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Title: SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction: A surface resulting from phase separation in a reducing environment

Abstract

Scanning tunneling microscopy (STM) was used to produce atomic resolution images of the Nb-doped SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction and its precursor surfaces. Low-energy electron diffraction was used to identify the surface structure in combination with the STM imaging. The ({radical}(5)x{radical}(5))-R26.6 deg. reconstruction was observed growing as an adlayer on top of the underlying terraces. The formation of this structure could routinely be detected by an increase in the crystal's electrical conductivity during ultrahigh vacuum (UHV) annealing at 1300 deg. C. This rise in conductivity occurs due to significant reduction of the crystal. Scanning electron microscopy (SEM) on these samples showed small islands with a rectangular base separated by flat regions. Auger electron spectroscopy showed titanium enrichment, and strontium and oxygen depletion of the surface region. The islands seen in the SEM images are TiO crystals, as reported by S. B. Lee et al. [Ultramicroscopy 104, 30 (2005)]. The flat regions between the islands are composed of a Sr adlayer on the surface that is consistent with the ({radical}(5)x{radical}(5))-R26.6 deg. adatom model of Kubo and Nazoye [Phys. Rev. Lett. 86, 1801 (2001)]. We propose that the TiO islands and the ({radical}(5)x{radical}(5))-R26.6 deg. Sr adlayer reconstruction form due to phase separation inmore » the surface region of the crystal. This phase separation is driven by oxygen depletion which occurs during high-temperature UHV annealing of the sample.« less

Authors:
; ;  [1];  [2]
  1. Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH (United Kingdom)
  2. Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ (United Kingdom)
Publication Date:
OSTI Identifier:
21045883
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 75; Journal Issue: 20; Other Information: DOI: 10.1103/PhysRevB.75.205429; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 36 MATERIALS SCIENCE; ANNEALING; AUGER ELECTRON SPECTROSCOPY; CRYSTALS; DOPED MATERIALS; ELECTRIC CONDUCTIVITY; ELECTRON DIFFRACTION; FERROELECTRIC MATERIALS; LAYERS; NEODYMIUM; OXYGEN; SCANNING ELECTRON MICROSCOPY; SCANNING TUNNELING MICROSCOPY; STRONTIUM; STRONTIUM TITANATES; SURFACES; TEMPERATURE RANGE 0400-1000 K; THIN FILMS; TITANIUM; TITANIUM OXIDES

Citation Formats

Newell, David T., Silly, Fabien, Castell, Martin R., and Harrison, Alastair. SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction: A surface resulting from phase separation in a reducing environment. United States: N. p., 2007. Web. doi:10.1103/PHYSREVB.75.205429.
Newell, David T., Silly, Fabien, Castell, Martin R., & Harrison, Alastair. SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction: A surface resulting from phase separation in a reducing environment. United States. doi:10.1103/PHYSREVB.75.205429.
Newell, David T., Silly, Fabien, Castell, Martin R., and Harrison, Alastair. Tue . "SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction: A surface resulting from phase separation in a reducing environment". United States. doi:10.1103/PHYSREVB.75.205429.
@article{osti_21045883,
title = {SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction: A surface resulting from phase separation in a reducing environment},
author = {Newell, David T. and Silly, Fabien and Castell, Martin R. and Harrison, Alastair},
abstractNote = {Scanning tunneling microscopy (STM) was used to produce atomic resolution images of the Nb-doped SrTiO{sub 3}(001)-({radical}(5)x{radical}(5))-R26.6 deg. reconstruction and its precursor surfaces. Low-energy electron diffraction was used to identify the surface structure in combination with the STM imaging. The ({radical}(5)x{radical}(5))-R26.6 deg. reconstruction was observed growing as an adlayer on top of the underlying terraces. The formation of this structure could routinely be detected by an increase in the crystal's electrical conductivity during ultrahigh vacuum (UHV) annealing at 1300 deg. C. This rise in conductivity occurs due to significant reduction of the crystal. Scanning electron microscopy (SEM) on these samples showed small islands with a rectangular base separated by flat regions. Auger electron spectroscopy showed titanium enrichment, and strontium and oxygen depletion of the surface region. The islands seen in the SEM images are TiO crystals, as reported by S. B. Lee et al. [Ultramicroscopy 104, 30 (2005)]. The flat regions between the islands are composed of a Sr adlayer on the surface that is consistent with the ({radical}(5)x{radical}(5))-R26.6 deg. adatom model of Kubo and Nazoye [Phys. Rev. Lett. 86, 1801 (2001)]. We propose that the TiO islands and the ({radical}(5)x{radical}(5))-R26.6 deg. Sr adlayer reconstruction form due to phase separation in the surface region of the crystal. This phase separation is driven by oxygen depletion which occurs during high-temperature UHV annealing of the sample.},
doi = {10.1103/PHYSREVB.75.205429},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 20,
volume = 75,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • The structure of the ({radical}(5)x{radical}(5))R26.6 deg. reconstruction of LaAlO{sub 3} (001) has been determined using transmission electron diffraction combined with direct methods. It has a lanthanum oxide termination with one lanthanum vacancy per surface unit cell. Density functional calculations indicate that charge compensation occurs by a fractional number of highly delocalized holes, and that the surface contains no oxygen vacancies and the holes are not filled with hydrogen. The reconstruction can be understood in terms of expulsion of the more electropositive cation from the surface and increased covalency.
  • Nb-doped SrTiO{sub 3} (111) samples are annealed in UHV at 850 deg. C for 30 min and investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). STM images show that both ({radical}(7)x{radical}(7))R19.1 deg. and ({radical}(13)x{radical}(13))R13.9 deg. reconstructions coexist on the surface. Step heights of 0.21{+-}0.02 nm on the surface are equivalent to the d{sub 111} lattice parameter, which is the distance between two adjacent, similar (111) planes in the bulk crystal. The calculated LEED pattern for this co-reconstruction corresponds to the observed LEED pattern, which resembles a six-petal flower. AES analysis indicates no detectablemore » impurities, and shows the surface to be Ti and Sr enriched and O deficient compared to the bulk stoichiometry. This change in surface composition is proposed to provide the stability for the polar surface.« less
  • We investigated infrared-active phonons of CaTiO3, SrTiO3, BaTiO3, and Ba0.5Sr0.5TiO3 thin films, and a (BaTiO3)5/(SrTiO3)5 artificial superlattice by a Fourier transform infrared spectrometer with a grazing angle (48{sup o}) incident reflectance method. The longitudinal phonon energies of the thin films were different from those of the bulk material due to the lattice strain of the substrate. The Ba0.5Sr0.5TiO3 thin film and (BaTiO3)5/(SrTiO3)5 superlattice showed different phonon modes due to structural discrepancies, even though their chemical compositions are similar. The Ba0.5Sr0.5TiO3 thin film showed a single phonon energy lying between the phonon energies of BaTiO3 and SrTiO3 thin films, while themore » (BaTiO3)5/(SrTiO3)5 superlattice well preserved the characteristic phonon modes of BaTiO3 and SrTiO3 thin films.« less
  • We have studied the properties of (C{sub 6}H{sub 5}){sub 5}C{sub 60}H in thick film form and adsorbed at two surfaces at the extremes of chemical reactivity - the highly reactive Si(111)-(7x7) and chemically passivated Ag:Si(111)-({radical}(3)x{radical}(3))R30 deg. surfaces - using photoemission spectroscopy (PES) and near-edge x-ray fine structure (NEXAFS) spectroscopy. Our results show that the phenyl groups produce dramatic changes in the electronic structure of the fullerene system, including a lifting of the degeneracy of electronic states and a widening of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) bandgap, resulting in changes in the chemistry of the fullerene cagemore » itself. The modification of the fullerene in this way also enhances the polarisation screening effect observed in fullerene systems. Adsorption at the Si(111)-(7x7) and Ag:Si(111)-({radical}(3)x{radical}(3))R30 deg. surfaces is mediated by two different mechanisms, the former involving formation of covalent bonds, and the latter largely van der Waals in character. Despite the lack of a strong chemical interaction, however, a 0.9 ML coverage of (C{sub 6}H{sub 5}){sub 5}C{sub 60}H on Ag:Si(111)-({radical}(3)x{radical}(3))R30 deg. leads to a shift of the Si 2p core-level spectrum by {approx}200 meV to higher binding energy, suggesting that a positive interface dipole contributes to the adsorption energy of the fullerene at this surface.« less
  • The degradation in magnetic properties in very thin film complex oxides is studied using SrTiO(3)(001)/La(1-x)Sr(x)CoO(3), providing unequivocal evidence for nanoscopic interfacial magnetic phase separation. Electron microscopy and spectroscopy reveal that this occurs due to inhomogeneity in local hole doping, driven by subtle, depthwise variations in the Sr and O stoichiometry. Simple thermodynamic and structural arguments for the origin of these variations are provided.