Hybrid structures containing semiconductor quantum dots and metal nanoparticles could lead to better light-emitting diodes and new nonlinear photonic devices. That is according to researchers in China, who have studied hybrids made of cadmium–telluride quantum dots and gold nanoparticle arrays. The amount of light emitted by these structures can be increased dramatically by simply tuning the plasma oscillations on the gold particles to resonate with transitions in the quantum dots.

Quantum dots are tiny semiconductor structures in which electrons are confined in all three dimensions. They have electronic and optical properties that can be controlled by adjusting the shape and size of the structures and have been intensively studied over the last two decades. In particular, quantum dots could be ideal for making a new generation of semiconductor lasers with lower threshold currents and higher efficiencies, as well as light-emitting diodes, solar cells and other photonic devices.

More recently, researchers have turned their attention to hybrid structures containing both semiconductor quantum dots and periodic arrays of metallic nanoparticles. Such systems have shown improved optical characteristics and might be useful for a variety of photonic applications, such as photocatalysis, light harvesting and all-optical switching. The improvements come thanks to the interactions between bound states of electrons and holes in the semiconductor – called excitons – and surface plasmons on the nanoparticles. Plasmons are collective oscillations of electrons on metal surfaces, and they interact strongly with light.

Improved optical response

In this latest work, Andrey Rogach and colleagues at the City University of Hong Kong and the Wuhan National Laboratory for Optoelectronics studied the optical properties of hybrid structures composed of cadmium–telluride quantum dots and gold nanoparticle arrays. The researchers found that they could dramatically increase the amount of light emitted by the structures by tuning resonance of the gold surface plasmon to the exciton transitions in the semiconductor quantum dots. "This is possible because the two materials are confined in the same small space – something that leads to the local electromagnetic field of the metal surface plasmon being enhanced," explains team member Ming Fu. "The interaction of the enhanced field and the excitons in the semiconductor dots can then improve the optical response of the entire system."

To date, the researchers have only studied the luminescence of the hybrid semiconductor–metal structures. They now plan to investigate the fluorescence behaviour of the materials using techniques such as confocal microscopy. "These experiments will hopefully help us glean more information about the interactions between metal particles and semiconductor quantum dots in general," says Fu.

The work is described in Applied Physics Letters.