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July 20, 2006

Nanoparticles self-assemble through chemical lithography



More advances in self assembly of nanoparticles and making more robust end products. One of the newest types of nanoparticle self-assembly – developed by scientists K. Prabhakaran and team from institutions in the U.S., Japan and Germany – is called “chemical lithography.” The process, the scientists demonstrate, can effectively form periodic arrays of very stable nanoparticles that don’t succumb to many of the defects and limitations of previous lithography techniques (which include, for example, atomic force microscope dip-pen lithography, laser lithography, electron beam lithography, embossing, etc.). Chemical lithography, instead, is a combination of techniques, where particle arrangement is controlled by differences in reactivity – a characteristic determined by exposing particles and surfaces to an assortment of chemical treatments.

The scientists believe that this this approach can be universal and extendable to more specific functionalities. This method has the potential to deliver tailored functional nanoarchitectures which will play a major role in realizing completely self-assembled nanodevices. Prabhakaran explained that it is possible to achieve precise “nanoarchitecturing” involving many sorts of applications with the ability to assemble particles that have a particular wavelength or magnetic property by selectively activating or sensing such properties.

The scientists used luminescent yttrium aluminum garnet (YAG) nanoparticles assembled on a silicon wafer, synthesizing the particles through doping and crystallization to determine their shape and composition. Before placing the particles on silicon wafers, the scientists pre-patterned the wafers using etching techniques based on a phenomenon called “atomic step movement.” Because atom-high steps innately exist on silicon surfaces, the scientists could move these steps during high-temperature treatments to fabricate a desired pattern. Chemical reactions (between the silicon, nitrogen and oxygen) caused very thin nitride linings to form in accordance with the atomic step boundaries, thereby pre-patterning the wafers.

Then, in order to align the particles along the pattern on the wafer, the scientists annealed the samples in an ultra-high vacuum chamber for several hours. After being heated at temperatures ranging from 1000 to 1500 degrees Fahrenheit (500-850 degrees Celsius), the scientists not only unveiled precisely aligned particles (see figures), but also another advantage. Measuring the nanoparticles’ alignment after the annealing process revealed the strength of the particles using this technique. Generally, many nanoparticles suffer from photobleaching, which is damage caused after exposure to high intensity light; these particles, on the other hand, remained intact after prolonged illumination of the scientists’ fluorescent imaging measurements.

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