White LED Incorporates Quantum Dots

Richard Gaughan

Quantum dots are nanometer-scale semiconductors that act as a potential well for electrons. They exhibit a broad absorption and relatively narrow emission spectrum, with a peak wavelength that is defined by their dimensions. By capping polyhedral ZnSe quantum dots with a blend of organic molecules, researchers at the Industrial Technology Research Institute in Hsinchu, Taiwan, have created an emitter based on quantum dots that produce white light when assembled in a layer above an InGaN ultraviolet LED.

A white LED employs quantum dots excited by ultraviolet radiation. Although not yet as efficient as other phosphors, the ease of manufacturing quantum dots makes them an attractive option for the production of white LEDs.

The scientists precipitate the quantum dots from a ZnO/Se colloidal suspension with stearic acid and trioctylphosphine oxide, which are incorporated into the outer layers of the ZnSe dots to modify their electronic band structure. A purely spherical 7.5-nm dot would produce an emission peak at 425 nm, but in these polyhedral quantum dots, surface strain modifies the lattice, altering the emission spectrum and creating a broader component with a peak at 510 nm. Interactions with electronic states in the organic capping molecules extend this broadband emission even further into the red. Together, the two emission peaks result in a broadband white-light emission with CIE coordinates of 0.38 and 0.41.

The investigators incorporate the capped quantum dots in an epoxy resin at approximately 6.5 percent by weight. They coat a 385-nm LED from Toyoda Gosei with the epoxy resin. The result is a device that emits white light.

According to Hsueh Shih Chen, quantum dots offer significant processing advantages over traditional phosphors. The manufacturing can take place at lower temperatures and in less time than it takes to produce red, green and blue phosphors. Moreover, because each quantum dot directly emits white light, the LEDs provide good color rendering without the need for careful balancing of various phosphors.

The efficiency is about 30 percent of that of devices that employ typical red, green and blue phosphors. Theoretically, a higher dot density would result in a greater coupling efficiency and higher output intensity, but would be above a level of 10 percent by weight in the resin, the quantum dots aggregate, reducing the efficiency.

The investigators hope to increase the dot density by using a different resin or surface modification. They also hope to tune the emission band by varying the size, shape and organic capping of the dots.

Applied Physics Letters, March 28, 2005, 131905.