Centimeter-Scale Ultrathin Platinum Monosulfide Films with Large Negative Temperature Coefficient of Resistance for Deformable Visible-to-Mid-Infrared Photodetectors

Broadband photodetectors covering a spectrum range of visible-to-mid-infrared (Mid-IR) are widely utilized for a range of applications, such as chemical sensing and medical devices. As their physical form factors evolve, a variety of photoresponsive electronic materials have been explored to adapt their demanded mechanical deformability. Herein, we report on a chemical vapor deposition (CVD) growth of centimeter-sized ultrathin (i.e., sub 10 nm) platinum monosulfide (PtS) films and their integration onto flexible polyimide (PI) substrates. Flexible devices composed of ultrathin PtS/PIs exhibit notable photoresponsiveness at a wide range of illumination wavelengths, i.e. visible spectra of 405 nm-to-940 nm to the mid-IR range of 4.6 μm, which is accompanied by a significant mechanical bendability. Furthermore, they exhibit temperature-variant transport characteristics of a p-type semiconductor involving a thermal generation of charge carriers; i.e., a significant increase of current with increasing temperature, yielding a large negativity of −0.62% °C^-1 for the temperature coefficient of resistance (TCR). The underlying mechanism for this mid-IR photoresponsiveness is attributed to the bolometric effect-driven carrier excitations facilitated by the midgap states of PtS films, as clarified through ultraviolet photoelectron spectroscopy (UPS) characterizations. Additionally, by leveraging the mechanical deformation-invariant photoresponsiveness, we demonstrate PtS/PI phototransistors able to biaxially stretch under modulated illuminations and gating conditions. This study is believed to offer opportunities for ultrathin semiconductors toward emerging photoelectronic devices with unconventional functionalities and configurations.