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Graphene

Laser-Induced Direct Graphene Patterning and Simultaneous Transferring Method for Graphene Sensor Platform Jae-Hyuck Yoo, Jong Bok Park, Sanghoon Ahn, and Costas P. Grigoropoulos*

General methods utilized in the fabrication of graphene devices involve graphene transferring and subsequent patterning of graphene via multiple wet-chemical processes. In the present study, a laser-induced pattern transfer (LIPT) method is proposed for the transferring and patterning of graphene in a single processing step. Via the direct graphene patterning and simultaneous transferring, the LIPT method greatly reduces the complexity of graphene fabrication while augmenting flexibility in graphene device design. Femtosecond laser ablation under ambient conditions is employed to transfer graphene/PMMA microscale patterns to arbitrary substrates, including a flexible film. Suspended cantilever structures are also demonstrated over a prefabricated trench structure via the single-step method. The feasibility of this method for the fabrication of functional graphene devices is confirmed by measuring the electrical response of a graphene/PMMA device under laser illumination.

1. Introduction Graphene is a two-dimensional honeycomb structure of carbon atoms. Due to its strong covalent bonds of hexagonal sp2 hybridized carbon atoms with its highly mobile pi (π) electrons, graphene possesses extraordinary impermeable,[1] protective,[2] mechanical,[3,4] optical,[5] electrical,[6,7] and thermal properties.[8,9] These properties have been intensively studied and demonstrated with application examples, including mechanical resonators,[10] photodetectors,[11] terahertz detectors,[12] and bolometers.[13] In addition, the ultrathin thickness of graphene provides high transparency and flexibility for soft electronics,[14] while the extremely high surface-to-volume ratio offers high sensitivity for single gas molecule detection.[15] These demonstrations were achieved by two pivotal technologies: the well-established chemical vapor deposition (CVD) technique[16] for large area graphene J.-H. Yoo, J. B. Park, S. Ahn, Prof. C. P. Grigoropoulos Department of Mechanical Engineering University of California Berkeley, CA 94720-1740, USA E-mail: [email protected] DOI: 10.1002/smll.201300990 small 2013, 9, No. 24, 4269–4275

synthesis and the conventional semiconductor fabrication technology for graphene device production. Generally, for a graphene device fabrication, a synthesized graphene layer on metal foil should be transferred to each substrate, and followed by patterning. However, these processes involving wet-chemical steps can unintentionally modify the graphene properties.[17,18] Furthermore, the efficiency in the use of the transferred graphene is not high because most area of the transferred graphene is etched away during the patterning step. Hence, the current graphene fabrication methodology has room for improvement. In this regard, new ways of making graphene devices were proposed, whereby micron-sized pattern arrays were pre-formed via the e-beam lithography and then site-specifically transferred to a target substrate using a micromanipulator.[19] Graphene patterns on dielectric surfaces in wafer scale were directly achieved by a rapid thermal process without a transferring step.[20] In addition, ultrafast pulsed laser processing techniques have been utilized for graphene by various groups due to its valuable attributes including scalability, reduced heat affected zone,[21] and maskless, programmable patterning functionality, resulting in fast prototyping. For example, direct graphene patterning was demonstrated by femtosecond laser ablation,[22–25] and direct graphene oxide reduction and

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used to yield graphene patterns on many different target substrates. We also emphasize that the LIPT method is completely different than the well-studied laser-based methods, including laser-induced forward transfer (LIFT)[27,28] and laser-induced thermal imaging (LITI)[29] techniques, whereby the laser-irradiated site itself is directly transferred. In these approaches, a thick film layer and vertically aligned carbon nanotubes were successfully transferred.[30,31] However, the application of these methods to graphene would not be straightforward, since graphene is atomically continuous and possesses a low ablation threshold.[32] In the LIPT process, the laser-irradiated spot is removed and the unaffected area from the laser interaction is transferred. Hence, we could obtain micron-scale continuous graphene patterns without laser damage. We demonstrated graphene/polymethyl methacrylate (PMMA) patterns on arbitrary substrates and measured the electrical response of a graphene/PMMA device under laser illumination in order to confirm the feasibility of our method toward the fabrication of functional graphene devices.

2. Results and Discussion 2.1. Laser-Induced Pattern Transfer (LIPT) Method Figure 1. Process flow of the laser-induced pattern transfer (LIPT) method. (a) Attachment step: The suspended graphene/PMMA layer structure is attached on a target substrate. The vapor treatment assists the attachment step by the surface tension driven self-flattening effect. (b) Direct patterning and simultaneous transferring step: Femtosecond laser ablation is performed to define graphene/PMMA patterns that are simultaneously transferred to the target substrate. (c) Detachment step: Due to no physical links between the transferred pattern and the suspended layer, the transferred patterns remain after the detachment. The detached suspended graphene/PMMA layer structure is repeatedly usable.

simultaneous patterning were performed by femtosecond laser annealing.[26] In the present study, we propose the laser-induced pattern transfer (LIPT) method for direct graphene patterning and simultaneous transferring. In addition to the novel attributes of ultrafast pulsed lasers, the LIPT method possesses advantages over conventional graphene fabrication techniques. First, it is fast and facile since the graphene transferring and subsequent patterning processes are operated via a single step. Second, arbitrary graphene patterns can be positioned and also precisely aligned to pre-existing structures on demand in contrast to utilizing the pre-patterned library.[19] Third, wet-chemical steps are not involved in the LIPT method that is performed under the ambient conditions; therefore, the chance of graphene contamination can be minimized. Fourth, a CVD grown graphene foil can be repeatedly

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The LIPT process flow is illustrated in Figure 1. The first step entails the graphene attachment on a target substrate. In order to handle atomically thin graphene, we devised the suspended graphene/PMMA layer structure in Figure 1(a). PMMA was chosen as a supporting material of the suspended configuration due to its high Young’s modulus. Furthermore, it is convenient to form PMMA layer on graphene via spin-coating. The key point of the attachment step is surface tension driven self-flattening of the graphene/PMMA layer on the target substrate. To this end, we applied water vapor on the substrate. The suspended graphene/PMMA layer structure was then gently located on the substrate and attached conformally to the substrate by itself. (see Figure S1 in the Supporting Information) The selfflattened graphene/PMMA layer facilitates maintaining the focal point of the laser beam across the entire layer during the following patterning step. Femtosecond laser pulses are delivered on the attached and flattened graphene/PMMA layer via an objective lens as depicted in Figure 1(b). Here, we employed a femtosecond laser whose pulse duration is shorter than the time scale for considerable energy transfer from electrons to phonons (a few picoseconds), and therefore


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Simultaneous Patterning and Transferring Method for Graphene

during the ablation process by introducing a large gap between the suspended graphene/PMMA layer and the target substrate, the transferred graphene/PMMA pattern was randomly displaced from the desired patterning position and not stably maintained on the substrate (see Movie S1 in Supporting Information) We note that the suspended graphene/ PMMA layer structure is repeatedly usable until no suspended layer remains. For example, with a 4 × 4 mm2 sized suspended graphene/PMMA layer, graphene patterns can be transferred over 1000 times on different substrates if the Figure 2. The suspended graphene/PMMA layer structures. (a,b) Process schematics and graphene pattern has the area of a 100 × 100 μm2. Without the capability of synphotographs of experimental examples of the copper foil frame type and the PI film frame type. (c) Process schematics for producing holes in the PI film frame (see Experimental thesizing large area graphene,[33,34] the Section for details). LIPT method in conjunction with the suspended layer structure might be a pracpatterning by laser ablation can be achieved with minimized tical alternative for graphene sensor device fabrication. Our lateral thermal damage.[21] Upon the irradiation of femto- experimental examples of the suspended graphene/PMMA second laser pulses, the focused spot in the graphene/PMMA layer structure and the process flows (also see the Experilayer is ablated. By translating a XY stage, the pulses are mental Section) for each type are shown in Figure 2. There spatially overlapped and arbitrary ablated paths are written are no design constraints for the suspended structure as long in the layer via motorized stage programming (see Movie S1 as graphene with supporting material can be suspended and in Supporting Information). After detaching the suspended ablated. For example, graphene/spin-coated polyimide (PI) structure from the substrate, graphene/PMMA patterns layer would be another good configuration. For the graphene/ remain on the substrate in the absence of a physical link PI layer case, additional frame part is not necessary since the between the pattern and the suspended layer (Figure 1(c)). PI layer is self-suspended.[35] In this study, all graphene patThe thermally and electrically insulating PMMA layer sup- terns were produced from the same suspended graphene/ ports the transferred graphene patterns upon comple- PMMA layer structure. tion of the LIPT process. Interestingly, we observed that the adhesion between the pattern and the target substrate was strengthened since the thermal field induced during 2.2. Transferred Labyrinth Graphene/PMMA Pattern the ablation process caused PMMA melting at the pattern edge that glued the pattern and the substrate together, or As a first demonstration of the LIPT method, we produced a “glue effect”. When the glue effect was not established a graphene/PMMA labyrinth pattern. Figure 3(a) presents the scanning electron microscope (SEM) image of the transferred pattern on a silicon substrate. The line width of the graphene/PMMA pattern is 16.35 μm and was achieved with a 50× objective lens (NA: 0.55) at the laser fluence of 0.68 J/cm2 and the stage translation speed of 0.1 mm/s. The repetition rate of the laser is 500 kHz, resulting in 1 Å shift between two subsequent pulses at the translation speed, and therefore more than 4000 laser pulses are irradiated in the same spot. The ablation threshold of PMMA is estimated as 0.17 J/cm2 via the relation, Φth(N) = Φth(1)Nξ-1, Figure 3. (a) SEM image of transferred labyrinth graphene/PMMA pattern on silicon substrate where Φ (N) is the ablation threshold for th by the LIPT method. The inset shows its reference design of the graphene/PMMA pattern. N laser pulses, N is the number of laser The line width of the graphene/PMMA pattern is 16 μm. The scale bar is 100 μm. (b) Raman pulses at the same spot, and ξ is the degree spectra from the white dashed area on the graphene/PMMA pattern and from PMMA layer [36] The incuwithout graphene on silicon substrate. The existence of single layer graphene is verified by of incubation for a material. the ratio of the peak intensity of the 2D band over the peak intensity of the G band. The D bation effect was caused by absorption behavior change during repetitive laser band (1350 cm−1) corresponding to graphene defects is not observed. small 2013, 9, No. 24, 4269–4275


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pulse irradiation. With the same experimental configuration, we could obtain 6 μm wide and 3 mm long graphene/PMMA line patterns. Smaller patterns would be achieved with a more intricate positioning and optical focusing system, and post direct laser ablation on graphene can further reduce the graphene pattern size.[32] In Figure 3(b), the two characteristic Raman peaks of graphene, i.e. the 2D band (2692 cm−1) and G band (1585 cm−1) are displayed. We verified the existence of single layer graphene by the ratio of the peak intensity of the 2D band over the peak intensity of the G band. The D band (1350 cm−1) corresponding to graphene defects from its breathing modes was not observed. Without direct exposure to a laser beam or chemicals that may cause damage or contamination, the graphene properties were maintained upon completion of the LIPT process. Raman peaks other than the G and 2D bands were observed from a spin coated 5 μm thick PMMA layer on a silicon substrate without graphene on it (Figure 3(b)) whose spectrum was consistent with the findings of a previous PMMA Raman study.[37]

2.3. Graphene/PMMA Patterns on Flexible ﬁlm and Graphene/PMMA Cantilevers Also, we performed the LIPT process on a 25 μm thick PI film using the same parameters as in the previous labyrinth case. Figure 4(a) shows the flexible PI film with twelve graphene/PMMA patterns wrapped over the curved body. The bottom inset shows that the surface of the PI film was rough and therefore the transferred pattern was not conformably attached to the film. Nevertheless, all patterns remained on the flexible film even after many bending cycles due to the glue effect developed along the graphene/PMMA pattern

edge during the patterning step. This demonstration implies the compatibility of the method to flexible substrates, and therefore the LIPT method can provide a route towards the fabrication of flexible applications. For instance, the method could be directly incorporated into a roll-to-roll system for soft electronics. We also demonstrated graphene/PMMA cantilevers in Figure 4(b). Generally, for fabricating suspended structures, the structures are pre-patterned using the lithography techniques and then the final releasing step is carried out via gas/ wet etching. Consequently, the whole process should be considered altogether, for example, material should be inert to the etching agent, and also the stiction issue at the releasing step should be resolved. In contrast, the LIPT method can be performed on a pre-fabricated trench structure, resulting in no design constraints over the whole fabrication process. In Figure 4(b), the suspended graphene/PMMA cantilevers were directly patterned and simultaneously transferred over a 400 μm wide trench. When the covering area of the graphene/ PMMA layer over a trench is large during the LIPT process, the height of the suspended layer can vary, and in general sags. If the height variation is greater than the laser focal depth, the laser focus on the entire layer might not be maintained during the patterning process. As a result, it is more challenging to make larger suspended structures. Figure 4(c) shows the optical microscope image of the transferred graphene/PMMA cantilevers over the trench, and its corresponding area of the suspended graphene/PMMA layer structure after the detachment is shown in Figure 4(d). The white cross shape is added to highlight that the cantilever produced by the LIPT method is aligned to the pre-existing trench structure. Since the suspended graphene/PMMA structure (see Figure S2 in the Supporting Information) can mechanically and thermally operate by itself and free from substrate effects, suspended configurations offer great advantages for sensor and actuator devices such as mechanical resonators,[10] photodetectors, and bolometers.[38] In addition, functionalizing the graphene/PMMA structure with quantum dots or metal nano particles, or replacing the PMMA supporting layer with more flexible polydimethylsiloxane (PDMS) or piezo materials may incorporate other interesting functionalities.[39,40]

2.4. Electrical Responses of Graphene/PMMA Device Under Light Illumination

Figure 4. (a) Optical microscopic image of transferred graphene/PMMA patterns on flexible PI film over curved surface. The image shows the area of the dashed area in the top inset. The bottom inset shows a detailed image of the pattern. The scale bar is 200 μm. (b) Tilted SEM image of graphene/PMMA cantilevers over trench in silicon substrate. The inset shows a SEM top view image of the cantilever pattern. The scale bar is 200 μm. Optical microscope images of (c) transferred graphene/PMMA cantilevers over trench and (d) its corresponding area of the suspended graphene/PMMA layer structure after the detachment. The white cross shape is added to highlight that the cantilever is aligned to the trench structure by the LIPT method. The scale bars are 400 μm.


Lastly, we investigated the electrical responses of the graphene/PMMA device in Figure 5. Although we could directly measure electrical signals from the graphene/PMMA pattern using a probe station, a silver paste was additionally applied to the both ends to protect the pattern from mechanical damage during the measurement. In Figure 5(a), the linear current–voltage (I–V) curve shows the characteristic response of graphene with a zero bandgap, acting as a semi-metal, and without the Schottky barrier. Li et al. reported that the sheet resistance of graphene on a PMMA film was lower than the value of graphene on a glass substrate.[41] We infer that reducing the remote oxide phonon


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Figure 5. (a) Linear I–V curve of graphene/PMMA device on glass slide. The optical microscopic image of the inset shows the graphene/PMMA device with probing pads. The sheet resistance is 0.95 kΩ/䊐. The scale bar is 200 μm. (b) Transient current response of the device at the bias voltage of 2.5 V under on/off event of laser pointer (402 nm, 8 mW). The inset shows the relative current response of the dashed region. Gas molecules from the atmosphere are adsorbed on a graphene surface and behave as electron acceptors. When light is illuminated, the adsorbed gas molecules are desorbed from the graphene surface by the energy of incident photons. This process reduces the doping concentration, and therefore the current decreases under the laser irradiation. (c) Relative current response of the device at the bias voltage of 2.5 V under laser illumination events (532 nm, 100 ns, and 100 kHz) at varied laser intensities. (d) Schematics of photo-induced electron-hole pair and their transports under biased potential. Due to the surface adsorbed molecules, the graphene channel is presented as a p-type. Under laser illumination, an electron-hole pair is created and they are transported in the opposite directions due to the biased potential, resulting in the current increase.

(ROP) scattering via inserting the PMMA layer results in the low sheet resistance of the graphene.[42] Therefore, the PMMA layer itself acts as a nice substrate. As a result, the graphene/PMMA configuration can be a simple, yet functional graphene device platform. The graphene/PMMA device with the sheet resistance of 0.95 kΩ/䊐 was subjected to light illumination. Since the device was fabricated on a glass slide rather than on a silicon substrate, photosensitive substrate effect[43] was excluded in the measurement. Figure 5(b) shows the current response of the device at the bias voltage of 2.5 V under laser irradiation over the entire device using a blue laser pointer (402 nm, 3.08 eV). In general, graphene operates as a p-type semiconductor with a positive Dirac point under the ambient conditions because the surface adsorbed gas molecules from the atmosphere behave as electron acceptors.[15,44] When light is illuminated, the adsorbed gas molecules are desorbed from the graphene surface by the energy of incident photons.[44,45] This process reduces the doping concentration, and therefore the current decreased under the laser irradiation. Reversely, when the laser was off, the current slowly returned to the initial small 2013, 9, No. 24, 4269–4275

value by re-adsorbed molecules, indicating no laser damage and full reversibility of the device. We expect that the graphene device combined with a compact laser diode could serve as a reusable gas molecule-detecting platform. We also employed a green laser beam (532 nm, 2.33 eV) that was irradiated via a 2× objective lens (NA: 0.055) on the middle of the graphene device at the bias voltage of 2.5 V. Figure 5(c) shows the increasing relative current response under laser irradiation. Interestingly, the increasing current response is opposite to the previous decreasing response under laser irradiation and the mechanism is illustrated in Figure 5(d). When the p-type graphene channel intercepts a photon, an electron–hole pair (EHP) is created; the electron and hole are then transported in opposite directions due to the electric potential exerted by the bias voltage, resulting in the increasing current.[46] We note that the two different mechanisms existed for both laser cases. (see Figure S3, S4 in Supporting Information). Since the wavelength dependent photoinduced molecular desorption is stronger at short wavelength,[44,45] the current response from the molecular desorption was dominantly observed in the blue laser case. In contrast, for the green laser case wherein the molecular effect was not dominant, the current response from the photoinduced EHPs could be measured.

3. Conclusion In conclusion, we have developed a new laser processing method that greatly reduced the complexity of graphene fabrication while also minimizing the chance of graphene contamination. As a proof of concept, we applied the LIPT method to produce graphene/PMMA patterns on various substrates, including a flexible film, and demonstrated precisely positioned, aligned, and suspended graphene/PMMA structures. We verified the Raman signature of single layer graphene from the transferred patterns and also investigated the electrical responses of a graphene/PMMA device under laser irradiation. The laser method in conjunction with the suspended graphene/PMMA structure was confirmed as an effective, practical solution for fabricating functional graphene devices. Lastly, we emphasize that the LIPT method is not just limited to graphene, but can be applied to various materials with relatively minor modifications. For example, by selecting a suitable material combination, thermally activating bimorph actuators or uncooled bolometer type infrared sensors can be produced using the LIPT method. Consequently, LIPT may



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provide flexibility and efficiency in general device fabrication, leading to novel architectures and improved performance.

4. Experimental Section Laser Ablation: All laser ablation processes in this study were carried out with laser (IMRA Inc.) of ∼500 femtosecond pulse duration, 532 nm wavelength, and 500 kHz repetition rate. The laser system is equipped with the XY translation stage (ANT130, Aerotech Inc.), a laser power attenuator (consisting of a half wave plate and a polarizing beam splitter), a mechanical shutter (Thorlabs, SC10), and in-situ monitoring setup. For focusing the laser beam, long working distance objective lenses (Mitutoyo) were used. Suspended Graphene/PMMA Layer Structure (Copper Foil Frame Type): In Figure 2(a), PMMA (495K-A8, Microchem) was spin-coated at 3000 RPM for 60 s on the both sides of the graphene/copper foil/graphene stack (CVD graphene, ACS Material) and dried on a hot plate at 120 °C for 5 min. The thickness of the coated PMMA layer was measured around 4 μm. Laser ablation via a 20× objective lens (NA: 0.42) at the laser power of 50 mW was then performed to create the etching through holes in the PMMA etching mask layer for selective copper layer etching through them. In order to shape the rectangular openings in the PMMA layer, we produced sequential ablated lines with a 10 μm inter distance by translating the stage at the speed of 5 mm/s. Then, the stack was put on a copper etchant (Sigma Aldrich) for 30 minutes at the room temperature, and rinsing in deionized water (DIW) was performed to remove residuals. After drying at 120 °C on a hot plate for 5 min, suspended graphene/PMMA layer over the copper foil frame was finally made in Figure 2(b). Suspended Graphene/PMMA Layer Structure (PI Film Frame Type): In Figure 2(c), PMMA was spin-coated on the one side of the single layer graphene on copper foil at 2500 RPM for 40 seconds, then a 50 μm thick PI film with rectangular holes was attached on the PMMA layer that was sticky before dried. Then, the PI/PMMA/ graphene/copper stack was put on a copper etchant at 60 °C on a hot plate for 20 min, and rinsing in DIW was performed to remove residuals. After drying at 120 °C on a hot plate for 5 min, the suspended graphene/PMMA layer over the PI film frame was finally made in Figure 2(b). In order to produce the rectangular holes in the 50 μm thick PI film, we performed laser ablation via a 20× objective lens at the laser power of 150 mW by translating the XY stage along rectangular contours at the speed of 1 mm/s (Figure 2(c)). Silicon Trench Structure of Graphene/PMMA Cantilevers: Using an e-beam evaporator, Cr (5 nm) and Au (10 nm) layers were deposited on a thermally grown SiO2 (100 nm) layer on a silicon substrate. For etching silicon, an opening in the metal/SiO2 mask layers was produced by laser ablation at the laser power of 50 mW with a 20× objective lens. For shaping the rectangular opening (400 μm × 2 mm), sequential ablated lines with a 10 μm inter distance were produced on the mask layers (Cr/Au/SiO2) by translating the XY stage at the speed of 5 mm/s. After tetramethylammonium hydroxide (10% TMAH in DIW) etching through the ablated opening for 1 h at 90 °C on a hot plate, rinsing in DIW, and drying, the silicon trench was formed. Measurements: The Raman spectra were obtained using a Renishaw Raman optical system with the laser of 532 nm wavelength and 3 mW power. The I–V plot and the current response of the


402 nm case were measured using a two-probe technique with a semiconductor parameter analyzer (HP 4155A, the data acquisition rate of 1 data/s). For the 532 nm case, a multimeter (Protek 608 Digital, the data acquisition rate of 2 data/s) was used for the current response.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Support to this work by the NSF SINAM NSEC is gratefully acknowledged. We would also 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.

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Received: March 28, 2013 Revised: May 8, 2013 Published online: July 11, 2013


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