Quantum-Dot Composite LED Emits in IR

Daniel S. Burgess

An electroluminescent composite that features quantum dots suspended in a semiconducting polymer promises to ease the development of advanced optical sources for applications such as telecommunications. Developed at the University of Toronto, a prototype LED built with the material generates near-IR radiation at wavelengths across the telecom spectrum, with the emission dependent on the size of the quantum dots.

Researchers at the University of Toronto have demonstrated electroluminescence at telecommunications wavelengths from PbS quantum dots embedded in a semiconducting polymer matrix. Courtesy of E.H. Sargent.

Commonly produced in emitters by Stranski-Krastanow growth in semiconductor stacks using epitaxial or chemical vapor deposition techniques, quantum dots have been called "artificial atoms" because they exhibit discrete energy spectra rather than quasi-continuous energy bands. Edward H. Sargent, the Nortel Networks-Canada research chair in emerging technologies at the university, explained that the size of a quantum dot determines the shape of the electronic wave function associated with it -- and the wavelength of the radiation produced by recombining exciton (electron-hole) pairs in the structure.

The wavelength of the emission from a quantum dot depends on the size of the particle. The photoluminescence spectra of three composites, each featuring quantum dots of a specific diameter, cover the telecommunications spectrum and beyond.

"Confining the electron wave in this manner ... influences the set of energies it can adopt," he said. "Just as changing the length of a guitar string changes its resonance frequency, changing the size of the quantum dot changes the electron's resonances. This in turn determines which colors of light are produced by the quantum dot."

For these experiments, the researchers produced lead sulfide quantum dots by a solution-phase organometallic technique at 150 °C. They accessed the infrared wavelengths used in fiber optic telecommunications by choosing a particle diameter of about 5 nm. They stabilized the particles by capping them with oleate or octylamine and dispersed the capped particles in toluene.

To produce the composite, they mixed this solution with MEH-PPV or CN-PPP matrix materials and then deposited 100- to 150-nm-thick layers of the mixture onto ITO-coated quartz or glass substrates, which served as the anode of the LED structure. They then employed vacuum evaporation to produce the upper cathode, which was composed of a layer of magnesium coated with silver.

The final devices displayed internal quantum efficiencies of electroluminescence as high as 1.2 percent. The researchers note that the performance of the material, which demonstrated a marked increase in electroluminescence activity at 3 V, is dependent on the thickness of the capping layers on the quantum dots.

The composite material offers several advantages for the fabrication of planar optical emitters for photonic integrated circuits. "Today's sources of light for fiber optic communications systems are made out of pure, perfect single crystals," Sargent said. "They are grown in vacuum at 600 to 800 °C. Our devices were made near room temperature and at ambient pressures in a simple, wet chemical laboratory. The materials are put onto a substrate -- it might be glass, silicon, plastic or perhaps another semiconductor -- by spin-coating or drop-casting."

Molecular engineering

Nevertheless, significant challenges remain before viable devices based on the composite can be realized. Notably, the researchers must improve the efficiency of the electroluminescence.

"The way to do this is intrinsically interesting and is at the heart of the spirit of nanotechnology," Sargent said. "It will involve engineering the molecules [that] surround the nanocrystals such that they maximize the efficiency of luminescence while still allowing energy to be transferred from the polymer matrix to the nanocrystals."