Strain Modulation\nin van der Waals Crystals Directly\nExfoliated onto PiezoceramicsApplication in Micropiezoreflectance\nSpectroscopy

The van der Waals (vdW) crystals and heterostructures,\nknown for\nmany intriguing physical properties, have garnered significant interest\nin recent years. Developing effective optical methods to study their\nproperties is still urgent for fundamental research as well as simple\nmaterial characterization. Among these methods, modulation spectroscopy\nstands out, as it offers the advantage of easily obtaining information\non direct optical transitions related to extended states while not\nbeing sensitive to defect-related states. In our work, we successfully\napplied piezoreflectance spectroscopy (strain modulation) to study\nvdW flakes and atomic-thick vdW layers with an approach that eliminates\nthe need to attach these crystals to the piezoceramics with an adhesive.\nThe strain variation in vdW flakes was achieved via direct exfoliation\nof crystal onto piezoceramic substrate and modulation of piezoceramic\ndimensions by an external voltage. The principle of operation of this\napproach is the electrostatic interaction between the vdW layer and\nthe piezoceramic substrate, which in the nanoscale can be treated\nas a vdW interaction between the atoms of the vdW layer and the atoms\nof the piezoceramic substrate. This work provides an effective method\nto study direct optical transitions in exfoliated vdW layers and reveals\nthe nature of the interaction between the vdW layer and the substrate.