Yun Wang

Bio: Yun Wang is an academic researcher from University of Waterloo. The author has contributed to research in topics: Nanorod & Dielectrophoresis. The author has an hindex of 4, co-authored 7 publications receiving 541 citations.

A Review of Carbon Nanotubes-Based Gas Sensors

Abstract: Gas sensors have attracted intensive research interest due to the demand of sensitive, fast response, and stable sensors for industry, environmental monitoring, biomedicine, and so forth. The development of nanotechnology has created huge potential to build highly sensitive, low cost, portable sensors with low power consumption. The extremely high surface-to-volume ratio and hollow structure of nanomaterials is ideal for the adsorption of gas molecules. Particularly, the advent of carbon nanotubes (CNTs) has fuelled the inventions of gas sensors that exploit CNTs' unique geometry, morphology, and material properties. Upon exposure to certain gases, the changes in CNTs' properties can be detected by various methods. Therefore, CNTs-based gas sensors and their mechanisms have been widely studied recently. In this paper, a broad but yet in-depth survey of current CNTs-based gas sensing technology is presented. Both experimental works and theoretical simulations are reviewed. The design, fabrication, and the sensing mechanisms of the CNTs-based gas sensors are discussed. The challenges and perspectives of the research are also addressed in this review.

A capacitive humidity sensor based on ordered macroporous silicon with thin film surface coating

Abstract: In this paper, we report a new approach for humidity sensing. The sensor is based on ordered macroporous silicon with a Ta2O5 thin film coating. The ordered macroporous silicon array has perfectly aligned pores and uniform pore size (4 μm). The 95 nm Ta2O5 thin film is uniformly deposited on the pore surface by atomic layer deposition (ALD), which acts as an adsorption enhancement layer. The sensor's capacitance is measured to RH changes. The sensor shows very high sensitivity and small hysteresis, especially at high RH levels. It also shows very good repeatability and long term stability.

Synthesis of aligned zinc oxide nanorods for humidity sensing

Abstract: Aligned zinc oxide nanorods thin film was synthesized by the hydrothermal method. The nanorods were vertically aligned on an indium tin oxide substrate. The length of the nanorods can be controlled by repetitive reactions. Preliminary results of the humidity sensing properties of the nanorods are reported. A 450 nm thick nanorods thin film was measured in this paper. The resistance response under changing Relative Humidity (RH) is characterized. In contrast to most of related reports of humidity sensing by ZnO, a resistance increase of the nanorods during was observed when they are exposed to water molecules.

Synthesis of Aligned ZnO Nanorods with Different Parameters and Their Effects on Humidity Sensing

Abstract: In this paper, we report the synthesis of aligned ZnO nanorods with different growing conditions and the effects of the growing parameters on the nanorods humidity sensing characteristics. Six different kinds of samples are prepared with different synthesis parameters, including the zinc source, growing time, with or without PEI and post annealing. All samples show increasing resistance response to increasing relative humidity level at room temperature. This can be explained by the formation of electron depletion layer on the nanorods surface due to the adsorption of water molecules. It is demonstrated that, ZnO nanorods grown from Zn(Ac)2 with shorter growing time and post annealing shows the best overall sensing characteristics. The nanorods without post annealing show much higher sensitivity and faster response, but very fast degradation and unstable response to RH changes.

Humidity sensing characteristics of laterally aligned ZnO nanowires by dielectrophoresis method

Abstract: In this study, the humidity sensing characteristics of zinc oxide (ZnO) nanowires are investigated experimentally. ZnO nanowires are grown by hydrothermal method at low temperature (95 °C) for 3–24 hours. Dielectrophoresis (DEP) force is then applied to assemble ZnO nanowires on interdigitated electrode (IDE) patterns to make connection between two electrodes. Prior to the assembling, thermal annealing is used and pure alcohol is dropped on the IDE pattern so that the adhesion of IDE patterns and substrate is improved effectively. The hydrogen sites of water molecules are thought to be positively charged due to the high electronegativity of oxygen compared to hydrogen. These charged hydrogen sites will capture the electrons of ZnO nanowires electrostatically. Thus, the electrical resistance of our sensor increases with increasing relative humidity (RH) level in the tests. Our sensors show extremely fast response time and they can reach 90% of the total change in 16 seconds when increasing RH level from 0% to 100%. These laterally aligned ZnO nanowires are expected to be promising for applications in commercial humidity sensors.

Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

Abstract: Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

Abstract: Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

Cu3(hexaiminotriphenylene)2: An Electrically Conductive 2D Metal–Organic Framework for Chemiresistive Sensing

Abstract: The utility of metal–organic frameworks (MOFs) as functional materials in electronic devices has been limited to date by a lack of MOFs that display high electrical conductivity. Here, we report the synthesis of a new electrically conductive 2D MOF, Cu3(HITP)2 (HITP=2,3,6,7,10,11-hexaiminotriphenylene), which displays a bulk conductivity of 0.2 S cm−1 (pellet, two-point-probe). Devices synthesized by simple drop casting of Cu3(HITP)2 dispersions function as reversible chemiresistive sensors, capable of detecting sub-ppm levels of ammonia vapor. Comparison with the isostructural 2D MOF Ni3(HITP)2 shows that the copper sites are critical for ammonia sensing, indicating that rational design/synthesis can be used to tune the functional properties of conductive MOFs.

A Simple, Low-Cost Conductive Composite Material for 3D Printing of Electronic Sensors

Abstract: 3D printing technology can produce complex objects directly from computer aided digital designs. The technology has traditionally been used by large companies to produce fit and form concept prototypes (‘rapid prototyping’) before production. In recent years however there has been a move to adopt the technology as full-scale manufacturing solution. The advent of low-cost, desktop 3D printers such as the RepRap and Fab@Home has meant a wider user base are now able to have access to desktop manufacturing platforms enabling them to produce highly customised products for personal use and sale. This uptake in usage has been coupled with a demand for printing technology and materials able to print functional elements such as electronic sensors. Here we present formulation of a simple conductive thermoplastic composite we term ‘carbomorph’ and demonstrate how it can be used in an unmodified low-cost 3D printer to print electronic sensors able to sense mechanical flexing and capacitance changes. We show how this capability can be used to produce custom sensing devices and user interface devices along with printed objects with embedded sensing capability. This advance in low-cost 3D printing with offer a new paradigm in the 3D printing field with printed sensors and electronics embedded inside 3D printed objects in a single build process without requiring complex or expensive materials incorporating additives such as carbon nanotubes.