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Piezoelectric two-dimensional (2D) van der Waals (vdWs) materials are highly desirable for applications in miniaturized and flexible/wearable devices. However, the reverse-polarization between adjacent layers in current 2D layered materials results in decreasing their in-plane piezoelectric coefficients with layer number, which limits their practical applications. Here, we report a class of 2D layered materials with an identical orientation of in-plane polarization. Their piezoelectric coefficients (e22) increase with layer number, thereby allowing for the fabrication of flexible piezotronic devices with large piezoelectric responsivity and excellent mechanical durability. The piezoelectric outputs can reach up to 0.363 V for a 7-layer α-In2Se3 device, with a current responsivity of 598.1 pA for 1% strain, which is one order of magnitude higher than the values of the reported 2D piezoelectrics. The selfpowered piezoelectric sensors made of these newly developed 2D layered materials have been successfully used for real-time health monitoring, proving their suitability for the fabrication of flexible piezotronic devices due to their large piezoelectric responses and excellent mechanical durability.

We demonstrate the strategies and principles for the performance improvement of layered semiconductor based photodetectors using multilayer indium selenide (InSe) as the model materials. It is discovered that multiple reflection interference at the interfaces in phototransistor device leads to thickness-dependent photo-response, which provide a guideline to improve the performance of layered semiconductor based phototransistors. The responsivity and detectivity of InSe nanosheets phototransistor can be adjustable using applied gate voltage. Our InSe nanosheets phototransistor exhibits ultrahigh responsivity and detectivity. An ultrahigh external photo-responsivity of ~10 4 AW -1 can be achieved during broad spectra ranging from UV to near infrared wavelength from our InSe nanosheets photodetectors. The detectivity of multilayer InSe devices is ~ 10 12 ~ 10 13 Jones, which surpasses that of currentlyexploited InGaAs photodetectors (10 11 ~ 10 12 Jones). This research shows that multilayer InSe nanosheets are promising materials for high performance photodetectors.

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