Narrow Photoluminescence Spectra Research Articles

Highly luminescent CsPbX3@MIL-53(Al) nanoarchitectonics with anomalous stability towards flexible emitting films

The poor stability of perovskite nanomaterials exposed in high temperature, moisture, light radiation, and polar solvent conditions severely restrict their commercial application. To address this issue, the growth in situ of CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) is carried out by using the stable aluminum (Al)-based metal-organic frameworks to create CsPbX3@MIL-53(Al) composites. Compared with pristine CsPbX3 NCs prepared using room temperature ligand assisted re-precipitation synthesis, the CsPbX3@MIL-53(Al) composites exhibit narrow photoluminescence (PL) spectra and greatly improved stability. The green emitting CsPbBr3@MIL-53(Al) composite exhibit narrowed PL spectrum (the full width at half-maximum of PL spectra: 20 nm for the composite and 28 nm for CsPbBr3 NCs) and a blue-shifted PL peak wavelength (517.6 nm for the composite and 527.6 nm for the CsPbBr3 NCs). The relationship of microstructure, components, PL, fluorescence lifetime, and stability are discussed systematically. Anomalous stability against heat-treatment, polar solvent and humidity conditions indicated the composites retained their initial PL intensity after storge more than 4 months in ambient conditions and heat-treated to 120ºC for 5 cycles. The emitting color of CsPbBr3@MIL-53(Al) composites are easily adjusted from blue, green to red by controlling the halide components of precursors. The composite was used to create emitting coating and highly bright flexible emitting films which also reveal high stability, suggesting that in situ encapsulation of CsPbX3 NCs in MIL-53(Al) would supply a novel platform for various emitting devices.

Crystallographic and Photophysical Analysis on Facet-Controlled Defect-Free Blue-Emitting Quantum Dots.

The burgeoning demand for commercializing self-luminescing quantum dot (QD) light-emitting diodes (LEDs) has stimulated extensive research into environmentally friendly and efficient QD materials. Hydrofluoric acid (HF) additive improves photoluminescence (PL) properties of blue-emitting ZnSeTe QDs, ultimately reaching a remarkable quantum yield (QY) of 97% with an ultranarrow peak width of 14nm after sufficient HF addition. The improvement in optical properties of the QDs is accompanied by a morphology change of the particles, forming cubic-shaped defect-free ZnSeTe QDs characterized by a zinc blende (ZB) crystal structure. This treatment improves the QD-emitting properties by facilitating facet-specific growth, selectively exposing stabilized (100) facets, and reducing the lattice disorders. The facet-specific growth process gives rise to defect-free monodispersed cubic dots that exhibit remarkably narrow and homogeneous PL spectra. Meticulous time-resolved spectroscopic studies allow an understanding of the correlation between ZnSeTe QDs' particle shape and performance following HF addition. These investigations shed light on the intricacies of the growth mechanism and the factors influencing the PL efficiency of the resulting QDs. The findings significantly contribute to understanding the role of HF treatment in tailoring the optical properties of ZnSeTe QDs, thereby bringing it closer to the realization of highly efficient and bright QD-LEDs for various practical applications.

Thermally Controllable Tuning of Emission Properties of Phenoxazine‐Substituted Acridones as One Step Forward to Efficient Organic Light‐Emitting Diodes Based on Crystalline Emitters

AbstractThe overwhelming majority of efficient organic light‐emitting diodes (OLEDs) are based on amorphous emitting layers. The devices with crystalline emitters usually show significantly worse performance. It is demonstrated that controlled thermal treatments of phenoxazine‐substituted acridones may lead to a spectacular improvement of photo‐ and electroluminescence of these luminophores. The three acridone derivatives studied, namely 2‐phenoxazine‐N‐hexylacridone (PHhA), 2,7‐bis‐(phenoxazine)‐N‐hexylacridone (bPHhA), and 2,7‐bis[3‐(phenoxazine)phenyl]‐N‐hexylacridone (PHPhhA), are characterized by relatively narrow photoluminescence spectra and photoluminescence quantum yields approaching 100%. These spectra can be reversibly tuned through appropriate thermal treatment of the deposited layers, undergoing shifts up to 92 nm upon transformation from amorphous to crystalline states. Strong thermally activated delayed fluorescence (TADF) observed for these compounds is manifested by a significantly higher reverse intersystem crossing rate of 1.92×106 s−1 as compared to the intersystem crossing rate of 5.68×105 s−1. These improvements are attributed to the annealing‐induced conformational equilibration due to crystallisation in combination with hosting. Adequacy of the proposed concept is demonstrated by the fabrication of devices exhibiting external quantum efficiencies reaching 20.7% – a record value for OLEDs with crystalline emitting layers. The presented approach of annealing‐induced conformational equilibration can lead to improved TADF properties for other fluorescent organic emitters presently considered inefficient.

Wide gamut white LED device using green CsPbBr3 quantum dots glass and red K2SiF6: Mn4+ phosphor

Wide gamut white LED device is fabricated by combining green CsPbBr3 quantum dots glass (QDG) and red K2SiF6: Mn4+ phosphor with blue LED. The green CsPbBr3 QDG shows steep absorption spectra, narrow and symmetrical photoluminescence (PL) spectra, broad band excitation spectra and bi-exponential fluorescence decay. PL peak, full width at half maximum and average lifetime are 515 nm, 18 nm and 94.9 ns respectively. The red K2SiF6: Mn4+ phosphor shows narrow red emission peaks of Mn4+, broad excitation peaks, and single exponential fluorescence decay with lifetime of 8.5 ms. The green CsPbBr3 QDG has strong temperature dependence of PL intensity. The red K2SiF6: Mn4+ phosphor has negligible temperature dependence. The white LED device has wide color gamut of 134% NTSC and high color purity of 98%, 89% and 99% for the blue, green and red lights. The white light of white LED device has been realized by controlling thickness of the green CsPbBr3 QDG and content of the red K2SiF6: Mn4+ phosphor. We have evaluated the chromatic coordinate stability at different operating powers and times, and the maximum chromatic changes are as small as (0.004, 0.012) and (0.009, 0.026) for (x, y) respectively in CIE 1931 (Commission International de l′Eclairage 1931) chromatic coordinate.

Polymeric Nanocarriers with Luminescent Colloidal Nanoplatelets as Hydrophilic and Non-Toxic Two-Photon Bioimaging Agents.

IntroductionSemiconductor nanoplatelets (NPLs) are promising materials for nonlinear optical microscopy since they feature good two-photon absorption (TPA) properties, narrow photoluminescence spectra and high quantum yields of luminescence. Nevertheless, the use of semiconductor NPLs is inevitably connected with concerns about heavy metal ion toxicity and their intrinsically hydrophobic character.MethodsOur contribution focuses on the design and engineering of coloidal bionanomaterial consisting of two-dimensional highly luminescent CdSe semiconductor NPLs loaded into spherical and homogeneous polymeric nanocarriers (NCs) based on poly(ethylene oxide) and poly(propylene oxide) block co-polymer. The biocompatibility and usefulness of the NPLs-loaded polymeric NCs in two-photon induced bioimaging was demonstrated invitroby cytotoxicity and two-photon microscopic studies using eukaryotic (normal fibroblasts and cancer ovarian) cells.ResultsThe encapsulated NPLs maintain their intensive and spectrally narrow photoluminescence, as well as preserve good TPA properties, while the surrounding polymer shell imparts hydrophilic character and non-toxicity towards eukaryotic cells. Specifically, TPA cross-sections of the colloidal NCs loaded with NPLs show large values reaching up to 2.0 × 108 GM, with simultaneously two-photon brightness reaching 2.2 × 107 GM at 870 nm. MTT proliferation assay performed on cell lines treated with encapsulated NPLs revealed at least 70% viability of normal human gingival fibroblast (HGF) and cancer ovarian (MDAH-2774) cells, while the results of multiphoton imaging of murine (L-929) fibroblasts suggest that the encapsulated NPLs are capable of labelling the target cells enabling their visualization.ConclusionAs a result, we obtained water dispersible and temporally stable hydrophilic NPLs-loaded NCs that offer excellent, both one- and two-photon excited fluorescence preserving optical properties of the raw hydrophobic and colloidal NPLs. The biological responses upon eukaryotic cells indicate that the encapsulation process protects cells from the toxic influence of cadmium simultaneously preserving the unique multiphoton properties of the active cargo which opens a promising perspective for its application in multiphoton cancer bioimaging excited at the “optical transmission window” of biological tissues in near-infrared range.

Short-wave infrared (SWIR) photodetection by InAs/GaAs quantum dot heterostructures grown on Ge (100) substrate without Migration Enhanced Epitaxy layer

Integration of III-V based optoelectronic devices with Silicon (Si) technology is the prime choice of researchers for the futuristic Si-photonics. Profusion research is going on for direct growth of III-V heterostructures on the Germanium (Ge) substrate. Also, monolithic integration of III-V based device structures on Si using Ge layer and Migration Enhanced Epitaxy (MEE) technique have been endeavored. In this study, InAs based quantum dot infrared photodetectors (QDIPs) are grown on Ge substrate without MEE layers. The GaAs/AlAs superlattice buffer (SLB) layers are used to suppress the defects and dislocations. Two different QDIPs with GaAs capped and In 0.15 Ga 0.85 As/GaAs capped InAs QD are grown and characterized using photoluminescence (PL), high-resolution X-Ray Diffraction (HRXRD), and Raman spectroscopy (RS). Red-shifted and narrow PL spectrum of the InGaAs/GaAs capped QDIP with respect to the GaAs capped QDIP attributes to formation of larger dots and homogeneous size distribution. Low strain and high activation energy in the InGaAs capped QDs signify better strain relaxation via the InGaAs strain reducing layer (SRL) and better carrier confinement. Interband-assisted near-infrared (NIR) photoresponse is obtained from both devices till room temperature. However, the InGaAs/GaAs capped QDIP shows shortwave infrared (SWIR) response, which promulgates the formation of QDs with better crystallinity due to the presence of InGaAs SRL. The room temperature (300 K) dark current density of the GaAs and InGaAs capped QDIP is 1.2 × 10 −1 A/cm 2 and 6.6 × 10 −3 A/cm 2 respectively with −2 V bias voltage. This work demonstrates the pathway to uncomplicated growth of high quality QDIP structures on Ge substrate. • A novel technique is used to grow defect free InAs QD structures on Ge substrate. • GaAs/AlAs SLB layers are used to suppress the defects and dislocations. • InAs QDs with GaAs and InGaAs/GaAs capping are investigated and compared. • Both structures show low dark current at room temperature. • Infrared photoresponse till room temperature is obtained for both QDIPs.

Luminance compensation in top-emitting organic light-emitting diodes with high color purity as quantum dots using horizontal dipole orientation

Abstract Organic light-emitting diodes (OLEDs) and quantum dots (QDs) are mainstream applications in large-sized displays. Owing to their narrow photoluminescence (PL), QDs offer a wide color gamut. Although OLEDs have a wider PL spectrum bandwidth than that of QDs, a high color purity can be obtained through an optical design of the top-emitting device. However, the luminance improvement from the strong cavity effect in a top-emitting geometry is not significant because of the wide PL spectrum. This insufficient luminance improvement can be compensated by applying organic light-emitting materials with horizontally oriented dipoles. Luminance compensation is color dependent. In a blue light-emitting structure, the luminance improvement by a narrow PL spectrum is more advantageous than by a horizontal dipole orientation. However, in green and red light-emitting structures, even if the full width at half maximum of the PL is about 20-nm-wide, luminance can be compensated using a horizontal dipole orientation while maintaining high color purity.

Planar Optical Waveguides Using Poly(styrene-ethylene-butylene-styrene) Activated with Inorganic Lead Halide Perovskite Nanoplates

Lead halide perovskite nanoplates support strongly quantum confined excitons, high photoluminescence quantum yield, fast and narrow photoluminescence spectra. Future use of these nanomaterials is s...

Pseudo-refractive index and excitonic features of single layer CdSe/CdS core–shell nanoplatelet films

Semiconductor CdSe/CdS core–shell nanoplatelets exhibit narrow and intense absorption and photoluminescence spectra in the visible range, which makes them suitable for numerous applications in optoelectronics. Of particular interest is the preparation and optical characterization of thin films with an accurately controlled amount of nanoplatelets. Here we report on the use of spectroscopic ellipsometry for investigating the optical properties of ultrathin films composed of a single layer of negatively charged CdSe/CdS core–shell nanoplatelets prepared by the electrostatic layer-by-layer deposition on SiO2/Si substrates. Combining the ellipsometric spectra with atomic force microscopy measurements, we were able to infer the nanoplatelet film extinction spectra which was found to exhibit the two characteristic exciton peaks albeit blueshifted relative to the colloidal nanoplatelets.

All-inorganic cesium lead halide perovskite nanocrystals for solar-pumped laser application

We demonstrate a low-concentrated solar-pumped laser (SPL) with natural sunlight by using an all-inorganic cesium lead halide perovskite (CsPbBrxI3−x) nanocrystal (NC) dispersed in toluene as a sensitizer. The perovskite NCs exhibit substantial advantages for SPL applications because of their broad absorption and narrow photoluminescence (PL) spectra with high quantum yield using inexpensive commercial precursors. We successfully synthesized CsPbBrxI3−x NCs with precisely tuned PL wavelengths from 581 to 612 nm by altering the I/Br ratio to achieve spectral overlap with Nd3+ ions, which have been widely used as a laser medium for SPLs. The measurement results show that the laser output power is highly sensitive to the peak PL wavelength of the NCs, and the highest laser output was obtained at the peak wavelength of 595 nm. Although the synthesized NCs have a wider absorption band, the laser output power obtained was much less than that of an organic dye of rhodamine 6G (R6G). The numerical analyses show that the optimal peak PL wavelength is 10 nm shorter than the absorption peak of Nd3+ ions because of the reflection property of the dichroic mirror coated on the input window. Moreover, we found that the concentration of NCs needs to be optimized in response to the peak PL wavelength. The calculations show that a laser output power 2.7 times greater than that of R6G can be obtained under an optimal peak PL wavelength of 575 nm and a concentration of 24 g/l for the CsPbBrxI3−x NCs.

Recent advances in nanowire quantum dot (NWQD) single-photon emitters

Future development of quantum technologies is dependent upon physical implementation of quantum systems. Photonic platforms have gained significant attention owing to the appealing properties that they offer, namely small probabilities of decoherence emergence, as well as being readily manipulable. In this regard, single-photon emitters play an integral role, and advancements in the design of single-photon sources, providing an anti-bunching photon correlation, have transitioned from its formerly proof-of-concept status to engineering attempts, and this has realized thanks to the ever-increasing improvements in the development of promising material platforms. In the race toward the realization of on-demand single-photon sources, solid-state host systems and in particular the quantum dot (QD)-based schemes, have very-well taken advantage of the recent headways in the bottom-up nanofabrication procedures. In this contribution, a review is presented on the recent progress and the existing challenges in fabrication of single-photon emitters, based on QD-containing nanowires (NWs) and their principles of operation. More specifically, we will consider how variation in values of different parameters, such as the purity, height and diameter of the NW waveguides, the electric-dipole orientation of the embedded QD, as well as different waveguide geometries, fabrication techniques and conditions exploited in their manufacturing, will affect the desired outputs in the system, i.e., the narrow photoluminescence spectrum, ideal single-photon emission of sufficient lifetime, and also the Gaussian profile in the far-field distribution. Additional focus is also given to different areas that the NWQD single-photon emitters have found application for quantum information processing purposes.

Compositional Grading for Efficient and Narrowband Emission in CdSe-Based Core/Shell Nanoplatelets

Colloidal semiconductor nanoplatelets exhibit exceptionally narrow photoluminescence spectra. This occurs because samples can be synthesized in which all nanoplatelets share the same atomic-scale t...

Manipulating the Transition Dipole Moment of CsPbBr3 Perovskite Nanocrystals for Superior Optical Properties.

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications.

Colloidal semiconductor nanocrystals (SCNCs) or, more broadly, colloidal quantum nanostructures constitute outstanding model systems for investigating size and dimensionality effects. Their nanoscale dimensions lead to quantum confinement effects that enable tuning of their optical and electronic properties. Thus, emission color control with narrow photoluminescence spectra, wide absorbance spectra, and outstanding photostability, combined with their chemical processability through control of their surface chemistry leads to the emergence of SCNCs as outstanding materials for present and next‐generation displays. In this Review, we present the fundamental chemical and physical properties of SCNCs, followed by a description of the advantages of different colloidal quantum nanostructures for display applications. The open challenges with respect to their optical activity are addressed. Both photoluminescent and electroluminescent display scenarios utilizing SCNCs are described.

Airy pattern on narrow photoluminescence spectrum of band to band recombination in CdTe:Te thin films

Semiconductor CdTe:Te films were deposited by means of rf sputtering on glass substrates. The excess of Te gave place to a high number of Cd-vacancies (VCd) producing p-type CdTe films. The density of carriers produced a high strength surface electric field which allowed obtain the bandgap value employing modulated transmittance spectroscopy. The obtained bandgap value of 1.40 ± 0.01eV was confirmed by absorption spectroscopy measures. The density of holes is so high that bandgap renormalization is observed. Photoluminescence (PL) measurements were carried out with the down-converted 883.2nm (1.403eV) line of the 441.6nm wavelength of a He-Cd laser. This energy allows to produce a resonant excitation of the CdTe:Te films, in such a way that electrons from the conduction band (CB) can be just excited to the valence band (VB). The resonant excitation produced a PL spectrum of band to band electron-hole recombination showing discrete energy emissions that follow the pattern of oscillations corresponding to the Airy model for a quantum triangular potential well. The average width of signals of the higher energy oscillations is 12 ± 3μeV and separation between energy levels is of the order of 12 ± 3μeV.

The red shift of the semiconductor quantum dots luminescence maximum in the hollow core photonic crystal fibers

Hollow core microstructural optical waveguides (photonic crystal fibers) can affect intensity and spectrum position of luminophores emission located in the hollow core. Most strongly this effects could be in case of luminescence semiconductor nanoparticles – quantum dots (QDs) due to the narrow and symmetrical photoluminescence spectra. Study of the spectral effects arising from the superposition of QDs luminescence band on the photonic band gaps in the fiber transmission spectrum is both of fundamental and applied interest. This paper demonstrates the red shift of photoluminescence maximum of CdSe/ZnS QDs with core-shell structures. Luminescence maximum shift can be more than 10 nm, depending on the position of the photonic band gaps in the fiber transmission spectrum.

Synthesis of Air-Stable CdSe/ZnS Core–Shell Nanoplatelets with Tunable Emission Wavelength

In the past few years, several protocols have been reported on the synthesis of CdSe nanoplatelets with narrow photoluminescence (PL) spectrum, high PL quantum efficiency, and short exciton lifetime. The corresponding core/shell nanoplatelets are however still mostly based on CdSe/CdS, which possess an extended lifetime and a strong red shift of the band-edge absorption and emission, in accordance with a quasi-type-II band alignment. Here we report on a robust synthesis procedure to grow a ZnS shell around CdSe nanoplatelets at moderate temperatures of 100–150 °C, to improve the optical properties of CdSe nanoplatelets via a type-I core/shell heterostructure. The shell growth is performed under ambient atmosphere, in either toluene or 1,2-dichlorobenzene. The variation of the shell thickness induces a continuous red shift of the PL peak, eventually reaching 611 nm. The PL quantum efficiency is increased compared to the original CdSe cores, with values up to 60% depending on the shell thickness. High-resolution transmission electron microscopy reveals a bending of the nanoplatelets caused by strain due to 12% lattice mismatch between CdSe and ZnS. The present procedure can easily be translated to other core/shell nanocrystals, such as CdSe/CdS and CdSe/CdZnS nanoplatelets.

Functional double-shelled silicon nanocrystals for two-photon fluorescence cell imaging: spectral evolution and tuning.

Functional near-IR (NIR) emitting nanoparticles (NPs) adapted for two-photon excitation fluorescence cell imaging were obtained starting from octadecyl-terminated silicon nanocrystals (ncSi-OD) of narrow photoluminescence (PL) spectra having no long emission tails, continuously tunable over the 700-1000 nm window, PL quantum yields exceeding 30%, and PL lifetimes of 300 μs or longer. These NPs, consisting of a Pluronic F127 shell and a core made up of assembled ncSi-OD kept apart by an octadecyl (OD) layer, were readily internalized into the cytosol, but not the nucleus, of NIH3T3 cells and were non-toxic. Asymmetrical field-flow fractionation (AF4) analysis was carried out to determine the size of the NPs in water. HiLyte Fluor 750 amine was linked via an amide link to NPs prepared with Pluronic-F127-COOH, as a first demonstration of functional NIR-emitting water dispersible ncSi-based nanoparticles.

Luminescence variations in europium-doped silicon-substituted hydroxyapatite nanobiophosphor via three different methods

Abstract This paper reports the first attempt for the synthesis of europium-doped Si-substituted hydroxyapatite (HA) nanostructure to achieve strong and stable luminescence of nanobiophosphor, particularly, by addition of different Eu dopants, Si substitutions, and application of optimum annealing temperatures of up to 1000 °C. The nanobiophosphor was synthesized by the coprecipitation, microwave, and hydrothermal methods. The nanoparticles demonstrated a nanowire to a spindle-like morphology, which was dependent on the method of synthesis. The photoluminescence (PL) intensity of the sample increases with the increase in Si substitutions and Eu dopants. The luminescent nanoparticles also showed the typical luminescence of Eu 3+ centered at 610 nm, which was more efficient for the annealed Eu-doped Si-HA nanoparticles than for the as-synthesized nanoparticles. Among the different synthesis methods, the hydrothermal method reveals the best light emission represented by high PL intensity and narrow PL spectra. These results suggest the potential application of Eu-doped Si-HA in stable and biocompatible nanophosphors for light emission and nanomedicine.

Growth of very large InN microcrystals by molecular beam epitaxy using epitaxial lateral overgrowth

Very thick InN (∼40 μm) was grown by molecular beam epitaxy using the epitaxial lateral overgrowth (ELO) technique. In some regions, the ELO of InN was observed as expected, indicating an important step toward fabricating quasi-bulk InN substrates. Interestingly, most parts of the sample consist of large flat-topped microcrystals and well-faceted microstructures. This is likely due to local growth condition variations during ELO, which is supported by an experiment where ELO of InN was performed on a substrate with various stripe mask patterns. TEM characterization of a flat top InN microcrystal revealed few stacking faults and only related threading dislocations. Defect-free small faceted microcrystals were also observed. The thick InN crystals show a narrow photoluminescence spectrum with a peak at 0.679 eV and linewidth of 16.8 meV at 4 K.