Portland, Ore. - Semiconducting aerogels have recently been demonstrated that cast quantum dots into a sparse, crystalline-like matrix with more space than substance. Such porous materials could enable supersensitive sensors and superefficient plastic photovoltaics, said the Wayne State University research team pursuing the project.
That's because the very porous structure, through which environmental molecules can waft, has the maximum possible amount of surface area, essentially enabling every embedded quantum dot to sense the environment independently.
"We have found that the best way to engineer nanoparticles with novel electronic properties is to replace the oxides that everybody else uses with a semiconductor," said professor Stephanie L. Brock, leader of the semiconducting-aerogel research group at Wayne State (Detroit). "Instead of an oxide, which is an insulator, we use a semiconducting material such as cadmium selenide or cadmium-, zinc- or lead sulphide."
Brock's group has created many varieties of semiconducting aerogels with the primary aim of enabling very sensitive sensors. "We put the quantum dots into a solution, then supply the conditions under which they self-assemble into a gel that is so porous, with so much surface area, that even though they are relatively densely packed, they still have all the good quantum confinement properties they have when isolated by oxides," said Brock. Oxides, by contrast, can bury many quantum dots inside the material.
Aerogels are spongelike silicon-based films that consist mostly of empty space. They are traditionally used to collect sample particles without damaging them.
In the Wayne State system, the quantum dots, which measure 2 to 5 nanometers in diameter, are separated by liquid molecules as big as 50 nm. After the liquid solvent is removed using carbon dioxide in a special supercritical state, the quantum dots spread out evenly, arranged in a matrix that enables the particles to act independently.
"Our architecture is akin to the arch," Brock said. "It encases an empty space like a doorway. Our material is engineered to assemble into a very porous network of such arches. We are putting them together in such a way as to define the empty spaces-the pores-thereby keeping the quantum dots very densely packed, but only in close proximity to a few neighbors."
Brock conceived the material to solve a problem with quantum dots. While they are able to fluoresce in response to specific compounds in the environment, such as air pollution, it is difficult to get the sample of air into contact with the tiny (nanoscale) quantum dots. After an aerogel is formed, however, all of the solvent used to support the network in suspension is removed, leaving behind a slender, porous network of quantum dots, each one of which is open to the environment.
"For detectors, we hope to use the pores to increase the sensors' surface area. But for photovoltaic-type devices, we will fill in the pores with a polymer, enabling electrons to conduct in one matrix and holes to conduct in another matrix," said Brock.
The completely room-temperature process involves premaking the nanoparticles at the precise sizes needed for a specific sensing application-say, having them turn "red" for air pollution. Then the nanoparticles are capped with thiolates and oxidized to remove surface groups in a very controlled manner that reveals sites into which the nanoparticles nestle. After nanoparticle condensation, Brock removes the solvent, maintaining the quantum dots inside a very porous architecture.
Next, Brock's group will begin to measure all the electronic properties of the material, to access its strengths and weaknesses. By fine-tuning their techniques, the researchers hope to provide a "cookbook" for creating large-area room-temperature thin films of quantum dots.
"Our initial interest is in sensing," said Brock, "so we have created a material that is mostly surface. We know the pores are going to let the molecules in and out, and since the color of the photoluminescent quantum dots is sensitive to what is on their surface, we expect to see profound changes in their color in response to, for instance, air pollution."
Brock thinks the group is three years away from announcing a full understanding of the material, though she promised to release periodic progress reports.