Graphene: the basics

With a thickness of only one atom and weighing around 0.77 milligrams per square meter, Graphene is one of the strongest materials known to man, with a breaking strength of 100 to 300 times greater than steel, and one of the most pliable compounds discovered. Firstly produced in 2004, graphene can conduct electricity and heat better than virtually anything else. It can even absorb a large amount of white light, which is useful in certain applications like mode-locking of laser fiber and spin transport.

Graphene is an hexagonal honeycomb lattice

Figure 1. Graphene is an hexagonal honeycomb lattice. (Photo courtesy of
Wikipedia)

Technically speaking, graphene is a crystalline allotrope of sp2 bonded carbon atoms with a bond length of 0.142 nanometer. Or, in simpler terms, it is a single layer of pure carbon, densely packed in a hexagonal honeycomb lattice. Graphene can be viewed like a chicken wire made from carbon atoms.

Extremely thin layers of graphite had been observed for several years but no atomically thin graphite layer was synthesized until 2004. The eventual production of graphene led to the Nobel Prize to Geim and Novoselov in 2010.

Seven major properties of graphene

As a “ultimate” material, graphene has outstanding properties: high strength, low weight, high elasticity, superb electrical and thermal conductivity, great optical absorption and cost-effectiveness.

1.Strength

As been pointed out, graphene is probably the strongest material known, and the reason for this may be attributed to the strength of its carbon-carbon bonds. Its tensile stiffness of 130 gigapascals is approximately 300 times greater than the 400 megapascals strength of A36 structural steel.

2. Low weight

Despite of its strength, graphene is very light. Its weight of 0.77 milligrams per square meter is 1000 times lighter than a square meter of paper. The most common analogy used to describe the strength and lightness of graphene is that one gram of a single sheet of graphene would be large enough to cover a whole football field. In other words, a cubic inch of graphene can be balanced on the edge of grass.

3. Elasticity

Another outstanding property of graphene is its elasticity. Graphene sheets of two to eight nanometers thick have a spring constant of one to five newtons per meter and the estimated Young’s modulus values of single- and bilayer graphene are 2.4 ± 0.4 and 2.0 ± 0.5 TPa, respectively.

4. Electrical conductivity

Figure 2 is a diagram representation of the atomic structure of a graphene sheet. A carbon atom has six electrons, two of which are located in the inner shell and the remaining four is at the outer shell, available for bonding. But in a graphene sheet, which is two dimensional, only three of these electrons bonded with other carbon atoms; leaving one electron free for electrical conduction. Thus, graphene has both holes and electrons charged carriers, increasing its conductivity.

Carbon-carbon bonds in a graphene sheet

Figure 2. Carbon-carbon bonds in a graphene sheet. Three of the four outer shells of carbon are bonded with other carbon atoms, leaving one free electron for electrical conduction.

A research team at Columbia University, headed by Professor James Hone, tested the electrical conductivity of graphene by straining the material. They were able to stretch the material by 20 percent of its initial size and still attain electrical conduction; whereas a silicon wafer can only be stretched by one percent without cracking.

5. Thermal conductivity

Virtually unlimited – this is how scientists describe the thermal conductivity of graphene. It has been found out that the thermal conductivity, usually regarded as a constant for other materials, with graphene varies with length. This exceptional property of graphene will be very beneficial in micro- and nano – electronics, where heat usually causes problems of leakage.

6. Light absorption

Typically, a material needs to be as thick as thousands of atoms in order to absorb light. But this is not the case for graphene. The idea of having a material only one atom thick that can absorb light has thrilled physicists and scientists. It has been discovered that the structural arrangement of the atoms in graphene is perfectly defined for optimal light absorption.

7. The greatest advantage

Finding a strong, light, flexible and highly conductive material is extremely rare, if not unique, in the history of chemistry. Graphene not only sums up all these properties but is also potentially very inexpensive. Carbon, which is the base material of graphene, is the fourth most abundant element on earth. This means, graphene can be a practical, economical and outstanding solution for a limitless number of applications. We list some of them here

A few potentially revolutionary applications for grapheme

  1. Mobile phones charged for more than a week and which need only 15 minutes to recharge.
  2. Foldable mobile phones as thin as paper that easily fitting into pockets.
  3. Extremely small sensors
  4. Bionic devices implanted directly in the human body and possibly integrated with the nervous system. Let` s not forget that, by being chemically inert, graphene is also bio-compatible.
  5. Electronic vehicles built on graphene that can be integrated with solar panels to recharge while driving.
  6. Extremely efficient water filters.
  7. Touch screens made of plastic.
  8. Super capacitors that will eventually render batteries obsolete.

Considering all the advantages and strong points that graphene has, why then graphene-based products have yet to hit the market?

Main issues with large-scale production of graphene

Though the carbon itself is inexpensive, the production of high quality graphene on a large scale is costly and requires complex technique as graphene is usually grown over a metallic substrate using toxic chemicals. Moreover, removing graphene layers from the metallic substrate without damage is challenging.

One of the most highly regarded processes of synthesizing graphene involves chemically extracting it from graphite oxide. Graphite is a three-dimensional array of carbon atoms composed of million layers of graphene. Graphite is oxygenated in order to expand its interlayer structure and at same time inherit it the ability to become hydrophilic. The oxygenated graphite is then exfoliated in water through sonication, producing a single layer or a few layers of graphene oxide. However, the sp2 bonds in graphene oxide are destroyed turning the material from being electrically conductor to insulator. In order to return the material to its honeycomb lattice formation and to regain its electrical conductivity, the oxygen content of graphene oxide is reduced by chemical functionalization. This method, while being probably the most straightforward to produce graphene in relatively high quantities, is not yet consistent and reliable in creating sheets of the same quality.

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