Achieving orthogonal upconversion luminescence in Er3+-doped Bi4O5Br2 for high-level anti-counterfeiting and optical thermometry

Er 3+ -activated upconversion (UC) luminescent materials show revolutionary potential in advanced anti-counterfeiting and non-contact temperature sensing by virtue of their excellent multiphoton absorption ability and color-tunable emission characteristics. However, it is difficult for the traditional system to achieve orthogonal UC luminescence of Er 3+ activator with color-tunable outputs in a simple nanostructure, which thus limits its multifunctional applications. To address this challenge, we designed Er 3+ -doped Bi 4 O 5 Br 2 nanosheets with tunable UC colors through smart dynamic manipulation of Er 3+ photon transition processes. Upon various excitation modes (e. g. non-steady-state, dual-wavelength-excitation, thermal excitation), the electron population among the intermediate states of Er 3+ ions can be precisely regulated through multi-photon upconverting processes, resulting in orthogonal luminescence with dynamically adjustable colors. Benefiting from exceptional emission performance (thermal-sensitive upconversion luminescence from thermally coupled levels Er 3+ : 2 H 11/2 / 5 S 3/2 and color-tunable orthogonal upconversion emission from green-emitting level Er 3+ : 2 H 11/2 / 5 S 3/2 and red-emitting state Er 3+ : 4 F 9/2 ), the as-obtained sample exhibits great potential in non-contact temperature measurement and multi-mode anti-counterfeiting via a series of concept verification experiments. This work gives a conceptual model to achieve Er 3+ orthogonal upconversion with multi-color outputs by modulating the emission dynamic in simple structural host, which will encourage researchers to develop multifunctional upconversion materials for the applications of high-level anti-counterfeiting and optical thermometry.