Article Cite This: ACS Appl. Nano Mater. 2018, 1, 3886−3894
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Pd-Functionalized, Suspended Graphene Nanosheet for Fast, LowEnergy Multimolecular Sensors Takamune Yokoyama, Takahisa Tanaka, Yoshihiko Shimokawa, Ryosuke Yamachi, Yuta Saito, and Ken Uchida*
ACS Appl. Nano Mater. 2018.1:3886-3894. Downloaded from pubs.acs.org by DURHAM UNIV on 08/28/18. For personal use only.
Electrical and Electronics Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ABSTRACT: To demonstrate the feasibility of a fast, lowenergy breath diagnosis method, hydrogen sensors utilizing Joule self-heating were developed. The sensors consisted of suspended graphene films functionalized with Pd nanoparticles as sensing layers and utilized self-heating to achieve fast responses and humidity robustness with low energy consumption. Thanks to nanoscale point contacts between the graphene and Au electrodes, heat dissipation to the electrodes was greatly suppressed. The application of an appropriate voltage bias increased the graphene temperature to 180 °C with a low power consumption per unit graphene width of 0.93 mW/μm. Because of the temperature increase caused by the Joule self-heating of the graphene, the sensors responded to parts per million level H2, and a response time of 15 s was achieved at a H2 concentration of 100 ppm. At temperatures over 100 °C, the sensor response realized by self-heating was lower than that by heating using an external heater. The response reduction was due to suppressed charged-carrier scatterings with H-induced potentials under high electric fields in the self-heated graphene. Finally, we demonstrated voltage-controlled multimolecular detection by self-heating. At a two-terminal voltage difference of 0.1 V, Pd-functionalized graphene responded not to H2 but to humidity. Meanwhile, at 0.9 V, the sensor responded to 10 ppm of H2 despite great humidity variations in the background. Temperature change by self-heating is much faster and requires less energy than that by heating using an external heater. Fast, low-energy multimolecule detection realized by self-heating paves the way for mobile, low-power, real-time molecular sensors for use in health diagnosis applications. KEYWORDS: graphene, hydrogen sensor, self-heating, low energy, palladium
A
Metal oxides have been the most widely used materials owing to their ability to detect low-concentration H2. However, they react with various components, or almost all organic compounds, and require high operation temperatures of 300−600 °C to maintain stable responses and to eliminate the effect of humidity.12 The utilization of external heaters prevents sensor-size and power-consumption reductions.13 Noble metals, such as Au, Pt, and Pd, are also used for gas sensors as catalytic metals to enhance the sensor response and selectivity.14 Among them, Pd has been used the most frequently for H2 sensing because of its high sensitivity to and selectivity of H2. The following reaction mechanisms between H2 and Pd have been reported.15−18 Hydrogen molecules dissociate on the Pd surface, and the dissociated H atoms diffuse into the Pd around the surface region to form Pd hydride. The change from Pd to Pd hydride affects material properties with respect to work function, grain size, and electrical resistance. Therefore, there have been various studies on Pd-based H2 sensors, including Pd-gated metal−oxide− semiconductor transistors, Pd thin film sensors, and Pd
nalysis of molecules in breath is a promising diagnostic technique because a number of compounds in human breath are related to various kinds of diseases.1−3 Breath diagnosis has gained increasing attention because of its safety, simplicity, and swiftness. The H2 concentration of breath is known to be a good indicator of disorders in small intestine including bacterial overgrowth, colonic fermentation, abnormal fermentation, and carbohydrate intolerance.1,4−6 The typical H2 concentration ranges from a few ppm in a fasting state to several hundred ppm after a meal. However, breath contains many disturbing substances, such as a high concentration of water.4,7 When the ambient around the sensor is changed from the atmosphere to expired air, the water concentration is changed much more greatly than H2 concentration is. Thus, to develop a H2-based breath diagnosis method that can be used anywhere at any time, H2 sensors are required to be capable of detecting low and wide ranges of H2 concentrations, to be small, and to exhibit humidity robustness and low power consumption. Therefore, numerous studies of H2 sensors have been reported on, and the corresponding sensors can be categorized into two groups: (1) metal-oxide-based sensors5,8−11 and (2) metal-based sensors, namely, metalcatalyst-decorated sensors in most cases and metal thin-film sensors in some cases. © 2018 American Chemical Society
Received: April 23, 2018 Accepted: July 3, 2018 Published: July 3, 2018 3886
DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
Article
ACS Applied Nano Materials
Figure 1. (a) Schematic diagram of the graphene hydrogen sensor. (b) Measurement setup of gas molecular sensing. (c) Typical time dependence of the resistance change. The sensor response was defined as the resistance change ratio. The response time was defined as the time necessary for the sensor response to reach half of its maximum value during the sensing duration. The sensing duration was 3 min.
nanowire sensors to detect low-concentration H2 .19,20 However, Pd-based sensors often exhibit a serious issue of vulnerability to humidity.21 This issue can be eliminated by raising the temperature of Pd. Therefore, high-temperature operation utilizing external heaters as well as Joule self-heating without external heaters, which is called self-heating, has been studied using various combinations with numerous materials. However, most related studies have focused on sensor response time and recovery time improvements.13,22−24 No attention has been paid to self-heating-induced changes in the physical properties and device parameters except for temperature. In electrical gas sensors, metal catalysts are usually combined with conductive materials, which transduce chemical reactions to electrical signals. Among the conductive materials decorated with metal catalysts, graphene has attracted growing interest, since it has unique electric properties suitable for electrical signal transducers: a high electrical conductivity with a low carrier concentration and a high surface-to-volume ratio. Furthermore, its thermal properties are outstanding and are expected to improve the sensor characteristics effectively. Since graphene has a small heat capacity, self-heating is an effective means of increasing its temperature with low power consumption. Therefore, graphene is a promising sensing material,25,26 and graphene-based sensors have been intensively studied.25,27−29 Pristine, or unfunctionalized, graphene exhibits responses to polar molecules, such as NO2 and NH3, and no response to nonpolar molecules, such as H2. However, graphene modified with Pd can detect nonpolar H2.30−32 Self-heating with graphene under constant humidity was already demonstrated for a NO2 sensor at a relatively high power consumption of 32 mW.33 However, no study is available on H2 sensors in self-heated graphene. In addition, it has been reported that in Pd-composite graphene sensors, the response to humid air is the same as that to 1% H2,34 suggesting that detection of hydrogen as low as 10 ppm under humidity variations is quite difficult. Thus, effectiveness of selfheating for eliminating the humidity effect even with substantial relative humidity (RH) variation should be
explored. In particular, if a low power-consumption suitable for health diagnosis applications anywhere at any time is required, H 2 detection becomes even more difficult. Furthermore, to design low-power and highly sensitive sensors, accurate understanding of the self-heating effects on the sensor properties is required. However, the effects of high electric fields in self-heated materials on the properties of sensors remain unexplored. In this work, we fabricated Pd-functionalized suspended graphene sensors to avoid heat transfer from the self-heated graphene to the substrate. The graphene was suspended on the electrodes using a soft lithography technique called the polydimethylsiloxane (PDMS) stamp method.35−39 The sensor responses to H2 were evaluated by changing H2 concentrations, substrate temperature, and applied bias conditions. The self-heating of the graphene was verified by comparing the response time in self-heated graphene with that in graphene heated by an external heater. Finally, we also explored the possibility of selective sensing of hydrogen/humidity by applying an appropriate bias voltage in realistic gas environments, in which small changes in the H2 concentration and large variations in the RH were induced.
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EXPERIMENTAL SECTION
Low energy consumption, small size, and humidity robustness are required for a breath H2 sensor accessible anywhere at any time. To meet these requirements, we fabricated a suspended graphene sensor functionalized by Pd nanoparticles (NPs). To reduce the sensor size and power consumption, a self-heating technique was utilized. In addition, we studied the difference between heating using an external heater and self-heating by comparing the response time of self-heated graphene with that of graphene heated by an external heater. Device Fabrication. The 300 nm-thick SiO2 thermally grown on the p-type Si substrate was cleaned in both acetone and 2-propanol for 5 min. To fabricate the patterned electrodes, lift-off resist (MicroChem LOR-3B) and photo resist (AZ Electronic Materials AZ5206E) were spin-coated onto the substrate at 4000 rpm for 25 s. The substrate was then exposed in a mask aligner and developed in AZ-300MIF for 20 s. Ti, Pt, and Au metals with thicknesses of 10, 100, and 40 nm were subsequently deposited by electron beam deposition at acceleration voltages of 6 kV, 6 kV, and 8 kV, 3887
DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
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ACS Applied Nano Materials
Figure 2. (a) Optical and SEM images of the suspended graphene sensor. (b) Raman spectrum of graphene on Au (black line) and graphene (red line). The Raman spectrum of graphene was obtained by subtracting the Raman spectrum of the Au from that of the graphene on the Au. (c) Cross-sectional TEM image of the Pd NPs on the graphene and Au. (d) AFM image of Au electrode without Pd deposition. (e) AFM image of Au electrode with transferred graphene and Pd deposition. In the lower areas, graphene was transferred onto Au electrode before the Pd deposition. mass flow controllers (MFCs). The concentration of H2 was changed from 2 to 100 ppm by adjusting the flow rates of the gases. In our experiments, the device was exposed to gases with a total flow rate of 500 sccm (standard cubic cm). As shown in Figure 1c, the sensor response was defined as14
respectively. Ti works as the adhesion layer to SiO2. Pt acts as the physically hard layer to stand against the needle probes for electrical measurement, since Mohs hardness of Pt (4.0) is greater than that of Au (2.5).40 However, Pt is a catalytic metal and may react with hydrogen. Therefore, Au layer is deposited to prevent a possible reaction of Pt with H2. Moreover, Au electrode is easier to transfer graphene onto the electrode using PDMS technique than Pt electrode is. The metal-deposited substrate was cleaned in acetone, 2-propanol, TMAH (tetramethylammonium hydroxide), and water with ultrasonication. Buffered HF was used to etch SiO2 in the areas without electrodes. The substrate was then cleaned in acetone and 2-propanol before graphene transfer. HOPG (430HP-AB grade ZYA, GE Advance Ceramics) was slowly exfoliated several times using Nitto tape (SPV224-R, Nitto). Then graphene on Nitto tape was placed on PDMS (Gel-Film WF-20 × 4 6-mil, Gel-Pak). To increase the number of graphene flakes on PDMS, Nitto tape was exfoliated quickly from PDMS. The PDMS was used to transfer the graphene on the electrodes. The graphene was transferred by PDMS stamping (also called soft lithography) with a three-axis stage. The transferred graphene was cleaned in acetone and 2-propanol and baked in a uniform temperature heat-treatment system (Thermo Riko GFA430VN) for 1 h at 300 °C in N2 atmosphere; then, 0.3 nmthick Pd (Kojundo Chemical Laboratory Co., Ltd.) was deposited by electron beam deposition and agglomerated in the same heattreatment system for 30 min at 400 °C in N2 atmosphere. As a result, Pd NPs were formed on the graphene. A schematic of the fabricated device is provided in Figure 1a. HOPG, Nitto tape, PDMS, and Pd used in this study are all commercially available. Characterization. Transferred graphene was verified by optical microscopy. Scanning electron microscope was operated at an accelerating voltage of 20 kV. Raman spectrum of the graphene was obtained with 532 nm wavelength. Pd on graphene and Au, and the interface between graphene and Au electrodes were explored by transmission electron microscopy (TEM). Sensing Measurement. The measurement setup is illustrated in Figure 1b. The resistance was obtained from the relationship between the drain voltage (VD) and drain current (ID). The temperature was controlled using the hot chuck of the probe station. The sensing performance was evaluated by exposing the reference and test gases alternately every 3 min. To evaluate the H2 sensing properties, dry or humid air was employed as the reference gas and H2 diluted with dry and humid air was used as the test gas. The flow rates of the 100-ppm of H2 balanced with dry air, dry air, and humid air were controlled by
sensor response =
R − R0 ΔR × 100% = × 100% R0 R0
(1)
where R0 represents the initial resistance when the device was exposed to the test gas and R represents the resistance during the sensor response measurements under the test gas. Figure 1c also shows the definition of the response time, which is the time necessary to reach half of the maximum sensor response. COMSOL Simulation. COMSOL multiphysics was used to evaluate the graphene channel temperature resulting from Joule selfheating. First, we obtained the sheet conductivity from the linear relationship of the ID−VD characteristics. The resulting sheet conductivity of 3.0 mS sq. was used in the COMSOL simulation. The electrical transport was obtained by applying the Poisson and current conservation equations, and the thermal transport was calculated by employing the Fourier law. The material parameters except for graphene we used in the simulations were the default parameters in the COMSOL library. Temperature was set to 27 °C. The thermal conductivity of graphene modified by Pd was assumed to be 200 W/m·K.41 The interfacial thermal resistance was used as a fitting parameter to match the experimental data of an electrically broken device referring to the previous investigations showing that graphene burns at around 600 °C in the atmosphere.42,43 Using these parameters, the graphene channel temperature was evaluated at various VD, and the channel temperature in the self-heated graphene was compared with that controlled by the hot chuck. The time dependence of the temperature during self-heating was also calculated. Monte Carlo Simulation. To demonstrate the difference between the sensor responses in low and high electric fields, an ensemble Monte Carlo simulation was performed. The time step was 0.1 ps, and the transports of 1 × 105 holes were simulated. For the band structure of graphene, linear energy dispersion relation was assumed. Ek = ℏvf |k| 3888
(2) DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
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ACS Applied Nano Materials
Figure 3. (a) ID versus VD characteristics of unfunctionalized and Pd-functionalized graphene. (b) Temperature dependence of the sensor response. The device was heated by an external heater (hot chuck). Initial resistances of R0 are 486, 463, 488, 508, 538) in the temperature ascending order. Sensor response as a function of time at VD (c) lower than and (d) greater than 0.8 V. Initial resistances of R0 are 481, 522, 525, 526, 526, 533, 548, 564) in the VD ascending order. (e) Relationship between sensor response and H2 concentration. The inset shows the time dependence of sensor response for various H2 concentrations, which were used to extract the relationship. where vf = 108 cm/s is the Fermi velocity, and k is the 2D wave vector from the Dirac point. In the simulation, intravalley and intervalley acoustic phonons, intervalley optical phonon, and impurity scattering were taken into account.44 The deformation potential and phonon energy were cited.45 The detailed formula of scattering ratio in the previous work46 is utilized. Because of the suspension of graphene, optical phonon originated from substrate was neglected. In our calculation, initial hole concentration and initial ionized impurity density of 3 × 1012 and 1 × 1012 cm−2 were assumed, respectively. The polarized H atoms adsorbed on the Pd NPs were treated as ionized impurities. Therefore, the adsorption of H atoms caused changes in the hole concentration and ionized impurity density of −Δp and Δp, respectively. The adopted value of Δp was 2 × 1012 cm−2. The simulated sensor response is defined by the ratio between the resistivity change by hydrogen and initial resistivity.
eight-layer graphene has the ability to work as a transducer of hydrogen-induced changes in external electric field. Figure 2d presents an atomic force microscopy (AFM) image of the Au electrode without Pd deposition. Figure 2e shows an AFM image of Au electrode with transferred graphene and Pd deposition. The upper area of the Figure 2e shows the Au electrode and the lower area corresponds to the graphene on the Au electrode. In the Pd-deposited Au film, a number of additional undulations with short correlation lengths were appeared as shown in Figure 2e. By comparing these figures, we consider that undulations with short correlation lengths correspond to Pd NPs. Therefore, the uniform distribution of Pd NPs on graphene is confirmed from the AFM image. We measured the sensor resistance before and after Pd deposition. The resistance was obtained from the relationship between the drain voltage (VD) and drain current (ID), which is shown in Figure 3a. The formation of the Pd NPs increased the channel resistance. The resistance after Pd deposition was 520 Ω. The resistance change should originate from two effects: hole doping and mobility reduction by the Pd NPs. Graphene in an air environment is originally hole doped with environmental oxygen. The graphene was further hole-doped by the Pd NPs because of the difference between the work functions of graphene and Pd, which are 4.5 and 5.6 eV, respectively.50,51 Therefore, hole doping by Pd NPs should result in a decrease in resistance. However, the carrier mobility was greatly reduced by the Pd NPs. As a result, ID, which is determined by the carrier concentration and mobility, was reduced, and the resistance was increased. Figure 3b depicts the sensor response to 100-ppm of H2 as a function of time at various operating temperatures from the room temperature RT to 180 °C. Resistance was increased (decreased) during H2 (dry air) exposure at any temperatures, which results from two phenomenon: a reduction in hole concentration and an increase in Coulomb scattering. H2 adsorbed on Pd dissociate into H atoms. H atoms diffuse through the Pd NPs and reach at the Pd/graphene interface, where ionized H atoms and
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RESULTS AND DISCUSSION Figure 2a shows optical and scanning electron microscopy (SEM) micrographs of the fabricated device. We confirmed the successful suspension of the graphene film on the electrodes. Figure 2b depicts the Raman spectrum (532 nm wavelength) of the graphene, which was obtained by subtracting the Raman spectrum of the Au from that of the graphene on the Au. The G (∼1580 cm−1) and 2D (∼2670 cm−1) intensities of the Raman spectrum indicate that the graphene consisted of multiple layers, and the small D band (∼1350 cm−1) demonstrates the high quality of the suspended graphene.47 We consider that the little D peak stems from weakening of C−C bonds, which is caused by Pd atoms adsorbed above the midpoint of C−C bonds.48 A transmission electron microscopy (TEM) image of the Pd on the graphene is shown in Figure 2c. The Pd was placed on graphene as NPs, and the size of the Pd NPs are about 8 nm. The number of graphene layers was eight, which was determined from the TEM image shown in Figure 2c. One might consider that eight-layer graphene could be too thick to work as a sensor material. However, it is reported that the electrical conductivity of eight-layer graphene can be well modulated by external electric field.49 Therefore, 3889
DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
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ACS Applied Nano Materials
Figure 4. (a) Normalized sensor responses of graphene sensors heated by self-heating (black) at VD of 0.9 V and the hot chuck (red) at 135 °C at VD of 0.01 V. During the self-heating measurements, the hot chuck temperature was kept at 300 K. Sensor responses normalized by the maximum values matched with each other. The close agreement suggests that the temperature increase by self-heating was successfully realized. (b) Dependences of the response time on the hot chuck temperature (blue crosses) and power used by self-heating (red squares). The close agreement indicates that the self-heated graphene temperature can be estimated from the response time. (c) Temperature as a function of self-heating power for the experimental data extracted from Figure 3b and simulation data. The simulation data were obtained using the finite element method simulator COMSOL Multiphysics. (d) Drain current versus drain voltage characteristics of suspended graphene functionalized with Pd NPs. Breakdown voltage was determined to be 1.5 V. (e) Simulated temperature distribution in suspended graphene shown in Figure 4d. The simulation was performed with COMSOL Multiphysics. The graphene temperature was increased to approximately 900 K when the interfacial thermal resistance was set to 2500 μm2·K/mW. (f) TEM images of Pd/graphene on an Au electrode. Numerous gaps were formed between the graphene and Au; the areas of contact between the graphene and Au were small. We expect the small contact areas to result in large thermal contact resistance between the graphene and Au. (g) Simulated position dependence of graphene channel temperature using the self-heating technique (red) and hot chuck (black).
dependent sensor response, which strongly suggests that VDinduced self-heating was successfully achieved. An improved sensor response and response time were confirmed in graphene with a high VD; a response time of 15 s was achieved with a VD of 0.7 V at a H2 concentration of 100 ppm. The sensor was more stable at a VD of 0.6 V than at 0.8 V because sensor response reduction occurred at a VD of around 0.8 V. Therefore, we explored the sensor response to H2 with various concentrations at a VD of 0.6 V, as shown in Figure 3e. The sensor response is linearly dependent on the square root of the H2 concentration. This relationship can be explained by Langmuir adsorption isotherm theory. Assuming that the sensor response is proportional to the coverage of H on the Pd surface, the relationship between the adsorption and desorption rates at equilibrium can be expressed as
image charge in Pd NPs form dipole directed toward graphene. As a result, carrier concentration was decreased by induced H+. In addition, holes are scattered by polarized H atoms on the Pd NPs. Therefore, resistance of the sensor was increased during H2 exposure. Whereas the resistance was recovered during dry air exposure because of hydrogen desorption. The sensor response increases as the operating temperature increases to 135 °C because of the promoted dissociative adsorption of H2. However, the sensor response is decreased at 180 °C, resulting from the impact of hydrogen desorption from Pd being greater than that of dissociation and adsorption at 180 °C. Figures 3c and 3d show the time dependence of the sensor response to 100-ppm of H2, where the device was operated at various VD from 0.01 to 1.1 V at RT. The sensor response increases as VD increases to 0.8 V, then gradually decreases as VD continues to increase from 0.9 to 1.1 V. This tendency of the VD-dependent sensor response is almost the same as that of the temperature-
kaP(1 − θ )2 = kdθ 2 3890
(3) DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
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ACS Applied Nano Materials
the electrical conductivity obtained by fitting the ID−VD characteristics of the device was used.52 In a previous report, the thermal conductivity was measured as a function of the area covered with Au NPs in suspended trilayer graphene.41 This information was utilized to determine the thermal conductivity of the present eight-layer suspended graphene covered with Pd NPs. The interfacial thermal resistance between the Au electrodes and graphene was determined as follows. We prepared another suspended Pd-functionalized graphene. The device was electrically broken by applying high drain voltage. Figure 4d shows ID versus VD characteristics of the broken device. The electrical breakdown was caused by burning out of the graphene in air at approximately 900 K.43 Therefore, the interfacial thermal resistance was determined to be 2500 μm2·K/mW to reproduce channel temperature of 900 K at a drain voltage of 1.5 V as shown in Figure 4e. The obtained interfacial thermal resistance between the graphene and Au was 100 times larger than the theoretical value of 25 μm2·K/mW.53,54 We consider this difference was because the interface between the graphene and Au was not uniform and was comprised of point contacts, as shown in Figure 4f. Thus, there were numerous large gaps between the graphene and Au. In addition, it has been reported that the graphene transferred by PDMS stamping is blistered,55 which may have produced additional gaps between the Au and graphene. Since gaps prohibit heat transfer between materials, a higher thermal resistance could result from these gaps. Moreover, the diameter of each point contact area is much less than the phonon mean free path of graphite. Therefore, phonons from graphene cannot enter Au electrodes easily, which could significantly increase the thermal contact resistance. Figure 4g shows the simulated temperature distribution of the graphene channel. The temperature at the center of the graphene is the highest because finite heat dissipation into the Au electrodes occurred near the electrodes. However, the channel temperature distribution realized by self-heating is almost constant and similar to that obtained using the hot chuck. Figure 5a shows the sensor response as a function of the temperature (using self-heating and the hot chuck). The sensor response with self-heating is smaller than that resulting from using the hot chuck at high temperatures. Although the temperature distribution obtained using self-heating differs from that resulting from using the hot chuck by 15 °C, as shown in Figure 4g, the sensor response obtained at 130 °C with self-heating is smaller than that at 115 °C on the hot chuck. Therefore, the sensor response reduction in self-heated graphene cannot be explained by the difference between the channel temperature distributions. To clarify the reason for the sensor response reduction, Monte Carlo simulations were performed. The self-heating effect, suspended structure, and electric field effects were considered in the simulations. H2 sensing was taken into account by introducing charged impurities. The sensor response was calculated from the conductivity changes. As shown in Figure 5b, the calculation data indicate that increasing the electric field decreased the sensor response. The VD of 1.1 V in self-heated graphene corresponds to a lateral electric field of approximately 400 kV/ μm. Although the experimentally observed response reduction is greater than that calculated, the difference is basically due to the simplified model used for the potentials induced by hydrogens. To illustrate the different effects caused by high and low electric fields, schematic diagrams are provided in Figure 5c. Hydrogens are dissolved and adsorbed on the Pd
Or θ = (KP)1/2 1−θ
(4)
where θ is the fractional surface coverage of adsorbed hydrogen atoms, ka and kd are the adsorption and desorption constants, respectively, K is an equilibrium constant defined as ka/kd, and P represents the partial pressure of H2. When the H2 concentration is low, the resistance change is given by ΔR ∝ θ ≈ (KP)1/2 R
(5)
The reaction between the H2 and Pd NPs on the graphene may be related to the diffusion of the H in the Pd NPs and the H coverage at the interface between the Pd NPs and graphene. However, the linear relationship between the sensor response and the square root of the H2 concentration suggests that the dissociation of H2 and adsorption of H on the Pd surface are the dominant mechanisms determining the sensor response. In the present experiments, we demonstrated that our sensor detected H2 with concentrations ranging from 2 to 100 ppm, which covers the important range of H2 concentrations in expired air. Although H2 concentrations of less than 2 ppm were not tested in our experimental setup, a detection limit of 60 ppb was derived. To calibrate the graphene channel temperature when the self-heating technique was utilized, the sensor responses to 100 ppm of H2 using the self-heating technique and hot chuck were compared. The present Pd-functionalized graphene sensors react with hydrogens and the sensor response shows temperature dependence because of temperature dependent reaction of hydrogens with Pd. Therefore, the comparison of sensor reactions on hot plate with negligibly small self-heating and those with prominent self-heating effects at room temperature enables us to evaluate operating temperature under self-heating conditions. As shown in Figure 4a, the normalized sensor response at a VD of 0.9 V at RT matches the response at a VD of 0.01 V at 135 °C with hot chuck heating. Although the sensor responses shown in Figure 3b (T = 135 °C) and c (VD = 0.9 V) are different in magnitude, their shapes matched with each other by normalizing sensor responses with their maximum values. The reason will be described later. We compared the response time in self-heated graphene with that in graphene heated by the hot chuck. Since the reaction between H2 and Pd is controlled mainly by temperature, the temperature realized by self-heating should be equal to that controlled by the hot chuck when the response times in both cases are the same, as shown in Figure 4a. The response times as functions of the input power and temperature are compared in Figure 4b, which clearly demonstrates the close agreement between the self-heated and heater-heated characteristics. From this result, the relationship between the temperature and input power was obtained and is depicted in Figure 4c, which shows that a graphene temperature of 180 °C was realized with an input power of 2.3 mW. It is worth noting that the temperature increase due to self-heating was negligibly small during the measurements on the hot chuck. The applied drain voltage of 0.01 V results in the self-heating-induced temperature increase of 0.014 K. We also verified the relationship between the input power and temperature in the graphene sensor using numerical simulations. The simulation data agree well with the experimental data as shown in Figure 4c. In the simulation, 3891
DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
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ACS Applied Nano Materials
Figure 5. (a) Temperature dependences of the sensor response using the self-heating technique (red squares) and hot chuck (black squares). The sensor response in the case of self-heating is smaller than that resulting from using the hot chuck. (b) Lateral electric field dependence of the sensor response calculated by Monte Carlo simulation. (c) Schematic diagrams showing the differences between the effects of Coulomb scattering in a low lateral electric field (left) and a high lateral electric field (right).
Figure 6. (a) RT time dependence of the sensor response to a RH change from 50% to 80% at a VD of 0.01 V. Initial resistance of R0 is 456 Ω. The humidity decreases the sensor resistance. (b) Sensor response to 10-ppm of H2 in humid air with constant RH of 0% (black line) and 50% (red line) as a function of time. Initial resistances of R0 are 519 and 520 Ω, respectively. (c) RT time dependence of the sensor response to 10 ppm of H2 with a RH increase from 50% to 80% at a VD of 0.1 V (upper) and 0.9 V (lower). Initial resistances of R0 when applied VD of 0.1 and 0.9 V are 471 and 520 Ω, respectively. Because of the RH increase, the sensor resistance is decreased at the lower VD of 0.1 V, which agrees with the data shown in Figure 5a. Meanwhile, at the higher VD of 0.9 V, thanks to the temperature increase induced by self-heating, the sensor resistance was increased by the H2. (d) Channel temperature change as a function of time following the VD change. Using the self-heating technique, the temperature both increases and decreases in 1 μs.
NPs during H2 sensing. In a low lateral electric field, the holes are scattered greatly by polarized H atoms on the Pd NPs. However, the holes are less scattered in a high lateral electric field because of their high kinetic energy obtained from the electric field. Thus, the response in self-heated graphene is smaller than that in graphene heated with a hot chuck in a high electric field. Finally, we demonstrated voltage-controlled multimolecule detection by self-heating. Figure 6a shows the time dependence of the sensor response to humidity at a VD of 0.01 V. In this measurement, the reference and test gases were humid airs with RHs of 50% and 80%, respectively. The sensor resistance decreased during exposure to the higher humidity of 80%, because water molecules were adsorbed on the oxidized Pd and acted as acceptors (hole donors) for the graphene. As a result, the graphene was hole doped, and the hole concentration increase led to a resistance decrease. Figure 6b shows the sensor responses to 10-ppm of H2 with constant RH of 0% and 50% at a drain voltage of 0.9 V. As shown in Figure 6b, the sensor responses were almost the same even under different RH conditions, thanks to the successful elimination of humidity around the Pd NPs by self-heating. To confirm the humidity robustness, we also explored the sensor response to 10-ppm of H2 under humidity variation. Figure 6c shows the sensor response to 10-ppm of H2 in humid air with a RH of 80% as the test gas. Again, humid air with a RH of 50% was used as the reference gas. As demonstrated in Figure 6c, the sensor function can be switched by changing VD for the same set of test and reference gases. The upper figure shows that at a VD of 0.1 V, around RT, the sensor responded to humidity. On the other hand, the lower figure shows that at a VD of 0.9 V, at an increased temperature of approximately 135 °C, the sensor responded to 10 ppm of H2 even when a large RH change from 50% to 80% occurred simultaneously. Therefore, self-heating successfully prevented the effect of humidity and resulted in the detection of a low concentration of H2. These results lead to the conclusion that the sensor function can be changed or
controlled using applied voltages, thanks to the self-heating effects. Further research is required to ensure high selectivity of H2 if other gas is mixed in humid air. We consider that the utilization of different temperatures realized by self-heating could be one of the efficient methods to distinguish one gaseous molecule to another using specific catalytic reactions at each temperature. Sensing properties of our sensors were compared with previous reports. As shown in Table 1, our sensor responded to low concentration of H2 even under great humidity variations. The multifunctionality realized with self-heating is advantageous in terms of the time necessary to change between functions. If a conventional external micro heater is used, the time necessary to change the sensor temperature should be much longer. Figure 6d shows the simulated temperature change as a function of time, revealing that a temperature increase to 460 K and a decrease to 300 K can each be realized within 1 μs. The short time necessary to change the functionality results from the small thermal capacitances of self-heated materials. 3892
DOI: 10.1021/acsanm.8b00667 ACS Appl. Nano Mater. 2018, 1, 3886−3894
Article
ACS Applied Nano Materials Table 1. Hydrogen Sensor Properties in Humid Ambienta material
H2 concentration
RH
reference gas
response time
recovery time
Pd56 Pt/AlGaN/GaN57 TiO258 CNT/Pd59 silicon nanowire22 graphene/Pd (this study)
400 ppm 500 ppm 300 ppm 300 ppm 5000 ppm 10 ppm
90% (const.) 100% (const.) 32% (const.) 80% (const.) 85% (const.) 50 → 80% (variation)
N2 N2 air air air air
