Two-dimensional semiconductors have attracted considerable interest for integration into emerging quantum photonic networks. Strain engineering of monolayer transition-metal dichalcogenides (ML-TMDs) enables the tuning of light-matter interactions and associated optoelectronic properties, and generates new functionalities, including the formation of quantum dots (QDs). Here, we combine spatially resolved micro-photoluminescence ($μ$-PL) spectroscopy from cryogenic (4$\text{-}$94 K) to room temperature with micro-Raman spectroscopy at room temperature to investigate the strain-dependent emission energies of thousands of individual QDs in ML-WS$_2$ and ML-WSe$_2$, integrated across multiple heterostructures and a piezoelectric device. Compared with delocalized excitons, QDs in both materials exhibit enhanced strain sensitivities of their emission energies $-$ approximately fourfold in WS$_2$ and twofold in WSe$_2$ $-$ leading to pronounced broadening of the ensemble emission linewidth. Temperature-dependent $μ$-PL spectroscopy combined with dynamic strain tuning experiments further reveal that the enhanced strain sensitivity of individual QDs originates from strengthened interactions with low-energy phonons induced by quantum confinement. Our results demonstrate a versatile strain-engineering approach with potential for spectral matching across solid-state, atomic, and hybrid quantum photonic networks, and provide new insights into phonon-QD interactions in two-dimensional semiconductors.
Quantum dots (QDs) in two-dimensional (2D) semiconductors have emerged as a promising solid-state nanophotonic platform for next-generation quantum photonic technologies [1,2]. Remarkable functionalities-including high-purity single-photon emission [3,4], entangled photon-pair generation [5,6], and Coulomb blockade of individual electrons and holes [7]-have been demonstrated in QDs hosted by atomically thin transition-metal dichalcogenides (TMDs), a widely studied 2D semiconductor platform. QDs in monolayer (ML) and few-layer TMDs typically exhibit either comblike emission spectra or a small number of spectrally isolated lines with widely varying wavelengths. This intrinsic spectral variability hinders the realization of multiple QDs with identical optical properties, which are essential for next-generation scalable quantum networks. Consequently, post-fabrication tuning strategies are required to achieve wavelength-tunable QDs in TMDs for developing hybrid quantum photonic networks that integrate TMD QDs with established solid-state III-V QDs and atomic systems.
To address this spectral inhomogeneity, various tuning approaches have been explored. These include, but are not limited to, temperature tuning [8], strain tuning [9], laser annealing [10], vertical electric field tuning [11], and magnetic field tuning [12]. As electric [13,14] and magnetic [15] fields play crucial roles in initialization, control and manipulation of electron or hole spins in QDs, strain tuning provides a complementary and effective route for wavelength control. Strain modifies the electronic band structure of TMDs and affects the light-matter interaction, influencing the exciton localization [16] and its emission wavelength [17,18].
Strain not only tunes the exciton emission wavelength but also reshapes the phonon landscape. The role of phonon interactions in determining the functionalities of QDs across a wide range of material platforms is well documented. For example, twophonon processes combined with other interactions [19] have been shown to account for long spin relaxation times-approaching 1 ms-of heavy holes confined in silicon QDs [20,21] as well as in III-V QDs [22][23][24][25]. More recently, chiral phonons in ML-WSe 2 have been demonstrated to provide a pathway for the generation of entangled photon pairs [6]. These advances suggest that the range of functionalities of QDs in ML-TMDs can be further extended through both strain engineering and phonon engineering, including the activation of chiral phonons [26].
Here, we investigate the pronounced strain sensitivity of QD emission energies in ML-TMDs using low-temperature (4 K), room-temperature (296 K), and temperaturedependent spatially resolved micro-photoluminescence (µ-PL) spectroscopy. A large number of individual QDs integrated into distinct van der Waals heterostructures, as well as a piezoelectric device, were investigated. QDs in ML-WS 2 and ML-WSe 2 are defined and tuned simultaneously by localized strain pockets created by placing ML-TMDs flakes on spherical SiO 2 nanoparticles (SNPs), e-beam-deposition-engineered SiO x nanoparticles (ENPs) and nanodroplets (NDs), or by unintentional wrinkles. We explored strain ranges of -0.10-0.75% for ML-WS 2 QDs and 0.05-0.20% for ML-WSe 2 QDs and show that, compared with delocalized excitons 2D-X 0 in these ML-TMDs, the emission energies of the QDs exhibit significantly larger strain-induced shift rates. This enhanced sensitivity leads to a pronounced broadening of the ensemble emission linewidth (full width at half maximum, FWHM). Dynamic strain tuning and temperature-dependent µ-PL measurements further reveal that this pronounced strain sensitivity-and the resulting broadening-are attributed to a quantum-confinementinduced enhancement of exciton-(low-energy acoustic) phonon interactions.
Highly redshifted QD in ML-WS 2 due to a shape engineered nanostressor: Our investigation begins with 4 K µ-PL spectra of ML-WS 2 acquired at individual SNP (SN) and ENP (EN2) locations (Figs. 1a-b; see optical micrographs in the inset of Fig. 1a marking SN and EN locations). The sharp emission lines observed in these PL spectra arise from localized excitons and are indicative of emission from a few QDs in ML-WS 2 [27,28]. Notably, the PL spectrum taken at the EN2 location shows emission extending beyond 700 nm, including a sharp emission line at 1.775 eV. This emission is strongly redshifted compared with a representative QD emission line at 2.067 eV at the SN location. These observations demonstrate that NP shape-engineering enables tuning of QD emission energies in ML-WS 2 over a maximum range of ≈ 292 meV.
We next examine spatially resolved PL emission to show that these highly redshifted QDs are present exclusively at ENP locations and are absent at SNP locations. Recent studies report that QD emission in ML-WS 2 occurs in the 605-655 nm wavelength range [27,28]. Consistent with this, we observe QD emission in ML-WS 2 at SNP
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