Layer-by-Layer AB-Stacked Bilayer Graphene Growth Through an

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Layer-by-Layer AB-Stacked Bilayer Graphene Growth Through an Asymmetric Oxygen Gateway Bing Liu, Yaochen Sheng, Shenyang Huang, Zhongxun Guo, Kun Ba, Hugen Yan, Wenzhong Bao, and Zhengzong Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01095 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on August 2, 2019

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Chemistry of Materials

Layer-by-Layer AB-Stacked Bilayer Graphene Growth Through an Asymmetric Oxygen Gateway Bing Liu1, Yaochen Sheng2, Shenyang Huang3, Zhongxun Guo2, Kun Ba1, Hugen Yan3, Wenzhong Bao2, Zhengzong Sun1* 1Department

of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, 2State Key Laboratory of ASIC and System and School of Microelectronics, and 3State Key Laboratory of Applied Surface Physics and Department of Physics, Fudan University, Shanghai 200433, P. R. China. ABSTRACT: Compared with the semi-metallic single layer graphene (SLG), large-sized bilayer graphene with desired stacking order is extremely useful in the twilight of two-dimensional (2D) electronics era. However, limited strategies in the existing chemistry arsenal have been developed to grow AB-stacked bilayer graphene (AB-BLG) film in a more controllable and scalable fashion, and this stagnancy is hindering the industry-level mass production of AB-BLG-based devices. Here we demonstrate a feasible method to synthesize large-area AB-BLG films by adopting an industry-compatible planar plasma-treated asymmetric SLG/Cu/Cu2O substrate. The Cu2O side serves as a catalytic oxygen gateway for continuous carbon diffusion through bulk Cu to grow the second graphene layer underneath the existing SLG template in a layer-by-layer manner. The as-grown graphene film can reach a BLG coverage of ~ 95% with an AB-stacking percentage up to ~ 99%.

INTRODUCTION Over the past several years, we have witnessed vigorous developments of graphene owing to its unique atom-thick layered structure and exceptional physical properties1-7. Especially, the abundant electronic variations built upon stacked graphene layers have aroused intense research interests, ranging from electronics to superconductivity fields. For example, twisted BLG with a ‘magic’ angle of 1.1° was recently discovered a superconductivity state upon electrostatic doping7. Beyond two layers of graphene, the stacking orders and relevant electronic properties are getting more complex. Common stacking orders are like AB stacking (ABAB), rhombohedral stacking (ABCABC), and random turbostratic stacking5. For most graphene structures, the semi-metallic (zerobandgap) feature constrains their applications in field-effect transistors, because the devices cannot be turned off at room temperature8. Luckily, we can resort to AB-stacked bilayer graphene (AB-BLG), which can be transformed into a semiconductor material under an applied electrical field8-10. Although many attempts have been taken to synthesize AB-BLG, still we have some critical problems that have not been addressed satisfactorily, especially on the scalable synthesis method and precise stacking control across wafer-sized BLG film. The growth strategy of AB-BLG slowly evolves into two mainstreams. The first one is to grow two graphene layers simultaneously from the same nuclei, called the co-nucleation method. This can be realized by precisely controlling the carbon feedstock’s concentration or partial pressure11-14, or fine-tuned temperature-dependent segregation on bulk Ni or Cu-Ni alloys15-17. In general, this approach is sensitive to growth parameters and usually lacks the precise control of layer number. Layer-by-layer growth stands for another prevalent AB-BLG growth strategy, in which two graphene layers grow in separate steps. Spatially, the second graphene layer can form above (epitaxial)18 or underneath (templated)19-21 the first graphene layer. In the epitaxial layer-bylayer method, the domain sizes are usually small, restricted to several microns18. Ruoff’s group21 employed a Cu ‘pocket’ to achieve large BLG single crystals in the templated layer-by-layer

method, however, which may not suitable for uniform and continuous BLG film growth. It also faces the challenge to scaleup the ‘Cu pockets’, in which case the carbon diffusion and masstransport may take control in a grand dimension. Obviously, a planar growth substrate will be more favorable for industry-level large-area AB-BLG growth. Here we adopted an asymmetric substrate, SLG-Cu-Cu2O, for the scalable and controllable growth of large-area AB-BLG films, with precisely controlled layer number and stacking order. Carbon atoms, produced at the Cu2O side from CH4 decomposition, diffuse from Cu2O (or oxygen) gateway into bulk Cu to segregate the second graphene layer underneath SLG template in a layer-by-layer manner.

RESULTS AND DISCUSSION In this layer-by-layer approach, we used large domain sized SLG films on Cu as the template for the second graphene layer growth. The domain size of SLG films can reach up to 800 μm (Fig. 1a & Fig. S1). As growth time is prolonged, SLG films can fully cover both Cu sides symmetrically, forming SLG-Cu-SLG, which would block the growth of additional graphene layers due to lack of catalytic Cu surface and carbon diffusion channels12. To open up the carbon diffusion gateway again, low-pressure air plasma (< 100 mTorr, 21% O2 and 78% N2) was employed to etch the SLG film on one Cu side (Fig. 1b) while SLG film on the other Cu side was protected with PET (polyethylene terephthalate) film. The plasma treatment lasted for 1 h to completely remove the unprotected SLG film and form a Cu oxide layer. After oxidative plasma, the shining surface of SLG-Cu-SLG changes into a brownish Cu oxide color (Fig. 1c). In X-ray diffraction (XRD) analysis of this oxide layer, the peaks at 29.6°, 36.5°, 42.4° and 61.6° (2θ) are attributed to the Cu2O (110), (111), (200) and (220) planes22, respectively (Fig. 1d, e). The thickness of this Cu2O layer was measured to be ~ 300 nm with scanning electron microscopy (SEM) as shown in Fig. 1f.

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Figure 1. Growth of AB-BLG on an asymmetric SLG-Cu-Cu2O substrate with an oxygen gateway. (a) Optical image of large domainsized SLG template on Cu. (b) Low pressure air plasma to etch SLG film on one Cu side and oxidize the exposed Cu to form Cu2O. (c) Colors of SLG-Cu-SLG before plasma and SLG-Cu-Cu2O after plasma. (d) XRD spectra of Cu2O surface. (e) Enlarged XRD spectra taken from (d). (f) False-colored SEM image of Cu2O on Cu after plasma. (g) Schematic illustration of the growth process of the second layer of AB-BLG. After SLG growth and etching steps, the symmetric SLG-Cu-SLG is transformed into the asymmetric SLG-Cu-Cu2O substrate with an oxygen gateway. Then this new asymmetric substrate was used to grow the second graphene layer of our BLG samples. The growth process and hypothesized mechanism are displayed in Fig. 1g. Throughout all the annealing and growth stages of the second layer, CH4 (0.5-1.0 sccm) and H2 (500 sccm) were continuously supplied into the CVD chamber, to protect the SLG template from etching and initiate the growth simultaneously. At 1,000 °C, methane molecules are decomposed at the oxygen-rich Cu2O side to form carbon atoms, which then diffuse from this Cu2O gateway into Cu bulk following an interstitial pathway23. Eventually, carbon atoms reach the interface between the existing SLG template and Cu surface, and start to segregate the second graphene layer underneath SLG template. With different growth time, snapshots of BLG samples transferred (PMMA-assisted) on 300 nm thick SiO2/Si substrate were investigated with optical microscope to study the second layer’s coverage and growth rate. After 1 hour’s growth, many isolated domains appear with an average size of ~ 3 μm (Fig. 2a). These small domains continues to grow larger in the second hour, and the average size reaches ~ 10 μm. As the growth time is prolonged to 3 hours, these domains coalesce with their neighbors

and form a continuous film with coverage up to ~ 95%. In Fig. 2b, the BLG coverage increases almost linearly except for the first hour. The average growth rate of the second layer is below 10 μm/h. In comparison, the SLG growth rate is as high as 2,400 μm/h in the same CVD system (Fig. S1d). This dramatic growth rate gap between SLG and the second layer of BLG suggests that different carbon diffusion mechanisms, gas phase vs. bulk phase, might play in these two growth scenarios. We performed independent Raman characterization on 1, 2 and 3 layered graphene areas transferred using PMMA-assisted (polymethyl methacrylate) method, and their Raman spectra were displayed in Fig. S2, which helps us identify AB-BLG samples. In Fig. 2c, the BLG samples show two main peaks, the 2D peak (~ 2,695 cm-1) and G peak (~ 1,580 cm-1). The full width at half maximum (FWHM) of 2D peak is ~ 55 cm-1, featuring the ABstacking order12, 24. The intensity ratio between 2D and G peaks (I2D/IG) is ~ 0.6, a bit lower than the reported value of exfoliated AB-BLG’s ~ 1.025, which may be caused by PMMA contamination during graphene transfer26. Raman mappings were also applied to investigate the uniformity of AB-BLG in a 20 μmⅹ20 μm region. In Fig. 2d, 95% of 2D peak’s FWHMs are between 55 cm-1 and 65 cm-1. The rest 5% are below 40 cm-1, coinciding with the optical image’s SLG coverage. In the I2D/IG map, 95% of the values are between 0.5 and


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Figure 2. Coverage and stacking orientation of BLG samples. (a) False-colored optical images of BLG film coverage after different growth time. (b) The relationship between BLG coverage and growth time. (c) Raman spectrum of BLG transferred on a 300 nm SiO2/Si substrate indicating AB-stacking order of BLG. (d, e) Raman maps of the FWHM of 2D peak and the I2D/IG of a selected BLG region indicating high percentage of AB-stacking order of BLG. (f) IR transmission spectra of SLG and BLG transferred on un-doped SiO2/Si substrates indicating AB-stacking order of BLG. (g) The image of BLG transferred on a TEM grid. (h) High-resolution TEM image of a selected region in (g) showing BLG edges. (i) SAED pattern of BLG chosen from a region in (h) indicating AB-stacking order of BLG. Inset: the intensity profile of diffraction spots in the dashed-line rectangular region in (i). 0.7, suggesting a uniform AB-stacked bilayer coverage (Fig. 2e). Statistic Raman spectra across the whole sample (2 cm ×2 cm) show that 99% of the BLG region holds AB-stacking order (Fig. S3). Comparing with current growth techniques listed in Table S1, our method is superior in both the critical growth parameters and the assessing metrics, making it more suitable for standard industrial growth. The high AB-stacking proportion can be ascribed to the template effect from a larger domain sized SLG film12, 21. In a compared experiment, smaller domain sized SLG shows a poor AB-stacking percentage (Fig. S4). In Fig. S5, the defect level of our AB-BLG samples was monitored with Raman map of ID/IG, with 98% region’s value lower than 0.03, indicating good quality of our AB-BLG samples. We also conducted infrared spectroscopy (IR) and transmission electron microscopy (TEM) characterizations to verify the ABstacking order of our BLG samples. IR transmission spectra in Fig. 2f shows no obvious adsorption band for SLG, while a broad adsorption band appear at ~ 3,000 cm-1 for AB-BLG27-28. The

strong adsorption for AB-BLG at ~ 3,000 cm-1 comes from the resonance of the infrared conductivity28. We transferred the BLG films onto TEM grids as shown in Fig. 2g. Close to the BLG films’ edges, a clear bilayer fringe is observed under high-resolution TEM, with an interlayer distance ~ 0.34 nm in Fig. 2h. Selected area electron diffraction (SAED) shows only one set of diffraction pattern with six-fold symmetry (Fig. 2i). The diffraction intensity profile shows a ratio of 0.5 between the inner (1100) plane and the outer (2110) plane, consistent with that of AB-BLG16, 18. To confirm the hypothesized growth mechanism of our AB-BLG samples, two control experiments were carried out. The first one was to grow the second graphene layer on a symmetric SLG-CuSLG substrate without Cu2O gateway for carbon diffusion. After 5 hours’ growth, no BLG domains were detected under optical microscope in Fig. S6a. Raman spectrum in Fig. S6b confirms the purity and integrity of SLG template, which is in line with the selfterminating growth mechanism for graphene growth on Cu catalyst16 as shown in Fig. S6c, where carbon atoms cannot diffuse



Figure 3. The layer-by-layer growth mechanism of AB-BLG through isotope labeling. (a, d) False-colored optical images of graphene transferred on a SiO2/Si substrate after growth on an asymmetric (12C-SLG)-Cu-Cu2O substrate. (b) Raman spectra taken from the blackand blue-dot regions in (a). (c) Schematic illustration of 13C diffusion to grow the second layer in a layer-by-layer mechanism. (e, f) Raman mappings of G peak at 1,585 ± 20 cm-1 (12C) and 1,525 ± 20 cm-1 (13C), respectively, taken from the same regions in (d). through SLG to grow additional graphene layers. We also introduced 13CH4 to grow the second graphene layer on asymmetric 12C-SLG-Cu-Cu O with a Cu O gateway. At first, independent 2 2 Raman study of different carbon isotope-labelled graphene samples are collected in Fig. S7 to help us identify these samples. After growth, similar coverage of BLG with that in Fig.2a was acquired after 3 hours’ growth (Fig. 3a). Raman spectra in Fig. 3b indicates that black-dot region maintains the characteristic signals of 12CSLG. However, the major blue-dot region) displays a much wider 2D peak (FWHM of ~ 103 cm-1) and two distinct G peaks (1,525 cm-1 for 13C and 1,585 cm-1 for 12C), which agrees with that of 12C /13C AB-BLG with a 12C layer and a 13C layer29. Raman mapping of G peak positions were also conducted on a junction area of BLG and SLG, as shown in Figure 3d-f. The G peak signature at 1,585 cm-1 spreads across the whole sample region (SLG + BLG), suggesting the first layer is grown entirely with 12C. In comparison, the G peak at 1,525 cm-1 (13C) only covers the whole BLG region. This result further favors the 12C/13C layout in AB-BLG and our layer-by-layer growth mechanism. The two control experiments indicate that carbon atoms diffuse from Cu2O gateway into bulk Cu to grow the second graphene layer as shown in Fig. 3c. In addition, it was reported that methane can decompose easily into carbon atoms on oxygen-rich Cu surface (Cu2O here), and thus a part of these carbon atoms would diffuse from this Cu2O gateway into bulk Cu at a rather high diffusivity of 1.3 ⅹ 10-7 cm2/s, to grow the second layer of BLG in a layer-by-layer method21. Critically, the integrity of the front side SLG template should be carefully preserved throughout the whole growth procedure. Otherwise, the second layer or few-layer domain in stripes could also grow at the broken regions of the damaged SLG template (Fig. S8). Other than the oxygen gateway, we also exploited other elements such as sulfur (S), telluride (Te), and positive result was observed on the sulfur gateway. Typically, a thin film of sulfur was evaporated onto one side of plasma-treated SLG-Cu-SLG substrate

(30 min air plasma followed by 30 min hydrogen plasma) to obtain asymmetric SLG-Cu-S substrate. The growth results in Fig. S9 confirmed the successful growth of the second graphene layer through the sulfur gateway. Apart from the aforementioned tools like Raman, IR and SAED to characterize AB-BLG, dual-gate field effect transistor devices (DG-FETs) can also help identify the stacking order of BLG via electrical transport measurement8-10, 30, 31. Therefore, we fabricated DG-FETs based on the AB-BLG samples, with a schematic device structure shown in Fig. 4a. Such DG-FET units are fabricated as an 8 ⅹ8 array in wafer-scale to confirm the sample homogeneity, as shown by an optical microscopic image in Fig. 4b. The zoom-in of one DG-FET unit is displayed in Fig. 4c. The AB-BLG sample is encapsulated between two dielectric layers, 300-nm-thick SiO2 as a back dielectric layer and 35-nm-thick HfO2 as a top dielectric layer. Au electrodes works as the source, drain and top-gate, and N-type heavily doped silicon wafer as a global bottom-gate. To investigate the gate modulation of this dual-gate architecture, the electrical resistance (R) is measured by sweeping both VTG and VBG to achieve a mapping result (Fig. 4d). A blue stripe with darker color at two ends indicates that the R of BLG at Dirac-point can be effectively tuned, by a fixed combination of VTG and VBG. By taking line traces in Fig. 4d, R is also plotted in Fig. 4e, as a function of VTG at different fixed VBG values. It is noticed that R gradually increases as the applied electrical displacement fields (D) is larger, which corresponds to the most left and right peaks in Fig. 4e . This is a firm evidence of the band gap opening. These results further confirm the AB-stacking order of these BLG samples8, 31. However, it should be noticed that the on/off ratio (defined as the ratio between the largest R and the smallest R when sweeping VTG at a fixed VBG and Vd) of our devices is smaller than those reported values of AB-BLG31, mainly because our measurement is performed at room temperature, and relatively lower displacement field is applied.


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Figure 4. Electrical measurements of dual-gate FET devices based on AB-BLG samples. (a) Schematic illustration of devices. (b) Images of device arrays. (c) Enlarged image of one device. (d) 2D color contour map of resistance (R) vs bottom-gate voltage (VBG) and top-gate voltage (VTG) at a source-drain voltage (Vd) of 10 mV. Inset: illustration of a tunable bandgap opening in a vertical electrical field. (e) Resistance as a function of VTG at different VBG.

CONCLUSION In summary, we have developed a layer-by-layer methodology to grow AB-BLG film on a planar asymmetric SLG-Cu-Cu2O substrate with an oxygen gateway, which favors high-yield bilayer thickness and precise control of AB-stacking-order. The Cu2O side works as a gateway for carbon diffusion into bulk Cu, and the segregation of the second graphene layer takes place under the guidance of the existing SLG template. This technique is suitable for wafer-sized AB-BLG films growth, which could lead to mass applications in consumer electronic products. It also has the potential to grow tri-layered (TLG) or more layered graphene samples by performing growth on BLG-Cu-Cu2O substrate or TLG-Cu-Cu2O substrate. There are still some challenges waited to be overcome in the future, like growing larger domain-sized ABBLG films at a faster growth rate.

assisted method. The XRD and SEM characterizations were conducted on Cu2O layer on SLG-Cu-Cu2O substrate to determine its thickness and crystallography. The instruments and test parameters are: Raman (Horiba LabRAM HR, 514 nm), XRD (Bruker AXS D8, 1,600 W, 40 kV, 40 mA, Cu target, Kα), SEM (Nova NanoSem 450), IR (Bruker 70v), and TEM (HT7700 EXALENS, 120 kV). The AB-BLG films were fabricated into dual-gate FET devices for transport characterization. The devices were fabricated with E-beam lithography, electron beam evaporation (EBE), and atomic layer deposition (ALD) techniques, with 35 nm thick Au as the source/drain (S/D) and top-gate electrodes, Si as the back-gate electrode, 300 nm thick SiO2 as the back dielectric layer, and 35 nm thick HfO2 as the top dielectric layer. The testing of these devices was carried out in a shielded platform equipped with four probes connected with Keysightb1500. (See details in Supporting Information Methods)

ASSOCIATED CONTENT

EXPERIMENTAL METHODS Fabrication of the asymmetric SLG-Cu-Cu2O substrate and growth of AB-BLG. First, we prepared a large domain sized SLGCu-SLG substrate through standard low pressure CVD method from 25-µm-thick electro-polished Cu foils. The growth temperature was 1,060 °C, with 500 sccm H2 and 100 sccm 1% CH4 in Ar under 3,000-3,500 Pa for 0.5-1.0 h. Then we used air plasma to etch one side of SLG film and further oxidize the exposed Cu into Cu2O, transforming the SLG-Cu-SLG substrate into the asymmetric SLG-Cu-Cu2O substrate. Finally, the AB-BLG was grown with the asymmetric substrate, with 500 sccm H2 and 0.51.0 sccm CH4 under 300 Pa, at 1,000 °C for 1.0-3.0 h. (See details in Supporting Information Methods) Characterizations. The AB-BLG films were transferred on pdoped 300 nm SiO2/Si substrates for Raman spectroscopy characterization, on undoped 300 nm SiO2/Si substrates for IR characterization, and on Au grids (Quantifoil, Ted Pella, Inc.) for TEM characterization, using a polymethyl methacrylate (PMMA)-

Supporting Information Detailed procedures to grow AB-BLG and fabricate the FET devices; Raman characterization; growth mechanism experiments; BLG growth on SLG-Cu-S substrate; summary of the existing BLG growth methods.

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCID Zhengzong Sun: 0000-0002-3710-4001



ACKNOWLEDGMENTS

FreeStanding High Quality Bilayer Graphene from Recycled Nickel Foil. Carbon 2016, 96, 268-275.

The authors are grateful to the financial support from the National Natural Science Foundation of China (21771040), the National Key Research and Development Program of China (2016YFA0203900, 2017YFA0207303) and the 1000 Plan Program for Young Talents. H.Y. is grateful to the financial support from the National Young 1000 Talents Plan, National Natural Science Foundation of China (grants: 11874009, 11734007), the National Key Research and Development Program of China (2016YFA0203900, 2017YFA0303504), Strategic Priority Research Program of Chinese Academy of Sciences (XDB30000000).

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