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Title: Scanning tunnelling microscopy of epitaxial nanomaterials on SrTiO3
Author: Gao, Yakun
ISNI:       0000 0004 7229 8176
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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This thesis focuses on the study of epitaxial nanomaterials grown on SrTiO3 substrates including their morphologies, their epitaxial relationship, the energetics of equilibrium crystals and the interactions with the substrates. The formation of ordered TiOx-rich dilines on the SrTiO3(001) surface was achieved following UHV annealing of sputtered surfaces. Upon extensive UHV annealing, leading to a significant TiOx surface enrichment, anatase TiO2(001) islands are nucleated on nanostructured SrTiO3(001). These (1 × 4) and (1 × 3) reconstructed islands are strained on the SrTiO3(001). Further Ti deposition encourages islands growth into a film with a relaxed (1 × 4) reconstruction. Both the dilines and the anatase film present close-packed rows aligning in the <100> direction and are suitable to study the epitaxially-grown Pd nanocrystals. Elongated Pd nanocrystals are formed at a low temperature and grow towards to an equilibrium hexagonal shape upon high temperature anneal. The adhesion energies are calculated to be 2.55±0.09 J/m2 and 2.05±0.05 J/m2 for Pd-dilines and Pd-anatase systems respectively. High temperature anneals (above 600°C) bring Pd-diline and Pd-anatase systems into the SMSI state, along with the formation of (7 × 7) wagon wheel and (4 × 4)R1.5° hexagonal superstructures attributed to moiré patterns. These moiré patterns are successfully interpreted by atomic models, via superposition of Ti layer on Pd(111). The moiré dots, resolved by STM at an atomic level, indicate seven-Ti atoms units in two configurations. Encapsulation of Pd nanocrystals by a TiOx layer can reduce the surface energy by 0.35±0.19 J/m2 for the dominant (111) facets from 1.64 J/m2 for bare Pd nanocrystals to 1.29±0.19 J/m2 for encapsulated Pd nanocrystals supported on anatase. Depending on the crystallographic interface of MoS2 with the SrTiO3 substrate, two shapes can be created: a regular triangle (MoS2(0001) ?SrTiO3(111)) and a right-angled triangle (MoS2(0001) ?SrTiO3(001)). MoS2 crystals grown on the symmetry-disparate (001) surface are more strained, as evidenced by the red-shift of the A peak in photoluminescence spectra and the softening of the E2g1 mode in Raman spectra. These findings provide an understanding of how epitaxy is achieved on SrTiO3 surfaces and of the interactions between these heterogeneous structures. This opens up a platform for the growth of tailored nanomaterials for electronics and catalysis applications.
Supervisor: Castell, Martin R. Sponsor: Chinese Scholarship Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available