New measurement method for thin films helping to advance promising technology

The dog days of summer can quickly transform a car’s interior into an oven, but imagine an automobile with high-tech windows that block out the heat, keeping its interior cool to the touch. Lutkenhaus

Such a product might be a step closer to reality thanks to research by Jodie Lutkenhaus, a chemical engineering professor at Texas A&M University whose work with thin-film coatings is helping advance the promising technology.

Specifically, Lutkenhaus has developed a new method for measuring the miniscule, physical changes of thin-film coatings in response to shifts in temperature. The method, which she says achieves new levels of accuracy and reliability, is detailed in the scientific journal ACS Nano. (Right: Lutkenhaus uses a "quartz crystal microbalance with dissipation" cell to perform precise measurements on the nanoscale thin films that she studies.)

The breakthrough measurement technique represents an important milestone for thin-film technology.

Made from a technique known as layer-by-layer assembly, these incredibly small films consist of alternating layers of polymers, long chains of atoms that have been linked together. Each polymer layer has either a positive or negative charge. As a layer surrounds the surface of the particular material it is coating, a new, oppositely charged layer is then added on top of it so that the layers adhere to one another until a relatively thick film is formed.

Thin films stand to impact a number of industrial and health-related applications, Lutkenhaus explains. In addition to heat-reflecting windows, thin-film coatings could potentially result in self-cleaning surfaces, flexible batteries, designer paint, light-refracting camouflage and even delivery systems that administer drugs to targeted areas inside the body. But before scientists can tap into the power of this emerging technology, they must first be able to measure the miniscule.

That’s where Lutkenhaus’ work comes in.

“When working with these films it’s important to know the temperature point at which their mechanical properties change – when they shift from being rigid to rubbery,” Lutkenhaus says. “This can be a problem. If you are designing something to exhibit mechanical integrity and it becomes rubbery, you really need to know at what temperature that is going to happen.”

That’s not a particularly easy task, considering the incredibly small size of the films with which Lutkenhaus works. An entire layer-by-layer assembly can be less than 100 nanometers thick, she says. To put that into perspective, a human hair is about 100,000 nanometers wide.

Because of this size issue, it’s been historically difficult to measure these physical changes exhibited by thin films with significant reliability and accuracy, and that, in turn, has hampered their design and use – but Lutkenhaus’ method is poised to change that.

Loosely based on how a common wristwatch works, Lutkenhaus’ method, known as “quartz crystal microbalance with dissipation,” detects the temperature at which these thin films begin to soften, something scientists call a “glass transition.”

A watch keeps time, Lutkenhaus explains, by providing power (via a battery) to a quartz crystal within the device that vibrates, producing a resonant frequency. Lutkenhaus expands on this concept by depositing a thin film on a quartz crystal and pulsing the crystal with energy. As might be expected, the vibrations of the crystal die down, or dissipate, at a different rate with the film deposited on the crystal.

Observing and comparing this rate of dissipation, Lutkenhaus can determine the mass of the film placed on the crystal and the mechanical properties of the film. This procedure is performed at different temperatures and the resulting response wave and its dissipation rate is measured. If the response wave decays differently, she explains, there has been a mechanical change to the film at that temperature.

To better understand the experiment, think of the crystal as a slinky and the film as a weight, Lutkenhaus says.

“Imagine a slinky that is pinged and moving back and forth,” she says. “Over time, the slinky will stop oscillating and return to its original position. We’re measuring how far that slinky bent over and how long it took to return to its original position. Now if we add a weight to the slinky and repeat this experiment, it is going to bend over differently and take a different amount of time to return to its original position. We’re building on this concept and applying a lot of equations so that we can tell how much that weight was and how rigid or rubbery that weight was -- all at different temperatures.”