Local manipulation of molecules with the optical nonlinearity just below the tips of Scanning Tunneling Microscopes

我们提出了一种利用扫描隧道显微镜（STM）中针尖和样品之间的隧道间隙产生的局域光来操纵单分子的新方法，取得了如下成果。(1)将激光器与STM相结合，在STM针尖和样品的隧道间隙处建立了一个局域非线性光学产生系统。(2)用FDTD方法计算了STM针尖的场增强效应。结果发现，一个巨大的增强，可以预期在STM的提示。(3)我们发现，由于STM样品针尖间隙的I-V非线性引起的光整流隧穿电流（即增强因子）在实验中根据针尖表面的情况而显著波动。非常稳定的STM尖端被发现是必要的，以利用非线性光学产生的STM尖端。(4)碳纳米管（CNT）具有优异的电学和力学性能，适合用作STM针尖。为了研究光整流隧道电流对碳纳米管场增强因子的影响，我们评估了几种材料作为碳纳米管STM观测的基底。据发现，尽管其平坦度，HOPG表面是不合适的，因为特殊的对比度，由于在石墨烯片的边缘处的电子状态。沉积在云母上的Au膜是合适的基底。(5)富勒烯（C60）分子的可逆聚合和解聚反应是通过从扫描隧道显微镜的尖端隧道注入空穴来诱导的，类似于隧道电子诱导的反应。空穴诱导反应和电子诱导反应的双向和双极性特征可以用一个统一的模型来解释，该模型考虑了[2+2]环加成和逆反应中的离子反应越过势垒。

We have proposed a new method to manipulate single molecule by a light locally generated at the tunnel gap of tip and sample in a Scanning Tunneling Microscope (STM).The achievements are as follows. (1) Combining Lasers and STM, we have constructed a system of local nonlinear optical generation at the tunnel gap of tip and sample in a STM. (2) Field enhancement effects at the STM tip were estimated with FDTD method. It was found that a huge enhancement can be expected at the STM tips. (3) We have found that the optical rectifying tunneling currents due to I-V nonlinearity of STM sample tip gaps (i.e.enhancement factors) fluctuate significantly in the experiments according to the condition of tip surfaces. Extremely stable STM tips are found to be necessary to utilize the nonlinear optical generation at the STM tips. (4) Carbon nanotubes (CNT) are suitable for use as STM tips, with its remarkable electronic and mechanical properties. To investigate the field enhancement factor of CNTs by optical rectifying tunneling currents, we assessed several materials as a substrate for CNT STM observations. It was found that despite its flatness, HOPG surfaces were not suitable because of peculiar contrasts due to electronic state at the edge of graphene sheets. Au films deposited on mica were suitable substrates. (5) Reversible polymerization and depolymerization reactions of fullerene (C60) molecules are induced by tunnel injection of holes from tips of scanning tunneling microscopes similarly to the reactions induced by tunneling electrons. The bi-directional and bi-polar features of the hole-induced and electron-induced reactions can be interpreted by a unified model considering ionic reactions over symmetry-imposed barriers in the [2+2] cyclo-addition and the reverse reactions.