title: Nanoscale probing of bandgap states on oxide particles using electron energy-loss spectroscopy.
abstract: Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2anatase nanoparticles using ultra-high energy resolution electron energy-loss spectroscopy (EELS) coupled to a scanning transmission electron microscope (STEM). This combination allows the surface electronic structure determined with spectroscopy to be correlated with nanoparticle size, morphology, facet etc. By acquiring the spectra in aloof beam mode, radiation damage to the surface can be significantly reduced while maintaining the nanometer spatial resolution. MgO and TiO2showed very different bandgap features associated with the surface/sub-surface layer of the nanoparticles. Spectral simulations based on dielectric theory and density of states models showed that a plateau feature found in the pre-bandgap region in the spectra from (100) surfaces of 60nm MgO nanocubes is consistent with a thin hydroxide surface layer. The spectroscopy shows that this hydroxide species gives rise to a broad filled surface state at 1.1eV above the MgO valence band. At the surfaces of TiO2nanoparticles, pronounced peaks were observed in the bandgap region, which could not be well fitted to defect states. In this case, the high refractive index and large particle size may make Cherenkov or guided light modes the likely causes of the peaks.
authors.affiliation: School for the Engineering of Matter, Transport and Energy, Arizona State University, 85287 AZ, USA.
authors.forename: Qianlang
authors.initials: Q
authors.lastname: Liu
authors.affiliation: Laboratoire de Physique des Solides, Bâtiment 510, Université Paris-Sud, 91405 Orsay Cedex, France.
authors.forename: Katia
authors.initials: K
authors.lastname: March
authors.affiliation: School for the Engineering of Matter, Transport and Energy, Arizona State University, 85287 AZ, USA. Electronic address: CROZIER@asu.edu.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Crozier
keywords: Bandgap states; Dielectric property; Electronic structure; Nanoparticle; Surface states