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Triboelectric Nanogenerator as a Self-Powered Communication Unit for Processing and Transmitting Information Aifang Yu, Xiangyu Chen, Rui Wang, Jingyu Liu, Jianjun Luo, Libo Chen, Yang Zhang, Wei Wu, Caihong Liu, Hongtao Yuan, Mingzeng Peng, Weiguo Hu, Junyi Zhai, and Zhong Lin Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07407 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 11, 2016

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Triboelectric Nanogenerator as a Self-powered Communication Unit for Processing and Transmitting Information

Aifang Yu, †,⊥ Xiangyu Chen,†,⊥Rui Wang,†,⊥ Jingyu Liu,† Jianjun Luo,† Libo Chen,† Yang Zhang,† Wei Wu,† Caihong Liu,† Hongtao Yuan, † Mingzeng Peng,† Weiguo Hu†,*, Junyi Zhai†,* and Zhong Lin Wang†,§,* †Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, China §School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245,United States. ⊥A.-F. Yu, X.-Y. Chen, and R. Wang contributed equally to this work. Address correspondence to [email protected](WGH), [email protected] (JYZ) and [email protected] (ZLW)

ABSTRACT In this paper, we demonstrate an application of a triboelectric nanogenerator (TENG) as a self-powered communication unit. An elaborately designed TENG is used to translate a series of environmental triggering signals into binary digital signals and drives an electronic-optical device to transmit binary digital data in real-time without an external power supply. The elaborately designed TENG is built in a membrane structure that can effectively drive the electronic-optical device in a bandwidth from 1.30 kHz to 1.65 kHz. Two typical communication modes (amplitude-shift keying and frequency-shift keying) are realized through the resonant response of TENG to different frequencies, and two digital signals, i.e., “1001” and “0110,” are successfully transmitted and received through this system, respectively. Hence, in this study, a simple but efficient method for directly transmitting ambient vibration to the receiver as a digital signal is established using an elaborately designed TENG and an optical communication technique. This type of the communication system, as well as the implementation method presented exhibits great potential for applications in the smart city, smart home, password authentication, and so on. 1

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KEYWORDS Triboelectric nanogenerator; self-powered communication; frequency selectivity; digital signals.

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The development of communication technology has tremendously promoted the progress and prosperity of human society. One of the most important advances of modern communication technology is information interaction between people, environments, and devices, which requires effective communication methods to transmit and received data.1-3 However, in existing portable communication devices, the energy used to harvest and transmit information mainly comes from batteries. Replacing and maintaining large batteries faces many challenges, and widely distributed batteries will inevitably cause environmental issues. Therefore, the construction of self-powered information interaction is urgently needed for the further development of communication technology. During the last decade, self-powered systems operating independently, sustainably, and wirelessly without the use of batteries have been developed intensively.4-8 One of the key elements of self-powered systems is the nanogenerator, which can scavenge various energies from the environment and store these energies in an energy storagemodule.7, 8 Subsequently, the collected power can be used to power other parts of the system for sensing information and transmitting data. Triboelectric nanogenerator (TENG) is a newly invented technology in the nanogenerator family, which can effectively convert mechanical energy into electricity and generate high output power based on conventional organic materials with very low costs.9-11 Ever since the first reported TENG in January 2012, the performance of TENG has increased dramatically. For example, the area power density of TENG has reached 1200 Wm-2, and the energy conversion efficiency has reached 50%–85%.12-14 As a new paradigm for energy harvesting, a TENG-based self-powered system can build up a series of sensor devices for detecting static and dynamic processes occurring in the environment.15-17 The sensor device is the basic unit of the information combination network. The successful development of the self-powered TENG sensor18-23 has inspired us to further consider a self-powered communication system to deliver information that people/things want to express. For this type of communication system, information identification and signal transmission are the key functions. Since the resonance phenomenon of a TENG device under vibration 3


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excitation can result in distinctive output performance in response to different frequencies,24-26 this may help us to establish some working bands for selectively identifying some special signals for delivering information. As for information transmission,

previous

studies

have

usually

considered

the

emission

of

electromagnetic signal to remote receivers,27 which requires sophisticated managing circuits and energy storage systems.28 In this case, an optical communication technique, which consumes less energy for signal emission, may be a promising alternative for realizing self-powered communication systems. In this paper, we designed a novel communication system by utilizing the resonance phenomenon of the TENG and an optical communication technique. An elaborately designed triboelectric nanogenerator (TENG) was demonstrated as a self-powered communication unit, which translated a series of environmental triggering signals into binary digital signals and drove the electronic-optical device to transmit binary digital data real-time without an external power supply. The elaborately designed TENG was built in a membrane structure that can effectively drive the electronic-optical device in a bandwidth from 1.30 kHz to 1.65 kHz. This working band can help TENG to identify specific trigger information and filter out other noise, which can lead to the selectivity of information transmission. The effective electrical power of the TENG reached a maximum value of 18.38 µW with a load resistance of 0.29 MΩ at 1.50 kHz. Typical amplitude-shift keying (ASK), and frequency-shift keying (FSK) methods were realized through the selective response of TENG to different frequencies, and two digital signals, i.e., “1001” and “0110,” were successfully transmitted and received through this system, respectively. Moreover, another cantilever TENG structure was designed to be driven by low frequency vibration signal with a working frequency band from 16 Hz to 22 Hz, thus further demonstrating the capability and feasibility of the TENG technique for communication. Our study established a simple but efficient method for directly transmitting ambient vibration to the receiver as digital signals by using the high frequency selectivity of TENG and an optical communication technique. The prototype of the communication system, as well as the implementation method 4


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exhibits a great potential for applications in smart city, smart home, password authentication, and so on.

RESULTS AND DISCUSSION In order to optimize the frequency selectivity of TENG and enhance its output performance, a membrane-structured TENG that operates in the contact-separation mode was designed and fabricated as shown in Figure 1a, which can collect both energy and information from sound waves. The insert figure of Figure 1a displays the corresponding photograph of the TENG. The membrane-structured TENG consisted of two triboelectric layers made by different materials. One contact face was a polytetrafluoroethylene (PTFE) film with a deposited copper (Cu) thin film as the back electrode, which was adhered onto an acrylic plate with a circular hole. The other contact face was a copper thin film deposited on top of a Kapton film, which was adhered onto a circular acrylic plate. The performance of TENG was highly related to the microstructure of the contact face, which were intentionally prepared through interfacial engineering techniques. The surface modification on PTFE was conducted using inductively coupled plasma (ICP) reactive ion etching in this paper. Different structures were generated under different conditions. Figure 1a displays the optimized uniformly distributed nanowire features, which can significantly increase the surface roughness and the effective surface area of the TENG for effective tribo-electrification. For comparison, the contact face of TENG with and without nanostructures is shown in Figure S1 and discussed in the Supporting Information. The electricity generation mechanism of the TENG is demonstrated in Figure1b. Because of the large difference in the ability to attract electrons, when the Cu contact face was in contact with the PTFE film, surface charge transfer took place in its original state (Figure 1b[I]). Because PTFE had a more triboelectric negative polarity than that of the Cu contact face, electrons were injected from the Cu contact face into PTFE, generating positive triboelectric charges on the Cu contact face side and negative charges on the PTFE side. Ambient sound waves or the noise signals generated from mechanical instruments can all work as trigger signals for this 5


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membrane-based TENG. In our experiment, an acoustic generator was used as the source of the trigger signal, where the frequency of the generated signal can be precisely modulated by software. Due to the wave character of sound propagation, a corresponding acoustic pressure separated the PTFE thin film away from the Cu contact face. As a result, the positive triboelectric charges and the negative ones no longer coincided on the same plane, and an inner dipole moment between the two contact surfaces was consequently generated, which drove free electrons to flow from the Cu electrode to the Cu contact face to screen the local electric field, producing positively induced charges on the Cu electrode (Figure 1b[II]) until it was fully offset. In using the membrane-structured TENG as a self-powered communication unit, the frequency response was of great importance for output performance and communication. Therefore, the frequency response of the TENG was evaluated and displayed. The relationship between frequency and electrical output of the as-fabricated TENG at bands from 0.02 kHz to 3.0 kHz is given in Figure S2. Since a high frequency can generate a greater information width, we only discuss here the frequencies from 1.10kHz to 3.0kHz in Figure 1c.It is also important to note that both resonant frequency and the frequency band can all be modified by changing the structure/parameters of the TENG.26, 29 From Figure 1c, it can be seen that both the voltage and current presented a gradual increase with the increase of frequency from1.10 kHz to 1.50 kHz. Then, both of them rapidly returned to their original values as the frequency increased from 1.50 kHz to 3.0 kHz. The resonant frequency of the system was approximately 1.50 kHz. The TENG only generated significant power in the bandwidth from1.10 kHz to 1.80 kHz. Therefore, the TENG can work as a band-pass filter, which is very important for information identification. It is also important to note that the receiver of this system was a photoresistor, which also had some threshold values for detecting light. Hence, the effective working band for the whole system was a little bit smaller than the working band of TENG alone, which was about 1.30 kHz to 1.65 kHz in our experiment. The bandwidth of this communication system was 0.35 kHz, which can be modified not only by changing the geometric parameters of the TENG but also by the surface structure configuration. 6


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As shown in Figure S3, the TENG with an optimized nanowire structure gave the largest bandwidth because of its high output deviating from the resonant frequency. For communication, the wider the bandwidth is, the faster the communication velocity is. In this study, the TENG was not only used for processing information but also as a power source for real-time transmission of harvested data directly. Accordingly, the performance of the TENG was examined, as shown in Figure 2. Figures 2a-d give the performance of the TENG measured under a frequency of 1.50 kHz. The peak-to-peak open-circuit voltage and short-circuit current were typically approximately 16.25 V and 20.62 µA, respectively (Figures 2a and 2b).The power output capability of the fabricated TENG was investigated by using resistors as external loads. As shown in Figure 2c, the output voltage increased quickly as the resistance increased from 0.001 MΩ to 9MΩ and then was saturated when the resistance was further increased. The output current followed a reverse trend compared to the output voltage. Therefore, the effective electrical power of the TENG was closely related to the external loadand reached a maximum value of 18.38 µW with a load resistance of 0.29 MΩ, which is shown in Figure 2d. It is also important to note that the increase of frequency made the energy transfer more efficient and elevated the maximum output power. From the point of view of the TENG, light power was closely related to the structure of TENG, which selected the correctly paired triboelectric materials, surface structure configuration, and functionalization. Figure S4 gives the electric output of the as-fabricated TENG with different microstructures. It can be seen an optimized nanowire structure gave the highest output because of the highly efficient tribo-electrification between the two contact faces. Of course, the high output can provide

more

accurate

information

transmission.

However,

this

type

of

communication system driven by the TENG with the output in Figure S4a does not normally work at all. The electro-optical device used in this experiment was a commercial LED with a luminescence wavelength of 550 nm. The light power of a single LED was 0.02 µw, and light generated by the LED was received by a photoresistor. The transmitted information was extracted by measuring the voltage of 7


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the matched resistor of the photoresistor. The entire integrated system is shown schematically in Figure 2e. Notably, the self-powered transmitter was very simple, low cost, and emitted low radiation, which merely included two communication units, i.e., a TENG and a common LED without the rectifier bridge and storage units. Moreover, this novel communication technology can be easily compatible with existing communication modes, such as infrared communication, visible light communication, and near field communication, which will expand the application fields of this communication method extensively. As shown in Figure 2e, a communication system was built to evaluate the capability of TENG as a self-powered communication unit in ambient environment. From Figure 1c, the TENG only responded to the trigger signal with a frequency inside the working band. This frequency selectivity can allow TENG to work as a band-pass filter to identify specific signals. Meanwhile, the different output amplitudes of the TENG can also represent two binary digital signals, i.e., “1” and “0,” in digital communication. Accordingly, a series of input trigger signals (with the frequency in or outside the working band) can be translated into binary digital signals for information transmission. Using different amplitudes to transmit information represents a typical ASK method, which requires that one of the two trigger signals must be out of the working band of the TENG. A demonstration of the ASK method for information transmission by using the TENG system can be found in Figure 3. On the other hand, if the frequencies of the trigger signals are all located in the working band of the TENG, the ASK method may not be able to clearly transmit the information. In this case, the different frequencies also can be used to deliver information. Since the received signals from the photoresistor always follow the output signal from TENG, the frequency of the trigger signal will be in good agreement with the received light signal. As seen in Figure 4, two different frequencies of the trigger signal can be considered “1” and “0” for information transmission, while the TENG-based communication system can also successfully transmit the information. This information transmission method is a typical FSK method. Therefore, when the amplitudes of the electrical signals have distinct 8


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differences, the ASK mode can be used to deliver information. By the same token, when the frequencies of electrical signals have distinct differences, the FSK mode can be used to deliver information. Figure 3 demonstrates that the system transmitted information under the ASK mode without a power supply. Different amplitudes represented different in formation. As shown in Figure 3a, a series of triggering signals with a combination of two different working frequencies (f=1.5 kHz and 2.6 kHz) was applied to the acoustic generator. The density of the input sound was approximately 95 dB. After receiving the triggering signal, TENG generated a voltage output with distinctive patterns, as shown in Figure 3b. Here, the triggering signal with a frequency of 1.5 kHz can induce a significant voltage output, while the triggering signal with a frequency of 2.6 kHz led to almost no output from TENG. These results demonstrated the filtering function of the TENG to selectively respond to specific trigger signals, which can provide two distinctive amplitudes for information communication. For information transmission, two amplitudes (a0 and a1) were defined as digital signals "0" and "1". A0 corresponded to TENG, and the photoresistor did not output when the trigger frequency (f = 2.6 kHz) was out of the operating bandwidth. A1 corresponded to TENG, and the photoresistor can work normally when the triggering frequency (f = 1.5 kHz) was located in the bandwidth. When the two trigger frequencies changed alternatively as shown in Figure 3a, the TENG generated corresponding electrical signals, which carried the information of “1001,” as displayed in Figure 3b. These electrical signals were transformed to light signals through the LED. The light propagated in the atmosphere and was received by the photoresistor. Figure 3c exhibits the electrical signals received by the photoresistor. According the signals in Figure3c, a binary encoding of “1001” can be transmitted and received completely by using this self-powered system. In binary-coded decimal, “1001” represents the decimal “9”. If “9” was defined as “on” or “off” beforehand, people or things that possessed the receiver will acquire this information and carry out the associated process. As mentioned in the Introduction, the triggering signal can be generated by people or things, and the working band of the designed TENG (1.30 kHz to 1.65 kHz) 9


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corresponded to the sound waves of humans speaking or noise signals in a daily environment. Therefore, this system has the potential to realize mutual communications of people to people, people to things, and things to things. Moreover, the frequency selectivity of the TENG can help us identify some sound waves with fixed frequencies, which can serve as an acoustic monitor and so on. Meanwhile, the working band of the TENG was related to the resonance frequency of the membrane on the TENG, meaning that we can adjust this working frequency band by altering the structural parameters of the TENG. When the frequencies of the trigger signal are all located within the bandwidth, the amplitudes of the output signal from TENG may not show significant differences. In this case, the frequency of the output signals can also be used to transmit information. Figure 4 demonstrates the transmission of information under the FSK mode in this communication system. First of all, two frequencies of 1.40 kHz and 1.58 kHz (f0 and f1) were selected as the working frequencies of the trigger signal. The trigger signal was applied to the acoustic generator to excite the TENG. Hence, the frequencies f0 and f1 were also the vibration frequencies that were received by TENG. For information transmission, f0 corresponded to the digital signal "0," and f1 corresponded to the digital signal "1" of a binary system. When the frequency of the trigger signal changed in accordance with that displayed in Figure 4a, the TENG will generate corresponding electrical signals which would carry the information of “0110,” as displayed in Figure 4b. These electrical signals drove the LED and were consequently

transformed

to

light

signals.

Accordingly,

the

TENG

had

self-sufficiently finished the collection and transmission of information. The light propagated in the atmosphere and was received by the photoresistor. Figure 4c exhibits the electrical signals received by the photoresistor. It can be seen that the frequency of the electrical signal received by photoresistor was consistent with that generated by TENG in Figure 5b. These results proved that the light signal generated by the TENG-driven LED was a reliable medium for information transmission, and the photoresistor can follow light signals. According to the signals in Figure 5c, a binary encoding of “0110” can be deduced, and the information was transmitted 10


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completely. In binary-coded decimal, “0110” represents the decimal “6”. If “6” was defined as “on” or “off” and as other meanings beforehand, people or things that possessed the receiver will acquire this information and carry out the associated process. Finally, if all frequencies of the trigger signal are out of the bandwidth of the TENG, the photoresistor will not receive any useful information. The environmental energy with frequencies out of the working band of TENG will all be filtered out through this system. Hence, in order to realize binary information transmission, at least one of the environmental trigger signals should have a frequency located inside the working band of TENG. In order to further verify the prototype of the communication system, as well as the implementation method, ASF was demonstrated at low frequencies. An elaborately designed TENG built in the cantilever structure was used to harvest low frequency vibration energies, which widely exist in the natural world. It had a frequency-selectivity with a bandwidth from 16 Hz to 22 Hz. The structure and relationship between frequency and electrical output of the as-fabricated TENG are shown in Figure S5. In order to improve the performance of TENG, the surfaces of the two contact faces were also further nano-configured with uniformly distributed nanowires (see Figure S5). Figure 5 demonstrates that the system successfully transmitted information under the ASK mode at low frequencies. A series of trigger signals with the combination of two different working frequencies (f = 21 Hz and 25 Hz) was generated by the vibration actuator. According the signals in Figure 5c, a binary encoding of “1001” can also be transmitted completely. The design flexibility of the TENG technique can allow this technique to transmit trigger signals with various frequencies. So far, typical FSK and ASF were realized through the response of TENG to different frequencies, which proved the feasibility of TENG as a self-powered communication unit. The high frequency selectivity of TENG combined with the optical communication technique can establish a simple but efficient method for directly transmitting ambient vibration to the receiver as a digital signal. Because the 11


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transmission of information is completed by the frequency selectivity of TENG, this system is difficult to hack. Thus, one potential application of this system can be used as a password authentication key in doors, cars, credit cards and other anti-theft systems. The novel information interaction between people and objects, objects and objects, as well as between people and people, can be useful for demonstrating the feasibility of self-powered communication systems and may promote the development of “The Internet of Things” in the future. A series of work is going to be conducted to further modify the performance and size of the device by using advanced micro or nano fabrication technology to meet the requirements of real applications.

CONCLUSION In conclusion, we demonstrated a self-powered communication unit based on two elaborately designed TENGs working in different frequency regions, where environmental trigger signals with specific frequencies were harvested as the energy supply for the whole communication system. The harvested trigger signals can also be translated into binary digital signals and directly transmitted by the electronic-optical device without an external power supply. The elaborately designed TENG for the high frequency signal was built in a membrane structure that can effectively drive the electronic-optical device in a bandwidth from 1.30 kHz to 1.65 kHz. Typical ASF and FSK were both realized through the selective response of TENG to different frequencies, and two digital signals of“1001” and “0110” were successfully transmitted and received through this system, respectively. FSK and ASF methods can coordinate with each other, which enabled the TENG-based communication system to effectively work with different frequency bands for harvesting and transmitting information. Another TENG design based on the cantilever structure driven by the low frequency vibration signal with a working frequency band from 16 Hz to 22 Hz further demonstrated the capability and feasibility of the TENG technique for information communication. The design flexibility of the TENG technique can allow for the transmission of trigger signals with various frequencies. Hence, in this study, a simple but efficient method for directly transmitting ambient 12


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vibration to the receiver as digital signals was established by using the high frequency selectivity of TENG and an optical communication technique. The prototype of the communication system, as well as the implementation method presented, exhibits a great potential for applications in smart city, smart home, password authentication, and so on.

EXPERIMENTAL METHODS Fabrication of the membrane-based triboelectric nanogenerator. One contact face was a PTFE film with a deposited copper thin film as the back electrode, which was adhered onto an acrylic plate with a circular hole. The tension of the PTFE film was closely related to the resonant frequency. The thickness of the acrylic plate was 20 mm, and the diameter of the circular hole was 55 mm. Subsequently, the microstructures on the PTFE surface were fabricated using inductively coupled plasma (ICP) reactive ion etching. In the optimized etching process, a Cu film with a thickness of approximately 40 nm was deposited on the PTFE surface as the mask. A mixed gas including Ar, O2, and CF4 was introduced in the ICP chamber, where the corresponding flow rates were 15.0, 10.0, and 30sccm, respectively. The nanostructure was etched under a power source of 200 W, which generated a large density of plasma, and the time was30 min. A piece of Kapton film with a deposited copper thin film as the other contact face was adhered onto a circular acrylic plate with a hole diameter of 60 mm. The distance between two triboelectric faces was regulated by a displacement table. Once the distance was optimized for output, two triboelectric faces were fixed by fastening two acrylic plates. Fabrication of the cantilever-based triboelectric nanogenerator. The first triboelectric layer was PDMS, which was coated on a polyethyleneterephthalate (PET, 100µm) film with a Cu electrode. The PET film was used as a cantilever with a thickness of 100µm, width of 2 cm, and length of 4 cm. Subsequently, the nanostructures on the polydimethylsiloxane (PDMS) surface were fabricated by using inductively coupled plasma (ICP) reactive ion etching. In the etching process, an Au film with a thickness of approximately 30 nm was deposited on the PDMS surface as 13


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the mask. A mixed gas including Ar, O2, and CF4 was introduced in the ICP chamber, where the corresponding flow rates were 15.0, 10.0, and 30 sccm, respectively. The nanostructure was etched under a power source of 200 W to generate a large density of plasma. Finally, the cantilever was fixed on an acrylic platform, and its height can be adjusted. For the other triboelectric layer, ZnO nanowires were grown on the PET substrate with a ZnO seed layer. For this growth, the PET substrate was immersed in a nutrient

growth

solution

with

1:1

ratio

of

zinc

nitrate

and

25mM

hexamethylenetetramine (HMTA) in an oven at 70℃ for 5 h. A 100-nm-thick Cu film was deposited on the ZnO nanowires by sputtering at a base pressure of 1.0×10-4 Pa. Finally, the second triboelectric layer was fixed on an acrylic supporter, and its slope angle can be adjusted. Characterizations and measurements. The output voltage of the device was measured using a Lecroy 610Zi oscilloscope with four channels and a load resistance of 10 MΩ. The output current of the device was measured using a low noise current preamplifier (Stanford Research SR570). The morphology of the nanostructures was characterized by scanning electronic microscopy (Hitachi SU8020). The light power of a single LED was measured by an optical power meter (Thorlabs PM100D).

Supporting information Support information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by NSFC 51472056，51503108, the “thousands talents” program for pioneer researcher and his innovation team, China, the Recruitment Program of Global Youth Experts, China. Thanks to Chao Yuan, Dr. Jin Yang, Dr. Xiong Pu, Limin Zhang, Jinzong Kou and Yudong Liu for technical support.

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Figures

Figure 1. (a) Structure, (b) mechanism, and (c) relationship between frequency and electrical output of as-fabricated TENG. The bar in Figure 1a is 1µm.

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Figure 2.Output performance of the as-fabricated TENG and the design of a communication system. (a) Typical voltage and (b) current signals of the TENG; Dependence of (c)output voltage, output current, and (d) output power density of the TENG as a function of the applied external load. (e) Schematic diagram of a communication system based on the TENG as a self-powered communication unit.

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Figure 3. Amplitude-shift keying (ASK) communication mode. (a) The trigger signal and the information will be transmitted. (b) The corresponding electrical output signal of TENG. (c) The received electrical signal by the photoresistor.

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Figure 4. Frequency-shift keying (FSK) communication mode. (a) The trigger signal and the information will be transmitted. (b) The corresponding electrical output signal of TENG. (c) The received electrical signal by the photoresistor.

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Figure 5. Amplitude-shift keying (ASK) communication mode under low frequencies(bandwidth from 16 Hz to 22 Hz).(a) The trigger signal and the information will be transmitted. (b) The corresponding electrical output signal of TENG. (c)The received electrical signal by the photoresistor.

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