Piezoreflectance Spectroscopy of Optical Transitions in van der Waals Layered Crystals

The goal of this protocol is to enable precise identification of direct optical transitions in van der Waals semiconductors by piezoreflectance spectroscopy. The method applies a small periodic strain modulation to a sample mounted on a piezoelectric transducer and detects the reflectance change with a lock-in amplifier, yielding derivative-like spectra in which a background signal is eliminated, and band-edge features become prominent. The workflow details assembly of a single-path, broadband optical setup (halogen source, monochromator, microscope optics, silicon photodiode), integration of a low-noise preamplifier and lock-in electronics, and safe high-voltage connection to the piezoceramic. The stepwise instructions cover exfoliation and transfer of flakes, application of ultrathin adhesive, silver-paste electrical contacts with room-temperature curing, and acquisition of the AC component (ΔR) and DC component (R) - reflectance referenced by chopper. Data processing includes baseline correction, computation of ΔR/R, and fitting with standard third-derivative line shapes to extract transition energies, broadenings, phases, and relative strengths. Representative settings and troubleshooting guidance optimize signal-to-noise ratio while preserving spectral accuracy. Compared with reflectance contrast and photoreflectance, this approach improves results for strain-responsive transitions and remains effective when intrinsic electric-field modulation is weak. The protocol supports micrometer-scale mapping and compatibility from thin layers to bulk, and it is encouraged to complement with photoluminescence or Raman measurements, providing a robust, generalizable route to optical characterization of layered semiconductors.