Graphene: The strongest material ever examined

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2009

This two-dimensional, single layer of graphite contains carbon atoms arranged in hexagonal structures. 

KEY CONCEPTS
• Graphene is a single layer of graphite that contains carbon atoms arranged in hexagonal structures.
• The mechanical strength of grapheme is determined by depositing pure monolayers onto a silicon substrate.
• Graphene is found to be the strongest material ever examined with a break strength 200 times the strength of steel.

Graphite is one of the best known solid lubricants. This material exhibits a lattice crystalline structure that contains layers or planes of carbon atoms. Jeffrey Kysar, associate professor of mechanical engineering at Columbia University in New York, says, “The atomic planes of carbon atoms in graphite easily decouple from each other, enabling them to easily slide, which generates very little friction.” This is the reason why graphite is such a good lubricant.

In the growing area of nanotechnology, one of the main substances of interest is carbon nanotubes, which are prepared from a single layer of graphite known as graphene. Kysar says, “Graphene is a molecule that consists of an individual atomic layer of carbon atoms. It is the fundamental building block of graphite. In addition, a sheet of graphene can be rolled up to form a carbon nanotube or used in the construction of a sphere such as a buckeyball.”

A graphene molecule contains carbon atoms arranged in hexagonal structures in which each atom is connected to three other atoms. The bond length between carbon atoms is approximately 0.142 nanometer. As such, graphene is a true two-dimensional material, which gives it special properties.

The carbon-carbon bonds in graphene are very strong, which enables such applications as the use of carbon fibers in advanced composites. One problem that researchers have faced is how to measure the intrinsic strength of grapheme, which is the strength of pristine graphene that contains no defects. Kysar comments, “There has been a good deal of difficulty in determining the actual intrinsic strength of graphene. Structural defects such as interstitial atomic vacancies and atomic dislocations and the presence of grain boundaries have made it very challenging to obtain a pure enough sample of graphene to evaluate.”

Graphene’s strength has been estimated by computer modeling theories. With the widening use of graphene in nanoscale devices, it is very important to determine this material’s actual strength. A recent set of experiments to characterize the intrinsic strength of graphene was performed by Kysar; his colleague James Hone, associate professor; Dr. Changguo Lee, a post-doctoral researcher; and Xiaoding Wei, doctoral candidate, all of the Mechanical Engineering Department at Columbia University.

NANOINDENTATION
Kysar and his research associates were able to isolate pure graphene by mechanically depositing graphite flakes onto a silicon substrate that were prepared by exfoliation from a graphite source. Kysar explains, “We prepared wells on the silicon wafer surface that were either 1 or 1.5 microns in diameter and had a depth of 0.5 micron.”

Graphite flakes were placed on the silicon wells and evaluated by an optical microscope to determine their composition. By using this approach, the researchers were able to distinguish the desired monolayer from bilayers and trilayers of graphene. Kysar says, “We encountered a random distribution of layers with various thicknesses. With practice, we were able to identify the monolayers and then confirm their structure using Raman spectroscopy.”

Noncontact atomic force microscope (AFM) imaging was used to confirm that the graphene layer was tightly stretched across the silicon wells. Kysar adds, “We found that the graphene adhered to the silicon epilayer because of van der Waals attraction.”

Each film was then indented using one of two different diamond tips connected to the cantilever of an AFM. The diamond tips have radii of 16.5 and 27.5 nanometers and were measured before and after indentation by transmission electron microscopy.

Kysar says, “The diamond tip was lined up to within 50 nanometers of the center of the graphene monolayer. We then pushed down on the center of the film and measured the force vs. displacement response several times. The data was found to be independent of the diameter of the radius of the diamond tip and of the monolayer tested.” Figure 2 shows a sketch of this process.


Figure 2. The strength of graphene is measured by pushing a diamond tip connected to a cantilever of an atomic force microscope onto the center of a monolayer film. (Courtesy of Columbia University)

The graphene films displayed good hysteresis and did not slip around the periphery of the silicon well. Given the geometry of the specimen, the researchers believed that only 5% to 10% of the atomic bonds in the immediate vicinity of the indenter tip were strained to extremely high levels.

In a second step in the experimentation, the graphene monolayers were indented until they broke. Kysar says, “We found that the stress needed to break graphene was 42 newtons per meter, which is a record for any material. This value was independent of the graphene tested and the diamond tip used.”

Based on the stress value obtained, the researchers could then determine the intrinsic strength of graphene. Kysar says, “We determined that graphene has a break strength of 130 gigapascals, which is 200 times the strength of steel, if one assumes the thickness of the graphene sheet is the same as that of the interplanar thickness in graphite.” From a real-world perspective, the researchers noted that a sheet of graphene with the thickness of Saran Wrap could support the weight of an elephant balanced on a pencil above it.

The researchers are very confident in the accuracy of the data. The graphene placed in the silicon wells was free of defects. By focusing on testing at the middle of the graphene monolayer, very reproducible results were obtained. A total of 67 test values were compiled from 23 separate films.

Kysar believes the mechanical properties of graphene will enable its use in many new applications that require materials with excellent strength. He noted that defect-free graphene could be used to develop the space elevator postulated by science fiction author Arthur C. Clarke.

Kysar says, “Clarke envisioned that a space elevator could be built to transport materials from the surface of the earth up 23,000 miles to satellites in geosynchronous orbit. Graphene is a substance that can be stretched out from that height above the Earth and can support its own weight. These characteristics could enable the use of graphene in a space elevator.”

The importance of this finding is that further use of graphene in nano-based applications will be better facilitated by more accurate information about the material’s mechanical properties. Further details on the work can be found in a recent article (1).

REFERENCES
1. Lee, C., Wei, X., Kysar, J., and Hone, J. (2008), “Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene,” Science, 321 (5887), pp. 385–388.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.