Quantum dots adapted to glow more brightly

18 Sep 2014

A thin layer of aluminum oxide layer makes quantum dots glow more brightly, promising diverse applications.

Researchers from the universities of University of Alabama in Huntsville and Oklahoma have discovered a new method of controlling the properties of quantum dots, which they say, promises a wide range of potential applications, from sensors, LEDs, and solar cells, to fluorescent tags for biomedical imaging and qubits in quantum computing.

A key property of quantum dots that makes them so useful is their fluorescence. Scientists can “tune” quantum dots to emit a specific color of light by adjusting their size: for example, small dots glow blue and large dots glow red. However, the dots’ ability to glow can change over time with exposure to light and air.

Seyed Sadeghi, a physicist at the University of Alabama, wondered if it would be possible to better control how quantum dots react to their environment. His team had previously found that placing quantum dots of a certain type on nanometer-thin layers of chromium and aluminum oxides significantly altered the dots’ behavior: the aluminum oxide increased their emission efficiency, while the chromium oxide increased the dots’ degradation rate when exposed to air. The researchers decided to extend their investigations to quantum dots with different structures.

Quantum dots come in a variety of shapes, sizes, and materials. For Sadeghi and his colleagues’ most recent studies, published in the Journal of Applied Physics, the researchers investigated the behavior of four different types of commercially available quantum dots. Some of the quantum dots had protective shells, while others did not.

The researchers found that ultrathin aluminum oxide could make quantum dots glow brighter and that the effect was much more significant for quantum dots without protective shells. They also found that while quantum dots with both binary and ternary cores shrink after reacting with the oxygen in air, ternary core dots placed on aluminum oxide glowed brighter despite the shrinkage.

Sadeghi commented, “The results of these studies can serve to enhance emission efficiency of quantum dots, which is an important feature for applications such as LEDs, sensors and detectors, photovoltaic devices, and for the investigation of various quantum and nano-scale physical phenomena.” As a next step in their own research, Sadeghi and his colleagues plan to investigate how metal oxides might affect the behavior of quantum dots when they are close to metallic nanoparticles.

Further details

In the Journal of Applied Physics article, Sadeghi and team report, “We investigated how the structure-dependent features of four different types of colloidal quantum dots can influence their photo-physical and photochemical properties using photoactive substrates consisting of 1nm layers of aluminum or chromium oxides.

“We have demonstrated that the COOH functionalized core/shell quantum dots were largely insensitive to enhancement processes (such as Coulomb blockade and surface passivation) while the amine-ligated CdSe and CdSe/ZnS dots were greatly influenced by these processes.

“Results for the high field intensity case were indicative of photo-oxidation and also revealed distinct spectral characteristics based on the type of quantum dot and photoactive substrate. Collectively, our results show the ability of photoactive substrates to interact uniquely with the QDs reveals their photophysical and photochemical properties.”