On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

Abstract

Unveiling the underlying mechanisms of properties of functional materials, including the luminescence differences among similar pyrochlores A2B2O7, opens new gateways to select proper hosts for various optoelectronic applications by scientists and engineers. For example, although La2Zr2O7 (LZO) and La2Hf2O7 (LHO) pyrochlores have similar chemical compositional and crystallographic structural features, they demonstrate different luminescence properties both before and after doped with Eu3+ ions. Based on our earlier work, LHO‐based nanophosphors display higher photo‐ and radioluminescence intensity, higher quantum efficiency, and longer excited state lifetime compared to LZO‐based nanophosphors. Moreover, under electronic O2−→Zr4+/Hf4+ transition excitation at 306 nm, undoped LHO nanoparticles (NPs) have only violet blue emission, whereas LZO NPs show violet blue and red emissions. In this study, we have combined experimental and density functional theory (DFT) based theoretical calculation to explain the observed results. First, we calculated the density of state (DOS) based on DFT and studied the energetics of ionized oxygen vacancies in the band gaps of LZO and LHO theoretically, which explain their underlying luminescence difference. For Eu3+‐doped NPs, we performed emission intensity and lifetime calculations and found that the LHOE NPs have higher host to dopant energy transfer efficiency than the LZOE NPs (59.3% vs 24.6%), which accounts for the optical performance superiority of the former over the latter. Moreover, by corroborating our experimental data with the DFT calculations, we suggest that the Eu3+ doping states in LHO present at exact energy position (both in majority and minority spin components) where oxygen defect states are located unlike those in LZO. Lastly, both the NPs show negligible photobleaching highlighting their potential for bioimaging applications. This current report provides a deeper understanding of the advantages of LHO over LZO as an advanced host for phosphors, scintillators, and fluoroimmunoassays.

Disorder driven asymmetry and singular red emission in doped Lu2Hf2O7 nanocrystals with no charge compensating defects

“…Faster decaying component (τ 1 , 364 μs, 17%) may be related to Eu 3+ ions which are near the defects or surfaces of the LuHOE NCs, whereas the slow decay process (τ 2 , 2.34 ms, 83%) is related to the Eu 3+ localized at Lu 3+ center [52]. And average fluorescence lifetime would be 1.79 ms. We clearly rule out any Eu 3+ ion occupying Hf 4+ site as have been observed in ideal pyrochlore lattices because Eu 3+ ion occupying Hf 4+ site is expected to offer lifetime in the range of few milliseconds [13,45,48]. As the high feasibility of Eu 3+ ion occupying Lu 3+ site based on the ionic radius and charge matching, Eu 3+ ion only occupies Lu 3+ site in the LuHOE NCs with a fraction being on the surface of the NCs as suggested by the lifetime spectra, so there is no CCDs present in the LuHOE NCs at all.…”

“…The emission spectrum of the air annealed LuHO NCs synthesized by the MSS method under 250 nm excitation (Fig. 3a [ 25,42,43,[45][46][47][48] It is the combined role of neutral and ionized oxygen vacancies which have been given farfetched explanation for the appearance of visible emission from above-mentioned pyrochlore lattices. Here it is believed that the defect level of neutral and ionized oxygen vacancies lies within the band gap of LuHO NCs.…”

One material, many possibilities via enrichment of luminescence in La2Zr2O7:Tb3+ nanophosphors for forensic stimuli aided applications

“…As a result, LZO pyrochlores were considered a crucial class of functional materials, which offers a wide range of applications, such as renewable energy, catalysis, nuclear waste hosts, scintillators, phosphors, thermal sensors, etc. 28 , 29 .…”

Design of Ni‐substituted La2(CeZrNi)2O7 Pyrochlore Catalysts for Methane Dry Reforming

“…Among them, pyrochlores, as a series of mixed metal oxides with a general formula of A 2 B 2 O 7 , have attracted a lot of attention in catalysis field, because of their high thermal phase stability, excellent oxygen mobility, and the existence of abundant oxygen vacancies. [14,15,16] In general, the A-site is occupied by trivalent rare-earth metal (La, Pr, Sm, Y, etc.) with relatively larger cation diameters, [14] while Bsite is occupied by tetravalent transition metal (Zr, Ce, Ti, etc.)…”