Collective Ecstasy: Molecules

Author: Daniel Webster

"What we see is energy transfer that's much faster than any semiconductor," Jakob Heier exclaims. The discovery that he and his team made together could spark interest in many areas, including sensor technology, optical data transmission, and organic solar cell fabrication. These are dye molecules that have an internal structure of perfection. These structures are known as J-aggregates by experts. They have been around for over 80 years. However, recent research has brought them back to the forefront of scientific inquiry. These dye islands have an electronic inner life that is unique.

Empa
Energy vibration: Jakob Heier with samples of his "antenna dye."

By Daniel Webster, dWeb.News

It is worth a quick look at the world of dyes to understand the findings of Heier and his colleagues. To make a dye glow, it must be activated first - with light. For example, optical brighteners found in detergents absorb ultraviolet light and emit visible light. This is why white clothes shine so brightly under the UV light from a club. Because part of the energy converted into vibrations (i.e., energy) is lost to the dye activation light, the emitted light has a lower energy level than the dye's light source. The dye molecule contains heat.

Energy antennas: Molecules are made of molecules

Surendra Anantharaman, Empa PhD student, and Heier studied J-aggregates. They behave differently to individual dye molecules. These molecular islands contain dye molecules that are very well organized and close together. It is similar to matches in a box. This constellation allows the dye molecule to not only glow but also "can" transmit its energy to another molecule.

There is one crucial difference between silicon-based semiconductors and the classic ones. In silicon semiconductors like solar cells, excitation energy is carried via charge carriers. For example, electrons "hop" through the material. J-aggregates on the other hand, have electrons that oscillate in the dye molecules only and never leave the material. Only oscillations can be transmitted, and not electrons. This is similar to the transmission and reception of antennas in the macroscopically-oriented world. J-aggregates are capable of "transmitting" energy at the smallest scale, which is extremely fast and across hundreds upon molecules.

For 80 years, high losses

Edwin E. Jelley, a US citizen, and Gunter Scheibe, a German resident, first noticed the phenomenon of J-aggregates. However, 95 percent of the radiation energy could not be transmitted and was lost. The system's construction errors were responsible. The molecules weren't perfectly aligned in reality. The energy transport was disrupted when the energy pulse came across one of these problems during its journey through J-aggregate. The transfer was stopped by an ordinary molecular vibration, and a little heat was produced.

Empa
Like matches in a box, dye molecules line up at the phase boundaries of a bicontinuous emulsion. This is the only way for signal transmission to succeed.

The perfect forest for antennas

With the support of researchers from ETH Zurich and EPF Lausanne as well as PSI, IBM Research Zurich, the Empa team has developed a dye system that re-emits up to 60% of the incoming sunlight. This means that 60 percent of the energy can now be transmitted without loss, compared to five percent previously. Perfectly constructed dye islands were created using a fine emulsion made of water and hexylamine. An emulsion can be described as a mixture of liquid droplets and another liquid. Mayonnaise or milk are two examples.

The Empa researchers found that no emulsion was suitable for the job. It had to be a bicontinuous emulsion. This means that the droplets in the outer liquid could not be separated from one another, but they must have combined to form streaky structures. Only then can the dye be formed without defects and "send" the absorbed energies over long distances with no loss. This is similar to a box of matches where dye molecules are aligned in a bicontinuous solution. Signal transmission can only succeed then.

Failures are part and parcel of the game

In good scientific tradition, the study now published mentions both the failures and the history of the success. The Empa team's experience should allow chemists and scientists around the globe to draw on it. It was impossible to crystallize dye as thin films on a hard surface. The transfer was ruined by too many defects in crystals. The dye is also not compatible with aqueous solutions. These solutions are made up of tiny droplets and the dye aggregates in them. Bicontinuous emulsions are the only ones that allow signal transmission. This is because there are no dye molecules in a liquid phase which can fill in holes or close gaps in J-aggregates.

What are the possibilities?

Researchers have much more work to do before the technical utility of what they have achieved with an emulsion is realized. Signal transmission by dyes could be used to penetrate many areas of our daily lives. It is possible, for example, to capture infrared light and convert it into digital signals using quantum dots. This is a benefit for both sensor technology and solar cell technology, which can provide electricity even under very low light levels. J-aggregates are also well-suited for applications in quantum computers or optical data transmission because of their unique properties.

The signal-conducting dyeaggregates could be useful in diagnosing living tissue. Infrared radiation or thermal radiation penetrates deep into tissue and does not damage cells. This radiation could be visible by J-aggregates, which can digitize it. This could greatly improve the quality of high-resolution microscope images of living tissue.