| AVS 61st International Symposium & Exhibition | |
| 2D Materials Focus Topic | Tuesday Sessions |
| Session 2D+AS+BI+PS+SS-TuM |
| Session: | 2D Materials: Surface Chemistry, Functionalization, Bio and Sensor Applications |
| Presenter: | MdAhsan Uddin, University of South Carolina |
| Authors: | M.A. Uddin, University of South Carolina A. Singh, University of South Carolina T. Sudarshan, University of South Carolina M.V.S. Chandrashekhar, University of South Carolina G. Koley, University of South Carolina |
| Correspondent: | Click to Email |
Graphene/Semiconductor Schottky devices attracted significant research attention due to wide range of applications from transistor to IR detector [1-2]. Such heterojunctions are also promising for sensing applications due to the molecular adsorption induced Schottky barrier height (SBH) change at the interface, affecting the junction current exponentially in reverse bias, which leads to ultrahigh sensitivity. Graphene/p-Si diode sensor [Device image, Raman spectra and I-V characteristics shown in fig. 1(a), (b) and (c)] has been developed with high bias-dependent sensitivity and low operating power.
Performance enhancement has been demonstrated by fabricating graphene chemiresistor and diode sensor on the same chip. The diode sensor exhibited 13 times higher sensitivity for NO2 [Fig. 2(a)] and 3 times higher for NH3 [Fig. 2(b)] in ambient condition, while consuming ~500 times less power for same applied voltage. Sensing tunability is achieved by operating the device in reverse bias, tuning the graphene work function and hence the SBH by the applied bias. The sensitivity varied from 268 to 574% for NO2 as the bias magnitude varied from -1 to -8V [Fig. 3(a)]. Optimized sensor design to detect particular analyte is also possible by careful selection of graphene/Si heterojunction SBH. For example, graphene/p-Si with larger SBH is better NO2 sensor while smaller SBH device has better NH3 sensitivity. The sensing mechanism based on SBH change has been confirmed by capacitance-voltage measurements [Fig. 3(b)]. The SBH decreased by 0.23eV for NO2 exposure while increased by 0.16eV for NH3. Variation in sensitivity with NO2 and NH3 concentration has also been demonstrated (Fig. 4).
Pd and Pt functionalization has been carried out to make the graphene/Si diode [Fig 5] sensitive to H2. Extrapolated SBH from the I-V characteristics, before and after few nm metal decoration, and H2 exposure showed initial SBH decrease after functionalization and subsequent increase in presence of H2, respectively [Fig. 6(a) and (b)]. Compared to graphene chemiresistor, the chemi-diode sensor offers more than one order of magnitude higher H2 sensitivity for both types of functionalization. Similarly, the reverse bias operation also enables low power consumption, tunable sensitivity and detection of H2 down to 1 ppm [Fig. 7(a)] in air which is close to the atmospheric background of 0.6 ppm [3]. Among the two metals, Pd-functionalization always exhibited better sensing response irrespective of the bias voltage [Fig. 7(b)]. Remarkably, for Pd-functionalization, the sensor response showed absolute exponential change with varying H2 concentration ranging from 2 to 1000 ppm [Fig. 7(c)].