May 12 2015
Scientists at the National Institute of Standards and Technology (NIST) have developed nanoscale whispering galleries for graphene electrons.
An international research group led by scientists at NIST has developed a technique for creating nanoscale whispering galleries for electrons in graphene. The researchers used the voltage from a scanning tunneling microscope (right) to push graphene electrons out of a nanoscale area to create the whispering gallery (represented by the protuberances on the left), which is like a circular wall of mirrors to the eletron. Credit: Jon Wyrick, CNST/NIST
This advancement would help create devices that would be capable of focusing and amplifying electrons similar to lenses that focus light and resonators that increase sound.
The whispering gallery phenomenon can take place in certain building structures where sound waves can travel along a curved wall. Typically, sound waves can travel further along a curved wall farther rather than a flat wall.
The St. Paul’s Cathedral in London has a dome structure, where even a faint sound that is made along any part of the curved wall can be heard by a person standing anywhere near this wall.
Based on the same principle, whispering galleries for light waves have been created. They have been used in a wide range of applications including laser frequency comb generation, communications, spectroscopy and sensing.
“The cool thing is that we made a nanometer scale electronic analogue of a classical wave effect,” said NIST researcher Joe Stroscio. “These whispering galleries are unlike anything you see in any other electron based system, and that’s really exciting.”
Graphene demonstrates many interesting chemical, magnetic and optical properties. It demonstrates remarkable strength and is a very good conductor of heat and electricity. Graphene which was created in 2004 is composed of a single atomic layer of carbon, where the atoms are arranged in the form of a honeycomb lattice.
Defects in graphene have acted as a deterrent in the study of electron behavior of graphene. However it is now possible to manufacture clean graphene with negligible defects. This would help researchers in ascertaining the full potential of graphene.
In traditional semiconductors, when electrons face a barrier, they require an increase in energy to continue their flow. This causes the electrons to get reflected most of the time. However, due to the tunneling effect, electrons could act as a wave and disregard the barrier. Graphene electrons possess light-like properties and they pass through the barrier when they hit it directly. These electrons have a tendency to tunnel, which makes it difficult for steering the electrons.
The research team created a graphene electron whispering gallery. They used a conductive plate to enrich the graphene with electrons. The graphene now had plenty of electrons, and some of these were pushed out using the voltage of a scanning tunneling microscope (STM). This led to creation of the whispering gallery. To the electron this gallery acts in the same manner as a circular wall of mirrors.
“An electron that hits the step head-on can tunnel straight through it,” said NIST researcher Nikolai Zhitenev. “But if electrons hit it at an angle, their waves can be reflected and travel along the sides of the curved walls of the barrier until they began to interfere with one another, creating a nanoscale electronic whispering gallery mode.”
Varying the voltage of the STM tip can help control the leakiness of the electronic whispering gallery. The STM probe has the capability to create and also detect whispering gallery modes.
The theory involved in graphene’s whispering gallery modes was developed by theoretical physicists from the Massachusetts Institute of Technology. The high mobility device was fabricated by NIST researchers Yue Zhao, Fabian Natterer and Jon Wyrick. They also performed the measurements.
This device was fabricated and measured at NIST’s Center for Nanoscale Science and Technology (CNST).
Graphene-based quantum electronic resonators and lenses could have significant uses which are not yet known. However, if the benefits of conventional optics are considered then they could find use in numerous applications.
This study has been published as a paper titled “Creating and probing electron whispering-gallery modes in graphene,” in the journal Science.