Kinetically stabilized aliovalent europium-doped magnesium oxide as a UV sensitized phosphor

Abstract

Doping of size mismatched aliovalent ions is challenging due to the associated elastic and electronic stress making the thermodynamics unfavorable. Despite such features, its utilization may be viable if such systems can be made metastable by suppressing the kinetics of phase segregation. In light of such a possibility, we utilize sol-gel synthesis for preparing a size mismatched trivalent europium doped MgO (Mg(1-x)Eu(x)O:(x/2)V"(Mg)) system, which can be potentially used in optical applications. It is found that such a doped system can be metastabilized and the extent of metastability can be correlated with critical temperature (Tc) required for phase segregation which decreases with the dopant concentration. For x = 0.005, 0.01, and 0.02, Tc is above 1200 °C, 500-800 °C and less than 500 °C, respectively. As the synthesis temperature is 500 °C, these trends in critical temperatures make it impossible to metastabilize europium in MgO with x > 0.01. Doping is evident from X-ray diffraction data, excitation spectra, high resolution emission spectra, and luminescence lifetimes. A characteristic strong red emission of Eu(3+) has been observed via energy transfer from the MgO matrix to Eu(3+). Density functional theory based simulations suggest stabilization of Eu(3+) in MgO at lower doping concentration through the formation of cation vacancies which is also evident from optical studies. Furthermore, thin films deposited using the e-beam evaporation technique from the Mg(1-x)Eu(x)O:(x/2)V"(Mg) (x = 0.005) system show UV sensitized emission with CIE coordinates (0.26, 0.21).

The antifungal effect of MgO and ZnO nanoparticles against powdery mildew disease of pepper (Capsicum annuum L.) under greenhouse conditions

“…Previous studies reported that MgO exhibit a broad absorption peak between 260-330 nm (Rastogi et al, 2015). The TEM results exhibited unsatisfactory dispensability of MgONPs due to agglomeration state, and the selected area electron diffraction (SAED) pattern confirmed crystalline nature and cubic shape of MgONPs.…”

Terbium Ion-Mediated Energy Transfer in WO3:Tb3+ and Eu3+ Phosphors for UV-Sensitized White Light Emission

“…For example, the compositions Lu 2 O 3 :Gd, Lu 2 O 3 :Ln (Ln = Eu, Sm, Ho, Er, Tm, or Yb), Lu 2 O 3 :Tb, and Lu 2 O 3 :Ln (Ln = Dy or Nd) exhibit host absorption (HA) due to interband transitions, acceptor-like charge transfer (CTT), donor-like ionization (IT), and f → fd transitions, respectively. The sensitization of Ln 3+ ions in most of the non-oxide hosts (LaSi 3 N 5 , GaN, ZnS, and NaYF 4 ) relies on the excitation mechanism such as charge transfer, self-excitation, or via sensitizer-assisted ET; nevertheless, in oxides such as MgO, ZnO, TiO 2 , and WO 3 , the host has strong ability to absorb UV radiation (due to the presence of inherent oxygen vacancies) and transfer it to doped Ln 3+ ions via an ET mechanism presented in Figure b . The presence of a broad and identical band in the excitation spectra of both WO 3 :Eu 3+ and WO 3 :Tb 3+ (discussed above) provides an important evidence in support of ET, thereby ruling out charge transfer.…”

“…In-depth understanding of ET from the sensitizer to activator Ln 3+ ions is vital for realizing novel luminescence characteristics. Several aspects such as doping levels, sensitizer–activator separation, and host crystallinity with relative band edge position (vis-à-vis ground and excited states of Ln 3+ ions) affect the ET mechanism. − A model proposed for the incorporation of Ln 3+ ions (e.g., Eu 3+ , Tb 3+ , Er 3+ , Nd 3+ , etc.) in semiconductor hosts (e.g., ZnS and TiO 2 ) suggests dependence of sensitization efficiency (or antenna effect) on the relative positions of Ln 3+ ground- and excited-state levels vis-à-vis valence and conduction band edges of the host material. ,, As an instance, TiO 2 :Eu 3+ nanoparticles do not display the antenna effect, while TiO 2 :Ln 3+ (Ln = Nd and Sm) produces strong host-sensitized emission due to favorable positioning of Nd 3+ /Sm 3+ energy states and the band edges of the host TiO 2 …”

Stabilization of non‐native polymorphs for electrocatalysis and energy storage systems

“…The addition of dopants greatly influences the NNP due to the introduction of the strain. The introduction of lower coordinating (Almonacid et al, 2016; Gangwar, Pandey, Sivakumar, Pala, & Parthasarathy, 2013; Rastogi et al, 2017) or high size‐mismatched dopants (J. Lai et al, 2003; Rastogi, Saha, Sivakumar, Pala, & Kumar, 2015; Rastogi et al, 2017) introduces large strain in the lattice structure. To decrease the strain, the crystal undergoes native ➔ non‐native transformation or stabilizes the NNPs at elevated temperatures.…”

“…When the valence charge densities of host ions and dopants are comparable, it is envisaged that atoms with smaller ionic radii used as dopant will favor the more close‐packed crystal polymorphs in similar reaction condition. However, a high size mismatch between host and dopant would also result in cluster formation of dopants or surface segregation (Rastogi et al, 2015). Similarly, J. Lai et al (2003) showed that in the case of γ‐Fe 2 O 3 was stabilized by Mn (III) dopant up to 600°C temperature.…”