Electrolyte-Gated Graphene Ambipolar Frequency ... - ACS Publications

Letter pubs.acs.org/NanoLett

Electrolyte-Gated Graphene Ambipolar Frequency Multipliers for Biochemical Sensing Wangyang Fu,* Lingyan Feng, Dirk Mayer, Gregory Panaitov, Dmitry Kireev, Andreas Offenhaü sser, and Hans-Joachim Krause Peter-Grünberg-Institute (PGI-8), Forschungszentrum Jülich, Jülich 52425, Germany S Supporting Information *

ABSTRACT: In this Letter, the ambipolar properties of an electrolyte-gated graphene field-effect transistor (GFET) have been explored to fabricate frequency-doubling biochemical sensor devices. By biasing the ambipolar GFETs in a common-source configuration, an input sinusoidal voltage at frequency f applied to the electrolyte gate can be rectified to a sinusoidal wave at frequency 2f at the drain electrode. The extraordinary high carrier mobility of graphene and the strong electrolyte gate coupling provide the graphene ambipolar frequency doubler an unprecedented unity gain, as well as a detection limit of ∼4 pM for 11-mer single strand DNA molecules in 1 mM PBS buffer solution. Combined with an improved drift characteristics and an enhanced lowfrequency 1/f noise performance by sampling at doubled frequency, this good detection limit suggests the graphene ambipolar frequency doubler a highly promising biochemical sensing platform. KEYWORDS: graphene, biosensors, frequency doublers, field-effect transistors, ambipolar, DNA

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istics were demonstrated as excellent building blocks for electronic circuits,17,18 including frequency doublers,19−21 phase detectors,22 and digital modulators.11 However, none of these ambipolar electronic circuits has been explored for biochemical sensing applications yet. For the first time, we demonstrate the operation of a graphene biochemical sensor in frequency-doubling mode, thus making use of the ambipolar properties of an electrolyte-gated GFET and providing a new degree of flexibility and tunability for the design of graphene biochemical sensors. When properly biased, an input sinusoidal voltage at frequency f at the electrolyte gate of a GFET can be rectified to a sinusoidal wave at frequency 2f at the drain electrode with high purity (more than 95% of the total output energy) and high gain (unity gain) due to the strong electrolyte−gate coupling. The graphene frequency doublers exhibit relatively high stability against drift and are expected to possess superior noise performance when being sampled at twice the signal frequency. As a proof-ofprinciple demonstration, we applied such a graphene frequency doubler for detecting the adsorption of negatively charged short single strand DNA (ssDNA) molecules on graphene surface. Our electrolyte-gated graphene frequency doubler utilizes a graphene conductive channel prepared by chemical vapor deposition (CVD) method on a Cu foil and transferred onto an undoped Si substrate with ∼120 nm dry SiO2 using the poly(methyl methacrylate) (PMMA) method.23 After removing

uring the past decade, highly sensitive label-free nanostructure-based electronic biochemical sensors have attracted a huge attention due to its applications in both fundamental research and healthcare diagnostics areas.1−5 Generally, the sensing principle is based on the detection of small modulations of the electrical conductance of the sensor channel upon external electric field, which is induced by a charged biomolecule adsorbed at the sensor surface.6 Among all of the solid-state materials, graphene possesses excellent electrical properties and is unique in that all the carbon atoms are on the surface.7,8 The combination of these demanded properties ensures the highest possible sensing response to environment stimulus, making graphene and graphene related nanomaterials a potential technological breakthrough. 6 At the same time, graphene electronic biochemical sensors have been developed to encompass the mechanical and optical properties of graphene for flexible and transparent sensor design,9−11 the quantum capacitance of graphene for wireless sensing,12,13 and even the low-frequency noise signature upon biomolecular adsorption for frequency domain detection,14 in addition to the classic conductance detection in real time. To date, these advanced graphene sensors offer an enormous design diversity in terms of taking advantages of the extraordinary properties and characteristics of graphene, which are either unique or surpass those of other materials.15 The ambipolar transfer behavior originates from the lack of an intrinsic band gap in graphene,7,16 representing another peculiar characteristics of graphene transistors. Graphene fieldeffect transistors (GFETs) possessing such ambiploar character© 2016 American Chemical Society

Received: November 19, 2015 Revised: February 26, 2016 Published: March 1, 2016 2295

DOI: 10.1021/acs.nanolett.5b04729 Nano Lett. 2016, 16, 2295−2300

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Figure 1. (a). Schematic presentation of an electrolyte-gated GFET frequency-doubling device. (b). Optical image of a fabricated GFET with liquid sealing. The graphene channel is indicated by the white dashed lines in the zoom-in panel. Scale bar: 20 μm. (c) Transfer curve VG(Vref) of the GFET-I (blue stars) and the corresponding parabolic fitting (red line) around the CNP. The working principle of the electrolyte-gated GFET frequency doubler is also illustrated in the diagram.

Figure 2. (a) Measured input and output waveforms of two electrolyte-gated GFET frequency doublers. The input frequency and amplitude (RMS) are 77.77 Hz and 100 mV, respectively, for GFET-I in the upper panel. For the lower panel, another GFET (from a different batch) is tested and the input frequency and amplitude are 7.777 kHz and 50 mV, respectively. (b) Power spectrum of GFET-I sampled at Vref = 50 mV (black line) and 120 mV (red line). (c) Amplitude of the second harmonic Aout (at the output of the GFET-I frequency doubler) versus liquid gate voltage Vref for different input amplitudes Ain ranging from 4 mV to 200 mV. The asymmetry is indicated by the black dashed arrows. (d) Maximum amplitude of the second harmonic ACNP (tested at the output of the GFET-I frequency doubler at Ain = 100 mV at CNP, VCNP = 120 mV) as a function of operation frequency. Inset: ACNP tested in (c) is plotted as a function of Ain. The black line is a parabolic fit: ACNP = 0.707b2Ain2 with b2 = 1.8 V−1.

the PMMA supporting layer by using acetone, this results in continuous, high-quality monolayer graphene over a large area. The graphene was patterned and metalized by using standard electron beam lithography (EBL) technique. As schematically sketched in Figure 1a, the electrolyte-gated GFET consists of source and drain metallic electrodes (made of 60 nm Pd)

bridged with a graphene channel. Figure 1b shows a fabricated GFET with a graphene channel size of 50 μm by 100 μm. Preliminary electrical tests performed at ambient conditions on altogether 20 devices revealed a fabrication yield of 100% with variations in the graphene channel resistance of less than 10% (2.2 ± 0.2 kΩ). In this manuscript, we present the experimental 2296


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low current bias of 3 μA. This impressive performance is consistent with our excellent transistor transconductance (normalized) of ∼1.2 mS/V, which is orders of magnitude higher than the previously reported maximum value of 25 μS/V for high-performance top-gated GFET frequency doublers.20 Strikingly, by biasing at a large current ∼80 μA, we achieved an unprecedented unity signal gain on another GFET device (from another batch with 10 μm × 20 μm graphene channel) operated at 15.554 kHz. This value of our devices is the best reported result for graphene-based frequency doublers to our knowledge.18−21 As plotted in the lower panel of Figure 2a, some delay in the phase and some distortion in the shape of the rectified sinusoidal wave (in light green) can be identified. These nonideal features can be ascribed to possible capacitive coupling at high frequencies (which can be efficiently suppressed by reducing the area of graphene). Ideally, as the regime of VG(Vref) close to the CNP can be represented by a parabolic approximation, the frequency doubling is realized with just a single graphene transistor device that gives a highpurity output spectrum (more than 95% of the total output energy) without any additional filtering. This can be seen more clearly by taking a closer look at the measured power spectrum of GFET-I in Figure 2b (tested at Ib = 2 μA, red line), where the fundamental mode is substantially attenuated at the maximum power of second harmonic. That is, the power intensity of the second harmonic at 155.54 Hz is ∼20 dB higher than the fundamental component (and much higher than all the other harmonics). On the one hand, it is clear that the electrolyte-gated GFETs can be applied as frequency-doubling devices if operated at or close to the CNP by making use of its ambipolar properties. On the other hand, the electrolyte gate can also be biased away from the CNP. The black line in Figure 2b represents the output power spectrum of electrolyte-gated GFET-I biased at Vref = 50 mV, around which the transfer curve is rather linear. The power intensity at 155.54 Hz (doubling the input reference frequency f in = 77.77 Hz) is now 26 dB weaker than its fundamental component at f in, which dominates the output power spectrum. In order to provide a comprehensive understanding of the behavior of an electrolyte-gated GFET frequency doubler, we have examined the output amplitude at the doubling frequency when sweeping the electrolyte gate voltage of GFET-I (by using lock-in technique). As shown in Figure 2c, the light blue curve depicts the liquid gate voltage dependent amplitude of the second harmonic tested at Ain = 100 mV. Close to the CNP, the amplitude reaches its maximum ACNP, which drops to its minimum at the linear regime. Afterward, the amplitude of the measured second harmonic goes up and down again (asymmetrically as indicated by the black dashed arrows) due to the curvature in the VG(Vref) far away from the CNP. At Vref = 50 (the black star in Figure 2c) and 120 mV (the red star in Figure 2c), the amplitude of the second harmonic deduced from the power spectrum of Figure 2b matches perfectly with the lock-in measurements, indicating a very good reproducibility. As shown in Figure 2c, the above trends have been further confirmed by measuring the Aout vs Vref with different Ain ranging from 4 mV to 200 mV. We also found that the maximum value of Aout biased at the CNP (ACNP) increases monotonously with increasing Ain (as expected) up to Ain = 200 mV. This trend of ACNP versus Ain is plotted at the inset (empty dots) of Figure 2d. Interestingly, we found that the measured ACNP(Ain) deviates from the parabolic relationship (black line, inset of Figure 2d), ACNP =

data collected mainly from four typical GFET devices denoted as GFET-I, -II, -III, and -IV. After wire bonding, a biocompatible, two-component epoxy was applied for sealing the GFETs against any possible leakage current in electrolyte environment between the metallic electrodes and the Ag/AgCl reference electrode, to which the liquid gate voltage Vref is applied. For electrical characterization, a current source Ib = 2 μA is connected to the drain electrode by applying a constant bias voltage Vb = 2 V across a large enough resistor of 1 MΩ (much larger than the graphene channel resistance of ∼2.2 kΩ). The corresponding voltage drop VG over the graphene channel is monitored at the drain electrode against the liquid gate voltage Vref and plotted as the transfer curve in Figure 1c. This VG(Vref) curve of GFET-I shows a rather symmetric ambipolar behavior, indicating that the charge carriers in the graphene channel can be continuously tuned from holes to electrons when sweeping the liquid gate voltage from negative to positive (blue stars).24 At the transition point, the drain voltage VG reaches its maximum value. This point is known as the Dirac point or the charge neutrality point (CNP).16 Using an interface capacitance of ∼2 μF/cm2,25 we estimate the fieldeffect mobility of this electrolyte-gated GFET-I to be ∼1200 cm2/(V s) for both hole and electron carriers. For the frequency-doubling test, the ambipolar GFET is operated in common-source configuration as shown in Figure 1a. The input sinusoidal voltage at frequency f is applied to the electrolyte gate. The rectified sinusoidal wave at frequency 2f is monitored at the drain electrode with an oscilloscope and analyzed by using a dynamic signal analyzer (HP85670) or a lock-in amplifier (SR830). Previously, GFETs have been operated as efficient frequencydoubling devices in back19 and top20 gating geometry at ambient conditions. Here, we achieve the first operation of an electrolyte-gated GFET frequency-doubling device in aqueous solution (1 mM PBS (Na2HPO4 and NaH2PO4 mixture) containing 20 mM NaCl and 1.6 mM KCl). The test is configured as shown schematically in Figure 1a. The GFET-I is operated in a common-source configuration with the Ag/AgCl reference electrode direct current (DC)-biased at VCNP = 120 mV modulated with a sinusoidal signal. In the following, we always use the root-mean-square (RMS) amplitude unless specified otherwise. The modulation signal amplitude was Ain = 100 mV at frequency f = 77.77 Hz, so Vin = √2Ainsin(2πf t) (sinusoidal wave in blue, Figure 1c and the upper panel of Figure 2a). The nonlinearity of the symmetric VG(Vref) curve (blue stars, Figure 1c) near the CNP (due to possible impurity doping, inhomogeneity, and short-range scattering in graphene)16,26 plays a key role in the applications. As shown by the fitted red line (Figure 1c), we can approximate the VG(Vref) relation of GFET-I around the CNP by using a parabolic function: VG = b0 − b2(Vref − VCNP)2, with b0 and b2 denoting the two fitting parameters. As illustrated by the red sinusoidal wave in Figure 1c, the corresponding rectified output at CNP can be described as VCNP =

2 Aout cos(4πft ) = b2A in 2 cos(4πft )

(1)

Using an oscilloscope, this rectified cosine function wave with doubled the frequency f = 2 × 77.77 = 155.54 Hz is experimentally recorded at the drain electrode as the red curve in Figure 2a (upper panel). A gain as high as 0.05, which equals the gain achieved previously by using a high-κ dielectric layer at a current bias of ∼70 μA,20 could already be achieved in our electrolyte-gated GFET-I frequency-doubling device at a very 2297


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Figure 3. (a) Schematic illustration of negatively charged ssDNA molecules adsorbed onto graphene surface. (b) Black squares and red dots: R(Vref) characteristics for electrolyte-gated GFET-II device recorded before and after ssDNA adsorption in 1 mM PBS buffer solution, respectively, showing the expected doping shift due to negative charged ssDNA. The gray triangles and stars show the corresponding sensing responses of the GFET before and after ssDNA attachment in 100 mM KCl solutions, respectively. (c) Corresponding Aout(Vref) characteristics for electrolyte-gated GFETII device through the steps of ssDNA adsorption in 1 mM PBS buffer solutions. (d) and (e) Changes in second harmonic signal of GFET-III and GFET-IV versus time upon the introduction (red arrows) of 1 nM and 100 pM ssDNA in 1 mM PBS buffer solution, respectively.

0.707b2Ain2 as given by eq 1 with b2 = 1.8 V−1 deduced from previous parabolic fitting in Figure 1c, and saturates quickly at larger Ain values, which can be ascribed to the deviation of the VG(Vref) curve from a parabolic shape away from the CNP. The above-discussed electrolyte-gated GFET-I frequency doubler was operated at relatively low frequency (155.54 Hz). In this way, we can avoid any spurious capacitive currents, thus achieving a reliable frequency-doubling device operation, although the GFET-I frequency doubler can be operated at much higher frequencies. Figure 2d shows the amplitude of the second harmonic measured in a frequency range from 18.222 Hz to 43.22 kHz. The observed roll-off at around 10 kHz can be understood by looking into the equivalent RC circuit of the electrolyte-gated GFETs, which consists of the interfacial capacitance CI and the series resistances RS consisting in part of the 1 mM PBS solution and the graphene device. The RS is dominated by the solution resistance and found to be about 100 kΩ for 1 mM PBS, whereas CI can be estimated as 2 μF/ cm2.25 As an order-of-magnitude estimation, the time constant of the system can be approximated as τ = 0.01 ms, which corresponds to a cutoff frequency around 16 kHz and explains our observations. We note here that if we apply 10 mM PBS buffer solution (ionic strength ∼200 mM) with a reduced series resistance RS of ∼10 kΩ, an area of ∼8 μm2 of the graphene channel is expected to support the operation of the frequency doubler devices up to ∼100 MHz.27 Such micrometer-sized graphene devices (0.8 μm in length L and 10 μm in width W, for example) are durable to moderate current bias up to ∼100 μA to guarantee the normal performance of the GFET, as graphene can sustain a very high current density (>1000 μA/ μm) before its breakdown due to Joule heating up to ∼1000 K.28 Here, we expect a negligible thermal effect (