Piezoelectric actuators are a class of actuators that precisely transfer input electric energy into displacement, force, or movement outputs efficiently via inverse piezoelectric effect-based electromechanical coupling. Various types of piezoelectric actuators have sprung up and gained widespread use in various applications in terms of compelling attributes, such as high precision, flexibility of stoke, immunity to electromagnetic interference, and structural scalability. This paper systematically reviews the piezoelectric materials, operating principles, representative schemes, characteristics, and potential applications of each mainstream type of piezoelectric actuator. Herein, we intend to provide a more scientific and nuanced perspective to classify piezoelectric actuators into direct and indirect categories with several subcategories. In addition, this review outlines the pros and cons and the future development trends for all kinds of piezoelectric actuators by exploring the relations and mechanisms behind them. The rich content and detailed comparison can help build an in-depth and holistic understanding of piezoelectric actuators and pave the way for future research and the selection of practical applications.

Twisted bilayer (TB) transition metal dichalcogenides (TMDCs) beyond TB-graphene are considered an ideal platform for investigating condensed matter physics, due to the moiré superlattices-related peculiar band structures and distinct electronic properties. The growth of large-area and high-quality TB-TMDCs with wide twist angles would be significant for exploring twist angle-dependent physics and applications, but remains challenging to implement. Here, we propose a reconfiguring nucleation chemical vapor deposition (CVD) strategy for directly synthesizing TB-MoS₂ with twist angles from 0° to 120°. The twist angles-dependent Moiré periodicity can be clearly observed, and the interlayer coupling shows a strong relationship to the twist angles. Moreover, the yield of TB-MoS₂ in bilayer MoS₂ and density of TB-MoS₂ are significantly improved to 17.2% and 28.9 pieces/mm² by tailoring gas flow rate and molar ratio of NaCl to MoO₃. The proposed reconfiguring nucleation approach opens an avenue for the precise growth of TB-TMDCs for both fundamental research and practical applications.

Micro-Opto-Electro-Mechanical Systems (MOEMS) accelerometer is a new type of accelerometer which combines the merits of optical measurement and Micro-Electro-Mechanical Systems (MEMS) to enable high precision, small volume and anti-electromagnetic disturbance measurement of acceleration. In recent years, with the in-depth research and development of MOEMS accelerometers, the community is flourishing with the possible applications in seismic monitoring, inertial navigation, aerospace and other industrial and military fields. There have been a variety of schemes of MOEMS accelerometers, whereas the performances differ greatly due to different measurement principles and corresponding application requirements. This paper aims to address the pressing issue of the current lack of systematic review of MOEMS accelerometers. According to the optical measurement principle, we divide the MOEMS accelerometers into three categories: the geometric optics based, the wave optics based, and the new optomechanical accelerometers. Regarding the most widely studied category, the wave optics based accelerometers are further divided into four sub-categories, which is based on grating interferometric cavity, Fiber Bragg Grating (FBG), Fabry-Perot cavity, and photonic crystal, respectively. Following a brief introduction to the measurement principles, the typical performances, advantages and disadvantages as well as the potential application scenarios of all kinds of MOEMS accelerometers are discussed on the basis of typical demonstrations. This paper also presents the status and development tendency of MOEMS accelerometers to meet the ever-increasing demand for high-precision acceleration measurement.

An ultrasensitive single-axis out-of-plane micro optical electro mechanical system (MOEMS) accelerometer with a relatively low cross-axis sensitivity based on a designated micromachined sensing structure and grating interferometry cavity is proposed. While seeking to achieve high performances, we investigated and optimized the design of the mechanical structure, with tensile stress in the flexure close to the ultimate yield strength of monocrystalline silicon. Expressions for acceleration-displacement sensitivity are given and compared with the results of finite-element-method simulations. The highly symmetric accelerometer was fabricated through a particular fabrication process on a specific double-device-layer silicon-oninsulate wafer. The acceleration-displacement sensitivity of the optimal sensing structure, as obtained from static acceleration measurement, was 158.20 μm/g and outperforms its previously reported counterparts. Experimental results revealed that the MOEMS accelerometer with an optimal design achieved an acceleration sensitivity of 2485 V/g with a resonant frequency of 34.5 Hz and a noise level of 185.8 ng/(Hz) ^0.5 , corresponding to a resolution of 1 μg, as well as a cross-axis sensitivity of 3.48%/0.1 g, which is one order smaller than the conventional one. The proposed design may pave the way for the applications that require extremely high sensitivity, low cross-axis sensitivity, and comparatively low-frequency response, such as seismology and microgravity detection.

Cross-axis sensitivity of single-axis optomechanical accelerometers, mainly caused by the asymmetric structural design, is an essential issue primarily for high performance applications, which has not been systematically researched. This paper investigates the generating mechanism and detrimental effects of the cross-axis sensitivity of a high resoluion single-axis optomechanical accelerometer, which is composed of a grating-based cavity and an acceleration sensing chip consisting of four crab-shaped cantilevers and a proof mass. The modified design has been proposed and a prototype setup has been built based on the model of cross-axis sensitivity in optomechanical accelerometers. The characterization of the cross-axis sensitivity of a specific optomechanical accelerometer is quantitatively discussed for both mechanical and optical components by numerical simulation and theoretical analysis in this work. The analysis indicates that the cross-axis sensitivity decreases the contrast ratio of the interference signal and the acceleration sensitivity, as well as giving rise to an additional optical path difference, which would impact the accuracy of the accelerometer. The improved mechanical design is achieved by double side etching on a specific double-substrate-layer silicon-on-insulator (SOI) wafer to move the center of the proof mass to the support plane. The experimental results demonstrate that the modified design with highly symmetrical structure can suppress the cross-axis sensitivity significantly without compromising the sensitivity and resolution. The cross-axis sensitivity defined by the contrast ratio of the output signal drops to 2.19% /0.1g from 28.28%/0.1g under the premise that the acceleration sensitivity of this single-axis optomechanical accelerometer remains 1162.45V/g and the resolution remains 1.325μg.