New approach to solid-state lighting

In a different approach to creating white light, several researchers at the Department of Energy's (DOE) Sandia National Laboratories have developed what is claimed to be the first solid-state white light-emitting device using quantum dots. In the future, the use of quantum dots as light-emitting phosphors may represent a major application of nanotechnology.

"Understanding the physics of luminescence at the nanoscale and applying this knowledge to develop quantum dot-based light sources is the focus of this work," says Lauren Rohwer, principal investigator, technology through nanoscience. The approach is based on encapsulating semiconductor quantum dots - nanoparticles approximately one billionth of a metre in size - and engineering their surfaces so they efficiently emit visible light when excited by near-ultraviolet (UV) light emitting diodes (LEDs). The quantum dots strongly absorb light in the near UV range and re-emit visible light that has its colour determined by both their size and surface chemistry.

This nanophosphor-based device is quite different from an alternative approach based on growth of blue, green and red emitting semiconductor materials that requires careful mixing of the primary colours to produce white illumination. Efficiently extracting all three colours in such a device requires costly chip designs, which likely cannot compete with conventional fluorescent lighting but can be attractive for more specialised lighting applications.

Rohwer and the quantum dot team - Jess Wilcoxon, Stephen Woessner, Billie Abrams, Steven Thoma and Arturo Sanchez - started on the project two and a half years ago. Subsequently their research has advanced significantly including recently reaching a major milestone of creating white and blue lighting devices using encapsulated quantum dots.

LEDs for solid-state lighting typically emit in the near UV to the blue part of the spectrum around 380-420 nm. Conventional phosphors used in fluorescent lighting are not ideal for solid-state lighting because they have poor absorption for these energies. So researchers worldwide have been investigating other chemical compounds for their suitability as phosphors for solid-state lighting.

Quantum dots represent a new approach. The nanometre size quantum dots are synthesised in a solvent containing soap-like molecules called surfactants as stabilisers. The small size of the quantum dots - much smaller than the wavelength of visible light - eliminates all light scattering and the associated optical losses. Optical backscattering losses using larger conventional phosphors reduce the package efficiency by as much as 50%.

Nanophosphors based on quantum dots have two advantages over the use of conventional bulk phosphor powders. First, while the optical properties of conventional bulk phosphor powders are determined solely by the phosphors' chemical composition, in quantum dots the optical properties such as light absorbance are determined by the size of the dot. Changing the size produces dramatic changes in colour. The small dot size also means that, typically, over 70% of the atoms are at surface sites so that chemical changes at these sites allow tuning of the light-emitting properties of the dots, permitting the emission of multiple colours from a single size dot.

"This provides two additional ways to tune the optical properties in addition to chemical composition of the quantum dot material itself," Wilcoxon says.

For the quantum dots to be used for lighting, they need to be encapsulated, usually in epoxy or silicone.

"Doing this we had to take care not to alter the surface chemistry of the quantum dots in transition from solvent to encapsulant," says Thoma, who worked on the encapsulation portion of the project.

Quantum dot phosphors are integrated with a commercial LED chip that emits in the near ultraviolet at 400 nm by encapsulating the chip with a dot-filled epoxy, creating a dome. The quantum dots in the dome absorb the invisible 400 nm light from the LED and re-emit it in the visible region - a principle similar to that used in fluorescent lighting.

However, a key technical issue in the encapsulation process had to be solved first. When altering the environment of the dots from a solvent to an encapsulant the quantum dots would 'clump up' or agglomerate, causing them to lose their light-emitting properties. By attaching the quantum dots to the 'backbone' of the encapsulating polymer they are close, but not touching. This allows for an increase in efficiency from 10-20% to an amazing 60%, Thoma says.

In the next year the researchers will increase the concentration of the quantum dots in the encapsulant to obtain further increases in light output while extending the understanding of quantum dot electronic interactions at high concentrations.