Surface studies of (110) terminated FE304 and SRTI03

21 March 2018

Abstract

In this thesis the surface reconstructions of (110) terminated single crystalline magnetite and strontium titanate are investigated by a host of surface region sensitive techniques, foremost of those being scanning tunneling microscopy and reflectance anisotropy spectroscopy. The anisotropic optical responses of bulk isotropic Fe3O4(110) and SrTiO3(110) are sensitive to the influence that the termination has on the bulk-like region in the vicinity of the termination; the responses are interpreted to originate from anisotropic strain gradients in the surface region which modify the bulk-like optical transitions. Additionally, this optical technique is extremely sensitive to the formation of an anisotropic conductance in vacuum annealed SrTiO3(110). This highlights the potential of reflectance anisotropy spectroscopy to monitor the formation of the electron gas which forms at the interface of insulating perovskites.

Scanning tunneling microscopy measurements reveal that the Fe3O4(110) surface termination is extremely sensitive to the preparation procedure; annealing alone results in the formation of the known {111}-nanofaceted row reconstruction while sputtering prior to annealing results in the coexistence of the row reconstruction and an atomically flat structure. Density functional theory calculations combined with the simulation of scanning tunneling microscopy images indicate that the atomically flat structure contains an ordered array of twofold oxygen surface vacancies. These vacancies and the correlated reduction of the surface iron's oxidation state reduces the surface charge of this polar termination. X-ray photoelectron spectroscopy measurements demonstrate that the {111}-nanofaceted row reconstruction exhibits increased Fe(2+)/Fe(3+) and Fe/O ratios.

However, the electronic properties are observed to differ depending on the annealing environment which is correlated to a slightly altered stoichiometry in the surface region. The observation of 1-dimensional electronic states and the row reconstruction's (1x3) periodicity are understood in terms of the anisotropic strain in the terminating layers identified by reflectance anisotropy spectroscopy measurements. Therefore, this work demonstrates the potential of combining surface sensitive scanning tunneling microscopy with near-surface-bulk sensitive reflectance anisotropy spectroscopy.

Citation

Walls, Brian Conor, Surface studies of (110) terminated FE304 and SRTI03, Trinity College Dublin. School of Physics. Physics, 2018

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Categories: Material & Chemical

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