Oxygen Plasma-Treated Graphene Oxide Surface Functionalization

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Oxygen Plasma Treated Graphene Oxide Surface Functionalization for Sensitivity Enhancement of Thin-film Piezoelectric Acoustic Gas Sensors Hongyuan Zhao, Shuangqing Fan, Yan Chen, Zhihong Feng, Hao Zhang, Wei Pang, Daihua Zhang, and Menglun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09547 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Applied Materials & Interfaces

Oxygen Plasma Treated Graphene Oxide Surface Functionalization for Sensitivity Enhancement of Thin-Film Piezoelectric Acoustic Gas Sensors Hongyuan Zhao, Shuangqing Fan, Yan Chen,† Zhihong Feng,† Hao Zhang,† Wei Pang,* Daihua Zhang,† and Menglun Zhang*

State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin 300072, China

KEYWORDS: graphene oxide (GO), oxygen plasma treatment, surface functionalization, acoustic wave gas sensor, sensitivity enhancement

ABSTRACT: In this work, we presented a thin-film piezoelectric acoustic gas sensor with enhanced sensitivity by a surface modification strategy of oxygen plasma treated graphene oxide (GO) functionalization. By exposing to ammonia vapor (NH3) of various concentrations at controlled temperature and humidity, the characteristics of the GO-coated acoustic sensor were investigated, i.e. sensitivity, linearity, response, and recovery time. Oxygen plasma treatment of

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the GO-coated sensor further enhanced the sensitivity compared with the freshly prepared GOcoated sensor. The mechanism of oxygen plasma treatment effect on the GO-coated sensor was discussed based on characterizations of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscope (SEM), and precise weighing of the acoustic sensor. It was found that the oxygen plasma treatment introduces numerous defects to GO flakes, which are uniformly distributed across the GO surface, providing more gas molecule binding sites.

1. INTRODUCTION Various kinds of sensors (such as optical, electrical and acoustic sensors) have been reported to improve their detection abilities by modifying the sensing interface with graphene or graphene oxide (GO) functional layers.1-4 Acoustic wave sensors,5-10 operating by the classic mass-loading effect, typically have better selectivity and sensitivity with functional layers. For example, GOcoated quartz crystal microbalances (QCMs) exhibited excellent humidity sensing performance, due to motional resistance change caused by both mass-loadings from water molecules and interlayer expansion stress of GO film derived from swelling effect in the high relative humidity (RH) range.11 QCM was also reported as ammonia gas (NH3) sensor when covered with GO/polymer nanocomposites.12,13 A graphene-like nano-sheets/LiTaO3 surface acoustic wave (SAW) sensor was used as the H2 and CO gas sensor, with detection limits being 0.06% and 60 ppm, respectively.14 In another research work, a graphene-coated QCM presented high sensitivity towards ethanol gases, mainly due to the adsorption and desorption of volatile organic compound (VOC) molecules on the defect sites of graphene sheets.15


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However, QCM and SAW sensors, among other conventional acoustic microsensors, are fabricated on bulky non-silicon crystal substrates.16-18 Since these crystals are not CMOScompatible, it is difficult to integrate signal conditioning and processing circuit with the sensor array on a single chip. Thin film piezoelectric acoustic sensor can be a good candidate to solve this issue. Thin film bulk acoustic resonator (FBAR) with its small-size and low-cost feature, is fully CMOS-compatible and easy for on-chip integration.19 By integrating multiple FBAR sensors on a chip into array format, the miniaturized chip enables high-throughput and multiplexed detection.20 Moreover, owing to its Giga-hertz resonant frequency, FBAR possesses the highest intrinsic mass sensitivity among all kinds of piezoelectric acoustic sensors.5,21 These features allow FBAR to detect ultra-low concentration of gases that are out-of-range for other acoustic wave sensors. Although FBAR has the above advantages over QCM, SAW and other acoustic wave sensors, graphene or GO modified FBAR gas sensor is still lacking. Moreover, selection of functional materials also needs to be reconsidered for acoustic sensors. Since commonly used top electrode material of an acoustic sensor is not selective or sensitive for gas sensing, the electrode surface needs to be modified with functional materials for effective adsorption of target gas molecules.22-24 Although graphene has been investigated as functional materials for sensitive gas sensors, a problem arises that its surface needs to be functionalized with polymers, proteins or other modifiers to enhance the chemisorption ability, rather than relying on its purely physical property.25,26 GO has the 2-D structure similar to graphene.27 Compared with graphene consisting of sp2 hybridized carbon atoms, however, GO is heavily oxygenated with epoxy and hydroxyl groups on sp3 hybridized carbon on the basal plane, as well as carbonyl and carboxyl groups at the flake edges on sp2 hybridized carbon.28 The presence of these oxidized dangling bonds greatly improves the ability of GO for more effective adsorption


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of gas molecules, making it a promising material over graphene for gas sensing. Although GO is found to be semi-insulating due to the presence of oxidized functional groups,29 which limits its use in electrical sensors such as FETs. The property, however, can be an advantage when GO is used as functional materials on the mass-sensitive acoustic sensors. Functional materials that are electrically conducting, e.g. graphene, needs to be patterned when covered on sensor electrodes, because electrical shorting of resonator results in device failure. The semi-insulating property of GO eliminates the complex and skillful patterning process, which is another advantage of GO over graphene. Besides, thin film piezoelectric acoustic sensors can retain their electrical performance when covered by GO films, owing to the light-in-weight property of GO. For the above aspects, GO is an excellent candidate as functional material for acoustic sensors in gas sensing applications. On the other hand, it is reported that oxygen plasma treatment on graphene or reduced graphene oxide would introduce functional groups, which enhances the ability to interact with other molecules.30-32 Therefore, oxygen plasma treated GO is expected to act as an performance-enhancing functional layer for acoustic gas sensors. In this paper, we propose a GO-coated, oxygen plasma treated thin-film piezoelectric acoustic sensor and demonstrate its use in gas sensing application. Freshly coated and plasma treated GO on solidly-mounted-type FBAR (SMR) are characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and Raman. The sensor response to NH3 is investigated and sensitivity enhancement of graphene and GO coatings are compared. Moreover, the strategy of oxygen plasma treated GO functionalization is proposed and discussed, which has various potential applications.33-35 2. EXPERIMENTAL


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2.1. Sensor fabrication The fabrication process of the sensor is shown in Figure 1 and described as follows. Firstly, alternative layers of silicon dioxide (SiO2) and aluminum nitride (AlN) were deposited on Si wafer to form the acoustic reflector by chemical vapor deposition (CVD) and sputtering, respectively. The acoustic reflector consists of alternative layers of SiO2, AlN, SiO2, AlN, SiO2, AlN (650 nm / 1000 nm / 1300 nm / 1000 nm / 700 nm / 1200 nm, from top to bottom). Then, a layer of Mo (600 nm) was deposited by sputtering and patterned to form the bottom electrode. Further, AlN film (1 µm) was deposited as the piezoelectric layer. To expose the bottom electrode for probe test or wire bonding, the AlN film was patterned by reactive ion etching (RIE). A thin adhesion Cr layer (60 nm, not shown in the figure) and Au layer (300 nm) were evaporated and patterned by lift-off process to form the top electrode. Finally, the sensor surface was modified with GO.


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Figure 1.

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Brief description of micro-fabrication process of the SMR sensor: (a) silicon

substrate; (b) deposition of Bragg reflector; (c) sputtering and patterning of bottom electrode; (d) patterning of sputtered piezoelectric layer; (e) patterning of gold electrode; (f) GO modification. 2.2. GO/graphene coating and plasma treatment According to the instruction, GO (2D Semiconductors) was produced by modified Hummer technique, ~62 mg/L concentration in ethanol solvent, with flakes around 100 nm to 5 µm in size. Since the original GO solution was too dense and sticky, we started by diluting it with isopropyl alcohol (IPA) by a factor of 10, and further agitating it using a probe sonicator for ~10 min to make the GO flakes uniformly dispersed. Once prepared, a droplet of diluted GO solution was placed on top of resonator, a thin film of GO was thus formed after IPA was evaporated in air for 20 min. Oxygen plasma treatments were carried out in an RIE chamber (Ministar-3c, Top Technology). The pressure was 20 mTorr, RF power was 15 watt, and resulting DC bias was 15 volts. Each round of the plasma treatment time was 15 min. The process of graphene functionalization is as follows. The monolayer graphene sheet (10×10 mm2) was transferred to the surface of SMR using a PMMA-assisted wet-transfer method. A heating process was followed to adhere to graphene sheet to substrate, and then the transferred device was dried in air. PMMA was later peeled off in acetone. Further, thin photoresist film was spin-coated and patterned by electron beam lithography. Graphene was etched in oxygen plasma for 2 min, leaving only film coverage on sensing area of SMR. Finally, thin photoresist was removed by acetone. 2.3. Measurement setup


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The design of measurement and gas-supply system takes into account multiple factors such as reliability, precision, convenience, and data collection. Figure 2 shows the schematic of the setup. The chip was mounted and wire-bonded onto a self-made RF evaluation board (EVB), which was specially designed for RF signal extraction. The ground and signal line on EVB were carefully designed to minimize parasitic capacitance and inductance. SMA connectors on EVB were connected to RF cable (Gore, Inc.), which was further connected to a vector network analyzer (VNA, Keysight, E5061B) for RF signal readout. The VNA was controlled by a program on PC for auto data extraction and recording. A self-designed steel chamber was used as the reaction chamber. The EVB was mounted inside a self-designed reaction chamber during measurement. The chamber was formed by clamping EVB using a bottom steel cap and an upper steel cap with a cavity. Two seal rings and three screws helped seal the chamber, leaving only gas inlet and outlet. The experiment was carried out in clean room at stable temperature of 21℃ and 47% RH. The chamber was purged with pure nitrogen (N2) for 30 min before tests began. NH3 gas was generated by bubbling N2 into strong aqua ammonia (NH3·H2O), then mixed with N2 from another pipe. Each pipe had an independent mass flow controller (MFC). Gases of different concentrations were controlled by varying the flow rates of each pipe while maintaining the overall flow rates.


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Figure 2. Schematic of the measurement setup. The chip (purple) was wire-bonded onto a selfmade RF evaluation board (orange) and then sealed inside a self-designed steel chamber (green) with gas inlet and outlet. Signal noise was suppressed to improve the resolution of SMR as a sensor and to extract signal without disturbance. S-parameters (e.g. S11) are often used for networks operating at microwave frequencies (e.g. ~GHz) where signal power are more easily quantified than currents and voltages. In order to evaluate the intrinsic SMR electrical performance, the input impedance of SMR was obtained by converting the measured S-parameter to Z-parameter. 2.4 Material characterization techniques The SEM photos were acquired using Inspect F50 (FEI company, now Thermo Fisher Scientific). The accelerating voltage and spot size were 10.00 kV and 2.5 nm, respectively. For low magnification (2,000×), the working distance was 12.4 mm; for high magnification (60,000× and


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120,000×), the working distance was 6.1 mm. The Raman spectra were acquired using Renishaw inVia Raman Microscope (532 nm laser excitation). The focused laser spot size was about 2 µm2 with a constant power of ∼1.38 mW. An integration time of 10 s was used during the measurement to reduce the heating effects induced by the laser. XPS measurements were performed in ultrahigh vacuum in an ESCALAB 250 Xi (Thermo Scientific) with a nominal spot size of 400 µm. The XPS system is equipped with a monochromatic X-ray source. The twin-crystal, microfocusing monochromator has a 500 mm Rowland circle and uses an Al anode. Three spots on the sample were measured to determine the XPS data. The data were analyzed and peaks were fitted by the software Thermo Avantage. The total mass of the GO on the active sensing area was estimated by

the mass-sensitive SMR. In advance, the mass sensitivity of the SMR was experimentally extracted to be 4.2 MHz/ng or 0.84 (MHz·cm2)/µg, by depositing a thin film of AlN (3.3 g/cm3) on the sensor surface and measuring the corresponding frequency shift. The GO mass characterization started with a SMR without GO (bare SMR), then coated GO on the SMR (GO/SMR), and finally treated the GO on the SMR (p-GO/SMR) by oxygen plasma. The characterization experiments were conducted on 3 SMR sensors, and each SMR underwent 3 rounds of treatments. Resonant frequency and mass load for each sample was extracted from S11 phase curve. 2.5 Gas sensing The gas concentration was monitored in real-time by tracing the shift of series resonant frequency. The sensors were exposed to NH3 for 160 s in each cycle and then purged with pure N2 gas. The exposure time was chosen empirically for substantial stable frequency shift in realtime measurement of each cycle of NH3 exposure. Here, response and recovery time were


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defined as the time where the sensor output reached 90% of the final value and 10% of the baseline value, respectively. 3. RESULTS AND DISCUSSION 3.1. Sensor structure The GO-coated SMR sensor is shown in Figure 3(a). The SMR is comprised of an upper sandwich structure and a bottom acoustic reflector. The sandwich structure consists of a top gold electrode, a bottom molybdenum electrode and an AlN piezoelectric layer between the two electrodes. The overlap region of the three layers defines the active sensing area. The pentagon shape of the active sensing area is for maximizing the Q value and reducing spurious modes. The pentagon active sensing area is designed with a side length of 110 µm and a corresponding area of 0.02 mm2. Three pairs of SiO2 and AlN layers on silicon substrate form the acoustic reflector. The thickness of each layer is optimized by simulations in order to balance the longitudinal and shear wave energy loss through the acoustic reflector.36 The RF electrical signal is applied on the electrodes and a Giga-hertz acoustic resonance is generated, which is very sensitive to the loaded mass on the sensor surface. As shown in Figure 3(b) and 3(c), the Butterworth Van Dyke (BVD) model indicates that the principle of SMR-based gas sensor lies in the frequency response to the tiny mass load of gas molecules, when gas molecules are absorbed on device surface.


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Figure 3. Schematics of the sensor structure and sensing principle. (a) SMR modified with baserich GO sheets to enhance surface gas adsorption. (b) The gas sensing BVD model. (c) The mass-loading effect from gas adsorption causes frequency shift of SMR. Compared with air-cavity type FBAR, SMR with its rigid bottom reflector is an advanced structure as a sensor in two aspects. First, the cavity-free structure of SMR effectively prevents possible disturbance from surrounding liquid, which occurs in the surface modification procedure. For air-cavity type FBAR, however, liquid infusion into cavity through release hole may cause device failure, since both sides of the resonator are damped. Second, its rigid surface allows for highly reliable graphene transfer. In contrast, the transfer process will usually damage the fragile suspended film of an air-cavity type FBAR. 3.2. Device characterization


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Figure 4. Measured four parameters of SMR. (a) impedance magnitude; (b) phase of S11; (c) Q value; (d) S11 on Smithchart. As shown in Figure 4(a), the impedance magnitude curve of SMR gives a neat valley, at which the frequency is the series resonant frequency. However, the valley is not sharp enough, which indicates a high level of noise floor and hence degraded sensing resolution. The slope of the valley is approximately 0.3 dB/MHz, in contrast to a desired value of 1 dB/MHz. Compared with impedance magnitude, the phase of S11 in Figure 4(b) shows a steeper roll-off at the resonant frequency, where phase transverse from -180 to +180 degrees. By recording the frequency at which S11 phase curve crosses zero as the sensing signal, the signal noise can be lowered.


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Device Q value also has a great impact on noise and resolution of SMR sensor, and a high Q value is appropriate for gas sensing. For SMR, the Q value is mainly determined by acoustic energy trapping ability of the bottom acoustic reflector. To obtain a SMR with high Q, careful design has been carried out on the Bragg acoustic stack to maximize the energy trapping ability, considering 1-D reflection of both longitudinal and shear modes, as well as transmission of lateral modes. Fabricated SMR with optimized acoustic reflector typically has a Qmax value of 500. However, the long test pads severely degrade the value due to large parasitics. As depicted in Figure 4(c), Qmax is 240, the curve exhibits a typical ‘Gaussian’ like distribution, and a 10 MHz band with Q above 150 is chosen for gas sensing. The sensing signal with Q well below 150 will have a severely degraded noise floor.5,8 S11 is plotted on Smithchart in Figure 4(d). A smooth circle in Smithchart indicates a main mode without any spurious modes. A spurious-free SMR is important for gas sensing application, because the existence of spurious modes will disturb the determination of resonant frequency point, causing failure in tracing the correct frequency value. The suppression of spurious modes is also attributed to the optimized design of the acoustic reflector, in which dispersion curve has been carefully tuned. 3.3. GO characterization XPS measurements on GO were conducted pre and post oxygen plasma treatments to determine the surface chemical changes. In Figure 5(a), the XPS survey spectra of GO and oxygen plasma treated GO (P-GO) show a predominant graphitic C1s peak at ca. 284 eV and an O1s peak at ca. 532 eV. The O1s peak increases after oxygen exposure from 30.6 at% to 32.9 at%. Figure 5(b) compares the high resolution C1s spectra of GO and P-GO. The C1s peak was split into four peaks at the binding energies of 284.9, 286.1, 287.6, and 289.2 eV, corresponding to C=C–C, COH, C=O and O–C=O, respectively. Tables 1 gives the information of various functional groups


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identified and elemental composition. The peak at 284.9 eV (corresponding to the graphitic structure C=C-C) has the highest composition of 45% in GO, which gets reduced to 28% by the oxygen plasma treatment. An increase from 10% GO to 20% P-GO from the peak at 286.1 eV (corresponding to C-OH) was observed in the case of P-GO.37,38 The other two peaks at 287.6 and 289.2 eV (corresponding to C=O and O–C=O) also increase from 35% GO to 41% P-GO and 10% GO to 11% P-GO. Raman spectra of GO and P-GO samples were performed. As shown in Figure 5(c), the Raman spectrum of GO displays a disorder D-band caused by the graphite edges at 1333 cm-1 and a broad in-phase vibration G-band approximately at 1590 cm-1. The G-band Raman intensity of GO increases significantly after plasma treatments. The G-band intensity increases as the GO thickness decreases due to oxygen plasma thinning and reaches a maximum at a few layers.39 On the other hand, D/G ratio remains essentially unchanged pre and post oxygen plasma treatment. Raman spectral information is the accumulation of all the layers within a detectable depth, which is approximately 50 nm for 532 nm laser.39 Because limited functional groups are introduced into GO surface by oxygen plasma treatment and the underneath is still not affected, the accumulation of all GO layer Raman spectra results in substantially constant ratio of the intensity of the D peak and G peak in the subsequent treatments. More importantly, the G band displays a shift to higher frequencies (blue-shift) from 1596 to 1600 cm-1, which implies that oxygen plasma treatment causes defects on GO.40 Therefore, the oxygen plasma etching and defectcausing effects on GO are further investigated.


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Figure 5. (a) XPS survey spectra of the GO and P-GO samples. (b) XPS high resolution C1s spectra for GO and P-GO samples. (c) Raman spectra of bare (SiO2), GO and P-GO (1st, 2nd, and 3rd oxygen plasma treatments) samples. (d) SEM photos (2,000× magnification) of GO and PGO (1st, 2nd, and 3rd oxygen plasma treatments) sample surfaces. (e) SEM photos with high magnification (60,000× and 120,000×) of GO and P-GO sample surfaces.


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Table 1. Elemental data and C1s XPS peak analysis of GO and P-GO C=C-C (%)

C-OH (%)

C=O (%)

O=C-O (%)

Sample

C (%)

O (%)

284.9 eV

286.1 eV

287.6 eV

289.2 eV

GO

69.4

30.6

45

10

35

10

P-GO

67.1

32.9

28

20

41

11

For the investigation of etching effect, a large amount of small GO flakes covering the whole surface area are removed and large GO flakes get thinned by the oxygen plasma treatment, as shown in Figure 5(d). As the treatment time gets longer, the GO etching is more pronounced. For a quantitative evaluation, the mass-sensitive SMR sensors are used to measure the GO total mass upon GO coverage and after plasma treatments. The average frequency shifts to the bare SMR and total GO mass are listed in Table 2. The average resonant frequency decreases by 6 MHz upon GO coverage, and then recovers after oxygen plasma treatment. The frequency change corresponds to ~1428 pg mass addition upon GO coverage, and ~124 pg GO remains on the sensor surface after 1st plasma treatment. Additional rounds of treatment would further etch the GO on the sensor surface. Table 2. Frequency shift and total GO mass after GO coating and plasma treatments

Stage

Average freq. shift (MHz)

Total GO mass (pg)

Bare SMR

0

0

GO/SMR

-6.0

1428

st

-0.53

124

nd

2 plasma

-0.32

75

3rd plasma

-0.15

35

1 plasma


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It has been reported that the highly activated oxygen would introduce detects on the surface of GO40 and graphene41. Accordingly, the structural detects on GO caused by oxygen plasma etching were also investigated. As shown in Figure 5(e), the oxygen plasma etches the GO surface into tiny hills and pits, which lateral dimension is on the order of nm. The grass-like structure is uniformly distributed across the GO surface. The defects are believed to be related with the fast gasification of carbon atoms which occurs homogeneously across the full face of GO.40 3.4. NH3 sensing Figure 6 illustrates the response of bare SMR, GO/SMR and p-GO/SMR sensors to different concentrations of NH3. To quantify the NH3 response, the frequency shifts of bare SMR, GO/SMR and p-GO/SMR sensors are plotted (inset of Figure 6). Gas sensitivity is obtained by fitting the response curve. Linear fit predicts the relationship between frequency shift and gas concentration in the range of 100~500 ppm. The Pearson correlation coefficients (Pearson's r) of the linear fit are 0.9759, 0.9997, 0.9846, 0.9846 and 0.9905 for bare SMR, GO/SMR and pGO/SMR after 1st, 2nd, 3rd rounds of treatment, respectively. The corresponding gas sensitivities are 5.9 Hz/ppm, 10.8 Hz/ppm, 27.0 Hz/ppm, 27.8 Hz/ppm, 25.2 Hz/ppm, respectively. Considering the noise of 300 Hz (S/N=3), the resolution in this linear range is ~12 ppm.


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Figure 6. Response of bare SMR, GO/SMR and p-GO/SMR sensors to different concentrations of NH3. 1st, 2nd and 3rd stand for the response of p-GO/SMR sensor after 1st, 2nd and 3rd rounds of plasma treatment, respectively. The linear fits of frequency shift to NH3 concentration are plotted in the inset. GO/SMR nearly doubles the sensitivity compared with bare SMR, and one cycle of oxygen plasma treatment of p-GO/SMR further improves the sensitivity of GO/SMR by a factor of 3. Theoretical and experimental research have shown that the introduction of structural defects and vacancies will act as the additional adsorption sites of gas molecules, and hence more gas molecules will bind to the surface of functional materials.43-45 The sensitivity enhancements are mainly due to the introduction of numerous detects on the GO surface by the oxygen plasma treatment. On the other hand, the contribution of functional groups introduced by the oxygen plasma treatment is limited, since the changes of elemental and binding environment percentages


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are minimal. The response can no longer be improved by further plasma treatment, as is observed that the response of the 2nd and 3rd rounds of treatments are similar to that of the 1st round, indicating that the increase of defects are saturated by the 1st round of plasma treatment. In addition, as Figure 6 shows, the recovery time of p-GO/SMR is longer than that of GO/SMR. The prolonged recovery time is consistent with other work, which is due to increase of the binding energy.43,45

Figure 7. Comparison of bare SMR, p-GO/SMR and p-G/SMR sensors in response to different concentrations of NH3. The linear fits of frequency shift to NH3 concentration are plotted in the inset. To prove the superiority of GO over graphene as a sensitive coating, the sensitivity of pGO/SMR and SMR with plasma treated graphene (p-G/SMR) were compared on NH3 detection. As shown in Figure 7, the response of p-G/SMR sensor (sensitivity of 12.5 Hz/ppm, Pearson's r


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of 0.9899) is larger than bare SMR sensor (sensitivity of 5.9 Hz/ppm, Pearson's r of 0.9759). The improvement is due to the large surface-to-volume ratio of 2-D graphene material, which provides more surface cites to attach the gas molecules. At low NH3 concentration around 100200 ppm, the p-G/SMR and p-GO/SMR sensors show similar response in magnitude. As the concentration goes over 300 ppm, the response of p-GO/SMR sensor is larger than that of pG/SMR sensor, and the gap becomes larger as the gas concentration increases. Since GO flakes are heavily decorated with more gas-sensitive chemical groups than graphene and defects can be more easily introduced into GO than graphene,41 the plasma treated GO have a larger absorption capacity than the plasma treated graphene. At low NH3 concentration, both graphene and GO are rich in sensing cites and they give similar response. As the concentration becomes higher, binding cites in graphene gradually becomes saturated, but the binding cites of GO are still available for further absorption of gas molecules. These results prove that GO surface functionalization of SMR NH3 gas sensor is better than graphene in terms of sensitivity. The GO coating and oxygen plasma treatment strategy is expected to enhance the sensing performance of other acoustic microsensors such as QCM, micro cantilever and so on. It is also believed to have wide implications for other types of micro/nano gas sensors with potentially improved sensitivity and other fields such as biosensing, biomedical imaging, catalytic and environmental applications. 4. CONCLUSIONS We proposed a chip-based SMR gas sensor with oxygen plasma treated GO surface functionalization. Owing to the large surface-to-volume ratio of GO and the rich chemical groups on its surface and edges, the sensitivity of the GO/SMR sensor to gas molecule absorption was


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improved. The resonant frequency of GO/SMR shifted linearly with increasing concentrations of NH3 gas. The response was further improved by oxygen plasma treatment on GO film by introducing numerous site-binding defects. The p-GO/SMR sensor achieves a sensitivity of ~27 Hz/ppm for NH3 detection. Finally, the response of p-GO/SMR and p-G/SMR was compared and discussed, further proving the superiority of GO as a coating material for acoustic microsensor over graphene. Since the SMR chip is compatible with CMOS process, high-throughput or multiplexed detection of gases can be expected in the future by integrating SMR array with selective coatings. Furthermore, the GO coating and oxygen plasma treatment strategy can be applied to other kinds of microsensors, and potential biomedical, energy and environmental applications as well.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (W.P.) *Email: [email protected] (M.Z.) Present Addresses †College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT The authors gratefully acknowledge financial support in part from the Natural Science Foundation of China (NSFC No. 51375341), the Program of Introducing Talents of Discipline to Universities (111 project No. B07014), and the National High Technology Research and Development Program of China (863 Program No. 2015AA042603). REFERENCES (1) Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192-200. (2) Stebunov, Y. V.; Aftenieva, O. A.; Arsenin, A. V.; Volkov, V. S. Highly Sensitive and Selective Sensor Chips with Graphene-Oxide Linking Layer. ACS Appl. Mater. Interfaces 2015, 7, 21727−21734. (3) Georgakilas, V.; Tiwari, J. N.; Christian Kemp, K.; Perman, J. A.; Bourlinos, A. B.; Kim, K. S.; Zboril, R. Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications. Chem. Rev. 2016, 116 (9), 5464–5519. (4) Potyrailo, R. A.; Surman, C.; Nagraj, N.; Burns A. Materials and Transducers toward Selective Wireless Gas Sensing. Chem. Rev. 2011, 111, 7315–7354. (5) Fu, Y.Q.; Luo, J.K.; Nguyen, N.T.; Walton, A.J.; Flewitt, A.J.; Zu, X.T; Li, Y.; McHale, G.; Matthews, A.; Iborra, E.; Du, H.; Milne, W.I. Advances in Piezoelectric Thin Films for


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TOC graphic


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Figure 1. Brief description of micro-fabrication process of the SMR sensor: (a) silicon substrate; (b) deposition of Bragg reflector; (c) sputtering and patterning of bottom electrode; (d) patterning of sputtered piezoelectric layer; (e) patterning of gold electrode; (f) GO modification. 218x160mm (150 x 150 DPI)


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Figure 2. Schematic of the measurement setup. The chip (purple) was wire-bonded onto a self-made RF evaluation board (orange) and then sealed inside a self-designed steel chamber (green) with gas inlet and outlet. 180x155mm (150 x 150 DPI)



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Figure 3. Schematics of the sensor structure and sensing principle. (a) SMR modified with base-rich GO sheets to enhance surface gas adsorption. (b) The gas sensing BVD model. (c) The mass-loading effect from gas adsorption causes frequency shift of SMR. 255x136mm (150 x 150 DPI)


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Figure 4. Measured four parameters of SMR. (a) impedance magnitude; (b) phase of S11; (c) Q value; (d) S11 on Smithchart. 197x147mm (150 x 150 DPI)



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Figure 5. (a) XPS survey spectra of the GO and P-GO samples. (b) XPS high resolution C1s spectra for GO and P-GO samples. (c) Raman spectra of bare (SiO2), GO and P-GO (1st, 2nd, and 3rd oxygen plasma treatments) samples. (d) SEM photos (2,000× magnification) of GO and P-GO (1st, 2nd, and 3rd oxygen plasma treatments) sample surfaces. (e) SEM photos with high magnification (60,000× and 120,000×) of GO and P-GO sample surfaces. 243x237mm (150 x 150 DPI)


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Figure 6. Response of bare SMR, GO/SMR and p-GO/SMR sensors to different concentrations of NH3. 1st, 2nd, and 3rd stand for the response of p-GO/SMR sensor after 1st, 2nd, and 3rd rounds of plasma treatment, respectively. The linear fit of frequency shift to NH3 concentration is plotted in the inset. 213x149mm (150 x 150 DPI)



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Figure 7. Comparison of bare SMR, p-GO/SMR and p-G/SMR sensors in response to different concentrations of NH3. The linear fits of frequency shift to NH3 concentration are plotted in the inset. 216x153mm (150 x 150 DPI)


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TOC graphic 35x15mm (300 x 300 DPI)


Oxygen Plasma-Treated Graphene Oxide Surface Functionalization

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