Thickness‐Dependent Elastic Softening of Few‐Layer Free‐Standing MoSe₂

Abstract

Few‐layer van der Waals (vdW) materials have been extensively investigated in terms of their exceptional electronic, optoelectronic, optical, and thermal properties. Simultaneously, a complete evaluation of their mechanical properties remains an undeniable challenge due to the small lateral sizes of samples and the limitations of experimental tools. In particular, there is no systematic experimental study providing unambiguous evidence on whether the reduction of vdW thickness down to few layers results in elastic softening or stiffening with respect to the bulk. In this work, micro‐Brillouin light scattering is employed to investigate the anisotropic elastic properties of single‐crystal free‐standing 2H‐MoSe2 as a function of thickness, down to three molecular layers. The so‐called elastic size effect, that is, significant and systematic elastic softening of the material with decreasing numbers of layers is reported. In addition, this approach allows for a complete mechanical examination of few‐layer membranes, that is, their elasticity, residual stress, and thickness, which can be easily extended to other vdW materials. The presented results shed new light on the ongoing debate on the elastic size‐effect and are relevant for performance and durability of implementation of vdW materials as resonators, optoelectronic, and thermoelectric devices.

Phonon transport in the gigahertz to terahertz range: Confinement, topology, and second sound

“…Nevertheless, the recent study of Babacic et al 14 on few- nanometer thick free-standing MoSe 2 membranes (Fig. 1a) shows that the intrinsic PE coupling of some low-dimensional materials can be exceptionally strong.…”

“…On the one hand, BLS can be used to reveal the elastic properties of nanomaterials like 0D nanospheres [41][42][43][44] , 1D nanowires ? or nanopillars 45,46 , and 2D nanosheets 14,47 . On the other hand, BLS can be used to study thermal transport [48][49][50][51] .…”

“…The conversion of light into motion is a separate challenge that can be addressed with various types of soft-matter photoactuators 87 . In the case of membrane-like systems, the additional merit of BLS is the ability to extract the strain from the dispersion relationship of flexural acoustic waves 14,47,65 (Fig. 2g).…”

“…The detection of acoustic phonons with light is a manifestation of optomechanical (OM) coupling. The OM coupling can be sufficiently strong for mechanical evaluation of single nanostructures [11][12][13][14] . Lightbased techniques can be categorized into frequency (GHz-THz) and time-domain (ns-ps) methods.…”

Excitation and detection of acoustic phonons in nanoscale systems

“…89 More recently, this technique has been applied to the investigation of other physical phenomena in diverse materials and structures. One of the main applications of BLS spectroscopy has been the detection of confined modes in a variety of nanostructures such as nanoporous alumina, 90 thin films, 17,91 nanospheres, 31 nanowires, 92 nanocubes, 93 2D materials, 94 and core-shell structures. 95 Additionally, there has been growing interest in the use of Brillouin scattering to investigate acoustic phonons in phononic crystals 29,30,96 and to detect guided modes in phononic waveguides.…”