Suppressed thermal transport in silicon nanoribbons by inhomogeneous strain

Nanoscale structures can produce extreme strain that enables unprecedented material properties, such as tailored electronic bandgap^1-5, elevated superconducting temperature^6,7 and enhanced electrocatalytic activity^8,9. While uniform strains are known to elicit limited effects on heat flow^10-15, the impact of inhomogeneous strains has remained elusive owing to the coexistence of interfaces^16-20 and defects^21-23. Here we address this gap by introducing inhomogeneous strain through bending individual silicon nanoribbons on a custom-fabricated microdevice and measuring its effect on thermal transport while characterizing the strain-dependent vibrational spectra with sub-nanometre resolution. Our results show that a strain gradient of 0.112% per nanometre could lead to a drastic thermal conductivity reduction of 34 ± 5%, in clear contrast to the nearly constant values measured under uniform strains^10,12,14,15. We further map the local lattice vibrational spectra using electron energy-loss spectroscopy, which reveals phonon peak shifts of several millielectron-volts along the strain gradient. This unique phonon spectra broadening effect intensifies phonon scattering and substantially impedes thermal transport, as evidenced by first-principles calculations. Our work uncovers a crucial piece of the long-standing puzzle of lattice dynamics under inhomogeneous strain, which is absent under uniform strain and eludes conventional understanding.