news digest ♦ LEDs applications.
“With quantum dots, the chemical environment that’s optimal for growth is usually not the environment that’s optimal for function,” says co- principal investigator Venkatesh Narayanamurti, Benjamin Peirce Professor of Technology and Public Policy at SEAS.
The ligands can interfere with current conduction, and attempts to modify them can cause the quantum dots to fuse together, destroying the properties that make them useful. Organic molecules can also degrade over time when exposed to UV rays.
Researchers would like to be able to use those ligands to produce the quantum dots in solution, while minimising the negative impact of the ligands on current conduction.
“The QD technologies that have been developed so far are these big, thick, multilayer devices,” says co-author Rafael Jaramillo, a Ziff Environmental Fellow at the Harvard University Centre for the Environment. Jaramillo works in the lab of Shriram Ramanathan, Associate Professor of Materials Science at SEAS.
“Until now, those multiple layers have been essential for producing enough light, but they don’t allow much control over current conduction or flexibility in terms of chemical treatments. A thin, monolayer film of quantum dots is of tremendous interest in this field, because it enables so many new applications.”
Harvard researchers have demonstrated a new design for LEDs by nestling quantum dots in an insulating structure that resembles an egg crate. (Stock image courtesy of Flickr user Cliff Muller.)
The quantum dots, each only 6 nanometres in diameter, are grown in a solution that glows strikingly under a black light.
The solution of quantum dots can be deposited onto the surface of the electrodes using a range of techniques, but according to applied physicist Edward Likovich, lead author of a paper describing the research, “That’s when it gets complicated.”
“The core of the dots is a perfect lattice of semiconductor material, but on the exterior it’s a lot messier,” he says. “The dots are coated with ligands, long organic chains that are necessary for precise synthesis of the dots in solution. But once you deposit the quantum dots onto the electrode surface, these same ligands make many of the typical device processing steps very difficult.”
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www.compoundsemiconductor.net November/December 2011
The new QD-LED resembles a sandwich, with a single active layer of quantum dots nestled in insulation and trapped between two ceramic electrodes. To create light, current must be funnelled through the quantum dots, but the dots also have to be kept apart from one another in order to function.
In an early design, the path of least resistance was between the quantum dots, so the electric current bypassed the dots and produced no light.
In an early design (left), the path of least resistance was between the quantum dots, so the current bypassed the dots and produced no light. Using the atomic layer deposition (ALD) technique (right), researchers were able to funnel current directly through the dots, creating a fully functional, single-layered QD- LED. (Image courtesy of Edward Likovich.)
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