The thin films of picene on the Cd(0001) surface are investigated by scanning tunneling microscopy (STM) in this work. Compared with conventional noble metal substrates such as Au, Ag, and Cu, Cd(0001) has low electronegativity and small work function , which can effectively weaken the molecule-substrate interactions, thereby promoting the intermolecular van der Waals attraction. The experiments are conducted by ultrahigh vacuum low-temperature STM combined with a molecular beam epitaxy system. The crystalline Cd(0001) films are grown on a surface of Si(111)-7×7 by depositing 15-20 monolayers of Cd atoms, followed by being annealed. The picene molecules are deposited on the Cd(0001) surface at 100–120 K, where monolayer (ML) is defined as the critical coverage before second-layer nucleation. All STM measurements are 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 show that the building block of molecular stripes is a picene dimer with the opposite dipole moments. More importantly, under a specific bias voltage, the first layer of molecular stripes exhibits electronic transmission: not only can the underlying Cd substrate atoms be observed, but also the standing waves of scattered electrons can be observed near the defects. When the coverage exceeds 1.0 ML, the second picene layer also forms 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 show 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.
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