Abstract: Luminescent phenomena have fascinated mankind since the earliest times. The light from the aurora borealis, glow worms, luminescent wood, rotting fish and meat are all examples of naturally occurring luminescence. E. Newton Harvey’s 770 page volume “A History of Luminescence: From the Earliest Times until 1900” is a classic which narrates interesting stories from ancient cultures to modern times. The earliest written account of a solid state luminescent material comes from a Chinese text published in the Song dynasty (960–1279 A.D.). The Buddhist sacred jewel, called "hashi-no-tama" in Japan, is alleged to be self-luminous and to shed a brilliant light on its surroundings. In the Svetasvatara Upanishad, probably recorded at some time before the sixth century BC, we find a mention of fire-flies as one of the manifestations of Brahma.
Abstract: Nanotechnology has led to a profound paradigm shift after the developments in recent years and after being recognised as one of the most important areas of impending technology. Nanomaterials are the basis of newly emerging nanotechnologies for various applications in sensors, photonics, drug delivery, proteomics, biomolecular electronics, and homeland security. Luminescent nanomaterials have attracted great interest worldwide because of their unusual structural, optical and electronic properties as well as efforts to prepare miniaturised devices. By understanding and manipulating these properties, the performance of the resulting optical structure can be tailored for desired end-use applications. Luminescence nanoparticles have tremendous potential in revolutionizing many interesting applications in today’s emerging cutting-edge optical technology such as solid state lighting. Solid-state lighting (SSL) relies on the conversion of electricity to visible white light using solid materials. SSL using any of the materials (inorganic, organic, or hybrid) has the potential for unprecedented efficiencies. The development of novel mercury-free inexpensive nanomaterials, that convert longer wavelength UV to blue light eventually into white-light and are eco-friendly with improved luminous efficacy, energy-saving, long-lifetime, and low-power consumption characteristics, is discussed. In this review, we present a general description of EL related to nanomaterials as the emitter and outlines basic research requirements that could enable solid-state lighting to achieve its potential. Continuing progress in the synthesis and purification of SSL materials are beginning to enable separation of extrinsic and intrinsic phenomena and improve device performance. This review mainly focuses on the basic mechanism, classification, synthesis and characterization of luminescent nanomaterials. The review also covers recent advances in lanthanide-based nanomaterials and photoluminescent nanofibers formed by combining electrospun polymeric nanofibers and quantum dots (QDs) for lighting applications. In spite of the remarkable scientific progress in preparation processes and applications of nanomaterials, they are still not widely used by the industry. Finally, we conclude with a look at the future challenges and prospects of the development of electroluminescence (EL) devices for lighting.Contents of Paper
Abstract: This article presents a broad review of long persistence (LP) materials that are a special kind of photon energy storage and conversion materials. They are also known as long afterglow phosphors or long decay phosphors (LDP). These phosphors can be readily excited by any ordinary household lamp, sunlight and/or ambient room lights and glow continuously in the dark for hours together without involving any radioactive elements. It is the modifications that are made to crystalline host lattice that exhibit these unusual properties related to persistence due to effective doping of some transition or rare-earth ions. A slight variation in the processing parameters such as type of reducing atmosphere, stoichiometric excess of one or more constituents, the nature of fluxes, and the intentional addition of carbon or rare-earth halides can drastically shift the emission colors and persistence times of the LP phosphors in the visible spectrum. Historically, Cu-doped ZnS phosphor had been a traditional LP material with its afterglow time less than an hour. The emission color of these LP phosphors was confined between green and yellow-green region only. However, synthesis of blue and red-emitting phosphors with long persistence times had been always a challenging task. This review article covers the recent advances in the blue, green and red-emitting LP phosphors/nanophosphors, persistence mechanism involved and the basic problems associated with their luminescence efficiency and persistence times. Modifications to existing nanosynthesis protocols to formulate a nontoxic Green Chemistry Route are also presented.Contents of Paper1. Long Afterglow Phosphors
Abstract: In recent years, the term ‘smart materials’ which means the potential of a material having novel functional abilities, has become a buzz word. Luminescent materials held a lion’s share amongst all functional materials. The research in these materials is nowadays becoming the frontline platform and has challenging options for the betterment of society. These materials have applications in diverse fields such as, radiation detection, monitoring and assessment, display devices (PDP, CRT, CTV, LCD, FPD, etc.), and Lighting devices (CFL, Hg Free lamps using VUV Xenon, Solid State lighting (SSL-LED blue chips, UV-LEDs, O-LEDs, etc.). The main and essential role is being played by the phosphors. The activity of the phosphor is also depending on the particle size and hence the nanophase phosphor developments are very important aspect of development of functional materials. It is also very much clear now that the method of synthesis controls/decides a particle size of the final product. A particular method is better for a specific phosphor for getting the desired particle size of the final product. Scaling is also very much essential for instituting the specific method of synthesis for desired phosphor. These aspects are important for commercial production.Looking at the applications specified in first paragraph, one thing is common which is red correction required for display devices or lighting devices. The red correction in full-colour display/white light emission is essentially done by using Y2O3:Eu3+ phosphor or Eu3+ doped yttrium based hosts. Red emission is of Eu3+ (5D0 →7Fj, where j could be predominantly 2, 1, 3) when symmetrically organized environment. In this context, the review of yttrium based phosphors is being presented. This also covers our experience in synthesis of yttrium based luminescent materials with different methods of synthesis and their comparative aspects with regard to luminescence properties.Contents of Paper
Abstract: The present paper reports both the experimental and mathematical aspects of elastico-mechanoluminescence (EML), plastico-mechanoluminescence (PML) and fracto-mechanoluminescence (FML) of coloured alkali halide crystals in detail, and thereby provides a deep understanding of the related phenomena. The additively coloured alkali halide crystals do not show ML during their elastic and plastic deformation. The ML emission during the elastic deformation takes place due to the mechanical interaction between bending dislocation segments and F-centres, and the ML emission during plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. The ML emission during fracture is also caused by the mechanical interaction between the moving dislocations and F-centres; however, in certain hard crystals like LiF, NaCl, NaF, etc., fracto ML also occurs due to the gas discharge caused by the creation of oppositely charged walls of cracks. The EML, PML, and solid state FML spectra of coloured alkali halide crystals are similar to their thermoluminescence spectra and afterglow spectra. However, the fracto ML spectra of certain hard crystals like LiF, NaCl, NaF, etc., also contain gas discharge spectra. The solid state ML spectra of coloured alkali halide crystals can be assigned to deformation-induced excitation of halide ions inV2-centres or in other hole-centres. Whereas, the intensity of EML and FML increases linearly with the applied pressure and the impact velocity, the intensity of PML increases quardratically with the applied pressure and the impact velocity because of the plastic flow of the crystals. Both Im and IT increase with the density of F-centres in the crystals and strain rate of the crystals; however, they are optimum for a particular temperature of the crystals. The ML of diminished intensity also appears during the release of applied pressure. Expressions are derived for the elastico ML, plastico ML and fracto ML of coloured alkali halide crystals, in which a good agreement is found between the experimental and theoretical results. Many parameters of crystals such as band gap between the dislocation band and interacting F-centre energy level, radius of interaction between dislocations and F-centres, pinning time of dislocations, work hardening exponent, velocity of cracks, rise time of applied pressure, lifetime of electrons in the dislocation band, lifetime of electrons in shallow traps, diffusion time of holes, critical velocity of impact, etc., can be determined from the ML measurements. The ML of coloured alkali halide crystals has potential for self-indicating method of monitoring the microscopic and macroscopic processes; mechanoluminescence dosimetry; understanding dislocation bands in crystals; interaction between the dislocations and F-centres; dynamics of dislocations; deformation bleaching of coloration, etc. The ML of coloured alkali halide crystals has also the potential for photography, ML memory, and it gives information about slip planes, compression of crystals, fragmentation of crystals, etc.Contents of Paper
Abstract: Ce3+ and Eu2+ activated BaAlSi5O2N7 phosphors have been prepared by the modified three step solid state diffusion method. Prepared phosphors are characterized by XRD, SEM, photoluminescence and thermoluminescence techniques. Upon excitation at 234nm, Ce3+ activated BaAlSi5O2N7 phosphor shows strong emission at 360nm. Upon excitation at 348nm, Eu2+ activated BaAlSi5O2N7 phosphor shows strong emission at 469nm in the blue region of the optical spectrum. After quenching at higher temperature and upon excitation at 363nm, BaAlSi5O2N7:Eu2+ phosphor shows broad band at 516nm in the green region of the optical spectrum. The trap parameters are studied using glow curve deconvolution. It was found that the obtained samples may be suitable for near UV excited white light emitting diodes.
Abstract: YAl3(BO3)4 doped with three rare earth (RE) ions namely Ce3 +, Dy3+ and Tb3+ were prepared by the conventional solid state diffusion method. The phosphors were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) techniques. PL spectra of Ce-doped samples showed the Ce3+ emission characteristic due to the 5d-4f transition. Dy-doped sample upon excitation at 351 nm, showed peaks corresponding to the 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions. PL spectra of Tb-doped sample showed peaks corresponding to the 5D3→7FJ transition. Upon excitation at 364 nm, the enhanced luminescence observed in Ce3+ co-doped YAl3(BO3)4:Tb3+/Dy3+ phosphors is understood as due to energy transfer from Ce3+ to Tb3+/Dy3. The color purity has also been verified by using the chromaticity diagram.
Abstract: Currently there is a great interest in II–VI semiconductor nanoparticles, particularly organically capped soluble particles of cadmium or zinc sulphide and selenide, for their ready to use application in devices. For electroluminescence (EL) devices, it is expected to cover a broad spectrum and to tune various specific colours by preparing Cd1-xZnx Se instead of CdSe and ZnSe. Ternary alloys have composition dependent properties; therefore Cd1-xZnxSe has attracted much attention in the fields of luminescence and optoelectronic devices. It has wide optical band-gap and good stability with respect to environment. In this study, Cd1-xZnxSenanoparticles have been synthesized by using starch as a capping agent through a chemical synthesis route at room temperature. Samples have been prepared varying composition factor ‘x’ in ternary alloy Cd1-xZnxSe. Cubic structure of all has been confirmed by XRD. Crystallite size calculated from XRD was found in the range of 3-5 nm and it was observed that size reduces on increasing Zn content in ternary compound. Optical absorption spectra showed the blue shift in absorption edge with increasing Zn content. Band gap has been obtained by absorption studies and increase in band gap observed on increasing Zn content in the compound. Electroluminescence studies reveal that lower threshold voltage is required for samples with lower ‘x’ value. The Brightness increases on increasing the voltage above threshold voltage and the variation pattern is almost exponential for all samples. The voltage-current curve represents ohmic nature of the EL cell. Impedance was found to increase on increasing ‘x’ value. The increase in EL intensity is faster for higher frequency. EL spectra revealed that light emission is in violet-green region corresponding to band gap for both Cd0.75 Zn 0.25Se and Cd0.5 Zn 0.5Se, with a slight blue shift on increasing Zn content. A ternary system Cd1–xZnxSe, may be engineered better for application purpose by suitably choosing the composition parameter ‘x’.Contents of Paper
Abstract: Sulfide based luminescent materials have attracted a lot of attention for a wide range of photo-and electroluminescence applications. Among the sulfides, ZnS is promising host material for development of phosphors in different visible emission bands. Doping of Ag can affect the electroluminescence of the host material. Incorporation of host ZnS into the polymer matrix is one of the best method to display their special functions, which stabilize the nanoparticles. Here we report a synthesis and electroluminescence of silver doped ZnS/PVK nanocomposites thin films. Reported films were prepared by using chemical route with varying Ag doping and ZnS loading in the composite. Structural and morphological characterization were carried out through XRD and SEM techniques, which confirmed the particles in nanoregime. Though optical absorption spectra and band gap of ZnS semiconductor nanoparticles in ZnS:Ag/PVK matrix were estimated, and using EMA model, particle size was calculated which supports the results of XRD. Electroluminescence of nanocomposite samples was studied and it was found that threshold voltage depends on doping of Ag and also on loading of ZnS. Voltage brightness characteristics support the production of EL by acceleration-collision mechanism.Contents of Paper