Sensitive Field-effect Transistor Sensors with Atomically Thin Black Phosphorus Nanosheets

Atomically thin phosphorene field-effect transistors (PFETs) have excellent potential for sensing applications. However, commercial scaling of PFET sensors is still in early stage due to various technical challenges, such as tedious fabrication, low response% caused by rapid oxidation, non-ideal response output (spike/bidirectional), and large device variation due to poor control over layer thickness among devices. Attempts have been made to address these issues. First, a theoretical model for response% dependence upon number of layers is developed to show the role of atomically thin phosphorene for better responses. A position-tracked, selected-area-exfoliation method has been developed to rapidly produce thin phosphorene layers with a narrow distribution (~1–7 layers), which can harness the excellent gate control over PFET channel. The typical current on/off ratio and sub-threshold swing (SS) are in the range of ~300–500 and 110–150 mV/dec, respectively. The cysteine-modified Al2O3-gated PFET sensors show high responses (~ 30–900%) toward a wide detection range (~1–400 ppb) of lead ions in water with typical response time of ~10–30 s. A strategy to minimize device variation is proposed by correlating PFETs’ on/off ratio with sensitivity parameters. Thickness variation of gate oxide is investigated to explain non-ideal and ideal response transient kinetics.