Since monolayer graphene was first efficiently separated from three-dimensional (3D) pure graphite by top-down mechanical exfoliation (1), graphene with its distinctive bodily and chemical properties has attracted nice consideration as an interesting platform for various purposes within the fields of electronics, optoelectronics, and different fields (2–4). However, mechanically exfoliated graphene nonetheless has limitations by way of dimension, yield, and thickness management, which isn’t appropriate for industrialization, leading to a powerful demand for different artificial approaches. As a substitute methodology for making ready graphene, the vapor-deposition methodology has been thought of to be one of many consultant bottom-up strategies for overcoming the aforementioned limitations of top-down mechanical exfoliation (5–7). Nevertheless, as a result of the inherent properties of graphene change considerably with the variety of layers and stacking order (8, 9), the event of progress strategies associated to their management has been tried (10, 11), however no excellent outcomes have but been reported. Above all, the preexisting bodily and chemical outcomes obtained with exfoliated graphene are nonetheless removed from being reproduced by as-grown graphene. Subsequently, if a way can overcome the problems of typical mechanical exfoliation, then it is going to be probably the most engaging artificial method for making ready graphene.
In the meantime, latest research have revealed that exfoliation of 2D supplies (2DMs) with the assistance of skinny metallic movies might be key to acquiring scalable skinny 2DM layers. Thus far, large-area monolayers of transition metallic dichalcogenides (TMDCs) have been obtained by completely different exfoliation approaches, e.g., direct exfoliation on Au substrates (12, 13), the layer-resolved splitting approach by managed crack propagation (14), and layer-by-layer exfoliation (15). These research have been profitable in acquiring large-area TMDCs however have been restricted to acquiring solely monolayer TMDCs. Why solely the topmost layer is peeled off isn’t but sufficiently understood, therefore limiting the appliance of the large-area exfoliation approach for numerous 2DMs, akin to graphene, and the thickness management of exfoliated crystals.
On this examine, we introduce the approach layer-engineered exfoliation (LEE) that not solely ensures large-area graphene but in addition gives management of the selective variety of graphene layers owing to the distinction within the interfacial binding power between the used metallic and graphene. A skinny Au movie is deposited on precleaved recent graphite to selectively peel off the topmost graphene monolayer, as its interfacial toughness with graphene is just like the interlayer binding power of graphite. By adjusting the interfacial toughness by way of the deposition of various metallic movies (Pd, Ni, and Co) which have increased interfacial toughness than the Au movie, large-area graphene with a managed variety of layers can also be obtained. The exfoliated graphene doesn’t exhibit any intrinsic defects or chemical contamination, as confirmed by spectroscopy and electron transport research. We consider that our exfoliation methodology generally is a promising method for constructing large-area 2D heterostructures, revisiting the standard exfoliation methodology that has been thought of removed from commercialization.
RESULTS AND DISCUSSION
LEE of graphene
As depicted within the schematic in Fig. 1A, to exfoliate large-area graphene with a selective variety of layers, we instantly evaporated a selective metallic movie onto a bulk graphite flake precleaved on the blue tape. The metallic movie was thermally evaporated as much as 60 nm as a stressor on the graphite flake, the place the tensile stress is given by the distinction within the lattice constants of the graphite and metallic movies (16). By exfoliating the graphite flake with spin-coated poly(methyl methacrylate) (PMMA) and thermal launch tape (TRT) as a dealing with layer, the exterior bending second creates a crack at area boundaries inside the graphite flake initiated by tensile stress, after which, the crack propagates parallel to the metal-graphene interface, leading to large-area exfoliation as a result of residual rigidity (14). Such spalling of the highest graphene layers happens alongside the out-of-plane path as much as the spalling depth, which is determined by the distinction between the binding power (γmetal-Gr) and interlayer binding power of graphite (γGr-Gr), as proven within the inset of Fig. 1A (14, 17, 18). The upper the distinction is, the deeper the spalling depth (experimental particulars might be mentioned later, in relation to Fig. 2). After the switch of exfoliated flakes onto a 300-nm SiO2/Si substrate, the TRT is eliminated, and the PMMA and metallic movie are subsequently chemically etched (see Supplies and Strategies for the detailed LEE course of).
Figure 1B exhibits a typical instance of exfoliated monolayer graphene obtained by following the described LEE process, the place a Au movie was used as a stressor. The small distinction in binding power between Au-graphene (γAu-Gr = 30 meV/atom) (19) and graphene-graphene (γGr-Gr = 21–25 meV/atom) (20, 21) allows separation of a monolayer. The monolayer graphene we exfoliated usually exhibits a lateral dimension of a millimeter with out bodily defects inside it (Fig. 1, B and C), which is believed to be the biggest exfoliated monolayer graphene ever reported (22). Right here, the dimensions of the exfoliated graphene is decided by that of the atomically flat domains of the mom graphite. Figure 1D illustrates the lateral dimension of particular person domains of pure graphite with atomic-level flatness. This dimension is roughly a number of hundred micrometers, just like the dimensions of the obtained monolayer graphene (Fig. 1, D and E, and figs. S1 and S2). Nevertheless, there are nonetheless further areas the place empty layers or multilayers are current as a result of not solely the small binding power distinction tends to peel off solely the Au movie with out graphene (23) but in addition pure graphite remains to be bumpy on the millimeter scale, respectively (fig. S3). Our complementary experiment on extremely oriented pyrolytic graphite (HOPG) confirms that the flat area dimension issues, because the floor of HOPG reveals many atomic steps that naturally happen throughout mechanical cleavage, resulting in the large-scale and nonuniform monolayer graphene flake (fig. S4).
First, we quantitatively analyze the standard dimension and density of exfoliated monolayer graphene to confirm the reliability of our LEE methodology for 25 samples (figs. S1 and S5). The common space of monolayer graphene obtained by the LEE methodology reaches 163,238 (±12,515) μm2, which is a rise of ~4200 occasions that of monolayer graphene exfoliated by the usual methodology, which yields 39 (±4.8) μm2 (Fig. 1, F and G). Our LEE methodology additionally exhibits higher outcomes than customary mechanical exfoliation by way of the density of the monolayer. When calculating the realm occupied by monolayer graphene in an arbitrary area (1 mm2 space) on the substrate, the density of the monolayer is measured to be solely 0.0096% for the usual exfoliation methodology (fig. S6). Nevertheless, as famous in Fig. 1H, the LEE methodology results in a monolayer protection of 34% (± 2.5) and as much as 58% at some factors. The reproducibility of the dimensions and density of monolayer graphene ensures that this method is a dependable methodology of exfoliating monolayer graphene in a managed method.
Then, to confirm how sturdy our methodology is in acquiring large-area layer-engineered graphene by adjusting the spalling depth, we deposited numerous metallic movies as a selective stressor on graphite flakes. Every Pd, Ni, and Co movie was evaporated on prepeeled graphite as a result of increased binding power with graphite than that of the Au movie, the place γPd-Gr = 84 meV/atom, γNi-Gr = 125 meV/atom, and γCo-Gr = 160 meV/atom (19). When pure graphite is exfoliated utilizing the Pd movie, bilayer graphene is noticed in a 1.5 mm by 1 mm area of the 300-nm SiO2/Si substrate (Fig. 2A). Exfoliation with the Ni and Co movies additionally leads to uniform multilayer graphene with a lateral dimension of a number of hundred micrometers (Fig. 2, B and C). The high-resolution optical microscopy (OM) photographs proven in Fig. 2 (D to F) clearly point out that the optical distinction of multilayer graphene modifications from gentle purple to blue with growing γmetal-Gr, as anticipated for thicker graphene layers. Atomic power microscopy (AFM) evaluation alongside the dashed traces in Fig. 2 (D to F) confirms that exfoliation with the Pd, Ni, and Co movie reproducibly leads to bilayer, 8-nm-thick multilayer, and 20-nm-thick multilayer graphene (Fig. 2G). Raman spectroscopy evaluation of the frequency and form of the 2D peaks additionally helps the robustness of our LEE methodology, as proven in Fig. 2H. Exfoliated graphene obtained with the assistance of the Pd movie exhibits the usual conduct of the 2D mode of bilayer graphene. Upon growing the variety of layers, a number of peaks seem within the 2D mode, and the middle frequency is shifted to increased values, as indicated by the dashed line in Fig. 2H (24). All these information point out that the variety of exfoliated graphene layers will increase with the usage of Au to Co movies as a stressor on graphite (fig. S7).
Spectroscopy evaluation of LEE of graphene
Raman spectroscopy measurements on LEE of graphene (LEE-graphene) assist our proposed spalling mechanism, as this method is delicate to the stress launched in the course of the spalling of graphene layers. Figure 3A exhibits the Raman spectra of LEE monolayer graphene, the place G and 2D peaks are clearly noticed. The G and 2D peaks within the Raman spectrum present data on the utilized pressure in graphene. Nevertheless, within the case of G peak evaluation, extracting significant data by utilizing the total width at half most (FWHM) of the G peak (ΓG) within the absence of a magnetic discipline is troublesome as a result of the FWHM strongly is determined by the exterior surroundings, akin to cost provider doping, screening, or digital broadening (25, 26). As anticipated, the FWHM of the 2D peak (Γ2D) doesn’t depend upon that of the G peak (ΓG), as proven in Fig. 3B. Nevertheless, we will extract the essential data as follows. In contrast with the Γ2D of monolayer graphene exfoliated by the usual mechanical methodology and encapsulated by two hexagonal boron nitrides (hBNs; for particulars, see Supplies and Strategies; fig. S8), the Γ2D of LEE monolayer graphene is bigger. In accordance with Γ2D broadening, we assume that an quantity of inhomogeneous pressure is launched in the course of the Au-assisted LEE (Au-LEE) course of. The worth of Γ2D, nevertheless, is probably going affected by cost doping, so we targeted on the correlation between the frequencies of the G and 2D peak positions (ωG and ω2D, respectively) by utilizing a easy vector mannequin as represented within the inset of Fig. 3C and fig. S9 (27). This mannequin consists of two vectors: OP = aeT + beH, the place a and b are constants and eT and eH are unit vector parts for tensile pressure and gap doping results, respectively. Right here, the preliminary slope values of eT = 2.2 and eH = 0.7 have been used (for particulars, see be aware S1). As well as, we set the origin, O (1582 and 2677 cm−1), because the intrinsic property of pristine graphene, the place the intrinsic frequencies of the G and 2D peaks are usually not affected by further cost and pressure (27). Because the pressure and cost doping range, ωG and ω2D transfer from O alongside eT and eH, respectively. In distinction, Q4 (Q1) is contributed by tensile (compressive) pressure mixed with the cost doping impact (Q2 and Q3 are usually not allowed as a result of cost doping ought to end in a rise in ωG). On the idea of the connection between the frequencies of the G (ωG) and 2D (ω2D) peaks, LEE-graphene exhibits a transparent linear relationship with a slope of two.2 (±0.1). This worth is in good settlement with earlier experimental and theoretical outcomes underneath uniaxial (2.02 to 2.44) or biaxial (2.25 to 2.8) stress (27–29). Though our experimental outcomes are in higher settlement with the uniaxial-strain scenario, the presence of biaxial pressure or a mixture of each can’t be excluded. All Raman factors are situated in Q4, which signifies that tensile pressure is utilized to LEE-graphene. When graphene is encapsulated by hBN crystals, Γ2D is decreased to ~23 (±2) cm−1 and (ωG and ω2D) strikes to close the origin O (ω0G and ω02D), as proven in Fig. 3C. The outcomes specify that tensile pressure in the course of the LEE course of is launched when graphene is lifted (indifferent) utilizing hBN crystals, recovering the pristine property of usually exfoliated graphene.
Additional microscopy and spectroscopy evaluation additionally verify the standard of our LEE-graphene. The absence of the disorder-related D peak close to 1350 cm−1 (field coloured in blue in Fig. 3A) and the symmetric Lorentzian line form of the 2D peak in Fig. 3A point out that the monolayer graphene obtained by Au-LEE is of top of the range (fig. S10). The AFM measurements point out that there aren’t any notable bodily defects, akin to cracks, folding, and tearing, on the graphene floor (Fig. 3D). The basis imply sq. roughness of LEE monolayer graphene is sort of just like that of usually exfoliated graphene on a silicon substrate (~0.8 Å distinction), which is generally as a result of nonflat floor of the SiO2/Si substrate (30). As well as, contemplating that monolayer graphene acquired by Au-LEE is uncovered to a chemical-based course of to take away the TRT, together with Au/PMMA layers, we confirmed that no important chemical contaminants, akin to PMMA and metallic etchant residues, stay through x-ray photoelectron spectroscopy (XPS; Fig. 3E and fig. S11). The XPS spectrum exhibits a pointy C 1s peak and is fitted by three dominant Lorentzian modes that originate from typical sp2-hybridized carbon: C═C (284.6 eV), C─O (286.3 eV), and C═O (287.4 eV). As soon as an extended chemical chain of PMMA is uncovered to graphene, it’s simply adsorbed on its floor, resulting in indelible polymer residues (31). We verified that the depth of amorphous carbon is negligible and that the depth of its associated peaks could be very weak. Thus, we consider that the metallic movie successfully protects the floor of graphene from natural residues throughout the entire LEE course of (16, 32).
Electron transport properties in LEE-graphene
To cross-check the standard of the LEE-graphene evidenced by the Raman spectroscopy and XPS outcomes, we carried out electron transport measurements on the monolayer graphene gadget. Our graphene was encapsulated between defect-free hBN crystals (33), after which, the mesa was ready by a sequential e-beam course of, as proven within the inset of Fig. 4A (see Supplies and Strategies for the detailed hBN encapsulation and gadget fabrication course of). Right here, the encapsulation of graphene allows us to analyze the standard of the electron transport traits within the monolayer graphene that we ready. hBN gives a flat and clear substrate for graphene, screens the cost fluctuations within the silicon substrate, and protects towards contamination after exfoliation (33, 34). Figure 4A exhibits the again gate–dependent longitudinal resistivity ρ at temperature T = 2 Ok, measured by utilizing a four-terminal configuration with the usual lock-in approach; a schematic of the wiring is proven in Fig. 4A. Right here, the place of the charge-neutral level (CNP) is on the back-gate voltage Vbg = 1.5 V (similar to provider density n = 9.0 × 1010 cm−2), which means that solely negligible charged impurities and pressure exist that can lead to the shift of the CNP place (35). Subsequent, to extract the extent of the potential fluctuation n*, we plot the longitudinal conductivity σ as a operate of n on a log scale (36). As proven in Fig. 4B, log(σ) stays fixed when the electron transport is dominated by the potential fluctuation arising from charged impurities, whereas it exhibits a linear dependence on log(n) at excessive n. The crossing level of every extrapolated log(σ) dashed line in Fig. 4B gives the n* of our graphene gadget, which is basically the identical as that from an enough exfoliated graphene gadget (34, 37) encapsulated between hBN. This once more demonstrates the suitable high quality of our graphene gadget, though the graphene was involved with the metallic movie in the course of the switch. We additionally decided the mobility μ (= σn−1e−1) of the gadget at T = 2 and 300 Ok to judge the standard of the gadget, the place e is the electron cost (Fig. 4C). The mobility at 300 Ok is increased than 20,000 cm2 V−1 s−1, which is increased than the reported mobility of exfoliated graphene (38) on a silicon substrate however the identical order of magnitude as that of the reported decent-quality graphene gadget (33) the place the graphene was exfoliated by the usual methodology. The mobility at 2 Ok is roughly double that at 300 Ok as a result of decreased momentum rest charge at low temperature. We additionally discovered a well-developed quantum Corridor impact (Fig. 4D) during which the Landau ranges begin to cut up from ~1 T at low n and the degeneracy of the Landau ranges is lifted at excessive magnetic fields, exhibiting filling components of −1, −2, −4, and −6, which we count on for high-quality graphene gadgets (39). The observations right here subsequently allow us to conclude that our LEE approach basically doesn’t degrade the standard of the graphene.
In abstract, the LEE method demonstrated right here gives terribly large-size and high-density graphene from pure graphite. Specifically, by utilizing completely different metallic depositions that merely management the spalling depth, layer-engineered graphene might be obtained on a big scale. In contrast to the usual exfoliation methodology that solely permits a single peeling course of, large-area graphene has been obtained a number of occasions from the identical graphite flake by repeating the deposition and tearing means of the metallic movie (see Supplies and Strategies for the detailed repeated exfoliation course of; fig. S12). Furthermore, we utilized the LEE method to hBN, confirming that the variety of layers of exfoliated hBN was additionally managed by the choice of stressor movies (fig. S13). We consider our outcomes exhibit that layer-engineered graphene might be exfoliated in a big space, paving the way in which for the event of a manufacturing-scale course of for future purposes based mostly on 2D heterostructure.
MATERIALS AND METHODS
LEE course of
First, bulk pure graphite (NGS Buying and selling & Consulting, Gra-bluck) have been caught and cleaved on adhesive blue tape (ELP BT-150E-KL). Subsequently, the chosen metallic skinny movies have been deposited on the graphite crystals by utilizing an E-beam evaporation system. To keep away from bodily harm of graphene, we lowered the deposition charge to as little as 0.1 Å/s for five nm. The inner stress of metallic movie (σmetallic) is estimated by utilizing the Stoney system, and the equation is given by
, the place R is the measured curvature of metallic, R0 is the measured curvature of the substrate wafer, Ys is the Younger’s modulus of the substrate, νs is the Poisson’s ratio of the substrate, ts is the thickness of the substrate, and t is the thickness of metallic (16). The measured stress ranges of every metallic (Au, Pd, Ni, and Co) ranged from 300 to 450 MPa. Then, PMMA was spin-coated at 1500 rpm for 1 min. 100–micrometer-thick TRT (Revalpha 3196, Nitto Denko, TRT) is connected to the highest of PMMA/metallic/graphite as a dealing with layer. Presently, the PMMA layer is used to reinforce conformal contact between the skinny metallic movie and TRT. By making use of mild power, TRT/PMMA/metallic/graphene was delaminated from bulk graphite and transferred onto 300-nm SiO2/Si substrates. The TRT misplaced its adhesion at 110°C. PMMA and metallic movie is totally eliminated by dipping it into acetone and acceptable metallic etchant (Au etchant: product no. 651818, Sigma-Aldrich; Pd, Ni, and Co etchant: product no. 034256, Transene). To wash up the residues of the meal etchant on the graphene, we soaked the LEE-graphene in a flowing deionized water tub over 20 min (fig. S14).
hBN encapsulation and gadget fabrication
To measure the intrinsic properties of LEE-graphene, hBN-encapsulated graphene samples have been ready. Monolayer graphene was exfoliated by our LEE methodology as described above, and hBN flakes have been obtained by the usual exfoliation methodology. We chosen flat and clear hBN flakes. To choose up and drop our samples, we used the polydimethylsiloxane (PDMS)/poly(propylene) carbonate (PPC) template described in (40). First, the PDMS/PPC template was put involved with the highest hBN flake on 300-nm SiO2/Si and lifted up at 60°C. Subsequently, we connected the PDMS/PPC/hBN to LEE-graphene and raised the substrate temperature to above 130°C to enhance the adhesion between graphene and hBN. After a couple of minutes, solely the PDMS was lifted, and the highest hBN remained on the LEE-graphene. In the identical method, after that, we picked up the highest hBN/graphene at 60°C and dropped it onto the underside hBN at 130°C utilizing the PDMS/PPC template, assembling an hBN-encapsulated graphene construction. The Corridor bar construction was patterned by customary e-beam lithography (width: 2.0 μm; size: 3.0 μm). Reactive ion etching in SF6/Ar plasma [40 standard cubic centimeters per minute (sccm)/40 sccm, 100 W, 1 min] was utilized to show the graphene edge (33). Subsequently, Cr/Au metallic movies (5 nm/45 nm) have been deposited by thermal evaporation and lifted off in acetone, leading to a 1D-edge metallic contact.
Calculation of the density and space of exfoliated monolayer graphene
To calculate the dimensions and density of exfoliated monolayer graphene on 300-nm SiO2/Si substrates, we used the open-source picture processing program ImageJ (https://imagej.net) (41). After acquiring OM photographs of arbitrary areas (1 mm by 1 mm) the place graphene flakes have been exfoliated, entire pixels of photographs have been extracted. Subsequently, the distinction worth (C) of every pixel was calculated by the next equation: C = (CR + CG + CB)/3, the place CR, CG, and CB are the R, G, and B values per pixel within the shade picture, respectively. Utilizing the above equation, we obtained the optical distinction of monolayer graphene (CML) and the substrate (CS) from the optical picture in fig. S5. The distinction distinction (CD; similar as CS − CML) between monolayer graphene and the 300-nm SiO2/Si substrate was calculated to be −4.7 (fig. S5D). Then, areas with the identical CD values have been chosen from the unique picture, and the variety of chosen pixels was calculated (fig. S5, A to C). The density of monolayer graphene was calculated by dividing the variety of pixels of the chosen monolayer graphene by the full variety of pixels. The realm was calculated by choosing particular person graphene domains within the type of a easy closed curve chosen by the ImageJ program (fig. S5E).
Repeated exfoliation course of
In contrast to the standard exfoliation methodology during which solely a single peeling course of is possible, large-area monolayers have been obtained a number of occasions from the identical graphite flake by repeating the deposition and tearing means of the Au movie. Three repeated exfoliations have been carried out for a similar area, and every course of result’s proven in fig. S12. Monolayer graphene with a lateral dimension of ~300 μm was obtained in the course of the first, second, and third repeated exfoliations. The realm of the obtained monolayer was calculated to be ~80,000 μm2 for the primary graphene monolayer. For a similar flake place, the areas of the second and third graphene monolayers have been ~74,000 and ~78,000 μm2. The standard of the repeatedly exfoliated monolayer graphene was confirmed by Raman evaluation. The depth ratios of the 2D peak and G peak, I2D/IG, of the primary, second, and third exfoliated graphene have been roughly 1.9, 2.0, and a pair of.1, respectively. As well as, no D peak was noticed in every spectrum, indicating that the bodily and chemical defect ranges of the monolayer graphene are low and that the crystallinity is excessive.
Floor morphology and thickness evaluation was performed by OM (Nikkon) and NX10 AFM (Park Programs Corp.) underneath tapping mode at a gradual scanning charge (~0.3 Hz). Raman measurements have been carried out utilizing a WITec Raman system [excitation wavelength of 2.33 eV (532 nm)] with a piezo stage, which was used to acquire large-area mapping information. The laser energy was lower than 1 mW to keep away from laser-induced heating of the 2D supplies. The Si peak at 520 cm−1 was used as a reference for wave quantity calibration. All of the peaks of the acquired Raman spectra have been fitted by Lorentzian. XPS evaluation was carried out to characterize the floor chemical state of graphene. It was carried out utilizing an ESCA2000 spectrometer with monochromatic Al Kα radiation (1468.6 eV). The height energies have been calibrated utilizing the C 1s peak at 284.6 eV.
Acknowledgments: Funding: This work was supported by the Nationwide Analysis Basis of Korea (grant nos. 2020R1A4A4079397 and 2019R1G1A1100363), POSCO Science Fellowship of POSCO TJ Park Basis, and Samsung Show Co. Ltd. Ok.W. and T.T. acknowledge assist from the Elemental Technique Initiative performed by the MEXT, Japan, grant no. JPMXP0112101001, JSPS KAKENHI grant no. JP20H00354, and the CREST (JPMJCR15F3), JST. Creator contributions: J.-Y.M., D.W., S.-Ok.S., and J.-H.L. conceived the concept for this analysis challenge. J.-Y.M. and S.-I.Ok. carried out the LEE of graphene, performed characterizations, and information analyses. M.Ok., S.X., and S.-Ok.S. made graphene gadgets and performed transport measurements. M.Ok., J.-H.C., and S.-Ok.S. carried out the Raman evaluation. D.S.P., J.S., and S.H.C. participated in graphene and hBN exfoliation and information analyses. Ok.W. and T.T. grew high-quality hBN single crystals. J.-Y.M., M.Ok., S.-Ok.S., and J.-H.L. wrote the manuscript. S.-Ok.S. and J.-H.L. supervised the analysis. All authors mentioned the outcomes and commented on the manuscript. Competing pursuits: The authors declare that they don’t have any competing pursuits. Knowledge and supplies availability: All information wanted to judge the conclusions within the paper are current within the paper and/or the Supplementary Supplies. Further information associated to this paper could also be requested from the authors.