Scientific breakthroughs are usually portrayed as being achieved by people conducting complicated experiments in equipment-filled labs.

While this picture is certainly true at least part of the time, many of the most important discoveries are also made by individuals working with nothing but pencil and paper. These scientists use their knowledge in any number of fields (including mathematics, physics, mechanics and many other disciplines) to uncover the nature and potential of new technologies and forms of matter. By revealing how something—from a naturally occurring chemical to a man-made machine—will behave under certain conditions, these individuals, known as theoreticians, provide a framework from which experiments can be designed and their results interpreted.

This is the role filled by Pradeep Sharma, assistant professor of mechanical engineering at the University of Houston Cullen College of Engineering. “For experimentalists, if you have theoretical work, you have a sense of what to expect,” he said. “Even if you get raw data, how are you going to interpret it without theoretical work? So instead of shooting in the dark, experimentalists often like to be guided by theories.”

One area Sharma and his research team are exploring is quantum dots. Quantum dots, which fall under the realm of nanotechnology, are tiny crystals made of semiconductor material that measure just a few nanometers wide (a nanometer is one-billionth of a meter).These dots can be constructed practically one atom at a time in highly specific formations, allowing scientists to tailor the amount of energy they produce and absorb or the wavelength of light they emit.

When used in different ways, quantum dots can impact everything from lighting systems, to medical diagnostics, to bioterrorism preparedness, to data storage for computers. For example, by manipulating the wavelength of light quantum dots emit, scientists are constructing blue-light lasers that, by their very nature, have smaller wavelengths than standard red lasers. This smaller wavelength allows data on compact disks to be stored in spaces that are physically smaller than the amount of space required by red lasers, thus greatly increasing the capacity of CDs and DVDs.

Sharma and his research team are now using mathematical modeling to predict how quantum dots will react in particular situations and under particular conditions, thus providing other scientists with the framework for performing their own research.

In the case of quantum dots, experimentalists need some understanding of how these nanoparticles will (or, at least, should) behave in order to most efficiently design their experiments, analyze the results and eventually design products. Many times such analysis reveals properties that are not at all intuitive but are present naturally at nanoscales. Experiments are then designed to verify the existence of such characteristics, which might lead to some unconventional applications.

Indeed, Sharma’s theories are currently being applied in research conducted by other researchers at UH. A research team led by Kirill Larin, assistant professor of biomedical and mechanical engineering, is developing a laser-based medical diagnostic tool that utilizes specially tailored quantum dots that reflect light at a particular spectrum when they bond to particular disease proteins, thereby informing doctors of the presence of a disease. (See article on page 12).

Professor Pradeep Sharma works with graduate students Mohamed Sabri Majdoub, Nikhil Sharma, Ravi Maranganti and Feng Shi to analyze the effects of length-scale on a material's behavior.

Not surprisingly, the process of formulating theories on the behavior of quantum dots is not performed in a typical “lab.” Rather, Sharma calls on the qualities he has crafted during his engineering career and his understanding of several different fields, including physics and mechanics, to devise these theories. He then works with his research team, comprised primarily of graduate students, to perform further calculations to determine the accuracy and implications of his ideas.

“Some part of (developing these theories) is creativity, some part of it is mathematical knowledge, some part of it is your knowledge of the field. It’s a mixture of things. You have to be knowledgeable and aware of what other people are doing in order to keep up to date,” he said.

Currently, one of the areas Sharma is paying particular attention to is the effect of mechanical strain on quantum dots.

Mechanical strain is simply the physical deformation of an object. When you stretch a rubber band, you are subjecting it to mechanical strain.

“Traditionally,” Sharma said, “scientists have held that impact of mechanical strain on the mechanical properties of an object do not change as the size of the object changes.” New research by Sharma and his team, however, shows that mechanical strain impacts materials differently at the nanoscale versus the macroscale, the realm in which most of the world operates. “In the bulk state, everything averages out,” said Sharma. “As you get smaller and smaller, though, the quantum effects become more prominent.”

These changes in mechanical properties, in turn, impact other attributes of materials. “How do changes in mechanical properties that occur with changes in size affect the electronic properties? That’s what we’re studying. We’re trying to come up with theories that predict those changes,” Sharma said. “More specifically, we are studying the impact of mechanical strain on quantum dots’ ‘bandgap,’ the property that dictates how much energy is needed to make a semiconductor conduct electricity.

In the bulk or macroscale, a material’s band structure is predetermined by nature. If researchers need a specific band structure to perform a task, then they must find another material that offers that property and hope that it meets their other needs, as well.

At the nanoscale, however, variations in size and mechanical strain can change a material’s band structure, in effect setting up a situation where scientists should be able to tune a bandgap to a specific frequency in order to meet their particular needs. This is where Sharma’s work is applied. His theories provide experimentalists with guidelines for applying mechanical strain to a quantum dot in order to attain a specific bandgap or more generally, a specific band structure.

With a tunable band structure, he says, researchers could construct more efficient and sensitive energy sensors that could be used to track missiles or make night-vision goggles, or more powerful lasers that improve data storage or act as tools for diagnosing diseases.

For any of these to become reality, though, the pioneers of experiments need an idea of what’s possible in the nanorealm, and that is exactly what Sharma gives them.

“Hopefully my work provides some guidelines for experimentalists,” said Sharma. “Theory and experiments have to go hand-in-hand.”