Nanomachines Based on Carbon Nanotubes

Abstract

Due to the hollow cylindrical structure of CNTs, they are employed in the construction of nanomachines. Different types of CNT-based nanomachines have been designed and fabricated, so far. In these CNT-based nanomachines, the transportations of cargos are available along the length of nanotubes. In other cases, carbon nanotubes have been utilized to build rotary molecular machines, in which we observe the rotation of molecular objects around the nanotubes axis. Moreover, the carbon nanotubes have the potential to be used as gigahertz oscillators. In this chapter, we review some basic ideas of using nanotubes in the structure of nanomachines. The controllable mechanical motions have been reported in the CNT-based nanomachines, through the experimental and computational studies. Achieving the favorable precise movements at nano-scale is one of the most fascinating topics in the field of nanotechnology.

1. Introduction

Nanomachine is defined as a set of molecular components that show certain mechanical movements in response to an external stimulus [1]. Carbon nanotubes (CNTs) have special mechanical properties that make them useful in a wide range of engineering applications [2, 3]. Sauvage et al. [4] were the first to start making tiny machines called nanomachines. They did this by linking together two small rings of molecules, which they called a catenane structure. Fraser Stoddart et al. [5] worked on making nanomachines out of molecules. In 1991, the group made a molecule that they called a “molecular shuttle” [6]. In this setup, a macrocycle is hooked onto a chain that looks like a dumbbell. The macrocycle can move between two fixed spots on the chain of molecules by adjusting the temperature. In 2016, Ben Feringa, Fraser Stoddart, and Jean-Pierre Sauvage won the Nobel Prize in Chemistry for inventing the first nanomachines [7]. Scientists have made different types of tiny machines in experiments [8]. In 2006, James Tour et al. [9] created the first nanorolling machine called “Nanocars”. These nanomachines are made to do special jobs, like moving things and energy around on surfaces [10]. Nanocars are like regular cars but much smaller. They have tiny wheels, axles, and chassis, just like the ones in bigger vehicles [11]. Many tests have been done to see how nanocars move on different surfaces [12, 13, 14, 15].

Carbon nanotubes have special properties that are useful for building these small machines. Nanotubes could also be used in nano-diodes [16], transistors [17], as tips for atomic force microscopes [18], in nano-tweezers [19], and as parts of tiny electrical switches [20]. All those applications use the special features of nanotubes, like their electrical and mechanical features. New ways to use double- or multi-wall nanotubes were discovered. The walls can move and wrap around the nanomaterial because they are only weakly attracted to each other through Van der Waals forces [21]. This feature allows us to create new nanomachines called “carbon nanotube-based nanomachines” or “carbon nanotube-based nanomotors”. These nanomachines made with carbon nanotubes move in different ways on the nano-scale, like moving along the CNT length [22], rotating around the nanotube [23], and oscillating inside the tube [24, 25]. In other words, carbon nanotubes are like tiny tracks that can carry things along them [26]. The tube-like shape of carbon nanotubes helps them rotate smoothly around their own axis [27]. Moreover, the nanotubes have free space inside them, so they can be used to make tiny motors that move back and forth [28]. In the next part of the chapter, the basic ideas for building tiny machines using carbon nanotubes will be introduced. In every case, the main purpose of inventing nanomotors is explained first. Then, the text describes the various components of nanomachines based on carbon nanotubes (CNTs). Next, we discuss in more detail how nanomotors move (their mechanism of the motion) and the important factors that influence their effectiveness. Additionally, there is an attempt to briefly outline the experimental or computational methods used in investigating CNT-based nanomachines.

2. CNT-based nanomachines

Because of special features in CNTs, they are used in tiny machines that move things around. These special features include the small size of CNTs [29], their unreactive nature [30], and their one-dimensional structure [31]. As mentioned before, the one-dimensional nature of CNTs makes them a favorable choice for limiting motion at the nano-scale. For instance, nanoparticles locked inside a carbon nanotube move in a spiral path [32]. Changing the way the nanotube is shaped can control how nanoparticles move inside it, making them move in spirals with different angles and speeds [33]. Essentially, when encapsulated nanoparticles interact with CNTs of varying chirality, it causes the encapsulated particle to move along multiple helical trajectories.

Scientists have created nanomachines that can move in different ways by combining the chiralities of two different CNTs [34]. This nanomachine has two nanotubes joined together. Figure 1 presents the design and actual image of the nanomachine setup. The longer CNT acts like a road for the shorter nanotube to move on. The mobile nanotube can move in different ways, from rotating in place to moving straight ahead, because of its mix of chiralities. The long CNT is connected to two chrome electrodes. The mobile nanotube moves when enough electric current (around 0.1 mA) passes through the longer CNT. A tiny gold particle is put onto the shorter carbon tube, like a payload for the nanomachine.

Figure 1.

(a) SEM images of a device with a central object attached to a nanotube (Scale bar: 300 nm). (b) Schematic of the nanotube-based motor where the outer nanotube moves in relation to the inner nanotube [34].

Arc discharge evaporation is used to produce multi-walled carbon nanotubes (MWCNTs) [35]. Barreiro et al. [34] used electron beam lithography to join the longer nanotube to the Cr electrodes. The same method was used to load gold onto a mobile nanotube. Passing a large electrical current through the MWCNT removes extra layers on the outside.

Different ways the cargo moves depend on the kinds of chiralities involved. The movement of the mobile CNT in this device faces different energy barriers for moving and rotating, and this depends on the types of chiral patterns it has [36]. Figure 2a–c shows how shorter carbon nanotubes move, with different chiral combinations such as: (5,5)/(10,10), (29,9)/(38,8), and (27,12)/(32,17).

Figure 2.

Energy barrier variations for the movement between two coaxial nanotubes: (5,5)/(10,10), (29,9)/(38,8), and (27,12)/(32,17) with inner tube diameters of 0.67, 2.7, and 2.7 nm, respectively. The minimum energy path is indicated by white arrows [34].

As shown in Figure 1, moving in the x-axis direction causes shorter CNT to slide along the longer ones. Moving in a certain way along the y-axis causes the mobile CNT to rotate. Based on the energy map, when the moving CNT gets enough energy, it switches from one minimum-energy point to another. The preferred directions, shown as white arrows in Figure 2, represent different ways the shorter CNT prefers to move. These preferred directions match with different chiral combinations.

Let us discuss about the mechanism of the motion of this nanomachine. When someone looks at it initially, it seems like the shorter carbon nanotube moves as a result of the electric current through the longer one, and electrons collide with moving particles (i.e., electromigration [37]). But in fact, the shorter CNT moves because the longer nanotube has a temperature difference along its length [38]. Barreiro et al. [34] found that when they switched the direction of the electric current, the CNT still moved in the same direction (Figure 3). This observation does not make sense if electro-migration is considered the cause of the translations. When you apply electric voltage to the longer nanotube, the middle of the nanotube gets higher temperature [39]. Both ends of the CNT are thought to have temperatures of 300 K. When there is a difference in temperature, the nanotube starts moving toward one end of the longer tube. It is important to observe that the gold cargo initially had a rectangular shape at the start of the experiment (see Figure 1a), but it transitioned into a spherical shape over time (see Figure 3). This change in shape shows that there is a temperature difference along the longer nanotube. The cargo got so hot while moving on the CNT track that it melted at 1300 K [40].

Figure 3.

Translational motion: Gold cargo moves along the nanotube in response to the current. The red arrow indicates the current direction. (a–b) Current: 0.11 mA (3.1 V). (c-d) Current: −0.13 mA (−3.6 V). Scale bar in (a): 400 nm [34].

The previous investigation explained in this chapter used carbon nanotube as a track for the motion of materials in one-dimensional direction. However, the rectilinear motion of CNT on the surface proposes the idea of using nanotubes in the fabrication of surface-rolling nanomachines. The surface motion of CNTs has been examined on the gold surface, by molecular dynamics (MD) simulations [41]. The effects of different properties of nanotubes have been evaluated on the surface motion, such as CNT’s diameter, length, and chirality.

The chirality of CNT has shown significant influence on the direction of the surface motion [42]. The trajectories of the motion of nanotubes with different chiralities are observable in Figure 4a, while the nanotubes lengths and diameters are similar. The zigzag (9,0) CNT indicates more directed motion on gold surface compared with other CNTs with different chiralities. At most of the time, the direction of the motion of (9,0) CNT is perpendicular to the axis of the nanotube. On the other hand, the armchair (5,5) carbon nanotube has log-range diffusive displacements. The potential energy surface (PES) analysis has been performed for (9,0) CNT, to understand the reason of the rectilinear motion in this nanotube (Figure 4b). In case of zigzag CNTs, the aromatic rings of nanotube are completely parallel to the gold atoms of the surface. According to the analysis of potential energy, the molecules with aromatic rings find the lowest energy when they are resting on their hexagonal faces [43, 44].

Figure 4.

(a) Trajectories of the motion carbon nanotubes on gold surface. The length and diameter of nanotubes are almost 16 and 7 Å, respectively; while the chiralities of CNTs are different. (b) The potential energy of (9,0) CNT at different points on the gold surface. The minimum energy occurs when the hexagonal faces of CNT are parallel to gold atoms of the surface [41].

The length of carbon nanotubes is another dominant factor, in the direction of the surface motion [45]. Kianezhad et al. [41] continued the investigation by simulating the zigzag (6,0) CNTs with different lengths. According to the results of the simulations, as the length of the nanotubes grows, CNTs show more directed movements on the surface. As we observe in Figure 5a and b, the motions of zigzag (6,0) nanotubes which are longer than 20 Å are completely rectilinear in the directions perpendicular to the CNTs axis. Figure 5c indicates the variations of potential energy of CNT during the rotation around the axis perpendicular to the surface (see the inset of Figure 5c). Based on the analysis of potential energy, there are smaller energy barriers against the rotation of shorter nanotubes. As a result, when the thermal energy is high enough, the shorter nanotubes overcome the barriers and change the direction of motion, while longer CNTs maintain the direction of motion.

Figure 5.

(a, b) Trajectories of the motion of (6,0) carbon nanotubes with different lengths, at the temperature of 300 K. (c) The variation of potential energy of (6,0) CNTs during the rotation of nanotubes around the axis perpendicular to the gold surface. The rotational movement is demonstrated in the inset [41].

As mentioned in the introductory section of the chapter, the nanocars are a group of nanomachines, which are similar to the conventional cars. They are equipped with wheels, axles, and chassis [46]. In the previous experimental studies, most of the nanocars were synthesized based on C60 [47, 48] or p-carborane [49, 50] molecules, which play the role of nanocars wheels. The symmetrical shape of these molecules leads to the random motion of the molecular machines on the surface [51, 52]. The rectilinear motion of CNTs on the surface makes them an appropriate candidate as the wheels of nanocars. Kianezhad et al. [41] propose the idea of the fabrication of nanocars with CNT wheels, which are expected to have directed movements on the surface (Figure 6).

Figure 6.

Nanocars based on C60 and p-carborane molecules are shown at the left side and at the middle of the figure, respectively. These nanomachines have been previously synthesized in experiments. The rectilinear motion of carbon nanotubes on the surface raises the idea of fabricating the molecular machines based on carbon nanotubes (the nanocar at right side) [41].

In the previously mentioned studies, scientists designed nanomachines to move stuff in straight lines or sideways. Single-walled carbon nanotubes were used to make nanomachines that can rotate [53]. Single-walled carbon nanotubes (SWCNTs) are thinner than multi-walled ones. So, SWCNT-based nanomachines find the ability to bend and twist [54, 55]. Meyer et al. [56] describe these spinning tiny machines as torsional pendulums. When the moving part twists, it bounces back to its starting position once we stop applying the electric field.

Figure 7 shows a picture of the torsional pendulum taken with an optical microscope. The gold particle is suspended on a single-walled carbon nanotube, and it can be seen at the center of the image. When you use an electric voltage between the pendulum’s support (electrode #1) and the lower electrode (electrode #2), the metal particle at the center starts spinning around the SWCNT’s axis. In this molecular pendulum, the carbon nanotube acts like a spring that twists and bounces back. When the electric field goes away in the nanomachine, the pendulum goes back to its starting angle. The pendulum swings to really big angles, like 180 degrees. Single-walled nanotubes have a benefit, because the outer layers of multi-walled nanotubes break at angles larger than 20 degrees [57].

Figure 7.

(a) Metal block suspended on one SWNT in an optical microscope. The electric voltage is applied between two electrodes (electrodes #1 and #2). (b) The suspended part rotates up to 70° in TEM due to the electron beam. (c) High-res TEM confirms single-molecule device. Scale bars: 2 μm (a), 200 nm (b), 5 nm (c) [56].

The swing of the pendulum has been tested with different forces. Figure 8 displays how a gold nanoparticle changes orientation at different voltages: 9.7, 7.4, 22, and 0 V. This picture shows that the pendulum swings more when the electric voltage goes up.

Figure 8.

TEM images of the rotation of gold nanoparticle at 0, 7.4, 9.7, and 22 V between electrodes, with a 30° tilt. Scale: 100 nm [56].

The gold particle shown in Figure 8 weighs about $m\approx 2\times {10}^{-16}kg$ and has a moment of inertia of roughly $J\approx 7\times {10}^{-30}kg.{\text{m}}^2$ around the axis of CNT. The torsional stiffness of the tube axis is $C=3\times {10}^{-18}\text{N}.\text{m}/\text{radian}$, found from the data in published literature [58, 59]. Hence, a torsional pendulum made with a SWNT can be moved by very small forces. To turn particle by 1 degree, you need a force of $5\times {10}^{-20}\text{N}.\text{m}$. This means there is a tiny force of 0.1 pN pushing on one end of the rotor, which is 500 nm away from the center. The resonance frequency [60] for torsional oscillation is calculated as$f=\frac{1}{2\pi }{\left(\frac{C}{J}\right)}^{0.5}\approx 0.1MHz$. At room temperature in the TEM, one can observe the pendulum’s edges appearing blurry due to these oscillations (Figure 7b). The amplitude of the vibration, given an energy of k_BT [61] (where k_B is a constant and T is temperature), measures 3 degrees in the device shown in Figure 7, which has a nanotube diameter of 2.4 nm.

Just like before, Fennimore et al. [62] created a nanomachine which shows rotational movements. It had a metal plate that could spin, and the rotational movements occur around a carbon nanotube. Figure 9 displays the basic design of the electromechanical rotational actuator. The spinning part (R) is a flat metal plate shaped like a rectangle. It is attached sideways to a hanging CNT. The nanotube’s ends are stuck into electrically conductive anchors (A1, A2), which sit on the oxidized surface of a silicon chip. The rotor plate is surrounded by three fixed stator electrodes. Two of them, called “in-plane” stators (S1, S2), are horizontally placed on the silicon oxide surface, while the third one, known as the “gate” stator (S3), is buried below the surface. Different voltage signals, including both direct current (d.c.) and appropriately phased alternating current (a.c.), are available to control the position, speed, and rotational direction of the rotor plate. The most important part of the assembly is the multi-walled carbon nanotube (MWNT). This tube does two main jobs: It holds up the rotor plate and lets electricity pass through to it. And most importantly, it allows the rotor plate to spin freely.

Figure 9.

(a) Nanoactuator concept: Metal plate rotor (R) attached to a multi-walled carbon nanotube (MWNT) acting as support and rotation axis. Electrical connection via MWNT and anchor pads (A1, A2). Three stator electrodes (S1, S2, and S3) for voltage control. SiO₂ surface etched for rotor mobility. (b) SEM image of nanoactuator pre-HF etching, scale bar: 300 nm [62].

This nanomachine made with CNT was built using lithography. Carbon nanotubes produced through a standard arc method [63] were mixed in 1,2-dichlorobenzene and applied onto silicon substrates that were heavily doped and coated with a 1 mm layer of SiO₂. The nanotubes were placed by finding marked spots on the SiO2 surface using either an atomic force microscope or a scanning electron microscope (SEM). The rest of the actuator parts, like the rotor plate, stators, anchors, and electrical leads, were made using electron beam lithography [64]. The Cr/Au was heated to 400°C to improve how well it sticks to the MWNT both electrically and mechanically. An HF etch [65] was utilized to remove approximately 500 nm of the SiO₂ surface, allowing clearance for the rotor plate once it underwent a 90-degree rotation.

Because the shells in multi-walled carbon nanotubes have a little friction, they can easily move relative to each other [66]. Numerous nanomachines have been developed using materials like MWCNTs or double-walled carbon nanotubes (DWCNTs), such as bearings [27] and motors [67], because of their unique properties. The DWCNT can be considered as a rotor (inner tube) and a stator (outer tube) [62]. The speed of the rotation in this molecular machine can be adjusted, by changing the chirality of the stator (outer CNT). To understand the effect of stator radius and chirality on the rotation of rotor, Cai and his team [68] studied eight motor models. These models all had the same spinning part but different stationary parts. They ran molecular dynamics simulations to figure it out.

In studying how rotary nanomachines work at a molecular level, authors used AIREBO potential [69] to describe how carbon atoms interact with each other in terms of their bonds and attraction over distances. The entire system followed a canonical NVT ensemble [70], which has been implemented by a Nose–Hoover thermostat [71]. After relaxing the DWCNT for 400 picoseconds, the atoms on the outer tube stopped moving, but the ones on the inner tube could still move freely. In the simulations, each step lasts for 1 fs. The authors created eight different motor models using double-walled carbon nanotubes, each with different sizes as shown in Figure 10. You can find the details of these models in Table 1.

Figure 10.

(a–h) Eight motor designed with identical (9, 9) rotors but varying stators [68]. (i) The schematics of the conventional electrical motor which consists of rotor and stator parts.

Model	Chirality	Radii difference (nm)	Lengths (nm)
(a)	(9, 9)/(23, 0)	0.295	2.993/2.016
(b)	(9, 9)/(21, 4)	0.305	2.993/2.003
(c)	(9, 9)/(16, 11)	0.315	2.993/1.972
(d)	(9, 9)/(18, 9)	0.325	2.993/1.987
(e)	(9, 9)/(19, 8)	0.335	2.993/2.011
(f)	(9, 9)/(20, 7)	0.345	2.993/2.006
(g)	(9, 9)/(21, 6)	0.355	2.993/2.050
(h)	(9, 9)/(19, 9)	0.365	2.993/1.975

Table 1.

Parameters and of DWCNTs motor models in Figure 10 [68].

Figure 11 demonstrates how the difference in sizes between the two tubes affects how the inner tube rotates. For instance, when the gap is only 0.295 nm (as shown in Figure 11a), the inner tube barely rotates. The walls being close together creates a large barrier, stopping the system from reaching an equilibrated state. Out of all the nanomachines, the inner tube spins fastest when the radius difference is 0.325 nm (shown by the green curve in Figure 11d). This distance is almost the same as the equilibrium distance between graphite sheets, which is 0.335 nm [72].

Figure 11.

Rotational frequency histories of inner tubes within motors, highlighting different radius differences (RD). The curves represent temperatures: 1000 K (green), 800 K (pink), 500 K (blue), 300 K (red), and 100 K (black). Positive values denote counterclockwise rotation, while negative values signify clockwise rotation [68].

At the temperature of 100 K, only one case shows the inner tube rotation (Figure 11e). In other words, initiating rotation of the rotor is challenging under low temperatures. Figure 11 shows that the inner tube’s rotation peak changes with the temperature in all models. For example, in Figure 11b, the inner tube spins fastest at about 100 GHz, when it is at 1000 K (green line). In the (9, 9)/(18, 9) motor, the highest frequency hits around 260 GHz at a temperature of 1000 K, shown by the green line in Figure 11d. It is the highest out of all cases. In the model (9, 9)/(21, 6), the highest rotational frequency hits around 220 GHz when the temperature is 800 K (shown by the pink curve in Figure 11g). At 500 Kelvin, the highest frequency is about 200 gigahertzes, shown by the blue line in Figure 11h. So, when the temperature is higher, it is easier for the inner tube to reach high rotational frequencies [73]. However, the fastest spinning of the rotor might not happen when the motor is hottest.

In Figure 11e and f, the rotor suddenly stops during the simulation. This happens at 3285 picoseconds for the green curve (1000 K) in Figure 11e and at 1679 picoseconds for the pink curve (800 K) in Figure 11f. Figure 12 displays where the two tubes were located in the (9, 9)/(20, 7) nanomotor, before and after the inner tube suddenly stopped. In Figure 12a, you can see that the yellow atom ($sp$ hybridization) on the stator moves more away from its normal position (the gray circle) than the other atoms at the stator’s end. Therefore, the yellow atom is more attracted to the rotor than the other atoms on the stator. At the 1671st picosecond, the two yellow atoms are near each other, but they are not very close. Therefore, the rotor keeps spinning steadily until the 1678th picosecond. Before the 1679th picosecond, the two yellow atoms on the rotor (blue atoms) and the stator (pink atoms) are not joined together. By looking at how the motor is set up at three different moments (i.e., 1679th, 1680th, and 1681st), we can see that an atom of the inner tube is bonded to the yellow atom on the stator. It shows that the rotor does not spin on average over time.

Figure 12.

Snapshots showing the left end of the system before and after the rotor experiences an abrupt halt within the (20, 7) stator. In (a), the gray circle illustrates the theoretical symmetric boundary of the left end of the stator [68].

The free space inside the nanotube allows for different applications [74, 75] such as building nano-oscillators. Legoas et al. [76] did the first study using molecular dynamics for these systems. Their research demonstrates that oscillating movements can be obtained by many different sizes and types of tubes, including armchair, zigzag, chiral, and mixed types. The structures of these nanomachines were made like this: A tube with closed ends was the core, and it could move. Then, one or more tubes with open ends (either single-wall or multi-walled) wrapped around the core tube, creating layers around (Figure 13). An initial velocity is assigned to the internal closed SWNT to start its movement.

Figure 13.

Oscillating behavior of a (9,0) nanotube within a longer (18,0) nanotube [76].

They ran MD simulations to study the nanomachine using classical mechanics. They used a common molecular force field [77] that considers different types of interactions between atoms. In each simulation, the system was minimized to find the energy differences less than $5\times {10}^{-3}kcal/mol$, and the maximum atomic displacements are less than$5\times {10}^{-5}\AA$ . The authors performed simulations under micro-canonical ensemble [78]. In NVE ensemble, the simulation system finds a constant number of particles, volume, and total energy. Legoas et al. [76] conducted the simulations with 1 femtosecond time step.

The authors findings revealed that both external SWNT and MWNT have similar overall dynamic features. When you add more layers on the outside of the multi-walled carbon nanotube, the layers inside become stiffer [79]. When three external nanotubes are used, the oscillatory frequency reaches its maximum level. In the rest of the chapter, we will exclusively present and discuss the outcomes pertaining to scenarios involving two shelled nanotubes. In these scenarios, the outer nanotube remains stationary while the inner one is unrestricted in its motion across all dimensions.

In Figure 14, the results of the simulations are depicted for cases where stable oscillatory motion occurs. Figure 14a and b displays the potential energy and forces of core nanotube, respectively. At first, the inner tube is moving at $1200m/s$ along the axis of outer nanotube. When the outside tube covers the inside one, there is no force (as shown in Figure 13a, and point A in Figure 14a and b). When the inner tube reaches the end of outer one, due to the increase of the potential energy, the core CNT slows down and stops moving (see Figure 13c, and point C in Figure 14a and b). After that, it starts moving back, and the force keeps pushing, now assisting the intrusion (shown in Figure 13d, and point D in Figure 14a and b), until the core tube is completely covered again. The same thing happens again on the other side (Figure 13f, and point F in Figure 14a and b) until the whole cycle finishes.

Figure 14.

Results of molecular dynamics simulations. In panels (a) and (b), they behave like perfect oscillator swinging back and forth smoothly. (e) this perfect oscillation is related to (9,0) and (18,0) carbon nanotubes. But when the coupling of nanotubes is not perfect (i.e., (5,5) and (19,0) nanotubes in panel (f)), the oscillation becomes less steady (panels (c) and (d)). Thicker lines show average forces [76].

The stable oscillation of the core nanotube (Figure 14a and b) is achievable when a perfect coupling is established between the pair tubes [80]. According to Figure 14e, the (9,0) and (18,0) carbon nanotubes are perfectly coupled. In other cases, the oscillating behavior is also observable in the pair nanotubes. However, this oscillation is not completely sustainable. Figure 14c and d represents the potential energy and force of the oscillation of (5,5) and (19,0) nanotubes, which are not perfectly coupled (Figure 14f).

3. Conclusions and outlook

In this chapter, we reviewed the basic ideas of using carbon nanotubes in the construction of nanomachines. These molecular machines perform the manipulation of materials at nano-meter scale. The cylindrical geometry of carbon nanotubes permits the transportation of materials along the length of nanotube or the rotations of nanoparticles around the CNTs axis. The hollow structure of CNT also provides this opportunity to obtain nano-oscillators, in the range of gigahertz. The fabrication of nanomachines based on carbon nanotubes leads to achieve the controllable motion of materials at nano-scale, which has several applications in the field of nanotechnology. As an example, these molecular machines can be employed to perform special tasks at nano-scale. On the other hand, the common methods of the manipulation of nanomaterial, such as scanning tunneling or atomic force microscopes, are limited at specific small dimensions [81]. The CNT-based nanomachines also permit the nano-scale transportation of material. This possibility can be utilized in the fabrication of nanostructures constructed through the bottom-up assembly of smaller nanoparticles. The mentioned applications of CNT-based nanomachines hold great promise for future nano-technological devices.

Conflict of interest

The author declares no conflict of interest.