Carbon nanotube nanomotor

However, such biological nanomotors are designed to work in specific environmental conditions (pH, liquid medium, sources of energy, etc.).

The vast differences in the dominant forces and criteria between macroscale and micro/nanoscale offer new avenues to construct tailor-made nanomotors.

Just fifteen years after making the world's first micrometer-sized motor, Alex Zettl led his group at University of California at Berkeley to construct the first nanotube nanomotor in 2003.

A few concepts and models have been spun off ever since including the nanoactuator driven by a thermal gradient as well as the conceptual electron windmill, both of which were revealed in 2008.

The quadratic scaling is alleviated by increasing the number of units generating the electrostatic force as seen in comb drives in many MEMS devices.

In the nano scale it can wreak havoc if not accounted for because the parts of a Nano-Electro-Mechanical-Systems (NEMS) device are sometimes only a few atoms thick.

Four independent voltage signals (one to the rotor and one to each stators) are applied to control the position, velocity and direction of rotation.

Empirical angular velocities recorded provide a lower bound of 17 Hz (although capable of operating at much higher frequencies) during complete rotations.

Gold with a chromium adhesion layer is thermally evaporated, lifted off in acetone and then annealed at 400 °C to ensure better electrical and mechanical contact with the MWNT.

The MWNT at this point displays a very high torsional spring constant (10−15 to 10−13 N m with resonant frequencies in the tens of megahertz), hence, preventing large angular displacements.

One simple way to accomplish this is by successively applying very large stator voltages (around 80 V DC) that cause mechanical fatigue and eventually shear the outer shells of the MWNT.

An alternative method involves the reduction of the outermost MWNT tubes to smaller, wider concentric nanotubes beneath the rotor plate.

The process is repeated on the opposite side to result in the formation of the short concentric nanotube that behaves like a low friction bearing along the longer tube.

[6] Due to the minuscule magnitude of output generated by a single nanoactuator the necessity to use arrays of such actuators to accomplish a higher task comes into picture.

Conventional methods like chemical vapor deposition (CVD) allow the exact placement of nanotubes by growing them directly on the substrate.

A Si substrate is coated with electron beam resist and soaked in acetone to leave only a thin polymer layer.

The substrate is selectively exposed to a low energy electron beam of an SEM that activates the adhesive properties of the polymer later.

The black carbonaceous deposit (a mixture of nanoparticles and nanotubes in a ratio of 1:2) is seen growing on the inside of the cathode while a hard grey metallic shell forms on the outside.

[8] There are several advantages of choosing this method over the other techniques such as laser ablation and chemical vapor deposition such as fewer structural defects (due to high growth temperature), better electrical, mechanical and thermal properties, high production rates (several hundred mg in ten minutes), etc.

However, at sufficiently high currents the nanotubes fail primarily due to rapid oxidation of the outermost shell.

Applying an increased bias displays multiple independent and stepwise drops in conductance (figure 1.4) resulting from the sequential failure of carbon shells.

This controlled destruction of shells without affecting disturbing inner layers of MWNTs permits the effective separation of the nanotubes.

The cargo moves towards the cooler electrode (Figure 2.2) due to the thermal gradient in the longer nanotube induced by the high current that is passed through it.

Just as the nanoactuator from the Zettl group, this enables low friction rotation and translation of the shorter nanotube along the axis of the longer tube.

[11] The interaction between the longer and shorter tubes generates an energy surface that confines the motion to specific tracks – translation and rotation.

Stray electric field effect could not be the driving factor because the metal plate staid immobile for high resistive devices even under a large applied potential.

On the other hand, experiments show that the displacement of the shorter tube is directly proportional to the thermal gradient (see Figure 2.5).

The nanodrill also comprises an achiral outer nanotube attached to a gold electrode but the inner tube is connected to a mercury bath.

The electron windmill model makes use of a new "electron-turbine" drive mechanism that obviates that need for metallic plates and gates that the above nanoactuators require.

By Newton's third law, this flux produces a tangential force (hence a torque) on the inner nanotube causing it to rotate hence giving this model the name – "electron windmill".

Figure 1.3: Cartoon showing the basic experimental setup for the arc-discharge technique of large scale carbon nanotube synthesis
Figure 1.4: (A) Graph showing remarkably discrete, constant drops in conductance for the removal of each subsequent carbon shell under constant voltage (B) Images of partially broken MWNTs show clear thinning, with a decrease in radius equal to the intershell spacing (0.34 nm) times the number of completed breakdown steps. The two segments of this sample were independently thinned by 3 and 10 shells, as depicted by the color overlays
Figure 2.1: Thermal gradient driven nanomotor. (A & B): SEM images of experimental setup. (C) Schematic of the nanomotor also displaying degrees of freedom
Figure 2.2: Motion of shorter nanotubes (red) along the longer tubes (yellow) from the hotter(top) section of the nanotube to the cooler (bottom) section of the nanotube carrying the metal cargo (gray)
Figure 2.3: Degree of translational and rotation are dependent on the chiralities of the two nanotubes
Figure 2.4: SEM images show the transformation of gold plate (left) into a ball (right) due to very high temperatures
Figure 2.5: Graph demonstrating the direct relationship between the thermal gradient and the displacement of the shorter tube / cargo
Figure 3.1: MWNT nanomotor (A) and nanodrill (B).