"Giant" Colloidal Quantum Well Heterostructures of CdSe@CdS Core@Shell Nanoplatelets from 9.5 to 17.5 Monolayers in Thickness Enabling Ultra-High Gain Lasing.

Semiconductor colloidal quantum wells (CQWs) have emerged as a promising class of gain materials to be used in colloidal lasers. Although low gain thresholds are achieved, the required high gain coefficient levels are barely met for the applications of electrically-driven lasers which entails a very thin gain matrix to avoid charge injection limitations. Here, "giant" CdSe@CdS colloidal quantum well heterostructures of 9.5 to 17.5 monolayers (ML) in total with corresponding vertical thickness from 3.0 to 5.8 nm that enable record optical gain is shown. These CQWs achieve ultra-high material gain coefficients up to ≈140 000 cm-1 , obtained by systematic variable stripe length (VSL) measurements and independently validated by transient absorption (TA) measurements, owing to their high number of states. This exceptional gain capacity is an order of magnitude higher than the best levels reported for the colloidal quantum dots. From the dispersion of these quantum wells, low threshold amplified spontaneous emission in water providing an excellent platform for optofluidic lasers is demonstrated. Also, employing these giant quantum wells, whispering gallery mode (WGM) lasing with an ultra-low threshold of 8 µJ cm-2 is demonstrated. These findings indicate that giant CQWs offer an exceptional platform for colloidal thin-film lasers and in-solution lasing applications.

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.

Dual-Resonance Nanostructures for Color Downconversion of Colloidal Quantum Emitters.

We present a dual-resonance nanostructure made of a titanium dioxide (TiO2) subwavelength grating to enhance the color downconversion efficiency of CdxZn1-xSeyS1-y colloidal quantum dots (QDs) emitting at ∼530 nm when excited with a blue light at ∼460 nm. A large mode volume can be created within the QD layer by the hybridization of the grating resonances and waveguide modes, resulting in large absorption and emission enhancements. Particularly, we achieved polarized light emission with a maximum photoluminescence enhancement of ∼140 times at a specific angular direction and a total enhancement of ∼34 times within a 0.55 numerical aperture (NA) of the collecting objective. The enhancement encompasses absorption, Purcell and outcoupling enhancements. We achieved a total absorption of 35% for green QDs with a remarkably thin color conversion layer of ∼400 nm. This work provides a guideline for designing large-volume cavities for absorption/fluorescence enhancement in microLED display, detector, or photovoltaic applications.

On the Rational Design of Core/(Multi)-Crown Type-II Heteronanoplatelets.

Solution-processed two-dimensional nanoplatelets (NPLs) allowing lateral growth of a shell (crown) by not affecting the pure confinement in the vertical direction provide unprecedented opportunities for designing heterostructures for light-emitting and -harvesting applications. Here, we present a pathway for designing and synthesizing colloidal type-II core/(multi-)crown hetero-NPLs and investigate their optical properties. Stoke's shifted broad photoluminescence (PL) emission and long PL lifetime (∼few 100 ns) together with our wavefunction calculations confirm the type-II electronic structure in the synthesized CdS/CdSe1-xTex core/crown hetero-NPLs. In addition, we experimentally obtained the band-offsets between CdS, CdTe, and CdSe in these NPLs. These results helped us designing hetero-NPLs with near-unity PL quantum yield in the CdSe/CdSe1-xTex/CdSe/CdS core/multicrown architecture. These core/multicrown hetero-NPLs have two type-II interfaces unlike traditional type-II NPLs having only one and possess a CdS ending layer for passivation and efficient suppression of stacking required for optoelectronic applications. The light-emitting diode (LED) obtained using multicrown hetero-NPLs exhibits a maximum luminance of 36,612 cd/m2 and external quantum efficiency of 9.3%, which outcompetes the previous best results from type-II NPL-based LEDs. These findings may enable designs of future advanced heterostructures of NPLs which are anticipated to show desirable results, especially for LED and lasing platforms.

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.

Gradient Type-II CdSe/CdSeTe/CdTe Core/Crown/Crown Heteronanoplatelets with Asymmetric Shape and Disproportional Excitonic Properties.

Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering. Moreover, these NPLs allow for switching between active channels with temperature, where lattice contraction directly affects the electron-hole (e-h) overlap. Dominated by the indirect transition channels over direct transitions, the lifetime of the NPLs starts to increase at 9 K, indicative of low dark-bright exciton splitting energy. The charge transfer states from the two type-II interfaces promote a large number of indirect transitions, which effectively increase the absorption of low-energy photons critical for nonlinear properties. As a result, these NPLs demonstrate exceptionally high two-photon absorption cross-sections with the highest value of 12.9 × 106 GM and superlinear behavior.

Optically detected magnetic resonance spectroscopic analyses on the role of magnetic ions in colloidal nanocrystals.

Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier-magnetic ion interactions. Various host materials, including the II-VI group, halide perovskites, and I-III-VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.

Observation of Phonon Cascades in Cu-Doped Colloidal Quantum Wells.

Electronic doping has endowed colloidal quantum wells (CQWs) with unique optical and electronic properties, holding great potential for future optoelectronic device concepts. Unfortunately, how photogenerated hot carriers interact with phonons in these doped CQWs still remains an open question. Here, through investigating the emission properties, we have observed an efficient phonon cascade process (i.e., up to 27 longitudinal optical phonon replicas are revealed in the broad Cu emission band at room temperature) and identified a giant Huang-Rhys factor (S ≈ 12.4, more than 1 order of magnitude larger than reported values of other inorganic semiconductor nanomaterials) in Cu-doped CQWs. We argue that such an ultrastrong electron-phonon coupling in Cu-doped CQWs is due to the dopant-induced lattice distortion and the dopant-enhanced density of states. These findings break the widely accepted consensus that electron-phonon coupling is typically weak in quantum-confined systems, which are crucial for optoelectronic applications of doped electronic nanomaterials.

Deep-Red-Emitting Colloidal Quantum Well Light-Emitting Diodes Enabled through a Complex Design of Core/Crown/Double Shell Heterostructure.

Extending the emission peak wavelength of quasi-2D colloidal quantum wells has been an important quest to fully exploit the potential of these materials, which has not been possible due to the complications arising from the partial dissolution and recrystallization during growth to date. Here, the synthetic pathway of (CdSe/CdS)@(1-4 CdS/CdZnS) (core/crown)@(colloidal atomic layer deposition shell/hot injection shell) hetero-nanoplatelets (NPLs) using multiple techniques, which together enable highly efficient emission beyond 700 nm in the deep-red region, is proposed and demonstrated. Given the challenges of using conventional hot injection procedure, a method that allows to obtain sufficiently thick and passivated NPLs as the seeds is developed. Consequently, through the final hot injection shell coating, thick NPLs with superior optical properties including a high photoluminescence quantum yield of 88% are achieved. These NPLs emitting at 701 nm exhibit a full-width-at-half-maximum of 26 nm, enabled by the successfully maintained quasi-2D shape and minimum defects of the resulting heterostructure. The deep-red light-emitting diode (LED) device fabricated with these NPLs has shown to yield a high external quantum efficiency of 6.8% at 701 nm, which is on par with other types of LEDs in this spectral range.

Self-Resonant Microlasers of Colloidal Quantum Wells Constructed by Direct Deep Patterning.

Here, the first account of self-resonant fully colloidal µ-lasers made from colloidal quantum well (CQW) solution is reported. A deep patterning technique is developed to fabricate well-defined high aspect-ratio on-chip CQW resonators made of grating waveguides and in-plane reflectors. The fabricated waveguide-coupled laser, enabling tight optical confinement, assures in-plane lasing. CQWs of the patterned layers are closed-packed with sharp edges and residual-free lifted-off surfaces. Additionally, the method is successfully applied to various nanoparticles including colloidal quantum dots and metal nanoparticles. It is observed that the patterning process does not affect the nanocrystals (NCs) immobilized in the attained patterns and the different physical and chemical properties of the NCs remain pristine. Thanks to the deep patterning capability of the proposed method, patterns of NCs with subwavelength lateral feature sizes and micron-scale heights can possibly be fabricated in high aspect ratios.

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.

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.

Room-Temperature Lasing in Colloidal Nanoplatelets via Mie-Resonant Bound States in the Continuum.

Solid-state room-temperature lasing with tunability in a wide range of wavelengths is desirable for many applications. To achieve this, besides an efficient gain material with a tunable emission wavelength, a high quality-factor optical cavity is essential. Here, we combine a film of colloidal CdSe/CdZnS core-shell nanoplatelets with square arrays of nanocylinders made of titanium dioxide to achieve optically pumped lasing at visible wavelengths and room temperature. The all-dielectric arrays support bound states in the continuum (BICs), which result from lattice-mediated Mie resonances and boast infinite quality factors in theory. In particular, we demonstrate lasing from a BIC that originates from out-of-plane magnetic dipoles oscillating in phase. By adjusting the diameter of the cylinders, we tune the lasing wavelength across the gain bandwidth of the nanoplatelets. The spectral tunability of both the cavity resonance and nanoplatelet gain, together with efficient light confinement in BICs, promises low-threshold lasing with wide selectivity in wavelengths.

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.

Giant Modal Gain Coefficients in Colloidal II-VI Nanoplatelets.

Modal gain coefficient is a key figure of merit for a laser material. Previously, net modal gain coefficients larger than a few thousand cm-1 were achieved in II-VI and III-V semiconductor gain media, but this required operation at cryogenic temperatures. In this work, using pump-fluence-dependent variable-stripe-length measurements, we show that colloidal CdSe nanoplatelets enable giant modal gain coefficients at room temperature up to 6600 cm-1 under pulsed optical excitation. Furthermore, we show that exceptional gain performance is common to the family of CdSe nanoplatelets, as shown by examining samples having different vertical thicknesses and lateral areas. Overall, colloidal II-VI nanoplatelets with superior optical gain properties are promising for a broad range of applications, including high-speed light amplification and loss compensation in plasmonic photonic circuits.

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

We proposed and showed strongly orientation-controlled Förster 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 (face-down), with near-unity surface coverage and across large areas over 20 cm2. 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 Förster 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.

Light Generation in Lead Halide Perovskite Nanocrystals: LEDs, Color Converters, Lasers, and Other Applications.

Facile solution processing lead halide perovskite nanocrystals (LHP-NCs) exhibit superior properties in light generation, including a wide color gamut, a high flexibility for tuning emissive wavelengths, a great defect tolerance and resulting high quantum yield; and intriguing electric feature of ambipolar transport with moderate and comparable mobility. As a result, LHP-NCs have accomplished great achievements in various light generation applications, including color converters for lighting and display, light-emitting diodes, low threshold lasing, X-ray scintillators, and single photon emitters. Herein, the considerable progress that has been made thus far is reviewed along with the current challenges and future prospects in the light generation applications of LHP-NCs.

Highly Stable, Near-Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero-Nanoplatelets Enabled by ZnS-Shell Hot-Injection Growth.

Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c-ALD) provides the ability to produce their core/shell heterostructures. However, as c-ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near-unity efficiency CdSe/ZnS NPLs are shown using hot-injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI-shell hetero-NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c-ALD shell-coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI-shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm-2 . Despite being annealed at 500 K, these ZnS-HI-shell NPLs possess low gain thresholds as small as 25 µJ cm-2 . These findings indicate that the proposed 573 K HI-shell-grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.

Light-Emitting Diodes with Cu-Doped Colloidal Quantum Wells: From Ultrapure Green, Tunable Dual-Emission to White Light.

Copper-doped colloidal quantum wells (Cu-CQWs) are considered a new class of optoelectronic materials. To date, the electroluminescence (EL) property of Cu-CQWs has not been revealed. Additionally, it is desirable to achieve ultrapure green, tunable dual-emission and white light to satisfy the various requirement of display and lighting applications. Herein, light-emitting diodes (LEDs) based on colloidal Cu-CQWs are demonstrated. For the 0% Cu-doped concentration, the LED exhibits Commission Internationale de L'Eclairage 1931 coordinates of (0.103, 0.797) with a narrow EL full-wavelength at half-maximum of 12 nm. For the 0.5% Cu-doped concentration, a dual-emission LED is realized. Remarkably, the dual emission can be tuned by manipulating the device engineering. Furthermore, at a high doping concentration of 2.4%, a white LED based on CQWs is developed. With the management of doping concentrations, the color tuning (green, dual-emission to white) is shown. The findings not only show that LEDs with CQWs can exhibit polychromatic emission but also unlock a new direction to develop LEDs by exploiting 2D impurity-doped CQWs that can be further extended to the application of other impurities (e.g., Mn, Ag).

Impurity incorporation and exchange interactions in Co2+-doped CdSe/CdS core/shell nanoplatelets.

The intentional incorporation of transition metal impurities into colloidal semiconductor nanocrystals allows an extension of the host material's functionality. While dopant incorporation has been extensively investigated in zero-dimensional quantum dots, the substitutional replacement of atoms in two-dimensional (2D) nanostructures by magnetic dopants has been reported only recently. Here, we demonstrate the successful incorporation of Co2+ ions into the shell of CdSe/CdS core/shell nanoplatelets, using these ions (i) as microscopic probes for gaining distinct structural insights and (ii) to enhance the magneto-optical functionality of the host material. Analyzing interatomic Co2+ ligand field transitions, we conclude that Co2+ is incorporated into lattice sites of the CdS shell, and effects such as diffusion of dopants into the CdSe core or diffusion of the dopants out of the heterostructure causing self-purification play a minor role. Taking advantage of the absorption-based technique of magnetic circular dichroism, we directly prove the presence of sp-d exchange interactions between the dopants and the band charge carriers in CdSe/Co2+:CdS heteronanoplatelets. Thus, our study not only demonstrates magneto-optical functionality in 2D nanocrystals by Co2+ doping but also shows that a careful choice of the dopant type paves the way for a more detailed understanding of the impurity incorporation process into these novel 2D colloidal materials.