Enhanced Chemical Reactivity of Graphene by Fermi Level Modulation

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Cite This: Chem. Mater. 2018, 30, 5602−5609

Enhanced Chemical Reactivity of Graphene by Fermi Level Modulation Myung Jin Park,*,†,⊥ Hae-Hyun Choi,‡ Baekwon Park,‡ Jae Yoon Lee,† Chul-Ho Lee,† Yong Seok Choi,‡ Youngsoo Kim,§ Je Min Yoo,‡ Hyukjin Lee,∥ and Byung Hee Hong*,‡

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†

KU-KIST Graduate School of Converging Science & Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Korea ‡ Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 08826, Korea § Graphene Square Inc. Inter-University Semiconductor Research Center, Seoul National University, 1 Gwanak-ro, Seoul 08826, Korea ∥ College of Pharmacy, Ewha Womans University, 11-1 Daehyun-Dong, Seodaemun-Gu, Seoul 120-750, Korea ⊥ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: Among various approaches to modify the electronic and chemical properties of graphene, functionalization is one of the most facile ways to tailor these properties. The rearranged structure with covalently bonded diazonium molecules exhibits distinct semiconducting property, and the attached diazonium enables subsequent chemical reactions. Notably, the rate of diazonium functionalization depends on the substrate and the presence of strain. Meanwhile, according to the Gerischer-Marcus theory, this reactivity can be further tuned by adjusting the Fermi level. Here, we precisely controlled the Fermi level of graphene by introducing the selfassembled monolayer (SAM) and investigated the degree of chemical reactivity of graphene with respect to the doping types. The n-doped graphene exhibited the highest reactivity not only for diazonium molecules but also for metal ions. The increased reactivity is originated from a remarkable electron donor effect over the entire area. In addition, the n-doped graphene enabled spatially patterned functionalization of diazonium molecules, which was further utilized as a growth template for gold particles that would be advantageous for enhanced electrochemical reactivity. engineering14 or by employing appropriate substrates with inhomogeneous charge fluctuations.15 Meanwhile, according to the Gerischer−Marcus theory, the electron transfer rate is proportional to the allowed redox potential gap between different reactants.11,16 Given that graphene has a linearly dispersed band structure, the Fermi level modulation by external condition can be achieved, which indicates that the electron transfer rate from graphene can be controlled. This unique band structure that shows limitations in logic device applications is still worth investigating in terms of graphene chemistry. We noted that the Fermi level of graphene is susceptible to the substrates and/or the presence of external molecules.17−20 Up to date, various doping methods influencing the Fermi level of graphene have been reported including electric field,21 self-assembled monolayer (SAM),22,23 and photoinduced

P

roviding new functions to graphene beyond its intrinsic properties is important as it can broaden the potential applications for electronic devices1,2 and biosensors.3,4 Studies on the chemical functionalization of graphene with diazonium molecules have been exploited for bandgap tuning5−7 and fluorescence probe/biological sensor development.8,9 For efficient chemical functionalization, proper understanding of chemical reaction between this gapless substance and other reactants must be preceded. In graphene, pi-electrons in the perpendicularly positioned pz orbital of sp2 hybridized structure can serve as an electron donor in chemical reactions. Previous works demonstrated that electron transfer from graphene to diazonium salt allowed covalent bond formation because the Fermi level of graphene and the vacant oxidation state of diazonium salt are well-matched.11 Other studies suggested that the chemical reactivity of graphene was more activated along the edge than the basal plane due to the existence of dangling bonds.10,12,13 Notably, the rate of graphene functionalization can be enhanced by strain © 2018 American Chemical Society

Received: April 18, 2018 Revised: August 1, 2018 Published: August 1, 2018 5602

DOI: 10.1021/acs.chemmater.8b01614 Chem. Mater. 2018, 30, 5602−5609

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Figure 1. (a) Schematic illustration of the overall reaction process (i.e., reaction with diazonium and gold chloride) through the electron transfer from n-doped G/APTES. Yellow region of the left image represents the SAM on SiO2/Si substrate. (b) Isd−Vg transport characteristics of graphene supported on different doping inducing substrates. Inset images indicate graphene samples transferred on each substrate with different Fermi levels. (c) Correlation between G and 2D peaks of G/OTS (black symbols), G/SiO2 (blue symbols), and G/APTES (red symbols). Inset: arrows marked as eC, eT, eH, and en represent vectors affected by compressive strain, tensile strain, hole doping, and electron doping effect, respectively. Green dot (zero point) represents the strain and doping free graphene. Linear slopes are for the strain (black color), hole doping (red color), and electron doping (dark yellow color), respectively, which were acquired from refs 21 and 26.

doping.24,25 Among these, the SAM method is particularly useful due to its stability even in the ambient condition. It can be directly applied to the oxide substrate through silanization and introduction of functional groups at the end of silane can effectively influence the Fermi level of graphene. To decide the Fermi level with respect to the doping type, substrate modifications were carried out with different SAMs: (1) an n-doped substrate by 3-aminopropyltriethoxysilane (APTES) with electron-donating lone pairs in −NH2, (2) a neutral substrate treated with octadecyltrichlorosilane (OTS) that blocks the undesired charge, and (3) a pristine SiO2 substrate that has p-doping character caused by charged impurities. The chemical reaction of graphene shows distinguishable behaviors for each doping type. The graphene on APTES (G/ APTES) with the highest Fermi level exhibits significantly higher reactivity not only for the diazonium but also for the gold chloride compared with those of graphene on OTS (G/ OTS) or SiO2 (G/SiO2). Our results suggest the potential of graphene to be utilized as a chemical platform in the 2D material chemistry. The electron transfer between graphene and diazonium is thermodynamically allowed because the vacant oxidation state of diazonium is below the Fermi level of graphene. The electron transfer rate (kET) can be defined by the equation from the modified Gerischer−Marcus theory,10,11,13−16

kET = ν

∫E

E F,G

εred(E)DOSG(E)Wox(E)dE

redox

where ν is the electron transfer frequency, EF,G is the Fermi level of graphene, Eredox is the redox potential of reactant, εred is the proportionality factor, DOSG is the density of state of graphene, and Wox(E) is the probability density function of vacant states in the diazonium. From this equation, we assume that the reaction rate of graphene is directly influenced by and proportional to the Fermi level. Schematic illustration in Figure 1a shows the representative overall reaction process through the electron transfer in ndoped G/APTES. The neutral and p-doping types are decided by with or without OTS treatment on the SiO2/Si substrate, respectively. Reaction for all the samples proceeded under the same condition. To investigate the charge density of graphene corresponding to the doping types, we analyzed the characteristics of the graphene field effect transistors (FET) and Raman spectra. Figure 1b shows the FET characteristics for each graphene. The Dirac point of G/SiO2 appears at the positive Dirac voltage (Vdirac = +45 V), which is shifted to Vdirac = +5 V and Vdirac = −165 V, respectively, for G/OTS and G/APTES. This indicates that the Fermi level of graphene is effectively adjusted by the underlying substrates. The spatial doping analysis of the 5603

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Figure 2. (a) Representative Raman spectra of graphene that was supported on each substrate before and after diazonium functionalization. (b−g) Spatial Raman mapping images of D/G intensity ratio (ID/G) corresponding to graphene supported on each substrate before (b, d, f) and after diazonium functionalization (c, e, g). Mapping images were collected from 225 spectra points for each sample. Same regions were measured to compare with before and after diazonium functionalization. All of the mapping areas are 30 μm × 30 μm, and scale bars are 10 μm.

Figure 3. Raman peak analysis extracted from mapping data before (O) and after (X) diazonium functionalization. (a) I2D/G as a function of G peak position. (b) fwhm of 2D peak as a function of 2D peak position. (c) fwhm of G peak as a function of G peak position. Dotted line was obtained from ref 21. (d) ID/G as a function of the modified Fermi level. Fermi level values for each sample are obtained from experimental Raman mapping data of before functionalization and those values for G/OTS and G/SiO2 were adjusted by considering the electron−hole puddle model.

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Figure 2a, the diazonium related peaks are definitely observed in all samples. This indicates that graphene can be doped by both physical and chemical bond formation of diazonium.12 Figure 3c is a graph for the G peak fwhm plotted against the G peak position. The solid line indicates the doped graphene referenced from ref 21 to differentiate doping and strain effect. The fwhm depends the doping effect before reaction, while it is influenced by the strain effect after reaction. This indicates that the surface of graphene lies on the mechanical deformation when graphene is exposed to diazonium. To investigate the surface morphology influenced by diazonium, G/APTES in before and after reaction were analyzed using atomic force microscopy (AFM) (Figure S2). The thickness of chemical vapor deposition (CVD) graphene was about 0.68 nm, which is close to the monolayer. The thickness increased to 1.48 nm through the APTES SAM treatment, and it was even more up to 4.02 nm by diazonium functionalization (Figure S2b,c). Surface roughness was also increased, which was confirmed by the height histogram for the entire region (Figure S2d). Interestingly, the diazonium functionalization made the wrinkled regions clearer, resulting in surrounding areas to be protruded (Figure S2c). This informs that the attachment of diazonium molecules is locally activated at the curvatured regions, leading to additional deformation. Figure 3d is a graph for D/G intensity ratio (ID/G) plotted against the Fermi level. Fermi level was modified by considering the electron/hole puddle model to explain the unexpected behavior in Figure 2, where ID/G of G/SiO2 was larger than that of G/OTS. Previous reports investigated that randomly fluctuated charge potential of SiO2 plays a negative role with undesired doping effect in graphene devices.31,32 This random potential was explained by the electron/hole puddle model, which was introduced to calculate the practical Fermi level used in the reaction.15 The modified Fermi level (EF,n) is derived from average Fermi level (EF,avg) (Figure S3). First, EF,avg is obtained from the IG/I2D and can be described by33

channel area for each sample was conducted by a Raman template that elucidates the correlation between strain and doping effects (Figure 1c).26 Hundreds of data points for G and 2D modes were acquired by Lorentzian fitting. When graphene is under strain (compressive and tensile) or hole doping effect, both G and 2D peaks are upshifted with different slopes; an uniaxial strain slope is (Δω2D/ΔωG)εuniaxial = 2.2, and a hole doping slope is (Δω2D/ΔωG)nhole = 0.75. The Raman points of G/SiO2 follow the uniaxial strain slope, moving toward the hole doping vector, which indicates that graphene is under p-doping (n: 4.8 × 1012 cm−2 ≈ Vdirac = 67 V) effect with compressive strain (εc: 0.1%). Those of G/OTS are converged near 1582.5 ± 0.7 cm−2 and 2679.2 ± 1.1 cm−2, which correspond under relatively low p-doping (n: 1.8 × 1012 cm−2 ≈ Vdirac = 25 V) with negligible strain. On the contrary, the points of G/APTES are deviated downward from the hole doping slope. Considering that the 2D peak of graphene is less sensitive to n-doping than p-doping and the peak is downshifted under the high doping level, this is consistent with the previous report.21 Overall, the spectral behaviors for each sample coincide with the doping types, even though the charge densities are somewhat larger than those values derived from the FET measurement due to the gate-induced p-doping effect.20,27 To investigate the correlation between the chemical reaction and the Fermi level height of graphene, each sample with different doping types was reacted in the 4-nitrobenzenediazonium tetrafluoroborate (4-NBD) solution at 30 °C for 4 h and analyzed through Raman peak information. In Figure 2a, differently doped samples had a sp3 bond related D peak with different intensities in the order of G/APTES, G/SiO2, and G/ OTS after functionalization, which was clearly verified in mapping images (Figure 2b−g and Figure S1). This indicates that electron transfer the most actively occurs in the n-doped graphene. However, the D peak intensity for G/SiO2 was higher than those of G/OTS, which is inconsistent with the reactivity being proportional to the Fermi level. This can be explained by the electron−hole puddle model and will be discussed later. We further investigated the spectral behavior with respect to the reaction through the G and 2D peak information from the mapping data in Figure 2. The 2D peak is noticeably sensitive to the doping density, leading to decrease of its intensity and increase of full width at half-maximum (fwhm) under high doping density. This is originated from the increased electron− electron collision and inelastic scattering rates during the double resonance process.28−30 In addition, the G peak position is upshifted proportionally to the doping density. Figure 3a is a graph for 2D/G intensity ratio (I2D/G) plotted against the G peak position. Before diazonium functionalization, the data points of I2D/G for each sample are decreased in the order of G/OTS, G/SiO2, and G/APTES, which is similar behavior to the Vdirac position in FET. After diazonium functionalization, I2D/G for all samples decreased due to the p-doping effect of the nitro group in diazonium.14 The data points of the 2D peak fwhm are also in agreement with the doping effect, which follows the order of doping density initially and further increases after functionalization. However, it is hard to differentiate the doping effect with respect to functionalization that causes the D peak. This is due to the physical adsorption tendency of diazonium molecules being able to form the charge transfer complexes on the basal plane of graphene without breaking sp2 bonds. In

IG = C(γe ‐ ph + f (ε)|E F,avg |) I2D

(1)

where γe‑ph (≈ 21 meV) is the average electron scattering energy of phonon emission at 514 nm and C (≈12.38 eV−1) is the constant modified from ref 33. f is a function depending on the dielectric constant (ε) and the calculated value is 0.075 for SiO2 (ε ≈ 4).28 EF,avg of each sample is adjusted as (−) for G/ APTES and (+) for the others, respectively, which was decided from the Vdirac position in the FET data. Modified Fermi level (EF,n) influenced by electron−hole puddle can be described by15 E F,n = E F,avg + α(ΔΓ2D)

(2)

where α (= 0.08 eV cm) is a proportionality constant and ΔΓ2D is the fwhm difference of the 2D peak between pristine graphene (≈ 23 cm−2)26 and doped graphene. The modified Fermi level (EF,n) was only applied to the G/SiO2 and the G/ OTS because the G/APTES showed n-doping behavior (Figure S3). Consequently, the data in Figure 3d derived from eqs 1 and 2 demonstrates that the reactivity is proportional to the Fermi level. The n-doped graphene showed catalytic character, which enabled selective functionalization (Figure 4). This selectivity clearly demonstrates that 5605

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Figure 4. Spatially controlled graphene functionalization. (a) Micropatterned OM image (50 × 50 μm2) fabricated by the electron-beam (E-beam) lithography. APTES is deposited on the micropatterned area (purple area). Subsequently, E-beam resist is removed by acetone. (b) OM image of transferred CVD graphene on the selectively patterned APTES-SAM substrate. (c) Mapping image of G peak position before functionalization. G peak position is upshifted at patterned area. (d) Mapping image of 2D peak position before functionalization. Patterned image is not differentiated because of the insensitivity of 2D peak under n-doping. (e) Mapping image of I2D/G before functionalization. The ratio is relatively lower along the patterned area, which indicates that doping density of APTES is higher than that of SiO2. (f) Mapping image of ID/G before functionalization. The ratio is almost 0 over the entire area, which indicates that the defect density of CVD graphene is negligible. (g) Mapping image of ID/G after functionalization. The ratio increases along the APTES patterned area. (h) ID/G height profile for a dotted line in (g). ID/G is higher along the patterned area where graphene is functionalized with diazonium.

the degree of concentration at G/OTS was comparable to G/ SiO2 or even more distinguishably higher in some samples, which is the opposite behavior where G/SiO2 shows higher reactivity than G/OTS in the diazonium solution (Figure 5a− i). Thus, we extended the existing electron−hole puddle model to account for this behavior (Figure 5j). In the diazonium reaction, each diazonium cation needs one electron to make a bond with graphene. On the other hand, in the gold ion reduction, the gold ion needs three electrons for the reduction of one atom (Au3+(aq) → Au0(s)), which is described by following formula.

the introducing functional molecules that directly affects the Fermi level height of graphene is more favorable to the chemical reaction than the pristine substrate. Selectively functionalized 2D region gives the versatility to the graphene to be utilized in the vertical direction as well as in the lateral direction. To further investigate the activated electron transfer of ndoped graphene, we observed that novel metal ions with high oxidation number are reduced on graphene. For example, introducing gold particles to graphene is known to increase the Fermi level of graphene.34 Assuming that gold particle is directly grown on graphene, it could provide additional ndoping. In fact, the gold particles were well-formed in all samples without any addition of reducing agents (Figure 5 and Supporting Information). Since the redox potential of gold ion (−5.5 eV) is lower than that of graphene (−4.4 eV), it is assumed that the electron transfer from graphene to gold ions is energetically favorable.35 The gold particle concentration was the highest at G/APTES, which is analogous to diazonium functionalization. However,

AuCl4 −(aq) + 3e− → Au 0(s) + 4Cl−(aq)

In this reaction driven by multi-electron transfer, the uniformity of the Fermi level height can be more important than the electron−hole puddle with high amplitude. Assuming that the distance between puddles for the electron transfer is large, electrons cannot be effectively supplied to Au3+ (Figure 5j). The uniformity of the Fermi level height was confirmed 5606

DOI: 10.1021/acs.chemmater.8b01614 Chem. Mater. 2018, 30, 5602−5609

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Figure 5. Optical microscopy (OM) and scanning electron microscopy (SEM) images for the reduction of gold ion on the graphene: (1) Bright field (a, d, g), (2) dark field (b, e, h), and (3) SEM (c, f, i) images. (a−c) are for the G/SiO2. (d−f) are for the G/OTS. (g−i) are for the G/ APTES. Scale bar: 50 μm (bright field and dark field images), 10 μm (SEM images), and 2 μm (inset images). (j) Schematic illustration of extended electron−hole puddle model in the gold ion/graphene reaction. Dirac cone images with different Fermi levels represent the doping type for each sample. Solid lines indicate the average Fermi level, and solid curves represent the degree of charge fluctuation. Graphene with high Fermi level in the low charge fluctuation is appropriate to the multielectrons transfer for the reduction of metal ion with high oxidation number. On the other hand, graphene with low Fermi level in the high charge fluctuation is inappropriate for the reduction of gold ions due to the long distance for the multielectrons transfer.

with the n-doped graphene that serves as a catalytic growth flatform.36,37 In conclusion, we demonstrated the chemical reaction of the graphene with respect to the Fermi level. The Fermi level was successfully modulated by applying self-assembled monolayers with different doping agents onto underlying SiO2 substrates. The degree of chemical reactivity on the graphene surface was directly governed by the type of doping. Among the samples with different doping types, the n-doped graphene exhibited the most remarkable reactivity toward organic molecules (4NBD) as well as metal ions (AuCl4−). This indicates that electron-rich n-doped graphene plays a pivotal role in the chemical reduction due to increased electron donating effect. Raman and FET analysis demonstrated that n-doped graphene is more reactive than graphene supported on SiO2 with charge fluctuation, which allowed the selective functionalization. Furthermore, gold particles formed on n-doped graphene enabled additional n-doping, which was demonstrated by the HER measurement. The n-doped graphene will provide the way to be utilized as a platform for the growth of functional materials. Our study suggests that the CVD graphene with modulated Fermi level can broaden the range of chemical applications by serving as an atomically thin catalytic platform in the 2D material chemistry.

from the Dirac point distribution in FET. We conducted the statistical analysis by measuring 12 FET devices per each doping type (Figure S4a). The Fermi level derived from FET measurement is described by E F = ℏνF πn , where ℏ is the plank constant, νF is the Fermi velocity, and n is the charge carrier density. The charge carrier density (n) is given by n = αVdirac, where α is 7.2 × 1010 cm−2 V−1 and Vdirac is the Dirac voltage.21 In Figure S4a, the Fermi level of G/SiO2 has a broad distribution compared to the other devices, which indicates that G/SiO2 is under the inhomogeneous charge fluctuation. This spatially fluctuated potential results in suppression of both electron and hole conductances, reducing the corresponding mobility (μ) (Figure S4b).38 In addition, we found that the reducing agent enhances the growth of gold particles. In Figure S5, the gold particles were further grown with dendrite shape by addition of ascorbic acid. To confirm the electrochemical reaction of graphene with respect to the Fermi level, we assessed the hydrogen evolution reaction (HER). Graphene transferred on the doping inducing substrate was masked by epoxy resin, leaving active area (1 × 1 cm2) and connecting as a working electrode to the standard three electrode system in a 0.5 M H2SO4 electrolyte. In polarization curves, the overpotential was lowered inversely proportional to the height of the Fermi level, in which the ndoped graphene with gold particles had the lowest value out of the samples (Figure S6a). This is attributed to the free electrons of gold particles that trigger additional n-doping, which facilitates electron transfer from graphene by lowering potential barrier for hydrogen reduction. Charge transfer impedance behaviors were also consistent with the polarization plots (Figure S6b). Electrocatalytic performance was not comparable to recent reports because we focused more on the doping dependent behaviors rather than overall catalytic performance. However, we believe that it has a potential to be improved through combining other functional materials

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METHODS

Chemicals. Poly(methyl methacrylate) (PMMA), ammonium persulfate (APS), 3-aminopropyltrimethoxysilane (APTES, −NH2 end group), octadecyltrichlorosilane (OTS, −CH3 end group), 4nitrobenzenediazonium tetrafluoroborate (4-NBD), sodium dodecyl sulfate (SDS), and gold(III) chloride trihydrate from Sigma-Aldrich; acetone from Samchun Pure Chemicals; deionized water from Purescience were used. CVD Graphene Growth and Transfer. The graphene was synthesized on 23 um thick copper foil by chemical vapor deposition (CVD) method in the mixture of CH4 (40 sccm) and H2 (4 sccm) 5607

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Chemistry of Materials gases with vacuum pumping at 1000 °C for 30 min. One side of graphene was protected by PMMA coating, and the other side was etched by reactive ion etcher (RIE). The Cu foil was etched with 80 mM of APS solution. After rinsing with distilled water, the graphene was transferred on target substrates and then dried under 80 °C for 5 h. Silanization and Device Fabrication. SiO2/Si substrates were under UV−ozone treatment to make silanol on the surface. Subsequently, substrates were immersed in APTES and OTS mixed toluene solution with different ratios (SAM:tolutene), 1:50 for APTES, and 1:100 for OTS, for 2 h, respectively. The reaction was conducted in the glovebox. After several rinsing with toluene, PMMA coated graphene was transferred on silanized substrates, subsequently, PMMA was removed by acetone. For the fabrication of graphene FET, Cr/Au (5 nm/40 nm) electrodes were deposited on the substrate using the prepatterned stencil mask. Channel areas (50 × 250 μm2 (length × width)) were isolated by e-beam lithography. Drain−source current (Ids) was measured at constant drain−source voltage (Vds = 10 mV). Patterning of APTES-SAMs. The SiO2/Si substrate was patterned by e-beam lithography. Silanization is conducted in deionized water (DW). After several rinses with DW, the e-beam resist is removed by acetone, followed by graphene being transferred on the prepatterned substrate. Diazonium Functionalization. Graphene transferred on different substrates were immersed in 4-NBD aqueous solutions (10 mM 4NBD with 0.5 wt % SDS) at 30 °C for 4 h. Figure 4 was reacted at 50 °C for 30 min to compare the reactivity. After several rinses with distilled water, samples were dried by nitrogen. Growth of Gold Particles on Substrates. Each sample (G/ APTES, G/OTS, G/SiO2) was soaked in gold chloride solution (0.25 mM of HAuCl4·3H2O in 20 mL of distilled water) at 30 °C for 1 h. Then, they were rinsed several times in distilled water and dried by nitrogen gas. Ascorbic acid was used as a mild reducing agent to see the additional reduction effect after the gold particle formation on the n-doped graphene. Sample Preparation for the Hydrogen Evolution Reaction. Graphene transferred on the SAM treated SiO2 substrate was prepatterned to deposit Cr/Au (5 nm/40 nm) electrode, leaving the active area (1 × 1 cm2). After electrode deposition, the entire region except for the active area was masked by epoxy resin (Loctite 9340). Characterization. Raman spectra was obtained by Renishaw 2000 (Excitation at 514.5 nm from an Ar ion laser). The power was maintained below 2 mW to avoid sample damage during measurement. AFM images were obtained by Park System XE-100 with noncontact mode. Agilent 2602 was used to measure the electrical properties of the FET devices in the ambient condition. SEM images were obtained by SUPRA 55VP field-emission scanning electron microscopy (Carl Zeiss). Bright-field and dark-field images were captured using optical microscopy (Nikon, Eclipse LV100ND). Electrochemical experiments were performed by an Ivium potentiostat (Ivium Technologies; Eindhoven, Netherlands, Compact-stat) with a three-electrode system using a Pt mesh as counter electrode and a saturated calomel reference electrode in a 0.5 M H2SO4 standard electrolyte solution. Electrochemical impedance spectroscopy (EIS) was conducted at −0.25 V vs RHE from 350 kHz to 0.1 Hz.

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*E-mail: [email protected] (M.J.P.). ORCID

Hyukjin Lee: 0000-0001-9478-8473 Byung Hee Hong: 0000-0001-8355-8875 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2012M3A7B4049807) and the KU-KIST School Project.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01614. Additional experimental details and figures (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.H.H.). 5608

DOI: 10.1021/acs.chemmater.8b01614 Chem. Mater. 2018, 30, 5602−5609

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DOI: 10.1021/acs.chemmater.8b01614 Chem. Mater. 2018, 30, 5602−5609