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Title: Characterisation of encapsulation grown nanowires
Author: Marks, Samuel R.
ISNI:       0000 0004 7425 5727
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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The push to decrease the size of electronic components has been pioneered over the last decade. This has led to electrodeposition being a primary nanoscale fabrication method. However, this technique is beginning to reach its physical limits with respect to the size of media that can be deposited. Therefore, alternative methods of sub 10 nm fabrication are being investigated. This thesis aims to explore: supercritical fluid electrodeposition as an encapsulated nanowire fabrication method, the phase transitions involved when heating said nanowires in situ and the encapsulation of materials within carbon nanotubes to form truly one-dimensional nanowires. All of this is undertaken using electron microscopy as the primary analysis technique. First, the supercritical fluid electrodeposition process is investigated as a nanowire fabrication technique. Initially, Ge planar films are deposited and shown to form electron beam sensitive crystallites embedded within an amorphous Ge matrix. This is expanded upon, with the deposition of Ge into 13 nm anodic alumina pores, also resulting in the formation of amorphous Ge nanowires. More advanced systems are explored, with crystalline CuTe nanowires forming in the P4/nmm space group. Further to this, the CuTeS system, the first supercritical fluid electrodeposited tertiary system, is proved to form in the Cu6Te3S structure. Sn is deposited into hierarchical alumina in attempts to decrease the encapsulated media size. This shows the ability for sub 10 nm nanowire formation from supercritical fluid electrodeposition. Next, the effects of in situ heating for both Te and Bi nanowires are presented. Here the Te system underwent a sublimation phase transition. The experimental rate of sublimation is imperfect and generates an evaporation coefficient of 2 x 10-3 as a multiplying factor. The effects of elemental contamination manifests in two forms during sublimation. The first as unmoving large masses that slow the rate of sublimation. The second, as a small atomic percentage that flows along the nanowire, at the sublimation front, before condensing in the end of the nanowire. Sublimation is not the only observed phase transition with Bi proven to melt in situ becoming an encapsulated liquid. Selected area diffraction, with radial distribution analysis, results in the first liquid Bi radial peak measured at 3.47 Å. This is akin to bulk neutron diffraction and XRD measurements, however, this is believed to be the first nanoscale measurement. Examination of the suspected pressure drop arising from the remnant alumina proves that the alumina coatings are non-continuous, as no experimental pressure drop is observed. Finally, the encapsulation of materials within carbon nanotubes is presented. It is demonstrated that the melt filling, from Ge and SbTe, will form crystalline bulk-like nanowires during encapsulation. The effects of electron beam interaction are visible, with energetic encapsulated crystallites. A study of the encapsulated SbTe indicates that for an 80kV electron beam, the threshold for amorphisation, due to electron beam heating, lies between a beam energy of 0.8 and 1.5 pA cm-2. Striving for higher filling percentages the sublimation filling technique is examined. For the case of Te, both a bulk-like helix and one-dimensional chain structure is observed across a range of nanowire diameters. Additionally, SnTe is formed generating a one-dimensional chain, for low dimension carbon nanotubes, and a zigzag structure within higher diameters. The chemical composition of both systems is examined using EELS. This proves the Te and zigzag SnTe chemical compositions, but suggests that the SnTe one-dimensional chain is SnI. In order to characterise the crystal structures ab initio random structure searches are performed for the first time. These result in a new level of structural understanding arising from this first order simulation process.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics