六方氮化硼/石墨烯异质结插层机制、多稳态及其转换驱动机制

Owing to the highly matched lattice of h-BN and graphene, a sandwiching hetero stacked structure can be fabricated. Such a way provides h-BN/graphene a tunable band-gap and electronic drag effect, implying an alluring application foreground in the fields of nano-fabrication and nano-processing. So far, the research is still in its infancy and there is no good understanding on the physical mechanism with regard to the formation of hetero stacked structure, especially the switch mechanism of multistable states of h-BN/graphene induced by external excitation which grievously obstructs its design and application. In the proposal, an atomistic-continuum multi-scale theoretical model, involving microstructure information of h-BN and graphene, is derived based on the higher-order Cauchy-Born rule and higher-order gradient theory. A novel mesh-free method based on moving Kriging interpolation is developed for numerical calculation. The influence of grain boundary on elastic property of h-BN and graphene is studied to determine the physical significance of size effect. The principal target is to study the formation mechanism of h-BN/graphene related to various factors involving material and geometrical nonlinearities, size effect, surface morphology, stacked style, lattice difference. According to the small in-plane bending stiffness of h-BN and graphene, we are going to explore the switch mechanism of multistable state s for h-BN/graphene induced by external excitation in order to seek out a new strategy for tuning electronic properties.

h-BN与石墨烯晶格高匹配性使二者能够紧密贴合形成交叠的异质结。通过这种插层方式使h-BN/石墨烯具有可调谐的带宽和电子拖曳效应，在微纳米器件制造加工有着诱人的应用前景。然而，h-BN/石墨烯的插层机制尚缺乏深入的认识，尤其是多稳态的可控形成及受激驱动下的相互转换，这些行为直接关系到h-BN/石墨烯的电学可调谐性，因此限制了该材料的设计和应用。本项目从h-BN和石墨烯微结构信息着手，基于高阶柯西-波恩准则和高阶梯度理论建立原子-连续多尺度理论模型，发展相洽的无网格数值算法。考虑小尺度晶畴的晶界对h-BN和石墨烯的力学性能的影响研究尺度效应，明确非局部等理论尺度因子的物理意义。研究材料和几何非线性、尺度效应、表面形貌、堆栈方式、晶格微差异等多因素下h-BN/石墨烯的插层机制；并探索基于h-BN和石墨烯小面内弯曲刚度特性，插层结构的多稳态及相互转换的驱动机制，为可调谐电学性能找到新思路。

类石墨烯结构的二维六方氮化硼是一类性能优异的纳米材料，适用于极端环境。利用有机溶剂的强表面张力能够剥离得到单层六方氮化硼，并与石墨烯复合在一起可以巧妙地得到六方氮化硼/石墨烯交替叠加的插层结构。这一现象在形貌操控、微纳米制造与加工有着诱人的应用前景。然而，到目前为止，对于六方氮化硼/石墨烯的插层机制尚缺乏深入的认识，尤其是多稳态的可控形成及受激驱动下的相互转换，这些行为直接关系到h-BN/石墨烯的电学可调谐性，因此限制了该材料的设计和应用。本项目从六方氮化硼的微结构信息着手，建立原子-连续多尺度理论模型并发展在无网格计算框架下的数值模拟方法研究六方氮化硼可控变形貌演和与石墨烯插层结构形成的若干基本问题。主要研究内容包括：1）尺度效应下氮化硼和石墨烯的弹性能；2）层内纳米材料的表面粘附特性；3）形貌操控及结构的稳定性。通过全原子模拟、多尺度理论和经典连续理论不同角度对六方氮化硼自由振动的研究，发现尺寸为4.2nm×2.5nm的氮化硼固有频率~0.18THz，并提出了准确测定单原子层六方氮化硼厚度的方法，0.0906nm，弯曲刚度1.42526eV。据此，解决了经典欧拉梁理论和铁木辛柯梁理论在预测氮化硼纳米管的弯曲变形时与全原子模拟结果存在差异的问题，并指出了广义连续梁理论中尺度因子引入的意义所在。通过圆状石墨烯的局部扭转，研究了石墨烯的表面褶皱操控。通过对石墨烯几何尺寸参数：内径和外径、扭转角等因素对褶皱波数和幅值等进行详细研究，发现石墨烯成皱的起始扭转角随着几何尺寸减小，并随扭转角度持续增大时，褶皱图案发生突变，伴随幅值骤然跳减。应变能曲线表明结构转变是可逆的，即石墨烯具有超弹性特性。相关研究成果促进了纳米材料在增强复合薄膜的开发，并阐释了纳米材料的可能迁移风险模式。因此，通过本项目开展建立的理论体系，解决了纳米材料的尺度效应、弯曲刚度、形貌操控等基本力学问题，为六方氮化硼在纳机电系统的工程应用奠定了理论基础和技术方法。