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Interdigitated electrodes (IDEs) were fabricated using Pt/Ti (150/30 nm), on a 4-inch SiO₂/Si substrate through photolithography. The distances between each electrode were 5 μm and 20 electrodes were placed in an area measuring 1 mm × 1 mm. The Pt-IDEs patterned substrates were cleaned with acetone, ethanol, and deionized water, after which they were dried under a flow of nitrogen gas.

Subsequently, electron beam evaporation was performed to deposit the SnO₂ and In₂O₃ nanorods at a glancing angle of 80o at a rotation speed of 15 rpm in off-axis mode. Shadow masks were used to deposit exclusively onto the area containing the electrodes, and the substrate was placed 30 cm from the crucible. The base pressure and growth rates were 5 × 10⁻⁶ mTorr and 1 Å s⁻¹, respectively. During the nuclei growth, the self-shadowing effect occurred depending on the incident angle of vapor flux. Next, the Pt, Au, and Pd catalysts were deposited by performing evaporation at a glancing angle of 15o with a rotation speed of 15 rpm in offaxis mode. The base pressure and growth rates were 5 × 10⁻⁶ mTorr and 0.1 Å s⁻¹, respectively. All the fabricated specimens were annealed at 550°C for 2 h in ambient air after the process of deposition.

The morphology of the fabricated SnO₂ and In₂O₃ nanorods was observed using a field-emission scanning electron microscope (SEM) (Inspect F50) with an acceleration voltage of 15 kV and a working distance of 10 mm.

The gas sensing properties were measured by external heating of the quartz tube in a box furnace. The gas flow rate was maintained at 1000 sccm using mass flow controllers, and the gas flow was varied from dry air to a calibrated target gas. The resistances were measured at a DC bias voltage of 1 V using a source measurement unit (Keithley 2401), and all the measurements were recorded on a computer using LabVIEW with general purpose interface bus (GPIB).

The sensing materials, SnO₂ and In₂O₃, are deposited directly on the PT IDE using the e-beam evaporator GAD method (Fig. 1(a)). At this time, it is possible to form well-aligned nanorods without a separate process due to the shadowing effect generated by the high tilt angle. The shadowing effect must be taken into consideration when evaporation equipment, such as e-beam evaporator with excellent straightness, is used, and the deposition process occurs at a high angle. It is deposited in the form of inclined nanorods, which can form straight nanorods through substrate rotation. Next, the low-angle GAD method can also be used for Pt, Au, and Pd catalyst deposition (Fig. 1(d)). While using the existing on-axis method, the catalyst cannot be dispersed on the entire surface of the straight nanorods and can be concentrated only on the upper part of the nanorods. This problem can be solved by dispersing the catalyst from a low angle off-axis to the inside of the nanorods.

Fig. 1. 
Schematic of (a) the fabrication procedures using GAD method via electron beam evaporation, (b) Nanorods deposited on Pt interdigitated electrode (IDE), and (c) catalyst decorated on nanorods using off-axis mode, (d) overall array gas sensor.

Figure 2 illustrates the top and cross-sectional SEM images of the well-aligned SnO₂ and In₂O₃ nanorods according to the fabrication procedure. All samples deposited with the same length of 300 nm and the surface changes depending on the catalyst can be confirmed through the top image.

Fig. 2. 
Top-view SEM images of (a) Pt or (b) Au-decorated SnO₂ nanorod and (d) Au or (e) Pd-decorated In₂O₃. Cross-sectional SEM images of (c) SnO₂ nanorod and (f) In₂O₃ nanorod.

The gas-sensing properties of individual samples were investigated by exposing all samples to 1 ppm of CH₃COCH₃ at 300°C, C₇H₈ or NO₂ at 200°C (Fig. 3(a)-(c)). The response is defined as (R_g/R_a − 1), where R_a and R_g denote the resistance of the sensors in the absence and presence of the target gas, respectively. Each sample exhibited different sensing properties depending on the decorated catalyst. In particular, Au-decorated SnO₂ produced a response of 8.9 for 1 ppm CH₃COCH₃, Audecorated In₂O₃ displayed a response of 2.7 for 1 ppm C₇H₈, and a Pd-decorated In₂O₃ displayed a sensitivity of 178 for 1 ppm NO₂.

Fig. 3. 
(a–c) Real time response to 1 ppm CH₃COCH₃, C₇H₈ and NO₂. (d–f) Sensor response as a function of gas concentration and theoretical detection of limits at 300°C or 200°C in dry atmosphere.

In addition, the gas detection characteristics were measured for various concentrations from 0.2–1 ppm (Fig. 3(d)-(f)) to measure the linearity and long-term stability of the sensor for each gas concentration. Through this, we confirmed through calculation that Au-decorated SnO₂, Au-decorated In₂O₃, and Pd-decorated In₂O₃ have detection limits of 781 ppt, 1.33 ppb, and 11.4 ppb respectively for CH₃COCH₃, C₇H₈, and NO₂.

Humidity and measurement temperature are the factors that significantly influence the sensing characteristics of a sensor. To investigate the gas detection characteristics based on the moisture and temperature, we measured 5 ppm CH₃COCH₃, C₇H₈, and NO₂, at various temperatures ranging from 150-350 °C at 40% relative humidity (Fig. 4). It was confirmed that the sensor sensitivity had high selectivity for a specific temperature and gas, and the sensitivity was slightly reduced compared to that in a dry environment, but it still had high selectivity and distinguishable sensitivity.

Fig. 4. 
Base resistance transients and response plot of SnO₂ nanorod and In₂O₃ nanorod to 5 ppm CH₃COCH₃, C₇H₈ and NO₂ as a function of operating temperature from 150-350°C in 40% relative humidity condition.

Oxygen adsorbed on the surface of the oxide semiconductor from the surrounding air forms a depletion layer and increases the resistance (Fig. 5(a)). When a catalyst is used, the catalyst is distributed over the entire surface of the semiconductor to move the electrons from the semiconductor to the catalyst, thereby promoting the formation of a depletion layer of the semiconductor and further increasing the resistance (Fig. 5(b)). At this time, oxygen species are adsorbed on the metal oxide surface in the form of O₂^-, O^-, or O^2- depending on the temperature. Afterwards, the reducing gases, CH₃COCH₃ and C₇H₈, react with oxygen on the surface of the semiconductor and cause the following changes:

This leads to desorbing oxygen on the surface of the semiconductor and reducing the resistance (Fig. 5(c)). In contrast, in the process wherein NO₂ is adsorbed on the semiconductor surface in the same way as oxygen, electrons are extracted from the semiconductor surface, forming a depletion layer and increasing the resistance (Fig. 5(d)). In the case of NO₂, the adsorbed oxygen has a higher reactivity at low temperatures because the adsorbed oxygen limits the space in which NO₂ is adsorbed.