应力致石墨烯类二维晶体奇异形变/相变特征及其机制研究

For graphene-like two-dimensional crystals with planar structure of only several monolayers in thickness, they should be attached onto some heterogeneous substrates, such as, silicon covered with hafnium dioxide, in practical applications. Stress can not be avoided in such a case due to thermal misfit, structural misfit as well as external loading, which will often induce the structural breakage and even the failure of device function. So it is indeed one of the fateful issues for the reliability and stability of these ultra-thin devices and, should be paid more attention. This proposal will focus on the stress induced anomalous plastic deformation/phase transformation as a result of the competition between bond rotation and fracture, in particular, the behaviors different from those of traditional three-dimensional materials. Based on the statics analysis and molecular dynamics (MD) simulation, we will emphasize on the nucleation, movement, multiplication and annihilation of Stone-Wales (SW) topological defects under different temperature and loading conditions, uncover the relevant thermal and kinetic inducements for brittle-to-plastic transformation, and understand the genaral mechanism and the influencing factors of the plastic deformation/phase transformation in these graphene-like two-dimensional materials. Taking the characteristic deformation structures obtained from MD simulation as references, the models will be established to calculate the phonon dispersion relation of lattice vibration and the energy band structure according to first-principle density functional theory. Based on the results, we will try to find the fingerprint signature between the characteristic deformation structures and the Raman scattering peaks as well as the scanning tunneling spectroscopy (STS), to develop the experimental method and technique to characterize the stress-induced plastic deformation/phase transformation in graphene-like two-dimensional crystals. According to the in-situ Raman spectroscopy measurement during micro-tensile loading process, ex-situ STS and HRTEM observation, we try to confirm the modelling results from the point of view of experiment. All these will help to the design of future nano-scale electronic devices.

石墨烯类二维晶体使用时通常要依附于异质基体表面，因热失配、结构失配或外载而引入的应力是此类超薄器件安全可靠服役的致命问题。本项目针对这种超薄二维体系，拟结合静力学分析和分子动力学模拟，考察因化学键旋转与断裂竞比可能引发的有别于传统三维晶体的应力致形变/相变行为；研究不同温度和外载条件下Stone-Wales(SW)拓扑缺陷的动态演化过程，揭示其脆性-塑性变形转变的热/动力学诱因，形成对二维晶体形变/相变机制及其制约因素的普适性认识；参照分子动力学模拟所观察到的特征变形结构，计算相对应的晶格振动声子谱和电子能带结构，建立特征形变/相变结构与Raman光谱特征峰和扫描隧道谱（STS）之间的"指纹"识别关系，发展和完善二维晶体形变/相变行为的实验表征方法；结合微拉伸加载过程的Raman光谱和STS表征及后续HRTEM观察，对理论模拟结果予以验证，为未来纳米电子器件的原理设计提供物理参照。

因奇特的物理特性和机械性能，石墨烯类二维晶体成为近十年来学术界高度关注的材料体系。使用时此类超薄材料通常要依附于异质基体表面。由于热失配、结构失配或外载而引入的应力是此类超薄器件安全可靠服役的致命问题。本项目以石墨烯为典型研究范例，深入考察了石墨烯类蜂窝状二维晶体在外载力场作引发的奇异形变/相变行为，已顺利完成了各项研究内容，达到预期目标。部分研究结果已在Carbon，Appl Phys Lett，Nanotechnology，RSC Adv，Phys Chem Chem Phys等国际期刊发表学术论文43篇，其中SCI检索41篇，EI检索2篇；申请国家技术发明专利2件，授权技术发明专利2件；培养博士生2名，硕士研究生5名；应邀参加相关学术会议并作特邀报告6人次。取得的重要进展有：(1) 阐明了石墨烯类二维晶体形变/相变机制与SW 拓扑缺陷集体协同运动的因果关系及其对材料特征尺寸、晶体取向和外载条件等因素的依赖。(2) 发现晶界附近沿扶手椅取向往往发生晶格剪切变形，且逐渐向晶内扩展，导致在局部区域发生由六角晶格到斜方晶格的转变；纳米晶石墨烯的断裂应力和断裂应变几乎与晶粒尺寸无关，而其弹性模量却随晶粒尺寸的减小而减小，表明晶界原子行为的决定性作用。(3) STM和Raman光谱表征发现，6H-SiC高温热解石墨烯时其表面缓冲层结构在保持长程有序性的前提下，其短程原子结构历经了从有序到无序再到有序的过程，且制备的外延石墨烯以扶手椅型边界为主，电子的干涉效应导致边界附近呈现周期性振荡条纹。(4) 针对石墨烯与金属交互作用的研究表明，金属原子趋于在SiC衬底上形核长大，Pb原子易形成高密度的小岛，Ag原子倾向于沉积在缓冲层上，Ag掺杂引起的缺陷倾向于分布在双层石墨烯区域。(5) 基于第一性原理计算发现，张应变会导致晶格振动模式的显著软化，热容随之而增大，热导却随之降低；发现石墨烯-氮化硼杂化结构费米能级附近出现杂化能级，H吸附导致杂化结构去磁化，其程度受石墨烯畴区形状的影响。本项目所取得的研究结果对于二维电子器件的设计与应用具有重要的理论指导价值。