The fabrication of coupled quantum dots (CQDs) may provide an excellent platform for the realization of high-end optoelectronic applications as well as quantum information processing. CQDs can be synthesized by the wellknown cation-exchange method and less explored "nanoparticle fusion" method, in which the latter involves the coupling between constituting facets of two different semiconductors. Herein, we elucidate the mechanistic formation pathway of different heterostructures with ZnS and CdSe quantum dots (QDs) with an emphasis on the formation of CQDs comprised of bicompartmental Janus structures (i.e., Janus structure consisting of two compartments with the two QDs coupled) via nanoparticle fusion. With the increase in the ratio of Cd/Zn from 0.9:1 → 1.3:1 → 2.5:1 → 4:1 → 12:1, we observe the evolution of the structure from CdZnSeS alloy → Acorn Janus → bicompartmental Janus A with ZnS zinc blende (ZB)−CdSe wurtzite (Wz) → bicompartmental Janus B (ZnS−CdSe-both ZB) and eventually to CdZnSeS alloy core−CdSe thick shell. Interestingly, the CQDs possess two distinct emission bands (570/ 630 nm) in which the 570 nm emission arises from the formation of new electronic states due to the strong coupling between the two QDs, whereas 630 nm is the characteristic CdSe emission. Further, the coupling enhances the exciton lifetimes of 570/630 nm emission (bicompartmental Janus A −36/31 ns and bicompartmental Janus B 41/94.8 ns), which can be exploited in various applications. Further, the DFT simulations provide the heuristics behind the formation of certain heterostructures with strain and interfacial energies of particular facets dictating the morphology during coupling of QDs.
The ordered SiO2:Tb(3+) inverse opal heterostructure films are fabricated through polystyrene spheres hetero-opal template using the convective self-assembly method to examine their potential for color purification. Their optical properties and photoluminescence have been investigated and compared with individual single inverse opals and reference (SiO2:Tb(3+) powder). The heterostructures are shown to possess two broad photonic stop bands separated by an effective pass band, causing suppression of blue, orange, and red emission bands corresponding to (5)D4 → (7)F(j); j = 6, 4, 3 transitions, respectively and an enhancement of green emission (i.e., (5)D4 → (7)F5). Although the suppression of various emission occurs because of its overlap with the photonic band gaps (PSBs), the enhancement of green radiation is observed because of its location matching with the pass band region. The Commission International de l'Eclairage (CIE) chromaticity coordinates of the emission spectrum of the heterostructure based on polystyrene sphere of 390 and 500 nm diameter are x = 0.2936, y = 0.6512 and lie closest to those of standard green color (wavelength 545 nm). In addition, a significant increase observed in luminescence lifetime for (5)D4 level of terbium in inverse opal heterostructures vis-à-vis reference (SiO2:Tb(3+) powder) is attributed to the change in the effective refractive index.
Tb3+ embedded silica inverse opal structures with different photonic stop bands have been fabricated by annealing the SiO2-polystyrene spheres (diameter 390 nm) opal template at 320-650 oC. The PSB tuning realized in the wavelength range 498 – 600 nm is shown to depend on annealing temperature and impending isotropic shrinkage of silica matrix. The impact of wide PSB shift on four Tb3+ ion emission bands (blue, green, yellow, and red at 486, 545, 580, and 620 nm, respectively) corresponding to 5D4→7Fj (j = 6,5,4,3) transitions have been investigated. The effect amounts to significant suppression of emission bands at 586, 545 and 486 nm in inverse opals, obtained by annealing opal template at 350, 400, and 650 oC, respectively. Further, luminescence lifetime of Tb3+ ion 5D4 state increases with shrinkage induced in inverse opal progressively and get enhanced up to 2.3 times vis-à-vis reference silica. The changes in refractive index caused by thermal annealing of opal template is found to be responsible for the observed improvement in 5D4 state lifetime.
Alfa (a)-NaYF 4 : Yb, Er nanoparticles embedded silica inverse opal heterostructure, with dual photonic stop bands and negligible local thermal effects, was fabricated. This material exhibited improvement in its upconversion efficiency and green emission purity. The heterostructure was designed and constructed with a stack of two inverse opals with different periodicity to possess dual photonic stop bands at wavelengths l % 410 and % 660 nm with a passband in-between ( % 545 nm). Although the two photonic stop bands suppress two distant emission bands (blue: 2 H 9/2 ! 4 I 15/2 and red: 4 F 9/2 ! 4 I 15/2 transitions) simultaneously, the passband enhances the green emission bands corresponding to 2 H 11/2 ! 4 I 15/2 and 4 S 3/2 ! 4 I 15/2 transitions of Er 3 + ions. The process leads to realization of a pure green color. In addition, the local thermal effects induced by laser irradiation at the surface of the nanoparticles are largely suppressed by effective dissipation of heat as a result of the microporous nature of the inverse opal heterostructures. The reduced thermal effect in embedded nanoparticles is advantageous and shown to be responsible for the enhancement of green emission, the increase or decrease in the luminescence lifetime of Er 3 + ion 2 H 11/2 and 2 H 9/2 or 4 S 3/2 and 4 F 9/2 states, a faster luminescence rise time, and almost the same green-to-red intensity ratio with variation in laser power.[a] V. power in 280-520 IOH when compared with bare a-NaYF 4 : Yb, Er nanoparticles. This provides further evidence for effective dissipation of heat in inverse opals. [53] Needless to say, local heat is generated on the emitter surface by laser irradiation. The porous structure of inverse opals facilitates batter heat dissipation leading to reduction in local temperature around the emitter efficiently.
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