title: Concentration enhanced upconversion luminescence in ZrO2:Ho3+, Yb3+ nanophosphors.
abstract: The upconversion luminescence properties of ZrO2:Ho3+ and co-doped ZrO2:Ho3+, Yb3+ nanophosphors with various concentrations of Yb3+ ions were synthesized via a solvothermal reaction method. Our samples have a nearby spherical shape and an average crystal size was about 80 nm. For low concentrations of Yb3+ ion, the crystalline structure changed from tetragonal to monoclinic phase as the Yb3+ concentration increased to 3 mol% Yb3+ ions. As the Yb3+ concentration increased to above 5 mol%, ZrO2 nanophosphors displayed a very stable tetragonal phase. The sample shows a strong green (550 nm) and weak red (660 nm) and near infrared (757 nm) upconversion emission corresponding to the transitions of Ho3+:5F4/5S2 --> 5I8, 5F5 --> 5I8 and 5S2 --> 5I7, respectively. The energy transfer (ET) processes between the Ho3+ and Yb3+ ions and the involved mechanisms have been investigated. Experimental results suggest that two-photon upconversion processes are taking place under excitation by a 975 nm.
authors.affiliation: Department of Physics, Pukyong National University, Busan 608-737, Korea.
authors.forename: Hyeon Mi
authors.initials: HM
authors.lastname: Noh
authors.forename: Hyun Kyoung
authors.initials: HK
authors.lastname: Yang
authors.forename: Byung Kee
authors.initials: BK
authors.lastname: Moon
authors.forename: Byung Chun
authors.initials: BC
authors.lastname: Choi
authors.forename: Jung Hyun
authors.initials: JH
authors.lastname: Jeong
authors.forename: Haeyoung
authors.initials: H
authors.lastname: Choi
authors.forename: Jung Hwan
authors.initials: JH
authors.lastname: Kim