Nondestructive Method Uses Lasers to Characterize Nanomaterials’ Properties

A noncontact measurement method that uses laser ultrasound so that the detailed elastic properties of nanostructured materials can be characterized will help engineers and scientists decide whether nanomaterials can be processed into certain components. These determinations depend on the materials’ mechanical properties.

Nanostructuring of materials can lead to the discovery of new, often surprising properties. While this opens doors to potential applications and technologies, the ability to determine their true mechanical properties — and, as a result, judge whether a certain material is a fit for a certain application — is difficult. The properties can change in the measurement process. Other deterministic tests can destroy the materials.

The method developed by a German-French research group led by Patrick Huber from DESY (Deutsches Elektronen-Synchrotron) and TU Hamburg used a pulsed UV laser to generate thermoelastic oscillations in porous silicon, without contact. The laser was a passively Q-switched frequency-tripled Nd:YAG (neodymium-doped yttrium aluminum garnet) laser, with an optical wavelength of 355 nm, pulse repetition of 500 Hz, pulse duration of 350 picoseconds with 25 μJ of energy per pulse, and a peak power of 60 kW. Pulses that last only a fraction of a billionth of a second excited a nanoporous silicon membrane to vibrate in the ultrasound range, in a way that is like droplets hitting a water surface. The oscillations, in the ultrasonic range, penetrate the entire monocrystalline material, Huber said. The distinct characteristics of the resulting surface and guided elastic waves in the volume of the material depend on the varying degrees of the direction-dependent elastic properties of the nanostructured silicon.

The group then used a laser interferometer (specifically, a Michelson interferometer) to detect these waves in a contactless and nondestructive manner to ensure that the mechanics could be derived with the help of a suitable model for elastic waves. The interferometer was equipped with a frequency-doubled Nd:YAG laser operating at an optical wavelength of 532 nm, the power of which the researchers set to 60 mW.

Similar to droplets hitting a water surface, short laser pulses lasting only a fraction of a billionth of a second excite a nanoporous silicon membrane to vibrate in the ultrasound range. The elastic wave propagation in the membrane, whose nanopores are filled with a liquid, is detected interferometrically with a second laser, so that a noncontact and nondestructive mechanical characterization of the hybrid nanomaterial is possible. The membrane illustration is based on electron micrographs of nanoporous silicon with a pore diameter of 7 nm. Illustration courtesy of TUHH/DESY/Künsting.

The signal analysis was a complex process, according to Huber and his group.

“Conventional ultrasound measurements have the disadvantage that the sample must be in direct mechanical contact with the ultrasound transmitters and receivers,” said Marc Thelen, first author of the study and a doctoral student in Huber’s research group. “This is done by applying a liquid coupling agent. Porous materials absorb these coupling liquids, so that classical ultrasound experiments are falsified. In contrast, we were able to study empty liquid-filled silicon and thus demonstrate that the mechanical behavior of porous materials functionalized with liquids can also be analyzed with ultrasound in detail.”

The nanostructured silicon samples that the group investigated contained parallel nanopores that had formed in a self-organized manner. Tests showed clear differences between the behavior of conventional silicon single crystals, as they are known as wafers from the semiconductor industry.

“On the one hand, we were able to demonstrate that the high nanoporosity causes a significant reduction in stiffness. However, it also leads to the fact that the direction-dependent elasticity present in the silicon crystal seems to be significantly canceled out,” said Claire Prada, the director of research at the Institut Langevin (Paris), who coordinated this German-French collaboration with Huber. “Surprisingly, we were also able to detect a slight change in the diameter (conicity) of the tubular pores on the nanoscale.”

Nanoporous solids, the pore space of which is filled with liquids (like the silicon membrane used in the work), have already led to the development of high-performance materials for biosensors, energy storage, and water treatment. Additionally, the researchers stated in their paper, nanoporous silicon in particular has drawn interest from different fields of research and applications. As it provides a monolithic single-crystalline medium with anisotropic pores, it can be used to study confinement effects on matter. Its optical, electrical, and thermal properties, they wrote, additionally support applications in optoelectronics, thermoelectronics, and MEMS.

Within the framework of the current collaboration, the group now plans to combine such laser-based ultrasound investigations with other measurement methods.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-23398-0).