Search
Menu
PI Physik Instrumente - Revolution In Photonics Align LW LB 3/24

Turning Quantum Dots into Tiny ‘Lightbulbs’

Facebook X LinkedIn Email
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.

Ohara Corp. - Optical Glass, Polish substrates 10-23

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.


Published: March 2007
Glossary
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum dots
A quantum dot is a nanoscale semiconductor structure, typically composed of materials like cadmium selenide or indium arsenide, that exhibits unique quantum mechanical properties. These properties arise from the confinement of electrons within the dot, leading to discrete energy levels, or "quantization" of energy, similar to the behavior of individual atoms or molecules. Quantum dots have a size on the order of a few nanometers and can emit or absorb photons (light) with precise wavelengths,...
Basic ScienceFeaturesLight SourcesMicroscopyMITnanophotonicsquantum dotsLEDs

We use cookies to improve user experience and analyze our website traffic as stated in our Privacy Policy. By using this website, you agree to the use of cookies unless you have disabled them.