Nanoscale reactions assisted by electron tunneling

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We have studied nanoscale reactions using scanning tunneling microscope (STM) on C60 films, deposited on HO PG(0001) substrates by the MBE method. We investigated desorption and polymerization of C60 molecules initiated by the STM needle. We found that the desorption reaction occurs when the bias voltage is higher than 2 V. The probability of the reaction is abruptly increased with increasing bias voltage. We speculate that the wavefunction of LUMO+1 in C60 overlaps with that of STM tip to form a bonding orbital, resulting in desorption of C60 from the surface. When repeating XY scan with a bias voltage of 2.5 V and tunneling current of 50 pA, some C60 molecules were found to be imaged with darker contrast. This is understandable by assuming that a surface COO molecule reacts with underneath ones and the distance between two molecules becomes shorter. In the darker molecules, we could see internal structures by STM, indicating that the molecule no longer rotates. This observation supports the above mentioned "vertical polymerization" mechanism.Scanning tunneling spectroscopy (STS) is a powerful to investigate local electronic structures in an atomic scale. However, STM measurements at room temperature suffer from thermal smearing, so that it is impossible to obtain energy resolution higher than 〜20 meV. In order to achieve higher energy resolution, we have developed a low temperature STM/STS instrument. We confirmed that the STM/STS has nano-scale spatial resolution together with high energy resolution of~0.1 meV from measurements of superconducting materials. Finally, we have applied the STM/STS to atomic scale observations of polymerized C60 molecules.