In operando SEM for the accelerated exploration of nanomaterials and their scalable device integration
The development of scalable manufacturing and device integration strategies are critical bottlenecks that currently hinder the commercial application of low-dimensional nanomaterials. Bottom-up approaches such as chemical vapour deposition are promising for meeting such needs. However, the reliable production of high-quality electronics-grade material remains non-trivial, owing to the large parameter spaces and complex reaction mechanisms underlying such processes. Traditional ex situ microscopies, which rely on post-mortem characterisation of nanomaterials, offer limited insight. Therefore, there is a need to establish a more suitable in situ microscopy platform that permits (1) operando monitoring of reactions and (2) rapid screening of different process conditions (temperatures and pressures). Herein, the development and implementation of an environmental scanning electron microscope (ESEM) that is suitable for operando imaging of nanoscale dynamics at high temperatures and pressures of reactive gases is outlined. Key to the microscope's operation is a gas injection nozzle that allows high gas pressures to be locally generated at targeted locations on a substrate. Since commercial gas injection systems are incompatible with the requisite high temperatures, this thesis first reports on the fabrication of custom nozzles based on tapered quartz capillaries with opening diameters of ~µms. The spatial confinement of gas fluxes is studied through test particle Monte Carlo simulations and verified experimentally through the localised thermal oxidation of nickel films. The ESEM is subsequently applied to study two low-dimensional nanoscale systems for which manufacturing and/or device integration challenges remain: 1D silicon nanowires and 2D tungsten disulfide. For the former, in situ vacuum annealing is performed to visualise the growth dynamics of nickel-silicide phases into silicon nanowires (`silicidation'), a candidate method for establishing high-quality electrical contacts. However, controlling the phase and length of the formed silicide remains challenging. Here, ESEM imaging is combined with a novel nanowire geometry incorporating lateral extensions, allowing for reaction- and diffusion-limited regimes to be dynamically followed over micron length-scales. Based on a thorough understanding of the silicidation mechanisms, different nanowire geometries are proposed and tested that permit the silicide length to be tuned. Next, high-temperature ESEM imaging is combined with gas injection to study the thermal oxidation of 2D tunstgen disulfide. Together with complementary operando X-ray photoemission spectroscopy, the oxidation reaction and in particular, the formation of oxidative etch pits is monitored in real-time. Importantly, a high-throughput experimental design is introduced, whereby individual tungsten disulfide domains are selectively oxidised at a time through local gas injection, permitting multiple oxidation conditions to be readily explored on a single substrate. In addition, an automated image processing workflow is introduced, permitting up to 1E3 etch pits to be dynamically traced, allowing for sets of `big data' of the oxidation kinetics to be obtained. Based on this in situ analysis, a kinetic model describing oxidation at the nanoscale is proposed and tested for the first time. Finally, an in situ investigation of the chemical vapour deposition of tungsten disulfide is presented. The focus herein is to establish suitable model systems with which the complex growth modes may be controllably and systematically studied. For this reason, simplified growth reactions involving the single-step sulfidation of pre-deposited tungsten using low-cost and low-toxicity dimethyl disulfide are selected. Two approaches are presented: (1) metal-organic chemical vapour deposition on gold foils and (2) on silicon dioxide substrates from molten sodium tungstate salts. For the former, the competitive deposition of amorphous carbon on gold is found to increase for higher dimethyl disulfide pressures, causing tungsten disulfide growth to terminate. For the latter, the unique growth modes on liquid substrates, such as the free rotation of tungsten disulfide domains, are confirmed for the first time via real-time imaging. The work described herein demonstrates a versatile in situ ESEM platform capable of high-throughput experimentation, representing a key stepping stone towards establishing scalable manufacturing and device integration approaches for nanomaterials.
low-dimensional nanomaterials, scanning electron microscopy
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