Amplified Spontaneous Emission and Lasing in Colloidal Nanoplatelets

Colloidal nanoplatelets (NPLs) have recently emerged as favorable light-emitting materials, which also show great potential as optical gain media due to their remarkable optical properties. In this work, we systematically investigate the optical gain performance of CdSe core and CdSe/CdS core/crown NPLs having different CdS crown size with one- and two-photon absorption pumping. The core/crown NPLs exhibit enhanced gain performance as compared to the core-only NPLs due to increased absorption cross section and the efficient interexciton funneling, which is from the CdS crown to the CdSe core. One- and two-photon absorption pumped amplified spontaneous emission thresholds are found as low as 41 μJ/cm² and 4.48 mJ/cm², respectively. These thresholds surpass the best reported optical gain performance of the state-of-the-art colloidal nanocrystals (<i>i.e.</i>, quantum dots, nanorods, <i>etc.</i>) emitting in the same spectral range as the NPLs. Moreover, gain coefficient of the NPLs is measured as high as 650 cm^–1, which is 4-fold larger than the best reported gain coefficient of the colloidal quantum dots. Finally, we demonstrate a two-photon absorption pumped vertical cavity surface emitting laser of the NPLs with a lasing threshold as low as 2.49 mJ/cm². These excellent results are attributed to the superior properties of the NPLs as optical gain media

Temperature-Dependent Emission Kinetics of Colloidal Semiconductor Nanoplatelets Strongly Modified by Stacking

We systematically studied temperature-dependent emission kinetics in solid films of solution-processed CdSe nanoplatelets (NPLs) that are either intentionally stacked or nonstacked. We observed that the steady-state photoluminescence (PL) intensity of nonstacked NPLs considerably increases with decreasing temperature, whereas there is only a slight increase in stacked NPLs. Furthermore, PL decay time of the stacked NPL ensemble is comparatively much shorter than that of the nonstacked NPLs, and this result is consistent at all temperatures. To account for these observations, we developed a probabilistic model that describes excitonic processes in a stack using Markov chains, and we found excellent agreement between the model and experimental results. These findings develop the insight that the competition between the radiative channels and energy transfer-assisted hole trapping leads to weakly temperature-dependent PL intensity in the case of the stacked NPL ensembles as compared to the nonstacked NPLs lacking strong energy transfer. This study shows that it is essential to account for the effect of NPL stacking to understand their resulting PL emission properties

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>i</i>.<i>e</i>., 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

High-Efficiency Optical Gain in Type-II Semiconductor Nanocrystals of Alloyed Colloidal Quantum Wells

Colloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSe_<i>x</i>Te_1–<i>x</i> quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 μJ/cm², long optical gain lifetimes (τ_gain ≈ 400 ps), and high modal gain coefficients (<i>g</i>_modal ≈ 930 cm^–1). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies

Colloidal Nanoplatelet/Conducting Polymer Hybrids: Excitonic and Material Properties

Here we present the first account of conductive polymer/colloidal nanoplatelet hybrids. For this, we developed DEH-PPV-based polymers with two different anchor groups (sulfide and amine) acting as surfactants for CdSe nanoplatelets, which are atomically flat semiconductor nanocrystals. Hybridization of the polymers with the nanoplatelets in the solution phase was observed to cause strong photoluminescence quenching in both materials. Through steady-state photoluminescence and excitation spectrum measurements, photoluminescence quenching was shown to result from dominant exciton dissociation through charge transfer at the polymer/nanoplatelet interfaces that possess a staggered (i.e., type II) band alignment. Importantly, we found out that sulfide-based anchors enable a stronger emission quenching than amine-based ones, suggesting that the sulfide anchors exhibit more efficient binding to the nanoplatelet surfaces. Also, shorter surfactants were found to be more effective for exciton dissociation as compared to the longer ones. In addition, we show that nanoplatelets are homogeneously distributed in the hybrid films owing to the functional polymers. These nanocomposites can be used as building blocks for hybrid optoelectronic devices, such as solar cells

Additional file 1: of Cd-free Cu-doped ZnInS/ZnS Core/Shell Nanocrystals: Controlled Synthesis And Photophysical Properties

Figure S1. (a) UV-visible and PL spectrum of ZnInS:Cu/ZnS CNCs by various Cu doping amounts. (b) PLE spectrum of ZnInS:Cu/ZnS CNCs at different emission wavelengths (500, 550, and 600 nm) acquired from PL spectrum (inset). Figure S2. UV-visible and PL spectrum of ZnInS:Cu (core) and ZnInS:Cu/ZnS (core/shell) CNCs with different Cu dopant percentages. Figure S3. UV-visible, PL, and PLE spectrum of ZnInS:Cu/ZnS CNCs. Table S1. Fluorescence decay components of the Cu-doped ZnInS (core) and ZnInS/ZnS (core/ shell) CNCs. Table S2. Fluorescence decay components of the Cu-doped ZnInS/ZnS CNCs. Table S3. Fluorescence decay components of the Cu-doped ZnInS/ZnS CNCs. Figure S4. EL spectra of G- and O-emitting ZnInS:Cu/ZnS CNCs integrated LED. Table S4. The CRI, luminous efficacy of optical radiation (LER), CCT, and CIE color coordinates of the as-fabricated WLEDs based on G- and O-Cu:ZnInS/ZnS CNCs blends with different weight ratios operated at different currents (mA). Figure S5. EL spectra of G-, Y-, O-emitting ZnInS:Cu/ZnS CNCs integrated LED. Table S5. The CRI, luminous efficacy of optical radiation (LER), CCT, and CIE color coordinates of the as-fabricated WLEDs based on G-, Y-, and O- Cu:ZnInS/ZnS CNC blends with different weight ratios operated at different currents (mA). (DOCX 1212 kb

CdSe/CdSe_1–<i>x</i>Te_<i>x</i> Core/Crown Heteronanoplatelets: Tuning the Excitonic Properties without Changing the Thickness

Here we designed and synthesized CdSe/CdSe_1–<i>x</i>Te_<i>x</i> core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe_1–<i>x</i>Te_<i>x</i> crown region from pure CdTe (<i>x</i> = 1.00) to almost pure CdSe doped with several Te atoms (<i>x</i> = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of ∼570 nm (for CdSe_1–<i>x</i>Te_<i>x</i> crown with <i>x</i> = 0.02) and ∼660 nm (for CdSe_1–<i>x</i>Te_<i>x</i> crown with <i>x</i> = 1.00). Furthermore, with the formation of the CdSe_1–<i>x</i>Te_<i>x</i> crown region, we observed substantially improved photoluminescence quantum yields (up to ∼95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from ∼49 up to ∼326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and -harvesting applications

Understanding the Journey of Dopant Copper Ions in Atomically Flat Colloidal Nanocrystals of CdSe Nanoplatelets Using Partial Cation Exchange Reactions

Unique electronic and optical properties of doped semiconductor nanocrystals (NCs) have widely stimulated a great deal of interest to explore new effective synthesis routes to achieve controlled doping for highly efficient materials. In this work, we show copper doping via postsynthesis partial cation exchange (CE) in atomically flat colloidal semiconductor nanoplatelets (NPLs). Here chemical reactivity of different dopant precursors, reaction kinetics, and shape of seed NPLs were extensively elaborated for successful doping and efficient emission. Dopant-induced Stokes-shifted and tunable photoluminescence emission (640 to 830 nm) was observed in these Cu-doped CdSe NPLs using different thicknesses and heterostructures. High quantum yields (reaching 63%) accompanied by high absorption cross sections (>2.5 times) were obtained in such NPLs compared to those of Cu-doped CdSe colloidal quantum dots (CQDs). Systematic tuning of the doping level in these two-dimensional NPLs provides an insightful understanding of the chemical dopant based orbital hybridization in NCs. The unique combination of doping via the partial CE method and precise control of quantum confinement in such atomically flat NPLs originating from their magic-sized vertical thickness exhibits an excellent model platform for studying photophysics of doped quantum confined systems

Near-Unity Efficiency Energy Transfer from Colloidal Semiconductor Quantum Wells of CdSe/CdS Nanoplatelets to a Monolayer of MoS₂

A hybrid structure of the quasi-2D colloidal semiconductor quantum wells assembled with a single layer of 2D transition metal dichalcogenides offers the possibility of highly strong dipole-to-dipole coupling, which may enable extraordinary levels of efficiency in Förster resonance energy transfer (FRET). Here, we show ultrahigh-efficiency FRET from the ensemble thin films of CdSe/CdS nanoplatelets (NPLs) to a MoS₂ monolayer. From time-resolved fluorescence spectroscopy, we observed the suppression of the photoluminescence of the NPLs corresponding to the total rate of energy transfer from ∼0.4 to 268 ns^–1. Using an Al₂O₃ separating layer between CdSe/CdS and MoS₂ with thickness tuned from 5 to 1 nm, we found that FRET takes place 7- to 88-fold faster than the Auger recombination in CdSe-based NPLs. Our measurements reveal that the FRET rate scales down with <i>d</i>^–2 for the donor of CdSe/CdS NPLs and the acceptor of the MoS₂ monolayer, <i>d</i> being the center-to-center distance between this FRET pair. A full electromagnetic model explains the behavior of this <i>d</i>^–2 system. This scaling arises from the delocalization of the dipole fields in the ensemble thin film of the NPLs and full distribution of the electric field across the layer of MoS₂. This <i>d</i>^–2 dependency results in an extraordinarily long Förster radius of ∼33 nm