Programmable motion and patterning of molecules on solid surfaces

Abstract

Adsorbed on a solid surface, a molecule can migrate and carry an electric dipole moment. A nonuniform electric field can direct the motion of the molecule. A collection of the same molecules may aggregate into a monolayer island on the solid surface. Place such molecules on a dielectric substrate surface, beneath which an array of electrodes is buried. By varying the voltages of the electrodes individually, it is possible to program molecular patterning, direct an island to move in a desired trajectory, or merge several islands into a larger one. The dexterity may lead to new technologies, such as reconfigurable molecular patterning and programmable molecular cars. This paper develops a phase field model to simulate the molecular motion and patterning under the combined actions of dipole moments, intermolecular forces, entropy, and electrodes.

Fig. 1.

A dielectric substrate has molecules adsorbed on its surface and an array of electrodes buried beneath the surface. The adsorbates carry electric dipole moments. Varying the voltage of the electrodes individually, one can program the motion of the adsorbates.

Fig. 2.

In the absence of the electrodes, the adsorbates self-assemble into islands. This time sequence starts with a lattice of circular islands with a diameter below the equilibrium island diameter. The islands coarsen to reach the equilibrium size.

Fig. 3.

Self-assembled pattern in the absence of electrodes. This time sequence starts with a lattice of islands with a diameter larger than the equilibrium island diameter. The pattern refines to approach the equilibrium feature size. Figures emerge and remain stable.

Fig. 4.

Two examples of electric field-guided patterning. The electrode voltage pattern is an array of stripes in Upper and an array of dots in Lower. In both cases, the amplitude of the electrode voltage is small, so that adsorbates still retain the natural pattern of fine stripes, and the electrode voltage pattern affects the overall layout. Note that the feature sizes of the electrode voltage pattern are different from the equilibrium features of the adsorbate patterns.

Fig. 5.

Reconfigurable patterning. Initially, the electrode voltage pattern is the letter P, and the adsorbates assemble accordingly. Then switch the electrode voltage pattern to the letter H, and the adsorbates reassemble. The amplitude of the electrode voltage is high, so that the electrode voltage pattern overwhelms the natural pattern of the adsorbates.

Fig. 6.

The effect of electrode voltage wave speed. (Upper) An adsorbate island driven by a slow electrode voltage wave at a low velocity. (Lower) An adsorbate island driven by a fast electrode voltage wave.

Fig. 7.

Splitting an island. The initial electrode voltage pattern is a circular dot. The voltage pattern is then divided into two halves and moved in opposite directions. The adsorbate island is split into two smaller islands.

Fig. 8.

The molecular car has a modular architecture. The binder and the pavement ensure that the car stays on the surface and moves fast. The electric dipole moment serves the function of the engine. The receptor serves the function of the passenger seat. The on-chip infrastructure consists mainly of electrodes that control the motion of the molecular car.