Shapelet-transformed multi-channel EEG channel selection

This article proposes an approach to select EEG channels based on EEG shapelet transformation, aiming to reduce the setup time and inconvenience for subjects, and to improve the applicable performance of Brain-Computer Interfaces (BCIs). In detail, the method selects top- EEG channels by solving a logistic loss-embedded minimization problem with respect to EEG shapelet learning, hyperplane learning, and EEG channel weight learning simultaneously. Especially, to learn distinguished EEG shapelets for weighting contributions of each EEG channel to the logistic loss, EEG shapelet similarity is also minimized during the procedure. Furthermore, the gradient descent strategy is adopted in the article to solve the non-convex optimization problem, which finally leads to the algorithm termed StEEGCS. In a result, classification accuracy, with those EEG channels selected by StEEGCS, is improved compared to that with all EEG channels, and classification time consumption is reduced as well. Additionally, the comparisons with several state-of-the-art EEG channel selection methods on several real-world EEG datasets also demonstrates the efficacy and superiority of StEEGCS

CenEEGs: valid EEG selection for classification

This article explores valid brain electroencephalography (EEG) selection for EEG classification with different classifiers, which has been rarely addressed in previous studies and is mostly ignored by existing EEG processing methods and applications. Importantly, traditional selection methods are not able to select valid EEG signals for different classifiers. This article focuses on a source control-based valid EEG selection to reduce the impact of invalid EEG signals and aims to improve EEG-based classification performance for different classifiers. We propose a novel centroid-based EEG selection approach named CenEEGs, which uses a scale-and-shift-invariance similarity metric to measure similarities of EEG signals and then applies a globally optimal centroid strategy to select valid EEG signals with respect to a similarity threshold. A detailed comparison with several state-of-the-art time series selection methods by using standard criteria on 8 EEG datasets demonstrates the efficacy and superiority of CenEEGs for different classifiers

Pulsed terahertz signal reconstruction

A procedure is outlined which can be used to determine the response of an experimental sample to a single, simple broadband frequency pulse in terahertz frequency time domain spectroscopy (TDS). The advantage that accrues from this approach is that oscillations and spurious signals (arising from a variety of sources in the TDS system or from ambient water vapor) can be suppressed. In consequence, small signals (arising from the interaction of the radiation with the sample) can be more readily observed in the presence of noise. Procedures for choosing key parameters and methods for eliminating further artifacts are described. In particular, the use of input functions which are based on the binomial distribution is described. These binomial functions are used to unscramble the sample response to a simple pulse: they have sufficient flexibility to allow for variations in the spectra of different terahertz sources, some of which have low frequency as well as high frequency cutoffs. The signal processing procedure is validated by simple reflection and transmission experiments using a gap between polytetrafluoroethylene (PTFE) plates to mimic a void within a larger material. It is shown that a resolution of 100 μm is easily achievable in reflection geometry after signal processing

Two-Dimensional van der Waals Materials with Aligned In-Plane Polarization and Large Piezoelectric Effect for Self-Powered Piezoelectric Sensors

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 1 order of magnitude higher than the values of the reported 2D piezoelectrics. The self-powered 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

Kirigami-inspired highly stretchable nanoscale devices using multi-dimensional deformation of monolayer MoS2

Two-dimensional (2D) layered materials, such as MoS2, are greatly attractive for flexible devices due to their unique layered structures, novel physical and electronic properties, and high mechanical strength. However, their limited mechanical strains (<2%) can hardly meet the demands of loading conditions for most flexible and stretchable device applications. In this paper, inspired from Kirigami, ancient Japanese art of paper cutting, we design and fabricate nanoscale Kirigami architectures of 2D layered MoS2 on a soft substrate of PDMS using a top-down fabrication process. Results show that the Kirigami structures significantly improve the reversible strechability of flexible 2D MoS2 electronic devices, which is increased from 0.75% to ~15%. This increase in flexibility is originated from a combination of multi-dimensional deformation capabilities from the nanoscale Kirigami architectures consisting of in-plane stretching and out-of-plane deformation. We further discover a new fundamental relationship of electrical conductance and large strain in MoS2 Kirigami structures through both experimental work and finite element simulation. Results show that the electrical conductance of the stretchable MoS2 Kirigami is closely related to its different stages of structural evolutions under strain: e.g., elastic stretching; then a combination of elastic stretching and out-of-plane buckling; and finally stretching and structural damage. This method provides a new opportunity to fabricate highly flexible and stretchable sensors and actuators using different types of 2D materials

Intrinsic Dipole Coupling in 2D van der Waals Ferroelectrics for Gate‐Controlled Switchable Rectifier

Miniaturization of device elements, such as ferroelectric diodes, depends on the downscaling of ferroelectric film, which is also crucial for developing high‐density information storage technologies of ferroelectric random access memories (FeRAMs). Recently emerged ferroelectric two‐dimensional (2D) van der Waals (vdWs) layered materials bring an additional opportunity to further increase the density of FeRAMs. A lateral, switchable rectifier is designed and fabricated based on atomically thin 2D α‐In2Se3 ferroelectric diodes, thus breaking the thickness limitation of conventional ferroelectric films and achieving an unprecedented level of miniaturization. This is realized through the interrelated coupling between out‐of‐plane and in‐plane dipoles at room temperature; that is, horizontal polarization reversal can be effectively controlled through a vertical electric field. Being further explored as a switchable rectifier, the obtained maximum value of rectification ratio for the α‐In2Se3 based ferroelectric diode can reach up to 2.5 × 103. These results indicate that 2D ferroelectric semiconductors can offer a pathway to develop next‐generation multifunctional electronics

Negative refracting materials at THz frequencies.

We demonstrate here, for the first time, the construction of artificial materials, which theoretically possess two separate pass-bands utilizing the difference between positive and negative refraction in the terahertz frequency regime. Experimental testing of these devices is currently being undertaken using both broadband pulsed sources and narrow frequency quantum cascade lasers