*3.2. Hydrothermal Method*

Hydrothermal synthesis is another cheap method to obtain high-quality nanocrystals. This process is usually carried out at high pressure and high temperature above the boiling point or even the critical point. A special container called autoclave is used. The main disadvantages are the opacity of the reactor and the lack of in-situ reaction process and mechanism research. On the other hand, the advantages of high crystallinity, good dispersion, and no post-treatment make it one of the most popular synthesis technologies of lanthanum-doped nanoparticles. Organic ligands/surfactants such as oleic acid [106], ethylenediamine tetraacetic acid [107,108], cetyltrimethylammonium bromide [107], and polyethyleneimine [109] are added together with precursors to achieve synchronous control of size, morphology, crystalline phase, and surface properties. For example, Zhao and colleagues reported the hydrothermal process of oleic acid mediated upconversion NaYF4 nanocrystal synthesis, which has different shapes and morphology, including nanotubes, nanorods, and flower patterned nano disks [110]. Liu et al., used Ga3+ doping method to control the crystal phase [111]. They found that after on adding Ga3+ (accurately controlling the concentration), the required reaction temperature and time were greatly reduced, and the ultra-small NaYF4 upconversion nanoparticles underwent a rapid cubic to hexagonal phase transition. Li et al., demonstrated a synthesis strategy of multiphase, interface controlled monodisperse nanoparticles based on Liquid-Solid-Solid transfer and separation mechanism [112]. With some improvements, this method is also used for the preparation of upconversion nanoparticles with different fluorine and oxyfluorine contents (Table 1).
