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 SnS_0.6Se_0.4 nanobelts with a high Curie temperature of approximately 600 K. The second-harmonic generation of SnS_0.6Se_0.4 nanobelts is approximately 300 times more intense than that of SnS nanobelts. The switchable spontaneous polarization of SnS_0.6Se_0.4 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.