[Show abstract][Hide abstract] 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
Full-text · Article · Jan 2015 · Defect and Diffusion Forum
[Show abstract][Hide abstract] ABSTRACT: A new ternary oxide Gd2CaZnO5 having interesting structural, mechanical, electronic and optical properties is synthesized and is studied in detail using density functional theory. The analysis revealed two polymorphs: orthorhombic and tetragonal; the orthorhombic phase was found to be the most stable structure under ambient conditions. A high-pressure (hydrostatic) phase transition to the tetragonal phase is predicted at about 4 GPa. This is one of very few reports that depict the phase transition of oxide materials under pressure. The calculated results are in agreement with the X-ray diffraction studies supported by Rietveld analysis. Analysis of the optical properties revealed both polymorphs to be direct-gap semiconductors with low dielectric constants. The calculated elastic constants of both phases satisfy the mechanical stability criteria. It is also identified that the half-filled 4f orbital of Gd induces a strong magnetic spin polarization in the host oxide lattice indicating that the material could be effectively used in versatile applications ranging from biomedical devices to light emitting diodes.
[Show abstract][Hide abstract] ABSTRACT: Blue emitting BaMgAl10O17:Eu2+ (BAM) phosphor is indispensable for Plasma Display panel and lighting because of high luminescence efficiency. However, thermal degradation (annealing in air at 500–600 °C) of BAM (upto ∼30%) remains an intriguing problem for display industry worldwide. In the present study, a systematic approach is pursued to develop highly efficient BAM phosphor that exhibits least degradation, understand the role of Eu2+ site occupancy in such BAM phosphor and encapsulate individual phosphor grains with a shell of silica nanoparticles. The approaches lead to highly efficient BAM:Eu2+ phosphor that showed no degradation against thermal baking (annealing at 500 °C in air) for both UV and VUV radiation under UV and VUV excitation. An optimum solid state chemical route including precursor phases, dopant concentration, and thermal regimes has been evolved to develop BAM. Emission from Eu2+ occupying three different sites is identified with energetically stable anti Beevers Ross as the dominant contributor. Coating by nano sized amorphous silica sol with subsequent sintering lead to uniform silica shell. This nano silica layer also helps to enhance the luminescence from phosphor grains.
Full-text · Article · Nov 2013 · Journal of Luminescence
[Show abstract][Hide abstract] ABSTRACT: Present work focuses on the effective doping of multi-walled carbon nanotube (CNT) in the ZnS:Cu phosphor nano-composite and thereafter improvement in the optical performance of electroluminescent (EL) device due to increased local ﬁeld effects. To facilitate doping of CNTs into the phosphor and decrease the operating voltage of the EL device, CNTs were shortened by milling and incorporated effectively using a ﬂux assisted solid-state annealing reaction. Interestingly shorter the length of CNTs used; greater was the local ﬁeld enhancement, improvement in brightness and efﬁciencies observed for the EL devices. When the ﬁeld is applied, adequate charge carriers are tunneled into the ZnS:Cu system through the tips of the CNTs by forming high energy hot spots thus enhancing the local ﬁeld. The improved device characteristics are due to ﬁeld enhancement and effective transfer of energy from hot spots to copper activator by impact ionization. The detailed electrical characterization of the novel EL device along with its brightness measurements are presented by considering the hot electron injection model.
Full-text · Article · Oct 2013 · Science of Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: There has been a stringent demand for blue (similar to 450 to 470 nm) absorbing and red (similar to 611 nm) emitting material systems in phosphor converted white light emitting diodes (WLEDs) available in the market. The conventionally used red-emitting Y2O3:Eu3+ phosphor has negligible absorption for blue light produced by GaInN based LED chips. To address this issue, a new red-emitting Gd2CaZnO5:Eu3+ (GCZO:Eu3+) nanophosphor system having exceptionally strong absorption for blue (similar to 465 nm) and significant red (similar to 611 nm) photoluminescence is presented. This is attributed to a dominant f-f transition (D-5(0) -> F-7(2)) of Eu3+ ions, arising due to an efficient energy transfer from the Gd3+ sites of the host lattice to Eu3+ ions. The external quantum yield (QY) measured at 465 nm absorption and 611 nm emission revealed that the GCZO: Eu3+ nanophosphor has better QY of 23% as compared to commercial Y2O3:Eu3+, which is <1%. X-ray diffraction and microscopy observations showed the nanocrystalline nature and slightly elongated morphology of the sample, respectively. While the energy dispersive X-ray analysis identified the chemical constituents of the GCZO: Eu3+ nanophosphor, the color overlay imaging confirmed the substitution of Eu3+ for Gd3+ ions. As seen from the QY statistics it is highly anticipated that the multifold absorption at similar to 465 nm would certainly improve the color rendering properties of existing WLEDs.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t We present a novel methodology to design a hybrid electroluminescent (EL) lamp by embedding carbon nanotubes (CNTs) inside the ZnS:Mn phosphor particles by conventional solid state diffusion technique. By doing so, the phosphor particles exhibited increase in EL brightness and efficiency at low operating voltages (<80 V AC). Interestingly, shorter the length of CNTs used, greater was the field enhancement effect and lower was the operating voltages to glow the EL lamps. The role of CNTs have been identified to form conductive paths inside the ZnS particle thereby triggering EL due to electron injection to lumi-nescent centers (Mn 2+) at nominal voltages. In addition, a detailed electrical characterization of the novel EL lamp along with its spectral energy distribution studies are presented. Ó 2013 Elsevier B.V. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: Present work focuses on the effective doping of multi-walled carbon nanotube
(CNT) in the ZnS:Cu phosphor system and thereafter improvement in the optical
performance of electroluminescent (EL) device due to the effect of increased
local field. To facilitate doping of CNTs into the phosphor and decrease the
operating voltage of the EL device, CNTs were shortened by milling and
incorporated effectively using a flux assisted solid-state annealing reaction.
Interestingly shorter the length of CNTs used, greater was the local field
enhancement, brightness and efficiency observed for the EL devices. When the
field is applied, adequate charge carriers are tunneled into the ZnS:Cu system
through the tips of the CNTs by forming high energy hot spots thus enhancing
the local field. The improved device characteristics are due to field
enhancement without flowing of undesired current in the EL device and effective
transfer of energy from hot spots to copper activator causing field-ionization.
The detailed electrical characterization of the novel EL device along with its
brightness measurements are also presented by considering the hot electron
[Show abstract][Hide abstract] ABSTRACT: We report a high ($94%) yield synthesis of intrinsic zinc oxide (ZnO) nanocrystal powders having crys-tallite sizes in the range 13–35 nm using a novel gel-incineration method with inexpensive precursor salts and citric acid as chelating agent. The influence of various precursor chemicals on the nanocrystal-lite size, morphology and luminescent properties has been studied in detail. It was identified that the ZnO nanocrystals prepared using organic precursor resulted the smallest crystallite size as compared to inor-ganic precursors. Reaction temperature was optimized to be $900 °C by simultaneous thermogravimetric analysis and differential scanning calorimetry studies. Morphology and microstructure of the ZnO nano-crystals have been studied using a scanning electron microscopy. Analysis of photoluminescence excita-tion and emission spectra enabled us to calculate the band gap energy and defect analysis of as prepared ZnO nanocrystals respectively. The stability of ZnO nanocrystals in water has been verified on time scale and its potential use has been successfully demonstrated for security marker applications. Ó 2013 Elsevier B.V. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: We present a novel methodology to design a hybrid electroluminescent (EL) lamp by embedding carbon nanotubes (CNTs) inside the ZnS:Mn phosphor particles. By doing so, the phosphor particles exhibited increase in EL brightness and efficiency at low operating voltages (<100 VAC). Interestingly shorter the length of CNTs used, greater was the field enhancement effect observed in the EL lamps. The role of CNTs have been identified to form conductive paths inside the ZnS particle thereby triggering EL due to electron injection to active centres at nominal voltges. In addition, a detailed electrical characterization of the novel EL lamp along with its spectral energy distribution studies are presented.
[Show abstract][Hide abstract] ABSTRACT: Manganese doped sodium zinc phosphate phosphor with exceptional features
having ultra-violet (UV) to visible absorption (300-470 nm), yellow-green (~543
nm) broad-band photoluminescence (PL) and appreciable color co-ordinates
(x=0.39, y=0.58) is reported. It has a crystal structure consists of discrete
PO4 tetrahedra linked by ZnO4 and NaO4 distorted tetrahedral such that three
tetrahedra, one of each kind, share one corner. The presence of UV sensitive
Zn-O-Zn bonds efficient energy transfer to Mn2+ ions resulted in brightest PL
and external quantum yield of 63% at 418 nm. Our experiment demonstrated the
possibility of producing inexpensive white-light emitting devices for future.
Full-text · Article · Nov 2012 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We present a novel, cost-effective and facile technique, wherein multi-walled carbon nanotubes (CNTs) were used to transform a photoluminescent material to exhibit stable and efficient electroluminescence (EL) at low voltages. As a case study, a commercially available ZnS:Cu phosphor (P-22G having a quantum yield of 65 ± 5%) was combined with a very low (∼0.01 wt%) concentration of CNTs dispersed in ethanol and its alternating current driven electroluminescence (AC-EL) is demonstrated. The role of CNTs has been understood as a local electric field enhancer and facilitator in the hot carrier injection inside the ZnS crystal to produce EL in the hybrid material. The mechanism of EL is discussed using an internal field emission model, intra-CNT impact excitation and the recombination of electrons and holes through the impurity states.
[Show abstract][Hide abstract] ABSTRACT: Highly luminescent-paramagnetic nanophosphors have a seminal role in biotechnology and biomedical research due to their potential applications in biolabeling, bioimaging, and drug delivery. Herein, the synthesis of high-quality, ultrafine, europium-doped yttrium oxide nanophosphors (Y(1.9) O(3) :Eu(0.1) (3+) ) using a modified sol-gel technique is reported and in vitro fluorescence imaging studies are demonstrated in human breast cancer cells. These highly luminescent nanophosphors with an average particle size of ≈6 nm provide high-contrast optical imaging and decreased light scattering. In vitro cellular uptake is shown by fluorescence microscopy, which visualizes the characteristic intense hypersensitive red emission of Eu(3+) peaking at 610 nm ((5) D(0) -(7) F(2) ) upon 246 nm UV light excitation. No apparent cytotoxicity is observed. Subsequently, time-resolved emission spectroscopy and SQUID magnetometry measurements demonstrate a photoluminescence decay time in milliseconds and paramagnetic behavior, which assure applications of the nanophosphors in biomedical studies.
[Show abstract][Hide abstract] ABSTRACT: There has been a stringent demand for blue (~450-470 nm) absorbing and red
(~611 nm) emitting material system in phosphor converted white light emitting
diodes (WLEDs) available in the market. Conventionally used red-emitting
Y2O3:Eu3+phosphor has negligible absorption for blue light produced by GaInN
based LED chip. To address this issue, a new red-emitting Gd2CaZnO5:Eu3+
(GCZO:Eu3+) nanophosphor system having exceptionally strong absorption for blue
(~465 nm) and significant red (~611 nm) photoluminescence (PL) is presented.
This is attributed to a dominant f-f transition (5D0\rightarrow7F2) of Eu3+
ions, aroused due to an efficient energy transfer from the Gd3+ sites of the
host lattice to Eu3+ ions. X-ray diffraction and microscopy observations
revealed the nanocrystalline nature and a bit elongated morphology of the
sample respectively. While the energy dispersive x-ray analysis identified the
chemical constituents of the GCZO:Eu3+ nanophosphor, the color overlay imaging
confirmed the substitution of Eu3+ for Gd3+ ions. It is highly anticipated that
the multifold absorption at ~465 nm would certainly improve the color rendering
properties of existing WLEDs.
[Show abstract][Hide abstract] ABSTRACT: In biological fluids, nanoparticles are always surrounded by proteins. As the protein is adsorbed on the surface, the extent of adsorption and the effect on the protein conformation and stability are dependent on the chemical nature, shape, and size of the nanoparticle (NP). We have carried out a detailed investigation on the interaction of bovine serum albumin (BSA) with polyethyleneimine-functionalized ZnO nanoparticles (ZnO-PEI). ZnO-PEI was synthesized using a wet chemical method with a core size of ~3-7 nm (from transmission electron microscopy). The interaction of BSA with ZnO-PEI was examined using a combination of calorimetric, spectroscopic, and computational techniques. The binding was studied by ITC (isothermal titration calorimetry), and the result revealed that the complexation is enthalpy-driven, indicating the possible involvement of electrostatic interaction. To investigate the nature of the interaction and the location of the binding site, a detailed domain-wise surface electrostatic potential calculation was performed using adaptive Poisson-Boltzmann software (APBS). The result shows that the protein surface can bind the nanoparticle. On binding ZnO-PEI, the protein gets destabilized to some extent, as displayed by CD (circular dichroism) and FTIR (Fourier transform infrared) spectroscopy. Chemical and thermal denaturation of BSA, when carried out in the presence of ZnO-PEI, also indicated a small perturbation in the protein structure. A comparison of the enthalpy and entropy components of binding with those derived for the interaction of BSA with ZnO nanoparticles explains the effect of hydrophilic cationic species attached on the NP surface. The effect of the NP surface modification on the structure and stability of BSA would find useful applications in nanobiotechnology.
[Show abstract][Hide abstract] ABSTRACT: In the present study, 11-mercaptoundecanoic acid-modified gold nanoparticles (∼7 nm) were conjugated with chloroquine to explore their potential application in cancer therapeutics. The anticancer activity of chloroquine-gold nanoparticle conjugates (GNP-Chl) was demonstrated in MCF-7 breast cancer cells. The MCF-7 cells were treated with different concentrations of GNP-Chl conjugates, and the cell viability was assayed using trypan blue, resulting in an IC(50) value of 30 ± 5 μg/mL. Flow cytometry analysis revealed that the major pathway of cell death was necrosis, which was mediated by autophagy. The drug release kinetics of GNP-Chl conjugates revealed the release of chloroquine at an acidic pH, which was quantitatively estimated using optical absorbance spectroscopy. The nature of stimuli-responsive drug release and the inhibition of cancer cell growth by GNP-Chl conjugates could pave the way for the design of combinatorial therapeutic agents, particularly nanomedicine, for the treatment of cancer.
Full-text · Article · Mar 2012 · Colloids and surfaces B: Biointerfaces
[Show abstract][Hide abstract] ABSTRACT: For the first time, we report a highly efficient, tunable and bright photoluminescence with external quantum yield up to $34% under 350 nm excitation from hydrophobic SiO 2 gel nanoparticles. This is achieved by suitably modifying the relative surface states of the gel network with trivalent (Al 3+ /Ga 3+) ions and emission from rare-earth (Eu 3+) dopant. This attractive property arises from the intrinsic separation of absorption and the emission energies due to a large Stokes shift and offers opportunities for the design of novel nano-phosphor based white light-emitting devices. Silicates and zeolites are well-known mesoporous materials having a wide range of applications 1,2 from catalysis, gas separation/sorption, and ion exchange to recently developed low-k materials 3,4 or zeolite-dye microlasers. 5 But unfortunately the efficient luminescence from these materials has not been investigated. Photoluminescence is one of the key properties by which the potential and applicability of a phosphor material, an important component of lighting and display technology, could be methodically estimated. 6,7 Current lighting technology uses UV-LEDs with triple-wavelength RGB phosphors or blue LEDs with a yellow-emitting YAG:Ce 3+ phosphor to produce white light. 8 In the latter case, there is a serious demand for the exploitation of advanced and novel yellow-emitting phosphors. At present, mixed luminescence from various sources has always been the route to white light, without other possibilities being advocated. Similarly single activator photoluminescence in a host lattice that emits multiple colours covering the entire visible part of the electro-magnetic spectrum has not yet been observed. Compared to those inorganic materials added with dye or transition metal ion activators, 9,10 intrinsic photoluminescence from a meso-porous silica (SiO 2) gel is noticeably weak. 11 Thereafter, some groups started serious efforts on sol–gel synthesis of optical silica glasses doped with rare-earth elements for lasing applications, but failed due to their negligible photoluminescence. 12 To overcome this, from Knobbe in 1993 to Boilot in 2010 many groups have been working on incorporation of rare-earth complexes 13 (instead of rare-earth ions) or luminescent nanocrystals into core–shell SiO 2 nanostructures, 14 and demonstrated many advanced practical applications. Significant research into the structural evolution of silica gels has been done in the last 20 years. 15 But almost no effort has been made to investigate and improve the intrinsic rare-earth ion photoluminescence from SiO 2 gel networks. 16 Since the optical properties of doped silica systems depend intimately on the local structure and bonding of dopant cations, a detailed understanding of these factors is important from a device engineering perspective. The introduction of the dopant acts as a perturbation of this well-studied system. Eu 3+ was selected as a representative rare-earth ion because its unique fluorescence prop-erties make it an ideal probe of local structure determination. As a research goal to pursue, we have successfully prepared highly lumi-nous hydrophobic gels and powders that have distinctive and tunable photoluminescence properties using Eu 3+ as a single activator. Herein, we report a new class of highly luminescent gels that individually emit from violet to deep-red luminescence with exposure to 350 nm UV light (Fig. 3). The advantages of the present protocol are numerous such as: 1) high surface-to-volume ratios with effective prevention of further aggregation of the nanoparticles (NPs), so as to retain a high luminescence yield; 2) the profitability of size-quantized, nanometre-sized hydrophobic SiO 2 particles with higher redox potential, 17 which in turn enhances the charge-transfer rates between the SiO 2 network and the activator (Eu 3+) ion; sometimes reducing the activator ion by host–activator interactions; 3) easier preparation than those obtain-able with usual NPs, which need further functionalization by silanes; 4) good dispersity and useful dimensions for possible coating over many substrates. In addition, we demonstrate that the SiO 2 :Eu 3+ NPs are excellent phosphors with high luminescence yields which could be useful for designing multi-colour-emitting novel light emitting diode (LED) structures including white LEDs. Highly luminescent, mesoporous and hydrophobic SiO 2 :Eu 3+ gels were prepared by a modified sol–gel technique 18 in the presence of a trivalent co-activator and thermolyzing the gels using supercritical solvothermal reaction in an autoclave (see ESI Fig. S1). The role of trivalent co-doping is to avoid clustering 19 and effectively disperse/ isolate Eu 3+ ions in the SiO 2 matrix; and the supercritical treatment increases the thermal diffusion of the activator ions and creates short-range crystallinity in the SiO 2 gel network. 16 The fine structural details of SiO 2 :Eu 3+ NPs were investigated by using electron microscopy. A typical scanning electron microscope
[Show abstract][Hide abstract] ABSTRACT: In the present work, we studied the particle size dependant antibacterial activity of zinc oxide (ZnO) nanoparticles (NPs) against Escherichia coli (E.coli) K-12 bacterial strain. ZnO particles of varying sizes were prepared via wet chemical route using zinc acetate as a precursor and lithium hydroxide as a reducing agent. X-ray diffraction measurement revealed hexagonal wurtzite structure of as synthesized ZnO NPs and the particle size was calculated by using Scherer's formula. The optical density measurement shows blue shift in lambda(max) of optical spectra with decreasing particle size and corresponding Tauc plot demonstrates increasing band gap in small size ZnO NPs. Minimum inhibitory concentration (MIC) measurements and growth kinetics studies were carried out to measure the antibacterial activity of ZnO NPs on E.coli. The enhanced bacterial growth inhibition by small sized ZnO NPs is attributed to their inherent ability of generating reactive oxygen species owing to their high surface area and large number of surface defects.
Full-text · Article · Jan 2012 · Science of Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: Graphene is an intriguing two-dimensional material, which could be modified for achieving tunable properties with many applications. Photoluminescence of graphene due to plasmonic emission is well-known, however, attempts to develop strong luminescent graphene have been difficult. Synthesis of a graphene-based material with a dual optical functionality, namely quenching the fluorescence of organic dyes while maintaining its own self-luminescence, is an interesting and challenging proposition. Here, we demonstrate this optical bifunctionality in a lattice-modified luminescent graphene, where europium(III) cations are complexed with graphene through oxygen functionalities. After excitation at 314 nm, a hypersensitive red emission is observed at 614 and 618 nm showing the complexation of europium(III) with graphene. We demonstrate dual functionality of this graphene by the quenching of luminescence of Rhodamine-B while displaying its own hypersensitive red emission. The decay lifetime observed through the time-resolved spectroscopy confirms its potential for applications in biosensing as well as optoelectronics.