Colloidal quantum dots (QDs) are nanoscale semiconductor crystals that exhibit size-dependent, tunable light emission. Their unique optical properties stem from quantum confinement effects, where the electron and hole wavefunctions are confined within the nanocrystal's dimensions, leading to discrete energy levels and size-dependent bandgaps. Core-shell heterostructuring, a technique where a shell of a different semiconductor material is grown epitaxially on a core, provides additional control over the QDs' photophysical characteristics, influencing properties such as luminescence efficiency, stability, and emission wavelength. Heteroepitaxy, the growth of one crystalline material on another with a different lattice constant, introduces strain at the interface. This strain, when carefully controlled, can significantly alter the electronic structure and optical properties of QDs. Asymmetric lattice strain, specifically, is known to lift the degeneracy of exciton states—the bound state of an electron and a hole—resulting in accelerated radiative decay, narrowed spectral linewidths, and reduced optical gain thresholds, crucial for various applications. Previous research has explored achieving asymmetric strain through different approaches, including asymmetrical shell growth or the use of compositionally graded shells. However, challenges remain in achieving consistent, defect-free QDs with precisely controlled strain profiles. The present study aims to overcome these limitations by employing a coherent pseudomorphic growth technique to produce strain-graded CdSe-ZnSe core-shell QDs with a compositionally abrupt interface. By controlling surface growth kinetics and carefully managing strain, the goal is to achieve highly stable, spectrally pure, and efficiently emitting QDs with tunable emission across the visible spectrum.

The field of colloidal quantum dots has witnessed significant advancements in controlling their optical properties through various techniques. Early work focused on core-shell structures to improve luminescence efficiency and photostability, with CdSe/CdS being a prominent example. Subsequent research explored the use of type-II QDs, where the electron and hole wavefunctions reside in different materials, leading to unique spectral properties. The influence of strain on QD properties has also been extensively investigated. Studies have shown that applying strain, either through external pressure or through the lattice mismatch in core-shell structures, can modify the bandgap, exciton fine structure, and phonon interactions. Asymmetrical strain, in particular, has attracted considerable attention due to its ability to lift the degeneracy of exciton states. Several methods have been explored to induce asymmetrical strain, including using non-spherical core shapes or employing compositionally graded shells. However, these methods often introduce complexities, resulting in challenges in controlling the strain profile and achieving high structural fidelity, limiting the stability and spectral purity of the resulting QDs. The development of methods to precisely control strain without introducing defects has remained a major goal in the field.

The researchers developed a coherent pseudomorphic growth technique to synthesize strain-graded (sg)-CdSe-ZnSe core-shell quantum dots (QDs). This involved carefully controlling the growth of a ZnSe shell on a CdSe core to minimize the formation of interfacial alloys and defects. Density functional theory (DFT) calculations were employed to guide the synthesis, predicting that using anion (Se)-terminated CdSe cores would prevent the formation of Cd_1-xZn_xSe alloys at the interface. The growth was carried out at elevated temperatures (≥340°C) to promote thermodynamic ZnSe growth, with the ZnSe shell thickness carefully controlled through a layer-by-layer approach. The growth rate was manipulated by regulating the cation-to-ligand stoichiometry and the feed rate of the Zn precursor. The resulting QDs were characterized using various techniques, including high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), to analyze the core-shell structure, elemental composition, and strain profile. Ensemble and single-dot spectroscopic analyses, including absorption and photoluminescence (PL) spectroscopy, were employed to investigate the optical properties of the sg-QDs. Time-resolved PL spectroscopy was used to measure radiative recombination rates, while polarization-resolved PL spectroscopy was performed on single QDs to determine the polarization characteristics of the emitted light. Chemical etching using benzoyl peroxide was used to manipulate the QD size and validate the role of strain in the observed effects. Finally, electroluminescent (EL) devices were fabricated using the sg-QDs as the emissive material to demonstrate their performance in photonic applications. The device structure included a ZnMgO nanoparticle electron transport layer and a 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) hole transport layer.

The researchers successfully synthesized strain-graded CdSe-ZnSe core-shell QDs with a compositionally abrupt interface and near-unity photoluminescence quantum yields (PL QYs). HR-TEM and EDS analyses confirmed the sharp interface and the absence of significant interfacial alloying. The QDs exhibited a significant strain gradient across the core-shell interface, resulting in an asymmetric compressive strain within the CdSe core. This asymmetric strain was significantly higher than in previously reported structures, reaching values as high as -3.08% along the [0002] axis and -4.33% along the [1120] axis in a CdSe (radius 2.5 nm)-ZnSe (shell thickness 5.0 nm) QD. The asymmetric strain led to a large energy splitting (up to 50 meV) between the heavy-hole and light-hole exciton states, a clear indication of the exciton state degeneracy lift. Furthermore, the radiative recombination rate was accelerated, with a decay time close to a purely radiative lifetime of 10 ns. Single-dot spectroscopy revealed narrow spectral linewidths (17.7 meV) and exceptionally low spectral diffusion (standard deviation 0.27 meV), demonstrating remarkable spectral purity and stability. The researchers observed that the magnitude of the heavy-hole-light-hole energy splitting was not solely determined by the compressive strain but also by the asymmetric stress distribution along crystal axes and the morphology of the ZnSe shell. By varying the CdSe core size and ZnSe shell thickness, they systematically investigated the relationship between QD structure and optical properties. Interestingly, the observed increase in energy splitting initially increased with increasing shell thickness but then decreased beyond a certain point due to non-linear changes in asymmetric strain. Single-dot measurements demonstrated the suppression of fluorescence intermittency (blinking) and Auger recombination processes, further confirming the stability and high efficiency of the sg-QDs. Polarization-resolved single-dot spectroscopy revealed that the two split exciton emissions were linearly polarized along orthogonal directions. This polarization behavior, consistent with theoretical predictions, is attributed to the anisotropic compressive strain. The researchers also observed this polarization behavior in higher exciton states, indicating that the degeneracy lift is not limited to the lowest exciton states. Finally, by expanding the synthesis to include Cd_0.25Zn_0.75Se cores, they demonstrated tunable emission across the visible spectrum with high PL QYs (above 90%) and narrow linewidths. They fabricated electroluminescent (EL) devices that showed narrow EL spectra (FWHM = 50 meV), high external quantum efficiency (EQE) of 21.6%, and high brightness. These results demonstrated the potential of strain-graded QDs for applications in advanced lighting and display technologies.

The findings of this study demonstrate the significant impact of precisely controlled asymmetric compressive strain on the optical properties of colloidal quantum dots. The coherent pseudomorphic heteroepitaxy approach employed enables the synthesis of high-quality, defect-free strain-graded QDs with exceptionally stable, spectrally pure, and highly efficient emission. The observed degeneracy lift of exciton states, combined with the suppression of nonradiative recombination pathways, results in superior optical performance compared to previously reported QDs. The narrow linewidths and highly polarized emission are particularly noteworthy, indicating the potential for advanced applications such as high-resolution displays and high-performance lasers. The tunability of the emission wavelength by controlling the core material composition and size opens the door to full-color display technologies. The success in fabricating high-efficiency EL devices confirms the practical applicability of these QDs in advanced lighting and display devices. The study's findings suggest that asymmetric compressive strain is a potent tool for enhancing the performance of QDs and advancing the field of semiconductor nanomaterials.

This work demonstrates a novel approach to synthesize strain-graded CdSe-ZnSe QDs with exceptional optical properties. The precise control over the strain profile, achieved through coherent pseudomorphic heteroepitaxy, leads to QDs with near-unity PL QYs, narrow spectral linewidths, and highly polarized emission. The tunable emission across the visible spectrum and the high performance of fabricated EL devices highlight the potential for widespread applications in advanced lighting and display technologies. Future research could focus on extending this method to other material systems, such as InP and Ag(In,Ga)S₂ QDs, and exploring techniques to align the transition dipoles of sg-QDs in solid-state devices for further improvement of light extraction efficiency.

While the study demonstrates the excellent optical properties of the strain-graded QDs, the synthesis method involves a complex multi-step process requiring careful control of growth parameters. Scaling up the synthesis to produce large quantities of QDs for commercial applications could present challenges. The study focuses primarily on CdSe-ZnSe and Cd_0.25Zn_0.75Se-ZnSe systems. Further research is necessary to determine the broader applicability of the coherent pseudomorphic heteroepitaxy method to other semiconductor material combinations. The long-term stability of the QDs in real-world device environments also requires further investigation.