O₂adsorption dependent photoluminescence emission from metal oxide nanoparticles

Abstract

Optical properties of metal oxide nanoparticles are subject to synthesis related defects and impurities. Using photoluminescence spectroscopy and UV diffuse reflectance in conjunction with Auger electron spectroscopic surface analysis we investigated the effect of surface composition and oxygen adsorption on the photoluminescence properties of vapor phase grown ZnO and MgO nanoparticles. On hydroxylated MgO nanoparticles as a reference system, intense photoluminescence features exclusively originate from surface excitons, the radiative deactivation of which results in collisional quenching in an O 2 atmosphere.Conversely, on as-prepared ZnO nanoparticles a broad yellow emission feature centered at hn Em = 2.1 eV exhibits an O 2 induced intensity increase. Attributed to oxygen interstitials as recombination centers this enhancement effect originates from adsorbate-induced band bending, which is pertinent to the photoluminescence active region of the nanoparticles. Annealing induced trends in the optical properties of the two prototypical metal oxide nanoparticle systems, ZnO and MgO, are explained by changes in the surface composition and underline that particle surface and interface changes that result from handling and processing of nanoparticles critically affect luminescence.

Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems

“…Then, at pressures p < 510 À6 mbar,t he sample temperature was raised to 1173 Ka nd kept at this temperature for 1huntil full dehydroxylation of the sample surface was achieved. [30] Powder characterization by X-ray diffraction and electron microscopy Before and after porphyrin adsorption, X-ray diffraction (XRD) measurements were performed on the MgO powders by using aB ruker AXS D8 Advance diffractometer with Cu Ka radiation (l = 154 pm). After each adsorption step, the alumina crucible was removed from the MgO powder,w hich, without breaking the vacuum, was transferred to the spectroscopic cell for UV/Vis diffuse reflectance (DR) measurements ( Figure S1, right).…”

Insight into the Role of Ligands in the Yellow Luminescence of Zinc Oxide Nanocrystals

“…Blue emission was observed for the toluene and chloroform solutions only, if the ZnO NCs were excited at wavelengths higher than but close to the band edge (λ exc 375 nm, Figure S1c). [40][41][42] [26] The origin of this blue emission of relatively low intensity is currently unclear and is the subject of controversies.…”

Defects in Metal Oxide Nanoparticle Powders

“…employing the Scherrer approach, assumes a homogeneous size distribution of crystalline domains. For the present case this would lead to erroneous results, since there exists a broad size distribution from small to large crystallites within one sample [11]. It is worth noting that 100 mg of a ZnO nanoparticle powder, i.e.…”

“…Only consistent values of crystallite domain size (X-ray powder diffraction), particle size concluded from surface determination (sorption analysis) and size distribution (electron microscopy in conjunction with workable statistics) can prove that the discussed particle system exhibits a sufficiently uniform size distribution, in order to start with a model system for physico-chemical powder studies. [11]. a Optical image of *100 mg nanoparticle powder; b a scanning electron micrograph (SEM) showing ZnO particle agglomerates with diameters in the range between 500 nm and 5 μm.…”

“…An ensemble of ZnO nanoparticles, which were obtained by metal organic chemical vapor synthesis and which were subjected to thermal processing thereafter, is shown in Fig. 9.2 [11]. After vacuum annealing up to 670 K the particle size distribution of related powder samples was narrow and peaks at an average particle size of 10 nm.…”