应变调制二维层状材料晶格振动谱的理论研究

The mechanical, thermal and electrical properties are closely related with lattice dynamics, which has been playing a key role in the research of twodimensional layered materials. Owing to their special geometry, layered materials are susceptible to various strains in experiments. Up to know, however, theoretical efforts focus on layered crystals without strain. Consequently, it’s urgent to carry out the theoretical research including strain effect. This project will apply many calculation strategies to investigate the lattice dynamics of layered materials under stretching, contracting, shearing and twisting strains. By applying strain on single and multilayer silicon, germanium SiC, ZnO and etc, we investigate the strain effect on the lattice dynamics from covalent to ionic crystals systematically. Similar with graphene, silicene and germanene also show the unusual electronic properties and lead to charge carriers resembling massless Dirac fermion. Because the two atoms in the primitive unit cell of silicene and germanene do not locate in the same plane, strain can dramatically change the electronic Fermi surface of silicone and germanene and give great influence on their electron-phonon interaction. Consequently the life time of phonons vary. Taking advantage of density-functional perturbation theory, force-constant model and bond- polarizability model, we intend to give a comprehensive research on the vibrational frequency, line shape, intensity, full with at half maximum and depolarization ratio of Raman and infrared spectra and then establish quantitative relation between strain and spectroscopic characterization of layered materials. Theory and experiment show that ultrathin films of wurtzite materials surprisingly transform into a stable graphite-like structure. Furthermore, the thickness range of stable graphitic films depends sensitively on epitaxial tensile strain. Around the transition from graphite-like structure to wurtzite structure, the vibrational frequency of out-of-plane dipole mode must be very low, which will result into high dielectric constant. We will also pay much attention on the polar vibrational modes of multi-layered semiconductors, and hope to discover high dielectric constant (high κ) materials in them. In conclusion, this project will further the understanding of lattice dynamics of two-dimensional materials and sheds light on the property modulation by strain.

晶格振动谱与材料的力学、热学和电学等性质密切相关，在二维层状材料的研究中占有极其重要的地位。由于特殊的几何结构，二维层状材料在实验中极易受到应变的作用。但目前的理论研究多以“无应变”的晶体作为研究对象，急需开展“应变对其作用”的系统研究。本项目拟利用多种计算方法，研究拉伸、压缩、剪切、扭曲等应变条件下二维层状材料的晶格振动谱及其相关性质。通过对硅烯、锗烯、SiC、ZnO等二维结构施加应变，系统研究从“共价晶体”到“离子晶体”的晶格振动谱随应变的变化规律；将密度泛函微扰法、力常数模型和键极化模型相结合，取长补短，全面、系统的揭示应变对“拉曼和红外光谱”的影响；揭示SiC、ZnO等层状半导体材料中“极性晶格振动模”随“堆垛层数”和“外延应变”的变化规律，从中找到“高介电常数”材料，为其应用提供理论支撑。本项目将进一步加深对二维层状结构晶格振动谱的认识，揭示应变对其进行调制的微观机制。

新型层状材料并以其奇特的性质迅速吸引众多科研机构和研究人员的关注，成为新材料中的佼佼者。在层状材料中，层间相互作用扮演着非常重要的角色。比如在双层石墨烯中，通过改变两层之间的转角，可以使双层石墨烯从半金属变为绝缘体，甚至变为超导体。这是最近邻层的相互作用导致的结果。我们发现，对于多层结构而言，次近邻层间的相互作用也会对材料的性质产生影响。我们使用力常数方法计算了最近邻和次近邻的层间相互作用对晶格振动谱的影响。结果表明，加入次紧邻层间相互作用后，“家族模式”依然保留。但是，“常数频率行为”不再成立。这说明，可以利用不同层数的振动频率来表征次紧邻相互作用的符号和大小。我们研究了双螺旋XY（X=Li, Na, K, Rb, Cs; Y=P, As, Sb）的结构及其晶格振动性质。如果固定阴离子，只改变阳离子，则类似于对结构施加轴向应变。同样，固定阳离子，只改变阴离子，也类似于对结构施加轴向应变。通过计算不同成分布里渊区中心点的振动频率，我们发现同相呼吸模、反相呼吸模的振动频率都表现出了规则的变化关系。MDABCO-NH4-X3 (X=Br, I)是最新合成的有机铁电体，其中完全不含金属离子。我们通过计算不同阴离子(X=Cl, Br, I)所对应的晶体结构，发现阳离子自身的电偶极矩非常小，因此该类材料的电偶极矩主要来自于阳离子中心与阴离子中心的偏移。通过计算，我们发现此类材料压电张量的分量dx5具有较大的数值，它与弹性系数s44有着密切的关系。