Portland, Ore. - Thin, 1-nanometer films of crystalline clay on germanium substrates are being developed at Purdue University (West Lafayette, Ind.) as a scaffolding for "smart" materials. The technique could enable faster, more sensitive sensors and materials that sense structural problems and alert users or even perform "self-healing" in remote applications such as robotic space exploration.
"We have demonstrated that we can control and manipulate clay monolayers, which we hope will lead to smart materials functionalized with organic molecules such as dyes, enzymes, proteins and polymers," said Purdue University professor Cliff Johnston, who is also a researcher at Purdue's Birck Nanotechnology Center in Discovery Park. Johnston collaborated on the work with Belgian scientists at Katholieke Universiteit (Leuven).
The researchers' method assembles a single crystalline layer of clay atop germanium substrates to create a rigid scaffolding for attaching molecules. Johnston's method makes it possible to pack long organic sensor molecules perpendicularly atop the clay, with their signal ends bonded to the clay and their sensor ends dangling in the environment.
"The structural integrity of the clay presents the organic molecules on a planar surface, preventing them from clumping up so that not all are accessible," said Johnston. "For instance, if you expose a sensor to a vapor or a solution, it will only have a limited cross-section of contact with the sensor molecules unless you are using our style of monolayer."
Such ultrathin hybrid films would not only make sensors more sensitive but also could enable them to respond orders of magnitude faster. Moreover, the wide range and multitude of organic molecules enable nearly anything to be sensed. "You can design an organic molecule to sense almost anything, and in response they can fluoresce or release a dye for us to see. Or in self-healing materials for space travel, the clay could sense a puncture and essentially move materials right in the skin itself to heal the rift," said Johnston.
Clay is a crystal that combines silicon, aluminum and oxygen in particles that are flat and planar. The thickness of one clay particle is about 1 nm. "A clay particle is shaped a lot like a sheet of paper-very thin but very wide-each measuring just 1 nm thick, but 1,000 nm wide," said Johnston.
Clay has traditionally been used to add strength to ceramics, and today advanced polymers are employing clay for lighter, stronger and more elastic materials. In newer materials such as nanoscale composites and catalysts, bulk clay particles are also being used. According to Johnston, there is a limit to how much bulk clay can be added in these applications before it begins to clump up, but depositing his films solves the clumping problem.
Sample under laser is 1 million times thicker than the 1-nm film.
"A lot of the application work [with clay films] is being done overseas, such as for making tunable LEDs, for electrochemical species, for the separation of chemicals and for many different sensor applications," he said. "The Japanese are active in the area of clay films too, for clay-modified electrodes, separation of chiral molecules, nonlinear optical materials and mesoporous solids."
The planar shape of the clay crystals makes them ideal for marriage to electronic substrates, and in Johnston's experiments, both saponite and montmorillonite clays showed an affinity for germanium and zinc-selenium substrates. Using a lab method called the Langmuir-Blodgett Balance, Johnston's group proved the concept that clay monolayers could be deposited on electronic substrates. The work also offers a peek at future hybrid clay-organic films.
"The difference with our work is that for the first time we have been able to verify that we actually have only a single layer of clay," said Johnston. "Basically what we have made is a hybrid film, comprised of an organic part and a clay part. The organic part is floated on the surface of water in a Langmuir trough with a positively charged head group that attaches itself to a single clay particle. Then we basically herd all those clay and organic molecules together by decreasing the area and carefully lifting the substrate up from below the water, with the film being deposited in a monolayer."
Cliff Johnston peers through a model of a 1-nm layer of clay at his Purdue University lab.
Johnston used atomic-force microscopy to observe the monolayer. Its thinness was measured to be a uniform .96 nm. Infrared-attenuated total reflection and polarized infrared reflection absorption spectroscopy were used to verify that the organic molecules were attached perpendicularly to the surface.
"Our work is unique in that we are working with single molecules of clay, which will ultimately allow us to build more complex functional materials. That's because in all areas of nanoscale materials, the more confidence you have about the material you are working with, the better, and in our case we know we are working with individual particles of clay," said Johnston. "What we are working on now is trying to attach more intelligent organic molecules on the surface."
Johnston's team is looking to pick particular organic sensor molecules, such as ones that change color in the presence of a toxic agent, and create hybrid clay-organic prototypes for specific applications. "We are in the preliminary stages of characterizing films that will respond in a particular way to a particular environment," he said.
"The next step is to add functionality to these organic molecules. You can have a dye that changes color, you can have a pH sensor, you could add a temperature-sensitive organic molecule," he said. "[The structure] could sense a certain type of explosive or a change in relative humidity."
Funding for the project was provided by the Fund for Scientific Research, a Flanders-Hungary Grant and the U.S. Department of Agriculture. Johnston also received a fellowship from Katholieke Universiteit.