micromachines

Article Understanding of the Mechanism for Laser Ablation-Assisted Patterning of Graphene/ITO Double Layers: Role of Effective Thermal Energy Transfer

1, 2, 3, 4 4 Hyung Seok Ryu †, Hong-Seok Kim †, Daeyoon Kim †, Sang Jun Lee , Wonjoon Choi , Sang Jik Kwon 1, Jae-Hee Han 2,* and Eou-Sik Cho 1,* 1 Department of Electronic Engineering, Gachon University, Gyeonggi-do 13120, Korea; [email protected] (H.S.R.); [email protected] (S.J.K.) 2 Department of Materials Science and Engineering, Gachon University, Gyeonggi-do 13120, Korea; [email protected] 3 Department of Energy IT, Gachon University, Gyeonggi-do 13120, Korea; [email protected] 4 School of Mechanical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Korea; [email protected] (S.J.L.); [email protected] (W.C.) * Correspondence: [email protected] (J.-H.H.); [email protected] (E.-S.C.); Tel.: +82-31-750-8689 (J.-H.H.); +82-31-750-5297 (E.-S.C.) These authors contributed equally to this work. †


Received: 3 August 2020; Accepted: 28 August 2020; Published: 29 August 2020


Abstract: Demand for the fabrication of high-performance, transparent electronic devices with improved electronic and mechanical properties is significantly increasing for various applications. In this context, it is essential to develop highly transparent and conductive electrodes for the realization of such devices. To this end, in this work, a chemical vapor deposition (CVD)-grown graphene was transferred to both glass and polyethylene terephthalate (PET) substrates that had been pre-coated with an indium tin oxide (ITO) layer and then subsequently patterned by using a laser-ablation method for a low-cost, simple, and high-throughput process. A comparison of the results of the laser ablation of such a graphene/ITO double layer with those of the ITO single-layered films reveals that a larger amount of effective thermal energy of the laser used is transferred in the lateral direction along the graphene upper layer in the graphene/ITO double-layered structure, attributable to the high thermal conductivity of graphene. The transferred thermal energy is expected to melt and evaporate the lower ITO layer at a relatively lower threshold energy of laser ablation. The transient analysis of the temperature profiles indicates that the graphene layers can act as both an effective thermal diffuser and converter for the planar heat transfer. Raman spectroscopy was used to investigate the graphite peak on the ITO layer where the graphene upper layer was selectively removed because of the incomplete heating and removal process for the ITO layer by the laterally transferred effective thermal energy of the laser beam. Our approach could have broad implications for designing highly transparent and conductive electrodes as well as a new way of nanoscale patterning for other optoelectronic-device applications using laser-ablation methods.

Keywords: graphene; ITO; laser ablation; thermal-energy transfer; temperature distribution; Raman spectroscopy

1. Introduction For the fabrication of transparent electronic devices, such as touchscreen panels (TSPs) and flat-panel displays (FPDs), it is essential to develop transparent conductive electrodes (TCEs) with

Micromachines 2020, 11, 821; doi:10.3390/mi11090821 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 821 2 of 11 high transmittance and low resistivity. Although indium tin oxide (ITO) has been mainly applied as a typical material for TCEs in electronic devices because of its high figure of merit and good compatibility with conventional semiconductor-processing technology [1–3], the demand for developing new TCE materials with better conductivity has been soaring as the sizes of TSPs and FPDs increase continuously [4–11]. Graphene has been widely attempted to be used as one of the promising candidates for TCEs in various optical and electronic device applications owing to its high transmittance and high flexibility [12–14]. These properties are known to stem from the extremely thin, one-carbon-atom thickness, uniform absorption of light causing a zero-energy bandgap, and high carrier mobility because of its two-dimensional (2D) sp2 electronic hybridization configuration [15–17]. On the other hand, it is difficult to apply graphene in industrial processes because of its relatively high sheet resistance as compensation for its extremely thin thickness and fragility. Therefore, it is necessary to solve the demerits of the graphene layer for applications such as TCEs. In this study, to realize a higher conductance of a TCE in an electronic device, a chemical vapor deposition (CVD)-grown graphene layer was transferred to indium tin oxide (ITO)-film-coated glass or polyethylene terephthalate (PET) substrates, and graphene on an ITO (graphene/ITO) double layer was suggested as a new TCE material. However, it is very difficult to produce fine electrode patterns because of the difference in etch selectivity between graphene and ITO layers. In the case of the wet etching process for the removal of the ITO layer, it is indeed challenging to maintain graphene-electrode patterns without damage. The electron-beam-lithography and plasma-etching techniques have been reported as some patterning methods for graphene [18–20]. On the other hand, such conventional lithography methods would be undesirable for patterning graphene/ITO because of their highly costly vacuum process. Therefore, for low-cost patterning with high throughput, we demonstrate a direct laser ablation method to pattern the graphene/ITO double-layered films on glass and PET substrates. In the fabrication of TSPs, laser ablation has been applied to various transparent oxides including graphene without a lithography mask [21–26]. As a method of relatively low-cost patterning, a pulsed laser with a wavelength of 1064 nm was used for direct laser patterning under various laser-beam conditions, and the experimental results were analyzed in terms of the laser ablation threshold of the graphene/ITO double layer for the optimization of the laser-patterning process.

2. Materials and Methods A-few layered graphene films were produced in a CVD furnace and transferred to soda-lime-glass and PET substrates [27–29], on which a 70 Å-thick ITO layer had been deposited. To fabricate either graphene/ITO/glass or graphene/ITO/PET substrates as a platform for the laser ablation studied in this work, the graphene layers were first synthesized as follows. After the copper (Cu) foil as a substrate for synthesis of graphene layers was heated in a furnace up to 1050 ◦C for 60 min in hydrogen (H2) at a flow rate of 60 sccm, a few-layered graphene film was grown with methane (CH4) feedstock gas at a flow rate of 5 sccm. Subsequently, polymethyl methacrylate (PMMA) was spin-coated over the CVD-grown graphene layer on the Cu foil; then, the whole PMMA/graphene/Cu foil structure with Cu foil downside was carefully floated on ammonium persulfate (APS, (NH4)2S2O8, Sigma-Aldrich Korea Ltd., Seoul, Korea)) solution to etch off the Cu foil [30,31], followed by rinsing the remainder using deionized water. Then, the remaining PMMA/graphene structure, still floating, was transferred to the target substrates, which were the ITO-film-coated glass or PET substrates. Finally, only the uppermost PMMA layer was immediately removed with acetone, resulting in the final structures of either the graphene/ITO/glass or graphene/ITO/PET substrates. Table S1 in Supplementary Materials lists the optical transmittance and sheet resistance of the ITO single and graphene/ITO double layers on glass and PET substrates. The transmittance with wavelengths ranging from 400 to 800 nm and sheet resistance were measured using a UV-visible spectrometer (Cary 100 UV-Vis, Agilent Technologies, Inc., Santa Clara, CA, USA) and 4-point probe (CMT-SR2000N, AIT Co., Ltd., Suwon, Gyeonggi-do, Korea), respectively. Compared to the ITO/PET substrate without a graphene Micromachines 2020, 11, 821 3 of 11

Micromachines 2020, 11, x FOR PEER REVIEW 3 of 11 layer, the 1-layer-graphene/ITO/PET or even the 2-layer-graphene/ITO/PET substrate showed similar graphene/ITO/PETtransmittance but a substrate trend towards showed a decrease similar intransm the sheetittance resistance but a trend with towards an increase a decrease in the number in the sheetof graphene resistance layers with [32 an], increase despite thein the contact number resistance of graphene between layers the [32], ITO desp and grapheneite the contact applied resistance (Table betweenS2). These the results ITO and confirm graphene that theapplied sheet (Table resistances S2). These obtained results were confirm appropriately that the measuredsheet resistances in this obtainedwork. In addition,were appropriately the data for measured the transmittance in this work for di. Infferent addition, ITO-coated the data substrates for the transmittance with and without for differenta graphene ITO-coated layer are alsosubstrates included with in and Figure without S1. a graphene layer are also included in Figure S1. A 1064 nm, Q-switched, diode-pumped, neod neodymium-dopedymium-doped yttrium vanadate (Nd:YVO4)) laser laser system (Miyachi, ML-7111A, ML-7111A, Miyachi Miyachi Korea Korea Co., Su Sungnam,ngnam, Gyeonggi-do, Korea) Korea) was was used for the direct patterning of the prepared graphene/ITO-layered graphene/ITO-layered filmsfilms (Figure(Figure1 1).). TheThe pulsepulse durationduration ((τ) of the the laserlaser beam was 10 ns forfor aa fullfull widthwidth atat halfhalf maximummaximum (FWHM),(FWHM), and and the the repetition repetition rate rate ranged ranged from from 0 0to to 200 200 kHz. kHz. The The samples samples were were laserlaser ablatedablated underunder variousvarious laser-beamlaser-beam conditions, such as different different repetition rates and scanning speeds. During During lase laserr ablation, ablation, the the beam beam energy energy per per laser laser pulse was adjusted from 44.6 to 266.5 μµJ according to thethe repetitionrepetition rate.rate.

Figure 1. SchematicSchematic of of the the experimental experimental setup setup for for laser laser ab ablationlation of of both both indium tin oxide (ITO) single and graphene/ITO graphene/ITO double layers on polyethylenepolyethylene terephthalate (PET) substrates.

3. Results Results and and Discussion Discussion Figure 22 showsshows thethe relationshiprelationship betweenbetween thethe squaredsquared laser-ablatedlaser-ablated spotspot sizesize onon thethe thinthin filmfilm andand the laser-beam fluence per etch pulse to determine the ablation threshold value of each layer of the the laser-beam fluence per etch pulse to determine the ablation threshold value of each layer of the 2 2 E0 graphene/ITO-double-layer structure [25,33–38]. Using the equation D = 2ω ln , from the graphene/ITO-double-layer structure [25,33–38]. Using the equation 𝐷 =2𝜔0 ×ln Eth, from the × ablated spot size data D per laser energy pulse E0, the laser beam radius ω0 can be obtained from the ablated spot size data D per laser energy pulse E0, the laser beam2 E radius ω0 can be obtained from the slope of the linear-fitted graph. Considering the equation Φ = × 2 , the ablation threshold values can π ω0× slope of the linear-fitted graph. Considering the equation Φ =× , the ablation threshold values be obtained by extrapolating the fitting lines to the intercepts of the× logarithmic axis of the laser-beam canfluence. be obtained From Figure by extrapolating2a, the ablation the thresholdfitting lines values to the of intercepts the laser-beam of the fluencelogarithmic obtained axis forof the both laser- the beamgraphene fluence. upper From and Figure ITO lower 2a, the layers ablation of the thresh grapheneold /valuesITO double-layer of the laser-beam structure fluence on glass obtained substrates for bothwere the 7.54 graphene and 0.86 upper J/cm2, respectively.and ITO lower An layers accurate of the value graphene/ITO of the ablation double-layer threshold structure for the ITO on single glass substrateslayer on glass were could 7.54 notand be 0.86 determined J/cm2, respectively. because the An ablation accurate threshold value of for the the ablation ITO layer threshold increases for when the ITOits thickness single layer is much on glass smaller could than not its be absorption determined length because [37 ].the Most ablation of the threshold laser beam for isthe expected ITO layer to increasestransmit throughwhen its both thickness the thin is ITOmuch and smaller glass substratethan its absorption instead of beinglength absorbed [37]. Most due of tothe the laser relatively beam islong expected wavelength to transmit of 1064 through nm employed both the thin in this ITO work. and glass Therefore, substrate the instead laser ablation of being process absorbed in due this toexperiment the relatively is expected long wavelength to be influenced of 1064 mainly nm employed by the thermal in this energy work. of Therefore, the laser beam. the laser Furthermore, ablation processwe also investigatedin this experiment the ablation is expected threshold to be values influenced for diff mainlyerent layers by the on thermal the PET energy substrates: of the the laser ITO beam.single Furthermore, layer and both we the also graphene investigated upper the and ablation ITOlower threshold layers values of the for graphene different/ ITOlayers double-layer on the PET substrates:structure on the each ITO PET single substrate layer have and the both values the of graphene 1.50, 0.30, andupper 1.31 and J/cm ITO2, respectively lower layers (Figure of 2theb). graphene/ITOCompared to thedouble-layer ablation threshold structure values on each with PET the su glassbstrate substrate have the (Figure values2a), of the 1.50, lower 0.30, values and 1.31 for J/cmeach2, layer respectively on the PET (Figure substrates 2b). Compared can be explained to the asablation a result threshold of the lower values thermal with conductivitythe glass substrate of PET (Figurethan glass 2a), substrate the lower [39 values,40]. for each layer on the PET substrates can be explained as a result of the lower thermal conductivity of PET than glass substrate [39,40].

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Figure 2.FigureSquared 2. Squared laser-patterned laser-patterned diameters diameters (line (line widths) widths) of ITOof ITO single single layer layer and and graphene graphene/ITO/ITO double layer ondouble (a) glass, layer whereon (a) glass, the where solid the red solid circles red circles and black and blac squaresk squares indicate indicate the the data for for graphene graphene and ITO layersand onITO ITO-coated layers on ITO-coated glass substrate, glass substrate, respectively, respectively, and and (b) ( PETb) PET substrates, substrates, wherewhere the the solid solid black black triangles and red circles indicate the data for graphene and ITO layers on ITO-coated PET triangles and red circles indicate the data for graphene and ITO layers on ITO-coated PET substrate, substrate, respectively, the blue squares are the ITO single layer on PET substrate. Each colored line respectively,Figure the 2. Squared blue squares laser-patterned are the diameters ITO single (line layer widths) on of PET ITO substrate. single layer Eachand graphene/ITO colored line shows shows a linear fit of the data. The laser-ablated line widths of Figure 2a,b were obtained from Figures double layer on (a) glass, where the solid red circles and black squares indicate the data for graphene a linear3 fitand of 4, therespectively. data. The laser-ablated line widths of Figure2a,b were obtained from Figures3 and ITO layers on ITO-coated glass substrate, respectively, and (b) PET substrates, where the solid and4, respectively. black triangles and red circles indicate the data for graphene and ITO layers on ITO-coated PET Optical images of the laser-ablated lines on the graphene/ITO double-layer structure on the glass substrate, respectively, the blue squares are the ITO single layer on PET substrate. Each colored line substrate with a laser scanning rate of 1000 mm/s at different pulse energies are shown in Figure 3. Opticalshows images a linear of the fit of laser-ablated the data. The laser-ablated lines on line the widths graphene of Figure/ITO 2a,b double-layer were obtained from structure Figures on the glass When the laser pulse energy was 266.5 μJ, both the graphene upper and ITO lower layers were found substrate with3 and a 4, laser respectively. scanning rate of 1000 mm/s at different pulse energies are shown in Figure3. When theto be laser etched pulse to form energy stripes was and 266.5 spots,µJ, respective both thely, graphene as shown upper in Figure and 3a. ITO In the lower case layersof 156.5 were μJ, found only small, etched spots on the ITO lower layer were found (Figure 3b), and furthermore, no ITO to be etchedOptical to form images stripes of the and laser-ablated spots, respectively, lines on the graphene/ITO as shown double-layer in Figure3 structurea. In the on case the glass of 156.5 µJ, layersubstrate appeared with ato laser be laser scanning ablated rate at of lower 1000 pulsedmm/s at energy, different 88.1 pulse and energies 44.6 μJ (Figureare shown 3c,d). in FigureAlthough 3. only small,theWhen laser etched the scanning laser spots pulse rate onenergy was the increased was ITO 266.5 lower to μ J,2000 both layer mm/s, the were graphene a similar found upper tendency (Figure and ITO to3 b),the lower andresult layers furthermore, of Figurewere found 3 was no ITO layer appearedobservedto be etched tofor be theto laserform graphene/ITO stripes ablated and doub at spots, lowerle-layer respective pulsed structurely, energy, as onshown the 88.1 PETin Figure and substrate 44.6 3a. In µ(Figure Jthe (Figure case 4). of The3 156.5c,d). largest μ AlthoughJ, the laseretchedonly scanning small, spots etched rate(~100 was μspotsm in increased ondiameter) the ITO to werelower 2000 found layer mm withwere/s, a thefound similar pulsed (Figure tendency energy 3b), ofand to266.5 furthermore, the μ resultJ. As the ofno pulsed Figure ITO 3 was observedenergylayer for appeared thewas graphenedecreased to be laserfrom/ITO ablated 266.5 double-layer to at 88.1 lower μJ, pulsedthe structure etched energy, spots, on 88.1 the even and PET on 44.6 the substrate μ ITOJ (Figure lower (Figure 3c,d). layer, Although 4were). The still largest observed, and no spots were found in the case of 44.6 μJ. The etched spot pattern of the graphene etched spotsthe laser (~100 scanningµm inrate diameter) was increased were to 2000 found mm/s, with a similar the pulsed tendency energy to the result of 266.5 of FigureµJ. As 3 was the pulsed upperobserved layer for in theFigure graphene/ITO 4a was attributed double-layer to the structure different onoverlapping the PET substrate rates as a(Figure result 4).of theThe scanning largest energyspeed wasetched decreased [22,40–42]. spots (~100 The from μ moverlapping in 266.5 diameter) to 88.1etched wereµ J,spot found the patterns etched with the of spots, thepulsed graphene even energy on upper of the 266.5 layer ITO μ J.in lower As Figures the layer, pulsed 3a and were still observed,4benergy could and was nobe due spotsdecreased to the were overlappingfrom found 266.5 to in of 88.1 thethe μ laser caseJ, the beam. etched of 44.6 spots,µJ. Theeven etchedon the ITO spot lower pattern layer, were of the still graphene upper layerobserved, in Figure and no4a spots was were attributed found in to the the case di offferent 44.6 μ overlappingJ. The etched spot rates pattern as a of result the graphene of the scanning speed [22upper,40–42 layer]. The in Figure overlapping 4a was attributed etched spotto the patternsdifferent overlapping of the graphene rates as upper a result layer of the in scanning Figures 3a and speed [22,40–42]. The overlapping etched spot patterns of the graphene upper layer in Figures 3a and 4b could be due to the overlapping of the laser beam. 4b could be due to the overlapping of the laser beam.

Figure 3. Optical microscopy images of the laser-patterned lines on the graphene/ITO double layer on a glass substrate produced with a scanning speed of 1000 mm/s at pulse energies of (a) 266.5, (b) 156.5, (c) 88.1, and (d) 44.6 µJ. White lines correspond to 100 µm. Micromachines 2020, 11, x FOR PEER REVIEW 5 of 11

Figure 3. Optical microscopy images of the laser-patterned lines on the graphene/ITO double layer Micromachineson a glass2020 substrate, 11, 821 produced with a scanning speed of 1000 mm/s at pulse energies of (a) 266.5, (b)5 of 11 156.5, (c) 88.1, and (d) 44.6 μJ. White lines correspond to 100 μm.

Figure 4. OpticalOptical microscopy microscopy images images of of the the laser-pattern laser-patterneded lines lines on on graphene/ITO graphene/ITO double double layer layer on on a PET substrate produced produced at at a a scanning scanning spee speedd of of 2000 2000 mm/s mm/s and and pulse pulse energies energies of of ( (aa)) 266.5, 266.5, ( (bb)) 156.5, 156.5, ((c) 88.1, and ( d) 44.6 μµJ. The white lines correspond toto 100100 µμm.

The difference difference in the laser-driven etched spot patterns between the graphene upper and ITO lowerlower layers layers can can be be explained explained as as a aresult result of of the the higher higher thermal thermal conduc conductivitytivity of ofgraphene graphene than than that that of theofthe ITO ITO layer layer [43]. [43 Therefore,]. Therefore, it is itpossible is possible to ex toamine examine the larger the larger etched etched spots spotson the on graphene the graphene layer andlayer the and higher the higher overlapping overlapping as the formation as the formation of stripes of [40]. stripes Compared [40]. Compared to the ITO to thesingle ITO layer, single a lower layer, ablationa lower ablationthreshold threshold and a larger and a largeretched etched spot spotwere were investigated investigated on onthe theITO ITO lower lower layer layer of of the graphene/ITOgraphene/ITO double-layer structure. structure. In In addition, addition, we we calculated calculated the the lateral lateral (or (or in-plane in-plane directional) directional) etch rate for for the the different different samples from our experime experimentalntal results (Figure S2). The The average average values values with with 2 standardstandard deviation deviation σσ ofof the the lateral lateral (or (or in-plane in-plane directional) directional) etch etch rates rates in unit in unit of mm of mm2/s for/s the for ITO the and ITO grapheneand graphene layers layers of the of graphene/ITO/glass the graphene/ITO /substratesglass substrates were found were to found be 0.02781 to be0.02781 (σ, 0.0064) (σ, and 0.0064) 0.27637 and (0.27637σ, 0.00905), (σ, 0.00905), respectively. respectively. Those of Those the graphene/ITO/PET of the graphene/ITO substrate/PET substrate were 0.18134 were 0.18134 (σ, 0.1035) (σ, 0.1035) and 0.56833and 0.56833 (σ, 0.2951), (σ, 0.2951), respectively. respectively. There There is a istendency a tendency that that in the in thesame same substrate, substrate, the thelateral lateral etch etch rate rate of grapheneof graphene is higher is higher than than that that of ofthe the ITO ITO layer. layer. This This could could be be because because of of the the difference difference in in the the thermal thermal conductivity as as well well as as film film thickness thickness between between the the two two layers. layers. For For the thesame same material, material, for example, for example, the graphenethe graphene layer layer with with PET PET shows shows a higher a higher value value with with increasing increasing σ σcomparedcompared to to that that with with a a glass substrate.substrate. This This result result would would be be possibly possibly due due to to the the lower lower thermal thermal conductivity conductivity [39,40] [39,40] as as well well as as the the possibility ofof aa less-uniformless-uniform distribution distribution of of the the heat heat developed developed during during the the laser-ablation laser-ablation process process for for the theformer former over over the the latter. latter. Figure Figure5a,b 5a,b show show the the expected expected e effectiveffective thermal-energy thermal-energy transfertransfer ofof the laser beam through through the the ITO ITO single single layer layer and and graphene/ITO graphene/ITO double-layer double-layer structure structure during during the the laser-ablation laser-ablation process, respectively.respectively. When When a lasera laser beam beam is incident is incident on the grapheneon the graphene layer with layer a Gaussian with distribution,a Gaussian distribution,its effective thermal its effective energy thermal is expected energy to is be expected transferred to be in transferred the lateral directionin the lateral through direction the graphene through theupper graphene layer [44 upper]. The layer thermal [44]. conduction The thermal from conduction the graphene from upper the graphene layer to the upper ITO lowerlayer to one the can ITO be lowerexplained one usingcan be a heat-flowexplained equation using a inheat-flow one-dimensional equation form in one-dimensional for the time-dependent form for temperature the time- dependentdistribution temperature in the laser-ablated distribution graphene in the laser-ab/ITO double-layeredlated graphene/ITO films [double-layered45]. The boundary films condition [45]. The at the interface between the graphene upper and ITO lower layers can be expressed for the heat boundary condition at the interface between th e graphene upper and ITO lower layers can be ∂T = ∂T expressedflow in the for equation the heatκ grapheneflow in∂ zthe graphene equationκITO 𝜅 ∂z ITO [46]. κ is the=𝜅 thermal conductivity, [46]. 𝜅 is the and thermalz is the distance into the vertical direction from the surface of the thin film to the substrate. From the above conductivity, and 𝑧 is the distance into the vertical direction from the surface of the thin film to the boundary condition, the conveyed effective thermal energy to the ITO lower layer of the graphene/ITO double-layer structure is expected to cause an abrupt change in temperature and ablate the ITO lower Micromachines 2020, 11, x FOR PEER REVIEW 6 of 11 Micromachines 2020, 11, 821 6 of 11 substrate. From the above boundary condition, the conveyed effective thermal energy to the ITO lower layer of the graphene/ITO double-layer structure is expected to cause an abrupt change in layertemperature more easily and ablate in a larger the ITO area lower than thelayer ITO more single easily layer in because a larger of area the than difference the ITO in thesingle thermal layer conductivitybecause of the of difference graphene in and the ITO, thermal as shown conductivity in Figure of5b. graphene and ITO, as shown in Figure 5b.

FigureFigure 5.5. Schematic diagram of the thermal energy transfer of laser beam through (a)) ITOITO singlesingle layerlayer andand ((bb)) graphenegraphene/ITO/ITO doubledouble layerlayer duringduring laserlaser ablation.ablation.

TheThe transienttransient temperaturetemperature distributiondistribution confirmsconfirms the didifferentfferent characteristicscharacteristics ofof thermal-energythermal-energy transporttransport onon graphenegraphene/ITO/ITO doubledouble andand ITOITO singlesingle layers,layers, enabledenabled by thethe laserlaser irradiationirradiation for 1010 nsns (Figure(Figure6 6).). TheThe analysisanalysis ofof thethe temperaturetemperature changeschanges waswas conductedconducted usingusing COMSOLomsol MMULTIPHYSICSultiphysics [47].[47]. First,First, the graphenegraphene/ITO/ITO doubledouble layerslayers showshow eeffectivffectivee thermal-energythermal-energy transport,transport, therebythereby facilitatingfacilitating heatheat dissipationdissipation on aa two-dimensionaltwo-dimensional plane, while the laserlaser beam is irradiated for only 10 ns,ns, asas shownshown inin FigureFigure6 6a.a. TheThe transienttransient changeschanges inin thethe temperaturetemperature profilesprofiles forfor everyevery 22 nsns along along the the white-dotted white-dotted line line moremore clearlyclearly presentpresent thethe extendedextended thermalthermal functionfunction ofof thethe graphene layerlayer on ITO film-coatedfilm-coated substratessubstrates (Figure(Figure6 6b).b). As As the the graphene/ITO graphene /ITO double double layer layer is expo is exposedsed in the in laser the irradiation, laser irradiation, it significantly it significantly absorbs absorbsthe produced the produced thermal thermalenergy, energy,and the andinstant the heat instant diffusion heat di inffusion the lateral in the direction lateral direction appears appears on the x–y on theplane. x–y The plane. higher The thermal higher thermalconductivity conductivity of the grap of thehene graphene layer could layer significantly could significantly enhance enhance the planar the planarthermal-energy thermal-energy transport, transport, despite despitethe low thethermal low thermalconductivity conductivity of the ITO of layer. the ITO Onlayer. the other On thehand, other the hand,thickness the of thickness the ITO oflayer the film ITO (~70 layer Å) film is over (~70 20 Å) times is over greater 20 times than greaterthat of thanthe graphene that of the layer graphene (~3 Å). layerWhile (~3 the Å). thermal While diffusion the thermal is guided diffusion in the is guidedplanar direction in the planar of the direction graphene of layer, the graphene the penetration layer, the of penetrationthermal energy of thermal passingenergy through passing the vertical through directio the verticaln is minimized, direction and is minimized,the desirable and thermal the desirable path via thermalthe graphene path vialayer the facilitates graphene highly layer facilitates anisotropic highly thermal anisotropic diffusion. thermal The different diffusion. trend The of di fftemperatureerent trend ofchanges temperature in the single changes ITO in layer the single enabled ITO by layer the enabledlaser irradiation by the laserclarify irradiation the distinct clarify functions the distinct of the functionsgraphene layer of the in graphene terms of the layer enhanced in terms planar of the th enhancedermal diffusion. planar The thermal laser-be diffamusion. irradiation The laser-beam for 10 ns irradiationon the ITO-layer for 10 film ns onforms the much ITO-layer narrower film formsthermal much diffusion narrower (~30 μ thermalm) along di theffusion planar (~30 directionµm) along than thethat planar on the direction graphene/ITO than that double on the layers graphene (Figure/ITO 6c). double The thermal layers diffusion (Figure6 c).length The thermalwith the digrapheneffusion lengthlayer (~70 with μm) the enhances graphene the layer thermal (~70 diffusionµm) enhances length by the more thermal than di twoffusion times. length This trend by more is observed than two in times.the transient This trend temperature is observed changes in the transientfor every temperature 2 ns, as well changes (Figure for 6d). every Although 2 ns, as well the (Figurelaser-beam6d). Althoughirradiation the increases laser-beam the irradiationpeak temperature increases at thethe peak central temperature area for both at the cases, central the area effective for both thermal cases, thediffusion effective in thermalthe planar diff directionusion in theonly planar increases direction in the only graphene/ITO increases in double the graphene layer./ ITOFurthermore, double layer. the Furthermore,maximum temperature the maximum on the temperature graphene/ITO on thedoub graphenele layer /ITOreaches double over layer 5000 reachesK, whereas over the 5000 ITO- K, whereassingle-layered the ITO-single-layered film only shows film about only 1500 shows K. about The 1500transient K. The analysis transient of analysis the temperature of the temperature profiles profilesindicates indicates that the thatgraphene the graphene layers can layers act as can both act asan botheffective an e ffthermalective thermal diffuser di andffuser converter and converter for the forplanar the planarheat transfer. heat transfer. In addition, In addition, despite despitethe relatively the relatively low melting low meltingtemperature temperature (523 K [48] (523 to K533 [48 K] to[49]) 533 of K the [49 PET]) of substrate the PET substratecompared compared to the peak to te themperature peak temperature (>1500 K) resulting (>1500 K) from resulting the simulation, from the simulation,it is reasonably it is reasonablyexpected that expected the PET that substrate the PET substratewould not would be completely not be completely melted with melted such with a short such alaser-ablation short laser-ablation duration duration (10 ns applied (10 nsapplied for both for the both experiment the experiment and simulation) and simulation) but rather but be rather cracked be crackedto release to the release stress the developed stress developed during the during process the process[39]. [39].

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Figure 6. TransientTransient analysis of temperature distribution on ( a,b) graphenegraphene/ITO/ITO double layers and (c,d) ITOITO singlesingle layers,layers, enabledenabled byby laser-beamlaser-beam irradiation.irradiation. The two-dimensional features of instant temperature profiles profiles with heating by laser-beam i irradiationrradiation for for 10 ns are shown in the x–y and x–z planes (top), and the correspondingcorresponding transient temperaturetemperature changes along the white dotted line are presented for every 2 ns (bottom).).

To confirm the removal of the graphene/ITO double layer after laser ablation, Raman spectroscopy To confirm the removal of the graphene/ITO double layer after laser ablation, Raman spectroscopy was performed to examine a laser-etched spot on the graphene/ITO double layer (Figure7a), where was performed to examine a laser-etched spot on the graphene/ITO double layer (Figure 7a), where the the regions 1, 2, and 3 were assigned to indicate the graphene upper layer, ITO lower layer, and PET regions 1, 2, and 3 were assigned to indicate the graphene upper layer, ITO lower layer, and PET substrate in sequence. On each part of the laser-ablated graphene patterns, the Raman spectra were substrate in sequence. On each part of the laser-ablated graphene patterns, the Raman spectra were compared, as shown in Figure7b, from 200 to 3150 cm 1. For the region 1, the three characteristic bands, compared, as shown in Figure 7b, from 200 to 3150− cm−1. For the region 1, the three characteristic D-, G-, and 2D-band, indicative of the existence of a graphene layer were found [50–57]. A D-band bands, D-, G-, and 2D-band, indicative of the existence of a graphene layer were found [50–57]. A D- peak (~1350 cm 1) of weak intensity and both G- (~1580 cm 1) and 2D-band peaks (~2700 cm 1) band peak (~1350− cm−1) of weak intensity and both G- (~1580 cm− −1) and 2D-band peaks (~2700 cm−−1) on the graphene layer were observed. When the Raman spectra were acquired along the edge of a on the graphene layer were observed. When the Raman spectra were acquired along the edge of a laser-etched spot, where the graphene upper layer was etched selectively but the ITO lower layer laser-etched spot, where the graphene upper layer was etched selectively but the ITO lower layer remained (region 2), very small traces of both the D- and G-band peaks were observed, but no 2D-band remained (region 2), very small traces of both the D- and G-band peaks were observed, but no 2D- peak was detected, indicating that most of the graphene upper layer had been removed as a result of band peak was detected, indicating that most of the graphene upper layer had been removed as a laserresult ablation. of laser ablation. Compared Compared to the Raman to the spectra Raman of spec a centertra of region a center of region the laser-etched of the laser-etched spot, on which spot, on no graphene-relatedwhich no graphene-related peaks were peaks observed were (regionobserved 3), (region some of 3), the some residual of the amorphous residual amorphous carbon on thecarbon ITO loweron the layerITO lower was found, layer whichwas found, is expected which tois haveexpect resulteded to have from resulted the incomplete from the heating incomplete and removalheating processand removal for the process ITO lower for the layer ITO by lower the laterally layer by transferred the laterally eff ectivetransferred thermal effective energy thermal of the laser energy beam. of the laser beam.

(a)

Micromachines 2020, 11, x FOR PEER REVIEW 7 of 11

Figure 6. Transient analysis of temperature distribution on (a,b) graphene/ITO double layers and (c,d) ITO single layers, enabled by laser-beam irradiation. The two-dimensional features of instant temperature profiles with heating by laser-beam irradiation for 10 ns are shown in the x–y and x–z planes (top), and the corresponding transient temperature changes along the white dotted line are presented for every 2 ns (bottom).

To confirm the removal of the graphene/ITO double layer after laser ablation, Raman spectroscopy was performed to examine a laser-etched spot on the graphene/ITO double layer (Figure 7a), where the regions 1, 2, and 3 were assigned to indicate the graphene upper layer, ITO lower layer, and PET substrate in sequence. On each part of the laser-ablated graphene patterns, the Raman spectra were compared, as shown in Figure 7b, from 200 to 3150 cm−1. For the region 1, the three characteristic bands, D-, G-, and 2D-band, indicative of the existence of a graphene layer were found [50–57]. A D- band peak (~1350 cm−1) of weak intensity and both G- (~1580 cm−1) and 2D-band peaks (~2700 cm−1) on the graphene layer were observed. When the Raman spectra were acquired along the edge of a laser-etched spot, where the graphene upper layer was etched selectively but the ITO lower layer remained (region 2), very small traces of both the D- and G-band peaks were observed, but no 2D- band peak was detected, indicating that most of the graphene upper layer had been removed as a result of laser ablation. Compared to the Raman spectra of a center region of the laser-etched spot, on which no graphene-related peaks were observed (region 3), some of the residual amorphous carbon on the ITO lower layer was found, which is expected to have resulted from the incomplete heating Micromachinesand removal2020 process, 11, 821 for the ITO lower layer by the laterally transferred effective thermal energy8 of of 11 the laser beam.

(a)

(a) 1.6 G peak

1 : ITO/Graphene 2 : ITO 1.2 3 : none(PET)

D peak 0.8 2D peak

Intensity (a.u) Intensity 0.4

0.

500 1000 1500 2000 2500 3000

Raman Shift (cm-1)

(b)

FigureFigure 7. 7.( a()a Expanded) Expanded optical optical microscopy microscopy images images of of Figure Figure4a. 4a. The The graphene graphene/ITO/ITO double double layer layer was was laserlaser ablated ablated at at a a pulse pulse energy energy of of 266.5 266.5µ Jμ withJ with a a scanning scanning speed speed of of 2000 2000 mm mm/s;/s; (b ()b Raman) Raman spectra spectra on on regionsregions 1, 1, 2, 2, and and 3 3 of of ( a(a).).

4. Conclusions In conclusion, we experimentally showed the laser-ablation-assisted patterning of graphene/ITO double layers for future TCE applications. To elucidate the underlying mechanism for the difference between ITO single and graphene/ITO double layers during the laser-ablation process, we also conducted a theoretical analysis of transient temperature spatial distribution, revealing thermal-energy transport on graphene/ITO double and ITO single layers. From the analysis, we demonstrated a role of effective thermal-energy transfer during laser ablation, indicating that the graphene layers can act as both an effective thermal diffuser and converter for the planar heat transfer. The analysis of Raman spectroscopy also supported this laser-ablation mechanism of the selective etching of graphene layers due to the effective thermal-energy transfer. Our study may provide insight into designing optimal electrode patterning using graphene layers, which would not only enable better electrode formation with high transparency and conductance for TCE applications but would also serve as a new way of nanoscale patterning for other optoelectronic-device applications using laser-ablation methods. Future work will be carried out towards the further reduction of the size of the pattern width as well as the development of complex shapes of pattern design. Micromachines 2020, 11, 821 9 of 11

Supplementary Materials: The following are available online at http://www.mdpi.com/2072-666X/11/9/821/s1. Table S1: Optical transmittance and sheet resistance of the ITO/PET and 1-layer-graphene/ITO/PET substrates; Table S2: The additional data for the optical transmittance, sheet resistance, and figure of merit (FOM) of the ITO/PET, 1-layer-graphene/ITO/PET, and 2-layer-graphene/ITO/PET substrates; Figure S1: A plot of the optical transmittance as a function of wavelength of incident light of the UV-visible spectrometer for different substrates; Figure S2: A plot of the etch rate in the lateral (or in-plane) direction for the different samples. Author Contributions: E.-S.C. and J.-H.H. conceived and designed the experiments. H.S.R., H.-S.K., D.K., and S.J.L. carried out the experiments and theoretical calculation, and E.-S.C., J.-H.H., S.J.K., and W.C. analyzed the data. E.-S.C., J.-H.H., and W.C. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded and conducted under [the Competency Development Program for Industry Specialists] of the Korean Ministry of Trade, Industry and Energy (MOTIE), operated by Korea Institute for Advancement of Technology (KIAT). (No. P0012453, Next-generation Display Expert Training Project for Innovation Process and Equipment, Materials Engineers). Conflicts of Interest: The authors declare no conflict of interest.

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