Nanodetectors Stretch Physics to Prevent Deadly Attacks

Primary Investigator: Dr. Abdennaceur Karoui, North Carolina Central University

It’s a terrifying scenario: extremists release deadly sarin gas into a crowded underground train station or into a military barracks. Invisible and odorless, the gas can kill people before they even know it’s there. Attackers did it twice in Japan in 1994 and 1995, killing a total of 20 people.

That’s why world health officials and the Department of Defense consider sarin and similar agents to be weapons of mass destruction, and a priority for research.

Now imagine a fleet of tiny drones that can detect a few molecules of toxic gas, patrolling quietly to give people time to flee a railway station or a battlefield before they get a lungful.

Abdennaceur Karoui of North Carolina Central University and colleagues are developing minuscule detectors that could be carried on these little drones, or loaded into cellphones for use in the field.

They are working at the very edge of applied physics, using quantum signal detection to make sensors that are more powerful than a mass spectrometer, yet small, light and inexpensive. He hopes to also make them user-friendly, so they can be handled by people with a minimum of training.

“Sarin must be detected as soon as the first molecules are around. For example, these molecules could be released from the outside of the container, before it is opened, or from the person who handled it, or the tools used to make that weapon,” Karoui said.

Karoui and a network of collaborators are using a process called rapid thermal chemical vapor deposition to make their quantum-level detectors, which sit on silicon chips. An array of them could be arranged on a chip 2.5 cm (1 inch) square.

The process involves putting a silicon chip onto a precisely controlled hot plate inside a vacuum chamber. After all impurities are removed, two gases are piped in: germane, a compound made of germanium combined with hydrogen; and silane, a silicon-hydrogen compound.

With the correct controlled heat and other factors, the gases deposit a layer of pyramid-shaped crystals on the surface of the wafer. “There is a sweet spot where they start to create the silicon germanium pyrimidal nanodots,” Karoui said.

The tip of each pyramid acts as a quantum dot. Each dot interacts with any molecules it encounters, trapping electrons. This atomic-level physical structure insulates the quantum dot and results in an action called quantum tunneling – in this case, electron tunneling. The spectrum that is generated tells the user when the targeted compound is interacting with the quantum dot.

“This is new science that needs to be built,” Karoui said.

“We are dealing with quantum information and that is not easy to understand.” One challenge is to gather the information before collapsing the quantum state of the system. Karoui says properly insulating the layers can do that.

His team has created the quantum dot layers on silicon wafers and is now working to test the spectra generated by various molecule models, because they cannot test real sarin gas or other toxins in non-specialized labs. The plan includes using microscopy techniques to image physical nanostructures.

Karoui envisions the mass production of these quantum dot wafers, tuned to detect various agents. It would not be difficult to make chips that could be inserted into cellphones for field use, or built into small drones that could continually patrol an area. His team includes several other physics professors a NCCU as well as at NC State University, Cornell University and elsewhere. They’re hoping their grant can be extended so they can continue the research.