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Transition from Reconstruction toward Thin Film on the (110) Surface of Strontium Titanate.

Wang Z, Loon A, Subramanian A, Gerhold S, McDermott E, Enterkin JA, Hieckel M, Russell BC, Green RJ, Moewes A, Guo J, Blaha P, Castell MR, Diebold U, Marks LD - Nano Lett. (2016)

Bottom Line: Such structures can also be viewed as ultrathin films, epitaxed on a substrate.Whereas SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n × 1 to 2 × n.At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium.

View Article: PubMed Central - PubMed

Affiliation: Institute of Applied Physics, TU Wien , Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria.

ABSTRACT
The surfaces of metal oxides often are reconstructed with a geometry and composition that is considerably different from a simple termination of the bulk. Such structures can also be viewed as ultrathin films, epitaxed on a substrate. Here, the reconstructions of the SrTiO3 (110) surface are studied combining scanning tunneling microscopy (STM), transmission electron diffraction, and X-ray absorption spectroscopy (XAS), and analyzed with density functional theory calculations. Whereas SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n × 1 to 2 × n. At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium. This transition from the n × 1 to 2 × n reconstructions is a transition from a pseudomorphically stabilized tetrahedral network toward an octahedral titania thin film with stress-relief from octahedral strontia units at the surface.

No MeSH data available.


Related in: MedlinePlus

Polyhedral representationsof the structures (color online), top normal to the surface and belowfrom the side. The 3 × 1 and 4 × 1 octahedra are brown andtetrahedra are golden; in all the others, octahedra in the outermostlayer are brown, dark blue are in the 2nd layerm while TiO5[] are purple in the top layer and light green in the second layer.
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fig3: Polyhedral representationsof the structures (color online), top normal to the surface and belowfrom the side. The 3 × 1 and 4 × 1 octahedra are brown andtetrahedra are golden; in all the others, octahedra in the outermostlayer are brown, dark blue are in the 2nd layerm while TiO5[] are purple in the top layer and light green in the second layer.

Mentions: Upon the basis of a structure similar to lepidocrocite,we now turn to an identification of the bright feature in the rows,as well the structure below the outermost layer that is invisibleto STM. With full occupancy of all the Ti sites in the second layer,the surface would be a metallic conductor, similar to previous reportsof lepidocrocite-like titania as cited earlier. Reducing the second-layeroccupancy of titanium leads to a valence-neutral and insulating surface.From Sr deposition experiments, there was a circumstantial link betweenthe bright features and the presence of extra Sr atoms. From a detailedDFT analysis of different occupancies, and coupling this with refinementsagainst the diffraction data, we isolated two homologous series of2 × n structures as shown in Figure 3, together with the 3 ×1 and 4 × 1 for reference. The first, 2 × na, contains two Sr atoms in the trench every unit cell along the “2”direction with a separation of (0,0,1), while the second 2 × nb contains one every other with a (0,0,2) spacing and abridging TiOx unit. Both are strictly2 × n structures, although the 2 × na family will not appear so in STM images. Careful analysisof the diffraction data (see Figure S5)indicated that the “2×” component was disordered.Because of this disorder, the Shelx code47 was used to refine the diffraction data against a combination ofeither 2 × 4a,b or 2 × 5a,b, that is, statistical fractional occupancy, yielding a crystallographicR1 of 15% with ∼1/4 coverage of 2 × 4a, and an R1 of 22% with 56 measurements for the 2 × 5 with also∼1/4 coverage of 2 × 5a, see Tables S1 and S2. While these are high for abulk structure, they are good numbers for a surface indicating thatone can have confidence in the structures.40 For completeness, the DFT calculations indicate the possibilityof additional higher-TiO2 coverage structures without anySr in the trenches but because we have no diffraction data to confirmthis we will not pursue these further here.


Transition from Reconstruction toward Thin Film on the (110) Surface of Strontium Titanate.

Wang Z, Loon A, Subramanian A, Gerhold S, McDermott E, Enterkin JA, Hieckel M, Russell BC, Green RJ, Moewes A, Guo J, Blaha P, Castell MR, Diebold U, Marks LD - Nano Lett. (2016)

Polyhedral representationsof the structures (color online), top normal to the surface and belowfrom the side. The 3 × 1 and 4 × 1 octahedra are brown andtetrahedra are golden; in all the others, octahedra in the outermostlayer are brown, dark blue are in the 2nd layerm while TiO5[] are purple in the top layer and light green in the second layer.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4834633&req=5

fig3: Polyhedral representationsof the structures (color online), top normal to the surface and belowfrom the side. The 3 × 1 and 4 × 1 octahedra are brown andtetrahedra are golden; in all the others, octahedra in the outermostlayer are brown, dark blue are in the 2nd layerm while TiO5[] are purple in the top layer and light green in the second layer.
Mentions: Upon the basis of a structure similar to lepidocrocite,we now turn to an identification of the bright feature in the rows,as well the structure below the outermost layer that is invisibleto STM. With full occupancy of all the Ti sites in the second layer,the surface would be a metallic conductor, similar to previous reportsof lepidocrocite-like titania as cited earlier. Reducing the second-layeroccupancy of titanium leads to a valence-neutral and insulating surface.From Sr deposition experiments, there was a circumstantial link betweenthe bright features and the presence of extra Sr atoms. From a detailedDFT analysis of different occupancies, and coupling this with refinementsagainst the diffraction data, we isolated two homologous series of2 × n structures as shown in Figure 3, together with the 3 ×1 and 4 × 1 for reference. The first, 2 × na, contains two Sr atoms in the trench every unit cell along the “2”direction with a separation of (0,0,1), while the second 2 × nb contains one every other with a (0,0,2) spacing and abridging TiOx unit. Both are strictly2 × n structures, although the 2 × na family will not appear so in STM images. Careful analysisof the diffraction data (see Figure S5)indicated that the “2×” component was disordered.Because of this disorder, the Shelx code47 was used to refine the diffraction data against a combination ofeither 2 × 4a,b or 2 × 5a,b, that is, statistical fractional occupancy, yielding a crystallographicR1 of 15% with ∼1/4 coverage of 2 × 4a, and an R1 of 22% with 56 measurements for the 2 × 5 with also∼1/4 coverage of 2 × 5a, see Tables S1 and S2. While these are high for abulk structure, they are good numbers for a surface indicating thatone can have confidence in the structures.40 For completeness, the DFT calculations indicate the possibilityof additional higher-TiO2 coverage structures without anySr in the trenches but because we have no diffraction data to confirmthis we will not pursue these further here.

Bottom Line: Such structures can also be viewed as ultrathin films, epitaxed on a substrate.Whereas SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n × 1 to 2 × n.At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium.

View Article: PubMed Central - PubMed

Affiliation: Institute of Applied Physics, TU Wien , Wiedner Hauptstrasse 8-10/134, 1040 Vienna, Austria.

ABSTRACT
The surfaces of metal oxides often are reconstructed with a geometry and composition that is considerably different from a simple termination of the bulk. Such structures can also be viewed as ultrathin films, epitaxed on a substrate. Here, the reconstructions of the SrTiO3 (110) surface are studied combining scanning tunneling microscopy (STM), transmission electron diffraction, and X-ray absorption spectroscopy (XAS), and analyzed with density functional theory calculations. Whereas SrTiO3 (110) invariably terminates with an overlayer of titania, with increasing density its structure switches from n × 1 to 2 × n. At the same time the coordination of the Ti atoms changes from a network of corner-sharing tetrahedra to a double layer of edge-shared octahedra with bridging units of octahedrally coordinated strontium. This transition from the n × 1 to 2 × n reconstructions is a transition from a pseudomorphically stabilized tetrahedral network toward an octahedral titania thin film with stress-relief from octahedral strontia units at the surface.

No MeSH data available.


Related in: MedlinePlus