Phononics and thermoelectric materials

A better understanding of the quantum nature of heat will advance energy research

Phonons are the quantum mechanical description of vibrations. Similar to electromagnetic waves, they also have characteristic frequencies spanning from a few hertz (1 Hz=1 cycle per second) to hundreds of terahertz (1 THz=1012 cycles per second). Low-frequency phonons, such as human sounds or mechanical vibrations, are generally well understood and can be well controlled, whereas the physics of high-frequency (above 100 GHz, 1 GHz=109 cycles per second) phonons are poorly understood. In fact, current technologies are unable to use high-frequency phonons, leading many to characterize them as “waste heat”.

At the Center for Condensed Matter Sciences, a unique center at NTU dedicated to concentrating professional knowledge at the frontiers of science, we have invented many new tools to begin charting several unexplored domains of research. These tools include tailored platforms to characterize the heat transfer properties of nanomaterials, sophisticated thermometers for precise temperature measurements, and advanced far-infrared spectroscopes to investigate the interactions between phonons and photons. Our recent discoveries include (1) the observation of ballistic thermal conduction over 5 micrometers in SiGe nanowires and (2) non-Fourier thermal conduction over 1 millimeter in carbon nanotubes. Remarkably, these experimental observations were made at room temperature, thus opening many possibilities to practically engineer the wave properties of heat in the near future.

The discovery of ballistic thermal conduction demonstrates that the thermal conductivity of microscale materials is lower than what was anticipated. Thus, this discovery has inspired new ideas to improve thermoelectric devices. Thermoelectric devices can convert waste heat into electricity, or they can be operated in reverse by converting electricity into refrigerating power. These devices will serve as new green energy resources as their operation does not emit greenhouse gases. Furthermore, because they are solid-state materials, they can be compact and durable.

At the Center for Condensed Matter Sciences at NTU, we hope to contribute new knowledge about phononics by using phonons for mass sensing, enhancing phononic interactions with electrons and photons, and introducing new unforeseen applications. However, research on thermoelectrics has a notorious history of poor reproducibility and a lack of standardization. Together with several faculty members at the Academia Sinica, we have formed a team to focus efforts toward fabricating, designing, and characterizing thermoelectric materials with rigorous methods. Our goal is to maintain the integrity of science even though scientists currently operate in a global, competitive market.


A tailored microscale platform for measuring phonon transport across an individual nanowire.

1. Tzu-Kan Hsiao, Hsu-Kai Chang, Sz-Chian Liou, Ming-Wen Chu, Si-Chen Lee and Chih-Wei Chang. (2013). Observation of room temperature ballistic thermal conduction persisting over 8.3μm in SiGe nanowires". Nature Nanotech. 8, 534–538. Published online 30 June 2013. DOI: 10.1038/nnano.2013.121.
2. Victor Lee, Chi-Hsun Wu, Zong-Xing Lou, Wei-Li Lee, and Chih-Wei Chang. (2017). Divergent and ultrahigh thermal conductivity in millimeter-long nanotubes", Physical Review Letters, 118, 135901. Published 30 March 2017. DOI: 10.1103/PhysRevLett.118.135901.

Dr. Chih-Wei Chang
Associate Research Fellow
Center for Condensed Matter Sciences