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Traditionally, many engineers and developers of solar cell technology have turned to crystalline silicon — a tried and tested material absorber capable of efficiently converting solar radiation to electricity at just four times the thickness of a strand of hair.

At up to a 100th the thickness of a hair strand, nano-thin metal films offer an even more cost-effective and flexible material alternative, holding promise in the future development of everything from solar power to sensor technology. 

However, metal nanofilms are currently more complicated to use as material absorbers due to complex processes that the material typically undergoes while the films are melted on glass panels or other solid substrates using intense heat generated by laser pulses. 

NJIT investigators are now shedding new light on the mysterious changes that occur to metal nanofilms during this complex evolutionary process.

A team of researchers led by Lou Kondic, professor of mathematics at NJIT, has devised a new computational model capable of tracking how extreme heat impacts the evolution of thin metal films on thermally-conductive solid substrates. The lab’s recent discovery of major thermal and fluid dynamic factors that drive the evolution of metal films could potentially aid future development of more efficient thin film technologies.

“Until now, we have not been able to precisely see how these films evolve to nanoparticles and arrange themselves under laser heat because the changes happen so fast,” said Kondic. “The idea of our research has been to use our new model to understand and measure the mechanisms that govern thin films and what they do on the nanoscale during this process.” 

In recent published research, featured in the AIP Publishing journal Physics of Fluids, Kondic’s lab used their model to accurately simulate fluid dynamics with spatial and temporal temperature changes in metal film and its substrate under nanoseconds-long pulses of laser irradiation. 

Challenging previous understanding in the field, Kondic’s team found that the evolution of the film in its liquid state is mainly influenced by time-dependent surface tension changes as it heats and cools over the course of each laser pulse, rather than by thermally-driven stresses created by spatial variation of liquid surface tension, otherwise known as the Marangoni effect.

Kondic’s lab accurately simulates a liquid metal strip evolving at the nanoscale under intense radiation from a pulse laser.

“The surprise was that we expected to show the Marangoni effect was crucial to the outcome of this process because other work had claimed that previously,” said Kondic. “It turns out that what is most crucial is that when you go from cold metal to hot metal, surface tension changes. At the beginning of each laser pulse we can see one surface tension, and later we can see a different one…the evolution of surface tension in time is what is really critical in determining the evolution of these films.”

Kondic’s team is now exploring how alloys involving more than one metal evolve under similar conditions. Kondic says learning more about how complex metal films evolve in a similar way may help boost development of alloys with even better radiation-absorbing properties. 

“If you can direct how these nanoparticles arrange themselves on top of silicon, then you could really do a lot of useful things and it is much more efficient and cost-effective,” said Kondic. “Developers of solar technology are looking into the physical mechanisms that are responsible for developing these structures, so ultimately I hope our work helps provide new details that could help them produce better materials.”

Kondic was recently elected as 2017 Fellow of the American Physical Society (APS) by the APS Council of Representatives in recognition for his “outstanding contributions to physics” and “understanding of complex fluid dynamics, from thin films to granular flows.”

In November 2017, Kondic was also awarded the Dr. Luis Federico Leloir Award to International Cooperation in Science, Technology and Innovation, which is awarded by the Ministry of Science, Technology and Productive Innovation of Argentina in recognition of “foreign experts who have contributed to the strengthening of international cooperation with our country.”