Journal of Materials Research


Critical shell thickness and emission enhancement of NaYF4:Yb,Er/NaYF4/silica core/shell/shell nanoparticles

Li Peng Qian, Du Yuan, Guang Shun Yia1 p1 and Gan Moog Chowa1 c1

a1 Department of Materials Science and Engineering, National University of Singapore, Kent Ridge, Singapore 119260, Republic of Singapore


Amorphous silica shells, used for functionalization of inorganic nanoparticles in bioapplications, were coated on chemically synthesized NaYF4:Yb,Er upconversion fluorescent nanoparticles via a reverse microemulsion method by using dual surfactants of polyoxyethylene (5) nonylphenylether and 1-hexanol, and tetraethyl orthosilicate as precursor. NaYF4:Yb,Er nanoparticles were equiaxed with a particle size of 11.1 ± 1.3 nm. The thickness of silica shell was ∼8 nm. NaYF4:Yb,Er/silica core/shell nanoparticles were well dispersed in solvents such as ethanol and deionized water. The emission intensities of NaYF4:Yb,Er/silica core/shell nanoparticles remained the same as that of uncoated nanoparticles after surface functionalization with an amine group using (3-aminopropyl)-trimethoxysilan. Silica, although providing a good barrier to the nonradiative relaxation between the upconversion nanoparticles and the environments, did not enhance the emission intensity of upconversion nanoparticles. To increase the emission intensity of NaYF4:Yb,Er/silica core/shell nanoparticles, an undoped NaYF4 shell (∼3-nm thick) was deposited on the upconversion nanoparticles before the silica coating. The total emission intensity of NaYF4:Yb,Er/NaYF4/silica core/shell/shell nanoparticles increased by 15 times compared to that without the intermediate NaYF4 shell. The critical shell thickness of NaYF4 was ∼3 nm, beyond which no further emission intensity enhancement was observed.

(Received July 01 2009)

(Accepted August 11 2009)

Key Words:

  • Core/shell;
  • Luminescence;
  • Nanostructure


c1 Address all correspondence to this author. e-mail:

p1 Present address: nanoBright Technologies Pte. Ltd., 8 Boon Lay Way, #05-03, 8@TradeHub21, Singapore 609964, Republic of Singapore.