The National Institute for Computational Sciences

Tiny Zaps, Big Results

Lasers Have a Unique Ability to Change Materials for Useful Purposes

The “small talk” of researchers in nanotechnology is extremely small. Their interest is in the physical phenomena occurring with things one-billionth of a meter, a million times shorter than the length of an ant, or up to 100,000 times thinner than a human hair. But the benefits to society of the science, engineering, and technology they’re doing at such tiny scales are huge.

In fact, more than 800 everyday commercial products rely on nanoscale materials and processes, according to the National Nanotechnology Initiative. A central aspect of nanotechnology is that it allows essential structures of materials to be tailored to achieve specific properties that improve a variety of applications in medicine, energy, information technology, and many other areas.

Leonid Zhigilei

One method of nanotechnology research involves the use of short laser pulses at minuscule fractions of a second to produce structural changes in thin, localized surface regions of various materials, such as gold, silver, or silicon. Leonid Zhigilei of the University of Virginia says that what attracted him to this type of research is the ability of lasers to excite and change materials in ways not possible with any other technique.

Short-pulse laser processing can, for example, transform a surface from very water attractant to very water repellant, thereby reducing friction, erosion, and contamination on items that need to be kept clean, examples being roof tiles and skyscraper windows.

Because laser-induced processes are complex and happen so fast, experimentation cannot provide a detailed understanding of the structural transformations triggered by the rapid laser energy deposition, Zhigilei says.

“Our atomistic simulations, on the other hand, provide clear visual representations, or ‘atomic movies,’ and are well-suited to reveal the relationships between the properties of laser-treated regions of the targets and the underlying microscopic mechanisms of laser-induced target modification,” adds Chengping Wu, a member of Zhigilei’s computational materials research group.

The group uses molecular dynamics, a computer simulation technique for studying the movements of atoms or molecules in a system.

“Unlike in real experiments,” Zhigilei explains, “the analysis of non-equilibrium processes in molecular dynamics can be performed with unlimited atomic-level resolution, providing complete information of the phenomena of interest.”

During 2015, Zhigilei’s group published two papers in the journal Physical Review with different collaborators.

Working with Henry Helvajian of The Aerospace Corporation, Los Angeles, they discovered they could use surface acoustic waves (SAWs) to move tiny particles of gold on a silicon surface. The paper (published on 30 June), titled “Strong enhancement of surface diffusion by nonlinear surface acoustic waves,” notes that the use of SAWs to bring about surface processes has broad implications for applications in which heating must be avoided.

Collaboration with the experimental group of Peter Balling of Aarhus University, Denmark, showed that the volume of a metal on a surface could be increased, providing new opportunities for tailoring surface properties to the needs of practical applications. The paper with Balling (published on 12 January) is titled “Generation of subsurface voids and nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target.” For perspective, a femtosecond is one-quadrillionth of a second.

An illustration of some of the results of large-scale atomistic computer simulations of laser-induced structural modification of silver targets irradiated by 200 femtosecond (one quadrillionth of a second) laser pulses. The simulations make predictions regarding the surface and explain the experimental observation of surface swelling. The experiments were done in the group of Peter Balling at Aarhus University, Denmark. Illustration courtesy of Leonid Zhigilei, the University of Virginia.

Zhigilei’s group is using the Darter supercomputer at the National Institute for Computational Sciences (NICS) and Stampede at the Texas Advanced Computing Center (TACC) for high-performance computing, which has played a crucial role in many of their projects because the systems they simulate can consist of up to a billion atoms.

The National Science Foundation’s eXtreme Science and Engineering Discovery Environment (XSEDE) made Zhigilei’s compute allocations on Darter and Stampede possible. XSEDE is a single virtual system that scientists can use to interactively share computing resources, data, and expertise. People around the world use these resources and services—things like supercomputers, collections of data and new tools—to improve our planet.

Zhigilei touts the value of XSEDE as a whole. “I think it [XSEDE] is important because we can use different computers,” he says. “When we write our allocation requests, we often ask for time on different computers, and we also take advantage of other XSEDE resources, of course. We use VisIt software [for visualizations]. In addition, many of my students enjoy the file transfer service, Globus Online, which is very efficient in moving large files that we are generating in our simulations.”

Zhigilei adds that some of his students have had the opportunity to attend educational events organized by XSEDE.

Going forward, Zhigilei’s group will study the effect of liquid environments on laser-induced processes and will explore the ability of lasers to modify the microstructure of complex multi-component alloys. And one of their proposals expresses an aim to study the ability of metal nanoparticles to efficiently convert laser energy absorbed at a surface to heat and mechanical work, a capability that is key to a growing number of imaging and therapeutic biomedical techniques, Wu says.

“For example, researchers have demonstrated that laser irradiation of gold or silver nanoparticles attached to gene markers and delivered to specific cells can be used for selective killing of cancer cells or bacteria,” Wu explains. “In the area of drug delivery, doping the walls of microcapsules with metal nanoparticles opens a way for the remote release of encapsulated material into specific cells by targeting metal nanoparticles with near-infrared laser irradiation.”

Thus, another case illustrates the big implications of nanoscale for humanity.

This research was conducted under XSEDE project grant TG-DMR110090.

Scott Gibson, science writer, NICS, JICS

Article posting date: 13 January 2016

About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and leading-edge engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers to be well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS operates the National Institute for Computational Sciences, NICS, one of the nation's leading advanced computing centers. NICS is co-located on the UT Knoxville campus and ORNL, home of the world's most powerful computing complex. The center's mission is to expand the boundaries of human understanding while ensuring the United States' continued leadership in science, technology, engineering, and mathematics. NICS is a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment (XSEDE).