NASA Technologist Mahmooda Sultana has been leading the development of tiny graphene sensors. Because of the material’s extreme sensitivity, graphene-based sensors have a wide range of possible space applications, including the detection of strain in composite materials and the discovery of trace gases in planetary bodies.
Sensor Technology spoke to Dr. Sultana this month to learn more about the nanotechnologies currently being built at Goddard Space Flight Center.
Sensor Technology: What makes graphene an ideal material for sensors?
Mahmooda Sultana: It’s the combination of graphene’s properties that makes it ideal for sensors. First of all, these two-dimensional materials have the highest surface-to-volume ratio. All the atoms are exposed to the surface. Along with that, graphene also has very low thermal noise and superior electrical properties, which allows us to measure the smallest of changes in electrical properties. For example, singlemolecule detection has been demonstrated with graphene. In addition, graphene is radiation hard due to the minute cross-sectional area, which makes it ideal for space applications.
Sensor Technology: What do these sensors look like?
Sultana: These are very small sensors. Our first prototype is about a 1-cm by 1-cm chip, with ten sensing elements. You can actually fit a lot more sensor elements than that; we haven’t optimized the real estate yet. These are tens-of-microns-sized sensors that have leads or contact lines. We basically wirebond those contacts to a printed circuit board. Then, you can use a pin connector to read off the data.
Sensor Technology: What is being measured?
Sultana: For these chemical sensors, we’re doing four probe measurements. We apply a small current and measure the voltage drop across the graphene element. The voltage drop is proportional to the resistance of graphene. When our target gas molecules adsorb onto graphene, the resistance changes. It’s a very simple electrical measurement, which makes these devices robust.
Sensor Technology: How can the sensors detect strain?
Sultana: We’re developing graphenebased sensors for strain sensing as well. These could be used for structural health monitoring of composite materials on spacecraft and cryotanks. If there is an impact during launch or while in space that affects our instruments, we want to know, so we can tell if we should still trust the data.
Sensor Technology: How are they placed on the structure?
Sultana: One of the advantages of these 2D sensors is that we can embed them in the composite structure. They are very thin, only about a few angstroms, so they can be easily integrated with composite materials. Due to the small size, we can actually put in a network of sensors, so we can tell which part of the structure suffered damage.
Sensor Technology: What kinds of chemical sensing applications are possible?
Sultana: I’m very interested in space applications related to planetary science, heliophysics, and earth science. For example, in planetary science, you can imagine spreading many of these tiny sensors over a planetary body and getting spatial/temporal data on various gases of importance. There are many ground-based applications as well, including in situ process monitoring, environmental pollutant monitoring, detection of hazardous gas in chemical plants or explosives in airports and public buildings, and medical diagnosis.
Sensor Technology: What about Mars exploration?
Sultana: The analytical tools sent on rovers can only measure one data point at a time. Typically, scientists pick a location to measure, and then the rover has to travel to that location to take measurements. The number of data points that you can measure this way is very limited, and you can’t track the concentration of gas species over an extended period of time.
However, with these tiny detectors, we can afford to send many of them at once, and spread a network across a planet like Mars. We’ll be able to simultaneously measure trace gases at various spatial locations over an extended period of time. Recent evidence indicates spatial and temporal variability of important gases, such as methane, on Mars. We can characterize such seasonal variations with a network of graphene sensors.
Sensor Technology: Can the sensors detect methane?
Sultana: Methane is one of our target gases, and we are currently working towards making our sensors selective to methane. It is a species of great interest on Mars, as well as other oxidized planetary environments, because it can be an indicator of photochemistry, hydrothermal activity, or microbial metabolism.
Sensor Technology: How can these sensors help reduce costs of space missions?
Sultana: These sensors could add significant value to missions at low cost because the cost of space missions is proportional to the size and mass of their payloads.
Sensor Technology: What are your next priorities for this sensor development?
Sultana: Although nanomaterials have great potential and offer a unique set of advantages, they are somewhat hard to process, because we don’t really have infrastructure to quickly develop devices based on nanomaterials.
Sensor Technology: What are the challenges?
Sultana: It is a challenge to make a large-area, high-quality crystalline structure that is just one atomic layer thick. The synthesis process of graphene depends on the process parameters, and the larger the reactor, the more difficult it is to control the process parameters uniformly across the reactor. In addition, it is difficult to transfer graphene from the substrate it is grown on to a more desirable substrate for device fabrication. Handling a material that is roughly 105-106 times thinner than human hair can easily damage it if not done with a lot of care. Finally, keeping graphene atomically clean is challenging.
I think that 3D-manufacturing techniques can simplify some of this processing. I am collaborating with Northeastern University to develop a process to make 3D-printed sensors. We actually developed a process to fabricate carbon nanotube gas sensors. We’re currently working on developing a process for graphene, molybdenum disulfide, and other 2D materials.
Sensor Technology: What are the benefits of 3D-printing the sensors?
Sultana: The device development time is much faster. The technique we are developing with Northeastern is about 1000 times faster than traditional 3D-printing techniques. Also, it eliminates a lot of the time-consuming and expensive microelectronics processing techniques we have to use for traditional fabrication of nanosensors.
Sensor Technology: What other applications are possible with graphene?
Sultana: In addition to sensors, there are a number of other applications of graphene that researchers around the world are looking at, including electronics, optics, filtration, interconnects, coatings, energy storage, photovoltaic cells, and composites. In terms of electronics, one of the lower hanging fruits is to use graphene as a transparent electrode. Samsung has already demonstrated a touchscreen using graphene. Graphene would essentially replace indium-tin oxide, which is the current material used as a transparent electrode.
Sensor Technology: How can graphene’s flexibility enable new applications?
Sultana: Flexible electronics is another area where graphene has shown significant promise. Graphene, although it’s a conductive material, is also flexible and can enable flexible gadgets such as rollable laptops or wristband monitors. You can bend graphene without damaging it, which is not the case for indium tin oxide or other metals. If you bend indium tin oxide, it breaks at the corners or bends.
Sensor Technology: What do you think is most exciting about this nanosensor development?
Sultana: I think the fact that these sensors have so many different possibilities and applications is exciting. Graphene’s unique set of properties can be used to develop new technologies, which in turn can enable new space missions. That to me is very exciting.
These sensors are currently in development. We only started working on them a few years ago; it’s still in the development phase. We’re hopeful that in the near future we can use them in space missions.
For more information, visit www.nasa.gov/centers/goddard/home/index.html .