This Small Business Technology Transfer (STTR) Phase II project will develop a low-cost, high sensitivity detector for nitrogen oxides (NOx). The detector will be based on the recently discovered material - graphene. In its most elemental form, graphene consists of a single layer of carbon atoms arranged in a hexagonal array. Since first isolated in 2004, scientists have been rapidly documenting the unusual physical and electrical properties of graphene, and the many potential commercial applications of this unique and multifunctional material. Gas detectors, such as the presently proposed NOx sensor, will be the first commercial application for graphene based devices. Graphene films can potentially detect down to a single molecule of an adsorbed gas.
The broader impacts of this research are that the availability of an effective, inexpensive NOx sensor will enable closed-loop control of engine conditions in auto and truck applications, allowing manufacturers to simultaneously optimize vehicle performance and fuel economy, while maintaining NOx emissions within standards. A portable version of the inexpensive NOx sensor will also enable emissions monitoring for a wide range of other industrial and regulatory applications. In addition, the technology developed will extend the knowledge base for graphene material processing and device applications. Many more applications for graphene are possible, ranging from high speed transistors to spintronic devices to radiation detectors (THz through infrared to optical) to NEMS devices. Graphene also offers the potential to combine these functions into a single device.
In this STTR program, Structured Materials Industries, Inc., (SMI) www.structuredmaterials.com, along with Cornell University (Cornell) www.cornell.edu and the University of South Carolina (USC) www.sc.edu, developed a low-cost, high sensitivity detector for nitrogen oxides (NOx). The detector is based on the recently discovered material - graphene. In its most elemental form, graphene consists of a single layer of carbon atoms arranged in a hexagonal array. Since the development of techniques for isolating graphene in 2004, scientists have been rapidly documenting the unusual physical and electrical properties, and the many potential applications of this truly unique and multifunctional material. Gas detectors are expected to be the first commercial application for graphene based devices. Due to their high surface to volume ratio, and affinity for electron transfer with adsorbed molecules, graphene films can potentially detect down to a single molecule of an adsorbed gas. During the Phase I project, the team of SMI, Cornell and USC demonstrated technical feasibility of a highly sensitive and highly selective graphene based NOx detector. We demonstrated growth of high quality, epitaxial graphene films on 6H-silicon carbide (SiC) substrates. We demonstrated the graphene film quality through characterization such as Raman spectroscopy and atomic force microscopy. Also during Phase I, we fabricated gas sensor elements from epitaxial graphene films, and measured the sensor response to a variety of gases. The Phase I results demonstrated high sensitivity to both NO and NO2. We also demonstrated high selectivity to interfering gases such as CO, CO2, NH3 and H2O. During Phase II, SMI, Cornell and USC expanded the graphene film deposition technology to low-cost substrates, including dielectrics such as sapphire and metals such as nickel and copper. Figure 1 shows a large area copper foil, with a uniform graphene film deposited by chemical vapor deposition (CVD) at SMI. We developed CMOS compatible fabrication technology for the graphene based NOx sensor, based on transfer of graphene films deposited on copper to processed silicon substrates. We also investigated several designs for the NOx sensor, including the back-gated graphene transistor. The graphene based NOx sensors are also compatible with standard device packaging, as shown in Figure 2. Also during Phase II, we performed extensive testing and analysis to characterize and understand the response of the graphene based sensor to various adsorbed gases. We analyzed the sensor response to a wide range of gases, and developed a fundamental understanding of how adsorbed gases effect graphene film conductivity, based on the electron donor or electron acceptor properties of the adsorbed molecules. At the conclusion of Phase II, the STTR team also demonstrated operation of the graphene based NOx sensor in actual automotive exhaust. Presently, SMI is preparing for commercialization of the graphene based NOx sensor. The initial market target is the manufacturers of passenger cars and light trucks, and the tier-1 suppliers to these manufacturers. NOx is the general notation for several gaseous compounds of nitrogen and oxygen, including nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O). NOx can be produced whenever combustion of fossil fuels occurs in air, such as in internal combustion engines. Since NOx emission can cause environmental problems, including smog and acid rain, many governments around the world have enacted legislation to limit NOx pollution. There is tremendous market pull for an accurate and reliable exhaust gas NOx sensor. This sensor will enable feed-back control of engine parameters, to simultaneously optimize vehicle performance and fuel economy, while maintaining NOx emissions within standards. Total annual sales of cars and light trucks in the US are presently around 14 million units per year. At an estimated selling price of $20 each, this represents an annual market of well over $200 million in the US alone. This market estimate does not include vehicle sales in Europe, which also has strict emissions standards, nor Asia and the third world, who are rapidly adopting strict emissions regulations.
