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A team from Cornell University has fabricated a miniature magnetic field sensor, using an ultrathin graphene "sandwich," that offers detection over a greater temperature change with enough sensitivity to sense subtleties in magnetic fields.

Graphene - 3D Ball and Stick Model
(Photo : Jynto via Wikimedia Commons)
Ball-and-stick model of graphene, a material made from a single layer of graphite, with many interesting properties and many potential uses in nanotechnology.

The research was led by Assistant Professor of Physics Katja Nowack, of Cornell's College of Arts and Sciences. Her university profile notes that Nowack's lab focuses on low temperature scanning probe microscopy, specializing in their Superconducting Quantum Interference Device, or SQUID microscopy - ideal for low temperature scanning of minute magnetic fields.

Results and details of their study were published in the journal Nature Communications on Thursday, August 20.

Utilizing the Hall Effect for Ultraclean Graphene

In physics, the Hall effect occurs when an electric current is applied through a conductor that is inside a magnetic field. The current through the conductor is "bent" by the magnetic field, exerting a force on the charge carrier that tends to push them to one side of the conductor. It creates a potential difference between the two sides, and therefore generates voltage.

 

Hall effect sensors are mostly used in semiconductors and electronics for different applications and industries - from mobile phones, to automotive braking systems, to robots. Most hall effect sensors use semiconductor materials silicon (Si) and gallium arsenide (GaAs).

The Cornell University team, however, is looking at a different material for their sensors. Graphene is one of the most popular carbon allotropes in the past few years, being used from pencil leads to semiconductor manufacturing - occurring as a 2D hexagonal lattice of single-layer carbon atoms.

Nowack's team overcame the graphene sheet's tendency toward "crumpling." When graphene sheets are placed directly on silicon substrate, the sheet crumples, which, in this study, is a phenomenon they must prevent from happening.

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To do so, they placed the graphene sheets in between layers of hexagonal boron nitride (BN) - a compound with notable resistance to thermal and chemical reactions. Its most stable and malleable form comes in the same hexagonal lattice as the graphene sheet, being the electrical insulator that keeps graphene from crumpling. 

Additionally, few-layer graphite (FLG) is added in the "sandwich," acting as the gate electrode - controlling the flow of electrons in the conductive graphene sheet. In the Cornell University press release, the stacking of layers is attributed to Lei Wang, a co-author in the study and a former postdoc researcher at the Kavli Institute at Cornell for Nanoscale Science.

A Novel Hall-Effect Sensor

"The encapsulation with hexagonal boron nitride and graphite makes the electronic system ultraclean," explained Nowack. She added that the setup now allows them to work with lower electron densities than what was formerly possible, noting that it boosts the Hall-effect signal they are targeting. 

The result was a micron-scale Hall sensor comparable to the best Hall sensors operating at room temperature. The experimental sensor from Cornell, however, reports far superior performance at extremely cold temperatures of 4.2 Kelvins, or approximately 452.11 degrees Fahrenheit.

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In terms of its Hall effect sensitivity, the Cornell sensor is so precise - detecting fluctuations from the target magnetic field against a background field (noise) that is a million times larger, or by six orders of magnitude.

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