Article

Controlled synthesis, formation mechanism, and great enhancement of red upconversion luminescence of NaYF4:Yb3+, Er3+ nanocrystals/submicroplates at low doping level.

Key Laboratory of Excited-State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, P. R. China.
The Journal of Physical Chemistry B (impact factor: 3.7). 12/2008; 112(49):15666-72.
Source: PubMed

ABSTRACT Strong red upconversion luminescence of rare-earth ions doped in nanocrystals is desirable for the biological/biomedical applications. In this paper, we describe the great enhancement of red upconversion emission (4F9/2 --> I15/2 transition of Er3+ ion) in NaYF4:Yb3+, Er3+ nanocrystals at low doping level, which is ascribed to the effectiveness of the multiphonon relaxation process due to the existence of citrate as a chelator and cross relaxation between Er3+ ions. The dissolution-recrystallization transformation, governing both the intrinsic crystalline phase (cubic and/or hexagonal phase) and the growth regime (thermodynamic vs kinetic), is responsible for the phase control of the NaYF4 crystals. The possible formation mechanism of the NaYF4 crystals and the role of trisodium citrate which acts as a chelating agent and shape modifier are discussed in detail. It is also found that the alpha --> beta phase transition is favored by the high molar ratio of fluoride to lanthanide and high hydrothermal temperature as well as long hydrothermal time.

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Keywords

chelating agent
 
dissolution-recrystallization transformation
 
Er3+ nanocrystals
 
growth regime
 
hexagonal phase
 
hydrothermal time
 
intrinsic crystalline phase
 
lanthanide
 
low
 
molar ratio
 
multiphonon relaxation process
 
nanocrystals
 
NaYF4 crystals
 
phase control
 
possible formation mechanism
 
rare-earth ions doped
 
red upconversion emission
 
Strong red upconversion luminescence
 
trisodium citrate