Analysis of spectral and temporal signatures of phototransients in iron oxide nanorods: implications on IoT/ IIoT sensor network

Abstract Modern Internet of Things (IoT) and Intelligent Internet of Things (IIoT) may involve integrating optical photodetector-based sensing for their operation in future. In this work, iron oxide nanorods (IONRs) have been synthesized that exhibit photosensing in a broad (UV to visible) range discusses phototransients that can act as spurious signals. Results show that the nature of transients is wavelength dependent with distinct dynamics: visible (red and green) light yields faster response than UV, while green produces periodic modulations. A 5–10 s delay between UV and visible response exists. While UV photoresponse shows slower growth, visible light response shows a faster but decay-like behavior. The theoretical modeling of the photocurrent’s decay curve yields two different lifetimes (∼ ms and s) suggesting two different mechanisms. While visible photons can transport electrons via surface hydroxyl groups or surface states, UV photons facilitate ionic conduction through inverse spinel lattice of IONRs and green photons show fluctuating defect-mediated response. The electrochemical impedance spectroscopy (EIS) is used to validate the above results. EIS, Nyquist and Bode plots confirm different charge carrier types, transport behavior and time domains. The EIS study strengthens the mechanism that electron conduction via surface hydroxyl groups contributes to faster conduction process and the ionic movement (Fe +2/+3 ) within the inverse spinel lattice is slower of the two processes. The 5–10 s delay in UV/visible light irradiation points to a critical vulnerability-window which can compromise the sensor reliability by missing a red/ green trigger or misinterpreting a UV trigger. Thus, in-operando transient analysis is crucial for evaluating material response and designing robust, wavelength-selective photodetectors for emerging IoT and IIoT technologies.