Remarkable Suppression of Thermal Transport by Inhomogeneous Strain

There is a mantra in the realm of material mechanics, “smaller is stronger”, the science of which took root in the 1950s and is vigorously developing today. As nanomaterials are mechanically much stronger, significantly higher strains can be applied to tune their physicochemical properties than is possible with traditional materials. Based on this, we can rationally engineer a spectrum of advanced functionalities ranging from transistors, solar cells and photodetectors to batteries, superconductors, and electrocatalysis. Despite extensive investigations into strain-engineered electronic properties, the intricate phonon transport mechanism subjected to inhomogeneous strain remains an issue of debate. This is especially frustrating given that precise thermal management stands as a critical bottleneck to device efficiency and lifespan.<br/><br/>One prevalent method for introducing strain involves growing thin epitaxial layers on substrates with lattice mismatch, such as Si on SiGe, and research has been conducted to investigate thermal transport through various epitaxial layers. However, although low thermal conductivity (κ) values even below their alloy counterparts have been observed, the effects of strain gradient through the epitaxial layers are experimentally difficult to decouple from the interfacial phonon boundary scattering effects, which presents a daunting challenge to draw a solid conclusion on the physical origin of the ultra-low κ. Similarly, although dislocations and vacancies could scatter phonons in various functional devices, it remains a formidable challenge to isolate their effects from that of long-range strain fields introduced by these defects, which could also impede thermal transport via an increase in vibrational anharmonicity. Consequently, questions regarding the cause of the unusual and somewhat perplexing thermal behavior in these functional materials have lingered unanswered.<br/><br/>To date, while elastic strain engineering often relies on highly inhomogeneous stress produced by nanoscale deformation (e.g., by epitaxial layer growth, defects and vacancies, or lithography patterning), most studies of strain effects on thermal transport have centered around materials under simplified condition of uniform stress. The major challenges in experimentally quantifying the effects of inhomogeneous strain on thermal transport include applying stress exclusively without introducing confounding factors (e.g., interfaces and defects), and combining thermal measurements with atomically resolved characterization of the strain field and phonon spectra. Here, we induce inhomogeneous strain through bending individual silicon nanoribbons on a custom-fabricated microdevice, and measure its effect on thermal transport while characterizing the local strain and vibrational spectra using atomically resolved electron energy-loss spectroscopy (EELS). Our results show that a strain gradient of 0.112% per nanometer could lead to a drastic thermal conductivity (κ) reduction of 37.4%, which is over 3-fold of previously demonstrated κ modulation under uniform strain. Taking advantage of recent progress in electron energy loss spectroscopy equipped with a monochromator in an aberration corrected scanning transmission electron microscope (STEM), we directly measured the local phonon modes and correlated them with the nanometer-scale strain gradient. Results show that the bending-induced lattice strain gradient significantly alters the vibrational states and broadens the phonon spectra. Coupled with ab initio theoretical modeling, this broadening effect is shown to enhance phonon anharmonicity and shorten phonon lifetimes, ultimately contributing to suppressed thermal conductivity.