In situ fabrication of self-filtered near-infrared Ti3C2Tx/n-Si Schottky-barrier photodiodes for a continuous non-invasive photoplethysmographic system

Two-dimensional (2D) MXenes have emerged as promising candidates to serve as Schottky contact electrodes for the development of high-performance photodiodes owing to their extraordinary electronic properties. However, it remains a formidable challenge to fabricate a large-area, uniform MXene layer for practical device application. Here, we develop a facile route to produce a large-area Ti3C2Tx layer by post-etching treatment of a pulsed laser-deposited Ti3AlC2 film, enabling the in situ construction of a back-illuminated Ti3C2Tx/n-Si Schottky-barrier photodiode. Significantly, the device exhibits excellent performance with a distinctive self-filtered near-infrared (NIR) photoresponse behavior in the range of 700-1100 nm. By avoiding disturbances caused by ambient light, the NIR photodiode-based transmission-type photoplethysmographic (PPG) measurement system is capable of more reliable detection of PPG waveforms than the commercial PPG sensors for continuously monitoring heart rate. This enables the accurate extraction of blood pressures using a PPG-only method. Our findings not only pave the way for fabrication of a high-quality large-area 2D MXene layer, but also provide a general design principle for developing high-performance MXene/Si photodiodes for health monitoring systems.