluminescence devices including LEDs, such novel light source can be controlled by high-frequency electric-field up to MHz. Moreover, a high-frequency ultrasonic transducer was also realized. The observed phenomena can be used to develop a dual-modal source combing light and ultrasonic signal. In conclusion, we have utilized the unique characteristics of ferroelectrics and piezoelectrics to rationally modulate the light emission from metal ions-doped phosphors. These findings will aid further investigation of fundamental research and the widespread applications of the phosphors.