Turning Quantum Dots into Tiny ‘Lightbulbs’

Hank Hogan

For anyone seeking a really small light source, researchers at MIT in Cambridge, Mass., have good news. The group has embedded quantum dots in structures similar to organic LEDs and has demonstrated light emission from single quantum dots at room temperature. “If you need a nanoscale light source — a lightbulb that is 5 nm big — we have it,” said MIT’s Vladimir Bulovic', an associate professor of electrical engineering and computer science.

He noted that the technique yields very small electrically excited items — the quantum dots — in very large area devices — the organic LEDs. The light source is versatile because quantum dots of a specific color can be employed and their density controlled, with a variety of both parameters possible.

Quantum dots emit over a narrow range, with the peak emission dependent on the size of the particle. They offer better emission performance than organic LEDs, which exhibit less pure colors. In addition, quantum dots are composed of inorganic materials such as cadmium and selenium, unlike the compounds found in organic LEDs, and so offer the possibility for different types of processing.

Previous efforts by the group to integrate quantum dots and organic LEDs involved monolayers rather than single dots. Other researchers have demonstrated electroluminescence in single quantum dots but only at low temperatures and only outside of an organic LED.

The MIT team used quantum dots with cadmium selenide cores and zinc sulfide shells that were made by Invitrogen Corp. of Carlsbad, Calif. These particles had an emission centered around 655 nm. The investigators put the dots in solution with N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD), spinning the solution onto a substrate of glass covered with indium tin oxide and then polyethylenedioxythiophene to create a 50-nm-thick film with TPD on the bottom and quantum dots on top. They then put down sequential layers, first 20 nm of the organic compound 2,4-triazole (TAZ), followed by a 20-nm-thick aluminum-containing organic layer, 100 nm of a silver magnesium alloy and a 20-nm-thick layer of silver.

The layers below the quantum dots acted as a conductor for holes, while the ones above acted as a conductor for electrons. The TAZ blocked holes, therefore keeping the two types of conductors apart wherever the TAZ was intact. However, from previous work, the group expected holes to form over the quantum dots. “Organic films that are grown on top of the quantum dots do not like the surface of the quantum dot and run away from it,” Bulovic' said.

The researchers measured the photoluminescence using a laser focused on the quantum dots via a Nikon objective. For electroluminescence, they turned off the laser and applied a voltage across the device. They used a CCD camera from Princeton Instruments to capture images.

They observed electroluminescence and photoluminescence from single quantum dots, which they attributed to thin or absent TAZ that lowered resistance at those points. Atomic force microscopy confirmed this hypothesis.

Although practical devices may be built with this technique someday, use in the near-term will be as physics test beds that will help scientists answer fundamental questions about the basic properties of such devices.

Applied Physics Letters, Jan. 8, 2007, 023110.