Imagine that everything in the world suddenly shrank to a size at the nano scale. Although it’s not intuitively apparent, many things would appear different. To cite examples used by the Royal Swedish Academy of Sciences, gold earrings would suddenly turn blue, while a gold ring would look ruby red. Why should two gold ornaments take different colours? Because their different sizes would dictate their properties in the mysterious quantum world — a fact discovered, researched and exploited by the three scientists chosen for the Nobel Prize for Chemistry on Wednesday.
Moungi G Bawendi of MIT, Louis E Brus of Columbia University, and Alexei I Ekimov of Nanocrystals Technology Inc, New York, shared the Nobel for the discovery and synthesis of “quantum dots” which are particles so small that their size determines their properties. Their work laid one of the foundations of nanotechnology, which has applications from electronics to surgery.
Size does matter The quantum world is known for quantum effects, a term to describe the strange phenomena that take place. As far back as 1937, the physicist Herbert Fröhlich predicted that nanoparticles would not behave like larger particles, and later researchers predicted various quantum effects dictated by size.
The reason size dictates the properties of nanoparticles lies in the movement of electrons. Most of the physical properties of a material are controlled by the movement of electrons and “holes”, which are gaps in energy positions once occupied by electrons. When this movement is constrained, it changes a property called band gap, which is otherwise unique for every material. This results in a change in various other properties, including colour.
“The effective spread of electrons and holes in typical materials is very small. When you reduce the particle to such a size comparable to these effective sizes of electrons and holes in any given material, they feel constrained — we call it confinement effect — and this moves the energies of the electrons and the holes significantly, changing the band gap and, therefore, the colour along with many other properties,” said D D Sarma, professor of chemistry at the Indian Institute of Science (IISc).
Creating quantum dots It was only in the late 1970s that quantum dots were actually created. Ekimov, then a physicist working at the SI Vavilov State Optical Institute in the Soviet Union, puzzled over the fact that a mixture of cadmium selenide and cadmium sulphide could turn glass either yellow or red, depending on the amount of heating and cooling. Why should the same mixture result in different two colours?
Ekimov prepared glass tinted with copper chloride, then X-rayed it and observed tiny copper chloride crystals. While larger particles behaved like copper chloride normally does, at the nano scale they began to absorb bluer and bluer light. This was the first observation of a size-dependent quantum effect. Ekimov published his findings in 1981.
On the other side of the Iron Curtain, Brus published his own findings in 1983. Working at Bell Laboratories in the US, Brus found that cadmium sulphide particles underwent a change in optical properties after he had left them on the bench for a while. He created smaller particles and found, like Ekimov had, that the smallest particles had size-dependent optical properties.
The nanoparticles produced by Brus often contained defects, and also varied in size. It was Bawendi, who began his postdoctoral training under Brus in 1988, who perfected the method after he joined MIT. In 1993, his research group developed easy-to-use methods that would produce almost perfect nanocrystals that showed quantum effects. This revolutionised the field.
Practical applications As research grew, so did the applications of quantum dots.
“One of the most popular applications of quantum dots in electronics,” said Mrinmoy De, professor of organic chemistry at IISc, whose research includes catalysis and biological applications using quantum dots.
“On an LED screen, the colour of the bright emissions depends on unique photophysical properties of the quantum dots. Another application is in biology. Functionalised quantum materials with suitable ligands can be good for high resolution cellular imaging, biological sensing and therapy,” he said.
