Strain Modulation in van der Waals Crystals Directly Exfoliated onto Piezoceramics─Application in Micropiezoreflectance Spectroscopy

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