**2.3. Sublimation and reconstruction from carbide**

of graphene oxide subsequently reduced. Liquid exfoliation can be accomplished through the use of solvents or ionic liquids with similar surface tension to graphene, which when sonicated exfoliate the bulk graphite into graphene sheets that can be subsequently centrifuged to create a supernatant and dispersed. [22–25] Probably the oldest known method for producing graphene is through the production of graphite oxide using Hummers' method, sonicating to create a dispersion and then reduction of the graphene oxide either through the introduction of hydrazine at elevated temperature or through the introduction of a quick burst of energy introduced either through a light burst as shown in Figure 5 (flash or laser) or a temperature

**Figure 5.** Reduction of graphene oxide using a LightScribe laser writing system ion, a standard DVD writer [28],

One of the more interesting liquid exfoliation methods utilizes sonicating graphite at the interface of two immiscible liquids, most notably heptane and water, producing macro-scale graphene films. [29] The graphene sheets are adsorbed to the high energy interface between the heptane and the water, where they are kept from restacking. [29] The graphene remains at the interface and the solvents may then be evaporated isolating the graphene flakes. [29]

Straightforward mechanical exfoliation methods have been able to produce high-quality graphene flakes that have been very beneficial for the investigation of the amazing character‐ istics of graphene, while liquid exfoliation (and reduction) methods have been utilized for the production of transparent conducting oxides, conductive inks, and electrodes for Li-ion batteries and super capacitors. Mechanical exfoliation, however, cannot be reliably scaled up to provide the reliable placement and large area high-quality graphene sheets desired for

As shown in Figure 6, graphene can be created by cutting open carbon nanotubes. [7] In one such method, multi-walled carbon nanotubes are cut open in a solution by action of potassium permanganate and sulfuric acid. [30] In another method, graphene nanoribbons were pro‐ duced by plasma etching of nanotubes partly embedded in a polymer film. [30] This method is useful for producing nanoribbons of graphene that induces a band gap in graphene through geometry breaking, which will be discussed in Section 3. However, the placement of the nanotubes on an integratable chip has been problematic, and thus this method once again is

only good for the production of test structures to probe graphene characteristics.

spike. [21, 26, 27]

64 Graphene - New Trends and Developments

transistor and device applications.

**2.2. Carbon nanotube unzipping**

Heating silicon carbide (SiC) or other carbide materials (TaC, NbCm ZrC, HfC, TiC) to high temperatures (>1,100°C) under low pressures (~10-6 torr) boils off the Si (from either the Si face or underlying Si from the C face) and reconstructs the C into a single layer graphene film, although multi-layer graphene has been produced through this approach as well. [19, 32] This process produces epitaxial graphene with dimensions dependent upon the size of the wafer.

**Figure 7.** Bonding of graphene at a SiC step edge [33].

The particular face of the SiC used for graphene formation, silicon- or carbon-terminated, highly influences the thickness, mobility, and carrier density of the resulting graphene, with the best results coming from a step edge in SiC that produces "floating" graphene attached to the SiC on the top and the bottom of the step edge as shown in Figure 7. [33] There has also been some work utilizing Ni and Cu bilayers to catalyze the production of graphene from SiC achieving growth at higher pressures and lower temperature. [34] The benefit of using graphene produced from SiC is that SiC is easily integratable with microelectronics processing technologies. The SiC is not desired for most electronics applications, making it desirable to transfer the graphene from its SiC substrate to a more standard substrate such as Si. The sublimation of graphene from SiC also creates a Si2O3 insulating under layer that could assist with the transfer process. Under high temperatures, a large variety of intercalant species can also be placed between the graphene and SiC layer that can potentially help with the exfoliation or the electrical modification/isolation. [19] Under normal conditions, the graphene SiC interface forms a Schottky contact; however, it has been shown that the oxide can be trans‐ formed to a nitrogen underlayer through a thermal annealing process in a nitride atmosphere modifying the electronic characteristics between the two. [35]

### **2.4. Growth through condensation after carbide formation**

Graphene growth utilizing a carbonaceous source material (such as methane introduced through a CVD process) differs from material to material with the carbon solubility in the metal and the growth conditions determining the deposition mechanism as shown by the phase diagrams in Figure 8. [7] For carbide producing metallic substrates (such as Ni), graphene growth occurs through a precipitation process during cooling from the carbide. [7] The solubility of C in the metal (Ni for example) is higher at higher temperatures, and thus during the furnace cooling phase carbon diffuses out of its Ni host. [7] The process of forming graphene on Ni has the fundamental limitation that single and few layered graphene is obtained over few to tens of micron regions and not homogeneously over the entire substrate. [7] The lack of control over the number of layers is attributed to the difference in out-diffusion of C from the grains and the grain boundaries of Ni creating non-homogeneous growth conditions.

**Figure 8.** The phase diagram for a carbide creating catalyst (Fe) and a non-carbide creating catalyst (Cu) [7].

### **2.5. Epitaxial growth of graphene on a non-carbide forming catalyst**

Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where there is registry between the two. In some cases, epitaxial graphene layers are coupled to surfaces weakly enough (by Van der Waals forces) to retain the 2D electronic band structure of isolated graphene. [31, 32] It is commonly accepted that the production of graphene through the surface absorption of carbon on a non-carbide producing metal (such as Cu or Ir) is an epitaxial process due to the registry between the underlying Cu (or Ir) crystal structure and the graphene layer.

Exceptional high-quality single layer graphene growth over large areas have been recently achieved on polycrystalline copper foils.[7] The growth on Cu or Ir is simple and straightfor‐ ward due to the metallic substrates not having a stable carbide material, thus the decomposi‐ tion of C is only reliant upon the grain orientation. [36] For example, Cu is an FCC lattice with three dominant grain orientations Cu(100), Cu(110), and Cu(111) along with high index facets which are made up of combinations of low index facets. [36] Cu(100), Cu(110), and Cu (111) have cubic, rectangular, and hexagonal geometries making the Cu(111) grain orientation able to support epitaxial growth. [36] Thus, grain growth on Cu(111) grains tend to be monolayered graphene sheets while Cu(100) and Cu(110) geometries prevent C diffusion causing compact multilayered C islands to form with higher index facets replicating the performance of the lower index grains. [36]

Despite the ability for Cu and other metal substrates to grow high-quality graphene flakes for device applications, graphene has to first be transferred onto a semiconducting or insulating substrate when using this growth method. [37] The transference process usually involves spinning on a polymer, etching off the catalyst metal layer, then transferring the graphene onto the desired substrate by placing it on the substrate, and finally etching off the substrate. [37] Both of these processes can produce contaminants on the graphene layer, reducing the mobility by adding scattering centers in the sheet. [1] Groups have been working on ways to reliably reduce these contamination effects; one group has utilized Ti sputtering along with a Ti etch to remove any remaining Cu, while another group has shown that by first spinning on a lift off resist before a normal polymer backing layer produces a much cleaner graphene layer. [38, 39] High-quality graphene has also been shown to be grown between a GaN and Ni interface where the Ni can be peeled off and the graphene layer is left on the GaN substrate. [40]
