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Nanotechnology based highly sensitive IoT capable sensing device
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 7 (2019) 894–903
www.materialstoday.com/proceedings
NanoFIS 2017
Nanotechnology based highly sensitive IoT capable sensing device Tanua, A. Daneshkhahc, M. El-Sharkawya, M. Agarwalb, c, M. Rizkallaa, c* a
Department of Electrical and Computer Engineering, IUPUI, Indianapolis, 46202, USA b Department of Mechanical Engineering, IUPUI, Indianapolis, 46202, USA c Integrated Nanosystems Development Institute, IUPUI, Indianapolis, IN 46202, USA
Abstract It is hypothesized that development of gold nanoclusters over monolayer graphene film changes its sensing properties. Monolayer graphene is utilized in a resistive configuration and gold nanoclusters are developed via sputtering deposition. An interface circuit is designed with 45 nm technology Cadence tools. An internet of things (IoT) device is developed to transmit the sensing parameter. Our results show that the monolayer graphene sensor’s resistance decreases by 1.7% in response to 5% CO2 while it increases by 0.84% after development of gold nanoclusters. Sensor’s fabrication, testing results, with simulation of interface circuit and results of IoT are presented and discussed. © 2019 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems. Keywords: graphene; gold nanoclusters; CO2 sensor; sensitivity; sputtering
1. Introduction Gas sensing technology has extensively been investigated due to its wide range of applications and inherent limitations [1–4]. Applications in areas such as industrial, medical, residential and homeland security make it a significant area of development. This draws attentions among industrial and academic researchers. Resistive based gas sensors are widely used to detect different gases and compounds through measuring the variation of electrical resistance of the sensing material. Different sensing materials such as metal oxide, conducting polymer, conducting
* Corresponding author. Tel.: +1-317-274-253; fax: +1-317-274-4567. E-mail address: [email protected]. 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems.
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polymer composites (CPCs), graphene, graphene oxide, carbon nanotubes (CNTs), monolayer protected gold nanoparticles (GNPs) have been used to detect different gases [2,5–11]. However, they may possess some inherent challenges like poor performance at high temperature, structural instability and degradation [5, 12–14]. Graphene and CNTs have been widely explored for gas sensing devices due to their unique electrical and mechanical properties, and high surface to volume ratio [11, 15]. Very high conductivity of graphene [16, 17] can be utilized with high reactivity property of gold nanoparticles to present new properties in gold decorated graphene sheets [18]. In this study, sensitivity of a monolayer graphene sheet to different CO2 concentrations, before and after development of gold nanoclusters has been investigated. 1.1. Importance of Gas Sensors For the comfort and health of human beings the level of harmful gases has to be controlled and maintained all the time. To detect, monitor, and control damage caused by bad air quality, reliable sensing systems are needed. A similar case of poor air quality revealed, the occupants of workplace premises could be suffering from a disease called “Sick building syndrome” [19]. The symptoms included headache, absenteeism and reduced work efficiency. This will have an economic impact on society. Avoiding the syndrome will help to reduce the costs associated with the treatments of the illness and improves the quality of the job conducted at the workplace. Gas sensing devices can be applied for homeland security, where early detecting of the poisonous gas in case of terror attacks or accidents concerning gas explosions could save lives [20]. Measurement of different concentrations of CO2 have recently drawn many attentions [3, 21, 22]. There is a risk of buildup of noxious gases such as CO2 in automobile cabins which may affect the ability of a driver to drive [21]. In the transport sector, emission control regulations are being enforced in different countries to bring the presence of air pollutants such as CO2 down [23]. Links between the volatile organic compounds (VOCs) and different health conditions have been reported in different studies [24, 25]. Breath CO2 level has been linked to different diseases and CO2 gas sensors can be used to provide valuable information about the patient’s health [22, 26, 27]. Gas sensors and CO2 sensors are used in all the mentioned cases to detect different concentrations of the gas. 1.2. Graphene as a Sensing Material In this work, graphene was chosen as the sensing material for development of the gas sensing device. Graphene consists of a single atomic sheet of conjugates sp2 carbon atoms which presents a high electrical conductivity, high surface to volume ratio, high electron transfer rate, unique mechanical properties, and high thermal stability [17, 28, 29]. Recent studies have shown that graphene presents a high sensitivity to VOCs such as CO2 [3, 11]. In this paper sensing properties of monolayer graphene film before and after decoration gold nanocluster in detection of CO2 has been investigated. 2. Material and Methods In this paper, nanotechnology material graphene has been used for development of highly sensitive sensing device. A gas-sensing device comprises of a sensing unit and a processing unit. Development of the sensing unit, design of interface circuit, and development of embedded device are investigated separately in this paper. 2.1. Sensor Unit Single layer CVD graphene on 90 nm SiO2/Silicon (p-doped) substrate was purchased from graphene supermarket (Calverton, NY, USA). Purified air (Air Medipure brand, grade USP) and CO2 (grade USP) were purchased from Praxair (Danbury, CT, USA). A Denton Vacuum Desk V turbo sputter coater (Moorestown, NJ USA) was used to develop the electrodes and also the gold nanoclusters over the graphene layer. A JSM-7800F field emission scanning electron microscope (JEOL USA, Peabody, MA, USA) was used to investigate the morphology of gold nanocluster decorated graphene film.
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Various sensing mechanisms and configurations can be used to measure change in different properties of a sensing material. Monolayer configuration of graphene was utilized to develop resistive based sensors. The sensor before and after development of gold nanoclusters were tested with different concentrations of CO2 in dry air. 2.1.1. Sensor Structure The structure of wafer comprises of a sensitive layer of monolayer graphene on 90 nm SiO2/Silicon (p-doped) substrate. Two gold electrodes have been developed by sputtering gold over the wafer while the gap between the electrodes has been adjusted to be 5 mm via a mask. The monolayer graphene based sensor structure is illustrated in the Fig. 1.
Fig. 1. Sensor Structure
2.1.2. Development of Gold Nanoclusters over Monolayer Graphene Sputtering is a thin film deposition process where target material is bombarded on a substrate by ionized gas such as argon in a vacuum chamber. A Denton Vacuum Desk V sputter coater was used to develop gold nanoclusters over the graphene layer. This gold sputtering was conducted for 15 seconds with process set point of 0.0003 Torr and sputter current set point of 20 milliamperes. 2.1.3. Testing Experimental Set-up The sensor is exposed to purified air (Air Medipure brand, grade USP, Praxair, Danbury, CT, USA) for period of one hour before the experiment starts to make sure the chamber is clean and no contamination is exposed to the sensor. CO2 is mixed with the air in a homogenizing flask and the concentration of the gas is adjusted with two mass flow controllers (MFCs). The total flow is set fixed at a flow rate of 400 SCCM at all the time during the experiment. The testing set up is presented in the Fig. 2.
Fig. 2. Gas Testing Setup
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2.2. Sensor Processing Unit To move toward development of a low power gas sensing device with high sensitivity an interface circuit is design. The processing unit comprises of an interface circuitry, an operational amplifier and a dynamic offset cancellation circuitry. The interface circuitry functions as energy conversion from the adsorption energy of gas into electrical energy in the form of voltage signal, reflecting the information described in ∆V. The second stage of the processing unit is the amplifier stage, which provides the noise suppression cross talk between the various signals of the assemblies, and the amplification factors of the signals. 2.2.1. Interface Circuitry Interface circuitry is a simple Wheatstone bridge. The analog sensing signal in form of resistance change is first caught by Wheatstone bridge which is converted into change in voltage. The small voltage change relayed ahead to the amplifiers. 2.2.2. Amplifier Operational amplifier in differential mode was used for amplifying the difference in voltage coming from the Wheat stone bridge. Schematic and layout of operational amplifier was designed on Cadence Virtuoso schematic editor and Virtuoso Layout suite respectively. Cadence Virtuoso Schematic Editor is a fast-easy design entry software. The component libraries at transistor level are available with wire-routing capabilities. The cadence generic process design kits (GPDK) provide device and semiconductor process level information for use with Cadence Virtuoso L, XL, and GXL products. PDK documentation covers layout design rules along with information about process technology to do device level design. A 45nm technology with a ’gpdk45’ cell library was used. 2.2.3. Dynamic Offset Cancellation Circuitry In CMOS technology, errors such as offset, drift and 1/f noise become dominant especially at low frequencies. In our gas sensing device, there was a need to bring offset noise to minimum since it can produce very low change in voltages which needs to be considered. In order to achieve a sensitive interface circuit, an offset cancellation circuit was incorporated in the operational amplifier as shown in Fig. 3. The schematic for the entire circuitry was designed using Cadence virtuoso schematic editor.
Fig. 3. Circuit demonstrating dynamic offset cancellation
Offset cancellation circuit uses non-overlapping clock generator to control the NMOS switches. It works in two phases: (1) when ph1 goes high, switches S1 and S2 are closed, the op-amp is in the offset storage mode, and the offset voltages of op amp stored across capacitor Cos. (2) when ph1 goes low and ph2 goes high, switch S3 is closed, and the circuit is in the offset cancellation mode. Pre-charged voltage stored in the capacitor Cos in one phase cancels the input offset voltage in the other phase. The two dummy transistors on either side of switch transistor are
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releasing and removing the charge present in the switch transistor when it is turned off. Those two are also controlled by clock signals in opposite phases, where ph1 and ph1b being clock signals of opposite phase. 2.3. IoT Implementation The gas-sensing device was made IoT capable using Freedom K64f development board. It is an ultra-low-cost development platform with various features including a 6-axis digital accelerometer and magnetometer, a tri-colored light emitted diode (LED), 2 user push-buttons for direct interaction, a microSD card slot, connectivity using on board Ethernet port and headers for use with Bluetooth and 2.4 GHz radio add-on modules. An IoT capable gas sensor was able to send gas sensing information to the user over internet. An overview of the system is shown in Fig. 4. Micro controller board receives analog voltage signal from the gas sensor at the analog input pin present in the board. If the input voltage is above the defined sensing threshold, the signal is sent to IBM IoT platform through Message Queue Telemetry Transport (MQTT). IBM IoT has two parts IBM Watson and IBM Bluemix. IBM Watson acts as a server which receives data in real time and its IoT services pass on the data packets to users via IBM Bluemix application. The application is designed using Node-Red to send the notification as well as data to client servers such as Gmail, Twitter, and Yahoo via Hypertext Transfer Protocol (HTTP).
Fig. 4. IoT system overview
2.3.1. Sending Data to the Cloud ARM mbed online compiler was used to write the code and compile the algorithm using C++ programming language. once successful Ethernet is established, IP address, MAC address and Device ID are utilized to register the device and organization ID, authentication token and Device type are used to validate the credentials and ensure secure connection to correct destination. The system is then ready to publish data to the cloud once gas sensing conditions are met. 2.3.2. Receiving Data from the Cloud The IBM Bluemix application designed using Node Red flow editor is shown in Fig. 5. The data in the cloud is fetched by the block ‘IBM IoT App-in’ and is relayed to the function block. Function block is coded with the defined conditions and once they are met, the data can be sent according to the client server and further in form of emails, messages and twitter. A ’msg.payload’ block for used for displaying data on the console window.
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Fig. 5. Node-Red Blocks for receiving the data from IBM Watson
3. Results and Discussions Field emission scanning electron microscopy (FESEM) is used to characterize and investigate development of gold nanoclusters over the film in Section 3.1. The response of monolayer graphene film before and after development of gold nanoclusters to different concentrations of CO2 are presented and discussed in the Section 3.2. Results of simulation for the interface circuit is presented in Section 3.3. Results related to the IoT is discussed in Section 3.4. 3.1. Sensor Characterization Formation of gold nanocluster over the monolayer graphene is investigated via FESEM analysis. The image of gold nanoclusters decorated graphene is illustrated in the Fig. 6. The morphology and uniform structure the film is demonstrated. The presented image illustrates the formation gold nanoclusters over the monolayer graphene on top of the substrate. The image was taken at 70,000x zoom and the scale bar is 100 nm.
Fig. 6. SEM imaging of gold nanocluster decorated monolayer graphene film
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3.2. Sensor Results The response of monolayer graphene film before and after development of gold nanoclusters to CO2 is investigated. The monolayer graphene sensor response to CO2 is presented in the Section 3.2.1 while the sensor response after development of gold nanoclusters is presented in Section 3.2.2. 3.2.1. Monolayer Graphene Sensor The monolayer graphene sensor was exposed to CO2 at different concentrations of 5%, 10%, 20% and 40% in dry air. Due to the interaction of CO2 and monolayer graphene the sensor resistance decreases [3]. The presented result in Fig. 7 shows that the sensor’s resistance has decreased by 1.7%, 2.5%, 3% and 3.5% in response to 5%, 10%, 20% and 40% of CO2 respectively. Interaction of dry air and the graphene results in desorption of CO2 from the monolayer graphene film. This causes the sensor resistance increases toward the initial values. The sensor’s response time at higher concentration of CO2 was measured to be as small as 140 seconds. The sensing mechanism of the sensor could be described based on the physical adsorption of CO2 to graphene monolayer [3, 30]. The charge transfer due to the interaction of CO2 and graphene is known as the main reason for increase of the graphene conductivity and decrease of sensor’s resistance [3].
Fig. 7. Resistance change response of Graphene sheet to different concentrations of CO2
3.2.2. Gold Nanocluster Decorated Graphene Sensor The monolayer graphene sensor with decorated gold nanoclusters was tested with different concentrations of CO2 in dry air. Fig. 8 shows that resistance of gold nanocluster decorated graphene sensor increases by 0.55%, 0.84%, 98%, 1.02% and 1.07% in a cycle in response to 2.5%, 5%, 10%, 20% and 40% concentration of CO2 gas respectively. This results (shown in Fig. 8) demonstrates that unlike the monolayer graphene film (shown in Fig. 7), the resistance of gold nanocluster decorated graphene increases in response to CO2. The sensing mechanism for this unusual behavior of monolayer graphene in the presence gold nanoclusters is unknown and further investigation needs to be conducted in the future works. One explanation could be that in monolayer graphene, the physical adsorption of CO2 to graphene monolayer has led to a charge transfer which increases the graphene conductivity and reduces the film resistance [3, 30]. However the iono-covalently attachment of CO2 to the gold nanoclusters reduces the number of carries in the graphene which increases the sensor’s resistance [31].
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Fig. 8. Percentage of resistance change of gold nanocluster decorated graphene sheet to different concentrations of CO2
3.3. Sensor Processing Unit The offset cancellation circuit incorporated with the operational amplifier (shown in Fig. 3) was simulated via Cadence Spectre circuit simulator. The simulation results shows that using the presented dynamic offset cancellation circuit, the offset voltage of 17 µV was achieved. 3.4. IoT Capable Sensing Using FRDM K64f As soon as the sensing conditions were met, the voltage data is published on the IBM IoT cloud. The information is then sent to the user via IBM Bluemix application designed. FRDM K64f has been coded for matching the values with the gas sensing signature array. According to the match, the gas name was sent to the receiver and received via email/message. 4. Conclusion and Future Work Graphene monolayer in resistive configuration have been used to detect different concentration of CO2. Development of gold nanoclusters over the graphene monolayer has changed the sensitivity and selectivity of the sensors. The resistance of monolayer graphene film decreases in response to the CO2 while the film resistance increases after development of gold nanoclusters over the film. This property of the graphene could be used in development of cross-selective sensors to detect type and concentration of different VOCs. Determining a mathematical model to explain this property is the immediate consideration for future work. The processing unit consisting of the bridge circuit, instrumentation/operational amplifiers with offset cancellation techniques have suggested a low input offset voltage that can be processed with operational amplifier and signal is accommodated for IoT capability. The embedded system device used in this study features Ethernet connectivity. The developed device may be fabricated in the array to identify the type and concentration of a gas from a gas mixture. Testing data for more gases can be collected and sensor array can be populated further. For device array system, cross talk may be a serious noise issue within a microchip. The study here elaborated on the use of guard rings to minimize the cross talk between the instrumentation amplifiers (processing units). The design of these guard rings and their fabrication is reserved for future considerations. Fig. 9 shows a processing unit with four nanoparticle sensing device array.
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Fig. 9. (a) Four processing units separated by guard rings; (b) Multi-NP sensing array
Acknowledgement The authors would like to acknowledge the Integrated Nanosystems Development Institute (INDI) for providing the facilities for this research, including the JEOL7800F Field Emission Scanning Electron Microscope (FESEM) awarded by NSF grant MRI-1229514. The authors also offer their appreciation to Dr. Dan Minner for his assistance with the FESEM instrument. References [1] Agarwal, M.; Daneshkhah, A.; Jafarian, H.; Shrestha, S.; Varahramyan, K.; Faiola, A. Low power wireless sensor system with ring oscillator ad sensors for use in monitoring of physiological data 2015. [2] Broza, Y. Y.; Haick, H. Nanomaterial-based sensors for detection of disease by volatile organic compounds. Nanomed. 2013, 8, 785–806, doi:10.2217/nnm.13.64. [3] Yoon, H. J.; Jun, D. H.; Yang, J. H.; Zhou, Z.; Yang, S. S.; Cheng, M. M.-C. Carbon dioxide gas sensor using a graphene sheet. Sens. Actuators B Chem. 2011, 157, 310–313, doi:10.1016/j.snb.2011.03.035. [4] Fam, D. W. H.; Palaniappan, A.; Tok, A. I. Y.; Liedberg, B.; Moochhala, S. M. A review on technological aspects influencing commercialization of carbon nanotube sensors. Sens. Actuators B Chem. 2011, 157, 1–7, doi:10.1016/j.snb.2011.03.040. [5] Daneshkhah, A.; Shrestha, S.; Siegel, A.; Varahramyan, K.; Agarwal, M. Cross-selectivity enhancement of poly(vinylidene fluoridehexafluoropropylene)-based sensor arrays for detecting acetone and ethanol. Sensors 2017, 17, 595, doi:10.3390/s17030595. [6] Daneshkhah, A.; Shrestha, S.; Agarwal, M.; Varahramyan, K. PPy/PMMA/PEG-based sensor for low-concentration acetone detection. In Proceedings of the SPIE 9107, Smart Biomedical and Physiological Sensor Technology XI, Baltimore, WD, USA, 7–9 May 2014; Volume 9107. ID 910712. [7] Daneshkhah, A.; Shrestha, S.; Agarwal, M.; Varahramyan, K. Poly(vinylidene fluoride-hexafluoropropylene) composite sensors for volatile organic compounds detection in breath. Sens. Actuators B Chem. 2015, 221, 635–643, doi:10.1016/j.snb.2015.06.145. [8] Agarwal, M.; Balachandran, M. D.; Shrestha, S.; Varahramyan, K. SnO2 nanoparticle-based passive capacitive sensor for ethylene detection. J Nanomater. 2012, 2012, 5:5–5:5, doi:10.1155/2012/145406. [9] Fu, K.; Chen, S.; Zhao, J.; Willis, B. G. Dielectrophoretic assembly of gold nanoparticles in nanoscale junctions for rapid, miniature chemiresistor vapor sensors. ACS Sens. 2016, 1, 444–450, doi:10.1021/acssensors.6b00041. [10] J.; Sterner, E. S.; Swager, T. M. integrated gas sensing system of swcnt and cellulose polymer concentrator for benzene, toluene, and xylenes. Sensors 2016, 16, 183, doi:10.3390/s16020183. [11] Meng, F.-L.; Guo, Z.; Huang, X.-J. Graphene-based hybrids for chemiresistive gas sensors. TrAC Trends Anal. Chem. 2015, 68, 37–47, doi:10.1016/j.trac.2015.02.008. [12] Chung, K. H.; Wu, C. S.; Malawer, E. G. Glass transition temperatures of poly(methyl vinyl ether-co-maleic anhydride) (PMVEMA) and poly(methyl vinyl ether-co-maleic acid) (PMVEMAC) and the kinetics of dehydration of PMVEMAC by thermal analysis. J. Appl. Polym. Sci. 1990, 41, 793–803, doi:10.1002/app.1990.070410326. [13] Ramgir, N.; Datta, N.; Kaur, M.; Kailasaganapathi, S.; Debnath, A. K.; Aswal, D. K.; Gupta, S. K. Metal oxide nanowires for chemiresistive gas sensors: Issues, challenges and prospects. Colloids Surf. Physicochem. Eng. Asp. 2013, 439, 101–116, doi:10.1016/j.colsurfa.2013.02.029. [14] Kumar, A.; Singh, R. K.; Agarwal, K.; Singh, H. K.; Srivastava, P.; Singh, R. Effect of p-toluenesulfonate on inhibition of overoxidation of polypyrrole. J. Appl. Polym. Sci. 2013, 130, 434–442, doi:10.1002/app.39182. [15] Abdelhalim, A.; Winkler, M.; Loghin, F.; Zeiser, C.; Lugli, P.; Abdellah, A. Highly sensitive and selective carbon nanotube-based gas sensor arrays functionalized with different metallic nanoparticles. Sens. Actuators B Chem. 2015, 220, 1288–1296, doi:10.1016/j.snb.2015.06.138. [16] Esmailpour, A.; Meshkin, H.; Saadat, M. Conductance of disordered strain-induced graphene superlattices. Phys. E Low-Dimens. Syst. Nanostructures 2013, 50, 57–60, doi:10.1016/j.physe.2013.02.014.
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ScienceDirect Materials Today: Proceedings 7 (2019) 894–903
www.materialstoday.com/proceedings
NanoFIS 2017
Nanotechnology based highly sensitive IoT capable sensing device Tanua, A. Daneshkhahc, M. El-Sharkawya, M. Agarwalb, c, M. Rizkallaa, c* a
Department of Electrical and Computer Engineering, IUPUI, Indianapolis, 46202, USA b Department of Mechanical Engineering, IUPUI, Indianapolis, 46202, USA c Integrated Nanosystems Development Institute, IUPUI, Indianapolis, IN 46202, USA
Abstract It is hypothesized that development of gold nanoclusters over monolayer graphene film changes its sensing properties. Monolayer graphene is utilized in a resistive configuration and gold nanoclusters are developed via sputtering deposition. An interface circuit is designed with 45 nm technology Cadence tools. An internet of things (IoT) device is developed to transmit the sensing parameter. Our results show that the monolayer graphene sensor’s resistance decreases by 1.7% in response to 5% CO2 while it increases by 0.84% after development of gold nanoclusters. Sensor’s fabrication, testing results, with simulation of interface circuit and results of IoT are presented and discussed. © 2019 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems. Keywords: graphene; gold nanoclusters; CO2 sensor; sensitivity; sputtering
1. Introduction Gas sensing technology has extensively been investigated due to its wide range of applications and inherent limitations [1–4]. Applications in areas such as industrial, medical, residential and homeland security make it a significant area of development. This draws attentions among industrial and academic researchers. Resistive based gas sensors are widely used to detect different gases and compounds through measuring the variation of electrical resistance of the sensing material. Different sensing materials such as metal oxide, conducting polymer, conducting
* Corresponding author. Tel.: +1-317-274-253; fax: +1-317-274-4567. E-mail address: [email protected]. 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or peer-review under responsibility of NanoFIS 2017 - Functional Integrated nano Systems.
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polymer composites (CPCs), graphene, graphene oxide, carbon nanotubes (CNTs), monolayer protected gold nanoparticles (GNPs) have been used to detect different gases [2,5–11]. However, they may possess some inherent challenges like poor performance at high temperature, structural instability and degradation [5, 12–14]. Graphene and CNTs have been widely explored for gas sensing devices due to their unique electrical and mechanical properties, and high surface to volume ratio [11, 15]. Very high conductivity of graphene [16, 17] can be utilized with high reactivity property of gold nanoparticles to present new properties in gold decorated graphene sheets [18]. In this study, sensitivity of a monolayer graphene sheet to different CO2 concentrations, before and after development of gold nanoclusters has been investigated. 1.1. Importance of Gas Sensors For the comfort and health of human beings the level of harmful gases has to be controlled and maintained all the time. To detect, monitor, and control damage caused by bad air quality, reliable sensing systems are needed. A similar case of poor air quality revealed, the occupants of workplace premises could be suffering from a disease called “Sick building syndrome” [19]. The symptoms included headache, absenteeism and reduced work efficiency. This will have an economic impact on society. Avoiding the syndrome will help to reduce the costs associated with the treatments of the illness and improves the quality of the job conducted at the workplace. Gas sensing devices can be applied for homeland security, where early detecting of the poisonous gas in case of terror attacks or accidents concerning gas explosions could save lives [20]. Measurement of different concentrations of CO2 have recently drawn many attentions [3, 21, 22]. There is a risk of buildup of noxious gases such as CO2 in automobile cabins which may affect the ability of a driver to drive [21]. In the transport sector, emission control regulations are being enforced in different countries to bring the presence of air pollutants such as CO2 down [23]. Links between the volatile organic compounds (VOCs) and different health conditions have been reported in different studies [24, 25]. Breath CO2 level has been linked to different diseases and CO2 gas sensors can be used to provide valuable information about the patient’s health [22, 26, 27]. Gas sensors and CO2 sensors are used in all the mentioned cases to detect different concentrations of the gas. 1.2. Graphene as a Sensing Material In this work, graphene was chosen as the sensing material for development of the gas sensing device. Graphene consists of a single atomic sheet of conjugates sp2 carbon atoms which presents a high electrical conductivity, high surface to volume ratio, high electron transfer rate, unique mechanical properties, and high thermal stability [17, 28, 29]. Recent studies have shown that graphene presents a high sensitivity to VOCs such as CO2 [3, 11]. In this paper sensing properties of monolayer graphene film before and after decoration gold nanocluster in detection of CO2 has been investigated. 2. Material and Methods In this paper, nanotechnology material graphene has been used for development of highly sensitive sensing device. A gas-sensing device comprises of a sensing unit and a processing unit. Development of the sensing unit, design of interface circuit, and development of embedded device are investigated separately in this paper. 2.1. Sensor Unit Single layer CVD graphene on 90 nm SiO2/Silicon (p-doped) substrate was purchased from graphene supermarket (Calverton, NY, USA). Purified air (Air Medipure brand, grade USP) and CO2 (grade USP) were purchased from Praxair (Danbury, CT, USA). A Denton Vacuum Desk V turbo sputter coater (Moorestown, NJ USA) was used to develop the electrodes and also the gold nanoclusters over the graphene layer. A JSM-7800F field emission scanning electron microscope (JEOL USA, Peabody, MA, USA) was used to investigate the morphology of gold nanocluster decorated graphene film.
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Various sensing mechanisms and configurations can be used to measure change in different properties of a sensing material. Monolayer configuration of graphene was utilized to develop resistive based sensors. The sensor before and after development of gold nanoclusters were tested with different concentrations of CO2 in dry air. 2.1.1. Sensor Structure The structure of wafer comprises of a sensitive layer of monolayer graphene on 90 nm SiO2/Silicon (p-doped) substrate. Two gold electrodes have been developed by sputtering gold over the wafer while the gap between the electrodes has been adjusted to be 5 mm via a mask. The monolayer graphene based sensor structure is illustrated in the Fig. 1.
Fig. 1. Sensor Structure
2.1.2. Development of Gold Nanoclusters over Monolayer Graphene Sputtering is a thin film deposition process where target material is bombarded on a substrate by ionized gas such as argon in a vacuum chamber. A Denton Vacuum Desk V sputter coater was used to develop gold nanoclusters over the graphene layer. This gold sputtering was conducted for 15 seconds with process set point of 0.0003 Torr and sputter current set point of 20 milliamperes. 2.1.3. Testing Experimental Set-up The sensor is exposed to purified air (Air Medipure brand, grade USP, Praxair, Danbury, CT, USA) for period of one hour before the experiment starts to make sure the chamber is clean and no contamination is exposed to the sensor. CO2 is mixed with the air in a homogenizing flask and the concentration of the gas is adjusted with two mass flow controllers (MFCs). The total flow is set fixed at a flow rate of 400 SCCM at all the time during the experiment. The testing set up is presented in the Fig. 2.
Fig. 2. Gas Testing Setup
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2.2. Sensor Processing Unit To move toward development of a low power gas sensing device with high sensitivity an interface circuit is design. The processing unit comprises of an interface circuitry, an operational amplifier and a dynamic offset cancellation circuitry. The interface circuitry functions as energy conversion from the adsorption energy of gas into electrical energy in the form of voltage signal, reflecting the information described in ∆V. The second stage of the processing unit is the amplifier stage, which provides the noise suppression cross talk between the various signals of the assemblies, and the amplification factors of the signals. 2.2.1. Interface Circuitry Interface circuitry is a simple Wheatstone bridge. The analog sensing signal in form of resistance change is first caught by Wheatstone bridge which is converted into change in voltage. The small voltage change relayed ahead to the amplifiers. 2.2.2. Amplifier Operational amplifier in differential mode was used for amplifying the difference in voltage coming from the Wheat stone bridge. Schematic and layout of operational amplifier was designed on Cadence Virtuoso schematic editor and Virtuoso Layout suite respectively. Cadence Virtuoso Schematic Editor is a fast-easy design entry software. The component libraries at transistor level are available with wire-routing capabilities. The cadence generic process design kits (GPDK) provide device and semiconductor process level information for use with Cadence Virtuoso L, XL, and GXL products. PDK documentation covers layout design rules along with information about process technology to do device level design. A 45nm technology with a ’gpdk45’ cell library was used. 2.2.3. Dynamic Offset Cancellation Circuitry In CMOS technology, errors such as offset, drift and 1/f noise become dominant especially at low frequencies. In our gas sensing device, there was a need to bring offset noise to minimum since it can produce very low change in voltages which needs to be considered. In order to achieve a sensitive interface circuit, an offset cancellation circuit was incorporated in the operational amplifier as shown in Fig. 3. The schematic for the entire circuitry was designed using Cadence virtuoso schematic editor.
Fig. 3. Circuit demonstrating dynamic offset cancellation
Offset cancellation circuit uses non-overlapping clock generator to control the NMOS switches. It works in two phases: (1) when ph1 goes high, switches S1 and S2 are closed, the op-amp is in the offset storage mode, and the offset voltages of op amp stored across capacitor Cos. (2) when ph1 goes low and ph2 goes high, switch S3 is closed, and the circuit is in the offset cancellation mode. Pre-charged voltage stored in the capacitor Cos in one phase cancels the input offset voltage in the other phase. The two dummy transistors on either side of switch transistor are
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releasing and removing the charge present in the switch transistor when it is turned off. Those two are also controlled by clock signals in opposite phases, where ph1 and ph1b being clock signals of opposite phase. 2.3. IoT Implementation The gas-sensing device was made IoT capable using Freedom K64f development board. It is an ultra-low-cost development platform with various features including a 6-axis digital accelerometer and magnetometer, a tri-colored light emitted diode (LED), 2 user push-buttons for direct interaction, a microSD card slot, connectivity using on board Ethernet port and headers for use with Bluetooth and 2.4 GHz radio add-on modules. An IoT capable gas sensor was able to send gas sensing information to the user over internet. An overview of the system is shown in Fig. 4. Micro controller board receives analog voltage signal from the gas sensor at the analog input pin present in the board. If the input voltage is above the defined sensing threshold, the signal is sent to IBM IoT platform through Message Queue Telemetry Transport (MQTT). IBM IoT has two parts IBM Watson and IBM Bluemix. IBM Watson acts as a server which receives data in real time and its IoT services pass on the data packets to users via IBM Bluemix application. The application is designed using Node-Red to send the notification as well as data to client servers such as Gmail, Twitter, and Yahoo via Hypertext Transfer Protocol (HTTP).
Fig. 4. IoT system overview
2.3.1. Sending Data to the Cloud ARM mbed online compiler was used to write the code and compile the algorithm using C++ programming language. once successful Ethernet is established, IP address, MAC address and Device ID are utilized to register the device and organization ID, authentication token and Device type are used to validate the credentials and ensure secure connection to correct destination. The system is then ready to publish data to the cloud once gas sensing conditions are met. 2.3.2. Receiving Data from the Cloud The IBM Bluemix application designed using Node Red flow editor is shown in Fig. 5. The data in the cloud is fetched by the block ‘IBM IoT App-in’ and is relayed to the function block. Function block is coded with the defined conditions and once they are met, the data can be sent according to the client server and further in form of emails, messages and twitter. A ’msg.payload’ block for used for displaying data on the console window.
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Fig. 5. Node-Red Blocks for receiving the data from IBM Watson
3. Results and Discussions Field emission scanning electron microscopy (FESEM) is used to characterize and investigate development of gold nanoclusters over the film in Section 3.1. The response of monolayer graphene film before and after development of gold nanoclusters to different concentrations of CO2 are presented and discussed in the Section 3.2. Results of simulation for the interface circuit is presented in Section 3.3. Results related to the IoT is discussed in Section 3.4. 3.1. Sensor Characterization Formation of gold nanocluster over the monolayer graphene is investigated via FESEM analysis. The image of gold nanoclusters decorated graphene is illustrated in the Fig. 6. The morphology and uniform structure the film is demonstrated. The presented image illustrates the formation gold nanoclusters over the monolayer graphene on top of the substrate. The image was taken at 70,000x zoom and the scale bar is 100 nm.
Fig. 6. SEM imaging of gold nanocluster decorated monolayer graphene film
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3.2. Sensor Results The response of monolayer graphene film before and after development of gold nanoclusters to CO2 is investigated. The monolayer graphene sensor response to CO2 is presented in the Section 3.2.1 while the sensor response after development of gold nanoclusters is presented in Section 3.2.2. 3.2.1. Monolayer Graphene Sensor The monolayer graphene sensor was exposed to CO2 at different concentrations of 5%, 10%, 20% and 40% in dry air. Due to the interaction of CO2 and monolayer graphene the sensor resistance decreases [3]. The presented result in Fig. 7 shows that the sensor’s resistance has decreased by 1.7%, 2.5%, 3% and 3.5% in response to 5%, 10%, 20% and 40% of CO2 respectively. Interaction of dry air and the graphene results in desorption of CO2 from the monolayer graphene film. This causes the sensor resistance increases toward the initial values. The sensor’s response time at higher concentration of CO2 was measured to be as small as 140 seconds. The sensing mechanism of the sensor could be described based on the physical adsorption of CO2 to graphene monolayer [3, 30]. The charge transfer due to the interaction of CO2 and graphene is known as the main reason for increase of the graphene conductivity and decrease of sensor’s resistance [3].
Fig. 7. Resistance change response of Graphene sheet to different concentrations of CO2
3.2.2. Gold Nanocluster Decorated Graphene Sensor The monolayer graphene sensor with decorated gold nanoclusters was tested with different concentrations of CO2 in dry air. Fig. 8 shows that resistance of gold nanocluster decorated graphene sensor increases by 0.55%, 0.84%, 98%, 1.02% and 1.07% in a cycle in response to 2.5%, 5%, 10%, 20% and 40% concentration of CO2 gas respectively. This results (shown in Fig. 8) demonstrates that unlike the monolayer graphene film (shown in Fig. 7), the resistance of gold nanocluster decorated graphene increases in response to CO2. The sensing mechanism for this unusual behavior of monolayer graphene in the presence gold nanoclusters is unknown and further investigation needs to be conducted in the future works. One explanation could be that in monolayer graphene, the physical adsorption of CO2 to graphene monolayer has led to a charge transfer which increases the graphene conductivity and reduces the film resistance [3, 30]. However the iono-covalently attachment of CO2 to the gold nanoclusters reduces the number of carries in the graphene which increases the sensor’s resistance [31].
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Fig. 8. Percentage of resistance change of gold nanocluster decorated graphene sheet to different concentrations of CO2
3.3. Sensor Processing Unit The offset cancellation circuit incorporated with the operational amplifier (shown in Fig. 3) was simulated via Cadence Spectre circuit simulator. The simulation results shows that using the presented dynamic offset cancellation circuit, the offset voltage of 17 µV was achieved. 3.4. IoT Capable Sensing Using FRDM K64f As soon as the sensing conditions were met, the voltage data is published on the IBM IoT cloud. The information is then sent to the user via IBM Bluemix application designed. FRDM K64f has been coded for matching the values with the gas sensing signature array. According to the match, the gas name was sent to the receiver and received via email/message. 4. Conclusion and Future Work Graphene monolayer in resistive configuration have been used to detect different concentration of CO2. Development of gold nanoclusters over the graphene monolayer has changed the sensitivity and selectivity of the sensors. The resistance of monolayer graphene film decreases in response to the CO2 while the film resistance increases after development of gold nanoclusters over the film. This property of the graphene could be used in development of cross-selective sensors to detect type and concentration of different VOCs. Determining a mathematical model to explain this property is the immediate consideration for future work. The processing unit consisting of the bridge circuit, instrumentation/operational amplifiers with offset cancellation techniques have suggested a low input offset voltage that can be processed with operational amplifier and signal is accommodated for IoT capability. The embedded system device used in this study features Ethernet connectivity. The developed device may be fabricated in the array to identify the type and concentration of a gas from a gas mixture. Testing data for more gases can be collected and sensor array can be populated further. For device array system, cross talk may be a serious noise issue within a microchip. The study here elaborated on the use of guard rings to minimize the cross talk between the instrumentation amplifiers (processing units). The design of these guard rings and their fabrication is reserved for future considerations. Fig. 9 shows a processing unit with four nanoparticle sensing device array.
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Fig. 9. (a) Four processing units separated by guard rings; (b) Multi-NP sensing array
Acknowledgement The authors would like to acknowledge the Integrated Nanosystems Development Institute (INDI) for providing the facilities for this research, including the JEOL7800F Field Emission Scanning Electron Microscope (FESEM) awarded by NSF grant MRI-1229514. The authors also offer their appreciation to Dr. Dan Minner for his assistance with the FESEM instrument. References [1] Agarwal, M.; Daneshkhah, A.; Jafarian, H.; Shrestha, S.; Varahramyan, K.; Faiola, A. Low power wireless sensor system with ring oscillator ad sensors for use in monitoring of physiological data 2015. [2] Broza, Y. Y.; Haick, H. Nanomaterial-based sensors for detection of disease by volatile organic compounds. Nanomed. 2013, 8, 785–806, doi:10.2217/nnm.13.64. [3] Yoon, H. J.; Jun, D. H.; Yang, J. H.; Zhou, Z.; Yang, S. S.; Cheng, M. M.-C. Carbon dioxide gas sensor using a graphene sheet. Sens. Actuators B Chem. 2011, 157, 310–313, doi:10.1016/j.snb.2011.03.035. [4] Fam, D. W. H.; Palaniappan, A.; Tok, A. I. Y.; Liedberg, B.; Moochhala, S. M. A review on technological aspects influencing commercialization of carbon nanotube sensors. Sens. Actuators B Chem. 2011, 157, 1–7, doi:10.1016/j.snb.2011.03.040. [5] Daneshkhah, A.; Shrestha, S.; Siegel, A.; Varahramyan, K.; Agarwal, M. Cross-selectivity enhancement of poly(vinylidene fluoridehexafluoropropylene)-based sensor arrays for detecting acetone and ethanol. Sensors 2017, 17, 595, doi:10.3390/s17030595. [6] Daneshkhah, A.; Shrestha, S.; Agarwal, M.; Varahramyan, K. PPy/PMMA/PEG-based sensor for low-concentration acetone detection. In Proceedings of the SPIE 9107, Smart Biomedical and Physiological Sensor Technology XI, Baltimore, WD, USA, 7–9 May 2014; Volume 9107. ID 910712. [7] Daneshkhah, A.; Shrestha, S.; Agarwal, M.; Varahramyan, K. Poly(vinylidene fluoride-hexafluoropropylene) composite sensors for volatile organic compounds detection in breath. Sens. Actuators B Chem. 2015, 221, 635–643, doi:10.1016/j.snb.2015.06.145. [8] Agarwal, M.; Balachandran, M. D.; Shrestha, S.; Varahramyan, K. SnO2 nanoparticle-based passive capacitive sensor for ethylene detection. J Nanomater. 2012, 2012, 5:5–5:5, doi:10.1155/2012/145406. [9] Fu, K.; Chen, S.; Zhao, J.; Willis, B. G. Dielectrophoretic assembly of gold nanoparticles in nanoscale junctions for rapid, miniature chemiresistor vapor sensors. ACS Sens. 2016, 1, 444–450, doi:10.1021/acssensors.6b00041. [10] J.; Sterner, E. S.; Swager, T. M. integrated gas sensing system of swcnt and cellulose polymer concentrator for benzene, toluene, and xylenes. Sensors 2016, 16, 183, doi:10.3390/s16020183. [11] Meng, F.-L.; Guo, Z.; Huang, X.-J. Graphene-based hybrids for chemiresistive gas sensors. TrAC Trends Anal. Chem. 2015, 68, 37–47, doi:10.1016/j.trac.2015.02.008. [12] Chung, K. H.; Wu, C. S.; Malawer, E. G. Glass transition temperatures of poly(methyl vinyl ether-co-maleic anhydride) (PMVEMA) and poly(methyl vinyl ether-co-maleic acid) (PMVEMAC) and the kinetics of dehydration of PMVEMAC by thermal analysis. J. Appl. Polym. Sci. 1990, 41, 793–803, doi:10.1002/app.1990.070410326. [13] Ramgir, N.; Datta, N.; Kaur, M.; Kailasaganapathi, S.; Debnath, A. K.; Aswal, D. K.; Gupta, S. K. Metal oxide nanowires for chemiresistive gas sensors: Issues, challenges and prospects. Colloids Surf. Physicochem. Eng. Asp. 2013, 439, 101–116, doi:10.1016/j.colsurfa.2013.02.029. [14] Kumar, A.; Singh, R. K.; Agarwal, K.; Singh, H. K.; Srivastava, P.; Singh, R. Effect of p-toluenesulfonate on inhibition of overoxidation of polypyrrole. J. Appl. Polym. Sci. 2013, 130, 434–442, doi:10.1002/app.39182. [15] Abdelhalim, A.; Winkler, M.; Loghin, F.; Zeiser, C.; Lugli, P.; Abdellah, A. Highly sensitive and selective carbon nanotube-based gas sensor arrays functionalized with different metallic nanoparticles. Sens. Actuators B Chem. 2015, 220, 1288–1296, doi:10.1016/j.snb.2015.06.138. [16] Esmailpour, A.; Meshkin, H.; Saadat, M. Conductance of disordered strain-induced graphene superlattices. Phys. E Low-Dimens. Syst. Nanostructures 2013, 50, 57–60, doi:10.1016/j.physe.2013.02.014.
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