Downsizing gas sensors based on semiconducting metal oxide: Effects of electrodes on gas sensing properties

Accepted Manuscript Title: Downsizing Gas Sensors based on Semiconducting Metal Oxide: Effects of Electrodes on Gas Sensing Properties Authors: Young Geun Song, Young-Seok Shim, Sangtae Kim, Soo Deok Han, Hi Gyu Moon, Myoung Sub Noh, Kwangjae Lee, Hae Ryong Lee, Jin-Sang Kim, Byeong-Kwon Ju, Chong-Yun Kang PII: DOI: Reference:

S0925-4005(17)30256-3 http://dx.doi.org/doi:10.1016/j.snb.2017.02.035 SNB 21766

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

15-10-2016 27-12-2016 4-2-2017

Please cite this article as: Young Geun Song, Young-Seok Shim, Sangtae Kim, Soo Deok Han, Hi Gyu Moon, Myoung Sub Noh, Kwangjae Lee, Hae Ryong Lee, JinSang Kim, Byeong-Kwon Ju, Chong-Yun Kang, Downsizing Gas Sensors based on Semiconducting Metal Oxide: Effects of Electrodes on Gas Sensing Properties, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.02.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Downsizing Gas Sensors based on Semiconducting Metal Oxide: Effects of Electrodes on Gas Sensing Properties

Young Geun Songa,b,†, Young-Seok Shima,†, Sangtae Kima, Soo Deok Hana,c, Hi Gyu Moona, Myoung Sub Noha,c, Kwangjae Leed, Hae Ryong Leee, Jin-Sang Kima, Byeong-Kwon Jub, and Chong-Yun Kanga,c,*

a

Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul, 02791, Republic of Korea

b

Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul, 02841, Republic of Korea c

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, 02841, Republic of Korea

d

Contents Convergence Research Center, Korea Electronics Technology Institute (KETI), Seoul, 03924, Republic of Korea e

Smart Game Platform Research Section, SW•Content Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon, 34129, Republic of Korea

†

These authors contributed equally.

*Corresponding author. Tel.: +82 2 958 6722; fax: +82 2 958 6720.

E-mail address: [email protected] (C. Y. Kang)

1

Research Highlights ► The sensing areas of Pt-interdigitated electrodes (IDEs) are varied to investigate the relation between sensing materials and electrodes. ► The effect of the interface resistance and the intergrain resistance are analyzed by linear regression. ► Not only the sensing properties of materials but also the interface between the electrodes and sensing materilas play a role to detemine the performance of the downsized gas sensors.

Abstract We report the highly sensitive and selective downsized gas sensors for the IoT application. Sensing areas of Pt-interdigitated electrodes (IDEs) was varied to investigate the relation between sensing materials and electrodes. In2O3 nanocolumns were deposited on the pre-patterned Pt IDEs using glancing angle deposition (GLAD). The effect of the interface resistance between electrodes and sensing materials, and the intergrain resistance between nanocolumns are analyzed by linear regression at different sensing area and incident angle of GLAD. In2O3 (angle: 85o, sensing area: 0.3 mm x 0.3 mm) nanocolumns with double Schottky barriers show the highest response and 2

selectivity with fast response time of 10 s to VOCs among the samples fabricated in this study. The analysis reveals that the intrinsic response of In2O3 (angle: 85o, sensing area: 0.3 mm x 0.3 mm) nanocolumns are dominantly affected by intergrain resistance, resulting in a high response. Our demonstration for the fundamental aspect of downsizing gas sensor makes an important contribution to the chemical sensor field with broad interest.

Keywords: Internet of Things, miniaturization, gas sensor, interdigitated electrodes, In2O3 nanocolumn

1. Introduction Recently, over 90% of modern people’s activity is conducted indoor, and the interior is often closed from the exterior to maintain comfortable temperature inside [1, 2]. According to World Health Organization (WHO), illnesses causes by poor indoor air quality result in approximately two million deaths per year, because building materials, furniture, and appliances contain chemical compounds which continuously emit hazardous gases [3]. Among the gases emitted, volatile organic compounds (VOCs) such as acetone, ethanol, toluene, benzene, and formaldehyde gases are extremely dangerous since they induce dizziness, paralysis, dyspnea, and 3

eventually lead to a comatose state [4–6]. Therefore, detecting and monitoring contamination of interior air become an important aspect of indoor design. The Internet of Things (IoT) is the interconneted network of physical devices embedded technology including electronics, software, sensors, and actuators, which offers the ability to measure, infer and understand environmental indicators anytime and anywhere [7, 8]. In particular, the IoT is effective in monitoring and controlling objects in the environments where people do not recognize potential, minor toxicity [9, 10]. Gas sensors linked with the IoT can offer continuous information regarding the present of state of indoor gas, especially the presence of unusual gaseous species in specific spaces [11, 12]. A suitable gas sensor for the IoT must fulfill several design requirements, including low power consumption, low cost, miniaturized size, and integration into electronic circuits and high sensing performance [13, 14]. As a promising candidate to meet these design needs for IoT, gas sensors based on semiconducting metal oxides are becoming strong candidates within the gas sensor society due to their outstanding advantages such as cost effectiveness, simplicity in fabrication, high sensitivity, and easy integration with electronic circuits [12–18]. Recent research efforts have focused largely on the use of catalysts, heterojunctions, and nanostructured materials, in order to enhance the gas sensing performances [19–23]. However, relatively few reports have studied the relation between the miniaturization of metal oxide gas sensor and the sensing properties, even though the sensing properties are strongly affected by electrode parameters (design, length, number of electrodes, and width between the electrodes) as well as the sensing area [24, 25]. Therefore, careful investigation on sensing properties of miniaturized gas sensors would effectively evaluate the possibility of applying gas sensors into the IoT.

4

In this work, we present a downsized gas sensor with high response, selectivity, and fast response time for VOCs. In order to investigate the interaction between the active layer and the electrode, In2O3 nanocolumns are deposited on the pre-patterned SiO2/Si substrates with active region varied between 1 mm × 1 mm, 0.5 mm × 0.5 mm, 0.3 mm × 0.3 mm, and 0.1 mm × 0.1 mm, with Pt-interdigitated electrodes (IDEs). The active sensing layer was deposited by glancing angle deposition (GLAD) with and electron beam evaporator. As the sensors get miniaturized, the effect of interface resistance and inter-grain resistance are analyzed carefully. To determine the appropriate downsizing condition, all samples are compared with various commercial gas sensors. In2O3 nanocolumns deposited at glancing angle of 85o, sensing area of 0.3 mm x 0.3 mm show a linear relation between the resistance changes and sensing areas, resulting in remarkable sensing properties with optimal resistance level to VOC gases including 50 ppm C2H5OH, C6H6, CH3COCH3, C7H8, and HCHO.

2. Experimental 2.1 Fabrication Pt/Ti (150 nm/30 nm thick) interdigitated electrodes (IDEs) were fabricated on a SiO2/Si substrate (1 μm/550 μm thick) using photolithography (followed by an etching procedure). The distances between the Pt/Ti IDEs was approximately 5 μm and the IDEs areas were 1 mm × 1 mm, 0.5 mm × 0.5 mm, 0.3 mm × 0.3 mm, and 0.1 mm × 0.1 mm, namely 1 mm2, 0.25 mm2, 0.09 mm2, and 0.01 mm2 as shown in Figure 1. Before depositing the sensing films, Pt/Ti IDE patterned SiO2/Si substrates were cleaned in acetone and ethanol followed by drying in nitrogen gas. For In2O3 thin film deposition, electron beam evaporator was utilized. The substrate was located 30 cm away from the crucible and shadow masks were used to deposit 5

only on the IDEs patterns. The base pressure and growth rate were 5 × 10-6 mTorr and 1 Å s1

, respectively. To synthesize In2O3 nanocolumns, deposition was carried out at a glancing

angles tilted of 78º, 82º, and 85º. The base pressure and growth rate were 5 × 10-6 mTorr and 1 Å s-1, respectively. All fabricated specimens were annealed at 550 ºC for 2 h in ambient air 2.2 Characterization An X-ray diffraction (DMax2500) was used to analyze deposited films with 2θ scan from 20 to 80o, where CuKα radiation (wavelength =1.5418 Å) was used for the X-ray source and the fixed incident angle of 2o. The morphology of the fabricated In2O3 nanocolumns was observed by a field emission scanning electron microscope (FESEM) using an acceleration voltage of 15 kV and a working distance of 10 mm. 2.3 Sensor property measurements Gas sensing properties of In2O3 dense thin films and In2O3 nanocolumns were measured in a quartz tube with external heating. The flow gas was changed from dry air to a calibrated target gas (balanced with dry air). A constant flow rate of 500 sccm was used for the dry air and target gas. The response was accurately determined by measuring the baseline resistance in dry air and the fully saturated resistance after exposure to the target gas.

3. Results and Discussion Generally, the Pt IDEs are used for gas sensors to obtain reliable sensing properties and appropriate electrical properties (resistance, current, and voltage) by controlling sensing area, namely, the number and length of IDE fingers [25]. To investigate the gas sensing properties as a function of sensing areas, we designed the Pt IDEs which consist of 22, 11, 7, and 3fingers into 1 mm × 1 mm, 0.5 mm × 0.5 mm, 0.3 mm × 0.3 mm, and 0.1 mm × 0.1 mm, 6

respectively (Fig. 1(a-d)). We then deposited In2O3 nanocolumns in the designed area to measure their sensing properties using GLAD technique which can easily synthesize various one dimensional (1D) nanostructures with high sensing properties such as nanorods, nanocolumns, nanohelixs, and nanozigzags [26–28]. Fig. 2 illustrates the fabrication procedure for the well-aligned In2O3 nanocolumns on SiO2/Si substrate and the change of selfshadowing region as a function of incident angles. Because the density and diameter of the 1D nanostructures are closely related to the accessibility of target gases, we deposited the In2O3 films at the substrate with a tilt angle of 0o, 78o, 82o, and 85o, respectively. Our previous study showed that approximately 300 nm length of the individual metal oxide nanocolumns had optimal gas sensing properties [29, 30]. Therefore, we directly deposited In2O3 films until the nanocolumn length reaches approximately 300 nm. Plain-view SEM images according to the fabrication procedure of well-aligned In2O3 nanocolumns are shown in Fig. 3(a-c). Although all samples were deposited to the same length, the nanocolumns diameter and porosity of each sample are increased as the incident angle increases. The nanocolumns were annealed at 550ºC for 2 h and characterized by X-ray Diffraction (XRD), exhibiting strong crystallinity, as presented in Fig. 3(d‒h). The diffraction peaks indexed as In2O3 (JCPDS no. 06-0416), Pt (JCPDS no. 04-0802), and the substrate (SiO2/Si) indicate that the In2O3 thin film and nanocolumns are polycrystalline. From the XRD results, no remarkable difference in crystallinity is observed for In2O3 nanocolumns deposited at different incident angles. In order to measure the gas sensing properties as a function of sensing area, we exposed all samples to 50 ppm C2H5OH at 300ºC. In air ambient, base resistance of all samples gradually increases with decreasing sensing areas because the designed Pt-IDEs are effectively 7

connected in the parallel circuit. Base resistances of all samples also increase with increasing incident angle since the necks between individual columns become narrower. The responses of In2O3 thin films and In2O3 nanocolumns deposited at 0o, 78o, 82o, and 85o as a function of sensing areas are shown in Fig. 4. Interestingly, the responses of all samples are improved upon exposure to 50 ppm C2H5OH, while it is conventionally accepted that the reduced sensing area in miniaturized gas sensors decreases the response to target gases. The reasons to the enhancement of response could be explained next equation. The numbers next to the curves indicate the response values, defined as the following: where



∝

(1)

is the measured resistance in ambient air,

is exposed to target gases, and

the resistance when the sensor

the resistance change, namely

or

. Response is directly proportional to resistance change. The resistance change can also be expressed as the following by using Ohm’s law: ∆ where

⋅

⋅

⋅

⋅

stands for the cross-section area of current paths,

ambient air and target gases, respectively, the electric field, and

⋅

(2)

∆

,

the electron charge,

the current density in the carrier mobility,

the carrier concentration. Since the carrier density changes upon and

target gas introduction, we express

∆ . Since

⋅

is either

constant or controlled, the sensor response directly scales with the following: ∝

(3)

∆

8

For low base resistance, there are abundant initial carriers and thus, ∆ compared to the initial carrier density

is relatively small

≫ ∆ . The response can be approximated to be

zero: ∝

(4)

∆

is small, the change in carrier density ∆

In contrast, if the initial carrier density

becomes significantly for high initial resistance samples. Correlations in the initial resistance are summarized as follows:

∆

∆



(5)

We also note that the increase in base resistance does not scale linearly with the sensing area, indicating the existence of additional factors present in the system. We linearly fit the measured base resistance with the calculated resistance as a function of the sensing area and the incident angle. Fig. 5 shows the linear fitting at different incident angles and the insets in Fig. 5 show the magnification of selected area. The calculated resistance was modelled as a simple parallel circuit with the number of IDE fingers and dimension of the sensitive layer between each electrode as shown in Fig. S1 in Supplementary data. The ratios of calculated resistance for all samples are 1, 4.6, 16.8, 486, which matches with the sensing area of 1 mm × 1 mm, 0.5 mm × 0.5 mm, 0.3 mm × 0.3 mm, and 0.1 mm × 0.1 mm, respectively. The coefficient of determination, denoted as R2, indicates the proportion of the variance in the dependent variable expectable from the independent variable. When the R2 equals 1, the regression perfectly fits the data [31]. R2 of In2O3 nanocolumns deposited at 0o, 78o, 82o, and

9

85o are 0.81281, 0.99917, 0.99838, and 0.99994, respectively. Increasing the incident angle during deposition results in linear regression. The non-linearity induced at a low incident angle can be explained using the schematics in Fig. 6. The two types of sensing layers used in this work, dense thin film and nanocolumns differ significantly in their resistance. Even though the sensing layers are polycrystalline in practical, double Schottky barriers between the grains were not considered for the case of dense thin film since the interacting gases cannot access the inner layer [32]. Thus, the interface resistance between electrodes and sensing materials become a dominant factor to the total resistance as shown in Fig. 6(a). In contrast, gases interact with the larger surface area and part of the inner layer via diffusion in the case of nanocolumns. Double Schottky barriers are developed between nanocolumns, making the inter-grain resistance the dominant factor to the total resistance, as illustrated in Fig. 6(b). The formation of a low-interface-resistance contact between the electrode and the sensing layer is essential for obtaining the high response properties, which is consistent with our previous study [25]. The total resistance (

) can be

expressed as the following: (6) is the resistance of the sensing layer,

where

sensing layer and the electrodes, and with Pt IDEs,

≫

and, thus,

the contact resistance between the

the resistance of the electrode. For the In2O3 sensors could be approximated as:

≅

(7)

Then, the measured response of the sensor with Pt IDEs is expressed as: ≅

⁄

(8)

⁄

10

≫

Since

for the nanocolumns, the measured response of the sensor

⁄

was close to the intrinsic response of the sensing material itself, that is: ≅ (9) In contrast, the interface resistance

⁄

takes a non-negligible portion in the dense thin

film case. This explains the decreased response of the sensor with Pt IDEs compared to the intrinsic response, ⁄ ⁄

(10)

Apparently, the low resistance between the electrode and the sensing layer blunted the detection of the sensor’s intrinsic response to a target gas because the change in the band bending only appears at the inter-grain interfaces, resulting in lower response values. For practical applications, we surveyed the properties of commercial sensors, such as their base resistance and sensing resistance, and tabulated them in Table S1 in Supplementary Materials. The properties of our sensors such as base resistance, sensing resistance, and the response (Table S2-4 in Supplementary Materials). Among the samples, studied in this work, the sensor designed on 0.5 mm × 0.5 mm sensing area at 85o deposition angle suits the resistance range for commercial gas sensors the most. In order to accurately investigate the gas sensing properties of the selected sample, including the response and selectivity, we exposed the selected sample to 50 ppm C2H5OH, C6H6, CH3COCH3, C7H8, HCHO, NH3, CO, and NO2 gases at the working temperature 300ºC. The response transients, responses and response times are illustrated in Fig. 7(a) and (b). Among the exposed gases, VOCs, such as C2H5OH, C6H6, CH3COCH3, C7H8, and HCOH, show the high response of over 300, and fast 11

response time within 10 s, which are superior to those of some commercial sensors. While the response with NO2 gas also was sufficiently high, the oxidizing gas shows a relatively slow response time. Based on the results and analysis, we find that the main factor for the extremely high response of In2O3 nanocolumns is the porous nanostructure with double Schottky barriers between grains, which surpasses the interface resistance. The underlying mechanism for the response enhancement is described with the following schematics in Fig. 8. Both the optimal filling density and the large surface-to-volume ratio of well-ordered In2O3 nanocolumns induce an enhancement of the transducer function and the utility factor which demonstrate how the response of each particle is transferred to that of the whole device and how the target gases are diffused through the surface of the particles, respectively. Thus, the samples deposited with 85o incident angle show much higher barrier potential than that deposited at 78o as illustrated in Fig. 8(b) and (c). These double Schottky barriers lead to more efficient modulation in the resistance upon exposure to target gases for 85o incident angle, resulting in much higher response than that deposited at 78o incident angle. In this regard, the incident angle of the In2O3 nanocolumns should be sufficiently high owing to the height of double Schottky barriers, leading to the ultrahigh response to VOCs gases. The selectivity of In2O3 sensor could be ascribed to two reasons. Firstly, In2O3 has an enhanced adsorption and catalytic capability towards VOCs. The many previous studies show that the In2O3 has high sensitivity and selectivity to the VOCs, although the properties of the In2O3 and the measurement conditions are different such as synthesis method, morphology, thickness, operating temperature, gas flow rate, and gas concentration [33–40]. Secondly, each gas has an optimal temperature range to dissociate and react with ionized oxygen species on the surface of the sensing material. The range of CO gas is approximately 350–400ºC [41– 12

42], NH3 and NO2 are 200–250ºC [42–45], and VOCs are 300ºC [46–48]. Hence, our In2O3 gas sensor represents a high selectivity to VOCs.

4. Conclusion A high efficiency, including high sensitivity, fast response/recovery time and stability, lowcost fabrication are essential for high performance gas sensors. In particular, miniaturization with low power consumption and easy integration with circuits are deemed as the most important factors for the gas sensors to be applied in the IoT. Here, we systematically investigate the gas sensing properties as a funtion of sensing areas. The relationship between sensing materials and electrodes are controlled the number and length of IDE fingers. The In2O3 nanocolumns are deposited on pre-patterned Pt IDEs and are optimized by controlling the incident angle of vapor flux. For In2O3 nanocolumns (angle: 85o, sensing area: 0.3 mm x 0.3 mm), the response dramatically increased compared with the other In2O3 nanocolumns. The response enhancement is explained by the well-aligned nanostructures (utility factor). Additionally, the intergrain resistance between the In2O3 nanocolumns with the optimal morphology enhanced the intrinsic response of sensing materials, resulting in high net response (transducer function). Upon exposure to various gases such as 50 ppm C2H5OH, C6H6, CH3COCH3, C7H8, HCOH, NH3, CO, and NO2, In2O3 nanocolumns (angle: 85o, sensing area: 0.3 mm x 0.3 mm) exhibited high sensitivity and selectivity to VOCs. Our experimental results suggest that not only the sensing properties of materials but also the interface between the electrodes and sensing materilas play a role to detemine the performance of the downsized gas sensors for IoT application.

Acknowledgements 13

This work was supported by the Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korean government (MSIP) (No. R0126-16-1050, Olfactory Bio Data Based Emotion Enhancement Interactive Content Technology Development).

References [1]

J. Lozanoa, J. Suárezb, P. Arroyo, J. Manuel, Wireless Sensor Network For Indoor Air

Quality Monitoring, Chem. Eng. 30 (2012) 319–324. [2]

R.A. Raso, M. Zeltner, W.J. Stark, Indoor Air Puri fi cation Using Activated Carbon

Adsorbers: Regeneration Using Catalytic Combustion of Intermediately Stored VOC, (2014). [3]

B.I. Stefanov, D. Lebrun, A. Mattsson, C.G. Granqvist, L. Österlund, Demonstrating

online monitoring of air pollutant photodegradation in a 3d printed gas-phase photocatalysis reactor, J. Chem. Educ. 92 (2015) 678–682. [4]

F. Rock, N. Barsan, U. Weimar, F. Röck, N. Barsan, U. Weimar, Electronic nose: Current

status and future trends, Chem. Rev. 108 (2008) 705–725. [5]

G. Peng, U. Tisch, O. Adams, M. Hakim, N. Shehada, Y.Y. Broza, S. Billan, R. Abdah-

Bortnyak, A. Kuten, H. Haick, Diagnosing lung cancer in exhaled breath using gold nanoparticles., Nat. Nanotechnol. 4 (2009) 669–673. [6]

H.G. Moon, Y. Jung, S.D. Han, Y.S. Shim, B. Shin, T. Lee, J.S. Kim, S. Lee, S.C. Jun,

H.H. Park, C. Kim, C.Y. Kang, Chemiresistive Electronic Nose toward Detection of Biomarkers in Exhaled Breath, ACS Appl. Mater. Interfaces. 8 (2016) 20969–20976. [7]

S. Chen, H. Xu, D. Liu, B. Hu, H. Wang, A vision of IoT: Applications, challenges, and

opportunities with China Perspective, IEEE Internet Things J. 1 (2014) 349–359.

14

[8]

L. Da Xu, W. He, S. Li, Internet of things in industries: A survey, IEEE Trans. Ind.

Informatics. 10 (2014) 2233–2243. [9]

J.-M. Jeon, Y.-S. Shim, S.D. Han, D.H. Kim, Y.H. Kim, C.-Y. Kang, J.-S. Kim, M. Kim,

H.W. Jang, Vertically ordered SnO2 nanobamboos for substantially improved detection of volatile reducing gases, J. Mater. Chem. A. 3 (2015) 17939–17945. [10]

C. Pérez-Garrido, F.J. González-Castaño, D. Chaves-Díeguez, P.S. Rodríguez-

Hernández, Wireless remote monitoring of toxic gases in shipbuilding., Sensors (Basel). 14 (2014) 2981–3000. [11]

S. Amendola, R. Lodato, S. Manzari, C. Occhiuzzi, G. Marrocco, RFID Technology for

IoT-based Personal Healthcare in SmartSpaces, IEEE Internet Things J. 1 (2014) 144–152. [12]

K. Arshak, E. Moore, G.M. Lyons, J. Harris, S. Clifford, A review of gas sensors

employed in electronic nose applications, Sens. Rev. 24 (2004) 181–198. [13]

K. Laubhan, K. Talaat, S. Riehl, S. Aman, A. Abdelgawad, K. Yelamarthi, A Low-Power

IoT Framework : From Sensors to the Cloud, (2016) 648–652. [14]

K. Chikkadi, M. Muoth, V. Maiwald, C. Roman, C. Hierold, Ultra-low power operation

of self-heated, suspended carbon nanotube gas sensors, Appl. Phys. Lett. 103 (2013). [15]

N. Yamazoe, K. Shimanoe, C. Sawada, Contribution of electron tunneling transport in

semiconductor gas sensor, Thin Solid Films. 515 (2007) 8302–8309. [16]

C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: Sensitivity and

influencing factors, Sensors. 10 (2010) 2088–2106. [17]

H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide

semiconductors: Overview, Sensors Actuators B Chem. 192 (2014) 607–627.

15

[18]

J. Zhang, X. Liu, G. Neri, N. Pinna, Nanostructured Materials for Room-Temperature Gas

Sensors, Adv. Mater. 28 (2016) 795–831. [19]

S.W. Choi, A. Katoch, J.H. Kim, S.S. Kim, Prominent reducing gas-sensing performances

of n-SnO2 nanowires by local creation of p-n heterojunctions by functionalization with p-Cr2O3 nanoparticles, ACS Appl. Mater. Interfaces. 6 (2014) 17723–17729. [20]

S. Park, G.-J. Sun, C. Jin, H.W. Kim, S. Lee, C. Lee, Synergistic Effects of a Combination

of Cr2O3-Functionalization and UV-Irradiation Techniques on the Ethanol Gas Sensing Performance of ZnO Nanorod Gas Sensors, ACS Appl. Mater. Interfaces. 8 (2016) 2805–2811. [21]

H.-S. Woo, C.W. Na, I.-D. Kim, J.-H. Lee, Highly sensitive and selective trimethylamine

sensor using one-dimensional ZnO-Cr2O3 hetero-nanostructures., Nanotechnology. 23 (2012) 245501. [22]

Y.S. Shim, D.H. Kim, H.Y. Jeong, Y.H. Kim, S.H. Nahm, C.Y. Kang, J.S. Kim, W. Lee,

H.W. Jang, Utilization of both-side metal decoration in close-packed SnO2 nanodome arrays for ultrasensitive gas sensing, Sensors Actuators, B Chem. 213 (2015) 314–321. [23]

D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for

gas sensing: A review, Sensors Actuators, B Chem. 204 (2014) 250–272. [24]

Y.-S. Shim, H.G. Moon, D.H. Kim, H.W. Jang, C.-Y. Kang, Y.S. Yoon, S.-J. Yoon,

Transparent conducting oxide electrodes for novel metal oxide gas sensors, Sensors Actuators B Chem. 160 (2011) 357–363. [25]

G.P. Alcantara, A short review of gas sensors based on interdigital electrode, IEEE,

ICEMI. 12 (2015) 1616–1621. [26]

M.M. Hawkeye, M.J. Brett, Glancing angle deposition: Fabrication, properties, and

applications of micro- and nanostructured thin films, J. Vac. Sci. Technol. A. 25 (2007) 1317. 16

[27]

K. Robbie, J.C. Sit, M.J. Brett, Advanced techniques for glancing angle deposition, J. Vac.

Sci. Technol. B Microelectron. Nanom. Struct. 16 (1998) 1115. [28]

K. Robbie, Sculptured thin films and glancing angle deposition: Growth mechanics and

applications, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 15 (1997) 1460. [29]

H.G. Moon, Y.R. Choi, Y.S. Shim, K. Il Choi, J.H. Lee, J.S. Kim, S.J. Yoon, H.H. Park,

C.Y. Kang, H.W. Jang, Extremely sensitive and selective NO probe based on villi-like WO3 nanostructures for application to exhaled breath analyzers, ACS Appl. Mater. Interfaces. 5 (2013) 10591–10596. [30]

H.J. Gwon, H.G. Moon, H.W. Jang, S.-J. Yoon, K.S. Yoo, Sensitivity Enhancement of

Nanostructured SnO2 Gas Sensors Fabricated Using the Glancing Angle Deposition Method, J. Nanosci. Nanotechnol. 13 (2013) 2740–2744. [31] Wikimedia Foundation, Inc., https://en.wikipedia.org/wiki/Coefficient_of_determination, 2016 (accessed 2016.10.13) [32]

N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: How to?,

Sensors Actuators, B Chem. 121 (2007) 18–35. [33]

K. Inyawilert, A. Wisitsora-At, A. Tuantranont, P. Singjai, S. Phanichphant, C. Liewhiran,

Ultra-rapid VOCs sensors based on sparked-In2O3 sensing films, Sensors Actuators, B Chem. 192 (2014) 745–754. [34]

S.J. Kim, I.S. Hwang, J.K. Choi, Y.C. Kang, J.H. Lee, Enhanced C2H5OH sensing

characteristics of nano-porous In2O3 hollow spheres prepared by sucrose-mediated hydrothermal reaction, Sensors Actuators, B Chem. 155 (2011) 512–518. [35]

L. Liu, T. Zhang, S. Li, L. Wang, Y. Tian, Preparation, characterization, and gas-sensing

properties of Pd-doped In2O3 nanofibers, Mater. Lett. 63 (2009) 1975–1977. 17

[36]

M. Seetha, S. Bharathi, D. Mangalaraj, D. Nataraj, Reducing gas response kinetics of

nanostructured indium oxide thin films, Thin Solid Films. 518 (2010) e125–e128. [37]

M. Seetha, D. Mangalaraj, Nano-porous indium oxide transistor sensor for the detection

of ethanol vapours at room temperature, Appl. Phys. A Mater. Sci. Process. 106 (2012) 137–143. [38]

V.S. Vaishnav, P.D. Patel, N.G. Patel, Indium tin oxide thin film gas sensors for detection

of ethanol vapours, Thin Solid Films. 490 (2005) 94–100. [39]

A. Vomiero, S. Bianchi, E. Comini, G. Faglia, M. Ferroni, N. Poli, G. Sberveglieri, In2O3

nanowires for gas sensors: morphology and sensing characterization, Thin Solid Films. 515 (2007) 8356–8359. [40]

J. Xu, Y. Chen, J. Shen, Ethanol sensor based on hexagonal indium oxide nanorods

prepared by solvothermal methods, Mater. Lett. 62 (2008) 1363–1365. [41]

S.S. Sharma, K. Nomura, Y. Ujihira, Mössbauer studies on tin-bismuth oxide CO selective

gas sensor, J. Appl. Phys. 71 (1992) 2000–2005. [42]

A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, Y. Ding, Z.L. Wang,

Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks, Appl. Phys. Lett. 88 (2006) 1–4. [43]

M. Penza, M.. Tagliente, L. Mirenghi, C. Gerardi, C. Martucci, G. Cassano, Tungsten

trioxide (WO3) sputtered thin films for a NOx gas sensor, Sensors Actuators B Chem. 50 (1998) 9–18. [44]

M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Modified zinc oxide thick film

resistors as NH3 gas sensor, Sensors Actuators, B Chem. 115 (2006) 128–133.

18

[45]

N. Van Hieu, V. Van Quang, N.D. Hoa, D. Kim, Preparing large-scale WO3 nanowire-

like structure for high sensitivity NH3 gas sensor through a simple route, Curr. Appl. Phys. 11 (2011) 657–661. [46]

B.H. Jang, O. Landau, S.J. Choi, J. Shin, A. Rothschild, I.D. Kim, Selectivity

enhancement of SnO2 nanofiber gas sensors by functionalization with Pt nanocatalysts and manipulation of the operation temperature, Sensors Actuators, B Chem. 188 (2013) 156–168. [47]

Z. Li, Q. Zhao, W. Fan, J. Zhan, Porous SnO2 nanospheres as sensitive gas sensors for

volatile organic compounds detection, Nanoscale. 3 (2011) 1646–1652. [48]

Q. Qi, T. Zhang, L. Liu, X. Zheng, Q. Yu, Y. Zeng, H. Yang, Selective acetone sensor

based on dumbbell-like ZnO with rapid response and recovery, Sensors Actuators, B Chem. 134 (2008) 166–170.

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Biographies Young Geun Song received his B.S. form Pusan National University, Korea, in 2016. He is currently a M.S. course student in the Department of Electrical Engineering, Korea University. His research interests include fabrication of metal oxide nanostructure, and chemical sensor applications. Young-Seok Shim received his Ph.D. degree from Department of Materials Science and Engineering of Yonsei University in 2016. Now he is a postdoctoral researcher in KIST. His research interests are the synthesis nanostructured oxide thin films and their applications to various devices including sensor and solar water splitting cells. Sangtae Kim received his Ph.D. from Department of Materials Science and Engineering at Massachusetts Institute of Technology in 2016. He is a researcher at KIST, pursuing research in energy harvesting, ferroelectrics and electrochemistry. Soo Deok Han received his BSc degree in Department of Materials Science and Engineering of Korea University. Now, he is a Ph.D. candidate at KU-KIST Graduate School of Converging Science and Technology in Korea University and KIST. His research interests concentrate on the nanostructured functional metal oxides and their applications. Hi Gyu Moon is studying for his Ph.D. in the Department of Materials Science and Engineering of Yonsei University. His research interests include the fabrication of metal oxide gas sensors and their electrical characteristics. Myoung-Sub Noh received his BSc degree in Department of Materials Science and Engineering of Suwon University. He is a Combined MS-Ph.D candidate at KU-KIST Graduate School of 20

Converging Science and Technology in Korea University. His doctoral research focuses on development of the flexible piezoelectric thin films using laser lift-off(LLO) and excimer laser annealing(ELA) methods. Kwangjae Lee received the B.S. degree in electronics engineering from Kookmin University, Seoul, Korea, in 2004. He received the unified course of master and doctoral degree at the Electronics and Computer Engineering, Korea University, Seoul, Korea, in 2014. Since 2012, he has been with Communication & Media R&D Division, Korea Electronics Technology Institute, Seoul, Korea. He has worked as an Engineer for application program, human–device interface, image compression, and processing. His research interests include human–device interface, lowpower digital circuit designs, and image signal processing. Hae Ryong Lee received his Ph.D. from the Department of Computer Engineering of Chungnam National University in 2005. Now he is a Principal Researcher in ETRI from 1993. His current research interests are twofold: pattern recognition techniques applied to electronic noses and usability of multimodal human-computer interfaces. Jin-Sang Kim received his Ph.D. from the Department of Materials Science and Engineering of Seoul National University in 1997. Now he is a principal research scientist in KIST. His research interests include synthesis of electronic materials and their applications into sensors and thermoelectric devices. Byeong-Kwon Ju received the B.S. and M.S. in Department of Electronic Engineering from University of Seoul, Seoul, Republic of Korea, in 1986 and 1988, respectively. He received the Ph.D. in Department of Electronic Engineering from Korea University, Seoul, Republic of Korea, 21

in 1995. In 2005, he joined Korea University, where he is currently pursuing developments of nanotechnologies in order to achieve various advanced electronics. Chong-Yun Kang received his Ph.D. from the Department of Electrical Engineering of Yonsei University in 2000. Now he is a Principal Research Scientist in KIST from 2000 and a professor of KU-KIST Graduate School of Converging Science and Technology in Korea University from 2012. His research interests include smart materials and devices, expecially, piezoelectric energy harvesting and actuators, electrocaloric effect materials, and nanostructured oxide semiconductor gas sensors.

22

Figure captions Fig. 1. Designs of Pt IDEs with sensing area for miniaturized gas sensors: (a) 1 mm x 1 mm, (b) 0.5 mm x 0.5 mm, (c) 0.3 mm x 0.3 mm, and (d) 0.1 mm x 0.1 mm.

Fig. 2. Schematic diagrams of the porous In2O3 nanocolumns on the SiO2/Si substrate with Pt IDEs utilizing glancing angle deposition using electron beam evaporator: (a) initial state and (b) growth state of nanocolumns. (c) 78o, (d) 82o, and (e) 85o incident angle.

Fig. 3. Plain-view SEM micrographs of In2O3 nanocolumns (a) 78º, (b) 82º, and (c) 85º tilt glancing angle deposition. XRD analysis of In2O3 nanocolumns (d) thin film, (e) 78o, (f) 82o, (g) 85o, and (h) Pt/Ti IDE on SiO2/Si.

Fig. 4. Response curves of In2O3 thin film and nanocolumns as function of incident angle and sensing area to 50 ppm C2H5OH at 300ºC.

Fig. 5. Linear plot curves of base resistance as a function of incident angle: (a) In2O3 thin film, (b) In2O3 78o nanocolumns, (c) In2O3 82o nanocolumns, and (d) In2O3 85o nanocolumns. The calculated resistance ratios are 1, 4.6, 16.8, and 486 as a function of sensing area: 1 mm x 1 mm, 05 mm x 0.5 mm, 0.3 mm x 0.3 mm, and 0.1 mm x 0.1 mm, respectively. 23

Fig. 6. Schematic representation of (a) thin film and (b) nanocolumn sensing layer with energy bands. Schematic representation of thin film and porous nanocolumns sensing layers with geometry and energetic bands, which shows the influence of electrode-sensing layers contacts. Rc resistance of the electrode-In2O3 contact, Rl1 resistance of the depletion of the thin film layer, R1 equivalent series resistance of Rl1 and Rc, equivalent series resistance of and Rc, Rgi average intergrain resistance in the case of porous nanocolumns layer, Eb minimum of the conduction band in the bulk, qVs band bending associated with surface phenomena on the layer, and qVc also contains the band bending induced at the electrod-In2O3 contact.

Fig. 7. (a) Gas response of the sensor based on In2O3 nanocolumns (85o) deposited on 0.3 mm x 0.3 mm sensing area for 50 ppm VOCs, NH3, CO, and NO2 under operating temperature of 300ºC, (b) Bar chart showing the gas response of In2O3 nanocolumns for 50 ppm VOCs gases under operating temperature of 300ºC.

Fig. 8. (a) Schematic illustration for the current path of the In2O3 nanocolumns on Pt IDEs. Schematic illustration for the enhanced double schottky barrier between (b) 78o and (c) 85o incident angle. Note that the double schottky barrier is improved by increasing the incident angle from 78o to 85o at the glancing angle deposition.

24

Fig. 1

25

Fig. 2

26

Fig. 3

27

(b)

104

1 mm x 1 mm 0.5 mm x 0.5 mm 0.3 mm x 0.3 mm 0.1 mm x 0.1 mm

In2O3 thin film

103

Resistance (Ω)

Resistance (Ω)

(a)

6.22 4.63 102

101

1.49 1.13 0

500

1000

1500

108

106 105 27.95

104

17.91 7.06 7.10

103 102 101

2000

In2O3 nanocolumns (78o)

107

0

500

In2O3 nanocolumns

107

(82o)

Resistance (Ω)

Resistance (Ω)

(d)

108

106 46.12

105

49.70 60.71

104 103

4.59

102 101

0

500

1000

1500

2000

Time (s)

Time (s)

(c)

1000

1500

108

Time (s)

2066

106

1301 1194 877

105 104 103 102 101

2000

In2O3 nanocolumns (85o)

107

0

500

1000

Time (s)

Fig. 4

28

1500

2000

(b)

1000 800

Thin films

Resistance (Ω)

Resistance (Ω)

(a)

R-Square = 0.81281

600 400

400

300 200

200

100 0

0

78o nanocolumns

2x104

2000

1x104

5

10

15

500 0

0

100 200 300 400 500

Resistance (Ω)

Resistance (Ω)

82o nanocolumns R-Square = 0.99838 2.0x104 1.5x104 1.0x104 5.0x103

0

0 0

0

5

10

15

5

10

15

20

100 200 300 400 500

Calculated resistance ratios

(d)

1x106

0

20

3x106 2x106

1500 1000

Calculated resistance ratios

(c)

R-Square = 0.99917

0

0

0

3x104

4x107 85o nanocolumns

3x107

R-Square = 0.99994

2x107

1.5x106 1.0x106

1x107 5.0x105

0

20

100 200 300 400 500

0 0

0

5

10

15

20

100 200 300 400 500

Calculated resistance ratios

Calculated resistance ratios

Fig. 5

29

Fig. 6

30

500

1000

1500

610

400

Time (s)

Fig. 7

31

3 0.28 22 5.03 18 166

320

600

150 100 50 0

Response time (s)

200

0

2000

335

800

200

0 0

1313

250

7

300

300

1000

3

600

1200

350 Response Response time

623

900

1400

7

Response

1200

C2H5OH C6H6 CH3COCH3 C7H8 HCOH NH3 CO NO2

7

1 V, 300ºC, 50 ppm

1130

(b)

1500

Response

(a)

Fig. 8

32