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 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⁺ into Ga₂O₃, investigations are limited only to the 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 the confinement into the mesopores as confirmed by HR-TEM, XRPD and the photoluminescence spectral shape. A detailed spectroscopic investigation in a temperature range between 16 to 500 K allowed to extrapolate the fundamental parameters in the framework of the Tanabe-Sugano diagram and to compare 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.