Connecting the (quantum) dots

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On December 14, 2004

Imagine if your office laser printer could print text the size of atoms, then embed that ink into a writing surface as light as air. That, in a sense, is what researchers at UMR have done with a new "laser writing" technique they have developed. But with this process, the "ink" is a semiconductor that could write a new chapter in the field of micro-computing.

Basing their work on photolithography, a technique commonly used by microchip makers to print circuitry on silicon wafers, the UMR researchers zapped isolated spots of a silica gel with a laser. In the process, they discovered that they could create tiny semiconducting materials known as quantum dots, which could lead to new advances in electronics, computing and materials science.

The UMR researchers, led by Dr. Massimo F. Bertino, an assistant professor of physics, report on their method in the Dec. 13 issue of the American Institute of Physics journal Applied Physics Letters. According to Bertino, this is the first time researchers have created quantum dots via photolithography.

The technique involves embedding tiny particles of semiconducting materials — the "ink" in this printing process — into the writing surface. Bertino and his team used cadmium sulfide as the semiconductor. Semiconductors are materials that have properties between metals, which can conduct electricity, and insulators, and are widely used in the electronics industry.

The "paper" in this case was a silica gel which, after further treatment, was turned into an aerogel, one of the lightest known materials. When isolated portions of the gel were zapped by an infrared laser, the result was the creation of particles so small they must be measured in "nanometers," or billionths of a meter.

"These particles are so small that the electrons are in the quantum confinement regime," says Bertino. That means the substances are "quantum dots" — specks that are only a few nanometers in size, says Bertino.

These nanoscale structures are of interest to the scientific community because they hold tiny puddles of electrons, which possess unusual optical properties. The cadmium sulfide dots Bertino and his team developed have interesting properties.

"Our cadmium sulfide quantum dots not only absorb light but they also emit light," he says. "By tuning the size of the particle, you can change the emission range."

While interesting from a theoretical and scientific viewpoint, such properties could also be of interest to scientists who hope to develop quantum computers, quantum-dot lasers or molecular-scale integrated circuits, Bertino says.

The UMR team originally set out to improve upon existing photolithographic methods, Bertino explains. More conventional photolithography, which involves a combination of chemical deposition and etching to apply materials to a surface, has been used with a few, mostly expensive, materials, such as silver and gold, Bertino says. But he and his colleagues wanted to find a way to use a broader palette of nanoscale semiconducting materials.

The researchers mixed cadmium nitrate and thiourea in a silica gel, which would later form the aerogel. The materials, mixed at room temperature, form the semiconductor cadmium sulfide.

After cooling the mixture to halt the chemical reaction, Bertino and his team placed the silica gel in front of the infrared laser. By honing the laser’s beam to a few microns, they heated tiny, isolated sections of the silica gel. The chemical reaction caused by the heat formed the semiconductor particles only in the heated regions.

The researchers plan to conduct similar experiments using ultraviolet laser beams and other materials, including a honeycomb structure that can be made with very small holes, as small as two nanometers in diameter. "Using an ultraviolet laser would allow us to work on materials that are sensitive to heat, such as polymers," Bertino says.

The researchers have also applied for a patent for their process, he adds.

The aerogel materials used by the researchers were developed earlier at UMR. That development was reported in the Sept. 12, 2002, issue of the American Chemical Society journal Nano Letters.

Working with Bertino on this research are Dr. John G. Story, associate professor of physics at UMR; Dr. Chariklia Sotiriou-Leventis, associate professor of chemistry at UMR; Dr. Akira Tokuhiro, assistant professor of nuclear engineering at UMR; Raghuveer R. Gadipalli, a UMR graduate student in physics; Chuck G. Williams of St. Louis, a senior in physics; Dr. G. Zhang, who recently received his Ph.D. in chemistry from UMR; Dr. Suchismita Guha, an assistant professor of physics at the University of Missouri-Columbia; and Dr. Nicholas Leventis of NASA’s Glenn Research Center in Cleveland.

Related Links

"Laser writing of semiconducting nanoparticles and quantum dots"
Applied Physics Letters
UMR physics department

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On December 14, 2004. Posted in Research