Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe_1–XTe_X Nanocrystals

Abstract

ZnSe 1−X Te X nanocrystals (NCs) are promising photon emitters with tunable emission across the violet to orange range and near-unity quantum yields. However, these NCs suffer from broad emission line widths and multiple exciton decay dynamics, which discourage their practicable use. Here, we explore the excitonic states in ZnSe 1−X Te X NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis of a series of ZnSe 1−X Te X NC samples with varying Te ratios coupled with computational calculations shows that, due to the distinct electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe 1−X Te X alloys create localized hole states spectrally distributed approximately 130 meV above the 1S h level of homogeneous ZnSe 1−X Te X NCs. This forms spatially separated excitons (delocalized electron and localized hole in trap), accounting for both inhomogeneous and homogeneous line width broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implications for photonic applications.

Compositional and Heterostructural Tuning in Red-Emissive Ternary ZnSeTe Quantum Dots for Display Applications

“…It is worth noting that the PL spectrum of blue ZnSeTe QDs (with Te/Se = 0.1) was asymmetric with the notable tail emission component at a longer wavelength region observed. The tail emission has been also commonly noticed from other less Te-alloyed blue ZnSeTe QDs in the literature, being attributed to the hole trap levels associated with lattice defects (likely induced by a large disparity in ionic radius between Se versus Te) , and nearest-neighbor Te pairs . On the other hand, other Te-richer (Te/Se ratio ≥0.5) QD samples maintained relatively symmetric PL shapes.…”

“…The tail emission has been also commonly noticed from other less Tealloyed blue ZnSeTe QDs in the literature, being attributed to the hole trap levels associated with lattice defects (likely induced by a large disparity in ionic radius between Se versus Te) 19,24 and nearest-neighbor Te pairs. 22 On the other hand, other Te-richer (Te/Se ratio ≥0.5) QD samples maintained relatively symmetric PL shapes. We conjecture that Te-induced hole trap states likely become buried below the valence band maximum (VBM) for such Te-richer ZnSeTe QDs due to their upshifted VBM levels, and thus, the recombination via the hole trap state will no longer become active.…”

“…ZnSeTe QDs alloyed with only small quantities of Te successfully yielded a more desirable blue color with emission wavelengths of ≥450 nm for display devices. 18−24 Blue-emissive ZnSeTe QDs with excellent PL QYs of ≥85% were synthesized in the heterostructural forms of core/shell/shell (i.e., ZnSeTe/ZnSe/ZnS) 20,24 and seed/ quantum well/shell (i.e., ZnS/ZnSeTe/ZnS and ZnSe/ ZnSeTe/ZnSe) 19,21,22 and subsequently exploited as efficient electroluminescent (EL) emitters for blue QLEDs having high external quantum efficiencies (EQEs) of >15%. 20,24 Green emissivity was also attainable by an appropriate increment of Te content in ZnSeTe QDs.…”

“…Alloying of ZnSe with a smaller- E g ZnTe (2.25 eV) in QD synthesis is an effective means to expand the emission coverage toward a longer wavelength. ZnSeTe QDs alloyed with only small quantities of Te successfully yielded a more desirable blue color with emission wavelengths of ≥450 nm for display devices. − Blue-emissive ZnSeTe QDs with excellent PL QYs of ≥85% were synthesized in the heterostructural forms of core/shell/shell (i.e., ZnSeTe/ZnSe/ZnS) , and seed/quantum well/shell (i.e., ZnS/ZnSeTe/ZnS and ZnSe/ZnSeTe/ZnSe) ,, and subsequently exploited as efficient electroluminescent (EL) emitters for blue QLEDs having high external quantum efficiencies (EQEs) of >15%. , Green emissivity was also attainable by an appropriate increment of Te content in ZnSeTe QDs. ,,− For instance, our group earlier reported synthesis of Te/Se ratio-varied green ZnSeTe core/shell QDs with tunable emission wavelengths of ca. 510–530 nm and high PL QYs up to 92%. , The emission wavelength can be further pushed to the red via more extensive Te alloying.…”

Broadband Tunable Optical Gain from Ecofriendly Semiconductor Quantum Dots with Near-Half-Exciton Threshold

“…The PL profile features a distinct dual-band emission, which can be decoupled into a dominant peak of the band-edge emission (P1) and a low-intensity peak at long-wavelength region (P2). Such a sub-band-gap P2 has been widely observed in ZnSe 1– x Te x alloys and proved to arise from the shallow localized hole-trap state as a result of the isovalent Te substitution of Se atoms (Figure d). − Figure S8 presents the overall charge density (OCD) distribution of ZnSe 1– x Te x by the first-principles calculation. The localized OCD around Te element agrees with the formation of localized state in ZnSe 1– x Te x .…”