二维铁电材料光致相变的机制研究

Ferroelectric materials hold great potential to serve as fast and high efficient phase change memory materials in the future, which have been receiving tremendous attention. With the emergent requirements of the miniaturization of high efficient devices with large data density, it is highly important to develop nanoscale two-dimensional ferroelectric materials and investigate their phase transition. One famous example of two-dimensional phase change materials is transition metal dichalcogenide monolayers, such as MoS2. Current theoretical and experimental studies showed that one can apply strain, carrier doping, or electrostatic gate voltage to realize phase transition of transition metal dichalcogenide monolayers. All these strategies need to either tips or electrochemical approaches, which would contact the samples and influence their properties. Therefore, it is intriguing to develop "non-contacting" schemes to trigger phase transition of ferroelectric materials, which is of great importance in the next-generation two-dimensional phase change materials. In this proposal, we propose a "non-contacting" opto-mechanical scheme to change the free energy of materials, based on thermodynamic theory. We will discuss optics driven phase transition mechanism and process of two-dimensional ferroelectric materials. Focusing on the experimentally realizable two-dimensional ferroelectric materials, we will perform first-principles density functional theory calculation as well as many body theory, to unravel the relationship between ferroelectricity and anisotropic optical response of these materials. We will also study the free energy variations under linearly polarized light exposure, and analyze phase transition energy barrier and kinetics. Based on these, we will establish mechanism of optics driven phase transition of two-dimensional ferroelectric materials, and design novel experimentally realistic phase transition of two-dimensional ferroelectric materials. This proposal could extend the knowledge of opto-mechanical driven phase transition, help us find novel structural phase change materials, motivate new experimental works in this field, and provide theoretical basis for novel phase change memory devices.

铁电材料承载着实现未来高效、快速相变存储器的可能性，一直以来受到人们的广泛关注。随着对器件微型化、高效率、高密度等方面的要求越来越迫切，对纳米尺度的二维铁电材料及其相变机制的研究具有至关重要的意义。现有对二维材料（如MoS2单层等）的相变主要通过应变、载流子掺杂、门电压等手段来实现，这些方法均需要与样品直接接触。本课题在热力学理论基础上，研究新型“非接触式”光致相变的机理和模型。我们将围绕实验上可行的铁电二维材料，通过第一性原理密度泛函和多体理论计算，揭示铁电极化导致的材料各向异性与材料对不同方向光响应的机理，研究线偏振光对铁电材料自由能的调控，探讨光照对相变能垒和相变动力学的影响，建立光致二维铁电材料的相变机理和相变过程，从理论上寻找和提出实验上可行的二维铁电相变材料及光控方案。本项目的开展，有利于获得新型结构相变材料，扩展光致相变的理论，为开发新型二维相变存储器材料提供理论基础。

铁电材料具有在下一代存储器件和传感器等多领域中的广泛潜在应用，近年来低维铁电材料在理论和实验研究上均取得了一系列有趣的进展。为了进一步探讨二维铁电材料的结构相变，人们通常使用电学手段来实现结构和极化的翻转。在本项目中，我们主要探讨二维铁电材料及其衍生材料的光学诱导相变方案，它可以避免电学手段带来的杂质和无序等问题。我们通过第一性原理计算了一系列二维硫族铁电材料的物理化学性质，包括结构稳定性，力学、电学、光学等方面的响应。这些计算均有助于进一步深入理解二维铁电材料，从而实现其潜在应用价值。具体来说，我们建立了非线性光学响应热力学理论，在热力学上计算了中等强度太赫兹-红外光照下这些材料的光电响应函数，阐明了光致材料应变和相变的微观过程，探索了兼具结构稳定性和强烈光-力-电耦合的结构模式，讨论了中等强度低频光照对材料形变和相变的可行性。我们的计算集中在具有共振型键结构的硫族化合物（如GeS、SnS、GeSe、SnSe、SnTe、MoTe2、WTe2、In2Se3等）的光致结构变形和相变。我们的理论分析和计算表明，中等强度的光照可以显著地改变这些材料的不同结构（稳定和亚稳）相的热力学相对稳定性，同时光照可以大幅度地降低相变过程中的能垒，从而导致皮秒级超快的形变和相变。另外，我们厘清了相变过程中的微观机理，指出这种低频光致相变的非线性光学过程，并且撰写了含时从头算分子动力学程序，证实了这一微观过程。该项目有助于人们理解二维铁电材料的光学响应以及非接触式信息存储材料的研究。