Large-Area All-Carbon Nanocapacitors from Graphene and Carbon

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Large-Area All-Carbon Nanocapacitors from Graphene and Carbon Nanomembranes Xianghui Zhang,*,† Emanuel Marschewski,† Paul Penner,† Thomas Weimann,‡ Peter Hinze,‡ André Beyer,† and Armin Gölzhäuser† †

Physics of Supramolecular Systems and Surfaces, Bielefeld University, 33615 Bielefeld, Germany Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany

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ABSTRACT: We report on the fabrication of large-area all-carbon capacitors (ACCs) composed of multilayer stacks of carbon nanomembranes as dielectrics sandwiched between two carbon-based conducting electrodes. Carbon nanomembranes (CNMs) are prepared from aromatic selfassembled monolayers of phenylthiol homologues via electron irradiation. Two types of carbon-based electrode materials, (1) trilayer graphene made by chemical vapor deposition and mechanical stacking and (2) pyrolyzed graphitic carbon made by pyrolysis of cross-linked aromatic molecules, have been employed for this study. The capacitor area is defined by the width of electrode ribbons, and the separation between two electrodes is tuned by the number of CNM layers. Working ACCs with an area of up to 1200 μm2 were successfully fabricated by a combination of bottom-up molecular self-assembly and top-down lithographic approaches. Then ACCs were characterized by Raman spectroscopy, helium ion microscopy, and impedance spectroscopy. A dielectric constant of 3.5 and an average capacitance density of 0.3 μF/cm2 were derived from the obtained capacitances. A dielectric strength of 3.2 MV/cm was determined for CNMs embedded in graphene electrodes with the interfacial capacitance being taken into account. These results show the potential of carbon nanomembranes to be used as dielectric components in next-generation environment-friendly carbon-based energy storage devices. KEYWORDS: carbon nanomembrane, dielectric nanocapacitors, graphene, impedance measurements, self-assembled monolayer gradable components have been prepared accordingly.7−9 Since carbon is a nonhazardous and abundant element, it is desirable to explore the potential as well as limitations of pure carbon-based capacitors. Self-assembly of amphiphilic organic molecules on surfaces leads to the spontaneous formation of self-assembled monolayers (SAMs),10,11 which are well-ordered dielectric films with defined physical and chemical properties.12−14 When being sandwiched between two carbon sheets acting as electrical conductors, molecular hydrocarbon arranged in SAMs could be considered as a dielectric layer for an allcarbon capacitor (ACC). However, a SAM is a layer of nonconnected molecules that lacks mechanical stability and needs a supporting metal substrate. SAMs also cannot grow on top of each other, so their layer thickness is limited to the length of a molecule (typically 1−2 nm). To overcome these limits, we utilize carbon nanomembranes (CNMs) from cross-

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here is significant research effort focused on sustainable energy and technologies designed to improve energy efficiency. Along this line, the fabrication of environmentally friendly energy storage devices such as capacitors with high energy density is of great interest. A capacitor is a passive two-terminal electrical component that stores potential energy in an electric field. Common capacitor types are dielectric capacitors that polarize a dielectric medium between two conducting electrodes, and electrochemical capacitors in which energy is stored by means of the electrical double layer effect. Capacitors are made from ceramics, polymers, rare earth metals, metal oxides, and electrolytes. Different approaches have been employed to achieve highenergy density in dielectric capacitors, for example, threedimensional nanoarchitectural electrode design for metal− insulator−metal (MIM) capacitors,1,2 polymer matrix filled with ceramic nanoparticles or core−shell architectures for thin film capacitors,3,4 and interface engineering of multilayers for ceramic capacitors.5,6 However, some capacitors even contain explosive or toxic substances whose use should be avoided. From this perspective, dielectric materials containing biode© XXXX American Chemical Society

Received: July 20, 2018 Accepted: August 24, 2018

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DOI: 10.1021/acsnano.8b05490 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. (A) Schematic illustration of the fabrication processes of all-carbon capacitors: (i) preparation of bottom graphene electrodes and metallic contacts; (ii) transfer and the subsequent patterning of the CNM stack; (iii) transfer of top graphene ribbons; (iv) deposition of metallic contact pads and disconnection of capacitors from one another. Optical images of building ACCs on a SiO2/Si substrate: (B) the bottom graphene electrodes and metallic contacts have been fabricated; (C) the CNM stack has been transferred and patterned; (D) the top graphene ribbons has been transferred; (E) the metallic contact pads have been fabricated and the capacitors have been separated from one another. Scale bars: 500 μm for upper images and 20 μm for lower ones.

linked aromatic SAMs via electron irradiation.15−17 The underlying mechanisms of cross-linking have been investigated using different surface-sensitive analytical techniques.18−21 The electronic transport through CNMs was studied by conductive probe atomic force microscope and eutectic Ga−In top contacts,22,23 which demonstrates the potential of CNMs for use as a dielectric material. Recently, CNMs prepared from pterphenylthiol were found to possess a high permeance and selectivity particularly for water molecules due to the presence of subnanometer channels.24 In addition, CNMs can be converted into conducting nanocrystalline graphene by pyrolysis.25,26 CNMs possess Young’s moduli of 10−20 GPa and a tensile strength of 600 MPa.27,28 The outstanding mechanical stability allows CNMs to be released from the original substrate and transferred onto another substrate to form a multilayer system simply by mechanical stacking.29−32 These results suggest the potential of multilayer CNMs for use as a dielectric material. The preparation of top conductive electrodes is a critical step in building an ACC. The direct deposition of metal atoms onto CNMs often leads to their diffusion into the dielectric. The presence of subnanometer channels in the CNMs could further account for short circuits in CNM-based capacitors. Several techniques have been employed to fabricate top electrodes in a nondestructive way, e.g., atomic layer deposition of a passivating layer,33 spin-coating a conductive polymer layer,34 and electron beam deposition of a carbon layer.35,36 Graphene, a single layer of carbon atoms arranged in a hexagonal lattice and showing a perfect electron−hole symmetry,37 has been proposed as multifunctional and conducting electrodes.38 Vertical heterostructures of CNM/ graphene can be readily made by means of mechanical stacking.39 A high yield of working SAM-based molecular junctions was achieved by utilizing graphene top electrodes.40,41 Therefore, graphene is a well-suited top electrode material for ACCs. In the following, we describe the fabrication and characterization of ACCs composed of CNM dielectric sandwiched between two conducting sheets that act as electrodes. The

construction of ACCs requires in general a combination of the bottom-up molecular self-assembly, the mechanical stacking and the top-down lithographic approaches. CNMs were formed from SAMs of phenylthiol homologues, i.e., biphenylthiol (BPT), p-terphenylthiol (TPT), and p-quaterphenylthiol (QPT). From each molecule, six-layer CNMs were used as the dielectric layer. Trilayer graphene made by chemical vapor deposition and mechanical stacking was used as top electrodes. In addition, carbon sheets composed of crosslinked aromatic molecules were pyrolyzed, which is called pyrolyzed graphitic carbon (PGC), and were also employed as top electrodes for ACCs. All in all, CNMs, graphene and PGC are pure carbon, so the ACCs contain no other chemical elements.

RESULTS AND DISCUSSION Fabrication of All-Carbon Capacitors. A scheme of the fabrication process of an ACC composed of graphene and CNM layers is illustrated in Figure 1A. First, monolayers of graphene were grown by low-pressure CVD of methane on copper foils.42 A direct layer-by-layer transfer method of graphene sheet that is free from any organic residue between the layers was used here.43 After the three-layer graphene stack had been transferred onto a SiO2/Si substrate, the patterning of bottom graphene electrodes was conducted by using electron-beam lithography and reactive ion etching, which is followed by the deposition of metal contact pads, as demonstrated in Figure 1B. Besides graphene electrodes, pyrolyzed graphitic carbon (PGC) electrodes with a thickness of 9 ± 1 nm were also employed as bottom electrodes in this study (Figure S1). In the next step, multilayer CNMs were transferred onto the bottom electrodes. Water was found to be trapped between the substrate and the CNM, which causes defects in the dielectric (Figure S2). Since CNMs exhibit extremely high thermal stability and the structural transformation does not occur below 450 °C,25 the trapped water can be removed by annealing the sample in a vacuum oven at 360 °C for 2 h (Figure S3). After the water has been removed, the multilayer CNM stack was patterned into small squares B


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Figure 2. (A) Capacitor consisting of six-layer-CNM dielectric and trilayer graphene electrodes imaged by helium ion microscopy. Scale bar: 10 μm. (B) Raman spectrum of the trilayer graphene electrode on a SiO2/Si substrate. (C) Impedance spectra of a graphene/CNM/ graphene capacitor with an area of 33.9 μm2. Black squares denote the impedance magnitude |Z| and the blue circles the phase angle. The red curve is the spectrum fitted with the inserted equivalent circuit. (D) The optical image of a capacitor consisting of six-layer-CNM dielectric and PGC electrodes. (E) Raman spectra obtained at four different spots marked in Figure 2D. (F) Impedance spectra of a PGC/ CNM/PGC capacitor with an area of 100 μm2.

with a dimension of 82 × 82 μm2, as shown in Figure 1C (also see Figure S4). To fabricate top electrodes for ACCs, a trilayer graphene or a PGC sheet was first transferred on a Si3N4/Si substrate and patterned into parallel ribbons with widths varying from 1 to 50 μm (Figure S5 and S6). The conductive ribbons were then released from the Si3N4/Si substrate and transferred onto the substrate with bottom electrodes and dielectric layers, as shown in Figure 1D. Afterward, Ti/Au top contact pads were deposited, and a final electron beam patterning step was performed to remove all unwanted top electrode materials that electrically connect individual capacitors. Figure 1E shows an overview of arrays of capacitors on a Si chip (the upper image) and a selected capacitor (the lower one). It is possible to identify graphene and CNMs by optical microscopy because the monochromatic contrast increases linearly with thickness for multilayer graphene and CNMs on SiO2/Si substrates.29,44 Each chip contains more than 500 electrically isolated capacitors: the dielectric thickness is defined by the number of layers and the capacitor area by the area of bottom and top electrodes that enclose the CNM dielectric. Helium Ion Microscopy and Raman Spectroscopy of ACCs. Graphene and CNM look quite similar when imaged by an optical microscope. However, their difference in conductivity allows them to be much clearer distinguished by a scanning helium ion microscope (HIM). HIM utilizes a focused beam of He+ ions to scan the sample surface and the intensity of the emitted secondary electrons is used to generate an image. Due to the emission of secondary electrons and the exposure to He+ ions, the imaging process leads to positive charging of an insulating sheet. Thus, the insulating sheet appears darker compared to a conducting sheet.45,46 Figure 2A shows a capacitor composed of trilayer graphene electrodes and a 6-layer TPT−CNM dielectric, where both conducting

graphene electrodes appear bright but the insulating CNM appears dark. Raman spectra were measured on trilayer CVD graphene on SiO2/Si substrates. Figure 2B shows two dominant peaks: the G band at ∼1586 cm−1 and the 2D band at ∼2720 cm−1. The G band is due to the doubly degenerate zone center E2g mode,47 whereas the 2D band is the second order of zoneboundary phonons.48 In addition, a weak band at 2445 cm−1 can be assigned to G* modes, which can be explained by the double resonance Raman model with an intervalley process but involving one in-plane transverse optical phonon and one longitudinal acoustic phonon.49 A detailed analysis of Raman spectra for CNM/graphene heterostructure has been reported.39 As a comparison, Figure 2D shows an optical image of a capacitor composed of PGC electrodes and a sixlayer TPT−CNM dielectric, where Raman spectra were obtained on four different spots of the capacitor. In Figure 2E, CNMs exhibit almost no Raman-scattering peaks (spectrum 1), indicating a lack of structural order.21 The Raman spectrum of PGC shows two peaks, i.e., the D band at ∼1350 cm−1 and the G band at ∼1600 cm−1. The D band is due to the breathing modes of the phenyl rings, which arises from the preserved sp2 hybridized carbon.50 Raman spectra measured both on the heterostructure CNM/PGC (spectrum 2) and PGC/CNM (spectrum 3) exhibit characteristics identical to the PGC/CNM/PGC region (spectrum 4). Impedance Spectroscopy of ACCs. To determine the dielectric characteristics of ACCs, impedance measurements were performed on intact capacitors. Figure 2C displays the impedance spectrum (Bode representation) of a graphene/6TPT/graphene capacitor. The impedance spectra were fitted with the EIS (electrochemical impedance spectroscopy) spectrum analyzer,51 where a simple three-element equivalent C


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Figure 3. (A) Obtained capacitance of 12 capacitors versus the capacitor area A, where graphene electrodes are used. (B) Obtained capacitance of 15 capacitors as a function of the capacitor area A, where PGC electrodes are used. (C) Plot of area/capacitance as a function of the separation d: its axis intercept gives rise to the interfacial capacitance and its slope the dielectric constant. (D) Leakage current density as a function of the capacitor area of two types of capacitors.

circuits and prevent the proper functioning of a capacitor.54 On the other hand, PGC electrodes do not show these impurities. The maximum active area achieved in the current work is only determined by the initial design. It is not a fundamental limit of the materials and capacitors with larger activate area up to cm2 are technically feasible. The dielectric constant εr can be calculated using a simple equation: Cmeas = ε0εr(A/d), where Cmeas is the measured capacitance, A is the capacitor area, and d is the separation. Two different values, i.e., 2.9 ± 0.1 for graphene/CNM/ graphene capacitors and 3.6 ± 0.1 for the PGC/CNM/PGC capacitors, are derived from the two plots in Figure 3A,B. This apparent difference can be due to a reduction of the capacitance because of intrinsic or extrinsic interfacial capacitances.55,56 It was found that quantum capacitance of graphene electrodes could contribute to the interfacial capacitance.57 Assuming that the interfacial capacitance of graphene/CNM gives rise to additional voltage drops at two interfaces, the apparent capacitance is given by

circuit (the inset of Figure 2C) was adopted here. The total impedance can be expressed as Z(ω) = R S + (RP−1 + (iωC P)−1)−1

(1)

where the series resistor RS arises from the bottom and top graphene ribbons between the capacitor part and the metallic pads, and the capacitor CP in parallel with the resistor RP models the ACC. From the fitted spectrum (red curve) a capacitance of 85 fF was obtained. As a comparison, Figure 2F shows the impedance spectrum of a PGC/6-TPT/PGC capacitor, from which a capacitance of 0.29 pF was obtained. Notice that phase angles of the graphene/6-TPT/graphene capacitor were nearly constant until 2.5 MHz and then dropped. In contrast, phase angles of the PGC/6-TPT/PGC capacitor first increased until 2 MHz and then dropped gradually. The sheet resistance of our trilayer graphene electrodes is determined to be 691 Ω/sq (Figure S10), which is in good agreement with the value of 2100 Ω/sq for a monolayer graphene and of 350 Ω/sq for four-layer graphene reported elsewhere.52 The sheet resistance of PGC electrodes is 10.6 kΩ/sq and the corresponding electrical resistivity is 9 mΩ·cm, which is comparable to the pyrolyzed photoresist films and glassy carbon.35,53 The different sheet conductivity of two types of electrodes could partly explain different phase angles of ACCs at higher frequencies. Dielectric Constant of CNMs. Parts A and B of Figure 3 show plots of the capacitance of all-carbon capacitors as a function of the capacitor area. With graphene electrodes, the maximum area of working capacitors containing six-layer-TPT CNMs was only 52 μm2. With PGC electrodes, operating nanocapacitors with active areas up to 1200 μm2 could be fabricated. This difference can be explained by the fact that CVD graphene suffers from metal impurities that lead to short

A A A 1 1 = + = + d Cmeas Cinter CCNM cinter ε0εr

(2)

where Cinter = Cinter/A is the interfacial capacitance, Cinter is the capacitance associated with the interfaces, and CCNM the capacitance only associated with the CNM dielectric. Figure 3C shows that the reciprocal areal capacitance A/Cmeas has a linear dependence on the electrode separation d with a nonzero y intercept, from which an interfacial capacitance of 1.48 μF/cm2 is obtained. This value is in agreement with the value of 1.3 μF/cm2 calculated from the electronic density functional theory for graphene.58 Moreover, we obtained an intrinsic dielectric constant of 3.5 for the CNM dielectric. This value is in good agreement with the obtained value for capacitors using D


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Figure 4. (A) Phase angle of two types of nanocapacitors as a function of bias voltage. (B) Dispersive behavior of a graphene/CNM/ graphene capacitor shown in the plot of the normalized capacitance as a function of DC bias voltages. (C) High DC bias voltage cause large fluctuations in the impedance and lead to the failure of two types of capacitors. (D) Breakdown of a graphene/CNM/graphene capacitor occurs at −4.8 V, after which a small residual barrier can be seen at 0 V.

graphene/6-TPT/graphene until a bias voltage of ±1 V and (2) the phase angles fluctuate between −85° and −75° for the PGC/6-TPT/PGC capacitor until a bias voltage of ±3.5 V. However, some capacitors exhibit asymmetric characteristics when a sweeping from negative to positive DC voltages was performed, i.e., the highest impedance magnitude was not located at 0 V, but in the range of 10−100 mV (Figure S12). This asymmetry could be accounted for by asymmetric configurations of the device and thus the charge impurities of graphene are screened differently for the bottom electrode (CNM/bottom-graphene/SiO2) and the top electrode (air/ top-graphene/CNM). To quantify the dispersive behavior of ACCs, the bias voltage dependent on capacitance was measured and the voltage coefficients of capacitance (VCCs) were determined by using the following equation

PGC electrodes. As a comparison: (1) dielectric constant of aliphatic SAMs ranges from 2.0 to 3.4,12−14,59 (2) dielectric constants of 2.88 and 2.98 were obtained for biphenyl and pterphenyl crystals using microwave cavity perturbation method,60 (3) a dielectric constant of 3.9 was predicted for the TPT−SAM using first-principles density functional theory (DFT) calculations.61 Figure 3D shows the leakage current density determined from the obtained resistance Rp at a bias voltage of 1 V. In general, the leakage current densities range from 0.5 to 5 mA/ cm2 for graphene/CNM/graphene nanocapacitors and from 2.5 to 20 mA/cm2 for PGC/CNM/PGC nanocapacitors. Some capacitors exhibit much higher leakage current densities, which is mainly attributed to the presence of defects. Energy Density, Power Density, and Voltage Coefficient of ACCs. We obtained an average capacitance density of 0.30 ± 0.06 μF/cm2 for all measured nanocapacitors. To calculate the energy density, we treat the ACC as a parallel plate capacitor and only consider the weight of the dielectric and both electrodes having the same areal dimension (see section 5 in the SI). The energy density and power density are 0.029 W·h/kg (0.19 J/cm3) and 6.4 × 105 W/kg, respectively, for a 26 μm2 graphene/6-TPT/graphene capacitor at a working voltage of 1.0 V. The two values fall within the range associated with conventional dielectric capacitors.62 In contrast, the energy density and power density are 0.135 W·h/ kg (0.66 J/cm3) and 1.4 × 107 W/kg, respectively, for a 100 μm2 PGC/6-TPT/PGC capacitor at a working voltage of 3.5 V. Figure S11 shows a Ragone plot with the values for both ACCs.62 The difference between two types of ACCs is mainly due to a higher working voltage of PGC/CNM/PGC capacitors compared to graphene/CNM/graphene capacitors, despite that the former possess thicker electrodes. Figure 4A compares the DC bias voltage effect on two types of ACCs: (1) the phase angles remain constant at −90° for the

C(V ) = C0(αV 2 + βV + 1)

(3)

where C0 is the capacitance at zero bias and α and β are the quadratic and linear coefficients of the capacitor. Figure 4B shows the normalized capacitance of a graphene/6-TPT/ graphene capacitor as a function of the DC bias voltages. We extracted values of 0.011 V−2 for α and 8.6 × 10−4 V−1 for β, respectively. The obtained α is about twice of HfO2-based MIM capacitors and 7 times higher than Al2O3-based MIM capacitors.63−65 To further compare the DC bias voltage effect on the performance of two types of ACCs, Figure 4C presents the variation of impedance magnitude as a result of the applied DC bias. It can be seen that the impedance of the graphene/ CNM/graphene capacitor changes dramatically at a bias voltage of 3.6 V, which leads to a sudden failure of the CNM dielectric. However, the impedance of the PGC/CNM/ PGC capacitor shows a remarkable fluctuation at 4.5 V, which leads to a sudden failure of the PGC electrode. For both types E


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equivalent circuit. A dielectric constant of 3.5 and an average capacitance density of 0.3 μF/cm2 were determined for carbon nanomembranes using both types of electrodes. Due to the dispersive effect, nanocapacitors show a certain degree of degradation in capacitor linearity. We also analyzed different paths that lead to dielectric failure of nanocapacitors. A dielectric breakdown voltage of 4.6 V was determined for the graphene/CNM/graphene capacitors. To fabricate nanocapacitors with higher energy densities, further experiments will be carried out by utilizing precursor molecules with higher polarizabilities and by modifying the interfaces either with organic complexes or with metal oxide nanoparticles. The ACC fabrication is scalable and allows the creation of functional hybrids connected to other materials to achieve lightweight and flexible carbon-based organic capacitors with high energy density and high performance.

of capacitors failure mode is associated with pronounced hysteresis loops (see Figures S13 and S14), which may arise from the local rearrangement of the charge distribution at interfaces and/or electrodes. Dielectric Breakdown of ACCs. The dielectric breakdown of ACCs was investigated by applying a variable DC bias voltage superimposed by an AC signal of 50 mV at 100 kHz and monitor a sudden change in the impedance magnitude and its phase angle. The obtained signal is actually a measure of the first derivative of the I−V curve. It was found that PGC/ CNM/PGC capacitors could withstand DC voltages up to 7 V, above which the capacitor becomes an open device. Having inspected some failed capacitors, we found that bottom PGC electrodes become insulating, which is associated with a color change (Figure S15). For graphene/CNM/graphene capacitors the breakdown occurs at the dielectric part and the corresponding breakdown voltage was determined to be 4.6 ± 0.2 V. A breakdown voltage of 0.68 V was reported for the HgBPT-SAM/BPT-SAM-Ag molecular junctions.66 This value indicates a good agreement with our measurements, as there are 4 phenyl rings and 1 interface across a Hg-BPT-SAM/BPTSAM-Ag junction and 24 phenyl rings and 7 interfaces across the graphene/6-layer-CNM/graphene capacitor, leading to a 7 times higher breakdown voltage. Moreover, our method allows us to gain insights into two possible breakdown processes: (1) the capacitor is completely shorted and then its frequency response resembles that of graphene electrodes; (2) the capacitor is not completely shorted, where a small residual barrier obtained at 0 V shows an exponential decay with increasing the maximum bias voltages until its complete disappearance, as shown in Figure 4D (also Figure S16). If we assume that the full bias voltage drop across the capacitor, the breakdown voltage corresponds to a dielectric strength of 4.1 MV/cm. However, if the interfacial capacitance is taken into account, the voltage drop across the CNM dielectric is estimated to be 3.6 V and thus an effective dielectric strength of 3.2 MV/cm is obtained. A comparison of both dielectric constant and dielectric strength of CNMs with other known dielectric materials was given in Table S3. Notice that the dielectric strength determined here is not only associated with the intrinsic strength of chemical bonding in the CNM, but also with the CNM−graphene interfaces and the topographic imperfections of electrodes. Local field enhancement at metallic and organic impurities, particularly on graphene electrodes, could initiate dielectric damages around irregularities and eventually lead to conduction paths through the CNM. The propagation of these conduction paths can be further associated with local decomposition of CNMs by joule heating, polarization, collision ionization, and electromigration of residual metal ions on graphene.

MATERIALS AND METHODS Synthesis of CVD Graphene and Fabrication of Trilayer Graphene. The single-layer graphene (SLG) sheets were grown on 25 μm thick Cu-foils (Alfa Aesar, 99.8%) cut into 2 × 2 cm2 pieces in a tube furnace (Gero F40-200) by low pressure chemical vapor deposition of methane (purity 4.5). The quartz glass tube of the furnace was loaded with copper foils, evacuated to 1 × 10−3 mbar with a rotary pump (Edwards RV5) and filled with hydrogen (purity 5.3, ∼1 mbar) under 50 sccm flow and heated to 1015 °C with a rate of 150 °C/h. The crystallinity of the copper foils had been improved by annealing for 30 min at 1015 °C and then the hydrogen flow was reduced to 10 and 70 sccm of methane were introduced at an overall pressure of ∼2 mbar for 15 min. Then the furnace was cooled with a rate of ∼250 °C/min to ∼250 °C and the gas flow was stopped. For the purpose of fabricating the trilayer graphene, the poly(methyl methacrylate) (PMMA) was spin-coated only on the first layer graphene and transferred onto the second layer graphene. After the copper foil below the second layer was dissolved, the bilayer stack was transferred onto a third graphene on copper foil. The stack of trilayer graphene was transferred onto a SiO2/Si substrate to form the bottom electrode of the nanocapacitor and onto a Si3N4/Si substrate to form the top electrode of the nanocapacitor. Preparation of Carbon Nanomembranes from Aromatic SAMs. Precursor molecules used in this study were bought from Sigma-Aldrich (biphenylthiol) or specially synthesized (p-terphenylthiol and p-quaterphenylthiol). For the preparation of biphenylthiol (BPT) SAMs, we used a 300 nm thermally evaporated Au layer on mica substrates (Georg Albert PVD-Coatings, Germany). The substrate was cleaned with a UV/ozone cleaner (UVOH 150 LAB FHR) for 5 min, rinsed with ethanol, and then blown dry under a nitrogen stream. Afterward, the substrates were immersed into a ∼10 mM solution of BPT in dry and degassed dimethylformamide (DMF) for 72 h in a sealed flask under nitrogen atmosphere at room temperature. For the preparation of p-terphenylthiol and pquaterphenylthiol SAMs, the same procedure was applied with the exception of SAM formation for 24 h in a sealed flask under nitrogen atmosphere at 70 °C. Cross-linking of SAMs was achieved in high vacuum (5 × 10−8 mbar) with an electron flood gun at an electron energy of 100 eV and with an electron dose of 50 mC/cm2 being applied. Electron-Beam Lithography and Reactive Ion Etching Processes. The patterning of trilayer graphene and multilayer CNM stack on SiO2/Si substrates and the deposition metallic contacts includes three lithographic steps. Step 1: spin-coating of a resist layer (PMMA) on the surface; pattern transfer into the layer by electron beam lithography (EBL) (Vistec EBPG 5000+) and resist development; reactive ion etching in an oxygen/argon plasma (Leybold Z401) of the nonprotected areas of graphene sheets; dissolution of PMMA in acetone. This step results in the patterning of the graphene electrodes or the multilayer CNM. Step 2: spin-coating of a new PMMA layer; electron beam lithography and development of

CONCLUSIONS To summarize, we demonstrated the successful fabrication of all-carbon nanocapacitors where the dielectric is made of carbon nanomembranes, and the electrodes are either made of CVD graphene or of pyrolyzed graphitic carbon from crosslinked aromatic molecules. The construction of ACCs requires a combination of the bottom-up molecular self-assembly, the mechanical stacking and the top-down lithographic approaches. Working capacitors with a capacitor area up to 1200 μm2 were demonstrated in this study. The frequency response of capacitors has been measured with an LCR meter and the impedance spectra were analyzed with a simple F


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ACS Nano the resist layer; subsequent vacuum evaporation (10−6 mbar) of a 10 nm adhesive Ti layer and a 40 nm Au layer; lift-off of the resist. Step 3: The quality of the metal/graphene contacts could be improved significantly if an additional 100 nm thick Au layer, having a direct contact to the device bars, was evaporated. This additional gold layer was fabricated by a similar procedure as described in step 2. Impedance Spectroscopy. The impedance measurements were conducted using an LCR meter (Agilent 4285A) at room temperature. The LCR meter has a frequency range of 75 kHz to 30 MHz, with a DC bias voltage up to 40 V or a DC bias current up to 10 mA. It employs the four-terminal pair configuration, and the measurement contacts were made using two Kelvin probes (two 5 μm radius tips separated by 25 μm) mounted on a probe holder (DCP-100, Cascade Microtech). Two BNC to SSMC cables with a length of 1 m were used to connect the Kelvin Probes and the LCR meter. A switch box allows the switching between the LCR meter and resistivity measurements for optimal contacts. The samples were mounted on a chip holder and fixed by an L-shaped PLEXIGLAS with three screws. The Si substrate was connected with the guards of the prober fixtures. Figure S8 shows detailed pictures of the experimental setup and the chip holder. Data acquisition was done by using a graphical user interface based on LabVIEW. Open/short/load compensations have been carried out for further data processing shown in Figure S9. Raman Spectroscopy. Raman spectra were acquired using a micro Raman spectrometer (LabRAM ARAMIS) operated in the backscattering mode. Measurements at 532 nm were obtained with a frequency-doubled Nd:YAG-Laser, a 50× long working distance objective (NA 0.50) and a thermoelectrically cooled CCD detector (2−3 cm−1 spectral resolution). During this measurement, the laser power and exposure time were kept very low to avoid local heating of the sample. Helium Ion Microscopy. A Carl Zeiss Orion Plus microscope was utilized to conduct the measurements. The helium ion beam was operated at an acceleration voltage of 33−38 kV with a beam current of 0.2−1.4 pA. A 10 μm aperture was used for all measurements. For the detection of secondary electrons an Everhart−Thornley detector with a grid voltage of 500 V was employed. The working distance ranged from 10 to 29 mm.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b05490. Additional information on materials and fabrication; experimental setup and calibrations; sheet resistances of electrodes; DC bias voltage effect; dielectric breakdown and a summary of capacitor specifications (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xianghui Zhang: 0000-0002-5544-5221 André Beyer: 0000-0002-9569-0344 Armin Gölzhäuser: 0000-0002-0838-9028 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the State of North Rhine-Westphalia and the Deutsche Forschungsgemeinschaft (SPP 1928) is acknowledged. We thank J. Christoffers for providing us with QPT molecules and N. Meyerbroeker for PGC sheets. We also thank D. Emmrich for HIM measurements and B. Völkel for technical support. G


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ACS Nano

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Large-Area All-Carbon Nanocapacitors from Graphene and Carbon

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