Piezoreflectance Spectroscopy of Optical Transitions in van der Waals Layered Crystals

My research developed piezoreflectance spectroscopy to identify direct optical transitions in layered semiconductors and understand strain-driven spectral changes. This method is suitable for micrometer scale optical studies of layered semiconductors, including bulk and few-layer van der Waals crystals. To begin, obtain the bulk material of a van der Waals's crystal such as tungsten disulfide or WS2 for optical characterization.

Thin the material by repeatedly cleaving it using adhesive tape until the preferred thickness is reached. Transfer the thin fragments from the tape to a polydimethylsiloxane, or PDMS stamp, by placing the stamp onto the tape. After approximately five minutes, peel back the PDMS so that flakes retained on the PDMS can be used for subsequent transfer.

Clean the piezoceramic surface by rinsing it with acetone and dry the surface using clean, dry air or nitrogen. Apply a very thin layer of an adhesive medium to the piezoceramic. Immediately place the PDMS, carrying the flake onto the wet adhesive on the piezoceramic.

Press gently to ensure contact. Wait approximately 30 seconds and peel off the PDMS so that the material remains on the piezoceramic. Verify adhesion by visual inspection or confirm under an optical microscope if needed.

Apply a silver paste to secure the piezoceramic and place the prepared piezoceramic containing the transferred sample on the stage to ensure electrical contact with the bottom electrodes of the piezoceramic. Form the top electrical contact using silver paste and copper wire. After applying the silver paste, allow it to dry and cure at room temperature for approximately 10 hours before proceeding.

Then insert the mounted sample into the optical setup. Connect the electrodes to the alternating current or the AC voltage generator and to the ground using insulated wires. Keep all exposed conductors insulated and positioned away from the optical path.

Turn on the halogen light source and set it to maximum output. Ensure that the optical chopper is not obstructing the monochromator entrance slit. Route the beam to the charge coupled device camera and align the spot on the selected region of the sample using the XYZ microcontrollers.

Then adjust the iris diaphragm aperture to set the spot diameter on the sample. Switch the signal path from the charge couple device preview to the silicon pin photo diode detector by removing the beam splitter from the optical path. Next, configure the pre-amplifier by setting the bias voltage, filter type, and cutoff.

Input offset, gain mode, and sensitivity to achieve a stable signal without saturation. Document the selected values. Configure the lock-in amplifier to display the in-phase X component.

Set the reference frequency for the AC voltage generator and select the internal reference source. Then choose an appropriate time constant and sensitivity. Enable the AC voltage generator and set the desired amplitude.

Verify that the signal reaches the piezoceramic without overdriving the device. Launch the acquisition application and select the measurement mode corresponding to the X lock-in output. Enter hardware consistent parameters, including a time constant representing the sampling rate, and specify the path to the output text file to save the data.

Define the spectral range in wavelength or photon energy and set the monochromator step size according to the instrument's limits. Initialize the devices, start the scan, and allow it to continue uninterrupted until the end of the programmed range. Estimate the total duration by multiplying the number of steps by the time constant, and plan accordingly.

After completion, turn off the AC voltage generator without disturbing the sample position. Confirm that the AC voltage generator remains off. Keep the beam position on the sample without alteration.

To ensure the focus is maintained, route the beam to the charge coupled device preview. Refocus and recenter the spot. Then return the signal path to the photo diode.

Start the optical chopper using its controller and set the frequency equal to the previously used modulation frequency. Verify that the chopper reference output is connected to the lock-in reference input. Configure the lock-in amplifier to display the referenced reflectance R, and select the chopper controller as the external reference source.

Adjust the time constant and sensitivity to maintain a stable signal while keeping the pre-amplifier settings unchanged. In the acquisition application, select the measurement mode representing lock-in R channel. Match the time constant to the lock-in setting and define a new output text file path.

Name the files to encode the sample, location, and acquisition parameters for pairing data sets. Set the same spectral range and monochromator step size that were used during the measurement of the delta R component for that spot on the sample. Start the scan and allow it to finish completely.

After completion, turn off the optical chopper. If no further measurements are planned, turn off the light source, monochromator, and other instruments. After labeling the data sets, compute and plot the normalized delta R divided by R spectrum versus photon energy to identify optical transitions.

Finally, fit the spectrum using the Aspnes line shape model by selecting an energy range that includes the resonance, iteratively refining parameters, and validating the fit based on residuals and stability. The strain modulated AC component delta R and the DC component proportional to reflectance R were measured for a single spot in tungsten disulfide. The baseline corrected Delta R divided by R spectrum showed clear exotonic resonances.

The Aspnes third derivative fit reproduced the delta R divided by R spectrum. The model was decomposed into four components and their sum was shown by a dashed line. The delta R line shape was preserved while the amplitude increased monotonically as the alternating current voltage was increased from 100 to 1, 200 volts.

The peak magnitudes at selected energies scaled linearly with the applied voltage amplitude. Increasing the lock-in time constant from three seconds to 10 seconds and 30 seconds reduced high frequency noise and revealed weaker features in molybdenum disulfide. It allows researchers to detect strain responsive direct optical transitions in those materials with enhanced sensitivity.

The key challenge is maximizing signal to noise ratio while ensuring efficient strain transfer and stable sample mounting for reliable optical measurements. Future studies can extend this method to monolayers and heterostructures to explore strain effects and optical transitions in more complex systems.