Photo-induced Force Microscopy Measures Nanoparticles With Light
A new technique called “photo-induced force microscopy,” which probes the optical properties of nanomaterials by measuring the physical force imparted by light, is being tested by scientists at Rice University.
Isabell Thomann’s primary research centers on using nanoparticles and sunlight to reduce the carbon footprint of power plants. A major focus is photocatalysis, a class of processes in which light interacts with high-tech materials to drive chemical reactions.
“Many experiments nowadays are done under high vacuum, but I want to run the reactor in my lab under more realistic conditions—normal temperature, normal pressure, in the presence of water—that will apply to capturing sunlight for photocatalysis,”
says Thomann, an assistant professor of electrical and computer engineering, materials science, nanoengineering, and chemistry at Rice University.
Ultrafast Laser Spectroscopy
Thomann has been working to develop new tools for measuring nanomaterials since arriving at Rice in 2012. She and her team are developing an ultrafast laser spectroscopy system that can read the optical signatures of short-lived chemical processes that are relevant to artificial photosynthesis.
“In a chemical reaction, there are reactants, which are the chemical inputs, and there are products, which are the outputs,” Thomann says. “Almost all reactions driven by light involve multiple steps where light is converted to quantum particulates such as electrons or phonons that need to be transported to surfaces to drive chemical reactions. It is very helpful to know exactly what these are, when they are made, and in what quantity, particularly if you are optimizing a process for industrial use.”
Thomann’s group designs light-activated nanoparticles that can capture energy from sunlight and use it to initiate chemical reactions.
The nanocatalysts, which can be tiny rods or discs of metal or other materials, interact with light due in part to their shapes and how closely they are spaced together. Thomann says that while engineers make every effort to produce uniform particles, small imperfections still exist and can have significant consequences on performance.
“Photocatalysts are often heterogeneous, which means they are not all exactly alike, and we need better tools for examining them with high spatial resolution in order to see these small differences,” she says. “We also need to follow the reaction processes with high temporal resolution, and we want to do all of this with much better spatial resolution than can be achieved with a normal optical microscope.”
In the photon-induced force microscopy experiments, Thomann’s team used a tiny tip from an atomic force microscope (AFM) to enhance the spatial resolution of measurements taken from gold nanorods and nanodiscs on glass surfaces.
The rods and discs, which are smaller than the wavelength of light used to measure them, would normally be blurry in an optical microscope due to a physical property called the diffraction limit.
To better resolve the nanoparticles, and the electromagnetic interactions between them, Thomann’s group shines light at the particles and uses an AFM tip to probe how these nanoparticles act as optical nanoantennas and concentrate the light.
“If we were trying to measure the reflected light, it would be very difficult because there are only a few scattered photons against a very busy background where light is bouncing all over the place, especially if these measurements were carried out in a liquid environment,” Thomann says. “But we are instead measuring the force exerted on the AFM tip, the slight pull on the tip when the optical nanoantennas are illuminated by light.
It turns out that measuring the force is a much more sensitive technique than trying to collect the few photons scattered off the tip.”
New Photocatalyst Materials
Thomann says the study provides theoretical understanding of how photo-induced force microscopy works and lays the groundwork for future studies of more complex photocatalyst materials her team hopes to create in the future. She credits her group’s improved understanding of the force-measuring technique to work by coauthor Xiao Yang, a Rice graduate student in the group of theoretical physicist and study coauthor Peter Nordlander.
Yang says the most difficult part of coming up with an explanation of the team’s experimental results was creating a solvable computational model that accurately described the real-world physics. For example, including the entire tip in the model made the mathematics impractical.
“I did try, at first, but it turned out it was impossible,” Yang says. “It would have taken an infinite time to reach convergence of the simulations.”
Yang eventually hit upon an idea, including just a portion of the tip in the model, that made the calculations both feasible and accurate.