Blue and Orange Photoluminescence and Surface Band-Gap Narrowing in Lithium-Doped MgO Microcrystals

• Published in: Journal of physical chemistry. C 2017-01, Vol.121 (3), p.1884-1892
• Main Authors: Soma, Haruka, Uchino, Takashi
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b12177

Structure and properties of Li-doped MgO have been extensively studied during the past decades in view of the effect of aliovalent doping on the catalytic, optical absorption and magnetic properties. However, the photoluminescence (PL) properties of Li-doped MgO have not been systematically investigated previously. In this work, we prepared micrometer-sized Li-doped MgO crystals using solid-phase redox reaction between B2O3 and metals of Mg and Li under argon atmosphere. The resulting Li-doped MgO microcrystals exhibit blue (2.8 eV or 440 nm) and orange PL (2.1 eV or 580 nm) emissions under excitation with photons of energies of ∼5 and ∼3 eV, respectively. The blue PL is attributed to the bulk oxygen vacancies, or F-type centers, whose emission energy levels are modified by the Li ions incorporated into the MgO structure. On the other hand, the orange PL presumably results from the oxygen vacancies at the near surface regions where a substantial bandgap narrowing down to visible range of ∼3 eV occurs. It is also found that the photoexcited electrons generated by surface interband transition can be transferred to the surface emission states via thermal activation, resulting in temperature antiquenching of the orange PL emission. Hence, the present method provides a simple and effective way to prepare visible luminescent Li-doped MgO microcrystals with an intriguing effect of aliovalent doping both on the bulk and surface electronic structures.