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The size (of the particles) does determine (their optical properties)

"If we break a gold coin in half," says Dr. Dan Oron, who recently joined the Department of Physics of Complex Systems at the Weizmann Institute of Science, "the two parts will have the same basic properties as the original coin. The situation is completely different when it comes to a gold particle that is only a few thousand atoms in size. In this case, among other things, the color of the parts that break from it will be different from that of the whole particle."

Dr. Dan Oron. Color changer
Dr. Dan Oron. Color changer
We see color when light rays are absorbed by objects or reflected from them, but when it comes to objects whose size does not exceed that of the wavelength of light, strange phenomena occur. In fact, when you go down to the nano dimensions (billionths of a meter), most of the properties of the materials we are familiar with change significantly.

Dr. Oron studies the properties of nano-crystals. He seeks to understand the laws governing the properties of pieces of matter the size of a protein molecule, more or less, and learns how to apply their laws and properties. "Traditional inorganic chemistry has given us a limited number of mechanisms to create new materials - mainly by changing the chemical composition and crystal structure of the material. "Nanoparticles, whose properties also depend on the size, shape and spatial structure, may open up the possibility for us to create a huge variety of materials with properties that cannot be achieved by other means," he says.

A quick look at two of Dr. Oron's recent works demonstrates the many possible uses of nanoparticles: in one, he creates unique nanoparticles that are able to illuminate molecules under a microscope; In the second, the nanoparticles form the basis of a new type of solar radiation collector.

Dr. Oron's nanoparticles are semiconductors. When they are disturbed, for example, through the impact of a photon (particle of light), a short excitation of an electron is caused, so that a "hole" with a positive charge remains in the material. When the excited electron goes back into the hole, light of a certain color is emitted. And what happens when you cause the excitation of two electrons in one nanoparticle? Do the electric charges inside the nanocrystal act on each other? Do they attract or perhaps repel each other? Dr. Oron discovered that it is possible to cause a strong repulsion by adding several atoms of another element into the nanoparticles. This addition creates a sort of "cage" that captures a single positive charge, thus repelling the other charge. In this state, the light emitted by the particle changes its color. In microscopes, nanoparticles that change color can be used for marking. Today, for the purpose of XNUMXD imaging, it is customary to use a certain type of microscopes which are based on the simultaneous absorption of two photons by a marker attached to the target bone - a cell or protein.

Following this, the marker emits a short spark of light. The light emitted in such a process from tiny nanoparticles, which are bound to cells or proteins, may be more stable and reliable. In addition, the ability to design a nanoparticle that will emit light of one color when the first photon hits it, and light of a different color when the second hit occurs, makes it possible, using both colors, to improve the separation ability of the microscope. Another possibility, which will also lead to a similar improvement, is to design a particle that will scatter light only when both photons hit it at the same time, a phenomenon known as non-linear scattering.

"The challenge in producing such tiny markers," says Dr. Oron, "is the need to control the production process to reach the necessary structure on the one hand, and the need to detect extremely weak optical signals on the other hand. However, through proper planning, it is possible, for example, to overcome the decrease in the nanoparticle's ability to scatter light as its size decreases." Such planning recently led to the creation of the smallest nanoparticle to date - its size does not exceed 15 nanometers - from which non-linear light scattering can be measured. "We were able to find a certain point where it works ten times better compared to a large block of material of the same type," he says.

Dr. Oron's solar cell research, on the other hand, is not based on the ability of the nanoparticles to emit light, but on their ability to absorb the light. The third generation light collectors, which are currently used in cheap solar cells, are based on organic dyes (as opposed to the expensive cells made of silicon). These cells are required to perform a difficult task: to absorb large amounts of light in a wide range of wavelengths, to separate the exciting electron from the " hole" that he leaves, and then take an electron back, in a process that repeats itself over and over again. Most organic dyes are able to separate charges well, but they are limited in the range of colors they absorb and in their chemical stability. On the other hand, semiconductor nanoparticles are able to absorb sunlight at most visible wavelengths, but they are not particularly effective in separating the charges. Dr. Oron, in collaboration with Dr. Arie Tsavan from Bar-Ilan University, came up with the idea of ​​dividing the work. They created microscopic devices where the nanoparticles act as a kind of antennas, which channel the solar energy to the paint molecules, where the charges are separated. Dr. Oron believes that after several improvements such integrated solar collectors may be very effective.


Dr. Dan Oron was born in Rehovot in 1974 and grew up near the Weizmann Institute of Science. "I spent a lot of time at the institute, and from the age of 11 I participated in various classes in the youth operations unit," he says. He completed his bachelor's and master's degrees as part of the "Talfiot" program. In the research work for his master's degree, at Ben-Gurion University, Oron studied the physics of the arbol. His PhD research in spectroscopy using short pulses of light was done in Prof. Yaron Zilberberg's group at the Weizmann Institute of Science. In his post-doctoral research, in the laboratory of Prof. Uri Benin at the Hebrew University of Jerusalem, Dr. Oron began working with nanoparticles. In 2007 he joined the Weizmann Institute of Science as a senior researcher. Dr. Dan Oron is married to Ruthi, and is the father of Hila, 7 years old.

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