Abstract
We report on a transition in a monolayer of C60 molecules deposited on a WO2/W(110) substrate. The transition from a static state, where the molecules are rigidly bound to the surface by a coordination bond, to a state where the molecules are loosely bound to the surface by van der Waals force and rotate continuously, has been studied using scanning tunnelling microscopy (STM). The separation between the molecules and the surface increases by 1.2 Å across the transition. The transition from the static state into the rotating state takes place at 259 K. The energy of the spinning state with respect to the lowest energy state, having a single coordinated bond, can be obtained from the statistics of the molecules switching. The binding energy of the molecule in the spinning state can be easily altered by changing the polarity of the bias voltage applied between the STM tip and the surface. The binding energy decreases by 80 meV when the bias polarity of the sample changes from positive to negative with respect to the tip. The results are consistent with the Coulomb blockade model: when electrons travel from the surface to the C60 molecule, and then to the tip; charge accumulates on the molecule due to the Coulomb blockade. This increases the electrostatic interaction between the molecule’s charge and a corresponding image charge generated on the metallic surface.
Highlights
We report on a transition in a monolayer of C60 molecules deposited on a WO2/W(110) substrate
The specific attachment can be identified on the basis of comparison between density functional theory (DFT) calculations and the scanning tunnelling microscopy (STM) images[2]
We have studied the transition of C60 molecules deposited on a WO2/W(110) substrate from a fixed state, where the molecules are bound to the surface by a well-defined bond, to another state, where the molecules are loosely bound to the surface by van der Waals forces
Summary
We report on a transition in a monolayer of C60 molecules deposited on a WO2/W(110) substrate. The molecules were found in conducting and nonconducting states, and our results indicated that such changes were associated with the molecule acquiring or losing a single electron, i.e. C60 on the surface could be charge-neutral, or it could gain/ lose one electron This effect can be understood in terms of Coulomb blockade blocking electron transport from the surface to the STM tip via the molecule. We demonstrate that at a certain temperature the molecules become loosely bound to the surface and at once initiate their rotational movement This change in the binding energy between the molecule and the surface is accompanied by the molecule acquiring a charge and this can be controlled via STM tip bias. The results can be explained on the basis of Coulomb blockade charging of the molecule, leading to a change in the Coulomb interaction force between molecule and substrate
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