The latest research from a Kansas State University chemical engineer may help improve humidity and pressure sensors¹, particularly those used in outer space. Vikas Berry, William H. Honstead professor of chemical engineering, and his research team are using graphene（石墨烯） quantum dots to improve sensing devices in a twofold project. The first part involves producing the graphene quantum dots, which are ultrasmall pieces of graphene. Graphene is a single-atom thick sheet of carbon atoms and has superior electrical, mechanical and optical properties. The second part of the project involves incorporating these quantum dots into electron-tunneling based sensing devices.

 

To create the graphene quantum dots, the researchers used nanoscale cutting of graphite to produce graphene nanoribbons. T.S. Sreeprasad, a postdoctoral researcher in Berry's group, chemically cleaved² these ribbons into 100 nanometers lateral³ dimensions.

 

The scientists assembled the quantum dots into a network on a hydroscopic microfiber that was attached to electrodes on its two sides. They placed the assembled quantum dots less than a nanometer apart so they were not completely connected. The assembling of dots is similar to a corn on the cob structure -- the corn kernels⁵（核心程序） are nanoscale quantum dots and the cob is the microfiber.

 

Several researchers -- including four 2012 alumni in chemical engineering: Augustus Graham, Alfredo A. Rodriguez, Jonathan Colston and Evgeniy Shishkin -- applied⁶ a potential across the fiber⁴ and controlled the distance between the quantum dots by adjusting the local humidity, which changes the current flowing through the dots.

 

"If you reduce the humidity around this device, the water held by this fiber is lost," Berry said. "As a result, the fiber shrinks and the graphenic components⁷ residing atop come close to one another in nanometer scale. This increases the electron transport from one dot to the next. Just by reading the currents one can tell the humidity in the environment."

 

Decreasing the distance between the graphene quantum dots by 0.35 nanometers increased the device's conductivity by 43-fold, Berry said. Furthermore, because air contains water, reducing air pressure decreased its water content and caused the graphene quantum dots to get closer together, which increased conductivity. Quantum mechanics suggests that electrons have a finite probability to tunnel from an electrode to a nonconnected electrode, Berry said. This probability is inversely⁸ and exponentially proportional to the tunneling distance, or the gap between the electrodes.

 

The research has numerous applications, particularly in improving sensors for humidity, pressure or temperature.

 

"These devices are unique because, unlike most humidity sensors, these are more responsive in vacuum," Berry said. "For example, these devices can be incorporated into space shuttles, where low humidity measurements are required. These sensors might also be able to detect trace amounts of water on Mars, which has 1/100th of Earth's atmospheric⁹ pressure. This is because the device measures humidity at a much higher resolution in vacuum."

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