We report the synthesis and systematic Raman study of twisted bilayer graphene (tBLG) with rotation angles from below 10° to nearly 30°. Chemical vapor deposition was used to grow hexagon-shaped tBLG with a rotation angle that can be conveniently determined by relative edge misalignment. Rotation dependent G-line resonances and folded phonons were observed by selecting suitable energies of excitation lasers. The observed phonon frequencies of the tBLG superlattices agree well with our ab initio calculation.

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The ion distribution at the charged three layer graphene–ionic liquid (1-butyl-3-methylimidazolium dicyanamide, [BMIM]­[DCA]) interface is probed by sum frequency generation spectroscopy (SFG), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG results show the presence of both [BMIM]+ cations and [DCA]− anions at the charged graphene surface, where the anion signal increased with the increasing external potential and the cation signal was little influenced by the external potential. The CV and EIS results suggest that ions form an electrically stable double layer at the charged graphene surface. Molecular dynamics simulations of [BMIM]­[DCA] near the charged graphene surface suggest that the interface consists of alternating layers of cations and anions that restructure with external potential variation. The experimental data can be interpreted by a double-layer model where the anionic and cationic layers interchange position with each other at the surface depending on the applied potential.

Sum frequency generation (SFG) vibrational spectroscopy and contact angle measurements of an ionic liquid, 1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA], at solid-liquid interfaces are reported. Bare solid single crystal BaF2 (111) surface, a single and few layer graphene-coated BaF2 (111) surface are used as the solid substrates. The SFG results indicate that both [BMIM](+) and [DCA](-) can be detected specifically on the graphene-coated BaF2 (111) surface, without coating only [DCA](-) are observed. [DCA](-) anions are attracted to the positively charged BaF2 (111) surface and occupy the first layer at the solid-liquid interface. The graphene coating shields the charged crystal surface and allows both cations and anions to exist at the interface. Furthermore, increase in the contact angle of BaF2 surface after graphene layers deposition suggests that the graphene coating lowers the surface energy.

Visualization of a Maze-Like Reconstruction of Graphene on a Copper Surface at the Atomic Scale * XIE Nan(谢楠) 1,2 , GONG Hui-Qi(宮会期) 2 , ZHOU Zhi(周智) 2 , GUO Xiao-Dong(郭晓东) 2 , YAN Shi-Chao(颜世超) 2 , SUN Qian(孙骞) 1 , XING Sirui(邢思锐) 3 , WU Wei(吴纬) 3 , PEI Shin-shem(白先慎) 3 , BAO Jiming(包吉明) 3 , SHAN Xin-Yan(单欣岩) 2 , GUO Yang(郭阳) 2** , LU Xing-Hua(陆兴华) 2**