Size effect on optical and vibrational properties of van der Waals layered In4/3P2S6

Metal thiophosphates have recently received increasing research interest, thanks to their diversity in regard to metal cation occupation within the P–S scaffold, which produces a broad array of functional properties. Among them, In4/3P2S6 belongs to a special case as regards the ordering of metal cations and vacancy sites, whose physical properties remain largely unexplored, especially in the atomically thin limit. In this work, we undertake a comprehensive investigation on the thickness-dependent optical and vibrational properties of In4/3P2S6 from the bulk scale down to a few-layer flakes. Both optical birefringence and anisotropic photoelectric response are observed, which are closely linked to the monoclinic symmetry of the crystal lattice. The vanishing of Raman, absorption, and photoluminescence intensities at about three to four layers signifies the enhanced structural disorder and lattice incoherence when approaching the monolayer limit. Before reaching this critical thickness, phonon softening due to weakened interlayer coupling and bandgap widening due to quantum confinement are accompanied by thickness reduction. These findings shed light on a general understanding of the finite size effect on the physical properties of metal thiophosphates and deliver possible device applications by exploiting the property of optical anisotropy.