Aerosol-Assisted Chemical Vapour Deposition of Inorganic Functional Materials

ABSTRACT:
Continued success of the electronic and energy sectors is driven by technological advances in the semiconductor industry. In layered combination, electron donating (n-type) and electron accepting (p-type) semiconductor materials form heterojunctions, which are the bedrock of transistor and photovoltaic devices. The miniaturisation of such devices has prompted a recent shift away from classical silicon-based materials, of which thin films are challenging to fabricate. As an alternative, the capabilities of both organic polymers and metal-based materials have been investigated, with the latter being favoured for their durability. Consequently, the fabrication of intricate layered architectures comprising inorganic semiconductors is of growing importance. Aerosol-assisted chemical vapour deposition (AACVD) is a promising fabrication technique. Aerosolisation of precursor solutions mitigates the volatility required for typical chemical vapour deposition methods, broadening the range of possible deposition precursors available.
It is proposed to survey the thermal properties of and thin films produced from, a series of metal chalcogenide molecular precursors for AACVD; thus to redress the relative lack in available precursors for formulation of inorganic p-type semiconductors. Initial attention will be focused upon the formulation of group 15 chalcogenide systems (M2X3, M = As, Sb, Bi and X = S, Se, Te), which are currently underdeveloped. More specifically, it is proposed initially to examine group 15 molecular complexes with xanthate-derived ligands, the deposition mechanism of which has been pre-characterised. The work may expand to other metals, for example tin or zinc. The aim is to yield a series of precursor chemicals viable for commercial AACVD scaleup,
to assist the construction of both constituents of an inorganic heterojunction. Future work can interrogate parameters controlling aerosol-mediated deposition, probing the influence of droplet size and solvent on film morphology.

Aerosol science has a significant impact on a broad range of disciplines, extending from inhaled drug delivery, to combustion science and its health impacts, aerosol assisted routes to materials, climate change, and the delivery of agricultural and consumer products. Estimates of the global aerosol market size suggest it will reach $84 billion/year by 2024 with products in the personal care, household, automotive, food, paints and medical sectors. Air pollution leads to an estimated 30-40,000 premature deaths each year in the UK, and aerosols transmit human and animal infections. More than 12 million people in the UK live with lung disease such as asthma, and the NHS spends ~£5 billion/year on respiratory therapies. Many of the technological, societal and health challenges central to these areas rely on core skills and knowledge of aerosol science. Despite this, an Industrial Workshop and online survey (held in preparation for this bid) highlighted the current doctoral skills gap in aerosol science in the UK. Participating industries reported that only 15% of their employees working with aerosol science at doctoral-level having received any formal training. A CDT in aerosol science, CAS, will fill this skills gap, impacting on all areas of science where core training in aerosol science is crucial.

Impact on the UK aerosol community: Aerosol scientists work across governmental policy, industrial research and innovation, and in academia. Despite the considerable overlap in training needs for researchers working in these diverse sectors, current doctoral training in aerosol science is fragmentary and ad hoc (e.g. the annual Fundamentals of Aerosol Science course delivered by the Aerosol Society). In addition, training occurs within the context of individual disciplines, reinforcing artificial subject boundaries. CAS will bring coherence to training in the core physical and engineering science of aerosols, catalysing new synergies in research, and providing a focal point for training a multidisciplinary community of researchers. Working with the Aerosol Society, we will establish a legacy by providing training resources for future researchers through an online training portal.

Impact on industry and public-sector partners: 45 organisations have indicated they will act as CAS partners with interests in respiratory therapies, public health, materials manufacturing, consumer and agricultural products, instrumentation, emissions and environment. Establishing CAS will deliver researchers with the necessary skills to ensure the UK establishes and sustains a scientific and technical lead in their sectors. Further, it will provide an ideal mechanism for delivering Continuing Professional Development for the existing workforce practitioners. The activity of CAS is aligned to the Industrial Strategy Challenge Fund (e.g. through developing new healthcare technologies and new materials) and the EPSRC Prosperity Outcomes of a productive, healthy (e.g. novel treatments for respiratory disease) and resilient (e.g. adaptations to climate change, air quality) nation, with both the skilled researchers and their science naturally translating to long-lasting impact. Additionally, rigorous training in responsible innovation and ethical standards will lead to aerosol researchers able to contribute to developing: regulatory standards for medicines; policy on air quality and climate geoengineering; and regulations on manufactured nano-materials.

Public engagement: CAS will provide a focal point for engaging the public on topics in aerosol science that affect our daily lives (consumer products, materials) through to our health (inhalation therapeutics, disease transmission and impacts of pollution) and the future of our planet (geoengineering). Supported by a rigorous doctoral level training in aerosol science, this next generation of researchers will be ideally positioned to lead debates on all of these societal and technological challenges.