Stabilizing cubic γ-Ga2O3:Cr3+ spinel nanocrystals by size confinement into mesoporous silica nanoreactor channels

Abstract

In recent years, Cr³⁺-activated phosphors have been attracting a lot of interest due to their unique optical features that can be exploited for applications ranging from lasers to optical thermometry, near-infrared (NIR) emitting phosphors for bioimaging and NIR-LEDs, to name a few. Despite the interesting optical properties shown by Cr³⁺-doped Ga₂O₃, investigations are limited only to β-Ga₂O₃ and α-Ga₂O₃ polymorphs. Using mesoporous silica particles with different pore sizes between 3 nm and 22 nm as nanoreactors, cubic γ-Ga₂O₃:Cr³⁺ spinel nanocrystals (NCs) are stabilized through confinement into the mesopores as confirmed by HR-TEM, XRPD and the photoluminescence spectral shape. A detailed spectroscopic investigation in the temperature range between 16 to 500 K allowed extrapolating the fundamental parameters in the framework of the Tanabe–Sugano diagram and comparing the values for Cr³⁺ in γ-Ga₂O₃ with the α- and β-phases. In addition, due to the confinement of the NCs into the silica structure, the phase transition to the β-phase, conventionally occurring at about 700 °C, is hindered up to 1000 °C, improving the stability of the γ-Ga₂O₃ phase.