Correlation between the optical absorption and twisted angle of bilayer graphene observed by high-resolution reflectance confocal laser microscopy
We report a systematic study of the optical absorption of twisted bilayer graphene (tBLG) across a large range of twist angles from 0° to 30° using a high-resolution reflectance confocal laser microscopy (RCLM) system. The high-quality single crystalline tBLG was synthesized via the efficient plasma enhanced chemical vapor deposition techniques without the need of active heating. The sensitivity of acquired images from the RCLM were better than conventional optical microscopes. Although the highest spatial resolution of RCLM is still lower than scanning electron microscopes, it possesses the advantages of beam-damage and vacuum free. Moreover, the high intensity-resolution (sensitivity) images firstly allowed us to distinguish the slight absorption differences and analyze the correlation between the optical absorption and twisted angle of tBLG after data processing procedures. A maximum absorption (minimum transmission) was observed at the stacking angle of tBLG from 10° to 20°, indicating the interplay between the laser and the electron/hole van-Hove singularities when tBLG oriented around the critical angle (θc∼13°). The twisted angle correlated optical absorption paves an alternative way not only to visibly identify the interlayer orientation of tBLG but also to reflect the characterization of the interlayer coupling via its band structure.
Optics Express, 29, 40481-40493 (2021)
Direct large-area growth of graphene on silicon for potential ultra-low-friction applications and silicon-based technologies
In summary, we have demonstrated the feasibility of a single-step method for direct growth of large-area graphene and graphene-based nanomaterials on silicon by means of PECVD without active heating. By proper control of the PECVD growth parameters, we can obtain a variety of graphene-based materials, including large-area graphene sheets and vertically grown graphene nano-walls. Correlation between the growth parameters and the resulting sample characteristics has been made by studying the Raman spectroscopy, XPS, UPS, SEM, optical transmission and AFM, which helps unveil the growth mechanism and optimize the growth quality of graphene on silicon. RGA studies during the growth process suggests that a key factor for successful PECVD growth of graphene on Si is the revelation of surface Si dangling bonds by plasma-induced surface Si-O and Si-H bond-breaking, which enables efficient reaction of Si with carbon atoms disassociated from CH4. Additionally, for Si substrates fully covered with multilayer graphene sheets, decreasing friction with increasing graphene layers has been demonstrated by AFM-based lateral force microscopy measurements. In particular, ultra-low friction with a load-independent frictional coefficient of ~0.015 has been achieved for an average of only ~14 layers of graphene on the Si surface. Thus, our demonstration of direct PECVD growth of large-area high-quality graphene on silicon suggests unprecedented opportunities for developing scalable and reproducible devices based on integrated graphene and silicon, which are not only promising for applications in areas of structural superlubricity but also for various silicon-based technologies such as optoelectronics and energy storage.
Nanotechnology, 31, 335602 (2020)