**2.1. Exfoliation**

applications that require fast response times, but not necessarily big on/off ratios such as RF

**Figure 3.** Diagram showing multiple mechanisms for inducing a band gap in graphene [19].

Since graphene's performance is very susceptible to contamination and structural defects (such as folds, grain boundaries, and pin holes) from processing or the transfer process, a review of graphene growth techniques should be done to determine the benefits and drawbacks of each. [1] Due to the sensitivity of graphene, the growth method must be chosen with the required quality, processing, scale, and device architecture in mind, making exfoliation good for small test structures but inadequate for a repeatable semiconductor targeted process. As shown in Figure 4 there are five major pathways for creating graphene sheets: exfoliation from bulk graphite, unzipping through etching a carbon nanotube, growth from sublimation and reconstruction of carbon from a carbide surface, epitaxial growth from a carbide forming catalyst layer by utilizing condensation during cooling, and the epitaxial growth utilizing a

electronics and IR detectors.

62 Graphene - New Trends and Developments

**2. Graphene fabrication**

non-carbide forming catalyst layer.

As of 2014, exfoliation methods produced graphene with the lowest number of defects and the highest electron mobility by the pioneers of graphene, Novoselov and Geim, using the adhesive tape method to isolate graphene from graphite.[1, 13, 20] The most common exfoli‐ ation method utilizes an adhesive tape to pull graphene films off a graphite crystal, which are subsequently thinned down by further strips of tape and finally rubbed against the desired substrate. This rather crude method creates a random array of single and double layer graphene flakes on the desired substrate that has been a key driver for investigating the many properties of graphene. Since graphene is susceptible to creating folds during this process, it cannot be produced with high accuracy and repeatability, so other mechanical and chemical exfoliation processes have been investigated. To address the difficulties of the scotch tape method, one group tried to exfoliate graphene from highly ordered pyrolytic graphite (HOPG) utilizing a sharp single-crystal diamond wedge penetrating into the graphite source to exfoliate layers. [21] This method has problems with defect initiation through shear stress and the reliable placement of the graphene flakes after exfoliation.

The other main exfoliation method is to utilize liquid-based techniques to create a dispersion of graphene or graphene oxide flakes that are drop-casted or ink-jet-printed, and in the case 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 spike. [21, 26, 27]

**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 transistor and device applications.
