TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Forms: Rare earth UCNPs are hexagonal phase. Preferred Parameters: Rare earth UCNPs are excited at 800 nm. Preferred Processes: Rare earth UCNPs of formulae AYF4:Yb,X@AYF4:Yb@ANdF4:Yb or AGdF4:Yb,X@AYF4:Yb@ANdF4:Yb are prepared by mixing trifluoroacetic acid rare earth salt (or rare earth fluoride salt) and trifluoroacetic acid alkali metal salt (or alkaline metal fluoride salt) in which stoichiometric molar ratio sodium (Na) (or potassium (K)):Y (yttrium) (or gallium (Ga)):Yb:X is 1:0.7-0.9:0.1-0.3:0.005-0.02 to obtain mixed powder, adding oleic acid and octadecylene, vacuum stirring, heating to 260-340 degrees C for 0.5-2 hours in the presence of argon gas, cooling, centrifuging, washing with ethanol, dispersing in non-polar solvent i.e. cyclohexane to obtain dispersion liquid 1, adding trifluoroacetic acid rare earth salt (or rare earth fluoride salt) and trifluoroacetic acid alkali metal salt (or alkaline metal fluoride salt) in which stoichiometric molar ratio Na (or K):Y:Yb is 0.3-0.6:0.2-0.5:0.03-0.06, adding oleic acid and octadecylene, vacuum stirring, heating to 260-340 degrees C for 0.5-2 hours in the presence of argon gas, cooling, centrifuging, washing with ethanol, dispersing in non-polar solvent i.e. cyclohexane to obtain dispersion liquid 2, adding trifluoroacetic acid rare earth salt (or rare earth fluoride salt) and trifluoroacetic acid alkali metal salt (or alkaline metal fluoride salt) in which stoichiometric molar ratio Na (or K):Nd:Yb is 0.5-1:0.5-1:0.05-0.1, adding oleic acid and octadecylene, vacuum stirring, heating to 260-340 degrees C for 0.5-2 hours in the presence of argon gas, cooling, centrifuging, washing with ethanol, and dispersing in non-polar solvent i.e. cyclohexane. Preferred Components: Rare earth UCNPs use 800 nm infrared laser as excitation light source. UCNPs substrate is sodium yttrium fluoride (NaYF4), sodium gadolinium fluoride (NaGdF4), potassium yttrium fluoride (KYF4), potassium gadolinium fluoride (KGdF4), yttrium(III) oxide (Y2O3), yttrium(III) fluoride (YF3), or lanthanum fluoride (LaF3). Nanoparticle matrix structure is of formula AYF4:Yb,X,AGdF4:Yb,X or Y2O3:Yb,X. Nanoparticle 1st shell (inner shell layer) is of formula AYF4:Yb, AGdF4:Yb or Y2O3:Yb. Nanoparticle 2nd shell (outer shell layer) is of formula ANdF4:Yb or Nd2O3:Yb. Upconversion nanoparticle specific structure is represented by formulae NaYF4:Yb,X@NaYF4:Yb@NaNdF4:Yb, NaYF4:Yb,X@NaYF4:Yb@Nd2O3:Yb, NaYF4:Yb,X@Y2O3:Yb@NaNdF4:Yb, NaYF4:Yb,X@Y2O3:Yb@Nd2O3:Yb, NaGdF4:Yb,X@NaYF4:Yb@NaNdF4:Yb,NaGdF4:Yb,X@NaYF4:Yb@Nd2O3:Yb, NaGdF4:Yb,X@Y2O3:Yb@NaNdF4:Yb, NaGdF4:Yb,X@Y2O3:Yb@Nd2O3:Yb, Y2O3:Yb,X@NaYF4:Yb@NaNdF4:Yb, Y2O3:Yb,X@NaYF4:Yb@Nd2O3:Yb, Y2O3:Yb,X@Y2O3:Yb@NaNdF4:Yb, or Y2O3:Yb,X@Y2O3:Yb@Nd2O3:Yb in which Na can be replaced by K.X=Er,Tm, or Ho; andA=Na or K.