Polarized and non‐polarized Raman spectroscopy of ZnO crystals: Method for determination of crystal growth and crystal plane orientation for nanomaterials

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Abstract
Analysis and determination of crystal orientation and exposed surface facets remain a challenge in nanomaterial science. In this work, we show that polarized and non‐polarized Raman spectroscopy can be useful tools to determine crystal plane orientation and conveniently be applied to spatial dimensions limited only by the diffraction limit of the excitation laser. The methodology is exemplified for wurtzite structured ZnO. Three different crystal facets, (0001), (1 00), and (11 0), of ZnO are investigated with angle‐resolved polarized Raman spectroscopy. The polarization direction dependences of the main Raman peaks are characterized and related to the experimental vibrational modes in the crystal lattice and corroborated by density functional theory (DFT) calculations using two different hybrid functionals. By exploiting the symmetry of the modes and differences in Raman intensity of the optically activated phonons, a simple model is derived for determining the relation between the polar and non‐polar crystal orientation. The results are generalized to allow peak intensity ratio analysis using Raman spectroscopy with a non‐polarized light source, making it compatible with Raman mapping, as well as to include a critical discussion on the ability to determine the crystal plane orientation and exposed crystal facets using this model for nanodimensional ZnO and equivalent models for other nanomaterials. As the approach allows for use of non‐polarized light sources, near‐field excitations and local plasmons can in an extension be utilized for the determination of crystal orientation and exposed planes in dimensions much smaller than the diffraction limit.
Funding Information
  • Svenska Forskningsrådet Formas (FORMAS‐2016‐00908)

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