title: Plasmonic enhanced emissions from cubic NaYF(4):Yb: Er/Tm nanophosphors.
abstract: A metal shell was used in this study to provide significant enhancement of the up-converted emission from cubic NaYF(4) nanoparticles, creating a valuable composite material for labeling in biology and other applications - use of the cubic form of the material obviates the need to undertake a high temperature transformation to the naturally more efficient hexagonal phase. The NaYF(4) matrix contained ytterbium sensitizer and an Erbium (Er) or Thulium (Tm) activator. The particle sizes of the as-synthesized nanoparticles were in the range of 20-40 nm with a gold shell thickness of 4-8 nm. The gold shell was macroscopically amorphous. The synthesis method was based on a citrate chelation. In this approach, we exploited the ability of the citrate ion to act as a reductant and stabilizer. Confining the citrate ion reductant on the nanophosphor surface rather than in the solution was critical to the gold shell formation. The plasmonic shell enhanced the up-conversion emission of Tm from visible and near-infrared regions by up to a factor of 8, in addition to imparting a visible color arising from the plasmon absorption of the gold shell. The up-conversion enhancement observed with Tm and Er were different for similar gold coverages, with local crystal field changes as a possible route to enhance up-conversion emission from high symmetry structural hosts. These novel up-converting nanophosphor particles combine the phosphor and features of a gold shell, providing a unique platform for many biological imaging and labeling applications.
authors.affiliation: Mechanical and Aerospace Engineering Department, University of California, Davis, 95616, USA.
authors.forename: L
authors.initials: L
authors.lastname: Sudheendra
authors.forename: Volkan
authors.initials: V
authors.lastname: Ortalan
authors.forename: Sanchita
authors.initials: S
authors.lastname: Dey
authors.forename: Nigel D
authors.initials: ND
authors.lastname: Browning
authors.forename: I M
authors.initials: IM
authors.lastname: Kennedy