Building Quantum Dots Slowly

Hank Hogan

Quantum dots are small, with sizes measured in nanometers. And the size is important because it determines optical properties such as the emission peak of the particle. Bigger quantum dots have redder emission peaks. The problem with engineering the particles has been controlling their growth during synthesis: Popular organo-metallic techniques — performed at
200 °C and above — tend to create the dots in a burst, making control of the final size difficult.

Now researchers Liping Liu, Qing Peng and Yadong Li from Tsinghua University in Beijing have demonstrated an approach in which small clusters of the particles are formed first at room temperature. That step is followed by slow particle growth at elevated temperatures. The reaction is slow enough that a desired optical response can be dialed in.

Growing a quantum dot slowly can help tune the optical results. Researchers prepared CdSe quantum dots using room-temperature injection for nucleation to provide slow — therefore controllable — growth. The results can be seen in these images of the quantum dot solution fluorescing when excited by a UV lamp (peak 365 nm). The evolution over time of the fluorescence results from the steady growth of the particles. Reprinted with permission from Inorganic Chemistry.

The key lies in the start of the process, Li said. “A suitable reactive system, capping ligand and the precursors are important for this room-temperature injection method.”

The investigators used CdSe quantum dots, building upon earlier work they had performed on preparing nanocrystals at room temperature, and they used oleic acid as a ligand, which means that the CdSe clusters formed at room temperature were small. Because of the conditions, they also did not grow.

They transferred the quantum dots to other solutions, keeping them at 40 to 150 °C during particle growth. Over the next few hours, the particles increased in size, and their peak photoluminescence wavelengths lengthened, as determined with a UV-visible absorbance spectrometer and a fluorescence spectrophotometer, both from Hitachi.

The photoluminescence peakswere a result of both time and temperature; e.g., after 10 h at
60 °C, the particles’ peak was 519 nm, and at 150 °C, it was 635 nm. As for time, the peak at
60 °C after 1 h was 499 nm. Some 49 hours later, the intensity was 531 nm. This effect is a consequence of the growth temperature determining the size of the quantum dots, which, in turn, determined the emission peak.

They noted that separating the nucleation from the growth — along with the mild reactive conditions — makes it possible to tune the optical properties of the particles. Li characterized it as being able to adjust their fluorescence leisurely.

To produce quantum dots with intensity peaks substantially below 500 nm, they used H₂O₂ as an etchant. This oxidized the particles and made them smaller, shifting their output to the blue while broadening it as well.

As for future research, Li noted that these CdSe quantum dots have a well-controlled bandgap structure, making them suitable for absorbing sunlight and converting it into electricity. “We are doing research on CdSe quantum dot-sensitized TiO₂ films for solar cells.”

Inorganic Chemistry, June 2, 2008, pp. 5022-5028.