Contemporary Challenges in van der Waals 2D Semiconductors

van der Waals (vdW) layered semiconductors have emerged as a unique class of quantum materials distinguished from their bulk counterparts by reduced dielectric screening, strong Coulomb interactions, large exciton binding energies, strong spin-orbit coupling, and pronounced thickness-dependent band structures. These fundamental attributes have enabled the exploration of exotic many-body physics and a broad spectrum of device applications, ranging from field-effect transistors and ferroelectric switches to optoelectronics, magnetic semiconductors, neuromorphic computing, and energy harvesting systems. Despite remarkable advances, critical challenges remain in the controlled synthesis of high-quality crystals, formation of low-resistance contacts, integration of stable and scalable gate dielectrics, and reliable device performance at the wafer scale. In this mega-review, we provide a comprehensive overview of contemporary challenges and future opportunities in vdW-layered semiconductors, structured across nine themes: growth and heterostructures of transition metal dichalcogenides, Ohmic contacts, emerging gate dielectrics, high-performance low-power field-effect transistors (FETs), diluted magnetic semiconductors, plasmonics and exciton propagation, hot-carrier solar cells, bioinspired neuromorphic computing, and electrocatalytic/photocatalytic energy conversion. By consolidating fundamental insights and device-level perspectives, this review aims to chart a roadmap for advancing vdW semiconductors from laboratory-scale discoveries to transformative technologies in electronics, optoelectronics, spintronics, and sustainable energy systems.