By decorating zinc sulfide quantum dots with two different organic molecules, researchers have made them emit pure white light (J. Phys. Chem. Lett. 2015, DOI: 10.1021/acs.jpclett.5b00295). The particles offer a new way to produce white light from quantum dots and could serve as nontoxic alternatives to quantum dots currently on the market.
Quantum dots are semiconductor nanocrystals that emit light over a short range of wavelengths. Electronics manufacturers want to use the materials to develop light-emitting diodes (LEDs) for lighting and displays because the devices would use less energy and produce richer, more vibrant colors than conventional LEDs.
Pure, bright white-light emission is key for these applications. Until now, researchers have created white light by combining the light from red-, green-, and blue-emitting nanocrystals. But making a single nanostructure that emits white light would yield uniform emissions and eliminate any unwanted tints in the white light. Plus, working with one material instead of three should make device fabrication easier, says Arun Chattopadhyay, a chemist at the Indian Institute of Technology, Guwahati.
So Chattopadhyay and his colleagues decided to carefully alter the chemistry of the zinc sulfide quantum dots they had been working on. They focus on these nanocrystals because they are nontoxic. “Most of the application-oriented quantum dots today use heavy metals,” which are toxic, Chattopadhyay says.
The team made manganese-doped zinc sulfide nanocrystals by heating a solution of zinc acetate dihydrate, manganese acetate tetrahydrate, and sodium sulfide. The crystals have exposed zinc and manganese ions on the surface. When the researchers shined 320-nm ultraviolet light on these quantum dots, they glowed orange at 588 nm.
Next, the researchers added a mixture of acetylsalicylic acid, or aspirin, and 8-hydroxyquinoline to the quantum dot suspension. The salicylic acid molecules react with the manganese and zinc ions to form complexes on the nanocrystal surface that emit 410-nm blue light when excited. Meanwhile, the 8-hydroxyquinoline reacts with zinc ions to form complexes that emit 500-nm green light.
The orange, blue, and green emissions mixed to create a pure white glow when the dots were excited with 320-nm light. The researchers could tune the particles to emit other colors by changing the ratio of the organic molecules added to the nanocrystal, Chattopadhyay says.
This is an original and interesting approach to generating white light from a single nanocrystal, says Marinella Striccoli, a physicist at the National Research Council Institute for Chemical & Physical Processes, in Bari, Italy. By playing with the concentrations of the ligands, the researchers could modify the relative intensity of light and tune the emitted color, she says, “opening the way to infinite color combinations.” However, to be used in LEDs, the quantum dots would need to be excited by electrical current, not light, she says.
Chattopadhyay says that his team plans to work on that problem as they pursue practical light-emitting devices.