Blue-violet Region Research Articles

Multi-Colour Nanowire Photonic Crystal Laser Pixels

Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form “super-pixels”. Conventional edge-emitting lasers and current surface-emitting lasers that require abrupt changes in semiconductor bandgaps or cavity length are not a viable solution. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission centre wavelength. We demonstrate this using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication. We obtain single-mode lasing in the blue-violet spectral region with a remarkable 60 nm of tuning (or 16% of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum.

A High Power InGaN-Based Blue-Violet Laser Diode Array with a Broad-Area Stripe

An array of high power InGaN/GaN multi-quantum-well laser diodes with a broad waveguide is fabricated. The laser diode structure is grown on a GaN substrate by metal-organic chemical vapor deposition. The laser diode array consists of five emitter stripes which share common electrodes on one laser chip. The electrical and optical characteristics of the laser diode array are investigated under the pulse current injection with 10kHz frequency and 100 ns pulse width. The laser diode array emits at the wavelength of 409 nm, which is located in the blue-violet region, and the threshold current is 2.9 A. The maximum output light peak power is measured to be 7.5 W at the wavelength of 411.8 nm under the current of 25 A.

Preparation and optical properties of Eu-doped Y2O3–Al2O3–SiO2 glass

Eu-doped Y2O3–Al2O3–SiO2 (YAS) glass with good transparency from ultraviolet to infrared region was prepared by furnace-cooling with a cooling rate (<0.5°C/s) much lower than those (>102°C/s) in conventional quenching approaches. The bulk density of Eu-doped YAS glass increased with increasing concentration of Eu2O3 and showed a linear dependence on the latter. Under ultraviolet excitations, the Eu-doped YAS glasses exhibited characteristic orange emissions of Eu3+ and the maximum emission intensity was obtained with Eu/(Y+Eu)=0.09. Under excitation at 363nm, a broad emission peak in violet-blue region was also observed, which was assigned to 4f–5d transitions of Eu2+.

Tetrahedral silicon-based luminescent molecules: Synthesis and comparison of thermal and photophysical properties by various effect factors

A series of luminescent molecules were presented employing the tetrahedral structural motif of the silicon atom, which further connected different N-containing heterocycle functional groups in their periphery using phenyl rings as the bridges. These compounds included three kinds of N-heterocycle functional silanes: imidazole derivatives (1a–h), pyrazole derivatives (2a–e) and benzimidazole derivatives (3a–b), and their structures were fully characterized by FT-IR, 1H NMR, 13C NMR and HRMS. The TGA results indicate that they all exhibit high thermal stabilities. The photophysical properties demonstrate that they are fluorescent in the violet-blue region and could be potentially applied as blue emitters for organic light-emitting diodes (OLEDs). The effect factors of sort, disposition and number of substituent groups and N-containing heterocycle functional groups on their thermal and photophysical properties were investigated. Molecular calculations were also performed to support the experimental results. Moreover, the computational results reveal that these compounds all exhibit relatively large HOMO–LUMO band gaps with the range from 4.82 eV (2d) to 5.19 eV (1a and 2c), making them become promising candidates as host materials for emitters and hole/electron blocking materials in OLEDs display.

Synthesis and characterization of bifunctional β-MnO2-based Pt/C photoelectrochemical cell for hydrogen production

A manganese dioxide (β-MnO2) photocatalyst for light-induced water splitting and hydrogen generation is studied via the impregnation and heat treatment method. The phase and fluorescence characterizations are examined by X-ray diffraction (XRD) and photoluminescence. A series of peaks for its (110) and (200) planes are identified as those for pure β-MnO2 crystals with lattice spacings of 3.11 and 2.19 Å, respectively. The photoluminescence shows that the primary signals are located in the blue-violet spectral region, corresponding to the band-edge emission of β-MnO2 (376 nm). Furthermore, two types of photoelectrochemical cell with added Pt are constructed to compare hydrogen production rates. A significant enhancement of light-induced hydrogen generation by water splitting is observed on Pt/β-MnO2/C, with a hydrogen generation rate of 194.67 μmol cm−1 h−1, greater than that on a Pt/TiO2/C photocatalyst, which can be attributed to the effective inhibiting of CO poisoning, thus maintaining the catalyst's surface area in methanol oxidation.

Application of Si Nanowires Fabricated by Metal-Assisted Etching to Crystalline Si Solar Cells

Reflection and transmission through a solar cell can be significantly reduced using light-trapping structures. This approach can be applied to both crystalline and thin-film solar cells to improve the light absorption and conversion efficiency of the cell. In this study, vertically aligned Si nanowires were fabricated over a large area via a metal-assisted etching technique. Following a detailed parametric study, nanowires were applied to industrial-size (156 mm × 156 mm) Si solar cells. The reflectivity from the device surface was reduced to less than 5% for the entire visible spectrum (350-750 nm), including the blue-violet region. Standard solar cell fabrication procedures were employed to fabricate cells with and without Si nanowires, and the results showed that the efficiencies of solar cells with nanowires were similar to those of standard pyramid-textured cells, revealing the potential of the proposed concept. A systematic study of the dependence of the solar cell parameters on the length of the nanowires was performed. The quantum efficiency of the cells exhibited relatively poor performance in the blue-ultraviolet range of the spectrum, and enhancement in carrier generation was observed in the red-infrared region especially for shorter nanowires.

Optical properties of silicon nanoparticles synthesized via electrical spark discharge in water

In this paper, we report a simple and low-cost technique for fabrication of silicon nanoparticles via electrical spark discharge between two plane silicon electrodes immersed in deionized water (DI). The pulsed spark discharge with the peak current of 60 A and a duration of a single discharge pulse of 60 μs was used in our experiment. The structure, morphology, and average size of the resulting nanoparticles were characterized by means of X-Ray Diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). TEM images illustrated nearly spherical and isolated Si nanoparticles with diameters in the 3–8 nm range. The Raman peaks of the samples were shifted to the lower wave numbers in comparison to this of bulk crystalline silicon indicating the existence of tiny particles. The optical absorption spectrum of the nanoparticles was measured in the violet–visible (UV–Vis) spectral region. By measuring of the band gap we could estimate the average size of the prepared particles. The silicon nanoparticles synthesized exhibited a photoluminescence (PL) band in the violet-blue region with a double peak at around 417 and 439 nm. It can be attributed to oxide-related defects on the surface of silicon nanoparticles, which can act as the radiative centers for the electron-hole pair recombination.

Size-dependent Luminescence of Colloidal ZnSe Quantum Dots

ZnSe quantum dots of three narrow size distributions are prepared through an environmentally friendly chemical route. Then the samples are systematically characterized by UV-Vis absorption spectroscopy and transmission electron microscopy. UV-Vis absorption spectra reveal blue shift of 0.56 eV, 0.19 eV and 0.02 eV respectively indicating strong quantum confinement while transmission electron micrograph displays images of ZnSe quantum dots in the samples in the ranges 2-5 nm, 3-7 nm and 4-8 nm respectively. Then optical emission is studied by fluorescence and electroluminescence spectroscopy. Both the luminescence studies are in good agreement with one another with output peak in and around violet-blue region. Hence, ZnSe quantum dots can be used as efficient LED with better compatibility and faster response. Keywords: ZnSe, Quantum dots, Quantum confinement, Fluorescence, Electroluminescence, LED, Size Quantization, diffraction, TEM, selenium powder, compatibility, oleic acid, phonon emission

Syntheses, X-ray Crystal Structures, and Optical, Fluorescence, and Nonlinear Optical Characterizations of Diphenylphosphino-Substituted Bithiophenes

A series of bithiophene derivatives that are either symmetrically disubstituted with two Ph(2)(X)P groups (X = O, S, Se) or monosubstituted with one Ph(2)(X)P group (X = O, S, Se) and an organic functional group (H, CHO, CH(2)OH, CO(2)Me) have been synthesized. The X-ray crystal structures of Ph(2)(Se)P(C(4)H(2)S)(2)P(Se)Ph(2), Ph(2)(O)P(C(4)H(2)S)(2)H, Ph(2)(S)P(C(4)H(2)S)(2)H, and Ph(2)(O)P(C(4)H(2)S)(2)CH(2)OH exhibit very different solid-state structures depending on the type of intermolecular π-π interactions that occur. The compounds have been characterized by electronic absorption and fluorescence studies. Of particular interest is that the quantum yields of Ph(2)(O)P(C(4)H(2)S)(2)H, Ph(2)(O)P(C(4)H(2)S)(2)P(O)Ph(2), Ph(2)(O)P(C(4)H(2)S)(2)CO(2)Me, and Ph(2)(O)P(C(4)H(2)S)(2)CH(2)OH are significantly larger than that of bithiophene (factors of 13, 14, 14, and 22, respectively). This behavior is quite different from that of analogously substituted terthiophenes in which substitution results in only modest increases in the quantum yields over that of terthiophene (factors of 0.94, 2.7, 1.3, and 1.5, respectively). DFT studies of the emission process suggest that modifying the Ph(2)(X)P group affects both the fluorescence and nonradiative rate constants while modifications of the organic substituents primarily affect the nonradiative rate constants. The higher quantum yields of the substituted bithiophenes make them promising for application in organic light-emitting devices (OLED). The optical power limiting (OPL) performances of these Ph(2)(X)P-substituted bithiophenes were evaluated by nonlinear transmission measurements in the violet-blue spectral region (430-480 nm) with picosecond laser pulses. The OPL performances are enhanced by heavier X groups and when by higher solubilities. Saturated chloroform solutions of Ph(2)(O)P(C(4)H(2)S)(2)H and Ph(2)(S)P(C(4)H(2)S)(2)H exhibit significantly stronger nonlinear absorption than any previously reported compounds and are promising candidates for use in broadband optical power limiters.

A Simple and Efficient Method for Synthesizing Te Nanowires from CdTe Nanoparticles with EDTA as Shape Controller under Hydrothermal Condition

We developed a simple and efficient method for synthesizing Te nanowires from CdTe nanoparticles with ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA) as shape controller under hydrothermal condition. The system could both complete the transformation to Te and reduce the interference of CdTeS by adjusting the concentration of EDTA, which was proved by inductively coupled plasma mass spectrometry, X‐ray diffraction patterns, and Raman spectra. It was found that the as‐prepared Te nanowires display strong fluorescence emission in the blue‐violet region. The nanowires exhibit a pretty good morphology with the average diameter of ca. 30 nm and a length up to micrometer scale. Moreover, a possible transformation mechanism of CdTe nanoparticles into Te nanowires is also discussed.

Design, Synthesis and Optical Properties of Tetrahedrally Silicon-Cored Phenyl-Linked Pyrazole Ligands

A novel series of tetrahedrally silicon-centered pyrazole derivatives, including bis(4-(pyrazol-1-yl)phenyl)dimethylsilane (1), bis(3-(pyrazol-1-yl)phenyl)dimethylsilane (2) and tri(4-(pyrazole-1-yl)phenyl)methylsilane (3) have been designed, synthesized and determined by FT-IR, 1H NMR, 13C NMR, 29Si NMR and mass spectroscopy. These compounds are stable in moisture and have high thermal stability. The optical properties reveal that these compounds show a fluorescent emission in the blue-violet region, which could be potentially applied as blue emitters for OLEDs and utilized to construct functional MOFs with luminescent properties.

Liquid crystal behavior induced in highly luminescent unsymmetrical borondifluoride β-diketonate materials

Highly luminescent and liquid crystal materials were successfully prepared by the strategic introduction of unsymmetrically alkyloxyphenyl substituted β-diketones, as ligands towards the BF2 group. So, four families of ligands and their boron derivatives were prepared, each of them containing invariable the alkyl chain at one of the substituents in the 1(3) position, with 12, 14, 16 or 18 carbon atoms, while for the other substituent at the 3(1) position, the number of alkyl carbon atoms was increased from 4 to 18. In this way, the influence of factors as asymmetry or molecular length was evaluated towards the liquid crystal behavior of the compounds.The photophysical characterization of the boron derivatives was also carried out, showing in all cases a high fluorescent emission in the blue–violet region in solution and in the green region in the solid state, with quantum yields near the unity. In addition, thermal studies by DSC, as well as the analysis by POM and XRD were determinant to characterize the compounds as liquid crystal materials, exhibiting B1 mesophases, in agreement with the banana molecular shape. In contrast the corresponding β-diketone precursors behave again as liquid crystals showing SmC and N mesophases.

Characterization of directed ammonium malate single crystals grown by Sankaranarayanan–Ramasamy method

Ammonium malate, an organic nonlinear optical single crystal, has been grown to a dimension of 50 mm length and 20 mm diameter by uniaxial solution-crystallization method of Sankaranarayanan–Ramasamy (SR). The growth conditions and experimental details of the grown crystals have been presented. The crystalline perfection of the grown crystals has been evaluated by high-resolution X-ray diffraction technique and the results reveal that the crystal quality of SR grown crystal is better than the conventional method grown crystal. The growth features in SR grown crystal have been analyzed by chemical etching technique and well-defined triangular etch hillocks have been observed. The mechanical property of the grown crystal has been analyzed by Vickers microhardness method and it reveals that the grown crystal is moderately harder material. Ammonium malate crystal possesses lower cut off wavelength at 230 nm. It enables good optical transmission of the second harmonic generation of Nd:YAG laser and other applications in the blue-violet region. The second harmonic generation of the grown crystal has been confirmed by the Kurtz powder technique.

Luminescence of Tb-doped Ca 3Y 2(Si 3O 9) 2 oxide upon UV and VUV synchrotron radiation excitation

Powders of calcium yttrium silicate, Ca 3Y 2(Si 3O 9) 2, containing 0.1–3% Tb 3+ were prepared using a sol-gel method and characterized with XRD, IR, UV–vis and UV–VUV spectroscopies at room temperature and 10 K. Structural analysis revealed pure monoclinic phase of Ca 3Y 2(Si 3O 9) 2 after heat-treatment at 1000 °C. Infrared spectroscopy showed that between 800 and 900 °C a short-range structural organization of the components proceeded, yet without crystallization. A strong emission of Tb 3+ had been observed both in the green part of the spectrum due to the 5 D 4→ 7 F J transitions and in the blue-violet region owing to the 5 D 3→ 7 F J radiative relaxation. The color of the light could be tuned from yellowish-green to bluish-white both by means of the dopant content and the temperature of synthesis. Efficient luminescence of Tb 3+-doped Ca 3Y 2(Si 3O 9) 2 phosphors could also be obtained upon stimulation with vacuum ultraviolet synchrotron radiation demonstrating that an energy transfer from the host to the Tb 3+ ions takes place.

Nonlinear optical absorption and fluorescence of phosphine-substituted bithiophenes in the violet-blue spectral region

The nonlinear optical absorptions of two 5,5′-bis(diphenylphosphino)-2,2′-bithiophene derivatives, Ph 2(X)P(C 4H 2S) 2P(X)Ph 2 (X = O, 1; S, 2), have been investigated by direct transmission measurement with both picosecond and nanosecond laser pulses from 420 nm to 480 nm. Saturated dichloromethane solutions of 1 and 2 exhibit strong nonlinear optical absorptions in this violet-blue spectral region with that of 2 being stronger at all wavelengths. In the picosecond regime, at 420 nm, the transmittance rapidly falls to 50% when the incident fluence is 0.22 J/cm 2 for 1 and 0.11 J/cm 2 for 2. Two-photon absorption appears to be the primary mechanism for this nonlinear absorption. The two-photon absorption coefficients β for 1 (2.1 cm/GW) and 2 (4.4 cm/GM) were obtained by fitting the measurement of transmittance as the function of incident beam intensity at 420 nm. These β values are comparable with some of the best results obtained for organic materials in the green, red and infrared spectral region. Both compounds also show fluorescence with an emission peak at 390 nm for 1 and 400 nm for 2. The fluorescence of 1 is considerably stronger than is that of 2. The combination of the wide band gap and strong fluorescence emission of 1 makes it a promising candidate as a host material for blue organic light emitting diodes.

Concentration and Annealing Effects on Luminescence Properties of Ion-Implanted Silica Layers

The development of optoelectronic or even photonic devices based on silicon technology is still a great challenge. Silicon and its oxide do not possess direct optical transitions and, therefore, are not luminescent. The remaining weak light emission is based on intrinsic and extrinsic defect luminescence. Thus the investigations are extended to ion implantation into silica layers, mainly on over-stoichiometric injection or isoelectronic substitution of both the constituents silicon or oxygen, that is, by ions of the group IV (C, Si, Ge, Sn, Pb) or the group VI (O, S, Se). The samples have been used were 500 nm thick thermally grown amorphous SiO2 layers, wet oxidized at 1100°C on a crystalline Si substrate. The ion implantations were performed with different energies but all with a uniform dose of 5 × 1016 ions/cm2. Such implantations produce new luminescence bands, partially with electronic-vibronic transitions and related multimodal spectra. Special interest should be directed to lowdimension nanocluster formation in silica layers. Implantations of group IV elements show a general increase of the luminescence in the violet-blue region and implantations of group VI elements lead to an increase in the yellow-red spectral region. Comparing cathodoluminescence, photoluminescence, and electroluminescence still too small luminescence quantum yields are obtained.

Microwave Hydrothermal Synthesis of GaN Nanorods

The GaOOH Nanocrystal rods were successfully synthesized by microwavehydrothermal method using Ga2O3 ,HNO3 and NH3·H2O as raw materials. Simple heat treatment of GaOOH in the flow of NH3 gas leads to the formation of submicron GaN rods even at 800°C through GaOOH. The resultant GaOOH and GaN nanomaterials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The growth mechanism of GaOOH and GaN was proposed. The results indicate that the as synthesized GaN were hexagonal nanorods with average aspects ratio of 5:1(diameter 800 nm and length about 4μm). Photoluminescence spectrum shows that a blue emission peak originates at 473.5 nm, indicating that the GaN nanorods displayed luminescence emission in the blue-violet region, which was related to crystal defects, and may be helpful for electro-optical applications of GaN material.

Synthesis, characterization and application of starburst 9-alkyl-1,3,6,8-tetraaryl-carbazole derivatives for blue-violet to UV OLEDs

A series of starburst carbazole derivatives, which comprise various aryl groups appended to the 1,3,6,8-positions of a 9-alkyl-carbazole core, are synthesized and characterized. These compounds exhibit good thermal stabilities with glass transition temperatures up to 199 °C and thermal decomposition temperatures ranging from 321 to 485 °C. The starburst configuration of the as-prepared compounds results in fluorescence emission in the UV to blue-violet region (λmax = 389–409 nm) with photoluminescence quantum efficiency (ΦPL) reaching 90% and narrow full width half maximum (FWHM) of 43–54 nm in solution. Light-emitting devices are successfully fabricated using these materials as emitters, and emit UV to blue-violet light. With a device structure of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/1,3,6,8-tetraaryl carbazole derivatives/1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al, maximum external quantum efficiency (EQEmax) of 2.05% and 3.4% have been obtained at UV wavelength of 394 nm with FWHM of 42 nm and blue-violet wavelength of 415 nm with FWHM of 46 nm, respectively.

One-pot synthesis of biocompatible Te@phenol formaldehyde resin core–shell nanowireswith uniform size and unique fluorescent properties by a synergized soft–hard templateprocess

One-pot hydrothermal process has been developed to synthesize uniform Te@phenolformaldehyde resin core–shell nanowires with unique fluorescent properties. A synergisticsoft–hard template mechanism has been proposed to explain the formation of the core–shellnanowires. The Te@phenol formaldehyde resin core–shell nanowires display uniquefluorescent properties, which give strong luminescent emission in the blue-violet andgreen regions with excitation wavelengths of 270 nm and 402 nm, respectively.

Electroluminescence from Solution Grown n-ZnO Nanorod/p-GaN-Heterostructured Light Emitting Diodes

To fabricate n-ZnO nanorod/p-GaN-heterostructured light emitting diodes, well-aligned ZnO nanorod arrays were synthesized on metallorganic chemical vapor deposited p-GaN layers on sapphire substrates using low temperature and low cost solution growth techniques. Current–voltage measurements showed the formation of a diode structure with a typical diode characteristic having a turn-on voltage of 4.8 V. Microstructure and room-temperature photoluminescence measurements confirmed the growth of ZnO nanorod arrays with a near-perfect microstructure, stoichiometry, and excellent optical quality. Room-temperature electroluminescence in the blue-violet region with a peak wavelength of was observed under a forward bias. No defect-related emission was observed in the deep visible region.