[Paper Review] Atomic resolution mapping of localized phonon modes at grain boundaries

Benedikt Haas, Tara M. Boland+6 more|arXiv (Cornell University)|Mar 9, 2023

Force Microscopy Techniques and Applications4 citations

TL;DR

This study demonstrates atomic-resolution mapping of localized phonon modes at grain boundaries in silicon using monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope, achieving meV energy resolution and sub-nanometer spatial precision. The results show strong suppression of the 60 meV optic phonon mode at grain boundaries with five- and seven-fold ring structures, with excellent agreement between experiment and first-principles calculations, confirming the existence of localized phonon modes and grain boundaries acting as waveguides for specific vibrational modes.

ABSTRACT

Phonon scattering at grain boundaries (GBs) is significant in controlling nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, meV energy resolution is needed with sub-nm 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 characterised by the presence of five- and seven-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.

Motivation & Objective

• To directly observe localized phonon modes at grain boundaries in silicon with atomic-scale spatial and energy resolution.
• To investigate the role of atomic-scale structural defects—specifically five- and seven-fold ring configurations—in modifying phonon behavior at grain boundaries.
• To validate theoretical predictions of localized phonon modes through high-resolution experimental measurements.
• To determine whether grain boundaries can act as waveguides for specific phonon modes, influencing thermal transport at the nanoscale.

Proposed method

• Employed monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to achieve sub-nanometer spatial resolution and meV-level energy resolution.
• Mapped the 60 meV optic phonon mode across grain boundaries in silicon, focusing on regions with five- and seven-fold ring defects.
• Compared experimental EELS intensity maps with calculated phonon densities of states (DOS) from first-principles simulations.
• Used atomic-scale structural models of grain boundaries to correlate specific topological defects with phonon localization.
• Performed spatially resolved EELS measurements along grain boundary interfaces to quantify intensity variations.
• Validated the experimental results against theoretical phonon dispersion and mode localization predictions.

Experimental results

Research questions

• RQ1 Do grain boundaries in silicon support localized phonon modes at the atomic scale, as predicted by theory?
• RQ2 How does the presence of five- and seven-fold ring defects at grain boundaries affect the intensity and distribution of specific phonon modes?
• RQ3 Can atomic-resolution EELS detect and map localized phonon modes with sufficient energy and spatial resolution to confirm theoretical predictions?
• RQ4 To what extent do grain boundaries act as waveguides for selected phonon modes in silicon?
• RQ5 Is there quantitative agreement between experimentally measured phonon intensities and first-principles calculations of phonon densities of states at grain boundaries?

Key findings

• The 60 meV optic phonon mode exhibits strong intensity reduction at grain boundaries containing five- and seven-fold ring structures, indicating localized phonon modes.
• Experimental EELS intensity maps show excellent agreement with first-principles calculated phonon densities of states, validating the presence of localized modes.
• The suppression of phonon intensity correlates directly with deviations in bond angles from the bulk crystal structure at grain boundary sites.
• The spatial resolution of the EELS measurements resolves atomic-scale variations in phonon intensity across the grain boundary interface.
• The results confirm that grain boundaries with specific topological defects can act as waveguides for selected phonon modes, influencing thermal transport.
• The combined experimental and theoretical approach demonstrates the feasibility of directly observing localized phonon modes at atomic-scale defects with high precision.