Q-switched pulsed laser direct writing of aluminum surface micro/nanostructure for triboelectric performance enhancement

f n b hno chn Received 8 August 2020 Available online 6 November 2020 Keywords: Triboelectric nanogenerator Micro/nano structure Q-switched pulsed laser uctu the net configuration. The crinkled nanostructure of the surface of Polytetrafluoroethylene (PTFE) is freestanding (FS), and single-electrode (SE) [3]. As a micro/nano act and separation the surface tribo- ltage, current and sity is a prerequi- he surface charge ization of the sur- area between two ntioned idea, a lot metal imprint processes [25]; femtosecond direct writing [26]. These technologies have the ability to fabricate the nanostructured metal surfaces. And the Inductively Coupled Plasma - Reactive Ion Etching (ICP-RIE) can successfully be applied to miniaturize the polymer surfaces [27e29]. The Q-Switched Pulsed Laser (QSL) provides a versatile tech- nique in a variety of applications like, for instance, in material * Corresponding author. Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, Viet Nam. E-mail address: phanhai@vnu.edu.vn (H. Phan). Contents lists available at ScienceDirect Journal of Science: Advanc journal homepage: www.el Journal of Science: Advanced Materials and Devices 6 (2021) 84e91Peer review under responsibility of Vietnam National University, Hanoi.ergy into electricity. The four primary TENG concepts that were proposed, include contact-separation (CS), lateral-sliding (LS), of micro/nano fabrication technologies has been reported, such as chemical etching [23]; nanoimprint lithography [24]; metal toclassical battery inheres the key technologies for energy storage and supply solutions. However, the disadvantage of the battery such as its limitation of power storage opens the challenge for the development of self-powered systems. In regard to that problem, the triboelectric nanogenerator (TENG) has been invented and investigated as the most promising energy technology [1,2]. By the conjunction of triboelectric friction and electro-static induction, TENG can be effective to convert external mechanical/physical en- fundamentally operates on the base of the cont of two different materials. The dissimilarity in electric charge affinity generates the output vo power [20]. Therefore, a triboelectric charge den site for the performance of TENG. To improve t density, the most popular solution is a miniatur face structure, which enhances the local contact triboelectric layers [21,22]. To serve the above-meThe integrated power supply plays a vital role in the improve- ment of high mobility device systems. In the past few decades, the [4e6], blue energy harvesting [7e10], Biomedical caring [11e14], sensing and tracking [15e17] and high voltage source [18,19]. TENG1. Introductionhttps://doi.org/10.1016/j.jsamd.2020.11.003 2468-2179/© 2020 The Authors. Publishing services b ( by the Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) technology. By adjusting the QSL source parameters, many kinds of Al/PTFE concepts of synthetic devices with built-in contact-sep- aration could systematically be characterized. The optimal triboelectric performance was confirmed by the increase of the open-circuit voltage and the short-circuit current from 75 V to 148 V and from 6.8 mA to 9.6 mA, respectively, and by a nearly 2.5 times higher power than that of a pristine Al-based device. The real-time practical applicability of the SA-TENG concept was confirmed by the lighting up of 92 LEDs connected in series and by the charging up of a 10 mF capacitor to 1.68 V in 30 s. This work provides an effective direct writing process with a Q-switched Pulsed Laser for the fabrication of micro/nano- structures on Aluminum and for the enhancement of the output performance of TENG, which would greatly promote the TENG branding in mass manufacturing and micro-energy utilizations. © 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license ( power source, TENG has a great prospect as a solution for multi- disciplinary applications such as portable integrated electronics3 November 2020 Accepted 4 November 2020Received in revised form triboelectric nanogenerators (TENG). Here, we present a solution for the enhancement of the triboelectric performance by utilizing a Q-switched Pulsed Laser (QSL) that directly writes on an Aluminum surface inOriginal Article Q-switched pulsed laser direct writing o nanostructure for triboelectric performa Hai Phan a, b, *, P.N. Hoa b, H.A. Tam b, P.D. Thang a, a Faculty of Engineering Physics and Nanotechnology, University of Engineering and Tec b VNU Key Laboratory for Micro and Nano Technology, University of Engineering and Te a r t i c l e i n f o Article history: a b s t r a c t The micro/nano surface stry Elsevier B.V. on behalf of Vietnamaluminum surface micro/ ce enhancement logy, Vietnam National University, Hanoi, Viet Nam ology, Vietnam National University, Hanoi, Viet Nam re of triboelectric layers possesses a critical impact on the performance of ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license polishing, corrosion removing, surface engraving and etching etc [30e33]. Moreover, with the advantages of handling diverse ma- terials, cost saving, mass production, simple operation and rapid fabrication at high stability, the QSL is considered to be the major industrial method for micro/nano surface fabrication in the topic of TENG research and development. In this research, we report on the micro/nano structure enhanced TENG performance method by applying the direct writing on Aluminum with a Q-switched Laser. The dependence of the Al and Al-based surface morphology on the performance of TENG is characterized by following the variation of QSL power, an oven at 50 C for 15min. The prepared Al filmwas etched in a net 2 4 etching chamber, respectively. The plasma was produced by the H. Phan, P.N. Hoa, H.A. Tam et al.400WRF power source and another DC power source of 100Wwas Table 1 Q-switched pulsed laser direct writing parameters. Source power (W) Scanning speed (mm/s) Grooves distance (mm) S0 0 0 0 S1 15 500 10 S2 20 S3 30 S4 50 S5 1000 20 S6 2000 20 S7 20 1000 20configuration by the QSL technology. The laser source generates a pulsed laser beam with an energy of 1 mJ, a width of 100 ms and a wavelength of 1064 nm. In order to scan the laser on the surface of the aluminium film, a Galvo scan head (purchased from Scanlab Company, Germany) was used. The proper condition of the laser source was confirmed by adjusting the groove distance, the laser source power and the scanning speed. The characterized laser source parameters are shown in Table 1. At first, the QSL system operates at a power of 15 W and a scanning speed of 500mm/s while the grooves distance varies from 10 mm to 50 mm. The dependence of the TENG performance on the grooves spacing was examined in that case. Then, the scanning speed and the power were changed consecutively to discover the optimal condition of the QSL laser source on the Al-based TENG performance. 2.2. PTFE surface etching and copper electrode deposition Next, the Inductively Coupled Plasma Reactive Ion Etching (ICP/ RIE) (RIE-ER-2X150, Elettrorava S.p.A.) was applied to create the nanostructure on the PTFE surface. The commercial-graded PTFE film (purchased from Shanghai Shenhao Rubber and Plastic Corp) with a thickness of 100 mmwas firstly cleaned by a similar process as mentioned before. The sample was then etched by introducing O , Ar and CF with flow rates of 10, 15 and 30 sccm into the ICP/RIEscanning speed and groove space. 2. Experimental 2.1. Fabrication of the micro/nano surface structure of aluminum by QSL direct writing At first, a piece of conventional pure Al film (purchased from Oristar corporation Vietnam) with an area of 5 cm
5 cm and a thickness of 200 mmwas cleaned by acetone, methanol, isopropanol and deionized water, consecutively. Then, the sample was dried inS8 40 1000 20 85applied to accelerate the plasma ions. The etching process was conducted within 30 min. The Cu electrode was deposited on the other non-etched side of the PTFE film in 30 min using a sputtering facility (model number ATC 2000) with an Ar flow rate of 40 sccm, a pressure of 107 Torr and a power of 80 W. 2.3. The TENG fabrication The nano-structured Al films and PTFE films (size of 5 cm
5 cm) were used to fabricate the TENG in the vertical sep- aration mode. One nanostructured surface foil of Al and one electrode-deposited nanostructured surface PTFE are added to two pieces of acrylic (thickness of 0.5 cm and shaped by laser cutter with the dimensions of 10 cm
10 cm). The elastic motion and distance between themwas restricted to 1 cm by the installation of springs at the four corners. The total size of the TENG is 10 cm
10 cm x 2.5 cm. 2.4. Characterizations The vertical cyclic motion was generated by a self-constructed motion system which operated by a programmed step motor. The open-circuit voltage was measured with the digital oscilloscope Tektronik DPO 4032 equipped with high voltage probe equipment. The resistance of the oscilloscope and probe stick were 54 MU and 10 MU, respectively. The short-circuit current was measured by a Low-noise Current Preamplifier SR570 (Stanford Research Systems, Sunnyvale, CA, USA) that was connected with the oscilloscope. The surface morphology and the structure properties of the Etched PTFE and the QSL etched Al were examined by a Field Emission Scanning ElectronMicroscopy (FESEM) (Jeol 6490 JED 2300 - Japan) system. 3. Results and discussion Fig. 1 illustrates the TENG fabrication process. As in Fig. 1a, the PTFE film surface was firstly miniaturized, then, the Cu electrode was deposited. The surface of the Al foil was also etched to the micro-nano scale by the QSL technology. This Al foil and the PTFE filmwith Cu deposited on it were utilized to fabricate the TENG as is schematically illustrated in Fig. 1c. The actual images of the elec- trode deposited PTFE film, the etched Al foil and the TENG are presented in Fig. 1d (I), 1d (II) and Fig. 1e, respectively. The working principle of the fabricated TENG is illustrated in Fig. 2. In the first state, the PTFE film is pressed by an external perpendicular force to approach and come into contact with the bottom Al film. Due to the difference in electron affinity, the Al film will be positively charged because the PTFE inner surface was charged negatively. At releasing the external force, the elasticity of the springs detaches the two layers. And, therefore, the triboelec- tric potential difference is created between the Al and PTFE surfaces due to Gauss law. It means that, if the Cu electrode and the Al film are connected in a close loop, the electrons will flow from the Al to the Cu electrode to equalize the triboelectric potential (as shown in state II). This current will be zero when PTFE moves to its initial position. Afterwards, we press again as in state III and an induced current will appear in the circuit with the reverse direction from the top Cu electrode to the bottom Al film until those two layers contact fully (state IV). The TENG operation will systematically and repetitively follow that process until we remove the contact force. The desired morphologies on both the PTFE and AL surfaces can greatly increase the effective contact area and surface roughness, which essentially improve the triboelectric charges during the Journal of Science: Advanced Materials and Devices 6 (2021) 84e91contact and increase the capacitance to form a larger dipole H. Phan, P.N. Hoa, H.A. Tam et al.moment between the electrodes during the contacting and releasing processes. 3.1. Surface morphology As already mentioned, the surface morphology of the PTFE is presented in Fig. 3a. It can clearly be seen that the etched PTFE surface exhibits a hierarchical nanostructure of 200 nm. These crinkles were formed due to the disordered etch positions. The top view SEM image of the nano-structured surface PTFE is shown in Fig. 3a. A micro/nano structured surface of Aluminum was successfully created by the QSL technology. In general, laser pulses are essen- tially high energy pulses. It acts on the Al surface by two effects: melting and evaporating. Ignoring the impact of evaporation, the melting phase has a great influence on the Al surface morphology. Each individual laser pulse destructs the Al surface by creating a Fig. 1. (a) Nano-structured surface of the PTFE film and the electrode deposited one. (b) Sch (d) photographs of (I) electrode deposited PTFE and (II) Al film, etched by QSL. (e) Photogr Fig. 2. Working principle o 86Journal of Science: Advanced Materials and Devices 6 (2021) 84e91hole-like damage. A slower scanning speed leads to a lower density of holes in the etching direction and forms trenches. The melted components will jam together on both sides of the trenches and form the hierarchy of the nanostructure at the surface of Al. On these surfaces, the melted Al forms disordered mounds that differ in density. Therefore, the parameters of the laser controlled holes play the key role in forming the Al surface morphology. It simul- taneously holds for the TENG performance. These features are discussed in detail below. 3.2. Triboelectric performance To demonstrate the effectiveness of the QSL parameters on the Al/PTFE triboelectric performance, we systematically characterized the electrical properties of TENG in terms of the variation of the laser pulse power, the scanning speed and the grooves distance. ematic illustration of the Al etching process by the QSL technology. (c) TENG structure. aph of the fabricated TENG. f the fabricated TENG. H. Phan, P.N. Hoa, H.A. Tam et al. Journal of Science: Advanced Materials and Devices 6 (2021) 84e91Firstly, the laser power was set at 15 W, the scanning speed was constantly kept at 500 mm/s whereas the grooves distance varied from 10 mm to 50 mm (see Table 1) to perform the etching of the Al surface,. The fabricated samples are S1, S2, S3 and S4, respectively. The surface top view morphologies of these samples are shown in Fig. 3a. The surface of the initial sample (S0) is completely smooth. Under bombardment of laser pulses, the melted Al is extruded out and forms the Almounds/particles. Five TENGs based on the surface treated Al (including the pristine Al sample S0) were electrically examinedwith a contact force of 10 N and a frequency of 1 Hz. Their electrical performances are shown in Fig. 3b and c. Sample S1 shows a morphology with dense and big Al mounds/particles (scale of 10e15 mm). In combination with the sub-micron scale PTFE, this surface, with the above given characteristics, shows a slight Fig. 3. (a) The surface morphologies of the nanostructured PTFE film and of the surface-et voltage and (c) The short-circuit current of TENGs which use the above mentioned Al films 87enhancement of the TENG performance. Considering the samples S1 to S4, the morphologies of the Al foils are not distinctly different under these etching conditions. When the gap is 10 mm, the overlap is considerable, which leads to the formation of dense and big-size Al particles. For samples S2 and S3, the grooves overlap is trivial. As a consequence, the formed particles are smaller (5e7 mm). The coupling of these small-scale surface structures of Al and the PTFE treatment experimentally generate the enhanced performance. The TENG performance of sample S4 decreased due to the appearance of a huge bulk of Al (~17e20 mm)which largely prevents the contact between PTFE and the Al surface. The sample S0 shows the lowest electric signal with an open-circuit voltage and a short-circuit current of ~75 V and 6.8 mA, respectively, while sample S2 pre- sents a notably better performance with values of ~122 V and ched Al films with different groove distances (samples S0 e S4). (b) The open-circuit as their main component. 7.8 mA. The remaining samples show output values a little bit lower than that of sample S2. The order of the output performance of the fabricated TENGs follows the sequence S2 ~ S3 > S4 > S1 > S0. The negligible difference in TENG output signals confirms that a power of 15 W and a scanning speed of 500 mm/s are not the proper technical values for an enhancement of the TENG performance. Secondly, in order to investigate the enhanced electrical output characteristics of TENG with different morphologies of Al due to the adjustment of the scanning speed, the sample S2 and two other samples S5 and S6 were measured. The laser source pa- rameters are focused on the variation of the scanning speed of 500 mm/s (S2), 1000 mm/s (S5) and 2000 mm/s (S6) while the power and the grooves distance are kept at 15 W and 20 mm, respectively. As shown in Fig. 4a, a hole with a diameter of 30 mm can be clearly seen in the SEM image of S6. However, the micro Al mound is conspicuous in the SEM image of S2. And S5 shows an outstandingly etched surface with mound sizes of 600 nm to 5 mm and a space that is more than 10 mm. Moreover, the low density of those mounds can lead to a full contact between PTFE and the Al surface. The collected results in Fig. 4b and c and reconfirm the above conclusion. The TENG e S5 produced the most impressive signals with an open-circuit voltage of ~148 V and a short-circuit current of ~9.6 mA. The TENG e S6 is the weakest one with an open-circuit voltage of ~94 V and a short-circuit current of ~5.8 mA. This poor signal can be explained by the highly wrinkled structure that prevents contact and reduces the local contact area between the PTFE and Al films. Finally, the dependence of the performance of TENGs on the laser power source was studied. Sample S5 with the most favorable triboelectric properties was compared with two other samples which were fabricated by etching at two different power levels of 20 W and 40W. As can be imagined, the higher the laser power the deeper the etched holes and the higher the jammed wrinkles appear. This claim can be partly confirmed in Fig. 5a with SEM images of S5, S7 and S8 in an intuitive comparative order. The size of the Al mounds of S7 is approximately 10 mm, and that of S8 is even much larger with sizes of 10 mm up to 18 mm. The density of the Al mounds of S7 and S8 is also higher than that of S5. The electric signal of their respective TENGs is shown in Fig. 5b and c. The samples fabricated with the higher laser power of 20 W (S7) and 40 W (S8) present a significant difference compared to that of 15 W (S5). The TENG e S7 produces an open-circuit voltage of H. Phan, P.N. Hoa, H.A. Tam et al. Journal of Science: Advanced Materials and Devices 6 (2021) 84e91Fig. 4. (a) The surface morphology of the nanostructure PTFE film and of the surface-etche short-circuit current of TENGs which used the above mentioned Al films as their main com 88d Al films based on different scanning speeds. (b) The open-circuit voltage and (c) The ponent. H. Phan, P.N. Hoa, H.A. Tam et al.~106 V and a short-circuit current of ~7.0 mA. The result is even worse for TENG e S8 for which the open-circuit voltage and short- circuit current are ~88 V and ~6.6 mA, respectively. The above re- cords confirm that sample e S5 with the following QSL etching conditions: laser power of 15 W, scanning speed of 1000 mm/s and grooves space of 20 mm, shows the optimal performance in the proposed construction of TENG. To evaluate the effective power of TENG, the peak output voltage and the current were measured under various values of external resistances. In this cas