Large Rectification Effect of Single Graphene Nanopore Supported by

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Large Rectification Effect of Single Graphene Nanopore Supported By PET Membrane Huijun Yao, Jian Zeng, Pengfei Zhai, Zongzhen Li, Yaxiong Cheng, Jiande Liu, Dan Mo, Jinglai Duan, Lanxi Wang, Youmei Sun, and Jie Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16736 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Applied Materials & Interfaces

Large Rectification Effect of Single Graphene Nanopore Supported By PET Membrane Huijun Yao†,*, Jian Zeng†, Pengfei Zhai†, Zongzhen Li†,§, Yaxiong Cheng†,§, Jiande Liu†, Dan Mo†, Jinglai Duan†, Lanxi Wang£, Youmei Sun†, Jie Liu†,*

† Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China §University of Chinese Academy of Sciences, Beijing 100049, China £ Science and Technology on Vacuum Technology and Physics Laboratory, Lanzhou Institute

of Physics, Feiyan Street 100, Lanzhou 730000, China

ABSTRACT:

Graphene is an ideal candidate for the development of solid state nanopores due to its thickness at the atomic scale, and high chemical and mechanical stabilities. A facile method was adopted to prepare single graphene nanopore supported by PET membrane (G/PET nanopore) within the three steps assisted by the swift heavy ion irradiation and asymmetric etching technology. The inversion of the ion rectification effect was confirmed in G/PET nanopore while comparing with bare PET nanopore in KCl electrolyte solution. By modifying the wall charge state of PET conical nanopore with hydrochloric acid from negative to positive, the ion rectification effect of G/PET nanopore was found to be greatly enhanced and the large rectification ratio up to 190 was obtained during this work. Moreover, the high ionic flux and high ion separation efficiency was also

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observed in G/PET nanopore system. By comparing the “on” and “off” state conductance of G/PET nanopore while immersed in the solution with pH value lower than isoelectric point of the etched PET (IEP, pH=3.8), the voltage dependence of the “off” conductance was established and it was confirmed that the large rectification effect was strongly dependent on the particularly low “off” conductance at higher applied voltage.

KEYWORDS: Graphene nanopore, PET membrane, Ion rectification effect, Inversion, Ion Irradiation

1. INTRODUCTION Solid state nanopores or nanochannels prepared in polymer1-2 and semiconductor films3-7 have shown interesting transport phenomena, because of their diameters at the nanoscale and positive or negative charges on their walls. Graphene is an ideal material for developing solid state nanopores not only due to its atomic scale thickness, high mechanical strength and chemical stabilities but also because of the impermeability of the pristine single layer graphene to all atoms and molecules except protons8. By creating nanometer-sized pores in graphene, it can be potentially used in ion filters9-10, gas separation11-12, protein sensors13-15, water desalination16-17, and DNA detection18 or sequencing19-21. Up to now, there are many groups have been taking effort on preparing graphene nanopores with focused ion- and electron-beams22-24. For example, the graphene nanopore with different diameters from 3 to 20 nm are drilled on the as-transferred graphene membrane in transmission electron microscope with focused electron beam energy above the carbon knockout potential (80 kV)23. Ion irradiation technology can also be utilized to introduce defects in single


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layer graphene sheets25-28, which could be enlarged into permeable pores subsequently by the oxidative etching for further research such as the ion selectivity28. All the important applications of graphene nanopores discussed above are required to meet one of the essential preconditions that a supporting substrate with certain mechanical strength and hollow area (or hole) is needed to support the prepared graphene nanopores of single atomic layer thickness. More importantly, the hollow area (or hole) should be smaller than 2 μm, otherwise the graphene covering the hole of large size will be easily broken because of surface tension29. Normally, the extremely hard SiN membrane10, 19-21, 23 is chosen as the substrate and a hole will be prepared to support the graphene nanopore. Electron beam lithography (EBL), optical lithography, and wet chemical etching procedures are often indispensable in preparing the hollow SiN substrate and these techniques are normally time consuming as compared to the drilling of a nanopore on graphene. Besides this, the current widely used method based on the electron-beam drilling fabrication cannot meet the requirement of the graphene nanopore for high ionic flux in the fields of ion separation, water desalination, etc., because the large sample area (at cm scale) and high pore density (>106 ions/cm2) are difficult to be realized. Considering the shortages in graphene nanopore preparing procedures, finding out a simple, fast and efficient technique for obtaining single- and multi- graphene nanopores with large area is highly desirable in the current days. Here, we demonstrate a facile method to fabricate porous substrate supporting graphene nanopore within three steps assisted by the heavy ion irradiation and asymmetric etching technology as shown in Figure 1. First, the single layer graphene grown with CVD method is transferred to a polyethylene (PET) membrane to form G/PET structure, showing in Figure 1(A). Secondly, the prepared G/PET is irradiated by the swift heavy ion which penetrate the G/PET 3 ACS Paragon Plus Environment


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structure and keep the ion energy loss in PET larger than homogeneous etching threshold along the entire ion trajectory. In this step, the nanopore on graphene can be obtained30 because of the ion bombardment with high electronic stopping power, and the latent track in PET membrane exactly underneath the graphene nanopore is also formed along the ion trajectory, as shown in Figure 1(B). In the last step as depicted in Figure 1(C), the irradiated G/PET undergoes asymmetric chemical etching with the etchant being placed on the PET membrane side and stopping solution being on the graphene side. In this case, a conical nanopore with the tip precisely supporting the graphene nanopore is formed in PET. In order to reduce the chance of possible unexpected ion leakage from the intrinsic defects out of graphene nanopore via sealing them with supporting PET membrane, the tip size of conical nanopore in PET can be controlled in the range of several nanometers in purpose, which is comparable to the diameter of graphene nanopore and much smaller than that in SiN membrane substrate (recently report as 200 nm21). Furthermore, the smaller supporting hole can also reduce the risk of the graphene nanopore broken caused by the high tension during the experiment. Up to now, with roll-to-roll method, the 30-inch G/PET structure 31-32 can be produced and make it possible to prepare G/PET nanopore with large area. The pore density in G/PET structure from single per sample to 106 cm-2 can also be reached by adjusting heavy ion irradiation fluence33-34 during the experiment. Our technique really meets the requirement of preparing porous membrane supported graphene nanopore with large sample area and high pore density (>106 ions/cm2) as required in the present days. As the single G/PET nanopore is the elementary building block for nanoporous membrane, the understanding and control of its individual ionic transport properties are also crucial in protein separation, water desalination, and bio-molecule detection using arrays of identical nanopores. In 4 ACS Paragon Plus Environment

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this study, the single G/PET nanopore was successfully prepared using our new method and the corresponding ionic transport properties were investigated in detail. It was found that not only the reverse rectification effect was confirmed but also the large rectification ratio up to 190 was obtained in KCl electrolyte solution. The results obtained here are beneficial to understand the mechanism of ion transport in graphene/PET nanopore and also to predict and design new bio-molecular sensors and nanofluidic devices in the future.

2. EXPERIMENTAL The monolayer graphene (1 cm ×1 cm, supplied by Nanjing Jcnano technology Co., LTD) with polymethyl methacrylate (PMMA) coating was transferred to polyethylene film (PET, 20 µm thick) from the original copper substrate according to the standard transfer procedure (S1). The prepared graphene/PET (G/PET) structure was irradiated by Bi ion at room temperature with ion initial kinetic energy of 9.5 MeV/u which were provided by two cyclotrons (HIRFL of IMP, Lanzhou). For single ion irradiation, a metal mask with a 0.5 mm aperture in the centre was placed in front of the sample mainly for the purpose of reducing the ion fluence and pre-determining the ion irradiating position. A scintillation radiation detector was fixed behind the sample that was used to detect the swift heavy ion after bombardment and penetration through the G/PET. In order to perform the single ion irradiation, the ion beam was shut off immediately once one ion was detected by the detector. During the irradiation, a reference sample of PET membrane (20 µm thick) was positioned in front of the G/PET structure. After irradiation, an asymmetric etching process was essential to get the conical pore in PET membrane that was used to support the graphene nanopore. Firstly, the irradiated G/PET was mounted in a home-made two-compartment electrolytic cell and the PMMA on graphene was



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removed completely by pouring acetone into the PMMA side of the cell three times (every time lasted for about 10 minutes). In order to get the desired conical pore in G/PET, 1 mol/L NaOH etching solution was poured into the compartment connected to PET membrane and 0.05 mol/L HCl stopping solution was filled into the reverse compartment which was connected to the graphene membrane. During the etching process, a voltage of 0.1 V was applied between the two Pt electrodes, which were immersed in the etchant and stopping solution separately, to monitor the transmembrance current. Once the pore opens, a sharp increase in the electrical current can be observed and the stopping solution will start flowing into the conical pore from the tiny end and stop the etching process (S3). The wide end of the conical pore was referred as the “base” and the narrow end as the “tip”. The reference PET sample was underwent asymmetric etching with the same procedure and etching conditions. The ion current rectification effect of PET supported graphene nanopore was measured by using picoammeter/voltage source (Agilent, B2902). The Ag/AgCl electrodes were utilized to detect ionic currents and minimize the effects of electrode polarization during the measurement. KCl solution was adopted to investigate the ion current rectification effect because K+ and Cl- ions have similar bulk nobilities, and therefore they exhibit negligible liquid junction potentials. The scanned voltage between electrodes was varied from -4 to +4 V. The extent of ion current rectification was evaluated by determining the rectification ratio (the current measured at positive voltage divided by the current measured at the corresponding negative voltage) and defining the applied electric filed direction from base to tip as the positive direction.

3. RESULTS AND DISCUSSION


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To characterize the graphene used for studying ion rectification effect, the graphene grown on copper substrate with spin coated PMMA layer was first transferred to porous PET membrane. Here, the transferred graphene size is 1 cm × 1 cm, as shown in Figure 2 (A) marked with dashed square. Figure 2 (B) provides the scanning electron microscopy (SEM) image of the graphene on porous PET membrane to confirm the integrity of supported single layer graphene in PET pore area. The dashed line indicates the graphene edge on PET membrane. The pores with diameter of around 1 μm can support the single layer graphene well without any broken area or any obvious deformation. This confirms that our transfer process can produce large-scale perfect single layer graphene on PET membrane and the PET nanopore in micrometer can acts as good substrate to support the graphene nanopore similar to the case of SiN nanopore. In order to further confirm the quality of the graphene used for nanopore preparation and ion rectification effect, Raman analysis was also used to examine the monolayer graphene after being transferred to SiO2 substrate. Several testing points were adopted to confirm the structure homogeneity of the monolayer graphene. The typical Raman spectra of the graphene is shown in Figure 2 (C) and the corresponding intensity ratios of 2D peak (2580 cm-1) to G peak (1584 cm-1) are found in the range 1.8-3.3 (more Raman data is provided in S2). It is seen that there is no discernable defect peak (D) at 1350 cm-1 that can be observed. These Raman features suggest that the graphene that we used in our experiment is single layer and possesses high structural quality. In this study, the nanopore structure of G/PET is important and essential for investigating the corresponding ion transport properties and rectification effects. For exploring the base diameter of conical nanopore, it can be easily determined from SEM images by preparing multi nanopores in G/PET with the same etching conditions as single conical nanopore. Here, the base side of the 7 ACS Paragon Plus Environment


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conical nanopore was observed with SEM as illustrated in Figure 3 (A) and (B) with pore density of 1×106 pore/cm2 and diameter of around 1.2 μm. Figure 3 (C) supplies the SEM image from top view of the G/PET nanopores for observing the nanopore tip side. A thin layer graphene with continues large area on PET membrane can be observed directly. The wrinkles on graphene are probably due to the relatively rough surface of PET membrane, which can introduce additional stress because of the adhesion34 during the transfer and drying processes. It is noticeable that the tip size of the conical nanopore in G/PET cannot be measured with routine procedure because of the ultra-narrow diameter of the tip that is only few nanometers which is beyond the SEM resolution for observing organic PET membrane. However, the tip size in single conical nanopore could be estimated with the conductometric method which has been widely used elsewhere 34, 36-39. The tip diameter can be calculated using the following formula: 𝑑𝑡𝑖𝑝 =

4𝐿𝐺

(1)

𝜋𝑘𝐷

Where dtip and D are the conical pore tip and base diameters, respectively, k is the specific conductivity of the electrolyte in the pore, which is 0.112 Ω-1cm-1 for 1 M KCl at 25℃ in our experiment, G is the corresponding ionic conductance of the electrolyte filled in the nanopore, which can be deduced from the linear I-V curve, and L is the pore length (i.e. the thickness of the membrane, 20 µm). In our case, the calculated tip diameter of the conical nanopore in reference PET sample is around 6.2 nm, while in G/PET sample it is around 8.4 μm. For the two conical nanopores in PET and G/PET obtained under the same irradiation and etching conditions, they should possess similar nanopore contours and sizes. The huge difference in the dip diameter mainly arises from the conductance variation during the I-V measurements. Figure 4 (A) shows the I-V curves for calculating the ionic conductance of the conical nanopore in PET and G/PET. 8 ACS Paragon Plus Environment

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The conductance deduced from the linear I-V curve is 2.4×10-9 S and 3.2×10-6 S for conical nanopore in PET and G/PET, respectively. There is substantial difference in the conductance in both cases which may be due to the enhanced electric field introduced by the graphene layer instead of the conical nanopore shape itself. During the I-V measurements, when a transmembrance potential was applied on the conical nanopore in G/PET, the applied electric field was asymmetrical along the pore axis because of the conical geometry2 and a significant potential drop formed in the tip area because of the edge effect of graphene nanopore40-41. The enhanced electric field in the graphene nanopore area thereby results in the enhanced ionic current and conductance. Eq. (1) is not suitable for the more complex G/PET conical nanopore in a certain extent. Therefore, we adopted 6.2 nm as the tip diameter for both PET and G/PET conical nanopore taking into consideration the same procedure during the entire experiment that was followed. According to the above discussion, the graphene nanopore plays a key role during the I-V measurements. For investigating the nanopore on graphene with TEM directly, the graphene was transferred to TEM grid and suffered higher ion irradiation fluence (1012 ions/cm2). The size of graphene nanopore we obtained was in the range 0.4-0.6 nm42, which is much smaller than the tip size in case of the PET conical nanopore. In order to investigate the difference of ion transport properties in case of G/PET and PET nanopore, the typical I-V measurement of KCl electrolyte solution in PET conical nanopore with different concentrations was carried out as shown in Figure 4 (B) firstly. All these measurements exhibit higher ionic current at negative voltage as compared to the positive applied voltage, which is same as reported previously33, 37, 43. The phenomenon of nonlinear I-V curves in PET conical nanopore called ion rectification effect has been studied extensively. The mechanism which 9 ACS Paragon Plus Environment


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underlies the rectification effect in PET conical nanopore is thought to include two key points: the potential in the tip area and the existing of diffusion double layer (DDL) which is comparable to the diameter of the tip. The presence of the potential has been confirmed in numerical simulations of the local electric field within the pores44-45, however its magnitude strongly depends on the surface charge and the applied potential. PET conical nanopore possesses a highly inhomogeneous electric field42, where the highest electric field exists in the pore tip area. The existing potential thus contributes to the ion-enrichment or ion-depletion in the conical nanopore37 as a conducting ion controller, which is the combination of the intrinsic electric field of the nanopore and the external applied electric field. The negative surface charge on PET conical nanopore wall due to the deprotonated carboxyls38, 43, 47 satisfies the condition of forming DDL after chemical etching. The conductance of the effective channel is determined by the DDL’s thickness and its profile, which are strongly associated with electrolyte concentration and the pore shape38, respectively. At negative potentials, the K+ flux is directed from the external bulk solution to the pore interior while Cl- ions move in the opposite direction. As the pore is cation selective because of the DDL, therefore the incoming Cl- ions are rejected by the negatively charged PET membrane surface because of the electrostatic repulsion. The K+ ions transport inside the conical nanopore is limited by the geometry, while the outer space near the tip area provides much faster transport of ions due to the larger access angle. At positive bias, the K+ ions are pushed from the nanopore and the faster mass-transport rate outside the tip opening leads to the depletion of ion concentration inside the pore, thereby giving rise to a low-conductance state and diminished ion current magnitude48. Because of the existence of negative charge and the DDL on conical wall surface, the PET conical nanopore presents cation selectivity properties in the form of exhibiting high conductance (“on” 10 ACS Paragon Plus Environment

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state) when the current (mainly contributed by cations) enters the cone tip at negative voltage while presenting low conductance (“off” state) when the current enters the cone base at positive voltage2 28. The same I-V measurements were performed on G/PET nanopore system and the obvious non-linear curves were confirmed with the KCl electrolyte concentration from 0.001 to 1 mol/L. In order to compare the results with PET conical nanopore, five typical I-V curves with KCl electrolyte concentrations ranging from 0.002 to 0.01 mol/L are picked out (shown in Figure 5A). Contrary to the results of PET conical nanopore, the G/PET nanopore system shows higher current at positive voltage and lower current at negative voltage surprisingly, which means that the reversal rectification effect was happened after combing PET conical nanopore with graphene nanopore. The possible reason for the ionic current rectification effect inversion is that the graphene nanopore rim and monolayer graphene on PET can easily be charged positively by HCl stopping solution (concentration of 0.05 mol/L, pH of 1.3) during the asymmetric etching. The presence of positive charges at the edge of graphene nanopore can remarkably impede the passage of K+ while enhance the transport of Cl-, which is an indication of the good ion selectivity for electrolytes9-10. When the graphene nanopore and PET conical nanopore tip overlapped each other in G/PET structure, the ion conducting channel will be mainly dominated by the positively charged graphene nanopore in the tip area, although the wall of PET conical nanopore possessing negative charge and thereby the ionic rectification effect can be reversed. Besides these, in case of same KCl electrolyte concentration, the conduct ionic current of G/PET at “on” state is found two orders higher than that of several nanoamperes in PET conical nanopore as can be seen in Figure 4 (B) and Figure 5 (A). The enhanced “on” ionic current may be caused by the graphene nanopore 11 ACS Paragon Plus Environment


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which has good electrical conductivity and introduces enhanced electric field in the nanopore area49. The rectification ratio as defined previously is used to evaluate the reversed rectify ability of G/PET nanopore. In Figure 5 (B), the rectification ratio as a function of KCl electrolyte concentration from 0.001 to 1 mol/L at different voltages was depicted. It can be seen that the rectification ratio increases with decreasing the electrolyte concentration from 1 to 0.01 mol/L and reaches a maximum value of 14 at 0.01 mol/L under the 4 V bias voltage. After this point, the rectification ratio begins to decrease with further decreasing the electrolyte concentration. It is well known that the variation of rectification ratio at different electrolyte concentrations is linked to the thickness of DDL50. The higher electrolyte concentration will result in thinner DDL thickness according to Debye-huckel theory

50

and the corresponding lower rectification effect.

Meanwhile, when the DDL begins to overlap in the pore tip area, the G/PET nanopore will exhibit the strongest ion rectification effect. In our case, the calculated thickness of DDL is around 3 nm with KCl electrolyte concentration of 0.01 mol/L50 and begins to overlap in the tip area (diameter of 6.2 nm). The DDL overlap effect on the nanopore conductance is more significant46 and that will influence the ionic conductance at the “on” and “off” states. As shown in Figure 5 (B), before the DDL overlapping, the rectification ratio in KCl exhibits a strong voltage dependence, i.e. the higher voltage was applied, the higher rectification ratio can be obtained. When decreasing the electrolyte concentration further, the larger DDL overlapping portion will be formed in the tip area and the voltage dependent rectification ratio is disappeared. Earlier simulation works52-53 on electrolyte conductance distribution profile in the nanopore proved that the ion depletion and ion enrichment in the pore tip could happen at different applied voltages. In case of DDL overlapping, 12 ACS Paragon Plus Environment

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if the electrolyte concentration was continuously decreased, the ion enrichment in the tip area caused by varying voltage will not eliminate the DDL overlapping state and the voltage dependent rectification effect will diminish. Meanwhile, the rectification ratio will decrease with the decreasing of electrolyte concentration and the increasing of DDL overlapping portion52, as shown in Figure 5 (B). Control of the surface charge on the nanochannel has been assessed as an effective method to modulate the ion transport through nanochannel and the current rectification of nanochannel54. Researchers have confirmed that electrolyte with pH values below 3 could adjust the distribution of surface charge37 54 by positively charging the wall of conical nanopore in PET and can achieve pH-reversed ionic current rectification without any modification of the PET membrane39. Since our G/PET conical nanopore has already achieved the inversion of ionic current rectification from cation selectivity to anion selectivity, we hopefully further enhance the reversed rectification effect by decreasing the electrolyte pH value. The effect of pH on rectification behavior was studied for a series of pH values and only hydrochloric acid was used to adjust the electrolyte pH from 2 to 5 in order to avoid introducing other anions in the system. In order to get better rectification effect in G/PET nanopore according to our previously results, three typical KCl electrolyte solutions with concentrations of 0.002, 0.02 and 0.2 mol/L are used for further studies. Figure 6 (A), (B) and (C) give the typical I-V curves of KCl electrolyte solution at different pH values with concentrations of 0.002, 0.02 and 0.2 mol/L, respectively. All the curves exhibit excellent ionic diode effect behavior in which the “on” state appears at positive voltage while the “off” state connects with negative voltage as we expected. It should be mentioned that although the maximum “on” ionic current exceeds one microampere in the single G/PET nanopore (mostly 13 ACS Paragon Plus Environment


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reported in nanoampere range), it still exhibits good rectification effect behavior. The slopes of the I-V curves at the positive voltage increased with decreasing the pH values, thereby demonstrating that the density of positive charge polarity on the nanochannel surface was highly adjustable. It can be seen clearly that the “on” ionic currents at the pH value 2 in Figure 6 (A) and (B) are much higher than other pH values. This is due to the reason that the introduced H+ and Cl- ion concentration in KCl electrolyte solution will reach 0.01 mol/L when the pH value is 2, which is comparable to or larger than the KCl electrolyte concentration of 0.02 and 0.002 mol/L used in the experiment. This means that there are more mobile ions conducting the “on” ionic current in case of pH 2 as compared to other pH values. However, the “off” ionic current does not increase with decreasing pH value obviously. In Figure 6 (C), it can be found that the “off” ionic current decreases as adding more hydrochloric acid, i.e. decreasing pH value of electrolyte. As known, the smaller “off” ionic current the nanopore has, the better ionic rectification effect it will exhibit. The rectification ratio was also used to evaluate the rectification effect of G/PET at different pH conductions. The rectification ratio as function of pH value is shown in Figure 7 (A), (B) and (C) that is related to KCl electrolyte concentrations of 0.002, 0.02 and 0.2 mol/L, respectively. As expected, the rectification ratio increases with decreasing pH value and shows a dependence on positive applied voltage. The maximum rectification ratios in Figure 7 (A), (B) and (C) are around 190, 130 and 60, respectively. To the best of our knowledge, the rectification ratio of 190 is the highest value obtained in single nanopore system up to now. In order to compare comprehensively, table 1 presents the key factors (materials, electrolyte, applied voltage, maximum “on” current, and rectification ratio) for studying the rectification effect both the previously reported values and the values obtained in our work. It should be pointed out that G/PET nanopore not only possesses 14 ACS Paragon Plus Environment

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high rectification ratio but also exhibits extremely high ionic “on” current, which indicates the high ionic flux and high ion separation efficiency. Due to the existence of isoelectric point (IEP) of polymeric surface (for track etched PET, IEP=3.8), if the PET membrane was immersed in the solution with pH higher than 3.8, the surface of PET conical nanopore is still negatively charged39, 43. But in the solution with pH lower than 3.8, carboxylate groups will be protonated and the surface of track etched PET conical nanopore can be positively charged55. Once the PET conical nanopore is positively charged, it will show the same ion selectivity as the positively charged graphene nanopore. Thus, the enhancement in rectification ratio at lower pH value (here is 3.8) will be contributed by both the charge screening of potassium ions moving into the nanopore and by the depletion of cations in double layer by positive charge at the surface of conical nanopore56. Although the large rectification ratio in G/PET nanopore we have obtained by positively charging PET conical nanopore with acidic solution in purpose, the ion transport properties in G/PET are still not clear. In order to clarify the mechanism of enhanced rectification ratio in G/PET nanopore, the conductance of G/PET conical nanopore as function of applied voltage at different pH values with KCl electrolyte concentrations of 0.002, 0.02 and 0.2 mol/L is plotted in Figure 8 (A), (B) and (C), respectively. The log scale is employed in Y-axis for observing the conductance variation more clearly. The left part (light yellow color) and the right part (light green color) in Figure 8 represent the “off” and “on” states, respectively. When positive voltage was applied, the G/PET will present “on” state and the ion conductance increases slightly with increasing the applied voltage. It can be observed that the solution acidity does not affect the ion “on” conductance substantially in case of 0.2 mol/L KCl, as shown in Figure 8 (C). The deviation 15 ACS Paragon Plus Environment


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of ion “on” conductance in Figure 8 (A) and (B) with pH 2 is because the concentrations of introduced H+ and Cl- ions are comparable to the KCl concentration and thereby increasing the electrolyte conductivity greatly, as discussed before. As far as the “off” state is concerned, two completely different “off” conductance varying trends can be found with increasing the absolute applied voltage as shown in Figure 8. For the solution with pH 4 and 5, the “off” conductance increases with increasing the negative applied voltage. For the KCl solutions with pH=2 and 3, the “off” conductance does not show any positive voltage relationship, but decreases with increasing negative voltage dramatically. In Figure 8 (C), it can also be deduced that the introduced hydrochloric acid can reduce the “leakage” current and decreases the “off” conductance. Still it should be kept in mind that the isoelectric point for etched PET conical nanopore is 3.8. Once the solution pH is lower than 3.8, the wall of PET conical pore will be positively charged. The magnitude of the surface charge (positive) density increases with decreasing pH that is expected because the concentration of H+ ions increases with decreasing pH, leading to the more positively charged functional groups39,

57.

It is known that the presence of surface charge can cause

preferential enhancement or depletion of mobile ions in conical pore. This effect can be reversed by changing the polarity of the surface charges, represented here as the lower pH solutions. When the negative voltage was applied on the positively charged conical wall, the cations will dominate the “off” ion current and the depletion effect of conducting cations will increase as the negative voltage increases58 as shown in Figure 8. Resultantly, as the more negative voltage is applied, the higher depletion effect and lower ionic conductance in G/PET nanopore will be obtained. While at higher pH values (> 3.8), the conical wall is still negatively charged and the cation enrichment happened in the conical nanopore to form DDL. In this case, as the higher negative voltage is 16 ACS Paragon Plus Environment

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applied on the membrane, the more cations will enter the G/PET nanopore and the higher “off” conductance will take place. In our current work, although the large rectification ratio up to 190 was obtained in G/PET nanopore in the acidic conditions, however, there are still some factors that can influence the ion rectification effect of G/PET nanopore, such as the tip and base size of the conical nanopore, graphene nanopore size, etc., which are still unclear and need to be investigated in detail in the future. It is hoped that by optimizing the G/PET structure parameters, even larger rectification ratios could also be obtained and can be utilized in the future nanofluidic devices.

4. CONCLUSIONS As graphene nanopore is a promising candidate to be used in various applications such as ion separation, biomolecule sensing, DNA sequencing, water desalination, etc., a facile method is established to fabricate single- and multi-graphene nanopores supported by PET membrane (G/PET nanopores) using swift heavy ion irradiation and asymmetric etching technology within three steps. It is found that the prepared graphene nanopore in our experiment possesses positive charged surface and exhibits good anion selectivity, which can cause the rectification effect inversion in G/PET nanopore as compared with PET nanopore. By positively charging the wall of G/PET conical nanopore with hydrochloric acid, the enhanced rectification effect is achieved and the large rectification ratio up to 190 is also obtained. The reason for the large rectification effect is the extremely low “leakage” ion current and low “off” conductance that happens at the higher negative voltage while the high “on” conductance is kept constant at the varying positive voltages. Furthermore, these surprising observations indicate that the atomically thin graphene nanopore can introduce quite good ion selectivity and should continue to be a rich platform for studying



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interactions of ions with surfaces and walls in model nanopores for further understanding electrostatics at the nanoscale as well as making better graphene nanopore sensors. In contrast to the isolated graphene nanopore, the multiple G/PET nanopores can also be prepared by increasing ion irradiating fluence for future practical applications on ion selectivity with sufficient pore densities acting in parallel over the macroscopic areas. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications

website at DOI:XX.XX.XX The detailed graphene transfer procedure used in this work, more Raman spectra of single layer graphene and the schematic illustration of G/PET asymmetric etching. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the finance support from the West Light project of the Chinese Academy of Sciences (CAS), the Youth Innovation Promotion Association of CAS under Grant No.2012298 and National Nature Science Foundation of China (Grant Nos. 11575261, 11275237, 11405229, 11505243 ,11304261 and 61405084). We would like to thank the members 18 ACS Paragon Plus Environment

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of the Materials Research Department and the accelerator staff of HIRFL of IMP for preparation and irradiation of polycarbonate templates. The authors would like to thank Dr. Maaz for useful discussion and modification on this paper.

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(A)

(B)

(C)

Figure 1. The sketch of preparing G/PET nanopore with heavy ion irradiation and asymmetric etching technology. (A) The single layer graphene is mechanically transferred to PET membrane to form G/PET structure. (B) The G/PET structure is irradiated with single heavy ion that resulted in single nanopore on graphene and a latent track exactly underneath the graphene nanopore is formed in PET membrane along the ion trajectory. The red part represents the latent track in PET. (C) The irradiated G/PET undergoes asymmetric etching to fabricate G/PET nanopore. The etching procedure is carried out only at the bottom of G/PET structure and it will be stopped once the conical nanopore is formed.

(A)

(B)

(C)

Figure 2. Characterization of graphene membrane. (A) The photograph of transferred graphene on porous PET membrane as marked with dashed square. A protection PMMA layer is still on the graphene membrane and makes the high contrast while transferring and operating process. The size of the graphene used here is 1 cm × 1 cm. (B) The SEM image of graphene transferred to porous PET membrane. Here, the protection PMMA membrane was removed by acetone and the pore in PET is around 1μm. The red dashed line is used for guiding the graphene edge on PET membrane. (C) Raman spectrum of the pristine monolayer graphene 23 prepared with CVD method. The intensity ratio of 2D/G is around 2 which indicates the single layer ACS Paragon Plus Environment

graphene possesses good structural qualities.


1 2 3 4 5 6 7 (A) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(A)

(B)

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(C)

Figure 3. SEM images of G/PET nanopore with bottom and top view. (A) and (B), The bottom view of conical nanopore in PET for observing base side with low and high magnification, respectively. (C) The top view of G/PET nanopore for characterizing the graphene and the tip side of conical nanopore in PET. The pore density and diameter on the base side are 1×106 cm-2 and 1.2 μm, respectively.

Figure 4. The I-V curves of PET and G/PET conical nanopore. (A) The I-V curves of PET concial nanopore and G/PET nanopore with small applied voltage from -0.2 to 0.2 V. The KCl electrolyte concentration is 1 mol/L and the ionic conductance deduced from the slops are 2.4×10-9 S and 3.2×10-6 S for PET conical nanopore and G/PET nanopore, respectively. (B) The typical I-V curves of PET conical nanopore with KCl electrolyte concentration from 0.002 to 0.01 mol/L with applied voltage from -4 to 4 V. The tip and base sizes of PET conical nanopore are around 6.2 nm and 1.2 μm, respectively. 24 ACS Paragon Plus Environment

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Figure 5. Rectification of G/PET nanopore. (A) current-voltage curves of G/PET nanopore with an applied voltage ranging from -4 V to 4 V in KCl electrolyte with different concentrations. (B) The ion rectification ratio as a function of KCl electrolyte concentration at different applied voltages.

Figure 6. Rectification of G/PET nanopore at different pH conditions. Current-voltage curves of G/PET nanopore with scan voltage from -4 V to 4 V and concentration of (A) 0.002 mol/L KCl; (B) 0.02 mol/L KCl; (C) 0.2 mol/L KCl. The KCl electrolyte at various pH values of 2 (black squares), 3 (green triangles), 4 (red circles) and 5 (blue diamonds).



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Figure 7. Rectification ratio of G/PET nanopore as a function of pH value at different voltage bias values and concentration of (A) 0.002 mol/L KCl; (B) 0.02 mol/L KCl; (C) 0.2 mol/L KCl.

Figure 8. The conductance of G/PET conical nanopore as function of the applied voltage at different pH values with KCl electrolyte concentration of (A) 0.002 mol/L; (B) 0.02 mol/L; (C) 0.2 mol/L. The left part (light yellow) shows the “off” state and the right part (light green) presents the “on” state. The dashed line is drawn only for guiding the naked eye.


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Table 1. Key factors of materials, electrolyte, applied voltage, maximum “on” current, and rectification ratio studying the rectification effect reported previously and in the current work. Nanopore Electrolyte

Applied voltage (V)

Maximum “on” current

Rectification ratio

Ref.

PET nanopore

0.2 M/L KCl

2

～250 nA

20.8

33

PET nanopore

0.05 M/L LiCl

1

～3.8 nA

～7.5

47

PET nanopore

0.2 M/L KCl

1

～2.5 nA

～7.9

34

Al2O3 nanoporem

0.1 M/L KCl

0.5

19 uA

～6.5

59

PEI coated glass nanopipette

0.01 M/L KCl

1

～7 nA

～27

50

Kapton nanopore

1 mM/L:1000mM/L KF

1

～20 nA

140

60

Quart nanopore

0.7 M/L KCl

0.5

～30 pA

2.2

61

Glass nanopore

1 M/L KCl

0.5

-

3

62

PET nanopore

0.01 M/L HCL

6

50 nA

～55

39

G/PET nanopore

0.2 M/L KCl

4

5 uA

60

This work

G/PET nanopore

0.02 M/L KCl

4

1.3 uA

130

This work

G/PET nanopore

0.002 M/L KCl

4

1.0 uA

190

This work

& Material

Note: m, the Al2O3 multipore sample includes 800 pores for investigation.



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TOC


Large Rectification Effect of Single Graphene Nanopore Supported by

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