Support-Free Transfer of Ultrasmooth Graphene Films Facilitated by

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Support-Free Transfer of Ultra-Smooth Graphene Films Facilitated by Self-Assembled Monolayers for Electronic Devices and Patterns Bin Wang, Ming Huang, Li Tao, Sun Hwa Lee, A-Rang Jang, BaoWen Li, Hyeon Suk Shin, Deji Akinwande, and Rodney S. Ruoff ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06842 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 23, 2015

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Support-Free Transfer of Ultra-Smooth Graphene Films Facilitated by Self-Assembled Monolayers for Electronic Devices and Patterns Bin Wang,†,# Ming Huang,†,‡,# Li Tao,§ Sun Hwa Lee,† A-Rang Jang,﬩ Bao-Wen Li,† Hyeon Suk Shin,﬩ Deji Akinwande,§ and Rodney S. Ruoff †,‡,﬩,*

†

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS),

Ulsan 689-798, Republic of Korea. ‡

School of Materials Science and Engineering, Ulsan National Institute of Science and

Technology (UNIST), Ulsan 689-798, Republic of Korea. §

Microelectronics Research Center, Department of Electrical and Computer Engineering, The

University of Texas, Austin, Texas, 78758, USA. ﬩

Department of Chemistry and Department of Energy Engineering, Ulsan National Institute

of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea. #

These authors contributed equally to this work.

*Address correspondence to [email protected]

KEYWORDS: Graphene, self-assembled monolayer, transfer, support-free, ultra-smooth, GFET, pattern

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ABSTRACT: We explored a support-free method for transferring large area chemical vapor deposition (CVD)-grown graphene films to various fluoric self-assembled monolayer (F-SAM) modified substrates including SiO2/Si wafers, polyethylene terephthalate (PET) films, and glass. This method yields clean, ultra-smooth and high-quality graphene films for promising applications such as transparent, conductive, and flexible films due to the absence of residues and limited structural defects such as cracks. The F-SAMs introduced in the transfer process can also modify a p-type graphene field-effect transistors (GFET) and benefit the carrier mobility of the graphene films, for which the gate modulation was as high as one fold (~12×). Clean graphene patterns can be facilely realized by utilizing the selective transfer of graphene onto the F-SAM modified surfaces.


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Transferring chemical vapor deposition (CVD)-grown graphene films from metal surfaces to target substrates while preserving their high quality is a major challenge, particularly for use in electronic devices.1-4 Thus far, some transfer techniques have been reported involving the use of support materials such as polymers, namely polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), poly (bisphenol A carbonate) (PC), thermal release tape (TRT), and also through ‘electrochemical delamination’.5-22 The use of pressure sensitive tape has recently been described by Kim et al., who stated that this is an effective method for transfer of graphene from Cu foil and to various substrates.23 However, the polymer/tapebased methods suffer from support material residues left on the surface of the transferred graphene films, and additional cleaning process(es) that may lead to cracking or folding/wrinkling of the graphene, resulting in a poorer performance of the graphene-based devices.24 Therefore, support-free transfer methods for CVD grown graphene films towards ultra-smooth graphene surfaces typically for electronic devices are highly desirable. Considering a high performance graphene field-effect transistors (GFET) device, it has been reported that the devices on SiO2 substrates have a lower carrier mobility than suspended devices or devices on hexagonal boron nitride (h-BN) substrates.25,

26

More

recently, the SAMs (molecular organosilane self-assembled monolayers) -modified SiO2 substrate have been reported to be efficient in modulating the electronic properties of graphene to make selectively doped FETs and boost field-effect mobility by attenuating surface polar phonon scattering,

27-30

and perhaps other factors such as improved surface

flatness and fewer wrinkles and so on. However, support materials were all included in the fabrication of these graphene/SAMs devices, and the resulting surface contaminations may influence the as-measured transport performances.


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Support-free transfer can be achieved by floating graphene films on the surface of solution through a slow and nearly static process or tactfully enhancing the adhesion between the graphene and target substrates using physical approaches involving electrostatic and/or mechano-electro-thermal forces.31-33 However, a novel and typical transfer method to generate ultra-smooth and high quality graphene without contamination from the support materials while synchronously be capable to modulate the electronic performance is specially required. Here we introduce a support-free method for transferring large area and ultra-smooth CVD-grown graphene films to various SAMs modified substrates including SiO2/Si wafers, polyethylene terephthalate (PET) films, and glass. The target substrate was first pre-treated to form a hydrophobic surface with the SAM (Figure 1). The graphene/Cu foil was then gently pressed on the modified substrate, followed by immersion in an aqueous copper etchant. Typically, graphene films tend to detach from the pristine SiO2/Si wafer and become suspended in water due to the water molecules diffusing between the graphene and substrate (Figure 1b). In contrast, the hydrophobic groups terminating the SAM hinder the insertion of water molecules and thus maintain the high adhesion between graphene and the modified SAM-coated SiO2/Si wafer during the process of Cu etching (Figure S1, ESI, in which an atomic force microscopy/AFM image shows the transferred graphene without water at the interface between it and the SAM/Si wafer).34, 35 This approach not only provides an efficient and clean route for transferring ultra-smooth CVD-grown graphene films onto different substrates, but also improves the electronic properties of the graphene devices due to the SAM on the substrate. RESULTS AND DISCUSSION


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In a typical experiment, a Si wafer with a 300-nm thick thermally grown oxide layer on the surface is used as the base substrate. For the untreated SiO2/Si wafer, Figure 2a, the water contact angle was 71.1°, indicating a somewhat hydrophilic surface. The direct transfer of graphene onto this substrate was investigated, and no graphene was observed on the wafer after etching the Cu foil. A well-known technique was used to modify the surface of the SiO2/Si wafer with SAMs, as shown in Figure S2. The SiO2/Si substrates were first treated with O2 plasma, forming a highly hydrophilic surface with a water contact angle of 7.8° (Figure S3). During this pre-treatment, silanol groups (-SiOH) were formed on the surface for the subsequent dechlorination reaction with silane coupling agents (see details in the Method section). Briefly, vapor deposition in vacuum was used to form the SAMs rather than solution-based methods to avoid possible contamination of the transferred graphene by the solvents, here typically toluene, hexane, and ethanol. Three types of coupling agents, namely, trichloro(octadecyl)silane,

trichloro(phenyl)silane,

and

trichloro(1H,1H,2H,2H-

perfluorooctyl)silane, were used to form the SAMs, with the substrates thus referred to as CH3-Si wafers, C6H5-Si wafers, and F-Si wafers, respectively. X-ray photoemission spectroscopy (XPS), water contact angle goniometry, ellipsometry and atomic force microscopy (AFM) measurements of thickness and roughness were carried out to evaluate each different SAM on the surface of the modified SiO2/Si wafers. In the XPS survey spectra of the substrates ranging from 0 to 1000 eV (Figure S4), the C1s peak with higher intensity compared with that of the pristine SiO2/Si wafer was observed, indicating the successful formation of the SAMs. The F1s peak was observed for the F-Si wafer, and no chlorine peak was observed for all three modified substrates, suggesting chemical bonding between these coupling agents and the silanol groups on SiO2. The water contact angles for the CH3-Si wafers, C6H5-Si wafers, and F-Si wafers were 5 ACS Paragon Plus Environment

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81.1°, 90.0°, and 105.2°, respectively. It is worth noting that the water contact angle of a wafer modified through the vapor deposition process is typically reported to be smaller than that obtained for SAMs deposited by the solution method, but it can provide a cleaner and smoother surface for the graphene transfer.36, 37 The thickness of the SAMs were measured by ellipsometry to be ~2.34 nm (CH3-), ~0.96 nm (C6H5-), and ~1.37 nm (F-), while the roughness (Rq) values were 0.42 nm, 0.51 nm, and 0.41 nm as measured by AFM, respectively, which are consistent with the reported values for monolayer SAM coverage.28, 37, 38

The three modified substrates were used to attempt graphene transfer under similar

conditions, as illustrated in Figure 1a. The F-Si wafer appears to maintain the most intact graphene film after transfer (Figure 2d), implying that the more hydrophobic surface affords a better transfer, or possibly other factors are also playing a role. A 6.5cm × 6.5cm graphene film was transferred onto a 4-inch F-Si wafer (Figure S5, Figure 3a). The optical image in Figure 3b shows the transferred graphene as being continuous and almost free of cracking, which is hard to be achieved using the PMMA method (Figure S6). As a comparison, the cracks on graphene films (SAM versus PMMA method, collected from a 1.4mm×1.1mm region, here we used the best graphene sample transferred by the PMMA method) were observed by optical microscopy and counted, and are presented in Figure S7, which shows a much smaller number of cracks in the case of graphene transferred onto the F-Si wafer. The uniformity and cleanliness of the transferred graphene film were evaluated by AFM (Figure 3c). The AFM data showed that the films were uniform and free of residue. The thickness of the monolayer region is around 1 nm, suggesting highly conformal contact between the graphene and the substrate. The surface roughness of the graphene films transferred respectively by PMMA and SAM methods were evaluated by AFM measurement. Ten graphene regions as clean as we 6 ACS Paragon Plus Environment

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could found with the equal size of 5*5 µm2 were selected and shown in Figure S8 as a comparison. The average surface roughness of graphene transferred by PMMA method is 1.20 nm (Table 1). The high surface roughness may be generated from the worse contact between graphene and the SiOx substrate formed during the scooping process, also relates to the polymer residues. In contrast, the Rq is as low as 0.26 nm when transferred by SAM method, even lower than the roughness of pure F-SAM. This observation may suggests the monolayer molecules under graphene films function as a cushion to release the surface tension of graphene, thus results in a ultra-smooth surface, which is essential for a promising GFET device. The optical images show that the CVD-grown graphene is mostly a monolayer but with randomly distributed bilayer patches, and the corresponding Raman spectra are shown in Figure 3d and Figure S9, respectively. For the monolayer graphene, the intensity of the 2D peak at 2683 cm-1 with a full width at half maximum (FWHM) of 28.7 cm-1 is roughly twice that of the G peak at 1596.1 cm-1, while the defect-related D-peak at 1345.3 cm-1 has a relatively low intensity, indicating the high quality of the transferred graphene film and that it is a monolayer. XPS measurements were performed to evaluate the surface cleanliness of the graphene transferred by the trichloro(1H,1H,2H,2H-perfluorooctyl)silane formed SAM (FSAM) method versus the PMMA method, Figure 3e. The C1s peak of the graphene film transferred using the F-SAM method is located at 284.5 eV, corresponding to the binding energy of the sp2 hybridized states, with a narrower bandwidth than the C1s peak for the graphene transferred using the PMMA method. The broadening of the C1s peak and a peak at 288.4 eV are attributed to the PMMA residue on the surface of the graphene.39 It is worth noting that the C1s peak at 291.5 eV for the F-SAM transferred graphene corresponds to the 7 ACS Paragon Plus Environment

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binding energy of the F-SAM on the wafer (Figure S10), and the apparent decrease in the intensity is due to the complete coverage of the graphene film. The XPS spot size is about 900 µm. The direct transfer of graphene onto transparent and/or flexible dielectric substrates can be achieved by a similar F-SAM transfer method. A similar F-SAM modification process as for the SiO2/Si wafer can be applied to a glass substrate. The monolayer graphene deposited on glass has a sheet resistance of ca. 500 Ω/□ and a transparency of ca. 97.0% at a wavelength of 550 nm, which is comparable to the theoretical transmittance of 97.7% for monolayer graphene at that wavelength. A PET film was treated with O2 plasma for 20 min, and was then exposed to the vapor agent to generate a hydrophobic surface with a high contact angle (Figure S11), resulting in the successful transfer of graphene (details in Experimental section). The transfer of graphene onto these two types of substrates is illustrated in Figure 3f. The bendable graphene/PET film in Figure 3f exhibits a slightly higher transparency of 97.3% than graphene on glass at a wavelength of 550 nm, which confirms the promise of the F-SAM method for transferring CVD-grown graphene onto glass and flexible substrates. Three-terminal back-gated graphene field effect transistors (GFETs) were fabricated to evaluate the electrical performance of the transferred graphene (Figure S12).40 A resist-free shadow mask process (Method section) was used to form devices without any potential polymer residue contamination to the graphene surface which is usual for a fabrication process involving lithography. These shadow-mask devices have a typical channel length and width both at the 200 µm scale and electrical measurements are performed at room temperature and ambient pressure. From Figure 4a, the graphene transferred by the F-SAM method shows a positive voltage shift of ca. 5 V compared with that transferred using the 8 ACS Paragon Plus Environment

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PMMA method, indicating it is more heavily p-doped, which is consistent with a reported graphene FET modified by the same F-SAM.28 The modulation of the charge carriers in graphene by the F-SAM molecules can be explained by a ‘built-in’ electric field superimposed on the externally applied gate field. The generated electric field inside the FSAM, was determined by Ein=N(µmol/ɛdmol), where N and dmol are the areal density (1-2×1014 cm-2) and length of the F-SAM molecules (1.1 nm), respectively, µmol is the dipole along the molecular axis (-2.202 Debye was calculated using density functional theory), and ɛ is the dielectric constant inside the F-SAM.

28, 38

The negative electric field determined by the

dipoles requires a positive shift of the voltage and thus agrees well with the p-doping of the graphene in the GFET device.28, 41 The gate modulation (Rmax/Rmin for each curve at Figure 4b) of F-SAM-GFET transferred graphene is more than one fold (~12×), whereas the gate modulation of PMMA-GFETs is ~7×. This indicates that F-SAM transferred graphene has fewer surface charge trapping sites, leading to improved gate control in accordance with improved field-effect mobility as seen from the following detailed discussion. To analyze the difference between the transferred graphene films (F-SAM versus PMMA methods) quantitatively, the carrier mobility (µ) and the residual carrier concentration (n0) of the samples can be extracted by using a well-accepted diffusive model.42 The carrier concentrations (electrons or holes) in the graphene channel ntotal can be approximated by ݊௧௢௧௔௟ = ඥ݊଴ ଶ + ݊ீ ଶ

(1)

where the residual carrier concentration (n0) is generated by charged impurities located either in the dielectric or at the graphene/dielectric interface,43 and nG is the carrier concentration resulting from the difference between VG and VDirac as determined from the following equation:


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ܸீ − ܸ஽௜௥௔௖ =

௘ ஼೚ೣ

݊ீ +

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ħ௩ಷ ඥగ௡ಸ ௘

(2)

where e is the electron charge, Cox the gate capacitance ~35 nF·cm-2 for 90 nm SiO2 (11 nF cm-2 for 285 nm SiO240), ħ the Planck constant 6.62 × 10-34 J s, and vF Fermi velocity 1.15 × 106 m s-1.44 The total resistance Rtotal of the device can be approximately estimated by 42, 44-47 ௅

ܴ௧௢௧௔௟ = ܴ௖ + ܴ௖௛ = ܴ௖ + ௐ

ଵ ௘ఓඥ௡బ మା௡ಸ మ

(3)

with Rc the contact resistance, Rch the graphene channel resistance, and µ the mobility. While nG is dependent on the applied gate voltage VG, the relevant parameters, µ and n0, can be extracted by fitting to the measured data, for instance, the typical plots in Figure 4b. The statistics of the carrier mobility extracted from a dozen different experimental devices is exhibited (Table S1). The highest field-effect mobility of graphene transferred by the PMMA method was 4563 cm2 V-1s-1 with n0 = 4.4× 1011 cm-2, while the average values of 2996 and 3022 cm2 V-1s-1 for holes and electrons, respectively, suggesting the carrier mobility value is influenced by the polymer residue and/or cracks in the graphene generated during the PMMA-facilitated transfer process (Figure 4c). In contrast, the highest value obtained in F-SAM-GFETs is µ = 10663 cm2 V-1s-1 with n0 = 5.2× 1011 cm-2, and the statistical mobility data give an average of 7293 cm2 V-1s-1 and 7173 cm2 V-1s-1 for holes and electrons, respectively. The average field-effect mobility of F-SAM transferred graphene is ~2.5 times that of the graphene transferred by the conventional PMMA-facilitated process, indicating a graphene of higher quality. The sheet resistances of graphene films on SiO2/Si and PET substrates transferred by F-SAM and PMMA methods were measured using a 4-point probe method to exclude the 10 ACS Paragon Plus Environment

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contact resistance. Figure 4d compares the statistics of the sheet resistance. The average values were respectively ~ 800 Ω/□ and ~700 Ω/□ for graphene on SiO2/Si and PET substrates transferred by F-SAM method, while the values increased to ~ 1500 Ω/□ and ~ 1800 Ω/□ when using the PMMA transfer method. The sheet resistances of graphene transferred by F-SAM are only half the values from PMMA transferred graphene, and this improvement can be attributed to the doping effect, lower roughness

48-50

and higher quality

of the F-SAM-transferred graphene. Inspired by the selective transfer of graphene onto F-SAM modified surfaces, we realized clean patterning of a graphene membrane on a SiO2/Si substrate without using the conventional photoresist or beam resist coating and removal processes. First, patterned Cu dots/ribbons were thermally evaporated onto a SiO2/Si substrate through a shadow mask (Figure 5a), then the F-SAM layer was deposited on this substrate in vacuum. By etching away the Cu patterns, a reverse patterned SAM was obtained. Afterwards, a graphene film was transferred onto the modified substrate and imitated the configuration of the SAM pattern to form a graphene pattern after removing the Cu foil. As shown in Figure 5b, graphene was not transferred to the square regions where no SAM existed except for some graphene fragments, thus forming a continuous graphene membrane with regular square holes. By using the shadow mask with arranged slits, graphene micro-ribbons were obtained as exhibited in Figure 5c. It should be noted that this patterning method combines the transfer and pattern into a one-step process, and does not use any polymer-resist that may contaminate the graphene membrane. CONCLUSION In summary, an effective method has been developed for the support-free transfer of CVD-grown graphene films using SiO2/Si wafer, glass or PET film substrates modified in a 11 ACS Paragon Plus Environment

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simple way with a hydrophobic SAM. This method yields clean, ultra-smooth and highquality graphene films for applications that require transparent, conductive, and flexible films, and for fundamental research on, e.g., the mechanics and chemistry of graphene, due to the absence of residues, limited structural defects such as cracks and ultra-smooth surfaces. The F-SAM introduced in the transfer process was also tailored for a p-type GFET and improved the carrier mobility of the graphene films. Clean graphene patterns can be facilely realized by using the selective transfer of graphene onto the F-SAM modified surfaces. This simple and cost-effective process appears to be scalable.

METHODS General SAM treatment of the substrates. Three coupling agents, CH3(CH2)16CH2SiCl3 (trichloro(octadecyl)silane), C6H5SiCl3 (trichloro(phenyl)silane), and CF3(CF2)5(CH2)2SiCl3 (trichloro(1H,1H,2H,2H-perfluorooctyl)silane), were used to form SAMs individually under as close to identical conditions as possible on SiO2/Si wafer, glass and PET film target substrates. Typically, these substrates were first exposed to O2 plasma for 10 min to modify the surface with silanol groups (-SiOH) for further chemical reaction with the molecules, while the PET films were treated twice. The agent solution (50 µL) was added to a vial and put into a vacuum desiccator together with the substrates, followed by keeping them in vacuum for 6 hours at room temperature. The obtained substrates were rinsed with ethanol and dried with nitrogen before use. Graphene transfer. Graphene films were grown on Cu foils as reported by Li et al.51 First, the graphene on one side was etched by exposing it to an O2 plasma for 10 min. After a rolling treatment to flatten the foil, the Cu foil/graphene was gently placed on the SAMsubstrates, and then immersed in an aqueous etchant (0.1 M (NH4)2S2O8). A roller was 12 ACS Paragon Plus Environment

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further used to squeeze out excess air in-between graphene/Cu and the substrate out to improve the contact. After the Cu foil was etched away, the substrates were thoroughly rinsed with deionized water and dried with nitrogen. The typical PMMA-based transfer method is comprised of the following steps: (1) spincoating a layer of PMMA on one side of a graphene/Cu foil followed by the removal of the other side of the graphene by an O2 plasma; (2) floating the film on aqueous (NH4)2S2O8 to etch away the Cu foil and (3) subsequently transferring the film to the target substrate, and removing the PMMA layer by acetone. Fabrication of graphene FETs. Au/Ti (60/2 nm) was deposited in an e-beam evaporator for the metal contact of the three-terminal graphene device in a vacuum of 2.0 × 10-6 Torr. A resist-free shadow mask process was used to define source and drain contacts, and the device isolation was achieved by probe-tip patterning. The devices had a channel length of 200 µm with a channel width of 200 µm. The process, which did not use lithography, was used to avoid the e-beam or photo resist contamination of the graphene surface, and the relatively large active region of the shaped graphene channel was for better macroscopic observation. The electrical measurements were performed at room temperature and ambient pressure on a Cascade® probe station with an Agilent® 4156 analyzer. Graphene patterns. Using a graphene mesh with square holes as an example, 100 nm patterned Cu dots (square, 150 µm * 150 µm) were first thermally evaporated onto a SiO2/Si substrate through a shadow mask. The F-SAM was then deposited on this substrate in vacuum for 6 hours. By etching away the Cu patterns, a reverse patterned SAM was obtained. Afterwards, a graphene film was transferred onto the modified substrate and imitated the configuration of the SAM pattern to form the graphene pattern after removing Cu foil. Finally, the obtained graphene pattern on the wafer was immersed in deionized water to remove the 13 ACS Paragon Plus Environment

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residues. Further sonication in water can be used to clean the surface without ruining the pattern. Characterization. The water contact angle was measured by a drop shape analyzer DSA100. Raman spectroscopy was performed with a Wi-Tec micro-Raman using 532 nm laser at ambient temperature. A ZEISS optical microscope (AxioCam MRc5) was used to characterize the morphology of the transferred graphene. The transparency of graphene on glass and a FET film was investigated by a Cary Series UV-Vis-NIR Spectrophotometer (Agilent Technologies). Bruker Dimension Icon AFM was used to analyze the thickness of the graphene and the roughness of the SAM. The thickness of the SAM was determined using an ellipsometer M-2000V (J. A. Woollam Co.). XPS was performed on a K-Alpha XPS System (Thermo Fisher Scientific, Inc.) with a 90 µm beam size. The electrical properties were measured with a 4200-SCS Semiconductor Characterization System (Keithley). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions #These authors contributed equally to this work Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT X. Chen, L. Colombo, P. Kim, and E. Tutuc commented on the manuscript. X. Zhang (Raman), D. Luo (AFM), and X. You (GFET) assisted with characterization. This work was 14 ACS Paragon Plus Environment

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supported by IBS-R019-D1. D. Akinwande acknowledges the support of the Office of Naval Research (ONR). Supporting Information Available: AFM image of single crystalline graphene, schematic of chemical modification, water contact angle of SiO2/Si wafer after O2 plasma, XPS surveys, image of Cu/graphene on treated wafer, optical image of graphene transferred by PMMA method, comparison of different transfer methods, comparison of AFM images, Raman spectrum of bilayer graphene, XPS of F-Si wafer, water contact angles of glass and PET, optical image of GFET, and the statistics of carrier concentration and mobility. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figures and Captions:

Figure 1. Illustration of transferring graphene to the SAM-modified SiO2/Si wafer. (a) Schematic of the transfer procedure. (b) SAM-modified substrates have a hydrophobic surface, which prevents the detachment of the graphene from the SiO2/Si wafer. In contrast, for the untreated wafer, the graphene film is normally damaged in aqueous solution during the etching of the Cu foil because water molecules are apt to go underneath the graphene and decrease its adhesion to the substrate.

Figure 2. Comparison of water contact angles and graphene transfer results of pristine and modified SiO2/Si wafers using three different SAMs.


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Figure 3. (a) A graphene film (6.5cm*6.5cm) transferred onto a 4 inch SiO2/Si wafer using the F-SAM method. (b) Optical image showing the continuous monolayer graphene film with randomly distributed bilayer regions. (c) AFM image showing a clean graphene film with a height of ca. 1 nm. (d) Raman spectrum of monolayer graphene. (e) XPS C1s spectra of graphene films transferred by the F-SAM and PMMA methods. (f) Graphene films transferred to glass and a PET film, and their transparency.


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Figure 4. Electrical characterization of fabricated graphene field effect transistors (GFET): (a) Gate-dependent conductance, Vd = 20 mV, (b) The normalized Rtot as a function of Vg - VDirac of GFETs where graphene were transferred by the PMMA and F-SAM methods. (c) Statistical value of extracted field-effect mobility from a dozen devices. (d) Sheet resistance of graphene transferred onto SiO2/Si and PET by F-SAM and PMMA methods, respectively.


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Bare

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Figure 5. Patterned transfer of graphene using the patterned F-SAM substrate. (a) A schematic of the patterning and transfer process. (b) Patterned continuous graphene membrane with regular square holes. c, Parallel graphene micro-ribbons on a SiO2/Si substrate.


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Table 1. The surface roughness (Rq) values of graphene transferred respectively by PMMA and SAM methods, corresponding to the AFM measurements in Figure S8. Samples

#1

#2

#3

#4

#5

#6

#7

#8

#9

#10

PMMA-G

1.02

1.32

1.44

1.22

1.19

1.59

1.39

1.29

1.12

1.39 1.20 nm

SAM-G

0.27

0.29

0.25

0.27

0.32

0.25

0.20

0.26

0.28

0.26 0.26 nm

Average


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