Team:Tec Guadalajara/Description

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At a glance

About Graphene

Isolated for the first time in 2004 by Nobel Prize laureates, Andre Geim and Konstantin Novoselov, the allotrope of carbon known as graphene has gathered an steadily increasing attention by scientists and industries alike. A bidimensional crystal stable at standard conditions, its structure confers it a number of unusual mechanical, thermal, and electric properties that has placed it over traditional materials such as silicon.

Some of graphene's intersting properties are:

Strong

Graphene is one of the hardest material known. A single layer of this carbon structure can achieve a Young's modulus of 1.0 TPa (Lee et al, 2008). This elastic modulus, which works as a measure of a material's stiffness, overmatches steel's by 5 (Halliday, Resnick and Walker, 2001).

Thin

Graphene is regarded as a 2D material because it is made up of a single layer of carbon atoms. This means it has an effective thickness around 3 Å (Cooper et al, 2012), that is about 300,000 times thinner than an average piece of paper (Sherlis, 2001).

Conductive

Graphene's exceptional thermal conductivity (up to 5000 ^W⁄_mK for a single sheet of graphene) could improve performance and reliability of electronic devices (Singh et al, 2011).

Stretchable

Thin graphene samples can fully adhere and follow the topography of an elastic substrate. This means that these kind of samples are highly sensitive to tension, thus they are easier to deform (Cooper et al, 2002).

Transparent

Graphene only absorbs 2.3% of incident light, indicating that it is a good candidate to build transparent electronic components such as electrodes, polarizers, and ultrafast lasers (Tsakmakidis, 2013).

Graphene Production

Graphene´s quality is important since any deformations produced by impurities, structural disorders or wrinkles can directly impact in its properties, resulting in a material with poor electronic and optical applicability.

Nowadays, there are two major bottlenecks when talking about production of high quality graphene:

1. First, its mass production, since it is very difficult to produce large amounts of graphene with needed qualities for electrical sectors given that it must have a high conductivity and optical transparency.
2. Secondly, actual methods to synthesize graphene involve the usage of hazardous chemicals, like hydrazine, that generate toxic gases and waste.

To address these issues, it is imperative to develop a greener process with an eco-friendly approach, since the demand of this material will increase, and as such, its pollutants if the process is not modified.

Some of the currents methods for graphene are summarized next:

Method	Final Product	Advantages	Disadvantages	Market
Liquid Phase Exfoliaton	Nanosheets (nm to a few μm)	High scalability Low cost	Low yield Moderate quality Impure	Polymer fillers Transparent electrodes Sensors
Chemical Reduction of Graphite Oxide	Nanoflakes or powder (nm to a few μm)	High scalability Low Cost	Low purity High defect Density	Conductive inks and paints Polymer fillers Battery Electrodes Supercapacitors Sensors
Micromechanical Exfoliation	Flakes (5 to 100 μm)	High quality	Small scale production High cost Uneven films	Research purposes
Epitaxial Growth	Thin films (>50 μm)	High quality	High cost Low yield High process temperature (1500 °C) Very expensive substrate	Transistors Circuits Interconnects Memory Semiconductors
Carbon Nanotube Unzipping	Nanoribbons (few microns)	High yield High quality Potentially low cost	Moderate scalability	FETs Interconnects NEMs Composites
Chemical Vapour Deposition (on Ni, Cu, Co)	Thin films (≤75 cm)	High quality	High process temperature (1000 °C) High cost Moderate scalability	Touch screens Smart windows Flexible LCDs & OLEDs Solar cells

From all these methods, the chemical reduction is the most likely to be used in mass production given that its disadvantages vary according to the reducing agent Actually, hydrazine is the reducing agent of choice, but its toxicity has pushed researchers to look for other alternatives

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Reducing Agents

Over the years, several reducing agents have been reported in the literature as replacements of hydrazine. The use of these substances and compounds has an impact on the final properties of the reduced material. In the table below, the electrical conductivity of rGO is shown as a function of the chemical involved in its synthesis.

Reducing agent	Electrical conductivity (S⁄m)	Reference
Hydrazine	9960	(Wang et al, 2008)
Ascorbic acid	7700	(Fernández-Merino et al, 2010)
Benzyl alcohol	4600	(Dreyer et al, 2011)
Isopropanol	1019	(Dreyer et al, 2011)
Pyrogallol	488	(Fernández-Merino et al, 2010)
Tea polyphenols	53	(Wang, Shi and Yin, 2011)
Yeast biomass	43	(Khanra et al, 2012)
Ethanol	1.8 × 10−4	(Dreyer et al, 2011)
Methanol	3.2 × 10−4	(Dreyer et al, 2011)

References