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

对气候变化和化石燃料消亡的担忧，不仅引起了人们对替代能源生产方式的关注，也引起了人们对减少能源消耗的战略的关注。据估计，建筑环境消耗了世界上30%-40%的一次能源，其中大部分用于制冷、供暖和照明。最近的研究表明，通过使用“智能”窗户，可以显著降低建筑物的能源利用率，这种窗户能够适应外部天气条件，最大限度地减少对供暖或空调的需求。实现这一目标的一项非常有希望的技术是在玻璃窗上涂上一层非常薄的改性钒氧化物(VO2)。这种氧化物在室温下不导电，但已知在68摄氏度以上的温度下会成为金属导体。这种转变可以通过引入一些杂质原子(如钨)在室温下进行，并且伴随着材料的光学性质的显著变化。因此，在炎热的天气，涂层是金属的，反射来自太阳的大部分红外辐射，保持室内凉爽，但仍允许大多数可见光通过。在较凉爽的天气中，窗户涂层会转换回低温阶段，允许更多的红外线辐射通过，减少对内部加热的需求。在这里，我建议使用先进的计算机模拟技术来研究与VO2基窗涂层的设计和功能相关的一组现象。我首先将重点放在这项技术的基本和尚未解决的设计问题上：如何在VO2薄膜中掺杂，使其不仅转变温度达到所需的值，而且薄膜的颜色和薄膜的光学性质都是商业上可以接受的。其他重要的相关现象也将被调查。例如，最近的实验表明，引入金纳米颗粒可以改变薄膜的颜色，这对于美学来说很重要，因为钨掺杂的VO2表现出相当不舒服的棕色/黄色。甚至有人提出，掺入金纳米颗粒可以降低薄膜的开关温度，这可能是由于电子转移到氧化物。我的目标是提供这些现象的微观描述。最后，我还想了解一下薄膜是如何附着在窗户玻璃上的。目前薄膜的附着力并不完美，这可能会限制其耐用性或应用范围。因此，我想深入了解控制附着力的微观因素，希望这些知识将导致更强大和通用的涂层技术。尽管计算机能力和理论算法的现代进步使许多材料的真实模型的研究成为可能，但VO2属于一类化合物，对计算建模特别具有挑战性。在这些材料中，主要包括过渡金属和稀土化合物，电子之间的相互作用如此强烈，以至于在固态计算中使用的典型的独立电子近似不能很好地工作。然而，在过去的几年里，在主流计算机代码中开发和实施了强大而高效的新方法，首次允许对这些强关联固体进行逼真的建模。使用这些工具，我将能够提供关于智能窗户涂层技术基础上令人兴奋的一系列现象的微观描述。

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.