Blog entry

How is graphene produced

By Christian Alvino,  Graphenea

Graphene is a two-dimensional system consisting of one (or few) layer(s) of graphite. Thanks to its remarkable properties (high charge mobility, high thermal conductivity, robustness to mechanical stress etc) it holds many promises in different areas of nanotechnology and for the discovery of new phenomena in solid state physics. It displays a number of peculiarities: it is a zero gap semiconductor with a degeneracy point of the electronic bands which gives an effective mass equal to zero for both type of charge carriers, its electrical transport properties are described by the Dirac equation and spin-orbit interaction is negligible. The latter is relevant for applications in spintronics.

However the production of this material is quite challenging: the first method that was used to isolate it was the mechanical exfoliation of graphite, the so-called "scotch tape method"1. As the name suggests, the graphene layers are exfoliated by scotch-tape from a graphite bulk sample. The top layers are removed from the graphite, further thinned and then deposited on a wafer with a suitable layer of SiO2/Si. In spite of its simplicity, this method still allows to obtain the best samples in terms of crystalline quality and electrical transport properties. Its limitations are: the low yield and the inability to produce graphene on a large scale.

In order to overcome this issue R&D moved on vapor deposition. The first successful method was the synthesize by the desorption of Si from SiC single-crystal surfaces, which results in a multilayered graphene structure that behaves like graphene2,3 . However, there is no method for the formation of a graphene layer that can be exfoliated and transferred from the graphene/SiC sample to an arbitrary substrate, but there is a way to grow and transfer graphene grown on metal substrates like nickel4-6, ruthenium7, iridium8 and copper9.

The technique used to grow graphene on metals is the Chemical Vapor Deposition. This method, widely used in semiconductor industry, consists in heating the substrate up to 1000 C in a controlled atmosphere (typically Argon and hydrogen) and then a flux volatile precursor like methane is introduced in the chamber. The methane dissociates by the catalytic effect of the substrate forming a graphene layer on the metal surface (1). In order to transfer the graphene to another substrate a polymer (PMMA) is spin-coated on top of the substrate in oder to support the graphene layer (2) and then the metal is etched (3). After a rinse in deionized water (4) the graphene layer can be transferred on the desired target (5). Later the polymer is dissolved in solvent leaving the graphene on the target (6).

 

Nickel and copper are the most used metal for graphene growth. Interestingly, the graphene grown on nickel appears to be limited by its small grain size, presence of multilayers due to the high solubility of carbon in nickel5,6. By the contrast graphene growth on copper foils is predominantly graphene with small area having two- and three-layer graphene flakes. More interestingly, the growth is self limited so the small multilayer flakes do not grow larger with time. It is also well known that annealing of Cu can lead to very large grains that is beneficial for industrial applications9.

Note added: the figure that illustrates this entry was taken from reference 10. 

 

Bibliography

  1. K. Novoselov et al., Science, 22 Oct 2004, Vol. 306, Issue 5696, pp. 666-669
  2. C. Berger et al., Science, 26 May 2006, Vol. 312, Issue 5777, pp. 1191-1196
  3. K. Emtsev et al., Nature Materials 8, 203 - 207 (2009)
  4. Q Yu et al., Appl. Phys. Lett.93, 113103 (2008)
  5. K. Kim et al., Nature 457, 706 - 710 (2009)
  6. A. Reina et al., Nano Lett.,2009, 9 (1), pp 30–35
  7. P. Sutter et al., Nature Materials 7, 406 - 411 (2008)
  8. J. Coraux et al., Nano Lett., 2008, 8 (2), pp 565–570
  9. X. Li et al., Science, 05 Jun 2009, Vol. 324, Issue 5932, pp. 1312-1314
  10. S. Goniszewski, J. Gallop,M. Adabi, K. Gajewski, Q. Shaforost, N. Klein, A. Sierakowski, J. Chen, Y. Chen, T. Gotszalk,
    L. Hao, Self-supporting graphene films and their applications, IET Circuits, Devices & Systems, 2015,Vol. 9, No.6, 420-
    427, DOI: 10.1049/iet-cds.2015.0149