Anomalous photoluminescence in CdSe quantum-dot solids at high pressure due to nonuniform stress

Surface- and Spatial-Regulated Cd-Free Quantum Dots for Efficient, Mechanically Stable, and Full-Color Flexible Light-Emitting Diodes

Flexible light-emitting diodes utilizing environmentally friendly cadmium (Cd)-free quantum dots (QDs) hold immense potential for next-generation wearable integrated displays. However, their overall performance lags behind Cd-based counterparts, and less research focuses on the suitability of QD layers in flexible devices. Herein, it is observed that the traditional surface oleate ligands on QDs readily detach under device operation after cycling bending, leading to increased surface defects and accumulated tensile stress in QDs layers, further diminishing their photoluminescence and electroluminescence performance. Based on these insights, a synergetic regulation strategy is developed employing a short-chain bidentate chelating ligand, diethyldithiocarbamate (DDTC), to strengthen the binding of QDs with ligands, minimizing ligand detaching and consequently inhibiting the non-radiative recombination of QDs; Meanwhile, the short-chain DDTC also reduces the inter-dot spatial distance and decreases the Young's modulus in QDs films, effectively dissipating stress localization and retaining the film morphology upon bending. Consequently, the resulting flexible devices based on blue ZnSeTe/ZnSe/ZnS QDs and green/red InP/ZnSe/ZnS QDs demonstrate the peak external quantum efficiencies of above 15% and maintain over 90% after 5000 bending cycles, rivaling state-of-the-art Cd-based flexible devices.

Tailoring Optical Properties of Luminescent Semiconducting Nanocrystals through Hydrostatic, Anisotropic Static, and Dynamic Pressures

Luminescent semiconductor nanocrystals are a fascinating class of materials because of their size-dependent emissions. Numerous past studies have demonstrated that semiconductor nanoparticles with radii smaller than their Bohr radius experience quantum confinement and thus size-dependent emissions. Exerting pressure on these nanoparticles represents an additional, more dynamic, strategy to alter their size and shift their emission. The application of pressure results in the lattices becoming strained and the electronic structure altered. In this Minireview, colloidal semiconductor nanocrystals are first introduced. The effects of uniform hydrostatic pressure on the optical properties of metal halide perovskite (ABX₃ ), II-VI, III-V, and IV-VI semiconductor nanocrystals are then examined. The optical properties of semiconductor nanocrystals under static and dynamic anisotropic pressure are then summarized. Finally, future research directions and applications utilizing the pressure-dependent optical properties of semiconductor nanocrystals are discussed.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

Effects of Hydrostatic Pressure on the Surface Plasmon Resonance of Gold Nanocrystals

The surface plasmon resonances of gold nanospheres and nanorods have been measured as a function of hydrostatic pressure up to 17 GPa in methanol-ethanol 4:1 solvent and up to 10 GPa in paraffin. Both the sphere resonance and the longitudinal rod resonance exhibit redshifts, whereas the transverse rod mode shows an extremely weak redshift or blueshift depending on the nanorod aspect ratio. Solidification of the solvent around 11 GPa causes some aggregation of the particles, readily identified through broadening of the surface plasmon band and further redshifting. Spectra collected during loading and unloading cycles exhibit only minimal hysteresis if the pressure remains below 11 GPa. The surface plasmon shifts are the result of two competing effects. Compression of the conduction electrons in the metals increases the bulk plasma frequency, which causes a blueshift. However, the increase in the solvent density under hydrostatic load leads to an increase in the solvent refractive index, which in turn leads to a redshift. We find that after accounting for the solvent contribution, we can spectroscopically determine the bulk modulus of the gold nanoparticles with a precision of 10%. The value obtained of K₀ = 190 GPa is significantly higher than the value for bulk gold (167 GPa). Furthermore, we show that pressure-induced solidification causes a significant broadening and anomalous shift of the surface plasmon band that we attribute to aggregation and nanorod deformation.

Single Semiconductor Nanocrystals under Compressive Stress: Reversible Tuning of the Emission Energy

The photoluminescence of individual CdSe/CdS/ZnS core/shell nanocrystals has been investigated under external forces. After mutual alignment of a correlative atomic force and confocal microscope, individual particles were colocalized and exposed to a series of force cycles by using the tip of the AFM cantilever as a nanoscale piston. Thus, force-dependent changes of photophysical properties could be tracked on a single particle level. Remarkably, individual nanocrystals either shifted to higher or to lower emission energies with no indications of multiple emission lines under applied force. The direction and magnitude of these reversible spectral shifts depend on the orientation of nanocrystal axes relative to the external anisotropic force. Maximum pressures derived from the applied forces within a simple contact-mechanical model lie in the GPa range, comparable to values typically emerging in diamond anvil cells. Average spectral shift parameters of -3.5 meV/GPa and 3.0 meV/GPa are found for red- and blue-shifting species, respectively. Our results clearly demonstrate that the emission energy of single nanocrystals can be reversibly tuned over an appreciable wavelength range without degradation of their performance which appears as a promising feature with respect to tunable single photon sources or the creation of coherently coupled particle dimers.

A Reversible Crystallinity-Preserving Phase Transition in Metal-Organic Frameworks: Discovery, Mechanistic Studies, and Potential Applications

A quenching-triggered reversible single-crystal-to-single-crystal (SC-SC) phase transition was discovered in a metal-organic framework (MOF) PCN-526. During the phase transition, the one-dimensional channel of PCN-526 distorts from square to rectangular in shape while maintaining single crystallinity. Although SC-SC transformations have been frequently observed in MOFs, most reports have focused on describing the resulting structural alterations without shedding light on the mechanism for the transformation. Interestingly, modifying the occupancy or species of metal ions in the extra-framework sites, which provides mechanistic insight into the causes for the transformation, can forbid this phase transition. Moreover, as a host scaffold, PCN-526 presents a platform for modulation of the photoluminescence properties by encapsulation of luminescent guest molecules. Through judicious choice of these guest molecules, responsive luminescence caused by SC-SC transformations can be detected, introducing a new strategy for the design of novel luminescent MOF materials.

Pressure-dependent optical behaviors of colloidal CdSe nanoplatelets

Two-dimensional (2D) colloidal anisotropic CdSe nanoplatelets (NPLs) have attracted a great deal of attraction within recent years. Their strong thickness-dependent absorption and emission spectra exhibit significant differences from those of other shaped CdSe nanocrystals (NCs) due to a unique atomically flat morphology. Based on their dielectric confinement effect and the large confinement energy, the 2D CdSe NPLs exhibit the best characteristics of optical and electronic properties as compared to the other CdSe nanocrystallite ensembles. Here, we systematically investigate the in situ high-pressure photoluminescence (PL), absorption, and time-resolved PL spectroscopy of CdSe NPLs with different thicknesses. The pressure-dependent optical behaviors of these NPLs exhibit several remarkable differences compared with those of other shaped CdSe NCs such as a higher phase transition pressure, irreversible PL and absorption spectra after the release of pressure, a narrower tunable range of absorption and PL peak energies, and minor changes in the ranges of PL decay time with increasing pressure. These phenomena and results are attributed to their unique geometric shape and distinctive soft ligand bonding on the surface.

Photoluminescence and time-resolved carrier dynamics in thiol-capped CdTe nanocrystals under high pressure

The application of static high pressure provides a method for precisely controlling and investigating many fundamental and unique properties of semiconductor nanocrystals (NCs). This study systematically investigates the high-pressure photoluminescence (PL) and time-resolved carrier dynamics of thiol-capped CdTe NCs of different sizes, at different concentrations, and in various stress environments. The zincblende-to-rocksalt phase transition in thiol-capped CdTe NCs is observed at a pressure far in excess of the bulk phase transition pressure. Additionally, the process of transformation depends strongly on NC size, and the phase transition pressure increases with NC size. These peculiar phenomena are attributed to the distinctive bonding of thiols to the NC surface. In a nonhydrostatic environment, considerable flattening of the PL energy of CdTe NC powder is observed above 3.0 GPa. Furthermore, asymmetric and double-peak PL emissions are obtained from a concentrated solution of CdTe NCs under hydrostatic pressure, implying the feasibility of pressure-induced interparticle coupling.

Strain-dependent photoluminescence behavior of CdSe/CdS nanocrystals with spherical, linear, and branched topologies

The photoluminescence of CdSe/CdS core/shell quantum dots, nanorods, and tetrapods is investigated as a function of applied hydrostatic and non-hydrostatic pressure. The optoelectronic properties of all three nanocrystal morphologies are affected by strain. Furthermore, it is demonstrated that the unique morphology of seeded tetrapods is highly sensitive to non-isotropic stress environments. Seeded tetrapods can thereby serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications including sensitive optomechanical devices and biological force investigations.

Determination of the exciton binding energy in CdSe quantum dots

The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.