The effect of electric field on a fullerene molecule on a metal surface by a nano STM tip

Abstract

Applying bias voltage generates the electric field between the sample and the probe tip of the scanning tunneling microscopy (STM). At the tunneling distance, individual fullerene molecule on a metal surface can be induced to diffuse into the region beneath the tip or desorb by the field strength. Finite Element Method Magnetics (FEMM) simulations were performed to investigate the electric field effect on a C60 molecule on a finite metal surface with a metallic STM tip. The field gradient has been simulated to study the effect of tip-apex size, 10, 30 and 50 nm and bias voltage, from +1.7 V to +4.8 at the calculated tunneling distance. This model appropriately defines the field strength just over the fullerene molecule as ∼6.8×109 V/m at +4.4 V and then exponentially decays with increasing radial distance from the tip about 69% in a range of 5 nm. Similar trends are obtained in between the molecule and surface gap but the field strength is enhanced by 2.5 times higher than the field over the C60 molecule with an effective range of 1 nm. The effective range of the field is enhanced by the bias voltage and the tip radii. For both the tip-molecule and molecule-sample gap, the E-field strengths show a linear increase with varying the bias voltage up to a maximum ∼17×109 V/m which is less than the threshold strength to yield a field evaporation of the surface adatoms. Independent from bias voltage polarity, the molecule can be diffused preferentially towards the tip-apex up to +3.0 V without thermal decomposition. Above +5.0 V voltage pulse, the desorption of the molecule is possible in the presence of the E-field. The van der Waals force with a range of 12–15 Å does not provide sufficient energy to induce either diffusion or desorption of the molecule.