Effect of Al2O3 Passive Layer on Stability and Doping of MoS2 Field-Effect Transistor (FET) Biosensors.

Molybdenum disulfide (MoS2) features a band gap of 1.3 eV (indirect) to 1.9 eV (direct). This tunable band gap renders MoS2 a suitable conducting channel for field-effect transistors (FETs). In addition, the highly sensitive surface potential in MoS2 layers allows the feasibility of FET applications in biosensors, where direct immobilization and detection of biological molecules are conducted in wet conditions. In this work, we report, for the first time, the degradation of chemical vapor deposition (CVD) grown MoS2 FET-based sensors in the presence of phosphate buffer and water, which caused false positive response in detection. We conclude the degradation was originated by physical delamination of MoS2 thin films from the SiO2 substrate. The problem was alleviated by coating the sensors with a 30 nm thick aluminum oxide (Al2O3) layer using atomic layer deposition technique (ALD). This passive oxide thin film not only acted as a protecting layer against the device degradation but also induced a strong n-doping onto MoS2, which permitted a facile method of detection in MoS2 FET-based sensors using a low-power mode chemiresistive I-V measurement at zero gate voltage (Vgate = 0 V). Additionally, the oxide layer provided available sites for facile functionalization with bioreceptors. As immunoreaction plays a key role in clinical diagnosis and environmental analysis, our work presented a promising application using such enhanced Al2O3-coated MoS2 chemiresistive biosensors for detection of HIgG with high sensitivity and selectivity. The biosensor was successfully applied to detect HIgG in artificial urine, a complex matrix containing organics and salts.