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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.


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STM imagesof the 4 × 1 (a) and two kinds of 2 × 5 surfaces (b,c).The arrows in between indicate that these structures can be switchedreversibly by depositing Ti/Sr followed by annealing. The Sr singleadatom on the 4 × 1 surface is labeled by an arrow. The insetsin (b,c) show magnified views, superimposed with simulated STM imagesof 2 × 5b in (b) and 2 × 5a in (c).
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fig1: STM imagesof the 4 × 1 (a) and two kinds of 2 × 5 surfaces (b,c).The arrows in between indicate that these structures can be switchedreversibly by depositing Ti/Sr followed by annealing. The Sr singleadatom on the 4 × 1 surface is labeled by an arrow. The insetsin (b,c) show magnified views, superimposed with simulated STM imagesof 2 × 5b in (b) and 2 × 5a in (c).

Mentions: Thelow-temperature samples were used for the STM (Figure 1) and for the XAS measurements and were checkedwith LEED in both cases (see Figure S1).Nb-doped (0.5 wt %) SrTiO3 (110) single crystal sampleswere purchased from MaTecK, Germany. In the low-temperature approach,the surfaces were initially sputtered and annealed at moderate temperature.By further deposition of Sr or Ti, followed by annealing at moderatetemperatures in O2, the surface structure could be preciselytuned according to the surface phase map shown in Figure S2 in Supporting Information and ref (23). Here the surface stoichiometry,and thus structure, was adjusted until a desired monophased surfacewas observed via STM and/or a sharp low-energy electron diffraction(LEED) pattern was present.29,30 For the results shownin Figure 1, the clean4 × 1 surface was obtained by cycles of Ar+ sputteringfollowed by annealing in 2 × 10–6 mbar oxygenat 900 °C for 1 h,31 and the 2 ×5 surface was obtained by depositing Ti at 600–700 °Cin 6 × 10–6 mbar O2. STM imageswere obtained in a SPECS UHV system with a base pressure of 1 ×10–10 mbar. XAS was performed at room temperatureat the I311 beamline at Max-lab,32 andstructures were ascertained by LEED. A Scienta electron analyzer wasused to measure the integrated intensity of the surface-sensitiveTi LM,M Auger lines with the detectorin a surface-grazing orientation, as well as the more bulk-sensitivetotal secondary electron yield. The Ti L2,3 spectra wereanalyzed using the CTM4XAS code.33


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)

STM imagesof the 4 × 1 (a) and two kinds of 2 × 5 surfaces (b,c).The arrows in between indicate that these structures can be switchedreversibly by depositing Ti/Sr followed by annealing. The Sr singleadatom on the 4 × 1 surface is labeled by an arrow. The insetsin (b,c) show magnified views, superimposed with simulated STM imagesof 2 × 5b in (b) and 2 × 5a in (c).
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Related In: Results  -  Collection

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fig1: STM imagesof the 4 × 1 (a) and two kinds of 2 × 5 surfaces (b,c).The arrows in between indicate that these structures can be switchedreversibly by depositing Ti/Sr followed by annealing. The Sr singleadatom on the 4 × 1 surface is labeled by an arrow. The insetsin (b,c) show magnified views, superimposed with simulated STM imagesof 2 × 5b in (b) and 2 × 5a in (c).
Mentions: Thelow-temperature samples were used for the STM (Figure 1) and for the XAS measurements and were checkedwith LEED in both cases (see Figure S1).Nb-doped (0.5 wt %) SrTiO3 (110) single crystal sampleswere purchased from MaTecK, Germany. In the low-temperature approach,the surfaces were initially sputtered and annealed at moderate temperature.By further deposition of Sr or Ti, followed by annealing at moderatetemperatures in O2, the surface structure could be preciselytuned according to the surface phase map shown in Figure S2 in Supporting Information and ref (23). Here the surface stoichiometry,and thus structure, was adjusted until a desired monophased surfacewas observed via STM and/or a sharp low-energy electron diffraction(LEED) pattern was present.29,30 For the results shownin Figure 1, the clean4 × 1 surface was obtained by cycles of Ar+ sputteringfollowed by annealing in 2 × 10–6 mbar oxygenat 900 °C for 1 h,31 and the 2 ×5 surface was obtained by depositing Ti at 600–700 °Cin 6 × 10–6 mbar O2. STM imageswere obtained in a SPECS UHV system with a base pressure of 1 ×10–10 mbar. XAS was performed at room temperatureat the I311 beamline at Max-lab,32 andstructures were ascertained by LEED. A Scienta electron analyzer wasused to measure the integrated intensity of the surface-sensitiveTi LM,M Auger lines with the detectorin a surface-grazing orientation, as well as the more bulk-sensitivetotal secondary electron yield. The Ti L2,3 spectra wereanalyzed using the CTM4XAS code.33

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