Wan Shu

Graphene-based gas sensor.

Graphene, a honeycomb structure with single layer sp2 hybridization carbon atom, has extraordinary mechanical and electrical properties, exhibiting great potential in sensing area. The ultrahigh electron mobility in room temperature as well as the ultrahigh specific surface area, endows graphene a promising candidate for ultrasensitive gas sensor. As a typical two dimensional material, every atom in graphene can be regarded as surface atom. Therefore, every atom will be able to interact with gas molecule, which provides an ultrahigh sensitivity and the ultralow detection limit (as low as for single molecule detection). The current researches for improvement of gas sensing mainly focus on two aspects: (1) Design of different working principle devices; (2) surface modification on graphene and composite with other materials (i.e . metal, metal oxide and organic polymer). The specific adsorption sites can be achieved, resulting the improvement of selectivity. This review will present and summarize recent achievements of graphene-based gas sensor in both aspects and also predict the potential research direction in the future. According to different working mechanism, graphene-based gas sensor can be classified as resistive type, field-effect-transistor (FET) type, mass sensitive type, and micro-electromechanical system (MEMS) type. Each kind has its advantages. Thanks to the simple manufacture process, resistive gas sensor is investigated broadly. Compared to passive resistive sensor, the active FET sensor exhibits better performance in sensitivity and stability generally. As for mass sensitive sensor, the unique mechanism can provide a new device structure different from the other three types. However, concerning the compatibility with integrated circuits (IC) manufacture, MEMS sensor can be a good choice. Materials with high specific surface area are able to provide more adsorption sites. Consequently, sensor performance can be improved sharply. Therefore, surface modification on graphene and composite with other materials attracts broad attention recently. By changing the density and the type of functional groups on graphene surface, specific adsorption sites can be achieved. Hence, the ability for selective gas detection is enhanced. Besides, graphene-based composite with other materials (i.e. metal, metal oxide and organic polymer) is another effective strategy to optimize sensor performance. In conclusion, lots of achievements and big breakthroughs for graphene-based gas sensor have been made in recent years. Especially the obvious enhancement for sensitivity (as low as for single molecule detection) and gas selective detection, graphene-based gas sensor exhibits great advantage compared with traditional sensors. However, the time-consuming process of gas adsorption and desorption hampers the real-time measurements due to the long response and recover time. Therefore, improvement in response and recover performance will be a potential research direction in the future. In addition, the integration of different kinds of gas sensors even other types of sensors will be a trend. Also, it will be a desirable fundamental research to give explanations for the sensing mechanism as well as to investigate the dynamic interaction between gas molecules and graphene. Recently, it is believed that with the help of special in - situ holder, researchers can launch experiments in transmission electron microscope (TEM) to explore this fundamental study.