(INVITED) Ultraviolet and visible luminescence from bismuth doped materials

Abstract

Bismuth is a non-toxic posttransition metal with a large number of possible valence states which, together with to its tendency to form clusters, make it a versatile but complex dopant for possible luminescence applications. The outer orbitals responsible for luminescence are unshielded and strongly influenced by the surrounding environment, so that their energies are host dependent. In this review article we surveyed models that have been developed to understand the optical properties of bismuth based materials and compiled a few case studies on the luminescence of bismuth doped materials, with some emphasis on trivalent Bi3+ and simple oxide hosts (e.g. CaO:Bi; Y2O3:Bi; ZnO:Bi; La2O3:Bi; SrO:Bi; YPO4:Bi and LaPO4:Bi). Special emphasis was given to the compilation of simplified energy diagrams under different conditions. For the most stable trivalent Bi3+ state it has been experimentally found by many researchers that the absorption spectra consist of medium, weak and strong bands in order of increasing energy (designated the A, B and C bands, respectively), which were interpreted in terms of transitions to the states associated with the 3P1, 3P2 and 1P1 levels of the excited 6s1p1 configuration. Traditionally above the C band energy, but before the host absorption, a further D band is generally encountered, which is attributed to a metal-to-metal charge transfer (MMCT) transition corresponding to the transfer of an electron from the Bi3+ ion to the bottom of the conduction band. Recent modelling of this MMCT band in different materials has clarified that it may occur at energies below the C band and be an important influence on excitation spectra at lower energies. The challenge of unambiguously identifying the nature of emissions is still an ongoing challenge, as illustrated by some of the case studies.