Phosphorene Modified Zinc Oxide for Extended-Gate Field-Effect Transistor pH Sensors
Monitoring pH levels is essential in a wide range of applications, including biomedical diagnostics, environmental monitoring, and industrial processing. One of the most efficient and scalable approaches for pH sensing is the use of transistor-based devices. Among these, the ISFET (Ion-Sensitive Field-Effect Transistor) and its modified version, the EGFET, have received significant attention. The EGFET configuration separates the sensing membrane from the transistor body, allowing for greater flexibility in sensor design and enhancing device robustness and durability.
In this study, zinc oxide, a wide bandgap semiconductor with high electron mobility (bandgap ~3.37 eV), was employed as the primary sensing material. Zinc Oxide is chemically stable, non-toxic, and also compatible with low-temperature processing, making it particularly suitable for biomedical and portable sensing applications. Zinc Oxide thin films were prepared using a sol-gel method followed by spin-coating onto cleaned fluorine-doped tin oxide (FTO) glass substrates.
Each coating was baked and repeated eight times before final annealing at 520°C to ensure film uniformity and crystallinity. Phosphorene (PP), a two-dimensional nanomaterial derived from black phosphorus, was introduced to enhance the sensing performance. Phosphorene offers excellent electronic properties, including a tunable direct bandgap (ranging from 0.3 to 2.0 eV), high carrier mobility, large surface area, and mechanical strength, which contribute to improved charge transfer and surface reaction efficiency.
Phosphorene nanosheets were synthesized using the Liquid-Phase Exfoliation (LPE ) method by ultrasonically dispersing black phosphorus powder in N-methyl-2-pyrrolidone (NMP). After centrifugation to remove unexfoliated particles, the purified nanosheets were redispersed in ethanol and mixed with the zinc oxide solution for film fabrication. The completed zinc oxide and phosphorene-modified zinc oxide (PP/ZnO) films were connected to a commercial Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) in an extended-gate configuration.
The sensor’s performance was evaluated using a semiconductor parameter analyzer for electrical characterization. Electrochemical Impedance Spectroscopy (EIS) was used to probe charge transfer characteristics.
Structural and morphological analyses confirmed the successful integration of phosphorene. Field Emission Scanning Electron Microscopy (FE-SEM) showed that phosphorene had a flake-like morphology. Raman spectroscopy exhibited characteristic peaks of black phosphorus, which showed slight shifts due to interlayer interactions, validating the exfoliation process. Surface analysis indicated that the addition of phosphorene increased the surface roughness and specific surface area of the zinc oxide film, thereby enhancing its reactivity in sensing applications.
Electrically, the unmodified zinc oxide EGFET sensor showed a sensitivity of 51.0 mV/pH with strong linearity. After modification with phosphorene, the sensor exhibited improved sensitivity of 62.5 mV/pH, surpassing the theoretical Nernst limit. This enhancement is credited to the synergistic effect of zinc oxide’s semiconducting properties and phosphorene’s superior conductivity and surface characteristics.
The PP/ZnO sensor demonstrated improved stability. Hysteresis voltage was reduced to 10 mV (forward) and 5 mV (reverse), while the drift rate dropped to 0.714 mV/h over 12 hours. EIS measurements revealed a significant reduction in charge transfer resistance from 80.33 Ω to 35.96 Ω, indicating enhanced electron mobility.
The integration of phosphorene into zinc oxide-based EGFET pH sensors resulted in notable improvements in sensitivity, stability, and electrical performance, highlighting its future potential for advanced pH sensing in biomedical and environmental applications.