Conventional phonon Raman spectroscopy is a powerful experimental technique for the study of crystalline solids that allows crystallography, phase and domain identification on length scales down to 1 mm.
Scientists from the US, University of Washington, State University of New York at Stony Brook, and Brookhaven National Laboratories [Berweger DOI: 10.1038/NNANO.2009.190] have demonstrated the extension of tip-enhanced Raman spectroscopy to optical crystallography on the nanoscale by identifying intrinsic ferroelectric domains of individual BaTiO3 nanocrystals through selective probing of different transverse optical phonon modes.
The technique is generally applicable for most crystal classes, and for example, structural inhomogeneities, phase transitions, ferroic order and related finite-size effects occurring on nanometre length scales can be studied with simultaneous symmetry selectivity, nanoscale sensitivity and chemical specificity.
Tip-enhanced Raman spectroscopy (TERS) achieves spatial resolution in the nanometre range. Although previously used in a specific tip-enhanced geometry to maximize the near-field contrast, the full potential offered by the symmetry selectivity of the Raman response has not yet been explored. In this study the scientists derive and experimentally demonstrate the tensor-based phonon Raman selection rules that will enable TERS to determine crystallographic orientation in general, and the ferroelectric order in particular, of crystalline solids, with nanometre spatial resolution.
As BaTiO3 has a variety of potential nanoscale devices based on its ferroelectric properties including piezoelectric actuators and non-volatile memory, Berweger and his team used this as a specific example.
The results demonstrate the capability of TERS for imaging crystalline nanostructures based on the optical phonon response. With the Raman tensor reflecting the crystal symmetry, selective probing of specific phonon modes in certain scattering geometries then allows for determination of the crystal orientation and associated ferroelectric order.
Ferroelectric domains arise from an interplay between the depolarization energy lowered by the formation of domains and the increased energy associated with domain walls. Results show that, despite crystal sizes of only several hundreds of nanometres and as such comparable to or smaller than typical domains found for bulk dielectrics, these nanocrystals are not necessarily single domain. Although a-domains have previously been artificially induced in chemically synthesized BaTiO3 nanorods our study provides evidence of spontaneous domain formation in such structures. This may indicate finite size effects on ferroelectric ordering due to the large surface-to-volume ratio by, for example, pinning due to surface defects, the possible influence of adsorbates, and the influence of the surface free energy in general and its spatial variation with crystal shape and size.