Saghir, Mohammad (2016) Topological insulators : a study of bulk crystalline and nanomaterials. PhD thesis, University of Warwick.

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Abstract

A new class of materials, TIs and TCIs, have been shown to exhibit exotic surface state properties that are protected by mirror or time-reversal symmetry. It is expected that the surface states will be easier to detect if the SAVR of the material increases. We report the experimental procedures to obtain high quality crystal boules of the TCI, Sn1-x InxTe, for 0 ≤ x ≤ 0:45,from which nanowires and microcrystals can be produced by the VLS growth technique. Detailed characterisation measurements of the bulk crystals, the superconducting properties as well as characterisation of the nanowires and microcrystals produced are presented. We also present optimised growth procedures to obtain high quality bulk crystals of the TCIs Pb1-xSnxTe and Pb1-xSnxSe, and nanowires from the bulk crystals, also using a VLS growth mechanism. Nanowires of Pb1-xSnxTe have been produced with a Sn composition of x = 0:25, at which a transition from trivial to non-trivial insulator is reported. The results obtained on the growth of nanomaterials of Pb1-xSnxSe are also described. Detailed characterisation of the bulk crystals and the nanomaterials through x-ray diffraction, microscopy techniques and EDX analysis are presented. Sb2Te3 is also a topological insulator which, under certain conditions, becomes superconducting. The growth methods and characterisation of Sb2Te3 crystal boules are discussed. The methods used to convert bulk samples of Sb2Te3 into nanomaterials and 2D layers on graphene are also presented. Investigating nanometer and micron sized materials thought to exhibit topological surface properties can present a challenge, as clean surfaces are a pre-requisite for band structure measurements when using nano-ARPES or laser-ARPES in UHV. We present the findings of an XPS study where various cleaning methods have been employed to reduce the surface contamination and preserve the surface quality for surface sensitive measurements. Microcrystals of SnTe were treated with atomic hydrogen, argon sputtering, annealing, as well as a combination of treatments. The samples were characterised using SEM, both before and after treatment. It was found that atomic hydrogen cleaning with an anneal cycle (200 [degrees]C) gave the best clean surface results.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Surfaces (Physics), Nanostructured materials
Official Date:	July 2016
Dates:	Date Event July 2016 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Balakrishnan, Geetha ; Wilson, Neil R.
Extent:	xxii, 116 leaves : illustrations (colour), charts
Language:	eng

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