High-Voltage Flexible Microsupercapacitors Based on Laser-InducedGrapheneXiaoqian Li,*,†,‡ Weihua Cai,†,§ Kwok Siong Teh,∥ Mingjing Qi,† Xining Zang,† Xinrui Ding,†

Yong Cui,† Yingxi Xie,† Yichuan Wu,† Hongyu Ma,† Zaifa Zhou,‡ Qing-An Huang,*,‡ Jianshan Ye,*,§

and Liwei Lin*,†

†Department of Mechanical Engineering, University of California, Berkeley, California 94709, United States‡Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, China§School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China∥School of Engineering, San Francisco State University, San Francisco, California 94132, United States

*S Supporting Information

ABSTRACT: High-voltage energy-storage devices are quitecommonly needed for robots and dielectric elastomers. Thispaper presents a flexible high-voltage microsupercapacitor(MSC) with a planar in-series architecture for the first timebased on laser-induced graphene. The high-voltage devices arecapable of supplying output voltages ranging from a few tothousands of volts. The measured capacitances for the 1, 3,and 6 V MSCs were 60.5, 20.7, and 10.0 μF, respectively,under an applied current of 1.0 μA. After the 5000-cycle charge−discharge test, the 6 V MSC retained about 97.8% of the initialcapacitance. It also was recorded that the all-solid-state 209 V MSC could achieve a high capacitance of 0.43 μF at a low appliedcurrent of 0.2 μA and a capacitance of 0.18 μF even at a high applied current of 5.0 μA. We further demonstrate the robustfunction of our flexible high-voltage MSCs by using them to power a piezoresistive microsensor (6 V) and a walking robot(>2000 V). Considering the simple, direct, and cost-effective fabrication method of our laser-fabricated flexible high-voltageMSCs, this work paves the way and lays the foundation for high-voltage energy-storage devices.

KEYWORDS: high voltage, laser-induced graphene, microsupercapacitors, microsensors, microrobots

■ INTRODUCTIONHigh-voltage power supplies are quite commonly required formany electronics and machines like small robots1−3 anddielectric elastomers.4−6 To obtain high-voltage output,batteries and supercapacitors are usually connected in seriesby cables because they could only offer less than 4 V for asingle cell. But such connection always suffers frominconvenience of operation by using plenty of conductingmetal wires.Supercapacitors have been viewed as a valuable alternative to

batteries because they can be charged and discharged muchmore rapidly with an almost infinite lifetime.7−13 Unlikebatteries that rely on electrochemical redox reactions, asupercapacitor stores charges via the adsorption anddesorption of ions within a thin electrical double layer thatexists between the solid electrodes and the bulk electro-lytes.14,15 This allows adsorbed ions to rapidly desorb from thesurface of the electrodes, enabling a supercapacitor to attainhigh power density by discharging and releasing power withina short time. Miniaturized supercapacitors, commonly knownas microsupercapacitors (MSCs), have become an active andintense area of research.16−18 This phenomenon is catalyzed bythe popularity and rise of miniaturized, self-poweringelectronics, which necessitates direct integration of mini-

aturized, on-chip energy-storage devices, such as super-capacitors, on a flexible platform.19−21 Increasingly, thesemicro energy-storage devices are designed to power micro-electronics, such as wireless microsensors networks22,23 andradio-frequency identification tags.24,25 Some of the micro-electronics are driven by high-voltage sources from a few tothousands of volts. Nevertheless, the intrinsic low outputvoltage (typically less than 4 V) of MSCs, critically limited bythe properties of electrode materials and electrolyte, presents aformidable challenge and obstacle in their use in directlypowering such microelectronics. To this end, a workablestrategy to make high-voltage MSCs is to assemble manyMSCs into a high-voltage energy stack via in-series connectionusing external conducting metal wires. The resultingassemblies, however, suffer from critical drawbacks inoperation safety and complex fabrication processes.26 There-fore, a safe, simple, and effective means to directly integratehigh-voltage MSCs with microelectronics, albeit difficult andelusive at present, remains important and desirable.

Received: June 20, 2018Accepted: July 13, 2018Published: July 13, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 26357−26364

© 2018 American Chemical Society 26357 DOI: 10.1021/acsami.8b10301ACS Appl. Mater. Interfaces 2018, 10, 26357−26364

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Herein, by using the in-series structure, flexible high-voltageplanar MSCs based on laser-induced graphene (LIG) cangenerate an output voltage of up to 209 V for the first time,which was the highest recorded voltage among all currentplanar MSCs. In other words, 210 LIG square electrodes werefabricated via a direct-write, ambient laser pyrolysis ofpolyimide film first and then 209 single-unit microsupercapa-citors were connected in series on a single-film substrate.Compared to the lithographic processes that may involvemultiple masking steps, direct laser writing provides a fast andlow-cost fabrication route that enables substantially fasterdesign iterations. Furthermore, to investigate the electro-chemical performance of high-voltage MSCs, cyclic voltam-metry (CV) and charge−discharge methods at high-voltagerange (0−209 V) were developed in this work. Besides, wesuccessfully applied the highly robust MSC in poweringpiezoresistive microsensors in the application of a walking step

counter (pedometer) for activity tracking. Besides, ten 209 VMSCs were also connected in series to drive small walkingrobots, which need the 2000 V power source.

■ RESULTS AND DISCUSSIONThe laser-assisted fabrication and electrolyte coating processesare schematically shown in Figure 1a. Briefly, the 209 V MSCpatterns were designed by using a pattern-generation softwareand transferred to a programmable CO2 infrared laser system.Next, a 125 μm thick Kapton (polyimide) substrate was rapidlyand irreversibly pyrolyzed by a laser beam (wavelength, 10.6μm; laser spot size, 200 μm) with an optimized scan rate of250 mm/s and a power of 8.0 W. Upon the transient laserheating, the surface of the Kapton substrate was instanta-neously transformed into a porous graphene network,27 whichexhibited good physical and electrical characteristics suitablefor potential energy-storage applications. To construct a 209 V

Figure 1. (a) Schematic diagram of a high-voltage planar MSC based on laser-induced graphene. The inset on the left shows the scanning electronmicroscope (SEM) image of the porous graphene electrode. The scale bar for SEM is 5 μm. (b) Temperature of a laser spot on the surface ofKapton film based on COMSOL Multiphysics software simulation. (c) Simulated temperature of laser spot and sheet resistance of graphene as afunction of laser output power. (d) Raman spectrum of laser-induced graphene. (e) X-ray photoelectron spectroscopy (XPS) image of laser-induced graphene. (f) High-resolution XPS image of C 1s. (g) High-resolution XPS image of O 1s.

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MSC, 210 porous LIG squares (3 mm × 3 mm for eachsquare) separated by 209 gaps (500 μm in distance) werelaser-patterned (inset on the left of Figure 1a). Moreover, eachgap and an estimated 30% area from one side of the LIGsquare, approximated from Figure S1, were covered with 1.0 MH2SO4−poly(vinyl alcohol) (PVA) electrolyte. The middlesection of each LIG square can act as the common conductiveelectrode for the in-series connections without the coating ofthe H2SO4−PVA electrolyte. The shear viscosity of H2SO4−PVA was as high as 5738 Pa s at room temperature (FigureS2), and a droplet-shape electrolyte was plotted at the gapusing a tiny brush (PRINCETON Lauren 4035R, Figure S3).The transient laser heating temperature (at 0.1 ms) on the

Kapton film (125 μm in thickness) was simulated byCOMSOL Multiphysics to be about 1609.8 °C when thelaser power output and scan speed were set at 8.0 W and 250mm/s, respectively, as shown in Figure 1b. Figure 1c illustratesthat as the laser power increased from 4.0 to 8.0 W, thesimulated temperature increased from 817.2 to 1609.8 °C,whereas the sheet resistance of the LIG decreased from 17.4 to9.3 Ω/sq. From the Raman spectrum in Figure 1d, threeprominent peaks with the D peak, G peak, and 2D peak wereobserved.28 The D peak at ∼1350 cm−1 induced by defects, theG peak at ∼1580 cm−1 resulting from the doubly degeneratezone center E2g mode,29 and the sharp 2D peak at ∼2700 cm−1

originating from second-order zone-boundary phonons in-dicated the existence of graphene after the laser pyrolysisprocess. As shown in the high-resolution transmission electronmicroscopy (TEM) image of LIG (Figure S4), the averagelattice space is about 0.337 nm, corresponding to the (002)planes in graphitic materials. From the TEM selected area

electron diffraction (SAED) pattern, the (002) planes werealso observed, indicating the formation of graphene sheets.27

The elemental composition and concentration of LIG wereinvestigated by X-ray photoelectron spectroscopy (XPS). Inthe survey spectrum (Figure 1e), the elements C and O couldbe observed. The binding energy values of C 1s and O 1s forthe sample were found to be 284.8 and 532.8 eV, respectively.The results obtained showed that the molar ratio of C/O isclose to 11:1. The high-resolution C 1s XPS image of LIGfurther indicated that LIG contained sp2 carbons with C−Cpeak being dominant at 284.5 eV (Figure 1f). For the high-resolution O 1s XPS image of LIG (Figure 1g), a broad peakappeared at 532.3 eV, indicating the combination of C−O(533.2 eV) and CO (531.8 eV) bonds, which is inagreement with the results previously reported.30

To verify the feasibility of an in-series structure for MSCs,MSCs with output voltage ranging from 1 to 6 V werefabricated, as schematically shown in Figure 2a. Twosymmetric electrodes separated by a gap were denoted asone unit of an MSC. The gap between two electrodes wascoated with the H2SO4−PVA electrolyte, leading to 1 V outputfor each unit. When three or six units are connected in series,MSCs with 3 or 6 V outputs, respectively, can be constructed.In fact, the voltage output scales linearly with the number ofunits of MSCs. The capacitance of a symmetric supercapacitorcell (Cdevice) can be estimated by the CV curves using eq 131

CQV Vv

i V V1

2( )d

V

V∫= =

−

+

(1)

where Q is the total charge obtained by integration of the CVcurve, i is the current, v is the scan rate, and V is the potentialwindow. The CV curves of the three different devices (1, 3,

Figure 2. (a) Schematic diagram showing MSCs with output voltages of 1, 3, and 6 V, respectively, from arrays of in-series electrodes bridged bysolid electrolytes. (b) Cyclic voltammetry curves of 1, 3, and 6 V devices at a scan rate of 100 mV/s. (c) Charged and discharged curves of 1, 3, and6 V devices at an applied current of 1.0 μA. (d) Cycling performance of a 6 V device at an applied current of 1.0 μA. The inset shows voltage−cycles profiles of the last 10 cycles in a 5000-cycle test.

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and 6 V) under a scan rate of 100 mV/s showed capacitancesof 58.1, 20.4, and 11.3 μF, respectively (Figure 2b),demonstrating the feasibility of the in-series structure forhigh-voltage planar MSCs.The galvanostatic charge−discharge method was used to

accurately characterize the capacitive properties of theseMSCs. The capacitance could be calculated from the followingequation32

CI t

V= ×

(2)

where I is the applied current, t is the discharge time of thesupercapacitors, and V is the electrochemical potentialwindow.For conventional capacitors, it is well known that the in-

series connections would result in the decay of capacitance.Supercapacitors also behave in a similar manner. The totalcapacitance of a high-voltage MSC connected in series couldbe calculated as

C C C C C C1 1 1 1 1 1

n i

n

i1 2 3 1

ikjjjjj

y{zzzzz∑= + + + ··· =

= (3)

where C is the total capacitance of a high-voltage MSC and Ci

is the capacitance of each unit. For first-order approximation,C1 = C2 = C3 = Ci, and this results in C = C1/n. Aftercalculating the charged and discharged results from Figure 2c,the capacitances for 1, 3, and 6 V MSCs were 60.5, 20.7, and10.0 μF, respectively, under an applied current of 1.0 μA.Supercapacitors typically have excellent cycling stability;

however, little is known about the cycling stability of high-voltage MSCs with an in-series electrode architecture. To shedlight on this crucial characteristic, a 5000-cycle charge−discharge test was conducted for a 6 V MSC, as shown inFigure 2d, and the 6 V MSC retained about 97.8% of the initialcapacitance. The inset shows charged−discharged profiles ofthe last 10 cycles in a 5000-cycle test. The capacitance decaycould possibly be attributed to the tiny variations of voltagesthat exist among different units, a result of the electrolytecoating process that could be further improved and optimized.

Figure 3. (a) Optical photo of a fabricated flexible planar 209 V MSC. (b) Magnified optical image of (a) the gaps between the graphene electrodesbeing filled with solid electrolytes. The scale bar is 1.5 mm. (c) Charging and discharging curves at different bending angles of a 209 V MSC. (d, e)Cyclic voltammetry curves of a 209 V MSC at various scan rates ranging from 0.1 to 10.0 V/s. (f) Device capacitance of a 209 V MSC as a functionof scan rates calculated from (d, e). (g, h) Charging and discharging curves of a 209 V MSC at various applied currents from 0.2 to 5.0 μA. (i)Device capacitance of a 209 V MSC at different applied currents obtained from (g, h).

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A digital image of a 209 V all-solid-state flexible MSC isshown in Figure 3a. As shown in Figure 3b, one corner of the209 V MSC was clearly observed with the uniform coating ofH2SO4−PVA electrolyte. To demonstrate the flexibility of thehigh-voltage MSCs, as shown in Figure 3c, a flexible planar 209V device was tested with negligible degradation of charge anddischarge curves at 2.0 μA when it was bent at different anglesranging from 0 to 180°, showing the good stability of theplanar high-voltage MSC. Instead of comparing the areal orvolumetric capacitances between MSCs with different outputvoltages, we use the capacitance derived from the designedMSC with a specific geometry. The volume of the 209 V MSCdevice is calculated to be 0.3 cm3 (Figure S5). Figure 3d−fshows the CV curves and the corresponding capacitance of the209 V device. The rectangular and symmetric shapes of the CVcurves indicated their capacitive properties with scan ratesranging from 0.1 to 10 V/s. The 209 V MSC was able toachieve 0.37 μF at 0.1 V/s when it was discharged from 209 to0 V and 0.19 μF at a scan rate of 10 V/s. The charged and

discharged data (Figure 3g−i) were further used to evaluatethe performance of the 209 V device with various appliedcurrents ranging from 0.2 to 5.0 μA. It can be seen that the all-solid-state 209 V MSC could achieve a high capacitance of 0.43μF at a low applied current of 0.2 μA and a capacitance of 0.18μF even at a high applied current of 5.0 μA. These chargecurves are relatively symmetric to their correspondingdischarge counterparts at different applied currents, furtherrevealing the capacitive behavior of the flexible all-solid-state209 V MSC. The maximum specific volumetric capacitance ofthe 209 V device was calculated from the charged−dischargedcurve to be 1.43 μF/cm3, which was comparable to the value(10.0 μF/cm3) of a commercial ceramic capacitor (X7R 1210)and 24 times larger than the value (0.06 μF/cm3) of acommercial Panasonic thin-film capacitor (Table S1). Mean-while, the 209 V MSC also maintained excellent flexibilitycompared to the fragile ceramic capacitor. For the micro-supercapacitors, they are often characterized and compared byusing the volumetric energy density instead of the mass-based

Figure 4. (a) Illustration of a 6 V MSC to power a piezoresistive microsensor that was made by transferring the laser-induced serpentine grapheneelectrodes to a PDMS substrate. (b) A microsensor pressed by a thumb at a pressure of about 60 kPa. Current profiles of a pressure microsensorworking under the press of a thumb with power supplied by (c) Gamry or (d) a 6 V MSC. (e) Microsensor placed inside a shoe to monitor thehuman walking activities. The current profiles of a pressure microsensor measuring heel strikes when powered by (f) Gamry or (g) a 6 V MSC. Theinsets show the zoomed-in characteristic signals for the corresponding profiles.

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energy density because the mass of the active materials wastiny.33 The volumetric energy density was calculated based onthe following equation: E CU V/1

22= , where C is the device

capacitance of the 209 V MSC, U is the working voltage of thedevice, and V is the volume of the 209 V MSCs. Therefore, thevolumetric energy density of 209 V MSCs was calculated as31.3 mWh/cm3 at 0.67 μA/cm3, which was much larger thanthe capacitance (5.7 mWh/cm3) from the previous work ofmicrosupercapacitors based on laser-processed graphene.34

The electrochemical impedance spectroscopy test wasconducted in a frequency range of 1 Hz to 1 MHz to furtherevaluate the electrochemical behaviors of the 209 V solid-statemicrosupercapacitor. As shown in the Nyquist plots in FigureS9a, the intercept of the Nyquist curve on the real axis wasabout 13.7 kΩ, indicating reasonable conductivity of the solid-state electrolyte and internal resistance of the device. Thesemicircle diameter referring to the charge-transfer resistance(Rct) was about 529 kΩ, which was acceptable for the high-voltage microsupercapacitors usually working in the μA currentranges. Moreover, the Bode plot is also recorded in Figure S9b,and it is be observed that the capacitor response frequency( f 0) at the phase angle of −45° was 4.64 Hz, which was similarto the values of solid-state supercapacitors based on carbonnanoparticles/MnO2.

35 The corresponding time constant τ0(=1/f 0) was 216 ms, which was close to that of laser-scribedgraphene supercapacitors (33 ms) but much shorter than thatof supercapacitors based on activated carbon (in the range of10 s).7 The phase angle of the 209 V MSC was about −77° at afrequency of 1 Hz, which is close to −90° for ideal capacitors.To demonstrate the potential of these high-voltage MSCs,36

a 6 V MSC was used to drive a wearable piezoresistive pressuresensor (Figure 4a), which was made by transferring the LIGonto a polydimethylsiloxane (PDMS) substrate based on amodified process from a previous report (see Figure S13).37

Briefly, a serpentine-shaped LIG was directly laser-written onKapton substrate. Next, liquid PDMS was poured onto the

patterns. After heating for 2 h in an oven, the serpentine LIGwas transferred onto the curing PDMS. To prevent possiblecontact with moisture, the surface of LIG was covered byanother layer of PDMS.Figure 4c shows the responses of the microsensor under an

applied voltage of 6 V powered by a Gamry workstation whilebeing finger-pressed at a pressure of about 60 kPa (Figure 4b)manually. The deformation of the sensor caused the increase inresistance and reduction in measured current as shown. Thedetailed signals were zoomed in the inset, and some variationswere observed due to the manual pressing operations. Whenthe power source was replaced by using a 6 V MSC, verysimilar data could be obtained in Figure 4d. However, thebaseline of current gradually decreased with time because ofthe continuous decrease of the supplied voltage from the 6 VMSC due to both the leakage and consumption of chargesduring the operations. The microsensor was further placedunder an insole inside a running shoe to monitor theambulatory motions in Figure 4e. By using the Gamryworkstation as a power source with 6 V output, a highlystable I−t curve could be achieved, as the microsensorrecorded the heel strikes during 1 min of walking (Figure4f). After replacing the power source by a 6 V MSC, a similarI−t curve (Figure 4g) with a constant decrease of baselinecurrent was observed, indicating excellent performance of theflexible planar high-voltage MSCs. The zoomed-in signals werenearly the same from two different power sources, validatingthe robustness and feasibility of our MSCs.Figure 5a shows the detailed structure of a crawling robot,

which was a 20 mm long and 47 mg weight prototype. Thecantilever beams together with the additional mass can beexcited to vibrate back and forth by a high-voltage direct-current power source (>2000 V) to impact the electrodes insequence and achieve self-sustained walking gaits, according toprevious reports.1,38 To demonstrate the feasibility of the high-voltage MSC, the walking robot was driven by 10 in-series-

Figure 5. (a) Designed structure of the walking robots; (b) microsupercapacitors stack with ten 209 V devices connected in series (>2000 V) forpowering the walking robots; (c) the walking robot began working at 0 s; and (d) the walking robot stopped working at 11 s.

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connected 209 V MSCs, as shown in Figure 5b. The crawlingmovement of a walking robot powered by a 2090 V MSC stackfor 11 s was recorded by a digital camera from Figure 5c,5d.These results proved that high-voltage MSCs with planar in-series structure could be a good option among energy-storagedevices.

■ CONCLUSIONSIn conclusion, this work reports a high-voltage flexible planarMSC with an in-series LIG electrode architecture via directlaser engraving of polyimide film. The high-voltage all-solid-state MSCs were flexible and capable of supplying outputvoltages ranging from a few to thousands of volts. Wedemonstrated the robust function of our flexible high-voltageMSCs by using them to power a piezoresistive microsensorthat was designed as a wearable step counter to track walkingactivities. Besides, a walking robot was also successfullypowered by a 2090 V MSC stack for 11 s. Considering thesimple, direct, and cost-effective fabrication method of ourlaser-patterned high-voltage flexible planar MSCs, this workpaves the way and lays the foundation for high-voltage energy-storage devices.

■ METHODSFabrication of the 209 V MSC. Kapton polyimide film

(McMaster-Carr; Cat. No. 2271K3; thickness: 125 μm) was used inthis work. The CO2 laser (wavelength, 10.6 μm) with an optimizedscan rate of 250 mm/s and an output power of 8.0 W was used to heatthe Kapton film with designed 210 sq. For each square electrode, theside length was 3.0 mm with a gap of 500 μm between every twoelectrodes. After the laser-heating process, the gaps were brush-coatedby electrolyte, leaving approximately 40% of each LIG square withoutthe coating of the electrolyte as the common conductive electrode forin-series connection. The electrolyte was made by adding 1.0 g ofPVA (Mw: 89 000−98 000) into 5.0 mL of 1.0 M H2SO4 solution,which was subsequently heated at 95 °C for 20 min until the solutionturned clear.Fabrication of the Pressure Microsensor. The Kapton film was

laser-heated into a conductive serpentine-shaped LIG first. After thelaser-heating process, PDMS (Sylgard 184, Dow Corning, mixing ratio15:1) was cast onto the LIG patterns. To remove the air from thesmall pores inside the porous graphene networks and allow the fullpenetration of PDMS liquids, a vacuum step was necessary. Thepolymer was cross-linked at 80 °C for 2 h in an electric oven. Afterfully cured, the PDMS/LIG microsensor was peeled off from theKapton film and further covered by another layer of PDMS.Characterizations. The electrochemical performances of 1, 3, and

6 V MSC were measured by an electrochemical workstation (GamryReference 600 potentiostat). The 209 V MSC was tested by Keithley2400. The I−t curves for the microsensors were obtained from theGamry workstation with a sample period time of 0.001 s. SEM imageswere taken with an FEI Quanta three-dimensional scanning electronmicroscope. The transmission electron microscopy (TEM) image andthe SAED pattern were recorded on an FEI Titan 80-8300. XPSinvestigations were conducted on a PHI 5600 X-ray photoelectronspectrometer. Raman spectra were recorded on LabRAM Aramis(HJY, France). The shear viscosity of electrolyte was characterized byAnton Paar at 25 °C.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b10301.

Schematic calculation of electrolyte-covered area; shearviscosity of electrolyte; digital image of a tiny brush;

TEM image of LIG; comparison of the high-voltageplanar MSC with commercial capacitors (PDF)Bending test of the 209 V MSC (Video S1) (AVI)Walking robot (Video S2) (AVI)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: lxq_seu@hotmail.com (X.L.).*E-mail: hqa@seu.edu.cn (Q.-A.H.).*E-mail: jsye@scut.edu.cn (J.Y.).*E-mail: lwlin@berkeley.edu (L.L.).ORCIDWeihua Cai: 0000-0003-3544-4309Yingxi Xie: 0000-0002-7961-3883Qing-An Huang: 0000-0002-0646-7635NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis project was supported in part by the National NaturalScience Foundation of China (Grant Nos. 61674034 and21372088), China Aviat ion Science Foundat ion(2015ZD51051), and Berkeley Sensor and Actuator Center.X.L. and W.C. are partially supported by fellowships from theChina Scholarship Council. The authors also thank MichaelLino, equipment technician at San Francisco State University,for his help in the preliminary laser-engraving experiments.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b10301ACS Appl. Mater. Interfaces 2018, 10, 26357−26364

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