Neck Barrier Engineering in Quantum Dot Dimer Molecules via Intraparticle Ripening

Abstract

Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed of fused core/shell semiconductor nanocrystals. The electronic coupling and wave function hybridization are enabled by the formation of an epitaxial connection with a coherent lattice between the shells of the two neighboring quantum dots where the shell material and its dimensions dictate the quantum barrier characteristics for the charge carriers. Herein we introduce a colloidal approach to control the neck formation at the interface between the two CQDs in such artificial molecular constructs. This allows the tailoring of the neck barrier in prelinked homodimers formed via fusion of multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature, and excess ligands are studied. The neck filling process follows an intraparticle ripening mechanism at relatively mild reaction conditions while avoiding interparticle ripening. The degree of surface ligand passivation plays a key role in activating the surface atom diffusion to the neck region. The degree of neck filling strongly depends also on the initial relative orientation of the two CQDs, where homonymous plane attachment allows for facile neck growth, unlike the case of heteronymous plane attachment. Upon neck filling, the observed red-shift of the absorption and fluorescence measured both for ensemble and single dimers is assigned to enhanced hybridization of the confined wave function in CQD dimer molecules, as supported by quantum calculations. The fine-tuning of the particle interface introduced herein provides therefore a powerful tool to further control the extent of hybridization and coupling in CQD molecules.

Nonadiabatic to Adiabatic Transition of Electron Transfer in Colloidal Quantum Dot Molecules

“…dp with an empirical form for k dp ∝ √ J. core diameters D core . We find that across the range of neck and shell dimensions that can be varied experimentally [13],…”

“…1(b)). Charge transfer in such systems is particularly interesting due to the flexibility in designing the donor and acceptor states by, for example, changing the width of the neck (D neck ) between two NCs and/or the diameter of each NC core (D core ), as well as controlling the shell thickness (D shell ) [12,13]. By continuously varying these parameters, the hybridization between the donor and the acceptor, J, can be tuned across a wide range of values while at the same time the reorganization energy, λ, and the typical vibrational frequency, ω c , change slightly.…”

Twisting Enabled Charge Transfer Excitons in Epitaxially Fused Quantum Dot Molecules

“…Herein, we have explored an alternative QD platform suited for realizing CT excitons, that is, epitaxially fused QD dimer molecule formed by fusing QDs via oriented attachment. − Both homodimer and heterodimer QD molecules can be synthesized in a controlled manner using the well-established wet-chemistry methods. − Besides inheriting all the superior optical properties of single colloidal QDs, they exhibit two exceptional features. The first is a nanoscale interface, which is necessary for the electron–hole charge separation.…”

Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules

“…26 CQDMs exhibit optical and electronic properties, which differ from their single QD building blocks as a result of the coupling. 18 Notably, the CQDMs' spectrum is red-shifted and broader, 20,27,28 the absorption cross-section is modified to be doubled at high energy and smeared out near the band gap, 24 their fluorescence decay lifetime is shorter, and their brightness is higher than their single QD constituents. 27 The optical properties of the CQDMs depend on the width of the interfacial area between the two fused QDs, or "neck", serving as a potential barrier.…”

“…Since the introduction of colloidal quantum dots (QDs) a few decades ago, their research is constantly developing, due to the intriguing quantum confinement effect that influences the electronic and optical properties as a function of the QD’s size and shape. , QDs are impressively already widely implemented in commercial displays and are of further relevance in additional applications including lasers, light emitting diodes (LEDs), , single photon sources, and photovoltaics. , The extensive study in this field established synthetic means to allow for better control over the size, morphology, and surface chemistry of QDs of various semiconductor materials, enabling improved quantum yields (QYs) and tunable emission and absorption spectra. − In recent years, further research has been carried out to synthesize more complex nanostructures with two or more coupled emission centers, thus launching the field of “nanochemistry”. − In particular, it was demonstrated that two QDs can be fused together via a process of constrained oriented attachment, forming a coupled QD molecule (CQDM). − …”