Graphene produced with chemical vapor deposition (CVD) will form the cornerstone of future graphene-based chemical, biological, and other types of sensors. The 2D nature of the material provides intrinsic advantages for sensing applications, because the entire material volume acts as a sensing surface. Furthermore, graphene provides excellent mechanical strength, thermal and electrical conductivity, compactness, and potentially low cost, which is necessary for competing on the crowded sensor market.
Graphene-based gas/vapor sensors have attracted much attention in recent years due to their variety of structures, unique sensing performance, room-temperature working conditions, and tremendous application prospects. Apart from water vapor, graphene has been used to sense gases such as NH3, NO2, H2, CO, SO2, H2S, as well as vapor of volatile organic compounds, resulting in a dramatic rise in scientific publication numbers on this topic. Graphene has also been used to detect traces of opioids in concentrations as low as 10 picograms per milliliter of liquid.
Histogram of scientific publications on the topic of graphene gas/vapor sensors (source: ISI Web of Knowledge).
The most simple and common configuration for graphene-based sensors is the graphene field-effect transistor (GFET), a sheet of graphene with a sensing area between two metal contacts. The carrier mobility can be tuned using the electric field effect with a back gate, yielding tunable sensitivity. Such a device was shown to have even single molecule detection capability (Nat. Mater. 6(9), 652–655 (2007)). The detection and working principle of GFET chemical sensors are also very simple: the electrical resistance of the device changes when something attaches to the graphene. Certain gases have shown up to 15% change in resistance, with others like methanol showing an easily detectable change of ~5% (see figure below).
Figure: Sensitivity of graphene sensors to various gases (Sens. Actuators B 163(1), 107–114 (2012)).
CVD graphene sensors have shown detection limits for ammonia on the ppb level, which is superior to commercially available devices (Appl. Phys. Lett. 100, 203120 (2012)). Ammonia not only contributes significantly to the nutritional needs of organisms, but also is a building-block for the synthesis of many pharmaceuticals, and is used in many commercial products. Although widely used, this gas (NH3) is both caustic and hazardous, and thus it is harmful to humans and causes environmental pollution. Detection of NH3 is thus a pressing societal requirement.
GFETs were also used to detect nitrogen dioxide. Although also an industrially relevant gas, NO2 is toxic to humans and the environment. Several reports have shown nitrogen dioxide sensors based on graphene with detection limits below ppm, high sensitivity, excellent selectivity and response speed (Nano-Micro Letters 8, 95 (2016)). All other graphene-based chemical sensors, such as those for hydrogen gas, carbon dioxide, carbon monoxide, methane, and sulfur dioxide, show detection limits better or on par with commercially available sensors.
A critical parameter in device sensitivity is the sheet resistance or carrier mobility in the graphene. Carrier mobility should be high to ensure small losses to heating. Thus, high-quality CVD graphene is a prime candidate for large-area, commercially viable GFET sensors.