Tiny Semiconductor Dots Respond Strongly to Material Strain

Researchers are increasingly focused on harnessing the potential of two-dimensional semiconductors for advanced photonic applications, and a new study led by Sumitra Shit and Yunus Waheed, from the School of Physical Sciences at the Indian Institute of Technology Goa, details a significant advancement in controlling their optoelectronic properties. Working with colleagues at the Research Center for Electronic and Optical Materials and the Research Center for Materials Nanoarchitectonics, both at the National Institute for Materials Science, the team investigated the strain sensitivity of quantum dots within monolayer tungsten diselenide and tungsten disulfide. Their findings reveal that these quantum dots exhibit markedly enhanced sensitivity to strain, up to four times greater than delocalized excitons, due to strengthened interactions with phonons, offering a versatile approach to spectral matching for a wide range of photonic devices and deepening our understanding of fundamental phonon-quantum dot interactions in these materials.

For decades, controlling light at the nanoscale has demanded ever-finer precision. Now, a method to sensitively tune the colour of quantum dots using simple stretching has been demonstrated. Promising a new way to build and connect tiny optical components for future technologies. Scientists are increasingly focused on two-dimensional semiconductors as building blocks for future quantum photonic networks.

These materials, particularly transition-metal dichalcogenides (TMDs), offer a pathway to create quantum dots (QDs), nanoscale semiconductors exhibiting unique optical properties. A central challenge lies in achieving uniform optical characteristics across many individual dots, a necessity for scalable quantum technologies. Current QD emission wavelengths vary considerably, hindering their integration into complex networks alongside other quantum systems.

Therefore, methods to precisely tune the emission wavelengths of these QDs are highly sought after. Scientists have demonstrated a flexible approach to control QD emission through strain engineering, manipulating the material’s structure to alter its optical properties. Investigations combining spatially resolved micro-photoluminescence (μ-PL) spectroscopy with micro-Raman spectroscopy reveal a pronounced sensitivity of QD emission energies to applied strain.

Experiments conducted at cryogenic temperatures (as low as 4 K) up to room temperature (296 K) examined thousands of individual QDs within both monolayer tungsten diselenide (ML-WSe₂) and tungsten disulfide (ML-WS₂), integrated into various heterostructures and a piezoelectric device. The sensitivity to strain isn’t uniform across all parts of the material.

QDs in both materials show approximately four times greater sensitivity to strain in WS₂ and twice the sensitivity in WSe₂ compared to broader, delocalized excitons. This heightened sensitivity results in a noticeable broadening of the overall emission spectrum. Further temperature-dependent μ-PL spectroscopy, coupled with active strain adjustments, indicates that this enhanced response stems from stronger interactions between the confined electrons within the QDs and low-energy phonons. Vibrational modes within the crystal lattice.

The team defined and tuned QDs by creating localized strain pockets using spherical silica nanoparticles (SNPs), e-beam-deposited silicon oxide nanoparticles (ENPs), and nanodroplets. Exploration of strain ranges from -0.10% to 0.75% for ML-WS₂ QDs and 0.05% to 0.20% for ML-WSe₂ QDs revealed that the emission energies of the QDs exhibit larger strain-induced shift rates than delocalized excitons. These findings suggest a pathway toward spectral matching across diverse quantum photonic platforms, opening possibilities for more complex and integrated quantum systems.

Quantum dot strain sensitivity correlates with nanoparticle positioning and broadens emission spectra

Thousands of individual quantum dots (QDs) in monolayer tungsten diselenide (WSe₂) and tungsten disulfide (WS₂) exhibited enhanced strain sensitivities compared to delocalized excitons. QDs in WS₂ displayed approximately fourfold greater sensitivity. Meanwhile, those in WSe₂ showed a twofold increase in their emission energies responding to strain — at low temperatures (4 Kelvin), spatial maps of micro-photoluminescence revealed highly redshifted QDs exclusively at locations with engineered nanoparticles. Indicating a direct correlation between nanoparticle placement and emission wavelength.

Ensemble emission spanning the 1.81, 2.08 eV range was observed across the four sample structures investigated. With Gaussian fits confirming the broad distribution of QD emission energies. Here, the full width at half maximum (FWHM) of the ensemble emission broadened considerably due to the increased strain sensitivity. Measurements from ML-WS₂ QDs on spherical SiO₂ nanoparticles showed an energy span of approximately 292 meV, demonstrating a substantial spectral range.

In turn, scientists determined that the heightened strain sensitivity of individual QDs originates from strengthened interactions with low-energy phonons, a consequence of quantum confinement. Exploration of strain ranges from -0.10% to 0.75% for ML-WS₂ QDs and 0.05% to 0.20% for ML-WSe₂ QDs revealed a direct link between applied strain and shifts in emission wavelengths.

Active strain tuning experiments and temperature-dependent μ-PL the pronounced strain sensitivity, and the resulting broadening of emission lines, are attributable to enhanced exciton-phonon interactions induced by quantum confinement. Histograms of QD emission energies illustrated the wide spectral distribution within samples containing engineered nanoparticles as nanostressors.

Cryogenic strain mapping via low-temperature micro-photoluminescence and nanoscale protrusion analysis

Micro-photoluminescence (μ-PL) spectroscopy served as the primary technique for examining strain-dependent emission energies from individual quantum dots (QDs) within monolayer transition-metal dichalcogenides. Measurements were performed across a cryogenic temperature range, spanning from 4.94 K to room temperature, and complemented by room temperature micro-Raman spectroscopy.

This combination allowed for detailed analysis of thousands of QDs in both monolayer tungsten diselenide (ML-WSe) and monolayer tungsten disulfide (ML-WS), integrated into various heterostructures and a piezoelectric device. By employing μ-PL, researchers could spatially resolve the emission from individual QDs, enabling precise determination of their energies under different strain conditions.

Quantifying strain under cryogenic conditions necessitated additional measurements. To assess thermoelastic strain relaxation upon cooling0.2D-X0 emission energies were measured at numerous nanoscale protrusions (NPs) and flat regions of ML-WS at 4 K for samples S1 and S4. Average temperature-induced X0 energy shifts were evaluated, comparing energies at 4 K and 296 K for both NP and flat regions.

Local strains at room temperature were estimated using μ-PL emission acquired at all NP locations in each sample. With micro-Raman spectroscopy confirming the PL-derived strain within a 0.18% standard deviation. Analysing the optical quality of excitons required careful consideration of layer structures. Different layer structures, incorporating ML-TMDs, were used to produce samples with varying average local strain at the NP locations.

PL intensity maps were obtained for samples S1 and C1, revealing multiple bright NP locations, each exhibiting several emission lines — the team directly measured strain relaxation by comparing average local strains at 4 K and room temperature for samples S1 and S4. Rather than relying solely on theoretical models. Bin counts from histogram plots of QD emission energies for samples S1, S2, S4, and C1 were fitted with Gaussian functions to extract ensemble peak energies and full width at half maximum (FWHM) linewidths.

Mechanical strain precisely tunes quantum dot emission for advanced photonics

Meanwhile, scientists are increasingly focused on manipulating light at the nanoscale. At the same time, this effort presents a refined method for controlling the colour of light emitted by tiny semiconductor crystals. To achieve precise control over these light-emitting quantum dots has been hampered by their sensitivity to even minor imperfections and environmental factors. Here, scientists have demonstrated a way to predictably tune the emission colour using mechanical strain, offering a pathway towards more stable and reliable photonic devices.

In turn, the implications extend beyond simply shifting colours. By carefully applying strain, these two-dimensional materials can be ‘matched’ to other light sources — creating opportunities for integrated photonic circuits and potentially even more efficient light-emitting diodes. A key challenge remains in scaling up this process, as working with individual quantum dots is precise. But building devices requires controlling many simultaneously.

Unlike previous approaches relying on material composition, strain offers an external tuning knob, potentially simplifying manufacturing. Meanwhile, the observed enhancement in strain sensitivity arises from the interaction between the quantum dots and vibrations within the material itself. Understanding these phonon interactions is vital for optimising the effect.

Better understanding of these interactions may allow for the design of materials with even greater strain sensitivity. At the same time, the degree of strain required to achieve substantial colour shifts remains a limitation, demanding strong mechanical systems. Beyond the immediate applications in photonics, this project opens avenues for exploring fundamental physics.

By probing the relationship between strain, phonons. Quantum dot behaviour, scientists can gain deeper insights into the behaviour of light and matter at the nanoscale. Future work could investigate similar effects in other two-dimensional semiconductors, potentially uncovering new and unexpected properties. The ability to dynamically control light emission with mechanical strain could become a standard technique in nanotechnology.