Towards a fundamental understanding of smart windows coating based on doped vanadium oxides

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Concerns about climate change and the extinction of fossil fuels have brought much recent attention to alternative ways of producing energy, but also to strategies to reduce energy consumption. It is estimated that the built environment consumes 30-40% of the primary energy in the world, most of which goes to cooling, heating and lighting. Recent research has demonstrated that it is possible to significantly reduce the energy utilisation in buildings by employing "smart" windows, which are capable of adapting to external weather conditions in a way that minimises the need for heating or air conditioning. A very promising technology to achieve this goal is based on coating glass windows with a very thin film of modified vanadium oxide (VO2). This oxide, which does not conduct electricity at room temperature, is known to become a metallic conductor at temperatures above 68 degrees Celsius. This transition can be tuned to take place at room temperature by introducing some impurity atoms (e.g. tungsten), and it is accompanied by a significant change in the optical properties of the material. Thus, in hot weather, the coating film is metallic and reflects most of the infrared radiation from the Sun, keeping the interior cool, but still allows most visible light to pass. During cooler weather the window coating transforms back to the low-temperature phase, which allows more of the infrared radiation to pass, decreasing the need for internal heating. In this way, large amounts of energy can be saved.I propose here to employ advanced computer simulation techniques to investigate a group of phenomena associated with the design and functioning of VO2-based window coatings. I will first focus on the fundamental and not-yet-resolved design problem for this technology: how to dope the VO2 films in a way that not only the transition temperature is shifted to the required value, but also the colour of the films and the optical properties of the film are acceptable for commercial use. Other important associated phenomena will also be investigated. For example, recent experiments have shown that the introduction of gold nanoparticles allows the modification of the colour of the films, which is important for aesthetic reasons, as tungsten-doped VO2 exhibits a rather unpleasant brown/yellow shade. It has even been suggested that doping with gold nanoparticles can decrease the switching temperature of the film, possibly due to electron transfer to the oxide. I aim to provide a microscopic description of these phenomena. Finally, I also want to understand how the films adhere to the window glass. The adherence of current films is not perfect, which can limit their durability or range of applications. So I want to gain insight into the microscopic factors controlling adhesion, with the hope that this knowledge will lead to more robust and versatile coating technologies.Although modern advances in computer power and theoretical algorithms have made possible the investigation of realistic models of many materials, VO2 belongs to a class of compounds which are particularly challenging for computational modelling. In these materials, which mainly include transition metal and rare earth compounds, the interactions between electrons are so strong that the typical independent-electron approximations employed in solid state calculations do not work well. However, in the last few years powerful and efficient new methods have been developed and implemented in mainstream computer codes, allowing for the first time a realistic modelling of these strongly correlated solids. Using these tools, I will be able to offer a microscopic description of the exciting range of phenomena at the basis of the smart windows coating technology.