A tunable green alkaline-earth silicon-oxynitride solid solution (Ca1? x Sr x )Si2O2N2:Eu2+ and its application in LED

A series of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ (x=0.0–0.97, y=0.03) phosphors were synthesized by high-temperature solid-state reaction. The XRD patterns confirm the formation of a solid solution of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺. An intense tunable green light is observed with the increasing ratio of Sr/Ca. With an increase in x, the excitation and emission spectra show a redshift and blueshift, respectively, due to large centroid shift and small Stokes shift. The temperature dependent luminescence is also investigated in the temperature range of 77–450 K. The Huang–Rhys factor and the thermal-quenching temperature are determined. Intense green LEDs were successfully fabricated based on the (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ phosphor and near-ultraviolet (∼395 nm) GaN/blue (460 nm) InGaN chips. All the results indicate that the solid solution (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ is a promising phosphor applicable to near-UV and blue LEDs for solid-state lighting.     

Synthesis of Eu x Si6? z Al z O z N8? z green phosphor and?its?luminescent properties

Rare-earth-doped oxynitride or nitride compounds have been reported to be photoluminescent and may then serve as new phosphors because of their good thermal and chemical stabilities. In this work, Eu²⁺-doped β-SiAlON phosphor with a composition of Eu _x Si_6−z Al _z O _z N_8−z (x=0.018, z=0.23) was prepared by gas-pressured solid state reaction. The crystallinity and particle morphology of the prepared phosphor were characterized. The Stokes shift and zero-phonon line were calculated mathematically and estimated from the spectral data. The temperature dependence of photoluminescence was measured from 25 to 250°C. The prepared Eu²⁺-doped β-SiAlON green phosphor showed superior thermal quenching property compared to silicate (SrBaSiO₄:Eu²⁺) green phosphor. The white light-emitting diode (LED) back-lighting unit (BLU) using the prepared β-SiAlON:Eu²⁺ green phosphor exhibited higher color gamut than a commercial silicate phosphor.     

Luminescent properties of Sr5(PO4)3 Cl:Eu2+, Mn2+ as?a?potential phosphor for UV-LED-based white LEDs

Eu²⁺ and Mn²⁺ co-activated Sr₅(PO₄)₃Cl phosphors with blue and orange color double emission bands, under a broad-band excitation wavelength range of 340–400 nm, were synthesized by the solid-state reaction. It was found that the processing parameters, including the fluxes, annealing time and activators concentrations, affect the emission intensity and other luminescent properties. Energy transfer between Eu²⁺ and Mn²⁺ was discovered and the transfer efficiency was also estimated based on relative intensities of Eu²⁺ and Mn²⁺ emission. Thus the relative strength of blue and orange emission intensities could be tuned by varying the relative concentration of Eu²⁺ and Mn²⁺. Since the photoluminescence excitation spectra of the newly developed Sr₅(PO₄)₃Cl:Eu²⁺, Mn²⁺ phosphors exhibit a strong absorption in the range of 340–400 nm, they are promising for producing UV-LED-based white LEDs.     

Volume structuring of high power LED encapsulates by femtosecond laser direct writing

We report on the micro-fabrication of diffractive optical elements (DOEs) such as 1D, 2D and concentric grating structures inside the volume of thin silicone films by femtosecond laser direct writing. In addition, we show that such structures can also be integrated into silicone films that act as encapsulation layers of high power light-emitting diodes. The latter strategy opens new possibilities to homogenize and to control the light emitted from such devices.     

Preparation of phosphors AEu(MoO4)2 (A?=?Li, Na, K and Ag) by?sol-gel method

A series of double molybdates phosphors AEu(MoO₄)₂ (A = Li, Na, K and Ag) have been prepared by sol-gel method. Their crystal structure and luminescent properties have also been investigated in a comparable way. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and size distribution of the phosphors. Samples except KEu(MoO₄)₂ showed tetragonal scheelite structure in the range of our experiments, and no phase transition appeared. Phosphor KEu(MoO₄)₂ possessed two structures, and the phase transition took place at about 800°C. All samples with high purity could be obtained at about 500°C for 5 hours, and they all showed intense red light peaked at 616 nm originated from ⁵D₀→⁷F₂ emission of Eu³⁺ under the excitation of 465 nm or 394 nm light. The excitation spectra of phosphors AEu(MoO₄)₂ (A = Li, Na, and K) are composed of a strong broad charge transfer (CT) band and some sharp lines, and the relative intensity of CT band, the two strongest absorption lines at 395 nm and 465 nm are comparative, so these three phosphors are good red phosphor candidates for violet or blue LEDs. For the excitation spectrum of phosphor AgEu(MoO₄)₂, intensities of CT band and the absorption line at 395 nm are much weaker than that of line at 465 nm, thus phosphor AgEu(MoO₄)₂ is only suit for GaN-based blue LED.     

Spectroscopic properties of Nd<Superscript>3+</Superscript>:CaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystal

The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd³⁺ ions in CaNb₂O₆ crystal were measured at room temperature. The peak absorption cross section was calculated to be 6.202×10⁻²⁰ cm² with a broad FWHM of 7 nm at 808 nm for E//a light polarization. The spectroscopic parameters of Nd³⁺ ions in CaNb₂O₆ crystal have been investigated based on Judd-Ofelt theory. The parameters of the line strengths Ω _t are Ω ₂=5.321×10⁻²⁰ cm²,Ω ₄=1.734×10⁻²⁰ cm²,Ω ₆=2.889×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. The fluorescence branch ratios are calculated to be β ₁=36.03%,β ₂=52.29%,β ₃=11.15%,β ₄=0.533%. The emission cross section at 1062 nm is 9.87×10⁻²⁰ cm².     

Upconversion in Ho<Superscript>3+</Superscript>-doped YbF<Subscript>3</Subscript> particle prepared by coprecipitation method

YbF₃ particles doped with Ho³⁺ were synthesized by coprecipitation method, from which the ultraviolet and visible emission bands of the Ho³⁺ and the 480 nm cooperative upconversion emission of Yb³⁺–Yb³⁺ are observed under 980 nm excitation. Under the same excitation power, the emission intensity of Ho³⁺ in coprecipitation method is enhanced by about two times comparing to that in solid-state reaction method. The novel ultraviolet and violet emissions of the Ho³⁺ are firstly obtained which are centered at 360 (⁵G₂→⁵I₈),391 (³K₇→⁵I₈),412 (⁵G₄→⁵I₈), and 446 nm (⁵G₅→⁵I₈). The luminescence decay profiles of 545 and 652 nm visible emissions were obtained with a 980 nm pulsed laser. The excitation power dependence of the emission intensity was also measured and intensity saturation was observed. Based on the level structures of Ho³⁺, two- and three-photon processes are suggested to perform populations of ⁵S₂ and ⁵G₃ (Ho³⁺) levels, respectively. The dominant upconversion mechanism may be attributed to a cooperative sensitization process of two excited states of Yb³⁺ and energy transfers from Yb³⁺ to Ho³⁺.     

Strong blue excitonic emission from CdS nanocrystallites prepared by LB technique

CdS nanocrystallites formed in ordered fatty acid LB multilayers exhibited strong surface states emission ∼550 nm and weak excitonic emission ∼400 nm. Treatment with aqueous CdCl₂ resulted in the suppression of surface states emission and enhancement of the blue excitonic emission. Subsequent annealing in air at 200°C caused an order of magnitude enhancement of excitonic emission. The growth of nanocrystallites during annealing as seen from the red-shift of excitonic absorption and emission is suppressed by the CdCl₂ treatment. The hindered growth of nanocrystallites, the significant enhancement of excitonic emission from CdS, and the suppression of surface states emission are attributed to surface passivation of CdS nanocrystallites by surface oxide formation.     

Controlled visible photoluminescence of ZnO thin films prepared by RF magnetron sputtering

ZnO thin films were prepared by RF magnetron sputtering. The photoluminescence dependence on the growth ambient and annealing temperatures and the atmosphere has been studied. Visible photoluminescence with blue, green, orange, and red emission bands has been demonstrated by controlling the preparation conditions. Complete suppression of the visible emission bands was also realized by annealing the O₂-ambient-grown samples in N₂ atmosphere at higher temperatures, which indicated the preparation of ZnO thin films with high optical quality.     

Enhanced photoluminescence from gallium arsenide semiconductor coated with Au nanoparticles

We demonstrate a method for improving photoluminescence of gallium arsenide semiconductor by simply coating a thin layer of Au nanoparticles on its surface. Further focused ion beam bombardment at the sputter-coated Au film was conducted to control the size, the distribution, and the morphology of the Au nanoparticles via the changes of the focused ion-beam irradiation conditions. Photoluminescence of GaAs coated with the Au nanoparticles with average size of 5 nm in diameter is enhanced to about threefold relative to that of pure GaAs. Numerical calculations were conducted based on finite-different time-domain method. Results indicated that the enhancement is mainly attributed to the contribution of local surface plasmon resonance of Au nanoparticles.     

The effects of chemical etching of porous silicon on Raman spectra

We have investigated the effects of chemical etching on Raman spectra of porous silicon. The as-anodized porous silicon consisted mainly of crystalline silicon, as indicated by the Raman spectra. The background in the spectrum was strong, indicating that the porous silicon surface was rough due to the presence of pores. When chemical etching was performed five times, the Raman spectrum revealed the presence of spherically shaped nanocrystalline silicon whose diameter was around 3.5 nm. Further chemical etching, however, extinguished the nanocrystallites, in addition to smoothing the surface morphology.     

Design of a polymer directional coupler electro-optic switch using two-section reversed electrodes

By using the coupled mode theory, electro-optic modulation theory, conformal transforming method and image method, the structure is designed, the parameters are optimized, and the characteristics are analyzed for a polymer directional coupler electro-optic switch with two-section reversed electrodes. Simulation shows that the designed device exhibits excellent switching functions. Under the operation wavelength of 1550 nm, the electro-optic coupling region length is 4751 μm, the cross-state and bar-state voltages are about 1.22 and 2.65 V, and the insertion loss and crosstalk are less than 2.21 and −30 dB, respectively. By slightly adjusting the state voltages, the blight of the fabrication errors on the switching characteristics can be easily eliminated. The calculation results of the presented technique are in good agreement with those of the beam propagation method (BPM).     

Light emitting diode as sensor for miniature multispectral radiometer

A methodology is proposed for studying the suitability of the light emitting diode (LED) as the optical sensor of a miniature multispectral radiometer. The optoelectronic properties of three commercially available LEDs of three different wavebands were standardized by measuring the open-circuit voltage and short-circuit currents at different light intensities and different wavelengths of incident radiation. A photoconductive measurement was found to be more useful for obtaining the appropriate optical response and was suggested as the suitable mode of sensing. The sensor performances were demonstrated for both solar photometry and surface reflectance measurements.     

Electroluminescence from ZnO nanowire-based p-GaN/n-ZnO heterojunction light-emitting diodes

Vertically aligned ZnO nanowires were successfully grown on the sapphire substrate by nanoparticle-assisted pulsed laser deposition (NAPLD), which were employed in fabricating the ZnO nanowire-based heterojunction structures. p-GaN/n-ZnO heterojunction light-emitting diodes (LEDs) with embedded ZnO nanowires were obtained by fabricating p-GaN:Mg film/ZnO nanowire/n-ZnO film structures. The current–voltage measurements showed a typical diode characteristic with a threshold voltage of about 2.5 V. Electroluminescence (EL) emission having the wavelength of about 380 nm was observed under forward bias in the heterojunction diodes and was intensified by increasing the applied voltage up to 30 V.     

Compact and efficient modulators for 100 Gb/s ETDM for telecom and interconnect applications

State of the art and prospects regarding semiconductor compact modulators and transmitters for on–off keying and more advanced modulations formats for output bitrates of 100 Gb/s and above are discussed. The implementation of a monolithically integrated transmitter comprising laser and light-intensity modulator is described and the prospects for a fully integrated transmitter for more advanced modulation formats elucidated, all for 100 Gb/s output bitrate     

Recombination zone and efficiency in bipolar single layer light-emitting devices: a numerical study

The efficiency of organic light-emitting devices (OLEDs) is closely related to the position and width of recombination zone (RCZ) in the emission layer. Based on the drift–diffusion theory of carrier motion in semiconductors, we developed a numerical model for the position and width of the RCZ in bipolar single layer OLEDs. The calculation results show that for a given operation voltage, the position and width of the RCZ are determined by the mobility difference of electrons and holes, and the energy barrier at the two contacts. When the anode and cathode contact are both ohmic, then RCZ will be near the electrode, from which the low-mobility carriers are injected, and the smaller the mobility difference, the wider the RCZ, and the width of RCZ will be maximal when the mobility of holes and electrons are equal. When the anode contact is Schottky, while the cathode contact is ohmic, then the position and width of RCZ will be determined by both the mobility difference and hole–injection energy barrier. When μ _p<μ _n, the RCZ will be at the anode side. When μ _p>μ _n, then RCZ will move away from the anode and become wider, with the increase of the hole injection barrier. For a given hole–injection barrier and mobility of holes and electrons, the position and width of RCZ change with the applied voltage.     

Microcontact printing of self-assembled monolayers to pattern the light-emission of polymeric light-emitting diodes

By patterning a self-assembled monolayer (SAM) of thiolated molecules with opposing dipole moments on a gold anode of a polymer light-emitting diode (PLED), the charge injection and, therefore, the light-emission of the device can be controlled with a micrometer-scale resolution. Gold surfaces were modified with SAMs based on alkanethiols and perfluorinated alkanethiols, applied by microcontact printing, and their work functions have been measured. The molecules form a chemisorbed monolayer of only ∼1.5 nm on the gold surface, thereby locally changing the work function of the metal. Kelvin probe measurements show that the local work function can be tuned from 4.3 to 5.5 eV, which implies that this anode can be used as a hole blocking electrode or as a hole injecting electrode, respectively, in PLEDs based on poly(p-phenylene vinylene) (PPV) derivatives. By microcontact printing of SAMs with opposing dipole moments, the work function was locally modified and the charge injection in the PLED could be controlled down to the micrometer length scale. Consequently, the local light-emission exhibits a high contrast. Microcontact printing of SAMs is a simple and inexpensive method to pattern, with micrometer resolution, the light-emission for low-end applications like static displays. Both authors (J.J. Brondijk and X. Li) contributed equally.     

UV excitable Y2? x ? y Gd y SiO5:Ce x phosphors for cool white light emission

Cool white light was realized in Y_2−x−y Gd _x SiO₅: Ce _y phosphor under near UV excitation, due to the occupation Ce³⁺ in Y³⁺ 1st and 2nd site, synthesized using solid state carbothermal reduction route. SEM with elemental analysis show the existence of Gd in Y₂SiO₅:Ce enhances the particles size in comparison to Y₂SiO₅:Ce phosphors alone. Gd³⁺ (0.00≤x≤0.75) and Ce³⁺ (0.02≤y≤0.10) concentration was optimized to 0.50 and 0.08 in Y₂SiO₅, respectively. The CIE chromaticity color coordinates (0.24, 0.20) are close to cool white light value which could be useful for the fabrication of cool white LED.     

MAPLE deposition of methoxy Ge triphenylcorrole thin films

Methoxy Ge Triphenylcorrole [Ge(TPC)OCH₃] has been recently synthesized and deposited as thin film by the Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. In the last few years, corroles have been the object of an increasing number of studies and MAPLE technique seems to be a very promising deposition method for organic and polymeric films, producing good results for applications in chemical gas sensing layers production. In this work Ge(TPC)OCH₃ thin films were deposited by both spin coating and MAPLE techniques for comparison. The morphology of the films was investigated by Atomic Force Microscopy (AFM), while their optical properties were analyzed by photoluminescence (PL) and UV-vis absorption measurements and were compared with the ones of the starting solution. The film absorption spectrum presented the same peaks with the same relative intensities of that recorded in solution. The luminescence spectra were acquired periodically to evaluate the aging effects and no detectable variations were recorded over a period of 1 month.     

Optical properties of polysiloxane hybrid thin films containing nano-sized Ag–As–Se chalcogenide clusters

Chalcogenide glasses are attractive for all-optical signal processing due to their outstanding optical properties, including large optical nonlinearity, a high refractive index and high photosensitivity. In device fabrication, a challenge lies in the difficulty of obtaining thin films with a high stability and good uniformity. In this paper, optical thin films containing nano-sized chalcogenide clusters in polysiloxane matrices are fabricated by a modified plasma deposition process. The optical absorption and luminescence emission properties of the hybrid thin films were characterized by UV-Vis-NIR and fluorescence spectroscopy. Luminescent emission from Ag–As–Se nano-sized clusters was observed for the first time in these nano-hybrid thin films, and the mechanism was discussed.