Anion Order Triggered Janus Ferroelectricity in Chalcogenides.

2D Janus structures break the mirror symmetry, thereby leading to the emergence of novel properties. Anion-ordered Janus structures hold potential for developing large spontaneous polarization ferroelectrics. However, van der Waals disordered alloys are generally more thermodynamically stable; anion-ordered Janus structures can only be realized in certain monolayer transition metal dichalcogenides so far. The polarization in these materials cannot be reversed; therefore, the realization of anion order triggered ferroelectricity remains a challenge. Here, we report that S/Se ordering induces Janus ferroelectricity in SnS0.6Se0.4 nanobelts with a high Curie temperature of approximately 600 K. The second-harmonic generation of SnS0.6Se0.4 nanobelts is approximately 300 times more intense than that of SnS nanobelts. The switchable spontaneous polarization of SnS0.6Se0.4 nanobelts was confirmed by piezoelectric force microscopy, representing the first realization of anion-ordered Janus ferroelectricity. This anion ordering induces a structural phase transition from the non-ferroelectric Pnma to the ferroelectric phase Pm. Our work has realized a periodically anion-ordered Janus structure with ferroelectricity in van der Waals materials and provided a candidate material for novel non-volatile memory devices and optoelectronic applications.