Influence Of Built-in Electric Field Modulation Based On Heterovalent Doping On The Upconversion Luminescence Of BiOCl:Er³⁺ Layered Semiconductors

In recent years,upconversion luminescence(UCL)of rare-earth ions(RE³⁺)doped nanophosphors has been widely used in biomedicine,3D display technologies and upconversion lasers,with their advantages of rich fluorescence band,high photochemical stability and long fluorescence life.But many defects in nanoparticles increase the nonradiative relaxation of RE³⁺ions,placing restrictions on UCL efficiency for nanomaterials.To solve those drawbacks,some strategies have been attempted to improve UC luminescence efficiency.However,most optical materials are three-dimensional(3D)crystal or network structure hosts,which is hard to tailor the electron transition and luminescence behavior of RE³⁺ions.In contrast,layered semiconductors can provide a different enhancement mechanism and design idea for UCL materials,due to the advantages of 2D material anisotropy,adjustable band structure,flexible crystal structure,easy to dope or dissociate and the excellent photoelectric properties of semiconductors.For layered polar semiconductors,spontaneous built-in electric field(IEF)can lead to the excitation field enhancement and the higher separation efficency of photocarriers.The ability of directional control on electron transition of RE³⁺ions may become a new and efficient method of upconversion enhancement and regulation.To confirm our idea,we tailor the internal polarization and the IEF via heterovalent doping and prove the effect and mechanism of the IEF on the upconversion luminescence of Er³⁺ions.According to the crystal structure of Bi OCl,we have used three different doping methods,such as heterovalent Ca²⁺cation substitution doping,heterovalent Li⁺lattice intercalation doping and heterovalent C anion substitution doping.The photoluminescence spectrum test,electrochemical characterizations and the first principle calculations were investigated.Firstly,we synthesized Ca²⁺doped Bi OCl:Er³⁺single-crystalline microsheet by solid phase method.Experimental results and first-principles calculations provide evidence that the replacement of Ca²⁺ions for Bi³⁺makes the charge distribution nonuniform and increases the dipole moment in the z-axis direction,which gives rise to strengthen macroscopic polarization and spontaneous IEF of Bi OCl.Under the excitation of 980 nm laser,the upconversion green and red emission of Er³⁺ions are greatly enhanced with Ca²⁺doping.This is because the enhanced IEF increases the excitation field around RE³⁺ions and promotes the separation of photogenerated electron-hole pairs effectively,resulting in a significant increase of UCL intensity and lifetime of Er³⁺ions.Secondly,we used the solid phase method to synthesize Li⁺and Er³⁺codoped Bi OCl microsheet.Experimental results and first-principles calculations show that Li⁺ions are doped into the interlayer of van der Waals force connection rather than the lattice gap.Although Li⁺intercalation doping reduced the crystallinity of Bi OCl,it can effectively improve the interlayer potential and spontaneous polarization IEF of Bi OCl.Compared with Ca²⁺doping,the UCL intensity still increases when the concentration of Li⁺doping increased to 30%.The probe results of Eu³⁺ions and electrochemical tests indicate that Li⁺doping can increase the strength of IEF continuously,thus improving the UCL intensity and lifetime of RE³⁺ions.In addition,we also verified this regulation method in Bi OCl nanocrystal with many defects.In this paper,the heterovalent C doped Bi OCl:Er³⁺nanosheets with C substituting Cl were prepared by hydrothermal method and heat treatment.Unlike microsheets,Under the doping of low C concentration,the visible UC emission of Er³⁺ions will even be quenched by C doping.When the doping concentration of C exceeding to 1mmol,it is observed that the UCL intensity increases with C doping remarkablely.The results show that for the nanosheet systems,the nonradiative relaxation of RE³⁺ions increases with the increase of defects under a low concentration of C heterovalent doping,which demonstrated that the quenching effect of crystal defects is greater than the positive effect of enhanced IEF on the UCL.As the doping level of C increases to a certain extent,the enhanced IEF by C doping exhibits stronger inhibition of photoelectron-hole pairs recombination.Therefore,for Er³⁺ions,due to the more efficient separation of photoholes,the recombination possibility of electrons on the intermediate excited state is deceased,resulting in the prolonged decay time of⁴I_13/2?⁴I_15/2 transition.Then,these electrons are promoted to reabsorb energy and transition to upper energy levels,leading to UCL enhancement.This work tailors RE³⁺upconversion luminescence in layered polar materials and reveals the synergistic effect of polarized electric field on rare earth luminescence.It will not only provide new insights for enhancing UC luminescence,but also help to promote the common development of 2D layered materials and rare earth luminescent materials,and provide a new direction for the design and preparation of more suitable and effective upconversion luminescent materials.