Design, Preparation And Properties Of Yttrium Vanadate Upconversion Luminescent Nanomaterials And Germanate-based Persistent Phosphors

Upconversion luminescence (UCL) is a nonlinear optical process that converts low-energy excitation, usually at near-infrared (NIR) wavelengths, to higher-energy emission, typically in the visible spectral region. Long-persistent luminescence (LPL) is a "self-sustained" luminescence phenomenon whereby luminescence can last for minutes to hours at room temperature after the stoppage of the excitation. Although the UCL and LPL processes exhibit different luminescence forms and mechanisms, they share similar implications for a variety of technologies. Particularly, both UCL nanophosphors and NIR LPL nanophosphors have attracted enormous attentions in recent years for the use as optical nanoprobes in bio-imaging applications, because the involvement of high-penetrating NIR light as either excitation source (for UCL) or imaging signal (for NIR LPL) can greatly reduce the optical interference (e.g., tissue autofluorescence) from issues, enabling significantly improved imaging sensitivity and depth.In recent years, significant progress has been made on upconversion luminescent nanomaterials using fluorides as host materials, with the representative ones including NaYF4, NaGdF4, KM11F3, and CaF2. However, the fluoride-based upconversion luminescent materials have deficiencies including poor chemical stability, high cost, and harsh preparation condition, which limit to some extent their wide applications. In consequence, there is still a strong demand ongoing for developing new host materials, espically the oxide hosts with good chemical stability. As for the long persistent luminescence, the past two decades have witnessed the significant achievements on the visible and near infrared (NIR) persistent phosphors, while the research and development of persistent phosphors in the ultraviolet (UV) spectral region (200-400 nm) and short-wave infrared (SWIR) region (900-1700 nm) are lacking due to the limit of suitable host materials and emitters. In addition, the LPL process usually requires high excitation energy (usually UV or blue light) to fill the traps in view of the high energy of trap level. The rigorous excitation condition limits many potential applications using the persistent phosphor (e.g., using persistent luminescence nanoparticle as in vivo imaging persistent probe for long observation window). Lower excitation-energy in the red or infrared spectral region for achieving the persistent luminescence is desired.In this dissertation, we first introduce the basic knowledge of luminescent materials, the current situation and development of upconversion luminescent nanomaterials and persistent luminescence materials. Then we focus on the preparation and luminescence properties of yttrium vanadate upconversion luminescent nanomaterials. Next, a series of novel germanate-based persistent phosphors were successfully developed and the corresponding persistent luminescence properties and mechanisms were systematically investigated. At last, a new conceptual luminescence process called upconverted persistent luminescence (UCPL) was proposed by combining the UCL and LPL processes, and the first NIR UCPL phosphor Zn3Ga2GeO8:1%Cr3+,5%Yb3+,0.5%Er3+ (ZGGO:Cr,Yb,Er) was designed and fabricated. The research results in the dissertation can be summarized as follows.(1) A group of YVO4:Yb3+,Er3+ upconversion luminescent nanomaterials with different morphologies and sizes were prepared by wet-chemical method. YVO4:Yb3+,Er3+ nanoparticles with size distribution between 30-50 nm have been successfully prepared via a facile hydrothermal technique in the presence of citric acid as a complexing agent followed by a subsequent heat treatment process. YVO4:Yb3+,Er3+ microspheres with diameters of 6-8 μm have been successfully prepared though a facile hydrothermal method assisted with disodium ethylenediaminetetraacetic acid and citric acid. The core-shell structured SiO2@YVO4:Yb3+,Er3+microspheres have been successfully prepared by coating monodisperse SiO2 microspheres with the YVO4:Yb3+,Er3+ up-conversion phosphors via a facile sol-gel technique. All the obtained YVO4:Yb3+,Er3+ upconversion luminescent nanomaterials show bright green upconversion luminescence corresponding to the 2H11/24â†'I15/2 and 4S3/2â†'4I15/2 transitions of Er3+ ions under the excitation of a 980 nm laser diode.(2) We further extend the persistent luminescence into the high-energy UV spectral region by developing a new UV persistent phosphor Sr2MgGe2C>7:Pb2+. The Sr2MgGe2O7:Pb2+ phosphor exhibits single-band persistent luminescence peaking at 370 nm and a long persistence time of more than 12 h after higher-energy (260-310 nm) light excitation. The Sr2MgGe2O7:Pb2+ persistent phosphor also exhibits a photo-stimulated persistent luminescence capability, in which the UV persistent luminescence in a pre-irradiated sample can be rejuvenated after short-time, low-energy visible or near-infrared light stimulation. We also conducted a comprehensive investigation on the trap transfer in the material using thermoluminescence technique to understand the underlying persistent luminescence mechanism.(3) We report a new function of Yb3+ ion-as an excellent emitting center for SWIR persistent luminescence. We have developed the first real SWIR persistent phosphor, Yb3+-doped MgGeO3, which exhibits a very-long SWIR persistent luminescence at around 1000 nm for longer than 100 h after UV light (<330 nm) excitation. The MgGeO3:Yb3+ phosphor also exhibits a photo-stimulated persistent luminescence capability, in which the SWIR persistent luminescence of UV pre-irradiated samples can be enhanced by low-energy white, red or near-infrared light stimulation. The oxygen vacancies associated with Ge4+ ions could be formed at high sintering temperature and act as electron traps in MgGeO3. We have also achieved SWIR persistent luminescence in Yb3+-doped gallate based phosphors.(4) We report short-wave infrared persistent luminescence in a series of Pr3+-doped persistent phosphors. Here we use MgGeO3:Pr3+ phosphor as an example, after ultraviolet light irradiation, the material exhibits three intense persistent luminescence peaks at 625 nm,900 nm and 1085 nm, respectively, lasting for more than 120 h. The above three persistent luminescence peaks originate from the same 1D2 state of Pr3+and share identical trap distributions in persistent luminescence process. In a biological tissue transmittance experiment using MgGeO3:Pr3+, the SWIR persistent luminescence emissions at 900 nm and 1085 nm transmit chicken breast meat more effectively than the red emission at 625 nm. The oxygen vacancies formed at high sintering temperature act as electron traps in persistent luminescence process.(5) By combining the unique features of UCL and LPL, we proposed a new conceptual luminescence process called upconverted persistent luminescence (UCPL), which enables to create persistent luminescence having emission energy higher than the excitation energy. Guided by the UCPL concept, we designed and fabricated the first UCPL phosphor Zn3Ga2GeO8:1%Cr3+,5%Yb3+,0.5%Er3+(ZGGO:Cr,Yb,Er). After excited by a low-energy 980 nm laser, the phosphor exhibits long-lasting (>24 hours) near-infrared persistent emission peaking at 700 nm.This work was supported by National Natural Science Foundation of China (Nos. 30870610 and 81171463), National Science Foundation (CAREER DMR-0955908, DMR-1403929) and China Scholarship council (CSC, File No.201206220027).