BOX 2.1

Graphene

Elemental carbon adopts a variety of crystalline forms—three-dimensional diamond, layered graphite, carbon nanotubes, and C60 fullerite. In 2005, a new form of carbon, graphene, was discovered. Graphene is a single sheet of hexagonally ordered carbon atoms.

The purely two-dimensional nature of graphene sheets gives rise to an astounding array of new phenomena, among which are the following:

  • Behavior that mimics the relativistic motion of particles in high-energy accelerators,

  • New states of matter in the quantum Hall regime (in Chapter 1 of this report, see the subsection entitled “Example in the Area of Thin Films: Gallium Arsenide-Based Heterostructures”),

  • Electronic conductivity at zero electron density, and

  • Extremely fast “ballistic” motion of electrons and holes even at room temperature.

The latter property suggests a new class of electronic devices with switching speeds much greater than those achievable in silicon complementary metal oxide semiconductors, perhaps reaching terahertz frequencies.

These properties and more result from an unusual electronic momentum-energy relationship. Electrons in the hexagonal crystal structure of graphene behave like massless relativistic electrons in a world with only two dimensions. Many materials possess a quasi-two-dimensional hexagonal structure in which the sheets interact slightly with neighboring sheets, such as in graphite itself, thus breaking the special momentum-energy relationship of electrons in graphene. What makes graphene special is that the sheets are only one atom or a few atoms thick.

The technical breakthrough that led to an explosion of research into graphene (now more than 1,000 papers per year; see Figure 2.1.1) was the discovery that crystals with a thickness of only a nanometer can be seen under an optical microscope when the crystals are placed on a Si wafer coated with a layer of silicon dioxide (SiO2) (see Figure 2.1.2). The SiO2 layer thickness must be precisely engineered—300 nm will not work, whereas 315 nm will—to produce interference contrast with the graphene crystal.

Thus, the discovery of graphene is an instance of the combining of a novel measurement approach with a prosaic “synthesis” technique. A large part of the future challenge for creating graphene-based devices will be that of replacing these techniques with a scalable manufacturing process that does not sacrifice the unique properties of this remarkable form of crystalline matter.



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