A charge injection material, poly(3,4-ethylene dioxythioohene)-poly(styrene sulfonic) acid, is introduced as a buffer layer between metal electrodes and a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer ultrathin film. The buffer layer not only prevents the reactions between P(VDF-TrFE) layer and the top electrode during metallization process, which leads to the loss of polarization, but also supplies the amount of charges needed for compensation of the ferroelectric dipoles to stabilize the domain during switching process so that the fatigue property was improved tremendously. The sandwiched structure shows prominent ferroelectric properties in a 50 nm P(VDF/TrFE) film. A coercive electric field of 52 MV/m and a remnant polarization of 88 mC/m2 are recorded. With two additional organic interlayers, 33 nm thick in total, the coercive field decreases. After more than 1×107 cycles of switching, the polarization remains as high as 68 mC/m2. Moreover, the cell still has good ferroelectric properties at 60 °C. Even as the thickness of P(VDF/TrFE) film is down to 25 nm, a remnant polarization of 54 mC/m2 is obtained.

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Two-dimensional transition metal chalcogenides (TMDCs) have attracted great interest due to their unique semiconductor properties. Among all TMDC materials, MoS2 and WS2 are promising for composing heterostructures. However, traditional TMDC heterostructure fabrication depends on transfer process, with drawbacks of interface impurity and small size. In this work, a two-step chemical vapor deposition (CVD) process was applied to synthesize large-scale WS2/MoS2 heterostructure. Surface morphology and crystal structure characterizations demonstrate the high-quality WS2/MoS2 heterostructure. The WS2/MoS2 heterostructure photodetector fabricated by photolithography exhibits an enhanced photoresponsivity up to 370 A W-1 in comparison with single WS2 or MoS2 devices. This study suggests a direct CVD growth of large-scale TMDC heterostructure films with clean interface. The built-in electric field at interface contributes to the separation of photo-generated electron-hole pairs, leading to enhanced photocurrent and responsivity, and showing promising potentials in photo-electric applications.

This paper solves the finite-time bounded tracking control problem for fractional-order systems. Firstly, by taking the fractional derivative on state equations and error signals, a fractional-order error system is constructed, and the error signal is taken as the output vector of the error system. Secondly, a state feedback controller is introduced into the error system, and the fractional derivative of disturbance signals and desired tracking signals are combined as the disturbance signal of the error system. Thus, the original problem is converted into the input-output finite time stability problem of the closed-loop error systems. Thirdly, based on the linear matrix inequalities (LMIs), the sufficient conditions which ensure the finite-time bounded tracking for the desired tracking signals are derived. Therefore, the finite-time bounded tracking controller of the original system is obtained. Finally, simulation results elucidate the effectiveness of the controller. INDEX TERMS Finite-time bounded tracking, fractional-order systems, error systems, linear matrix inequalities.