Orientation-Controlled Nonradiative Energy Transfer to Colloidal Nanoplatelets: Engineering Dipole Orientation Factor

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.

Boosting Excited-State Energy Transfer by Anchoring Dipole Orientation in Binary Thermally Activated Delayed Fluorescence/J-Aggregate Assemblies

Förster resonance energy transfer (FRET) has been widely applied in fluorescence imaging, sensing and so on, while developing useful strategy of boosting FRET efficiency becomes a key issue that limits the application. Except optimizing spectral properties, promoting orientation factor (κ2) has been well discussed but rarely utilized for boosting FRET. Herein, we constructed binary nano-assembling of two thermally activated delayed fluorescence (TADF) emitters (2CzPN and DMAC-DPS) with J-type aggregate of cyanine dye (C8S4) as doping films by taking advantage of their electrostatic interactions. Time-resolved spectroscopic measurements indicated that 2CzPN/Cy-J films exhibit an order of magnitude higher kFRET than DMAC-DPS/Cy-J films. Further quantitative analysing on kFRET and kDET indicated higher orientation factor (κ2) in 2CzPN/Cy-J films play a key role for achieving fast kFRET, which was subsequently confirmed by anisotropic measurements. Corresponding DFT/TDDFT calculation revealed strong "two-point" electrostatic anchoring in 2CzPN/Cy-J films that is responsible for highly orientated transitions. We provide a new strategy for boosting FRET in nano-assemblies, which might be inspired for designing FRET-based devices of sensing, imaging and information encryption.

Gradient Alloy Shell Enabling Colloidal Quantum Wells Light-Emitting Diodes with Efficiency Exceeding 22

Colloidal quantum well (CQW) based light emitting diodes (LEDs) possess extra-high theoretical efficiency, but their performance still lags far behind conventional LEDs due to severe exciton quenching and unbalanced charge injection. Herein, we devised a gradient composition CdxZn1-xS shell to address these issues. The epitaxial shell with gradient composition was achieved through controlling competition between Cd2+ and Zn2+ cations to preferentially bind to the anions S2-. Thus, exciton quenching was suppressed greatly by passivating defects and reducing nonradiative recombination, thereby achieving near-unity photoluminescence quantum yield (PLQY). The gradient energy level of the shell reduced the hole injection barriers and increased the hole injection efficiency to balance the charge injection of LEDs. As a result, the LEDs achieved a high external quantum efficiency (EQE) of 22.83%, luminance of 111,319 cd/m2 and a long operational lifetime (T95@100 cd/m2) over 6,500 h, demonstrating the state-of-the-art performance for the CQW based LEDs.

Light-sensitive monolayer-thick nanocrystal skins of face-down self-oriented colloidal quantum wells

Colloidal quantum wells (CQWs), a quasi-two-dimensional, atomically-flat sub-family of semiconductor nanocrystals, are well suited to produce excellent devices for photosensing applications thanks to their extraordinarily large absorption cross-sections. In this work, we propose and demonstrate a new class of light-sensitive nanocrystal skins (LS-NS) that employ a monolayer of face-down orientation-controlled self-assembled CQWs as the active absorbing layer in the UV-visible range. This CQW LS-NS platform enables non-conventional photosensing operation that relies on the strong optical absorption of the monolayered assembly of CQWs and the subsequent photogenerated potential build-up across the device, allowing for self-powered operation. Here such self-oriented CQWs reduce the surface roughness in their monolayer-thick film, essential to high device performance. Owing to their ease of fabrication and low cost, these devices hold great promise for large-scale use in semi-transparent photosensing surfaces.

Optical anisotropy of CsPbBr3 perovskite nanoplatelets

The two-dimensional CsPbBr3 nanoplatelets have a quantum well electronic structure with a band gap tunable with sample thicknesses in discreet steps based upon the number of monolayers. The polarized optical properties of CsPbBr3 nanoplatelets are studied using fluorescence anisotropy and polarized transient absorption spectroscopies. Polarized spectroscopy shows that they have absorption and emission transitions which are strongly plane-polarized. In particular, photoluminescence excitation and transient absorption measurements reveal a band-edge polarization approaching 0.1, the limit of isotropic two-dimensional ensembles. The degree of anisotropy is found to depend on the thickness of the nanoplatelets: multiple measurements show a progressive decrease in optical anisotropy from 2 to 5 monolayer thick nanoplatelets. In turn, larger cuboidal CsPbBr3 nanocrystals, are found to have consistently positive anisotropy which may be attributed to symmetry breaking from ideal perovskite cubes. Optical measurements of anisotropy are described with respect to the theoretical framework developed to describe exciton fine structure in these materials. The observed planar absorption and emission are close to predicted values at thinner nanoplatelet sizes and follow the predicted trend in anisotropy with thickness, but with larger anisotropy than theoretical predictions. Dominant planar emission, albeit confined to the thinnest nanoplatelets, is a valuable attribute for enhanced efficiency of light-emitting devices.

High External Quantum Efficiency Light-Emitting Diodes Enabled by Advanced Heterostructures of Type-II Nanoplatelets

Colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), are exciting material systems for numerous photonic applications, including lasers and light-emitting diodes (LEDs). Although many successful type-I NPL-LEDs with high device performance have been demonstrated, type-II NPLs are not fully exploited for LED applications, even with alloyed type-II NPLs with enhanced optical properties. Here, we present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and systematic investigation of their optical properties, including their comparison with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSe_xTe_1-x core/crown heterostructures, here the proposed advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in a high quantum yield (QY) of 83% and a long fluorescence lifetime of 73.3 ns. These type-II transitions were confirmed experimentally by optical measurements and theoretically using electron and hole wave function modeling. Computational study shows that the multi-crowned NPLs provide a better-distributed hole wave function along the CdTe crown, while the electron wave function is delocalized in the CdSe core and CdSe crown layers. As a proof-of-concept demonstration, NPL-LEDs based on these multi-crowned NPLs were designed and fabricated with a record high external quantum efficiency (EQE) of 7.83% among type-II NPL-LEDs. These findings are expected to induce advanced designs of NPL heterostructures to reach a fascinating level of performance, especially in LEDs and lasers.

Highly-Directional, Highly-Efficient Solution-Processed Light-Emitting Diodes of All-Face-Down Oriented Colloidal Quantum Well Self-Assembly

Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800 cd m^-2 with a long operational lifetime of 247 h at 100 cd m^-2 as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV at a current density of 1 mA cm^-2 and a high J₉₀ of 99.58 mA cm^-2 . These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.

Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nanoplatelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr₃ nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable-temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

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.

Disentangling the Orientations of Spectrally Overlapping Transition Dipoles in Dense Dye Layers

The transition dipole orientations of dye assemblies in heterostructures have a crucial impact on the efficiency of novel optoelectronic devices such as organic thin-film transistors and light-emitting diodes. These devices are frequently based on heterojunctions and tandem structures featuring multiple optical transitions. Precise knowledge of preferred orientations, spatial order, and spatial variations is highly relevant. We present a fast and universal large-area screening method to determine the transition dipole orientations in dye assemblies with diffraction-limited spatial resolution. Moreover, our hyperspectral imaging approach disentangles the orientations of different chromophores. As a demonstration, we apply our technique to dye monolayers with two optical transitions sandwiched between two ultrathin silicate nanosheets. A comprehensive model for dipole orientation distributions in monolayers reveals a long-range orientational order and a strong correlation between the two transitions.

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.

Double-crowned 2D semiconductor nanoplatelets with bicolor power-tunable emission

Nanocrystals (NCs) are now established building blocks for optoelectronics and their use as down converters for large gamut displays has been their first mass market. NC integration relies on a combination of green and red NCs into a blend, which rises post-growth formulation issues. A careful engineering of the NCs may enable dual emissions from a single NC population which violates Kasha’s rule, which stipulates that emission should occur at the band edge. Thus, in addition to an attentive control of band alignment to obtain green and red signals, non-radiative decay paths also have to be carefully slowed down to enable emission away from the ground state. Here, we demonstrate that core/crown/crown 2D nanoplatelets (NPLs), made of CdSe/CdTe/CdSe, can combine a large volume and a type-II band alignment enabling simultaneously red and narrow green emissions. Moreover, we demonstrate that the ratio of the two emissions can be tuned by the incident power, which results in a saturation of the red emission due to non-radiative Auger recombination that affects this emission much stronger than the green one. Finally, we also show that dual-color, power tunable, emission can be obtained through an electrical excitation.

Nanocrystals are desirable light sources for advanced display technologies. Here, the authors report on double-crowned 2D semiconductor nanoplatelets as light downconverters that offer both green and red emissions to achieve a wide color gamut.

Generating and Capturing Secondary Hot Carriers in Monolayer Tungsten Dichalcogenides

It remains challenging to capture and investigate the drift dynamics of primary hot carriers because of their ultrashort lifetime (∼200 fs). Here we report a new mechanism for secondary hot carrier (∼25 ps) generation in monolayer transition metal dichalcogenides such as WS₂ and WSe₂, triggered by the Auger recombination of trions and biexcitons. Using ultrafast photocurrent spectroscopy, we measured and characterized the photocurrent stemming from the Auger recombination of trions and biexcitons in WS₂ and WSe₂. A mobility of 0.24 cm² V^-1 s^-1 and a drift length of ∼3.8 nm were found for the secondary hot carriers in WS₂. By leveraging interactions between exciton complexes, we envision a new mechanism for generating and controlling hot carriers, which could lead to efficient devices in photophysics, photochemistry, and photosynthesis.

Stacking-Dependent Electrical Transport in a Colloidal CdSe Nanoplatelet Thin-Film Transistor

Here, we report an exceptional feature of the one-dimensional threadlike assemblies of a four-monolayer colloidal CdSe nanoplatelet (NPL)-based thin-film transistor. A series of different lengths of threads (200-1200 nm) was used as an active n channel in thin-film transistors (TFTs) to understand the change in mobility with the length of the threads. The film with the longest threads shows the highest conductivity of ∼12 S/cm and electron mobility of ∼14.3 cm² V^-1 s^-1 for an applied gate voltage of 2 V. The mobility trends with the length seem to be driven mostly by the lower defects in threads, where the loss of electron hopping is less. Furthermore, our results show the mobility trends in stacking-dependent CdSe NPL threads and provide a new insight into fabricating high-mobility TFTs with the use of colloidal CdSe NPLs.

Nanocarriers Made of Proteins: Intracellular Visualization of a Smart Biodegradable Drug Delivery System

This work analyzes the intracellular fate of protein-based nanocarriers along their endolysosomal pathway by means of correlative light and electron microscopy methods. To unambiguously identify the nanocarriers and their degradation remnants in the cellular environment, they are labeled with fluorescent, inorganic nanoplatelets. This allows tracking the nanocarriers on their intracellular pathway by means of electron microscopy imaging. From the present data, it is possible to identify different cell compartments in which the nanocarriers are processed. Finally, three different terminal routes for the intracellular destiny of the nanocarriers are presented. These findings are important to reveal the degradation process of protein nanocapsules and contribute to the understanding of the therapeutic success of an encapsulated drug.

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.

Multicolor Patterning of 2D Semiconductor Nanoplatelets

Nanocrystal micro/nanoarrays with multiplexed functionalities are of broad interest in the field of nanophotonics, cellular dynamics, and biosensing due to their tunable electrical and optical properties. This work focuses on the multicolor patterning of two-dimensional nanoplatelets (NPLs) via two sequential self-assembly and direct electron-beam lithography steps. By using scanning electron microscopy, atomic force microscopy, and fluorescence microscopy, we demonstrate the successful fabrication of fluorescent nanoarrays with a thickness of only two or three monolayers (7-11 nm) and a feature line width of ∼40 nm, which is three to four NPLs wide. To this end, first, large-area thin films of red-emitting CdSe/Zn_yCd_1-yS and green-emitting CdSe_1-xS_x/Zn_yCd_1-yS core/shell NPLs are fabricated based on Langmuir-type self-assembly at the liquid/air interface. By varying the concentration of ligands in the subphase, we investigate the effect of interaction potential on the film's final characteristics to prepare thin superlattices suitable for the patterning step. Equipped with the ability to fabricate a uniform superlattice with a controlled thickness, we next perform nanopatterning on a thin film of NPLs utilizing a direct electron-beam lithography (EBL) technique. The effect of acceleration voltage, aperture size, and e-beam dosage on the nanopattern's resolution and fidelity is investigated for both of the presented NPLs. After successfully optimizing EBL parameters to fabricate single-color nanopatterns, we finally focus on fabricating multicolor patterns. The obtained micro/nanoarrays provide us with an innovative experimental platform to investigate biological interactions as well as Förster resonance energy transfer.

Near-Field Energy Transfer into Silicon Inversely Proportional to Distance Using Quasi-2D Colloidal Quantum Well Donors

Silicon is the most prevalent material system for light-harvesting applications; however, its inherent indirect bandgap and consequent weak absorption limits its potential in optoelectronics. This paper proposes to address this limitation by combining the sensitization of silicon with extraordinarily large absorption cross sections of quasi-2D colloidal quantum well nanoplatelets (NPLs) and to demonstrate excitation transfer from these NPLs to bulk silicon. Here, the distance dependency, d, of the resulting Förster resonant energy transfer from the NPL monolayer into a silicon substrate is systematically studied by tuning the thickness of a spacer layer (of Al₂ O₃ ) in between them (varied from 1 to 50 nm in thickness). A slowly varying distance dependence of d^-1 with 25% efficiency at a donor-acceptor distance of 20 nm is observed. These results are corroborated with full electromagnetic solutions, which show that the inverse distance relationship emanates from the delocalized electric field intensity across both the NPL layer and the silicon because of the excitation of strong in-plane dipoles in the NPL monolayer. These findings pave the way for using colloidal NPLs as strong light-harvesting donors in combination with crystalline silicon as an acceptor medium for application in photovoltaic devices and other optoelectronic platforms.

Study of Complex Optical Constants of Neat Cadmium Selenide Nanoplatelets Thin Films by Spectroscopic Ellipsometry

Knowledge of tunability of complex optical constants of colloidal CdSe nanoplatelets (NPLs) thin films is essential for accurate modeling and design of NPL-containing optoelectronic devices. Here, dielectric functions, complex optical conductivities, and absorption coefficients of a series of CdSe NPL films with a varying number of atomic layers were investigated in a combination of spectroscopic ellipsometry techniques and transmittance measurements over a broad spectral range. Fine electronic structures were deciphered from the dielectric functions. Oscillator strengths at the lowest exciton resonance up to 0.62 for a series of CdSe NPL films were also determined. From our results, increasing the number of monolayers was found to boost the complex optical constants and the amplitude of the coupling strength of the fundamental exciton state mainly due to higher inorganic volume filling factors and pronounced surface passivation. Our work gives insights into both the interpretation and improvements of performance of CdSe NPL-based photoelectronic applications.

Optical Gain in Ultrathin Self-Assembled Bi-Layers of Colloidal Quantum Wells Enabled by the Mode Confinement in their High-Index Dielectric Waveguides

This study demonstrates an ultra-thin colloidal gain medium consisting of bi-layers of colloidal quantum wells (CQWs) with a total film thickness of 14 nm integrated with high-index dielectrics. To achieve optical gain from such an ultra-thin nanocrystal film, hybrid waveguide structures partly composed of self-assembled layers of CQWs and partly high-index dielectric material are developed and shown: in asymmetric waveguide architecture employing one thin film of dielectric underneath CQWs and in the case of quasi-symmetric waveguide with a pair of dielectric films sandwiching CQWs. Numerical modeling indicates that the modal confinement factor of ultra-thin CQW films is enhanced in the presence of the adjacent dielectric layers significantly. The active slabs of these CQW monolayers in the proposed waveguide structure are constructed with great care to obtain near-unity surface coverage, which increases the density of active particles, and to reduce the surface roughness to sub-nm scale, which decreases the scattering losses. The excitation and propagation of amplified spontaneous emission (ASE) along these active waveguides are experimentally demonstrated and numerically analyzed. The findings of this work offer possibilities for the realization of ultra-thin electrically driven colloidal laser devices, providing critical advantages including single-mode lasing and high electrical conduction.

Thickness-Tunable Self-Assembled Colloidal Nanoplatelet Films Enable Ultrathin Optical Gain Media

We propose and demonstrate construction of highly uniform, multilayered superstructures of CdSe/CdZnS core/shell colloidal nanoplatelets (NPLs) using liquid interface self-assembly. These NPLs are sequentially deposited onto a solid substrate into slabs having monolayer-precise thickness across tens of cm² areas. Because of near-unity surface coverage and excellent uniformity, amplified spontaneous emission (ASE) is observed from an uncharacteristically thin film having 6 NPL layers, corresponding to a mere 42 nm thickness. Furthermore, systematic studies on optical gain of these NPL superstructures having thicknesses ranging from 6 to 15 layers revealed the gradual reduction in gain threshold with increasing number of layers, along with a continuous spectral shift of the ASE peak (∼18 nm). These observations can be explained by the change in the optical mode confinement factor with the NPL waveguide thickness and propagation wavelength. This bottom-up construction technique for thickness-tunable, three-dimensional NPL superstructures can be used for large-area device fabrication.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

In recent years, impurity-doped nanocrystal light-emitting diodes (LEDs) have aroused both academic and industrial interest since they are highly promising to satisfy the increasing demand of display, lighting, and signaling technologies. Compared with undoped counterparts, impurity-doped nanocrystal LEDs have been demonstrated to possess many extraordinary characteristics including enhanced efficiency, increased luminance, reduced voltage, and prolonged stability. In this review, recent state-of-the-art concepts to achieve high-performance impurity-doped nanocrystal LEDs are summarized. Firstly, the fundamental concepts of impurity-doped nanocrystal LEDs are presented. Then, the strategies to enhance the performance of impurity-doped nanocrystal LEDs via both material design and device engineering are introduced. In particular, the emergence of three types of impurity-doped nanocrystal LEDs is comprehensively highlighted, namely impurity-doped colloidal quantum dot LEDs, impurity-doped perovskite LEDs, and impurity-doped colloidal quantum well LEDs. At last, the challenges and the opportunities to further improve the performance of impurity-doped nanocrystal LEDs are described.

Kinetic Control over Self-Assembly of Semiconductor Nanoplatelets

Semiconductor nanoplatelets exhibit spectrally pure, directional fluorescence. To make polarized light emission accessible and the charge transport effective, nanoplatelets have to be collectively oriented in the solid state. We discovered that the collective nanoplatelets orientation in monolayers can be controlled kinetically by exploiting the solvent evaporation rate in self-assembly at liquid interfaces. Our method avoids insulating additives such as surfactants, making it ideally suited for optoelectronics. The monolayer films with controlled nanoplatelets orientation (edge-up or face-down) exhibit long-range ordering of transition dipole moments and macroscopically polarized light emission. Furthermore, we unveil that the substantial in-plane electronic coupling between nanoplatelets enables charge transport through a single nanoplatelets monolayer, with an efficiency that strongly depends on the orientation of the nanoplatelets. The ability to kinetically control the assembly of nanoplatelets into ordered monolayers with tunable optical and electronic properties paves the way for new applications in optoelectronic devices.

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

Fluorescent emitters like ions, dye molecules, or semiconductor nanoparticles are widely used in optoelectronic devices, usually within densely packed layers. Their luminescence properties can then be very different from when they are isolated, because of short-range interparticle interactions such as Förster resonant energy transfer (FRET). Understanding these interactions is crucial to mitigate FRET-related losses and could also lead to new energy transfer strategies. Exciton migration by FRET hopping between consecutive neighbor fluorophores has been evidenced in various systems but was generally limited to distances of tens of nanometers and involved only a few emitters. Here, we image self-assembled linear chains of CdSe nanoplatelets (colloidal quantum wells) and demonstrate exciton migration over 500 nm distances, corresponding to FRET hopping over 90 platelets. By comparing a diffusion-equation model to our experimental data, we measure a (1.5 ps)^-1 FRET rate, much faster than all decay mechanisms, so that strong FRET-mediated collective photophysical effects can be expected.

Emergent Optoelectronic Properties of Mixed-Dimensional Heterojunctions

ConspectusThe electronic dimensionality of a material is defined by the number of spatial degrees of confinement of its electronic wave function. Low-dimensional semiconductor nanomaterials with at least one degree of spatial confinement have optoelectronic properties that are tunable with size and environment (dielectric and chemical) and are of particular interest for optoelectronic applications such as light detection, light harvesting, and photocatalysis. By combining nanomaterials of differing dimensionalities, mixed-dimensional heterojunctions (MDHJs) exploit the desirable characteristics of their components. For example, the strong optical absorption of zero-dimensional (0D) materials combined with the high charge carrier mobilities of two-dimensional (2D) materials widens the spectral response and enhances the responsivity of mixed-dimensional photodetectors, which has implications for ultrathin, flexible optoelectronic devices. MDHJs are highly sensitive to (i) interfacial chemistry because of large surface area-to-volume ratios and (ii) electric fields, which are incompletely screened because of the ultrathin nature of MDHJs. This sensitivity presents opportunities for control of physical phenomena in MDHJs through chemical modification, optical excitation, externally applied electric fields, and other environmental parameters. Since this fast-moving research area is beginning to pose and answer fundamental questions that underlie the fundamental optoelectronic behavior of MDHJs, it is an opportune time to assess progress and suggest future directions in this field.In this Account, we first outline the characteristic properties, advantages, and challenges for low-dimensional materials, many of which arise as a result of quantum confinement effects. The optoelectronic properties and performance of MDHJs are primarily determined by dynamics of excitons and charge carriers at their interfaces, where these particles tunnel, trap, scatter, and/or recombine on the time scales of tens of femtoseconds to hundreds of nanoseconds. We discuss several photophysical phenomena that deviate from those observed in bulk heterojunctions, as well as factors that can be used to vary, probe, and ultimately control the behavior of excitons and charge carriers in MDHJ systems. We then discuss optoelectronic applications of MDHJs, namely, photodetectors, photovoltaics, and photocatalysts, and identify current performance limits compared to state-of-the-art benchmarks. Finally, we suggest strategies to extend the current understanding of dynamics in MDHJs toward the realization of stimuli-driven responses, particularly with respect to exciton delocalization, quantum emission, interfacial morphology, responsivity to external stimuli, spin selectivity, and usage of chemically reactive materials.

All-optical control of exciton flow in a colloidal quantum well complex

Excitonics, an alternative to romising for processing information since semiconductor electronics is rapidly approaching the end of Moore's law. Currently, the development of excitonic devices, where exciton flow is controlled, is mainly focused on electric-field modulation or exciton polaritons in high-Q cavities. Here, we show an all-optical strategy to manipulate the exciton flow in a binary colloidal quantum well complex through mediation of the Förster resonance energy transfer (FRET) by stimulated emission. In the spontaneous emission regime, FRET naturally occurs between a donor and an acceptor. In contrast, upon stronger excitation, the ultrafast consumption of excitons by stimulated emission effectively engineers the excitonic flow from the donors to the acceptors. Specifically, the acceptors' stimulated emission significantly accelerates the exciton flow, while the donors' stimulated emission almost stops this process. On this basis, a FRET-coupled rate equation model is derived to understand the controllable exciton flow using the density of the excited donors and the unexcited acceptors. The results will provide an effective all-optical route for realizing excitonic devices under room temperature operation.

Record High External Quantum Efficiency of 19.2% Achieved in Light-Emitting Diodes of Colloidal Quantum Wells Enabled by Hot-Injection Shell Growth

Colloidal quantum wells (CQWs) are regarded as a highly promising class of optoelectronic materials, thanks to their unique excitonic characteristics of high extinction coefficients and ultranarrow emission bandwidths. Although the exploration of CQWs in light-emitting diodes (LEDs) is impressive, the performance of CQW-LEDs lags far behind other types of soft-material LEDs (e.g., organic LEDs, colloidal-quantum-dot LEDs, and perovskite LEDs). Herein, high-efficiency CQW-LEDs reaching close to the theoretical limit are reported. A key factor for this high performance is the exploitation of hot-injection shell (HIS) growth of CQWs, which enables a near-unity photoluminescence quantum yield (PLQY), reduces nonradiative channels, ensures smooth films, and enhances the stability. Remarkably, the PLQY remains 95% in solution and 87% in film despite rigorous cleaning. Through systematically understanding their shape-, composition-, and device-engineering, the CQW-LEDs using CdSe/Cd_0.25 Zn_0.75 S core/HIS CQWs exhibit a maximum external quantum efficiency of 19.2%. Additionally, a high luminance of 23 490 cd m^-2 , extremely saturated red color with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.715, 0.283), and stable emission are obtained. The findings indicate that HIS-grown CQWs enable high-performance solution-processed LEDs, which may pave the path for future CQW-based display and lighting technologies.

Mutual Energy Transfer in a Binary Colloidal Quantum Well Complex

Förster resonance energy transfer (FRET) is a fundamental process that is key to optical biosensing, photosynthetic light harvesting, and down-converted light emission. However, in total, conventional FRET in a donor-acceptor pair is essentially unidirectional, which impedes practical application of FRET-based technologies. Here, we propose a mutual FRET scheme that is uniquely bidirectional in a binary colloidal quantum well (CQW) complex enabled by utilizing the d orbital electrons in a dopant-host CQW system. Steady-state emission intensity, time-resolved, and photoluminescence excitation spectroscopies have demonstrated that two distinct CQWs play the role of donor and acceptor simultaneously in this complex consisting of 3 monolayer (ML) copper-doped CQWs and 4 ML undoped CQWs. Band-edge excitons in 3 ML CQWs effectively transfer the excitation to excitons in 4 ML CQWs, whose energy is also harvested backward by the dopants in 3 ML CQWs. This binary CQW complex, which offers a unique mutual energy-transfer mechanism, may unlock revolutionary FRET-based technologies.

Determination of the In-Plane Exciton Radius in 2D CdSe Nanoplatelets via Magneto-optical Spectroscopy

Colloidal, two-dimensional semiconductor nanoplatelets (NPLs) exhibit quantum confinement in only one dimension, which results in an electronic structure that is significantly altered compared to that of other quantum-confined nanomaterials. Whereas it is often assumed that the lack of quantum confinement in the lateral plane yields a spatially extended exciton, reduced dielectric screening potentially challenges this picture. Here, we implement absorption spectroscopy in pulsed magnetic fields up to 60 T for three different CdSe NPL thicknesses and lateral areas. Based on diamagnetic shifts, we find that the exciton lateral extent is comparable to NPL thickness, indicating that the quantum confinement and reduced screening concomitant with few-monolayer thickness strongly reduces the exciton lateral extent. Atomistic electronic structure calculations of the exciton size for varying lengths, widths, and thicknesses support the substantially smaller in-plane exciton extent.