Up-conversion (UC) process, in which high-energy photons are emitted upon absorption of low-energy photons, has been observed in compounds doped with Lanthanides (Ln) elements and UC nanomaterials have drawn tremendous attention in recent years for their potential applications in lasers, next-generation lighting and three dimensional (3-D) displays, optoelectronic devices, labels in biological assays and imaging etc. Their growing research and applications concern their comprehensive advantages, which can be generalized into three major aspects: 1) rare earths (REs) as dopant, which have narrow emission bands and thus long luminescence lifetime, unique emission lines and multi-colors; 2) fluorides as hosts, which have low non-radiative decay and thus high UC intensity, low photo-bleaching and chemical degradation and biocompatibility; 3) infrared (IR) light as pump source, which has low damage and autofluorescence background, deep penetration into human tissues, high spatial-temporal resolution, and also eliminates the requirement of expensive pulsed lasers . In this work, one mainobjective is to synthesize the high purity hyperfine UC powder nanomaterials using a microwave-assisted method which is relatively mild, to optimize the synthesis conditions, to investigate the UC emission mechanism, and finally to explore their applications in various fields, eg.biological fields and solar cells. The other mainobjective is to investigate the synthesis of such UC nanomaterials with different host materials (such as Barium Fluorides and Sodium Fluorides) via a hydrothermal method in the presence of different solvents as reaction medium, in order to investigate the influence of solvents on the morphology and the effects of morphology on the emission ability of the UC nanomaterials, and also to compare their emission ability of the UC nanomaterials with different host materials. Chapter 1 introduces the mechanism of UC, the host-dopant selection standard, the properties of UC materials and also gives an overview of the current synthesis methods of UC nanomaterialsand the various applications of UC nanomaterials. Chapter 2 reports how colloidal hydrophilic BaYF5:Yb/Er nanoparticles (NPs) are synthesized directly by a facile microwave-assisted method for the first time. The composition, morphology, surface information and optical properties are investigated by X-ray powder diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAED), transmission electron microscopy (TEM), Perkin Elmer Model FTIR-1600 spectrometer and fluorolog-3 FL3-12 instrument with an external light source of 980 nm laser diode, respectively. The as-prepared BaYF5 NPswith the tetragonal phase are almost mono-dispersed and have a narrow size distribution of 33±1 nm, showing intense green emissions under the excitation of 980 nm diode laser. The UC mechanism research reveals that two-photon process dominates the UC process. Besides, the as-obtained NPs can be evenly dispersed in water by some polar organic solvents, suggesting that such NPs hold great promise for applications in bio-imaging, bio-labeling and drug delivery. In Chapter 3, the fast-synthesized colloidal hydrophilic BaYF5:Yb/Er nanoparticles (NPs)are utilized in the Infrared (IR) solar cells (polymer solar cells and dye-sensitized solar cells (DSSCs)) in order to improve their IR response values. In the UC process, the transmitted IR light of the solar spectrum for solar cells is transferred to the visible light, which can be reabsorbed by the polymer in polymer solar cells and the dye in DSSCs. The as-prepared NPs are finally demonstrated to be promising IR photoanodes for photovoltaic solar cells, especially DSSCs, enhancing the power conversion efficiency from 0.63 to 5.98 by comparing the photoanodes without and with UCNPs layers. However, there is no obvious improvement of the efficiency in polymer solar cells, which is probably caused by the quality of polymers and the preparation processes. In Chapter 4, the fast-synthesized colloidal hydrophilic BaYF5:Yb/Er nanoparticles (NPs) are further investigated by evaluating their biological co-toxicity and demonstrating their UC emission ability in rubbers. The co-toxicity evaluation results reveal that the as-prepared UCNPs have low co-toxicity and are good for the various biological applications after the further modifications or functionalization. Their dispersibility and emission ability in rubber shows their potential applications in new-type 3-D displaying field. Chapter 5 discusses the synthesization of Na-based and Ba-based UCNPsby hydrothermal methods with the assistant of various solventsas reaction medium. The influence of solvents on the morphology and the effects of morphology on the emission ability of the UC nanomaterials are investigated, and their emission abilities of the UC nanomaterials with different host materials are compared. Chaper 6 summarizes the research results and discusses the possible areas for further study.