The two-dimensional material represented by graphene has excellent electrical properties and optical properties, and is therefore expected to be used for the development of a new generation of electronic components, transistors, and photovoltaic devices that are thinner and have faster conduction speeds. Stacking graphene can give multilayer graphene.

In addition to the multi-layered graphene with the same Bernal stacking (ie, AB stacking) as the bulk graphite, it is also possible to prepare or synthesize graphene sheets with different random orientations in the laboratory, ie, multiple corners. Graphene. The difference in the number of layers in each subsystem of the corner multilayer graphene and the difference in the rotation angle between the various subsystems will make it infinitely more likely, which undoubtedly greatly enriches the research object and research content of the graphene material, such as a single layer. The difference in the stacking method of multilayer graphene may lead to different interlayer coupling of the graphene sheet, thereby affecting the electronic energy band structure thereof, so that the corner multilayer graphene has various photoelectrics corresponding to its stacking method. nature.

The study of Raman spectra of phonon vibration modes is one of the most effective techniques for characterizing graphene materials. The interlayer shear and breath phonon mode is a unique phonon vibration mode that distinguishes multilayer graphene materials from monolayer graphene.

In recent years, Tan Pingheng, a researcher at the Semiconductor Superlattice State Key Laboratory of the Institute of Semiconductors at the Chinese Academy of Sciences, has observed interlaminar shear in multi-layered graphenes and multi-layered graphenes using the ultra-low wave number Raman technology developed by him. Cut Mode [Nature Materials 11, 294-300 (2012), Nat. Commun. 5:5309 (2014)]. However, due to symmetry and weak electro-phonon coupling, scientists have not been able to observe multilayered graphene interlayer breathing patterns at room temperature. In addition, the monoatomic chain model proposed by Tan Pingheng's research group has been able to perfectly describe the physical properties of the inter-layer shear mode of multi-layer graphene when considering only the nearest-neighbor interaction. However, whether this model can be applied to the inter-layer respiratory module can also be applied. Not sure.

Recently, Wu Jiangbin, a research student and Tan Pingheng, a researcher, and others collaborated with Professor Ji Wei of the Renmin University of China and Professor Ferrari of the University of Cambridge to use resonance Raman spectroscopy to systematically study the interlaminar respiratory pattern of the corner multilayer graphene. . They found that the inter-layer respiratory coupling at the corner multi-graphene interface is comparable to that of normal Bernal stacks of multi-graphene. This result is significantly different from the inter-layer shear coupling, where the interlaminar shear coupling at the corner multilayer graphene interface is reduced to 20% of normal Bernal stacking multilayer graphene. The first-principles calculations show that the symmetry breaking at the corner of the multi-graphene interface is the main cause of the different behavior of the shear mode and the respiratory mode.

The study also shows that the relationship between the frequency of respiratory pattern of multilayer graphene and its number of layers can not be correctly described by the single atom chain model considering only the nearest neighbor interaction. However, if the next-nearest neighbor inter-layer respiratory coupling is added, the experimentally observed inter-layer respiratory pattern frequency of the multi-angle graphene is interpreted, and the second-nearest neighbor inter-layer respiratory coupling of the multi-layer graphene is fitted to the nearest neighbor 9 %. This is the first observation of this second-nearest neighboring inter-layer respiratory coupling in van der Waals two-dimensional crystal material. The results of this study indicate that the number of layers of the Bernal stacking subsystems in the corner multilayer graphene can be detected by observing the interlaminar shear mode, and the total number of layers in the Beral stacking system can be detected using an inter-layer breathing module.

This research work has received strong support from the National Natural Science Foundation of China and was recently published online on the ACS Nano, an academic journal of the American Chemical Society. Tan Pingheng and Ji Wei are co-authors of the paper. This discovery is not only an important advance in the field of graphene Raman spectroscopy, but it can also be extended to other two-dimensional heterojunctions that are stacked from different two-dimensional crystalline materials, making ultra-low-frequency Raman spectroscopy the second An important characterization method for inter-layer heterojunction coupling.

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