Nanowire‐Based Sensors

Nanowires are important potential candidates for the realization of the next generation of sensors. They offer many advantages such as high surface‐to‐volume ratios, Debye lengths comparable to the target molecule, minimum power consumption, and they can be relatively easily incorporated into microelectronic devices. Accordingly, there has been an intensified search for novel nanowire materials and corresponding platforms for realizing single‐molecule detection with superior sensing performance. In this work, progress made towards the use of nanowires for achieving better sensing performance is critically reviewed. In particular, various nanowires types (metallic, semiconducting, and insulating) and their employment either as a sensor material or as a template material are discussed. Major obstacles and future steps towards the ultimate nanosensors based on nanowires are addressed.     

Charge‐Trapping‐Induced Non‐Ideal Behaviors in Organic Field‐Effect Transistors

Organic field‐effect transistors (OFETs) with impressively high hole mobilities over 10 cm² V^?1 s^?1 and electron mobilities over 1 cm² V^?1 s^?1 have been reported in the past few years. However, significant non‐ideal electrical characteristics, e.g., voltage‐dependent mobilities, have been widely observed in both small‐molecule and polymer systems. This issue makes the accurate evaluation of the electrical performance impossible and also limits the practical applications of OFETs. Here, a semiconductor‐unrelated, charge‐trapping‐induced non‐ideality in OFETs is reported, and a revised model for the non‐ideal transfer characteristics is provided. The trapping process can be directly observed using scanning Kelvin probe microscopy. It is found that such trapping‐induced non‐ideality exists in OFETs with different types of charge carriers (p‐type or n‐type), different types of dielectric materials (inorganic and organic) that contain different functional groups (? OH, ? NH₂, ? COOH, etc.). As fas as it is known, this is the first report for the non‐ideal transport behaviors in OFETs caused by semiconductor‐independent charge trapping. This work reveals the significant role of dielectric charge trapping in the non‐ideal transistor characteristics and also provides guidelines for device engineering toward ideal OFETs.     

A Nanoelectronic Enzyme‐Linked Immunosorbent Assay for Detection of Proteins in Physiological Solutions

Semiconducting nanowires are promising ultrasensitive, label‐free sensors for small molecules, DNA, proteins, and cellular function. Nanowire field‐effect transistors (FETs) function by sensing the charge of a bound molecule. However, solutions of physiological ionic strength compromise the detection of specific binding events due to ionic (Debye) screening. A general solution to this limitation with the development of a hybrid nanoelectronic enzyme‐linked immunosorbent assay (ne‐ELISA) that combines the power of enzymatic conversion of a bound substrate with electronic detection is demonstrated. This novel configuration produces a local enzyme‐mediated pH change proportional to the bound ligand concentration. It is shown that nanowire FETs configured as pH sensors can be used for the quantitative detection of interleukin‐2 in physiologically buffered solution at concentrations as low as 1.6 pg mL^?1. By successfully bypassing the Debye screening inherent in physiological fluids, the ne‐ELISA promises wide applicability for ligand detection in a range of relevant solutions.     

A Highly Aligned Nanowire‐Based Strain Sensor for Ultrasensitive Monitoring of Subtle Human Motion

Achieving highly accurate responses to external stimuli during human motion is a considerable challenge for wearable devices. The present study leverages the intrinsically high surface‐to‐volume ratio as well as the mechanical robustness of nanostructures for obtaining highly‐sensitive detection of motion. To do so, highly‐aligned nanowires covering a large area were prepared by capillarity‐based mechanism. The nanowires exhibit a strain sensor with excellent gauge factor (≈35.8), capable of high responses to various subtle external stimuli (≤200 µm deformation). The wearable strain sensor exhibits also a rapid response rate (≈230 ms), mechanical stability (1000 cycles) and reproducibility, low hysteresis (<8.1%), and low power consumption (<35 µW). Moreover, it achieves a gauge factor almost five times that of microwire‐based sensors. The nanowire‐based strain sensor can be used to monitor and discriminate subtle movements of fingers, wrist, and throat swallowing accurately, enabling such movements to be integrated further into a miniaturized analyzer to create a wearable motion monitoring system for mobile healthcare.     

3D‐Structured Stretchable Strain Sensors for Out‐of‐Plane Force Detection

Stretchable strain sensors, as the soft mechanical interface, provide the key mechanical information of the systems for healthcare monitoring, rehabilitation assistance, soft exoskeletal devices, and soft robotics. Stretchable strain sensors based on 2D flat film have been widely developed to monitor the in‐plane force applied within the plane where the sensor is placed. However, to comprehensively obtain the mechanical feedback, the capability to detect the out‐of‐plane force, caused by the interaction outside of the plane where the senor is located, is needed. Herein, a 3D‐structured stretchable strain sensor is reported to monitor the out‐of‐plane force by employing 3D printing in conjunction with out‐of‐plane capillary force‐assisted self‐pinning of carbon nanotubes. The 3D‐structured sensor possesses large stretchability, multistrain detection, and strain‐direction recognition by one single sensor. It is demonstrated that out‐of‐plane forces induced by the air/fluid flow are reliably monitored and intricate flow details are clearly recorded. The development opens up for the exploration of next‐generation 3D stretchable sensors for electronic skin and soft robotics.     

Secondary Sensitivity Control of Silver‐Nanowire‐Based Resistive‐Type Strain Sensors by Geometric Modulation of the Elastomer Substrate

A secondary method for modulation of the sensitivity in silver nanowire (AgNW) resistive‐type strain sensors without the need to change the material or coating process in the sensory layer is demonstrated. Instead of using a planar elastomer (polydimethylsiloxane is used in this study) substrate, diverse relief structures are introduced to induce nonuniform and complex strain within the elastic substrate and thereby different distributions of the crack density of the AgNWs upon stretching, which plays an important role in the modulation of the gauge factor (GF). Analysis of the sensory layer and mechanical studies reveal that a lower height ratio and greater number of trenches enhance the sensor sensitivity, for example, reaching a GF of 926 at 9.6% in this study. The demonstration of wrist‐motion sensors using the technology illustrates the feasibility of using relief structures for various types of sensors and sensitivity ranges using an identical sensor layer.     

Gas Sensors Based on Nano/Microstructured Organic Field‐Effect Transistors

Benefiting from the advantages of organic field‐effect transistors (OFETs), including synthetic versatility of organic molecular design and environmental sensitivity, gas sensors based on OFETs have drawn much attention in recent years. Potential applications focus on the detection of specific gas species such as explosive, toxic gases, or volatile organic compounds (VOCs) that play vital roles in environmental monitoring, industrial manufacturing, smart health care, food security, and national defense. To achieve high sensitivity, selectivity, and ambient stability with rapid response and recovery speed, the regulation and adjustment of the nano/microstructure of the organic semiconductor (OSC) layer has proven to be an effective strategy. Here, the progress of OFET gas sensors with nano/microstructure is selectively presented. Devices based on OSC films one dimensional (1D) single crystal nanowires, nanorods, and nanofibers are introduced. Then, devices based on two dimensional (2D) and ultrathin OSC films, fabricated by methods such as thermal evaporation, dip‐coating, spin‐coating, and solution‐shearing methods are presented, followed by an introduction of porous OFET sensors. Additionally, the applications of nanostructured receptors in OFET sensors are given. Finally, an outlook in view of the current research state is presented and eight further challenges for gas sensors based on OFETs are suggested.     

扫描Kelvin探针力显微镜:工作原理及在材料腐蚀研究中的应用

扫描Kelvin探针力显微镜(SKPFM)是在原子力显微镜(AFM)的基础上应用扫描Kelvin探针(SKP)技术开发的检测表征手段,它能够在获取材料表面纳米级分辨率形貌的同时,原位得到样品表面高分辨率的接触电势差分布图,为揭示腐蚀反应机理提供了崭新的思路,近年来发展迅速。本文介绍了SKPFM两种工作模式的基本原理,总结了SKPFM在应用中的问题,并讨论了SKPFM和传统扫描Kelvin探针技术(SKP)的优缺点,重点综述了SKPFM在腐蚀科学研究中的应用,最后展望了SKPFM的发展方向与应用前景。