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Title: The surface structure and reconstruction of SrTiO3 (001).
Author: Newell, David T.
ISNI:       0000 0001 3443 0744
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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SrTi03 is a metal oxide with a perovskite structure which can support numerous surface reconstructions. On its (001) surface more than 7 reconstructions have been reported previously and substantiated with atomic scale imaging. These reconstructions include surface structures with periodicities of (2 xl), (2 x 2), c(4 X 4), c(4 X 2), c(6 X 2), (J5 x J5) - R26.6° and (V13 x V13) - R37.7°. Each of these reconstructions can be produced in a controlled and repeatable manner simply by changing the time, temperature, and environment in which the sample is annealed. This thesis provides the most comprehensive study of the SrTi03(00l) surface to date and provides STM imaging and chemical characterisation for the majority of the surface reconstructions which have been reported, as well as the conditions required to generate each of the structures. In addition to this work, data is shown for new surface reconstructions which have not been previously reported and include structures with periodicities of (1 xl), (4 X 4), (4V2 x V2) - R45°, and c(4V2 x 2V2) - R45° It was found that upon annealing SrTi03(001) samples at increasingly higher temperatures, the first structures to form are reconstructions which have a Ti02 double layer termination. These structures have periodicities of (2 xl), c(4 x 4) and (2 x 2) and are stable at temperatures up to 950°C. Annealing SrTi03(001) samples above 950°C results in the formation of a Ti-rich adlayer with a c(4 x 4) periodicity. This c(4 x 4) ordered surface exists up to temperatures of 1250 °c and its structure is different to that of the Ti02 double layer which forms at lower temperatures. Above anneal temperatures of 1250 °c, reduction of the sample can pass a critcal stage where the SrTi03 perovskite structure is no longer stable. Phase separation occurs giving rise to TiO islands and a surface Sr adlayer with (J5 x J5) - R26.6° periodicity. Re-annealing (J5 x J5) - R26.6° reconstructed samples below the critical temperature causes the Sradlayer to oxidise becoming a SrO-(l x 1) terminated surface. For Nb doped SrTi03(001) - (J5 x J5) - R26.6° and (1 x 1) reconstructed samples, the clectonic effect of dopants can be seen in STM images as bright four point clusters. These bright clusters arise from the perturbation of surface Sr atoms due to charge transfer from Nb atoms to Ti atoms in the atomic layer immediately beneath the Sr adlayer. For SrTi03(001) samples doped with La, the dopant atoms substitute with Sr atoms and are incorporated into the surface adlayer. This incorporation of La frustrates the formation of the (J5 x J5) - R26.6° reconstruction and leads to line structures arising on the surface which are aligned with the direction of the (J5 x J5) - R26.6° structure. SrTi03 samples annealed in highly reducing conditions, achieved by annealing in low pressures of H2 gas, formed surfaces terminated by a c(4 x 2) structure. SrTi03(001) samples pre-annealed in flowing oxygen produced structures with periodicities of (4V2 x V2) - R45°, (2 x 2), (4 X 4), and c(4V2 x 2V2) - R45°. Chemical characterisation of the reconstructions observed on the surface of SrTi03 samples indicated that samples annealed under reducing conditions had surfaces that were Ti enriched, whilst samples annealed under oxidising conditions had surfaces that were Sr enriched. The experimental work undertaken would suggest that the driving force for the reconstruction of the SrTi03 surface is the amount of oxygen that is in the surface region.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available