Ultraviolet upconversion spectra of sonochemically synthesized doped NaYF4 crystals

Abstract

A facile sonochemical method was developed for the production of alpha- and beta-forms of Yb3+(20%)/Tm3+(2%) or Er3+(2%) doubly doped NaYF4 crystals. The reactions took place in aqueous solutions at low temperatures and employed oxide or salt precursors. Micrometer sized beta-NaYF4 and nanometer sized alpha-NaYF4 were obtained with upconversion property. The required activation energy for the solid-solid phase transition was overcome with the energy provided by cavitations. Upconversion emission properties of the Yb3+(20%), Er3+(2%) or Tm3+(2%) doubly doped, alpha- and beta-NaYF4 were investigated. The alpha-form with emission in the ultraviolet region had higher efficiency contradicting the literature. Several new peaks in the spectra, most likely due to crystal defects, were temporarily assigned.