Piezoresistive Effect in Plasma-Doping of Graphene Sheet for High

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Piezoresistive Effect in Plasma-Doping of Graphene Sheet for HighPerformance Flexible Pressure Sensing Application M. A. S. M. Haniff,*,† S. M. Hafiz,*,‡ N. M. Huang,*,§ S. A. Rahman,∥ K. A. A. Wahid,† M. I. Syono,† and I. A. Azid⊥ †

Advanced Devices Lab, MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur 57000, Malaysia Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia § Faculty of Engineering, Xiamen University of Malaysia, Jalan Sunsuria, Bandar Sunsuria, 43900 Sepang, Selangor Darul Ehsan, Malaysia ∥ Low Dimensional Materials Research Centre, Physics Department, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia ⊥ Mechanical Section, Universiti Kuala Lumpur Malaysian Spanish Institute, Kulim Hi-TechPark, Kedah 09000, Malaysia ‡

S Supporting Information *

ABSTRACT: This paper presents a straightforward plasma treatment modification of graphene with an enhanced piezoresistive effect for the realization of a high-performance pressure sensor. The changes in the graphene in terms of its morphology, structure, chemical composition, and electrical properties after the NH3/Ar plasma treatment were investigated in detail. Through a sufficient plasma treatment condition, our studies demonstrated that plasma-treated graphene sheet exhibits a significant increase in sensitivity by one order of magnitude compared to that of the unmodified graphene sheet. The plasma-doping introduced nitrogen (N) atoms inside the graphene structure and was found to play a significant role in enhancing the pressure sensing performance due to the tunneling behavior from the localized defects. The high sensitivity and good robustness demonstrated by the plasma-treated graphene sensor suggest a promising route for simple, low-cost, and ultrahigh resolution flexible sensors. KEYWORDS: graphene, piezoresistive, doping, nitrogen plasma, pressure sensor

1.0. INTRODUCTION Graphene is an emerging two-dimensional carbon nanostructure composed of sp2-hybridized carbon atoms arranged in a hexagonal lattice. Since the discovery of graphene by Novoselov and Geim in 2004, immense efforts have been focused on its potential applications in optoelectronics, flexible electronics, transparent conducting electrodes, and sensors because of its extraordinary properties such as high heat conduction, great elasticity, good electrical conduction, and transparency at visible light wavelengths.1 Therefore, many efforts have been devoted to developing diverse approaches to fabricate graphene. However, different graphene growth techniques have their advantages and disadvantages which satisfy the diverse demands of specific applications. Recently, progress in graphene technology has been achieved by implementing flexible mechanical sensors to imitate the tensile, bending, and tactile sensing capabilities of human skin.2,3 Typical pressure sensors use the piezoresistive effect to transduce the pressure imposed on the sensor to a resistance signal, which has been widely used because of the feasible device preparation and easy signal collection. Up to now, many © 2017 American Chemical Society

kinds of graphene-based pressure sensors have been reported in the literature by implementing the piezoresistive effect of graphene.4−6 Although a large number of studies have already been published, the piezoresistive effect of graphene is not yet completely understood. Theoretically, this effect can be separated into two parts: the modulation of the band structure and the geometrical effect.7 First, for the modulation of the band structure that appears in semiconductor materials, a lattice deformation would result in changes in the carrier distribution and average effective carrier mass, thus changing the electrical resistance.8 Nevertheless, the absence of a band gap in graphene may set its limitations in piezoresistive sensor applications.9 However, several mechanisms for engineering the band gap of graphene have been reported, which can be classified into three aspects. First, with quantum size effects, it is possible to modulate the bandgap gap by tweaking the width and specific edge of the strained graphene (e.g., zigzag-type or Received: February 26, 2017 Accepted: April 18, 2017 Published: April 18, 2017 15192

DOI: 10.1021/acsami.7b02833 ACS Appl. Mater. Interfaces 2017, 9, 15192−15201

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ACS Applied Materials & Interfaces

temperature of 1000 °C in a H2 atmosphere with a flow rate of 50 sccm, which was maintained for 20 min to simultaneously remove the native oxide layer and enlarge the Cu grain boundaries. At this temperature, a CH4/H2 gas mixture (50 and 10 sccm, respectively) was introduced in the furnace tube for 30 min, and the samples were then slowly cooled to room temperature at a rate of ∼2 °C min−1 in an inert N2 atmosphere. 2.2. Graphene Transfer and Device Fabrication. By using a typical wet etching, the graphene film synthesized on the Cu foils was transferred to a polyimide substrate incorporated with an interdigitated electrode (IDE) (Supporting Information, Figure S1) as a pressure sensor device and also to a thermally oxidized Si(100) substrate for further characterization. The transfer was initiated with a spin-coating of 100 nm-thick poly(methyl methacrylate) (PMMA) on the as-grown graphene film, followed by cutting the samples into small pieces (4 × 4 mm) and immersing the whole samples in 0.1 M ferric chloride (FeCl3) for a sufficiently long time to remove the Cu substrates. The resultant graphene with PMMA in the form of a transparent sheet was immersed in the deionized water to clean the Cu residues and the FeCl3 etching agents. Then, the transparent sheet was transferred to the target substrate, where the remaining PMMA was removed in a boiling acetone solution to form the graphene sheet. 2.3. NH3/Ar Plasma Treatment Modification of Graphene. All of the transferred graphene samples were annealed at 100 °C for 1 h with H2 plasma (H2: 10 sccm, pressure: 300 mTorr, power: 10 W) to further remove any possible residues. In order to initiate the surface modification of graphene, the samples were carefully plasma-treated in the vacuum chamber of the plasma enhanced chemical vapor deposition (PECVD) system (Nanofab700, Oxford Instruments) with a NH3/Ar gas mixture (60 and 40 sccm, respectively) at a pressure of 1000 mTorr, 40 W of power, and a temperature of 200 °C for a 20 min treatment. The Ar gas was used as a carrier gas because of its relatively high atomic mass (39.9 amu) to further enhance the removal rate of carbon atoms from the in-plane graphene, thereby allowing the nitrogen doping effectively. This process is inferred to increase the piezoresistive effect due to the plasma surface modification process. 2.4. Characterization. The morphologies of the transferred graphene were observed by a field emission electron scanning microscope (FESEM, JEOL JSM 7500F) operated at an accelerating voltage of 2.0 kV and an atomic force microscope in a semicontact mode (NTEGRA Spectra, MT-MDT). The Raman spectra were recorded using the NTEGRA spectra system (MT-MDT) with 473 nm laser excitation to evaluate the structure of the graphene. In addition, the chemical properties of the graphene were confirmed using X-ray photoemission spectroscopy (XPS) performed at beamline BL3.2U(a) of the Synchrotron Light Research Institute in Thailand. The XPS system is equipped with a Thermo VG Scientific CLAM2 electron spectrometer and was operated under the conditions of maximum photon energy of 600 eV with 0.1 eV kinetic energy steps for a narrow scan. For the device characterization, current−voltage (I− V) measurements were carried out by applying a voltage of 0−1.0 V at a temperature of 299 K. The electromechanical characterizations of the device were conducted by two different experiments: (i) pressure bulge and (ii) uniaxial strain test. First, the piezoresistive responses of the fabricated device to an applied differential pressure of 0−10 kPa were characterized by applying an inert N2 gas pressure on the backside of the device to bend up both the substrate and graphene sheet in a drum-like fashion, as shown in Figure 1a. The device was completely sealed all around on a test jig by epoxy adhesives (Araldite) to prevent the gas streaming out over the graphene that may affect the output responses. A pressure sensor (gas pressure sensor, Vernier) was used to verify the accuracy of the pressure measurement inside the test chamber. Second, for the application of strain, the tensile strain on the graphene sheet was loaded by stretching the device in one direction, as illustrated in Figure 1b. The resistance changes were simultaneously recorded using an LCR meter (EA4980A, Agilent Technologies) under ambient conditions.

armchair-type graphene nanoribbons).10,11 Second, it is also possible for the symmetry breaking of the graphene lattice to open the band gap (such as through the interaction between the graphene and silicon carbide substrate or by the application of an external electronic field on bilayer graphene).12,13 Third, boron (B) or nitrogen (N) doping could be used for the graphene layer, which is also capable of opening the band gap of graphene.14−16 However, with the difficulty of specifically controlling the edge of graphene, the complexity of electrically gated bilayer graphene, and the specific substrate of graphene placement, the doping technique is considered to possibly be the simplest, most versatile, and cost-effective method for the modulation of the band gap at the K (K′) point in the graphene electronic structure for a flexible pressure sensor application. In addition, to the best of our knowledge, the use of the piezoresistive effect of graphene through the doping process for a flexible pressure sensor application has not yet been reported. Second, the geometrical effect which commonly appears in metallic and semiconductor nanomaterials is related to a change in the geometrical dimensions due to an applied external force, which results in a change in the electrical resistance. One of the pioneer studies was reported by Zhu et al., who demonstrated this effect using high-quality, multilayer, polycrystalline graphene obtained in the CVD process.6 Even though graphene structure deformation is often used for graphene-based flexible pressure sensors, there have been reports on the use of the imperfections in the graphene network to increase the piezoresistive effect upon deformation.17 Recently, Tian et al. reported a significant increase in the pressure sensing range from 10 to 50 kPa and a high sensitivity at 5.0 × 10−3 kPa−1 using patterned laser-scribed reduced graphene oxide (LSG).18 Two layers of LSG were stacked on each other with the patterns arranged perpendicular to each other to form a crossbar structure. Upon the application of pressure, a compressive deformation could enhance the contact between the two LSG lines and reduce the interlayer LSG distance, resulting in more electrical pathways through the crossbar structure. The pressure ranges exerted by a human in the form of a gentle touch and object manipulation will be less than 10 kPa and 10−100 kPa, respectively.18 In this study, we focused on a pressure sensor with a working area corresponding to the lowest range of human perception (0−10 kPa).19 Hence, the morphology, structure, chemical composition, and electrical properties of a graphene sheet were systematically studied in relation to the effect of the NH3/Ar plasma treatment on the graphene piezoresistive effect. The results were incorporated to produce a highly sensitive graphene-based pressure sensor performance. With the confirmation of the increasing piezoresistive effect due to the plasma-doping process, the sensor was further evaluated at an extremely small pressure of 15 Pa, and the recognition of resistance changes was shown. Interestingly, a sensitive response to this subtle-pressure regime enables mimicking the physical sensing properties of natural skin in upcoming development of wearable artificial skins or electronic skins (e-skins) for potential applications in advanced robotics and biomedical and health monitoring technologies.20

2.0. EXPERIMENTAL DETAILS 2.1. Graphene Growth. A high-quality graphene film was synthesized using Cu foils (25 μm thick, Alfa Aesar) as the substrate, through a custom-made hot filament temperature chemical vapor deposition (HFTCVD) system.21 Prior to the graphene deposition, a plasma treatment was performed on the surface of the Cu foil at a 15193


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20 min of plasma treatment in the NH3/Ar atmosphere (see Figure 2c and d), the surface became coarsened with the nanocrystallite size of less than ∼10 nm. This could have been because of the mild plasma treatment along with the duration of the exposure to the etching effect of the NH3+/Ar+ ion bombardment. It is also worth noting that no significant crack formation was observed after the plasma treatment at a relatively slow plasma etching/doping reaction, and the continuity of the graphene sheet could remain unchanged. However, a few wrinkles and large impurities were apparently seen in both the UG and plasma-treated graphene (PG) sheet, which may have originated from the imperfection during the graphene growth.22,23 Typical atomic force microscopy (AFM) image of the PG sheet, as shown in Figure 2f, further confirmed the presence of a coarser surface with the particulate structure compared to that of the UG sheet (see Figure 1e). Here, the average roughness of the UG and PG sheet was evaluated to be 0.46 and 2.23 nm, respectively. The increased surface roughness after plasma treatment has also been observed by other groups, and it was attributed to graphene sample damage and conglomeration of graphitic carbon clusters.24 As in our case, NH3/Ar plasma treatment process may lead to nitrogen atoms to substitute the carbon atoms in the form of pyridinic-N, pyrrolic-N and graphitic-N configuration (see Figure 2g). The Raman spectra of the PG sheet were examined in comparison with the spectra of the UG sheet, and the typical spectra are shown in Figure 3a. The Raman spectra (λ = 473.0 nm excitation) of both samples show three intense graphene features: the G-band at ∼1587 cm−1, D-band at 1350 cm−1, and

Figure 1. Schematic representation of the electromechanical characterizations of the device by (a) pressure bulge and (b) uniaxial strain test.

3.0. RESULTS AND DISCUSSION 3.1. Morphological and Structural Properties. FESEM was employed to examine the morphological evolution of a graphene sheet before and after plasma treatment. As shown in Figure 2a and b, the untreated graphene (UG) sheet initially has a smooth surface in the morphology of a monolayer (lighter) and also shows the dark islands on the underneath monolayer, which can be identified as a multilayer region. After

Figure 2. Morphological evolution of graphene sheet. (a and b) FESEM images of the UG sheet and (c and d) the PG sheet on SiO2/Si substrate at low and high resolutions. (e and f) AFM images of the UG and PG sheets on SiO2/Si substrate and their height profiles plot along the white line. (g) Schematic representation of the plasma surface modification of graphene sheet by NH3/Ar plasma. Nitrogen atoms are expected to substitute the carbon atoms in the form of pyridinic-N, pyrrolic-N, and graphitic-N configurations. 15194


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Figure 3. Raman spectra of (a) the UG and (b) the PG sheet. The increase in the D-band intensity, decrease in the 2D-band intensity, and the blueshift of the 2D- and G-bands for the PG sample were associated with the effect of disorder and/or doping in the graphene lattice.

Figure 4. (a−d) XPS studies of the PG sheet with C 1s and N 1s orbitals. The inset shows the C 1s XPS spectra of the UG sheet. The deconvolution of N 1s peak revealed that plasma treatment produces ∼2% nitrogen doping with pyridinic-N, pyrrolic-N, and graphitic-N configurations inside the graphene lattice. This leads to nonequivalence of the π and π* states at the Dirac points, which can be observed with the absent of π−π* bonds in the deconvoluted C 1s peak (as compared to UG sheet).

2D-band at ∼2700 cm−1, which originated from the twophonon double resonance process. For the UG sheet, the Dband is seen to have a very weak intensity from the inset of Figure 3a, which becomes active with the presence of minor defect structures (i.e., grain boundaries or edges). However, after plasma treatment, a significantly higher intensity of the Dband is apparently observed for the PG sheet as a result of the prominence of intervalley scattering, indicating the presence of significant disorder in the sp2-hybridized carbon (i.e., single/ multiple vacancies, substitutional dopants), which were caused by the NH3+/Ar+ ion bombardment. Another important characteristic feature in the PG sheet is the 2D-band, which has an intensity relatively lower than that of the UG sheet due to an increase in the electron−hole pair scattering rate from lattice vacancies and/or dopants.25−27 In addition, the disorder quantification of both samples was further examined by analyzing the ID/IG intensity ratio between the D-band and

G-band. It was found that the ID/IG intensity ratio of the PG sheet (ID/IG = 2.19) was much higher than that of the UG sheet (ID/IG = 0.10). On the basis of the general Tuinstra− Koenig relation,26 a very rough estimation of the in-plane crystallite size (La) was obtained for both the UG and PG sheets using the following empirical formula: La = (2.4 × 10−10)λ4(ID/IG)−1, where λ is the laser excitation wavelength used for the Raman measurements. The La values of the UG and PG sheets were ∼120.1 and ∼5.5 nm, respectively, where the PG sheet exhibited a significant reduction in crystallite size by almost ∼22 times compared to that of the UG sheet. In this case, the reduction of La is strongly related to the in-plane defects. We also evaluated the defect density of the graphene sheet, which is given by27 nD = ((1.8 ± 0.5) × 1022)λ−4(ID/IG). Assuming the graphene sheet is continuous on the substrate, here we estimate that maximum limits of the defect density of UG and PG sheets were ∼0.46 × 1011 and ∼1.0 × 1012 cm−2, 15195


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lower N-doping concentration.14,31 On the basis of the tightbinding model, the introduction of N on one of the graphene sublattices will break the potential symmetry of two neighboring carbon sites (i.e., producing localized defect states), which then results in the nonequivalence of the π state and π* state at Dirac points and thus opens the band gap.15 3.3. Conduction Mechanism. To understand the behavior of charge carrier transport in both the UG and PG sheet, an analysis using the temperature-dependent Hall-effect characteristic was performed on a 1 × 1 cm sample. At room temperature (T = 299 K), the extracted Hall coefficient of UG sheet is 0.35 (positive value), while that of the PG sheet is −0.43 (negative value). The positive and negative values of the Hall coefficient indicate the p-type and n-type behavior, respectively. It is expected that the UG sheet exhibits p-type conduction because of the typical adsorption of oxygen or water in air. Meanwhile, the PG sheet with the nitrogen binding configurations exhibits n-type conduction as the N atoms act as donors that provide excess electrons to the carbon network. Figure 5a shows the temperature dependence of conductance

respectively. Figure 3b also shows another important characteristic of the PG sheet, in which the 2D-band and G-band have shifted to higher frequencies compared to that of the UG sheet, with average blueshifts of ∼18 and ∼3 cm−1, respectively. It should be noted that the blueshifts of these bands may be associated with the effect of the disorder and/or doping in the graphene lattice.28 In addition, both samples also exhibit a relatively small value of IG/I2D ratio of ∼0.35, indicating a single layer of graphene. 3.2. Elemental Composition. To further understand the chemical properties of the UG and PG sheets, X-ray photoemission spectroscopy studies were carefully performed, and the results of wide scan spectra for both the UG and PG sheets are shown in Figure 4a. Both samples show the presence of typical C 1s peak located at ∼284 eV, while the additional peak of N 1s located at ∼399 eV was observed for the PG sheet only. Note that the XPS spectrometer was calibrated using a clean polycrystalline Au foil, and a narrow scan of the Au 4f7/2 and Au 4f5/2 peaks position found 84.0 ± 0.1 eV and 88.0 ± 0.1 eV, respectively. Therefore, all of the measured binding energies (BE) for the C 1s and N 1s peaks were accurate on an absolute scale within 0.1−0.2 eV. A Shirley background subtraction was then applied, and Gaussian−Lorentzian product functions were used to approximate the line shapes of the fitting components. By analyzing the values of BE, we further evaluated the nature of the disorder and/or doping in the graphene sheet due to the NH3+/Ar+ ion bombardment. For the UG sheet, the core level XPS spectra of C 1s were generally fitted with four components, as shown in Figure 4b, which consist of the main peak at ∼284.4 eV corresponding to the CC sp2 and other lower peaks with higher binding energies at ∼285.9, 286.7, and 288.1 eV corresponding to the C−OH, CO, and C−OOH bonds present at the graphene plane and edges, respectively. After the plasma treatment, we found that the main peak at ∼284.4 eV associated with the CC sp2 bonds became much weaker than that of the untreated graphene, as shown in Figure 4c. In addition, other new prominent peaks appeared at ∼285.1, 286.7, and 288.1 eV, corresponding to the C−C sp3, N-sp2, and N-sp3 bonds, respectively, which may have originated with defects in the form of lattice vacancies or substitutional N atoms inside the graphene sheet. Next, there were three possible components obtained for the core level N 1s spectra at ∼398.6, 400.1, and 401.5 eV, which corresponded to pyridinicN, pyrrolic-N, and graphitic-N, respectively, as shown in Figure 4d. For the pyridinic-N, the N atom contributes one p-electron to the system and has a lone-pair electron in the plane of the ring. As for the pyrrolic-N, the N atom contributes two pelectrons to the system, has two lone-pair electrons, and also contains N−H bonds. Meanwhile, graphitic-N refers to the N atoms that replace the C atoms within the graphene plane. The atomic percentage of nitrogen doping (N-doping) in the PG sheet was estimated by XPS to be about ∼2.0%. The high concentration of pyridinic-N and pyrrolic-N bonds (as shown from the deconvolution of the N 1s peak) would lead to a significant disorder in the sp2-hybridized carbon bonds in terms of lattice vacancies, which correspond well with the results of the Raman analysis. However, there was only a small amount of substitutional dopant from graphitic-N, which would retain the graphene crystal structure, but it was high enough to introduce extra electron carriers and turn it into an n-type graphene material.29,30 Furthermore, in the theoretical and experimental study, an obvious band gap was observed on N-graphene with a

Figure 5. (a) The temperature-dependent Hall-effect characteristic plot of G versus T. The positive (UG sheet) and negative (PG sheet) values of the Hall coefficient indicate the p-type and n-type behavior, respectively. (b) The carrier transport behavior of UG and PG sheets according to the VRH model plot using ln(G) versus T−1/3. From fitting the data of the PG sheet between 100 ≤ T ≤ 350 K, a signature of VRH transport between the localized sp2 states is clearly shown. Note that all of the measurements were done for transferred graphene on SiO2/Si substrate.

for the UG and PG sheets. The UG sheet has metallic behavior (dG/dT < 0), which is quite consistent with the pristine graphene.32 In contrast to the UG sheet, the PG sheet here has much lower conductance at 100 K due to the localization of carriers induced by the high density of defects, but the conductance values tend to increase as the temperature increases up to 350 K, indicating that a thermal excitation effect dominates the transport behavior. The direct thermal excitation behavior in the PG sheet was further examined through the linear relationship between the conductance and quadratic temperature at 100−260 K in the inset of Figure 5a. It should be noted that the exposure of NH3/Ar plasma on the graphene not only introduces the N-substitutions but also produces the lattice defects that would result in strong localization of carriers. According to the variable-range hopping (VRH) model33 G(T) = G0exp(−(T0/T)1/3), we further 15196


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Figure 6. I−V characteristics of the fabricated pressure sensors with (a) the UG sheet and (b) the PG sheet at T = 299 K. (c) Possible resistance components of the device. The linear I−V curve indicates the ohmic contact characteristics of the graphene sheet incorporated on the device. (d) Schematic representation of the fabricated device showing the deformation of both the substrate and graphene under pressure loading. (e and f) Relative change in resistance in response to applied differential pressure utilizing the UG and PG sheets as functional piezoresistive materials.

created by the pyridinic-N, pyrrolic-N bonds, or lattice vacancies Rtunneling; and (iv) the resistance due to the metal electrode−graphene contacts Rcontact. To evaluate the pressure sensitivity of the fabricated sensors with both the UG and PG sheet, inert N2 gas at specific pressures ranging from 0 to 10 kPa were applied to the back surface of the sensors (see Figure 6d), and the input and output response were simultaneously monitored using a source meter and a reference pressure sensor under ambient conditions. The working principle of the sensors is based on the uniform pressure applied on one side of the substrate, which consequently leads to the deformation of both substrate and graphene to generate the electrical output signal. The relative changes in resistance (ΔR/R0) as a function of the change in applied pressure (ΔP) for both sensors are shown in Figure 6e and f. A linear variation in ΔR/R0 with an increase in ΔP was observed for both sensors. Here, the sensitivities of the sensors were evaluated based on the slopes of the fitted lines. Interestingly, the fabricated sensor with the PG sheet exhibited a sensitivity of 4.5 × 10−2 kPa−1, which is about 1 order of magnitude higher than the sensitivity obtained from the UG sheet (1.2 × 10−3 kPa−1) as well as the controllable defectivegraphene sheet from our previous work (2.5−4.5 × 10−3 kPa−1).34 Note that the pressure sensing properties of fabricated sensors were also dependent on the mechanical properties of the substrate, in which the applied pressure causes the strain or bending displacement of the substrate and eventually in turn determines the pressure sensitivity. From the variation of device structures (e.g., size, thickness, materials properties, sensor configuration, etc.), it is therefore necessary to further investigate their gauge factor (GF) to compare the piezoresistive effect of the materials, which is expressed by the following equation: GF = (ΔR/R0)/ε, where ε is the strain induced to the sensor. The GF is a key parameter to determine

examine the carrier transport behavior by analyzing the relationship of ln(G) versus T−1/3 of both the UG and PG sheets, as shown in Figure 5b. It can be noticed that a linear plot for the PG sheet shows a signature of VRH transport between the localized sp2 states, while this behavior cannot be found in the UG sheet due to the absence of localization. This observation suggests that the lattice defects with N-doping from the controlled plasma treatment could alter the electronic properties of the graphene and lead to the observed transport gap. Therefore, it induces the modulation of transport properties of graphene from semimetallic to semiconducting behavior. 3.4. Device Characterization. Prior to testing the electromechanical response of the fabricated sensor devices, the I−V characteristics of the fabricated sensors with UG and PG sheets were measured using a standard probe station. The plots in Figure 6a and b show linear curves and ohmic I−V characteristics for both the UG and PG sheets recorded at a temperature of 299 K, showing the good quality of the contacts between the graphene sheet and metal electrodes without any significant Coulomb blockade effect. The extracted resistance values R0 of the devices that incorporated the UG and PG sheets were 44.712 ± 0.05 Ω and 1.6056 ± 0.005 kΩ, respectively. It is expected that the R0 for the PG sheet would be relatively higher than that of the UG sheet because of the larger charge carrier scatterings, which may originate from the tunneling barriers or the structural disorders caused by the NH3/Ar plasma treatment. The total resistance of the fabricated device with the PG sheet can be possibly contributed by the four components (see Figure 6c): (i) the intrinsic resistance of the metal electrode Relectrode; (ii) the intrinsic resistance of the graphene sheet itself by the modulation of the graphene band structure created from the graphitic-N bond Rintrinsic; (iii) the tunneling resistance caused by the disorders 15197


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Figure 7. (a) Relative change in resistance upon applied tensile strain for the fabricated sensors with the UG and PG sheet. On the basis of the slope of linear fitted lines, the gauge factors of UG and PG sheets were estimated to be ∼5.1 and ∼56, respectively. (b) Raman spectra of the unstrained and strained UG and PG sheet showing the 2D-band and G-band. (c) ln(R/R0) versus strain ε for the PG sheet.

height between neighboring nanocrystallite graphene. Assuming that the tunneling distance changes from d0 to d0 + Δd when the PG sheet was deformed with the strain ε = Δd/d0. Under strain below 0.2%, according to eqs 1 and 2, the resistance changes dependency on the strain can be described as37 ln(R/R0) = ln(1 + ε) + Xd0ε. Figure 6c is a linear plot of experimental ln(R/R0) versus ε, showing reasonable agreement with this equation model. From the linear fitted line, the initial tunneling distance d0 can be estimated to be ∼5.4 nm, which is slightly larger than the value reported by Zhao et al. when X = ∼10 nm.39 The observation suggests that the larger tunneling distance with the higher density of nanocrystallite graphene for the PG sheet would lead to the significant changes of the percolation path in response to the applied strain and eventually results in a better sensitivity with a GF of ∼56 compared to the previous work with a gauge factor of ∼37.39,40 In addition, because the PG sheet consists of multilayer islands on the underneath monolayer region, the interlayer tunneling effect between the stacked multilayer islands also plays an important role as conductive pathway for the strain sensing. Under the tensile strain loading, it possibly induces the slippage of overlapping nanocrystallite graphene and eventually results in an increase in resistance. Furthermore, investigation on the effect of NH3/Ar plasma treatment time on the GF was also performed to further examine the piezoresistive effect. The results show that the gauge factor increases when the NH3/Ar plasma treatment time increases (Supporting Information, Figure S2). It was observed that the high GF of ∼150 can be achieved at 40 min of plasma treatment, but the substantial etching effect of the graphene sheet contributes to the highresistance and the high-noise level of the device (R0 = 4.35 ± 0.5 MΩ) because of the large interdistances between the adjacent graphene islands and eventually limits its practical application. Therefore, as a good trade-off between sensitivity and resistance, only the electromechanical response of PG sheet at 20 min plasma treatment was considered for further characterization. To investigate its practical application for sensing, the physical robustness and reliability of the fabricated sensor with the PG sheet were evaluated by subjecting it to a series of deformations under repeated pressure loading and unloading. The time-dependence of the resistance changes in the sensor is displayed in Figure 8a with a relative hysteresis of less than ∼3.0%, suggesting a stable output response of the sensor. In addition, it also showed that the pressure sensor incorporated with the PG sheet was quite robust and could tolerate a cyclic

the gauge of the piezoresistive material itself and it is commonly used to describe the performance of sensors, in which higher GF value improves the sensitivity of sensors by generating higher resistance changes under an identical loading. Figure 7a shows the electromechanical characterization of the fabricated sensors with UG and PG sheet under applied tensile strain up to 0.2%. It is seen that the resistance increases linearly with the applied strain, and the GF values of ∼5.1 and ∼56 can be calculated from the slope of fitted lines for the UG and PG sheets, respectively. It is expected that the GF of the UG sheet is very small and comparable to the GF values of pristine graphene (from 1.9 to 6.1) reported by previous works as the piezoresistive effect of pristine graphene is limited by the lattice distortion that would cause changes in the carrier distribution and average effective mass carrier and eventually changes the electrical resistivity.6,35−38 As for the PG sheet, a much higher GF with enhanced piezoresistive effect was obtained when the same strain is applied. The piezoresistive effect of the PG sheet is mainly related to the modification of percolation paths in the small nanocrystallite graphene network, and the mechanism of strained PG sheet is quite different from the view of strained UG sheet. Through the Raman spectra results as shown in Figure 7b, it was noticed that the 2D-band and G-band positions show significant redshift (14 and 6 cm−1, respectively) for the strained UG sheet at ε =∼ 0.2% because of the elongation of the carbon−carbon bonds, thus allowing the modulation of the band gap. In contrast, no significant shift of 2D-band and G-band was found for the strained PG sheet under the same applied condition. The enhanced piezoresistive effect in the PG sheet by one order of magnitude higher than that of the UG sheet is associated with the tunneling-resistancedominated conduction through the localized defects (e.g., lattice vacancies, pyridinic-N, and pyrrolic-N impurities) that act as tunneling barriers.38,39 The relationship between the resistance R and the average tunneling distance d can be described by the following expression:37 R=

⎛ 8πhL ⎞ Xd ⎜ ⎟e ⎝ 3A2 XdN ⎠

(1)

X=

4π (2mφ)1/2 h

(2)

where N is the number of conducting paths, L is the number of particles within a conducting path, m is the electron mass, h is the Plank constant, A is the effective area, and φ is the barrier 15198


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possible to achieve a modulation of the sensor sensitivity of 1 order of magnitude from 1.2 × 10−3 to 4.5 × 10−2 kPa−1 with a GF of ∼56, which showed an enhanced piezoresistance effect in graphene. It was deduced that this large piezoresistance effect and high sensitivity at an extremely low pressure regime largely came from the tunneling behavior between the adjacent nanocrystallite graphene.

4.0. CONCLUSION In summary, we successfully fabricated a high-performance flexible pressure sensor by plasma-doping a graphene sheet under an NH3/Ar atmosphere. The plasma-doping process formed pyridinic-N, pyrrolic-N, and graphitic-N bonds inside the graphene structure. It was deduced that the ∼2.0% of Ndoping turned the graphene into an n-type behavior. In terms of pressure sensing measurement, the PG sheet showed a significantly enhanced sensitivity performance in a working area corresponding to the lowest range of human perception (0 to 10 kPa) compared to that of the UG sheet. It was found that the sensitivity of the fabricated sensor with the PG sheet was one order of magnitude higher than that of the UG sheet. Moreover, we also demonstrated its excellent sensing capabilities by sensing an extremely low pressure of 15 Pa. The mechanism of the enhanced piezoresistance effect of the PG sheet was deduced to largely come from the tunneling behavior between the adjacent nanocrystallite graphene islands. To the best of our knowledge, this straightforward surface modification through the NH3/Ar plasma-doping process suggests a promising route for a simplified, versatile, and cost-effective method for the fabrication of high-performance flexible pressure sensor devices compared to other methods.

Figure 8. Measurement of durability and time-dependent responses. (a and b) Multicycle test of repeated loading and unloading and the response and recovery time at applied differential pressure of ∼10 kPa. (c) Real-time response to the application of a piece of paperboard with a weight of 38.5 mg loaded on the backside of a sensor of 0.25 cm2 (corresponding to ∼15.0 Pa) at an applied bias voltage Vb = 1 mV and frequency f = 100 kHz.

test under pressure loading (∼10 kPa) and unloading (∼0 kPa). The response and recovery times of the sensor were important features to further evaluate the pressure sensing performance. From the results shown in Figure 8b, the sensor response and recovery times were estimated to be about ∼0.7 and ∼0.8 s, respectively. The results obtained here were higher and not comparable to the previous findings.9,41 In this case, this could be due to the slow on−off venting of the gas line. Interestingly, as shown in Figure 8c, the fabricated pressure sensor with the PG sheet could also detect a minimum pressure of ∼15.0 Pa, which was demonstrated by attaching a piece of paperboard with a weight of 38.5 mg to the sensor. The realtime response of the resistance changes exerted by the paperboard was carefully recorded, and the ΔR/R0 was about ∼7.0 × 10−4. This indicated that the incorporation of the PG sheet on the sensor made it sensitive to the extremely low pressure regime, which could be further expanded for practical sensor application in tactile sensing. This type of sensing can be used in mimicking human in-hand object manipulation such as contact and release of an object, lift and replacement of an object, and detecting tangential forces due to the weight and shape of the object to prevent slip. The sensor will be arranged with diaphragm-based configuration with the sensing material placed at points of maximum deflection of a diaphragm. Kim et al. presented tactile sensing in a variation of polyimide layer thickness.42 They found that with a thicker layer, a higher deflection of the device is possible with higher sensitivity range. However, in this case, the spatial resolution and mechanical flexibility of the sensor is somewhat decreased. In case of a large area object, fabrication of sensor arrays is needed to show pressure distribution measurements that are applied with a normal force to the sensing material. Furthermore, we demonstrated that the plasma-doping treatment made it

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02833. Fabrication process of graphene-based pressure sensor on a flexible substrate (Figure S1), effect of plasma treatment time on the gauge factor and the device resistance (Figure S2), and comparison of sheet resistance and contact resistance for the UG and PG sheets (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: aniq.haniff@mimos.my. *E-mail: syedhafi[email protected]. *E-mail: [email protected]. ORCID

M. A. S. M. Haniff: 0000-0003-3980-3460 Author Contributions

M.A.S.M.H. and S.M.H. directed the project and wrote the main manuscript text. The sample preparation and characterization were conducted by M.A.S.M.H. and S.M.H.; the fabrication of the sensor and electromechanical characterization were done by M.A.S.M.H., and N.M.H., S.A.R., K.A.W., and M.I.S. provided support in the verification and interpretation of the results. All of the authors reviewed and approved the final manuscript. M.A.S.M.H. and S.M.H. contributed equally to this work. 15199


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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a High Impact Research Grant by the Ministry of Higher Education of Malaysia (Grant UM.C/ 625/1/HIR/MOHE/SC/06), Fundamental Research Grant Scheme (FRGS) FP011-2015A, and Newton-Ungku Omar Fund (Grant 6386300-13501) from the British Council and MIGHT. Special thanks to the scientists of the Photoemission Spectroscopy (PES) of synchrotron beamline BL3.2U(a) at the Synchrotron Light Research Institute, Thailand, for their technical assistance and beam-time support.

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