This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Letter Cite This: Nano Lett. 2019, 19, 9029−9036 pubs.acs.org/NanoLett Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene Xiaoyu Jia,†,‡ Min Hu,§ Karuppasamy Soundarapandian,∥ Xiaoqing Yu,† Zhaoyang Liu,†,# Zongping Chen,†,∇ Akimitsu Narita,† Klaus Müllen,† Frank H. L. Koppens,∥ Jun Jiang,§ Klaas-Jan Tielrooij,⊥ Mischa Bonn,† and Hai I. Wang*,† † Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ‡ The Graduate School of Excellence Materials Science in Mainz, University of Mainz, Staudingerweg 9, 55128 Mainz, Germany § Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China ∥ ICFO - Institut de Ciéncies Fotóniques, Mediterranean Technology Park, Castelldefels, Barcelona 08860, Spain ⊥ Catalan Institute of Nanoscience and Nanotechnology (ICN2), BIST and CSIC, Campus UAB, Bellaterra, Barcelona 08193, Spain * S Supporting Information ABSTRACT: Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion- sensing material. KEYWORDS: Graphene, terahertz spectroscopy, doping, ionic permeation raphene finds extensive use in various electrochemical G applications, including ionic sensing,1,2 biochemical detection,3,4 lithium-ion batteries,5 dye-sensitized solar cells,6 carrier density (related to the Fermi energy via E F = ℏνF(π |n|)1/2 where νF is the Fermi velocity) and μ is the charge carrier mobility. Although it is apparent that the and supercapacitors,7,8 due to its outstanding electrical and mechanical properties, large surface-to-volume ratio, and presence of (aqueous) electrolyte solutions affects the excellent chemical stability. Despite its critical importance for conductivity of graphene, it is typically not straightforward to these applications, the effect of electrolyte solutions on the disentangle how μ and n are affected. In the case of polymer electronic properties of graphene has remained poorly understood. There is particular debate regarding the micro- Received: October 2, 2019 scopic mechanism underlying the effect of aqueous solutions Revised: November 8, 2019 on graphene’s conductivity, σ = neμ, where n is the charge Published: November 19, 2019 © 2019 American Chemical Society 9029 DOI: 10.1021/acs.nanolett.9b04053 Nano Lett. 2019, 19, 9029−9036 Nano Letters Letter Figure 1. Concept of cation-induced Fermi level shift in graphene and its measurement. (a) Illustration of cation permeation through the graphene sheet to the graphene−SiO2 interface. Graphene is initially hole-doped due to the presence of negative surface charges (indicated in yellow) on the SiO2 surface. Intercalated cations lead to a shift of the Fermi level of graphene toward the Dirac point. (b) Schematic picture of optical pump−THz probe spectroscopy on the graphene/electrolyte system, with the graphene supported by fused silica (SiO2). First, graphene is photoexcited by a femtosecond laser pulse (in red), followed by a THz probe pulse (in light gray). The change in THz transmission, ΔE = E − E0, is directly proportional to the change in THz conductivity, Δσ. (c) Illustration of typical Fermi energy-dependent THz photoconductivity, −ΔE/E0, in graphene. The conductivity is positive for graphene with the Fermi energy close to the Dirac point, whereas it becomes increasingly negative upon increasing the Fermi energy.25−28 This distinct sign change allows us to assign the effect of cations on the graphene conductivity to a change in Fermi level. electrolyte solutions, it was found that they are extremely through a strong electrostatic effect that shifts the Fermi level effective at inducing electron/hole doping in graphene through toward the Dirac point (ΔEF ∼ 200 meV). Through kinetic ions located in the Debye layer on top of graphene.9 The effect measurements, we find that it takes tens of minutes for the of cations has been primarily ascribed to cations absorbed on Fermi energy shift to complete, where the time scale of this the surface of graphene, where they also act as charge- process depends critically on the solvation radius of the scattering centers,10−12 thus affecting μ. On the other hand, cations. We rationalize the concentration- and size-dependent ions and water molecules have previously been reported to be cation-induced Fermi level shift that we observe as follows: able to diffuse between the interlayer space in van der Waals cations permeate through pores or atomic defects in graphene structures,13−16 and permeate through the graphene layer via (as schematically illustrated in Figure 1a) and diffuse further atomic defects.17−19 Although these processes have been between the graphene−SiO2 interface. Through electrostatic demonstrated to be useful for molecular separation and ionic effects, most likely involving screening of the negatively sieving,17,20 its impact on the electronic and optoelectronic charged SiO2 surface by the intercalated cations, the p-type properties of graphene has not been considered and doping effect from the substrate is counteracted. Based on first- investigated. In addition, for graphene in contact with aqueous solutions, ion-induced variations of the charge carrier density, principles calculations, we find that, in line with our n, the role of defects in graphene, and the supporting substrate experimental observations, cations favor residing between have remained elusive or mostly unexplored. graphene and SiO2 rather than on top of graphene. Our Here, employing optical pump−terahertz (THz) probe report reveals the process of cation permeability through (OPTP) spectroscopy as a noncontact, all-optical method to graphene and its impact on the electronic properties of determine the conductivity, we investigate the impact of a graphene, highlighting the importance of the interfacial series of cations (based on chloride salts XCl or YCl2, with X = chemistry for graphene’s applications in electrochemistry and Li+, Na+, K+, and Y = Ca2+) in aqueous solution on the electronics. These results have implications for graphene-based electrical conductivity of graphene supported by SiO2. We ion-sensing devices, graphene device processing, and possible demonstrate that metal cations affect the conductivity mainly applications of graphene in desalination membranes. 9030 DOI: 10.1021/acs.nanolett.9b04053 Nano Lett. 2019, 19, 9029−9036 Nano Letters Letter Figure 2. Effect of cations on the THz photoconductivity and Raman spectrum of graphene. (a) Cation (Na+) concentration-dependent THz conductivity (−ΔE/E0) dynamics. The dynamics are recorded when the signal is saturated after graphene is in contact with electrolyte. (b) Peak value of the pump-induced THz conductivity for various ions (Li+, Na+, K+, Ca2+) as a function of ion concentration. All data points are normalized to the THz conductivity value in Milli-Q water (shown as the gray dashed line). (c) Raman spectroscopy of graphene in the electrolyte. The G peak positions are 1592.1 cm−1 for graphene in 0.01 M Na+ and 1598.2 cm−1 for Milli-Q. This indicates a shift of the Fermi level toward the Dirac point, as shown in the schematics on the right. Results. We study the electrolyte effect on the electronic Epump − E0 (where Epump and E0 are the transmissions with and properties of graphene using monolayer graphene grown by without photoexcitation, respectively), is proportional to the chemical vapor deposition (CVD) that is transferred onto a conductivity change (−ΔE ∝ Δσ) of graphene. Whereas this fused silica (SiO2) substrate (details in the Supporting technique is a powerful tool to study the ultrafast charge Information). As typically observed, this leads to hole-doped dynamics and conductivity in graphene,25−28,30−34 it also graphene. The origin of this p-doping effect has been widely allows for obtaining information on the steady-state Fermi studied. Employing first-principles simulations, Nistor et al. energy in a rather straightforward manner. The reason for this identified the (negatively charged) dangling oxygen bonds is that the sign and magnitude of the pump-induced THz present in the SiO2 substrate as one of the main sources.21 photoconductivity (−ΔE/E0 or Δσ) are directly correlated to Other origins, including hole-donating impurities either in the the Fermi energy in graphene, as we illustrate in Figure 1c: substrate lattice or on the graphene surfaces (such as molecular When the Fermi level is close to the Dirac point in graphene (| O2), have been reported and proposed.22−24 We characterize EF| ≲ 0.10 eV), photoexcitation leads to graphene becoming the initial Fermi level of graphene on silica (without more conductive; that is, the THz conductivity is positive electrolyte) by Raman spectroscopy and find it to be |EF| = following photoexcitation, whereas with the Fermi level away 220−280 meV (below the Dirac point). We then bring the from the Dirac point (|EF| ≳ 0.15 eV), graphene becomes more graphene in contact with electrolytes by sandwiching the resistive after the optical pump, so the THz conductivity is electrolyte of interest between the graphene (supported by negative.25−28,30,32,33 Thus, by monitoring the sign and SiO2) and a top SiO2 glass; we limit the thickness of the magnitude of the changes in THz conductivity, we readily electrolyte by a Teflon spacer to be ∼10 μm to ensure obtain the modulation of the Fermi energy in graphene. sufficient THz transmission. In our study, the fluence of the Importantly, this enables us to unambiguously disentangle laser pump is chosen such that we observe no change of THz changes in carrier density from changes in carrier mobility. conductivity of graphene over 1 h of measurements, as shown To interrogate the effect of the electrolyte or, more in Figure S1. This indicates that our technique is noninvasive specifically, the role of hydrated cations in aqueous solution and does not create new defects or increase the size of existing on the carrier conductivity in graphene, we study four different defects. metal chloride solutions with various cation sizes: KCl, NaCl, Size-Dependent Cation Induced Doping Effect in Gra- LiCl, and CaCl2. To quantitatively study the effect of cations phene. To monitor how cations affect the graphene on the Fermi level, we fix the anion to Cl− and the solvent to conductivity, we employ OPTP spectroscopy. In a typical H2O, thus focusing on the effect of cations. First, we OPTP measurement, an 800 nm laser pulse with sub-100 fs investigate the effect of ionic concentration on the pump- duration photoexcites graphene. Subsequently, we record the induced THz conductivity (referred to as “THz photo- THz transmission change induced by this pump as a function conductivity” below) of graphene for the four different of delay time29 between the optical excitation and THz pulse electrolytes. The electrolyte solutions do not absorb the (Figure 1b). The change in the transmitted THz field, ΔE = excitation light and therefore show no differential terahertz 9031 DOI: 10.1021/acs.nanolett.9b04053 Nano Lett. 2019, 19, 9029−9036 Nano Letters Letter Figure 3. Time evolution of the cation-induced Fermi level shift in graphene. (a) Kinetic sodium ion (Na+)-induced Fermi level shift in graphene, as revealed by monitoring the pump-induced change in THz transmission depending on the contacting time of sodium ion solution, 0.5 M NaCl aqueous solution. All measurements are conducted in the same spot of a graphene sample. (b) Peak value of the pump-induced THz change, −ΔE/ E0, as a function of contacting time for metal cations including K+, Na+, Li+, and Ca2+. All data points are normalized to the same starting point for a better comparison. (c) Doping time is plotted as the function of the radius of the hydrated ion. (d) Archived final Fermi levels (representing by the cation-induced THz conductivity changes in graphene) vs the hydrated cation radius. response. We show typical results for NaCl in Figure 2a: the photoconductivity of graphene (by taking the peak values electrolyte gives rise to a decrease in the magnitude of the charge carrier dynamics shown in Figure 2a) for a series of negative conductivity in graphene, using the THz photo- cations with varying ionic concentration, as shown in Figure conductivity of graphene in contact with Milli-Q water as a 2b. Whereas all electrolytes show qualitatively the same effect reference. At sufficiently high concentrations, the sign of the (less negative photoconductivity and thus a Fermi energy shift photoconductivity becomes positive (>0.1 M for NaCl, as toward the Dirac point), the achieved final Fermi level in presented in Figure 2a). Following the discussion related to graphene displays an obvious cation size effect with a trend Figure 1b, and given the fact that the initial doping of our from the largest shift for K+ (with the smallest solvation radius graphene is p-type (∼220−280 meV below the Dirac point), of ∼3.3 Å among all cations studied here35) to the smallest we conclude that the NaCl electrolyte induces a shift of the change for Ca2+ (with the largest solvation radius of ∼4.1 Å for Fermi level toward the Dirac point (i.e., less hole doping) by the cations used35). Sodium and lithium ions with similar size an amount ΔEF ∼ 200 meV. of hydrated radius have comparable effects at high concen- As an independent verification of the observations using tration. In our study, we use the hydrated ion (solvation) OPTP measurements, we perform Raman measurements, radius (defined as the radius of the hydration sphere of the which allow for extracting the change of Fermi level in ions) rather than the ion radius because the solvation radius is graphene. As shown in Figure 2c, we observe a shift of the G relevant for transport in an aqueous solution. band from 1598.2 cm−1 for graphene in contact with 0.01 M Tracking the Cation Induced Doping Kinetics in NaCl electrolyte to 1592.1 cm−1 for graphene in contact with Graphene. To further investigate the underlying mechanism Milli-Q water, corresponding to a shift of Fermi level of ∼150 for the observed size-dependent cation effect on the Fermi meV toward the Dirac point, in agreement with our OPTP level, we monitor in situ the kinetics of the Fermi level changes results (∼200 meV upshift of the Fermi level in graphene by by tracking the THz photoconductivity as a function of 0.5 M NaCl electrolyte). graphene−electrolyte contacting time. In Figure 3a for In the following, we vary the cation concentration and study graphene in contact with a 0.5 M sodium chloride solution, how this affects the THz photoconductivity with respect to the ion-induced Fermi level shift is not an instantaneous that of graphene in contact with Milli-Q water as a reference. process. Instead, it takes ∼80 min for the THz conductivity of We summarize the cation-induced effect on the THz graphene to gradually turn from negative to positive 9032 DOI: 10.1021/acs.nanolett.9b04053 Nano Lett. 2019, 19, 9029−9036 Nano Letters Letter Figure 4. Simulation of cation effect at the graphene/SiO2 interface. (a) System energetics for Na adsorbing at graphene’s top surface, the energy barrier for Na permeation through a graphene double vacancy defect, and Na residing at the graphene−SiO2 substrate interfaces. (b−d) Modeled atomic structures in cross-sectional views corresponding to the three energy states in (a). When Na is at graphene’s top surface and at SiO2’s surface beneath graphene, the calculated Fermi energies of the system are −0.93 and −0.49 eV, respectively (details in Figure S6). (e) Na residing at graphene−SiO2 substrate interfaces after it permeates through a double vacancy defect (highlighted as a yellow polygon). (corresponding to a change from hole-doped graphene to dependent kinetics and degree of Fermi energy shift induced graphene with a Fermi energy in the vicinity of the Dirac by cations (as shown in Figure 3c,d) is highly reproducible for point). We also study these cation-induced dynamics for all samples, we find that the absolute changes (e.g., doping graphene in contact with cations of different concentrations time and magnitude) vary from sample to sample, presumably and sizes. First, we measure the ionic effect induced by 10−2 due to the variations in initial doping and defect densities of and 0.5 M NaCl electrolyte at the same sample spot of a graphene in different samples (see the extended discussion in graphene sample, as demonstrated in Figure S2a. The time the next section). required for reaching the final Fermi energy is much shorter for Ionic Permeation and Interfacial Doping of Supported increased concentrations (∼80 min vs 150 min to change the Graphene. The observed cation-induced change in Fermi THz conductivity sign from negative to positive for 0.5 M in energy (more specifically, the reduced hole density) challenges comparison to 10−2 M NaCl, respectively). Second, we the intuitive and straightforward view that the cations are compare concentration-dependent THz conductivity measure- located on the top surface of graphene (which would not give ments for K+, Li+, and Ca2+. For a reliable comparison, we rise to the observed slow, size- and time-dependent doping conduct all the THz measurements in a single spot of a processes), and that they act only passively as scattering graphene sample and fix the cationic concentration to be 0.5 M centers, solely modifying the carrier mobility. Instead, our for all cases (for all original data, see Figure S2b in the results here can be explained by an ionic process that involves Supporting Information). We describe the normalized time- concentration-, size-, and time-dependent permeation of metal dependent dynamics of the photoconductivity change by a cations through atomic defects in graphene.17−19 The single-exponential process, as indicated by the solid lines in intercalated cations between the graphene and substrate Figure 3b. We plot the inferred exponential time scales and the surfaces then electrostatically screen the p-doping effect of final photoconductivity as a function of the solvation radius of the SiO2 substrate, thus resulting in graphene with reduced p- the cations in Figure 3c,d. It appears that all three small cations doping. In principle, an atomically perfect graphene lattice is (Li+, Na+, K+) used here can effectively modify the carrier impermeable for all ions except for protons (H+), thus acting density of graphene. However, cations with a larger ionic as an ideal sieve to protons from other cations.20 This picture, solvation radius (K+ < Na+ < Li+) require an increasingly however, does not hold for large-area graphene with inevitable longer time to achieve the Fermi level shift. For the largest atomic defects, such as missing carbon atoms resulting in hydrated ions, Ca2+, we observe a rather slight change in nanopores. Indeed, permeation of ions and water molecules graphene’s conductivity and, thus, Fermi energy. Finally, it is through graphene atomic defects has previously been worth commenting that although the trend of the size- reported,17−19 and selective cation transport through graphene 9033 DOI: 10.1021/acs.nanolett.9b04053 Nano Lett. 2019, 19, 9029−9036 Nano Letters Letter nanopores has also been demonstrated.17−19,36−38 Moreover, and the isolated parts, suggesting high stability. We then add a based on atomic force microscopy measurements of the CVD Na atom (consisting of Na+ ion plus an electron) into the graphene used here (see Figure S3), we find grain boundaries model system to investigate the cation effect on graphene. To and holes in the size range of up to several micrometers, which simplify the calculation, here, only Na+ is included in the can be the initial sites for cation intercalation. As such, cation simulation, and charge neutralization is realized by adding an permeation is kinetically possible in our CVD graphene and is electron instead of a Cl− anion. Despite the simplicity of our thermodynamically favorable (which is further supported by model, it captures the essence of the cation−graphene the simulation discussion in Figure 4). The cations tend to interaction and explains our experimental data well (as move to the supported silica surfaces and remain intercalated shown below). First, we investigate the energetics for two between graphene and the silica substrate, due to the negative configurations of Na+ either on top of graphene or residing surface potential of silica at neutral pH. The permeated cations between graphene and the SiO2 support. As shown in Figure can subsequently diffuse in-plane, away from the defect 4a, our results clearly illustrate that it is energetically more centers, along the graphene−SiO2 interface. favorable for the cation to reside between graphene and SiO2 For a certain size and amount of atomic defects in graphene, than to be adsorbed at the top surface of graphene, in good ionic permeation, as well as the subsequent diffusion of ions agreement with the conclusions from the experiments. intercalated between graphene and SiO2, depends critically on Furthermore, we explore the kinetic Na+ permeation processes the size of the hydrated ions.18,19,36−38 Therefore, we expect for graphene with (e.g., single vacancies and double vacancies) cationic permeation and in-plane diffusion to speed up with smaller solvated ion size, as we indeed observe. When the size and without atomic defects (detailed in the Supporting of hydrated cations or a hydrated cation complex (as for the Information). The permeation of Na+ through a perfect case of Ca2+39) further increases to be comparable or larger graphene layer is associated with a prohibitively high energetic than the defects or nanopore size in graphene, ionic blockade barrier (>10 eV). The presence of a double vacancy reduces can take place,36 also in accordance with our experiment. the barrier to 0.69 eV (3.22 eV with a single vacancy, as shown To further support our explanation and model of ionic in Figure S5), which is comparable to the penetration barrier permeation and the resultant shift of the Fermi energy of of 1.2−1.6 eV for a proton to migrate through the perfect graphene by the intercalated cations, we highlight two graphene monolayer.20 Finally, to investigate the impact of additional experimental observations. First, following the intercalating cations on graphene, we compute the density of equilibration of the graphene/silica system with a specific states (DOS) of the complex with and without Na+ (Figure electrolyte solution, we exchange that solution with pure water S6). The Fermi level appreciably shifts toward less negative and investigate the reverse process of the Fermi level shift values (from −0.93 to −0.49 eV) when Na+ is included in the (“redoping”) (Figure S2c). We find that substrate-induced complex. Figure S7 shows the partial DOS of the C atoms hole doping is recovered after rinsing away the solutions around Na+. Substantial p-orbitals of these carbon atoms containing the cations Li+, Na+, and K+. The recovery of the emerge in the vicinity of the up-shifted Fermi level, suggesting doping level in the absence of ions exhibits distinct size- that the new filled state originates from electrons in the p- dependent kinetics (detailed discussion in Figure S2). If the orbitals of the C atoms in the vicinity of interfacial Na+. The cations would not intercalate between SiO2 and graphene, we shift of Fermi level in the complex system induced by Na+ is in would expect the ions to dissolve very quickly back into the line with the experimental results. water when the graphene is contacted with pure water. Instead, Summary. To summarize, employing optical pump−THz we observe a time scale on the order of tens of minutes and a probe spectroscopy as a contact-free, all-optical conductivity clear dependence on ion hydration size, thus corroborating our measurement, we investigate the role of a series of cations with picture of intercalated cations. Second, we find that the relative various solvation radii (based on chloride salt XCl and YCl2 change in THz conductivity (and thus Fermi level shift) for a with X = Li+, Na+, K+, and Y = Ca2+) on the electrical given ionic composition and concentration is much more conductivity of graphene. We observe a cation-induced shift of pronounced when the graphene contains fewer polymer the Fermi level in excess of 200 meV toward the Dirac point in residues (by comparing two sets of graphene samples using graphene. Combined with theoretical calculations, such an different post-transfer treatments; details in Figure S4). We ionic effect in graphene appears to involve cationic permeation rationalize that the Fermi level shift depends crucially on the through defects in graphene, which is kinetically controlled by sample cleanness; that is, when the cation permeation pathway the interplay of the hydration radius of cations and the size of is blocked by the polymer residuals, a smaller change in carrier density in graphene is achieved. atomic defects in graphene. Simulations. We now present first-principles calculations by These findings not only provide a new tool to monitor the which we explore the interaction between SiO2-supported ionic dynamics at graphene and potentially other materials graphene and electrolyte solutions. The simulations are based interfaces but also offer new insights into the role of cation on spin-polarized density functional theory calculations as permeability through graphene and the influence of ion implemented in the Vienna ab initio simulation package,40,41 interfacial dynamics on electronic properties. Finally, these using Perdew−Burke−Ernzerhof42 functionals and the plane- results are of interest for electrochemical charge and energy wave projector augmented wave43 method with van der Waals storage, graphene device processing (where, for example, interactions. etching chemical or electrolytes are extensively in use), In the optimized structure of supported graphene, the Si−O desalination membranes, and ionic-sensing devices. Regarding bond lengths are in the range of 1.60−1.62 Å, in good the last case, controlling the size and density of atomic defects agreement with those in previous studies.44,45 The binding in graphene, as well as the nature of surface charges in energy of −0.84 eV for a unit cell of the complex system is supporting substrates, can serve as effective and novel calculated as the energy difference between the bonded system approaches to enhance the ionic sensitivity and specificity. 9034 DOI: 10.1021/acs.nanolett.9b04053 Nano Lett. 2019, 19, 9029−9036 Nano Letters Letter ■ * ASSOCIATED CONTENT S Supporting Information European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349. ICN2 is The Supporting Information is available free of charge at supported by the Severo Ochoa program from Spanish https://pubs.acs.org/doi/10.1021/acs.nanolett.9b04053. MINECO (Grant No. SEV-2017-0706). F.H.L.K. acknowl- edges financial support by Fundacio Cellex Barcelona, support Optical pump−THz probe spectroscopy; graphene from the Spanish Ministry of Economy and Competitiveness, transfer; electrolyte contacting; computational methods through the “Severo Ochoa” Programme for Centres of and details; stability of graphene in laser excitation; Excellence in R&D (SEV-2015-0522), and support by the evolution of the cation doping dynamics on graphene; Generalitat de Catalunya through the CERCA program. ■ surface characterization of graphene on SiO2; compar- ison between less-clean graphene and clean graphene on fused silica; energy landscape and modeled structures for REFERENCES Na + ion penetrating vacancies; density of state (1) Fu, W.; Jiang, L.; van Geest, E. P.; Lima, L. M. C.; Schneider, G. calculation result of graphene−SiO2 interface; calculated F. Sensing at the Surface of Graphene Field-Effect Transistors. Adv. 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