Emerging two-dimensional metal thio(seleno)phosphates crystals: Challenges and opportunities

Two-dimensional (2D) metal thio(seleno)phosphates (MTPs) have recently emerged as a versatile and rapidly expanding family of functional materials that bridge the gap between conventional semiconductors and multifunctional quantum materials. Compared with mainstream 2D transition metal dichalcogenides (TMDCs), MTPs exhibit substantially richer chemical compositions and lower structural symmetry, enabling access to emergent functionalities that are difficult to realize in binary layered systems. Distinguished by their compositional diversity and structural tunability, MTPs encompass a wide range of ternary and quaternary compositions with rich coordination environments and stacking motifs, giving rise to flexible control over symmetry breaking and interlayer coupling. The structural diversity of MTPs engenders a broad spectrum of intertwined order parameters, including giant nonlinear optical responses, robust spontaneous polarization, ion–electron coupled states, tunable charge transport, and intrinsic photoelectric effects, rendering MTPs promising candidates for 2D integrated optoelectronic circuits and multifunctional heterostructure platforms. This Review provides a comprehensive overview of the crystal structures, electrical and optical properties, and device applications of 2D MTPs. Emphasis is placed on their structure and property relationships, mechanisms of ferroelectric and ionic dynamics, and the synergistic coupling between polarization, charge, and light. The challenges and opportunities lie ahead ranging from scalable synthesis and domain engineering to device integration and theoretical modeling toward the realization of MTPs based multifunctional technologies in optoelectronic, energy, and neuromorphic systems.