Forging dream materials with semiconductor quantum dots

(Nanowerk News) Researchers from the RIKEN Center for Emergent Matter Science and collaborators have successfully created a “superlattice” of semiconductor quantum dots that can behave like metals, potentially imparting exciting new properties to this popular class of materials.

Semiconductor colloidal quantum dots have garnered tremendous research interest because of their special optical properties, which arise from the effect of quantum confinement. They are used in solar cells, where they can improve energy conversion efficiency, biological imaging, where they can be used as fluorescent probes, electronic screens, and even quantum computing, where their ability to trap and manipulate individual electrons can be exploited.

However, getting semiconductor quantum dots to efficiently conduct electricity has been a huge challenge, hindering their full use. This is mainly due to the lack of orientation order in the assembly. According to Satria Zulkarnaen Bisri, principal investigator on the project, who conducted research at RIKEN and now at the Tokyo University of Agriculture and Technology, “making it metallic would allow, for example, the display of a quantum dot that is brighter but uses less energy than current devices.

Now, the group has published a study in Nature Communications (“Enabling Metal Behavior in a Two-Dimensional Superlattice of Semiconductor Colloidal Quantum Dots”) that can make a major contribution to achieving that goal. The group, led by Bisri and Yoshihiro Iwasa of RIKEN CEMS, have created a lead sulfide semiconductor quantum dot superlattice that displays the electrically conducting properties of metals.

The key to achieving this is getting the individual quantum dots in the lattice to attach to each other directly, “epitaxially”, without a ligand, and doing this by orienting their facets in the right way.

The researchers tested the conductivity of the material they had made, and when they increased the carrier density using an electrical double-layer transistor, they found that at some point it became a million times more conductive than what is currently available from quantum dot displays. . Importantly, the quantum confinement of the individual quantum dots is still maintained, meaning that they do not lose their function despite their high conductivity.

“Quantum dot semiconductors have always been promising for their optical properties, but their electronic mobility has been a challenge,” said Iwasa. “Our research has shown that precise orientation control of the quantum dots in an assembly can lead to high electronic mobility and metal behavior. This breakthrough could open up new avenues for using semiconductor quantum dots in new technologies.”

According to Bisri, “We plan to carry out further studies with this class of materials, and believe it can lead to major improvements in the quantum dot’s superlattice capabilities. As well as enhancing current devices, this could lead to new applications such as all-QD true direct electroluminescence devices, electrically driven lasers, thermoelectric devices, and highly sensitive detectors and sensors, previously outside the scope of quantum dot materials.”