The photo‐mechanical response characteristics of carbon nanocoil‐based cantilever

Due to their natural tiny size, large aspect ratio and excellent mechanical property, 1D nanotubes and nanowires have attracted much attention for their promising applications in micro-actuators, such as micro/nano electro-mechanical systems, motors, and biomimetic robots. Carbon nanocoil (CNC), which has outstanding mechanical, electrical and thermal properties, is a kind of quasi-1D carbon nanofiber with unique helical morphology. The excellent flexible and stretchable properties of CNCs benefit their applications in the fields of strain sensors [1, 2] and biological probes [3]. Because of the large aspect ratio, CNCs have been used as cantilevers for mass sensing [4] and mechanical resonators [5–7]. However, the small size of CNCs reduces the operation efficiency while increasing the possibility of mechanical invasiveness. Compared to conventional mechanical manipulation, remote actuation is a desirable methodology for the activation of nanoscale actuators under complex or special conditions. With the advantages of good controllability, low power consumption and non-invasion, photo-actuation is one of the most powerful approaches for contactless control of nanoactuators. In recent years, laser has been widely used for characterization, manipulation and assembly of micro/nanowires [8–12]. Some progress, which focuses on the photo-actuation of CNCs, has been made. It is found that CNCs show notable photo-mechanical response under laser irradiation [13, 14]. Our previous work has realized the photo-induced vibration of CNC cantilever [15]. In addition, CNC is a good backing material for nanocomposites due to its large specific area. Photo-driven nanocomposites based on CNC/TiO2 and CNC/VO2 with multi-motion modes have been developed for photocatalytic nanomotors and infrared micro-detectors, respectively [16–18]. Although considerable progress is achieved, there are some problems restricting the applications of CNC-based photoactuators. The preparation of CNC-based nanocomposites is expensive and time-consuming, while the small photo-induced amplitude of pure CNC is small. Due to the small optical force (in the scale of 10−11 N), the amplitude-to-length ratio of pure CNC cantilever is approximately 0.05 [15], which cannot meet the actual needs. Specifically, the mechanism of light-CNC interaction is still not clear.


FIGURE 1 (a) Optical image of a single CNC cantilever. (b) SEM image of a single CNC cantilever
In this paper, the static photo-mechanical response characteristics and dynamic photo-induced vibration of CNC are investigated. It is found that the photo-mechanical behaviour of CNC is not only attributed to the optical pressure, but also affected by the photothermal coupling between CNC and surrounding environments, which comes from the excellent photothermal ability of CNCs. Moreover, for a curving CNC, through regulating the irradiation point from free end to the fixed end, the vibration amplitude of CNC cantilever increases approximately five-fold. The photo-thermal and photo-mechanical behaviours help CNCs find potential applications on nano-scale photocontrolled thermal generators, mechanical resonators and other opto/electro-mechanical switches.

EXPERIMENTAL DETAILS
CNCs were synthesized by a chemical vapor deposition (CVD) method [19,20]. The prepared CNCs have unique helical morphology with an average coil diameter of 830 nm and an average pitch of 420 nm. A single CNC was fixed to a tungsten microtip with silver paste and extracted from the prepared CNCs cluster. Figure 1 shows the optical image and the SEM image of a single CNC. It is found that an individual CNC has extremely large aspect ratio to be considered as a natural cantilever. An optical circuit was set up for observing, stimulating and measuring the photo-mechanical response behaviour of CNC (the experimental schematic is shown in [15]). A focused laser is used to apply photo-mechanical interaction on CNC, the diameter of the laser spot is approximately 4 µm. Through an

The static photo-mechanical response characteristics of CNCs
Previous researched have demonstrated that a CNC cantilever, no matter exposed to air or water, shows lateral deformation under the irradiation of a focused laser beam, namely a CNC shows mechanical response to laser radiation pressure [13,15,21]. The mechanism behind is attributed to the momentum exchange between photons and the CNC.
As shown in Figure 2(a), when a laser beam is focused between two adjacent CNCs, which are exposed to air, the distance between two CNCs becomes larger. Intuitively, the position variations of the two CNCs exactly come from the laser radiation pressure. However, compared with optical irradiation on a single CNC, an "enhancement of photo-mechanical response (EPR)" effect is found. The displacements of the free end of CNC1 and CNC2 (marked as Δs 21 and Δs 22 , respectively) are larger than the displacements when they are irradiated separately (marked as Δs 1 and Δs 2 , respectively) at the same position without another CNC. It is found that the EPR effect is closely related to the distance between the two CNCs (D), the cross length (L) and the laser power (P). As shown in Figure 2(bd), the displacements of CNC1 and CNC2 both have positive relationship with L and P, while negative relationship with D, respectively. During the experimental process, keep CNC2 and laser point still while changing the value of D and L by moving CNC1.
The enhancement factor (N i ), which is used to describe the interaction between two CNCs, is written as below: where Δs 2i (i = 1, 2) are the displacements of CNCs when they are irradiated together, Δs i is the displacement of CNC when it is irradiated solely. For both two CNCs, N i ≥ 1. With the distance between two CNCs (D) increasing, the laser radiation pressure of CNC1 decreases rapidly, Δs 1 tends to be 0. However, even CNC1 is far enough away from CNC2 (D > 10 µm or L < −10 µm), Δs 21 is still available. Thus, N 1 goes from about 2 to ∞ (which is not shown in Figure 2). For CNC2, N 2 is also strongly affected by the close relationship between two CNCs. With D increasing or L decreasing, N 2 tends to be 1, as shown in Figure 2(b,c), namely, the EPR effect disappears. Conversely, with CNC1 closing to CNC2, the EPR effect becomes more notable. The detailed process is shown in Movies S1 and S2, Supporting Information. The cause of EPR effect is considered as a result of photoinduced electrical repulsion at first. However, the EPR effect still exists even if the CNCs are electrically grounded or neutralized. Another reasonable hypothesis is the photo-mechanical coupling between CNCs, photons and air. As a kind of black carbon nanomaterial, CNCs have been demonstrated to have excellent photothermal performance. Our previous work has found that the photothermal efficiency of CNCs is approximately 60%, and laser-irradiated CNC absorbs photo energy while releasing thermal energy, which could induce high-speed water flow [22]. In this case, the air around the CNC is heated, especially the air between the two CNCs expands rapidly, which results in an extra repulsive force on the two CNCs. Thus, the practical opto-force of CNC, F opto , can be described as below: where F photon and F air are photon impact force and air propulsion force, respectively. For an individual CNC irradiated alone, F photon is dominant relative to F air , although the CNC is exposed to laser asymmetrically. For two CNCs irradiated simultaneously, F air cannot be ignored. Approximately, the enhancement factor is written as below: where F photon and F air both depend on the distance between the two CNCs (D) and the laser power (P). When the two CNCs are far away from each other, CNC2 is irradiated directly while CNC1 is dark. With D increasing or L decreasing, F photon1 tends to be 0, while F air1 is still available due to the photothermal conversion of CNC2; thus N 1 goes to infinity first and then decreases to 0 (see details in Movies S1 and S2, Supporting Information). For CNC2, F photon2 keeps constant while F air2 decreases with CNC1 moving farther away; thus, N 2 decreases from nearly 2 to 1, namely the EPR effect is gradually weakening. Although the photo-mechanical coupling between two CNCs are strongly modulated by the laser power, the enhancement factor (N 2 ) is not much influenced, as shown in Figure 2(d), which indicates that F photon and F air vary almost equally under laser modulation. Moreover, according to Equations (2) and (3), F air is approximately equal to F photon , which means the surroundings have great influence on the photo-mechanical interaction between CNCs and laser.
Based on the EPR effect, CNCs are fabricated as an optically actuated tweezer for manipulation of tiny objects, such as cell trapping. As shown in Figure 3, a yeast cell is trapped by optical tweezer firstly, and then moved closing to CNC-based tweezers. The CNC-tweezer is actuated by laser in an opening state forcell. Combined with optical tweezer technology, using CNCbased nano-tweezers for cellular physiological characteristics is a further subject.

The dynamic photo-induced vibration characteristics of CNCs
Our previous work has realized the photo-induced vibration of CNC cantilever using a wave-chopped laser irradiating on the free end, and the amplitude is found to show a positive relationship with laser power [15]. However, the vibration characteristics with the irradiation point of laser have not been investigated. As shown in Figure 4(a-c), for a given CNC with a length of 47 µm, under a laser power of 78 mW, the fixed-end-actuated amplitude is much higher than the free-end-actuated amplitude. Figure 5(a) shows the amplitude-frequency curves of CNC with different irradiation points. It is found that the maximum fixed-end-actuated amplitude reaches 11.4 µm, which is approx- The amplitude-frequency and amplitude-to-length curves of CNC with different irradiation points. (a) The relationships between actuation frequency and amplitude. The disappearance of the resonance peak comes from the large air damping (the damping factor of CNC is bigger than 1). (b) The relationships on irradiation point versus amplitude and amplitude-to-length ratio imately five-fold higher than free-end-actuated amplitude (2.4 µm). Figure 5(b) indicates that the amplitude-to-length ratio of CNC cantilever reaches as high as 0.24 under fixed-end actuation, which is also five times higher than free-end actuation. Obviously, longer CNC shows larger vibration amplitude. However, longer CNC suffers from greater air damping (generally, the damping factor becomes more than 1 when the length of CNC reaches 90 µm [5,15]), which would reduce the amplitude. Accordingly, the vibration characteristics of CNC cantilever is dominated by the balance between the length and damping force.
In addition, it is found that for laser irradiation point induced modulation of amplitude generates on curving CNCs, while no such phenomenon occurs on an exactly straight CNC. Our experiments have found the amplitude of straight CNC under chopped laser irradiation decreases monotonously to 0 with the irradiation point moving to the fixed end. A possible hypothesis is that the flex point of curving CNC acts as a non-fixed lever fulcrum; the flex point reduces the effective length of CNC cantilever. Previous work has demonstrated that the resonance frequency is proportional to the squared length of CNC, and CNC show larger amplitude under low-frequency actuation [5,6]. Thus, free-end-actuated CNC cantilever shows a lower effective resonance frequency and a higher-amplitude vibration. A theoretical analysis and finite analysis simulation for describing the vibration characteristics of curving CNCs is a further project.

CONCLUSION
The photo-mechanical response characteristics of an individual CNC cantilever is investigated. Due to the excellent photothermal ability of CNCs, an "EPR" effect is found when two adjacent CNCs are irradiated by a laser, which results from the photothermal coupling between CNCs and surroundings. Moreover, a chopped-laser-induced vibration of curving CNC with tunable amplitude is realized. The fixed-endactuated amplitude-to-length ratio is approximately five-fold higher than free-end-actuated ratio, which reaches as high as 0.24. Because of the excellent photothermal ability and photomechanical response, CNCs have promising applications in the fields of micro/nano photo-activated thermal generators, photo-actuators and photo-controlled bio-tweezers.