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Elad Harel of Michigan State University with laser equipment used to “paint” crystals. | Credits: Paul Henderson, Finn Gomez / College of Natural Sciences.
Crystals used in applications as diverse as lasers, light-emitting diodes, and semiconductors used in sensors in astronomical instruments may someday be “drawn” rather than “grown,” increasing performance and lowering costs.
The team, led by Elad Harel of Michigan State University, used a laser to heat gold nanoparticles, which then causes crystals to form in a solution of lead halide perovskite. Therefore, by moving the gold nanoparticle and again using lasers, it is theoretically possible to “paint” the crystals exactly where they need to be in an electronic device.
Crystals used in electronics are traditionally made using a variety of methods, such as vapor diffusion, where the crystal is deposited from solution, or by planting a “seed” of the crystal and watching it grow. However, such methods are imprecise, resulting in crystals that are somewhat random and not always in the right place or the right shape or size.
“The device may require very small amounts of crystalline material placed in very specific locations,” Harel told Space.com.
Harel’s new technique, which uses a phenomenon called “plasmonic heating,” may regain some control over crystal formation. In laboratory experiments, Harel’s team fired a 660-nanometer laser wavelength in a gold nanoparticle in a reaction chamber filled with a lead halide perovskite precursor solution over a borosilicate glass substrate on which the crystal would be “pulled”.
A gold nanoparticle is tiny, less than one-thousandth the width of a human hair. Therefore, the entire procedure must be extremely precise and can be observed in real time using high-speed microscopes with sub-millisecond frame rates.
“The reason we use gold nanoparticles is that they act as little heaters,” Harel said. “When a laser irradiates a particle at the right frequency, it causes electrons to oscillate in the gold, which generates heat’.
This is plasmonic heating and induces crystallization of the precursor solution in the locations Harel’s team wants.
Lead halide perovskite crystals have high performance in solar cells and light-emitting diodes, but they are not the only type of crystal used in electronics. For example, Mid-Infrared Instrument (MIRI) semiconductors James Webb Space Telescope contains arsenic doped silicon crystals. Harel hopes that this plasmonic heating technique can be applied to other such crystals, but it works for lead halide perovskites because they have quite unusual properties.
“The peculiarity of these perovskites is that the solubility decreases with increasing temperature, which promotes crystallization,” he said. “Most substances do not have this property of retrograde solubility; generally, as temperature increases, solubility increases.”
This high-speed microscope footage captures a laser hitting a gold nanoparticle, triggering crystallization. | Credit: Harel Lab at MSU
However, there may be a way around this, in the hidden, excited electrons. According to Harel, electrons not only produce heat, but can also participate directly in the chemistry of crystal formation, promoting crystal formation.
“We need to do more to generalize this concept to other materials, but we believe it will work,” he said.
The advantages of cheaper, faster and more accurate crystal formation are obvious. Crystals are used in everything from touch screens, smoke alarms, solar panels, medical imaging devices, and many optoelectronics and photodetectors.
“It’s a very simple method using cheap lasers,” Harel said. “There are also significant savings in production costs because the crystal can be placed exactly where and when it is needed.”
Given the importance of crystals in astronomical sensing, the technique of drawing them could allow for cheaper instruments to be launched on future space missions.
The next step is to use multiple lasers at different wavelengths to “paint” more complex crystal patterns, and then start testing them in real devices to see if they really offer a better standard of performance at a lower cost. “We’re working on that right now,” Harel said.
This new technique for “drawing” crystals is published in the journal ACS nano.