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John O | July 2017

Researchers use white graphene to build thermally stable, small sensors


A team of researchers from Delft University of Technology (TU Delft) in the Netherlands and the University of Cambridge (U.K.) has created tiny sensors with two-dimensional sheets of hexagonal boron nitride (h-BN), known as white graphene, suspended over holes in a silicon substrate that could be used in harsh environments like space and in future electronics.

 


A look at the "white graphene" drum created at TU Delft. (TU Delft)

 

According to a report from the university, h-BN is more chemically and thermally stable than graphite because it acts as an insulator rather than a conductor despite having graphite’s honeycomb lattice structure.

 

Studies of h-BN have been limited because of the difficulty in creating the material. Layers of h-BN stick to themselves and other materials more easily than graphene. Cambridge researchers used chemical vapor deposition to grow monolayers of h-BN on pieces of iron foil.

 

The article explained, “The Delft researchers transferred the sheet of transparent white graphene onto a silicon substrate containing tiny circular cavities. By doing so, they created microscopic ‘drums’. These drums function as mechanical resonators and could be used as infinitesimal gas or pressure sensors, for instance in mobile phones.”

 

The process of moving the layers of h-BN to silicon substrate also contaminated the white graphene with various polymers. Researchers solved this problem using ozone gas to clean the drums. All of the organic polymers were removed and only PMMA, a polymer with inorganic compounds, remained.

 

A report on this development was published in 2D Materials and Applications. The abstract stated:

 

“Hexagonal boron nitride is a 2D material whose single-layer allotrope has not been intensively studied despite being the substrate for graphene electronics. Its transparency and stronger interlayer adhesion with respect to graphene makes it difficult to work with, and few applications have been proposed.

 

“We have developed a transfer technique for this extra-adhesive material that does not require its visual localization, and fabricated mechanical resonators made out of chemical vapor-deposited single-layer hexagonal boron nitride. The suspended material was initially contaminated with polymer residues from the transfer, and the devices showed an unexpected tensioning when cooling them to 3 K.

 

“After cleaning in harsh environments with air at 450 °C and ozone, the temperature dependence changed with f0Qproducts reaching 2 × 1010 Hz at room temperature.

 

“This work paves the way to the realization of highly sensitive mechanical systems based on hexagonal boron nitride, which could be used as an alternative material to SiN for optomechanics experiments at room temperature.”

 

A video showing the drums in action can be viewed at https://d1rkab7tlqy5f1.cloudfront.net/News/2017/06_June/TNW/whitegrapheneresonator.mp4.

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