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

Alloyed Heterostructures of CdSe_<i>x</i>S_1–<i>x</i> Nanoplatelets with Highly Tunable Optical Gain Performance

Here, we designed and synthesized alloyed heterostructures of CdSe_<i>x</i>S_1–<i>x</i> nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSe_<i>x</i>S_1–<i>x</i> core NPLs as a seed, we prepared CdSe_<i>x</i>S_1–<i>x</i>/CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSe_<i>x</i>S_1–<i>x</i> cores, we achieved enhanced photoluminescence quantum yield (PL-QY) (reaching 60%), together with increased absorption cross-section and improved stability without changing the emission spectrum of CdSe_<i>x</i>S_1–<i>x</i> alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSe_<i>x</i>S_1–<i>x</i>/CdS core/shell NPLs. Depending on the sulfur composition of the CdSe_<i>x</i>S_1–<i>x</i> core and thickness of the CdS shell, CdSe_<i>x</i>S_1–<i>x</i>/CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560–650 nm. Finally, we studied the optical gain performances of different heterostructures of CdSe_<i>x</i>S_1–<i>x</i> alloyed NPLs offering great advantages, including reduced reabsorption and spectrally tunable optical gain range. Despite their decreased PL-QY and reduced absorption cross-section upon increasing the sulfur composition, CdSe_<i>x</i>S_1–<i>x</i> based NPLs exhibit highly tunable amplified spontaneous emission performance together with low gain thresholds down to ∼53 μJ/cm²

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

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

Low-Molecular-Weight Dipeptide Nanogel Containing Plasmonic Gold Nanoparticles for Drug Release Applications

The control of the release behavior of a drug by an external stimulus, controllability from outside the body, reduced toxicity, and other side effects can provide benefits in delivery systems used in cancer treatment. However, the preparation and development of systems that control the dosage of the periodically released drug by an external stimulus are still a clinically needed approach. Here, we developed near-infrared (NIR) laser-enabled peptide nanogel systems by preparing Fmoc-diphenylalanine (Fmoc-FF) nanogels with a simple dispersion approach and preparing gold nanoparticles (AuNPs) decorated or gold nanostars (AuNSs) embedded in this system. The morphological properties and sizes of the prepared AuNS-embedded and AuNP-decorated Fmoc-FF nanogels were investigated by dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. Structural and thermal characterizations were performed with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC), respectively. Epirubicin (EPI) release behaviors of the prepared AuNS-embedded and AuNP-decorated Fmoc-FF nanogels were investigated under 808 nm NIR laser irradiation. In vitro cytotoxicity and genotoxicity behaviors of the prepared particles were examined, and their effects on laser control release behaviors were also evaluated. It has been observed that EPI release can be controlled by laser irradiation in nanogels containing embedded or decorated plasmonic gold nanoparticles. In addition, it is understood from the in vitro results that the prepared nanogel systems are more effective synergistically under 808 NIR laser irradiation. Our results showed that Fmoc-FF peptide nanogel systems prepared as plasmonic AuNP embedded or decorated have great potential for controlled drug delivery 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