Raman and Far-Infrared Synchrotron Nanospectroscopy of Layered Crystalline Talc: Vibrational Properties, Interlayer Coupling, and Symmetry Crossover

Abstract

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 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 on 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.

Phyllosilicates as earth-abundant layered materials for electronics and optoelectronics: Prospects and challenges in their ultrathin limit

“…Several experimental techniques are used to verify that exfoliated LMs were not structurally damaged when compared to the parent bulk material. 21,22,31,35,40 Mechanical exfoliation has been successfully applied to a variety of phyllosilicates, such as biotite, 37,40 serpentine, 31 phlogopite, 20 clinochlore, 27 talc [23][24][25][26][41][42][43] , muscovite, [44][45][46][47][48] and others, 17,37 reaching stable single or FL-flakes onto different substrates. Thus, we foresee that novel vertical vdWHs will be made by…”

“…The mechanical properties of talc have already been determined using first-principles methods, and properties such as breaking strength (σmax), elastic modulus, Poisson's ratio, and flexural stiffness were able to be obtained and compared with experimental results and other LMs. 23,26 From these calculations, it was possible to determine that talc has a σmax only 17% lower than graphene, the elastic modulus is approximately half of the value calculated for graphene, and 1L-talc is 30 times stiffer than graphene, with respect to flexural deformations. In another approach, the mechanical properties of clinochlore were also determined through hydrostatic compression and axial deformation analysis.…”

“…In this context, near-field optical microscopy (SNOM) techniques enabled the characterization of vibration modes in few to single layer crystals of conductors, semiconductors, and insulators LMs, as well as their vdWHs. 26,27,35,[134][135][136][137] Besides surpassing the light diffraction limit, SNOM-based techniques for optical nanospectroscopy are capable of signal amplification. Moreover, in the case of synchrotron IR nanospectroscopy (SINS), out-of-plane modes are accessed in single-crystals due to the specific light-matter interaction through the tipenhancement mechanism of signal.…”

“…Moreover, in the case of synchrotron IR nanospectroscopy (SINS), out-of-plane modes are accessed in single-crystals due to the specific light-matter interaction through the tipenhancement mechanism of signal. 24,26,35,137 Moreover, by combining point spectra with hyperspectral images with 25 nm resolution (typical tip size), the existence of impurities and defects could most likely be identified. Therefore, we foresee the use of more sophisticated SNOM-based tools in combination with first-principles calculations to assign the complete fingerprint of these LMs as a function of layers.…”

“…[17][18][19] Phyllosilicate minerals are wide bandgap insulators, stable in different environments, and abundant in nature. [17][18][19][20][21][22][23][24][25][26][27] They present a layered structure that enables their exfoliation down to 1L. 20,23,28,29 The variety of phyllosilicate specimens arises from ionic substitutions with different geometric arrangements of atoms within the crystal.…”