Controlling Water Intercalation Is Key to a Direct Graphene Transfer

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Controlling Water Intercalation Is Key to a Direct Graphene Transfer Ken Verguts, Koen Schouteden, Cheng-Han Wu, Lisanne Peters, Nandi Vrancken, Xiangyu Wu, Zhe Li, Maksiem Erkens, Clement Porret, Cedric Huyghebaert, Chris Van Haesendonck, Stefan De Gendt, and Steven Brems ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12573 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Applied Materials & Interfaces

Controlling water intercalation is key to a direct graphene transfer Ken Verguts,†,‡ Koen Schouteden,§ Cheng-Han Wu,‡ Lisanne Peters,†,‡ Nandi Vrancken,‡ Xiangyu Wu,‡ Zhe Li,§ Maksiem Erkens,§ Clement Porret,‡ Cedric Huyghebaert,‡ Chris Van Haesendonck,§ Stefan De Gendt,*,†,‡ and Steven Brems*, †

Departement Chemie, KU Leuven, Celestijnenlaan 200F, B3001 Leuven, Belgium ‡

§

‡

imec vzw, Kapeldreef 75, B3001 Leuven, Belgium

Laboratorium voor Vaste-Stoffysica en Magnetisme, KU Leuven, Celestijnenlaan 200D, B3001 Leuven, Belgium

AUTHOR INFORMATION Corresponding Author *Address correspondence to [email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The key steps of a transfer of two-dimensional (2D) materials are the delamination of the asgrown material from a growth substrate and the lamination of the 2D material on a target substrate. In state-of-the-art transfer experiments, these steps remain very challenging and transfer variations often result in unreliable 2D material properties. Here, it is demonstrated that interfacial water can insert between graphene and its growth substrate despite the hydrophobic behavior of graphene. It is understood that interfacial water is essential for an electrochemistry based graphene delamination from a Pt surface. Additionally, the lamination of graphene to a target wafer is hindered by intercalation effects, which can even result in graphene delamination from the target wafer. To circumvent these issues, a direct, support-free graphene transfer process is demonstrated, which relies on the formation of interfacial water between graphene and its growth surface, while avoiding water intercalation between graphene and the target wafer by using hydrophobic silane layers on the target wafer. The proposed direct graphene transfer also avoids polymer contamination (no temporary support layer) and eliminates the need for etching of the catalyst metal. Therefore, recycling of the growth template becomes feasible. The proposed transfer process might even open the door for the suggested atomic-scale LEGO® based stacking of different 2D materials, which will enable a more reliable fabrication of Van der Waals heterostructure based devices and applications.

KEYWORDS Graphene; direct transfer; chemical vapor deposition; platinum; bubble transfer; electrochemical delamination; interfacial water


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INTRODUCTION The transfer of two-dimensional (2D) materials remains an important bottleneck for the fabrication of reliable 2D based devices. This transfer is definitely unavoidable for graphene and hexagonal boron nitride (h-BN) when these 2D materials are integrated in devices, since optimized chemical vapor deposition (CVD) growth processes of graphene and h-BN are typically done at high temperatures (> 400 °C) on top of a sacrificial catalyst layer. For the other members of the family of 2D materials (MX2 materials like MoS2, MoSe2,...) although a catalyst layer is not needed, an epitaxial template for oriented grain growth at high temperature does result in improved 2D material quality (i.e. larger and oriented MX2 grains). This also implies that the development of a reliable transfer for these 2D materials will be difficult to avoid. Typical problems associated with state-of-the-art 2D material transfers include polymer residues,1 metal contamination,2 additional wrinkle formation caused by a wet transfer,3 macroscopic cracks caused by handling of the 2D material, presence of interfacial water (IFW) or solvents,4,5 2D material delamination issues,6,7 etc. As a result, it is clear that more in-depth understanding is absolutely required for the development of an optimized transfer. Several 2D material release and transfer procedures have been proposed based on different mechanisms including etching of the substrate or catalyst layer,8,9 bubble based transfer methods (i.e. electrochemistry based,10–14 ultrasound based15 and bubble formation as a result of a chemical reaction16), peeling methods,17–19 and wedging based transfer.20–22 Also, the majority of the transfer methods rely on an intermediate support layer, apart from a few exceptions.23,24 However, these support-free transfer methods still rely on etching of the catalyst layer, which results in metal contamination and avoids the reuse of the catalyst template.


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An electrochemical ‘bubble’ transfer has been demonstrated for graphene grown on Pt,10,12,13 Ir14 and Cu11,25,26 surfaces. In this paper, graphene delamination from Pt surfaces using electrochemical methods will be discussed in detail. It is shown that water intercalation between graphene and a Pt surface is critical to achieve a successful graphene delamination using an electrochemical method. It is also demonstrated that intercalation effects can occur between graphene and a target wafer, resulting in unwanted graphene delamination effects. The intercalation based electrochemical transfer method will form the basis of the presented optimized direct graphene transfer method. This transfer method does not rely on catalyst template etching and does not make use of a temporary support layer. Such a transfer process would be very beneficial, since it completely avoids polymer residues and minimizes 2D material handling and metal contamination. Here, graphene has been grown on Pt foil and Pt(111) thin films deposited on C-plane sapphire. Platinum remains an interesting material for CVD graphene growth, since it is an excellent catalytic material for the growth of high quality monolayer graphene27 and millimeter sized graphene islands.10 Furthermore, a growth-etchregrowth CVD technique can be used to minimize defect sites and the edge orientation of graphene grains can be controlled using this growth-etch technique.28

Electrochemical delamination of graphene from Pt surfaces Graphene delamination from Pt surfaces using electrochemical methods has been discussed in several articles.10,12,13 A polymer (often PMMA) is spin coated on top of graphene and the sample is immersed in an electrolyte solution (often NaOH). Next, a potential of a few volts is applied to the graphene sample, which serves as the cathode, while a Pt anode wire is often used as the counter electrode. The formation of a large number of H2 bubbles at the interface between


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graphene and the Pt surface is expected to detach the PMMA/graphene layers from the Pt surface.10 However, when evaluating this specific electrochemical graphene delamination method, the process yield turned out to be very variable. When PMMA is spin coated on top of graphene immediately after the CVD graphene growth process, the delamination process always resulted in macroscopically broken graphene. Electrochemical delamination with a sample voltage of -3 V in 0.2 M NaOH using PMMA and PDMS as a support was always unsuccessful (see movie 1 in SI). When the voltage was increased to -4.5 V, delamination was only partially possible when the PDMS was pulled manually. However, the graphene sheet was heavily damaged during this procedure (see movie 2 in SI). The same holds when the voltage is further increased to -10 V or when the support layer consists only of a PMMA spin coated polymer layer (not shown). This unexpected tough graphene delamination behavior from Pt surfaces is caused by the absence of intercalated water between graphene and Pt. Figure 1a shows an ultra-high vacuum (UHV) low temperature scanning tunneling microscopy (LT-STM) image of graphene grown on an Al2O3(0001)/Pt(111) sample for which the sample exposure to ambient conditions was kept minimal.


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Figure 1. Low temperature STM image of graphene on a Al2O3(0001)/Pt(111) substrate. Pt(111) terraces are clearly visible (a). Topographic cross-sections of the STM image indicate the presence of Pt(111) terrace steps (height 2.1 Å) and smooth graphene on the Pt(111) surface (b). The triangular terraces are very flat (less than a few Å variation, see Fig. 1b) and the measured terrace step heights are equal to 2.1 Å , which is in agreement with the theoretical value of 2.25 Å for a monoatomic fcc Pt(111) step (lattice constant of Pt is 3.913 Å).29 When a similar Al2O3(0001)/Pt(111)/graphene sample is exposed for approximately 14 days to ambient conditions and measured again with LT-STM, the terraces are still present (see Fig. 2a), but a meander type pattern is visible on top of the terraces with a peak-to-valley distance of approximately 7 Å (see Fig. 2b,c).


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Figure 2. Low temperature STM image of graphene on a Al2O3(0001)/Pt(111) substrate which has been exposed for 2 weeks to ambient conditions. Triangular patterns can still be visualized (a), but the roughness on top of the terraces is clearly increased during the graphene exposure to ambient conditions (b), leading to height variations of approximately 7 Å (c). Low temperature STM measurement performed after an annealing step of 3 h in UHV at 400 °C. The inset shows the measured Moiré pattern in one of the valleys (I = 1 nA and Vbias = 0.17 V) (d). Low temperature STM image obtained after an additional anneal at 500 °C for 4 h in UHV. The meander pattern is still present after the annealing procedure (e). This increase in roughness cannot be explained by adsorbed water on top of graphene, since the pattern remains visible in an in situ LT-STM measurement after annealing in UHV at 400 °C


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for 3 h (see Fig. 2d). The observed pattern corresponds well with STM measurements of CVD graphene transferred on mica using a standard wet transfer.4 This same fingerprint has been observed with atomic force microscopy measurements after the transfer of a graphene flake on a mica substrate and was explained as IFW between graphene and mica.5,30 As reported in literature, a high temperature anneal in UHV could not remove the topography caused by IFW and the height of 2 monolayers IFW was approximately 7 Å.4,31 Also in our case, IFW has been observed between graphene and the substrate after exposure to ambient conditions. After a UHV high temperature anneal, the meander pattern remains present, indicating that either the IFW is still present, or that the graphene sheet is not relaxing back to the situation immediately after CVD growth. It is possible to observe Moiré patterns in the valleys of the LT-STM image (see inset Fig. 2d) and the shown Moiré pattern corresponds well with a graphene sheet and a Pt(111) surface rotated over 19.7° (see Fig. S1). Since it is possible to model the observed Moiré pattern in the valleys, it is clear that the Pt(111) surface did not undergo any surface reconstruction. A Moiré pattern is never observed on top of the patterns, which is expected when IFW is present between graphene and Pt(111). Additional UHV annealing for 4 h at 500 °C did increase the valley area, but the meander pattern remains clearly visible (see Fig. 2e). When millimeter sized graphene islands are grown on Al2O3(0001)/Pt(111), and exposed to ambient conditions for approximately 2 days, a different scanning electron microscopy (SEM) contrast is always observed at the edges of the graphene islands (see Fig. 3a).


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Figure 3. SEM image of the edge of a millimeter sized graphene island. A clear band with different contrast is visible at the edge of the island (a). Raman spectroscopy mapping at approximately the same position of the SEM image indicates a clear shift of the G peak position (b) and the 2D peak position (c). Raman spectroscopy mapping performed at the edge of the graphene islands indicates a clear shift in the G peak and 2D peak position (see Fig. 3b,c). A very large blue shift of approximately 75 cm-1 of the 2D peak is observed at the edge of the graphene islands. After several days, the SEM contrast band at the edge of the graphene islands widens, gets more blurry and gradually disappears. Furthermore, the width of the bands appears to depend on the orientation between the


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graphene and the Pt(111) surface (see Fig. S2). It is our believe that this SEM contrast corresponds to IFW, which slowly intercalates between graphene and Pt(111). Since graphene is impermeable to water, this intercalation starts at the border of the graphene island and at the graphene defect sites (see Fig. S3). In order to mimic this intercalation process, a closed graphene sheet grown on a Pt foil (2x2 cm2) was kept in ultra-pure water (UPW) at 50 °C for 16 h. A SEM image of a Pt foil/graphene sample immediately after growth is shown in Fig. 4a. The observed color contrast corresponds mainly to different Pt grain orientations, which is related to an electron channeling contrast.32 The electron channeling contrast is formed as a result of electron backscattering effects which occur in a thin layer at the sample surface, which makes it a surface sensitive technique. When the electron beam penetrates further into the Pt foil, elastic scattering effects decollimate the initially highly collimated electron beam and inelastic scattering reduces the energy of the electrons. Further scattering effects lead to the development of the bulk backscattering contribution, which is insensitive to crystal orientation, since the beam electron trajectories are effectively randomized.33 A few multilayer graphene spots are also visible in Fig. 4a and are indicated with white dashed lines.


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Figure 4. SEM image of graphene on a Pt foil measured immediately after the CVD growth process. A clear SEM contrast due to the polycrystalline nature of the Pt foil is observed. Dashed lines indicate graphene multilayer areas (a). Raman spectroscopy measurements at random positions immediately after CVD graphene growth on a Pt foil are presented in the top panel, while measurements at random positions after submerging the sample in 50 °C UPW for 16 h are presented in the lower panel (b).


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Raman spectroscopy shows a clear blue shift of the G and 2D peaks after the water treatment procedure (see Fig. 4b), which has also been observed at the edge of the millimeter sized graphene islands. The Raman peak intensities are low, but this is consistent with a previous Raman spectroscopy report of CVD graphene on Pt surfaces.34 The small additional peaks at 1556 cm-1 and 2328 cm-1 originate from the Raman mode of O2 and N2 gas, respectively.35,36 These gases are present in the ambient surrounding the sample. The results show that after the water treatment (graphene sample submerged for 16 h in 50 °C UPW), graphene is still present on the surface, but IFW is intercalated between the Pt foil and the graphene sheet. A room temperature STM image obtained immediately after the water treatment procedure, which was performed straightaway after the CVD growth on Al2O3(0001)/Pt(111), clearly shows a meander pattern (see Fig. 5 and S4). As a result, the intercalation of water between graphene and Pt via ambient exposure can be accelerated by submerging the graphene sample in hot water. It takes approximately 16 h to completely intercalate a graphene/Pt sample in 50 °C UPW. The intercalation time can be reduced further to 3 h in 80 °C UPW (not shown).


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Figure 5. STM measurement obtained under ambient conditions. After CVD growth, the Al2O3(0001)/Pt(111)/graphene sample was straightaway submerged in 50 °C UPW for 16 hours. The STM measurement was performed immediately after the water treatment and is acquired using a bias voltage of 105 mV and a current of 0.2 nA.

X-ray

Photoelectron

Spectroscopy

(XPS)

measurements

are

performed

on

a

Al2O3(0001)/Pt(111)/graphene sample with limited exposure to ambient conditions and after submerging the sample for 16h in 50 °C UPW to elucidate the chemical composition of the interface (see Fig. S5a). The Pt-4f spectra do not show any difference in chemical state, but a larger fraction of C-O bonds is found on the Al2O3(0001)/Pt(111)/graphene sample after the water treatment. Relative depth plots (see Fig. S5b) indicate that the majority of oxygen is positioned above the graphene sheet on the pristine sample and in between the graphene sheet and the Pt(111) surface after the water treatment procedure. Hydrogen cannot be detected with XPS, but the relative depth plots are an additional evidence that water intercalates between graphene and the Pt surface. The observed IFW is of prime importance for a successful graphene delamination from the Pt surface using electrochemical methods. Electrochemical delamination of intercalated graphene on Pt surfaces with PMMA/PDMS as a support layer is very reproducible at -3 V in 0.2 M NaOH (see movie 3 in SI in which graphene delamination was initiated 2 min after applying a -3 V sample bias). The easy graphene delamination is in strong contrast to the same process applied immediately after graphene growth. Graphene delamination tests further indicate that a sample voltage of -3 V for 10 s is already sufficient to delaminate graphene from Pt surfaces when IFW is present. Dollekamp et al. recently demonstrated the electrochemical reduction of IFW between mica and graphene, which resulted in the formation


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of interfacial bubbles that promoted graphene delamination.37 This result confirms that water intercalation is of prime importance, and this conclusion is valid for several 2D material transfer methods, including the use of ultrasonics, wedging methods as well as some peeling methods.19 It is even possible to peel graphene with PMMA/PDMS when IFW is present. However, macroscopic defects are often observed using a direct peeling method. These macroscopic defects are not observed when an electrochemical delamination method is used in combination with a water intercalation step.

Lamination of graphene on a target wafer with a support layer After the delamination of graphene from the Pt surface, the subsequent step of the graphene transfer procedure is the lamination of the graphene sheet on a target wafer. The control of top and bottom graphene surfaces is of critical importance. Therefore, a wet transfer method has to be avoided. Typically, the PDMS/PMMA/graphene sheet is laminated on a Si/SiO2 (90 nm) wafer using a home-made bonding setup (see Fig. S6). A process vacuum is used when laminating the 2D material on top of Si/SiO2 and subsequently, an additional 200 kPa pressure is applied to the stack for 10 min. Next, the PDMS layer is peeled off from the PMMA support layer at a speed of 50 µm s-1 using a motorized stage while keeping the sample at 130 °C, which is above the glass transition temperature (Tg) of the PMMA layer. Optical microscopy images before the dissolution of the PMMA layer are shown in Fig. 6.


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Figure 6. Optical microscopy image during a graphene transfer procedure on a Si/SiO2(90 nm) wafer. A PMMA layer is still present on top of graphene (see inset). Multilayer graphene areas are present at the bottom left. A few bilayer graphene areas (BLG) are visible, which indicate the successful graphene delamination from the growth substrate and graphene lamination on the Si/SiO2 target wafer. However, graphene scrolling is often observed when the PMMA support layer is dissolved in room temperature acetone (see Fig. S6). This graphene scrolling has also been observed by S. Karamat et al.38,39 for CVD graphene which was transferred from Pt foils on Si/SiO2 wafers. The same scrolling effect has been reported for transferred graphene flakes on hydrophilic Si/SiO2 surfaces, where the monolayer graphene scrolling probability was very high when the transferred sample was submerged in water for approximately 1 s.7 The scrolling probability decreased considerably with increasing flake thickness. The same kind of graphene flake scrolling has been observed when graphene flakes deposited on Si/SiO2 samples are submerged in IPA. It was reported that graphene scrolling was only observed when the polymer contamination of the graphene flakes was limited.6 In all cases, graphene scrolling was attributed to an intercalation effect of water or isopropyl alcohol (IPA), this time between graphene and the target Si/SiO2


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surface. Graphene delamination during the discussed transfer is very often observed for monolayer graphene, but this scrolling is less of an issue for bi- and multilayer graphene (see Fig. S7). It has to be taken into account that the bending rigidity of monolayer graphene (~1.4 eV)40 is much lower than the bending rigidity of bilayer graphene (~35.5 eV),40,41 which explains this behavior. In order to verify the influence of intercalation, the hydrophobicity of the target substrate was varied, using a perfluorodecyltrichlorosilane (FDTS) treated Si/SiO2 (water contact angle of 109.5°) and a NH4OH/H2O2/H2O (1:1:3 volume ratio) cleaned Si/SiO2 (water contact angle of < 10°) target wafer. Optical microscopy images after the graphene transfers are shown in Fig. 7.

Figure 7. Optical microscopy image after a graphene transfer on top of a Si/SiO2/FDTS wafer (left) and a NH4OH/H2O2/H2O cleaned Si/SiO2 wafer (right). Graphene scrolling is clearly observed on the FDTS treated wafer, even multilayer graphene areas are delaminated. During the PMMA removal step using acetone, graphene scrolls completely on FDTS coated SiO2 surfaces (even bi- and multilayer areas are scrolling), while graphene remains intact on hydrophilic Si/SiO2 surfaces. A Raman spectroscopy measurement of monolayer graphene on a hydrophilic Si/SiO2 surface is shown in Fig. S8. The low defect peak demonstrated the high graphene quality. It has to be mentioned that FDTS on SiO2 is stable in acetone. The observed graphene scrolling can be explained taking into account the intercalation effect of acetone between graphene and the hydrophobic FDTS treated SiO2 surfaces. So, it is shown that


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graphene can be transferred on hydrophilic Si/SiO2 wafers when the PMMA layer is dissolved with acetone. However, a strong reduction of graphene mobility has been observed when graphene is transferred to hydrophilic Si/SiO2 surfaces,42 which renders this surface less suitable for transferring graphene. The obtained graphene mobility value after a graphene transfer on a hydrophilic Si/SiO2 wafer using bottom Pd contacts and bottom gate graphene field effect transistors is approximately 1,600 cm2 V-1 s-1 (see Fig. S9). In order to achieve a successful high quality graphene transfer, a mechanism to avoid solvent and/or water intercalation between graphene and a target wafer will be definitely beneficial. Direct transfer of graphene without a support layer B. Wang et al.24 recently demonstrated a direct transfer of CVD graphene grown on a Cu foil to

a

hydrophobic

Si/SiO2

surface,

which

was

treated

with

a

silane

layer

(perfluorooctyltrichlorosilane, FOTS – water contact angle 105.2°). This transfer process was successful, since Cu is dissolved in an aqueous solution and water intercalation is prevented between the hydrophobic silane layer and the hydrophobic graphene sheet. However, this transfer process still makes use of a rough Cu foil and is based on Cu etching. Figure 8 shows the scheme of our proposed direct graphene transfer, based on water intercalation between graphene and the Pt surface, electrochemical delamination and the use of a hydrophobic silane layer (FDTS) on a Si/SiO2 surface in order to prevent water intercalation.


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Figure 8. Scheme of a direct transfer method of graphene grown on top of a Pt surface to a hydrophobic Si/SiO2 surface treated with an FDTS silane layer. The first step is the formation of IFW between graphene and the Pt surface. Next, a dry bonding step to a hydrophobic target sample is implemented, while applying a pressure of 200 kPa. Next, an electrolysis step is done in 0.2 M NaOH to detach graphene from the growth substrate and the electrolyte solution is replaced with water until the pH reached a value of 7. The last step is the gradual reduction of the applied pressure.

The first step of the transfer procedure is the immersion of the Pt/graphene sample in UPW at 50 °C for 16 h to achieve water intercalation between graphene and the Pt(111) surface. Next, a Si/SiO2 wafer coated with FDTS is bonded to the Al2O3(0001)/Pt(111)/IFW/graphene substrate and a pressure of 200 kPa is applied to the sample stack. Next, 0.2 M NaOH was added in a modified home-made bonding setup (see Fig. S10) and -3 V was applied for 2 min to the Pt/graphene sample in the electrochemical bonding setup. The aqueous NaOH was rinsed with UPW until the pH of the solution reached 7 and finally the pressure in the bonder was gently released. Figure 9a-c shows the optical microscopy, SEM and AFM measurements after graphene transfer.


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Figure 9. Optical microscopy image after a direct graphene transfer. A graphene wrinkle is indicated (a). SEM image (b) and AFM image (c) after a direct graphene transfer. Wrinkles can be observed after a direct graphene transfer in (c), but polymer contamination is completely absent. The results illustrate that a direct graphene transfer on a hydrophobic surface without making use of etchants is feasible. The advantages of this transfer procedure are: 1) the growth template wafer can potentially be recycled, 2) handling of the 2D material is minimized, and even more important, 3) polymer residues are completely avoided. Figure 10a,b shows a Raman map of the FWHM of the 2D peak (Γ2D = 27.3±2.2 cm-1) and the 2D/G peak ratio (1.22±0.11) after the room temperature direct graphene transfer procedure.


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Figure 10. Raman spectroscopy maps of the FWHM of the 2D peak (a) and the 2D/G peak ratio (b) after a room temperature direct graphene transfer on an FDTS treated Si/SiO2(90 nm) wafer. Raman spectroscopy maps of the FWHM of the 2D peak (c) and the 2D/G peak ratio (d) after a direct graphene transfer and a subsequent anneal at 150 °C for 1 h in a nitrogen atmosphere. A decrease of the 2D/G peak ratio after annealing can be observed. The obtained Γ2D value is comparable to a wet graphene transfer on h-BN flakes and is lower than values typically measured after an etch based wet graphene transfer on SiO2.43 The low 2D/G peak ratio is a consequence of graphene induced p-doping by the FDTS silane layer.44 Furthermore, it is known that Γ2D is very sensitive to strain inhomogeneities on a nanometer length scale in the graphene sheet.45 These strain fluctuations are believed to be a main contributor to charge carrier scattering in graphene46 and a direct correlation between the carrier mobility and Γ2D has been demonstrated.43,46,47 As a result, Γ2D is an easily accessible quantity for


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classifying nanometer scale flatness as well as the local electronic properties of graphene. The low Γ2D value is a good indication of transfer improvements using the direct graphene transfer process. The obtained Γ2D value (27.3±2.2 cm-1) corresponds well with the reported Γ2D value after an etch based direct transfer of graphene on FOTS (28.7 cm-1).24 Annealing the sample at 150 °C for 1 h in a nitrogen atmosphere decreases the 2D/G peak ratio to 1.08±0.11, as indicated in Fig. 10d, while Γ2D remains almost unchanged (see Fig. 10c). The results demonstrate the feasibility of transferring graphene by making use of IFW between graphene and the growth substrate, and avoiding water intercalation between the target substrate and the graphene sheet. It has to be mentioned that controlling the surface roughness of the growth template is critical. Performing this transfer procedure using graphene grown on Pt foils resulted in macroscopically broken graphene, due to the surface roughness of the Pt foil. Quite some graphene wrinkles can still be observed after transfer (see Fig. 10), which are mainly coming from the growth process. Furthermore, it is important to note that although it is possible to grow almost centimeter sized graphene islands on top of Pt(111) surfaces, a single growth orientation of the different graphene islands on Pt(111) surfaces is still lacking and graphene wrinkles can still be observed. In this respect, it has to be mentioned that wrinkle-free monolayer graphene has been demonstrated on a Si/SiO2/Pt(110) growth template wafer, which could further improve the direct graphene transfer.48 Also, oriented graphene growth has been achieved on Ir(111), and recently, an electrochemistry based transfer method in NaOH has been demonstrated for graphene growth on Ir surfaces.14 Moreover a successful graphene transfer based on the intercalation of tetraalkylammonium ions between graphene and iridium surfaces has been established.49 This reveals that the presented direct graphene transfer could also work for graphene grown on iridium surfaces, but wrinkle formation might still be an issue since the


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thermal expansion coefficient mismatch between graphene and Ir is significant.50 Moreover, this ion intercalation based transfer method was recently extended to CVD graphene grown on platinum and copper surfaces.51 It is clear that interfacial molecules are of prime importance for 2D material transfers, but the understanding of the exact intercalation mechanism and the structure and dynamics of confined molecules are still in its infancy.52 To end, it should be noted that the bonding step of the direct transfer was not performed in vacuum, which makes that still some gas and/or water molecules could be trapped between graphene and the hydrophobic layer. Further development of the transfer process is needed for reliable integration of 2D materials in semiconductor processing facilities. CONCLUSION Exposing a Pt/graphene sample for several days to ambient conditions or submerging it in warm UPW gradually changes the graphene surface morphology and varies the chemical composition of the graphene/Pt interface. The results are consistent with water intercalation, which progressively develops at the interface. This intercalation process is essential to obtain a successful electrochemical graphene delamination procedure. Furthermore, a direct graphene transfer process has been established based on IFW between graphene and the CVD growth substrate, and on avoiding water intercalation between the hydrophobic target wafer and graphene. Such a direct graphene transfer avoids polymer contamination (no temporary support layer) and etching of the catalyst metal. As a result, recycling of the growth template becomes feasible. Since the interaction between graphene and water is comparable to the interaction between MoS2 and water,53 a transfer procedure based on controlling water intercalation might also work for other 2D materials like MoS2. As a result, the proposed transfer process might even


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open the door for the predicted atomic-scale LEGO® stacking of different hydrophobic 2D materials.54


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EXPERIMENTAL METHODS Preparation of the Al2O3(0001)/Pt(111) template C-plane double-sized polished 2” Czochralski grown sapphire wafers (Roditi International Corporation) are used as a substrate material (surface misorientation is ≤ 0.3°). To remove polishing scratches, the as-received wafers are cleaned using a 3:1 concentrated acid mixture of H2SO4:H3PO4 at 300 °C for 20 min followed by a 3 min UPW rinse at room temperature. The acid clean leaves the sapphire surface Al-terminated and the subsequent UPW rinse hydroxylates the surface, which leads to an OH-termination. Next, a 500 nm thick Pt(111) layer was deposited on the cleaned sapphire wafers using e-beam evaporation. Platinum is deposited in a Pfeiffer PLS 580 unit (base pressure is 10-5 mbar). The acceleration voltage of electrons is -10 kV and the deposition rate is 9 nm min-1. The substrate temperature is kept around 500 °C to ensure a Pt(111) growth.55 CVD growth of graphene on Pt Graphene is grown both on Al2O3(0001)/Pt(111) (cf infra) and Pt foils (Alfa Aesar, 50 µm thick, 99.99% metals basis). Growth is performed at 750 mbar in a vertical cold-wall AIXTRON Black Magic Pro 6” CVD system. The reactor is heated in 850 sccm hydrogen atmosphere (pressure of 750 mbar) till 1080 °C, measured with a thermocouple directly connected to the susceptor plate. Next, 6 sccm methane is introduced into the chamber for 30 min. Finally, the reactor is cooled down to room temperature at a cooling rate of 15 °C min-1 under a methane and hydrogen ambient with a methane to hydrogen ratio of 3:850. Direct transfer of graphene A Si wafer with 90 nm of thermal SiO2 is exposed to an ozon clean for 15 min to activate the SiO2 surface. Subsequently, the target wafer is loaded in an oven at 140 °C and exposed to FDTS


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for 60 min. The sapphire/Pt(111)/graphene sample is submerged in 50 °C UPW for 16 h to achieve water intercalation. Before bonding, both samples are placed for 5 min on a hotplate of 120 °C to remove adsorbed water on the surface. Next, the FDTS coated target wafer is bonded to the Al2O3(0001)/Pt(111)/IFW/graphene substrate and a pressure of 200 kPa is applied to the sample stack in a homemade bonding setup. Next, 0.2 M NaOH was introduced into the bonder and -3 V was applied for 2 min to the Pt/graphene sample in the electrochemical bonding setup. The aqueous NaOH was rinsed with UPW until the pH of the solution reached 7 and finally the pressure in the bonder was gently released. Characterization techniques Graphene is characterized using Raman spectroscopy (Horiba Labram HR with a green laser (λ = 532 nm) and a grating of 600 gr/mm), SEM (FEI Nova 200), AFM (Bruker Dimension Edge) and STM measurements are performed either under UHV (10-11 mbar) and low temperature (4.5 K) conditions (Omicron Nanotechnology), or under ambient conditions (NaioSTM).

ASSOCIATED CONTENT Supporting Information. Movies S1-S3 show the effect of IFW on the delamination of graphene using the described electrochemical delamination method. Figures S1-S10 present additional information on the characterization of graphene and the schemes of the homemade bonding setups used to transfer graphene are presented. This material is available free of charge via the Internet at http://pubs.acs.org. Notes


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The authors declare no competing financial interests. ACKNOWLEDGMENTS The authors would like to thank K. Teo (Aixtron) for the Joint Development Project on graphene growth and transfer. We would also like to show our gratitude to B. Wang and R.S. Ruoff for sharing their knowledge on the direct graphene transfer and for their invitation to their facilities. The research is funded by a Ph.D. grant (K.V.) of the Agency for Innovation & Entrepreneurship (VLAIO) and the European Graphene Flagship. K.S. and Z.L. acknowledge additional support from the Research Foundation - Flanders (FWO). REFERENCES (1)

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TOC GRAPHICS


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Abstract figure 221x100mm (300 x 300 DPI)



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Figure 1. Low temperature STM image of graphene on a Al2O3(0001)/Pt(111) substrate. Pt(111) terraces are clearly visible (a). Topographic cross-sections of the STM image indicate the presence of Pt(111) terrace steps (height 2.1 Å) and smooth graphene on the Pt(111) surface (b). 190x254mm (96 x 96 DPI)


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Figure 2. Low temperature STM image of graphene on a Al2O3(0001)/Pt(111) substrate which has been exposed for 2 weeks to ambient conditions. Triangular patterns can still be visualized (a), but the roughness on top of the terraces is clearly increased during the graphene exposure to ambient conditions (b), leading to height variations of approximately 7 Å (c). Low temperature STM measurement performed after an annealing step of 3 h in UHV at 400 °C. The inset shows the measured Moiré pattern in one of the valleys (I = 1 nA and Vbias = 0.17 V) (d). Low temperature STM image obtained after an additional anneal at 500 °C for 4 h in UHV. The meander pattern is still present after the annealing procedure (e). 190x254mm (96 x 96 DPI)



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Figure 3. SEM image of the edge of a millimeter sized graphene island. A clear band with different contrast is visible at the edge of the island (a). Raman spectroscopy mapping at approximately the same position of the SEM image indicates a clear shift of the G peak position (b) and the 2D peak position (c). 190x254mm (96 x 96 DPI)


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Figure 4. SEM image of graphene on a Pt foil measured immediately after the CVD growth process. A clear SEM contrast due to the polycrystalline nature of the Pt foil is observed. Dashed lines indicate graphene multilayer areas (a). Raman spectroscopy measurements at random positions immediately after CVD graphene growth on a Pt foil are presented in the top panel, while measurements at random positions after submerging the sample in 50 °C UPW for 16 h are presented in the lower panel (b). 190x254mm (96 x 96 DPI)



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Figure 5. STM measurement obtained under ambient conditions. After CVD growth, the Al2O3(0001)/Pt(111)/graphene sample was straightaway submerged in 50 °C UPW for 16 hours. The STM measurement was performed immediately after the water treatment and is acquired using a bias voltage of 105 mV and a current of 0.2 nA. 195x195mm (96 x 96 DPI)


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Figure 6. Optical microscopy image during a graphene transfer procedure on a Si/SiO2(90 nm) wafer. A PMMA layer is still present on top of graphene (see inset). Multilayer graphene areas are present at the bottom left. 190x254mm (96 x 96 DPI)



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Figure 7. Optical microscopy image during a graphene transfer procedure on a Si/SiO2(90 nm) wafer. A PMMA layer is still present on top of graphene (see inset). Multilayer graphene areas are present at the bottom left. 190x254mm (96 x 96 DPI)


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Figure 8. Scheme of a direct transfer method of graphene grown on top of a Pt surface to a hydrophobic Si/SiO2 surface treated with an FDTS silane layer. The first step is the formation of IFW between graphene and the Pt surface. Next, a dry bonding step to a hydrophobic target sample is implemented, while applying a pressure of 200 kPa. Next, an electrolysis step is done in 0.2 M NaOH to detach graphene from the growth substrate and the electrolyte solution is replaced with water until the pH reached a value of 7. The last step is the gradual reduction of the applied pressure. 190x254mm (96 x 96 DPI)



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Figure 9. Optical microscopy image after a direct graphene transfer. A graphene wrinkle is indicated (a). SEM image (b) and AFM image (c) after a direct graphene transfer. Wrinkles can be observed after a direct graphene transfer in (c), but polymer contamination is completely absent. 190x254mm (96 x 96 DPI)


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Figure 10. Raman spectroscopy maps of the FWHM of the 2D peak (a) and the 2D/G peak ratio (b) after a room temperature direct graphene transfer on an FDTS treated Si/SiO2(90 nm) wafer. Raman spectroscopy maps of the FWHM of the 2D peak (c) and the 2D/G peak ratio (d) after a direct graphene transfer and a subsequent anneal at 150 °C for 1 h in a nitrogen atmosphere. A decrease of the 2D/G peak ratio after annealing can be observed. 190x254mm (96 x 96 DPI)


Controlling Water Intercalation Is Key to a Direct Graphene Transfer

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