In situ real-time monitoring of upconversion emission is applied to study the reaction mechanism for the synthesis of β-NaYF4:17% Yb,3% Er nanoparticles in oleic acid and octadecene via the heat-up method. Transmission electron microscopy is used to correlate the spectroscopic signature of the reaction mixture with its composition. The power of real-time spectroscopic monitoring to precisely time the duration of the various stages of the reaction, and to accurately identify the transitions between those stages, including the completion of the reaction, is demonstrated. During the heat-up stage, the initial precipitate present is transformed into small α-phase nanoparticles. In the second and longest stage of the reaction, a period of relative stasis is maintained, during which there is at most a slight change in the size distribution of the α-phase nanoparticles formed during heat-up. In the third stage, a relatively rapid conversion of small α-phase nanoparticles to the larger β-phase product nanoparticles is observed. The size distribution of the β product remains relatively constant during the phase-transition stage, indicating that, once the β particles begin to form, they grow very rapidly to the final product size. A significant variation in the time required to complete the reaction is observed, which is due almost entirely to the variation in the duration of the second reaction stage. The time required to complete the reaction does not appear to affect the size or uniformity of the product β-NaYF4:17% Yb,3% Er nanoparticles.

We implement a model for nanocrystal growth and crystallographic phase transition during the synthesis of β-NaYF4. In this model, the size of the α-phase nanoparticles, formed during the heating of the precursor materials, grows slightly in mean diameter and broadens in distribution width until some particles reach a size at which the β phase is thermodynamically favored. Individual particles crossing this threshold convert to the β phase, and then, being less soluble than the α phase, grow at the expense of dissolving α-phase particles. Implementing a straightforward kinetic formalism for individual particle growth and a variable phase definition depending on particle size, the model reproduces, in a quantitative fashion, the experimentally observed growth dynamics of β- NaYF4:Yb,Er. This work supports a hypothesis that the β-particle seeds arise from a phase transition in individual α-phase particles. The model also suggests that the great variability observed in the duration of the stage during which the α particle ripen, before β particles begin to appear, may be attributed to rather small differences in the size distribution of the α particles formed during the heating of precursor material.