No abstract

We proposed and showed strongly orientationcontrolled Forster resonance energy transfer (FRET) to highly anisotropic CdSe nanoplatelets (NPLs). For this purpose, we developed a liquid−air interface self-assembly technique specific to depositing a complete monolayer of NPLs only in a single desired orientation, either fully stacked (edge-up) or fully nonstacked (facedown), with near-unity surface coverage and across large areas over 20 cm 2 . These NPL monolayers were employed as acceptors in an energy transfer working model system to pair with CdZnS/ZnS core/shell quantum dots (QDs) as donors. We found the resulting energy transfer from the QDs to be significantly accelerated (by up to 50%) to the edge-up NPL monolayer compared to the face-down one. We revealed that this acceleration of FRET is accounted for by the enhancement of the dipole−dipole interaction factor between a QD-NPL pair (increased from 1/3 to 5/6) as well as the closer packing of NPLs with stacking. Also systematically studying the distance-dependence of FRET between QDs and NPL monolayers via varying their separation (d) with a dielectric spacer, we found out that the FRET rate scales with d −4 regardless of the specific NPL orientation. Our FRET model, which is based on the original Forster theory, computes the FRET efficiencies in excellent agreement with our experimental results and explains well the enhancement of FRET to NPLs with stacking. These findings indicate that the geometrical orientation of NPLs and thereby their dipole interaction strength can be exploited as an additional degree of freedom to control and tune the energy transfer rate.

Colloidal semiconductor nanoplatelets (NPLs) are highly promising luminescent materials owing to their exceptionally narrow emission spectra. While high-efficiency NPLs in non-polar organic media can be obtained readily, NPLs in aqueous media suffer from extremely low quantum yields (QYs), which completely undermines their potential, especially in biological applications. Here, we show high-efficiency watersoluble CdSe/CdS@Cd 1−x Zn x S core/crown@shell NPLs formed by layer-by-layer grown and compositiontuned gradient Cd 1−x Zn x S shells on CdSe/CdS core/crown seeds. Such control of shell composition with monolayer precision and effective peripheral crown passivation, together with the compact capping density of short 3-mercaptopropionic acid ligands, allow for QYs reaching 90% in water, accompanied by a significantly increased photoluminescence lifetime (∼35 ns), indicating the suppression of nonradiative channels in these NPLs. We also demonstrate the controlled attachment of these NPLs without stacking at the nanoscale by taking advantage of their 2D geometry and hydrophilicity. This is a significant step in achieving controlled assemblies and overcoming the stacking process, which otherwise undermines their film formation and performance in optoelectronic applications. Moreover, we show that the parallel orientation of such NPLs achieved by the controlled attachment enables directed emission perpendicular to the surface of the NPL films, which is highly advantageous for light extraction in light-emitting platforms. † Electronic supplementary information (ESI) available: Details of synthesis procedures, experimental set-up, theoretical modelling and additional figures. See

Solution-processed type-II quantum wells exhibit outstanding optical properties, which make them promising candidates for light-generating applications including lasers and LEDs. However, they may suffer from poor colloidal stability under ambient conditions and show strong tendency to assemble into face-to-face stacks. In this work, to resolve the colloidal stability and uncontrolled stacking issues, we proposed and synthesized CdSe/CdSe 1−x Te x /CdS core/multicrown heteronanoplatelets (NPLs), controlling the amount of Te up to 50% in the crown without changing their thicknesses, which significantly increases their colloidal and photostability under ambient conditions and at the same time preserving their attractive optical properties. Confirming the final lateral growth of CdS sidewalls with X-ray photoelectron spectroscopy, energy-dispersive analysis, and photoelectron excitation spectroscopy, we found that the successful coating of this CdS crown around the periphery of conventional type-II NPLs prevents the unwanted formation of needle-like stacks, which results in reduction of the undesired scattering losses in thin-film samples of these NPLs. Owing to highly efficient exciton funneling from the outmost CdS crown accompanied by the reduced scattering and very low waveguide loss coefficient (∼18 cm −1), ultralow optical gain thresholds of multicrown type-II NPLs were achieved to be as low as 4.15 μJ/cm 2 and 2.48 mJ/cm 2 under one-and two-photon absorption pumping, respectively. These findings indicate that the strategy of using engineered advanced heterostructures of nanoplatelets provides solutions for improved colloidal stability and enables enhanced photonic performance.

Here, we designed and synthesized alloyed heterostructures of CdSe x S1–x nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSe x S1–x core NPLs as a seed, we prepared CdSe x S1–x /CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSe x S1–x cores, we achieved enhanced photoluminescence quantum yield (PL-QY) (reaching 60%), together with increased absorption cross-section and improved stability without changing the emission spectrum of CdSe x S1–x alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSe x S1–x /CdS core/shell NPLs. Depending on the sulfur composition of the CdSe x S1–x core and thickness of the CdS shell, CdSe x S1–x /CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560–650 nm. Finally, we studied the optical gain performances of different heterostructures of CdSe x S1–x alloyed NPLs offering great advantages, including reduced reabsorption and spectrally tunable optical gain range. Despite their decreased PL-QY and reduced absorption cross-section upon increasing the sulfur composition, CdSe x S1–x based NPLs exhibit highly tunable amplified spontaneous emission performance together with low gain thresholds down to ∼53 μJ/cm2.