Defect controlled UV-to-visible down conversion in Bi³⁺-doped Y₂Zr₂O₇ monoclinic-like phosphors

Cost-effective, defect-controlled, lanthanide-free and stable down-conversion UV-visible phosphors play an important role in solid-state lighting and optoelectronic applications. Down-conversion phosphor materials transform intense UV light into visible emission. The development of defect-controlled phosphors is challenging because it is commonly seen in many reported UV-visible phosphors that disorder increases with dopant concentration, which results in the creation of coulombic traps and scattering centres. These traps impede further electron and hole transport. To overcome this kind of challenge, Bi³⁺-doped Y₂Zr₂O₇ distorted monoclinic-like phosphors were synthesized by a straightforward and affordable conventional high-temperature solid-state reaction method. X-ray diffraction confirms the formation of a distorted monoclinic-like phosphor with a slight variation in lattice parameter. The morphological study was done using scanning electron microscopy, which confirms agglomerated, submicron-sized particles, spherical in shape. Electronic states and oxidation states were calculated using XPS, which suggests the formation of oxygen vacancy defect states and the successful doping of Bi³⁺ in the host. Optical band gap and Urbach energy analysis, derived from UV diffuse reflectance spectroscopy, revealed a narrow bandgap and stabilized disorder with increasing dopant concentration in the host. PL confirms broadband visible light under 350 nm excitation via a UV-to-visible down-conversion emission mechanism, which is facilitated via Bi³⁺ level emission and defects states associated with an oxygen vacancy mechanism. Chromaticity analysis reveals stable CIE coordinates near standard coordinates with a high correlated color temperature (CCT) value, which is the characteristic of a cyan-blue emission. Overall, the results show that Bi³⁺-doped Y₂Zr₂O₇ distorted monoclinic-like phosphors had good UV-to-visible down conversion capabilities, which could be good candidates for optoelectronic and solid-state lighting applications.