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Posted: Jul 21, 2015
Fabricating patterned graphene from a solid source of polystyrene
(Nanowerk Spotlight) Since the first 'Scotch tape' method – i.e. mechanical peeling – of making graphene was reported in 2004, researchers have come up with a variety of techniques for producing graphene. Since simply using the as-produced graphene flakes is not good enough for use in sophisticated applications, intricate patterning processes are essential for the development of the required graphene structures for use in nanoelectronic and optical devices.
"Common patterning methods require several steps such as lithographic techniques and wild etching process, resulting in damages and poor quality of graphene," Jin Kon Kim, a professor in the Department of Chemical Engineering at Pohang University of Science and Technology, and Director of National Creativity Research Initiative Program for Smart Block Copolymers, tells Nanowerk. "To overcome this problem, we studied a bottom-up approach based on CVD graphene growth from solid source."
"For example" he elaborates, "heteroatoms such as boron nitride and aluminum oxide are used as a barrier material under the growth process of graphene. These approaches, however, have complicated fabrication steps and lead to reduced device performance resulting from diffusion of heteroatom."
Therefore, Kim and his team utilize that barrier material and graphene are individually yet simultaneously grown from a single polymer, resulting in directly micro and nano patterned graphene with high quality.
In this process, graphene is successfully grown from neat polystyrene regions, while patterned cross-linked polystyrene (CPS) regions turn into amorphous carbon (a-C) because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least 2 orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene region.
"Furthermore, measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface," Kim points out.
Optical image of controlled cross-linked pattern of polystyrene; Light purple regions are neat PS, and the others are CPS (left side of the upper figure). Optical image of graphene/a-C transferred on SiO2/Si wafer (right side of the upper figure). Illustration of growth mechanism for graphene/a-C heterostructures (the lower figure). Since CPS reveals higher thermal stability than PS, CPS converts into a-C which acts as an effective barrier preventing graphene growth, while PS converts into high quality patterned graphene. (Image: Center for Smart Block Copolymers, Pohang University of Science and Technology)
Kim notes that, in this study, the resistivity difference between graphene and a-C is not large – two-orders of magnitude difference. However, the team has identified a way to increase further the resistivity of a-C. As the growth temperature decreases, the resistance of a-C is increased due to its temperature-dependent phase transition from nanocrystalline carbon network to amorphous carbon. On the other hand, the quality of graphene is generally reduced when it grows at lower temperature.
"Therefore, by using proper choice of alloy catalyst and optimization of the process, one can grow simultaneously both a-C with high resistance and graphene with high quality," Kim concludes.
(a) Optical microscope (OM) image of as-grown graphene/a-C hexagonal pattern on copper foil. The six sides of hexagons are a-C, and the others are graphene. (b-d) OM image, SEM image and dark field OM image of graphene/a-C hexagonal pattern transfer to SiO2/Si wafer. (e) Raman spectra from graphene and a-C. (f and g) Raman maps of graphene/a-C line pattern based on the Raman frequency of ωG and ω2D. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
The results of this work could be directly applied to graphene-based flexible devices. The team found that for a bending radius up to 8 mm, corresponding to a tensile strain value of 3%, both Dirac voltage and mobility of their graphene/a-C heterostructure are comparable to those of unstrained graphene.
Moreover, the heterostructures fabricated in this study could be easily extended to nanoscale patterned graphene once templates with nanosized patterns – for instance, block copolymers – are used.
Two-dimensional (2D) materials, such as graphene, h-BN and MoS2, are very promising materials for applications electronics and optics. As this work by the Korean researchers indicates, in order to integrate these 2D materials into atomically thin systems, the direct synthesis of patterned graphene from polymer pattern could be further exploited to prepare versatile heterostructures based on 2D materials.