Energy security and the scale of change needed to meet net-zero targets are top of the global political agenda. Up to 70% of worldwide energy is currently lost as heat and this is expected to improve to just 50% by 2030, representing vast quantities of wasted energy and unnecessary greenhouse gas emissions. This creates an urgent need for technologies to improve energy efficiency, which would make low-carbon energy go further and provide "breathing space" to develop and scale-up other net-zero technologies such as electric vehicles, grid storage and carbon capture. Thermoelectric generators (TEGs) harness the Seebeck effect in a thermoelectric material to recover waste heat as electricity, and are a front-running technology for improving the efficiency of energy-intensive processes. TEGs are flexible and can, with the right materials, be used at scales from powering wearable devices to recovering waste heat from industrial furnaces. However, despite established applications in the aerospace industry and an estimated $1.2bn global market by 2027, large-scale thermoelectric power is currently not feasible due to limited efficiency and the scarcity and toxicity of the materials used. High-performance thermoelectric materials must balance a large Seebeck coefficient and electrical conductivity with a low thermal conductivity. This makes optimising the thermoelectric figure of merit, ZT, a complex interdisciplinary materials-design challenge. Furthermore, a high-performance thermoelectric not only requires high efficiency (large ZT), but must also meet cost and sustainability requirements for the intended application. With this in mind, progress in thermoelectrics has historically been limited by a poor understanding of thermal conductivity and how to engineer it, which has led to unsustainable materials made from rare and/or toxic elements such as the current industry standards Bi2Te3 and PbTe. The initial part of the Future Leaders Fellowship has made use of state-of-the-art computational materials modelling to address two key questions: What are the key microscopic processes that suppress or enhance heat transport in materials, and how can we use materials design and engineering to control them? In doing so, the Fellow and team have established a cutting-edge research programme on thermal conductivity and thermoelectrics, which has provided new fundamental insight into the heat transport in thermoelectrics together with modelling tools to make accurate predictions of the thermal conductivity, electrical properties and ZT of a wide range of thermoelectric materials. Over the three years of the FLF Renewal period, we will exploit these developments to design efficient, cost-effective and sustainable materials for thermoelectric power and related renewable-energy applications including photovoltaics (solar cells). By working with experimental collaborators and industrial partners, we will further seek to progress these materials from predictions to prototype devices. In parallel, we will also consolidate the developments from across the FLF to make them as widely accessible as possible to the project beneficiaries, and thus maximise the impact of the research programme. The continuation of the research programme will deliver thermoelectric materials suitable for widespread commercialisation and enable the UK to reap the economic benefits of the $560bn global market for energy efficiency while contributing to the UN Sustainable Development Goals of affordable and clean energy, climate action, and reducing poverty. The improved ability to control heat transport enabled by this programme will also benefit other technologies, for example more efficient solar cells, better thermal management in batteries and improved power electronics and silicon chips, providing considerable scope to maintain and grow our research beyond the Fellowship.

Hexagonal boron nitride (hBN) is currently attracting international attention due to its technological potential for deep ultraviolet (UV) photonics, single photon emission and its incorporation into van der Waals (vdW) heterostructures. hBN is a layered material in which strong covalent bonds between boron and nitrogen atoms stabilise a planar honeycomb atomic arrangement. In its bulk form hBN consists of many such planes stacked on top of each other, and, like graphite, layers of hBN with thickness down to a single monolayer can be exfoliated from bulk crystals. There have been many demonstrations showing that exfoliated hBN layers can be combined with other layered materials, for example graphene, to form 'van der Waals heterostructures' in which hBN acts as a tunnel barrier, substrate or gate dielectric. The interest in hBN has motivated many groups to explore the growth of thin films and monolayers of hBN using various techniques, but it has proved difficult to reproduce the optical and electrical properties of the highest-quality mm-scale bulk hBN crystals, which are grown by our Project Partners in Tsukuba (Japan). In a recent breakthrough, we demonstrated that hBN can be grown using high-temperature molecular beam epitaxy (HT-MBE) and that layers grown using this technique have unprecedented optical quality with strong luminescence in the deep UV region with a photon energy, for monolayer thickness, of 6.08 eV. This high photon energy offers the prospect of solid-state devices emitting light in the UV-C range, which is known to be relevant to water purification and surface sterilisation. In collaboration with Australian academics, we have also shown that single photon emitters can be formed in our hBN material. In addition, we have demonstrated the growth of lateral heterostructures of graphene and boron nitride, in which the composition varies within a single monolayer. These structures are predicted to have novel electronic and magnetic properties. In order to build on our promising early results and realise the technological potential of hBN, we now propose to advance our understanding of the relevant growth mechanisms and explore, both in Nottingham and through our network of international collaborations, the technological opportunities provided by high quality hBN monolayers and thin films. Our hypothesis is that HT-MBE provides a route to the scalable growth of high-quality hBN layers, which have the potential for technological exploitation in the areas of deep UV photonics, single photon sources and vdW heterostructures, as well as the exploration of the electronic properties of hBN edge states and lateral heterojunctions. In our research programme we will investigate and optimise HT-MBE growth of hBN. In addition, we will explore doping of hBN and the formation of simple optoelectronic devices, as well as the growth of hBN-based alloys of BNC and BNSi as a route to the spontaneous formation of phase separated nanostructures and band gap engineering. In addition, we will establish the relationship between growth parameters and the formation of carbon-induced single photon emitters in hBN. To determine the potential of HT-MBE-grown hBN for deep UV photonics, we will fabricate prototype devices operating at UV-C wavelengths. We will also utilise epitaxial hBN to study the formation and structure of lateral hBN/graphene heterojunctions and investigate the emergence of novel electronic and magnetic effects in these structures due to electron-electron interactions. An important further objective is to demonstrate the scalable growth of hBN on large area substrates, which are commercially available, for example sapphire and silicon carbide, so that the hBN layers are compatible with processing and fabrication techniques, which are used widely in industry.

Light emitting semiconductor materials and devices dominate many aspects of everyday life. Their influence is all pervasive providing the sources which enable the internet, large area displays, room and street lighting to give just a few examples. Their existence relies on the high quality semiconductor structures which may be prepared by advanced crystal growth and sophisticated nanofabrication. Our proposal aims to capitalise on the advanced growth and fabrication to achieve similar advances in the quantum world where often counter-intuitive behaviour is governed solely by the laws of quantum mechanics. Our overall aim is to explore the behaviour of nano-devices operating in regimes where fundamentally new types of quantum-photonic phenomena occur, with potential to underpin the next generation of quantum technologies. We focus on two complementary systems: III-V semiconductors with their highly perfect crystal lattices, proven ability to emit photons one by one and long coherence quantum states, and atomically-thin graphene-like two dimensional (2D) semiconductors enabling new band structures, stable electron-hole bound states (excitons) and easy integration with patterned structures. The combination of the two material systems is powerful enabling phenomena ranging from the single photon level up to dense many-particle states where interactions dominate. A significant part of our programme focusses on on-chip geometries, enabling scale-up as likely required for applications. The semiconductor systems we employ interact strongly with photons; we will achieve interactions between photons which normally do not interact. We will gain entry into the regime of highly non-linear cavity quantum electrodynamics. Excitons (coupled electron-hole pairs) and photons interact strongly, enabling ladders of energy levels leading to on-chip production of few photon states. By coupling cavities together, we will aim for highly correlated states of photons. These advances are likely to be important components of photonic quantum processors and quantum communication systems. In similar structures, we access regimes of high density where electrons and holes condense into highly populated states (condensates). We aim to answer long-standing fundamental questions about the types of phase transitions that can occur in equilibrium systems and in out-of-equilibrium ones which have loss balanced by gain. We will also study condensate systems up to high temperatures, potentially in excess of 100K, and of the mechanisms underlying phase transitions to condensed states. The condensed state systems, besides their fundamental interest, also have potential as new forms of miniature coherent light sources. Nanofabrication will play a vital role enabling confinement of light on sub-wavelength length scales and fabrication of cavities for photons such that they have very long lifetimes before escaping. The ability to place high quality emitters within III-V nanophotonic structures will receive enhancement and potential world lead from a crystal growth machine we have recently commissioned, specially designed for this purpose, funded by the UK Quantum Technologies programme. Similar impact is expected from our ability to prepare 2D heterostructures (atomically thin layers of two separate materials placed one on top of the other) under conditions of ultrahigh vacuum free from contamination, enabling realisation of bound electron-hole pair states of very long lifetime, the route to condensation to high density states. The easy integration of 2D heterostructures with patterned photonic structures furthermore enables nonlinear and quantum phenomena to be studied, including in topological structures where light flow is immune to scattering by defects. Taken all together we have the ingredients in place to achieve ground-breaking advances in fundamental quantum photonics with considerable potential to underpin next generations of quantum technologies.

Quantum technologies exploit the intriguing properties of matter and light that emerge when the randomizing processes of everyday situations are subdued. Particles then behave like waves and, like the photons in a laser beam, can be split and recombined to show interference, providing sensing mechanisms of exquisite sensitivity and clocks of exceptional accuracy. Quantum measurements affect the systems they measure, and guarantee communication security by destroying cryptographic keys as they are used. The entanglement of different atoms, photons or circuits allows massively powerful computation that promises complex optimizations, ultrafast database searches and elusive mathematical solutions. These quantum technologies, which EPSRC has declared one of its four Mission-Inspired priorities, promise in the near future to stand alongside electronics and laser optics as a major technological resource. In this 'second quantum revolution', a burgeoning quantum technology industry is translating academic research and laboratory prototypes into practical devices. Our commercial partners - global corporations, government agencies, SMEs, start-ups, a recruitment agency and VC fund - have identified a consistent need for hundreds of doctoral graduates who combine deep understanding of quantum science with engineering competence, systems insight and a commercial head. With our partners' guidance, we have designed an exciting programme of taught modules to develop knowledge, skills and awareness beyond the provision of traditional science-focused PhD programmes. While pursuing leading-edge research in quantum science and engineering, graduate students in the EPSRC CDT for Quantum Technology Engineering will follow a mix of lectures, practical assignments and team work, peer learning, workshops, and talks by our commercial partners. They will strengthen their scientific and engineering capabilities, develop their computing and practical workshop skills, study systems engineering and nanofabrication, project and risk management and a range of commercial topics, and receive professional coaching in communication and presentation. An industrial placement and extended study visit will give them experience of the commercial environment and global links in their chosen area, and they will have support and opportunities to break their studies to explore the commercialization of research inventions. A QT Enterprise Club will provide fresh, practical entrepreneurship advice, as well as a forum for local businesses to exchange experience and expertise. The CDT will foster an atmosphere of team working and collaboration, with a variety of group exercises and projects and constant encouragement to learn from and about each other. Students will act as mentors to junior colleagues, and be encouraged to take an active interest in each other's research. They will benefit from the diversity of their peers' backgrounds, across not just academic disciplines but also career stages, with industry secondees and part-time students bringing rich experience and complementary expertise. Students will draw upon the wealth of experience, across all corners of quantum technologies and their underpinning science and techniques, provided by Southampton's departments of Physics & Astronomy, Engineering, Electronics & Computer Science, Chemistry and its Optoelectronics Research Centre. They will be given training and opening credit for the Zepler Institute's nanofabrication facilities, and access to the inertial testing facilities of the Institute of Sound & Vibration research and the trials facilities of the National Oceanography Centre. Our aim is that graduates of the CDT will possess not only a doctorate in the exciting field of quantum technology, but a wealth of knowledge, skills and awareness of the scientific, technical and commercial topics they will need in their future careers to propel quantum technologies to commercial success.