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Nanocrystal Films Promising for Photonic Circuits
Aug 2012
CAMBRIDGE, Mass., Aug. 24, 2012 — A new process for patterning defect-free nanocrystal films with nanometer precision could advance photonic devices and basic physics research.

Semiconductor nanocrystals hold promise for a range of applications, including high-resolution display screens, biomolecule detectors, solar cells, and photonic or electronic circuits. However, controlling the placement of nanocrystals on a surface to make films that are structurally uniform has proved difficult. Typically, nanocrystal films also have cracks that limit their usefulness.

Now, MIT researchers have found a way to make defect-free nanopatterned films of nanocrystalline material that fluoresce in various colors based on their size, even though they are all made of the same material. The work builds on research conducted by Moungi Bawendi, the Lester Wolfe Professor of Chemistry at MIT and a co-author of the paper published online in Nano Letters.

“We’ve been trying to understand how electrons move in arrays of these nanocrystals,” which has been difficult with limited control over the formation of the arrays, said physicist Marc Kastner, the Donner Professor of Science, dean of MIT’s School of Science and senior author of the paper.

Images of nanopatterned films of nanocrystalline material produced by the MIT research team. Each row shows a different pattern produced on films of either cadmium selenide (top and bottom) or a combination of zinc cadmium selenide and zinc cadmium sulfur (middle row). The three images in each row are made using different kinds of microscopes: (from left), scanning electron microscope, optical (showing real-color fluorescence), and atomic force microscope. (Image: Mentzel et al, from Nano Letters)

Initially, nanoscale patterns that emitted light in the infrared range were produced. Later, semiconductor nanocrystal patterns were made to glow with visible light, making it detectable through optical microscopes.

“Even though the nanoscale patterns are below the resolution limit of the optical microscope, the nanocrystals act as a light source, rendering them visible,” said postdoc Tamar Mentzel.

The defect-free films achieved electrical conductivity approximately 180 times that of conventional cracked films. In addition, the MIT process made it possible to create patterns on a silicon surface that are just 30 nm across. These tiny patterns of defect-free films were achieved by coating a silicon dioxide substrate with a thin polymer layer before depositing the nanocrystal layer on top of it.

“The trick was to get the film to be uniform, and to stick” to the silicon dioxide substrate, Kastner said.

Because of their ability to both emit and absorb a wide spectrum of colors, the nanocrystal patterns could be used for broad-spectrum solar cells.

The researchers plan to study the fundamental processes of solids using the arrays. The work has already enabled new research on how electrons move in the films.

The material could be used also to develop sensitive detectors for tiny amounts of certain biological molecules, either as screening systems for toxins or as medical testing devices, they said.

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The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
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...
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