Three researchers have been awarded this year’s Nobel Prize in Chemistry for discovering quantum dots–a class of materials so small that their size alone alters their optical and electronic properties.
Moungi Bawendi of the Massachusetts Institute of Technology, Louis Brus of Columbia University, and Alexei Ekimov of Nanocrystals Technology Inc. will share a prize of roughly $1 million for demonstrating the abilities of the tiny crystals and showing how to produce them reliably. In doing so, they “planted an important seed for nanotechnology”, the Royal Swedish Academy of Sciences said in a press release this morning. The dots have found applications in TV displays, lighting, and medical imaging. One day, quantum dots could be used to create flexible electronics, tiny sensors, improved solar cells, and quantum communication.
During a press conference today, Johan Åqvist, chair of the Nobel chemistry committee, displayed several flasks of liquid each containing nanocrystals with differing sizes. Each fluoresced brilliantly in a different color. “When faced with particles this small, quantum mechanics starts to play all kinds of tricks. The nanoparticles in each flask are made of exactly the same simple substance. So how can they differ in color?” he asked. “Well, that, ladies and gentlemen, is a quantum effect.”
The possibility of making such tiny structures was proposed as long ago as the 1930s, as physicists were discovering the implications of quantum mechanics, the physical laws that govern the atomic world. Crystals a millionth the size of a pinhead would act like a box, confining electrons in a way that alters their properties. Thinking of the electron as a wave, a smaller box means the wave is compressed to a shorter wavelength; the shorter the wavelength, the higher the electron’s energy. If the electrons are excited by an outside source of light, a smaller quantum dot will emit bluer, higher energy light. A larger dot emits lower energy yellow or red light.
In the late 1970s, Ekimov, then at the Vavilov State Optical Institute, first managed to make nanometer-size crystals of copper chloride in molten glass. He showed that different sizes of dots would fluoresce in different colors. A few years later, Brus, then at Bell Labs, was looking for catalysts to capture the energy of sunlight in a chemical reaction. When Brus’s team crystallized particles of cadmium sulfide out of a solution, they noticed that the larger ones reacted to light differently than the smaller ones and realized it was the same quantum phenomenon.
Defects in these quantum dots hampered their use. In 1993, Bawendi and his team, also at Bell Labs, devised a way to make high-quality crystals of a well-defined size. This involved injecting quantum dot ingredients into a hot solvent to immediately form crystal seeds, and then stopping their growth by rapidly cooling and diluting the solvent. To make dots of a required size, the solvent is warmed again to continue crystal growth in a more controlled way. “It was just an exciting time,” says David Norris, a materials engineer at ETH Zürich who was a former graduate student of Bawendi’s. “It was a really great environment that Moungi had created in those early days.”
The process devised by Bawendi’s team led to the wide commercialization of quantum dots, with many companies competing to produce the nanocrystals cheaply. For instance, some 8% of the global TV market now relies on quantum dots to soak up light from LED displays and add brilliant colors. Speaking on the phone during the press conference, Bawendi said the research community realized in the mid-1990s the real-world potential of quantum dots. “There’s still a lot of exciting work to be done in this field, that’s for sure.”
