The result shows that the one-phase/two-phase envelope extends from the point at 100% water plus ethanol to the point at 26.23% water plus ethanol, 20.45% OA, and 53.32% n-hexane, and the two-phase part is located in the lower OA region. Obviously, with an increase of the ratio of OA/(water plus ethanol), more n-hexane can be dissolved into their mixtures to form a stable system. The actual point (point B) we used is located in the right-bottom region, where the oil-in-water microemulsions are formed.

Figure ​Figure22 shows the characterization data for the NaYF₄:20% Yb³⁺, 2% Er³⁺ sample. The TEM image (Figure ​(Figure2A)2A) demonstrates that the synthesized particles are roughly spherical, monodisperse with the size uniformity of about 20 nm in diameter. The X-ray powder diffractometer (XRD) pattern (Figure ​(Figure2B)2B) shows well-defined peaks, indicating the high crystallinity of the synthesized material, and the peak positions and intensities from the experimental XRD pattern match closely with the calculated pattern for cubic phase of NaYF₄ (JCPDS card, No. 77-2042). From the line broadening of the diffraction peaks, the crystallite size of the sample was determined to be approximately 18 nm using the Debye-Scherrer formula, which corresponds to the particle size determined from the TEM result.

The NaYF₄:Yb³⁺, Er³⁺ UCNPs can easily be dispersed in nonpolar solvents (such as n-hexane, cyclohexane) to form homogenous colloidal solutions. Figure ​Figure3A3A shows images of a 1 wt.% solution of NaYF₄:20% Yb³⁺, 2% Er³⁺ UCNPs in n-hexane, demonstrating its transparency. The visible upconversion luminescence can be observed when the solution is excited at 980 nm with a power density of 1.2 kW/cm² (Figure ​(Figure3B).3B). The corresponding upconversion luminescence spectrum is also shown in Figure ​Figure3C.3C. There are three major emission bands at 520-530 nm (green light), 540-550 nm (green light), and 650-670 nm (red light), which are assigned to the ²H_11/2 to ⁴I_15/2, ⁴S_3/2 to ⁴I_15/2, and ⁴F_9/2 to ⁴I_15/2 transitions of Er³⁺ ion, respectively. Under 980 nm excitation, the absorption of the first photon can elevate Yb³⁺ ion to the ²F_5/2 level from ground state, and then it can transfer the energy to the Er³⁺ ion. This energy transfer can promote Er³⁺ ion from ⁴I_15/2 level to the ⁴I_11/2 level and from the ⁴I_11/2 level to the ⁴F_7/2 by another energy transfer upconversion process (or a second 980 nm photon) if the ⁴I_11/2 level is already populated. Then, the Er³⁺ ion can relax nonradiatively to the ²H_11/2 and ⁴S_3/2 levels, and the green emissions occur (²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2). Alternatively, the ion can further relax and populate the ⁴F_9/2 level leading to the red emission (⁴F_9/2 → ⁴I_15/2) [8,22]. The curve also shows that red emissions are much stronger than green emissions, so the products present light of orange color on the whole (Figure ​(Figure3B3B).

It is noted that the as-prepared nanoparticles are cubic phase, whose fluorescence efficiency is at least one-order of magnitude less than that of the hexagonal phase [8]. A thermal treatment at ca. 400-600°C was reported to transform the cubic phase to the hexagonal phase, but which led to undesirable particle growth and agglomeration [2]. We carried out the annealing of the as-prepared nanoparticles under N₂ atmosphere by heating them to 600°C, and maintaining this temperature for 5 h. After annealing, the particles aggregated into larger clusters (Figure ​(Figure4A),4A), and the XRD pattern (Figure ​(Figure4B)4B) shows that hexagonal NaYF₄:Yb³⁺, Er³⁺ phase emerged in addition to the already existing cubic pattern (marked with asterisks), which implies that the particles transformed partially from cubic phase to hexagonal phase by annealing. In addition, upconversion luminescence emission spectrum (Figure ​(Figure5)5) was obtained after ultrasonic dispersion of a 1 wt.% solution of the products in n-hexane, compared with the spectrum of nanoparticles before annealing, its green emission plays a dominant role, and the overall emissions are much stronger than those for cubic phase products.

In a typical synthetic route, sodium hydroxide (400 mg) was dissolved in a mixture of water (20 mL) and ethanol (30 mL), followed by the addition of oleic acid (7.4 mL) and n-hexane (4 mL); this formed a bright yellow transparent solution. Then, two separate aqueous solutions (5 mL) of Ln(NO₃)₃ (0.8 mmol, Y:Yb:Er = 78:20:2) and sodium fluoride (3.2 mmol) were added to the above microemulsions one after the other with vigorous stirring. Then, the solution was transferred to a Teflon-lined stainless steel autoclave and heated at 180°C for 6 h. When the autoclave was cooled down to room temperature, the products were found deposited at the bottom. Then, n-hexane (30 mL) was added to destroy the one-phase solution and form a two-phase mixture, so the hydrophobic colloidal NaYF₄:20% Yb³⁺, 2% Er³⁺ UCNPs were extracted into the upper layer (n-hexane region). With precipitation by additional ethanol, and highspeed centrifugation, the white products (yield: 85%) were re-dispersed in n-hexane to bring out a transparent colloidal solution.