Focus on Nanoscience

Nanoscience has been around for years, yet it is still a relatively new field.  As the field develops, nanomaterials are finding uses in a variety of applications including display devices, photovoltaic devices for sustainable energy, and solid-state lighting; as well as the recent development of biomedical applications. One problem with producing nanomaterials is controlling the thermodynamics of growth that are governed by the process of crystallization in analogy to the formation of rock candy from sugar, often referred to as “nucleation theory”. Due to thermodynamics, the process of crystallization proceeds to form macroscopic materials rather than materials at the nanoscale. Researchers can control this growth through the manipulation of the chemical interactions at the atomic level through the application of microwave energy fire heating, choice of solvent, and reactants. Microwave energy can heat materials uniformly, but more importantly extremely quickly.  Since controlling material growth is directly related to the controlled application of heat to drive a chemical reaction, the ability for a microwave to provide an instant on (heating)/instant off (cooling) energy source, the rate of growth can be directly controlled. These qualities make microwave energy especially well-suited for producing nanomaterials.

Professor Geoffrey F. Strouse, Professor of Chemistry at Florida State University in Tallahassee specializes in nanoscience and has successfully introduced microwave energy into his laboratory.

Strouse:   From a perspective of material synthesis, one of the first things we realized was that it’s not good enough just to make a nanomaterial; you have to make materials at a level in which a company can apply the new technology. Being able to control the growth and quality through the application of microwave methodology could revolutionize the ability to prepare and utilize this class of materials. Although nanoscience has been around for a long time, nanomaterials and their application of nanomaterials has been limited to largely academic pursuits due the available synthetic methods. How do you make a gram of material? How do you make a kilogram of material? How do you make these materials in an environmentally responsible manner? How do we maximize reaction efficiency and minimize the waste stream? These are the questions we asked when we developed new synthetic methodology for high-throughput synthesis of nanomaterials. In fact, these questions led to a joint effort on solid-state phosphors with Mitsubishi Chemical Corporation and the development of microwave chemistry for nanomaterials.

Interviewer: What is that?

Strouse:   Solid-state phosphors offer a unique material for the replacements for fluorescent light bulbs. Most people don’t recognize that fluorescent light bulbs have 5-25 mg of mercury per tube. So, in a given room there could be hundreds of milligrams of mercury and this is pretty toxic, more for children than adults. Japan and Europe are reducing mercury in lighting and efforts to minimize heavy metal waste are leading to the desire to reduce heavy metal uses in industry. The U.S. probably isn’t that far behind. Mitsubishi came to me with the problem and said they wanted to produce solid-state lighting that looks like a normal tube, but without the mercury. Our answer was quantum dots or nanomaterials, semiconductor particles that have a specific color of light with a high quantum efficiency. We developed not only the material chemistry, but the methods for the microwave as well, so that we get selective heating of the reactants in a solvent that doesn’t absorb the microwave energy. The rational behind this is that the solvent effectively acts as a moderator for the growth. Because we’re trying to beat thermodynamics, we want to heat them up faster and cool them down faster; something the microwave does very effectively.

Interviewer: How is microwave energy beneficial in this type of reaction?

Strouse:   From a perspective of chemistry, from a perspective of industrial science, the typical reaction for formation of a nanomaterial is best described as almost like setting off a bomb. You take a reaction in an organic solvent, such as a phosphine, at 330 degrees and inject varied intermediates to produce nanomaterials. The result is a large distribution in material quality and little control.  In the microwave, we are able to take this reaction and by controlling the microwave power and interaction time, we control the nanomaterial growth directly.  No more high-temperature injections; no more toxic solvents.   By use of the microwave cavity, what used to take hours (or even days), we can do in seconds.  We can be more environmentally conscious, since we reduce energy consumption and the waste stream.  We can use simple precursors instead of these highly reactive ones and the reason is that the microwave dumps so much energy into those reactants; you get instantaneous material formation, or nucleation, followed by rapid growth. This gives us an unprecedented level of control and the ability to produce high-quality materials.

Why a microwave? Well, when trying to create high quality materials, the immediate focus goes to controlling the nucleation process. What we mean by nucleation is controlling the size of the initial particle that’s formed. This is going to depend on several things. How high or how fast can you get the reaction to temperature? How uniform is the energy field that this reaction is being done in? What about thermal gradients and things like that? Microwave chemistry has some unique advantages there. It heats the reactant. It has no problem with thermal gradients and you can do it in a very controlled way. What do you need? You need focused microwave energy via a single-source microwave capable of operating at above 300 °C. This isn’t a household microwave. This isn’t like an industrial microwave, which are multimode. They fail. We tested several instruments before working with CEM. The breakthrough for us was figuring out how to harness the microwave for nanomaterial growth, how to take advantage of non-traditional solvents, such as hexane or octane, and use it as a heat source or as a moderator of the growth because alkanes do not absorb microwaves, only the reactant does.

Interviewer:   Your research group is working in several different areas of nanoscience. Can you tell me about their projects?

Strouse:   When you look at my research group, it is composed of a large synthetic team that develops new nanomaterials with optical properties for a specific color. Mitsubishi’s lighting is an example of that. We want a high quantum yield. We want very crystalline materials. We want very specific sizes, because in a quantum dot the size of the material is directly related to the energy, so if you have a 3nm cadmium selenide, you get a material that emits in the green. If you have a 6nm CdSe, you have a material that absorbs in the red, so you actually control color by size. To do that, you have to have a very narrow distribution, and to do that, you have to have very controlled reaction conditions. Once again, that pretty much leaves you the idea that microwave is a better way to do it. The second part of my group does extensive optical measurements of these materials, so we produce materials that are either classic semiconductors or are non-classical semiconductors where we intentionally put a defect ion into it and by putting a defect ion into it, we can make it a magnet. We can make it have specific properties that we want. The last part of the group is taking these materials that we’ve produced and using them in biological studies, so we actually do targeted drug deliveries to cells. These simple ideas that started with making materials rapidly led into other applications. We produce materials in the microwave now that we’re using not only to stain a cancer cell, but to actually deliver a reactive organic, or reactive chemical, or a drug into the cell to kill the cancer cell.

It’s kind of interesting how a very simple question that a company asked me years ago — How do we make quantum dots on a large scale? —  basically led down this path. You have to think of a reaction, what drives it, what’s the mechanism, what plays a limiting role in the reaction and how do you reproducibly produce the materials. It’s easy to make milligrams, but how do you make a kilogram of exactly the same material? This led to the idea that we could use a flow-through or a stop-flow microwave system, such as the CEM Voyager™, and that this would be commercially viable.  After all, for industrial applications, the goal is to reproduce materials day-to-day and do it continuously. So, we’re taking advantage of the flow-through system. We’ve been able to pump through materials and produce large quantities. From an industrial viewpoint, it really opens up the world to using nanomaterials for technology.