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本文利用低温扫描隧道显微镜(STM)研究了苉分子在Cd (0001)量子阱薄膜上的自组装和薄膜生长。在亚单层的覆盖度，苉分子会铺满整个衬底表面并形成无序的二维分子气，表明苉分子之间存在静电排斥力。随着覆盖度的增加，第一层分子会发生无序-有序相变，形成由平躺分子组成的分子带平行阵列。高分辨的STM图表明，分子带的基本结构单元是携带相反电偶极矩的苉二聚体。更重要的是，在特定的偏压下第一层分子带会出现电子透射现象：可以透过分子层看到下面的Cd衬底原子。当覆盖度超过1.0 ML，第二层苉分子也会形成分子带阵列，其基本单元是一个平躺分子和一个侧立分子组成的二聚体，类似于苉晶体中的(110)面。有趣的是，第二层苉分子带也出现了电子透射现象。以上结果表明，Cd (0001)薄膜中的量子阱态上的电子具有超强的穿透能力，其竖直隧穿长度达到了两个苉分子层的距离。We report the scanning tunneling microscopy (STM) studies of the thin films of picene on the Cd(0001) surface. Compared to conventional noble metal substrates such as Au, Ag, and Cu, the low electronegativity and small work function of Cd(0001) can effectively weaken the molecule-substrate interactions, thereby promoting the intermolecular van der Waals attraction.
The experiments were conducted in a ultrahigh vacuum low-temperature STM combined with a molecular beam epitaxy system. The crystalline Cd(0001) films were grown on a Si(111)- 7×7 by depositing 15-20 monolayers of Cd atoms with subsequent annealing. The Picene molecules were deposited onto the Cd(0001) surface held at 100–120 K, where one monolayer (ML) was defined as the critical coverage preceding second-layer nucleation. All STM measurements were acquired in constant-current mode.
It is observed that, in the submonolayer regime, the picene molecules occupy the entire substrate surface and form disordered two-dimensional molecular gas, indicating the existence of electrostatic repulsive interaction among picene molecules. With the coverage increasing, the first layer of molecules undergoes the disorder-order transition, forming the parallel array of molecular stripes of flat-lying molecules. The high-resolution STM images indicates that the building blocks of molecular stripes is a picene dimer with the opposite dipole moments. More importantly, under specific bias voltages, the first layer of molecular stripes exhibits electronic transmission: not only the underlying Cd substrate atoms but also the standing waves of scattered electrons can also be observed nearby the defects. When the coverage exceeds 1.0 ML, the second picene layer also form the stripe array composed of picene dimers of a flat-lying and a side-on molecules, similar to the (110) plane in picene crystals. The above results indicate that, the electrons from the quantum-well states of Cd (0001) thin film have very strong penetration ability, and their vertical tunneling length reaches to the distance of two molecular layers.-
Keywords:
- Picene molecules /
- STM /
- Dipole-dipole interaction /
- Electronic transmission
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