Atomic-scale wetting and growth dynamics of encapsulated metallic nanowires

Thesis

Encapsulation within carbon nanotubes (CNTs) offers a route to stabilise metallic nanowires against oxidation, coarsening, and structural degradation, while providing a confined environment that enables unique one-dimensional (1D) phenomena. Yet, the fundamental mechanism of vapour-phase nanowire growth inside CNTs remains poorly understood - particularly the role of wetting under nanoscale confinement, where classical capillarity models break down. This thesis investigates how atomic-scale wetting governs the vapour-phase encapsulation of Sn within the core of multi-walled CNTs, with the aim of establishing a predictive framework for confined and continuous metallic nanowire growth.

A vapour-phase pathway was developed using SnO precursors, where oxygen enhances interfacial wetting and promotes the formation of nanoscale droplets that act as growth reservoirs. In situ atomic-resolution transmission electron microscopy (ARTEM) was employed to capture condensation, wetting, and solidification events directly at atomic scale. To extend these observations beyond single events, a convolutional neural network (CNN) was trained to classify liquid, solid, and crystalline intermediate SnₓO phases across hundreds of high-resolution TEM frames, enabling quantitative, statistical mapping of growth dynamics across multiple SnO nanowires.

The results reveal that nanowire formation follows a two-stage mechanism: nucleation at CNT openings, facilitated by curvature-enhanced condensation, followed by capillarydriven elongation sustained by continuous vapour influx. Growth requires the establishment of a wetting interface (contact angle, q < 90°), demonstrating that nanoscale wetting, rather than bulk surface tension, dictates whether encapsulation proceeds. These findings highlight the limitations of Kelvin and Lucas–Washburn models, showing instead that nucleation kinetics, vapour flux, and interfacial condensation govern time-dependent filling. By unifying in situ imaging, AI-assisted analysis, and mechanistic modelling, this thesis establishes a framework for predicting nanowire growth under confinement.

 Beyond Sn nanowires, this framework provides a generalisable pathway for the synthesis of 1D nanostructures with tailored crystallinity, morphology, and stability, offering new opportunities for the scalable fabrication of these materials for energy storage, catalysis, nanoelectronics, and quantum technologies.

Authors: Tebbutt, G

• DOI: 10.5287/ora-erjno7rn4
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: encapsulation mechanism; carbon nanotubes; metallic nanowires; carbon fibers; in situ transmission electron microscopy
• Subjects: Carbon fibers; Nanowires; Carbon nanotubes; Transmission electron microscopy