Operando Raman spectroscopic evidence of electron–phonon interactions in NiO/TiO2 pn junction photodetectors

The pn junctions significantly affect the responsivity of photodetectors (PDs). However, the enhancement mechanism of the pn junction is still unclear. Herein, operando Raman spectroscopy was employed to study PDs with NiO/TiO2 pn junctions composed of p-NiO nanoparticles (NPs) and n-TiO2 nanotube arrays (TNAs). The results suggest that the built-in potential field of the NiO/TiO2 interface decreases the charge transfer resistance and changes the vibrational frequency of the phonon modes of TiO2, which is attributed to the electron–phonon coupling effect. Operando Raman spectroscopy is proved to be a powerful tool for manufacturing highly responsive PDs.