Graphene

The big brown bottle of hydrogen peroxide (H₂O₂) is a staple of the modern medicine cabinet, always on hand for first aid needs. Lesser known uses of hydrogen peroxide include disinfecting hospital equipment and fueling spacecraft. Yet as common and beneficial of a substance as it is, hydrogen peroxide is surprisingly hard to produce and transport. Currently, hydrogen peroxide is made through what’s known as the “anthraquinone process.” This method is energy-intense, requires large-scale production, and produces large quantities of carbon dioxide (CO2) as a byproduct. While directly reacting hydrogen and oxygen to make hydrogen peroxide would be ideal, thermodynamics prefers to form the more stable water (H2O) over hydrogen peroxide.

So the challenge becomes: does a material exist that can be used to selectively, reliably, and efficiently form hydrogen peroxide whenever and wherever it’s needed, so that transporting it isn’t necessary? A team of researchers from Carnegie Mellon University has set out to meet that difficult challenge. Associate Professors Venkat Viswanathan (mechanical engineering) and Tzahi Cohen-Karni (biomedical engineering/materials science and engineering) are leading an effort to develop a cheap, renewable, and sustainable method of creating hydrogen peroxide. The team has published a paper in ACS Catalysis on the work.

In a letter published in the February 2017 issue of Nature Nanotechnology, Ben Hunt and his collaborators at the Massachusetts Institute of Technology, the University of California Santa Barbara, and the National Institute for Materials Science in Tsukuba, Japan describe how they engineered a graphene electron–hole bilayer device into a helical 1-dimensional (1D) conductor and characterized its transport properties. In a helical 1D conductor, electrons moving in opposite directions also have opposite spin polarizations, and such helical states can be obtained by combining two quantum Hall (QH) edge states with opposite spins and opposite momenta relative to the magnetic field (i.e. opposite chiralities).

“My colleagues at MIT came up with this ingenious way of producing helical edge states from two decoupled graphene layers, and then they proved their idea worked with a series of powerful transport experiments,” says Hunt. “I was thrilled to be able to make a contribution to the experiment by using capacitance measurements to help prove that the unique helical states they observe really are edge states.”

The remarkable transport properties of graphene, such as the high electron mobility, make it a promising material for electronics. However, unlike semiconductors such as silicon, graphene's electronic structure lacks a band gap, and a transistor made out of graphene would not have an “off” state. Ben Hunt and his colleagues modulated the electronic properties of graphene by building a heterostructure consisting of a graphene flake resting on hexagonal boron nitride (hBN), which has the same honeycomb structure as graphene, but consists of alternating boron and nitrogen atoms instead of carbons. The natural mismatch between the graphene and hBN lattices led to a moire pattern with a large wavelength, causing the opening of a band gap, the formation of an elusive fractional quantum Hall state, and, at high magnetic fields, a fractal phenomenon in the electronic structure called the Hofstadter butterfly.