Gate-Free Hydrogel-Graphene Transistors As Underwater Microphones

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Functional Inorganic Materials and Devices

Gate-Free Hydrogel-Graphene Transistors As Underwater Microphones Shichao Li, Jinglin Zheng, Jia Yan, Zhanjun Wu, Qin Zhou, and Li Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14034 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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ACS Applied Materials & Interfaces

Gate-Free Hydrogel-Graphene Transistors As Underwater Microphones Shichao Li,†,‡,§ Jinglin Zheng,§ Jia Yan,†,‡ Zhanjun Wu,*,†,‡ Qin Zhou,*,§,⊥ and Li Tan*,§,⊥ †School ‡State

of Aerospace, Dalian University of Technology, Dalian 116024, China.

Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of

Technology, Dalian 116024, China. §Department ⊥Nebraska

of Mechanical & Materials Engineering, University of Nebraska, Lincoln, NE.

Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE.

Correspondence and requests for materials should be addressed to *L.T. (email: [email protected]), *Q.Z. (email: [email protected]), or *Z.W. (email: [email protected]). KEYWORDS: graphene, hydrogel, electric double layer, transistor, acoustics ABSTRACT A perfect impedance match from water-rich hydrogels to an oceanic background makes hydrogel microphones ideal for long-distance, underwater acoustic reception with zero reflection. A novel hydrogel-graphene transistor is thus designed to work under a gate-free mode, in which a sheet of graphene directly converts mechanical vibrations from a microstructured hydrogel into electrical current. This work shows that the quantum capacitance of graphene plays an important role in determining the shift of Fermi level in graphene, and subsequently the amplitude of the current signal. Once employed underwater, this device provides a response to sound waves with high stability, low noise, and high sensitivity in a much-needed low frequency domain.

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1. INTRODUCTION Soft tissues in animals and humans are made with hydrogels – a network of polymer chains in which water is dispersed and solidified.1-4 These networked structures bear multiple functions, including being a structural support,5,6 reacting under nerve pulses (actuation),7 and forwarding external stimuli to the central nervous system (sensing).8,9 These fascinating mechanical and electromechanical behaviors, plus a molecule-level control in preparing an artificial hydrogel, incite widespread research interests. Many works from tissue engineering and biomedical fields focused on enriching the soft material with a structural adapting capability10-13 or improving its strength and fracture resistance;14-16 other works paid attention to engineer soft hydrogels as flexible conducting wires17 or smart membranes.18,19 Ultimately, a combination of these two disciplines could provide ideal candidates for wearable / implantable devices. A nearly untapped potential from hydrogel, however, resides in underwater acoustics. Though being a solid, acoustically, hydrogels behave almost the same as water. Shear waves damp quickly and longitudinal waves propagate with nearly identical acoustic impedance as water. If made into underwater acoustic sensors (or hydrophones), hydrogels are not easily detectable, because they have near zero sound reflection. This is in great contrast with the state-of-art hydrophone technology, which often uses piezoelectric ceramics20 and thus the majority of incoming acoustic energies are reflected. In order to use hydrogels as complementary acoustic devices of those hydrophones, a different transduction mechanism is needed to convert mechanical vibration into electrical signal. For example, hydrogels are electrically conductive, so hydrogel strain sensors have been reported based upon a change in resistance due to pure geometrical effects (i.e. the soft slab gets longer and thinner under stretching, so its resistance increases).21 However, this effect is very small – a sound pressure at a high level of 120 dB @

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100 Hz could merely generate ~3×10-10 relative resistance change (Note 1, Supporting Information).

To effectively detect sound, a more sensitive mechanism is required.

One

promising option is to take advantage of an intrinsic feature of hydrogel, i.e., the extremely large capacitance value from electric double layers (EDLs). EDLs form when an electrode is brought into contact with hydrogel, where free charged ions in the hydrogel move to the nanometer vicinity of the electrode.22,23 When a bias voltage is applied between the electrode and hydrogel, the amount of charge varies, rendering hydrogels effective as capacitors. Unlike a large gap between oppositely charged electrodes in dielectric capacitors, the narrow gap in EDLs enable more than five orders of magnitude higher specific capacitance.19 In this work, we present a hydrogel microphone by interfacing hydrogel with a monolayer graphene. The conductance in this monolayer graphene is tuned by the dynamic EDLs that were triggered from acoustic vibrations and formed at the graphene-hydrogel interface. Unlike earlier works which used graphene as mechanical diaphragms,24-26 heating elements for thermoacoustic waves,27-29 or electrodes for piezoelectric drivers,30 in this work, dynamic EDLs induce a large number of free carriers in the graphene, giving the new device a high sensitivity.

This

mechanism also differs significantly from previous graphene-based pressure sensors where the graphene serves as a contact resistance-variable conductor,31,32 presenting at least one order of magnitude higher pressure sensitivity. This operation with dynamic EDLs also differs from previous tuning method of using a bias voltage in graphene transistor, which does not exhibit pressure sensitivity at all.33 Furthermore, the high electron mobility of graphene and large capacitance change with EDLs offers our device significantly higher electric current generation per unit pressure compared to the ones using organic semiconductor channels and dielectric membranes.34 We also find that, since the ion affinity of graphene directly induces dynamic

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EDLs, the gate electrode can be completely eliminated. This gate-free architecture avoids a direct metal-hydrogel contact, as well as a bias voltage that could induce electrochemical reactions. In comparison, previous transistor architectures often need over 100 V to operate.34 The only metal electrodes in our device are the source and drain electrodes on graphene, which are not in direct contact with the hydrogel, and have a very small bias voltage on the order of 0.1 V. This ensures a significantly improved lifespan and noise figure of the device. 2. RESULTS AND DISCUSSION Hydrogel and large-area graphene were used to cast the underwater microphone. In contrast to a rigid solid like ceramics, hydrogels are soft materials that, being mostly water, have almost perfect acoustic impedance matching with water. Polar functional groups from hydrogel molecules allow them to absorb a large amount of water without leaking. In this work, we used polyacrylamide (PAAm) as the backbones of the polymer network. We also introduced NaOH or NaCl ions in the gel network. As these ions can move freely in the gel, they can form EDLs when being placed next to a conducting electrode, such as graphene. In our device, a sheet of chemical vapor deposition (CVD) grown graphene (0.2 × 1.2 cm2) on a silicon substrate with 300 nm thermally grown oxide35,36 was obtained via two steps, i.e., graphene growth and graphene transfer (details see Figures S1 and S2, Supporting Information), the hydrogel, on the other hand, was polymerized inside a 3D printed mold (Figure S3, Supporting Information) to reveal microstructures on one side of its surface (Figure 1a). The deformability of these hydrogel slabs was tuned by using three different batches of PAAm with their crosslink densities being ranged from high (Young’s modulus, 218.45 ± 24.38 kPa), medium (53.35 ± 2.34 kPa), to low (7.01 ± 0.39 kPa, see Figure S4).

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Next, the hydrogel was used to cast devices as shown in Figure 1b, in which a graphenecarrying substrate (additional fixtures can be added to ensure a reliable contact, but we omit them

Figure 1. Device design and performance highlights. (a) Illustrative microstructured hydrogel slab with four domains of pyramids. The width of all pyramids is 400 μm and the spacing is also 400 μm. The heights of pyramids in each domain differ from the center to outward, respectively 1000, 600 and 300 μm; (b) device layout of resulting gate-free hydrogel-graphene transistor and (c) its operating mechanism through a pressure-induced formation of dynamic electric double layer (EDL), followed by subsequent free carrier modulation inside the monolayer graphene; (d) thermally grown graphene monolayer strip was transferred from a copper foil to the surface of a silicon wafer, followed by depositing two gold patches at both ends of the strip as source and 5



drain electrodes; (e) soft hydrogel slab cast from a 3D printed mold was peeled to reveal patterned features on one side; (f) placing the hydrogel slab between the source and drain electrodes and over the surface of graphene completed the fabrication of a gate-free hydrogelgraphene transistor; (g) optical micrograph of the cross sectional view of hydrogel-graphene interface; (h) frequency response of the gate-free transistor towards underwater acoustic sound from 20 to 3000 Hz; and (i) underwater acoustic waves (100 Hz) were applied to three different devices every 3 min to track device stability over long periods of time. These devices are respectively, a hydrogel capacitor with two identical electrodes of chromium/gold (orange triangles), a hydrogel capacitor with two different electrodes (one chromium/gold and another silver) (red diamonds), and a gate-free hydrogel-graphene transistor (filled blue circles). Salt concentration (NaOH) was kept at 1 M in all these devices. The last device shows the best stability, due to the chemical inertness of the graphene and the gate-electrode free setup. Note the error bars in this graph were all less than 1% of the value of measured data, making them barely noticeable behind the markers.

for now) was directly supporting a slab of microstructured hydrogel. Both ends of the graphene sheet were contacted by patches of evaporated chromium (2 nm) and gold (20 nm), respectively serving as the source and drain electrodes and leaving a channel length of 0.7 cm and width of 0.2 cm (Figure 1d). Then, soft hydrogel slab (Figure 1e) was cut to the size of the graphene channel, and carefully transferred onto the graphene-carrying substrate to cover the channel (Figure 1f). Figure 1g shows the optical micrograph of the cross-sectional view of the hydrogelgraphene interface. When the device is exposed to acoustic waves, the channel current in the graphene is converted to voltage by the resistor RL in a standard common-drain amplification configuration (Figure 1b, c; the channel current can also be fed into a current amplifier to get larger signals). The transduction mechanism for converting acoustic vibration to graphene channel current is illustrated in the inset of Figure 1c. Briefly (details discussed later), there are four steps: (1) The pyramid-like microstructures convert the acoustic vibration to a change of contact area between the hydrogel and graphene. (2) Free charged ions in the hydrogel migrate to (away from) the nanometer vicinity of the newly formed (disappeared) contact surfaces.37 (3) The ions induce 6


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electric current carriers (electrons or holes) with an opposite polarity in graphene. (4) When the graphene channel is biased, these induced carriers contribute to additional drift current. Figure 1h shows the performance of a prototype device. When a bias of 0.1 V is applied between the source and drain electrodes, this gate-free device shows clear responses toward underwater sound waves from 20 to 3000 Hz, with higher signal strength at a low frequency of 20 Hz but lower value at 3000 Hz. Figure 1i further demonstrates the advantage of a gate-free operation in terms of device reliability. We see the acoustic sensors with biasing electrodes (non FET-based device architecture with device layout from our previous work) indeed degrade.38 In contrast, the sensitivity of the new gate-free sensor has little change over prolonged operations. When sensitivity of our gate-free device was compared with commercial hydrophone, our device had a much better response at low frequencies (Figure S5, Supporting Information). Next, we elaborate the operating principle, explain the role of quantum capacitance from graphene to sense the dynamic EDLs in hydrogel, and show an extra layer of thin liquid (silicone oil) between the hydrogel and graphene enables the device’s deepwater compatibility.

In

contrast to transistors based on dielectric capacitance tuning,39 quantum capacitance (CQ)40 of graphene plays an important role when interfaced with a hydrogel. Quantum capacitance results from the change of Fermi level EF in graphene when carriers are added or removed. The effect is negligible in metals due to their large density of states; and in graphene FET with dielectric gating, the effect is also often neglected, due to the relatively small amount of carrier fluctuation. For example, the quantum capacitance of graphene is

𝐶𝑄 =

2𝑒2𝐸𝐹

2𝑒2

𝑛 = 𝜋ℏ2𝑣2𝐹 ℏ𝑣𝐹 𝜋

7


(1)


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where ℏ is the Planck constant, e the electron charge, vF ≈ c/300 the Fermi velocity of electrons in the linear band dispersion, and n the electron density. In a typical graphene sample, the intrinsic doping of graphene from impurities readily contributes n on the level of ~1012 cm-2,40 which corresponds to CQ ~ 3 μF/cm2. For comparison, a typical dielectric gating by, e.g., 1 μm of elastomer gives a dielectric capacitance of 2.2 nF/cm2, which is significantly smaller. Therefore, when gating through a dielectric material, most voltage drops across the dielectric capacitor, and the contribution of quantum capacitance can thus be neglected. In contrast, EDL capacitance from hydrogel is on the level of tens of μF/cm2,41 which makes the quantum capacitance nontrivial. We will show quantum capacitance in the gate-free device above by looking at a gated counterpart first, as shown in Figure 2a, where the equivalent circuit is illustrated along its right hand side. In this sketch, a biasing (gate) electrode is added on the hydrogel, so that the electrical potential between the hydrogel and graphene can be experimentally regulated. Essentially, under an external bias, the quantum capacitance (CQ) in graphene and two EDL capacitances (C1 & C2) in hydrogel will form, with one of the EDLs along the top gate electrodehydrogel interface and another along the pointed protrusions and the graphene layer. Even though the hydrogel slab is soft and easily deformable, EDL1 will remain intact (C1 = constant) but both the EDL2 and quantum capacitance will change upon external pressure or load. Briefly, contact area in the EDL2 will increase under an applied pressure, giving rise to an increased capacitance value in C2 (Note 2, Supporting Information). Likewise, extra charges from the increased EDL2 will tune the charge distribution in the graphene. capacitance will also change according to Equation 1.

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As such, the quantum

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Figure 2. The device transduction mechanism is demonstrated as two separate processes: the pressure induced EDL capacitance change and the EDL induced graphene conductance change. The gate electrodes have been added here to provide an additional knob for EDL tuning. (a) Schematic of a gated hydrogel-graphene transistor, its equivalent circuit, and charge distribution at the graphene-hydrogel interface due to external pressures; (b) graphene-free, hydrogel capacitors with pyramid patterns respond to external pressures with a large change in absolute capacitance (unfilled circles), whereas hydrogels with straight ridges show much weaker responses that are highly dependent on the rigidity of the hydrogel (from top to bottom, medium, low, and high crosslink density); (c) response details of hydrogel capacitors with pyramid patterns toward external loads; (d) source-drain current (Isd) of hydrogel-graphene transistor under different levels of pressures (bottom to top: no external pressure, 2.24, and 5.10 kPa); (e) the gate voltage variation can be converted to the charge carrier concentration variation in graphene (the graphene’s quantum capacitance plays an important role and cannot be neglected). The graphene mobility can then be extracted from the conductance versus carrier concentration curve. Here, the hydrogel is in complete contact with graphene; and (f) analytical fittings largely follow experimental results, implying dynamic EDL and the quantum capacitance at the 9



deformed hydrogel-graphene interface a major player for the high source-drain current (upper curve) whereas the dielectric capacitance along the suspended hydrogel-graphene interface is responsible for the low value in current (lower curve).

To investigate the change of EDL2 with respect to applied pressure, we simplify the device layout by sandwiching the hydrogel slab between two metal electrodes to remove the effect of quantum capacitance. Hydrogel slabs with two different surface patterns are designed, with the patterns placed on one side and the other side flat. Either pyramid shaped protrusions or straight ridges are adopted (Figure S3 ， Supporting Information), with the purpose to seek the largest capacitance change under a given external pressure. Other than the geometry of the patterns, rigidity in the hydrogel will also affect the deformability or limit the change of the contact area in EDL2. We pattern hydrogel surfaces with straight ridges as a test model to select the optimal rigidity from a series of compositions having the crosslink density vary from a low percentage to a medium and then to a high value (Experimental Section; Figure S6 and Table S1 in Supporting Information). After the hydrogel slab is sandwiched between two conducting pads, we measure its capacitance using an LCR meter (10 mV, 1000 Hz, parallel mode). Figure 2b shows these hydrogel-based capacitors with three different softness levels responding to the same external pressures, where the absolute sensitivity (ΔC/ΔP) reaches its maximum value of 50 nF/kPa under a medium crosslink density. After we fix the amount of crosslinker at the medium loading level, we change surface features to pyramids and obtain an absolute sensitivity approaching 900 nF/kPa (or a relative sensitivity of 10 kPa-1), which was four times of our own record38 and was also higher than the previously reported values, such as pressure sensor using microstructured rubber (0.55 kPa-1),34 graphene-polyurethane sponge (0.26 kPa-1),42 gold nanowires coated tissue fibres (1.14 kPa-1),43 SWNTs/PDMS (1.80 kPa-1).44 However, the value of our device was lower

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than that of micropatterned ionic gels pressure sensor (41.0 kPa-1).45 Figure 3c gives details of their responses toward static loads, with higher capacitance values gained under a larger load and then back to its original low value after the load is removed. Overall, this investigation tells us the range of capacitance change that is solely from EDL. To explain the interaction between the EDL capacitance and quantum capacitance, we wire a hydrogel-graphene transistor as in Figure 2a and plot source-drain current (Isd) vs. gate voltage in Figure 2d. When no pressure is applied over the hydrogel, we receive a relatively flat “V”shaped source-drain current curve under a gate voltage from -1 to 1 V, with the minimum current at the bottom of the “V” (Dirac point; Vg,min = -0.2 V). When external pressures are applied on the soft slab, the source-drain current curves become sharper and show minimum current at the same Dirac point, only with smaller current inside a small voltage window (Vg,min ± 0.1 V); beyond this narrow window, current increases with the increasing of external pressures. Overall, we see two linearly increasing source-drain current curves on each side of this Dirac point, with a value gradually saturated over the shoulder of the “V”. As the source-drain current originates from carrier flows under an electric field, where 𝑗 = 𝑒 ∙ 𝜇 ∙ 𝐸 ∙ 𝑛′ (j is the current density per unit cross sectional area of graphene, E the electric field crossing the channel length [L], and n’ the charge carrier concentration per unit volume [n’ = n/t; t is the monolayer thickness of graphene]), we can calculate the charge mobility and carrier concentration. The curve with the steepest slope in Figure 2d is from hydrogel that is in almost full contact with graphene (under a pressure of 5.10 kPa), and we first use this dataset to calculate the graphene mobility μ. With full contact, the relation between the gate voltage Vg and the charge carrier concentration n is:41,46

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|𝑉𝑔 ― 𝑉𝑔,𝑚𝑖𝑛| =

ℏ ∙ 𝑣𝐹 ∙ 𝜋𝑛 𝑒

+

𝑛𝑒 𝐶𝐸𝐷𝐿

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(2)

where the first item on the right side corresponds to the gate voltage drop due to the quantum capacitance, and the second is for the voltage drop over the two EDL capacitors. Comparing the measured Isd data with the carrier concentration determined by Equation 2, we calculate the electron mobility in the range between 1000 to 1400 cm2/V·s in our devices. This matches well with other graphene devices with an EDL gating.41 In contrast, when graphene is laid on top of a silicon wafer with a thin layer of dielectric material (SiO2) in between and further wired up as a back-gate transistor, we receive a higher charge mobility of 2300 cm2/V·s (Figure S7, Supporting Information) in this hydrogel-free transistor. Using the extracted mobility data and the minimal conductance, we can also figure out the impurity charge concentration nimp ~ 1.9 × 1013 cm-2.47 After the carrier concentration (n) inside the graphene layer is calculated as in Figure 2e, we can plug the values back into the first and second items in Equation 2 to backtrack the contribution of quantum capacitance to the gate voltage drop. For an easy comparison, we consider the ratio between those two voltage-drops as

Δ𝑉𝑔,𝑄

Δ𝑉𝑔,𝐸𝐷𝐿 = ℏ𝑉𝐹𝐶𝐸𝐷𝐿 𝜋 𝑒2 𝑛. When n varies from

low to high, this ratio gradually diminishes from more than 50 to 2 (Table S2, Supporting Information). Nevertheless, this nontrivial ratio suggests, when hydrogel is in full contact with graphene, that gate voltage is largely distributed over the interface through the contribution of quantum capacitance. The relatively large EDL capacitance compared to the quantum capacitance helps to improve the sensor stability against factors such as ion concentration fluctuation in the hydrogel (two

12


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capacitors are connected in serial, so the total capacitance is dominated by the smaller capacitor). As a demonstration, we intentionally reduce the ion concentration in hydrogels by 100 times to 10 mM, which will lower the EDL capacitance (proportional to square root of concentration) by approximately ten times. Recorded source-drain current vs. gate voltage indeed shows minimal variation in the shape and little change in the magnitude of the current (Figure S8, Supporting Information). These acquired data, however, cannot be directly applied for cases in partial contact mode, when the charge carrier distribution becomes nonhomogeneous among patches in contact with hydrogel and patches not in contact. We can use an analytical model to describe the configuration where these patches are electrically connected together and prove that the change of current in graphene channel is caused by the change in contact area between hydrogel and graphene, as well as the different charge mobilities (Note 3, Supporting Information). In the end, the features of device outputs measured in the experiment (the shape of “V” curves with different slopes under different pressures) can be nicely fitted as shown in Figure 2f. To examine the detailed response of the source-drain current toward external pressures, three ISD vs. Vg curves before (filled red), during (filled blue), and after (open blue) a 5.1 kPa pressureloading test are recorded in Figure 3a-left. The two curves before and after the loading test almost overlap each other, suggesting minimal residue left on the graphene after the contactseparation cycle between the graphene and hydrogel. The relative positions of the curves show that the gate voltage can tune the device responses. For example, when the gate voltage is fixed at -0.8 V (labeled  in red), applying a pressure results in a large increase of the source-drain current. This can also be seen in Figure 3a-right, where the current of two loading cycles was recorded. When the gate voltage is fixed at +0.1 V (labeled  in green), the response is smaller with the polarity reversed. We attribute the measured response to two factors: the charge carrier

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Figure 3. Response of the hydrogel-graphene transistor to static and dynamic load. (a-left) Three ISD vs. Vg curves before (filled red), during (filled blue), and after (open blue) a 5.10 kPa pressure loading test; (a-right) the time evolutions of ISD during the loading cycle at different fixed gate voltages indicate the device response amplitude and polarity can be set by the gate voltage; (b-left) gate metals with different electrochemical potentials shift the Dirac point in the hydrogel-graphene transistor; (b-right) source-drain current variation in Al-gated transistor before (lower curve) and after the load is applied (upper curve); (c) At zero-gate-voltage, the Algated device response to acoustic waves from 1.5 to 48.2 Pa (Note: gate-free data can be found in both Figure 1h and Figure S10); and (d) Output signal amplitude ΔIsd vs. incoming sound pressure ΔP. A sensitivity of approximately 6.0 μA/kPa can be extracted (data in blue, dashed line). The device can also be prepared for high-pressure operation (e.g. under deep ocean) by filling the void volumes between the microstructured hydrogel and graphene with silicone oil (data in red, showing reduced device sensitivity of ~2.0 μA/kPa).

variation Δn and electron mobility variation Δμ in graphene upon graphene-hydrogel contact. The output current, determined by the graphene conductance, is proportional to the product of the charge carrier concentration and electron mobility or (n + Δn)∙(μ + Δμ) ≈ n∙μ + n∙Δμ + μ∙Δn. 14


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At larger gate voltages, the pressure induced carrier number variation Δn is larger (Δn = Vg ∙ ΔC, where ΔC is determined by contact area change due to the applied pressure), which makes the third item dominating or an increased conductance of graphene (step-up current pulse). However, at lower gate voltages, the pressure induced carrier number variation (Δn) is smaller; the final conductance is now dominated by the reduction of electron mobility Δμ from the second item, where more areas of graphene is now in contact with hydrogel (likely to provide more electron scattering centers). In measured responses, the current reversed its direction by showing a step-down current pulse. We also find that the gate electrode material can strongly affect the device behaviors (Figure 3b). This is due to the establishment of EDLs around the gate electrode, where different gate electrode materials will have different electrochemical potentials and therefore different voltage drops from the gate electrode to hydrogel. Because the total gate voltage equals to the voltage drop from gate electrode to hydrogel plus another voltage drop from hydrogel to graphene, changing the gate electrode material will shift the response curves. For example, as shown in the right panel of Figure 3b, it is possible to achieve high sensitivity at zero-gate voltage if we choose aluminum as the gate electrode material (absolute sensitivity = 6~7 μA / kPa (or a relative sensitivity of 0.55 kPa-1)), which was higher than the coplanar-gate GFET pressure sensor (0.12 kPa-1);31 but the value of relative sensitivity was lower than that of polymer FET pressure sensor (8.4 kPa-1).39 However, this polymer FET pressure sensor needs a higher gate voltage (40 V) to achieve its higher relative sensitivity. To evaluate the device response to dynamic load, we pack the aluminum gated device with a thin layer of food wrap, electro-magnetically shield in a copper-mesh Faraday cage, immerse in water, and use a loudspeaker that is also placed in water to generate acoustic waves of 100 Hz 15



(Figure S9, Supporting Information; Note: the food wrap has negligible effect on the transmission of acoustic waves because its thickness is much smaller than the wavelength of interest. For enhanced durability, any other polymer laminate could also be used for the same purpose). Figure 3c illustrates the response under different levels of sound pressures. Clearly, the signal output of our device closely follows the stimuli. The measured voltage signal can be converted to the source-drain current (ΔIsd) by dividing it by the load resistor (RL, see Figure 1c) of 4000 Ω. Figure 3d shows a plot of such against a series of sound pressures, from 1.5 to 48.2 Pa. The performance of this hydrogel-graphene transistor roughly falls into two regimes, with a higher sensitivity regime above a sound pressure of 30 Pa but a lower sensitivity below this value. One additional issue to use the device as a hydrophone is that the void volumes between the microstructured hydrogel and the graphene can be annihilated under high environment pressure (such as in deep ocean), and device will then lose its pressure-sensing capability. To overcome this issue, we can fill the cavities with silicone oil, with a hope to help the microstructured hydrogel to withstand high-pressure environment. The result in Figure 3d shows a reduced sensitivity of approximately two-thirds (6.0 μA/kPa →2.0 μA/kPa), probably because filling the cavities with oil reduces the hydrogel deformation. To visualize what happens after the silicone oil is inserted between the patterned hydrogel and the single-atom-thickness graphene, a finite element model (Note 4, Supporting Information) comprised of three key components is developed - a rigid graphene electrode, a patterned hydrogel slab and the fluid filled in the pockets between the graphene and hydrogel, as shown in Figure 4a. The slab deforms under an acoustic wave of a frequency of 100 Hz and amplitude of 48.2 Pa incoming at the free end, and the colormap shows the y-displacement field. In Figure 16


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Figure 4. Modeled response of the hydrogel-graphene transistor toward dynamic load. (a) Axisymmetric finite element model with graphene modeled as a rigid plane. Silicone oil is filled in the space between the graphene and hydrogel. There is an incoming excitation toward the hydrogel at one end. The color map shows the resultant displacement in y direction; (b) the contact interface between the graphene and hydrogel is deformed under acoustic pressure. More elements are brought into contact with the graphene, leading to an increased contact area and a larger device capacitance; (c) Modeled normalized contact area modulation in two consecutive periods under 48.2 Pa acoustic excitation. The amplitude is reduced by 68% when silicone oil is filled in the space between the graphene and hydrogel; and (d) contact area modulation amplitude from the finite element model match well with the test data from Figure 3d.

4b, we zoom in on the contact interface between the graphene and hydrogel and show how this deformation changes the contact profile.

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It is obvious that elements on the free edge of the hydrogel pyramid move up and get into contact with graphene, therefore contributing to the total contact area and hence the device capacitance. In Figure 4c, we plot the simulated contact area modulation in the time domain. The simulated time duration is 0.2 s and only the last two time periods (0.02 s) are shown. It is observed that the contact area oscillates under acoustic excitation with the same frequency (100 Hz). With silicone oil filled in the space between the graphene and hydrogel, the modulation amplitude decreases significantly. This is due to the additional pressure arising in the oil when being compressed. This pressure prevents the interface from being further deformed and reduces the contact area modulation. By applying acoustic excitations of different levels ranging from 0 – 48.2 Pa, we get a series of contact area modulation amplitudes and directly compare them with experimental results, as shown in Figure 4d (normalized to the maximum response).

The

modulation amplitude almost increases linearly with the acoustic excitation due to the small deformation range (on the order of nanometers) as compared to the overall dimension of the hydrogel slab (on the order of millimeters). The predicted response with oil filled in the interface is about 32% of the case without oil, which also agrees well with test data. The successful operation of the device at zero-gate voltage encourages us to operate the device with the gate completely eliminated, with the device layout and performance highlighted back in Figure 1. We note that zero-gate and gate-free are different: gate-free essentially leaves the gate floating, instead of fixing the gate potential at zero. When an electrode (or graphene) is brought into contact with hydrogel but without being wired to the external circuit, the interactions between the electrode and the ions in the hydrogel result in the formation of EDLs and built-in potential drops from the electrode to the hydrogel. While the driving force for EDL formation might appear to be lacking without an external gate voltage, we note that EDL always appears on

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the surface of an electrode when it is exposed to a free-ion-containing media such as hydrogel. The first layer of EDL is called the surface charge, and it is formed by the adsorption of ions onto the surface of the electrode, which depends on factors such as electrode materials, ion types, and ion concentration. A built-in potential drop is established after the EDL formation. The charges in EDL and the related potential drop can be modified by externally applied voltage, but they exist even without an externally applied voltage. Thus, the total gate voltage drop Vg, from the gate electrode to the hydrogel then to the graphene, mainly happens at the electrode-hydrogel interface VEH and the hydrogel-graphene interface VHG. Therefore we have Vg = VEH + VHG (Similar to a battery: its voltage output equals the sum of electrode potential of the two half cells; except here no redox reaction is involved). In a gated mode, the original built-in potential of VEH and VHG can be changed when the gate voltage is fixed externally at a certain value, in which process Vg = VEH + VHG holds. At zero-gate, the new potentials should follow VEH’ + VHG’=0. Therefore, the potential across the hydrogel-graphene interface VHG’ is different from VHG, the gate free case. Due to such a difference, the amount of free charge carriers inside graphene induced by the EDLs will be different in the zero-gate and gate-free situations. Nevertheless, when the gate electrode is left floating or completely eliminated, our device shows good sensitivity and the major advantage is the removal of noise that is becoming evident in a gated device under a slightly elevated gate bias (Figure S10, Supporting Information). In this gate-free configuration, the built-in potential VHG is difficult to measure and not tunable, but the pressure induced dynamic formation process of EDLs still effectively tunes the charge carrier concentration in graphene. 3. CONCLUSIONS

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In conclusion, pressure induced EDL capacitance change is used to directly induce charge carriers in graphene.

The atomic thinness of graphene makes the interface tuning highly

efficient, and the high electron mobility of graphene ensures fast read-out with much improved sensitivity (in terms of output current per applied pressure). Compared to conventional dielectric capacitance tuning, the EDL capacitance tuning used in this work provide orders of magnitude larger capacitance variation, resulting in low voltage operation and strong current signal. Since this tuning in EDL capacitance can be conveniently executed by varying ion concentrations, fabricating a nanometer-thick hydrogel membrane to achieve a large capacitance becomes unnecessary. This could potentially reduce the manufacturing difficulty and cost. We also demonstrated that, due to the interactions between graphene and the ions in hydrogel, the formation of EDLs does not require a bias voltage, and as a result, our device works even without a gate electrode. The elimination of the metal electrode in contact with hydrogel avoids electrochemical reactions, which reduces the sensor noise and improves the sensor lifespan. The small on/off ratio of graphene transistor would pose a major obstacle in utilizing it in a logic circuit. However, for analog signal transduction, this is not a problem at all – it merely generates a small background signal. In fact, with its high mobility and extremely low Johnson and excess noise, graphene is considered as among the best materials for sensors, allowing even single gas molecules to be detected.48 One would also argue that evaporation of water can affect the performance of this device. Without any packaging, the evaporation of water is indeed a major problem, but this can be solved by sealing the device with a thin layer of moisture-blocking layer or by changing the chemical composition of hydrogel.49 However, using an ionic material like hydrogel to fabricate a sensor does bring in issues of non-selectivity. This includes sensor response being affected by temperature and hydrostatic pressure. All the non-selectivity issues

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need to be addressed aggressively before the sensor could find value or important application in a practical application. 4. EXPERIMENTAL SECTION 4.1. Chemicals. The chemicals used in this current work were purchased from the following vendors unless otherwise specified and used as received without further purifications. Acrylamide (AAM), N,N –methylenebisacrylamide (MBAA), ammonium persulfate (APS) and N,N,N’,N’, -tetramethylethylenediamine (TEMED) are from Sigma Aldrich. The copper foils (99.8% purity) of 25 µm in thickness are from Alfa Aesar. 4.2. Synthesis / Fabrication of Polyacrylamide Hydrogel Slabs. The polyacrylamide (PAAm) hydrogels of three different crosslink densities (high, medium and low) were synthesized following a conventional recipe by mixing 10 ml of an aqueous precursor solution of AAM (2.2 M, 1.564 g), MBAA, APS (0.05 mol%, 0.003g) and TEMED (10 µL). Concentrations of MBAA were fixed at 10.0 mol% (0.34 g, high density), 1.0 mol% (0.034 g, medium density) and 0.1 mol% (0.0034 g, low density). Meanwhile, plastic molds containing two different microstructures (Pyramid shape and straight ridges) were prepared by 3D printing using a Stratasys (Objet500 Connex3) 3D printer, and the rigid resin (VeroMagenta) was used as the mold material (for details see Figure S3). The prepared precursor solution was then poured into plastic molds with recessed pyramid or straight-line microstructures, kept in a vacuum chamber for 10 minutes to remove trapped air, and then quickly covered with a 2 mm thick glass plate. The entire package was then allowed to cure the gel precursor at 50 oC for 1.5 hour. Finally, the prepared hydrogel slab was immersed in an aqueous solution of NaOH or NaCl (the concentration was fixed respectively at 10 mM, 100 mM, and 1.0 M) for more than 12 h.

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4.3. Growth of Graphene and Its Transfer to Silicon Wafer. (Graphene Growth) 35 Largearea graphene was grown by chemical vapor deposition (CVD) using a gas mixture of hydrogen and methane and by placing a copper foil inside a home built quartz tube furnace. At first, Cu foil was cleaved into a narrow strip (15 × 25 mm2) and thoroughly cleaned with the following solvents: acetone (15 sec), isopropanol (IPA, 15 sec), copious de-ionized (DI) water rinsing, acetic acid at 45 oC (25 min), copious DI water rinsing, acetone (15 sec) and then IPA (15 sec). Then, the copper foil was loaded into the quartz tube and pumped to a base pressure of 10 mTorr. Subsequently, a constant flow of H2 (320 sccm) was introduced into the chamber. And the tube was heated to 1020 oC. After the copper foil was thermally cleaned and annealed at this temperature for 10 minutes, a constant flow of methane (71.5 sccm) was introduced to trigger the growth of the graphene monolayer. Growth lasted 10 minutes, after which the furnace was cooled down to room temperature over an hour (for details see Figure S1). (Graphene Transfer) Two graphene monolayers were grown on copper foil, one on each side. To keep one of them for device construction, a 4.0 wt% polymethylmethacrylate (PMMA) solution in anisole was spun coated on one side of the copper foil at 3000 rpm for 30 sec. Then oxygen plasma was used to etch the extra graphene from the other side of the copper foil. Graphene / PMMA coated copper foil was immersed in a copper-etching solution (APS, 0.1 g·ml-1) with the PMMA side facing up. After the copper was removed completely, PMMA / graphene membrane was scooped into DI water. Later, this bilayer was floated with clean DI water six times to remove any inorganic residue. Finally, the bilayer was fished out using a piece of silicon wafer (wafer coated with 300 nm of silicon oxide), briefly dried about an hour in ambient conditions, and then deeply dried in a vacuum oven at 100 oC for 24 hr. The PMMA layer was removed by soaking the bilayer sample in acetone, followed by copious IPA rinsing. Random but multiple locations of the

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graphene monolayer were checked with Raman spectroscopy to confirm the monolayer thickness and integrity of the graphene (see additional data in Figure S2). 4.4. Fabrication of Hydrogel Capacitor. The patterned hydrogel slab was cut into the desired dimensions (10 × 10 mm2) and then sandwiched between two pieces of gold-coated glass slides, forming a capacitive sensor. The gold-coated glass was made by physical vapor deposition of a chromium adhesion layer (2 nm) and gold (20 nm) onto a clean 0.16 mm-thick cover glass. Afterwards, each gold surface was glued with a thin copper wire (12 gauge) using a silver paste (Electron Microscopy Sciences), followed by capping with a droplet of epoxy glue (Loctite Quick Set Epoxy) to secure a firm contact for later measurements. 4.5. Fabrication of Hydrogel-Graphene Transistor. Graphene / PMMA were cut into rectangular strips (2 × 10 mm2) and transferred onto silicon wafers with a 300 nm of silicon oxide. Gold source and drain electrodes were thermally evaporated with a thin layer of chromium (2 nm) and gold (20 nm) through a shadow mask to give a channel length of 7 mm and a width of 2 mm. Copper wires were attached to the gold source and drain with silver paste, then covered with epoxy glue. The patterned hydrogel slab (microstructure side facing graphene), graphene and gate electrode were laminated by pressing against each other. The goldcoated glass slide, copper plate (0.05 cm in thickness), and aluminum plate (0.03 cm in thickness) were used as the gate electrode. For the fabrication of the transistor filled with silicone oil among the cavities, the gate electrode and patterned hydrogel slab (flat side facing the gate) were laminated with each other. Then, 100 μL silicone oil was spread over the surface (microstructure side) of the hydrogel slab (10 × 10 mm2). Finally, the hydrogel slab with the liquid was covered by graphene and its substrate.

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4.6. Hydrogel Capacitor and Hydrogel-Graphene Transistor Characterizations. The capacitances of the hydrogel capacitors were measured with a Gwinstek 800 Precision LCR meter, with experimental data exported to an LCRVIEW program. A frequency of 1 kHz and an AC bias of 20 mV (parallel mode) were used to measure the capacitance, due to its relative insensitivity to static loads and minimal interference to electrolytes inside the hydrogel membrane. Hydrogel-graphene transistors are characterized in a top-gate, bottom-contact geometry. Device performance was measured in ambient conditions using a Keithley 2400 series source meter. Static loads were applied to hydrogel devices (capacitor or transistor) by placing different weights onto the device surface. The load contact area was defined by the size of the hydrogel slab (1 × 1 cm2). Experimental data were exported to Matlab for later analysis. 4.7. Underwater Acoustic Wave Detection. A plastic wrap was used to insulate the hydrogelgraphene transistor from its aqueous environment. Then, the device was placed inside a homemade metal mesh cage and immersed in water, by connecting the leads of the transistor with an external resistor (100 kΩ) via a home-designed circuit to convert source-drain current change into voltage output. A loudspeaker (Dayton Audio, DAEX25VT-4 Vented 25mm Exciter 20W 4 Ω) driven by an amplifier (Lepai, LP-2020A + Tripath TA2020 Class-T Hi-Fi Audio Amplifier) served as the acoustic wave generator. A network analyzer (Hewlett Packard 3577A) was used as a signal generator connected with the amplifier LP-2020A to control the waveform, frequency, and sweep time of the input signal. Audio input was in the form of a sine sweep with a constant amplitude over frequencies from 20 Hz to 3 kHz (for details see Figure S11). For frequency response measurement, the network analyzer was also used as a signal receiver, with the experimental data collected by a customized LabVIEW program. An oscilloscope (PicoScope

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5000 Series) was used to record the voltage output of our transistor. The signal was recorded by a PicoScope program and analyzed for the frequency content by Fourier transform analysis. 4.8. Calibration of Underwater Sound Pressure with Hydrophone. To determine the local sound pressure applied on the hydrogel-graphene transistor, a commercial hydrophone (SQ 26 Cetacean Research Technology, Seattle, WA) was used to replace the hydrogel device at the same location under identical acoustic impact conditions (settings controlled by the sound card on the computer and the amplification ratio). Once voltage output from the hydrophone was recorded by an oscilloscope, sound pressure with a unit of Pa was calculated using peak-to-peak voltage and a sensitivity map of the hydrophone (see Note 5). ASSOCIATED CONTENT Supporting Information Heat treatment / gas flow procedures for graphene growth, design of patterned hydrogel slabs, tensile stress-strain curves of hydrogel slabs, sensitivity of hydrogel capacitors with different rigidities, performance of a back-gate graphene transistor, gate voltage distribution over the graphene-hydrogel interface, calibration of acoustic pressures, the analytical and finite element model to analyze the device response toward underwater sound, as well as signal / noise from gate-free and gated devices are all included in the Supporting Information (Figure S1−S11, Table S1−S2 and Note 1−5) (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] 25



*E-mail: [email protected] ORCID Shichao Li: 0000-0002-7370-2607 Li Tan: 0000-0003-1510-6476 Qin Zhou: 0000-0001-6534-809X Author Contributions L.T., Q.Z., and Z.W. conceptualized the work. S.L. carried out most of the device development and experimental work. J.Z. helped with finite element analysis. L.T. wrote the first draft of the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements L.T. gratefully acknowledges the financial support from the National Science Foundation (Grant number: CMMI 1098652 and IIA 1338988) and the J.A. Woollam Foundation. L.T. also appreciates the Hai’tian scholarship received from the Dalian University of Technology. References (1)

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TOC

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Gate-Free Hydrogel-Graphene Transistors As Underwater Microphones

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