New electronic wonder electronic material is “made in Manchester”

Dr Matthew Halsall – Microelectronics and Nanostructures

School of Electrical and Electronic Engineering

In October 2010, the scientific world was abuzz with the news that two researchers at

Manchester University had been awarded this year’s Nobel prize in Physics for their

discovery of a 21st century wonder material- Graphene.

The two professors involved -Andre Geim and Kostya Novoselov had been working on a well

known form of carbon - Graphite (best known for its application at the sharp end of pencils!).

It had been predicted for many years that very thin layers of carbon could have interesting

properties, in particular that electrons could move very fast in them. However, obtaining such

thin layers had proved problematic, graphite is brittle material and tends to fragment into

clumps when broken and not split neatly into layers as one might split slate for instance.

In 2004 Geim and Novoselov found a solution in the most unlikely of places, in fact the same

place that most people would find the graphite- in a stationary cupboard! They knew that

graphite could be cleaned effectively by applying sellotape to its surface and pealing a layer

of graphite off, they simply wondered what the material left adhering to the tape looked like.

After a few trials and repeated attachment

and detachment of the tape, they found

that they had produced the holy grail of

carbon researchers, single atomic layers of

hexagonally arranged carbon atoms (artist

impression shown right). They

immediately got to work testing out their

theories. In rapid time they demonstrated

that the material had truly remarkable

properties.

Physically graphene is 200 times stronger than steel (for an equivalent thickness) and is being

actively studied for reinforcement of polymers for applications in spacecraft, aircraft, sports

equipment and any area were carbon fibres currently dominate. However it is in the field of

electronic were its effects are most likely to be felt. Electrons moved faster in graphene then

any other material, showing quantum effects like the “quantum hall effect” at room

temperature (previously temperatures close to absolute zero were necessary).

Researchers in the school of electrical and electronic engineering have contributed to this

work since the first reports of the isolation of this remarkable material.

In the Microelectronics and nanostructure group in the School we have been studying the

optical properties of the materials supplied by Profs Geim and Novoselov. Currently silicon

dominates the semiconductor market, in many senses the dominance of silicon can be put

down not to the properties of silicon, but to those of its oxide.

When processing a silicon wafer into chips it is a very simple process to form an insulating

layer (essential to make components such as field effect transistors) by oxidation of the silicon

to form an insulating oxide. Graphene has the capacity to revolutionise the electronics

industry, maintaining Moore’s law on the speed of computers for several more decades.

Researchers have already demonstrated graphene based transistors that can operate at speeds

of 100’s GHz.

However if graphene is to find its way into everyone’s desktop, it needs to be processed as

silicon can be, in particular a way needs to be found to convert it into an insulator using

simple chemical processes. Last year members of my research group were involved in

studying layers of graphene that had been exposed to a hydrogen plasma in Professor Geim’s

group in Physics. We used an optical technique known as Raman scattering to study the

effects of the processing. The Raman Effect is where light incident on a material has its

wavelength altered as the result of the interaction of the light with the atoms of a material.

The Raman shift is a measure of the frequency of vibration of the atoms.

In the figure below the left hand spectrum shows that of normal graphene, the right hand side

after its exposure to hydrogen, the inset is a picture of the now transparent graphene layer

(taken from ref [1]).

The Raman spectra on the right shows a more intense D’ line, the presence of this line is

related to disorder in the crystal caused by the hydrogen atoms bonding to the surface of the

graphene. The changes were found to be reversible, disappearing after high temperature

anneal to remove the hydrogen. This was conclusive proof that they had formed a new

material “graphane” which was insulating and could be used to play the role of silicon oxide

in a graphene based electronic system.

Although there is much still be done before we find it used in all our electronic products, the

work on graphene is now growing almost exponentially, not bad for a new material “made in

Manchester” only 6 years ago!

[1] “Control of graphene's properties by reversible hydrogenation"

R.Nair, T.M.G.Mohiuddin, S.V,Morozov, D.Celias, P.Blake, M.P.Halsall, A.C. Ferrari,

D.W.Boukhalov, M.Katsnelson, A.Geim, and K.S.Novoselov SCIENCE Volume: 323

Issue: 5914 Pages: 610-613 Published: 2009