Saturday, June 4, 2011

Introduction to Graphene and Graphene Nanoribbons-Part2

Graphene Production

Graphene was first produced by mechanical exfoliation of graphite. This method provided a small amount of high quality samples for fundamental studies. Later on, several methods have been utilized to synthesize graphene sheets which might be categorized into bottom-up and top-down approaches with carbon containing molecules and graphite as initial materials, respectively. Cost, throughput, size of sheets, quality of sheets, chemical modification, and compatibility with commercial chip fabrication process are among the most notable considerations in selecting a method for synthesizing graphene. Some of the groundbreaking efforts in synthesizing graphene are Mechanical exfoliation, Supported growth (thermal decomposition of carbides or epitaxial growth by CVD and wet chemical routes)

· Mechanical exfoliation: (top- down approach)

With an inter-layer van der Waals interaction energy of about 2 eV/nm2, the order of magnitude of the force required to exfoliate graphite is about 300 nN/lm2 . This extremely weak force can be easily achieved with an adhesive tape as experienced each time one refreshes a graphite crystal substrate for AFM or STM imaging,

Micromechanical exfoliation remains the best method in terms of electrical and structural quality of the obtained graphene, primarily because it benefits from the high-quality of the starting single crystalline graphite source.

· Supported growth:

It has been known since the early 1970s that graphene could be grown directly on solid substrates and two different mechanisms can be exploited: the thermal decomposition of carbides or the epitaxial growth of graphene on metallic or metal carbide substrates by chemical vapor deposition of hydrocarbons.

The thermal treatment of silicon carbide at about 1300 C under vacuum results in the sublimation of the silicon atoms while the carbon-enriched surface undergoes reorganization and, for high enough temperatures, graphitization. The careful control of the sublimation has recently led to the formation of very thin graphene coatings over the entire surface of SiC wafers, with occasionally only one graphene layer being present.

Growth of few-layer graphene on Ni films, by CVD at atmospheric pressure is reported.

Some of the groundbreaking efforts in synthesizing graphene are summarized in Table 1

Graphene Nanoribbons (GNRs)

Graphene is a zero gap semiconductor, so that a field effect transistor (FET) will not have an ‘‘off’’ state unless a forbidden gap is created. Such a gap can be produced confining the electronic wave functions by etching narrow graphene nanoribbons (GNRs) typically of a few nanometers in width and with well defined crystallographic orientation.

So GNRs are 1D while Graphene layer is a 2D allotrop of carbon.

-Post process:

A suitable lithographic technique will allow the patterning of the ultimate, one atom thick nanoelectronics, that can operate initially most likely in combination with semiconductor- based circuitry, but with the potential that shortly graphene-only circuitry will become possible. To make possible the exploitation of the advantages arising from the possibility of lithographic patterning, two conditions have to be fulfilled: (i) the width of the GNRs has to be controlled down to a few nanometers in order to be able to achieve gap values that will allow room temperature operation; (ii) the crystallographic orientation of the GNRs has to be controlled equally precisely as a misorientation of only a few degrees can completely ruin the gap.

To be continued...