Nuclear magnetic resonance (NMR) is a powerful scientific tool used in medical imaging and in probing the chemical structure of molecules and compounds, but its inherently low sensitivity has precluded broader usage. That could change soon. New research from Brown University describes a technique that helps adapt NMR to study the physical properties of thin films, two-dimensional nanomaterials, and exotic states of matter.
"NMR is a very useful technique, but the signal you get is very weak," said Vesna Mitrovic, an associate professor of physics and the senior author of the research, which was published in Review of Scientific Instrument last month. "To get a usable signal, you need to detect a lot of spins, which means you need a lot of material, relatively speaking. So much of the work we're doing now in physics is with thin films that are part of small devices or materials that have tiny crystals with odd shapes, and it's really difficult to get an NMR signal in those cases."
Part of the problem has to do with the geometry of the probe used to deliver the radio pulses and detect the associated signal. It's usually a solenoid, a cylindrical coil of wire inside of which the sample is placed. The NMR signal is strongest when a sample takes up most of the space available inside the cylinder. But if the sample is small compared to the volume of the cylinder—as thin films and nanomaterials would be—the signal weakens to nearly nothing.
But for the past few years, Mitrovic's lab has been using flat NMR coils for a variety of experiments aimed at exploring exotic materials and strange states of matter. Flat coils can be placed directly on or very close to a sample, and as a result they don't suffer from the signal loss of a solenoid. These types of NMR coils have been around for years and used for some specific applications in NMR imaging, Mitrovic says, but they've not been used in quite the same way as her lab has been using them.
For this latest research, Mitrovic and her colleagues showed that flat coils are not only useful in boosting NMR signal, but that different geometries of flat coils can maximize signal for samples of different shapes and in different types of experiments.
The ability to get a signal at varying magnetic field orientations is important, Mitrovic says. "There are exotic materials and interesting physical states that can only be probed with certain magnetic field orientations," she said. "So knowing how to optimize our probe for that is really helpful."
Image: Researchers have shown how flat NMR probes, as apposed to cylindrical ones, can be made useful in studying the properties of nanomaterials. Image courtesy of Mitrovic lab/Brown University