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.

Highly Efficient Light-Emitting Diodes Based on Self-Assembled Colloidal Quantum Wells

Nanocrystal-based light-emitting diodes (Nc-LEDs) have immense potential for next-generation high-definition displays and lighting applications. They offer numerous advantages, such as low cost, high luminous efficiency, narrow emission, and long lifetime. However, the external quantum efficiency (EQE) of Nc-LEDs, typically employing isotropic nanocrystals, is limited by the out-coupling factor. Here efficient, bright, and long lifetime red Nc-LEDs based on anisotropic nanocrystals of colloidal quantum wells (CQWs) are demonstrated. Through modification of the substrate's surface properties and control of the interactions among CQWs, a self-assembled layer with an exceptionally high distribution of in-plane transitions dipole moment of 95%, resulting in an out-coupling factor of 37% is successfully spin-coated. The devices exhibit a remarkable peak EQE of 26.9%, accompanied by a maximum brightness of 55 754 cd m-2 and a long operational lifetime (T95 @100 cd m-2 ) over 15 000 h. These achievements represent a significant advancement compared to previous studies on Nc-LEDs incorporating anisotropic nanocrystals. The work is expected to provide a general self-assembly strategy for enhancing the light extraction efficiency of Nc-LEDs based on anisotropic nanocrystals.

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.

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.

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.

Ultralow-Threshold and High-Quality Whispering-Gallery-Mode Lasing from Colloidal Core/Hybrid-Shell Quantum Wells

The realization of efficient on-chip microlasers with scalable fabrication, ultralow threshold, and stable single-frequency operation is always desired for a wide range of miniaturized photonic systems. Herein, an effective way to fabricate nanostructures- whispering-gallery-mode (WGM) lasers by drop-casting CdSe/CdS@Cd_{1- x} Zn_x S core/buffer-shell@graded-shell nanoplatelets (NPLs) dispersion onto silica microspheres is presented. Benefiting from the excellent gain properties from the interface engineered core/hybrid shell NPLs and high-quality factor WGM resonator from excellent optical field confinement, the proposed room-temperature NPLs-WGM microlasers show a record-low lasing threshold of 3.26 µJ cm^-2 under nanosecond laser pumping among all colloidal NPLs-based lasing demonstrations. The presence of sharp discrete transverse electric- and magnetic-mode spikes, the inversely proportional dependence of the free spectra range on microsphere sizes and the polarization anisotropy of laser output represent the first direct experimental evidence for NPLs-WGM lasing nature, which is verified theoretically by the computed electric-field distribution inside the microcavity. Remarkably, a stable single-mode lasing output with an ultralow lasing threshold of 3.84 µJ cm^-2 is achieved by the Vernier effect through evanescent field coupling. The results highlight the significance of interface engineering on the optimization of gain properties of heterostructured nanomaterials and shed light on developing future miniaturized tunable coherent light sources.

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.

Origin of Low Temperature Trion Emission in CdSe Nanoplatelets

Colloidal semiconductor nanoplatelets (NPLs) are a scalable materials platform for optoelectronic applications requiring fast and narrow emission, including spin-to-photon transduction within quantum information networks. In particular, three-particle negative trions of NPLs are appealing emitters since, unlike excitons, they do not have an optically "dark" sublevel. In CdSe NPLs, trion emission dominates the photoluminescence (PL) spectrum at low temperature but using them as single photon-emitting states requires more knowledge about their preparation, since trions in these materials are not directly optically accessible from the ground state. This work demonstrates, using power-dependent time-resolved transient absorptions (TA) of CdSe NPLs, that trions form via biexciton decay in 1.6 ps. The scaling of the trion population and formation lifetime with excitation power indicates that they do not form through collisional mechanisms typical for 2D materials, but rather by a unimolecular hole transfer. This work is a step toward deterministic single photon emission from trions.

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.

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.

Intersubband Relaxation in CdSe Colloidal Quantum Wells

The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied via pump-push-probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (e.g., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant intraband relaxation in colloidal quantum dots (up to hundreds of picoseconds), intersubband relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.

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.

Colloidal CdSe Quantum Wells with Graded Shell Composition for Low-Threshold Amplified Spontaneous Emission and Highly Efficient Electroluminescence

Semiconductor nanoplatelets (NPLs) have emerged as a very promising class of colloidal nanocrystals for light-emitting devices owing to their quantum-well-like electronic and optical characteristics. However, their lower photoluminescence quantum yield (PLQY) and limited stability have hampered the realization of their outstanding luminescent properties in device applications. Here, to address these deficiencies, we present a two-step synthetic approach that enables the synthesis of core/shell NPLs with precisely controlled shell composition for engineering their excitonic properties. The proposed CdSe colloidal quantum wells possess a graded shell, which is composed of a CdS buffer layer and a Cd_xZn_1-xS gradient layer, and exhibit bright emission (PLQY 75-89%) in the red spectral region (634-648 nm) with a narrow emission line width (21 nm). These enhanced optical properties allowed us to attain low thresholds for amplified spontaneous emission (down to ∼40 μJ/cm²) under nanosecond laser excitation. We also studied the electroluminescent performance of these NPLs by fabricating solution-processed light-emitting diodes (LEDs). In comparison to NPL-LEDs with CdSe/CdS core/shell NPLs, which exhibit an external quantum efficiency (EQE) value of only 1.80%, a significantly improved EQE value of 9.92% was obtained using graded-shell NPLs, the highest value for colloidal NPL-based-LEDs. In addition, the low efficiency roll-off characteristics of NPL-LEDs enabled a high brightness of up to ∼46 000 cd/m² with an electroluminescence peak centered at 650 nm. These findings demonstrate the paramount role that heterostructure engineering occupies in enhancing the optoelectronic characteristics of semiconductor NPLs toward practically relevant levels.

Giant Alloyed Hot Injection Shells Enable Ultralow Optical Gain Threshold in Colloidal Quantum Wells

As an attractive materials system for high-performance optoelectronics, colloidal nanoplatelets (NPLs) benefit from atomic-level precision in thickness, minimizing emission inhomogeneous broadening. Much progress has been made to enhance their photoluminescence quantum yield (PLQY) and photostability. However, to date, layer-by-layer growth of shells at room temperature has resulted in defects that limit PLQY and thus curtail the performance of NPLs as an optical gain medium. Here, we introduce a hot-injection method growing giant alloyed shells using an approach that reduces core/shell lattice mismatch and suppresses Auger recombination. Near-unity PLQY is achieved with a narrow full-width-at-half-maximum (20 nm), accompanied by emission tunability (from 610 to 650 nm). The biexciton lifetime exceeds 1 ns, an order of magnitude longer than in conventional colloidal quantum dots (CQDs). Reduced Auger recombination enables record-low amplified spontaneous emission threshold of 2.4 μJ cm^-2 under one-photon pumping. This is lower by a factor of 2.5 than the best previously reported value in nanocrystals (6 μJ cm^-2 for CdSe/CdS NPLs). Here, we also report single-mode lasing operation with a 0.55 mJ cm^-2 threshold under two-photoexcitation, which is also the best among nanocrystals (compared to 0.76 mJ cm^-2 from CdSe/CdS CQDs in the Fabry-Pérot cavity). These findings indicate that hot-injection growth of thick alloyed shells makes ultrahigh performance NPLs.

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).

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.

Lateral Size-Dependent Spontaneous and Stimulated Emission Properties in Colloidal CdSe Nanoplatelets

Here, we systematically investigated the spontaneous and stimulated emission performances of solution-processed atomically flat quasi-2D nanoplatelets (NPLs) as a function of their lateral size using colloidal CdSe core NPLs. We found that the photoluminescence quantum efficiency of these NPLs decreases with increasing lateral size while their photoluminescence decay rate accelerates. This strongly suggests that nonradiative channels prevail in the NPL ensembles having extended lateral size, which is well-explained by the increasing number of the defected NPL subpopulation. In the case of stimulated emission the role of lateral size in NPLs influentially emerges both in the single- and two-photon absorption (1PA and 2PA) pumping. In the amplified spontaneous emission measurements, we uncovered that the stimulated emission thresholds of 1PA and 2PA exhibit completely opposite behavior with increasing lateral size. The NPLs with larger lateral sizes exhibited higher stimulated emission thresholds under 1PA pumping due to the dominating defected subpopulation in larger NPLs. On the other hand, surprisingly, larger NPLs remarkably revealed lower 2PA-pumped amplified spontaneous emission thresholds. This is attributed to the observation of a "giant" 2PA cross-section overwhelmingly growing with increasing lateral size and reaching record levels higher than 10(6) GM, at least an order of magnitude stronger than colloidal quantum dots and rods. These findings suggest that the lateral size control in the NPLs, which is commonly neglected, is essential to high-performance colloidal NPL optoelectronic devices in addition to the vertical monolayer control.

Stacking in colloidal nanoplatelets: tuning excitonic properties

Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Förster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Förster radius of ∼13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening.