Measuring optical properties of individual SnO 2 nanowires via valence electron energy-loss spectroscopy
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Robert E. Williams Materials Science and Engineering Department, The Ohio State University, Columbus 43210, Ohio, USA; and Center for Electron Microscopy and AnalysiS (CEMAS), The Ohio State University, Columbus 43212, Ohio, USA
Sheikh A. Akbar and Pat A. Morris Materials Science and Engineering Department, The Ohio State University, Columbus 43210, Ohio, USA
David W. McComba) Materials Science and Engineering Department, The Ohio State University, Columbus 43210, Ohio, USA; and Center for Electron Microscopy and AnalysiS (CEMAS), The Ohio State University, Columbus 43212, Ohio, USA (Received 23 February 2017; accepted 14 April 2017)
For the first time, valence electron energy-loss spectroscopy (VEELS) was applied to individual single-crystalline SnO2 nanowires to investigate the dielectric function, band gap, and optical absorption coefficient. The results are compared with data from optical techniques such as spectroscopic ellipsometry and UV-Vis, and theoretical calculations from variations of density functional theory. The data obtained agree well with the standard optical and theoretical techniques. The dielectric function and optical absorption coefficient are given up to 20 eV, which otherwise requires a synchrotron source and large single crystals via optical methods. The energy loss function is given up to 40 eV, which gives a useful comparison to previous theoretical studies in an energy range that cannot be achieved via optical measurements. The comparison gives confidence in the accuracy of this method for exploring spatially-resolved measurements in individual nanoparticles or more complex nanostructures that are otherwise difficult to measure accurately using optical techniques.
I. INTRODUCTION
SnO2 is a wide band-gap and high dielectric constant (high-k) n-type semiconductor often used in transparent electrodes,1 chemical sensors,2 and catalysts.3 SnO2 is easily doped to high carrier concentrations and has lately garnered much interest for optoelectronic devices such as light-emitting and laser diodes.4–6 Furthermore, researchers have found many ways to synthesize high quality nanoparticles and nanowires from SnO2 optimized for use in these applications.7 However, the optical and dielectric properties of nanomaterials are difficult to accurately measure. Any technique using a photon source has a spatial resolution much larger than a single nanoparticle, yielding data that is an average of many nanoparticles. This makes it challenging to understand subtle changes in properties between nanoparticles, especially locally within a single nanoparticle. Furthermore, nanomaterials are typically grown in optically rough, porous
Contributing Editor: Gary L. Messing a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.173
thin films which scatter light and lack good boundary conditions with which to carry out the necessary calculations for accurately determining the absorption coefficient and dielectric function. Valence electron energy-loss spectroscopy (VEELS) is increa
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