Controllable Synthesis Of Lanthanide Phosphate Fluorescent Nanomaterials And Their Optical Properties

Lanthanide based fluorescent materials have attracted increasing interest because of their unique optical properties. They have been largely used in light emitting diode (LED), lasers, glass fibers for optical communication, etc. Recently, there is a rising interest on the lanthanides based materials used for upconversion. Lanthanide materials have been found to be of low toxicity when applied in biological studies. More importantly, lanthanide based upconversion materials, normally excited by near infrared (NIR) which is a lower energy light source, have unique advantages, compared with conventional UV excited fluorescent materials. Nanomaterials, arising from their small size, high specific surface area and quantum effect etc., provide them with unique properties different from their bulk phase. The study of nanosized fluorescent materials is one of the most interesting research areas. It may not only improve the properies, but also encourage the application of fluorescent materials in the area like bio-medicine and so on.However on the other hand, their optical properties would be greatly influenced after the materials are nanosized. Nanosized fluorescent materials have not only considerably high surface energy, but also expose large quantity of atoms on the surface which have different surrounding environment from the bulk atoms. And this may enormously impact their physical and chemical properties.Taking into consideration of the industrial production and later on application needs, to simplify the synthesis condition, to avoid the use of toxic organic solvents, high temperature and high pressure, and also to prevent the fluorescence from decreasing caused by nanosized materials and maintain good optical properties, in this thesis, we focus on controllable synthesis of well defined lanthanide phosphate (LnPO4) nanomaterials in mild conditions and the improvement of their optical properties. It includes (1) controllable synthesis of LnPO4 nanomaterials using different preparation methods, and study the impact of morphology on the optical properties; (2) Room temperature growth of LnPO4 nanomaterials via heterogeneous nucleation process, and study the impact of this protocol on the structure and optical property of the obtained nanomaterials; (3) study of the fluorescence maintenance and their functions of LnPO4 in multifunctional nanomaterials and other nanocomposite systems. We have made some progress in the following aspects:Using sodium tripolyphosphate (TPP) as phosphate source, PO43- was released slowly when heated and reacts with Ln3+ resulting in LnPO4 nanoparticles. Solid nanoparticles (8-10nm) and hollow nanoparticles (16-25nm) with holes ranging from 3-6nm in diameter can be obtained at different synthesis times.At room temperature,1-dimensional LnPO4 nanomaterials were successfully synthesized by "process intensification" such as spinning disc processing (SDP) and rotating tube processing (RTP). The result shows that, the aspect ratios of the prepared samples can be easily tuned by adjusting the spinning/rotating speed and/or feeding concentration. The high spinning/rotating speed of both platforms results in high aspect ratio and narrow size distribution of the products, while high feeding concentration has dichotomy effects for different methods (SDP or RTP). Moreover, the optical characterization of the obtained products shows that the fluorescence intensity increases with the increase of aspect ratio.Magnetic Fe3O4 nanoparticles and fluorescent LaPO4:Ce3+:Tb3+nanorods can be assembled into koosh nanoball structure with the help of p-sulfonato-calix[6]arene (SC[6]) as binder. Although there is the fluorescence quenching effect from water molecules and Fe3O4 nanoparticles, the prepared Fe3O4@SC[6]-LaPO4:Ce3+:Tb3+ nanocomposite shows quite bright light emission with the magnetism maintained. Thus a bifunctional nanocomposite has been successfully synthesized with both magnetism and fluorescence expressed.Quantum dots (QDs) with very short synthesis time have been obtained though narrow channel reactor (NCR). Using RTP cooperated with NCR, QDs@CePO4 nanocomposite can be synthesized. The particle size of QDs can be tuned by choosing different synthesis time, resulting in a change of the emission wavelength. With the increase of particle size of QDs, both QDs itself and QDs@CePO4 nanocomposite show a red shift in the emission spectra. Also, QDs@CePO4 has one more excitation peak than QDs, which largely increases the Stokes shift in QDs@CePO4 nanocomposite.We have synthesized single walled carbon nanotube (SWCNT) strung ultrafine LnPO4 nanoparticles by supressing the 1-dimensional preferential growth of LnPO4 with SWCNT serving as heterogeneous nuclei centres. Compared with LnPO4 nanowires, ultrafine LnPO4 nanoparticles show improved fluorescence with the quantum yield reaching up to 89% which is close to the corresponding bulk material. Also, we have got upconversion nanocomposite by growing LnPO4 nanocrystals on graphene sheets, and its high green to red ratio in the emission spectrum indicates very high upconversion efficiency.