title: Improved multiphoton ultraviolet upconversion photoluminescence in ultrasmall core-shell nanocrystals.
abstract: Near-infrared to ultraviolet multiphoton upconversion photoluminescence in ultrasmall Tm3+/Yb3+-codoped CaF2 nanocrystals (∼6.7  nm in size) was observed and further significantly enhanced by growing an active shell of NaYF4:Yb3+. Owing to the active shell, the lanthanide emitters inside the core are effectively prevented from the surface quenchers, and the excitation energy is absorbed more efficiently via the additional luminescence sensitizer Yb3+embedded in the shell. The details of underlying physics were investigated and discussed. The results present a good ultrasmall luminescent material system for achieving efficient multiphoton upconversion, which shows great potential in versatile industrial and biological applications.
authors.forename: Lili
authors.initials: L
authors.lastname: Tao
authors.forename: Bo
authors.initials: B
authors.lastname: Zhou
authors.forename: Wei
authors.initials: W
authors.lastname: Jin
authors.forename: Yang
authors.initials: Y
authors.lastname: Chai
authors.forename: Chun-Yin
authors.initials: CY
authors.lastname: Tang
authors.forename: Yuen H
authors.initials: YH
authors.lastname: Tsang
mesh_terms: D002124:Calcium Fluoride / Q000737:chemistry*; D008163:Luminescent Measurements; D053758:Nanoparticles; D010316:Particle Size; D017785:Photons; D013932:Thulium / Q000737:chemistry; D014466:Ultraviolet Rays; D015018:Ytterbium / Q000737:chemistry
chemical_list: D013932:Thulium; D015018:Ytterbium; D002124:Calcium Fluoride