Lift-Off Assisted Patterning of Few Layers Graphene

Graphene and 2D materials have been exploited in a growing number of applications and the quality of the deposited layer has been found to be a critical issue for the functionality of the developed devices. Particularly, Chemical Vapor Deposition (CVD) of high quality graphene should be preserved without defects also in the subsequent processes of transferring and patterning. In this work, a lift-off assisted patterning process of Few Layer Graphene (FLG) has been developed to obtain a significant simplification of the whole transferring method and a conformal growth on micrometre size features. The process is based on the lift-off of the catalyst seed layer prior to the FLG deposition. Starting from a SiO2 finished Silicon substrate, a photolithographic step has been carried out to define the micro patterns, then an evaporation of Pt thin film on Al2O3 adhesion layer has been performed. Subsequently, the Pt/Al2O3 lift-off step has been attained using a dimethyl sulfoxide (DMSO) bath. The FLG was grown directly on the patterned Pt seed layer by Chemical Vapor Deposition (CVD). Raman spectroscopy was applied on the patterned area in order to investigate the quality of the obtained graphene. Following the novel lift-off assisted patterning technique a minimization of the de-wetting phenomenon for temperatures up to 1000 °C was achieved and micropatterns, down to 10 µm, were easily covered with a high quality FLG.


Introduction
In the last years, Single Layer or Few Layer Graphene (SLG-FLG) have been widely exploited to obtain novel high performance devices for different types of applications from electronics [1] to energy storage [2], sensors [3], biomedical implants [4] and others [5,6]. The quality of the SLG or FLG has been found to be a critical issue for the functionality of the devices and hence it is fundamental to improve the production and synthesis steps to avoid defects. To develop an SLG and FLG based device, the typical technological processes involved are: Chemical Vapor Deposition (CVD) on metal catalyst seed layers such as Cu or Ni foils [6]; the transferring step on polymer,-that is, poly(methyl methacrylate) (PMMA)-by spin coating and wet etching; the deposition on active regions, that is, metallic electrodes on SiO 2 finished Si substrates; and finally, the patterning step by plasma etching, laser ablation or other techniques [7]. The transfer of graphene onto arbitrary substrates is generally accomplished by polymer-assisted procedures. The transfer process consists of removing graphene from the growing substrate with the aid of a sacrificial polymer layer. For this purpose, several polymers like poly(methyl methacrylate) and polyvinylidene fluoride (PVDF) are widely employed as sacrificial supports [7,8]. Removing of the growing catalyst substrate is accomplished by chemical

Graphene Deposition on Pt Film
Graphene was grown on Si/SiO2/Al2O3/Pt substrates by a cold-wall chemical vapor deposition (CVD) reactor operating at low pressure. To remove contaminants from the surface, a two-step annealing of the substrates was performed: 2 min at 900 °C under reducing flow of Ar at 190 sccm and H2 at 10 sccm (flow control regime) and then 30 s at 1000 °C in an atmosphere composed of Ar 90% and H2 10% at 10 torr (pressure control regime). The growth of graphene was then carried out at 1000 °C for 300 s, in a mixed atmosphere of Ar (80%), H2 (10%) and CH4 (10%) at 10 torr. The samples were finally cooled down to 200 °C under a reducing flow of Ar + H2 (190 sccm and 10 sccm) and then to room temperature in Ar atmosphere.

Characterization
Un-patterned Al2O3/Pt thin films were characterized with field emission scanning electron microscopy (FESEM) after an annealing treatment at 900 °C, 1000 °C and 1050 °C to investigate the high temperature effect related to the CVD graphene growth process. Images were obtained with FESEM ZEISS Supra 40 (Oberkochen, Germany). For this purpose, the annealing was performed in the same atmosphere and time duration previously described for the growth of graphene but excluding CH4 in the gas mixture.
X-Ray Diffraction (XRD) was performed on un-patterned Si/SiO2/Al2O3/Pt substrates with the twofold aim of analysing the corresponding crystal structure and orientation and evaluating the effect of the thermal annealing at 1000 °C. XRD patterns were collected using a Panalytical X'Pert Diffractometer (PANalytical, Almelo, The Netherlands) in Bragg-Brentano configuration, equipped with a Cu Kα radiation as X-ray source (λ = 1.54059 Å).
Pt/FLG substrates were characterized by means of a Renishaw InVia Reflex micro-Raman spectrometer (Renishaw plc, Wottonunder-Edge, UK), equipped with a cooled CCD camera. The Raman source was a laser diode (λ = 514.5 nm) and samples inspection occurred in backscattering light collection through a 50× microscope objective for all the single spectra acquisition. The spectra

Graphene Deposition on Pt Film
Graphene was grown on Si/SiO 2 /Al 2 O 3 /Pt substrates by a cold-wall chemical vapor deposition (CVD) reactor operating at low pressure. To remove contaminants from the surface, a two-step annealing of the substrates was performed: 2 min at 900 • C under reducing flow of Ar at 190 sccm and H 2 at 10 sccm (flow control regime) and then 30 s at 1000 • C in an atmosphere composed of Ar 90% and H 2 10% at 10 torr (pressure control regime). The growth of graphene was then carried out at 1000 • C for 300 s, in a mixed atmosphere of Ar (80%), H 2 (10%) and CH 4 (10%) at 10 torr. The samples were finally cooled down to 200 • C under a reducing flow of Ar + H 2 (190 sccm and 10 sccm) and then to room temperature in Ar atmosphere.

Characterization
Un-patterned Al 2 O 3 /Pt thin films were characterized with field emission scanning electron microscopy (FESEM) after an annealing treatment at 900 • C, 1000 • C and 1050 • C to investigate the high temperature effect related to the CVD graphene growth process. Images were obtained with FESEM ZEISS Supra 40 (Oberkochen, Germany). For this purpose, the annealing was performed in the same atmosphere and time duration previously described for the growth of graphene but excluding CH 4 in the gas mixture.
X-Ray Diffraction (XRD) was performed on un-patterned Si/SiO 2 /Al 2 O 3 /Pt substrates with the twofold aim of analysing the corresponding crystal structure and orientation and evaluating the effect of the thermal annealing at 1000 • C. XRD patterns were collected using a Panalytical X'Pert Diffractometer (PANalytical, Almelo, The Netherlands) in Bragg-Brentano configuration, equipped with a Cu Kα radiation as X-ray source (λ = 1.54059 Å).
Pt/FLG substrates were characterized by means of a Renishaw InVia Reflex micro-Raman spectrometer (Renishaw plc, Wottonunder-Edge, UK), equipped with a cooled CCD camera. The Raman source was a laser diode (λ = 514.5 nm) and samples inspection occurred in backscattering light collection through a 50× microscope objective for all the single spectra acquisition. The spectra of the patterned structures were obtained by focusing the laser spot on their centre, while a Raman map of the 10 µm-wide circle was collected by scanning, by means of a long working distance 100× objective, a 16 µm × 16 µm area, with a 0.5 µm step. The spectral map analysis was performed by means of the Renishaw WiRE 3.2 software. To collect both the single spectra and the map, 50 mW laser power, 60 s of exposure time and 4 accumulations were employed.
Optical images were acquired with a Nikon Eclipse ME600 microscope (Nikon, Tokyo, Japan).

Results
Lift-off assisted patterning of FLG has been successfully obtained ( Figure 2) on Al 2 O 3 /Pt catalyst film. of the patterned structures were obtained by focusing the laser spot on their centre, while a Raman map of the 10 µm-wide circle was collected by scanning, by means of a long working distance 100× objective, a 16 µm × 16 µm area, with a 0.5 µm step. The spectral map analysis was performed by means of the Renishaw WiRE 3.2 software. To collect both the single spectra and the map, 50 mW laser power, 60 s of exposure time and 4 accumulations were employed. Optical images were acquired with a Nikon Eclipse ME600 microscope (Nikon, Tokyo, Japan).

Results
Lift-off assisted patterning of FLG has been successfully obtained ( Figure 2) on Al2O3/Pt catalyst film.

Morphologica Characterization of De-Wetting Dynamic
The temperature effects were evaluated by thermal annealing tests on Al2O3/Pt layer at 900 °C, 1000 °C and 1050 °C ( Figure 3).

Morphologica Characterization of De-Wetting Dynamic
The temperature effects were evaluated by thermal annealing tests on Al 2 O 3 /Pt layer at 900 • C, 1000 • C and 1050 • C ( Figure 3). of the patterned structures were obtained by focusing the laser spot on their centre, while a Raman map of the 10 µm-wide circle was collected by scanning, by means of a long working distance 100× objective, a 16 µm × 16 µm area, with a 0.5 µm step. The spectral map analysis was performed by means of the Renishaw WiRE 3.2 software. To collect both the single spectra and the map, 50 mW laser power, 60 s of exposure time and 4 accumulations were employed. Optical images were acquired with a Nikon Eclipse ME600 microscope (Nikon, Tokyo, Japan).

Results
Lift-off assisted patterning of FLG has been successfully obtained ( Figure 2) on Al2O3/Pt catalyst film.

Morphologica Characterization of De-Wetting Dynamic
The temperature effects we  FESEM images demonstrate that the Al 2 O 3 adhesion layer allows for controlling the de-wetting process (see supplementary information) to achieve a FLG growth temperature up to 1000 • C. It can be noticed that a detrimental effect appears at 1050 • C, where a discontinuous film is formed, making it impractical to use for most technological applications.

XRD Characterization
The diffraction spectrum obtained by XRD investigation (Figure 4) shows a comparison between as-grown Al 2 O 3 /Pt samples and the 1000 • C annealed one. Apart from the contribution coming from the Si substrate (2θ-69.2 • ), a single diffraction peak is detected at 40.1 • in both cases and ascribed to the family of Pt(111) crystal planes (JCPDS Card 04-0802). After annealing, the crystal quality of the Pt layers turns out to be improved, as demonstrated by the higher Pt(111) peak intensity. Moreover, the (111) crystal orientation is also highly desirable for promoting graphene growth [23]. This characterization validates Al 2 O 3 as adhesion layer for this kind of application; indeed, with respect to previous work on a similar process [31], Al 2 O 3 prevents a premature de-wetting for e-beam evaporated Pt film. FESEM images demonstrate that the Al2O3 adhesion layer allows for controlling the de-wetting process (see supplementary information) to achieve a FLG growth temperature up to 1000 °C. It can be noticed that a detrimental effect appears at 1050 °C, where a discontinuous film is formed, making it impractical to use for most technological applications.

XRD Characterization
The diffraction spectrum obtained by XRD investigation (Figure 4) shows a comparison between as-grown Al2O3/Pt samples and the 1000 °C annealed one. Apart from the contribution coming from the Si substrate (2θ-69.2°), a single diffraction peak is detected at 40.1° in both cases and ascribed to the family of Pt(111) crystal planes (JCPDS Card 04-0802). After annealing, the crystal quality of the Pt layers turns out to be improved, as demonstrated by the higher Pt(111) peak intensity. Moreover, the (111) crystal orientation is also highly desirable for promoting graphene growth [23]. This characterization validates Al2O3 as adhesion layer for this kind of application; indeed, with respect to previous work on a similar process [31], Al2O3 prevents a premature de-wetting for e-beam evaporated Pt film.

Raman Characterization of Patterned Pt
The patterned Al2O3/Pt was characterized by Raman spectroscopy to evaluate the quality of the grown graphene, which according to Wang et al. was about 2-3 layers [38].
The analysis was performed with the aim to evaluate the selective growth of FLG on Pt patterns with respect to SiO2 and the correlation between the FLG defectivity and the patterns sizes. Furthermore, growing temperature effect was investigated. Raman spectra of FLG on both un-patterned (red curve) and patterned Pt samples ranging from 5 to 100 µm wide strips were reported ( Figure 5). For the patterned Pt samples, the Raman spectra were collected on a 1-2 µm wide area, far from the edges. The intensity (I), position and shape of D, G and 2D peaks (centred at ~1350, 1580 and 2700 cm −1 respectively) are similar for the 100 µm patterned and un-patterned areas but the intensity of the D peak differs on the 5 µm pattern. The presence of an ubiquitous peak at ~2324 cm −1 can be related to atmospheric N2 gas fundamental vibration-rotation as previously reported [39].

Raman Characterization of Patterned Pt
The patterned Al 2 O 3 /Pt was characterized by Raman spectroscopy to evaluate the quality of the grown graphene, which according to Wang et al. was about 2-3 layers [38].
The analysis was performed with the aim to evaluate the selective growth of FLG on Pt patterns with respect to SiO 2 and the correlation between the FLG defectivity and the patterns sizes. Furthermore, growing temperature effect was investigated. Raman spectra of FLG on both un-patterned (red curve) and patterned Pt samples ranging from 5 to 100 µm wide strips were reported ( Figure 5). For the patterned Pt samples, the Raman spectra were collected on a 1-2 µm wide area, far from the edges. The intensity (I), position and shape of D, G and 2D peaks (centred at~1350, 1580 and 2700 cm −1 respectively) are similar for the 100 µm patterned and un-patterned areas but the intensity of the D peak differs on the 5 µm pattern. The presence of an ubiquitous peak at~2324 cm −1 can be related to atmospheric N 2 gas fundamental vibration-rotation as previously reported [39].
For the FLG growth on the un-patterned Pt sample, the G peak only differs from the patterned ones in shape and intensity with respect to the D band: a narrower and more symmetric band and an I D /I G ratio of~0.20 are observable. Moreover, the calculated I D /I G is~0.31 on the 100 µm pattern and~0.73 on the 5 µm pattern suggesting that the presence of the microstructures could induce a more disordered superposition of the graphene few layers. As pointed out in the review by Ferrari and Basko [40], other factors confirm the disorder induced in the case of patterned Pt/FLG with respect to plain film; these include: the increased dispersion of the G peak; the small elbow at the right of the G peak which could be associated with a small D' peak and the noise at the left of 2D which could be associated with the D" peak. On the other hand, the shape and position of the 2D band for both samples (un-patterned and patterned) indicate that the number and the quality of the deposited graphene sheets are quite comparable, as the peak, though symmetric, cannot be fitted by one Lorentzian and it has a FWHM of~77 cm −1 , 64 cm −1 and 84 cm −1 respectively for un-patterned, 100 µm and 5 µm patterns, which are compatible with the characteristics of FLG grown on a nickel-coated SiO 2 /Si substrate, previously reported by Park et al. [41]. For the FLG growth on the un-patterned Pt sample, the G peak only differs from the patterned ones in shape and intensity with respect to the D band: a narrower and more symmetric band and an ID/IG ratio of ~0.20 are observable. Moreover, the calculated ID/IG is ~0.31 on the 100 µm pattern and ~0.73 on the 5 µm pattern suggesting that the presence of the microstructures could induce a more disordered superposition of the graphene few layers. As pointed out in the review by Ferrari and Basko [40], other factors confirm the disorder induced in the case of patterned Pt/FLG with respect to plain film; these include: the increased dispersion of the G peak; the small elbow at the right of the G peak which could be associated with a small D' peak and the noise at the left of 2D which could be associated with the D'' peak. On the other hand, the shape and position of the 2D band for both samples (un-patterned and patterned) indicate that the number and the quality of the deposited graphene sheets are quite comparable, as the peak, though symmetric, cannot be fitted by one Lorentzian and it has a FWHM of ~77 cm −1 , 64 cm −1 and 84 cm −1 respectively for un-patterned, 100 µm and 5 µm patterns, which are compatible with the characteristics of FLG grown on a nickel-coated SiO2/Si substrate, previously reported by Park et al. [41]. Patterned Pt/FLG grown at 900 °C, 1000 °C and 1050 °C was further characterized by Raman spectroscopy (Figure 6). It can be noticed that at 1050 °C graphene quality improves although Pt thin film undergoes de-wetting effect and, with the increasing temperature, the ratio between 2D and G peaks also increases indicating a reduction in the number of layers. Moreover, both D peak intensity reduction and G peak sharpness indicate a minimization of the graphene defects. But from the comparison with morphological analysis (Figure 3), at 1050 °C the de-wetting of the Pt film has relevant detrimental effect. Patterned Pt/FLG grown at 900 • C, 1000 • C and 1050 • C was further characterized by Raman spectroscopy (Figure 6). It can be noticed that at 1050 • C graphene quality improves although Pt thin film undergoes de-wetting effect and, with the increasing temperature, the ratio between 2D and G peaks also increases indicating a reduction in the number of layers. Moreover, both D peak intensity reduction and G peak sharpness indicate a minimization of the graphene defects. But from the comparison with morphological analysis (Figure 3 Figure 7 shows FLG Raman spectra from the centre of a 5 µm strip to the border of the same pattern and then in a region 2.5 µm far from the edge. A transition from graphene to graphitic carbon residual is observed as the developing of D and G peaks indicate the presence of sp 2 carbon with a consistent number of defects as previously reported [42]. In order to verify the homogeneity of the FLG distribution and possible physical boundary effects, the scanning of a 16 × 16 µm 2 area, including circle-shaped patterns (10 µm in diameter), was performed. The collected spectral Raman map ( Figure 8) highlights that, in the inner region of the microstructure, the intensities of the D, G and 2D bands are quite constant in distribution and mutual ratio. Then, all the peak intensities increase by approaching the edge of the micro-circle, suggesting an accumulation of more defective and lower quality graphene sheets within such regions. Beyond the microstructure boundaries, no Raman features related to FLG are present, in accordance with blank spectrum (black curve) of Figure 6. This demonstrates the high selectivity of the growing process. Regarding defects accumulation on the edges, it is possible to assume that the discontinuities on the catalyst can affect the formation of graphene crystal domains thus leading to a more disordered growth.  Figure 7 shows FLG Raman spectra from the centre of a 5 µm strip to the border of the same pattern and then in a region 2.5 µm far from the edge. A transition from graphene to graphitic carbon residual is observed as the developing of D and G peaks indicate the presence of sp 2 carbon with a consistent number of defects as previously reported [42].
In order to verify the homogeneity of the FLG distribution and possible physical boundary effects, the scanning of a 16 × 16 µm 2 area, including circle-shaped patterns (10 µm in diameter), was performed. The collected spectral Raman map ( Figure 8) highlights that, in the inner region of the microstructure, the intensities of the D, G and 2D bands are quite constant in distribution and mutual ratio. Then, all the peak intensities increase by approaching the edge of the micro-circle, suggesting an accumulation of more defective and lower quality graphene sheets within such regions. Beyond the microstructure boundaries, no Raman features related to FLG are present, in accordance with blank spectrum (black curve) of Figure 6. This demonstrates the high selectivity of the growing process. Regarding defects accumulation on the edges, it is possible to assume that the discontinuities on the catalyst can affect the formation of graphene crystal domains thus leading to a more disordered growth.

Discussion
The reported analysis demonstrates that the lift-off assisted patterning is a valid method to obtain good quality FLG on Pt layer. A significant time reduction with respect to traditional process was achieved since the typical transferring steps were completely skipped. In addition, this method is not affected by the contamination of supporting polymers as PMMA. The obtained optimal repeatability on micrometric patterns allows for covering Pt film with every layouts and, more

Discussion
The reported analysis demonstrates that the lift-off assisted patterning is a valid method to obtain good quality FLG on Pt layer. A significant time reduction with respect to traditional process was achieved since the typical transferring steps were completely skipped. In addition, this method is not affected by the contamination of supporting polymers as PMMA. The obtained optimal repeatability on micrometric patterns allows for covering Pt film with every layouts and, more

Discussion
The reported analysis demonstrates that the lift-off assisted patterning is a valid method to obtain good quality FLG on Pt layer. A significant time reduction with respect to traditional process was achieved since the typical transferring steps were completely skipped. In addition, this method is not affected by the contamination of supporting polymers as PMMA. The obtained optimal repeatability on micrometric patterns allows for covering Pt film with every layouts and, more important, Pt can be deposited with common techniques such as sputtering or e-beam evaporation and then easily integrated in a full device fabrication process. Pt represents an optimal metal selection for electrodes in chemical/biological sensors [5] as well as for high temperature micro-hotplates in Micro Electro Mechanical System (MEMS) [6], due to its high chemical and temperature stability and hence the implementation of this method is of high relevance for a wide range of applications from biosensing to neuronal stimulation.

Conclusions
This FLG was grown directly on the patterned Pt seed layer by Chemical Vapor Deposition (CVD). The use of a proper adhesion layer, Al 2 O 3 , for the Pt film allows for raising the FLG growth temperature up to 1000 • C. The lift-off process of the catalyst, obtained by a standard photolithographic step, leads to a significant time reduction and consequent costs, of the graphene patterning since the typical transferring and etching steps were completely skipped, moreover an optimal repeatability on micrometric patterns can be easily obtained. The Raman characterization shows that the micropatterning was effective, and an accumulation of defects was mostly observed on the edges due to the discontinuity of the patterns. Since Pt is one of the most used materials for electrochemical or gas sensors due to its high thermal and chemical stability, the presented patterning approach has a potential high impact on the fabrication of graphene-based devices, when high quality graphene is required on noble metal electrodes. Moreover, the presented process can be applied to fabricate microelectrodes directly decorated with graphene on a whole wafer of any size avoiding the constraints correlated to polymer-assisted graphene transfer and etching.
Supplementary Materials: The following are available online at http://www.mdpi.com/2072-666X/10/6/426/s1, Figure S1: FESEM images at different magnifications of graphene growth on Pt at 900 • C. Figure S2: FESEM images at different magnifications of graphene growth on Pt at 1000 • C. Figure S3: FESEM images at different magnifications of graphene growth on Pt at 1050 • C. Table S1: Percentage of Pt coverage to evaluate de-wetting.