Self-limiting Growth Mechanisms for Stable Monolayer Films of Non-van-der-Waals Oxides

高科技应用的进步和突破，从信息通信技术、照明到能源储存/发电和催化，都建立在材料沉积技术的基础上，在过去的几十年里，材料沉积技术已经从块状发展到薄膜，再到纳米技术。一种材料的单层代表了最薄的薄膜，从层状范德瓦尔斯（vdW）材料中分离和自下而上生长的原子薄单层突出了单层材料设计对基础研究和应用的巨大影响。虽然这种实验分离的二维层的目录一直在扩大，但焦点几乎完全集中在自然相分层的vdW固体上。在可及性和功能方面仍然存在较大的差距，例如在2.3-6 eV之间缺乏电子带隙。本提案的动机是探索新的科学和方法，如何实现非vdW材料的选择性成核和各向异性晶体生长，真正自我限制在单层，从而为迄今为止仅限于vdW材料的材料设计和原子水平上的性能定制开辟了新的可能性。迄今为止报道的方法一般分为两类：(1)通过与衬底（外延）或与两个界面（受限生长）的强相互作用来强迫材料的二维相，(2)通过在特定晶体结构中选择性蚀刻平面来实现二维分层，然后进行液体剥落（例如MAXene到MXene）。许多表面科学研究已经探索了方法(1)，其中最先进的方法是由于强支撑相互作用形成小（<50 nm）的准二维畴。这种生长模式既不是基本的自我限制，也没有提供从衬底上去除形成薄膜的可行途径，材料相在隔离状态下不稳定。所有关于方法(2)的文献都是基于薄片生产，薄片仍然是多层的，质量有限。鉴于其工业重要性，本提案将重点关注高k氧化物材料作为模型系统，不仅关注此类材料的基础研究，如晶体生长和非vdw单层的弛豫路线，而且还关注发现与未来几代电子产品技术相关的可扩展生产方法。在新的混合原子层/化学气相沉积（ALD/CVD）过程中，将使用In-operando方法直接探索氧化晶体膜的成核，相变和畴生长行为，然后通过远程外延进行转移和独立隔离。确定了非vdw材料的自限制单层的形成机制，将为全新一类材料的目标结构和性能设计开辟新的领域，包括相互掺杂和相互合金化策略。所提出的新型氧化物单层可用于各种各样的应用，包括非均相催化，如汽车转换器，自旋电子学的超薄屏障，量子阱荧光粉，集成气体传感器，光学涂层和电子产品中的高k介电体。

Progress and breakthroughs in high-tech applications, ranging from ICT, lighting to energy storage/generation and catalysis, builds on material deposition technology, which over the last decades has moved from bulk, to thin films to nanotechnology. A monolayer of a material represents the ultimate thinnest film possible, and the isolation and bottom-up growth of atomically-thin monolayer from layered van-der-Waals (vdW) materials highlight the huge impact both on fundamental research and applications that material design at the monolayer level can have. While the catalogue of such experimentally isolated 2D layers has been expanding, the focus has been almost exclusively on vdW solids for which the natural phase is layered. There remain large gaps in accessible properties and functionalities, such as the lack of electronic band gaps between 2.3-6 eV. The motivation for this proposal is to explore new science and approaches how to achieve selective nucleation and anisotropic crystal growth that is truly self-limiting to a monolayer for non-vdW materials, and thus open a new horizon of possibilities for materials design and tailoring of properties at the atomic level that hitherto have been limited to vdW materials. Reported approaches for this to date generally fall into two categories: (1) forcing a 2D phase of the material by strong interaction with the substrate (epitaxy) or with two interfaces (confined growth), (2) achieving a 2D layering by selective etching of planes in a specific crystal structure, followed by liquid exfoliation (e.g. MAXene to MXene). A number of surface science studies have explored approach (1) with the state-of-the-art being the formation of small (<50 nm) quasi-2D domains due to strong support interactions. Such growth modes are neither fundamentally self-limiting, nor do they offer viable routes to remove the as-formed film from the substrate, with the material phase not stable in isolation. All literature on approach (2) is based on flake production, which remain multi-layered and of limited quality. Given their industrial importance, this proposal will focus on high-k oxide materials as model system, not only on the fundamental studies of such materials, such as crystal growth and relaxation routes of non-vdW monolayers, but also on discovering scalable production approaches that are technologically relevant for future generations of electronics. In-operando methodology will be used to directly explore the nucleation, phase changes and domain growth behaviour of oxide crystal films during new hybrid atomic layer/chemical vapour deposition (ALD/CVD) processes, followed by their transfer and free-standing isolation through remote epitaxy. The identified mechanisms for the formation of self-limiting mono-layers of intrinsically non-vdW materials will open up a new playground for targeted structure and property design for a whole new class of materials, including inter-doping and inter-alloying strategies. The proposed novel oxide mono-layers can be exploited in a rich variety of applications, including in heterogeneous catalysis such as automotive converters, ultrathin barriers for spintronics, quantum-well phosphors, integrated gas sensors, optical coatings and high-k dielectrics in electronics.