Structural Asymmetry and Chiral-Induced Spin Selectivity in Chiral Palladium-Halide Semiconductors.

Chiral Pb-free metal-halide semiconductors (MHSs) have attracted considerable attention in the field of spintronics due to various interesting spin-related properties and chiral-induced spin selectivity (CISS) effect. Despite their excellent chemical and structural tunability, the material scope and crystal structure of Pb-free chiral MHSs exhibiting the CISS effect are still limited; chiral MHSs that have metal-halide structures of octahedra and tetrahedra are only reported. Here, we report a new class of chiral MHSs, of which palladium (Pd)-halides are formed in 1D square-pyramidal structures or 0D square-planar structures, with a general formula of (( R / S -MBA) 2 PdBr 4 ) 1- x (( R / S -MBA) 2 PdCl 4 ) x (MBA = methylbenzylammonium; x = 0, 0.25, 0.5, 0.75, and 1) for the first time. The crystals adopt the 1D helical chain of Pd-halide square-pyramid (for x = 0, 0.25, 0.5, and 0.75) and 0D structure of Pd-halide square-plane (for x = 1). All the Pd-halides are distorted by the interaction between the halide and the chiral organic ammonium and arranged in a noncentrosymmetric position. Circular dichroism (CD) for (( R / S -MBA) 2 PdBr 4 ) 1- x (( R / S -MBA) 2 PdCl 4 ) x indicates that chirality was transferred from chiral organic ammonium to Pd-halide inorganics. (( R -MBA) 2 PdBr 4 ) 1- x (( R -MBA) 2 PdCl 4 ) x ( x = 0, 0.25, 0.5, and 0.75) shows a distortion index of 0.127-0.128, which is the highest value among the previously reported chiral MHSs to the best of our knowledge. We also find that ( R / S -MBA) 2 Pd(Br 1- x Cl x ) 4 crystals grow along the out-of-plane direction during spin coating and have high c -axis orientation and crystallinity, and ( R / S -MBA) 2 Pd(Br 1- x Cl x ) 4 ( x = 0 and 0.5) crystals exhibit a CISS effect in polycrystalline bulk films. These results demonstrate the possibility of a new metal-halide series with square-planar structures or square-pyramidal structures for future spintronic applications.