Trap-Mediated Sensitization Governs Near-Infrared Emission from Yb3+-Doped Mixed-Halide CsPbClxBr3-x Perovskite Nanocrystals.

Understanding the photosensitization mechanisms in Yb3+-doped perovskite nanocrystals is crucial for developing their anticipated photonic applications. Here, we address this question by investigating near-infrared photoluminescence of Yb3+-doped mixed-halide CsPbClxBr3-x nanocrystals as a function of temperature and revealing its strong dependence on the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, we developed a theoretical model in which energy transfer from the perovskite matrix to Yb3+ ions occurs through intermediate trap states situated beneath the conduction band of the host. The developed model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and near-infrared photoluminescence quantum yield as a function of the anion composition. Our findings establish trap-mediated energy transfer as a dominant photosensitization mechanism in Yb3+-doped CsPbClxBr3-x nanocrystals and open up new ways of engineering their optical properties for light-emitting and light-harvesting applications.

Nonparabolicity of size-quantized subbands of bilayer semiconductor quantum wells with heterojunction.

This paper presents a theory of size quantization and intersubband optical transitions in bilayer semiconductor quantum wells with asymmetric profile. We show that, in contrast to single-layer quantum wells, the size-quantized subbands of bilayer quantum wells are nonparabolic and characterized by effective masses that depend on the electron wave number and the subband number. It is found that the effective masses are related to the localization of the electron wave function in the layers of the quantum well and can be controlled by varying the chemical composition or geometric parameters of the structure. We also derive an analytical expression for the probability of optical transitions between the subbands of the bilayer quantum well. Our results are useful for the development of laser systems and photodetectors based on colloidal nanoplates and epitaxial layers of semiconductor materials with heterojunctions.

Luminescence enhancement of alloyed quantum dots bound to gold nanoparticles by mercaptocarboxylic acids in colloidal complexes.

The understanding of the physical mechanisms of the nanoobjects interaction within the nanostructured complex materials is one of the main tasks for the development of novel materials with tunable properties. In this work, we develop a formation procedure of the colloidal complexes based on alloyed CdZnSe/ZnS quantum dots and gold nanoparticles where the various mercaptocarboxylic acids are used as the binding molecules. The QD photoluminescence enhancement (up to ×3.1) can be achieved by the control of the interparticle distance in colloidal solutions. We provide a detailed discussion on the influence of the linking molecules on the nanoparticle complexes optical parameters through the steady-state and time-resolved spectral measurements.

Enhanced stability of the optical responses from all-inorganic perovskite nanocrystals embedded in a synthetic opal matrix.

Nanostructured luminescent materials based on perovskite nanocrystals (p-NCs) are attractive since their optical properties can be tuned in a wide spectral range with high luminescence quantum yields and lifetimes, however, they lack stability. In this work, the optical properties of highly luminescent colloidal p-NCs (CsPbX3, where X = Cl/Br, Br, I) embedded in porous opal matrices are presented. It is shown that the photoluminescence of the p-NCs embedded into opal matrices possess increased longtime stability of its spectral and kinetic parameters under ambient conditions. LEDs based on the developed materials show pure color p-NC emission with stability of its parameters. The results of this work may expand the knowledge of interactions between luminescent nanoparticles within multicomponent nanostructured materials for further photonic applications.

Optical properties of ordered superstructures formed from cadmium and lead chalcogenide colloidal nanocrystals.

The optical properties of three-dimensional ordered superstructures formed on glass substrates by self-assembly of cadmium selenide or lead sulfide nanocrystals (NCs) are investigated and compared to the optical properties of the initial NC colloidal solutions. The formation of the superstructures is strongly correlated to the presence of oleic acid molecules on the surface of the NCs. It is found that the absorption band of the NCs in the superstructures is broadened and shifted to shorter wavelengths in comparison with the absorption band of the NCs in solution. The luminescence spectra of the NCs in the superstructures also differ from the spectra of the NCs in solution. The observed modification of optical properties of superstructures is a manifestation of interactions between the NCs and the chemical environment within the superstructures.

Self-organization of colloidal PbS quantum dots into highly ordered superlattices.

X-ray structural analysis, together with steady-state and transient optical spectroscopy, is used for studying the morphology and optical properties of quantum dot superlattices (QDSLs) formed on glass substrates by the self-organization of PbS quantum dots with a variety of surface ligands. The diameter of the PbS QDs varies from 2.8 to 8.9 nm. The QDSL's period is proportional to the dot diameter, increasing slightly with dot size due to the increase in ligand layer thickness. Removal of the ligands has a number of effects on the morphology of QDSLs formed from the dots of different sizes: for small QDs the reduction in the amount of ligands obstructs the self-organization process, impairing the ordering of the QDSLs, while for large QDs the ordering of the superlattice structure is improved, with an interdot distance as low as 0.4 nm allowing rapid charge carrier transport through the QDSLs. QDSL formation does not induce significant changes to the absorption and photoluminescence spectra of the QDs. However, the luminescence decay time is reduced dramatically, due to the appearance of nonradiative relaxation channels.

Near-infrared two-photon excited photoluminescence from Yb3+-doped CsPbClxBr3-x perovskite nanocrystals embedded into amphiphilic silica microspheres.

Nonlinear absorption of metal-halide perovskite nanocrystals (NCs) makes them an ideal candidate for applications which require multiphoton-excited photoluminescence. By doping perovskite NCs with lanthanides, their emission can be extended into the near-infrared (NIR) spectral region. We demonstrate how the combination of Yb3+ doping and bandgap engineering of cesium lead halide perovskite NCs performed by anion exchange (from Cl- to Br-) leads to efficient and tunable emitters that operate under two-photon excitation in the NIR spectral region. By optimizing the anion composition, Yb3+-doped CsPbClxBr3-x NCs exhibited high values of two-photon absorption cross-section reaching 2.3 × 105 GM, and displayed dual-band emission located both in the visible (407-493 nm) and NIR (985 nm). With a view of practical applications of bio-visualisation in the NIR spectral range, these NCs were embedded into silica microspheres which were further wrapped with amphiphilic polymer shells to ensure their water-compatibility. The resulting microspheres with embedded NCs could be easily dispersed in both toluene and water, while still exhibiting a dual-band emission in visible and NIR under both one- and two-photon excitation conditions.

Editorial for Special Issue "Luminescent Colloidal Nanocrystals".

The field of luminescent colloidal nanocrystals and the numerous nanosystems based on them has recently made a rapid breakthrough from initial basic research to real applications and devices [...].

Ytterbium-Doped Lead-Halide Perovskite Nanocrystals: Synthesis, Near-Infrared Emission, and Open-Source Machine Learning Model for Prediction of Optical Properties.

Lead-halide perovskite nanocrystals are an attractive class of materials since they can be easily fabricated, their optical properties can be tuned all over the visible spectral range, and they possess high emission quantum yields and narrow photoluminescence linewidths. Doping perovskites with lanthanides is one of the ways to widen the spectral range of their emission, making them attractive for further applications. Herein, we summarize the recent progress in the synthesis of ytterbium-doped perovskite nanocrystals in terms of the varying synthesis parameters such as temperature, ligand molar ratio, ytterbium precursor type, and dopant content. We further consider the dependence of morphology (size and ytterbium content) and optical parameters (photoluminescence quantum yield in visible and near-infrared spectral ranges) on the synthesis parameters. The developed open-source code approximates those dependencies as multiple-parameter linear regression and allows us to estimate the value of the photoluminescence quantum yield from the parameters of the perovskite synthesis. Further use and promotion of an open-source database will expand the possibilities of the developed code to predict the synthesis protocols for doped perovskite nanocrystals.

Kinetics of pulse-induced photoluminescence from a semiconductor quantum dot.

Optical methods, which allow the determination of the dominant channels of energy and phase relaxation, are the most universal techniques for the investigation of semiconductor quantum dots. In this paper, we employ the kinetic Pauli equation to develop the first generalized model of the pulse-induced photoluminescence from the lowest-energy eigenstates of a semiconductor quantum dot. Without specifying the shape of the excitation pulse and by assuming that the energy and phase relaxation in the quantum dot may be characterized by a set of phenomenological rates, we derive an expression for the observable photoluminescence cross section, valid for an arbitrary number of the quantum dot's states decaying with the emission of secondary photons. Our treatment allows for thermal transitions occurring with both decrease and increase in energy between all the relevant eigenstates at room or higher temperature. We show that in the general case of N states coupled to each other through a bath, the photoluminescence kinetics from any of them is determined by the sum of N exponential functions, whose exponents are proportional to the respective decay rates. We illustrate the application of the developed model by considering the processes of resonant luminescence and thermalized luminescence from the quantum dot with two radiating eigenstates, and by assuming that the secondary emission is excited with either a Gaussian or exponential pulse. Analytic expressions describing the signals of secondary emission are analyzed, in order to elucidate experimental situations in which the relaxation constants may be reliably extracted from the photoluminescence spectra.

Improved One- and Multiple-Photon Excited Photoluminescence from Cd2+-Doped CsPbBr3 Perovskite NCs.

Metal halide perovskite nanocrystals (NCs) attract much attention for light-emitting applications due to their exceptional optical properties. More recently, perovskite NCs have begun to be considered a promising material for nonlinear optical applications. Numerous strategies have recently been developed to improve the properties of metal halide perovskite NCs. Among them, B-site doping is one of the most promising ways to enhance their brightness and stability. However, there is a lack of study of the influence of B-site doping on the nonlinear optical properties of inorganic perovskite NCs. Here, we demonstrate that Cd2+ doping simultaneously improves both the linear (higher photoluminescence quantum yield, larger exciton binding energy, reduced trap states density, and faster radiative recombination) and nonlinear (higher two- and three-photon absorption cross-sections) optical properties of CsPbBr3 NCs. Cd2+ doping results in a two-photon absorption cross-section, reaching 2.6 × 106 Goeppert-Mayer (GM), which is among the highest reported for CsPbBr3 NCs.

Revealing the nature of optical activity in carbon dots produced from different chiral precursor molecules.

Carbon dots (CDs) are light-emitting nanoparticles that show great promise for applications in biology and medicine due to the ease of fabrication, biocompatibility, and attractive optical properties. Optical chirality, on the other hand, is an intrinsic feature inherent in many objects in nature, and it can play an important role in the formation of artificial complexes based on CDs that are implemented for enantiomer recognition, site-specific bonding, etc. We employed a one-step hydrothermal synthesis to produce chiral CDs from the commonly used precursors citric acid and ethylenediamine together with a set of different chiral precursors, namely, L-isomers of cysteine, glutathione, phenylglycine, and tryptophan. The resulting CDs consisted of O,N-doped (and also S-doped, in some cases) carbonized cores with surfaces rich in amide and hydroxyl groups; they exhibited high photoluminescence quantum yields reaching 57%, chiral optical signals in the UV and visible spectral regions, and two-photon absorption. Chiral signals of CDs were rather complex and originated from a combination of the chiral precursors attached to the CD surface, hybridization of lower-energy levels of chiral chromophores formed within CDs, and intrinsic chirality of the CD cores. Using DFT analysis, we showed how incorporation of the chiral precursors at the optical centers induced a strong response in their circular dichroism spectra. The optical characteristics of these CDs, which can easily be dispersed in solvents of different polarities, remained stable during pH changes in the environment and after UV exposure for more than 400 min, which opens a wide range of bio-applications.

Near-Infrared Emission of HgTe Nanoplatelets Tuned by Pb-Doping.

Doping the semiconductor nanocrystals is one of the most effective ways to obtain unique materials suitable for high-performance next-generation optoelectronic devices. In this study, we demonstrate a novel nanomaterial for the near-infrared spectral region. To do this, we developed a partial cation exchange reaction on the HgTe nanoplatelets, substituting Hg cations with Pb cations. Under the optimized reaction conditions and Pb precursor ratio, a photoluminescence band shifts to ~1100 nm with a quantum yield of 22%. Based on steady-state and transient optical spectroscopies, we suggest a model of photoexcitation relaxation in the HgTe:Pb nanoplatelets. We also demonstrate that the thin films of doped nanoplatelets possess superior electric properties compared to their pristine counterparts. These findings show that Pb-doped HgTe nanoplatelets are new perspective material for application in both light-emitting and light-detection devices operating in the near-infrared spectral region.

Engineering the Optical Properties of CsPbBr3 Nanoplatelets through Cd2+ Doping.

Lead halide perovskite nanoplatelets (NPls) attract significant attention due to their exceptional and tunable optical properties. Doping is a versatile strategy for modifying and improving the optical properties of colloidal nanostructures. However, the protocols for B-site doping have been rarely reported for 2D perovskite NPls. In this work, we investigated the post-synthetic treatment of CsPbBr3 NPls with different Cd2+ sources. We show that the interplay between Cd2+ precursor, NPl concentrations, and ligands determines the kinetics of the doping process. Optimization of the treatment allows for the boosting of linear and nonlinear optical properties of CsPbBr3 NPls via doping or/and surface passivation. At a moderate doping level, both the photoluminescence quantum yield and two-photon absorption cross section increase dramatically. The developed protocols of post-synthetic treatment with Cd2+ facilitate further utilization of perovskite NPls in nonlinear optics, photonics, and lightning.

Beyond Charge Transfer: The Impact of Auger Recombination and FRET on PL Quenching in an rGO-QDs System.

PL intensity quenching and the PL lifetime reduction of fluorophores located close to graphene derivatives are generally explained by charge and energy transfer processes. Analyzing the PL from PbS QDs in rGO/QD systems, we observed a substantial reduction in average PL lifetimes with an increase in rGO content that cannot be interpreted solely by these two processes. To explain the PL lifetime dependence on the rGO/QD component ratio, we propose a model based on the Auger recombination of excitations involving excess holes left in the QDs after the charge transfer process. To validate the model, we conducted additional experiments involving the external engineering of free charge carriers, which confirmed the role of excess holes as the main QD PL quenching source. A mathematical simulation of the model demonstrated that the energy transfer between neighboring QDs must also be considered to explain the experimental data carefully. Together, Auger recombination and energy transfer simulation offers us an excellent fit for the average PL lifetime dependence on the component ratio of the rGO/QD system.

Chiral carbon dots based on L/D-cysteine produced via room temperature surface modification and one-pot carbonization.

Since chirality is one of the phenomena often occurring in nature, optically active chiral compounds are important for applications in the fields of biology, pharmacology, and medicine. With this in mind, chiral carbon dots (CDs), which are eco-friendly and easy-to-obtain light-emissive nanoparticles, offer great potential for sensing, bioimaging, enantioselective synthesis, and development of emitters of circularly polarized light. Herein, chiral CDs have been produced via two synthetic approaches using a chiral amino acid precursor l/d-cysteine: (i) surface modification treatment of achiral CDs at room temperature and (ii) one-pot carbonization in the presence of chiral precursor. The chiral signal in the absorption spectra of synthesized CDs originates not only from the chiral precursor but from the optical transitions attributed to the core and surface states of CDs. The use of chiral amino acid molecules in the CD synthesis through carbonization results in a substantial (up to 8 times) increase in their emission quantum yield. Moreover, the synthesized CDs show two-photon absorption which is an attractive feature for their potential bioimaging and sensing applications.

Methanol-induced fast CsBr release results in phase-pure CsPbBr3 perovskite nanoplatelets.

Formation of a non-emissive wide bandgap CsPb2Br5 component often accompanies the synthesis of CsPbBr3 perovskites, introducing undesired energy states and impeding the charge transport. Here, we demonstrate that a small amount of a methanol additive can promote the CsBr release rate, facilitating CsPbBr3 formation and suppressing CsPb2Br5 formation. Some of the methanol ionizes into CH5O+ and CH3O-, which act as surface ligands and change the crystallization environment, inducing shape evolution from spherical nanocrystals to rectangular nanoplatelets (NPLs), leading to monodispersed and phase-pure 8 unit-cell-thick CsPbBr3 NPLs. Meanwhile, nonradiative recombination processes are inhibited as a result of NPL surface passivation. Bright CsPbBr3 NPLs with a photoluminescence quantum yield reaching 90% were employed as emitters for electroluminescent light-emitting devices.

Band Structure and Intersubband Transitions of Three-Layer Semiconductor Nanoplatelets.

This paper presents the first general theory of electronic band structure and intersubband transitions in three-layer semiconductor nanoplatelets. We find a dispersion relation and wave functions of the confined electrons and use them to analyze the band structure of core/shell nanoplatelets with equal thicknesses of the shell layers. It is shown that the energies of electrons localized inside the shell layers can be degenerate for certain electron wave vectors and certain core and shell thicknesses. We also show that the energies of intersubband transitions can be nonmonotonic functions of the core and shell thicknesses, exhibiting pronounced local minima and maxima which can be observed in the infrared absorption spectra. Our results will prove useful for the design of photonic devices based on multilayered semiconductor nanoplatelets operating at infrared frequencies.

Tunable Mie Resonances of Tin-based Iodide Perovskite Islandlike Films with Enhanced Infrared Photoluminescence.

The ability of light manipulation at a sub-wavelength scale of metal halide perovskite-based nanostructures through nanophotonic design were employed for advanced optical and optoelectronic applications. While these nanostructures could be efficiently tuned in the visible spectral range, their operation at infrared wavelengths is still challenging. Herein, we illustrate that islandlike films of lead-free CH3NH3SnI3 can generate strong and tunable Mie-type resonances in the near-infrared spectral range. Two critical factors contribute to the Mie resonance properties-the microscale geometry is crucial for the initiation of Mie resonances in the particles, while the concentration of free holes formed via the oxidation of Sn2+ to Sn4+ modulates the spectral position of Mie resonances. Moreover, coupling the Mie resonances to the photoluminescence peak wavelength results in the enhancement of the photoluminescence intensity. This study offers a platform for the implementation of optically resonant perovskite nanostructures as tunable light emitters for infrared photonics and optoelectronics.

Ligand-Assisted Formation of Graphene/Quantum Dot Monolayers with Improved Morphological and Electrical Properties.

Hybrid nanomaterials based on graphene and PbS quantum dots (QDs) have demonstrated promising applications in optoelectronics. However, the formation of high-quality large-area hybrid films remains technologically challenging. Here, we demonstrate that ligand-assisted self-organization of covalently bonded PbS QDs and reduced graphene oxide (rGO) can be utilized for the formation of highly uniform monolayers. After the post-deposition ligand exchange, these films demonstrated high conductivity and photoresponse. The obtained films demonstrate a remarkable improvement in morphology and charge transport compared to those obtained by the spin-coating method. It is expected that these materials might find a range of applications in photovoltaics and optoelectronics.