Name: Piezoreflectance Spectroscopy of Optical Transitions in van der Waals Layered Crystals (Video) | JoVE Journal
Uploaded: 2026-05-22T12:50:00.000+00:00
Duration: 9 min 6 s
Description: Learn piezoreflectance spectroscopy for identifying direct optical transitions in van der Waals semiconductors using strain modulation. This method enables precise band-edge characterization with enhanced sensitivity and micrometer-scale mapping.

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5 minutes

This study was supported by a grant from the Polish National Science Center (PRELUDIUM BIS 4&#160;,
Project No. 2022/47/O/ST3/01965).

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<li>Philipp, H. R., Dash, W. C., Ehrenreich, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezoreflectance+in+Ge%5BTitle%5D)+AND+%22Phys+Rev%22%5BJournal%5D)">Piezoreflectance in Ge.</a> Phys Rev. 127, 762-765 (1962).</li>
<li>Pollak, F. H., Cardona, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezo-Electroreflectance+in+Ge%2C+GaAs%2C+and+Si%5BTitle%5D)+AND+%22Phys+Rev%22%5BJournal%5D)">Piezo-Electroreflectance in Ge, GaAs, and Si.</a> Phys Rev. 172, 816(1968).</li>
<li>Merski, J., Eckhardt, C. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezomodulation+spectroscopy+of+molecular+crystals.+I.+Methods+and+principles%5BTitle%5D)+AND+%22J+Chem+Phys%22%5BJournal%5D)">Piezomodulation spectroscopy of molecular crystals. I. Methods and principles.</a> J Chem Phys. 75, 3691-3704 (1981).</li>
<li>Mathieu, H., Allègre, J., Gil, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezomodulation+spectroscopy%3A+A+powerful+investigation+tool+of+heterostructures%5BTitle%5D)+AND+%22Phys+Rev+B%22%5BJournal%5D)">Piezomodulation spectroscopy: A powerful investigation tool of heterostructures.</a> Phys Rev B. 43, 2218(1991).</li>
<li>Yu, C. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Multiple+confined-state+transitions+within+surface+quantum+dots+by+a+piezomodulation+reflectance+study%5BTitle%5D)+AND+%22Phys+Status+Solidi+%28A%29+Appl+Mater+Sci%22%5BJournal%5D)">Multiple confined-state transitions within surface quantum dots by a piezomodulation reflectance study.</a> Phys Status Solidi (A) Appl Mater Sci. 204, 1171-1177 (2007).</li>
<li>Engeler, W. E., Fritzsche, H., Garfinkel, M., Tiemann, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High-Sensitivity+Piezoreflectivity%5BTitle%5D)+AND+%22Phys+Rev+Lett%22%5BJournal%5D)">High-Sensitivity Piezoreflectivity.</a> Phys Rev Lett. 14, 1069(1965).</li>
<li>Garfinkel, M., Tiemann, J. J., Engeler, W. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezoreflectivity+of+the+Noble+Metals%5BTitle%5D)+AND+%22Phys+Rev%22%5BJournal%5D)">Piezoreflectivity of the Noble Metals.</a> Phys Rev. 148, 695(1966).</li>
<li>Sell, D. D., Kane, E. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezoreflectance+of+Germanium+from+1.9+to+2.8+eV%5BTitle%5D)+AND+%22Phys+Rev%22%5BJournal%5D)">Piezoreflectance of Germanium from 1.9 to 2.8 eV.</a> Phys Rev. 185, 1103(1969).</li>
<li>Gerhardt, U., Beaglehole, D., Sandrock, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezo-Optical+Constants%2C+Deformation+Potentials%2C+and+the+Electronic+Structure+of+Copper%5BTitle%5D)+AND+%22Phys+Rev+Lett%22%5BJournal%5D)">Piezo-Optical Constants, Deformation Potentials, and the Electronic Structure of Copper.</a> Phys Rev Lett. 19, 309(1967).</li>
<li>Balslev, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Apparatus+for+Combined+Static+and+Dynamic+Uniaxial+Stress%5BTitle%5D)+AND+%22Rev+Sci+Inst%22%5BJournal%5D)">Apparatus for Combined Static and Dynamic Uniaxial Stress.</a> Rev Sci Inst. 38, 1528-1529 (1967).</li>
<li>Sell, D. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Review+of+piezomodulation+spectroscopy%5BTitle%5D)+AND+%22Surf+Sci%22%5BJournal%5D)">Review of piezomodulation spectroscopy.</a> Surf Sci. 37, 896-913 (1973).</li>
<li>Sebayang, K., Sigiro, M. Piezoreflectance study of Nb-doped MoS2 single crystals. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aSebayang%2C+K+%22IOP+Conf+Seri+Mater+Sci+Eng%22">IOP Conf Seri Mater Sci Eng</a>. , Institute of Physics Publishing. (2017).</li>
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<li>Gerhardt, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Piezoreflectance+in+Si%5BTitle%5D)+AND+%22Phys+Lett%22%5BJournal%5D)">Piezoreflectance in Si.</a> Phys Lett. 9, 117-118 (1964).</li>
<li>Kopaczek, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Temperature+Dependence+of+the+Indirect+Gap+and+the+Direct+Optical+Transitions+at+the+High-Symmetry+Point+of+the+Brillouin+Zone+and+Band+Nesting+in+MoS2%2C+MoSe2%2C+MoTe2%2C+WS2%2C+and+WSe2Crystals%5BTitle%5D)+AND+%22J+Phys+Chem+C%22%5BJournal%5D)">Temperature Dependence of the Indirect Gap and the Direct Optical Transitions at the High-Symmetry Point of the Brillouin Zone and Band Nesting in MoS2, MoSe2, MoTe2, WS2, and WSe2Crystals.</a> J Phys Chem C. 126, 5665-5674 (2022).</li>
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 <a target="_blank" href="https://www.piceramic.com/en/expertise/piezo-technology/piezoelectric-materials">https://www.piceramic.com/en/expertise/piezo-technology/piezoelectric-materials</a> (2026).</li>
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The authors have nothing to disclose.

Here we present piezoreflectance (PzR) spectroscopy for van der Waals crystals, a protocol that uses periodic strain from a piezoceramic transducer to produce derivative-like reflectance spectra and isolate critical points with high contrast. Compared with reflectance contrast (RC), the method sharpens weak or overlapping resonances and extends to cases where RC provides ambiguous contrast, while retaining a single broadband optical path and compatibility from monolayers to bulk; the primary tradeoffs are the lack of abs...

Piezoreflectance spectroscopy (PzR) is a strain&#8209;modulation variant of modulation spectroscopy that exploits well&#8209;established light-matter interactions to extract with high precision excitonic and band-to-band transitions energies1. In PzR, the sample is mounted on a piezoelectric transducer; an applied alternating voltage generates a small periodic strain (&#948;a/a) that perturbs the crystal lattice and modulates the electronic band structure, primarily the band gap. This induces synchronous changes in the complex dielectric function and hence in the reflectance, &#916;R, which are detected with phase&#8209;sensitive (lock&#8209;in) methods to yield derivative&#8209;like line shapes with the slowly varying background strongly suppressed. The objective is to enable quantitative optical characterization of van der Waals (vdW) crystals and semiconductor microstructures - including superlattices, quantum wells, and heterojunctions - by revealing weak band&#8209;edge features that are often invisible in conventional reflectance contrast (RC) and by providing a controlled handle on strain-exciton coupling relevant to advanced electronic and optoelectronic applications2,3,4,5,6. The protocol is compatible with micrometer&#8209;scale flakes and can be co&#8209;registered with photoluminescence or Raman measurements for multimodal analysis under tunable strain. In addition, we are investigating an adhesive&#8209;free configuration in which vdW crystals are exfoliated directly onto piezoceramics to measure micropiezoreflectance (&#181;PzR) in micron-sized vdW flakes7.
Compared to reflectance contrast, usually applied to vdW crystals and heterostructures8,9, piezoreflectance offers several practical and analytical advantages. Because the lock-in detected PzR signal is intrinsically normalized to &#916;R/R, the spectral line shape is preserved even when absolute throughput drifts1, provided the photon flux is sufficient for a good signal-to-noise ratio. The derivative nature of modulation spectra suppresses slowly varying backgrounds and sharpens optical transitions related to a critical point in the total optical density of states, which improves energy resolution and enables reliable extraction of even weak interband excitonic and band-to-band transitions11,12. PzR measurements selectively probe only those transitions whose parameters (transition energy E, intensity I, or broadening &#915;) respond to the applied strain; consequently, spectral regions that are strain-insensitive yield no signal, while truly responsive transitions stand out with high contrast1,13,14. On the other hand, RC relies on subtracting two independently measured reflectance spectra (sample and reference usually coming from substrate), so any small mismatch or drift translates into a large, nonzero baseline across the entire spectral range; weak transitions can therefore be obscured by this background, a limitation explicitly demonstrated in materials where RC fails to resolve interband features that remain prominent in PzR (e.g., FePS&#8323; and NiPS&#8323;), while strong transitions in MoS&#8322; remain visible by either method7.
As a stress-modulated counterpart to conventional reflectance, PzR defines a distinct branch of modulation spectroscopy in which periodic uniaxial or coplanar stress isolates derivative-like features directly linked to deformation potentials and symmetry selection rules1,15,16,17,18,19,20,21,22,23,24. Situated within six decades of modulation spectroscopy, piezomodulated reflectance has evolved from its 1960s first application in measurements of group-IV/III&#8211;V semiconductors into a versatile probe of deformation potentials, carrier character, and excitonic structure across bulk crystals, heterostructures, and vdW materials1,15,16,18,19,25,26,27. PzR emerged in the early 1960s with landmark demonstrations of piezoreflectance in Ge15 and Si28, as well as piezo-electroreflectance across Ge, GaAs, and Si. Foundational theory/experiment in the early 1970s consolidated the formalism and provided comprehensive reviews1,25. A subsequent renaissance established PzR as a direct probe of deformation potentials in the elastic limit and as a vibronic-state-sensitive technique17, which catalyzed applications to strained heterostructures in the 1990s &#8211; resolving electron/hole character and spatial localization in epilayers, quantum wells, and superlattices under controlled uniaxial or coplanar stress geometries18. In the 2000s and beyond, the scope extended to semiconductor nanostructures and van der Waals (vdW) materials - capturing confined-state transitions in surface quantum dots19 and probing electronic band structures in TMDs and alloys26,27. A unifying thread across this literature is the application of a well-controlled, periodically modulated stress: via piezoelectric transducers on single crystals, on evaporated metal films21,22, or by low-frequency bending of ionic and metallic crystals (e.g., KI, KBr, Cu)23, complemented by an apparatus enabling combined static and dynamic uniaxial stress24. Building on this corpus, our method targets open opportunities for vdW crystals and heterostructures &#8211; probing excitonic transitions that can be weak or invisible in reflectance contrast, enabling stress coupling, and integrating PzR with PL measurements or Raman spectroscopy for multimodal, strain-resolved characterization19,26,27. Therefore, readers can position PzR alongside and in combination with established optical probes when judging suitability for their systems.

<table><tbody><tr><td>Acetone (analytical grade)</td><td>Sigma-Aldrich / Merck</td><td>650501</td><td></td></tr><tr><td>CCD Camera</td><td>e.g. Hamamatsu Photonics / Princeton Instruments / Thorlabs</td><td>N/A</td><td></td></tr><tr><td>Computer with control and data acquisition software (e.g., LabVIEW)</td><td>National Instruments / Custom setup</td><td>N/A</td><td></td></tr><tr><td>DSP Lock-In Amplifier</td><td>Stanford Research Systems (SRS)</td><td>SR810</td><td></td></tr><tr><td>EKE Halogen Lamp, 150 W 21 V (GX5.3 base)</td><td>OSRAM</td><td>93638</td><td>Used as the replacement bulb for the Thorlabs OSL1-EC Fiber Illuminator.</td></tr><tr><td>Gel-Film PDMS-based viscoelastic film, 4.0&amp;Prime;&amp;times;4.0&amp;Prime; sheet</td><td>Gel-Pak</td><td>WF-40&amp;times;40-0060-X4</td><td></td></tr><tr><td>High Intensity Fiber Light Source (halogen lamp) - 150 W</td><td>Thorlabs</td><td>OSL1-EC&amp;nbsp;</td><td>The model is listed as discontinued (superseded by OSL2) as of 07 Jan 2014.</td></tr><tr><td>High Voltage Rectangular Signal Generator</td><td>Custom-made in our laboratory</td><td>HVG-3</td><td></td></tr><tr><td>Low-Noise Current Preamplifier</td><td>Stanford Research Systems (SRS)</td><td>SR570</td><td></td></tr><tr><td>M Plan APO NIR 50&amp;times; Objective Lens</td><td>Mitutoyo Corporation</td><td>378-825-17</td><td>Other objectives with suitable magnification and spectral transmission can also be used depending on the optical setup.</td></tr><tr><td>Monochromator / Imaging Spectrograph, focal length 320 mm - Zolix Omni-&amp;lambda;300i</td><td>Zolix Instruments Co., Ltd.</td><td>Omni-&amp;lambda;300i</td><td></td></tr><tr><td>Optical Chopper Controller</td><td>Stanford Research Systems (SRS)</td><td>SR540</td><td></td></tr><tr><td>Optomechanical and optical components (lenses, mirrors, irises, optical chopper, beam splitters, optical fibers, and mounts)</td><td>Thorlabs / custom-made</td><td>Various</td><td></td></tr><tr><td>Piezoelectric Ceramic Disk (&amp;Oslash;30 &amp;times; 1 mm, PZT-5O, silver electrodes)</td><td>He-Shuai Ltd.</td><td>N/A</td><td></td></tr><tr><td>PVC Surface Protection Low-Adhesion Cleanroom Tape (blue)</td><td>Cleanroom Supply</td><td>N/A</td><td></td></tr><tr><td>Silicon PIN Photodiode</td><td>e.g. Hamamatsu Photonics / Thorlabs&amp;nbsp;</td><td>N/A</td><td>Other types of photodetectors can be employed depending on the spectral range of interest.</td></tr><tr><td>Silver conductive varnish (&amp;ldquo;Leitlack&amp;rdquo;)</td><td>Busch GmbH &amp;amp; Co. KG</td><td>5900</td><td></td></tr><tr><td>Transparent nail polish</td><td>Revlon, Inc.</td><td>N/A</td><td>Used as an adhesive layer for fixing vdW flakes. Any clear nail polish of similar composition can be used as an alternative.</td></tr><tr><td>Tungsten Disulfide (WS&lt;sub&gt;2&lt;/sub&gt;), CAS 12138-09-9</td><td>HQ Graphene</td><td>N/A</td><td>sample material</td></tr></tbody></table>

piezoreflectance spectroscopy of optical transitions in van der waals layered crystals

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 (&#916;R) and DC component (R) &#8211; reflectance referenced by chopper. Data processing includes baseline correction, computation of &#916;R/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.

Lock-in piezoreflectance (PzR) yields derivative-like signatures at direct optical transitions across a broad set of van der Waals (vdW) crystals and semiconductor microstructures. Figure 6 shows typical raw output from a single spot in WS₂ acquired with the workflow in Sections 4–6: strain-modulated AC reflectance ΔR, reference DC component proportional to reflectance R (Figure 6A), and the baseline-corrected ratio ΔR/R (Figure 6B). With a piezoceramic disk...

We present a reproducible strain-modulation piezoreflectance spectroscopy protocol that enables measurement of &#916;R/R with lock-in detection for van der Waals crystals transferred onto piezoceramics, and results with characterization of direct optical transitions through &#916;R/R spectra utilizing third-derivative fitting with the Aspnes formula.

The goal of this protocol is to enable precise identification of direct optical transitions in van der Waals semiconductors by piezoreflectance ...

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.

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Research

JoVE Journal

Engineering

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