Synthesis and characterization of tunable color upconversion luminescence β-NaGdF4:Yb3+,Er3+ nanoparticles

The visible green and red upconversion luminescence hexagonal phase Er3+ and Yb3+ doped NaGdF4 nanoparticles of different sizes and morphologies were prepared through high temperature RE3+ ions/oleic acid based organometals and hydrothermal methods, respectively. Their microstructural characterizations were accomplished using X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy analyses. High resolution transmission electron microscopy (HR-TEM) images suggested spherical ultra-small nanocrystals of 6.4 nm for the green luminescence β-NaGdF4:Yb3+,Er3+ and oval/mostly rod shapes particles of ~ 40 nm width and ~ 100 nm length for the red emitting one. The same hexagonal (β-NaGdF4) local structure was confirmed from the selected area electron diffraction (SAED), in corroboration with XRD patterns for both sizes and shapes nanoparticles. Raman scattering result exhibited red Raman shifts of lattice peaks and anomalous line narrowing with decreasing the particle size from ~ 100 to ~ 6.4 nm. The photoluminescence spectroscopy manifested higher intensity 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 (green emission) transitions of Er3+ ion for ultra-small nanocrystals and dominant 4F9/2 → 4I15/2 (red emission) one for the large size nanorods. The change in size of the particles might be account for the observed tuning in upconversion emission.