Blue Photoluminescence Emission Research Articles

Synergy of photoluminescence emission and antibacterial activity of Ag-Cu2O nanocomposite

Conglomerate nanocomposites comprising metal and metal oxide hold significant potential for exhibiting properties that surpass the combined characteristics of their individual components, owing to the interactions occurring at the interfaces between the metal and metal oxide elements. In this study, we present the synthesis of Cu2O nanoparticles (NPs) (with diameters ranging from 40 to 53 nm) and Ag-Cu2O nanocomposites using an aqueous solution method at room temperature, employing varying concentrations of Ag NPs. Through optical absorption studies, we determined the optical band gap of Cu2O NPs in 0.5Ag-Cu2O, 1Ag-Cu2O, and 1.5Ag-Cu2O nanocomposites samples to be 2.13 eV, 2.25 eV, 2.34 eV, and 2.41 eV, respectively. The x-ray diffraction data are analysed using the Williamson–Hall technique and revealed noteworthy variations in microstructural characteristics such as strain and stress within the Cu2O nanocrystallites fabricated under different Ag concentrations. The nanocomposites amplified the intensities of violet-blue, blue, and green photoluminescence (PL) emissions, attributable to the interfaces between Ag and Cu2O, lattice mismatches, and the induced microstructural parameters lattice strain, and stress of Cu2O nanocrystallites. The enhanced PL intensities can be attributed to the influence of the local electric field on the Ag core composites. The Ag-Cu2O nanostructure exhibits potential applications in water purification technologies, while the PL emission properties and low band gap (∼2.13 eV) hold promising applications in optoelectronic devices. The antibacterial activities of Cu2O and Ag-Cu2O nanomaterials against Enterococcus faecalis and Escherichia fergusonii are examined using MH agar well plate diffusion methods. Here, Ag NPs enhance bactericidal effectiveness through enhanced interaction with bacteria and the release of Ag+ ions, while the Cu2O shell discontinuity on Ag NPs contributes to their unique antibacterial properties.

Luminescence Properties of Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> Phosphors Powder with Solid State Method

The low-temperature solid-state method is utilized to prepare Cs3Cu2I5 phosphors powder by using the cesium iodide (CsI) and cuprous iodide (CuI) as the raw materials. The phase structures of samples were investigated by using X-ray spectrum and steady-state fluorescence spectrometer. The experimental results show that all of samples were crystallized in the orthorhombic structure with pnma space group at the annealing temperature of 100-400°C. Cs3Cu2I5 phosphors exhibits a strong blue photoluminescence emission with peak at 440nm under excitation at 310nm. With the increase of the annealing temperature in the range of 100-400°C, the photoluminescence quantum yield (PLQY) of Cs3Cu2I5 powder achieved the 79.95%. It is revealed that the prepared Cs3Cu2I5 powder phosphors potentially have the promising application in the blue light emitting materials.

Structural and spectroscopic correlation in barium-boro-tellurite glass hosts: effects of Dy2O3 doping

In this study, a series of barium-boro-tellurite glass hosts with varying concentration of Dy2O3 doping (0 to 1.25 mol%) were made by melt-quenching method. A study was conducted to investigate how Dy2O3 dopants affect the physical and spectroscopic traits of glasses. Raw materials including barium oxide (BaO), tellurium dioxide (TeO2), boron oxide (B2O3), and dysprosium oxide (Dy2O3) were used to produce these glasses. XRD patterns of the samples showed a broad hump and absence of long-range periodic lattice arrangements, indicating their amorphous nature. The Raman spectral analyses displayed the various vibration modes where the most intense band caused by BaO vibrations at 300 cm-1 and 450 cm-1 corresponding to the symmetric stretching vibration mode of Te–O–Te intra-chain bridges. The peak at 750 cm-1 was due to TeO4 and Te-O-Te vibration modes. The value of optical band gap energy was decreased from 3.155 to 2.1894 eV and then increase at higher Dy2O3 level (0.75 to 1.25 mol%). At Dy3+ contents between 0.25 to 1.25 mol% seven absorption bands were observed at 390, 424, 452, 750, 797, 895 and 1092 nm due to the electronic transitions in Dy3+. The glass refractive indices were raised from 2.3563 to 2.6584 and then decreased at higher Dy2O3 contents which was mainly because of the generation of more bridging oxygen (BO) in the glass matrix. The value of glass electronic polarizability and oxide ions polarizability calculated using LorentzLorenz equation showed a decrease with the rise of Dy2O3 contents, which was ascribed to the presence of fewer non-bridging oxygen (NBO). The optical basicity of the proposed glass hosts was calculated using Duffy and Ingram equation which was decreased with the increase of doping contents. In addition, the optical transmission was increased and reflection loss was reduced with increasing Dy+3 levels. The value of metallization parameter below 1 proved the true amorphous nature of the prepared samples. All the glasses revealed blue and yellow photoluminescence emission peaks due to 4F9/2→ 6H15/2, and 4F9/2 →6H13/2 transitions in Dy3+, respectively. The proposed glass compositions may be beneficial for the advancement of solid-state lasers.

Two bimetallic compounds with attractive structures and properties

Two new bimetallic lanthanide-mercury isonicotinic acid compounds were successfully synthesized through classical hydrothermal reactions using DyCl3·6H2O or LuCl3·6H2O, HgCl2, isonicotinic acid, and distilled water. The resulting compounds, [Dy2(IA)6(H2O)2(H3O)2(Hg2Cl5)2]n(3nHgCl2) (1; IA refers to isonicotinic acid anions) and [Lu(HIA)(IA)2(H2O)2Hg2Cl5]2n[3n(HgCl2)]·6nH2O (2; HIA refers to isonicotinic acid), exhibit intriguing crystal structures and photophysical properties. The crystal structures of compounds 1 and 2 were determined through single crystal X-ray diffraction, revealing their two-dimensional (2D) nature. Compound 1 displays upconversion yellow photoluminescence, attributed to the 4F9/2 → 6H13/2 electronic transition of the trivalent dysprosium ion. In contrast, compound 2 exhibits blue photoluminescent emission. The Commission Internationale de I'Éclairage (CIE) chromaticity coordinates of (0.4874, 0.5115) for 1 and (0.1225, 0.0678) for 2. The correlated color temperatures (CCT) are 2960 K and 5503 K for compounds 1 and 2, respectively. Furthermore, the semiconductor band gaps for compounds 1 and 2, determined by solid-state diffuse reflectance experiments, are 2.92 eV and 4.75 eV, respectively. Additional insights into their thermal stability are provided via thermogravimetric analysis (TG) results.

Colloidal synthesis and optical properties of Cs2ZnBr4 and Cs3ZnBr5 nanocrystals

A framework to understand the effects of an oxide-ligand environment on the photoluminescence (PL) of rare-earth-element-doped oxynitride compounds is well established, whereas the effects of halide coordination on crystal field splitting and the associated PL emission are rarely studied. In this paper, we report a synthetic route to alkali metal halide NCs of Cs2ZnBr4 and Cs3ZnBr5 with a broad blue PL emission and large Stokes shift. In particular, the zero-dimensional Cs3ZnBr5 phase, in which the isolated [ZnBr]42− units were completely separated by surrounding Cs+ cations, exhibited a strong blue PL emission at 437 nm with a PL quantum yield of 8.92 %. The formation of additional trap states or defects in the Cs3ZnBr5 crystalline lattice can be associated with a relatively considerable bands at 345 and 400 nm of the optical band edge, resulting in singlet-to-singlet transition of localized excitons with a short lifetime of 3.74 ns. Therefore, herein we demonstrate that tunable alkali metal halide NCs have considerable potential for optoelectronic applications.

A Luminescent MOF Based on Pyrimidine-4,6-dicarboxylate Ligand and Lead(II) with Unprecedented Topology

In the present work, we report on a 3D MOF of {[Pb5(μ3-OH)(μ3-NO3)3(μ6-pmdc)3]·H2O}n formula (pmdc = pyrimidine-4,6-dicarboxylate) synthesized by an oven-heated, solvent-free procedure. The large connectivity afforded by the three ligands in their coordination to lead(II) ions grows cubic building units characterized by a central Pb atom with an unusual coordination index of 12 and 6 pmdc ligands occupying the faces. These cubic units are linked to one another giving rise to a quite condensed structure that represents an unprecedented topology showing the (4·62)6(43)2(45·610)3(45·68·82)6(46·69)6(612·83) point symbol. The crystalline material has been characterized by routine physico-chemical techniques to confirm its purity, and its thermal behaviour has been also studied by thermogravimetric and thermodiffractometric analyses. The solid presents a greenish blue photoluminescent emission based on pmdc ligands, as revealed by time-dependent density-functional theory (TDDFT) calculations, which is substantially more intense than in the free H2pmdc ligand according to its improved quantum yield. The emissive capacity of the material is further analysed according to decreasing temperature of the polycrystalline sample, finding that sizeable, long-lasting phosphorescence is present.

Structural, optical and temperature-sensing properties of LaOF:Yb3+, Tm3+ nanophosphor prepared by microwave-assisted hydrothermal synthesis

Luminescence thermometry forms a unique tool for remote temperature sensing by exploiting the variations in the luminescence properties of the probe. Thermometry based on the fluorescence intensity ratios (FIR) is a widely practiced approach due to them being a self-referenced measure. The quest for efficient temperature probes for thermometry led to the discovery of LaOF:Yb3+,Tm3+ upconversion nanoparticles, which were synthesized using a microwave-assisted hydrothermal synthesis method. Blue and near-infrared upconversion photoluminescence (UCPL) emission was observed under 980 nm laser excitation. The temperature-dependent UCPL was studied for their temperature sensing application. FIR of thermally coupled and non-coupled emission bands were calculated in the temperature range of 303 to 473 K. The thermometric characteristics such as absolute sensitivity, relative sensitivity and temperature uncertainty were also determined. The thermal stability of the UCNPs was established under continuous laser exposure operating at maximum power (2500mW). The reproducibility of the UCNPs was also determined by temperature cycling the UCNPs.

Correlation between structural and optical properties of Dy[formula omitted] doped BaGd[formula omitted]O[formula omitted] phosphors intended for solid-state lighting applications

Highly crystalline and single phase BaGd2−xDyxO4 (0.00 ≤x≤ 0.16) phosphors, with an average crystallite size around 126 nm, have been synthesised using solid-state reaction technique. The structural and optical properties of these phosphors have been studied in detail to establish an unambiguous correlation between these properties. High-angle annular dark field (HAADF) images have confirmed that the constituent elements are homogeneously distributed in the particles, and their elemental composition has been established using X-ray photoelectron spectroscopy (XPS). The tuning of optical band gap with x has been achieved, which is a rare achievement in these phosphors. Also, the optimum concentration of Dy3+ ions has been found to be 0.8 mol%, which is the lowest among the Dy3+ doped BaGd2O4 phosphors reported so far. This concentration quenching effect has been discussed on the basis of a combination of decay curve analysis, calculation of average critical distance between the Dy3+ ions and integrated intensities of photoluminescence (PL) emission bands. The average crystallite size and optical band gap has also been found to decrease after x = 0.016, from which their correlation with concentration quenching effect has been investigated. The asymmetry ratio between the integrated intensities of yellow and blue PL emission bands has been observed to be greater than 2 throughout x, which confirmed the preferential lattice site for Dy3+ ions in these phosphors with present synthesis conditions. The variation of asymmetry ratio and Gd3+-dominated IR-active lattice vibrations with x, and Vegard’s law pertaining to the volume of a unit cell confirms that the local bonding environment in the lattice of these phosphors gets modified at x = 0.016. The photometric parameters for these phosphors reveal their suitability for fabrication of warm light orange LEDs on appropriate UV chips.

PH-Sensitive Fluorescence Emission of Boron/Nitrogen Co-Doped Carbon Quantum Dots

Carbon quantum dots (CQDs) with their strong photoluminescence (PL) activity, high biocompatibility, robust stability, low cytotoxicity, and flexible surface structures have been employed in many fields including chemical sensing, biosensing, photocatalyst, energy storage, and biomedical applications. Of note, CQDs present an intrinsic pH-sensitive PL nature indicating their intense potential for pH-mediated sensing and imaging. Despite the numerous studies performed in the last two decades, the pH-sensitive PL mechanism of CQDs is still under debate and must be clarified to overcome the limitations in practical applications. Therefore, in this report, we performed a systematical study to determine the pH-sensitive PL nature of boron/nitrogen co-doped CQDs (B/N CQDs). In the first part, B/N CQDs with a strong blue emission were fabricated via a hydrothermal synthesis procedure. B/N-CQDs showed a strong blue PL emission with high quantum yield and excitation-dependent nature. Under the low pH conditions (pH 3), B/N-CQDs exhibited a robust green fluorescence emission with a significant red-shift (48 nm) and the loss of the excitation-dependent nature. The change in PL nature originated from the protonation of surface groups, a decrease in negative surface charge (from −20.6 to −1.23 eV), and finally, aggregation of the nanostructure (the size of CQDs from 4.8 to 7.5 nm). However, in the case of alkaline conditions, the deprotonation surface groups significantly enhanced the surface charge and led to the emergence of a negative ‘protective shell’ with a zeta potential of −71.3 eV. In a high pH medium (pH 13), PL spectra showed the loss of excitation-dependent features and a red-shift (35 nm) in emission peak maxima with lower intensity. This report provides significant progress in the clarification of the pH-sensitive PL mechanism of CQDs. We envision that the proposed CQDs would provide unique opportunities in the fabrication of novel pH sensor systems and fluorescence imaging where a wide range of pH sensitivity is required.

Intense Blue Photo Emissive Carbon Dots Prepared through Pyrolytic Processing of Ligno-Cellulosic Wastes

In this work, Carbon Dots with intense blue photo-luminescent emission were prepared through a pyrolytic processing of forestry ligno-cellulosic waste. The preparation path is simple and straightforward, mainly consisting of drying and fine grinding of the ligno-cellulosic waste followed by thermal exposure and dispersion in water. The prepared Carbon Dots presented characteristic excitation wavelength dependent emission peaks ranging within 438-473 nm and a remarkable 28% quantum yield achieved at 350 nm excitation wavelength. Morpho-structural investigations of the prepared Carbon Dots were performed through EDX, FT-IR, Raman, DLS, XRD, and HR-SEM while absolute PLQY, steady state, and lifetime fluorescence were used to highlight their luminescence properties. Due to the wide availability of this type of ligno-cellulosic waste, an easy processing procedure achieved photo-luminescent properties, and the prepared Carbon Dots could be an interesting approach for various applications ranging from sensors, contrast agents for biology investigations, to photonic conversion mediums in various optoelectronic devices. Additionally, their biocompatibility and waste valorization in new materials might be equally good arguments in their favor, bringing a truly "green" approach.

Optimized triple core blue emitters based on anthracene–pyrene–anthracene chromophores for organic light-emitting diodes

Three new fluorescent triple-core emitters were newly synthesized by attaching an optimized electron-donating group to an anthracene–pyrene–anthracene core moiety, resulting in 1,6-di(anthracen-9-yl)pyrene (DAP), 10,10′-(pyrene-1,6-diyl)bis(N,N-diphenylanthracen-9-amine) (DPA–DAP–DPA), and 4,4′-(pyrene-1,6-diylbis(anthracene-10,9-diyl))bis(N,N-diphenylaniline) (TPA–DAP–TPA). The photophysical, thermal, and electroluminescence properties of these synthesized materials were examined experimentally and theoretically. The triple-core derivatives were found to exhibit high thermal stability, with decomposition temperatures greater than 433 °C. DAP, DPA–DAP–DPA, and TPA–DAP–TPA showed deep-blue, sky-blue, and real blue photoluminescence emission, with maximum wavelengths of 430, 497, and 459 nm, respectively, in the solution state. Among the synthesized materials, TPA–DAP–TPA showed an excellent photoluminescence quantum yield of 77%. The device doped with TPA–DAP–TPA showed a high efficiency of 5.68 cd/A, CIE coordinates of (0.143, 0.151), and an external quantum efficiency of 4.64% at 10 mA/cm2, indicating that the device has potential applications in mobile phones.

Analysis of structural defects with the chemical composition of rGO/GaN nanocomposites using Raman spectroscopy

In this work, Raman spectroscopy was employed to examine the interface properties of chemically synthesized Gallium Nitride decorated reduced graphene oxide (rGO/GaN) nanocomposites. From the Raman analyses, a redshift of 6.68 cm−1 was detected in the E2 (high) phonon mode of GaN which indicates that GaN has tensile stress in the rGO network. As calculated from intensity ratios of the D to G band, there was the detection of fewer structural defects in the rGO/GaN nanocomposites while increasing the amount of GaN from 1 to 20 wt%. Moreover, from the analysis of the width of the 2D peak, less stacking of graphene sheets was discovered with more association of GaN nanoparticles. Photoluminescence (PL) studies revealed the absolute intensity corresponding to both the near band edge (NBE) GaN and blue PL emission of rGO increases with more incorporation of GaN. This implies that the radiative charge carrier recombination on rGO/GaN surface was increased thus improving the optical quality of the rGO/GaN nanocomposites. rGO/GaN nanocomposites have been reported to be useful in energy storage applications and can further be extended to applications such as LEDs, solar cells, photodetectors, UV photodiodes, and gas sensors in the prospect.

Modular design for constructing narrowband deep-blue multiresonant thermally activated delayed fluorescent emitters for efficient organic light emitting diodes

The simultaneous achievement of high efficiency and deep blue narrowband emission in multi resonant thermally activated delayed fluorescence (MR-TADF) materials are crucial and challenging. Herein we report two deep blue MR-TADF emitters, namely, TPD4PA and tBu-TPD4PA, using double boron, three nitrogen and two oxygen atoms. The design is based on amalgamating the high charge transfer (CT) characteristic moiety into MR-type fragments towards efficient MR-TADF emitters with improved CT characteristics. Both the materials show deep blue photo luminescent emissions of ∼450 nm with a high photoluminescence quantum yield (PLQY) of ∼90 %. These materials showed very small singlet–triplet gap (≤0.06 eV) and a high rate of reverse intersystem crossing of ∼2.5 × 105 s−1. The TADF devices based on TPD4PA and tBu-TPD4PA showed maximum external quantum efficiencies of 30.7 and 32.5 %, respectively. Furthermore, both devices exhibited narrow band deep blue emissions and corresponding CIE y coordinates 0.06 and 0.07, which match near NTSC and BT2020 blue color requirements.

Scalable and Blue Photoluminescence Emissions of (C4H9NH3)2PbBr4 2D Perovskite Fabricated by the Dip-Coating Method Using a Co-Solvent System

The improved efficiency of perovskite-related photovoltaic devices, such as light-emitting diodes (LEDs), is related to film uniformity, the compactness of each layer, and thickness. Herein, we improved the traditional single-solvent, solution-processed method and developed a co-solvent method to prepare a two-dimensional (2D) (C4H9NH3)2PbBr4 perovskite film for blue photoluminescence (PL) emissions. A poor film-forming uniformity was observed for the use of the single-solvent, dimethylformamide (DMF) method. In adding 1,2-dichlorobenzene (ODCB) of a smaller polarity to DMF, the co-solvent engineering dramatically changed the film-forming properties. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometer (XRD), and time-resolved PL (TR-PL) spectroscopy analyses confirmed that the perovskite film prepared by the co-solvent system had a good crystallinity, fewer defects, and a longer carrier lifetime. These experimental results show a simple, scalable (1.23 × 1.23 cm2), and stable reproducibility method for preparing 2D perovskite of 415 nm wavelength PL emissions that might be beneficial for the development of ultraviolet (UV) photodetectors, blue LEDs, and high-resolution displays.

A facile strategy to synthesize high colour purity blue luminescence aluminium-doped CsPbBr3 perovskite quantum dots

Recently, metal halide perovskite materials (CsPbX3, X = Cl, Br, and I) have attracted extensive attention for designing optoelectronic devices owing to their tuneable bandgap and high photoluminescence quantum yields (PLQYs). However, the corresponding study of CsPbX3 perovskite quantum dots (QDs) with blue luminescence still lags far behind that of their red and green counterparts. Herein, we developed aluminium (Al3+) ion-doped CsPbBr3 perovskite QDs through a supersaturated recrystallization synthetic approach at room temperature. The crystal structure and optical properties of the as-prepared QDs were systematically characterized. The results reveal that the particle size and lattice spacing of Al3+ ion-doped CsPbBr3 QDs decrease as the Al/Pb molar ratio increases, which is attributed to the radius of Al3+ ions being much smaller than Pb2+ ions. A significant blueshift in the luminescence from 503 nm to 465 nm of Al3+ ion-doped QDs is observed as the Al/Pb molar ratio increases. Moreover, high colour purity (wide full-width at half-maximum of 83.3 meV) blue photoluminescence emission at 465 nm with a PLQY of 65.7% is achieved when the Al/Pb ratio reaches 1.0 in the Al3+ ion-doped CsPbBr3 QDs, which shows good stability under ambient atmosphere. These as-prepared high colour purity blue luminescence Al3+ ion-doped CsPbBr3 QDs show promising potential in blue optoelectronic fields.

Cu-CDs as dual optical and electrochemical nanosensor for βME detection

Thiols play critical roles in biological processes. Here, a facile hydrothermal synthesis approach was used for fabrication of flourescent Cu-carbon dots (Cu-CDs), to be employed as accurate thiol Nanosensors. The obtained Cu-CDs were characterized using transmission electron microscopy, UV–visible, photoluminescence and X-ray photoelectron spectroscopy scannings. The highest-occupied- and lowest-unoccupied- molecular-orbitals and the pertaining energy band gap were achieved using analytical chemistry density functional theory. Electrochemical performance of Cu-CDs showed the capability of the fabricred nanoparticles for accurate, sensitive, and selective nanosensing of thiols, as both electrochemical and optical detection tools. The electrochemical peak of Cu-CDs were selectively, rationally and intensively suppressed by thiols, due to the strong electrostatic interaction and covalent bonding between the NH2 functional groups of the Cu-CDs and -SH groups of thiols. On the other hand, the blue photoluminescence emission of Cu-CDs rationally increased, due to detachment of Cu ions from the edges of Cu-CDs in the existance of thiols. According to the above description, the limit of detections (LODs) of 1 nM was achieved, introducing such-prepared Cu-CDs nanosensors, as simple, fast, low-cost, sensitive and selective tools for thiol detection.

Green synthesis of Spirulina-based carbon dots for stimulating agricultural plant growth

In this study, photoluminescent carbon quantum dots (CQDs) were synthesized by thermal pyrolysis of the microalgae spirulina without any surface passivating agent. The CQDs showed ~10 nm average size and exhibited blue photoluminescence emission around 450 nm under 340–400 nm excitation wavelength. A quantum yield of 23.5% was attained at 300 °C for 2 h. Thermodynamic and kinetic investigations about pyrolysis reaction showed that CQDs synthesis occurs slowly and non-spontaneously, with a mean activation energy of 192.6 kJ/mol. The final product showed a broad range of solubility, besides being efficient for lentil seed growth when dispersed in 0.05–0.1 mg/mL aqueous solutions. The CQDs reported here may be considered a promising renewable bio-based fertilizer for crop species in the sustainable agriculture field.

Achieving Narrow FWHM and High EQE Over 38% in Blue OLEDs Using Rigid Heteroatom‐Based Deep Blue TADF Sensitized Host

AbstractSimultaneously obtaining high efficiency and deep blue emission in organic light emitting diodes (OLEDs) remains a challenge. To overcome the demands associated with deep blue thermally activated delayed fluorescence (TADF) emitters, two deep blue TADF materials namely, DBA–BFICz and DBA–BTICz, are designed and synthesized by incorporating oxygen‐bridged boron (DBA) acceptor with heteroatoms, oxygen and sulphur‐based donors, BFICz and BTICz, respectively. Both TADF materials show deep blue photoluminescence emissions below 450 nm by enhancing the optical band gap over 2.8 eV through deeper highest occupied molecular orbital (HOMO) level of heteroatom based donor moieties. At the same time, the photoluminescence quantum yields (PLQYs) of both TADF materials remain over 94%. The TADF device with DBA–BFICz as an emitter exhibits a good external quantum efficiency (EQE) of 33.2%. Since both new TADF materials show deep blue emissions and high efficiencies, hyperfluorescence (HF) OLED devices are fabricated using ν‐DABNA as a fluorescence dopant. DBA–BFICz as a TADF sensitized host in HF–OLED reveals an outstanding EQE of 38.8% along with narrow full width at half maximum of 19 nm in the bottom emission pure blue OLEDs. This study provides an approach to develop deep blue TADF emitters for highly efficient OLEDs.

Photoluminescent surface-functionalized graphene quantum dots for spontaneous interfacial homeotropic orientation of liquid crystals

An interfacial molecular array of octadecylamine-functionalized graphene quantum dots (GQDs) was fabricated to simultaneously realize photoluminescence characteristics and molecular orientation for liquid crystals (LCs). The functionalized GQDs were synthesized using a simple bottom-up method by first grafting them with octadecylamine. The subsequent interfacial self-assembly of these functionalized GQDs in an LC medium formed a nanostructured molecular array on an indium tin oxide (ITO) electrode. Experiments revealed that the hydrophobic GQDs spontaneously covered the hydrophilic ITO surface through polar intermolecular interactions, and the resulting GQD layer induced the uniform vertical orientation of the LC molecules at the ITO interface owing to van der Waals interactions of long side chains. In addition, the unique photoluminescence of pure GQDs in the blue region was maintained even in an LC mixture with a low concentration of functionalized GQDs. A new type of homeotropically oriented LC device based on the nanostructured layer of functionalized GQDs exhibited superior electro-optical properties and a 375% improvement in response time for turn-off switching of the device relative to those of a conventional polymer layer. This functionalized GQD-based device simultaneously achieved the spontaneous alignment of the LCs, blue photoluminescence emission, and high-speed response performance for turn-off switching of the device. In addition, our method is cost-effective and encompasses a simple fabrication process for developing functional controllable surfaces without requiring a tedious multi-step manufacturing process.

Zinc sulfide quantum dots coated with PVP: applications on commercial solar cells

In this study, we report the synthesis and characterization of zinc sulfide quantum dots (ZnS QDs) coated with different concentrations of polyvinylpyrrolidone (PVP), as well as their deployment as luminescent films on the window side of previously characterized commercial silicon solar cells to quantify their influence on the power conversion efficiency (PCE). The synthesis of the semiconductor nanoparticles was carried out by the reaction of zinc nitrate with sodium sulfide in an aqueous solution at room temperature. XRD measurements indicated a cubic sphalerite phase of the QDs crystal structure, which was not modified by the addition of PVP in the synthesis process. However, the PVP concentration was a key parameter to modulate the size distribution and the luminescent intensity of the QDs, suggesting that an increase in the PVP concentration produced a slight decrease in the QDs size and improved their luminescent properties desired for the photovoltaic applications. The obtained nanoparticles presented great absorption of photons with energies above 3.72 eV and a broad intense blue photoluminescent emission centered at around 450 nm, under excitation of ultraviolet light of 325 nm. The implementation of the synthesized ZnS QDs as spectral response enhancer produced improvements on the performance of solar cells, leading to increases of 0.7%, 1.9%, and 6.1% on the efficiencies of commercial polycrystalline solar cells after the deposition of ZnS QDs synthesized without PVP, with 0.05 mM PVP and with 0.10 mM PVP, respectively.