Higher Photoluminescence Intensity Research Articles

Synthesis and scintillation properties of cerium-doped Gd2SiO5 nanopowders under alpha radiation and the importance of selecting the appropriate calcination temperature

In this study, single phase cerium-doped Gd2SiO5 (GSO:Ce) nanopowders were successfully synthesized by a Pechini sol-gel process, and the effect of calcination temperature on the crystallinity and purity of GSO phase was investigated by XRD and FTIR analyses. The suitable temperature for calcination of nanopowders and the formation of single phase GSO is estimated to be about 1250 °C. The morphology of samples, elemental composition, and nanoparticle size distribution of the single phase sample were examined by using FESEM, EDX, and DLS analyses. In addition to crystallite size and crystal lattice strain, the crystal lattice parameters of this sample were obtained using a series of equations, the results of which are very close to those obtained from the Rietveld refinement method. Studies show that unlike undoped GSO single crystal, the optical absorption spectra of synthesized GSO nanopowders are continuous and doping with 2 mol% cerium causes the highest photoluminescence intensity at 435 nm under excitation at 345 nm. The scintillation pulse height spectra of these samples were also investigated under alpha radiation. Finally, the importance of selecting the appropriate calcination temperature and its effect on the size of the nanoparticles, photoluminescence intensity, and scintillation pulse height spectrum has been studied.

NUV light-induced visible green emissions of Erbium-doped hierarchical Bi2Zr2O7 structures

We reported a facile strategy to synthesis of Er3+ doped Bi2Zr2O7 nanophosphors via hydrothermal route using Aloe Vera gel as a bio - surfactant. The photoluminescence spectra exhibit intense peaks centered at ~524, 547 and 660 nm, which were attributed to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ions, respectively. The highest photoluminescence intensity was achieved in 5 mol % of Er3+ ions. The electric dipole-quadrupole (d-q) interaction was responsible for the concentration quenching. The fluorescence intensity ratio technique was utilized for optical thermometric properties of the nanophosphors in the temperature range of 303–463 K. At lower temperature (282 K) the relative sensor sensitivity of the optimized nanophosphor was found to be ~0.0621 K−1. Thermoluminescence glow curves of 5 mol % Er3+ ions doped nanophosphor irradiated with γ-rays showed two peaks at ~120 and 203 °C. A thermoluminescence glow peak at ~120 °C showed good linearity with dose, simple glow curve structure, minimal fading and good reproducibility. The above results clearly showed that the optimized nanophosphor is quite useful in WLED's, TL dosimetry and optical thermometry applications.

Enhanced photoluminescence through Forster resonance energy transfer in Polypyrrole-PMMA blends for application in optoelectronic devices

Polypyrrole-PMMA (PPy-PMMA) blends have been prepared by chemical oxidative polymerization method. The band gap of all the blends lies in semiconductive range though PMMA is insulating in nature. Spherical agglomerated structure of the blends is observed from microstructural images. High photoluminescence (PL) intensity is obtained in PPy-PMMA blends even if PPy has low PL emission efficiency. The enhancement of PL emission intensity of the blends is due to Forster resonance energy transfer. Here an energy transfer occurs from donor PMMA to acceptor PPy which enhances the emission intensity of PPy. PL spectra show the increase in emission intensity with increase in concentration of PMMA in the blend up to a certain limit. Donor-donor interaction is also observed in the blend due to which the emission intensity of the blend having excess amount of PMMA suddenly falls down. Thus by tuning the amount of PMMA added in PPy matrix the PL intensity of the blend can be tuned and the blend can be used as a promising material for optoelectronic devices.

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaOx buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm−2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaOx semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Stability and dispersion improvement of quantum-dot films by hydrosilylation between quantum-dot ligands and a siloxane matrix.

Quantum-dot (QD) ligands were modified and hydrosilylated with a siloxane matrix to improve the quantum efficiency and stability of the QDs. Conventional oleic acid (OA) ligands were exchanged with vinyl ligands without any reduction in the quantum yield. After ligand modification, hydrosilylation was induced between the vinyl ligands on the QDs (vinyl QDs) and a siloxane matrix, resulting in a uniform QD dispersion in the matrix. The hydrosilylated QDs in siloxane showed 23% higher photoluminescence intensity than OA QDs blended in siloxane after storage for 30 days at 85 °C under 85% relative humidity. The QDs also showed 22.3% higher UV/thermal stability than OA QDs in siloxane after 29 h under a high LED photon flux. This study demonstrates that the chemical reaction of QD ligands with polymer matrices can improve the QDs' dispersion and stability.

Wide-wavelength-range control of photoluminescence polarization in closely stacked InAs/GaAs quantum dots

We developed a method of growing closely stacked InAs/GaAs quantum dots (QDs) to control the photoluminescence (PL) polarization characteristics in a wide wavelength range. The emission wavelength of the closely stacked QDs redshifted with decreasing substrate temperature during stacking growth, while the PL polarization characteristic was controlled by the GaAs spacer layer thickness and the number of QD layers. A unified rule for the optimum GaAs spacer layer thickness that both enhances the transverse magnetic (TM)-polarized component and achieves a high PL intensity for all growth temperatures was revealed. 30-layer stacked QDs with the optimum spacer layer thickness grown at substrate temperatures from 430 to 480 °C exhibited TM-enhanced polarization characteristics in the 1.15–1.3 μm band. Moreover, we studied the one-dimensional electronic states in the closely stacked QDs with the optimized GaAs spacer layer thickness by time-resolved PL.

Ligand induced switching of the band alignment in aqueous synthesized CdTe/CdS core/shell nanocrystals

CdTe/CdS core/shell quantum dots (QDs) are formed in aqueous synthesis via the partial decomposition of hydrophilic thiols, used as surface ligands. In this work, we investigate the influence of the chemical nature (functional group and chain length) of the used surface ligands on the shell formation. Four different surface ligands are compared: 3-mercaptopropionic acid, MPA, thioglycolic acid, TGA, sodium 3-mercaptopropanesulfonate, MPS, and sodium 2-mercaptoethanesulfonate, MES. The QD growth rate increases when the ligand aliphatic chain length decreases due to steric reasons. At the same time, the QDs stabilized with carboxylate ligands grow faster and achieve higher photoluminescence quantum yields compared to those containing sulfonate ligands. The average PL lifetime of TGA and MPA capped QDs is similar (≈20 ns) while in the case of MPS shorter (≈15 ns) and for MES significantly longer (≈30 ns) values are measured. A detailed structural analysis combining powder X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) indicates the existence of two novel regimes of band alignment: in the case of the mercaptocarboxylate ligands the classic type I band alignment between the core and shell materials is predominant, while the mercaptosulfonate ligands induce a quasi-type II alignment (MES) or an inverted type I alignment (MPS). Finally, the effect of the pH value on the optical properties was evaluated: using a ligand excess in solution allows achieving better stability of the QDs while maintaining high photoluminescence intensity at low pH.

Unusual Stability and Temperature-Dependent Properties of Highly Emissive CsPbBr3 Perovskite Nanocrystals Obtained from in Situ Crystallization in Poly(vinylidene difluoride).

All-inorganic cesium lead halide perovskite nanocrystals (CsPbX3, X = Cl, Br, or I) present broad applications in the field of optoelectronics due to their excellent photoluminescence (PL), narrow spectral bandwidth, and wide spectral tunability. However, their poor stability limits their practical application. In this work, we successfully use an in situ crystallization strategy for growing and cladding CsPbBr3 perovskite nanocrystals in poly(vinylidene difluoride) (PVDF). The CsPbBr3 nanocrystals in the as-fabricated CsPbBr3@PVDF composites have an average diameter of 16-18 nm and a strong PL emission (537 nm), with a photoluminescence quantum yield exceeding 30%. In addition, the fabricated CsPbBr3@PVDF composites present improved resistance to heat and water preserving with remarkable optical performance, owing to the effective protection of PVDF. Moreover, the CsPbBr3 nanocrystals generated in PVDF can withstand temperatures up to 170 °C and can be completely immersed in water for 60 days while still retaining high PL intensity, which facilitate the practical application of CsPbBr3 perovskite nanocrystals. These CsPbBr3@PVDF composite films with remarkable optical performances and superior anti-interference ability have broad application prospects in optoelectronics as well as good potential as temperature sensors in mechanical engineering.

Probing Facet-Dependent Surface Defects in MAPbI3 Perovskite Single Crystals

Halide perovskites such as methylammonium lead iodide (MAPbI3) currently attract considerable attention because of their excellent optoelectronic properties and performance in solar cell devices. Despite tremendous research efforts to elucidate their fundamental properties, ion migration with the presence of ionic defects is still not fully understood. Here, various types of ionic defects for specific (100) and (112) lattice facets in single-crystal MAPbI3 have been investigated systematically. Our measurements reveal significant anisotropic properties. Photoluminescence (PL) and electrical transport measurements show that the (100) facet has higher PL intensity and over 1 order lower trap density compared to that of the (112) facet. We find that the facet-dependent variations of contact potential difference measured with Kelvin probe force microscopy under different bias voltages and light illuminations provide insights into different types of ionic defects on the surface of MAPbI3 single crystals. We also observe a completely different ion migration behavior on specific crystal facets through nanoscale scanning probe microscopy investigations. Our results indicate that the (100) facet exhibits an n-type behavior dominated with I– vacancies, whereas the (112) facet exhibits a p-type behavior with MA+ or Pb2+ vacancies. The findings on the facet-dependent configuration of ionic defects provide deeper understanding on facet-dependent optoelectronic properties in single-crystal MAPbI3.

Edge Effect on the Population of Free Carriers and Excitons in Single‐Crystal CH3NH3PbBr3 Perovskite Nanomaterials

AbstractThe spatial distribution of free carriers and excitons in perovskite micro‐/nanostructures is currently a major topic of debate. They have been reported to coexist with spatial separation in individual perovskite grains and films. It is revealed that free carriers and excitons in single‐crystal CH3NH3PbBr3 perovskite microplatelets can spatially overlap; however, the distribution is such that the edges can be entirely occupied by the former, while the interior is occupied only by the latter. This is further confirmed in single‐crystal CH3NH3PbBr3 perovskite microstrips, in which only free carriers are observed. These results may be attributable to electrostatic potential fluctuations on the edges, which can promote electron–hole separation so that they do not bind to form excitons. Microscopic I–V measurements demonstrate that the microplatelet edges and the microstrips show current densities of more than 260 and 340 A cm−2 in a dark environment under 1.2 V bias voltage, respectively. It is suggested that these findings mean that such microstructures hold promise for the development of novel perovskite micro‐/nanostructure‐based optoelectronic devices with interesting photoconductive properties such as high photoluminescence intensity, longer excited‐state lifetime, high conductivity, large saturation current densities, and suppressed hysteresis effects (smaller open voltages).

Identification of surface-passivating ligands and core-size-dependent CdSe/CdZnS with highly emitting for cell labeling

Different organic ligands capped CdSe quantum dots (QDs) are synthesized. Our results show that CdSe QDs exhibit high photoluminescent intensity when the surface dangling bonds are passivated by trioctylphosphine and oleylamine together. FT-IR vibrational spectra reveal that the oleylamine mainly bond to the metal sites. In the process of synthesis, oleylamine can make CdSe more uniform by controlling the rate of nucleation and growth. Because of size-dependent band gap, core/shell QDs emitting desired color can be obtained by coating different sized cores. The photoluminescence peaks of core/shell QDs cover a wide range from 580 to 615 nm. The core/shell QDs are successfully transferred into aqueous solution through ligand exchanging with reduced glutathione and used to label HeLa cells.

Dark-exciton valley dynamics in transition metal dichalcogenide alloy monolayers

We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo1−xWxSe2 alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe2 monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe2, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin. We find the critical value of W-concentration, xc, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around xc show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.

A CF3-substituted diphosphine dioxide ligand that enhances both photoluminescence intensity and solubility of Eu(III) complexes

The author developed the di(3-trifluoromethylphenyl)[3-(dioctylphosphinyl)propyl]phosphine oxide (DPDO-mCF3) ligand in order to prepare Eu(III) complexes with high photoluminescence intensity and solubility. By coordinating the DPDO-mCF3 ligand to Eu(III), photoluminescence intensities were increased to 6.4 and 4.3 times those of tris(4,4,5,5,6,6,6-heptafluoro-1-(2-naphthyl)-1,3-hexanedionato)europium(III) (Eu(III)(hfnh)3; 4) and tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III) (Eu(III)(fod)3; 5), respectively. Coordination of the DPDO-mCF3 ligand also increased the asymmetry ratio, total photoluminescence quantum yield (ΦTOT), and absorbance of Eu(III) complexes. ΦTOT of Eu(III)(hfnh)3(DPDO-mCF3) (1) was 0.60 in ethyl acetate at a concentration of 1 × 10−3 mol/L and 0.82 in the solid state. In addition, ΦTOT of Eu(III)(fod)3(DPDO-mCF3) (2) is 0.39 in ethyl acetate at a concentration of 1 × 10−3 mol/L and 0.56 in the solid state. Solid-state quantum yields were larger than solution-state quantum yields across a wide wavelength region. Furthermore, the solubility of 2 in 2,3-dihydrodecafluoropentane (54 μmol/g) was far higher than that of 5 (7.2 μmol/g). DPDO-mCF3 is thus an innovative ligand for Eu(III) complexes, and Eu(III) complexes bearing a DPDO-mCF3 ligand are promising candidates for many applications, including LED devices, secure media, and sensing devices.

Different Roles of Ce3+ Optical Centers in Oxyorthosilicate Nanocrystals at X-ray and UV Excitation

Luminescence properties of Lu2SiO5:Ce3+ and Y2SiO5:Ce3+ nanocrystals were studied using photo- and X-ray luminescence techniques. The crystal structure of Re2SiO5 nanocrystals (P21/c space group) differs from the crystal structure of Re2SiO5 bulk crystals (C2/c space group) with 9- and 7-oxygen-coordinated cation positions instead of 6- and 7-coordinated ones observed for Re2SiO5 bulk crystals. Two optical centers (Ce1 and Ce2) were observed for Re2SiO5:Ce3+ nanocrystals originating from cerium ions substituting 9- and 7-oxygen-coordinated cation sites. Preferential substitution of larger cation sites by cerium ions leads to higher photoluminescence intensity of Ce1 centers, however, Ce2 centers are the main centers for electron-hole recombination, so only Ce2 band is observed in X-ray luminescence spectra. The features of oxygen coordination of Ce1 and Ce2 centers and high content of oxygen vacancies in Re2SiO5:Ce3+ nanocrystals can provide preferential trapping of electrons near Ce2 centers, and therefore, the dominant role of Ce2 band in X-ray luminescence spectra.

Optical, electrical and luminescent studies of CuO/MgO nanocomposites synthesized via sonochemical method

CuO/MgO nanocomposites (NCs) were synthesized by energy efficient sonochemical method using glucose as surfactant. The structural properties were carried out by means of Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Diffused Reflectance Spectra (DRS). In addition to this, Photoluminescence (PL), Electrical conductivity and Thermoluminescence (TL) properties were also discussed in detail. The diffraction peaks indicates the coexistence of CuO/MgO composite. PXRD studies revealed CuO and MgO exhibit monoclinic and cubic phases respectively. PL studies of the composites showed peaks at 478, 538, 593 and 714 nm upon excited at 431 nm wavelength. Among the composites, the highest PL intensity was recorded for 70:30 ratio of CuO/MgO. The CIE chromaticity coordinates and Correlated Color Temperature (CCT) results clearly evident that the optimized composites were quite useful for display applications. Enhanced electrical conductivity and dielectric properties were observed for 90:10 CuO/MgO NCs. The TL results clearly indicate, single prominent glow peak of composites recorded in the range of 114–120 °C along with shouldered peak at ∼150 °C. The Kinetics parameters (τ, S, b) and dosimetric properties of the composites were thoroughly investigated. A good conformity was obtained between the results obtained from various analysing methods. The present results addresses that CuO/MgO nanocomposite is a promising candidate for use as a radiation dosimeter.

CVD growth of monolayer WS2 through controlled seed formation and vapor density

Large area, single layer WS2 has a high potential for use in optoelectrical devices with its high photoluminescence intensity and low response time. In this work, we demonstrate a systematic study of controlled tungsten disulfide (WS2) monolayer growth using chemical vapor deposition (CVD) technique. With a detailed investigation of process parameters such as H2 gas inclusion into the main carrier gas, growth temperature and duration, we have gained insight into two-dimensional (2D) WS2 synthesis through controlling the seed formations and the radical vapor density associated with WO3. We confirm that H2 gas, when included to the carrier gas, is directly involved in WO3 reduction due to its reductive reagent nature, which provides a more effective sulfurization and monolayer formation process. Additionally, by changing the CVD growth configuration, hence, increasing the tungsten related vapor density and confining the reactant radicals, we succeed in realizing larger WS2 monolayers, which is still a technological challenge in order to utilize these structures for practical applications. Further optimization of the growth procedure is demonstrated by tuning the growth duration to prevent the excess seed formations and additional layers which will possibly limit the device performance of the monolayer flakes or films when applied.

Morphological and Optical Properties of Polypyrrole Nanoparticles Synthesized by Variation of Monomer to Oxidant Ratio

Abstract Among the conducting polymers, polypyrrole (PPY) has drawn a lot of interest due to its excellent environmental stability, ease of preparation and high conductivity. Here we have used chemical oxidative polymerization to synthesize PPY from its monomer pyrrole. We have studied morphological and optical properties of PPY samples by varying the monomer to oxidant molar ratio as 1:1.25 and 1:2 and denoted the samples by 1125ppy and 1200ppy respectively. The prepared samples are characterized by FTIR, UV-Vis, photoluminescence (PL) spectroscopy and FESEM. FTIR spectra reveal the required functional groups that should be present in PPY. From UV-Vis spectra, it is clear that the band gap of PPY changes due to the variation of monomer to oxidant ratio. There are two major absorption bands among which one is at about 350 nm for both the samples due to π-π* transition and another absorption band is at about 600nm for 1125ppy due to polaron band transition. The polaron band transition for 1200ppy is observed at about 650 nm. This redshift is due to the increased extent of oxidation induced by increasing the amount of oxidant. From the FESEM image, it is observed that agglomeration of particles increases with thedecreased monomer to oxidant ratio. As the ratio decreases the rate of oxidation increases and more cross-linked structure of polypyrrole is formed which resultsin more agglomerated structure. PL Spectra shows that 1125ppy has higher PL intensity than that of 1200ppy. Since agglomeration quenches PL intensity, the process is supported by our SEM results. So, PL intensity decreases with decreasing monomer to oxidant ratio

Synthesis and electroluminescence of novel white fluorescence quantum dots based on a Zn-Ga-S host.

White-light-emitting Ag, Mn: Zn-Ga-S/ZnS quantum dots (QDs) with a gratifying photoluminescence (PL) quantum yield (QY) of up to 90% were prepared, and shown to be ultra-stable, maintaining a high PL intensity at 300 °C or for 32 h of UV illumination. These QDs were employed in a white QD-LED displaying a color rendering index (CRI) of 72 and maximum current efficiency of 0.22 cd A-1.

A highly luminescent quantum dot/mesoporous TiO2 nanocomplex film under controlled energy transfer.

We have prepared a highly luminescent quantum dot (QDs)-TiO2 nanocomplex film by the dip coating method. Because QDs with 3-mercaptopropionic acid as a ligand adsorb ionized Ti+ cations on the TiO2 particle, the average distance between the QDs can be changed through controlling the porosity in the film. The porosity is controlled using ethyl cellulose (EC). EC is the best material for well dispersing the hydroxyl functional group (-OH) in the chemical structure, and forming pores inside the film under the decomposition temperature (above 698 °F). As the porosity decreases, the average decay time controlled by the porosity increases to the maximum 91.2 ns. On the other hand, the amount of QDs decreased to 50%, hindering the increase of photo-luminescence (PL) intensity. Through this result, we have found that the PL intensity of the QD films is strongly related to the amount of the QDs and the resolution of aggregation. Consequently, we have optimized the porosity of the film and obtained high PL intensities up to approximately 17 times.

Enhancing the Performance of Inverted Perovskite Solar Cells via Grain Boundary Passivation with Carbon Quantum Dots.

Nonradiative recombination, the main energy loss channel for open circuit voltage ( Voc), is one of the crucial problems for achieving high power conversion efficiency (PCE) in inverted perovskite solar cells (PSCs). Usually, grain boundary passivation is considered as an effective way to reduce nonradiative recombination because the defects (uncoordinated ions) on grain boundaries are passivated. We added the hydroxyl and carbonyl functional groups containing carbon quantum dots (CQDs) into a perovskite precursor solution to passivate the uncoordinated lead ions on grain boundaries. Higher photoluminescence intensity and longer carrier lifetime were demonstrated in the perovskite film with the CQD additive. This confirmed that the addition of CQDs can reduce nonradiative recombination by grain boundary passivation. Additionally, the introduction of CQDs could increase the thickness of the perovskite film. Consequently, we achieved a champion device with a PCE of 18.24%. The device with CQDs retained 73.4% of its initial PCE after being aged for 48 h under 80% humidity in the dark at room temperature. Our findings reveal the mechanisms of how CQDs passivate the grain boundaries of perovskite, which can improve the efficiency and stability of PSCs.