Europium Effect on the Electron Transport in ... - ACS Publications

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Europium Effect on the Electron Transport in Graphene Ribbons Alfredo D. Bobadilla,†,‡ Leonidas E. Ocola,§ Anirudha V. Sumant,§ Michael Kaminski,‡ Narendra Kumar,† and Jorge M. Seminario*,† †

Department of Chemical Engineering, Department of Electrical and Computer Engineering, Department of Material Sciences and Engineering, Texas A&M University, College Station, Texas 77843, United States ‡ Nuclear Engineering Division and §Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: We report in this complementary theoretical− experimental work the effect of gating on the electron transport of graphene ribbons when exposed to very low concentration of europium in an aqueous solution. We find a direct correlation between the level of concentration of europium ions in the solvent and the change in electron transport in graphene, observing a change of up to 3 orders of magnitude at the lowest level of concentration tested (0.1 mM), suggesting a possibility that graphene ribbons can be used for detecting very low concentrations of europium in liquid solutions.

1. INTRODUCTION Identification of low concentrations of heavy metals belonging to the lanthanide and actinide series includes applications in environmental sampling,1,2 water quality,3,4 nuclear processing,5,6 and nuclear forensics and safeguards.7−9 Current techniques are complex and costly10−12 and require special laboratory conditions.13,14 A key need is therefore a portable, low-cost device to detect trace heavy metals. In the present work, we analyze the change in the electrical characteristic of graphene upon interaction with trivalent europium dissolved in water. Graphene has unique plasmonic characteristics that may permit ultrasensitive detection of sorbed species.15−17 Ab initio molecular orbital calculations predicted a strong, characteristic response of graphene nanoribbons to uranyl complexes sorbed to the surface.18−22 To improve surface complexation, research has focused on functionalized forms of graphene oxide employing hydroxyl, carboxyl, carbonyl, and epoxy moieties23−27 or more complex ligands and chelators.28−33 Graphene oxide was reported as adsorbent for the preconcentration of U(VI) ions from aqueous solutions;25−27,34 the pHpzc (point of zero charge) value of the graphene oxide nanosheets was 3.89. The surface charge was positive at pH < pHpzc and was negative at pH > pHpzc. With an increasing pH, the surface charge becomes more negative, and the electrostatic interactions between the U(VI) ions and the graphene oxide nanosheets become stronger and thereby result in an increase in U(VI) sorption on the graphene oxide nanosheets. The sorption of U(VI) ions on the graphene oxide nanosheets was strongly dependent on pH and independent of the ionic strength, indicating that the sorption is mainly dominated by inner-sphere surface complexation rather than by outer-sphere surface complexation or ion exchange.34 In an inner-sphere © XXXX American Chemical Society

complex, ions bind directly to the surface with no intervening water molecules. Intrinsically charge-neutral graphene can be easily doped electrically or chemically.35−41 As the speciation of lanthanide and actinide metals in solution is almost exclusively as cationic complexes in acidic waters,42 we test a corollary method of inducing cationic sorption onto graphene by using electrical gate electrodes to induce a negative charge on the graphene.43−48 Therefore, we fabricate graphene electron devices whose response is modulated by a negatively charged graphene and a layer of counterions (cations) at the graphene/ water interface.

2. METHODOLOGY Single-layer graphene is grown via chemical vapor deposition (CVD) of methane gas on copper foil at 1000 °C, 50 sccm, and 1 Torr. Before introducing methane gas, the copper foil (Alfa Aesar 0.025 mm thick, 99.8% metal basis, ∼3.5 cm × 4 cm) is annealed in hydrogen gas at 800 °C for 30 min. Details about the graphene growth process can be found elsewhere.49 2.1. Transfer Process. Graphene is transferred from the copper foil to an oxide wafer by a PMMA-assisted method (Figure 1). Briefly, during the CVD process graphene grows on both sides of the copper foil. After the CVD growth, one side of the graphene/copper foil is spin-coated with a poly(methyl methacrylate) (PMMA) A8 e-beam resist at 3000 rpm for 60 s and air-dried overnight (∼12 h) in a vented hood to obtain a uniform film. The unprotected side is etched in a 1:10 mixture Received: July 7, 2015 Revised: September 1, 2015

A

DOI: 10.1021/acs.jpcc.5b06499 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

etchant solution, the PMMA/graphene membrane is almost transparent. The PMMA/graphene membrane is then transferred to a beaker with DI water. Then, another four transfers to DI water are performed every 3 min to clean the PMMA/ graphene membrane. Afterward, the PMMA/graphene membrane is lifted from solution with an oxide wafer. The SiO2 thickness was 300 nm, which ensures the best contrast to visualize graphene under the light microscope. After transfer to the oxide wafer, a water layer is present between PMMA/graphene membrane and the oxide substrate. To effectively dry the membrane with minimum wrinkles, the sample is left overnight (∼12 h) in a vented hood. Alternatively, baking at 37 °C for 2.5 h works equally well. If the sample is not properly dried, the yield of flat graphene is poor. After the sample fully dried, the PMMA resist covering graphene is removed in acetone for 3 h, rinsed in isopropyl alcohol (IPA) for 10 min, and dried with a nitrogen gun. The sample is then baked on a hot plate at 150 °C for 15 min to ensure a strong adhesion of graphene to the oxide substrate and fewer defects (primarily in the form of holes on graphene). This baking process will also be critical to the integrity of interface electrodes during the liftoff process described next. 2.2. Graphene Ribbon Device Fabrication. Metal electrodes (60 nm Au/2 nm Cr) are deposited on top of graphene by e-beam vapor deposition and patterned in a fourelectrode configuration with two electrodes acting as an

Figure 1. Depiction of the PMMA-assisted transfer process of CVD graphene: (a) CVD graphene is covering a copper foil and protected with a PMMA resist thin film, (b) the PMMA/graphene/copper is placed into a diluted copper etchant solution, (c) after copper etching is complete, the PMMA/graphene membrane is nearly optically transparent; (d) graphene is transferred to a beaker with distilled water to further dilute impurities from the etching process; (e) after rinsing with water, the PMMA/graphene membrane is lifted from solution with an oxide wafer.

of nitric acid and DI water (10 mL/100 mL) for 10 min. Deionized (DI) water is from laboratory supply (16 MΩ.cm). Then a 1 cm2 sample of graphene/copper foil is immersed in a 1:5 copper etchant (iron chloride solution diluted in DI water) for 6 h. After the copper completely dissolved in the

Figure 2. Schematic representation of the four-electrode configuration for measurements of the electrical response of graphene ribbons: (a) top view; (b) side view. Two of the electrodes, drain (D) and source (S), act as an interface to graphene while the other two (G) act as gating mechanisms. (c) Light microscope image of the device showing a set of five graphene ribbon devices. (d) Graphene ribbon in four-electrode configuration. Interface electrodes (horizontal pair) are covered with resist. Scale bar is 10 μm. In a typical experiment, a small drop of ∼0.1 μL is deposited on the device. The diameter of the drop is 1−2 mm. Note the circular mark left on the device after the drop dried. B


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minimum bias voltage needed to produce hydrogen by water electrolysis depends on the electrodes being used and on the electrodes configuration, but we certainly expect generation of hydrogen ions since we work with a relatively large bias voltage, Vg, in the range 0−20 V. 2.5. Atomistic Calculations. The detection mechanism is studied with molecular dynamics simulations using the CHARMM force field54 as implemented in the program LAMMPS.55 These simulations allow us to analyze the strength of nonbonded interactions between solution ions and graphene. We consider a maximum negative charge of −0.2 (all charge units are in electrons in this work) per carbon atom in graphene.45 Lennard-Jones parameters for europium(III) are from Veggel and Reinhoudt (1999),56 while Lennard-Jones parameters for the hydrogen ion are from the TIP3P model for water.57 Lennard-Jones parameters for interaction between water and graphene are from Zhang and Wang.58 Molecular dynamics simulations are performed with nonperiodic boundary conditions and with inner and outer cutoff distances of 20 and 22 Å for nonbonded interactions. A time step of 0.2 fs is used and the energy equilibration is performed at room temperature (300 K) in an NVT ensemble. In addition and in order to complement the experimental work, a Kohn−Sham (KS)59 molecular structure theory combined with a Green’s function transport procedure, GENIP,60−66 is used to analyze the electron transport properties of the graphene ribbon device addressed by gold nanotips. Density functional theory (DFT) electronic structure calculations are performed using the Gaussian-09 program.67 For the gold atoms, the basis set and effective core potential used is the LANL2DZ,68,69 which yields acceptable theoretical predictions. For europium, we use the quasi-relativistic Stuttgart effective core potential and associated basis set ECP28MWB_SEG,70,71 whereas the 6-31G(d)72 basis set is used for the other remaining atoms. The graphene nanosheet we use has a molecular formula C50H18 with two gold atoms replacing two H atoms at each end connecting to the electrodes. This geometry of a graphene nanojunction is fully optimized using the hybrid functional B3PW91.73,74 We also obtain the optimized structure of the graphene nanojunctions Au−C50H16−Au under the presence of hydrogen and europium ions.

interface to graphene and the other two electrodes for a gating mechanism. Patterning is performed by direct-write optical lithography using Shipley resist (S1805). The liftoff is performed by immersion in acetone at room temperature for 3 h, rinsing by pipet with acetone for 2 min, and sonication in acetone for 5 s. Adhesion of interface electrodes and graphene to the substrate is not very strong, and sonication for 1 min or longer can cause detachment. We also avoid exposing the edges of graphene during metal deposition to avoid a low yield of working devices. To selectively pattern graphene ribbons, a resist mask is fabricated by direct-write optical lithography using Shipley resist (S1805). Under the light microscope, we can view the graphene and the electrodes through the transparent optical resist and position the ribbon mask pattern with respect to the metal electrodes. After the resist is developed, rinsed, and dried, the sample is exposed to oxygen plasma (24 sccm, 160 mTorr, 20 W, 5 min) to etch the exposed zones of graphene. Once the etch process is complete the resist is removed using acetone for 3 h at room temperature, rinsed in IPA for 10 min, and dried with a nitrogen gun. Acetone is an effective resist remover that does not damage the graphene layer, while a conventional resist remover (e.g., Shipley microposit remover 1165) would remove graphene. The width of graphene ribbon devices is ∼10 μm (5 devices) or ∼25 μm (7 devices). 2.3. Electrodes Protection and Sample Preparation. To test a liquid sample, an optical resist coating patterned by direct-write optical lithography acts as a barrier protection between the aqueous sample and interface electrodes. A thin resist is chosen (S1805, 0.5 μm thick) to minimize the amount of resist impurities after the development process. The resist coating for interface electrodes avoids interference from the electrical impedance of the aqueous sample (Figure 2). Europium nitrate is prepared by dissolving 0.025 g of europium nitrate pentahydrate (99.9% trace metals basis, from SigmaAldrich) in 5 mL of DI water. Four different types of samples are tested: water and three solutions at different levels of concentration of europium nitrate (0.1, 1, and 10 mM) made by serial dilution of the europium nitrate stock. The 0.5 μm thick resist barrier is not effective to block a 0.5 μL drop from invading the area of a neighboring ribbon device. To test the electrical response of graphene ribbons to europium nitrate solution, a small fraction of an ∼0.5 μL drop of the liquid sample is deposited into each graphene ribbon device with the aid of a micropipet. The drop typically covers a circular area of ∼1 mm diameter (Figure 2c). The typical time for a sample drop to evaporate is about 2 min, and the time to perform a current−voltage measurement is a few seconds. 2.4. Electrical Measurements. The electrical measurements are performed in a four-electrode configuration. Two gate electrodes (G) are used to drive europium ions toward the graphene ribbon and the other pair (D and S) to drive the ionmodulated electrical current through graphene (Figure 2). We expect ions will modulate the drain-source (I) current. The drain-source bias voltage of graphene is Vds = ∼10 mV (dc), and the gating voltage is Vg = ∼2−20 V (dc). Thus, Vds ≪ Vg. The bias voltage between graphene and the gate electrodes creates a negative charge on graphene that drives cations toward graphene and modulates its electrical transconductance. Furthermore, a bias voltage larger than ∼2 V induces water electrolysis,50−53 the splitting of water into hydrogen and hydroxyl ions. And a bias voltage as small as 1.5 V from an ordinary AAA battery can produce water electrolysis.53 The

3. RESULTS AND DISCUSSION 3.1. Electrical Measurement Results. We measure the I− Vds and I−Vg response of graphene ribbon for each drop sample. I−Vds is performed before and after sample deposition and always in dry conditions, while I−Vg is performed right after sample drop deposition in wet conditions. Therefore, I− Vds provided the electrical conductance of graphene in dry conditions and I−Vg the dynamics of ion-modulated graphene transconductance during wet conditions. I−Vg is always performed at constant Vds of 10 mV, and we use the I/Vds values at dry conditions to find a correlation with the effect of europium on graphene. We perform initial measurements at low gate voltages, Vgs = 0−2 V, and in a different sample at high gate voltages, Vgs = 0− 10 V. In both cases, we observe a nonlinear characteristic in the I−Vg curve (Figures 3 and 4). A high bias gate voltage is required for a change to occur in the electrical conductance of graphene ribbon at dry conditions (Figure 4). The I−Vg nonlinear characteristic at low gate voltage (0−2 V) is similar to that observed in graphene-based field-effect transistor (FET) C


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Figure 3. (a) Electrical response of graphene ribbon device at low bias gate voltage, Vg = 0−2 V, in DI water. (b) Measurements in dry conditions; “pristine” and “drop dried” correspond to measurements before sample deposition and after the sample drop dried, respectively. Points in the plots are connected by straight lines as a help to the eye. These measurements are performed at a constant Vds = 10 mV.

devices (Figure 3). However, the currents on both samples are not similar for the same gate voltages. The gate electrode in aqueous media or in electrolyte is expected to be much more effective than the back gate solidstate FET approach in tuning the electrical conductance of graphene. This is because the applied gate voltage will fall across the double layer formed at the graphene−solution interface. The double layer thickness is determined by ion sizes (∼1 nm), which is 2 orders of magnitude thinner than those of the oxide (∼100 nm) used in a back gate FET configuration.43,75−77 Initial tests (Figures 3 and 4) are performed only in the range Vg = 0−10 V; in this range and under wet conditions, there are always oscillations in the electrical current as shown in Figure 4a. These initial tests are performed to have a qualitative idea of the electrical response of graphene in water. For a bilayer graphene ribbon device, we measure the I−Vg before water sample deposition and after sample deposition (Figure 5) to offer further insights into the oscillations in the electrical current in the I−Vg curve when Vg < 10 V. We observe that switching all voltages, Vg and Vds, off does not produce a marked change in the I−Vg characteristic, i.e., no memory effects. In initial measurements at low gate voltage (Figure 3) we do not observe a correlation or difference in the electrical characteristic of graphene when interacting with distilled water (Figure 3) or europium solution (not shown). However, at high gate voltages (Figure 4a), we observe a clear change in the electrical conductance of graphene at dry conditions (Figure 4b). We therefore performed further measurements at high gate voltage (0−20 V) and look for a correlation between the level of concentration of europium solution and a change in the electrical conductance of graphene. A change in the electronic structure of graphene occurs during I−Vg measurements; therefore, we considered it important to reach a similar

Figure 5. I−Vg of bilayer graphene ribbon device before sample deposition (I1); a first measurement of ribbon device in water (I2); a second measurement in water (I3) after disconnecting and reconnecting Vg and Vds; and another I−Vg in dry conditions (I4) after the water sample dried out.

steady state (constant level of current in the I−Vg characteristic) in all cases. At a maximum gate voltage of 10 V, it was still possible to observe oscillations in the electrical current (Figure 4a), but a gate bias voltage of 20 V was high enough to always observe the electrical current reaching a close to steady state in the I−Vg characteristic. And as the concentration of europium increases, the electrical conductance of graphene increases, and the ratio g2/g1 increases (Figures 6 and 7), where “g1” and “g2”

Figure 6. g1 and g2 represent the electrical conductance of graphene before and after, respectively, exposure to europium aqueous solution under a bias voltage (Vds = 10 mV and Vg = 0−20 V). Data at 0 mM correspond to results with DI water. Three graphene devices were tested with DI water and nine graphene devices with europium(III) nitrate aqueous solution.

Figure 4. Electrical response of graphene ribbon device (different to the one used in Figure 3) at high gate bias voltage, Vg = 0−10 V in DI water. I− Vg (a) measurement is performed at a constant Vds = 10 mV and in wet conditions, while I−Vds (b) measurement is performed in dry conditions and with no gate voltage Vg applied. “Pristine” and “drop dried” correspond to measurements before sample deposition and after the sample drop dried, respectively. Points in the plot are connected by straight lines as a help to the eye. D


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level of concentration, but sometimes that is not the case, for example, data in Figure 5 at 0.1 and 1 mM show g2/g1 values of ∼10−2 and ∼1.2 × 10−2, respectively. Notice that both axes in Figure 6 are in logarithmic scale. The imperfection in the trend of g2/g1 versus EuNO3 [mM] (Figure 6) could be due to defects on graphene from the synthesis, transfer, or device fabrication processes. The aqueous samples are also not stable since they evaporated in about 2 min; therefore, a fluidic reservoir or microfluidic channel might help to obtain a more accurate probing of the aqueous sample. Alternatively, the point at which the electrical current is a minimum in the I−Vg curve may be correlated with the level of concentration of the aqueous sample.75,78 However, we do not observe a reproducible I−Vg response at the same level of concentration. For example, for measurements in a europium aqueous sample at 1 mM (Figure 7c,d), the current minimum for “device 1” is around Vg = 2 V while for “device2” the minimum is observed around Vg = 1 V. Figures 6 and 7 are for the range Vg = 0−20 V; we find that the electrical current reaches, in most cases (9 out of 12), a steady state around Vg = 20 V. Therefore, Figures 6 and 7 are used to draw the conclusions. We believe that at a positive bias gate voltage larger than 2 V, hydrogen ions generated by electrolysis, given by

Figure 7. Electrical responses of graphene ribbon to [1 mM] europium solution. (a) I−Vg curve. (b) A decrease in electrical conductance of graphene is observed at comparing the I−Vds curve before exposure to the aqueous sample (pristine graphene) and after the sample drop dried. (c) I−Vg for a small range of Vg = 0−2.8 V. (d) I−Vg.

represents the electrical conductance of graphene before and after exposure to europium solution. After electrical measurements in DI water and after the sample drop dried, the current through graphene is zero (I < 10 pA) during I−V ds measurements, and therefore the electrical conductance of graphene is effectively zero. The minimum current the semiconductor analyzer can measure is ∼10 pA. The current through graphene is typically ∼μA (at Vds = 10 mV), and after electrical breakdown the current reaches ∼10 pA at the same bias voltage. Therefore, after the electrical breakdown induced by water, the g2/g1 ratio is actually ∼10−5 (Figure 6). We observe a trend in the change of electrical conductance of graphene (g2/g1) versus the level of concentration of the europium solution (Figure 6). However, the trend is not perfect; the resultant change in electrical conductance (g2/g1) should ideally be at least 1 order of magnitude different at each

H 2O → H+ + OH− are driven toward the negatively charged graphene ribbon surface (Figure 2b). Hydrogen ions are neutralized by graphene charges, recombining then to form hydrogen gas. We do not discard at the same time the event of hydrogen covalently binding to graphene. Hydrogen and hydroxyl ions are constantly being generated by electrolysis while the amount of europium is fixed. Trivalent europium preferentially binds to graphene due to its higher charge density than hydrogen ions, partially blocking hydrogen interaction with graphene. We suggest the interaction of graphene with ions gives origin to a change in graphene ribbon structure or functionalization and the observed change in electrical conductance of graphene. Further studies are needed to determine the change in

Figure 8. Binding of solution ions to negatively charged graphene (−0.2 per carbon atom) at room temperature. Atoms are color coded: europium ion (green), hydrogen ion (purple), graphene (gray), water (cyan and pink). Average distances are Eu−C ∼ 2.6 Å and H---C ∼ 1.6 Å. E


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charge reaches a maximum value of −0.2, the bond distance decreases an additional 0.3 Å for both ions (Table 1); however, the relative change is larger for the hydrogen ion since its bond distance to graphene is always much smaller than that of europium to graphene (Table 1). This would enhance the effect of the hydrogen ion on the electrical properties of graphene. Electronegativity is not only an intuitive concept (Pauling scale) but can also be treated as a quantum chemical parameter (Mulliken scale).82,83 And the electronegativity parameter has been shown to be in good agreement with chemical behavior of europium complexes in diverse applications.84−88 An ionic bond between two atoms contains partial covalent character, depending on the relative difference of their electronegativities. The smaller the difference, the larger the covalent character of the ionic bond. Therefore, we consider that the ionic bonding of hydrogen to graphene has a strong covalent character while the binding of europium to graphene is close to purely ionic (Table 2). This implies that the ionic binding of hydrogen ion to negatively charged graphene is capable of altering the electronic structure of graphene.

graphene ribbon structure upon application of a high bias gate voltage in a microfluidic device. 3.2. Atomistic Calculations Results. A water solvation layer normally surrounds europium ions in aqueous solution, and in this shell, the ion−water interaction is much stronger than the water−water one.79−81 At small negative charges in graphene, −0.02 per carbon atom, the electrostatic attraction is not strong enough to induce binding of solution ions to graphene. We analyze the same system under a maximum charge for graphene of −0.2 per carbon atom and find the electrostatic attraction between graphene and ions strong enough to overcome the water solvation layer normally surrounding ions in solution (Figure 8). We observe that the ion stabilizes around the center of a benzene ring on the graphene surface. For a maximum charge of −0.2 per carbon atom, the negative charge on the benzene ring totals −1.2, which effectively neutralizes a hydrogen ion charge of +1 and would not prevent other hydrogen ions from binding to graphene on adjacent sites. For a europium ion, even a maximum charge of −0.2 is not enough to neutralize the cation (+3), and the total charge of the ionic complex would continue being positive, preventing by electrostatic repulsion, the binding of other cations to graphene in adjacent atomic sites. To analyze the strength of ion binding to graphene, we consider a hypothetical case of an ion positioned 3 Å above graphene as the initial structure (Figure 9) and analyze how the

Table 2. Electronegativity in Pauling Scale of Atoms Participating in the Ionic Bonding

atomic charge of graphene affects the bond distance of ions to graphene. To predict the stable structure, we perform an energy minimization and then an energy equilibration at 0.1 K, under an NVT ensemble. This is followed by a calculation of the average bond distance between the ion and the six carbon atoms of the closest six-member ring from the graphene surface. We find a significant change in the ionic bond distance for the hydrogen ion (Table 1). When the atomic charge of graphene is −0.025, the bond distance decreases ∼0.8 Å for europium and ∼0.4 Å for hydrogen ion. When the atomic Table 1. Average Ion−Carbon Bond Distance after Binding of Ions to the Negatively Charged Graphene C---H+ (Å)

C−Eu3+ (Å)

0.000 −0.025 −0.050 −0.100 −0.200

2.21 1.81 1.72 1.63 1.56

3.71 2.92 2.81 2.73 2.61

electronegativity

hydrogen europium carbon

2.20 1.20 2.55

Europium ions can have an effect on the electrical conductance of graphene ribbons. We suggest the effect depends on the strength of the ionic bonding of europium to graphene. Europium can show affinity to other molecules, and in that scenario, the strength of binding to graphene can be weakened. For example, Förster et al.89 reported the nature of Eu adsorption on graphene depends strikingly on the Eu coverage; a single adatom of europium dopes graphene with a charge of 1e, but as the layer of europium atoms covering graphene increases, the charge transfer per europium atom reduces to 0.3e; since europium prefers binding to other europium atoms rather than to graphene. And due to the small density of states of graphene, charge transfer to graphene causes an upward shift of the Fermi level.90 We observed a strong charge transfer between Eu adatom and graphene when the total charge was zero. The NBO analysis yields a charge of +0.86e on Eu adatom and the electronic configuration of 6s0.934f6.995d0.166p0.07, and the Eu adatom is found to be located at a distance of 2.5 Å from the basal plane; these results are consistent with the theoretical− experimental work (6s14f7) by Förster et al.89 using a plane waves basis set to analyze europium growth on graphene. In case of Eu+ adsorbed on graphene, the NBO yields a charge of +0.89e on Eu adatom with very similar electronic configuration. When the total charge was set to +3e, i.e., assuming the Eu(III) ion, the total charge on Eu increased to +1.79e with an electronic configuration of 6S0.014f7.005d0.196p0.01 (or 6s04f7). For aqueous systems, Kumar et al.20 reported a change in the electrical conductance of graphene ribbons when europium nitrate bonds to graphene. However, the change in the electrical conductance of graphene is practically negligible when the europium ion is coordinated by three nitrate ions and water molecules, yielding a more stable complex than the cases with only one or two nitrate counterions.

Figure 9. Structure for analyzing dependence of ion−graphene bond distance on graphene atomic charges.

qC (e)

atom type

F


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Figure 10. Density-functional theory optimized structure of (a) graphene−hydrogen ion (GH+) X and (b) graphene−europium ion (GEu+) complexes, and (c) the corresponding electron-transport characteristic compared to that of pristine graphene (G). The hydrogen ion (+1e), highlighted in a dotted circle, is directly bonded to a carbon atom (C−H bond distance ∼1.12 Å), The europium ion (+1e) is stable around the center of graphene (average Eu−C bond distance ∼2.95 Å). Atoms are color coded: gold (yellow), carbon (gray), hydrogen (white), europium (cyan). Gold atoms act as an interface to bulk gold electrodes (purple). A bias voltage (V) drives the electrical current through graphene.

4. CONCLUSIONS We study in this fundamental research, both theoretically and experimentally, the effects of europium on the electron transport properties of graphene. Since precise comparisons and validations can be done only for very small and trivial systems, which are used in most cases to validate each other methods, the importance of combining theory and experiment centers on the search of complementary information as it is clear that when something is difficult to obtain experimentally usually is easier theoretically and vice versa. For more realistic systems, we are limited to look for trends. A scheme is suggested to test the response of graphene in the presence of europium solutions of different concentrations. Under a fourelectrode configuration, we find different behaviors at low (2 V) bias gate voltage. At low voltage, we find a FET-like electrical characteristic for graphene, but europium ions have no effect on the FET characteristic. At high bias voltage, the change in electrical conductance of graphene has a direct relation with the level of concentration of europium ions. We attribute the behavior at high voltage to water electrolysis, graphene interaction with hydrogen ions, and a passivation of this effect proportional to the level of concentration of europium ions. We suggest this passivation is due to the binding of europium ions to a negatively charged graphene, which partially prevents hydrogen ions from binding to graphene. Finally, graphene−ion interactions affect graphene ribbon structure and functionalization yielding changes in electrical conductance of graphene, providing us with a potential opportunity for the fabrication of nanosensors. In

Changes in the electrical conductance of graphene at dry conditions are attributed to the changes in the competing effects of two ions, europium and hydrogen. On the basis of the findings by Förster et al.,89 we consider the europium ion to be charged +1e. Therefore, we performed electron transport calculations of a graphene ribbon device and upon interaction with europium and hydrogen ions (Figure 10). We obtain a HOMO−LUMO gap (HLG) of 2.39 eV for our molecular graphene nanojunction. This is expected due to the very small size of the graphene sheet. We also noted that as the number of carbon atoms in the graphene sheet increases, the HLG reduces from 2.39 eV for Au−C50H16−Au to 1.08 eV in the case of Au− C58H18−Au. The interaction of H+ with graphene is covalent with partially ionic character while the interaction of Eu+ with graphene is purely electrostatic. The electrical conductance corresponds to the slope of the current−voltage characteristic; the electron transport calculations (Figure 10c) predict a significant decrease in the electrical conductance of graphene upon interaction with hydrogen ion and only a slight increase in the electrical conductance upon interaction with europium ion. This agrees with the observed larger change in electrical conductance of graphene at a decreasing level of concentration of europium aqueous solution (Figure 6) and a corresponding negligible change of the electrical conductance of graphene at the highest level of concentration of europium tested, 10 mM. In addition, the current−voltage curve for Eu(III) adsorbed on graphene shows a conductance 5.6 times higher than the current on pristine graphene. G


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any I−V type of sensor, the shape of the I−V is what determines the nature of the agent to be sensed. However, on the question of how selective is graphene for the detection of Eu, we have reported20 the effect on the I−V from several possible moieties such as Eu2O3, Eu(NO3)3, Eu(NO3)3(H2O)4, Eu(NO3)2(H2O)5+, and Eu(NO3)(H2O)72+, showing different shapes of I−V curves; thus, we expect that any other moiety will most likely show different I−V behavior. Certainly, for specific applications, further studies are needed to determine the change in graphene ribbon structure upon application of a high bias gate voltage in an aqueous environment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract DE-AC0206CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. J.M.S. also acknowledges high-performance computing support provided by the Texas A&M Supercomputer Facility and the Texas Advanced Computing Center (TACC) as well as the financial support from Argonne National Laboratory’s Laboratory-Directed Research and Development Strategic Initiative.

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