Two-dimensional indium selenide (InSe): fundamentals, synthesis strategies, and next- generation optoelectronic device architecture

Two-dimensional (2D) layered semiconductors have garnered significant interest as promising candidates for next-generation optoelectronic applications, owing to their tunable electronic band structures, high surface-to-volume ratios, and pronounced light-matter interactions. Among this growing family of materials, van der Waals indium selenide (InSe) has emerged as a particularly attractive candidate for future high-mobility optoelectronic devices, such as high-performance photodetectors, polarisation-sensitive devices, and chemical sensors. This is primarily due to its thickness-dependent bandgap, high intrinsic carrier mobility, and strong optical absorption. Compared with other extensively investigated 2D materials, InSe combines a comparatively high electron mobility with a layer-tunable band gap and spectral response extending from the visible to near-infrared region, whereas graphene lacks a band gap and MoS 2 shows weak near-infrared absorption. Its practical advantage, however, depends on encapsulation, contact quality, and phase control. This review provides a comprehensive and critical assessment of recent advances in InSe-based photodetectors, with particular emphasis on their crystal structure, electronic properties, synthesis methodologies, and device engineering strategies. Various fabrication techniques, including mechanical exfoliation, liquid-phase exfoliation, chemical vapor deposition, and physical vapor deposition routes (physical vapor transport, pulsed laser deposition, molecular beam epitaxy, and thermal evaporation) are systematically discussed in relation to their impact on material quality and device performance, together with the transfer and device fabrication steps that link the material to the photodetector. Furthermore, the fundamental photodetection mechanisms, including photoconductive, photovoltaic, and photogating effects, are analyzed in detail to elucidate charge generation and transport processes. Recent developments in device architectures, particularly heterostructure-based photodetectors, are also highlighted to demonstrate performance-enhancement strategies. Moreover, key challenges, such as environmental instability and the need for scalable synthesis approaches, are critically examined. Future research directions aimed at achieving high-performance, stable, and commercially viable InSe-based optoelectronic devices are also outlined.