Fig. 2: Emission properties, printing and colour analyses.

a Emitted intensity quantified by the RGB colour coordinates for dU6Tb as a function of the number of printouts under 254 nm excitation and for dU6Eu under 365 nm excitation. Images were obtained with a smartphone camera. The printed luminescent QR code images under b daylight excitation, c 254 nm excitation presenting an intense green colour and d 365 nm emitting a bright red colour. e Absorption extinction coefficient and f emission spectra excited at 350 and 370 nm for dU6Tb and dU6Eu, respectively. g CIE diagram with the colour coordinates for dU6Eu and dU6Tb calculated from the emission spectra and with the colour coordinates for the images in (a) calculated from their RGB colour coordinates.