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Metal-coated nanostructures print images at 100,000dpi

Posted: 12 Apr 2013 ?? ?Print Version ?Bookmark and Share

Keywords:colour printing? hydrogen silsesquioxane? plasmons?

Singapore's A*STAR research team has developed a printing technique that can print images at 100,000 dots per inch (dpi), the highest possible resolution for a colour image. This images, A*STAR says, can possibly be used as minuscule anti-counterfeit tags or to encode high-density data.

Commercial laser printers typically produce pin-sharp images with spots of ink about 20?m apart, resulting in a resolution of 1,200dpi. The researchers shrank the separation to just 250nmroughly 100 times smallerand produced images at 100,000dpi.

To print the image, the team coated a silicon wafer with insulating hydrogen silsesquioxane and then removed part of that layer to leave behind a series of upright posts of about 95nm high. They capped these nanoposts with layers of chromium, silver and gold (1-, 15- and 5nm thick, respectively), and also coated the wafer with metal to act as a backreflector.

Each color pixel in the image contained four posts at most, arranged in a square. The researchers were able to produce a rainbow of colors simply by varying the spacing and diameter of the posts to between 50nm and 140nm.

When light hits the thin metal layer that caps the posts, it sends ripplesknown as plasmonsrunning through the electrons in the metal. The size of the post determines which wavelengths of light are absorbed, and which are reflected.

Light hits metal layer

Variation in post size and spacing in the metal array alters which incoming wavelength of light (red, green or blue) is reflected back.

The plasmons in the metal caps also cause electrons in the backreflector to oscillate. "This coupling channels energy from the disks into the backreflector plane, thus creating strong absorption that results in certain colors being subtracted from the visible spectrum," said Joel Yang, who led the team of researchers at the A*STAR Institute of Materials Research and Engineering and the A*STAR Institute of High Performance Computing.

Printing images in this way makes them potentially more durable than those created with conventional dyes. In addition, colour images cannot be any more detailed: two adjacent dots blur into one if they are closer than half the wavelength of the light reflecting from them. Since the wavelength of visible light ranges about 380-780nm, the nanoposts are as close as is physically possible to produce a reasonable range of colours.

Although the process takes several hours, Yang suggested that a template for the nanoposts could rapidly stamp many copies of the image. "We are also exploring novel methods to control the polarization of light with these nanostructures and approaches to improve the color purity of the pixels," he added.

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