This week's graphene stories include the mapping of electron flow in graphene by a University of Melbourne team, and a mechanical testing study at MIT quantifying graphene's ability to resist pressure.
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Mapping the flow of electrons in graphene
The electrical conductivity of graphene is a well-documented phenomenon that has led to a range of proposed application such as biological engineering, optical electronics and photovoltaic products1.
This advantageous property is directly related to the location of each atom within the carbon lattice of this material. In graphene, each carbon atom is connected to three other carbon atoms to form its unique two-dimensional (2D) plane, which allows for one electron to remain freely available in the third dimension to provide ample opportunity for electrical conductance to occur2.
Graphene also exhibits a abundance of electrical transport phenomena that contribute to its conductibility, and this property has been widely studied for its contribution to future electronic and sensing devices.
To study graphene’s electron transport, resistivity transport measurements have provided important information; however, these approaches are unable to distinguish edge effects or other defect-induced aspects of the material from its bulk properties, making it challenging to accurately determine its electrical resistivity3.
The presence of cracks and defects within ultra-thin materials, such as graphene, can greatly affect its electron flow, therefore limiting its full potential in devices. In an effort to develop a method that eliminates the limitations often associated with traditional resistivity measurements, a group of researchers from the University of Melbourne’s Centre for Quantum Computation & Communication Technology have mapped the microscopic flow of electrons within graphene, which is the first time such an endeavor has successfully been achieved3.
Led by Professor Lloyd Hollenburg, the team of researchers have developed a noninvasive method of integrated quantum imaging to analyze the flow of electrons present on the surface of a given graphene material.
To do so, graphene ribbons and metallic contacts are placed directly onto a diamond chip, in which the center of the chip contains atom-sized magnetic sensors in the form of nitrogen-vacancy (NV) centers. This fabrication process grows graphene directly onto the diamond chip through chemical vapor deposition (CVD), which is then followed by electron beam lithography (EBL) and plasma etching to create the graphene-diamond platform.
To visualize this sheet, the NV layer is illuminated with a green laser beam, resulting in a red photoluminescence (PL) layer to be imaged onto a computer screen3. By measuring this response of the red light from the graphene-diamond chip, the researchers were able to gain valuable information on the impurities, grain boundaries and/or ripples present within the chip that contribute to its overall electrical conductibility3.
The team of researchers is hopeful that this diamond-based imaging approach could provide important information for not only graphene materials, but also other 2D electronic systems.
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Burst testing graphene membranes
As the strongest material on Earth, graphene’s robust strength and pressure-resistance properties are attributed to a variety of properties including the granular structure of the polycrystalline material within graphene, the presence of van der Waals (vdW) structures within the thin layers of the material to allow for adhesion, and other relating structural properties4.
This strength is directly related to the ability of graphene to resist high pressures, as well as avoid possible deformations to the material, thereby allotting this material to be of primary interest for a number of device applications. To understand this property further, a team of MIT researchers led by Rohit Karnik from the Department of Mechanical Engineering looked at just how far they could test the tolerance of graphene’s fascinating ability to resist pressure.
Through utilization of the “burst test,” which involves measuring the pressure at which a membrane fails as it is suspended across a defined crevice present in the material. Once the researchers successfully grew graphene sheets of graphene through CVD, they then placed the sheets onto a porous polycarbonate track-etched membrane (PCTEM), whose pores measure between 30 nm and 3 micrometers (mm) in diameter4.
The burst test measured the failure of the pressure of the material by monitoring changes in the rate of the gas flow across the entire membrane. While graphene’s micromembrane makes it difficult to measure the precise pressure drop of the material, measuring the gas flow rate across the surface of graphene was sufficient to determine the overall pressure resistance of the material.
It was determined that the graphene placed over the PCTEM pores withstood pressures of 100 bars, which is nearly twice as strong as common pressures utilized for desalination purposes. In addition to this remarkable discovery, the researchers also found that graphene that contained wrinkles failed the pressure test, or “burst,” at pressures as low as 30 bars, while those that were void of any wrinkles reached the 100 bars mark.
By understanding the impact of wrinkles on graphene’s strength, the researchers concluded that the application of graphene for desalination and other water purification systems is a realistic goal for the future. While this is true, further research into creating graphene containing a uniform porous membrane must be achieved in order to fully determine the pressure-withstanding capabilities of its addition into these types of systems5.
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References
- Uses and Applications for Graphene in Electronics, Energy, Filtration and Advanced Composites
- “Graphene Applications & Uses” – Graphenea
- Quantum imaging of current flow in graphene” J. Tetienne, N. Dontschuk, et al. Science Advances. (2017). DOI: 10.1126/sciadv.1602429.
- “Single-Layer Graphene Membranes Withstand Ultrahigh Applied Pressure.” L. Wang, C. Williams, et al. Nano Letters. (2017). DOI: 10.1021/acs.nanolett.7b00442.
- “Graphene holds up under high pressure.” MIT News.
- Image credit: Shutterstock / Anna Kireieva
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