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Transfer-Free Growth of Few-Layer Graphene by Self-Assembled Monolayers Hyeon-Jin Shin, Won Mook Choi, Seon-Mi Yoon, Gang Hee Han, Yun Sung Woo, Eun Sung Kim, Seung Jin Chae, Xiang-Shu Li, Anass Benayad, Duong Dinh Loc, Fethullah Gunes, Young Hee Lee,* and Jae-Young Choi* Graphene is an ideal 2D planar structure with an electron mobility that reaches 200 000 cm2 V−1 s−1, an ideal theoretical sheet resistance of 30 Ω sq−1, and an excellent transmittance of 97.5% per layer.[1–3] Recent development of large area graphene synthesis on a metal layer by chemical vapor deposition opened the possibility for a wide range of applications.[4–8] Although a Ni metal layer provided an efficient way of producing graphene, controlling the number of layers has not been realized and the graphene layers are not uniform.[4–6] Cu foil has been used to produce monolayer graphene by the self-limiting growth but controlling the number of graphene layers has never been accessible.[8] However, controlling the number of graphene layers with high uniformity is a prerequisite for numerous applications. For instance, the bandgap is opened in bilayer graphene, which is useful for transistors.[9] Furthermore, these growth methods on a metal layer involve an inevitable transfer step of large area graphene that creates defects, impurities, wrinkles, and cracks and has been a bottleneck for science and technology innovation.[10] For this reason, transfer-free synthesis of graphene film directly on substrate has been recently investigated by several research groups. To obtain a direct patterned graphene film on a substrate, the catalytic metal layer located underneath graphene was partially etched away in a short time using a photoresist mask to invoke side etching.[11] Graphene was allowed to stick onto the original SiO2/Si substrate by a wet etching process or evaporation of a metal catalyst at high temperature.[12,13] All these approaches have been limited to small-area graphene. To

Dr. H.-J. Shin, Dr. W. M. Choi, S.-M. Yoon, Dr. Y. S. Woo, Dr. J.-Y. Choi Graphene Center Samsung Advanced Institute of Technology Yongin, Gyeonggi, 446-712, Republic of Korea E-mail: [email protected] G. H. Han, E. S. Kim, S. J. Chae, D. D. Loc, F. Gunes, Prof. Y. H. Lee BK21 Physics Division Department of Energy Science Sungkyunkwan Advanced Institute of Nanotechnology Sungkyunkwan University Suwon 440-746, Republic of Korea E-mail: [email protected] X.-S. Li, Dr. A. Benayad AE Center Samsung Advanced Institute of Technology Yongin, Gyeonggi, 446-712, Republic of Korea

DOI: 10.1002/adma.201102526

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make matters worse, the possibility of forming wrinkles and tears cannot be avoided in these processes. A new approach of large-area graphene preparation without creating such artificial defects is necessary to facilitate more profound unique science and technical applications in an ideal 2D graphene system. Here, we report a facile synthesis method for transfer-free growth of few-layer graphene on a dielectric substrate by pyrolysis of a self-assembled monolayer (SAM) of carbon materials squeezed between a catalytic metal and a substrate. The metal/ SAM/substrate structure provides advantages not only for the robust removal of the top-most catalyst after growth, but also in preventing the SAM material from being evaporated at high temperature, which is crucial for high-quality graphene growth without creating additional defects, wrinkles, and tears. Furthermore, we choose three SAM materials to characterize the effect of the structure of the carbon source (aliphatic SAM versus aromatic SAM) and the relationship between the amount of carbon atoms in the SAM material and the number of graphene layers formed. The uniformity and number of graphene layers were well controlled. The graphene formed by this process indicated a high carrier mobility of ≈4400 cm2 V−1 s−1 at an electron density of 2 × 1012 cm−2. Figure 1a is a schematic illustration of transfer-free graphene growth. In order to form an octyl-SAM on 300 nm SiO2/Si substrates, the substrate was dipped into a 10 mM precursor solution of trichloro-octylsiloxane in hexane for 20 min. A nickel layer of 300 nm was then deposited onto the surface of the octyl-SAM using an electron beam evaporator. The SAM carbon materials were successfully converted to graphene layers by pyrolysis at high temperature. After graphene formation, the top-most metal layer was etched away while the remaining underlying graphene layer remained intact. Figure 1b shows the surface roughness and thickness of an octyl-SAM material acquired by tapping-mode atomic force microscopy (AFM). The surface of the octyl-SAM material was flat without large pore formation. Root mean square (RMS) roughness of this film was 0.36 nm, which affected the graphene film uniformity. The thickness of the octyl-SAM was 1.3 nm, close to a theoretical value of 1.2 nm.[14] The cross-sectional high-resolution transmission election microscopy (HR-TEM) image provided further insight into the existence of a graphene layer between the substrate and Ni catalyst after heat treatment (Figure 1c). By removing the top-most metal layer by wet etching, some grain boundary on the graphene surface on SiO2 wafer was observed in the AFM image (Figure 1d). The formation of a grain boundary on graphene is attributed to the trace left from the grain boundary

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COMMUNICATION Figure 1. a) Schematic illustration for transfer-free growth of graphene on a substrate. Carbon SAM materials are squeezed between the top metal layer and the substrate, where the top-most metal layer is etched after pyrolysis. b) AFM image of octyl-SAM on a SiO2/Si substrate. The tapping mode was used to measure the thickness of SAM. A step was formed by removing half of the SAM layer with UV/ozone cleaning. c) Cross-sectional HR-TEM image after pyrolysis of octyl-SAM, showing bilayer graphene. d) AFM image of graphene originated by octyl-SAM on the substrate after metal etching.

of the metal. It has been known that wrinkles are formed on graphene due to the thermal stress caused by the difference in the thermal expansion coefficients of the Ni layer and graphene and/or furthermore during transfer process.[15] Interestingly, no wrinkles were observed in our case, in good contrast with the transferred sample (see Figure S1, Supporting Information). The location of the SAM material was a key control factor to make transfer-free graphene layers in this study. In Ni/phenylSAM/substrate structure (inset of Figure 2a), the carbon atoms of the SAM material were directly transformed into high-quality graphene layers with increasing temperature, which was evident from the development of a C(002) peak in the X-ray diffraction (XRD) pattern (Figure 2a). Heat treatment for graphitization of the SAM material was executed at 500, 700, and 900 °C under Ar atmosphere for 10 min. High-temperature treatment improved the graphitization process of carbon atoms. Heat treatment at high temperature also minimized the D-band intensity in Raman spectroscopy; in other words, it improved the crystallinity of the graphene layer, as shown in Figure 2b. The presence of a graphene layer between the metal layer and the SiO2/Si substrate was proven from the depth profile of X-ray photoelectron spectroscopy (XPS), which was composed of a mixture of carbon, oxygen, silicon, and nickel. Nickel metal was changed to nickel oxide with a thickness of ≈200 nm from the bottom surface (Figure 2c). Even though a small amount of nickel carbide existed at the interface between Ni and SiO2, most carbon atoms remained on the SiO2 surface (Figure 2d). A clear sp2-like C 1s peak on the SiO2 surface was observed after removing the metal layer (Figure 2e). Furthermore, the carbon in the phenyl-SAM material was nearly converted to a

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graphene layer through suppression of the evaporation of the SAM material by passivation of Ni metal (see Figure S2, Supporting Information) These behaviors similarly appear without species of the SAM materials. On the other hand, in phenylSAM/Ni/substrate structure, molecules of the SAM material were evaporated before the graphitization reaction took place (see Figure S3a, b, Supporting Information). Even though the unit molecule of the SAM material has a low boiling point near 200 °C, the formation of graphene layers squeezed between the metal layer and substrate limits the diffusion of molecules within the limited interlayer space to suppress their evaporation during heat treatment at high temperature. The intercalated structure of the SAM material between the metal catalyst and substrate can directly synthesize a graphene layer on the substrate surface by preventing the SAM material from being evaporated at high temperature. Therefore, the key roles of the top-most metal layer are not only the robust removal of the topmost catalyst after growth but also an encapsulation of the SAM material to facilitate graphitization without evaporation at high temperature, which is crucial for high-quality graphene growth. Previously amorphous carbons and polymers with high molecular weight, such as polystyrene (PS), poly acrylonitrile (PAN), and poly(methyl metacrylate) (PMMA), have been used as a carbon source to synthesize graphene.[16,17] Even though the amount of solid carbon source was controlled by the thickness of the carbon materials, most of the carbons were deeply diffused into the metal top surface, where the graphene layers were located on the metal top surface and the film quality was affected by the chosen molecular structure. This work implies that not only the thickness of carbon source but also stacking



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Figure 2. a) XRD pattern and b) Raman spectrum at 514 nm of graphene films with the structure Ni/phenyl-SAM/substrate according to annealing temperature. XPS data after 900 °C heat treatment of c) depth profile from Ni top surface to SiO2 bottom surface, d) C 1s peak converted by phenylSAM according to the depth, and e) C 1s peak of graphene originating from phenyl-SAM on SiO2 after removing the metal.

of the molecular structures may play an important role to synthesize high-quality graphene layers only at the interaction between the bottom metal surface and the carbon source. To verify this, we chose three SAM materials to characterize the effect of the structure of carbon source (aliphatic SAM versus

aromatic SAM) and the relationship between the amount of carbon atoms in the SAM material and the number of formed graphene layers. Table 1 summarizes the structures of the aliphatic SAM and aromatic SAM. Optical images of a graphene film converted from several SAM materials on 300 nm SiO2/Si

Table 1. Calculation of the number of graphene layers from SAM materials. Graphene (GR)

Aliphatic-SAM

Aromatic-SAM

Unit structure

Octyl2

C atoms per nm

Theoretical number of possible GR layers Number of GR layers observed by HR-TEM (standard deviation)

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Octadecyl-

Phenyl-

38.2

78.1

175.7

58.6

monolayer

bilayer

tetra- or pentalayer

mono- or bilayer

-

bilayer (±1.8 layer)

pentalayer (±1.7 layer)

trilayer (±1.2 layer)


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COMMUNICATION Figure 3. Characterization of the graphene films of several SAM materials. Optical images of graphene films from a) octyl-, b) octadecyl-, c) phenylSAM and the corresponding Raman spectra. d–f). Excitation energy of 514 nm was used in the micro-Raman system. Black, red, and blue colors in the spectra were obtained from the positions with the same colors in (a–c). HR-TEM image of graphene obtained by pyrolysis from g) octyl-, h) octadecyl-, and i) phenyl-SAM. Insets of (g,h) show the corresponding electron diffraction patterns and the inset of (i) shows the interlayer distance.

wafer are shown in Figure 3a–c. Various morphologies of graphene layers with pores and islands were observed from aliphatic SAM materials (Figure 3a,b), whereas a uniform layer without pores was observed from the aromatic SAM material (Figure 3c). This uniformity was confirmed by the similar G′/G intensity ratio of the Raman spectra (Figure 3d–f). In the case of aliphatic SAM, Raman spectra of the dark area in the optical image indicated thicker layers than others (black circle in Figure 3a,b). Otherwise, there is no Raman signal at the bright area in optical image (blue circle in Figure 3a,b). In the case of the aromatic SAM, all of the Raman spectra show a similar signal as shown in the optical image. Furthermore, in confocal Raman mapping (532 nm wavelength) for an area of 20 × 20 μm2, these Raman signal distributions reflected the uniformity (see Figure S4a,b, Supporting Information). The HR-TEM image provides further insight into the number of graphene layers. The number of graphene layer was confirmed by the

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observation of the film edge, as shown in Figure 3g–i. Although the graphene films originating from the aliphatic SAM have a varied morphology, as confirmed by optic and Raman spectroscopy, bilayer and pentalayer graphene films were formed dominantly from octyl-SAM and octadecyl-SAM, respectively (see Figure 3g,h and Figure S5, Supporting Information). On the other hand, a mostly trilayer graphene film was formed from the phenyl-SAM. The interlayer distance of the trilayer graphene was obtained from the intensity profile of the lines at the edges, as shown in the inset of Figure 3i. The intensity profile shows that the interlayer distance was extended to nearly 3.8 Å, compared to 3.4 Å of graphite.[18] To examine the stacking order between layers, the diffraction pattern was measured. The randomly stacked structure implies the formation of turbostratics (see the insets of Figure 3g,h and Figure S6, Supporting Information). The turbostratic structure could be the origin of the large interlayer distance of 3.8 Å. In general,



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followed by a graphene transfer onto another heavily doped Si wafer and subsequent formation of the source and drain electrodes with Au (700 nm). The channel length and widths were 25 μm and 1000 μm, respectively (inset of Figure 4a). The current–voltage (I–V) curve measured under ambient conditions shows p-type behavior and the upshift of the Dirac point at a gate voltage (Vg) = 100 V. Carrier densities (n) and mobilities (μ) can be extracted by applying a plane capacitor model (Drude model, μ = (neρ)−1, where e is the electric charge and ρ is the resistivity)), as shown in Figure 4b. The tunable carrier density (ns) corresponds to the estimated charge induced by the gate voltage, ns = Cg Figure 4. a) Electron transfer characteristics of the graphene film from phenyl-SAM materials obtained from a gate voltage scan at source-drain voltage (Vsd) = 1 mV measured under (Vg – Vdirac)/e, where Vdirac is the voltage assat ambient conditions. The inset is an optical image of the device. b) Graph of the mobility and dirac point and Cg is the gate capacitance and charge carrier density of graphene. assuming a Cg of 115 aF μm−2 obtained from geometrical considerations.[24] The mobility the mobility of multilayer graphene with a tubostratic structure of graphene film by phenyl-SAM was ≈4400 cm2 V−1 s−1 at –2 × is similar to that of monolayer, which is different from Bernal 1012 cm−2. The high mobility of the trilayer graphene device is [ 19 , 20 ] structure. This mobility effect in our multilayer graphene attributed to the turbostratic structure, which releases the layer with turbostratic structure is discussed in Figure 4. interaction. The number of graphene layers is strongly correlated to In conclusion, few-layer graphene was directly synthesized the number of carbon atoms in the SAM material. As shown on a SiO2 substrate without involving transfer processes by in Table 1, the number of carbon atoms in graphene is facilitating the metal/SAM/substrate multilayer structure. The 38.2 per nm2. We chose three SAM materials: trichlorocatalytic structure not only helped graphene formation without octylsilane, trichloro-octadecylsilane, and trimethoxyphenylsivaporization of a SAM material but also induced direct growth lane. When the SAM molecules are formed on the SiO2 subof graphene on the substrate. Pyrolysis of aromatic SAM matestrate, a stable cyclotetrasiloxane ([R–Si–O]4, R = octyl, octadecyl, rial provided formation of uniform graphene layers compared phenyl) unit is assembled as a unit structure, as shown in Table 1. to aliphatic SAM material. The number of graphene layers was By presuming that the SAM unit is perfectly close-packed, all modulated by a precise control of number of carbon atoms in the carbon atoms in the unit structure will be converted into the the SAM material. This inverted catalytic structure and growth graphene layer without being precipitated inside the Ni layer process provide a robust method for transfer-free graphene and the expected number of graphene layers is 2, 4–5, or 1–2 growth with uniform thickness. layers, respectively. This prediction is in congruent with the values obtained in the experiments, as shown in HR-TEM of Figure 3g–i. In the case of phenyl-SAM, the obtained number of Experimental Section graphene layers was the triple layer, which is slightly larger than Preparation of Graphene Film: Trichloro-octylsilane (boiling point (b.p.) = the calculated value. The surface of aliphatic SAMs are more 233 °C at 731 mm Hg), trichloro-octadecylsilane (b.p. = 223 °C at hydrophobic with a contact angle of 106° and 112°[14,20] (see 10 mm Hg), and trimethoxy phenylsilane (b.p. = 233 °C at 760 mm Hg) Figure S7, Supporting Information), preferring monolayer foras a precursor of the SAM were purchased from Sigma-Aldrich. To form mation of the aliphatic SAM. On the other hand, the surface of SAM materials on 300 nm SiO2/Si substrates, the substrate treated with phenyl-SAM is less hydrophobic with a contact angle of 83°.[21] piranha solution (H2SO4:H2O2 = 3: 1) was dipped into a 10 mM precursor This allows more phenyl-SAM molecules to be adsorbed within solution in hexane for 20 min and then baked at 120 °C for 20 min to crosslink it perfectly. A nickel layer of 300 nm was then deposited onto the unit structure.[22] As a consequence, the observed number the surface of the SAM with an electron beam evaporator. This film was of graphene layers is slightly larger than the theoretical value. moved into the chamber and Ar gas of 50 sccm was flowed at a pressure We emphasize that aromatic SAM provides uniform multilayer of 200 mTorr. The temperature was increased from room temperature to graphene compared to aliphatic SAM. Although the current 900 °C in 10 min and then maintained at 900 °C for 10 min under the studies are limited to producing trilayer graphene, additional same atmospheric pressure. The chamber was then cooled to 500 °C for adsorption of phenyl SAM molecules can be prohibited by low 10 min. After synthesis, the substrate was dipped into Ni etchant (FeCl3) in order to etch away the Ni metal. When the Ni was completely etched temperature SAM processes, which is a well known SAM techaway, the graphene sheet on the substrate was rinsed in deionized water nique.[23] Therefore, an ideal mono- or bilayer graphene could several times to wash away etchant residues. No appreciable Ni-related be obtained. peak was observed in the XPS data. Figure 4a shows the transfer characteristics of a graphene film Characterization of Graphene Film: The morphology of the SAM and obtained from phenyl-SAM. Due to the oxide leakage, which graphene was measured using tapping-mode AFM (Dimension V, Veeco was invoked by the high growth temperature, the original SiO2/ Co.). The cross-sectional HR-TEM (JEM 2100F, JEOL) image and the Si substrate was etched away by buffered oxide etchant (BOE), depth profile of the XPS (QUANTUM 2000, Physical electronics) provided



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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This research was supported by the star-faculty (No. 2010-0029653), World Class University (WCU) program (2008-000-10029-0), and the International Research & Development Program (2010-00429) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea. Received: July 3, 2011 Revised: July 26, 2011 Published online: August 23, 2011 [1] K. I. Bolotin, K. J. Sikes, Z. Jiang, G. Fundenberg, J. Hone, P. Kim, H. L. Stormer, Solid State Commun. 2008, 146, 351. [2] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozyilmaz, J.-H. Ahn, B. H. Hong, S. Iijima, Nat. Nanotechnol. 2010, 5, 574.

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further insight into the existence of the graphene layer between the substrate and Ni catalyst after heat treatment. HR-TEM was also used to investigate the interlayer distance of graphene and the diffraction pattern by fast Fourier transform (FFT). Samples for HR-TEM were prepared on SiO2/Si substrates as described and then directly transferred onto Cu grids by etching the SiO2 layer in diluted HF solution followed by a further dilution. XRD patterns (D8 FOCUS 2.2 KW, Bruker AXS), Raman spectroscopy (Renishaw, RM-1000 Invia, 514 nm, Ar+ ion laser), and Raman mapping (WITec, 532 nm) were used to characterize graphene films on SiO2/Si substrates. Measurement of Electrical Properties: Device fabrication simply employed graphene film transferring by BOE etching of the original SiO2/ Si substrate on another heavily doped Si wafer, followed by deposition of source and drain electrodes with Au (700 nm) by an electron beam evaporater with a shadow mask. The channel length and widths were 25 μm and 1000 μm, respectively. Graphene devices were then measured in a two-probe configuration to current between source and drain (Ids) with applied gate bias (Vg) from –150 V to 150 V at Vsd = 1 mV at room temperature under ambient conditions. Carrier density (n) and mobilities (μ) were extracted by applying a plane capacitor model.

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