An important component of any chemiresistive gas sensor is the way in which the resistance of the sensing film is interrogated. The geometrical structure of an electrode can enhance the performance of a gas-sensing device and in particular the performance of sensing films with large surface areas, such as carbon nanotubes. In this study, we investigated the influence of geometrical structure on the performance of gas sensors, combining the characteristics of carbon nanotubes with a novel gas sensor electrode structure based on fractal geometry. The fabricated sensors were tested with exposure to nitric oxide, measuring both the sensor resistance and capacitance (RC) of the sensor responses. Experimental results showed that the sensors with fractal electrode structures had a superior performance over sensors with traditional geometrical structures. Moreover, the RC characteristics of these fractal sensors could be further improved by using different test frequencies that could aid in the identification and quantification of a target gas.Chemosensors 2019, 7, 0031 2 of 11 greater than that of the material itself [6]. Therefore, increasing a specific surface area [7] and improving the structure of sensor are beneficial to improving the sensitivity of a sensor [8]. However, most studies had adopted traditional Euclidean geometry for sensor shape, with simple electrode structures that do not interrogate all of the sensing film and are typically formed of two parallel electrodes [9,10]. Later designs and the majority of MEMS (micro-electro-mechanical-systems) gas sensors that employ micro-hot plates, operated interdigitated fingers to try and increase the volume of the sensing film interrogated and reduce the measured resistance of the sensors [11]. However, with optimization, further improvements in the sensing film interrogation (and potentially sensitivity) could be achieved through more sophisticated electrode structures.The term "fractal" was first proposed in 1975 by Benoit Mondelbrot. He was both a researcher at the Physics Department of the IBM Research Center (United States) and a Professor of Mathematics at Harvard University. For more than 30 years fractal theory has influenced various disciplines and have been applied in biology, geophysics, physics, chemistry, astronomy, materials science, and other fields. Many traditional scientific problems have been solved by the introduction of fractal theory. As a new concept and method, fractal is being applied and explored in many fields. The well-known American physicist Wheeler said: Who is not familiar with fractals in the future, who cannot be called a scientific cultural man [12].Here, we report on the design of a new gas sensor structure based on fractal geometry. This sensor has a dual capacitance and resistance read out that are based on fractal geometry. The basic circuit model for this sensor has also been analyzed. Since carbon nanotubes (CNTs) are good gas sensing materials with a fast response-time, good repeatability, are able work at room temperature...
