Narrowing the optical gap of CdPS3 single crystal via chemical intercalation using liquid ammonia method

Abstract

The transition metal phosphorus trichalcogenides are fascinating two dimensional (2D) materials with expansive potential in optoelectronic devices, catalysis and energy storage. The van der Waals gap in structure gives a permit for rich intercalation chemistry, which could induce novel properties. In this work, we report the chemical intercalation in CdPS3 single crystals utilizing liquid ammonia method. It is demonstrated that the co-intercalation of alkali metal (Li or K) and NH3 molecule occurs during the intercalation reaction, leading to significantly enlarged layer distances (9.347–9.438 Å). The oxidation state of Cd in intercalated phases is clearly deceased compared to the parent CdPS3, which is ascribed to the charge transfer from the s orbitals of alkali metals to the 3d orbitals of Cd during intercalation. The charge transfer also largely narrows the optical gap of CdPS3 from 3.32 eV to 1.50 eV in K-intercalated phase, inducing visible-light response in this layered material. It is interesting to note that for the first time we report the successful chemical intercalation in transition metal phosphorus trichalcogenides with the single crystal form, instead of the polycrystalline sample. The intercalated products obtained by liquid ammonia method are still in single crystal form, which will facilitate the further mechanical or chemical exfoliation to synthesize new 2D materials.