Radiative lifetime-encoded unicolour security tags using perovskite nanocrystals

Abstract

Traditional fluorescence-based tags, used for anticounterfeiting, rely on primitive pattern matching and visual identification; additional covert security features such as fluorescent lifetime or pattern masking are advantageous if fraud is to be deterred. Herein, we present an electrohydrodynamically printed unicolour multi-fluorescent-lifetime security tag system composed of lifetime-tunable lead-halide perovskite nanocrystals that can be deciphered with both existing time-correlated single-photon counting fluorescence-lifetime imaging microscopy and a novel time-of-flight prototype. We find that unicolour or matching emission wavelength materials can be prepared through cation-engineering with the partial substitution of formamidinium for ethylenediammonium to generate "hollow" formamidinium lead bromide perovskite nanocrystals; these materials can be successfully printed into fluorescence-lifetime-encoded-quick-read tags that are protected from conventional readers. Furthermore, we also demonstrate that a portable, cost-effective time-of-flight fluorescence-lifetime imaging prototype can also decipher these codes. A single comprehensive approach combining these innovations may be eventually deployed to protect both producers and consumers.

Fig. 1. Colloidal perovskite NCs and their optical properties.

Structural and morphological comparison of CsPbBr₃ and {en}FAPbBr₃. a, c Schematic of the structural difference between the undoped and {en}-doped (hence hollow) perovskite NCs; octahedral tiltings in CsPbBr₃ are omitted for simplicity. b, d TEM images of CsPbBr₃ NCs and {en}FAPbBr₃ NCs (scale bar is 20 nm). e Absorption and PL spectra and f time-resolved photoluminescence traces for CsPbBr₃ (blue) and {en}FAPbBr₃ (red) NCs.

Fig. 2. Optical spectroscopy characterization of {en}FAPbBr3 NC film.

2D pseudo-colour plots for fast decay range: a transient absorption, 0–5 ns and b transient PL, 0–1.5 ns of; c 2D pseudo-colour plot of temperature dependence of PL emission, data points and fitting curve (abscise axis is on top) – bandwidth parameter of Lorentzian fit for the PL spectra at the corresponding temperature; d 2D pseudo-colour plots for time-resolved PL for long decay range, 0–50 ns.

Fig. 3. EHD printing.

a Scheme representing the EHD printing setup used to print the colloidal perovskite CsPbBr₃ NC inks. A voltage is applied between a gold-coated capillary nozzle and a grounded substrate, which drives the EHD ejection of ink droplets. b An example of the high-resolution printed pattern (~12,000 dpi) showing low dot-to-dot variation, the distance between dots is 2 µm, the scale bar is 20 µm.

Fig. 4. Unicolour FLQR tags.

a A true-colour image of unicolour FLQR code that was printed as two separate images (QR code and its inverse) from perovskite NC inks (scale bar is 800 µm); b zoomed-in image of a containing regions of CsPbBr₃ (blue) and {en}FAPbBr₃ (red) inks (scale bar is 30 µm). c The spatially resolved PL spectra from the corresponding regions of (blue and red rectangles on b), whose PL spectra overlap to form one apparent emission band (green).

Fig. 5. FLI microscopy (FLIM) imaging and binarization of FLQR tags.

a Zoomed-in view of the central portion of the unicolour FLQR code printed with CsPbBr₃ and {en}FAPbBr₃ inks. b TCSPC-FLI microscopy pseudo-colour image of the same region (scale bars are 400 µm). c The two inks exhibit two distinct fast-lifetime (see Supplementary Note 6) histograms that can be defined as white (lifetime ~6.5 ns, blue) and black (lifetime ~9.5 ns, red) to create a machine-readable QR code d (pattern size is identical to that in Fig. 4a). Please try to scan it with your smartphone.

Fig. 6. ToF-FLI principle and imaging.

a Principle for ToF-based LIDAR where x is distance and Δϕ is the corresponding change in phase. b The analogous approach for ToF-based TRPL where Δϕ is replaced by the fluorescent lifetime, τ. c Lifetime pseudo-colour map of FLQR as recorded by the ToF-FLI imager (scale bar is 1 mm). d Black-and-white machine-readable QR code after binary-discretization of (e), Please try to scan it with your smartphone.