Out-of-plane ferroelectricity with a high transition temperature in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing electrostatic fields and interfacial chemical bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the experimental discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin ${\mathrm{In}}_2{\mathrm{Se}}_3$ crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane electric polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temperature ($\sim 700\mathrm{K}$) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh density memory devices.

We demonstrate the strategies and principles for the performance improvement of layered semiconductor based photodetectors using multilayer indium selenide (InSe) as the model material. It is discovered that multiple reflection interference at the interfaces in the phototransistor device leads to a thickness-dependent photo-response, which provides a guideline to improve the performance of layered semiconductor based phototransistors. The responsivity and detectivity of InSe nanosheet phototransistor can be adjustable using applied gate voltage. Our InSe nanosheet phototransistor exhibits ultrahigh responsivity and detectivity. An ultrahigh external photo-responsivity of ∼10⁴ A W⁻¹ can be achieved from broad spectra ranging from UV to near infrared wavelength using our InSe nanosheet photodetectors. The detectivity of multilayer InSe devices is ∼10¹² to 10¹³ Jones, which surpasses that of the currently exploited InGaAs photodetectors (10¹¹ to 10¹² Jones). This research shows that multilayer InSe nanosheets are promising materials for high performance photodetectors.

Two-dimensional (2D) semiconductors are limited to graphene analogues of layered materials, so it is extremely challenging to fabricate 2D non-layered materials with thicknesses of only a few atomic layers. Here, we report the successful fabrication of 2D Ga₂O₃ from the corresponding GaSe nanosheets and a solar blind photodetector based on 2D Ga₂O₃. The as-prepared 2D β-Ga₂O₃ is polycrystalline and has a thickness of less than 10 nm. Furthermore, we demonstrate a photodetector based on 2D β-Ga₂O₃, which show a sensitive, fast and stable photoresponse to ultraviolet radiation (254 nm). The responsivity, detectivity and external quantum efficiency of the photodetector are 3.3 A W⁻¹, 4.0 × 10¹² Jones and 1600%, respectively, indicating that the 2D Ga₂O₃ has great potential for application for solar-blind photodetectors.