Abstract
Mesoscale thermal transport is of fundamental interest and practical importance in materials such as thermoelectrics. Coherent lattice vibrations (acoustic phonons) govern thermal transport in crystalline solids and are affected by the shape, size, and defect density in nanoscale materials. The advent of hard x-ray free electron lasers (XFELs) capable of producing ultrafast x-ray pulses has significantly impacted the understanding of acoustic phonons by enabling their direct study with x-rays. However, previous studies have reported ensemble-averaged results that cannot distinguish the impact of mesoscale heterogeneity on the phonon dynamics. Here we use Bragg coherent diffractive imaging (BCDI) to resolve the 4D evolution of the acoustic phonons in a single zinc oxide rod with a spatial resolution of 50 nm and a temporal resolution of 25 picoseconds. We observe homogeneous (lattice breathing/rotation) and inhomogeneous (shear) acoustic phonon modes, which are compared to finite element simulations. We investigate the possibility of changing phonon dynamics by altering the crystal through acid etching. We find that the acid heterogeneously dissolves the crystal volume, which will significantly impact the phonon dynamics. In general, our results represent the first step towards understanding the effect of structural properties at the individual crystal level on phonon dynamics.
Highlights
In crystalline solids, atomic positions deviate from their equilibrium positions under external stimuli such as strain and temperature
The real space image is complex: the amplitude is proportional to the diffracting or Bragg electron density[22] and the phase is proportional to a projection of the atomic displacement field onto the measured scattering vector[15, 23, 24]
The different orientations of the crystals on the substrate allow Bragg peaks from individual crystals to be isolated on an x-ray sensitive area detector (Fig. 1b)
Summary
Atomic positions deviate from their equilibrium positions under external stimuli such as strain and temperature. Bragg coherent diffractive imaging (BCDI) with hard x-rays[15, 16] is uniquely suited to probing atomic motions, thereby resolving acoustic phonons, at the individual crystal level with nanometer resolution. The real space image is complex: the amplitude is proportional to the diffracting or Bragg electron density[22] and the phase is proportional to a projection of the atomic displacement field onto the measured scattering vector[15, 23, 24]. The Bragg electron density can be used to identify crystal regions of a different phase[25, 26] while the atomic displacement field can be used to identify dislocations and reveal the strain distribution[26,27,28,29,30,31].
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