Detecting Electric Dipoles Interaction at the Interface of Ferroelectric

Jan 6, 2017 - ... from DEME-TFSI ionic liquid (top-gate) and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT, back-gate) to probe the interface dipoleâ...
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Detecting Electric Dipoles Interaction at the Interface of Ferroelectric and Electrolyte Using Graphene Field Effect Transistors Chunrui Ma,*,†,‡ Rongtao Lu,‡ Guangliang Hu,§ Jinsheng Han,§ Ming Liu,§ Jun Li,∥ and Judy Wu*,‡ †

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Shannxi 710049, P. R. China Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States § School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China ∥ Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States ‡

S Supporting Information *

ABSTRACT: Graphene was inserted into the interface between electric dipole layers from DEME-TFSI ionic liquid (top-gate) and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT, back-gate) to probe the interface dipole− dipole interaction in response to DC and pulsed gate voltages. A highly complicated behavior of the interface dipole−dipole interaction has been revealed as a combination of electrostatic and electrochemical effects. The interfacial polar molecules in the DEME-TFSI electrical double layer are pinned with assistance from the PLZT back-gate in response to a DC topgate pump, leading to strong nonlinear electrochemical behavior. In contrast, depinning of these molecules can be facilitated by a faster pulsed top-gate pump, which results in a characteristic linear electrostatic behavior. This result not only sheds light on the dynamic dipole−dipole interactions on the interface between functional materials but also prototypes a unique pump and probe approach using graphene field effect transistors to detect the interface dipole−dipole interaction. KEYWORDS: graphene, ferroelectric thin film, ionic liquid, dipole interaction, electrochemical effect polarized ferroelectric thin film at the interface between the ferroelectric film and ionic liquid electrolytes.11,12 Understanding such dipole−dipole interactions, especially in a dynamic fashion, can provide critical information for highspeed electronic applications such as field effect transistor,13−16 ferroelectric nonvolatile memories,17−19 sensors, and various electrochemical devices. It is known that an graphene atomic layer is very sensitive to electric charge and can detect an electrical dipole switch in the configuration of a graphene field effect transistor (GFET).20,21 Therefore, in this paper, we inserted graphene into the interface between ferroelectric (back-gate) and ionic liquid (top-gate), forming a double-gate GFET configuration shown in Figure 1a, to investigate the dipole interaction. Many inorganic/organic ferroelectric materials have been employed in field effect transistors (FETs) as gate dielectrics considering that their large dielectric constants can provide a higher local electrostatic field in the FET channel than that produced by traditional dielectric gates with low dielectric constants. In addition, ferroelectric hysteresis can be utilized for nonvolatile memory devices. 22−24 Here, an epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) ferroelectric film was selected

1. INTRODUCTION The interface between two functional materials, especially ferroelectric and ionic liquid electrolyte, is of particular interest because both can serve as high efficiency gates in field-effect devices and a comparable, high areal dipole density can be achieved at a small gate voltage of a few volts. At this interface, the behaviors of electrical dipoles may differ dramatically from the cases of bulks. On oxide ferroelectric films, for example, it is well-known that a layer of oxygen deficiency typically forms on the surface at a few nanometers in thickness and has a profound effect on the devices, such as Schottky diodes fabricated atop, resulting in a strong screening effect at the interface.1−3 On the other hand, the electric field of ionic liquid electrolytes primarily concentrate at the interface of a charged solid surface and the electrolyte, i.e. so-called electrical double layer (EDL), whose thickness is typically on the order of a few nanometers (as represented by Debye length).4−6 The electric dipole behavior of the EDL in ionic liquids not only reflects shortrange molecular reconfiguration but also longer-range ion diffusion, which is important to a large array of devices including lithium ion batteries, supercapacitors, catalysis, biosensors, to name a few.7−9 Recently, there were several reports showing that inorganic/organic ferroelectric dipoles can dynamically interact with ionic liquid electrolytes. For example, the ionic liquid can induce ferroelectric polarization in poly(vinylidene fluoride),10 and an EDL can be formed by © 2017 American Chemical Society

Received: November 10, 2016 Accepted: January 6, 2017 Published: January 6, 2017 4244

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) A photo of the real double-gate GFET with PLZT back-gate and DEME-TFSI top-gate and its schematic diagram. (b, c) ID−VBG curves without (black solid) and with (black open) ionic liquid (IL) DEME-TFSI gate at VTG = 0, and the ID−VTG curve (red open) at VBG = 0 of the device with the n-doped and p-doped initial state, respectively. The inset is the comparison of the ID−VTG curve (red) multiplied by a factor and the ID−VBG curve (black). (d−f) Schematic diagram of double-gate GFET with residual VDirac = 0, VDirac < 0, and VDirac > 0, respectively. (g) Schematics of the electric field across the ionic liquid top-gate thickness with two EDLs shown at the GFET and the Pt top-gate electrode. laser (wavelength of 248 nm and pulse width of 25 ns). The average laser energy density was 2 J/cm2, and repetition rate was 5 Hz. After deposition, the PLZT films were in situ annealed at 600 Torr oxygen for 30 min to reduce oxygen vacancies in it before cooling to room temperature.28 2.2. Graphene Fabrication and Transfer. Single-layer graphene sheets grown using chemical vapor deposition (CVD) were transferred onto the PLZT films using a procedure reported in our previous work.20,31,32 Briefly, graphene was grown at ∼1000 °C in a CVD system on commercial polycrystalline copper foils of 25 μm. To transfer the graphene onto a PLZT film, poly(methyl methacrylate) (PMMA) was spin-coated on the graphene sample before it was submerged in copper etchant (CE100) for removing the copper foil. After the samples were rinsed with deionized (DI) water for multiple times, the PLZT film was slowly immersed into DI water to engage one side of the suspended graphene. The graphene/PLZT assembly was carefully lifted from the water to smoothly engage the entire graphene sheet onto the PLZT surface. After the transfer, the samples were baked in air at 150 °C for 1 h in an oven to eliminate moisture, followed by immersing them in acetone to remove the PMMA on the graphene. Isopropyl alcohol rinse was employed afterward to remove residues on the surface of the graphene. 2.3. Double-Gate GFETs Fabrication. Source and drain electrodes were defined in the first photolithography, followed with electron-beam evaporation of 2 nm titanium/88 nm gold and liftoff. In the second photolithography, GFETs were fabricated with reactive ion etch (RIE) in oxygen plasma at 20 W RF power under 6.7 mTorr oxygen partial pressure. The RIE time was 150 s. The GFET devices have the same channel width and length of 20 μm. Then the GFETs were cleaned in high vacuum at more than 5 × 10−6 Torr for an extended period of more than 24 h, which was found effective and sufficient to remove contaminants and extrinsic polar molecules adsorbed to the graphene channel possibly during the GFET fabrication process. Finally, the ionic liquid DEME-TFSI was placed on the graphene channel using a micropipette. The gate voltage was applied to the PLZT back-gate through the conductive Nb:SrTiO3 substrate and to the DEME-TFSI top-gate using a platinum wire (0.25 mm in diameter) immersed in the ionic liquid.

as the back-gate and ionic liquid DEME-TFSI (N,N-diethyl-N(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide) as the top-gate due to their comparable high gate efficiency, which is critical to the operation of the double-gate devices in the same voltage range. At a thickness on the order of a few hundreds of nanometers, a small gate voltage range of ∼±0.6 V is adequate to induce sufficient charge density in the GFET channel for observation of electric dipole alignment and switch in the ferroelectric PLZT gate because of its high permittivity (>1000) and low coercive field (∼12 kV/cm).20,25−28 On the other hand, DEME-TFSI exhibits a high specific capacitance of ∼2.0−2.5 μF/cm2 and a high gating field at a moderate topgate voltage within its electrochemical window of ∼±1−3 V because of a multilayer EDL forming at the graphene/DEMETFSI interface.4,29,30 Therefore, in this paper, we investigate the dipole interaction between ferroelectric PLZT and ionic liquid DEME-TFSI by measuring the GFET source-drain current vs gate voltage (ID−VG) characteristics and probe its dynamic behavior by measuring ID−VG in response to a “pump” consisting of a pulsed gate voltage of alternating polarities and varying amplitudes from one gate while under different DC bias from the other. In the PLZT/DEME-TFSI double-gate GFET, we have found that the interface dipole−dipole interaction pumped by DC gate voltages exhibits both electrostatic and electrochemical mechanisms, while the quantitative effects of these mechanisms depend on the initial doping state (n-doping or p-doping) of the device, resulting from the residual interface dipole puddles of a high concentration. A pump of the pulsed gate can effectively reduce or diminish the nonlinear electrochemical contribution, leading to a characteristic linear electrostatic behavior.

2. EXPERIMENTAL SECTION 2.1. Epitaxial PLZT Film Fabrication. Epitaxial PLZT film of 300 nm thickness was fabricated at 650 °C under 225 mTorr oxygen partial pressure on conductive (001) Nb:SrTiO3 substrates (as gate electrodes) by using a pulsed laser deposition with KrF excimer 4245

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

ACS Applied Materials & Interfaces

3. RESULTS AND DISCUSSION The ID of a representative sample is depicted in Figure 1b as a function of the PLZT back-gate voltage VBG at a fixed sourcedrain voltage VSD = 10 mV before (black solid) and after (black open) the DEME-TFSI top-gate was introduced on the GFET channel. The minimum GFET channel conductance remains approximately a constant, suggesting that no additional charge scattering mechanism was introduced by the DEME-TFSI. One subtle difference, however, is visible upon application of the DEME-TFSI. The Dirac point (VDirac) of GFETs shifts from about −0.24 V (n-type) to −0.09 V (n-type). In fact, all GFET samples on the PLZT gate employed in this work are n-doped before application of the DEME-TFSI top-gate as shown in Figure 1c for another sample. After the ionic liquid top-gate was applied, the molecular charges in the ionic liquid with opposite polarity to the residual interfacial charges on GFET/PLZT move toward the GFET channel. Neutralization of the charges from polar molecules attached on GFET channels and hence a shift of the Dirac point toward 0 V is anticipated in ideal cases as schematically depicted in Figure 1d. Interestingly, a small none-zero value of VDirac is present, which could be either negative (Figure 1b, black open) or positive (Figure 1c, black open) and varies from sample to sample. While the origin of the residual interface electric dipoles remains unclear, we hypothesize that it is due to localized polar molecules that cannot be neutralized upon the EDL formation in the DEMETFSI top-gate. As depicted in Figure 1e,f, residual dipoles may exist if the areal density of the polar molecules is higher than that achievable in the EDL. The net residual dipoles under a GFET channel determine the location of VDirac on either ndoping or p-doping, respectively. Intriguingly, as we will show later, this initial doping state of the GFET correlates quantitatively to the gate field range where the electrochemical effect is observed. After the application of the DEME-TFSI top-gate, the ID− VTG curves were also measured at zero VBG and the results are included in Figure 1b,c (red open) for the two representative GFET samples with negative and positive VDirac values, respectively. While the Dirac points derived from ID− VTG and ID−V BG differ slightly in the both samples, the polarity of VDirac remains the same. It should be noted that the slope of the ID−VTG curve is smaller than that of the ID−VBG curve in the same sample, indicating that the efficiency of top-gate is lower than that of the back-gate. This may be explained schematically in Figure 1g as a consequence of the two different EDL layers in series: one is between the Pt electrode and the ionic liquid, and the other is between the ionic liquid and the graphene channel. In our devices, the capacitance at the Pt electrode is about 1.5 times that of the GFET channel, because 60% of the total voltage drop VTG occurs between the GFET channel and the ionic liquid, which is the quantity that can be directly compared with the voltage of the ferroelectric back-gate. By multiplying a factor of ∼0.6 to the top-gate voltage VTG, the ID−VTG curve indeed coincides reasonably well with ID−VBG curve as shown in the insets of Figure 1b and 1c, respectively, for the two samples displayed. The interaction between the interface electric dipoles in the ferroelectric PLZT back-gate and the ionic liquid DEME-TFSI top-gate was investigated by measuring a series of DC ID−VBG shown in Figure 2 left column (ID−VTG, shown in Figure 2, right column) curves under different fixed VTG (VBG) values of the opposite gate, at an increment of 50 mV. Figure 2a,b

Figure 2. (a) ID−VBG (left) curves taken at different DC VTG bias voltages and ID−VTG curves (right) taken at different VBG bias voltages with a fixed source-drain voltage VSD = 10 mV on the n-doped sample shown in Figure 1b. (b) ID−VBG (left) curves taken at different DC VTG bias voltages and ID−VTG curves (right) taken at different VBG bias voltages with a fixed source-drain voltage VSD = 20 mV on the p-doped sample shown in Figure 1c.

summarizes the results measured respectively on the n-doped sample and p-doped sample. In the widest voltage range of the opposite gate, the ID−VG characteristics remain the same as in the zero opposite gate bias case except that the Dirac point VDirac is shifted by the opposite gate VTG and VBG, respectively. The VDirac shift in ID−VBG scans is fairly linear with the VTG bias for both n-doped (Figure 3a) and p-doped PLZT/DEME-TFSI double-gate GFETs (Figure 3c), except for a ∼0.6 reduction factor on VTG as explained earlier in Figure 1g, and the minimum GFET channel conductance of graphene remains unchanged under different VTG bias values, revealing that the additional charge doping from the EDLT inducing oxygen in the PLZT film to migrate into the interface is negligible.33,34 However, in the case of the VBG bias, the linear VDirac shift persists only within a certain range of the VBG biases and depends on the initial doping state of the device. For the ndoped device, it is primarily on positive VBG biases (Figure 3b), while for the p-doped case, it is primarily on negative VBG biases (Figure 3d). The linear VDirac shift with the opposite VG bias implies that the interface dipole−dipole interactions is “electrostatic” as a response to the superposition of applied VTG and VBG with one providing the fixed bias and the other the sweeping electric field. This is shown schematically in Figure 4. In the dipole “tail to tail” (VTG < 0 and VBG < 0, Figure 4a) or “head to head” (at VTG > 0 and VBG > 0, Figure 4b) cases, the induced interfacial charges from the two gates add up, resulting in a higher local field. On the other hand, in the configurations of “head to tail” at VTG > 0 and VBG < 0 (Figure 4c), or VTG < 0 and VBG > 0 (Figure 4d), the charges from the two gates are opposite and hence cancel each other out to yield a weaker total local field, which decreases the doping in graphene. The question arises on why the electrostatic behavior does not persist in certain ranges of the VBG bias and why the range depends on the initial doping state of the device? For the ndoped PLZT/DEME-TFSI double-gate GFETs, the nonlinear behavior occurs in the range of negative VBG biases (Figure 3b) while for the p-doped case, it is positive VBG biases (Figure 3d). Basically, a much reduced shift of VDirac was observed in the 4246

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) ID−VBG curves taken at different VTG bias voltages with 0.3 V interval and (b) ID−VTG curves taken at different DC VBG bias voltages with 0.2 V interval on the n-doped sample. (c, d) ID−VBG and ID−VTG curves taken at different VTG and VBG bias voltages with 0.4 V interval on the p-doped sample, respectively.

with the polarity of VDirac at zero bias or the type of initial doping of the PLZT/DEME-TFSI double-gate GFETs. Considering these observations, we hypothesize that a most probable mechanism is the electrochemical effect reported previously in carbon nanotubes and single-gate GFET devices involving ionic liquid gates or electrodes.35,36 The electrochemical effect is attributed to the polar molecules from the ionic liquid physically adsorbed on the carbon surface. These molecules differ from those in a regular EDL because they do not respond promptly and proportionally to the external electric field due to the “pinning” on the carbon surface. This argument is supported by the following observations. First, the nonlinear VDirac shift with applied gate voltages only occurs when VTG is the sweeping gate or the driving force to change the orientation of the polar molecules in the EDL on the GFET channel. This indicates that the pinned polar molecules are located on the top side of the GFET in the ionic liquid. In addition, the pinning of polar molecules is most probably associated with the residual localized polar molecules present in the GFET channel, which are responsible for the initial GFET doping shown in Figure 1b,c. In the n-doped (p-doped) case, a negative (positive) VBG applied from the back-gate will have the same polarity of the residual (net) interface charge at the interface (Figure 1e for n-doped and Figure 1f for p-doped), which enhances the local electric field near those charge puddles with the net negative (positive) charges. As shown in Figure 1e (Figure 1f), the polar molecules will be pinned over the charged islands and the net effect of the pinning is that a portion of the polar molecules pinned on negatively (positively) charged islands in the EDL on GFET will no longer be able to respond to the sweeping VTG. The portion of the pinned polar molecules increases with the absolute value of VBG; hence, further deviation of the VDirac−VBG curve from the linear trend is anticipated at higher absolute values of VBG. To shed light on the dynamic process of the interface dipole−dipole interaction, especially by depinning the pinned

Figure 4. (a−d) Schematics of dipole−dipole configuration from topgate and back-gate at four different combinations of VBG and VTG polarities.

nonlinear VBG range, and the reduction is more severe at high absolute value of VBG. For example, in Figure 3d, a 50% reduction in the shift of VDirac (from red to black) occurs as VBG is varied from 0.4 to 0.8 V, as compared to the 0.37−0.38 V shift in response to the same ΔVBG = 0.4 V in the range of lower VBG. The nonlinear VDirac shift with VBG bias indicates that the interface dipole−dipole interaction is no longer solely electrostatic in these VBG bias ranges. The question is what additional mechanism(s) sets in and why it becomes important only in certain VBG bias ranges. Answering these questions is important to understanding the interface dipole−dipole interaction at an atomic scale. There are two key pieces of evidence related to the nonlinearity of the VDirac shift: the reduced response (or VDirac shift) only occurs at certain VBG biases when VTG is the sweeping gate, and the polarity of the VBG biases at which the nonlinear behavior of the VDirac shift was observed correlates 4247

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) Schematics of the PLZT/DEME-TFSI double-gate GFET with pulsed VBG. (b−h) Measured ID with pulsed VBG of increasing amplitude (black) under an applied VTG DC bias of −1 V, −0.6 V, −0.2 V, 0 V, 0.2 V, 0.6 V, and 1 V, respectively. The responding ID at DC VBG (red dot) are included. (i) VDirac at DC gate voltage (red) and pulse gate voltage (black) as a function of the DC VTG bias.

further illustrated in the agreeable DC and pulsed VDirac−VTG curves depicted in Figure 5i. A similar result was obtained by repeating the same measurement on the p-doped sample, and the results are shown in Figures S2 and S3. The situation is remarkably different when the roles of VBG and VTG are switched as depicted schematically in Figure 6a. Figure 6b−h depicts the pulsed ID−VTG curves (top) in response to VTG pulses (bottom) under different DC VBG biases on the n-doped sample. A direct comparison of the pulsed (black) and DC (red) ID−VTG curves is shown in Figure S4 for a set of DC VBG biases. While the DC and the pulsed ID−VBG curves have similar characteristics and overlapping hole branches, a systematic shift of their electron branch is clearly seen. In particular, the gap between the two electron branches increases with the absolute value of VBG, which results in ID values measured with pulsed sweeping VTG considerably larger than its DC counterpart (red dot) at negative DC VBG biases as shown Figure 6b−h. Consequently, the pulsed VDirac−VBG curve (black) deviates considerably from its DC counterpart (red) in the negative VBG range as illustrated in Figure 6i. A similar behavior was also observed for the p-doped sample as summarized in Figure 7, and the direct comparison of the pulsed (black) and DC (red) ID−VTG curves is shown in Figure S5. It should be noted that the DC VDirac−VBG curve deviates

polar molecules that contribute to the electrochemical effect, a series of ID−VG curves were taken in the similar voltage range with one of the two gates being pulsed alternatively between the positive and negative polarity while the other, providing a DC bias (Figure 5a). Figure 5b−h depicts the pulsed ID−VBG curves (top) in response to VBG pulses (bottom) under seven different DC VTG biases measured on the n-doped PLZT/ DEME-TFSI double-gate GFETs. The rising and falling time for all the VBG pulses shown in Figure 5 is 86 ms. Six pairs of VBG pulses of alternating polarities and increasing amplitudes at 0.1 V increment from 0.0 V were applied at biases of VTG on the opposite gate of −1.0 V, −0.6 V, −0.2 V, 0 V, +0.2 V, +0.6 V, and +1.0 V. Overall, ID responded to the VBG pulses instantaneously as illustrated in the synchronized patterns of time dependence of the ID and VBG pulses, which is not surprising based on the reported high-speed in GFETs.37,38 A direct comparison of the pulsed (black) and DC (red) ID−VBG curves is shown in Figure S1 for a set of DC VTG biases. In addition, the ID values obtained at the DC VBG values corresponding to the same pulse amplitudes and the DC VTG biases are overlaid in Figure 5b−h (red dot). Overall, a reasonable agreement between the ID−VBG characteristics (Figure S1) and ID values were observed with the same DC and pulsed sweeping VBG at the same DC VTG biases. This is 4248

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) Schematics of the PLZT/DEME-TFSI double-gate GFET with pulsed VTG. (b−h) the measured ID with pulsed VTG of increasing amplitude (black) under an applied VBG DC biases of −0.6 V; −0.4 V; −0.2 V; 0 V; 0.2 V; 0.4 and 0.6 V, respectively. The responding ID at DC VTG (red dot) are included. (i) VDirac at DC gate voltage (red) and pulse gate voltage (black) as a function of the DC VBG bias.

Hz.5 The localized depinning processes in the ionic liquid topgate of the GFET in this study may be associated with this time scale (∼86 ms or lower) but faster than the bulk rearrangement. This observation illustrates the complexity of dipole− dipole interactions at the PLZT/DEME-TFSI interface, and the net effect cannot be treated as a simple superposition of the DC and AC electrical fields applied. This result is, therefore, important to application of the GFET high-speed devices because extrinsic polar molecules adsorbed on graphene may retard and alter the device response in a way similar to that of molecular cations and anions in the ionic liquids.

from the linear trend in the negative VBG range for the n-doped sample in Figure 6i (positive VBG range for p-doped sample in Figure 7i) and the deviation becomes more significant at a larger absolute value of VBG. If the electrochemical effect is the main mechanism responsible for this nonlinear behavior as we argued earlier, the VTG pulses with alternative polarities are expected to activate those polar molecules pinned originally on the interfacial charge puddles, leading to a weakened or eliminated electrochemical effect. Indeed as shown in Figure 6i for n-doped and Figure 7i for p-doped double-gate GFETs, the pulsed VDirac values are more aligned with the linear trend expected from the electrostatic effect only. The slight overshoot (undershoot) of the pulsed VDirac−VBG curve in Figure 6i (Figure 7i) may be attributed to the total number of the depinned polar molecules in the EDL layer as a consequence of the total net force composed of the driving force of pulsed sweeping VTG and pinning force by the DC VBG bias together with the residual charges of the interface. Such effects may be obscured in the DC gated experiments which involve longer time to allow the EDL to re-establish the equilibrium with the bulk solution. In ionic liquids, it is known that re-establishing EDLs requires a rather slow spatial arrangement of the component ions and thus the specific capacitance significantly drops when the frequency of the AC bias is higher than 102

4. CONCLUSIONS PLZT/DEME-TFSI double-gate GFETs have been employed as an atomically thin probe to investigate the dynamic dipole− dipole interactions at the interface between DEME-TFSI ionic liquid top-gate and PLZT ferroelectric back-gate. The GFET source-drain current ID was measured as a function of the superposition of the top- and back-gate voltages, from which the behavior of the interfacial dipole−dipole interactions was extracted. In response to DC gate voltages, an electrostatic behavior was observed for a sweeping VBG under different VTG biases as manifested in a linear shift of VDirac with VTG and the 4249

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

ACS Applied Materials & Interfaces

Figure 7. Data from the p-doped sample. (a) Schematics of the PLZT/DEME-TFSI double-gate GFET with pulsed VTG. (b−h) Measured ID with pulsed VTG of increasing amplitude (black) under an applied VBG DC biases of −1.0 V; −0.6 V; −0.2 V; 0 V; 0.2 V; 0.6 and 1.0 V, respectively. The responding ID at DC VTG (red dot) are included. (i) VDirac at DC gate voltage (red) and pulse gate voltage (black) as a function of the DC VBG bias.

GFET channel conductivity predictable from the superposition of VBG and VTG. In contrast, the electrostatic behavior persists only in a certain range of VBG bias when VTG served as the sweeping gate. Beyond these ranges, a nonlinear and reduced VDirac shift with VBG was observed. Intriguingly, the range, in which this nonlinear behavior occurs relates to the initial doping state of the double-gate GFETs, which may be attributed to the polar molecule pinning in the ionic liquid assisted by the net residual interfacial charge puddles when VBG with compatible polarity was applied. Further confirmation of this so-called electrochemical effect using pulsed sweeping VTG of alternating polarities and increasing amplitude reveals that such a pinning effect can be overcome. These results not only shed light on the dipole−dipole interactions at an atomic-scale interface between functional materials but also provide a unique pump and probe approach using GFET as an atomically thin probe to detect the interface dipole−dipole interaction.

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Additional information on the pulsed and DC data from p-doped sample, comparison of ID−VBG (ID−VTG) curves measured on the n-doped and p-doped sample at DC and pulsed gate voltages with different DC bias VTG (VBG) values (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chunrui Ma: 0000-0002-7824-7930 Ming Liu: 0000-0002-4392-9659 Jun Li: 0000-0002-3689-8946 Author Contributions

C.R.M. and R.T.L. fabricated the double-gate GFETs. C.R.M. and G.L.H performed the electric measurement. G.L.H., M.L., C.R.M., J.S.H., J.L., and J.W. worked on data analysis and figures. J.W. designed the experiments and directed the project. C.R.M. and J.W. led the effort in writing the paper. All authors discussed the results and helped revise the manuscript.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14380.

Notes

The authors declare no competing financial interest. 4250

DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252

Research Article

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ACKNOWLEDGMENTS The authors acknowledge support in part by NASA contract NNX13AD42A, ARO contracts W911NF-12-1-0412 and W911NF-16-1-0029, and NSF contracts NSF-DMR-1105986, NSF-DMR-1337737, and NSF-DMR15094. This research also was supported by Natural Science Foundation of China no. 51390472, China Postdoctoral Science Foundation no. 2015M582649, and the Fundamental Research Funds for the Central Universities.

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DOI: 10.1021/acsami.6b14380 ACS Appl. Mater. Interfaces 2017, 9, 4244−4252