An unusal strategy of Ca2+ heterovalent doping enabled upconversion enhancement of Er3+ in bismuth oxychloride layered semiconducting crystals

Abstract Substitutional heterovalent doping has been considered as an effective method to control optical properties of lanthanide doped phosphors in the past, but it focuses only on three-dimensional and insulating crystal hosts. This study aimed to report the efficient upconversion enhancement of Er3+ induced by a new strategy of Ca2+ heterovalent doping in the layered semiconducting host based on BiOCl crystals concentrated by van der Waals force. The surface photovoltages (SPV) tests, theoretical calculations and optical probing effect of Eu3+ ions provided evidence that the replacement of Bi3+ with Ca2+ strengthened the macroscopic polarization and the corresponding spontaneous inter electric field (IEF) along the [001] of the BiOCl crystal sheet. The upconversion (UC) red and green emission intensities of the sample co-doped with 10 mol% Ca2+ ions were enhanced 39 and 12 times, respectively, compared with those of the Ca2+-free sample. Moreover, this study showed that the higher separation efficiency of photocarriers led to the prolongation of the decay time of excited energy levels under the strengthened intrinsic IEF by Ca2+-heterovalent doping, which was partially responsible for the UC photoluminescence (PL) enhancement of Er3+. This work not only provided new insight into the method for the engineering of UC emissions but also enhanced the understanding of layered semiconducting materials to modify the transition of lanthanide ions.