Conduction Mechanism Could Lead to New Forms of Energy Conversion

Conduction mechanisms using recent innovations in 2D materials could eventually lead to new forms of energy conversion and high-resolution scanning machines, including those used in quality control for manufacturing and also in airports.

Large portions of the electromagnetic spectrum have been harnessed by humans for a wide range of technologies, from X-rays to radios, but a large part of the spectrum has remained out of reach. This is known as the terahertz gap, which exists between radio waves and infrared radiation, two spectrum parts used in everyday technologies, such as toasters, TV remotes and cell phones.

Felix Bloch, late Stanford professor and Nobel laureate, developed a theory suggesting that a customized structured material allowing electrons to oscillate in a specific manner might have the potential to perform these terahertz signals that are in great demand.

Decades after Bloch’s theory, Stanford physicists may have produced materials that enable these theorized oscillations, and this could lead to futuristic advancements in technologies ranging from solar cells to airport scanners. The findings were published by the group in the Sept. 30 issue of Science.

Innovations in Superlattice Materials

Researchers have suggested that materials with repeating spatial patterns on the nanoscale could enable Bloch’s oscillations, but technology is only just catching up to theory.

This type of material requires electrons to travel long distances without deflection, where even the tiniest imperfection existing in the medium where the electrons travel can put them off their usual path, similar to a stream attempting to wind over and around fallen trees and rocks.

This type of material can become a reality due to the growing research in two-dimensional materials and superlattices. Superlattices are known as semiconductors produced by layering ultra-thin materials whose atoms are positioned in a periodic lattice design.

A two-dimensional superlattice was developed by the researchers for this study. This was achieved by sandwiching a sheet of atomically thin graphene in between two electrically insulating boron nitride sheets. The atom spacing differs in the boron nitride and the graphene, so they produce a particular wave interference pattern known as moiré pattern when they are piled on top of each other.

New uses for Electrons

Electrons in the graphene are protected from contaminants and air by boron nitride below and above. These electrons travel through smooth paths without deflection, which is exactly what the earlier theory recommended to conduct terahertz signals.

The researchers succeeded in sending electrons through the graphene sheet, followed by collecting these electrons on other side and then using them to understand the activity of the electrons all through the entire way.

Generally, when a voltage is used across a crystal, electrons are constantly accelerated in the electric field’s direction until they are deflected. David Goldhaber-Gordon, physics Professor and co-author of the study, stated that the researchers demonstrated the possibility of confining the electrons to narrower bands of energy in this moiré superlattice.

Combined with prolonged time lines between deflections, this will result in electrons oscillating in place and emitting radiation in the terahertz frequency range. This is considered as an initial success towards the development of monitored emission and sensing of terahertz frequencies.

As part of this mission to bring Bloch’s theory closer to reality, the researchers discovered a totally unexpected change in their superlattice material’s electronic structure.

In semiconductors, like silicon, we can tune how many electrons are packed into this material. If we put in extra, they behave as though they are negatively charged. If we take some out, the current that moves through the system behaves as if it’s instead composed of positive charges, even though we know it’s all electrons.

David Goldhaber-Gordon, Professor, Stanford University

However, a new twist is brought about by this superlattice. The addition of extra electrons develops particles of positive charge, and a negative charge is obtained when an increasing amount is taken out.

In the future this change in the behavior of the electrons could occur in the form of extremely efficient p-n junctions, which are significant building blocks to almost all semiconductor electronic devices such as transistors, LEDs and solar cells. Generally, if light is shined on a p-n junction, sending out a single electron for every single photon that is absorbed is regarded as an exceptional performance.

However, these new junctions could emit an increasing number of electrons for each photon, harvesting the energy of the light in a much efficient manner.

Terahertz and Stanford, Past and Future

Even though a Bloch oscillator is yet to be developed, scientists have attained the initial step by demonstrating that the velocity and momentum of an electron can be preserved for a prolonged time period and over long distances with this superlattice, explained Menyoung Lee, co-author of the study who conducted the research as a graduate student in the Goldhaber-Gordon Group.

We apply the very first original lessons of solid-state physics that Felix Bloch figured out a long time ago, and it turns out we can use that to drive unique conduction phenomena in novel engineered materials.

Menyoung Lee, Graduate Student, Stanford

Terahertz frequency technology could be considered as an advancement over existing technologies. Today, microwaves are used by U.S. airports to scan passengers at security checkpoints. Microwaves break through nonmetal materials in order to expose concealed metal objects.

Goldhaber-Gordon explained that similar transmission properties and shorter wavelength are found in terahertz exhibiting even nonmetal concealed objects at high resolution. He further stated that terahertz scanners could also be used to identify defects such as hidden cavities existing in objects on a manufacturing assembly line.

This study demonstrated a clean electronic conduction, which also enhances the understanding of the ways in which electrons communicate with each other and flow. Goldhaber-Gordon stated that his lab is planning to use these insights for developing majorly narrow beams of electrons to pass through superlattices. He referred to this new field as “electron optics in 2-D materials” as these beams pass through straight lines and follow laws of refraction corresponding to light beams.

This is going to be an area that opens up a lot of new possibilities, and we’re just at the start of exploring what we can do.

David Goldhaber-Gordon, Professor, Stanford University

Additional authors of this paper, “Ballistic Miniband Conduction in a Graphene Superlattice,” are Patrick Gallagher of Stanford University, John R. Wallbank and Vladimir I. Fal’ko of University of Manchester, and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Tsukuba, Japan.

Funding was obtained from the Air Force Office of Scientific Research and the Gordon and Betty Moore Foundation, and was performed in part in the Stanford Nano Shared Facilities.