## 4 Results
### Sensor Characterizations with Interface Electronics  
The fabricated sensor was first measured using a network analyzer. As predicted by COM modeling, it exhibited a center frequency of 222 MHz, low insertion loss (14 dB), and minimal parasitic conductance in the high-frequency range (Figure 9). Owing to the transferred sensing material, a slight downshift in the center frequency and an increase in insertion loss were observed. To preserve the accurate characteristics of our developed interface electronics, the insertion loss must remain below 18 dB. Figure 8: (**a**) Resistance variation of the temperature sensor as a function of externally applied temperature, (**b**) calibrated output of the temperature sensor, (**c**) response and recovery times of the temperature sensor resistance as a function of applied voltage to the microheater, (**d**) temperature of the microheater as a function of applied voltage, measured at the temperature sensor location and surface, and (**e**) heat distribution on the sensor surface induced by a 12.5 V DC voltage applied to the microheater, captured using an infrared camera. Two sensors with different sensing materials are fabricated and characterized to compare their properties as a function of the sensing material's volume (thickness). The first sensor is a heterostructure combining graphene and MoS\({}_{2}\) flakes grown by CVD, while the second sensor is a heterostructure combining graphene and MoS\({}_{2}\) film grown by RF magnetron sputtering. The sensors are placed in a chamber, where a 5 V DC voltage is applied to the oscillator, and the output frequency is stabilized for an extended period in an N\({}_{2}\) environment. Two operating temperatures are tested, room temperature (RT) and 90 \({}^{\circ}\)C. The maximum temperature is set to 90 \({}^{\circ}\)C to minimize changes in the piezoelectric properties of LiNbO\({}_{3}\) and to prevent degassing from the sensor. NO\({}_{2}\) gas is injected into the chamber through an MFC, while N\({}_{2}\) gas is used as the control (blank) gas. Figure 10 shows the sensor characteristics, including the graphene/MoS\({}_{2}\) flake sensing material, in response to varying NO\({}_{2}\) concentrations (2-5 ppm) at both RT and 90 \({}^{\circ}\)C. Despite the low NO\({}_{2}\) concentration (2 to 5 ppm), a substantial frequency change and fast response time are observed at 90 \({}^{\circ}\)C, owing to the combined effects of significant changes in the conductivity of the monolayered 2D material and the mass loading effect. As the temperature rises to 90 \({}^{\circ}\)C, both the frequency change and response time improve markedly. At RT, the frequency change for 1 ppm (sensitivity) was 31 Hz/ppm, while at 90 \({}^{\circ}\)C, the frequency change significantly increased to 578 Hz/ppm. Figure 10: (**a**) Response and recovery of 5 ppm NO\({}_{2}\) on graphene/MoS\({}_{2}\) flakes SAW sensor, (**b**) repeatability of 5 ppm NO\({}_{2}\) on SAW sensor, (**c**) response to 2-5 ppm NO\({}_{2}\) on SAW sensor, (**d**) calibration plot for different NO\({}_{2}\) concentrations on SAW sensor, and (**e**) selectivity of SAW sensor. Figure 9: The S\({}_{21}\) scattering parameter as a function of frequency, measured using a network analyzer. The evaluated sensor's response time to reach 90% of the saturation value at 90 \({}^{\circ}\)C for 5 ppm was 2.5 s. These characteristics represent the highest performance reported to date among SAW-based sensors (see Table 3), to the best of my knowledge. While NO\({}_{2}\) sensors typically exhibit long recovery times, the recovery time in this study to reach 10% of the saturation value at 90 \({}^{\circ}\)C for 5 ppm was measured at 8 s. This rapid recovery is attributed to the minimized defects in the NO\({}_{2}\) sensing material, the weak interaction between the sensing material and the NO\({}_{2}\) gas, and the increased desorption energy provided by the microheater.