Monodisperse alpha-NaYF4:Yb,Er and beta-NaYF4:Yb,Er nanocrystals with controlled size, chemical composition, and surface state were synthesized from trifluoroacetate precursors in hot surfactant solutions via a unique delayed nucleation pathway. The growth mechanisms of both alpha- and beta-phase nanocrystals and the underlying alpha ->beta phase transition process were uncovered by upconversion (UC) spectroscopy, transmission electron microscopy, and X-ray diffraction techniques. The UC emission was sensitive to the growth process of the NaYF4:Yb,Er nanocrystals; that is, the UC intensity is sensitive to the nucleation and phase transition process, and the ratio of green to red emission is also sensitive to the crystallite size and phase, which makes it easy to distinguish various nanocrystal growth stages and phase transition modes with UC spectroscopy. Differently sized monodisperse alpha-NaYF4:Yb,Er nanopolyhedra (5-14 nm) were readily obtained via only prolonging the reaction time. During the reaction, four consecutive nanocrystal growth stages including nucleation in a delayed time, particle growth by monomer supply, size shrinkage by dissolution, and aggregation were identified. The essential of the delayed nucleation was likely due to the requisite in accumulating enough NaF (from the decomposed Na(CF3COO)) for the nanocrystal growth. Monodisperse beta-NaYF4:Yb,Er nanocrystals with tunable sizes in a broad range from 20 to 300 nm were obtained from alpha-NaYF4:Yb,Er monomers by restricting or enhancing the Ostwald-ripening process, in which the alpha ->beta transition happened in a delayed time.
