Michigan State University uses laser pulses to impact gold nanoparticles for crystal growth

To make crystals suitable for use as optoelectronic materials, the key is to precisely control the crystallization, but this control is difficult.

Producing lead halide perovskites, promising components for next-generation solar cells and photodetectors, has proven particularly challenging, with slow growth rates and uncontrolled nucleation being common issues.

A project at Michigan State University (MSU) has now developed a new way to stimulate crystal growth using laser pulses, which could accelerate the development of these advanced next-gen technologies.

Described in ACS Nano, the seed-free plasmonic heating-driven approach could mean that "the traditionally tricky crystal-growing process is turned on its head."

Growth potential: controlled crystallization

"With this method, we can essentially grow crystals at precise locations and times," said Md Shahjahan from MSU. "It's like having a front-row seat to watch the very first moments of a crystal's life under a microscope, only here we can also steer how it develops."

The technique leverages plasmonic heating in gold nanoparticles, and the ability of a laser to precisely control the temperature in the immediate vicinity of a nanoparticle's surface. This localized thermal gradient can influence supersaturation conditions in specific areas, and effectively control nucleation and growth.

This offers researchers the ability to "draw" crystals with levels of control that could transform fields ranging from clean energy to quantum technologies, said the project. It could also help expand the understanding of how crystals form, providing "a unique opportunity for real-time visualization of the crystallization process with sub-millisecond resolution using high-speed microscopy."

Optical properties maintained

In trials using methyl-ammonium lead bromide (MAPbBr3) perovskites, the team employed a 660-nanometer laser, tuned to match the localized surface plasmon resonance (LSPR) behavior of the gold nanoparticles.

Unlike many other solutes, MAPbBr3 exhibits a decrease in solubility with rising temperature, so the laser's localized heating causes the precursor solution to become supersaturated near the surface, driving the formation of stable crystal nucleii which then act as seeds for further growth.

"We found that in a narrow range around 60 mW laser power, there is an optimal thermal environment at the focal spot, whereby single crystals nucleate and continue to grow steadily," wrote the project in its paper. The crucial optical properties of the resulting crystals were later found to be comparable to naturally grown counterparts.

The project's next steps will include using multiple lasers of different colors to draw even more intricate crystal patterns, and attempting to create entirely new materials that can't be made through conventional methods.

"Now that we can 'draw' crystals with lasers, the next step is to make larger and more complex patterns, and to test how these crystals perform in real devices," said Elad Harel from the MSU DeepSpec Lab. "We're just beginning to scratch the surface of what’s possible."

Source: optics.org