Nanoscale Rotary Motors Driven by Electron Tunneling

We examine by semiclassical molecular dynamics simulations the possibility of driving nanoscale rotary motors by electron tunneling. The model systems studied have a carbon nanotube shaft with covalently attached “isolating” molecular stalks ending with “conducting” blades. Periodic charging and discharging of the blades at two metallic electrodes maintains an electric dipole on the blades that is rotated by an external electric field. Our simulations demonstrate that these molecular motors can be efficient under load and in the presence of noise and defects.

Figure 1

Tunneling-driven nanoscale motors with three (left) and six (right) fullerene blades. In an external homogeneous electric field $\mathcal{E}$ oriented along the vertical $z$ direction, the electron tunneling from the neutral electrodes to the blades maintains an electric dipole $p$ on the rotor, which is on average orthogonal to the field direction. This dipole is unidirectionally rotated by the electric field (movie in Ref. 29).

Figure 2

The efficiency $\eta$ for the 3-fullerene and 6-fullerene motors at different damping $c_d$. At larger $c_d$ the efficiency grows and becomes stabilized, but ${\eta }_{3\mathrm{\text{−}}fu}>{\eta }_{6\mathrm{\text{−}}fu}$. In the presence of mistakes ($c_d=0.8\mathrm{nN}\mathrm{ps}$ and $\mathcal{E}=0.05\mathrm{V}/\mathrm{nm}$), the 3-fullerene motor has $\eta =48.38\%$ (one mistake in charging per rotation), the 6-fullerene has $\eta =34.32\%$ (two nonconsecutive mistakes in charging per rotation) and $\eta =27.89\%$ (perfect charging, but one of the six blades is missing). The efficiency of the more complex dual-blade system ${\eta }_{\mathrm{dip}}$ is even smaller.

Figure 3

Tunneling-driven molecular motor with three dual blades. The inset (left down) shows the blade in detail. (Right) The activity of the blades during the dual-tunneling process. (Top) Before the first tunneling, where both half-blades are attracted; (middle) after the first tunneling, where the electrode interacts with a local dipole $p_{\mathrm{loc}}$ formed by the blades; (bottom) after the second tunneling, where both blades are repulsed forward (movie in 29).