Indirect evidence of Bi3+ valence change and dual role of Bi3+ in trapping electrons and holes for multimode X-ray imaging, anti-counterfeiting, and non-real-time force sensing

Discovering bismuth based smart materials that can respond to thermal, mechanical, and wide range X-ray to infrared photon excitation remains a challenge. Such materials have various uses like in advanced information encryption. In this work, valence state change between Bi ²⁺, Bi ³⁺, and Bi ⁴⁺, and the dual role of Bi ³⁺ in trapping electrons and holes have been studied in Bi ³⁺ or/and Ln ³⁺ (Ln=Tb or Pr) doped LiScGeO ₄ family of compounds by vacuum referred binding energy (VRBE) diagram construction, thermoluminescence, and spectroscopy. Electron release from Bi ²⁺ has been evidenced. It can be used to experimentally determine the VRBE in the Bi ^{2+ 2}P _1/2 ground state and to realize Bi ³⁺ negative quenching luminescence. Particularly, a new force induced charge carrier storage phenomenon has been discovered for non-real-time force recording. Wide range of emission tailorable afterglow, unique Bi ³⁺ ultraviolet-A, white, and infrared afterglow have been demonstrated by using Bi ³⁺ as a hole trapping and recombination center and using energy transfer processes from Bi ³⁺ to Tb ³⁺, Pr ³⁺, Dy ³⁺, or Cr ³⁺ . Proof-of-concept advanced anti-counterfeiting, information encryption, and X-ray imaging will be demonstrated. This work not only develops smart storage phosphors, but more importantly unravels the valence change between Bi ²⁺, Bi ³⁺, or Bi ⁴⁺ and how it can affect the trapping and release of charge carriers with thermal, optical, or mechanical excitation. This work therefore can promote the discovery and development of Bi ³⁺ based smart materials for various applications.