Tag Archives: excitons

University of Vermont and the ‘excitons’ of an electron superhighway

This story starts off with one of the current crazes, folding and bendable electronics, before heading off onto the ‘electron highway’. From a Sept. 14, 2015 news item on ScienceDaily (Note: Links have been removed),

TV screens that roll up. Roofing tiles that double as solar panels. Sun-powered cell phone chargers woven into the fabric of backpacks. A new generation of organic semiconductors may allow these kinds of flexible electronics to be manufactured at low cost, says University of Vermont physicist and materials scientist Madalina Furis.

But the basic science of how to get electrons to move quickly and easily in these organic materials remains murky.

To help, Furis and a team of UVM materials scientists have invented a new way to create what they are calling “an electron superhighway” in one of these materials — a low-cost blue dye called phthalocyanine — that promises to allow electrons to flow faster and farther in organic semiconductors.

A Sept. 14, 2015 University of Vermont news release (also on EurekAlert) by Joshua E. Brown, which originated the news item, describes the problem the researches were trying to solve and the solution they found,

Hills and potholes

Many of these types of flexible electronic devices will rely on thin films of organic materials that catch sunlight and convert the light into electric current using excited states in the material called “excitons.” Roughly speaking, an exciton is a displaced electron bound together with the hole it left behind. Increasing the distance these excitons can diffuse — before they reach a juncture where they’re broken apart to produce electrical current — is essential to improving the efficiency of organic semiconductors.

Using a new imaging technique, the UVM team was able to observe nanoscale defects and boundaries in the crystal grains in the thin films of phthalocyanine — roadblocks in the electron highway. “We have discovered that we have hills that electrons have to go over and potholes that they need to avoid,” Furis explains.

To find these defects, the UVM team — with support from the National Science Foundation — built a scanning laser microscope, “as big as a table” Furis says. The instrument combines a specialized form of linearly polarized light and photoluminescence to optically probe the molecular structure of the phthalocyanine crystals.

“Marrying these two techniques together is new; it’s never been reported anywhere,” says Lane Manning ’08 a doctoral student in Furis’ lab and co-author on the new study.

The new technique allows the scientists a deeper understanding of how the arrangement of molecules and the boundaries in the crystals influence the movement of excitons. It’s these boundaries that form a “barrier for exciton diffusion,” the team writes.

And then, with this enhanced view, “this energy barrier can be entirely eliminated,” the team writes. The trick: very carefully controlling how the thin films are deposited. Using a novel “pen-writing” technique with a hollow capillary, the team worked in the lab of UVM physics and materials science professor Randy Headrick to successfully form films with jumbo-sized crystal grains and “small angle boundaries.” Think of these as easy-on ramps onto a highway — instead of an awkward stop sign at the top of a hill — that allow excitons to move far and fast.

Better solar cells

Though the Nature Communications study focused on just one organic material, phthalocyanine, the new research provides a powerful way to explore many other types of organic materials, too — with particular promise for improved solar cells. A recent U.S. Department of Energy report identified one of the fundamental bottlenecks to improved solar power technologies as “determining the mechanisms by which the absorbed energy (exciton) migrates through the system prior to splitting into charges that are converted to electricity.”

The new UVM study — led by two of Furis’ students, Zhenwen Pan G’12, and Naveen Rawat G’15 — opens a window to view how increasing “long-range order” in the organic semiconductor films is a key mechanism that allows excitons to migrate farther. “The molecules are stacked like dishes in a dish rack,” Furis explains, “these stacked molecules — this dish rack — is the electron superhighway.”

Though excitons are neutrally charged — and can’t be pushed by voltage like the electrons flowing in a light bulb — they can, in a sense, bounce from one of these tightly stacked molecules to the next. This allows organic thin films to carry energy along this molecular highway with relative ease, though no net electrical charge is transported.

“One of today’s big challenges is how to make better photovoltaics and solar technologies,” says Furis, who directs UVM’s program in materials science, “and to do that we need a deeper understanding of exciton diffusion. That’s what this research is about.”

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

Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films by Z. Pan, N. Rawat, I. Cour, L. Manning, R. L. Headrick, & M. Furis. Nature Communications 6, Article number: 8201 doi:10.1038/ncomms9201 Published 14 September 2015

This is an open access article.

Could there be a quantum internet?

We’ve always had limited success with predicting future technologies by examining current technologies. For example, the Internet and World Wide Web as we experience them today would have been unthinkable for most people in the 1950s when computers inhabited entire buildings and satellites were a brand new technology designed for space exploration not bouncing communication signals around the planet. That said, this new work on a ‘quantum internet’ from Eindhoven University of Technology is quite intriguing (from a Dec. 15, 2014 news item on Nanowerk),

In the same way as we now connect computers in networks through optical signals, it could also be possible to connect future quantum computers in a ‘quantum internet’. The optical signals would then consist of individual light particles or photons. One prerequisite for a working quantum internet is control of the shape of these photons. Researchers at Eindhoven University of Technology (TU/e) and the FOM foundation  [Foundation for Fundamental Research on Matter] have now succeeded for the first time in getting this control within the required short time.

A Dec. 15, 2014 Eindhoven University of Technology (TU/e) press release, which originated the news item, describes one of the problems with a ‘quantum internet’ and the researchers’ solution,

Quantum computers could in principle communicate with each other by exchanging individual photons to create a ‘quantum internet’. The shape of the photons, in other words how their energy is distributed over time, is vital for successful transmission of information. This shape must be symmetric in time, while photons that are emitted by atoms normally have an asymmetric shape. Therefore, this process requires external control in order to create a quantum internet.

Optical cavity

Researchers at TU/e and FOM have succeeded in getting the required degree of control by embedding a quantum dot – a piece of semiconductor material that can transmit photons – into a ‘photonic crystal’, thereby creating an optical cavity. Then the researchers applied a very short electrical pulse to the cavity, which influences how the quantum dot interacts with it, and how the photon is emitted. By varying the strength of this pulse, they were able to control the shape of the transmitted photons.

Within a billionth of a second

The Eindhoven researchers are the first to achieve this, thanks to the use of electrical pulses shorter than nanosecond, a billionth of a second. This is vital for use in quantum communication, as research leader Andrea Fiore of TU/e explains: “The emission of a photon only lasts for one nanosecond, so if you want to change anything you have to do it within that time. It’s like the shutter of a high-speed camera, which has to be very short if you want to capture something that changes very fast in an image. By controlling the speed at which you send a photon, you can in principle achieve very efficient exchange of photons, which is important for the future quantum internet.”

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

Dynamically controlling the emission of single excitons in photonic crystal cavities by Francesco Pagliano, YongJin Cho, Tian Xia, Frank van Otten, Robert Johne, & Andrea Fiore. Nature Communications 5, Article number: 5786 doi:10.1038/ncomms6786 Published 15 December 2014

This is an open access paper.

ETA Dec. 16, 2014 at 1230 hours PDT: There is a copy of the Dec. 15, 2014 news release on EurekAlert.

“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.