In this paper

discrete Yb

3+

-Tm

3+

co-doped LaF

3

nanoparticles with controlled chemical composition and size distribution were readily synthesized using reverse micelles with microemulsions as confined reaction media.The morphology of the final product was characterized by the FESEM and HRTEM images.When the solid sample disperses in the ethanol without ultrasonic treatment

it is maize cob-like aggregator.The average diameter and length of the maize cob are about 110 and 575 nm

respectively.We suggested that the nanoparticles assemble into well-defined superstructures arising from the solvent evaporation

molecular cross-linking or interaction from surfactant molecules attached to specific nanoparticles crystal faces.After ultrasonic treatment for 30 min

the maize cob-like aggregator turns into a lot of nanoparticles owing to ultrasonic breach above a series of factors.The HRTEM image taken from individual nanoparticles indicates the crystalline nature

and the interplanar spacing of ～0.366 nm corresponding to the planes of pure hexagonal phase LaF

3

.The TEM image of the samples annealed at 300℃ for 30 minutes displays the average diameter of NCs increased to about 35 nm and is in good agreement with XRD result.In addition

the NCs exhibit nearly monodisperse property

which is in favor of optimizing luminescence of the sample.The nanoparticles present a bright blue upconversion luminescence under the 978 nm excitation from a laser diode

which provides a promising upconversion phosphor for optoelectronic or biological applications.