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“If you break a piece of gold in half,” says Dr. Dan Oron of the Institute’s Physics of Complex Systems Department, “Each of the halves will have the same basic properties as the original. But the same won’t be true if that piece of gold is just a few thousand atoms in size: Among other things, the color of the halves will be different.”
We see color when light waves are absorbed or reflected from objects, and once the size of those objects gets down to the length of light waves or smaller, strange things begin to happen. In fact, many of the properties of familiar materials change dramatically at the nanometer scale (a billionth of a meter).
Oron has been investigating the characteristics of nanocrystals – seeking to understand the rules governing bits of matter around the size of protein molecules, and learning how to put those rules and properties to use. “Conventional inorganic chemistry gives us one kind of mechanism for creating materials – one with a limited set of ‘knobs.’ Nanoparticles could give us radically new machinery that could help us realize previously unattainable material properties.”
Just how versatile nanoparticles are can be seen in two of Oron’s recent research projects: In one, unique nanoparticles he is working on might be used to light up molecules under the microscope; in the other, they might power a new kind of solar collector.
Oron’s nanoparticles are semiconductors. When they’re disturbed – say by a photon (a light particle) striking them – an electron is briefly excited, leaving behind a positively charged hole. When the excited electron and the hole recombine, light is re-emitted. What happens when two electrons are excited in a single nanoparticle? Do the electrical charges occupying the nanocrystal interact, repelling or attracting each other? Oron has found that he can induce strong repulsion between the two holes by adding just a few atoms of a different element to the nanoparticles. This creates a cage that traps a single positive charge, repelling the second one as it does so and causing the light to be re-emitted from the particle in a different color.
In microscopes, such color-altering nanoparticles could be used as markers. Some of today’s advanced microscopes fire two photons at the same spot in rapid succession, causing the material to emit a brief flash of light. But light from tiny nanoparticles attached to cells or proteins could prove to be more stable and reliable. In addition, a nanoparticle could be designed to emit one color the first time it was struck by a photon and another color the second; or, to enhance resolution, it might scatter light only after being struck by two photons simultaneously. The challenge in creating such ultra-tiny markers, says Oron, lies on the one hand in controlling their fabrication to attain the desired structure and on the other hand in detecting extremely weak optical signals. The ability of nanoparticles to scatter light, for example, drops drastically as they shrink. These challenges, however, can be overcome. "We have recently succeeded in creating the smallest nonlinear light-scattering nanoparticle yet – less than 15 nanometers across. We managed to find a ‘sweet spot’ – one that works about ten times better than the equivalent bulk material,” he says.
Oron’s solar collector research relies not on the nanoparticles’ light-emitting skills but rather on their ability to absorb light. The organic light absorbers in dye-based solar cells have a tough job: They must absorb quantities of light of a wide range of wavelengths, separate the electron from the “hole” it leaves, and then accept an electron back, over and over again. Oron and Dr. Arie Zaban of Bar-Ilan University knew that semiconductor nanoparticles could absorb sunlight much more easily almost across the entire visible spectrum but were much less efficient at splitting the charges. Many organic dyes, on the other hand, can separate charges reliably but are limited in the wavelengths of light they can take in, as well as in their long-term stability. Oron and Zaban had an idea: Why not divide the workload? The researchers created microscopic devices in which nanoparticles act as antennae, funneling the sun’s energy to dye molecules, where the charge separation takes place. Oron believes that with some tweaking, such combination solar collectors could be highly efficient.
Rehovot born and bred
Dr. Dan Oron was born in Rehovot in 1974 and grew up right around the corner from the Weizmann Institute. “I was at the Institute all the time and participated in many extracurricular activities here; my first was at age 11,” he recalls. He completed his B.Sc. and M.Sc. through the elite Talpiot army study program, the latter at Ben-Gurion University of the Negev, where he investigated the physics of turbulence. His doctoral research on spectroscopy using ultra-fast pulses of light was conducted at the Weizmann Institute in the group of Prof. Yaron Silberberg. Oron began working with nanoparticles during his postdoctoral research in the lab of Prof. Uri Banin at the Hebrew University of Jerusalem. He joined the Weizmann Institute in 2007.
Dan is married to Ruti; they have a daughter, Hila.
Dr. Dan Oron's research is supported by the Phyllis and Joseph Gurwin Fund for Scientific Advancement; the Wolfson Family Charitable Trust; Yossie Hollander, Israel; and Daniel S. Shapiro, UK.
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