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1. (School of Information Science and Engineering, Shenyang University of Technology, Shenyang, 110870, China)
2. (Department of Electrical and Computer Engineering and Inter-University Semiconductor Research Center, Seoul National University, Seoul, 08826, Korea)

CO2 sensor, room temperature, FET, SWNT, PEI

## I. INTRODUCTION

CO2 is one of the critical greenhouse gases. Concentrations of CO2 in Earth’s atmosphere have risen rapidly from 315 ppm in 1958 to about 410 ppm today[1]. It has important influence on the respiration of bio organisms and the photosynthesis of plants[2]. Moreover, the excessively high indoor CO2 content will induce inattention, headaches, nausea and other symptoms. Therefore, it is necessary to well monitor and control concentrations of CO2 in rooms, and low-cost portable CO2 sensors with high sensitivity and selectivity are desired[3]. So far, many researches on various kinds of sensors and sensing materials have been conducted to detect CO2. Nowadays, carbon-based materials, like CNT and graphene, have attracted many attentions because they can detect CO2 even at room temperature[4]. Most of reported sensors with carbon-based materials measure their resistance changes during target gas detection[5-7]. However, they usually suffer significant drift in the sensing signals which greatly diminishes the sensing accuracy[6,7]. Hence, further improvement is required to effectively reduce the drift and obtain reliable sensing results. In this paper, the sensing performance of an FET-type CO2 gas sensor, which has a horizontal FG, is investigated at room temperature. The layer-by-layer PEI-coated SWNT (PEI-SWNT) sensing materials are formed by inkjet printing. I-V transfer characteristics and DC sensing properties are investigated. With PEI coating, I-V hysteresis of the sensor is significantly improved. Moreover, the drift of sensing signal is suppressed by applying a pulse measurement.

## II. EXPERIMENTAL

The structure of the proposed FET-type CO2 sensor is shown in Fig. 1. Fig. 1(a) and Fig. 1(b) show the microscopic images of sensors with printed SWNT network and PEI-SWNT network, respectively. Fig. 1(c) shows the microscopic image of the MOSFET platform. Fig. 1(d) and Fig. 1(e) show the 2D schematic cross-sectional views of sensors cut along line A-A’ in Fig. 1(c), which have random SWNT network and PEI-SWNT network, respectively. The interdigitated FG and CG are formed horizontally to increase the coupling ratio between them. In this paper, pMOSFET sensor platform is adopted. The length and width of the channel of the MOSFET are 2 μm and 2.4 μm, respectively. The detailed fabrication processes of the sensor platform can be found in our previous work[8] and main fabrication steps are described as follows. First, the active region of the MOSFET is defined. A 550-nm-thick isolation oxide is grown by local oxidation of silicon (LOCOS) technique. After growing 10-nm-thick gate oxide, an n+-doped poly-silicon FG is formed and patterned. Next, the source and drain regions are created by ion implantation process. The SiO2 (10 nm)/Si3N4 (20 nm)/SiO2 (10 nm) (ONO) insulator layers are formed to cover the whole substrate for preventing any contaminations. After that, stacked metal layers of Ti (20 nm)/TiN (30 nm)/Au (30 nm) are deposited consecutively to serve as the electrodes. Finally, an SU-8 passivation layer is formed and patterned to only expose the interdigitated CG-FG area and the electrode contact pads.

Fig. 1. (a) and (b) show the microscopic images of the sensors with printed only SWNT network and PEI-SWNT network, respectively, (c) shows the microscopic top view of the MOSFET platform, (d) and (e) show the 2D schematic cross-sectional views cut along line A-A’ in (c). (d) is the sensor with only random SWNT network and (e) is the sensor with PEI-SWNT network.

The sensing material, PEI coated SWNT, is formed layer by layer using ink-jet printing process. Semiconducting SWNT ink (1mg 98%) is purchased from NanoIntrgris (Canada) and printed on the platforms without further processing. Then the device is annealed under vacuum at 250 ºC for 3 hours to evaporate the solvent. Since the ink having a viscosity of 1 to 10 cps is suitable for inkjet printing[9], when producing PEI ink, a Wt. 50% branched PEI aqueous solution (number average molecular weight 1200) purchased from Sigma-Aldrich (USA) is diluted to 10-3 mol/L with deionized water. As a result, the viscosity of the solution is reduced. After printing the PEI, the sensor is heated at 60 ºC for 2 hours and then cooled in air for 1 hour. The CO2 sample gas is prepared by mixing 1% CO2 diluted in N2 with dry synthetic air (20 vol. % of O2 and 80 vol. % of N2). The sensing characteristics are tested by switching between the sample target gas and the reference gas. A humid gas sample is obtained by running the dry gas through a bubbler filled with deionized water. All electrical measurements are carried out with an Agilent B1500A at room temperature.

## III. RESULTS AND DISCUSSION

Fig. 2 shows double-sweep DC I-V curves of the SWNT sensor and the PEI-SWNT sensor at room temperature. The SWNT sensor has larger I-V hysteresis than the PEI-SWNT sensor. According to literature[10], as the SWNTs contact silicon dioxide, a great number of traps are generated at the interface and induce a significant I-V hysteresis due to charge trapping and detrapping. Whereas, by coating the SWNT with PEI, nanotubes are wrapped and dispersed by PEI[11,12]. It reduces the contact of SWNT network with the oxide, resulting in less interface traps. Therefore, the I-V hysteresis is reduced.

Fig. 2. Double-sweep DC I-V curves of the SWNT sensor and the PEI-SWNT sensor at room temperature.

The FET sensor platform can be electrically programmed or erased by applying pulses to the CG because it has an FG that can store the charge. As a result, the threshold voltage of the sensor can be calibrated. Fig. 3(a) and Fig. 3(b) show the double-sweep pulsed I-V curves of the programmed SWNT sensor and PEI-SWNT sensor, respectively. The $V_{th} s$ of both sensors are shifted to positive direction after programming compared to the results in Fig. 2. The pulse pattern for measuring I-V is shown in the inset, where the pulse width and period are 50 μs and 0.1 s, respectively. Pulsed gate biasing limits the time for charge trapping and detrapping, resulting in the suppression of I-V hysteresis. In Fig. 3, the I-V hysteresis of the PEI-SWNT sensor is removed by pulse measurement but the I-V hysteresis of the SWNT sensor still exists. By coating SWNT network with PEI and applying the pulse measurement, the I-V hysteresis in the sensor is nearly eliminated.

Fig. 3. Double-sweep pulsed I-V curves of the SWNT sensor and the PEI-SWNT sensor at room temperature after programming.

Fig. 4 shows the dry CO2 sensing properties of the PEI-SWNT sensor measured by applying DC bias at room temperature after one day (Fig. 4(a)) and after 2 weeks (Fig. 4(b)) after depositing PEI coated SWNTs on the sensor platform. In Fig. 4(a), the drain current decreases when the sensor is exposed to CO2 target gas. During the sensing measurement, as CO2 acts as a kind of reducing gas[13,14], the adsorbed CO2 molecules transfer electrons to SWNTs, reduce the work-function of the sensing material, and result in the decrease of the absolute drain current. However, after two weeks, the sensor has no response to dry CO2 gases. This may be attributed to the effect of moisture on CO2 detection. It seems that the water left in the sensing material is fully evaporated within two weeks and the response to CO2 is degraded. Therefore, it is necessary to investigate the effect of moisture on CO2 gas detection.

Fig. 4. Dry CO2 sensing properties of the PEI-SWNT sensor measured by applying DC bias at room temperature after one day (a) and two weeks (b) after the sensing material is deposited.

Fig. 5 shows wet CO2 sensing properties measured by applying a DC bias to the PEI-SWNT sensor with the sensing material formed two weeks before the measurement (the same device measured in Fig. 4). Before measuring, we humidify the sensing material by blowing humid air into the sensor for 3 hours. The relative humidity (RH) of the gas sample is set to be 30%. With the help of water vapor in the target gas and the humidification, the sensor responds to CO2 again. The drain current decreases with the increase of CO2 concentration. It indicates that the PEI-SWNT sensor requires moisture for the detection of CO2 at room temperature. CO2 gas and water vapor together react with the sensing material, change the threshold voltage of the sensor and decrease the drain current. Some research groups also reported the effect of moisture on the reliability of PEI-based sensing material. In literature[15], PEI-PANI sensing material was investigated. As the dissolution and base-catalyzed hydration of CO2 in the PEI layer require water, the response of the sensor to CO2 is strongly dependent on the relative humidity[15]. According to their results, at 10% RH, barely no response was observed at low concentrations of CO2 ( < 2500 ppm), while at 35% RH, the response was dramatically increased (about 5% at 2500 ppm according to the figures). Note that, moisture places an important role for the detection of CO2 not only when using PEI-based materials but also some other sensing materials, like metal oxides[16,17], even though the sensing mechanisms are different. In the case of metal oxides, the water molecules will be pre-adsorbed on the surface of metal oxides forming the OH- group, so that CO2 molecules can be absorbed with the help of the OH- group[17].

Fig. 5. Wet CO2 detection characteristics of the PEI-SWNT sensor measured by applying DC bias at room temperature. The relative humidity (RH) is 30%.

As shown in Fig. 4 and Fig. 5, there is drift in the drain current over time due to hysteresis. During the DC sensing measurement, traps in the sensing material and interface traps will be charged over time. It has a significant impact on the accuracy of CO2 detection. Hence, a pulse scheme for transient measurement is introduced to obtain reliable measurement results without drift. The pulse scheme is shown in Fig. 6(a). An appropriate pre-bias ($V_{pb}$) and read bias ($V_{r}$) are applied sequentially to the CG. Note that when $V_{pb}$ is applied to the CG, $V_{DS}$ is set to 0 V simultaneously to remove unwanted power consumption. In read biasing period, both $Vr and$VDS are synchronized. During the read period, the drain current of the sensor will be read out and recorded as the sensing signal. Here, the width of $V_{pb}$ pulse ($t_{w}$) and $V_{r}$ pulse ($t_{r}$) are set to 1 s and 50 µs, respectively. Fig. 6(b) shows wet CO2 (30% RH) sensing characteristics of the PEI-SWNT sensor obtained by applying the pulse scheme at room temperature. The drift is removed by the pulse measurement method. The response of the sensor to CO2 gas is defined as

Fig. 6. (a) Voltage pulse scheme for measuring sensing characteristics, (b) Wet CO2 detection characteristics of the PEI-SWNT sensor measured by applying the voltage pulse at room temperature.

where $I_{D_B}$ and $I_{D_G}$ represent the drain currents of the sensor exposed to the reference gas and the target gas mixture, respectively. We define response time ($t_{RES}$) as the rise time of |$I_{D}$| to 90% of its maximum value, and recovery time ($t_{REC}$) as the fall time to 10% of the difference between the maximum and reference currents. In Fig. 6, our sensor shows a response of 16%, a $t_{RES}$ of 81 s and a $t_{REC}$ of 202 s for 2000 ppm of CO2 at room temperature. Table 1 lists the CO2 sensing characteristics of some recently reported sensors based on various sensing materials[17-20]. The information of the proposed sensor in this work is also included for comparison, which shows that the proposed sensor has a reasonable response and competitive response and recovery times.

Table 1. CO2 sensing characteristics of the recently reported sensors based on various sensing materials

 Sensing material Classification T Sensing condition Response Response time Recovery time Ref. SnO2 thick-film nanopowders Resistor 200℃ 20000 ppm, 14% RH 2%* 150s* 100s* [17] La1-xSrxFeO3 (0 $\leq$ x $\leq$ 0.3) nanocrystalline powders Resistor 380℃ 2000 pp, (for La0.8Sr0.2FeO3) 1.25% 11 min 15 min [18] naphthalene-based $\pi$-conjugated amine and ZnO nanohybrids Resistor RT 500 ppm 9.16% 206 s 254 s [19] nanocomposite of diphenyl-ethylenediamine (DPED) and P-type MWNT Capacitor RT ppm level (for DPED/0.5 wt.% P-MWNT) - 75 s 312 s [20] PEI-SWNT FET RT 2000 ppm, 30% RH 15% 81 s 202 s This work

*The values were taken directly form the figures and may be approximate; −, date not available; T, temperature; RT, room temperature.

## IV. CONCLUSIONS

In this work, an FET-type CO2 sensor working at room temperature has been investigated. It was shown that the PEI coating reduces the traps at the interface between SiO2 and SWNT network resulting in less hysteresis in I-V characteristics. In the I-V curves of the PEI-SWNT sensor, the hysteresis is greatly reduced, but the finite hysteresis under DC bias conditions leads to drift of the drain current, degrading the sensing characteristics. In transient sensing characteristics, drift was removed by applying voltage pulses. It was confirmed that moisture plays an important role in detecting CO2 gas when PEI-SWNT is used as a sensing material. The response, response time, and recovery time of our FET-type PEI-SWNT gas sensor are 16%, 81 s and 202 s, respectively, for 2000 ppm CO2 at room temperature.

### ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea (NRF-2016R1A2B3009361), and the Brain Korea 21 Plus Project in 2018.

### REFERENCES

1
National Oceanic and Atmospheric Administration , Earth System Research Laboratory , Global Monitoring Division , May 22. 2018, Trends in atmospheric carbon dioxide, https://www.esrl.noaa.gov/gmd/ ccgg/trends/graph.html
2
Yamazoe N., Miura N., Jun. 1995, Development of gas sensors for environmental protection, Components, Packaging, and Manufacturing Technology: Part A, IEEE Transactions on, Vol. 18, No. 2, pp. 252-256
3
Morio M., et al , Jun. 2009, Effect of macrostructural control of an auxiliary layer on the CO2 sensing properties of NASICON-based gas sensors, Sensors and Actuators B: Chemical, Vol. 139, No. 2, pp. 563-569
4
Wang Y., et al , Jun. 2017, Functionalized horizontally aligned CNT array and random CNT network for CO2 sensing, Carbon, Vol. 117, pp. 263-270
5
Mittal M., Kumar A., Nov. 2014, Carbon nanotube (CNT) gas sensors for emissions from fossil fuel burning, Sensors and Actuators B: Chemical, Vol. 203, pp. 349-362
6
Penza M., et al , Nov. 2011, Pt-modified carbon nanotube networked layers for enhanced gas microsensors, Thin Solid Films, Vol. 520, No. 3, pp. 959-965
7
Penza M., et al , Feb. 2010, Metal-modified and vertically aligned carbon nanotube sensors array for landfill gas monitoring applications, Nanotechnology, Vol. 21, No. 10, pp. 105501
8
Wu M., et al , Apr. 2018, An FET-type gas sensor with a sodium ion conducting solid electrolyte for CO2 detection, Sensors and Actuators B: Chemical, Vol. 259, pp. 1058-1065
9
Feyssa B., et al , Jul. 2013, Patterned immobilization of antibodies within roll-to-roll hot embossed polymeric microfluidic channels, PloS ONE, Vol. 8, No. 7, pp. e68918
10
Park R. S., et al , Apr. 2016, Hysteresis in carbon nanotube transistors: measurement and analysis of trap density, energy level, and spatial distribution, ACS Nano, Vol. 10, No. 4, pp. 4599-4608
11
Freeman D. D., et al , Nov. 2012., N-type thermoelectric performance of functionalized carbon nanotube-filled polymer composites, PloS ONE, Vol. 7, No. 11, pp. e47822
12
Kyrylyuk A. V., van der Schoot P., Jun. 2008, Continuum percolation of carbon nanotubes in polymeric and colloidal media, Proceedings of the National Academy of Sciences, Vol. 105, No. 24, pp. 8221-8226
13
Shivananju B. N., et al , Jun. 2013, CO2 sensing at room temperature using carbon nanotubes coated core fiber Bragg grating, Review of Scientific Instruments, Vol. 84, No. 6, pp. 065002
14
Lin Z. D., et al , Dec. 2015, CO2 gas sensors based on carbon nanotube thin films using a simple transfer method on flexible substrate, IEEE Sensors Journal, Vol. 15, No. 12, pp. 7017-7020
15
Srinives S., et al , May. 2015, A miniature chemiresistor sensor for carbon dioxide, Analytica chimica acta, Vol. 874, pp. 54-58
16
Laubender E., et al , Jan. 2016, Ceria-zirconia mixed oxide prepared through a microwave-assisted synthesis for CO2 sensing in low power work function sensors, Materials Today: Proceedings, Vol. 3, No. 2, pp. 429-433
17
Wang D., et al , May. 2016, CO2-sensing properties and mechanism of nano-SnO2 thick-film sensor, Sensors and Actuators B: Chemical, Vol. 227, pp. 73-84
18
Fan K., et al , Feb. 2013, CO2 gas sensors based on La1−xSrxFeO3 nanocrystalline powders, Sensors and Actuators B: Chemical, Vol. 177, pp. 265-269
19
Mandal B., et al , π-Conjugated amine-ZnO nanohybrids for the selective detection of CO2 gas at room temperature, ACS Applied Nano Materials, advance online publication
20
Rahimabady M., et al , May. 2017, Dielectric nanocomposite of diphenylethylenediamine and P-type multi-walled carbon nanotube for capacitive carbon dioxide sensors, Sensors and Actuators B: Chemical, Vol. 243, pp. 596-601

## Author

##### Meile Wu

received the B.S. degree in Electronic Science and Tech-nology from Shenyang University of Technology, Shenyang, China in 2012, the M.S. degree in Micro-electronics and Solid-State Electro-nics from Shenyang University of Technology, Shenyang, China in 2015, and the Ph.D. degree in Electrical and Computer Engineering from Seoul National University (SNU), Seoul, Korea in 2019.

She is now a postdoctoral researcher at Shenyang University of Technology, Shenyang, China. Her current research interest is FET-based sensors.

##### Yoonki Hong

received the B.S. degree in Electrical and Computer Engineering from Seoul National University (SNU), Seoul, Korea in 2013.

He is currently working toward a combined master's and doctorate program in the Department of Electrical and Computer Engineering at SNU.

He is also with the Inter-University Semiconductor Research Center, SNU.

His current research interests include MOSFET-based gas sensors and humidity sensors.

##### Dongkyu Jang

received the B.S. degree in Material Science and Engineering from Korea University, Seoul, Korea in 2008 and the M.S. degree in Electrical Engineering from Korea University, Seoul, Korea in 2010.

He is currently pursuing the Ph.D. degree in the Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea.

He is also with the Inter-University Semiconductor Research Center, SNU. His current research interest is CMOS pressure sensors.

##### Xiaoshi Jin

received the B.S. degree in Physics from Dalian University of Technology, Dalian, China, in 2004, the M.S. degree in Physics from Gyeongsang National University, Jinju, Korea, in 2006 and the Ph.D. degree in Semiconductor and Display Engineering from Kyungpook National University, Daegu, Korea, in 2010.

He works in the School of Information Science and Engineering, Shenyang University of Technology as an associate professor.

He has authored or coauthored more than 30 papers published in refereed journals and has been granted more than 20 patents in this area.

His research interests include semiconductor physics and device modeling, design of advanced semiconductor devices and ICs.

##### Jong-Ho Lee

received the B.S. degree from Kyungpook National University, Daegu, Korea, in 1987 and the M.S. and Ph.D. degrees from Seoul National University, Seoul, in 1989 and 1993, respectively, all in electronic engineering.

In 1993, he worked on advanced BiCMOS process development at ISRC, Seoul National University as an Engineer.

In 1994, he was with the School of Electrical Engineering, Wonkwang University, Iksan, Chonpuk, Korea.

In 2002, he moved to Kyungpook National University, Daegu Korea, as a Professor of the School of Electrical Engineering and Computer Science.

Since September 2009, he has been a Professor in the School of Electrical and Computer Engineering, Seoul National University, Seoul Korea.

From 1994 to 1998, he was with ETRI as an invited member of technical staff, where he worked on deep submicron MOS devices, device isolation.

From August 1998 to July 1999, he was with Massachusetts Institute of Technology, Cambridge, as a postdoctoral fellow, where he was engaged in the research on sub–100 nm double-gate CMOS devices.

He has authored or coauthored more than 216 papers published in refereed journals and over 326 conference papers related to his research and has been granted 85 patents in this area.

He received 18 awards for excellent research papers and research excellence.

He invented bulk FinFET, Saddle FinFET (or bCAT) for DRAM cell, and NAND flash cell string with virtual source/drain, which have been applying for mass production.

His research interests include CMOS technology, non-volatile memory devices, thin film transistors, sensors, neuromorphic technology, and device characterization and modeling.