Unveiling the mechanisms of defects induced upconversion luminescence enhancement in Er3+-sensitized 2D Bi3O4Br nanosheets

Abstract

Lanthanide (Ln3+) ions doped upconversion (UC) nanosheets have attracted tremendous attention such as displays, sensing, bioimaging and lasers etc, which was benefitting from the intriguing optical characters of Ln3+. However, the field of UC nanosheets has been hindered by low UC conversion efficiencies associate with nonradiative relation (NR) occurring by defect, the existence and influence of defects still cannot be eliminated completely. In this work, we design introduce the impurity energy level by doping Er3+in Bi3O4Br:Er3+ nanocrystal materials, which was closed with the intermediate band (IB) formed by oxygen vacancies defects. The density functional theory calculations confirm the IB energy level was closed with the intermediate excited states of Er3+, which provided the potential to tailored the ground state carriers transition from matrix semiconductor to Er3+ and thus tool to counteract the effect of NR and even enhance the UC luminescence performance. The photo-current results evidenced that the photocarrier success transition from IB to Er3+ intermediate excited states energy level leads to a sharp decrease in the surface carrier, on the contrary, the electron population on the excited state energy level of Er3+ have increased. As a result, compared with unmodified sample the UC emission intensity under excited by 980 nm of green and red is enhanced by 7 and 4 times respectively. This work paves the way to design efficient UC nanosheets through by energy transfer (ET) combine matrix semiconductor with RE and greatly enriches the understanding about the ET behavior of RE.