Triphenylamine and quinoline-containing polyfluorene for blue light-emitting diodes

A novel blue light-emitting polyfluorene-based copolymer PTHD containing electron-rich triphenylamine and electron-poor phenylquinoline side chains in the C-9 position of fluorene unit is described. By comparison of the solution and thin film photoluminescence (PL) spectra of PTHD, a considerable red-shift of Δλ = 10-15 nm was observed in the thin film PL spectrum. The emission intensity of the shoulder peak appeared in dilute solution was also significantly enhanced in the thin film. In contrast to the reference polymer poly{[9,9-dihexylfluorene]-alt-[9,9-di(2,4-diphenylquinoline)fluorene]}, PTHD exhibits higher HOMO energy level, and higher maximum brightness with the PLED device configuration of ITO/PEDOT:PSS/polymer_70% + PBD_30%/TPBI/LiF/Al.     

Fabrication and characterization of hexagonal boron nitride powder by spray drying and calcining-nitriding technology

Hexagonal boron nitride (hBN) powder was fabricated prepared by the spray drying and calcining-nitriding technology. The effects of nitrided temperature on the phases, morphology and particle size distribution of hBN powder, were investigated. The synthesized powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Fourier transformed infrared spectrum, ultraviolet-visible (UV-vis) spectrum and photoluminescence (PL) spectrum. UV-vis spectrum revealed that the product had one obvious band gap (4.7 eV) and PL spectrum showed that it had a visible emission at 457 nm (λ_ex=230 nm). FESEM image indicated that the particle size of the synthesized hBN was mainly in the range of 0.5-1.5 μm in diameter, and 50-150 nm in thickness. The high-energy ball-milling process following 900 °C calcining process was very helpful to obtain fully crystallized hBN at lower temperature.     

Effect of a perfluorocyclopentene core unit on the structures and photoluminescence of fluorene- and anthracene-based compounds

A novel series of blue luminescent compounds, in which three identical functional groups, such as fluorene, anthracene, and spiro-bifluorene, are linked distortedly around a perfluorocyclopentene core, have been synthesized and characterized. The introduction of a perfluorocyclopentene linkage into the molecular framework leads to an enhancement of the photoluminescence (PL) efficiency and thermal stability. All compounds exhibit intense blue photoluminescence, which has been attributed to fluorene- or anthracene-based π→π* transitions. The maximum emission wavelengths of all compounds at room temperature are in the region of 420-480 nm, with higher PL quantum efficiencies than in 9,10-diphenylanthracene. The electroluminescent (EL) properties of compound 4, 1,2-bis(9,9′-spirobifluoren-2-yl)-3,3,4,4,5,5-hexafluorocyclopentene, were investigated. A multilayer EL device with the configuration of ITO/2TNATA(60 nm)/NPB(20 nm)/ADN:2%-compound-4(35 nm)/Alq₃(20 nm)/LiF(2 nm)/Al has been successfully fabricated.     

A low-temperature synthesis of ultraviolet-light-emitting ZnO nanotubes and tubular whiskers

We have successfully synthesized single-crystal ZnO nanotubes and tubular whiskers by employing Zn(NO₃)₂·6H₂O, NH₃·H₂O as the starting materials in the presence of polyethylene glycol (PEG, M_w=2000) at ambient pressure and low temperature (70 °C). Characterizations are carried out by X-ray powder diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM, HRTEM) and photoluminescence (PL) measurement. The results show that the as-prepared ZnO are tubular textures, which have average cross-sectional dimensions of 200-300 nm, lengths of 2-3.5 μm, and wall thickness of 80 nm. These tubular products demonstrate a sharp ultraviolet excitonic emission peak centered at 385 nm at room temperature. A possible growth mechanism and the influence of the reaction temperature on the formation of crystalline ZnO are presented.     

Synthesis, photoluminescent and electrochromic properties of new aromatic poly(amine-hydrazide)s and poly(amine-1,3,4-oxadiazole)s derived from 4,4′-dicarboxy-4″-methyltriphenylamine

A series of new poly(amine-hydrazide)s I were prepared from the dicarboxylic acid 4,4′-dicarboxy-4″-methyltriphenylamine with terephthalic dihydrazide (TPH) and isophthalic dihydrazide (IPH), respectively, via the Yamazaki phosphorylation reaction. Polymers I were readily soluble in many common organic solvents, and could be solution cast into transparent, tough, and flexible films with good mechanical properties. Differential scanning calorimetry (DSC) indicated that the hydrazide polymers had T_g’s in the range of 222-223 °C and could be thermally cyclodehydrated into the corresponding oxadiazole polymers in the range of 300-400 °C. The resulting poly(amine-1,3,4-oxadiazole)s II exhibited T_g’s in the range of 269-283 °C, 10% weight-loss temperatures in excess of 511 °C, and char yield at 800 °C in nitrogen higher than 63%. These poly(amine-hydrazide)s I exhibited strong UV-Vis absorption bands at 351-355 nm in NMP solution. Their photoluminescence spectra in NMP solution and film showed maximum bands around 459-461 nm in the blue region for I series. The hole-transporting and electrochromic properties are examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the poly(amine-hydrazide)s I prepared by casting polymer solution onto an indium-tin oxide (ITO)-coated glass substrate exhibited one reversible oxidation redox couples at 1.32-1.33 V vs. Ag/AgCl in acetonitrile solution. All obtained poly(amine-hydrazide)s I revealed excellent stability of electrochromic characteristics, changing color from original pale yellowish to blue.     

Self-assembly and aggregation of ATRP prepared amphiphilic BAB tri-block copolymers contained nonionic ethylene glycol and fluorescent 9,10-di(1-naphthalenyl)-2-vinyl-anthracene/1-vinyl-pyrene segments

Two novel amphiphilic BAB-type block copolymers, ADN-PEG3400-ADN and Py-PEG3400-Py containing deep blue and bluish-green fluorescent moieties were prepared using atom transfer radical polymerization (ATRP) (where, ADN = poly(9,10-di(1-naphthalenyl)-2-vinylanthracene), Py = poly(1-vinyl pyrene) and PEG3400 = poly(ethylene glycol) with M_n = 3400 g/mol). The GPC number averaged molecular weights (MW) of the block copolymers were M_n = 9600 and 13,800 g/mol, respectively, based on polystyrene MW standards. The PEG3400 segment has a melting temperature (T_m peak) at 64–65 °C, whereas the glass transition temperatures (T_g midpoint) of the ADN and Py segments were found to be 230 °C and 193 °C, respectively, and are similar to their respective homopolymers indicating complete microphase segregration. The photoluminescence (PL) emission of the copolymers ADN-PEG3400-ADN exhibited two maxima at 423.5 nm and 441.5 nm while Py-PEG3400-Py has a maximum at 488.5 nm. Both copolymers form individual spherical micelles with diameter from 30 to 90 nm for Py-PEG3400-Py and 40–160 nm for ADN-PEG3400-ADN. The micelles, however, transform into cross-linked pearl-necklace-like aggregates at polymer concentrations above 1000 ppm, which may be attributed to the physical cross-linking between adjacent spherical micelles caused by the PEG3400 segments.     

Novel cyclopenta[def]phenanthrene based blue emitting oligomers for OLEDs

Novel blue emitters, oligo-MCPPs (tri-MCPP, tetra-MCPP, and penta-MCPP), have been synthesized and characterized. The introduction of cyclopenta[def]phenanthrene (CPP) units into the structure of oligo-MCPPs gave LEDs with high efficiency and pure blue emission. UV-visible absorption spectra of the thin films of these compounds appear at 333-354 nm, and their maximum PL emission at 416-447 nm. Multilayer organic EL devices with oligo-MCPPs as an emitting layer showed the turn-on voltage of about 4.8 V, the maximum brightness of 1076 cd/m² (at 8.2 V), the maximum luminescence efficiency of 0.81 cd/A, and the CIE coordinates of (0.17, 0.14) with blue color.     

Investigation of structural, morphological, luminescent and thermal properties of combusted aluminium-based iron oxide

Nanocomposites of aluminium integrated hematite α-Fe₂O₃ are synthesized by combustion route using aqueous solutions of AR grade ferric trichloride and aluminium nitrate as precursors. The influence of aluminium incorporation on to the morphology, XPS, photoluminescence and thermal properties has been investigated. The FESEM and AFM micrographs depict that the samples are compact and have homogeneously distributed grains of varying sizes (∼20-60 nm). Chemical composition and valence states of constituent elements in hematite are analyzed by XPS. In room temperature photoluminescence (PL) study, we observed strong violet emission around 436 nm without any deep-level emission and a small PL FWHM indicating that the concentrations of defects are responsible for deep-level emissions. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant. We studied interparticle interactions using complex impedance spectroscopy. We report a new potential candidate for its possible applications in optoelectronics and magnetic devices.     

Photoluminescence in the CaxSr1−xWO4 system at room temperature

In this work, a study was undertaken about the structural and photoluminescent properties, at room temperature, of powder samples from the Ca_xSr_1−xWO₄ (x=0-1.0) system, synthesized by a soft chemical method and heat treated between 400 and 700 °C. The material was characterized using Infrared, UV-vis and Raman spectroscopy and XRD. The most intense PL emission was obtained for the sample calcined at 600 °C, which is neither highly disordered (400-500 °C), nor completely ordered (700 °C). Corroborating the role of disorder in the PL phenomenon, the most intense PL response was not observed for pure CaWO₄ or SrWO₄, but for Ca_0.6Sr_0.4WO₄. The PL emission spectra could be separated into two Gaussian curves. The lower wavelength peak is placed around 530 nm, and the higher wavelength peak at about 690 nm. Similar results were reported in the literature for both CaWO₄ and SrWO₄.     

Visual and fluorescent detection of acetamiprid based on the inner filter effect of gold nanoparticles on ratiometric fluorescence quantum dots

In this work, we develop a simple and rapid sensing method for the visual and fluorescent detection of acetamiprid (AC) based on the inner-filter effect (IFE) of gold nanoparticles (AuNPs) on ratiometric fluorescent quantum dots (RF-QDs). The RF-QDs based dual-emission nanosensor was fabricated by assembling green emissive QDs (QDs_539 nm, λ_em = 539 nm) on the surface of red emissive QDs (QDs_661 nm, λ_em = 661 nm)-doped silica microspheres. The photoluminescence (PL) intensity of RF-QDs could be quenched by AuNPs based on IFE. Acetamiprid can adsorb on the surface of AuNPs due to its cyano group that has good affinity with gold, which could induce the aggregation of AuNPs accompanying color change from red to blue. Thus, the IFE of AuNPs on RF-QDs was weakened and the PL intensity of RF-QDs was recovered accordingly. Under the optimized conditions, the PL intensity of the RF-QDs/AuNPs system was proportional to the concentration of AC in the range of 0.025–5.0 μg mL⁻¹, with a detection limit of 16.8 μg L⁻¹. The established method had been used for AC detection in environmental and agricultural samples with satisfactory results.     

Composition dependent frequency upconversion of Er/Yb-codoped lead chloride tellurite glasses

The green and red upconversion luminescence of Er³⁺ in lead chloride tellurite glasses excited at 980 nm is investigated. Three intense emission bands centered at 530, 545, and 658 nm corresponding to the transitions ⁴S_3/2→⁴I_15/2, ²H_11/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2, respectively, were simultaneously observed at room temperature. With increasing PbCl₂ content, the intensity of green (530 nm) emissions increase slightly, while the green (545 nm) and red (658 nm) emissions increase significantly. The results indicate that PbCl₂ has more influence on the green (545 nm) and red (658 nm) emissions than the green (530 nm) emission. The dependence of upconversion intensities on excitation power and possible upconversion mechanisms are discussed and evaluated.     

Visible and infrared luminescence properties of Er-doped Y2Ti2O7 nanocrystals

Er³⁺-doped Y₂Ti₂O₇ nanocrystals were fabricated by the sol-gel method. While the annealing temperature exceeds 757 °C, amorphous pyrochlore phase Er_xY_2−xTi₂O₇ transfers to well-crystallized nanocrystals, and the average crystal size increases from ∼70 to ∼180 nm under 800-1000 °C/1 h annealing. Er_xY_2−xTi₂O₇ nanocrystals absorbing 980 nm photons can produce the upconversion (526, 547, and 660 nm; ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively) and Stokes (1528 nm; ⁴I_13/2→⁴I_15/2) photoluminescence (PL). The infrared PL decay curve is single-exponential for Er³⁺ (5 mol%)-doped Y₂Ti₂O₇ nanocrystals but slightly nonexponential for Er³⁺ (10 mol%)-doped Y₂Ti₂O₇ nanocrystals. For 5 and 10 mol% doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption. Much stronger intensity of red light relative to green light was observed for the sample with 10 mol% dopant. This phenomenon can be attributed to the reduced distance between Er³⁺-Er³⁺ ions, resulting in the enhancement of the energy-transfer upconversion and cross-relaxation mechanisms.     

An improved method for ratiometric fluorescence detection of pH and Cd using fluorescein isothiocyanate–quantum dots conjugates

In this study, thioglycolic acid capped-CdTe quantum dots (QDs) were modified by polyethylenimine (PEI), and then combined with fluorescein isothiocyanate (FITC) to fabricate FITC–CdTe conjugates. The self-assembly of FITC, CdTe and PEI was ascribed to electrostatic interactions in aqueous solution. The resulting conjugates were developed toward two routes. In route one, ratiometric photoluminescence (PL) intensity of conjugates (I_FITC/I_QDs) was almost linear toward pH from 5.3 to 8.7, and a ratiometric PL sensor of pH was favorable obtained. In route two, firstly added S²⁻ induced remarkable quenching of QDs PL peak (at the “OFF” state), which was restored due to following addition of Cd²⁺ (at the “ON” state). In the conjugates, successive introduction of S²⁻ and Cd²⁺ hardly influenced on FITC PL peaks. According to this PL “OFF-ON” mode, a ratiometric PL method for the detection of Cd²⁺ was achieved. Experimental results confirmed that the I_FITC/I_QDs exhibited near linear proportion toward Cd²⁺ concentration in the range from 0.1 to 15 μM, and the limit of detection was 12 nM. Interferential experiments adequately testified that the proposed sensors of pH and Cd²⁺ were practicable in real samples and complex systems. In comparison with conventional analytical techniques, the ratiometric PL method was simple, rapid, economic and highly selective.     

Sol-gel deposition and luminescent properties of LaMgAl11O19:Ce/Tb phosphor films

Rare earth ions (Ce³⁺, Tb³⁺)-doped LaMgAl₁₁O₁₉ phosphor films were deposited on quartz glass substrates by Pechini sol-gel and dip coating method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), atomic force microscopy (AFM), field emission scanning electronic microscopy (FESEM), photoluminescence (PL) spectra, and lifetimes were used to characterize the resulting films. The results of XRD indicated that the magnetoplumbite structure LaMgAl₁₁O₁₉ phase can be obtained at 1200 °C on quartz glass substrates. This was further verified by the results of FT-IR and TG-DTA. AFM study showed that uniform films have an average grain size of 150 nm and a root mean square (RMS) roughness of 4 nm. The thickness of the films characterized by FESEM is about 340 nm. LaMgAl₁₁O₁₉:Ce³⁺ film showed the parity and spin allowed 5d-4f band emission of Ce³⁺ with a maximum at 350 nm. Ce³⁺, Tb³⁺-codoped LaMgAl₁₁O₁₉ films showed the band emission of Ce³⁺ and characteristic emission of Tb³⁺, namely, ⁵D_3,4-⁷F_J (J=6, 5, 4, 3) due to an efficient energy transfer from Ce³⁺ to Tb³⁺ in the host.     

Efficient photoluminescence of Dy at low concentrations in nanocrystalline ZrO2

Nanocrystalline ZrO₂:Dy³⁺ were prepared by sol-gel and the structural and photoluminescence properties characterized. The crystallite size ranges from 20 to 50 nm and the crystalline phase is a mixture of tetragonal and monoclinic structure controlled by dopant concentration. Strong white light produced by the host emission band centered at ∼460 nm and two strong Dy³⁺ emission bands, blue (488 nm) and yellow (580 nm), under direct excitation at 350 nm were observed. The highest efficiency was obtained for 0.5 mol% of Dy³⁺. Emission is explained in terms of high asymmetry of the host suggesting that Dy³⁺ are substituted mainly into Zr⁴⁺ lattice sites at the crystallite surface. Luminescence quenching is explained in terms of cross-relaxation of intermediate Dy³⁺ levels.     

Novel powellite-based red-emitting phosphors: CaLa1−xNbMoO8:xEu for white light emitting diodes

We report the photoluminescence properties of a novel powellite-based red-emitting phosphor material: CaLa_1−xNbMoO₈:xEu³⁺ (0.01, 0.03, 0.05, 0.1) for the first time. The photoluminescence investigations indicated that CaLa_1−xNbMoO₈:xEu³⁺ emits strong red light at 615 nm originating from ⁵D₀→⁷F₂ (electric dipole transition) under excitation either into the ⁵L₀ state with 394 nm or the ⁵D₂ state with 464 nm, that correspond to the two popular emission lines from near-UV and blue LED chips, respectively. When compared with emission intensity from a CaMoO₄:Eu³⁺, the emission from CaLaNbMoO₈:Eu³⁺ showed greater intensity values under the same excitation wavelength (394 nm). The enhanced red emission is attributed to the enhanced f-f absorption of Eu³⁺. These materials could be promising red phosphors for use in generating white light in phosphor-converted white light emitting diodes (WLEDs).     

New confined p-phenylenevinylene (PPV)-type polymer analogue of poly(phenylene sulfide)

A new confined p-phenylenevinylene (PPV)-type polymer (PPVS) has been synthesized using Wittig condensation. The chemical structure of the polymer was well defined by ¹H NMR, ¹³C NMR, and FTIR spectroscopic analysis. PPVS contains oligomeric PPV units separated by sulfide bridges in the main chain; it is fully soluble in common organic solvents and has a number-average molecular weight of 3500 g mol⁻¹. Thermogravimetric analysis and differential scanning calorimetry indicate that PPVS is amorphous, stable up to 360 °C in air and displays a glass transition temperature of 98 °C. The optical properties of the polymer were investigated by UV-visible absorption and photoluminescence spectroscopies. The polymer film absorbs at 375 nm and emits at 517 nm with a narrow emission spectrum. From the cyclic voltammetry analysis, the electrochemical bandgap was estimated to be 2.78 eV. A single-layer diode device of the configuration indium-tin oxide/PPVS/aluminium has been fabricated and has a relatively low turn-on voltage of 3.4 V. An electroluminescent emission similar to photoluminescence is demonstrated in a multilayer device.     

Comparative study of photoelectric properties of a copolymer and the corresponding homopolymers based on poly(3-alkylthiophene)s

As semiconducting materials in organic light-emitting devices (OLEDs), a novel, highly soluble poly[(3-octylthiophene)-co-(3-(2-ethyl-1-hexylthiophene))] (P3OTIOT) and the corresponding homopolymers (poly(3-octylthiophene) (P3OT) and poly(3-isooctylthiophene) (P3IOT)) were prepared by an FeCl₃-oxidative approach to compare their photoelectric properties. Characterization of the polymers included FT-IR, ¹H NMR, gel permeation chromatography (GPC), thermo-gravimetric analysis (TGA), UV-vis spectroscopy, photoluminescence (PL) and electroluminescence (EL). P3OTIOT and P3OT depicted excellent solubility in common organic solvents. TGA studies showed that all of the materials exhibited very good thermal stabilities, losing 5% of their weight on heating to 300 °C. The optical property investigations showed that the band-gap energy of P3OTIOT was similar to that of P3OT (2.43 eV) at 2.45 eV and 6% lower than that of P3IOT (2.6 eV) in CHCl₃ solution. In PL spectra, the emission maxima of P3OTIOT and P3IOT were 50 nm and 130 nm blue-shifted with respect to that of P3OT, respectively. However, the PL intensity of P3OTIOT was seven times higher than that of P3OT. Single layer polymer light-emitting devices (PLEDs) with the ITO/polymer/Ag configurations were fabricated by the spin-coating method with P3OT, P3IOT and P3OTIOT as the EL materials, which exhibited red (650 nm), orange-red (610 nm) and yellow-green (525 nm) EL, respectively. The external EL quantum efficiencies (QE) of P3IOT and P3OTIOT devices are 6.4 × 10⁻³% and 5.1 × 10⁻³% which are about five and four times higher than that of the P3OT device (1.2 × 10⁻³%), respectively. The turn-on voltage of the P3OTIOT device (5 V) is between that of the P3OT (4.5 V) and P3IOT (6 V) devices. These results indicated that the P3OTIOT combined the photoelectric properties of P3OT and P3IOT with excellent solubility, processability, low band-gap energy, high QE and low turn-on voltage in the PLEDs, and they might be excellent polymeric materials for applications in organic light-emitting diodes, light-emitting electrochemical cells and polymer solar cells.     

Sonochemical synthesis and luminescence properties of single-crystalline BaF2:Eu nanospheres

BaF₂ nanocrystals doped with 5.0 mol% Eu³⁺ has been successfully synthesized via a facile, quick and efficient ultrasonic solution route employing the reactions between Ba(NO₃)₂, Eu(NO₃)₃ and KBF₄ under ambient conditions. The product was characterized via X-ray powder diffraction (XRD), scanning electron micrographs (SEM), transmission electron microscopy (TEM), high-resolution transmission electron micrographs (HRTEM), selected area electron diffraction (SAED) and photoluminescence (PL) spectra. The ultrasonic irradiation has a strong effect on the morphology of the BaF₂:Eu³⁺ particles. The caddice-sphere-like particles with an average diameter of 250 nm could be obtained with ultrasonic irradiation, whereas only olive-like particles were produced without ultrasonic irradiation. The results of XRD indicate that the obtained BaF₂:Eu³⁺ nanospheres crystallized well with a cubic structure. The PL spectrum shows that the BaF₂:Eu³⁺ nanospheres has the characteristic emission of Eu^{3+ 5}D₀-⁷F_J (J=1-4) transitions, with the magnetic dipole ⁵D₀-⁷F₁ allowed transition (590 nm) being the most prominent emission line.     

A novel blue-emitting phosphor LiSrPO4:Eu for white LEDs

A novel blue-emitting phosphor, LiSrPO₄:Eu²⁺, was prepared by the solid-state reaction and X-ray powder diffraction (XRD) analysis confirmed the formation of LiSrPO₄:Eu²⁺. Photoluminescence (PL) results showed that the phosphor can be efficiently excited by UV-visible light from 250 to 440 nm, and exhibited bright blue emission. The effects of the doped-Eu²⁺ concentration in LiSrPO₄:Eu²⁺ on the PL were investigated in detail. The results showed that the relative PL intensity increases with Eu²⁺-concentration increasing until a maximum intensity is reached, and then it decreases due to concentration quenching and a red-shift appears, which are explained satisfactorily with the luminescent theory. Upon excited with 396 nm light, the present synthesized phosphor has higher emission intensity than that from the commercial blue phosphor, BaMgAl₁₀O₁₇:Eu²⁺. Bright blue light-emitting diodes were fabricated by the combination of the synthesized LiSrPO₄:Eu²⁺ with ∼397 nm emitting InGaN-based chips.