Topotactic Reaction Enabled Luminescence From Bismuth Containing Materials

With the arrival of the information age,the fast development of computer network and telecommunication technology has put forward higher demand for near-infrared?NIR?optical sources.Traditional rare-earth ions-doped optical fiber amplifiers will be abandoned due to the 4f-4f forbidden transition of rare-earth ions with narrow emission band.Whereas,NIR luminescent materials containing bismuth is a promising active medium for optical amplifiers and lasers because of its super broadband emission in telecommunication bands,which have widely potential applications in future optical communication system.At first,this paper introduces the research progress of bismuth-containing luminescent materials,and select the main scientific problems in this field,1)the techniques available for the synthesis of this class of materials are rather limited and specific;2)the origins of the NIR luminescence are still controversial.We found that topochemical reduction reactions can be adopted as a universal method to topotactically convert Bi-doped phosphors into NIR-emitting ones.This is a generic approach on the rational manipulation of luminescence?PL?from typical Bi-doped visble-emitting systems to those emitting in the NIR region.Moreover,we found that the local environmental of the Bi ions strongly changed in the atomic scale.It is proposed that the NIR PL is derived from the luminescence center formed by Bi in the under-coordinated Bi-O polyhedral.So,we have found a novel generic approach to create unconventional Bi-based emitters to those emitting in the NIR region,and made progress in understanding the nature of Bi-related NIR active centers.We anticipate that this low temperature topochemical route can be extended to tune the properties of other types of optoelectronic materials.The main research conclusions of this paper are as follows:Firstly,a topochemical reduction strategy was introduced into the control of property of the bismuth-containing luminescent crystal material.This approach is demonstrated by using Bi³⁺-doped Y₂O₃ as a model system.By utilizing calcium hydride?CaH₂?as a low-temperature reducing agent,the local environment of Bi is markedly influenced by the oxygen de-intercalation,further evidencing that oxygen vacancies are randomly distributed in the yttria matrix.More interestingly,we demonstrate that the low-temperature topotactic reduction strategy can be applied to the conversion of Bi³⁺-doped VIS-emitting systems into NIR-luminescent materials.Secondly,we have demonstrated that Bi²⁺-doped red-emitting BaBPO₅ crystal can be converted into NIR-emitting systems enabled by utilizing Al powders as a low-temperature reducing agent.At the same time,we firstly identified that a minority of Bi atoms occupy the Ba²⁺site based on the XANES and EXAFS analyses,and most of Bi atoms occupy the P⁵⁺and/or B³⁺sites in the as-synthesized product.We also found that the existence of oxygen vacancies changes the local environment of Bi at P⁵⁺/B³⁺sites,and the created[BiO_x]units which contribute to the NIR emission.As a result,such a topotactic transformation not only enrich the strategies for the controlled synthesis of novel Bi-activated NIR-luminescent materials bearing unusual Bi-O units,but also help us gain unprecedented insight into PL mechanisms of Bi-doped systems.