Nano diamonds forever

Professor Alon Hoffman from the Shulich Faculty of Chemistry tells how he benefited from unexpected phenomena on the way to discovering the formation mechanism of nanodiamonds

by Noam Berkowitz

About sixty years ago, researchers made the announcement, which at the time sounded completely unfounded, that they had succeeded in growing tiny diamond crystals under conditions of subatmospheric pressure and a temperature of several hundred degrees by depositing carbon atoms from the gaseous phase on a surface. The accepted method at the time for the production of monocrystalline artificial diamonds required enormous pressures of about 70,000 Kg/cm² and a temperature of over 2300 degrees Kelvin, and because of that the discovery was received with the indifference reserved for impossible discoveries and was almost completely forgotten.

Over twenty years passed until Japanese scientists were able to repeat the experiment and remove the doubt about the feasibility of the method. Since then, many studies have been devoted to clarifying the processes of formation and growth of polycrystalline diamond layers under conditions of relatively low temperature and pressure on different materials. Some of the discoveries in the field - including the discovery of the mechanism by which the formation nuclei of diamonds are formed from electrical breakdown - were made in the laboratory of Professor Alon Hoffman from the Faculty of Chemistry.

The ceramic material of the future

Professor Hoffman came to grow diamonds from the field of surface research and thin layers in the solid state. A thin layer is defined as a solid with a thickness of several tens to several thousand atomic layers. Understanding and controlling surface properties is of great importance because the world around us consists of encounters of intermediate surfaces, and the stability of materials is largely determined by their surface properties.

The strength and durability of the diamond make it a preferred candidate as a coating and protection material for surfaces facing extreme conditions of mechanical wear and chemical corrosion. Hoffman won a special award from General Motors for developments that would allow steel to be coated with a thin layer of diamond crystals and thereby increase its resistance to abrasion. Hoffman predicts that the use of polycrystalline diamond layers will expand to the electronics industries, to biomedical applications and to the space field and that diamond will be the ceramic material of the future.

The process of growing layers of diamond grains from the gaseous phase as carried out in the Faculty of Chemistry at the Technion is schematically described in the diagram below:

The filament is at a temperature of about 2000⁰ And the surface about a centimeter below it is at a temperature of about 700⁰. Crystallization nuclei are first formed on the surface, and due to the differences in the energy state between the radicals in the area of ​​the filament and the growth surface, carbon atom adsorption processes are obtained which, in the presence of the active hydrogen, cause the growth of the diamond nuclei. Continuous polycrystalline diamond layers grow at a rate of one micron per hour.

In order to coat a certain surface with a uniform and smooth diamond layer, it is necessary to achieve a high density of the growth nuclei, and Hoffman and his team investigated how this parameter can be controlled. They tried to develop a chemical-physical method for attaching growth nuclei to the surface intended for coating, and tested, among other things, the use of an ultrasonic bath. In this process, the object intended for coating is placed into an alcohol mixture containing nanodiamond powder and particles of other materials and an ultrasonic field is applied to it.

Professor Hoffman and his team noticed that the process greatly increases the density of the growth nuclei, but they received variable results that did not advance the understanding of the process, until by chance one of the team members - Rosa Ahbaldiani - introduced large alumina grains into the mixture, which is a very hard and durable material. The result was beyond all expectations - the density of the nuclei increased by three orders of magnitude from 10⁸ To 10¹¹ Kernels per cmr. The surprised researchers realized that this was not a chemical process because the alumina is inert under the experimental conditions, and concluded that a mechanical process had taken place - the large alumina grains vibrated under the influence of the ultrasonic field and struck like hammers the tiny diamond particles floating in the solution, forcefully attaching them to the surface.

The discovery was born out of an accidental action, but Hoffman says that an unexpected result in an experiment or a result that seems illogical at first often leads to breakthroughs and discoveries in science. He always makes sure to convey this message to his students. There is no substitute for planning and hard work, but it is also important not to ignore unusual events and results that at first seem contrary to conventional wisdom, because it is precisely in them that the important discovery is often hidden, explains Professor Hoffman.

A discovery of the twilight hour

Another extraordinary moment of this kind, in which a fortuitous result led to the solution of a great mystery, was used by Hoffmann to contribute to the clarification of the mechanism underlying the deficit process, or nucleation, of diamonds from energetic particles.

The discovery happened when Hoffman and his team tested the effect of an electric field on the process of creating nanodiamonds after it was hypothesized that the formation nuclei are formed under the influence of such a field. Here, too, the experiment initially provided results that did not add up, but everything changed thanks to one moment of coincidence and scientific intuition. It was dusk, so darkness prevailed in the laboratory. Research student Irina Guzman called Hoffman to test the facility and activated the process as she had done many times before. She turned on the electric field, but this time she forgot to turn on the filament. In the darkness that prevailed in the laboratory, the researchers noticed to their surprise a spark inside the experimental facility that also remained dark due to the absence of the filament.

True to his method, Hoffman focused on the unexpected find and realized that he had discovered something important. The team continued to check the phenomenon until the image became sharper and clearer. It turned out that the electric field is the one that causes the creation of an electric breakdown in which energetic particles are formed that contribute to the creation of the nanodiamonds that are primary formation centers. Under these conditions, carbon and hydrogen ions are accelerated at high speed to a surface where they form the initial nucleus of the diamond. The process of creating the initial nucleus and growing it to the dimensions of a nano-diamond was subsequently explained by Yeshayahu Lifshitz (currently a professor in the Faculty of Materials Engineering at the Technion), Hoffman and others and was published in the prestigious journal Science in 2002.

Diamonds in space

Hoffman is now taking the research to new heights and testing the behavior of polycrystalline diamond layers in space. At an altitude of 200 to 500 km above the Earth, where satellites orbit, there are relatively high concentrations of atomic oxygen and UV radiation that corrode various materials very efficiently. In laboratory experiments, polycrystalline layers of diamonds were found to be very resistant in the harsh environment of space. Recently, layers of diamonds grown in Hoffman's laboratory were sent with the space shuttle Atlantis, for a one-year stay in space. When they return, the effect of the environment on them will be tested and whether they are suitable for use as a coating and protection material for parts of satellites, thus significantly extending their durability and lifespan.

The findings described in this short article are part of the many research works that have been done in the Faculty of Chemistry in the last two decades by many research students and a research team under the direction of Professor Hoffman and his partners.