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• Monodisperse ZrO 2 nanocrystals were controllably prepared by a phase-transfer route. • The as-prepared ZrO 2 nanocrystals have tunable shapes, crystal forms and sizes. • DFT calculation was used to study the root causes of underpinning growth mechanisms. • Fluorescent monodispersed ZrO 2 :Yb3+/Er3+ nanocrystals could be similarly prepared. Colloidal nanocrystals can offer exquisite control of physical and chemical properties for technological applications by tuning their sizes, shapes and crystal forms. Herein, a convenient phase-transfer route was presented to controllably prepare transparent dispersions of monodispersed ZrO 2 nanocrystals in an aqueous-organic two-phase medium. By adjusting the surfactants, solvents and basicity, the controllability of crystal forms from monoclinic to tetragonal phases, the particle length from 5 to 20 nm, and the different shapes including spindle, cobblestone, sphere, branch, rod and cube can be achieved. The as-prepared ZrO 2 nanocrystals can be readily dispersed in nonpolar solvents, thereby forming the ultra-highly concentrated (60 wt%), highly stable (>18 months) and highly transparent nanodispersions. Density functional theory (DFT) calculations were further used to elucidate the root causes of underpinning growth mechanisms. This work not merely enriches the methodology of controllable fabrication of monodispersed sub-10 nm ZrO 2 nanocrystals, but also enables a platform to synthesize rare-earth-doped nanophosphors using ZrO 2 as the host matrix. The fluorescent ZrO 2 :Yb3+/Er3+ nanocrystals, which can be easily monodispersed in organic solvents, were also prepared with visible luminescene properties under near-infrared irradiation. [ABSTRACT FROM AUTHOR]