Nano-diamonds are forever

Professor Alon Hoffman of the Schulich Faculty of Chemistry tells how he exploited unexpected results on his way to uncovering the mechanism for nano-diamond formation from energetic species

Professor Alon Hoffman in his lab

By Noam Bercovitz

About sixty years ago, scientist declared that they had succeeded in growing minute diamond crystals - which at that time was perceived as a totally outrageous claim - under sub-atmospheric pressure and relatively low temperatures by depositing carbon atoms onto a surface. The standard way of forming synthetic single crystal diamonds, at the time, required tremendous pressure of approximately 70,000 kg/cm² and very high temperatures over 2300°K, and accordingly, the discovery met with the indifference usually reserved for impossible discoveries and was almost completely forgotten.

Over twenty years passed until Japanese scientists successfully replicated the experiments and removed the doubt concerning the method's feasibility. Much research was dedicated since then to clarifying the formation and growth processes of poly-crystalline diamond films under conditions of relatively low temperatures and pressure on different materials. Some of the discoveries in this area - among which was the discovery of the mechanism through which nano-diamond crystal seeds are formed via electrical discharge - occurred in the laboratory of Prof. Alon Hoffman of the Schulich Faculty of Chemistry.

The Future Ceramic Material

Prof. Hoffman came to the field of diamond growth from the area of solid state surface and thin film research. A film is defined as a solid with a thickness of several tens up to several thousands of atomic layers. The understanding and control of surface properties is very important because the world that surrounds us is composed of the convergence of surfaces and interfaces between solid, liquid and gaseous phases.   The stability of many solids,  for example, is to a large extent, determined by their surface characteristics.

The properties of diamonds makes them the preferred option as a coating and protective agent for use on surfaces that must withstand extreme conditions of mechanical wear and chemical corrosion.   Hoffman won a special prize from the General Motors Corporation for developments that have enabled the coating of steel with an extremely thin film of diamond crystals, thereby  enhancing  the steel's properties such as wear resistance and chemical stability. Hoffman anticipates that the use of diamond films will also be taken up by the electronics industries, utilized in bio-medical applications and in the field of space technology, and that diamonds are destined to become the ceramic material of the future.

The process of growing poly diamond films from the gaseous phase as executed in the Schulich Faculty of Chemistry at the Technion appears schematically in the following diagram:

The filament is heated to 2000° and the surface, which lies about a centimeter beneath it, has a temperature of about 700°.  Diamond crystal grow from seeds as a result of differences in the radicals  energetic states between the hot filament and the growing surface. Carbon atoms are adsorbed onto the growing surface, which in the presence of active hydrogen cause the diamond seeds to grow. Continuous poly crystal diamond layers grow at a rate of one micron per hour.

In order to coat a particular surface with a uniform and smooth layer of diamonds, the growing seeds must be highly dense. Hoffman and his staff studied how to control this parameter. They tried to develop a physico-chemical method for attaching the growing seeds to the surface needing to be coated, and among others, they tested an ultrasonic bath. In this process the object to be coated is inserted into an alcohol mixture containing nano-diamond dust and various different metal and ceramic particles, and then undergoes ultrasonic activity.  

Prof. Hoffman and his team saw that the process greatly increases the density of the growing seeds, but their results were inconsistent. They were unable to understand the process, until unintentionally a team member (Roza Akhvlediani) added relative large alumina particles, which is a very hard and durable material, to the mixture. The result was beyond what anyone had expected: the density of the growing seeds increased by three orders of magnitude from 10⁸ to 10¹¹ seeds per cm². The surprised researchers understood that this wasn't a chemical process because the alumina is inert the under experimental conditions, and suggested that a mechanical process had taken place - the larger alumina particles were agitated by the ultrasonic field and hit the minute diamond particles floating in the solution fixing them to the surface.

The discovery was the result of a mistake, but Hoffman explains that an unexpected outcome in an experiment or a result that at first appears irrational often produces scientific breakthroughs. This is a message he always makes sure to convey to his students. There is no substitute for planning and hard work; nonetheless, it is also important not to ignore unusual events and results that appear at first contrary to common wisdom, because it is these incidents especially that often screen important discoveries, explains Prof. Hoffman.

A Dusk Discovery

Another such unusual occasion, during which an accidental result produced the solution to a great mystery, helped Hoffman to contribute to the clarification of the mechanism of diamond nucleation from energetic species.

The discovery occurred when Hoffman and his team investigated the effect of an electrical field on the formation process of nano-diamonds.  Here too the experiment initially provided incompatible findings, but everything changed thanks to one moment of serendipity and scientific intuition. It was about dusk, so that the light in the lab was dim. A research student (Irina Gouzman) called Hoffman to check the deposition system and then started the process as she had already done many times before. She activated the electrical field but this time forgot to turn on the extremely hot wire. In the shadowy light of the lab, to their surprise the researchers noticed a spark within the experimental device that had remained dark because the heated wire had not been turned on.

True to his method of operation, Hoffman focused on the unexpected result and understood that something very important had happened. The team continued looking into the phenomenon until the picture sharpened and came into focus. It turned out that the electrical field is what caused the electrical discharge and this event created the energetic species that contribute to the formation of the nano-diamonds that constitute the initial crystallization clusters. Under these conditions the carbon and hydrogen ions accelerate rapidly to the surface in which they are implanted and form the initial diamond nucleus. The mechanism by which the diamond nuclei is formed and growth up to the size of nano-diamond crystallites was later explained by Yeshayahu Lifshitz (now professor at the department of Materials Engineering at Technion), Hoffman and others and published in the prestigious journal Science in 2002.

Diamonds in Space

Hoffman is now aiming his research at new heights and is investigating the behavior of poly crystal diamond films in space. At a height of 200 to 500 km above the Earth, where satellites orbit, there are relatively high concentrations of atomic oxygen and UV rays that corrode different materials very efficiently. In a lab experiment it was found that  poly crystal diamond film are very resilient in the harsh simulated space environment. Recently diamond films that were grown in Hoffman's lab were sent up into space on the Atlantis space shuttle. They will be there for about a year and when they return, the effect of this environment on them will be checked. Scientists will be looking to see if they are appropriate for use as a coating and protective material for satellite parts and if they can, therefore, significantly extend the parts' lifespan and durability.

The results described in this short article are only a short part of the much larger research work that has been going on in the Schulich Faculty of Chemistry for over two decades by many research students,  research team members under the supervision of Prof. Hoffman and collaborators.