Controlled synthesis, properties, and applications of rare earth based photoluminescent nanomaterials.

• Main Author: Teng, Xue.
• Other Authors: School of Chemical and Biomedical Engineering
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Chemical engineering::Industrial photochemistry; DRNTU::Engineering::Bioengineering
• Online Access: http://hdl.handle.net/10356/55193
• Institution: Nanyang Technological University

Photon upconversion (UC) is a kind of anti-Stokes type optical process characterized by the successive absorption of several pump photons followed by the emission of output radiation at a shorter wavelength than the pump wavelength. UC nanoparticles have opened up the opportunity for exploring new applications in diverse research areas such as displays, solar cells, and biological assays, which makes it one of the most promising research topics. UC nanoparticles are typically composed of an inorganic host lattice and lanthanide dopant ions embedded in the host lattice. This thesis focuses on the synthesis and tailoring of different kinds of UC nanoparticles. In this thesis, several novel upconversion host materials are introduced, including NaGdF4, LixNayYF4, NaxScF3+x, and AxScF3+x (A is chosen to be one of the alkali metal elements, Li, Na, K, Rb, or Cs). Chapter 2 focuses on the study of how different experiment conditions affect the synthesis of hexagonal shaped monodisperse NaGdF4: Yb3+/Er3+ UC nanoparticles. Extensive experiments are performed to test the influence of different OA/ODE ratio, temperature, and reaction time. Chapter 3 studies the enhancement effect resulted from the co-doping of Li+ and Na+ in the UC nanoparticles. NaYF4 has been generally regarded as the most efficient upconversion host for practical applications due to its fine crystal structure and good UC photo luminescence performance. Recently, LiYF4 has also been reported to show promising potential as an upconversion host. It gives enhanced fluorescent emission signals compared to that of Na+ based lanthanide-doped nanomaterials. However, studies on the co-doping of Na+ and Li+ as host material is still absent. Continuous crystal structure control as well as UC emission tuning are achieved by lithium-sodium host co-doping of yttrium fluoride-based nanoparticles in an oleic acid/1-octadecene solvent system. A crystal structure evolution from hexagonal NaYF4 to monoclinic Li0.5Na0.5YF4, then to tetragonal LiYF4 is observed when Li/Na ionic ratio is altered from 0 to 1. A crystallographic model that represents the Li+-doping induced continuous evolution of lanthanide active site geometry is proposed. Another important part of this research lies in the development of a novel host material, NaxScF3+x. Scandium (Sc, atomic number 21) is a silvery-white metallic transition metal, which has historically been classified as a rare earth element, together with yttrium and the lanthanoids. Rare earth-based nanomaterials have drawn impressive attention due to their unique energy upconversion (UC) capabilities. However, studies on Sc3+-based nanomaterials are still absent. Chapter 5 reports the synthesis and fine-control of NaxScF(3+x) nanocrystals by tuning the ratio of oleic acid (OA, polar surfactant) to 1-octadecene (ODE, nonpolar solvent). When the OA:ODE ratio increases from low (3:17) to (3:7), the nanocrystals change from pure monoclinic Na3ScF6 to pure hexagonal phase NaScF4, via a transition stage at intermediate OA:ODE ratio (3:9) where not only the mixture of nanocrystals in monoclinic and hexagonal phases was obtained, the coexistence of both phases inside individual nanocrystals was also observed. More significantly, due to the small radius of Sc3+, NaxScF(3+x):Yb3+/Er3+ nanocrystals show different UC emission from that of NaYF4: Yb3+/Er3+ nanocrystals, which broadens the applications of rare earth-based nanomaterials ranging from optical communications to disease diagnosis. We further extend the study on Sc based UC nanoparticles in Chapter 5, including size control based color tuning and Sc/alkali-metal co-doped host material. Traditionally, the color tuning of UC nanoparticles is performed by monitoring the amount of dopant (rare earth; e.g. Yb3+, Er3+). We propose a new method to achieve color tuning by changing host structure (size). The size of the nanocrystals can affect the emission spectra and decay rates and thus affect the photoluminescence property. Another part of my work is about the synthesis of AxScF3+x (A= Li, Na, K, Rb, Cs) Nanocrystals. Experimental results show that Li/Na/K based host materials possess clear crystal structure and thus good photoluminescence property. However, Rb/Cs based lanthanide doped UC nanoparticles show poor crystal structure and very limited photoluminescence property. Finally, in Chapter 6 a novel synthesis method is proposed for the preparation of upconversion nanoparticles. The proposed method is much more efficient than the traditional coprecipitation (CPT) method, which requires much less reaction time and starting materials.