Nanoscale probing of bandgap states on oxide particles using electron energy-loss spectroscopy

• Published in: Ultramicroscopy 2017-07, Vol.178 (C), p.2-11
• Main Authors: Liu, Qianlang, March, Katia, Crozier, Peter A.
• Format: Article
• Language: English
• Subjects: Bandgap states; Dielectric property; Electronic structure; Nanoparticle; Surface states
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2016.06.010

Surface and near-surface electronic states were probed with nanometer spatial resolution in MgO and TiO2 anatase 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 TiO2 showed 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 TiO2 nanoparticles, 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. •Bandgap states detected with aloof beam monochromated EELS on oxide nanoparticle surfaces.•Dielectric theory applied to simulate the spectra and interpret surface structure.•Density of states models also be employed to understand the surface electronic structure.•In MgO, one states associate with water species was found close to the valence band edge.•In anatase, two mid-gap states associated with point defects were found.