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Graphene from Nickel

June 11, 2014

Most materials solidify into crystals, which are regular arrays of atoms. That's a simple consequence of thermodynamics, since such a regular arrangement of atoms is a low entropy state; and, at least for ions such as those of sodium and chlorine, such an arrangement leads to a higher bonding energy between atoms.

Essentially, you're minimizing the Gibbs free energy G, which is a function of both reaction enthalpy (the bonding energy) H and the entropy S at a given absolute temperature T; viz., G = H - TS.

Figure captionFodder for science textbooks, the crystal structure of sodium chloride (NaCl, halite, sodium = red, chlorine = blue)

This is a face-centered cubic crystal, 0.564 nm on an edge. Halite was an early standard for Xray crystallography.

(Illustration via Wikimedia Commons, modified)

In a perfect world, atoms would always arrange themselves in neat arrays, but in the real world, everything has defects, and atoms move around. Atoms will remain in placed at absolute zero, essentially because atomic movement is how temperature is defined. Atoms remain in place until about 80% of a material's melting point in absolute temperature units (kelvin). Above that, atom motion is used to beneficial effect in annealing, which removes internal strain.

The industrial revolution was built not only by steam, but also by steel. Iron is a plentiful and inexpensive material, and the 1855 Bessemer process, which had actually been practiced for quite a time before Henry Bessemer's patent, allowed production of very pure iron in large quantity. Pure iron is soft and ductile, but a pinch of carbon transforms iron to higher strength steel.

Carbon is a small atom (77 picometers (pm) when bonded as sp3) compared with iron (126 pm), so carbon atoms are relatively mobile in iron. Since steel is such an important material, the diffusion (movement) of carbon atoms in iron has been thoroughly researched. Carbon diffusion in iron follows an Arrhenius-type law,

 Arrhenius equation for carbon diffusion in iron

where D is the diffusion coefficient (measured in cm2/sec), the activation energy is 0.873 electron volts, kB is the Boltzmann constant and T is the absolute temperature. Because of this is an exponential function, the diffusion coefficient varies by many orders of magnitude between room temperature and the melting point of iron, 1538 °C.

Nickel, like iron, is in the first transition metal series of the Periodic Table, and it's separated from iron by just one chemical element, cobalt. The crystal structure of nickel (face-centered cubic) has its atoms more closely spaced than the body-centered cubic structure of iron, so the diffusion of carbon in nickel is much smaller.

At high temperatures, the diffusivity of carbon in nickel is more than a million times smaller than that for iron.[1] However, if your goal is to transport carbon through a very thin layer of nickel, the diffusivity is more than enough. The solubility of carbon in nickel is also a hundred times less than that of carbon in iron.[1]

Scientists from the Department of Mechanical Engineering, the University of Michigan (Ann Arbor, Michigan), the Department of Mechanical Engineering, the Massachusetts Institute of Technology (Cambridge, Massachusetts), and Guardian Industries Corporation (Carleton, Michigan) have used these properties of carbon in nickel to devise a process for production of large area graphene sheets on silica.[2-3]

The first process for creation of graphene was the use of Scotch tape to exfoliate atomically thin graphene sheets from graphite. It was a crude process, but it was good enough to win the 2010 Nobel Prize in Physics for its discoverers, Andre Geim and Konstantin Novoselov. It would be difficult to commercialize graphene devices using this technique.

Another process for production of graphene is to grow it on a metal, such as nickel or copper, but it's only useful when removed from the metal. Although the growth of large areas of graphene has become common, removal of the graphene from its growth substrate is a major issue.[3] That's the problem tackled in this latest study.

The research team, led by A. John Hart of MIT, developed a process for production of graphene on silica glass (SiO2) by using chemical vapor deposition (CVD) to grow graphene from ethylene on both sides of a nickel film deposited on the glass substrate. A subsequent dry mechanical delamination using adhesive tape removes the nickel layer and the top graphene layer to leave a layer of graphene on glass (see figure).[2]

Figure captionThe graphene-on-glass process.

Graphene can be grown on both sides of a nickel film on a glass substrate.

(Original image by the study authors, redrawn for clarity, via MIT)

The graphene is produced in micrometer-sized, monolayer to multiple-layer domains. There's greater than 90% coverage across a centimeter dimension substrate, which was the size limit for their CVD system.[2] An important part of the removal process is a strain-relief anneal of the nickel film prior to graphene deposition. Although the nickel film remains adherent to the glass even after formation of the graphene, it can be mechanically removed after deposition.[2]

Says MIT's Hart,
"We still need to improve the uniformity and the quality of the graphene to make it useful... The ability to produce graphene directly on nonmetal substrates could be used for large-format displays and touch screens, and for 'smart' windows that have integrated devices like heaters and sensors."[3]
The work was supported by the National Science Foundation, the Air Force Office of Scientific Research, and Guardian Industries.[3]

References:

  1. J. J. Lander, H. E. Kern and A. L. Beach, "Solubility and Diffusion Coefficient of Carbon in Nickel: Reaction Rates of Nickel‐Carbon Alloys with Barium Oxide," J. Appl. Phys., vol. 23, no. 12 (December 1, 1052), p.1305ff., DOI:10.1063/1.1702064.
  2. Daniel Q. McNerny, B. Viswanath, Davor Copic, Fabrice R. Laye, Christophor Prohoda, Anna C. Brieland-Shoultz, Erik S. Polsen, Nicholas T. Dee, Vijayen S. Veerasamy and A. John Hart, "Direct fabrication of graphene on SiO2 enabled by thin film stress engineering," Scientific Reports, vol. 4 (May 23, 2014), Article no. 5049 (doi:10.1038/srep05049). This is an open access article with a PDF file available, here.
  3. David L. Chandler, "A new way to make sheets of graphene," MIT Press Release, May 23, 2014.

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