Graphene/h-BN Layer Composites:the First Principle Calculations

Since the discovery of graphene, the study of two-dimensional materials has become a topic of growing interest. Now graphene remains the focus of the research of layered materials since most of the other layered materials have more or less relationship with graphene. The research of layered materials has extended to the graphene-like materials such as hexagonal boron nitride (h-BN), silicene, germanene and WS2. Different carbon allotropes have been proposed by changing the framework or the bonding features of carbon atoms. Revealing the relationship between the unique properties and the atomic structures of these layered materials becomes the aim of recent theoretical and experimental works. Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy-momentum dispersion relations which cross at the Dirac point. Substrate materials that offer mechanical support to graphene without interfering with its electrical properties are quite crucial for achieving such applications, because the disorder arising from the substrate will leave the graphene with local microscopic electron and hole puddles which reduces the device performance. The source of the disorder includes corrugation effects of graphene, charge traps, and dangling bonds. Recent works showed that hexagonal boron nitride (h-BN) is a promising substrate material yielding high-quality graphene devices. This BN polymorph has much in common with graphite:(1) They have the same atomic arrangement but h-BN has a lattice constant slightly longer (~1.8%) than graphite.(2) Weak van der Waals interactions hold the sheets together, allowing layers to cleave readily. The electronic properties, however, exhibit clear differences. Due to the ionic characteristics of the B-N bonds, h-BN is a wide-band-gap (6.0eV) electric isolator. Therefore, the planar structure of h-BN cleaves into an ultra-flat surface and the ionic bonding of h-BN should leave it free from dangling bonds and charge traps at the surface. This reduces the disorder and charge inhomogeneity of the graphene on h-BN substrate.In this thesis we mainly studied the energy landscape and band structure variation during the interlayer movement of G/BN by the first principle calculations method based on the density functional theory (DFT). Then we analyzed the result of the calculations and compared the result with the experiment data. The main content of the study is as followings:(1) We studied the landscapes of sliding energy surface and energy band gap of graphene on BN substrate within a lattice-matched approximation from first-principles calculations. We show that the sliding energy surface is rather smooth with the AA and AB stacking modes being the global maximum and minimum in energy, respectively. The energy difference between the two modes is only22.5meV/cell. There is a saddle point that joins two AB stacking modes together, corresponding to an energy barrier of15.6meV/cell. Such features can be well reproduced using a registry index (R1) method, implying the capability of the simple geometric model in capturing the essence of the interlayer-interaction-related properties of more complex systems. For most stacking patterns, the interlayer interactions open a band gap at the Dirac point of graphene. Most interestingly, there are special stacking patterns that preserve the Dirac cones of graphene, which can be ascribed to the charge redistribution during the sliding process. The existence of zero-band-gap stacking modes hints the complexity of the electronic structures of the long periodic graphene-BN moire structure arising from small lattice mismatch. These first-principles landscapes of sliding energy surface and band gap offer not only benchmarks for developing empirical strategies dealing with long-periodic graphene-h-BN moire structures but also useful pictures for understanding the morphology and electronic properties of graphene on BN substrate.(2)The Moire structure of graphene on h-BN substrate can also be generated by the rotation between the two lattices. The periodicity of the Moire pattern is closely related to rotation angle. The Dirac cone feature of graphene monolayer is well preserved in these Moire patterns regardless of rotation angle. The Fermi velocities isolated graphene monolayer. This theoretical result is in good agreement with experimental findings.