Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous electric polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calculations to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelectric materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap reduction in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.

. We find that in bilayer MoS 2 the circularly polarized photoluminescence can be continuously tuned from −15% to 15% as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS 2 the photoluminescence polarization is gate independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values through symmetry control.The Dirac-valley degree of freedom has recently been considered for new modes of electronic and photonic device operation 4,5,[9][10][11][12][13][14][15][16][17] following the arrival of atomically thin two-dimensional (2D) electronic systems 6,7,18,19 (Fig. 1a). In this context, phenomena such as valley polarization and anomalous valley-and spin-Hall effects have been discussed for the +K and −K Dirac valleys at opposite corners of the Brillouin zone in hexagonal systems [9][10][11][12]15 . The realization of these effects hinges on achieving control of valley contrast, that is, of properties that differ between the two valleys, in particular the magnetic moment (m) and Berry curvature ( ). Time-reversal symmetry dictates that each pseudovector, m as well as , has the same magnitude but opposite sign in the two valleys, whereas inversion symmetry requires them to have the same sign. Therefore, a necessary condition for valley-contrasting m and is inversion symmetry breaking 4 . Monolayer MoS 2 lacks structural inversion symmetry (Fig. 1a), and thus m and are non-zero, having equal magnitude but opposite signs in the two ±K valleys owing to timereversal symmetry. One direct consequence of non-zero m is valley-contrasting optical dichroism 5,8,9 , whereby charge carriers in the two valleys can be selectively excited by circularly polarized optical fields [9][10][11] . This effect permits optical generation of valley polarization, as recently demonstrated using polarized