Quantum Dots Made To Order

Researchers have found a cheap, simple way to arrange quantum dots of any size in any pattern on a surface (Nano Lett., DOI: 10.1021/nl204233r). The technique could ease the use of quantum dots in sensors, solar cells, and electronic devices, the researchers say.

Carefully tailoring the size and chemical makeup of quantum dots, which are semiconductor nanocrystals, changes the color of light they emit. But incorporating them into devices has been challenging because it requires placing them at specific spots on a surface so to combine with electronic circuitry. Existing methods to assemble quantum dots, which typically start with creating them in a solution, do not offer precise control over the crystals’ size or placement.

To achieve that control, Chad A. Mirkin, a chemistry professor at Northwestern University, and his colleagues turned to a lithography technique that they first developed for patterning metal nanoparticles. The method uses a fast scanning probe and block copolymers, which are materials that link blocks of chemically different polymers. In this case, Mirkin and his colleagues dip an atomic force microscope (AFM) tip into a solution of block copolymers and cadmium chloride. One polymer block in their copolymer attaches to the cadmium ions, while the other helps the solution flow from the tip.

The researchers use the AFM tip much like a pen to deposit the solution on a silicon surface. The longer the tip rests on one location, the more cadmium ions end up on that spot. The computer-controlled tips can create complex designs and arrange dots of solution just a few nanometers apart. The researchers can also use an array of tips, creating millions of dots simultaneously.

After they’ve laid out the solution in a pattern, the researchers then expose the surface to hydrogen sulfide gas, creating an arrangement of cadmium sulfide nanoparticles. They use various transmission electron microscopy and diffraction methods to verify that the nanoparticles are crystalline. Finally, they expose the samples to oxygen plasma to remove the block copolymer.

“You can effectively create little nanoscale reactors where you want them to be,” Mirkin says. “You can control the location of where the reaction takes place and you can precisely dial in the number of atoms that go into making the individual particle.”

Elena V. Shevchenko, a nanoscientist at Argonne National Laboratory, says that other researchers have used electron-beam lithography to make custom patterns of nanoparticles, but that technique is labor intensive and expensive. Because of its relative ease, the new lithography technique is an important step in nanofabrication, she says.

Mirkin says the technique could be compatible with other nanoscale materials. He says the method also offers researchers a powerful way to build “libraries of nanostructures that can be screened for all sorts of purposes from catalysis to optics.”