Bandgap Engineering of Scandium Microspheres for Anti-Counterfeiting and Multicolor Imaging

Bandgap engineering plays an important role in the regulation of the photophysical properties of semiconductor materials. Developing the facile and powerful strategy to achieve bandgap tunability is highly desired. Herein, the bandgap tunability of scandium microspheres prepared by the hydrothermal treatment of Sc³⁺ ions/l-glutamine (Sc³⁺/Gln) complexes is achieved. The continuously decreased bandgap is directly attributed to the increase of carbon element and the decrease of scandium element in scandium microspheres controlled by the Gln ligand, which causes the red-shifted room temperature phosphorescence from Sc/Gln-0.15 (3.17 eV, green) to Sc/Gln-0.25 (2.53 eV, yellow). Meanwhile, scandium microspheres have an excitation-dependent emission ranged from green to red color, revealing their multi-color imaging capability. Further increase of Gln ligand can further reduce the bandgap to 1.62 eV (Sc/Gln-0.3), realizing the transition from photoluminescence to photothermal properties of scandium microspheres. Because of their superior optical properties, scandium microspheres can be well used for advanced anti-counterfeiting and multicolor imaging. This new finding not only offers a deeper insight into the photophysical structure of rare earth semiconductor materials, but also provides a scalable method for the preparation of scandium microspheres with the desired bandgaps.