By Saren Johnston
Cai-Zhuang Wang and Kai-Ming Ho tend to view materials as marvelous collections of atoms and electrons. As theoretical physicists, they rarely investigate a solid chunk of material sitting on a workbench, preferring instead to explore the material at the atomic level -- a world teeming with activity.
The two Ames Laboratory researchers devote their efforts to describing this activity and understanding why atoms interact the way they do in specific situations. The atomic simulations they develop help explain the "why" behind what experimentalists observe in the laboratory.
Over the last 10 years, Wang and Ho have developed a method for doing quantum molecular dynamics simulations of practical materials with far more ease, speed and economy than is possible with conventional first principles methods. The technique, called tight-binding molecular dynamics, is a computationally efficient means of studying the structures, dynamics and electronic properties of complex systems at the atomic level. And TBMD can accommodate changes in bonding within a material due to electronic excitation, a capability that most atomic simulation methods don't possess.
Wang explains that the interactions between atoms originate from the electrons, so the interactions are best described by quantum mechanics. But full quantum mechanics calculations are too complicated for atomic systems containing large numbers of atoms.
"The idea of TBMD is to keep the spirit of quantum mechanics but simplify the process by developing a model for molecular dynamics simulations that can describe atomic interactions accurately, yet is fast enough to treat systems with large numbers of atoms," says Wang.
Wang's and Ho's research on TBMD won a Department of Energy Materials Sciences Award in 1996. Using TBMD, they have created simulations to investigate such phenomena as carbon clusters and buckyballs, and amorphous forms of carbon and silicon. They have successfully developed other TBMD schemes to simulate how carbon atoms behave in different environments and under different circumstances.
Iowa State University researchers, lead by mechanical engineering professor Pat Molian, wanted to better understand what they were observing in their laboratory experiments on laser ablation of diamond. So Molian decided to contact Wang and Ho. He was hoping to interest the two physicists in doing some unique TBMD simulations that would describe what was going on at the atomic level during diamond ablation with laser pulses of different durations.
Diamond is popular within the microelectronics and tool-making industries because of its unique properties. It is the hardest material known and has the highest thermal conductivity. It also has a large bandgap that makes it transparent to light in the ultraviolet range. However, the very properties that make diamond such a useful material also make it extremely difficult and expensive to machine -- a predicament that Molian thinks could be remedied with pulsed laser technology.
Pulsed lasers provide what scientists call peak power -- the amount of power delivered during a single pulse of laser energy. Pulse width, or duration, greatly affects the peak power of a laser because it defines the amount of time the material being studied interacts with the light. Pulse widths of longer durations are generally associated with lower intensity lasers and longer periods of light exposure, while pulse widths of shorter durations are associated with higher intensity lasers and shorter light exposure, characteristics essential for high-precision laser machining.
Molian and his colleagues have been investigating the use of ultrashort pulsed lasers for micromachining diamond in their laser ablation experiments. They observed that ablation with nanosecond (one billionth of a second, or 10-9 second) or longer laser pulses caused graphite formation and contaminated surfaces. The researchers discovered the graphite, or "diamond dirt," by using Raman spectroscopy, which detects the vibrations in a material. Before nanosecond laser ablation of diamond, the Raman spectra displayed characteristic peaks. But the peaks had disappeared in the spectra produced following nanosecond laser ablation, indicating that the surface was no longer diamond in structure.
"Graphite formation is a big issue for many applications, especially electronics, where they don't want any contamination," says Molian. "So we moved on from nanosecond to femtosecond laser technology." (Femtosecond pulses are one quadrillionth of a second, or 10-15 second long.)
The switch to the ultrashort femtosecond pulse proved beneficial. The Raman spectra produced both before and after femtosecond laser ablation showed similar peaks, indicating clean surfaces free of graphite contamination. The femtosecond laser pulse had made a clean sweep, removing the top layers of diamond atoms.
"We were very anxious to learn why the femtosecond lasers did so much better than the nanosecond lasers. We could see what was happening, but we didn't know why," says Molian.
Ho says, "They came to us to see if we'd do some simulations that would tell them what was happening atomically -- what was taking place on the diamond surface and why the femtosecond and nanosecond laser ablation processes resulted in different structures."
Wang explains, "It usually takes a picosecond for the energy from the laser-excited electrons to reach the atoms. (A picosecond is one trillionth of a second, or 10-12 second.) When diamond is irradiated with nanosecond laser pulses, the laser is intense at 10-9 second, and the atoms can be thermally excited. This leads to melting and the formation of light-absorbing graphite residues that contaminate the surface layers."
On the other hand, the femtosecond laser pulse is much shorter than the time required for an atom to vibrate, so it is able to eject the electrons into highly excited states while the atoms are still thermally cool. The energy stays in the electron system. The TBMD simulation shows that femtosecond laser ablation peels atoms from the diamond surface in a layer-by-layer fashion through a non-thermal mechanism, leaving a smooth diamond surface.
"The femtosecond laser leaves a very clean cut," says Ho. "With the different amount of energy that is input, there is more surface removed."
The conventional view of material removal by laser is that the material is heated, melts, becomes a liquid or a gas and goes away, Wang explains. "However," he continues, "the physics involved with the femtosecond laser is different than the normal melting picture because the time scale is not long enough to allow the atoms to vibrate. That is some new physics, and the reason why Dr. Molian and his colleagues came to us for the research and TBMD simulations that would help them better understand that process. Our models correlate very well with their experimental findings."
For more information:
Cai-Zhuang Wang, (515) 294-6934
wangcz@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
Last revision: 9/15/00 sd
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