Tag Archives: Michaël Sarrazin

“Care to swap excitons?” asked one graphene layer to the other layer

Belgian science does not often make an appearance here perhaps due to language issues or the direction that science research has taken in that country or something else. In any event, a Feb. 3, 2014 news item on Nanowerk highlights some graphene research taking place in Belgium (Note: A link has been removed),

Belgian scientists have used a particle physics theory to describe the behaviour of particle-like entities, referred to as excitons, in two layers of graphene, a one-carbon-atom-thick honeycomb crystal. In a paper published in EPJ B (“Exciton swapping in a twisted graphene bilayer as a solid-state realization of a two-brane model”), Michael Sarrazin from the University of Namur, and Fabrice Petit from the Belgian Ceramic Research Centre in Mons, studied the behaviour of excitons in a bilayer of graphene through an analogy with excitons evolving in two abstract parallel worlds, described with equations typically used in high-energy particle physics.

I found the previous description a little more confusing that I’d hoped but do feel that this line present in the Jan. 21, 2014 EPJ B news release (also on EurekAlert but dated Feb. 3, 2014) helped clarify matters,

Equations used to describe parallel worlds in particle physics can help study the behaviour of particles in parallel graphene layers

One of the problems with skimming through material as I often do is that more complex sentences cause confusion and whoever removed the first line from the news item was relying on me (the reader) to carefully read through some 70 to 80 words before revealing that the scientists had created two parallel virtual worlds to test their theory. Once that was understood, this made more sense (from the news release),

The authors used the equations reflecting a theoretical world consisting of a bi-dimensional space sheet—a so-called brane—embedded in a space with three dimensions. Specifically, the authors described the quantum behaviour of excitons in a universe made of two such brane worlds. They then made an analogy with a bilayer of graphene sheets, in which quantum particles live in a separate space-time.

They showed that this approach is adapted to study theoretically and experimentally how excitons behave when they are confined within the plane of the graphene sheet.

Sarrazin and his colleague have also theoretically shown the existence of a swapping effect of excitons between graphene layers under specific electromagnetic conditions. This swapping effect may occur as a solid-state equivalent of known particle swapping predicted in brane theory.

To verify their predictions, the authors suggest the design for an experimental device relying on a magnetically tunable optical filter. It uses magnets whose magnetic fields can be controlled with a separate external magnetic field. The excitons are first produced by shining an incident light onto the first graphene layer. The device then works by recording photons in front of the second graphene layer, which provide a clue of the decay of the exciton after it has swapped onto the second layer from the first.

Here’s a link to and a citation for the paper,

M. Sarrazin and F. Petit (2014), Exciton swapping in a twisted graphene bilayer as a solid-state realization of a two-brane model, European Physical Journal B, DOI 10.1140/epjb/e2013-40492-5

Clicking on the link will lead you directly to this open access paper.

Fireflies and their jagged scales lead to brighter LEDs (light emitting diodes)

According to the Jan. 8, 2013 news item on ScienceDaily, scientists have used an observation about fireflies to make brighter LEDs (light emitting diodes),

The nighttime twinkling of fireflies has inspired scientists to modify a light-emitting diode (LED) so it is more than one and a half times as efficient as the original.

Researchers from Belgium, France, and Canada studied the internal structure of firefly lanterns, the organs on the bioluminescent insects’ abdomens that flash to attract mates. The scientists identified an unexpected pattern of jagged scales that enhanced the lanterns’ glow, and applied that knowledge to LED design to create an LED overlayer that mimicked the natural structure. The overlayer, which increased LED light extraction by up to 55 percent, could be easily tailored to existing diode designs to help humans light up the night while using less energy.

The Optical Society of America’s Jan. 8, 2013 news release, which originated the news item, describes how the scientists came to make their observations,

“The most important aspect of this work is that it shows how much we can learn by carefully observing nature,” says Annick Bay, a Ph.D. student at the University of Namur in Belgium who studies natural photonic structures, including beetle scales and butterfly wings.  When her advisor, Jean Pol Vigneron, visited Central America to conduct field work on the Panamanian tortoise beetle (Charidotella egregia), he also noticed clouds of twinkling fireflies and brought some specimens back to the lab to examine in more detail.

Fireflies create light through a chemical reaction that takes place in specialized cells called photocytes. The light is emitted through a part of the insect’s exoskeleton called the cuticle.  Light travels through the cuticle more slowly than it travels through air, and the mismatch means a proportion of the light is reflected back into the lantern, dimming the glow. The unique surface geometry of some fireflies’ cuticles, however, can help minimize internal reflections, meaning more light escapes to reach the eyes of potential firefly suitors.

In Optics Express papers, Bay, Vigneron, and colleagues first describe the intricate structures they saw when they examined firefly lanterns and then present how the same features could enhance LED design. Using scanning electron microscopes, the researchers identified structures such as nanoscale ribs and larger, misfit scales, on the fireflies’ cuticles. When the researchers used computer simulations to model how the structures affected light transmission they found that the sharp edges of the jagged, misfit scales let out the most light. The finding was confirmed experimentally when the researchers observed the edges glowing the brightest when the cuticle was illuminated from below.

“We refer to the edge structures as having a factory roof shape,” says Bay.  “The tips of the scales protrude and have a tilted slope, like a factory roof.” The protrusions repeat approximately every 10 micrometers, with a height of approximately 3 micrometers. “In the beginning we thought smaller nanoscale structures would be most important, but surprisingly in the end we found the structure that was the most effective in improving light extraction was this big-scale structure,” says Bay.

Here’s how the scientists applied their observations to LEDs (from the news release),

Human-made light-emitting devices like LEDs face the same internal reflection problems as fireflies’ lanterns and Bay and her colleagues thought a factory roof-shaped coating could make LEDs brighter. In the second Optics Express paper published today, which is included in the Energy Express  section of the journal, the researchers describe the method they used to create a jagged overlayer on top of a standard gallium nitride LED. Nicolas André, a postdoctoral researcher at the University of Sherbrooke in Canada, deposited a layer of light-sensitive material on top of the LEDs and then exposed sections with a laser to create the triangular factory-roof profile. Since the LEDs were made from a material that slowed light even more than the fireflies’ cuticle, the scientists adjusted the dimensions of the protrusions to a height and width of 5 micrometers to maximize the light extraction.

“What’s nice about our technique is that it’s an easy process and we don’t have to create new LEDs,” says Bay.  “With a few more steps we can coat and laser pattern an existing LED.”

Other research groups have studied the photonic structures in firefly lanterns as well, and have even mimicked some of the structures to enhance light extraction in LEDs, but their work focused on nanoscale features. The Belgium-led team is the first to identify micrometer-scale photonic features, which are larger than the wavelength of visible light, but which surprisingly improved light extraction better than the smaller nanoscale features. The factory roof coating that the researchers tested increased light extraction by more than 50 percent, a significantly higher percentage than other biomimicry approaches have achieved to date. The researchers speculate that, with achievable modifications to current manufacturing techniques, it should be possible to apply these novel design enhancements to current LED production within the next few years.

For those who care to investigate further,

Both articles (HTML version) are open access; PDF versions were not checked.