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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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