Research Projects

The proposal addresses a critical technological gap identified in high-temperature photonics: the absence of a thermally resilient photodetector capable of sensing light across the visible to near-infrared (NIR) spectrum, while reliably operating at temperatures above 150 °C. The demand for in-situ, real-time optical detection in harsh thermal environments is rapidly growing in industrial process monitoring, aerospace propulsion diagnostics, geothermal energy, and space exploration. Conventional silicon or InGaAs photodetectors fail to operate reliably above 150 °C due to high dark currents and interconnect material degradation. Wide bandgap materials (h-BN, Ga₂O₃, GaN or SiN) based devices show thermal resilience, but operate only in the UV range. Alternatives like fiber-optic or sapphire window-guided detection, or thermal detectors (bolometers, pyroelectric sensors) are either inefficient or lack sensitivity and speed. To address these limitations, the project aims to fabricate a self-powered photodetector based on a hybrid p-n heterojunction formed by few-layer graphene (FLG) and superionic cuprous sulfide (Cu₂S) nanocrystals, enabling broadband detection from 400 – 980 nm across a broad temperature range (25 °C to 500 °C). The rationale behind the device architecture can be explained as follows: below 103.5 °C, bulk Cu₂S exist in a low-chalcocite (γ) phase. Upon heating above 104 °C, Cu₂S undergoes an abrupt, solid-solid phase transition to high-chalcocite (β), which exhibits superionic conductivity, where the Cu⁺ cations moves freely in the rigid S²⁻ anionic framework. A recent article (Nat. Commun.,10, 3285, 2019) showed that Cu₂S nanocrystals can exhibit room temperature superionic behavior, when the size is reduced to sub-7 nm or with excessive iodine-doping. We have performed DFT simulations on Cu₂S material system, which yielded several key insights: (A) At room temperature, partial deposition of a FLG channel with bulk Cu₂S, converts the coated part to p-type, while the uncoated region remains as n-type – naturally forming a p-n junction; (B) with suitable iodine-doping levels, Cu₂S nanocrystals can exhibit a stable superionic phase across 25 °C to 495 °C; (C) Deposition of iodine-doped Cu₂S nanocrystals on FLG, induces a p-type behavior via dynamic bound states between mobile Cu+ cations and FLG electrons, with this bound-state sustained over the full temperature range, 25 °C to 495 °C. Based on the DFT results, to explore the possibility of realizing a thermally-resilient, hybrid p-n photodetector, we fabricated proof-of-principle devices using commercially available bulk Cu₂S (particle size larger than 1 µm) and graphene-based conducting ink. In a typical configuration, the conducting channel was partially coated with bulk Cu₂S powder, and contacted using silver paste; the incident light was directed at the heterojunction region. The devices registered responsivity of ~ 0.95 μA/W at room-temperature (400 – 808 nm); with enhanced performance from 105 °C to 150 °C. However, erratic behaviour was observed between 80 – 104 °C, due to the absence of superionic conduction in bulk Cu₂S (details are provided in Part-B). Our preliminary results are highly promising and address a significant unmet need for broadband (vis-to-NIR) detection, over a wider temperature range. However, the challenge of the erratic photocurrent behaviour between 80 – 104 °C, remains unsolved. In the proposed project, we will harness the superionic conductivity of specifically engineered, iodine-doped Cu₂S nanocrystals across a broad temperature range (25 °C – 500 °C), to develop high-performance photodetectors operating from 400 – 980 nm. The culmination of this work will be a rugged, self-powered broadband photodetector for extreme environments—supporting India's strategic objectives in defense, energy, and space. The project directly aligns with the ANRF–ARG’s mission to promote applied research of national importance.