An electrical current flowing through a wire typically involves billions upon billions of electrons moving as one. But as electronic devices become ever smaller, the current flowing through them also shrinks to a trickle, approaching the quantum regime where electrons suddenly behave as individual particles and display very different properties.
A team led by Principal Investigator Kuan Eng Johnson Goh and Dharmraj Kotekar-Patil at A*STAR’s Institute of Materials Research and Engineering (IMRE) saw this happen in their prototype nanoribbon transistors, demonstrating that they had inadvertently manufactured quantum dots in the process.
“Quantum dots are nanostructures which confine electrons in all three dimensions,” explained Dr Dharmraj Kotekar-Patil, a scientist in Goh’s team and first author of the study. “They are small enough for electrons to travel through them one at a time. In the process, the electrons repel other electrons and prevent them from passing through at the same time, leading to a ‘Coulomb blockade’ that reduces the current flow.”
Kotekar-Patil and his team were studying monolayer molybdenum sulphide nanoribbons, tiny semiconductor strips just one to a few molecules high and a few dozen molecules wide. At room temperature the nanoribbons functioned as transistors, smoothly increasing their current as the voltage was increased. But when the nanoribbons were cooled to ultra-low temperatures, the electrical current initially disappeared at low voltages, and then jumped in a stepwise fashion as the voltage was increased—tell-tale signs of quantum behavior.
“This Coulomb blockade showed the existence of discrete energy levels that only allowed single electron transport,” Kotekar-Patil said. This in turn demonstrated that quantum dots had been formed, either at the jagged edges of the nanoribbon or in the rough surface underneath.
These quantum dots could be a blessing or a curse. Nanoribbon transistors are meant to transmit a smooth electric current, and their performance could be degraded if electrons get trapped instead of passing through smoothly. “We could eliminate these quantum dots by using atomically flat surfaces under the nanoribbon, or chemically smoothing out the nanoribbon edges,” Kotekar-Patil said.
However, Kotekar-Patil also believes that these quantum dots could be harnessed for quantum computing, a new paradigm that could lead to much faster and more powerful computers. “We could manipulate confined single electrons using additional contacts on top of the nanoribbon and encode quantum information using their spin states,” he continued. “This would contribute to the development of quantum computing, which would be much faster than existing classical computers at several important computational tasks.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).