Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties

Abstract

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, singlenanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron−hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals

“…Even for conventional semiconductor colloidal NCs, there are possibilities to combine different materials into core–shell heterostructures of various sizes and shapes [ 7 ], thus expanding the possibilities for new phenomena and applications. Recently, the family of CdSe colloidal NCs comprising nearly spherical nanocrystal quantum dots, nanorods, nanoplatelets, and tetrapods was replenished by cube-shaped nanocrystals [ 11 ]. Besides the shape, one can choose the desired crystal structure of the nanocrystals.…”

Ultrastable and High-Efficiency Deep Red QLEDs through Giant Continuously Graded Colloidal Quantum Dots with Shell Engineering

“…However, the emission wavelength of red QLEDs with record EQE and operation lifetime had been limited to the range of 600–630 nm due to the challenge of synthesizing high-quality CdSe QDs emitting beyond 630 nm. ,,,,− One reason is the difficulty in increasing their size while maintaining the high PLQY. This difficulty is caused by the decrease in radiative decay rate with increasing size, while the defect-related nonradiative decay rate remains nearly unchanged. − Additionally, the number of internal and surface defects acting as nonemitting centers is proportional to the QDs’ volume and surface area. − Another challenge is the growth of a high-quality, defect-free passivating shell around larger seeds. Due to the large lattice mismatch between CdSe and ZnSe (6.3%), the reported values of PLQY from deep red CdSe/ZnSe QDs are considerably lower than those with red or green emission .…”

Toward Surface Chemistry of Semiconductor Nanocrystals at an Atomic-Molecular Level

“…12 Consequently, this means their interesting size-dependent properties cannot be fully redeemed, 13 because of the extremely large surface atom ratio in the nanosize regime (Figure 1a). Experimental results confirm that electron and hole wave functions in either strongly confined 14 or weakly confined 15 size regime are significantly delocalized to the nanocrystal surface. Consequently, nearly all interesting properties and technical applications of colloidal semiconductor nanocrystals are affected by their surface.…”

“…Results in Figures 6b−e confirm that the surface chemistry design discussed above can indeed enable formation of monodisperse CdSe, CdS, and CdSe/CdS core/shell nanocubes, each of which is terminated with six zinc-blende (100) facets (Figure 6f). 3,15 The nanocrystals grown without chloride ligands deviated significantly from the cube-shape structure (Figure 6d), but the mixed ligands of alkanoate and chloride ions can convert them to cube-shaped ones through facet reconstruction, indicating robust facet-ligands pairing between a zinc-blende (100) facet and a properly mixed alkanoatechloride ligand system. Furthermore, the irreversible (Figures 6d and 6e) and reversible (Figures 3a−d) facet reconstructions suggest that facet-controlled�or broadly shape-controlled� synthesis of colloidal nanocrystals is not dictated by the growth process alone.…”