SYNTHESIS AND LUMINESCENT PROPERTIES OF MICRO- AND NANOSIZED INDIUM ORTHOBORATES DOPED WITH Cr(III) IONS

Micro- and nanosized samples of indium orthoborate doped with Cr3+ ions were obtained by solid-state reaction method and by thermolysis of an amorphous precursor, respectively. The samples were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and luminescent spectroscopy. The XRD results indicated that the final products were formed in homogenous form and their XRD patterns were well matched with data from JCPDS database for InBO3. According to the obtained SEM images, the microsized samples were composed of irregular shape crystallites with a relatively low degree of agglomeration. The crystallite sizes range from 1 to 3 μm. The samples prepared by thermolysis of an amorphous precursor were found to consist of submicron aggregates (300–900 nm) of nanosized crystallites. The emission spectra of Cr3+ ions in the micro- and nanosized InBO3 samples are shown to consist of a broad band in the infrared (IR) region with a maximum at about 810 nm, which is due to the spin-allowed 4T2 → 4A2 transition. In contrast to the Cr3+ emission intensity, the spectral positions of the maxima of the Cr3+ emission band and corresponding excitation bands were found to be practically independent on the particle size. At 293 K, the luminescence quantum efficiency (η) of the microsized InBO3:Cr3+ samples is high as 0.70, whereas η is only 0.26–0.30 for the nanosized samples. This effect is attributed to the quenching of Cr3+ luminescence due to interaction with imperfections of the crystal lattice, including surface defects, the role of which increases with a decrease in the particle size. Also, the bulk and surface disorder of the nanosized samples induces local crystal-field fluctuations and is expected to result in an inhomogeneous broadening of the emission and excitation spectra of Cr3+ ions, and the absence of fine structure in the emission spectra at low temperatures. It is concluded that under the condition of optimization of the synthesis procedure and an increase in the luminescence quantum efficiency, nanosized InBO3:Cr3+ can be of interest for the development of IR LEDs for various purposes.