*2.4. Cross-Relaxation (CR)*

The CR process involves two identical or different ions. In the CR process, ion 1 transfers part of the energy of the E2 energy level to ion 2, causing ion 2 to transfer to a higher excited state, while ion 1 returns to a lower energy level through a non-radiative relaxation process (Figure 2d). Although CR is related to the concentration quenching effect, it can be used to adjust the emission spectra of UCNPs. The related studies currently reported include Y2O3: Yb3+/Er3+ [55], GdPO4:Sm3+ [56], KYF4:Tb3+, Yb3+ [57], etc.

## *2.5. Photon Avalanche (PA)*

PA includes ESA process and CR process. Photon avalanche (PA) in Figure 2e is a process of upconversion above a certain excitation power threshold. Once excited, the upconversion fluorescence luminescence intensity will increase by orders of magnitude. In addition, the excited state energy levels of rare earth ions are also required to have a relatively high lifetime. In this process, the CR process mentioned above is also required. Ion 2 transitions from E2 back to E1 and releases energy at the same time. Ion 1 absorbs the energy and transitions from E1 to E2, and then transfers the energy to the E1 energy level of ion 2. At this time, the E1 energy level of ion 2 will absorb photons through the ESA process, which increases the population of the E1 energy level exponentially, leading to the PA process. This photophysical process is often uncommon because of the higher excitation power density required. Er3+/Yb3+ co-doped NaBi(WO4)2 phosphor produces strong green upconversion luminescence through the photon avalanche process [58]. Under excitation at 980 nm, the seven-photon PA upconversion (UC) behavior of Er3+ ions and four-photon NIR emission [59].

#### **3. Synthesis Strategy and Surface Modification of UCNPs**

The luminescence properties of upconversion nanomaterials are closely related to their preparation methods. Different preparation methods will affect the size, morphology, and corresponding microstructure of luminescent materials, making their application directions more diversified. Among them, thermal decomposition, hydrothermal decomposition, and co-precipitation are the three most commonly used methods, and their advantages and limitations are shown in Table 1. Other synthesis methods, including sol-gel method and combustion method, are also discussed for comparison [60–63].


**Table 1.** Advantages and disadvantages of upconversion nanoparticles synthesized by different methods and examples.


**Table 1.** *Cont.*

## *3.1. Thermal Decomposition Method*

The thermal decomposition method is based on the high-temperature decomposition of organometallic precursors (such as metal trifluoroacetate) in high-boiling organic solvents (such as 1-octadecene). Surfactants are long-chain hydrocarbons and functional groups, such as -COOH, -NH2 or -PO3H (such as oleic acid, oleyl amine, trioctyl phosphine and trioctyl phosphine oxide), which are used as ligands, thus preventing the aggregation of nanoparticles. This method was originally developed by Professor Yan and others for the synthesis of LaF3 nanoparticles [64], and later extended to the synthesis of highquality NaYF4 upconversion nanoparticles [65]. Capobianco et al. used trifluoroacetate precursors to synthesize Yb, Er and Yb, Tm co-doped NaYF4 nanoparticles [101,102]. The asymmetric molecular structure of octadecene (with a high boiling point of 315 ◦C) is used as a solvent, and oleic acid is used as a passivation ligand. Based on the separation of nucleation and growth of nanocrystals, an improved method has been developed [100], that is, the precursor is slowly added, followed by heating, to synthesize highly monodispersed nanoparticles. Using the same strategy, Murray and colleagues prepared hexagonal NaYF4:Yb,Er nanoparticle bodies, and precisely controlled their morphology and size [103]. Lim et al. The NaGdF4:Er3+/Yb3+ colloidal particles with an average particle size of 32 nm were successfully synthesized by the thermal decomposition method. The prepared phosphor shows bright green upconversion luminescence under a 976 nm semiconductor laser. Phosphor particles increase the scattering of optical coherence tomography (OCT) scanning radiation, thereby observing higher image contrast [104]. So far, this method has been widely used to synthesize a series of upconversion fluoride and oxyfluoride nanoparticles (Table 1). Although this method can produce high-quality upconversion nanoparticles, it also has some disadvantages, including expensive materials, air-sensitive precursors, and the production of toxic byproducts (such as hydrogen fluoride). Recently, the Pu team reported a safe and environmentally friendly method to replace octadecene (ODE) with paraffin liquid as a high-boiling non-coordinating solvent [105]. This method is biologically cheaper and sustainable.
