Freestanding Triboelectric Nanogenerator Enables Noncontact Motion

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Freestanding Triboelectric Nanogenerator Enables Noncontact Motion-Tracking and Positioning Huijuan Guo, Xueting Jia, Lu Liu, Xia Cao, Ning Wang, and Zhong Lin Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00140 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Freestanding Triboelectric Nanogenerator Enables Noncontact Motion-Tracking and Positioning Huijuan Guo,†, ‡ Xueting Jia,§, ‡ Lu Liu,† Xia Cao,*,†,§Ning Wang,*,§ Zhong Lin Wang*,†,┴ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National

Center for Nanoscience and Technology (NCNST), Beijing, 100083, China §

Research Center for Bioengineering and Sensing Technology, University of Science and

Technology Beijing, Beijing, 100083, China ┴

School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia

30332-0245, United State * Corresponding author. E-mail: [email protected]; [email protected]; [email protected]

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KEYWORDS: triboelectric nanogenerator, noncontact sensor, self-powered positioning and motion-tracking system

ABSTRACT: Recent development of interactive motion-tracking and positioning technologies is attracting increased interests in the many areas, such as wearable electronics, intelligent electronic and internet of things. For example, the so-called somatosensory technology can afford users strong empathy of immersion and realism due to their consistent interaction with the game. Here, we report a noncontact self-powered positioning and motion-tracking system based on freestanding triboelectric nanogenerator (TENG). The TENG was fabricated by nano-engineered surface in the contact-separation mode with the use of a free moving human body (hands or foot) as the trigger. The poly (tetrafluoroethylene) (PTFE) arrays based interactive interface can give an output of 222 V from casual human motions. Different from previous works, this device also responses to a small action at certain height of 0.01m~0.11m from the device with a sensitivity of about 315 V·m-1, so that the mechanical sensing is possible. Such a distinctive noncontact sensing feature promotes wide range of potential applications in smart interaction systems.

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Virtual reality has recently gained significant attention in a variety of fields, including those of medicine, gaming, and rehabilitation.1-3 To enable interaction between the user and a virtual environment and bring users senses of immersion efficiently, orders and feedback must be provided quantitatively according to the user’s force sensation and motion control in responding to a change in environment.4 Accordingly, there is a significant need in the field of interactive somatosensory technology with a total feeling of control and immersion of the game by using gesture and movement. 5, 6 For example, somatosensory controllers enable the user to interact with a computer by hands, feet, arms and even eyes. Composed of both a depth of field camera and a RGB camera, these systems can simultaneously capture color images, infrared images, and the user's gestures, thus identifying the distance and action in real time, making it possible for fabricating intelligent robotics.5, 7 However, these devices operate on the base of optical cameras, which may not be fully capable to collect basic information of the object and user such as distance and moving actions in dark, especially in 3D.8 Besides the optical limitation, another question comes from their power source. Nowadays electrochemical batteries are mostly used for driving these devices. The limited capacity is becoming impractical along with the fast increase of number, distribution and complex functions of diverse sensors for non-contact feedback.9-11 Triboelectric nanogenerator (TENG) can collect various mechanical energies from environment, which are then converted to electricity to power mobile devices directly base on the coupled triboelectric and electrostatic effects.12-14 TENG was initially developed for collecting mechanical energy from ambient environment, which will otherwise be wasted. Recently, they also find their way for wide range of applications in mobile/wearable/portable electronics,15,

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biomedical devices,17 sensor networks,18 environmental protection,19-21 infrastructure inspection

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and defense security due to their intrinsic sensitivity to force and motion.22 It is also effective for position detection and motion trajectory, which are indispensable functions for logistics, transportation, and Internet of things.23, 24 Now TENGs can be designed as of miniaturization, high precision, and simple structure.23 In theory, a TENG is like a flat panel capacitor.19 As designed in a double-electrode mode, information on the incoming objects can be conveniently read from the output signal of the TENG due to the electrostatic induction effects. Such TENG-based sensors have also additional advantages for producing electricity, so these devices are especially suitable for developing self-driven electronics and find applications in the recently emerged somatosensory interactive systems.25,

26

However, no reports on the TENG-based sensors can operate in the

noncontact mode. If used in somatosensory interactive systems for location, distance and motion sensing, the TENG-based self-driven system may render an innovative and effective approach toward the above-mentioned internet of things. 23, 27 Herein we report a TENG-based vertical positioning and motion tracking device for somatosensory interaction in a freestanding triboelectric-layer mode. This device can collect mechanical energies from human activities as well as environment. Interestingly, it can also simultaneously provide information on the vertical position and distance of a approaching object. The etched poly-(tetrafluoroethylene) (PTFE) nanoparticle arrays significantly increased the effective contact surface and triboelectric density. A sound output of 222 V and 18 μA were obtained from an etched PTFE film (2 cm × 2 cm). We observed a wide linear range in response to vertical height of the motion objects. The sensitivity is of about 315 V·m-1 and sensing range is of from 0.01 m to 0.11 m. With advantage of low cost, easy fabrication and high efficiency, this non-contact position-sensing device holds great potential for applications in somatosensory interaction and internet of things.

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RESULTS AND DISCUSSIONS Figure 1a illustrates the structure of the as-designed TENG. Due to easy handling and excellent impact strength, poly (methyl methacrylate) (PMMA) covered with Cu film was used as the substrate. The copper (Cu) film acts as both an electrode and triboelectric layer. Figure 1b shows the assembling process and Figures 1c-e illustrate the morphology of PTFE surface. Nanoarrays on the PTFE surface can be clearly seen from scanning electron microscopy (SEM) images (Figures 1d, e) compared to original PTFE (Figure 1c), and it was reported that the output performance of TENG can be improved.28, 29

Figure 1. (a-b) Structure and assembling process of the as-designed TENG. (c-e) SEM images of the PTFE, the original PTFE surface(c) and nanoparticle arrays structure of PTFE after etching(d, e).

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The working principle of the as-designed TENG as an energy collector is illustrated in Figures 2a-e. Figure 2a shows the original position without external force where the initial gap distance is defined as d, and no charge is generated. Figure 2b-e show a cycle of electricity generation process based on triboelectric and electrostatic effects. As sketched in Figure 2b, when exerting a force upon the surface, contact or friction occurs between the PTFE and copper film. At the molecular level, triboelectric effect happens at this time due to the adhesion, attraction and other complex activities between the contact molecules. Though unpredictable, the charge quantity and polarity can be roughly depicted and judged by the triboelectric series.30-32 In this case, electrons are injected from Cu film 1 to PTFE and equivalent charges are created on the two surfaces. Once the force is removed, the contacting surfaces are separated due to inertia. At this time, the Cu2 has a higher potential than the Cu1 and thus the electric potential difference drives free electrons from the Cu1 to the Cu 2 to reach an electrostatic equilibrium (Figure 2c). These electrons stop flowing along with the increase of the gap distance until back to initial state (Figure 2d). As the force once again is applied, the electrons flow in a reversed direction until the two surfaces fully contact with each other, starting another cycle. In principle, TENG is very similar with a flat-panel capacitor. 19, 33-35 If we define the electric potential of the bottom side (Vbottom) as zero, the electric potential of the top side (Vtop) can be expressed by

Vtop =

 d' 0

(1)

where σ is the triboelectric charge density, ε0 is the vacuum permittivity, and d’ is the gap distance between the two panels. The open-circuit voltage (Voc) is equal to the Vtop, which can be expressed by

Vtop =Voc =

 d' 0

(2)

Therefore, the Voc increases along with the increasing distance. The Voc reaches the maximum

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when the distance reaches the original value.

Figure 2. Schematic diagram (a) and operation principal of the as-designed TENG (b-e). (f-g) Operation principle of detecting position and distance of a moving hand. Figures 2f, g illustrate the working mechanism of the as-fabricated TENG for vertical positioning system. In brief, as the Cu electrode 1 reverts to the initial position, the self-powered TENG stops working. At this time, an electric potential difference is produced between these two electrodes. When the hand with some negative charges is approaching, the distance between the

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top plate and hand reaches the critical distance (Dc) (We define the maximum distance between the hand and top plate of TENG as critical distance where TENG stop producing electrical signals). When the hand continues to approach, the negative charges on the hand will affect the inductive charges on the top plate. As a result, inductive charges on the bottom plate are also reduced. We can then get a reduced voltage signal from the reduced electric potential difference. As the distance decreases, so does the voltage. The height of the object in the vertical direction is quantitatively detected by changing the electric potential of the TENG. The electrical output signals of the as-fabricated TENG triggered by a hand with stable applied force are shown in Figure 3. In this experiment, sizes of the TENGs are 4 cm × 4 cm and d is set as 1.0 cm. Figures 3a–c display the open-circuit voltage (Voc), short-circuit current (Isc), and the amount of transferred charges (Q). Here a stable performance was observed (Voc = 222 V, Isc = 18 µA, and Q = 0.08 μC). Considering a contacting area of around 16 cm2, the current density was nearly 1.2 μA·cm-2 (Figure 3d). The peak value of Isc (18 μA) corresponds to the press at half cycle (inset of Figure 3b). The positive voltage of 222 V is got because of an instant charge separation (inset of Figure 3a). The voltage remained unchanged until another half cycle starts.

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Figure 3. Output performance of the as-designed TENGs: (a) Voc, (b) Isc, (c) transferred charges, and (d) current density. To explore the potential applications of coupled triboelectric effect and electrostatic induction, a self-powered positioning device for measuring the height and distance of a moving object was designed (Figure 4). Here human hand acts as an active object. The real-time of Vtop that we obtained is the peak of voltage at highest point and the value of Voc is peak to peak value of voltage. Figure 4 (I) shows that the TENG was triggered by hand patting and Voc is 222 V. As the impact withdrew, the TENG reverted to the original position. The Vtop was 222 V at this time. When the hand approached the top plate of TENG within the critical distance, the Vtop decreased. Test results show that the critical distance is 11 cm in this case. Figure 4 (II) shows the output voltage signals

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when the hand moved up and down in a range from 0 to 11 cm. The Vtop changed distinctively because the inductive charges on the top plate were partially screened when the hand contacted with the top plate. There was no contact between the top/bottom plates, thus the Vtop did not reach zero. Figure 4 (III) shows the output voltage signals when the hand moved up and down with the distance between the TENG and hand from 1 to 11 cm, and the distance of the hand moved decreases gradually. There was no contact with between the hand and the top plate. As a result, Vtop changed slightly compared with Vtop in Figure 4 (II). This result discloses a noncontact position sensing technology, that can be used in somatosensory system to bring users senses of immersion by quantitatively responding according to the user’s force and motion. In addition, the lightemitting diode was lightened up by human hand movements which illustrates an electric signal can be obtained by user's movement and gesture, as shown in the Video S1 (Supporting Information). This technology hints us the scene in “Iron Man 2”, where the computer was turned on with a wave of hand.

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Figure 4. Detecting process of position system. (I: the output voltage signals by hand patting to trigger the TENG, II: output voltage signals when the hand moved up and down with the distance between the TENG and hand rising from 0 ~ 11 cm. III: the output voltage signals when the hand moved up and down with the distance between the TENG and hand rising from 1 ~ 11 cm.) To characterize the TENG’s response to motion quantitatively, electromechanical details of the TENG were collected by moving hand around and upright. Figures 5a-e show the output voltage signals when the hand moved at different heights, that is, 11, 10, 8, 4 and 2 cm, respectively. The corresponding range of motion are 11 ~ 1 cm, 10 ~ 1 cm, 8 ~ 1 cm, 4 ~ 1 cm, and 2 ~ 1 cm. As the height and the motion distance decrease, so do the Vtop and ΔVoc. As shown in Figures 5ae, the Vtop are 222, 218, 211, 198 and 194 V, and the ΔVoc are 33, 29, 23, 16 and 11 V, correspondingly. Figure 6f shows the positive correlation of Vtop with the height (H) of the motion

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object. As expanded in Figure 5f, the Vtop increases linearly with the height in the range of 1 ~ 11 cm, with a correlation coefficient (R2) of 0.991 and a sensitivity of about 315 V·m-1. Figure 5f also shows that the ΔVoc rises linearly up with motion distance (D) in the range from 11 to 1 cm, with a correlation coefficient (R2) of 0.979 and a sensitivity of about 232 V·m-1. These results demonstrate that the TENG is a good candidate for noncontact positioning system. In addition, because this selective sensitivity toward the distance, we can even define the size and even morphology of the objects by dividing the unit electric density in a pixelated pattern in the future.

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Figure 5. Detecting details of position system. (a-e) The output voltage signals when the hand moved up and down at different heights. (f) Fitting curve of output voltage signals responding to heights and motion distances in the range of 1 ~ 11 cm. The influences of several important parameters (the gap distance between the two plates and working hours) on the sensing performance of the TENG-based sensor were further studied to

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aquire a deeper understanding of work principle. According to the formula (2), the gap distance d2 has a great influence on the output voltage. Firstly, we explored the influence of gap distance between two plates on the Voc and the critical distance of TENG. The as-fabricated TENG was mechanically triggered by a linear motor with the same impact rate of 0.02 cm·s-1 to get stable and uniform contacting force. When the d is 1, 2, 4 and 8 cm, the Voc is 40, 25, 16 and 5 V and Dc is 25, 19, 15 and 11 cm, respectively, as shown in Figure 6a-b. The results show the Voc and Dc increased with increase of d and it verifies that the self-powered positioning performance was improved along with the increase of gap distance.

Figure 6. (a-b) The output voltage and the critical distance with different gap distances.(c) Duration of output voltage signals for working 30 s, 60 s and 90 s. (d) Decay fitting curve of the Voc . Finally, the stability and durability of the TENG-based positioning system was tested by

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investigating the decay rate. A Voc of 222 V was obtained by hand patting for 30 s. Next, the TENG was placed in an independent space and the output voltage signals of the device were measured. It can be seen that the output voltage signals of the fabricated TENG can be kept for a long time. As sketched in the Figure 6c, the duration of output voltage signals that TENG produced is 120, 200 and 260 min when the TENG worked for 30, 60 and 90 s, respectively. Figure 6d showed that the duration of output voltage is 120 min and the decay rate is 0.0287 V·s-1 after working for 30 s. Therefore, the self-powered motion and distance tracking device can have both the good sensitivity and potential for the self-powered positioning. CONCLUSIONS In summary, we have demonstrated that a TENG can serve not only for harvesting mechanical energy from objects motions but also for non-contact positioning and motion tracking sensor. The as-fabricated TENG presents excellent output performance and accurate position detection by utilizing nanostructured PTFE film. The dual-electrode TENG shows much stronger triboelectric and electrostatic induction effects in comprison with its single-electrode conterpart. The hand-driving output voltage is of 222 V and the sensitivity is of about 315 V·m-1 within the critical distance of 11 cm of the motion object. In this protocol, we can conveniently define the size and even morphology of the objects by dividing the unit electric density in a pixelated pattern in the future. This self-powered noncontact positioning system has significant advantages such as low cost, real-time monitoring, easy assembly processing and represents a great advance of TENGs for somatosensory interaction system. METHODS

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Fabrication of Nanoarrays on PTFE Surface. In brief, a PTFE thin film (0.08 mm) was consecutively cleaned with alcohol, deionized water, and blown dry with nitrogen gas. Then, a thin copper film (about 10 nm) was deposited onto the PTFE surface by using e-beam evaporator (EBeam, Denton, USA). Surface nanostructure was obtained by inductively coupled plasma reactive ion etching (ICP-RIE). In the reactive ion etching stage, Ar, O2 and CF4 was sent into the ICP chamber with flow rates of 15.0, 10.0 and 30.0 sccm, respectively. The etching time was about 60 s, the pressure is 2 Pa and the substrate temperature is 20℃. One power of 100 W was used to accelerate the plasma ions. Fabrication of the triboelectric nanogenerator. The PTFE layer with nanoparticle arrays was fixed onto a Cu film, and a PMMA substrate was used to fabricate the PTFE/Cu/PMMA electrode. The counter electrode was fabricated by gluing a thin copper thin film onto another PMMA. Next, the two electrodes waere assembled in a sequence of PMMA/Cu/PTFE-Cu/PMMA and separated by springs. The gap distance d was set as 1 cm, 2 cm and 4 cm, respectively. Subsequently, the two electrodes were connected to collect electric signals, as shown in Figure 1. Characterization. The morphology of the PTFE was characterized by a scanning electron microscope (SEM 450, FEI, Nova Nano). External forces were applied by a linear mechanical motor (LinMot-Talk 6). The output electrical signals were collected by a Keithley 6514 electrometer. Related pictures and videos are taken with the digital camera (Canon). Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information, videos (AVI). AUTHOR INFORMATION

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Corresponding Author *(X. C.) E-mail: [email protected] *(N. W.) E-mail: [email protected] *(Z. L. W.) E-mail: [email protected] Author Contributions The manuscript was prepared through the contribution of all coauthors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT We thank the financial support from National key R and D project from Minister of Science and Technology, China (2016YFA0202702), the National Natural Science Foundation of China (NSFC No. 21575009, 21173017, 51272011, 21275102, 51432005 and Y4YR011001), the science and technology research projects from education ministry (213002A).

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