Electrode-Free Anodic Oxidation Nanolithography of Low

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Electrode-free anodic oxidation nanolithography of low-dimensional materials Hongyuan Li, Zhe Ying, Bosai Lyu, Aolin Deng, Lele Wang, Takashi Taniguchi, Kenji Watanabe, and Zhiwen Shi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04166 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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Electrode-free anodic oxidation nanolithography of low-dimensional materials Hongyuan Li1, 2, Zhe Ying1, 2, Bosai Lyu1, 2, Aolin Deng1, 2, Lele Wang1, 2, Takashi Taniguchi3, Kenji Watanabe3, Zhiwen Shi1, 2* 1Key

Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China. 2Collaborative

3National

Innovation Center of Advanced Microstructures, Nanjing, China.

Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan.

ACS Paragon Plus Environment

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KEYWORDS: Scanning probe lithography, electrode-free local anodic oxidation, highfrequency AC voltage, graphene, low-dimensional materials,

ABSTRACT:

Scanning probe lithography based on local anodic oxidation (LAO) provides a robust and general nanolithography tool for a wide range of applications. Its practical use, however, has been strongly hampered due to the requirement of a pre-fabricated micro-electrode to conduct the driving electrical current. Here we report a novel electrode-free LAO technique, which enables in-situ patterning of as-prepared low-dimensional materials and heterostructures with great flexibility and high precision. Unlike conventional LAO driven by a DC current, the electrode-free LAO is driven by a high-frequency (>10 kHz) AC current applied through capacitive coupling, which eliminates the need of a contacting electrode and can be used even for tailoring insulating materials. Using this technique, we demonstrated flexible nanolithography of graphene, hexagonal boron nitride (hBN), and carbon nanotubes (CNTs) on insulating substrates with ~10-nanometer precision. In addition, the electrode-free LAO exhibits high etching quality without oxide residues left. Such an in-situ and electrode-free nanolithography with high etching quality opens up new opportunities for fabricating ultraclean nanoscale devices and heterostructures with great flexibility.


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TEXT: Nanolithography is widely used in fabricating functional devices in nanoscience and nanotechnology1-10. Compared with conventional optical/E-beam lithography with multiple steps, scanning probe lithography (SPL) based on various chemical/physical mechanisms provides a simpler and more flexible way2-7. Local anodic oxidation (LAO) lithography has been one of the most robust and versatile SPL methods10-17. Conventional LAO relies on a spatially confined electrochemical reaction driven by a DC voltage applied between the tip (cathode) and the sample (anode). The sharp tip apex can cause a localized strong electric field (>107 V/m) in the tip-sample gap, which has two main functions18. First, it can attract polar H2O molecules from air and form a nanometer-sized water bridge connecting the tip and sample surface19, 20. Second, the strong electric field can help generate ions (e.g. H+, OH- and O2-) by decomposing water molecules and drive the oxygen-containing radicals (e.g. OH- and O2-) to the sample surface to achieve oxidation13,

21.

Conventional LAO has been demonstrated on various

conductive materials, including Si22, graphene13,

14, 17, 23, 24,

transition metal dichalcogenides

(TMDs)25 and carbon nanotubes (CNTs)26. However, the application of conventional LAO is seriously limited due to its complex pretreatment, low etching quality, and selection of conductive samples. Especially, to apply a DC voltage to a small sample of micrometer size, one needs to first fabricate a micro-electrode connected to the sample using other nanolithography techniques, such as E-beam lithography and UV lithography.

In addition, patterning via

conventional LAO must follow a strict order to ensure the sample is not disconnected from the electrode, which also limits its application. Here, we report an electrode-free LAO (EFLAO) technique, which can realize high-quality nanolithography for low dimensional materials and heterostructures on insulating substrates


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without micro-electrode connection. Unlike conventional LAO driven by a DC voltage, the new EFLAO is instead driven by a pure AC voltage. With a specially selected substrate conducting high frequency AC current, we are able to perform LAO without electrodes connected to the sample. In this letter, we first introduce how the EFLAO works through patterning a graphene sample. Then we demonstrate the high quality of the fabricated graphene nanostructures. We further show that this technique is also suitable for patterning other low-dimensional materials and heterostructures, such as graphene/hexagonal boron nitride (hBN) heterostructure and CNTs. A simple model is proposed to illustrate the mechanism of EFLAO, and some key factors that impact the etching quality are investigated. We first introduce how the EFLAO works through fabricating a monolayer graphene nanoribbon (GNR) array on a SiO2/Si substrate, the schematic of which is shown in Fig. 1A. The experiment was performed on a standard atomic force microscope (AFM) platform in atmosphere. A high frequency (40 kHz) AC voltage was applied between a gold-coated AFM tip and the conductive Si layer of the substrate. Then the tip slowly approached the sample and then scanned along a designed path to achieve a controlled nano-etching. More experimental details are included in the methods section. Fig. 1B shows the topography of a GNR array fabricated by EFLAO, which displays high uniformity and reproducibility (corresponding optical image is shown in Fig. S1). Note that no micro-electrode is connected to the sample here. The highfrequency AC current can penetrate the SiO2 layer through a capacitive coupling effect, which will be discussed later.


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Figure 1. Illustration of electrode-free local-anodic oxidization (EFLAO). (A) Schematic of the EFLAO. The electrochemical reaction is driven by an AC voltage applied through the SiO2/Si substrate without any electrode directly connected to the sample. The electrochemical reaction is localized within the nano-sized water bridge. (B) An array of graphene nano-ribbons fabricated using EFLAO. The inset shows the zoom-in of the white dash box. RH = 60%; f = 40 kHz.

We then demonstrate the high quality of the graphene nanostructures fabricated by EFLAO. To directly compare the etched edge with an exfoliated natural edge, a tapered graphene ribbon with both types of edges were fabricated. As shown in Fig. 2A, little distinction in topography can be observed between the two edges. Moreover, the corresponding infrared scanning near-field optical microscopic (IR-SNOM, see Supporting Information for more details) image in Fig. 2B displays almost identical fringes of surface plasmon polariton (SPP) near both edges. Line profiles of SPP (Fig. 2C) across the two edges show a symmetric feature, indicating that the etched edge owns a high quality comparable to the natural edge. Otherwise, if there exists graphene oxide area near the etched edge, the SPP reflection would happen at the grapheneoxide /graphene interface, and the SPP interference pattern will shift and show asymmetry. In fact, graphene edges etched via conventional DC-LAO usually contain oxide residues13-15,

27,


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which show either irregular fringes or simply no IR response (See Supporting Information). Fig. 2D shows the topography of a uniform array of 20nm-wide GNRs, and the inset shows that those GNRs own very smooth edges. Fig. 2E displays an ultra-thin graphene nanoribbon with width ~10 nm and edge roughness less than 2 nm, which has already reached the resolution limit of our AFM. Owing to its high resolution and etching quality, the new technique is suitable for fabricating high-quality nano-devices, such as plasmonic waveguides, Hall bar structures. Furthermore, we show that 1D carbon nanotube and 2D heterostructures can also be etched using EFLAO. With a 10 kHz AC driving voltage applied between the tip and the Si layer, one can directly cut off a nanotube on hBN/SiO2/Si substrate. The obtained segments and the original complete nanotube are displayed in Fig. 2F. With this technique, one can achieve nanotube with desired lengths, which could be useful for fabricating nanotube-based electronic and photonic devices. Next, we show that heterostructures consist of two different materials can also be tailored by EFLAO. We successfully etched an array of trenches on an hBN/graphene heterostructure consisting of a 10-nm-thick hBN layer on top and a monolayer graphene layer on bottom, as shown in Fig. 2G. The etching depth is measured to be 10 nm, revealing that the heterostructure is fully etched through. An additional example of etching hBN can be found in Supporting Information. Note that hBN is an insulator with bandgap of ~ 6 eV, which cannot be etched with conventional DC-LAO. However, a high-frequency AC current is able to penetrate the hBN flake and induce a novel anodic etching effect. With the assistance of an underlying graphene layer, a voltage drop can be formed between the tip and graphene, and hence a sufficiently strong electric field is able to be applied to the hBN/water interface and enable the anodic etching of hBN. The oxidation products of hBN could be NxOy and B2O3. The former as gas can escape easily, while the latter can be dissolved in water and form H3BO3, which can


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further vaporize28. The capability of the EFLAO to pattern 2D heterostructures could largely simplify the procedure of fabricating devices based on 2D heterostructures.

Figure 2. Demonstration of the high etching quality and capability of EFLAO. Topography (A) and IR-SNOM image (B) of a tapered graphene ribbon with one etched edge and one natural edge. (C) Line profiles of plasmons at the line cuts in (B). All the plasmon profiles show highly symmetric feature, indicating that the sample quality near the etched edge is as good as that near the natural edge. (D) Topography of a uniform array of ultra-thin GNR with width~20nm. The inset shows the GNRs owns quite smooth edges. (E) Topography of an ultra-thin GNR with width~10nm and edge roughness of ~2nm. (F) Topography of nanotube segments cut by FELAO. Cutting positions are labeled by white dash arrows. The inset shows the original complete nanotube. RH = 40%; f = 10 kHz. (G) Topography of an array of hBN/graphene nano-trenches fabricated by EFLAO, the depth of which is ~10nm. The inset shows a schematic of the original hBN/graphene heterostructure on SiO2 substrate. RH = 45%; f = 140 kHz.


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To confirm that the etching is indeed due to anodic oxidation, we systematically examined the impact of humidity on the etching effect. Conventional DC-LAO features a strong dependence on relative humidity (RH)10-12, since RH will affect formation of the water bridge20. Similar RHdependence is observed in our EFLAO experiment. More than 90 etching lines were performed on a large graphene flake at different humidity. Etching results at RH = 10%, 39%, 60%, and 85% are shown in Fig. 3A. The etching failed completely at RH = 10% and worked well at RH = 60%, 85%. The statistics of the success rates, defined as the ratio of the successfully etched length to the total scanned length, is shown in Fig. 3B, which starts from zero for low RH < 25%, and reaches about 100% for RH > 45%. The observed RH-dependence confirms unambiguously that the etching effect results from anodic oxidation. It is noteworthy that a pure AC voltage can also induce an anodic oxidation. We first point out that the period of the AC voltage applied (~10 μs) is far longer than the time scale of the water bridge formation (~100 ps)19. Therefore, the AC voltage-driven electrochemical reactions can be regarded as a quasistatic process, and be simply divided into two stages within each AC period. In stage 1, the graphene is positively charged (anode). Anodic oxidation of graphene will start when the voltage exceeds a threshold value as that in conventional DC-LAO. This stage is similar to conventional DC-LAO. Sufficient oxidation of graphene will generate gaseous product CO or CO2 and lead to the etching effect. In stage 2, the graphene is negatively charged (cathode) and the tip is positively charged (anode). The tip is protected by a layer of gold from been oxidized, and the graphene may be hydrogenated by reactive hydrogen atoms or positive hydrogen ions13. However, the hydrogenation is weak and could be cancelled in the stage 1, so


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that the impact of this stage is negligible. Therefore, the net effect induced by a pure AC voltage on the graphene is anodic oxidation. A significant advantage of the AC-driven EFLAO is its simplicity with no requirement of prefabricated micro-electrodes, as AC current can flow through dielectric substrates in the form of displacement current. We now introduce how the AC current is conducted in the EFLAO in details. The structure of graphene/SiO2/Si multilayer shown in Fig. 3C can be regarded as a capacitor with impedance of 1/𝑗2𝜋𝑓𝐶, where j is the imaginary unit, C the capacitance and f the frequency. The nano-sized water bridge can be regarded as a resistor R. The effect of tipgraphene capacitive impedance can be neglected here, because it is in parallel with the water bridge resistor and its value is about two orders of magnitude larger than the water bridge resistance at a typical driving frequency of 10 kHz (see Supporting Information for more details). The water bridge resistance R and the graphene/SiO2/Si capacitive impedance 1/𝑗2𝜋𝑓𝐶 are in series. When an AC voltage U is applied between the Si layer and the tip, the voltage drop across the water bridge UR, which is also the voltage between the AFM tip and graphene sample, can be easily calculated as 𝑈𝑅 =

R ⋅𝑈 R + 1/𝑗2𝜋𝑓𝐶

(1)

UR will monotonically increase with f, and trigger the electrochemical reaction when it reaches a threshold value. More quantitatively, we provide an estimation on the magnitudes of 1/𝑗2𝜋𝑓𝐶 and R. For a 10 μm2 graphene flake on a 300 nm-thick SiO2 layer, 1/𝑗2𝜋𝑓𝐶 is in the order of 1010 Ω at f = 10 kHz. The DC resistance of water bridge R is measured to be ~1010 Ω when performing EFLAO etching (see Supporting Information for more details). Theoretically calculated value for R using the ideal resistivity of deionized water is ~1011 Ω. The comparison of various impedances at f = 10 kHz is shown in Fig. 3D. It can be found that R is roughly


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comparable to 1/𝑗2𝜋𝑓𝐶 at f = 10 kHz, which means the applied voltage can efficiently acts on the water bridge and enable the electrochemical reaction at such a high frequency.

Figure 3. The principle of EFLAO. (A) AFM images of typical etching results obtained at different relative humidity (RH) of air. f = 40 kHz. Scale bar: 400nm. (B) Statistics for the success rate at different RH values. The success rate is close to unity when RH is higher than 50%. Pink dash curve shows a guiding line. The strong RH-dependence of etching success rate confirms unambiguously that the etching is due to anodic oxidation (C) Schematic of the equivalent electrical circuit for EFLAO. The graphene/SiO2/Si multilayer can be regarded as a


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capacitor. (D) Impedances of the G/SiO2/Si capacitor, water bridge resistor and direct tipgraphene (Gr.) contact. For water bridge, both experimental and theoretical values are displayed. (E) AFM images of typical EFLAO etching results obtained at different driving frequencies. RH = 57~60%. Scale bar: 200nm. (F) Statistics for the success rate and the amount of residues left.

The dependence of EFLAO on the driving frequency was investigated, with the results displayed in Fig. 3E and 3F. At f = 0 Hz, all etching lines are completely failed; at f = 625 Hz, all the etching lines are terminated near the starting points; at f = 2.5 kHz, the etching is overall successful but yields many oxide residues; at f = 20 kHz, all the etching lines get success and display smooth edges without residues. The statistics of the success rate and the amount of residues (see the exact definition in Supporting Information) shown in Fig. 3F reveal that a 10 kHz frequency and above can yield ideal etching results with 100% success rate and few residues. The frequency-dependent success rate is attributed to frequency-induced change of potential drop 𝑈𝑅 across the water bridge, which agrees well with our capacitor-resistor model. The frequency-dependent generation of oxide residues is more complicated and will be discussed later in details. It is noteworthy that during the EFLAO etching the AFM tip never directly contacts the graphene (as illustrated in Fig. 3C. See experimental evidence in Supporting Information), otherwise the EFLAO will be failed because the tip-graphene contact resistance (~104 Ω) is too small to obtain sufficient driving voltage. The use of high-frequency AC voltage leads to high etching quality with no oxide residues, which is another merit of this technique. As shown in Fig. 2, Fig. 3E and 3F, the EFLAO etching with a sufficiently high driving frequency (> 10 kHz) can result in smooth edges free of oxide residues. One should not simply attribute such a result to a pure frequency effect, because the driving voltage UR is coupled with the frequency f according to Eq. (1). In order to distinguish


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the roles of f and UR, we performed AC-LAO etching on a micro-electrode connected graphene sample, which enables independently tuning of frequency f and voltage drop UR. With an increasing f and a fixed UR at 10V, the amount of residues decreases gradually while the success rate keeps at 100% (Fig. 4A and 4B). The increased frequency also leads to a narrower etching trench. With an increasing UR and a fixed f at 40 kHz, only the success rate increases while all line-etchings are free of residues (Fig. 4C and 4D). The two groups of controlled experiments show clearly that the amount of residues is only related to the driving frequency. A likely mechanism for the elimination of graphene oxide and the narrower etching trench in the high-frequency AC-LAO is again based on a resistor-capacitor voltage divider model, as illustrated in Fig. 4E. The impedance of an electrochemical cell typically results from the electrolyte and the two electrode/electrolyte interfaces. The electrolyte has only resistance, whereas the interfaces own both resistance and capacitance. The capacitance is due to the electric double layer (EDL)29, 30. Only the voltage drops at the two interfaces are effective in driving the electrochemical reactions. At high frequency, the effective impedance of the interfaces will decrease relative to resistance of the electrolyte, and the oxidation process will be cut off. This effect dominates for relatively long water bridge, i.e. points away from the tip apex, where the electrolyte resistance is large. In other words, it requires the collective motion of more ions to establish the electrostatic equilibrium for long water bridge, which would cost more voltage drop. For short water bridge, i.e. points very close to the tip apex, the voltage drop at the interfaces is still sufficient to drive the reactions. That explains why anodic oxidation etching driven by high frequency AC current has a smaller etching width than that driven by DC current. Shorter water bridge can also induce stronger electric field, which is beneficial for fully oxidizing graphene to gaseous CO/CO2. Additionally, even if graphene oxide is generated during


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the etching, it can be further oxidized to gaseous CO/CO2, because oxide residues can still conduct high frequency AC current.

Figure 4. The mechanism for elimination of oxide residue in EFLAO etching. AFM images of typical AC-LAO etching results obtained with different driving frequencies at 10V voltage (A) and with different voltages at 40 kHz frequency (C), and their corresponding statistic results (B, D). The AC-LAO etching was performed by directly applying an AC voltage between a microelectrode connected graphene and the tip to ensure that the voltage and frequency can be independently controlled. RH = 50~55%. Scale bars: 400nm (A) and 300nm (C). (E) Schematic of the resistor-capacitor voltage divider model and corresponding effective circuit. Red and blue areas represent the electric double layers (EDL). The electrolyte has only a resistance while the EDL owns both resistance and capacitance. At high frequency, longer water bridge (at the left and right side) results in larger electrolyte resistance and lower voltage drop at the interfaces.


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Only the area close to the tip apex can obtain sufficient voltage drop at the interfaces to drive the chemical reactions (central area). Shorter gap distance in the central area can also induce stronger electric field, which may be helpful for fully oxidizing graphene.

In conclusion, we have reported a novel electrode-free anodic oxidation nanolithography driven by high-frequency AC voltage, and demonstrated its ability in performing high-quality tailoring of low-dimensional materials and heterostructures in a single step with no need of prefabricated micro-electrodes connected to those samples. The non-pretreatment, high etching quality, as well as in-situ operation and characterization of the EFLAO should greatly facilitate its application in nanolithography. The EFLAO developed here provides a simple and efficient way to pattern low dimensional materials, and has the potential to become a standard nanolithography technique.

Methods: Silicon based substrates with a layer of 300nm-thick thermal oxide SiO2 on top were used for the EFLAO etching. To perform etching, an AC voltage with amplitude of 10 V and frequency range of 10 ~ 1000 kHz (graphene: 40 ~ 50 kHz (if not specified); graphene/hBN: 140 kHz; carbon nanotube: 10 kHz) is applied between the AFM tip and the silicon substrate. Then the tip slowly approaches the sample. The AFM is operated in contact mode, with a lift-down force of ~1500nN. The RH of the air is maintained in


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the range of 40~75 % (graphene: 55~75%, graphene/hBN: 45%, nanotube: 40%) if not specified, and the room temperature is kept at 20°C when conducting the experiments. The tip moving velocity was kept around 1~4 μm/s during the etching.

ASSOCIATED CONTENTS

Supporting Information

(1) Optical image of the GNRs fabricated by EFLAO, (2) IR scanning near-field optical microscopy measurement, (3) Comparison between AC- and DC-driven LAO, (4) A square hBN flake fabricated by EFLAO, (5) Estimation of the tip-graphene capacitance, (6) Measurement of the water bridge and tip-graphene contact resistances, (7) Method for quantitatively characterizing the amount of residues, and (8) Evidence for no direct tip-graphene contact.

AUTHOR INFORMATION

Corresponding Author


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* Email: [email protected] (Z.S.)

Author Contributions

H.L. and Z.S. conceived the project. H.L. and B.L. performed the EFLAO lithography. Z.Y., B.L. and A.D. helped on preparing the samples. H.L. and Z.S. analyze the data. All authors discussed the results and contributed to writing the manuscript.

Notes The authors declare no financial competing interests.

ACKNOWLEDGMENT

This work was mainly supported by National Key Research and Development Program of China (grant number 2016YFA0302001) and National Natural Science Foundation of China (grant number 11574204, 11774224). Z.S. acknowledges support from the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai


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Institutions of Higher Learning, and support from the National 1000 Young Talents Program and Shanghai 1000 Talents Program.

ABBREVIATIONS LAO, local anodic oxidation; EFLAO, electrode-free LAO; hBN, hexagonal boron nitride; CNT, carbon nanotube; GNR, graphene nanoribbon; AFM, atomic force microscope; RH, relative humidity; IR-SNOM, infrared scanning near-field optical microscope.

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Figure 1. Illustration of electrode-free local-anodic oxidization (EFLAO). (A) Schematic of the EFLAO. The electrochemical reaction is driven by an AC voltage applied through the SiO2/Si substrate without any electrode directly connected to the sample. The electrochemical reaction is localized within the nano-sized water bridge. (B) An array of graphene nano-ribbons fabricated using EFLAO. The inset shows the zoom-in of the white dash box. RH = 60%; f = 40 kHz.


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Figure 2. Demonstration of the high etching quality and capability of EFLAO. Topography (A) and IR-SNOM image (B) of a tapered graphene ribbon with one etched edge and one natural edge. (C) Line profiles of plas¬¬mons at the line cuts in (B). All the plasmon profiles show highly symmetric feature, indicating that the sample quality near the etched edge is as good as that near the natural edge. (D) Topography of a uniform array of ultra-thin GNR with width~20nm. The inset shows the GNRs owns quite smooth edges. (E) Topography of an ultra-thin GNR with width~10nm and edge roughness of ~2nm. (F) Topography of nanotube segments cut by FELAO. Cutting positions are labeled by white dash arrows. The inset shows the original complete nanotube. RH = 40%; f = 10 kHz. (G) Topography of an array of hBN/graphene nanotrenches fabricated by EFLAO, the depth of which is ~10nm. The inset shows a schematic of the original hBN/graphene heterostructure on SiO¬¬2 substrate. RH = 45%; f = 140 kHz.


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Figure 3. The principle of EFLAO. (A) AFM images of typical etching results obtained at different relative humidity (RH) of air. f = 40 kHz. Scale bar: 400nm. (B) Statistics for the success rate at different RH values. The success rate is close to unity when RH is higher than 50%. Pink dash curve shows a guiding line. The strong RH-dependence of etching success rate confirms unambiguously that the etching is due to anodic oxidation (C) Schematic of the equivalent electrical circuit for EFLAO. The graphene/SiO2/Si multilayer can be regarded as a capacitor. (D) Impedances of the G/SiO2/Si capacitor, water bridge resistor and direct tipgraphene (Gr.) contact. For water bridge, both experimental and theoretical values are displayed. (E) AFM images of typical EFLAO etching results obtained at different driving frequencies. RH = 57~60%. Scale bar: 200nm. (F) Statistics for the success rate and the amount of residues left.


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Figure 4. The mechanism for elimination of oxide residue in EFLAO etching. AFM images of typical AC-LAO etching results obtained with different driving frequencies at 10V voltage (A) and with different voltages at 40 kHz frequency (C), and their corresponding statistic results (B, D). The AC-LAO etching was performed by directly applying an AC voltage between a micro-electrode connected graphene and the tip to ensure that the voltage and frequency can be independently controlled. RH = 50~55%. Scale bars: 400nm (A) and 300nm (C). (E) Schematic of the resistor-capacitor voltage divider model and corresponding effective circuit. Red and blue areas represent the electric double layers (EDL). The electrolyte has only a resistance while the EDL owns both resistance and capacitance. At high frequency, longer water bridge (at the left and right side) results in larger electrolyte resistance and lower voltage drop at the interfaces. Only the area close to the tip apex can obtain sufficient voltage drop at the interfaces to drive the chemical reactions (central area). Shorter gap distance in the central area can also induce stronger electric field, which may be helpful for fully oxidizing graphene.


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Electrode-Free Anodic Oxidation Nanolithography of Low

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