Long Range Energy Transfer in Self-Assembled Stacks of Semiconducting Nanoplatelets

Plasmon Mediated Single Photon Emission from a Nanocrystal Ensemble

Quantum photonic devices require robust sources of single photons to perform basic computational and communication protocols. Thus, developing scalable, integrable, and efficient quantum light sources has become crucial for the realization of quantum photonic devices. Single quantum dots are promising sources of quantum light due to their tunable emission wavelength. Here, we show the emergence of quantum-emitter-like antibunched emission behavior when multiple quantum dots are located in the vicinity of plasmonic particles. To evaluate the robustness of this phenomenon, we consider both monometallic and bimetallic particles. We find that the photoluminescence intensity of the plasmon coupled quantum dots fits well to a single sublinear power law exponent that is distinct from the behavior of CQD aggregates. Significantly, we find that plasmon coupling results in reduced flickering, thus enabling the realization of a more stable and reliable single photon source. Possible roles of emergent excitonic interactions in the coupled system are discussed.

Orientation-Dependent Photoconductivity of Quasi-2D Nanocrystal Self-Assemblies: Face-Down, Edge-Up Versus Randomly Oriented Quantum Wells

Here, strongly orientation-dependent lateral photoconductivity of a CdSe monolayer colloidal quantum wells (CQWs) possessing short-chain ligands is reported. A controlled liquid-air self-assembly technique is utilized to deliberately engineer the alignments of CQWs into either face-down (FO) or edge-up (EO) orientation on the substrate as opposed to randomly oriented (RO) CQWs prepared by spin-coating. Adapting planar configuration metal-semiconductor-metal (MSM) photodetectors, it is found that lateral conductivity spans ≈2 orders of magnitude depending on the orientation of CQWs in the film in the case of utilizing short ligands. The long native ligands of oleic acid (OA) are exchanged with short-chain ligands of 2-ethylhexane-1-thiol (EHT) to reduce the inter-platelet distance, which significantly improved the photoresponsivity from 4.16, 0.58, and 4.79 mA W-1 to 528.7, 6.17, and 94.2 mA W-1, for the MSM devices prepared with RO, FO, and EO, before and after ligands exchange, respectively. Such CQW orientation control profoundly impacts the photodetector performance also in terms of the detection speed (0.061 s/0.074 s for the FO, 0.048 s/0.060 s for the EO compared to 0.10 s/0.16 s for the RO, for the rise and decay time constants, respectively) and the detectivity (1.7 × 1010, 2.3 × 1011, and 7.5 × 1011 Jones for the FO, EO, and RO devices, respectively) which can be further tailored for the desired optoelectronic device applications. Attributed to charge transportation in colloidal films being proportional to the number of hopping steps, these findings indicate that the solution-processed orientation of CQWs provides the ability to tune the photoconductivity of CQWs with short ligands as another degree of freedom to exploit and engineer their absorptive devices.

Chiroptical Properties of Semiconducting Nanoplatelets Functionalized by Tartrate Derivatives

Inducing chirality in semiconductor nanoparticles is a recent trend motivated by the possible applications in circularly polarized light emission, spintronics, or stereoselective synthesis. However, the previous reports on CdSe nanoplatelets (NPLs) exclusively rely on cysteine or its derivatives as chiral ligands to induce optical activity. Here, we show a strong induction of chirality with derivatives of tartaric acid obtained by a single-step synthesis. The ligand exchange procedure in organic solvent was optimized for five-monolayer (5 ML) NPLs but can also be performed on 4, 3, and 2 ML. We show that the features of the CD spectra change with structural modification of the ligands and that these chiral ligands interact mainly with the first light-hole (lh1) band rather than the first heavy-hole (hh1) band, contrary to cysteine. This result suggests that chiroptical properties could be used to probe CdSe nanoplatelets' surface ligands.

Luminescence Dynamics of Single Self-Assembled Chains of Förster (FRET)-Coupled CdSe Nanoplatelets

Self-assembled linear chains of CdSe nanoplatelets are known to exhibit highly efficient Förster resonant energy transfer (FRET) leading to fast exciton diffusion between platelets. Here, we compare the luminescence decay dynamics of single nanoplatelets, clusters of a few platelets, and self-assembled chains. As the number of stacked platelets is increased, we show that the luminescence decay becomes faster, which can be interpreted as the FRET-mediated effect of quenchers: excitons may diffuse to nearby quenchers so that their decay rate is increased. On the other hand, a minor slow decay component is also observed for single platelets, corresponding to trapping-detrapping mechanisms in nearby trap states. The contribution of the slow component is enhanced for the platelet chains. This is consistent with a FRET-mediated trapping mechanism where the excitons would diffuse from platelet to platelet until they reach a trap state. Finally, we develop toy models for the FRET-mediated quenching and trapping effects on the decay curves and analyze the relevant parameters.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Atomically flat semiconductor nanoplatelets for light-emitting applications

The last decade has witnessed extensive breakthroughs and significant progress in atomically flat two-dimensional (2D) semiconductor nanoplatelets (NPLs) in terms of synthesis, growth mechanisms, optical and electronic properties and practical applications. Such NPLs have electronic structures similar to those of quantum wells in which excitons are predominantly confined along the vertical direction, while electrons are free to move in the lateral directions, resulting in unique optical properties, such as extremely narrow emission line width, short photoluminescence (PL) lifetime, high gain coefficient, and giant oscillator strength transition (GOST). These unique optical properties make NPLs favorable for high color purity light-emitting applications, in particular in light-emitting diodes (LEDs), backlights for liquid crystal displays (LCDs) and lasers. This review article first introduces the intrinsic characteristics of 2D semiconductor NPLs with atomic flatness. Subsequently, the approaches and mechanisms for the controlled synthesis of atomically flat NPLs are summarized followed by an insight on recent progress in the mediation of core/shell, core/crown and core/crown@shell structures by selective epitaxial growth of passivation layers on different planes of NPLs. Moreover, an overview of the unique optical properties and the associated light-emitting applications is elaborated. Despite great progress in this research field, there are some issues relating to heavy metal elements such as Cd²⁺ in NPLs, and the ambiguous gain mechanisms of NPLs and others are the main obstacles that prevent NPLs from widespread applications. Therefore, a perspective is included at the end of this review article, in which the current challenges in this stimulating research field are discussed and possible solutions to tackle these challenges are proposed.

Layer-by-Layer Deposition of 2D CdSe/CdS Nanoplatelets and Polymers for Photoluminescent Composite Materials

Two-dimensional (2D) semiconductor nanoplatelets (NPLs) are strongly photoluminescent materials with interesting properties for optoelectronics. Especially their narrow photoluminescence paired with a high quantum yield is promising for light emission applications with high color purity. However, retaining these features in solid-state thin films together with an efficient encapsulation of the NPLs is a challenge, especially when trying to achieve high-quality films with a defined optical density and low surface roughness. Here, we show photoluminescent polymer-encapsulated inorganic-organic nanocomposite coatings of 2D CdSe/CdS NPLs in poly(diallyldimethylammonium chloride) (PDDA) and poly(ethylenimine) (PEI), which are prepared by sequential layer-by-layer (LbL) deposition. The electrostatic interaction between the positively charged polyelectrolytes and aqueous phase-transferred NPLs with negatively charged surface ligands is used as a driving force to achieve self-assembled nanocomposite coatings with a well-controlled layer thickness and surface roughness. Increasing the repulsive forces between the NPLs by increasing the pH value of the dispersion leads to the formation of nanocomposites with all NPLs arranging flat on the substrate, while the surface roughness of the 165 nm (50 bilayers) thick coating decreases to R_a = 14 nm. The photoluminescence properties of the nanocomposites are determined by the atomic layer thickness of the NPLs and the 11-mercaptoundecanoic acid ligand used for their phase transfer. Both the full width at half-maximum (20.5 nm) and the position (548 nm) of the nanocomposite photoluminescence are retained in comparison to the colloidal CdSe/CdS NPLs in aqueous dispersion, while the measured photoluminescence quantum yield of 5% is competitive to state-of-the-art nanomaterial coatings. Our approach yields stable polymer-encapsulated CdSe/CdS NPLs in smooth coatings with controllable film thickness, rendering the LbL deposition technique a powerful tool for the fabrication of solid-state photoluminescent nanocomposites.

Deciphering modes of long-range energy transfer in perovskite crystals using confocal excitation and wide-field fluorescence spectral imaging

Excitation energy migration beyond mesoscale is of contemporary interest for both solar photovoltaic and light-emissive devices, especially in context of organometal halide perovskites (OMHPs) which have been shown to have very long (charge carrier) diffusion lengths. While understanding the energy propagation pathways in OMHPs is crucial for further advancement of material design and improvement of opto-electronic features, the simultaneous existence of multiple processes like carrier diffusion, photon recycling, and photon transport makes it often complex to differentiate them. In this study, we unravel the diverse yet dominant excitation energy transfer mode(s) in crystalline MAPbBr3 micron-sized 1-D rods and plates by localized (confocal) laser excitation coupled with spectrally-resolved wide-field fluorescence imaging. While rarely used, this technique can efficiently probe excitation migration beyond the diffraction limit and can be realized by simple modification of existing epifluorescence microscopy setups. We find that in rods of length below ~2 microns, carrier diffusion dominates amongst the various energy transfer processes. However, the transient non-radiative defects severely inhibit the extent of carrier migration and also temporarily affect the radiative recombination dynamics of the photo-carriers. For MAPbBr3 plates of several tens of micrometers, we find that the photoluminescence (PL) spectral characteristics remain unaltered at short distances (< ~3 μm) whilst at a larger distance, the spectral profile is gradually red-shifted. This implies that carrier diffusion dominates over small distances, while photon recycling, i.e., repeated re-absorption and re-emission of photons propagates excitation energy transfer over extended length scales with assistance from wave-guided photon transport. Our findings can potentially be used for future studies on the characterization of energy transport mechanisms in semiconductor solids as well as for organic (molecular) self-assembled microstructures.

Unique orientation of 1D and 2D nanoparticle assemblies confined in smectic topological defects

New collective optical properties have emerged recently from organized and oriented arrays of closely packed semiconducting and metallic nanoparticles (NPs). However, it is still challenging to obtain NP assemblies which are similar everywhere on a given sample and, most importantly, share a unique common orientation that would guarantee a unique behavior everywhere on the sample. In this context, by combining optical microscopy, fluorescence microscopy and synchrotron-based grazing incidence X-ray scattering (GISAXS) of assemblies of gold nanospheres and of fluorescent nanorods, we study the interactions between NPs and liquid crystal smectic topological defects that can ultimately lead to unique NP orientations. We demonstrate that arrays of one-dimensional - 1D (dislocations) and two-dimensional - 2D (grain boundaries) topological defects oriented along one single direction confine and organize NPs in closely packed networks but also orient both single nanorods and NP networks along the same direction. Through the comparison between smectic films associated with different kinds of topological defects, we highlight that the coupling between the NP ligands and the smectic layers below the grain boundaries may be necessary to allow for fixed NP orientation. This is in contrast with 1D defects, where the induced orientation of the NPs is intrinsically induced by the confinement independently of the ligand nature. We thus succeeded in achieving the fixed polarization of assemblies of single photon emitters in defects. For gold nanospheres confined in grain boundaries, a strict orientation of hexagonal networks has been obtained with the 〈10〉 direction strictly parallel to the defects. With such closely packed and oriented NPs, new collective properties are now foreseen.

Electron-Hole Binding Governs Carrier Transport in Halide Perovskite Nanocrystal Thin Films

Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer in NPL films. To realize efficient devices, it is imperative to understand how exciton transport progresses therein. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films between 15 °C and 55 °C. At room temperature (RT), we find the diffusion length to be inversely correlated to the thickness of the nanocrystals (NCs). With increasing temperatures, exciton diffusion declines for all NC films, but at different rates. This leads to specific temperature turnover points, at which thinner NPLs exhibit higher diffusion lengths. We attribute this anomalous diffusion behavior to the coexistence of excitons and free electron hole-pairs inside the individual NCs within our temperature range. The organic ligand shell surrounding the NCs prevents charge transfer. Accordingly, any time an electron-hole pair spends in the unbound state reduces the FRET-mediated inter-NC transfer rates and, consequently, the overall diffusion. These results clarify how exciton diffusion progresses in strongly confined halide perovskite NC films, emphasizing critical considerations for optoelectronic devices.

Curvature and self-assembly of semi-conducting nanoplatelets

Semi-conducting nanoplatelets are two-dimensional nanoparticles whose thickness is in the nanometer range and controlled at the atomic level. They have come up as a new category of nanomaterial with promising optical properties due to the efficient confinement of the exciton in the thickness direction. In this perspective, we first describe the various conformations of these 2D nanoparticles which display a variety of bent and curved geometries and present experimental evidences linking their curvature to the ligand-induced surface stress. We then focus on the assembly of nanoplatelets into superlattices to harness the particularly efficient energy transfer between them, and discuss different approaches that allow for directional control and positioning in large scale assemblies. We emphasize on the fundamental aspects of the assembly at the colloidal scale in which ligand-induced forces and kinetic effects play a dominant role. Finally, we highlight the collective properties that can be studied when a fine control over the assembly of nanoplatelets is achieved.

Tailoring Experimental Configurations to Probe Transition Dipoles of Fluorescent Nanoemitters by Polarimetry or Fourier Imaging with Enhanced Sensitivity

Probing the transition dipoles responsible for the luminescence of a nanoemitter is essential to understanding its physical properties, its interactions with its environment, and its potential applications. Various methods in photoluminescence microscopy, based on polarimetry or Fourier imaging, have been developed to measure an emitter's dipole properties: the number of radiating dipoles, the oscillator strength ratio between them, and their orientation. In this article, we model the most used of these protocols and show that their sensitivity depends crucially on the experimental conditions: substrate material, presence of another lower or upper layer, and objective numerical aperture. We develop guidelines to optimize the measurement sensitivity by tailoring the experimental conditions, depending on the type of protocol used and the dipole property to be measured.

Super-resolution Imaging of Energy Transfer by Intensity-Based STED-FRET

Förster resonance energy transfer (FRET) imaging methods provide unique insight into the spatial distribution of energy transfer and (bio)molecular interaction events, though they deliver average information for an ensemble of events included in a diffraction-limited volume. Coupling super-resolution fluorescence microscopy and FRET has been a challenging and elusive task. Here, we present STED-FRET, a method of general applicability to obtain super-resolved energy transfer images. In addition to higher spatial resolution, STED-FRET provides a more accurate quantification of interaction and has the capacity of suppressing contributions of noninteracting partners, which are otherwise masked by averaging in conventional imaging. The method capabilities were first demonstrated on DNA-origami model systems, verified on uniformly double-labeled microtubules, and then utilized to image biomolecular interactions in the membrane-associated periodic skeleton (MPS) of neurons.

Coupled Colloidal Quantum Dot Molecules

Electronic coupling and hence hybridization of atoms serves as the basis for the rich properties for the endless library of naturally occurring molecules. Colloidal quantum dots (CQDs) manifesting quantum strong confinement possess atomic-like characteristics with s and p electronic levels, which popularized the notion of CQDs as artificial atoms. Continuing this analogy, when two atoms are close enough to form a molecule so that their orbitals start overlapping, the orbitals energies start to split into bonding and antibonding states made out of hybridized orbitals. The same concept is also applicable for two fused core–shell nanocrystals in close proximity. Their band edge states, which dictate the emitted photon energy, start to hybridize, changing their electronic and optical properties. Thus, an exciting direction of “artificial molecules” emerges, leading to a multitude of possibilities for creating a library of new hybrid nanostructures with novel optoelectronic properties with relevance toward diverse applications including quantum technologies.

The controlled separation and the barrier height between two adjacent quantum dots are key variables for dictating the magnitude of the coupling energy of the confined wave functions. In the past, coupled double quantum dot architectures prepared by molecular beam epitaxy revealed a coupling energy of few millielectron volts, which limits the applications to mostly cryogenic operation. The realization of artificial quantum molecules with sufficient coupling energy detectable at room temperature calls for the use of colloidal semiconductor nanocrystal building blocks. Moreover, the tunable surface chemistry widely opens the predesigned attachment strategies as well as the solution processing ability of the prepared artificial molecules, making the colloidal nanocrystals as an ideal candidate for this purpose. Despite several approaches that demonstrated enabling of the coupled structures, a general and reproducible method applicable to a broad range of colloidal quantum materials is needed for systematic tailoring of the coupling strength based on a dictated barrier

This Account addresses the development of nanocrystal chemistry to create coupled colloidal quantum dot molecules and to study the controlled electronic coupling and their emergent properties. The simplest nanocrystal molecule, a homodimer formed from two core/shell nanocrystal monomers, in analogy to homonuclear diatomic molecules, serves as a model system. The shell material of the two CQDs is structurally fused, resulting in a continuous crystal. This lowers the potential energy barrier, enabling the hybridization of the electronic wave functions. The direct manifestation of the hybridization reflects on the band edge transition shifting toward lower energy and is clearly resolved at room temperature. The hybridization energy within the single homodimer molecule is strongly correlated with the extent of structural continuity, the delocalization of the exciton wave function, and the barrier thickness as calculated numerically. The hybridization impacts the emitted photon statistics manifesting faster radiative decay rate, photon bunching effect, and modified Auger recombination pathway compared to the monomer artificial atoms. Future perspectives for the nanocrystals chemistry paradigm are also highlighted.

Exciton diffusion exceeding 1 µm: run, exciton, run!

Exciton diffusion lengths reaching the micrometer length scale have long been desired in solution-processed semiconductors but have remained unattainable using conventional materials to date. Now halide perovskite nanocrystal films show unprecedented exciton migration with diffusion lengths approaching 1 µm owing to the efficient combination of radiative and nonradiative energy transfer.

Efficient interlayer exciton transport in two-dimensional metal-halide perovskites

Two-dimensional (2D) metal-halide perovskites are attractive for use in light harvesting and light emitting devices, presenting improved stability as compared to the more conventional three-dimensional perovskite phases. Significant attention has been paid to influencing the layer orientation of 2D perovskite phases, with the charge-carrier transport through the plane of the material being orders of magnitude more efficient than the interlayer transport. Importantly though, the thinnest members of the 2D perovskite family exhibit strong exciton binding energies, suggesting that interlayer energy transport mediated by dipole-dipole coupling may be relevant. We present transient microscopy measurements of the interlayer energy transport in the (PEA)₂PbI₄ perovskite. We find efficient interlayer exciton transport (0.06 cm² s^-1), which translates into a diffusion length that exceeds 100 nm and a sub-ps timescale for energy transfer. While still slower than the in-plane exciton transport (0.2 cm² s^-1), our results show that excitonic energy transport is considerably less anisotropic than charge-carrier transport for 2D perovskites.