  Talc is an insulating layered material that is stable at ambient conditions
and has high-quality basal cleavage, which is a major advantage for its use in
van der Waals heterostructures. Here, we use near-field synchrotron infrared
nanospectroscopy, Raman spectroscopy, and first-principles calculations to
investigate the structural and vibrational properties of talc crystals, ranging
from monolayer to bulk, in the 300-750 cm-1 and <60 cm-1 spectral windows. We
observe a symmetry crossover from mono to bilayer talc samples, attributed to
the stacking of adjacent layers. The in-plane lattice parameters and
frequencies of intralayer modes of talc display weak dependence with the number
of layers, consistent with a weak interlayer interaction. On the other hand,
the low-frequency (<60 cm-1) rigid-layer (interlayer) modes of talc are
suitable to identify the number of layers in ultrathin talc samples, besides
revealing strong in-plane and out-of-plane anisotropy in the interlayer force
constants and related elastic stiffnesses of single crystals. The shear and
breathing force constants of talc are found to be 66% and 28%, respectively,
lower than those of graphite, making talc an excellent lubricant that can be
easily exfoliated. Our results broaden the understanding of the structural and
vibrational properties of talc at the nanoscale regime and serve as a guide for
future ultrathin heterostructures applications.
