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

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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.