Increase In Photoluminescence Intensity Research Articles

The effects of various molecular weight of passivator on the photoluminescence properties of graphene quantum dots

Here, we realize manipulation of photoluminescence (PL) intensity of GQDs through various molecular weight of surface passivator. With the increase of passivator molecular weight, only the PL intensity increases. However, the peak positions and half-peak width of spectral line remain unchanged. Our results reveal that surface passivation with larger molecular weight only weakens the chemical activity of graphene quantum dots (GQDs) and the π-π stacking interaction between GQDs, but exerts less impact on GQDs intrinsic energy gap. Therefore, the weaker π-π stacking interaction and chemical activity induce the weaker non-radiative recombination, leading to enhanced fluorescence from radiative recombination.

Photoluminescence of two-dimensional plasmonic structures: enhancement, spectral and lifetime peculiarities below the lasing threshold

We study the interplay between optical properties and photoluminescence spectra of structures based on square lattices of silver nanodiscs or nanoholes in silver embedded in or adjoined to a dye-doped polymer waveguiding layer. The lattices and the waveguiding gain layer were designed to overlap the spectral features of such two-dimensional (2D) plasmonic structures. By varying the lattice constant, we demonstrate a condition for a significant increase of photoluminescence intensity and compare lifetime reduction for both nanodiscs- and nanoholes-based structures, when pumping them below lasing threshold. These enhancements are reached by matching the dark modes of the structure with the gain medium fluorescence spectrum.

Self‐modulated photoluminescence of CrBr 3 flake

The layered van der Waals magnetic insulator chromium bromine (CrBr3) has excellent optical properties and it provides an interesting platform for investigating light–matter interactions. Here, we focused on its self-modulated photoluminescence (PL) spectrum. We find that there are interference fringes superposed on the PL curves due to the Fabry–Perot resonance between the front and rear interfaces of CrBr3 flakes. Moreover, there is a huge Stokes shift between the PL spectrum and lowest absorption band, which can prevent reabsorption of PL and is a benefit for enhancing the resonant intensity of the PL spectrum. In addition, we investigate the PL spectrum variation under suppressing thermal vibration with decreasing temperature from 300 to 80 K, which shows the significant increase of PL intensity and blue shift of PL maximum peak. This work gives details of CrBr3 self-modulated PL properties and creates opportunities for using CrBr3 in optical devices and lasing components in future applications.

Octadecylamine-capped CdSe/ZnS quantum dot dispersed cholesteric liquid crystal for potential display application: Investigation on photoluminescence and UV absorbance

ABSTRACT In the present investigation, we have undertaken the photoluminescence and UV absorbance study of a newly synthesised cholesteric liquid crystal (CLC) and CLC– octadecylamine-capped CdSe/ZnS quantum dots (QDs) composites. Three different CLC–QDs composites are prepared by varying the concentration of QDs in CLC. The thermal behaviour of both the pure CLC as well as three different CLC–QDs composites are performed by using Polarising Optical Microscopic (POM) textural studies. The photoluminescence (PL) study indicates that the PL intensity increases after the dispersion of QDs into the pure CLC. Decrement in the FWHM parameter also supports this increased PL intensity. However, the UV absorbance decreases for the QD dispersed CLC system when compared to the pure CLC. Optical band gap (by the Tauc plot method) suggests that the observed band gap reduces after the dispersion of QDs into CLC. The outcome of these investigations may be useful in the liquid crystal displays (LCDs) and other opto-electronic devices, which require less band gap materials.

Water Passivation of Perovskite Nanocrystals Enables Air‐Stable Intrinsically Stretchable Color‐Conversion Layers for Stretchable Displays

Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. An air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs) is proposed. PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of a compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. A display that can be stretched up to 180% is demonstrated; it is composed of an air-stable SCCL that down-converts the SELD's blue emission and reemits it as green. The work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays.

Photoluminescent Detection of Human T-Lymphoblastic Cells by ZnO Nanorods.

The precise detection of cancer cells currently remains a global challenge. One-dimensional (1D) semiconductor nanostructures (e.g., ZnO nanorods) have attracted attention due to their potential use in cancer biosensors. In the current study, it was demonstrated that the possibility of a photoluminescent detection of human leukemic T-cells by using a zinc oxide nanorods (ZnO NRs) platform. Monoclonal antibodies (MABs) anti-CD5 against a cluster of differentiation (CD) proteins on the pathologic cell surface have been used as a bioselective layer on the ZnO surface. The optimal concentration of the protein anti-CD5 to form an effective bioselective layer on the ZnO NRs surface was selected. The novel biosensing platforms based on glass/ZnO NRs/anti-CD5 were tested towards the human T-lymphoblast cell line MOLT-4 derived from patients with acute lymphoblastic leukemia. The control tests towards MOLT-4 cells were performed by using the glass/ZnO NRs/anti-IgG2a system as a negative control. It was shown that the photoluminescence signal of the glass/ZnO NRs/anti-CD5 system increased after adsorption of T-lymphoblast MOLT-4 cells on the biosensor surface. The increase in the ZnO NRs photoluminescence intensity correlated with the number of CD5-positive MOLT-4 cells in the investigated population (controlled by using flow cytometry). Perspectives of the developed ZnO platforms as an efficient cancer cell biosensor were discussed.

Gamma irradiation of graphene quantum dots with ethylenediamine: Antioxidant for ion sensing

Due to the low consumption of chemicals, the absence of toxic residual side products, the procedure simplicity and time-saving aspects, gamma irradiation offers advantages over the classical chemical protocols. We successfully employed gamma irradiation in order to introduce N-atoms in Graphene Quantum Dots (GQDs). By irradiating GQDs water dispersions in the presence of isopropyl alcohol and ethylenediamine, at doses of 25, 50 and 200 kGy, we attached amino groups onto GQDs in a single synthetic step. At the same time, a chemical reduction is achieved, too. Selected conditions induced incorporation of N-atoms within GDQs atomic lattice (around 3 at%), at all applied doses. Additionally, the C-atoms percentage was highly increased, from 63 to 79 at% or higher. The zeta potential of dots changed from −34.6 to +9.1 mV, due to the modification of functionalizing groups localized at the surface. Produced chemical changes lead to the desired alteration of the GQDs optical properties, such as an increased photoluminescence intensity, a higher photoluminescence quantum yields (from 2.07 to 18.40%) and a narrowing of the spectral features in the emission spectra. The ability of gamma-irradiated GQDs to quench free radical species was investigated and positively assessed; additionally, non-enzymatic optical detection of Cu(II) ions using GQDs as a sensor was studied and the detection limits are herein reported. These results suggest that GQDs can be potentially applied as smart photoluminescent sensors for metal cations.

Dual sensing of tetracycline and l-Lysine using green synthesized carbon dots from Nigella sativa seeds

In this study, carbon dots (CDs) that exhibit bright-blue fluorescence were hydrothermally synthesized from Nigella sativa seeds and employed for selective and quantitative detection of tetracycline (TC) and l-lysine (L-Lys). Specifically, TC addition led to quenching of CDs’ fluorescence intensity, while the enhanced fluorescence was spotted in the presence of L-Lys. CD quenching was attributed to both dynamic and static mechanisms, which is different from earlier-reported studies. In contrast, the introduction of L-Lys supported the emergence of hydrogen bonds between CDs and L-Lys, and consequently resulted in an increase in photoluminescence (PL) intensity. PL quenching was linearly correlated with TC concentration ranged from 0.01 to 40 μM with 14 nM detection limit. A linear correlation was observed for L-Lys in the concentration range of 0–500 μM and displayed a limit of detection of 453 nM. CDs showed TC detection in the range of 95.2–102.5% in tap water, river water, urine, and powdered milk. Likewise, CDs have been successfully utilized for detecting L-Lys in serum samples with a recovery range of 98.9–100.3%. The study describes the use of a dual sensing strategy for TC and L-Lys using green synthesized CDs. This study will provide an in-depth insight into the mechanism of TC and L-Lys detection by CDs, which will help to understand the role of different nanoparticles in sensing applications.

Chirality-pure carbon nanotubes show distinct complexation with recognition DNA sequences

Pure-chirality single-wall carbon nanotubes (SWCNTs) that are non-covalently complexed with recognition DNA sequences exhibit unique interaction behavior and hybrid stability in aqueous environments. The complexation of DNA-wrapped SWCNTs was found to be a strong function of both the DNA sequence and SWCNT chiral structure, highlighted by the distinct coating displacement of the same recognition DNA sequence from a pair of (6,5) enantiomers by a strong surfactant. A broad range of changes were observed for different DNA/SWCNT recognition pairs with surfactant exchange including the increase in nanotube photoluminescence intensity in the near-infrared (NIR) from 1.3 to 14.7-fold and time constants deduced from DNA displacement kinetics ranging from 9 s to 230 s. A large time constant of 230 s and a relatively small 4.4-fold increase in NIR emission intensity were obtained for the CTC3TC-(7,6) hybrid highlighting the vast potential of short DNA sequences for improved nanotube sorting and hybrid stability in aqueous environments. Additionally, CTC3TC-(7,6) was identified as the only hybrid to exhibit an increase in NIR fluorescence intensity in serum-containing cell culture media among all samples tested. Our results demonstrated unique optical properties and hybrid stability of DNA/SWCNT recognition pairs, providing a foundation for developing applications of chirality-pure SWCNTs.

Enhanced Photoluminescence of Hydrogenated Amorphous Silicon Carbide Thin Films by Means of a Fast Thermal Annealing Process.

In this paper, the photoluminescence (PL) of hydrogenated amorphous silicon carbide (a-Si1−xCx:H) thin films obtained by Plasma Enhancement Chemical Vapor Deposition (PECVD) is reported. Strong PL is obtained after a fast annealing process for 60 s at temperatures of 200, 400, 600, and 800 °C. The thin films are characterized using Fourier Transform Infrared spectroscopy (FTIR), PL spectroscopy, and Energy-Dispersive X-ray Spectroscopy (EDS). According to the results of the structural characterization, it is deduced that a structural rearrangement of the amorphous matrix is carried out during the fast annealing process, which results in different degrees of oxidation on the a-Si1−xCx:H films. The PL peak position shifts towards higher energies as the temperature increases. The sample deposited with a silane/methane flux ratio of 37.5 at an Radio Frequency (RF) power of 6 W experiences an increase in PL intensity of more than nine times, with a displacement in the peak position from 2.5 eV to 2.87 eV, at 800 °C. From the PL analysis, we observe two emission bands: one centered in the near infrared and other in the visible range (with a blue peak). This study opens the possibility to use such thin films in the development of optoelectronics devices, with potential for application in solar cells.

The Optical and Microstructural Characterization of the Polymeric Thin Films with Self-Assembly Nanoparticles Prepared by Spin-Coating Techniques

anhydride-poly(ethylene glycol) co-polymer (A-PEGCP) has been synthesized from maleic anhydride, poly(ethylene glycol) and bisphenol-A diglycidyl ether without using any organic solvent. The thin films produced from A-PEGCP solution were spin-coated on ITO-coated glass. The nanoparticles are observed in the thin films. It is proposed that the nanoparticle is built by a self-assembly process with bisphenol-A aggregates and poly (ethylene glycol) moieties. The effects of concentration, thermal annealing, excitation wavelength and moisture on the optical and nanostructured characterization of the thin films are investigated in this study. Photoluminescence (PL) spectrum of the thin film on ITO-coated glass has a peak of about 450 nm that extends from 360 to 550 nm under 325 nm excitation. The increase in PL intensity is accompanied by a red shift of PL spectrum as concentration increases. Moreover, the slightly red shift of PL spectrum is also observed as annealing temperature increases. Meanwhile, PL intensity negligibly decreases with annealing temperature. The degradation in PL intensity is apparent due to moisture. The excitation-wavelength dependent photoluminescence (EWDP) is observed in the thin film. UV-Vis absorption spectra of the thin films are red-shifted with concentration due to more molecular aggregation. The highest occupied molecular orbital (HOMO) energy is −9.52 eV. The optical band-gap energy is 4.09–4.44 eV.

Performance analysis of anomalous photocatalytic activity of Cr-doped TiO2 nanoparticles [Cr(x)TiO2(1−x)

We report the synthesis and characterisation of pristine and chromium (Cr) metal ion-doped titanium dioxide nanoparticles [Cr(x)TiO2(1−x)] to study the anomalous effect of Cr doping on the photocatalytic property of TiO2. The presence of dopants generates more number of recombination pairs and increases surface coverage sites which decreases photocatalytic activity. We study the structural morphology of the synthesised Cr(x)TiO2(1−x) samples using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy analysis. The effect of Cr3+ ions on the optical properties of TiO2 has been studied using various imaging and spectroscopic techniques. Further, the effect of doping of Cr on the photocatalytic activity of TiO2 has been analysed in detail. The concentration of Cr in TiO2 has been chosen as 0, 1, 5 and 10% by weight. It has been observed that the pristine TiO2 exhibits better photocatalytic activity as compared to Cr-doped TiO2 irrespective of the Cr concentration. This can be attributed to the fact that due to Cr doping in TiO2, the number of available adsorption sites for malachite green reduces which degrades its photocatalytic activity. It is also confirmed by photoluminescence (PL) and time-resolved photoluminescence spectroscopy. PL intensity increases, and lifetime decreases with increase in doping concentration. Radiative recombination of electron and hole pairs of Cr3+ in TiO2 degrades its photocatalytic activity. The degradation efficiency is found to be 96% in the case of pristine TiO2 which reduces to 12% when doped with x = 10% concentration of chromium. Therefore, it is observed that in comparison with Cr-doped TiO2, pristine TiO2 exhibits an improved photocatalytic activity which shows the anomalous effect of Cr doping on the photocatalytic property of TiO2.

Photoluminescence properties of l-cysteine-derived carbon dots prepared in non-aqueous and aqueous solvents

Fluorescent carbon dots (CDs) can be produced from amino acids such as l-cysteine by an intermolecular dehydration reaction between NH2 and COOH groups, followed by carbonization. This dehydration reaction may be hindered in aqueous media because the amino acid molecules are surrounded by water; therefore, we used a non-aqueous reaction media to facilitate the dehydration reaction. In the present study, l-cysteine was heated at 230 °C for 30 min in a non-aqueous solvent, diphenyl ether, which has a high boiling point of 258 °C, to yield CDs-NA. For comparison, we also hydrothermally prepared CDs-A from l-cysteine dissolved in water, with the use of an autoclave and microwave heating system under the same temperature and time conditions. The maximum photoluminescence (PL) intensity of the water dispersion of CDs-NA was 1.5 times as high as that of the CDs-A. The absolute PL quantum yield (QY) of CDs-NA was 4.2%, which was 1.4 times as high as that of CDs-A, 3.0%. Characterization of the elemental compositions, particulate properties, and chemical bonding states confirmed that the non-aqueous solvent not only facilitated CD formation via the dehydration reaction but also contributed to the formation of sulfonic acid groups by oxidation of thiol groups. The rapid particle growth and formation of the additional functional groups might contribute to the increased PL intensity and absolute PLQY of CDs-NA. Through the optimization of the synthesis conditions in a non-aqueous solvent, the absolute PLQY of CDs-NA was increased from 4.2% to 10.2%.

Investigation of the Role of the Environment on the Photoluminescence and the Exciton Relaxation of CsPbBr3 Nanocrystals Thin Films

In this work, we present a detailed optical investigation of the effects of the environment on the photoluminescence (PL) spectra and the relaxation dynamics of pristine and aged CsPbBr3 nanocrystal (NC) thin films. We demonstrate that, contrary to previous results on similar NCs, the PL intensity of pristine NCs is higher when the sample is in wet air than in vacuum, due to the passivation of defects reducing the free exciton trapping and the bound excitons non-radiative relaxation. The aged NCs show a PL intensity increase in wet air nine times stronger than the pristine ones, due to an interplay between static and dynamic effects, increasing the number of emitting NCs and reducing the non-radiative recombination rate of free excitons. These results improve the understanding of the possible interactions between perovskite NCs and the environment, which could be relevant for the development of optical gas sensors exploiting perovskite NCs.

Enhanced luminescent performance with surface wrinkled Al-doped ZnO films

To improve the photoluminescence (PL) properties of ZnO films, researchers generally preferred cations doping such as Al, but the effects of Al doping on the increase in PL intensity of ZnO films were dissatisfactory usually when surface of films was smooth. In this work, a new and simple process was proposed for enhancing the PL intensity in the range of 375–600 nm of ZnO films. We prepared surface wrinkled Al-doped ZnO (AZO) films by sol–gel method with a quickly pre-annealing process. The wrinkle morphology was found to provide a larger absorption and luminescence area. Results showed that the wrinkle surface as compared with the smooth surface could raise the PL intensity of AZO films to 4 times, which made ZnO further close to the application for ultraviolet and visible luminescent devices.

The role of excess MgO in the intensity increase of red emission of Mn4+-activated Mg2TiO4 phosphors

The influence of magnesium oxide (MgO) content on the intensity of red photoluminescence (PL) of Mn4+ ions in Mn-doped phosphors Mg2TiO4:Mn produced by solid-state reaction at 1200 °C has been investigated by PL, optical absorption, X-ray diffraction, and electron paramagnetic resonance methods. The phosphors synthesized with excess MgO show an increase of Mn4+ red emission compared with those of stoichiometric composition. The magnitude of this increase depends on both MgO and Mn content. The largest increase of PL intensity is found for the phosphors synthesized under 3:1 molar ratios of MgO to TiO2. For these phosphors, the PL intensity increases from time 1.1 to time 3 when Mn concentration decreases from 1.0 to 0.0001 mol%. The phosphors produced under 6:1 molar ratios demonstrate a decrease of PL intensity at any Mn concentration. It is shown that excess MgO promotes stabilization of Mg2TiO4 phase against decomposition, hinders formation of Mn2+ centers, and enhances Mn4+ ions incorporation in the Mg2TiO4 crystal lattice. The latter together with reduced concentration quenching are supposed to be the main reasons of PL enhancement, which leads to the conclusion that excess MgO is necessary to produce an efficient red phosphor.

Interlayer‐Sensitized Linear and Nonlinear Photoluminescence of Quasi‐2D Hybrid Perovskites Using Aggregation‐Induced Enhanced Emission Active Organic Cation Layers

AbstractA concept of interlayer‐sensitized photoluminescence (PL) of quasi‐2D hybrid perovskite (PVK) with a π‐conjugated optically interacting organic cation layer is introduced and demonstrated. A rod‐shaped aggregation‐induced enhanced emission (AIEE) organic cation (BPCSA+), well fitted into the lattice size of 2D PVK layers, is designed and synthesized to prolong the exciton lifetime in a condensed layer assembly in the PVK. The BPCSA+ promotes the PL of this hybrid PVK up to 10‐folds from that of a non‐π‐conjugated organic cation (OA) 2D PVK. Notably, different from PL of OA 2D PVK, the increased PL intensity of BPCSA 2D PVKs with an increase of the BPCSA ratio in the PVK indicates a critical photon‐harvesting contribution of BPCSA. The films of BPCSA 2D PVKs are incredibly stable in ambient environments for more than 4 months and even upon direct contact with water. Additionally, due to the strong two‐photon absorption property of BPCSA, the BPCSA 2D PVK displays superior emission properties upon two‐photon excitation with a short wavelength IR laser. Thus, the AIEE sensitization system for quasi‐2D PVK hybrid system can make a drastic improvement in performance as well as in the stability of the PVK emitter and PVK based nonlinear optical devices.

Composition-, Size-, and Surface Functionalization-Dependent Optical Properties of Lead Bromide Perovskite Nanocrystals.

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on surface passivation. We present a study on the temperature-dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by surface passivation, and a postsynthesis ligand exchange with a quaternary ammonium bromide (QAB) results in more stable passivation over a larger temperature range. The PL intensity is highest from 50 to 250 K, which indicates that ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), surface passivation still needs to be improved to achieve maximum emission intensity in nanocrystal films.

Investigation of polycrystalline GaxIn1 − xP for potential use as a solar cell absorber with tunable bandgap

There is ongoing interest in developing a stable, low-cost, 1.6–1.8 eV top-cell material that can be used for two-junction (tandem) solar cells, particularly in combination with a silicon bottom cell. In this work, polycrystalline GaInP is grown and characterized to explore its properties and use for this purpose. The film composition and deposition temperature are varied to determine their effects on grain size, morphology, and photoluminescence (PL) over a range of bandgaps from 1.35 to 1.7 eV. An Al-assisted post-deposition treatment for 1.7-eV polycrystalline GaInP results in a 90-fold increase in peak photoluminescence (PL) intensity, a 220-fold increase in integrated PL intensity, and increased time-resolved PL lifetime from <2 ns to 44 ns. The increase in PL intensity and lifetime is attributed to a reduction of nonradiative minority-carrier recombination at the top surface, and at grain boundaries near the surface, due to the formation of a higher-bandgap AlGaInP alloy. These materials provide a viable path toward increased minority-carrier concentration under illumination and improved recombination properties needed for high-efficiency tandem solar cells.

Formation of SnO and SnO2 phases during the annealing of SnO(x) films obtained by molecular beam epitaxy

SnO and SnO2 films were obtained on the SiO2 surface by the molecular-beam epitaxy method. The initial films are in the polycrystalline phase. The annealing of SnO(x) films at a temperature of 300 °C resulted in the formation of the tetragonal SnO phase. Three vibration modes Eg, A1g, and B1g with the frequencies of SnO bond vibrations of 113, 211 and ~360 cm−1, respectively, which correspond to the SnO phase, were first observed by the Raman spectroscopy method. The orthorhombic SnO2 films were obtained by increasing the annealing temperature to 500 °C. Based on the valence band XPS (X-ray photoelectron spectroscopy) spectrum, several features with the binding energy approximately 5 eV, 7.5 eV and 11 eV, which are the same with the valence band of SnO2, were identified. The refractive index and absorption coefficient were investigated by the spectral ellipsometry technique. The high absorption coefficients correspond to the high Sn content. The film dielectric properties were revealed at the temperature higher than 300 °C. The refractive index values lie in the range of 1.5–2.6 for the visible spectral region. The pronounced absorption edges at 2.85 eV and 3.6 eV corresponding to those of stannous oxide (SnO) and stannic oxide (SnO2) were observed. The photoluminescence (PL) from the SnO(x) films was observed at room temperature. The increase of the annealing temperature resulted in the increase of PL intensity. Such PL intensity behavior is likely due to the Sn nanoislands.