Electrostatically Controlled Pyrophototronic Effect Enabled Accident Alert System using a Strain-Polarized WS2 Phototransistor

Abstract: Event-based dynamic light detection, specifically in low illumination power environments, is a critical requirement in autonomous vehicles. This work reports low optical power photodetection through the dynamic pyrophototronic effect in an ultra-thin 2D WS2 phototransistor. A four-stage pyrophototronic photoresponse has been realized through biaxial strain-polarization of the non-centrosymmetric (5-layer) WS2 channel using a sub-wavelength, nanopatterned hBN gate dielectric. Presence of strain in WS2 has been verified through extensive spectroscopic characterization and that of strain-induced charge polarization through density functional theory calculations as well as piezo force microscopy. The pyrophototronic effect boosts dynamic photoresponsivity (0.7 A/W) and detectivity (1.2x10^15 Jones cm^-1) by up to 8x and enhances the photodetection speed by 3x over the non-patterned (unstrained) phototransistor, demonstrating a path to ameliorating the responsivity-speed trade-off in 2D photodetectors. Analysis of gate voltage, wavelength, and optical power dependence of the pyrophototronic current through measurements and band physics highlights its prominence under low channel population of electrostatically- or optically-induced free carriers. Gate tunability of the pyrophototronic current has been leveraged to design an optical spike-triggered dynamic accident alert system with speed-specific control for self-driving applications under low light conditions.