Matt Chang
University: Penn State University
Gradation Date: May 2011
Hometown: Live in
My Project: An Atom Thick: Graphene Based Quantum Hall Resistance Standard
Once claimed to be scientific myth, graphene, a single atom-thick sheet of carbon, turned impossibility into reality when it was exfoliated from graphite in 2004. Known as the building block of all graphitic forms, this 2-D crystal has shown a multitude of fascinating properties; in particular, electrons on its surface conduct at relativistic speeds and consequently do not behave like traditional electrons – instead they are known as massless Dirac fermions. One such result of this and graphene’s two-dimensional nature is that the Quantum Hall Effect (QHE), the quantum mechanical version of the classical Hall Effect, can be witnessed on graphene’s surface even at room temperature. The significance of the QHE lies in the fact that the Hall Resistance in the QHE is exactly quantized, highly reproducible, and depends solely on the invariant, fundamental constant h/e2. Moreover, the accuracy of the quantized Hall resistance is two orders of magnitude better than the current SI standard of the ohm, making the QHE a new and valuable standard for electrical resistance.
As of now, the QHE is primarily measured in gallium arsenide (GaAs) heterostructures at liquid-helium temperatures and high magnetic fields. Only a handful of GaAs devices are of high enough metrological quality to be used as a standard. Consequently, besides national institutions such as NIST, labs across the world do not have direct access to the accuracy of the QHE resistance standard. Graphene inherently provides the2-D electron gas system needed for the QHE, eliminating the difficulty of creating a 2-D electron gas in a 3-D GaAs device. This shortcut, along with graphene’s ability to display the QHE at room temperature, makes graphene based QHE devices much more feasible for other labs.
My project focuses on the isolation of graphene from graphite through mechanical exfoliation followed by the fabrication of a graphene based quantum Hall bar. If the resulting measurements are as accurate as those of GaAs devices, this will be the first step in a long-term goal in which QHE graphene devices, and thus a highly accurate standard of the ohm, will be disseminated to labs across the United States. In conjunction with the Josephson Effect, which provides a highly accurate standard of the volt, a more widespread QHE via graphene devices will work towards the final goal of a worldwide-stable electrical system of units completely based on invariant, fundamental constants.
A primary graphene based device, which can be used to make Hall measurements
A massive piece of graphene (center) still attached to a thicker piece. Often pieces do not fully exfoliate, which is why we’re sometimes left with attached graphene pieces. Graphene is only visible, using white light and no filters, when placed upon ~300 nm thick SiO2 (background). The barely visible contrast can be seen with the naked eye.
A typical quantum Hall resistance measurement. The red line, which represents Hall Resistance plateaus at certain values of the magnetic field. These plateaus are highly accurate and reproducible – offering a valuable new standard for resistance.
