Progress of two-dimensional ferroelectrics in physical and chemical applications

Ferroelectric materials are intriguing and technologically useful because the spontaneous electric polarization is electrically switchable, leading to fascinating properties and wide range of applications such as field effect transistor, non-volatile memories and sensors. In general, the emergence of ferroelectricity requires an asymmetric structure. Previous studies of ferroelectrics have mainly focused on ABO3 perovskite compounds such as BaTiO3, PbTiO3. For technological demands, the size of the devices is decreasing dramatically that conventional ferroelectric materials will suffer from many constraints at nanoscale. For example, when the thickness of ABO3 film falls below a critical value, the ferroelectricity would probably disappear. Therefore, it is essential to search for the low-dimensional materials with stable ferroelectricity. Atomically thin two-dimensional (2D) materials which exhibit abundant electronic, optical and mechanical properties would be ideal candidates and a variety of 2D ferroelectric materials have been proposed accordingly. The first part of this article reviews the recent theoretical and experimental progress of 2D ferroelectric materials. In contrast to conventional 2D materials, 2D ferroelectrics have reversible spontaneous electric polarization which would give rise to an electrostatic potential from the bottom to the top surface. The electrostatic potential difference on two surfaces would lead to distinct band edge alignments and different electron transfer when they are respectively integrated with other materials, thus changing the properties of adjacent layers. On the other hand, switching the polarization direction could change the surface’s charge density, making it possible to modulate the binding strength of adsorbed species. According to the Sabatier principle, the adsorbate-surface binding strength must be optimal to achieve the maximum efficiency. A ferroelectric surface with tunable activity can provide a platform to enhance specific catalytic performance. What’s more, the intrinsic electric field in 2D ferroelectric materials is also beneficial to separate the photo-generated carriers and inhibit their recombination, which are of great importance in photocatalytic reactions. It is worth noting that, the opposite electric polarizations in ferroelectric materials are bistable, which means the electrical switching of these two states is nonvolatile. The states will remain even the external electric field is retracted. Therefore, it is expected that this intrinsic electric field would be an effective way to achieve nonvolatile control of the properties of adjacent materials and reaction activity. To this end, in the middle part, we focus on four respects, namely the effects of ferroelectric polarization on magnetism, electronic properties, gas adsorption and photocatalytic water splitting to show the applications of 2D ferroelectric materials. Up to now, great efforts have been devoted in ferroelectric materials with out-of-plane spontaneous polarization. Compared to out-of-plane ferroelectricity, in-plane ferroelectrics have unique advantage in terms of preventing vertical perturbations. With the rapid progress of the experimental techniques, 2D ferroelectric materials would hold great potential to be applied in a broader range of applications. At last, a brief overview for 2D ferroelectric materials is presented.