Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO–MgO Particulate Systems

Abstract

The rational design of composite nanoparticles with desired optical and electronic properties requires the detailed analysis of surface and bulk contributions to the respective overall function. We use flame spray pyrolysis (FSP) to generate nanoparticles of the ternary Zn-Mg-O system the compositions of which range from solid solutions of Zn(2+) ions in periclase MgO to phase separated particle mixtures which consist of periclase (cubic) MgO and wurtzite (hexagonal) ZnO phases. The structure and composition of the composite Zn(x)Mg(1-x)O (0 ≤ x ≤ 0.3) particles are investigated using X-ray diffraction and high-resolution transmission electron microscopy, whereas UV diffuse reflectance and photoluminescence (PL) spectroscopy are used for the investigation of their optical properties. Vacuum annealing has been carried out to track the effects of stepwise elimination of surface adsorbates on the photoexcitation and PL emission properties. We demonstrate that for Zn(0.1)Mg(0.9)O particles, the admixed ZnO suppresses the MgO specific surface excitons and produces a PL emission band at 470 nm. Although gaseous oxygen partially reduces the emission intensity of hydroxylated particles, it leads to entire quenching in completely dehydroxylated samples after vacuum annealing at 1173 K. Consequently, surface hydroxyls at the solid-gas interface play a significant role as protecting groups against the PL-quenching effects of O(2). The obtained results are relevant for the characterization of ZnO-based devices as well as for other metal oxide materials where the impact of the surface composition on the photoelectronic properties is usually neglected.

Synthesis of Zn2+-doped MgO nanoparticles using substituted brucite precursors and studies on their optical properties

“…-doped MgO samples are shifted toward higher values on the introduction of larger Zn 2? ions in the MgO lattice, and this has been reported by other authors, too [13,15]. Typical SEM images of pure MgO and Zn 2?…”

“…In a similar way, intensity of ZnO emissions (452, 470 and 485 nm) increases on increasing the concentration of Zn 2? [13]. The bumps at around 325 nm are attributed to F-aggregate centers, formed by the association of cation and anion vacancies on the surface [35].…”

“…can be explained by ''diadochy'' mechanism proposed by Komarneni et al [37], and this leads to the easy synthesis of Zn 2? -doped MgO nanoparticles compared to other highenergy consuming routes [10][11][12][13][14][15]. The synthesis of Zn 2?…”

“…which is incorporated in 10 mmol of MgO nanoparticles at 1000°C. Among the methods known to prepare solid solutions such as solid-state reaction [10,15,39], thermal evaporation [12], flame spray pyrolysis [13] and chemical vapor deposition [14], the use of Zn 2? -substituted brucite precursors for the synthesis of Zn 2?…”

“…The Zn 2? -doped MgO solid solutions (Zn X Mg 1-X O) have been synthesized by physical methods such as solid-state reaction [10,15], thermal evaporation [12], flame spray pyrolysis [13], chemical vapor deposition [14] and chemical methods such as sol-gel method [11]. Yamamoto et al [10] have reported the synthesis of Zn X Mg 1-X O solid solutions with different Mg 2?…”