Quantum Dot-Based Displays Benefit from Misty Deposition
Lynn M. Savage
LED displays made with nanocrystal quantum dots are enticing because they may offer higher efficiency, brightness and color saturation than existing LED technologies. However, although they have been under development for a couple of years, one major hurdle must be overcome before quantum dot LEDs become ubiquitous: Depositing and patterning layers of quantum dots remain inexact processes.
Shown here is an image of a 6 × 6 matrix of pixels composed of 5-nm-diameter CdSe/ZnS quantum dots (green) and 8-nm-diameter quantum dots (red) under UV irradiation. Images reprinted with permission of Applied Physics Letters.
Spin coating commonly is used to deposit quantum dots, but only monochromatic arrays of the particles can be created this way, leaving out the possibility of constructing RGB displays. Likewise, ink-jet and screen printing can produce organic LED structures, but they cannot deposit layers of quantum dots with the precision needed for monatomic thickness, which is critical for optimal emission efficiency.
Now researchers from Pennsylvania State University in University Park, led by professors Jian Xu and Jerzy Ruzyllo, and from Ocean NanoTech LLC in Fayetteville, Ark., in collaboration with Primaxx Inc. of Allentown, Pa., have used mist deposition to form an LED array with no defects and with uniform brightness.
Shown above are the normalized spectra of patterned films comprising the quantum dots. NQD = nanocrystalline quantum dots.
Following mist deposition techniques developed in the microelectronics industry to form fine layers of dielectrics and other materials on substrates, the investigators dispersed 5-nm nanocrystalline CdSe quantum dots in a liquid precursor, toluene. In a mist deposition module supplied by Primaxx (now part of Sumitomo Precision Products Co. Ltd. of Amagasaki, Japan), a stream of nitrogen gas carried the solution through an atomizer with three stages designed to break the droplets down until they reached a diameter of ∼0.25 μm. On average, there were five quantum dots in each drop of mist that exited the atomizer.
The researchers applied an electric field across the face of the substrate during placement of the quantum dots, enabling them to precisely control the deposition rate. After the process was completed, they thermally cured the quantum dot film to ensure a homogeneous coverage of the substrate surface. They successfully created monolayers of the quantum dots as well as films that were five layers, or 25 nm, thick.
The scientists next tested the emission quality of the layers in two ways: first, by depositing red-emitting CdSe-core/ZnS-shell quantum dots onto a substrate; second, by depositing a set of 5-nm quantum dots on a substrate and — using a shadow mask — depositing a second set of 8-nm particles to form red and green arrays of LED emitters with a pixel size of 500 μm2.
Applied Physics Letters, Jan. 14, 2008, Vol. 92, 023111.
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