Multi-scale graphene patterns on arbitrary substrates via laser-assisted transferprinting process J. B. Park, J.-H. Yoo, and C. P. Grigoropoulos Citation: Applied Physics Letters 101, 043110 (2012); doi: 10.1063/1.4738883 View online: http://dx.doi.org/10.1063/1.4738883 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Transfer patterning of large-area graphene nanomesh via holographic lithography and plasma etching J. Vac. Sci. Technol. B 32, 06FF01 (2014); 10.1116/1.4895667 Selective edge lithography for fabricating imprint molds with mixed scale patterns J. Vac. Sci. Technol. B 31, 06FB03 (2013); 10.1116/1.4827814 Laser printed micron-scale free standing laminate composites: Process and properties J. Appl. Phys. 108, 083526 (2010); 10.1063/1.3492708 Fast three-dimensional nanostructure fabrication by laser-assisted nanotransfer printing Appl. Phys. Lett. 89, 113103 (2006); 10.1063/1.2347693 Direct femtosecond laser nanopatterning of glass substrate by particle-assisted near-field enhancement Appl. Phys. Lett. 88, 023110 (2006); 10.1063/1.2163988
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APPLIED PHYSICS LETTERS 101, 043110 (2012)
Multi-scale graphene patterns on arbitrary substrates via laser-assisted transfer-printing process J. B. Park, J.-H. Yoo, and C. P. Grigoropoulosa) Department of Mechanical Engineering, University of California Berkeley, California 94720-1740, USA
(Received 30 May 2012; accepted 9 July 2012; published online 24 July 2012) A laser-assisted transfer-printing process is developed for multi-scale graphene patterns on arbitrary substrates using femtosecond laser scanning on a graphene/metal substrate and transfer techniques without using multi-step patterning processes. The short pulse nature of a femtosecond laser on a graphene/copper sheet enables fabrication of high-resolution graphene patterns. Thanks to the scale up, fast, direct writing, multi-scale with high resolution, and reliable process characteristics, it can be an alternative pathway to the multi-step photolithography methods for printing arbitrary graphene patterns on desired substrates. We also demonstrate transparent strain C 2012 American devices without expensive photomasks and multi-step patterning process. V Institute of Physics. [http://dx.doi.org/10.1063/1.4738883]
Capability of producing high-quality graphene patterns of various sizes and shapes at a high resolution on arbitrary substrates is an essential component for promoting various graphene-based applications such as flexable transparent electronics.1–3 Recent advances in large-scale synthesis of high-quality graphene sheets on metal films using chemical vapor deposition (CVD)4 are expected to enable the various applications, since they have outstanding optical,5 mechanical,6 electrical,7 and thermal8 properties that can be transferred to arbitrary large substrates using roll-to-roll CVD graphene processes.9 Currently, conventional photolithography is the most reliable method for forming graphene patterns on a transferred CVD-grown graphene sheet.10 However, conventional photolithography approaches require several steps and expensive photomasks. Photoresists that are used during photolithography processes may not be completely removed from the graphene features after the patterning process.11 Furthermore, the design is practically difficult to change due to the expensive photomasks that are used for fabricating various shapes and sizes of graphene patterns. Additionally, it is hard to implement into roll-to-roll processes due to multi-step patterning and flat-based process configurations. For these reasons, the development of a fast, direct patterning, maskless, and scalable process with access to a wide range of milli/micro/nanometer resolution in order to print graphene patterns on arbitrary substrates without using photolithographic processes has attracted wide attention in recent years. There are various non-lithographic graphene patterning methods including transfer printing and direct-write processes.11–14 However, laser-assisted techniques are promising for one-step scalable direct patterning, since a focused laser beam with high energy density can fabricate arbitrary patterns of varying resolution on target substrates via a laser scanning process within a very short time. Recently, direct laser patterning of transferred graphene on target substrates has been studied by several groups.15,16 However, direct laser a)
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ablation of a transferred graphene sheet on a target substrate frequently entails possibility of substrate damage, since highquality single-layer graphene is optically transparent over a wide range of wavelengths (>97%).17 Although graphene patterns can be fabricated without damage on the transferred substrates, the laser power density window is too narrow for industrial applications as the damage threshold of transferred substrates has to be higher than that of a transparent graphene sheet to avoid the substrate damage. Moreover, the patterning of the transferred graphene is difficult to apply for various applications such as bottom-up interconnections and suspended graphene processes.18 Therefore, there is strong need for developing a reliable laser-assisted graphene printing process on arbitrary substrates for various applications. Herein, as an alternative to conventional photolithography based graphene printing processes, we introduce fast laser-assisted graphene printing on arbitrary substrates whereby various patterns of multi-scale resolution (from tens of microns to 100 nm) were fabricated on a copper foil at high scanning speeds. The fabricated graphene patterns were transferred from the sacrificial copper foil onto arbitrary substrates. We also demonstrate a design of graphene-based transparent strain devices without requiring expensive photomasks and involving multi-step patterning processes owing to the fast direct writing characteristic. In the experiment, CVD grown graphene on a copper foil was irradiated by a focused femtosecond laser beam (100 fs, 400 nm, and 1 kHz) to fabricate various graphene patterns, thereby allowing the graphene patterns on the copper foil under ambient environment. Power density around 0.5 J/cm2 was used on the sample surface. Objective lenses (Mitutoyo NIR 5, Nikon LU plan 100) were used for focusing and in situ monitoring of the sample surface. The scan speeds of a motorized stage were increased up to 10 000 lm/s depending on the focused beam size. The power density was adjusted with a half wave plate and a polarizing beam splitter. A mechanical shutter was inserted in the laser beam path to control laser beam irradiation. A motorized X-Y stage (ANT130, Aerotech Inc.) was used to fabricate various graphene patterns on graphene/Cu foils.
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C 2012 American Institute of Physics V
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To transfer graphene patterns on arbitrary substrates, wet-based transfer procedures were carried out using PMMA (poly(methyl methacrylate)) and PDMS (polydimethylsiloxane). In case of the graphene pattern/PMMA transfer, the graphene pattern on a copper foil was spin-coated with PMMA at 3000 rpm. The copper foil was then wet etched using a copper etchant (Alfa Aldrich Co.) for 10 min, resulting in a graphene pattern/PMMA film floating on the surface of the etchant. This film was then collected and rinsed in deionized (DI) water before transferring it onto the target substrates. To fabricate transparent strain devices, patterned graphene/copper foils were transferred onto PDMS substrates (transparent strain devices). The patterned graphene foil directly attached to PDMS substrates and the copper foil etched by copper etchant. Figure 1 shows an illustration of the laser-assisted graphene printing procedure on arbitrary substrates. CVD grown graphene on copper foil (Fig. 1(a)) is patterned by femtosecond laser ablation (Fig. 1(b)). A femtosecondpulsed laser with high peak power can effectively remove graphene without incurring significant heating effects, hence making possible to fabricate high-resolution patterns. It is important to note that it is hard to remove single-layer of graphene without substrate damage using continuous wave (CW) or nanosecond pulsed laser irradiation.19 Additionally, it is difficult to achieve high resolution patterns because of the long heat diffusion length. PMMA or PDMS is coated on the patterned graphene/metal substrate (Fig. 1(c)) for the transfer process. Roll-to-roll transfer processes could be applied for large-scale transfer instead of wet processes.9 For graphene transfer onto a flexible transparent PDMS substrate (Fig. 1(d)), the patterned graphene/copper foil is etched by a copper etchant after the transfer to PDMS. In the case of graphene transfer onto arbitrary substrates using PMMA, the PMMA coated on the graphene/copper foil can be transferred to arbitrary substrates after a copper etching step as shown in Fig. 1(e). The PMMA layer is removed to obtain graphene patterns on arbitrary substrates as shown in Fig. 1(f). Figure 2 shows microscopic images of various shapes of graphene patterns that were formed by the fast laser-assisted
Appl. Phys. Lett. 101, 043110 (2012)
transfer-printing process. Figure 2(a) shows graphene patterns on a sacrificial copper substrate after femtosecond laser scanning (0.5 J/cm2 and 10 000 lm/s). The exposed copper foil appears darker than the graphene patterns. The patterning process has a wide range of process conditions because damage to the sacrificial copper foil, which is removed during etching process, is of no concern (the fluence for Cu damage: 0.1 J/cm2). The patterned graphene on copper foil was transferred to a SiO2/Si substrate with a 280 nm thick SiO2 layer as shown in Fig. 2(b). In the optical microscope image of Fig. 2(b), the purple color patterns are single-layer graphene and the other area shows a SiO2/Si substrate. Scanning electron microscope (SEM) images of embossed and engraved graphene patterns with the text “graphene laser printing” are shown in Figs. 2(c) and 2(d), respectively. The dark patterns are single-layer graphene. The patterns can be fabricated at high scanning speeds up to 10 000 lm/s. The scan speed can be increased by increasing the focused beam size and decreasing the resolution of the patterns. Figure 3 shows the structural information of a typical graphene pattern on a SiO2/Si substrate. Figure 3(a) shows an optical micrograph of the embossed graphene pattern with a honeycomb lattice shape. Figure 3(b), depicting Raman spectra of regions “A” and “B” in Fig. 3(a), clearly shows the difference between a single-layer graphene pattern and the SiO2 substrate. In the Raman spectrum of single-layer graphene in Fig. 3(b), both the symmetric 2D-band (2691 cm1) with a FWHM of 35 cm1 and the high ratio of the peak intensities of 2D- to G-band (1582 cm1) (ratio ¼ 2 3) confirm the presence of a single-layer graphene pattern. The small D-band in this spectrum also confirms the high quality of the graphene pattern. Figures 3(c) and 3(d) show the intensity mapping of the G-band (green) and the 2D-band (red) as shown in Fig. 3(b), respectively. The Raman mapping in Figs. 3(c) and 3(d) shows that the intensities of the G- and 2D-band signals are clearly distinguished from the irradiated areas (dark region), thus demonstrating the precisely controlled localization of the graphene pattern. The Raman intensity distributions of the graphene patterns are relatively uniform in the patterned areas, also demonstrating the uniformity of the fabricated graphene patterns.
FIG. 1. Schematic of the laser-assisted transfer-printing procedure on arbitrary substrates. (a) Graphene on a metal substrate. (b) Laser patterning on a graphene/ metal substrate. (c) PMMA or PDMS coating on the patterned graphene/metal substrate. (d) The graphene pattern on PDMS (flexible transparent substrate) after metal etching. (e) PMMA/graphene layer transfer onto arbitrary substrates after metal etching. (f) Graphene on arbitrary substrates after PMMA removal.
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FIG. 2. Optical micrograph of (a) various shapes of graphene patterns on a graphene/copper substrate after femtosecond laser scanning and (b) transferred graphene patterns on a SiO2/Si substrate. Note that purple color patterns are a single-layer graphene. SEM images of (c) embossed and (d) engraved patterns with the text “graphene laser printing.” The inserted scale bars are 100 lm.
Figure 4 shows graphene micro/nano ribbons formed by the laser assisted transfer printing process. Figures 4(a) and 4(b) show optical and scanning electron microscopic images of micro graphene ribbons with a 30 lm width and 20 lm spacing. The micro patterns were fabricated at a high scan speed of 2 000 lm/s by using low magnification (5) objective lens with a large laser spot size. Figures 4(c) and 4(d) show various micro/nano graphene ribbons with 1 lm spacing on SiO2 substrates. Graphene micro-ribbons with various widths (2 9 lm) are demonstrated in Fig. 4(c). Graphene nano-ribbons with various widths (100 nm 1 lm) were also fabricated in Fig. 4(d). The resolution limit (100 nm) was obtained at a low scan speed of 100 lm/s and at the ablation threshold by a high magnification (100) objective lens (1 lm focused spot). It is recommended that the experiment is carried out near the ablation threshold to reduce thermal degradation of the edge of the graphene patterns. Therefore, the highly controllable printing process can access a wide range of feature resolution. Within a device
FIG. 3. (a) Optical micrograph of a printed graphene pattern on a SiO2/Si substrate and (b) its Raman spectrum from the “A” and “B” regions. Raman intensity mapping of (c) G-band (1582 cm1) and (d) 2D-band (2691 cm1).
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FIG. 4. Printed micro/nano ribbons on a SiO2 substrate. (a) Optical micrograph and (b) Scanning electron microscope images of micro ribbon with 30 lm width and 20 lm spacing. (c) Graphene micro ribbons with variable widths 29 lm and 1 lm spacing. (b) Graphene nano ribbons with variable widths 100 nm1lm and 1 lm spacing.
fabrication sequence, one can easily switch between highand low-resolution modes depending on the specific application requirements. For a demonstration of the technique introduced in this work, transparent strain devices were fabricated without photomasks and multi-step processes. Figure 5(a) shows different aspect ratios of millimeter graphene patterns on copper substrates fabricated by femtosecond laser scanning. Different device patterns were directly transferred onto flexible transparent substrates (PDMS) as shown in Fig. 5(b). The transparent stretchable devices were precisely tested on the micro-strain measurement setup in Fig. 5(b). During the strain test, we used an indium-eutectic liquid as contact electrode to reduce contact resistance. The resistance and resistance ratio characteristics of stretchable devices with respect to 1% strain cycles are shown in Figs. 5(c) and 5(d), respectively. The variation of the resistance is repeatable during 1% strain cycles. The resistance variation of high aspect ratio devices is more sensitive than that of low aspect ratio ones with respect to the strain variation. The variation of the resistance ratio is 0.02 0.04 at 1% strain. This means that the transparent strain devices are highly stretchable compared to other metal based strain devices. In conclusion, we have developed a reliable, fast, multiscale, high resolution, scalable, maskless direct graphene patterning process that is transferrable to arbitrary substrates. The short pulse nature of a femtosecond laser on a graphene/ copper sheet enables fabrication of high-resolution graphene patterns. Machined graphene patterns on the copper foil can be directly transferred to the desired substrates for various applications. The fast laser-assisted transfer printing process for a wide range of milli/micro/nanometer graphene printing on arbitrary substrates is remarkably free of geometrical limitations and provides a true alternative to conventional multistep photolithography techniques. This direct writing process can also be readily incorporated into roll-to-roll CVD graphene manufacturing sequences. It is, therefore, expected that the laser-assisted transfer printing process will provide an innovative pathway for graphene-based printing electronics.
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Appl. Phys. Lett. 101, 043110 (2012)
FIG. 5. (a) Graphene patterns with different aspect ratios(x-y) on copper substrates (b) Fabrication process of transparent strain devices and strain measurement setup. (c) The resistance and (d) resistance ratio characteristics of the transparent strain devices with respect to 1% strain cycles.
This research work was financially supported by the King Abdullah Univesity of Science and Technology (KAUST). We would like to thank Professor Junqiao Wu’s group from the Materials Science and Engineering Department of the University California, Berkeley for supporting the Raman scattering measurements. 1
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