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NaYF4:Yb3+,Er3+ nanoparticle has attracted a tremendous amount of attention due to its potential applications in solid-state lasers,three-dimensional displays,and especially in biomedical areas.Shell coating is a significant method to improve the upconversion luminescence efficiency of NaYF4:Yb3+,Er3+ nanoparticle.In this work,the advantages and disadvantages of two types of shell(active shell and inert shell)are systematically compared.For the first time,we proposed a bidirectional energy transfer process between core and active shell.Furthermore,an optimal core/active shell/inert shell structure is designed to control the bidirectional energy transfer process.Compared with one-layer shell structure,the double-shell structure has strongest upconversion luminescence,which offers exciting opportunities for important biological and energy applications.