Enhancing the emission color sensitivity of bulk microcrystals in response to excitation power density and NIR wavelength by introducing Li+/Lu3+ for anti-counterfeiting

Dynamic tuning upconversion color is the most convenient approach through modulation of pump power density and NIR wavelength for anti-counterfeiting. However, most of bulk upconversion systems suffer from the insensitive response to excitation power and NIR wavelength (980 nm and 1550 nm). Herein, we demonstrate that the excitation power density sensitivity and NIR wavelength sensitivity of bulk microcrystals could be improved via co-doping Li + /Lu 3+ ions. More interestingly, co-doping ions with different valences affect the emission color tendency at 1550 nm excitation, which is contrary to previous reports. The emission color of bulk NaYF 4 :Er/Yb/Li/Lu microcrystals transform from yellow to bright green color through adjusting pump power density and pump wavelength (980 nm and 1550 nm). Moreover, the mechanism of the results has been systematically discussed. The tunable emission colors of bulk NaYF 4 :Er/Yb/Li/Lu microcrystals can be easily distinguished with naked eyes and difficultly counterfeited. Therefore, the upconversion materials have a promising application in transparent anti-fake printing.