Atom-sized Quantum Dots

INDIVIDUAL SILICON ATOMS can exhibit electronic properties typical of quantum dots composed of thousands of atoms, according to researchers at Canada's National Institute for Nanotechnology and the University of Alberta, Edmonton (Phys. Rev. Lett. 2009, 102, 046805). The discovery of atom-sized quantum dots and the associated room-temperature control over individual electrons may lead to a new type of circuitry that drives miniature, low-power computers.

Semiconductor nanocrystals are often called quantum dots because, among other reasons, their electronic energy levels are quantized and discrete—like an atom's. Researchers are studying that property, which ties the color of colloidal nanocrystals to their size for applications such as computing. The idea is to use the electronic states of quantum dots, which are far smaller than today's transistors, to represent the zeros and ones of digital processing.

At present, the working material for that research thrust is some the smallest quantum dots readily available—crystals of about 1,000 atoms. But to use them for computations, researchers must chill the nanocrystals to millikelvin temperatures to prevent them from flipping spontaneously between electronic states. The Edmonton team, led by Robert A. Wolkow, has discovered a way to sidestep those limitations.

Using a scanning tunneling microscope to probe hydrogen-blanketed silicon, the group showed that kicking out a hydrogen atom bonded to silicon results in a negatively charged Si atom that behaves like a quantum dot. Kicking out another nearby hydrogen leads to an artificial Si₂ molecule with one electron shuttling between the two atoms (similar to H₂⁺). Then by using a large charge, the team showed that this "bonding" electron can be pins at will, thereby locking the quantum dot into a given electronic state that remains stable even at room temperature.

"These results are remarkable," says Craig S. Lent, professor of electrical engineering at the University of Notre Dame. Wolkow's group has demonstrated a new binary switching mechanism that operates at room temperature and at a truly atomic size scale, he explains. Lent adds "The world is looking for a new binary switch to replace the transistor when size scaling ends. This just might be it."