Nanoscale reactions assisted by electron tunneling

我们利用扫描隧道显微镜（STM）研究了用MBE方法沉积在HO PG（0001）衬底上的C60薄膜的纳米级反应。我们研究了由STM针引发的C60分子的解吸和聚合。我们发现当偏置电压高于2 V时，会发生解吸反应。随着偏置电压的增加，反应发生的可能性突然增加。我们推测C60中LUMO+1的波函数与STM尖端的波函数重叠形成成键轨道，导致C60从表面脱附。在偏置电压为2.5 V、隧道电流为50 pA的条件下，对C60分子进行重复扫描，发现部分C60分子的成像对比度较暗。假设表面的COO分子与下面的COO分子发生反应，两个分子之间的距离变短，这是可以理解的。在颜色较深的分子中，我们可以通过STM看到内部结构，表明分子不再旋转。这一观察结果支持了上述“垂直聚合”机理。扫描隧道光谱（STS）是研究原子尺度局域电子结构的有力手段。然而，室温下的STM测量受到热涂抹的影响，因此不可能获得高于~ 20 meV的能量分辨率。为了实现更高的能量分辨率，我们开发了一种低温STM/STS仪器。通过对超导材料的测量，我们证实了STM/STS具有纳米尺度的空间分辨率和~0.1 meV的高能量分辨率。最后，我们将STM/STS应用于C60聚合分子的原子尺度观察。

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.