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

对气候变化和化石燃料灭绝的关切最近使人们更加关注生产能源的替代方式，但也关注减少能源消耗的战略。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，通过采用“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，从而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前途的技术是在玻璃窗上涂上一层非常薄的改性氧化钒（VO 2）薄膜。这种氧化物在室温下不导电，已知在68摄氏度以上的温度下变成金属导体。这种转变可以通过引入一些杂质原子（例如钨）在室温下进行调节，并且伴随着材料光学性质的显着变化。因此，在炎热的天气下，涂层膜是金属的，并反射来自太阳的大部分红外辐射，保持内部凉爽，但仍然允许大部分可见光通过。在较冷的天气，窗户涂层转变回低温阶段，这允许更多的红外辐射通过，减少了对内部加热的需要。在这里，我建议采用先进的计算机模拟技术来调查一组现象与VO 2基窗户涂料的设计和功能。我将首先关注这项技术的基本和尚未解决的设计问题：如何掺杂VO 2薄膜的方式，不仅转变温度转移到所需的值，而且薄膜的颜色和薄膜的光学特性是商业用途可接受的。还将研究其他重要的相关现象。例如，最近的实验表明，金纳米颗粒的引入允许改变薄膜的颜色，这对于美学原因是重要的，因为钨掺杂的VO 2表现出相当令人不愉快的棕色/黄色阴影。甚至有人提出，掺杂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。我的目的是提供这些现象的微观描述。最后，我还想了解薄膜是如何粘附在窗户玻璃上的。当前薄膜的粘附性并不完美，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多功能的涂层技术。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研究成为可能，但VO 2属于一类化合物，这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是如此之强，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，在过去的几年中，强大而有效的新方法已经开发并在主流计算机代码中实现，第一次允许这些强相关固体的逼真建模。使用这些工具，我将能够在智能窗户涂层技术的基础上提供一系列令人兴奋的现象的微观描述。

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.