Atomic-Resolution Mapping of Localized Phonon Modes at Grain Boundaries

Abstract

Phonon scattering at grain boundaries (GBs) is significant in controlling the nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, milli-electron volt (meV) energy resolution is needed with subnanometer spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterized by the presence of 5- and 7-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides.

Nanoscale Localized Phonons at Al2O3 Grain Boundaries

“…Therefore, it is expected that general GBs in Al 2 O 3 , even with different geometry configurations, would exhibit similar defect phonon DOS. Our present findings differ from those GB phonon structures in SrTiO 3 and Si reported so far. , For example, Hoglund et al’s study of the SrTiO 3 GB provided evidence in correlating the coordination, chemistry, bonding, and electronic states with vibrational modes, while our study found that for Al 2 O 3 the change in bond length is the dominant one. Furthermore, our findings in Al 2 O 3 also differ from that in Si defects, where it is reported that bond angle dominates GB vibrational modes .…”

“…Our present findings differ from those GB phonon structures in SrTiO 3 and Si reported so far. , For example, Hoglund et al’s study of the SrTiO 3 GB provided evidence in correlating the coordination, chemistry, bonding, and electronic states with vibrational modes, while our study found that for Al 2 O 3 the change in bond length is the dominant one. Furthermore, our findings in Al 2 O 3 also differ from that in Si defects, where it is reported that bond angle dominates GB vibrational modes . Therefore, it seems that the dominant factor to determine the local defect phonon can be very different in different materials.…”

“…For bulk materials, the phonons can be measured using infrared-Raman (IR) spectroscopy, high-resolution electron energy loss spectroscopy (HREELS), inelastic neutron scattering (INS), inelastic X-ray scattering (IXS), scanning near-field optical microscopy (SNOM), etc . However, the limited spatial resolution of these methods makes it challenging to probe the nanoscale defects like dislocation and GBs, not to mention simultaneously correlating with the local atomic arrangement. − Recent advances in monochromated EELS and aberration-corrected STEM enable phonon measurement (namely, vibrational EELS spectrum measurement) for nanoscale defects, atomic-scale heterointerfaces, or even single point defects. − Here, we used STEM-EELS to study the local phonons of Al 2 O 3 GBs. Al 2 O 3 exhibits high thermal conductivity, chemical stability, and mechanical strength and, thus, is widely used as heat dissipation materials and light windows in the fields of semiconductor and optical devices. − Such properties in the Al 2 O 3 ceramics would be largely affected due to the existence of GB through the local phonon interactions, while the knowledge of such localized phonon structures is still lacking …”