Laser Zone Annealing Technique Enables Rapid Self-Assembly at the Nanoscale

Researchers from the Brookhaven National Laboratory of the US Department of Energy have succeeded in developing a laser-based method for executing self-assembly at the nanoscale with excellent ease and efficiency.

The amazing billionth-of-a-meter qualities exhibited by nanoscale materials are capable of transforming everything from data storage to energy generation.

The efficiency of a nanostructured solar cell is undoubtedly excellent, however the precision is tough to attain at industrial levels. The solution for this is self-assembly or teaching the molecules to knit themselves together into high-performance configurations.

“We design materials that build themselves,” said Kevin Yager, a scientist at Brookhaven’s Center for Functional Nanomaterials (CFN). “Under the right conditions, molecules will naturally snap into a perfect configuration. The challenge is giving these nanomaterials the kick they need: the hotter they are, the faster they move around and settle into the desired formation. We used lasers to crank up the heat.”

Yager and Brookhaven Lab postdoctoral researcher Pawel Majewski designed a unique machine that sweeps a focused laser-line across a sample for generation of instantaneous and instant temperature spikes. This novel technique, known as laser zone annealing (LZA) drives self-assembly at rates over 1,000 times faster than conventional industrial ovens. The results are recorded in the ACS Nano journal.

“We created extremely uniform self-assembled structures in less than a second,” Majewski said. “Beyond the extraordinary speed, our laser also reduced the defects and degradations present in oven-heated materials. That combination makes LZA perfect for carrying small-scale laboratory breakthroughs into industry.”

The researchers formulated the materials and constructed the LZA instrument at the CFN. The samples were then evaluated using advanced electron microscopy at CFN and x-ray scattering at Brookhaven’s now-retired National Synchrotron Light Source (NSLS), both of which are DOE Office of Science User Facilities.

“It was enormously gratifying to see that our predictions were accurate—the enormous thermal gradients led to a correspondingly enormous acceleration!” Yager said.

This can be compared to baking a complicated cake, but the cake is not baked in an oven however a laser barrage creates it perfectly almost instantly. Using optimum cooking conditions, the ingredients will be blended into a picture-perfect dish. This nanoscale recipe is also highly effective and produces extraordinary results.

The researchers concentrated on block copolymers, which are molecules having two linked blocks with distinct chemical properties and structures. Complicated and robust nanoscale structures are created as the blocks tend to repel each other.

“The price of their excellent mechanical properties is the slow kinetics of their self-assembly,” Majewski said. “They need energy and time to explore possibilities until they find the right configuration.”

Conventionally, to initiate self-assembly of block copolymers, the materials are subjected to heat in a vacuum-sealed oven. For 24h the sample is baked so that sufficient amount of kinetic energy is created to snap the molecules into place, which is definitely too long a period in terms of commercial viability.

Cracks and imperfections are created throughout the sample because the long exposure time to high heat results in thermal degradation.

The LZA process offers sharp heat spikes to excite the polymers rapidly without the energy which is damaging to the material.

“Within milliseconds, the entire sample is beautifully aligned,” Yager said. “As the laser sweeps across the material, the localized thermal spikes actually remove defects in the nanostructured film. LZA isn’t just faster, it produces superior results.”

Temperatures higher than 500°C are generated by LZA however the thermal gradients can be over 4000°/mm. Thermal gradient is the change in temperature with regards to the location and direction in a material. Though it is a known fact that high temperature can improve self assembly, this is the first time that extreme gradients have caused such a drastic improvement.

“Years ago, we observed a subtle hint that thermal gradients could improve self-assembly,” Yager said. “I became obsessed with the idea of creating more and more extreme gradients, which ultimately led to building this laser setup, and pioneering a new technique.”

The researchers required considerable technical expertise and world-class facilities to shift the LZA from the proposal stage to the execution stage.

“Only at the CFN could we develop this technique so quickly,” Majewski said. “We could do rapid instrument prototyping and sample preparation with the on-site clean room, machine shop, and polymer processing lab. We then combined CFN electron microscopy with x-ray studies at NSLS for an unbeatable evaluation of the LZA in action.”

Added Yager, “The ability to make new samples at the CFN and then walk across the street to characterize them in seconds at NSLS was key to this discovery. The synergy between these two facilities is what allowed us to rapidly iterate to an optimized design.”

A novel microscale surface thermometry technique known as melt mark analysis was developed by researchers to determine the precise heat that laser pulses generate and adjust the instrument based on that.

“We burned a few films initially before we learned the right operating conditions,” Majewski said. “It was really exciting to see the first samples being rastered by the laser and then using NSLS to discover exactly what happened.”

The LZA symbolizes a breakthrough in scale-up of nanotechnology and is the first of its kind machine globally. It is also possible to use the laser to draw structures across the surface implying that the nanostructures are capable of assembling in precise patterns. This amazing manner of synthesis could pave the way to complicated applications such as electronics.

“There’s really no limit to the size of a sample this technique could handle,” Yager said. “In fact, you could run it in a roll-to-roll mode—one of the leading manufacturing technologies.”

The researchers are working on further advancement of the technique for the creation of multilayer structures with instant effect on improved solar cells, reflective coatings and sophisticated electronics.

The DOE Office of Science funded the research and operations at NSLS and CFN. The Office of Science of the U.S. Department of Energy supported the Brookhaven National Laboratory.