Scientists are working to find ways to control nanoparticles to bring nanotechnology into practice. A conversation with Professor Richard Robinson who is a guest at the Hebrew University in Jerusalem/Ethan Crane
The article was published with the approval of Scientific American Israel and Ort Network
Professor Richard Robinson arrived to the laboratory of Professor Uri Banin at the Institute of Chemistry in Jerusalem because of a hamburger sandwich. If it were made of bread and meat, it would be the smallest hamburger in the world. However, the "bun" in this case is made of zinc sulfide (ZnS) and the "hamburger" itself is made of a thin layer of copper sulfide (Cu2S) which can reach a thickness of only a few atoms. The way to create this sphere, which is about 20 billionths of a meter in diameter, or 20 nanometers, may lead to a new and intriguing field of inorganic chemistry. To discuss these processes, I visited the Hebrew University in Jerusalem to talk with Robinson, who came from Cornell University in the USA with the assistance of a Fulbright scholarship from the US-Israel Education Foundation.
One of the most striking things about Robinson's biography is the diversity: he received his bachelor's and master's degrees in mechanical engineering from Tufts University; He received his doctorate in applied physics from Columbia University and did his postdoctoral work at the University of California at Berkeley in nanochemistry. He is currently a researcher in the Department of Materials Science at Cornell University. "In my research I do combine all these fields," Robinson tells me with a self-deprecating smile. according to him,his research group Trying to bridge the nanoscopic world and the real world in order to create devices based on nanotechnology. To this end, the members of the group are engaged in chemical modifications of nanoparticles that are used as building blocks in devices on a larger scale and characterizing them using physical methods. The physical characterization tells them how to make further chemical changes, and God forbid. "In order to create batteries, supercapacitors and chemical catalysts, I have to combine all the fields of knowledge I have been involved in: engineering, physics and chemistry," says Robinson.
Robinson compares the state of nanotechnology today to the study of plastic materials in the 50s. Plastic polymers were known much earlier, but only then did chemists and engineers learn to control their composition and structure in order to control their chemical and physical properties and apply them on a large scale. In recent years, chemists have learned to prepare a wide variety of nanoparticles, says Robinson. They know how to make particles with a precise composition and a precise and uniform size. Particle size is extremely important. In normal chemistry, the thing that most affects the properties of the substance is the structure of the molecule or lattice. In nanochemistry, on the other hand, particles with the same composition, but different size and shape, will have completely different physical properties: color, electrical conductivity, magnetic properties, and more. "Today they know how to create a 'stamp album' of nanoparticles: spheres, prisms, stars and hexagons, and so far they have mainly dealt with this," says Robinson. We know how to add many new types of "stamps" to this album, but we do not have proper means to integrate them into the standards. In addition, we have not yet been able to find the way to scale-up the processes for the production of nanoparticles.
New chemistry?
(Courtesy of Prof. Richard Robinson)
As mentioned, the first part of solving the problem is a precise chemical change of the nanoparticles. The materials that Robinson works on are ionic compounds of semiconductors. In normal chemistry, these are crystalline materials, three-dimensional structures enormous in size, compared to nanoparticles, in which the ions are arranged in cycles of alternating positive and negative ions, in columns and rows. If you put such a crystal, once formed, into a solution containing other ions, the only way it can react with them is on the surface. "Foreign ions can enter and replace the ions in the crystal, but only for a very short distance," explains Robinson.
Crystals with a similar structure, but in nanoscopic dimensions, behave in a completely different way. For example, if you introduce nanoparticles in which the positive ion is cadmium, (Cd+2) for a solution containing other positive ions, such as silver (+Ag) ions, a very rapid exchange of the ions occurs (despite the charge differences between them). Since the surface area of the nanoparticles is huge, relative to their volume, the reaction rate becomes very fast. "The exchange is so fast that despite the change in chemical structure, the shape of the particles is preserved," explains Robinson. Moreover, the ions penetrate the nanoparticle in certain directions: on both sides of a sphere, or at constant intervals along the length of a sphere. No one understands exactly these mechanisms, and in many respects this is a new and fresh branch of chemistry.
Robinson's group does something new: it stops the process in the middle. This is how structures are formed consisting of two different materials in the same particle. This brings us to the "hamburger": Robinson and his colleagues put the copper sulfide balls into a solution containing zinc ions, and when they replaced the copper ions, the researchers stopped the reaction at set times (see figure). The process is so regular that you can get uniform sandwiches, the thickness of the "hamburger" stuck in them depends on the reaction time. When the reaction is stopped right at the end, a thin layer of copper with a thickness of a single atom is obtained. And the properties of this layer are tested by Robinson in Israel.
(Courtesy of Prof. Richard Robinson)
Meet the Israeli experts
And here in Jerusalem work some of the world's leading experts in the field of nanoparticle characterization. In the laboratory of Professor Benin, says Robinson, there is spectroscopic equipment, which was built here in Israel and which is one of the best of its kind. Uri Benin is very experienced, and specializes in structural and spectroscopic characterization of nanoparticles. "I came here to advance my career and establish a collaboration with the researchers in Jerusalem," says Robinson. "They are the experts, and I am the junior." The second expert that Robinson refers to in his words isProfessor Oded Milo from the Rakeh Institute of Physics at the Hebrew University. Milo is an expert on the characterization of nanoparticles usingScanning tunneling microscope (STM). "There is no big and expensive equipment in Israel," describes Robinson. "But the level of science here, and the expertise, is equal to that in the US or even exceeds it." The Israeli students he works with, and whom he hears in seminars, are at least as good as the students he knows in the US. "The scientists here are definitely breaking boundaries," he says.
look up
The research that Robinson is engaged in in Israel is pure research, which, as mentioned, may lead to a new field in chemistry. But the research has many practical purposes. One example deals with catalysts: substances capable of accelerating certain chemical reactions and directing them. The biochemistry that controls our lives is based on catalytic proteins, called enzymes, which are capable of reaching an incredibly precise level of control. Although the modern chemical industry is far from biological precision, it is increasingly based on catalysts. The field allows for shortcuts, significant energy savings and the use of more environmentally friendly materials. The nanoparticles, with their huge surface area will certainly play an important role. Cobalt oxide, for example, is at the center of research looking for ways to break down water using light - with the aim of imitating photosynthesis. Partially replacing the oxygen ions with sulfur ions in the cobalt oxide nanoparticles, Robinson says, improves their ability to catalyze the reaction. The reason for this is probably due to the destabilization of the crystal structure. Oxygen ions and sulfur ions are negative ions, which by nature are larger than positive ions. Small positive ions may insert into the crystal structure without breaking it, but the exchange of negative ions destabilizes the structure. And precisely the unstable structure may improve the activity.
But, as mentioned, the integration of nanoparticles in products is still in its infancy. There are already displays that incorporate nanoparticles, Robinson tells me, and they display richer colors, but they're also more expensive.
And perhaps, the breakthrough will be made possible by the cooperation between Jerusalem and Cornell.
Professor Richard Robinson is staying in Israel as part of the Fulbright program which promotes academic-scientific cooperation between the US and Israel and supports outstanding researchers and research students. Israel's participation in this program is managed by the US-Israel Education Foundation. The interview was conducted with the assistance of Hila Ovadia-Akerman.
Prof. Richard Robinson is a University Visiting Professor at the Institute of Chemistry of The Hebrew University of Jerusalem
