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 rotation dynamics of single magnesium porphine molecules on an ultrathin NaCl bilayer is investigated with low-temperature scanning tunneling microscopy and density functional theory calculations. It is observed that the rotational oscillation between two stable orientations can be turned on and off by the molecular charge state, which can be manipulated with the tunneling electrons. The features of the charge states and the mechanism of molecular rotational on/off state control are revealed at the atomic scale. The dependence of molecular orientation switching rate on the tunneling electron energy and the current density illustrates the underlying resonant tunneling excitation and single-electron process. The drive and control of molecular motion with tunneling electrons demonstrated in this study suggests a novel approach toward electronically controlled molecular rotors and motors.

The influence of the carrier concentration on the elasticity is measured for a microscale silicon resonator. UV radiation is used to generate a surface charge that gates the underlying carrier concentration, as indicated by the device resistance. Correlated with the carrier concentration change is a drop in the resonant frequency that persists for 60 hours following exposure. Model calculations show that the change in resonant frequency is due to the modification of the elastic modulus in the near-surface region. This effect becomes increasingly important as device dimensions are reduced to the nanometer scale, and contributes an important source of instability for micro and nano scale electro mechanical devices operating in radiation environments.