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  • ‘Egg Crate’ Design for QD-LEDs Optimizes Performance
Nov 2011
CAMBRIDGE, Mass., Nov. 16, 2011 — A new quantum-dot LED "egg crate" design is turning troublesome ligand molecules into a critical element that creates a more versatile QD-LED structure that could have applications in displays, lighting, and lasers.

QDs are tiny crystals that glow with bright, rich colors when stimulated by an electric current. Ligands — organic molecules that dangle from quantum dots — are essential to QD growth, but can interfere with current conduction and cause functional problems later on.

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)

Researchers at Harvard School of Engineering and Applied Sciences (SEAS) discovered an alternative that uses ligands to build a more versatile single-layer, egg-crate structure that better controls the flow of electric current — optimizing the QD-LED’s performance.

QDs are grown in a solution that glows strikingly under a black light. The solution can be deposited onto an electrode surface, but because the ligands are attached, the process gets complicated, the researchers say.

“The core of the dots is a perfect lattice of semiconductor material, but on the exterior it’s a lot messier,” said Edward Likovich, who conducted the research as a doctoral candidate in applied physics at SEAS. “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.”

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 also can degrade over time when exposed to UV rays.

“The QD technologies that have been developed so far are these big, thick, multilayer devices,” said Rafael Jaramillo, a Ziff Environmental Fellow at Harvard's Center for the Environment. “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.”

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 is funneled through the quantum dots, but the dots also must be kept apart from one another 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 funneled current directly through the dots, creating a fully functional, single-layer QD-LED. (Image courtesy of Edward Likovich)

Abandoning the traditional evaporation technique they had been using to apply insulation to the device, the researchers instead used atomic layer deposition (ALD), which involves jets of water. ALD takes advantage of the water-resistant ligands on the quantum dots, so when the aluminum oxide insulation is applied to the surface, it selectively fills the gaps between the dots, producing a flat surface on the top and allowing more effective control over the flow of electrical current.

“Exploiting these hydrophobic ligands allowed us to insulate the interstices between the quantum dots, essentially creating a structure that acts as an egg crate for quantum dots,” said Kasey Russell, a postdoctoral fellow at SEAS. “The benefit is that we can funnel current directly through the quantum dots despite having only a single layer of them, and because we have that single layer, we can apply new chemical treatments to it, moving forward.”

Through Harvard’s Office of Technology Development, Likovich and his colleagues have applied for a provisional patent on the device. Beyond the aforementioned applications, the technology could one day be used in field-effect transistors or solar cells.

The research was published in Advanced Materials; Likovich was lead author.

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quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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