Advances and new directions in gas-sensing devices
Introduction
Society has benefited greatly, over the past century, from advances that have come about in the energy, transportation, communication and medical fields. More recently, society has become increasingly concerned with the unintended consequences of these advances, including global warming, pollution of air and water, and destruction of the ozone layer and forests. Many of these destructive forces can be tied to increased emissions from power plants, home and factory heating units, and vehicles, which derive nearly 90% of their energy from fossil-fuel combustion processes. Add to this various often toxic and/or combustible gaseous and liquid products generated at chemical and materials processing plants, and the need to insure security at airports and other public sites, and it becomes obvious that the means for tracking and controlling such emissions or chemical analytes are required. This article concerns itself, therefore, with gas sensors, reviewing their principles of operation, the progress that has been ongoing in refining their operation and the trends defining where progress is likely to take us in the future.
Sensitivity is the primary property that comes to mind when discussing sensors. This follows from the fact that certain chemical species, even at ppm or lower levels, can be toxic to humans, contribute to corrosion of critical components (e.g. nuclear reactors) and/or poison catalysts essential in emissions control or to the chemicals industry. This brings into play another key sensor property, selectivity, which reflects the often-enormous challenge of selectively detecting small numbers of a specified molecule suspended in a sea of other chemical species, e.g. the surrounding atmosphere. Another important sensor parameter is speed of response. For example, an automotive exhaust sensor must respond within the order of 10 ms to a change in gas composition in order to enable feedback control of the air-to-fuel ratio needed for proper operation of the catalytic convertor. The last of the four key properties is stability, without which reliable sensor readings become impossible. This latter property is becoming more challenging to achieve, as we increasingly require sensors to operate under harsh temperature and environmental conditions. The analysis of the four S’s—sensitivity, selectivity, speed and stability—is thus essential in any discussion of chemical sensor development.
For many years, high sensitivity/selectivity sensor systems were limited to the laboratory. More recently, the trend has been away from such large stand-alone analytical chemistry systems (mass spectrometers, chromatographs, IR spectrometers, etc.) that lack portability, require skilled operators and are costly, towards miniature devices, often embedded as part of a sensor array. These lower-cost devices are portable, draw considerably reduced power, and when integrated with appropriate software, provide a level of selectivity impossible with single-sensor-based devices. These advances have been made possible by leveraging corresponding advances in microelectronic and microelectromechanical (MEMS) processing. At the same time, it must be remembered that while silicon-based chips normally operate at or near room temperature, wrapped in packaging designed to isolate the device from the environment, chemical sensors, on the other hand, commonly operate at elevated temperatures to accelerate kinetic processes and in often harsh chemical environments. This has required the integration of materials not common in the microelectronics field and the modification of the substrates and metallizations capable of operating under such conditions. These efforts have been aided by the need for high-power (electric vehicles, power grid controls) and high-temperature electronics (automotive and jet engine controls) where wide band gap materials such as SiC and AlGaN have been introduced and continue to be refined.
Many means for detecting chemical analytes are possible and have been investigated, including those based on chemically induced modulation of the electrical, electrochemical, electromechanical or optical properties of materials. Examples of each will be given here. The bulk of this review, however, will focus on progress being made on so-called chemoresistive sensors, which are particularly attractive, given their conceptually simple structure, ease of fabrication and low cost, coupled with high sensitivity. While both organic and inorganic materials are being investigated as the basis of chemical sensors, in this article we focus on inorganic refractory materials, given that many of the applications that we consider require exposure to harsh environments. Under those circumstances such materials, most commonly semiconducting metal oxides (SMOs), satisfy the requirement of stability.
The introduction of nanodimensioned materials has had a particularly striking impact on the field over the past decade. This follows both from the high surface areas of such structures, but often more importantly, the matching of the modulation depth induced by the adsorbed chemical analytes, with the cross-sectional dimensions of the nanosized particles or nanowires (NWs) that make up the device. Advances in materials processing has enabled the fabrication of tailored structures and morphologies offering, at times, orders of magnitude improvements in sensitivity, while high-resolution analytical methods have enabled a much improved examination of the surface structure and chemistry of these materials. Progress along these lines is reviewed in this article.
The operation of chemical sensors inevitably relies on an understanding of a number of disciplines. The bulk transport properties of, for example, SMOs requires an understanding of both solid-state physics and defect thermodynamics and kinetics. The chemisorption of molecular and atomic species on the surface of such semiconductors relies on charge transfer processes that involve an understanding of semiconductor junction physics, electrochemistry and catalysis. It is no wonder that a detailed understanding of the operation of chemical sensors remains, in many cases, lacking. In this paper, we review progress being made in understanding these phenomena.
Finally, we end this article by describing the directions that research is likely to take in the coming years with respect to new sensor materials platforms, advanced processing and characterization approaches, light and electric field modulation, and the modeling of sensor materials and their operation.
Section snippets
Electrochemical devices
There are, in principle, many ways to detect chemical species in the environment. Most commonly, the sensing device takes the form of a chemical to electrical transducer. Classically, this would be in the form of electrochemical cells, operating either in the potentiometric or amperometric mode. Indeed, the sensors installed in tens of millions of new automobiles per year, for the purpose of monitoring the oxygen partial pressure, , of the exhaust gas, are potentiometric devices utilizing
Response mechanisms of chemisorptive SMO gas sensors: advances in understanding
SMOs such as SnO2 and TiO2 often display remarkable changes in their electrical properties, e.g. their work function and electrical conductivity, upon exposure to O2, CO, NO2 and other reactive gases. This phenomenon underlies their application as gas and oxygen sensors. Gas sensors are typically operated in air at temperatures between 100 and 400 °C. Under these conditions, the surface of the sensing layer, or the particles within the sensing layer, in the case of porous materials, are covered
Novel nanostructured architectures
The application of novel nanostructured architectures, as mentioned above, has contributed to significant progress in the development of highly sensitive and selective chemical sensors. These architectures include one-dimensional (1-D) and quasi-1-D building blocks, carefully assembled by a combination of top-down controlled chemical etching and various bottom-up nanofabrication processes [79], [80], [81], including hollow spheres [82], [83], [84] and hemispheres [85], [86], [87], [88], with
Field effect transistor-based chemical sensors
SMOs have been used as the active channel layer in field effect transistors (FETs). Field-induced current modulation can lead to further improvements in chemical sensing characteristics. NW-based FETs using ZnO [160], [161], In2O3 [114], [162], SnO2 [163], [164] and Zn2SnO4 [165] have been reported with superior NO2, CO and VOC sensing behavior. Although FET-based sensors using single NW channels have very good sensing performance, pick-and-place methods for positioning of the channel layer,
Future directions
The authors have attempted to demonstrate that the field of solid-state gas-sensitive devices is a very dynamic and fast evolving field, and that it can be expected to continue to grow rapidly with advances in materials and device fabrication, characterization and modeling. Under these circumstances, it is very likely that the authors will miss certain important future trends. Nevertheless, it is useful to consider which developments may deserve careful scrutiny.
A key challenge for
Acknowledgements
H.L.T. thanks the following organizations for support of his research on topics related to this work: National Science Foundation, Division of Materials Research, Materials World Network (DMR-0908627), Korea Research Council Industrial Science and Technology (B551179-10-01-00), The Brazil-MIT program, Department of Energy, Basic Energy Sciences (DE SC0002633). I.D.K. acknowledges the support by the Engineering Research Center (ERC-N01120073) program from the Korean National Research Foundation.
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