New Approach to Manipulate Crystal Vibrations Could Pave Way for More Energy-Efficient Devices

An atom’s vibrational motion in a crystal spreads to adjacent atoms, which results in wavelike propagation of the vibrations all over the crystal. The way in which these natural vibrations move through the crystalline structure define the major properties of the material. For instance, these vibrations establish how well heat and electrons can navigate the material, and how the material interacts with light.

Currently, researchers have demonstrated that by swapping out just a tiny fraction of a material's atoms with atoms of a varied element, they can regulate the frequencies and speed of these vibrations. This finding, reported in Applied Physics Letters, by AIP Publishing, offers a potentially simpler and inexpensive way to tweak a material's properties, allowing for a broad range of new and more efficient devices, such as in electronics and solid-state lighting.

The normal vibrations of a crystalline material travel as particles known as phonons. These phonons transport heat, scatter electrons, and impact the electrons’ interactions with light. Earlier, scientists regulated phonons by dividing the material into smaller pieces whose boundaries can scatter the phonons, restricting their movement. Of late, scientists have built nanoscale structures, such as nanowires, into the material to control phonons’ frequencies and speed.

Scientists from the University of California, Riverside and the University of California, San Diego have recently discovered that by doping - adding diverse elements into the material - one can manipulate phonons. The scientists doped aluminum oxide with neodymium, which substitutes a few of the aluminum atoms. As neodymium is larger and more enormous than aluminum, it modifies the vibrational properties of the material, altering the way phonons can travel.

"It introduces distortion to the lattice, which persists over a large distance compared to the atomic size, and affects the whole vibrational spectrum," said Alexander Balandin of the University of California, Riverside.

Using a new technique of creating evenly doped crystals and new sensitive tools to measure the phonon spectrum, the team demonstrated, for the first time, that even a small amount of specific dopants can have a major impact. "This approach provides a new way of tuning the vibrational spectrum of materials," Balandin said.

Earlier, researchers supposed that any important effect on phonons would necessitate an extremely high concentration of dopants. But, the researchers learned that doped aluminum oxide with a neodymium density of merely 0.1% was sufficient to lower the phonon frequency by a few gigahertz and the speed by 600 m/second.

Improving phonon speeds promotes a material's thermal conductivity, allowing miniature transistors to cool sooner. Decelerating phonons, on the other hand, would be beneficial in creating more efficient thermoelectric devices, which change electricity into heat and vice versa. Additionally, in optical devices such as light-emitting diodes, decelerating phonons and suppressing phonon interactions with electrons would translate into more energy used to create photons (light) and less energy lost as heat.

The team is currently applying their approach to other dopants and materials, such as gallium arsenide, with a focus on creating energy-efficient devices, Balandin explained.

The U.S. Department of Energy funded the research.

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