Mechanism Of Light Emission Research Articles

Comparison study of electrochemiluminescence of boron-dipyrromethene (BODIPY) dyes in aprotic and aqueous solutions

Boron-dipyrromethene (BODIPY) dye molecules are well known for their excellent absorption and emission optical properties. Here, we report the results of an electrochemical and electrochemiluminescence (ECL) study of an organic-soluble and a water-soluble BODIPY dye to explore their effectiveness as ECL luminophores. A combination of UV–visible, fluorescence, and spooling ECL spectroscopy was used to study the mechanisms of ECL light emission. It was discovered that excimers, the excited states of analyte dimers, contributed significantly to the production of ECL. These BODIPY molecules exhibited relatively weak ECL efficiencies following the annihilation mechanism. However, ECL was enhanced with co-reactants such as persulfate, and tri-n-propylamine. Spooling ECL spectroscopy was utilized to gain analytical insight into light emission mechanisms. It is shown that while these two BODIPY compounds can form photoinduced excited monomers, excimers were generated mainly in their co-reactant ECL pathways.

Origin of Dual Photoluminescence States in ZnS–CuInS2 Alloy Nanostructures

ZnS–CuInS2 (ZCIS) alloy nanostructures are becoming increasingly important materials because of their photoluminescence properties. Here we explore the emission properties of ZCIS quantum dots (QDs) capped with dodecanethiol, which exhibit Zn:Cu-dependent emission properties. Absorption and photoluminescence excitation spectra indicate a single, composition-independent light absorbing state. The emission spectra point out the existence of two emissive states with lifetimes of ∼10 ns and ∼100 ns. The photoluminescence and time-resolved emission analysis provide insight into the synergy between the two intraband states and the possibility of modulating the emission through variation in the Zn/Cu ratio. Better understanding of light absorbing and emission mechanisms in alloyed nanostructures is essential for future development of photoelectric and display devices.

Cathodoluminescence properties of Pr, Tm co-implanted GaN thin films

In this work, Pr and Tm co-doped GaN thin films were prepared by ion implantation. After a thermal annealing treatment for the lattice recovery and ions activation, temperature-dependent cathodoluminescence (CL) spectroscopy was applied to investigate the luminescent properties. At room temperature, the intensity ratio of blue emission to infrared emission of Tm ions was decreased as the implantation dose of Pr ions was increased, which is due to the energy transfer between Pr and Tm ions. In addition, the 467 nm blue emission of Tm and the 528 nm green emission of Pr were observed only at low temperature. A model was proposed to illuminate the light emission mechanism of Pr and Tm co-implanted GaN thin films.

(Invited) Silicon Nanostructures: A Versatile Material for Photonics

The increased control over nanoscale materials structuring is reshaping a number of applications and technologies at a fast pace. Semiconducting materials are playing an increasingly important role and Silicon is so far the most investigated one thanks to its broad availability and its fundamental role in microelectronics. Despite all its favorable properties, a major drawback of silicon is the lack of an efficient light emission mechanism that prevents its use in active photonics. Quantum size effects has been demonstrated a viable strategy to overcome this limit and to achieve light emission from silicon nanostructures. Si nanostructures are well controlled and can be tuned to achieve the desired properties, often exploiting the mature CMOS technological platform. Simultaneously several synthetic strategies have been developed in order to fabricate silicon quantum dots from chemical synthesis. Silicon chemistry is (rather) well known and permits to synthesize nanoparticles and to functionalize their surface using a broad number of reactions. Such flexibility allows for the grafting of both organic and inorganic species onto the Si QDs surface, to add functionalities and to protect the nanoparticle from their natural oxidations that, in turn, quenches the photoluminescence properties. Thus, despite the initial research was focused on the possibility to fabricate inorganic light sources that might rival with III-V or II-VI semiconductors. More recently a large research effort has been devoted to the use of Silicon nanostructures in biological-related applications. In such research topics Silicon has several properties take renders it an ideal candidate: it is considered a biocompatible material; it is readily dissolved in biological environment without releasing toxic byproducts; its red and tunable luminescence make it an interesting fluorescent tag able to exploit the tissue transparency window. During the presentation I will make a brief review of the fabrications methods proposed so far and I will discuss in details some of the recent research activities performed in our group in ICT- as well as in bio-related topics, all sharing the common denominator of Si QDs as active optical element. About the ICT-related topics I will discuss two applications of Si QDS. The first is about the use of Si QDs in quantum technologies. Data encryption and cryptography are core technologies hidden in our daily life and their importance is constantly growing. We have developed a robust physical random number generator that exploits Si QDs as a random bit source. The preliminary results are promising and the random sequences pass the standard NIST test suite. Furthermore the system is fully compatible with CMOS technology and can be scaled up to achieve large bit-rate generation. The second case in point is about the large optical nonlinearities possessed by Si QDs and their exploitation to develop integrated photonic devices. As a bio-related application I will discuss how nanostructured silicon microparticles can be used as fluorescent tags and are successfully up-taken by human dendritic cells without stimulating apoptosis up to a very large concentration. Porous silicon micro-particles (micro-pSi) with size in the range of 1-10 um are functionalized obtaining an interface that exposes either positive or negative charges at neutral pH. Their porous structure enables their use as drug carriers. Moreover, the surface modification stabilizes their intense photoluminescence and suggests their use for simultaneous imaging and drug delivery. The few cases discussed underline the great potentialities of Silicon nanostructures in different technologies and suggest its possible implementation as a functional interface between organic and inorganic systems.

Cosmologically Strengthening Hydrogen Atom in Black Hole Universe

With reference to the earlier proposed black hole model of cosmology, the authors proposed a unified model mechanism for understanding the light emission mechanism in cosmologically ‘strengthening hydrogen atom’. In this proposed model, characteristic cosmic mass, characteristic nuclear charge radius, Avogadro number and possible quantum states of electron seem to play a major role. Throughout the cosmic evolution, Planck’s constant seems to be a constant whereas the currently believed ‘reduced Planck’s constant’ seems to be a cosmological decreasing variable.With this new proposal - Hubble’s redshift interpretation, Super novae dimming and currently believed cosmic acceleration can be reviewed at fundamental level and a correct model of cosmology can be confirmed.

Structure and photoluminescent properties of green-emitting terbium-doped GdV1-x Px O4 phosphor prepared by solution combustion method.

Terbium-doped gadolinium orthovanadate (GdVO4 :Tb(3+) ), orthophosphate monohydrate (GdPO4 ·H2 O:Tb(3+) ) and orthovanadate-phosphate (GdV,PO4 :Tb(3+) ) powder phosphors were synthesized using a solution combustion method. X-Ray diffraction analysis confirmed the formation of crystalline GdVO4 , GdPO4 ·H2 O and GdV,PO4 . Scanning electron microscopy images showed that the powder was composed of an agglomeration of particles of different shapes, ranging from spherical to oval to wire-like structures. The chemical elements present were confirmed by energy dispersive spectroscopy, and the stretching mode frequencies were determined by Fourier transform infrared spectroscopy. UV-visible spectroscopy spectra showed a strong absorption band with a maximum at 200 nm assigned to the absorption of VO4 (3-) and minor excitation bands assigned to f → f transitions of Tb(3+) . Four characteristic emission peaks were observed at 491, 546, 588 and 623 nm, and are attributed to (5) D4 → (7) Fj (j = 6, 5, 4 and 3). The photoluminescent prominent green emission peak ((5) D4 → (7) F5 ) was centred at 546 nm. The structure and possible mechanism of light emission from GdV1-x Px O4 :% Tb(3+) are discussed. Copyright © 2016 John Wiley & Sons, Ltd.

In situ investigation of energy transfer in hybrid organic/colloidal quantum dot light-emitting diodes via magneto-electroluminescence.

Energy transfer (ET) and charge injection (CI) in the hybrid organic/colloidal quantum dot light-emitting diodes (QD-LEDs) have been investigated by using magneto-electroluminescence (MEL) as an in situ tool. The feasibility and availability of MEL as an in situ tool were systematically demonstrated in the typical QD-LEDs based on CdSe-ZnS core-shell QDs. Our results suggest that the ET and CI processes can be well discerned by MEL measurements since these two processes exhibit distinct responses to the applied magnetic field. Through measurement of the MEL and current efficiency, we indicated that ET would be the main mechanism for light emission in the present hybrid QD-LEDs. This study strongly suggests that MEL could be a highly sensitive fingerprint for ET, which provides a facile and efficient method for the in situ investigation of fundamental processes in hybrid organic/colloidal QD-LEDs and other organic/inorganic composites.

Confocal electroluminescence investigations of highly efficient green InGaN LED via ZnO nanorods

The light emission mechanism of green InGaN-based light-emitting diodes (LEDs) coated with ZnO nanorods (ZnO-nanorods/InGaN LEDs) was investigated by confocal scanning electroluminescence (EL) spectroscopy (CSEM). The ZnO nanorods were grown by a hydrothermal method on the surface of fully fabricated InGaN LEDs. The vertical alignment and high crystallinity of the ZnO nanorods were confirmed by scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, micro-Raman spectroscopy, and high-resolution transmission electron microscopy. Measurement of the light output as a function of the injection current, far-field radiation pattern, and current versus voltage curves revealed that the light output power of the green InGaN LEDs with ZnO nanorods was enhanced by approximately 60% compared to that of conventional green InGaN LEDs without ZnO nanorods. However, the electrical properties of the ZnO-nanorods-coated device were not appreciably affected by the nanorods. To analyse the light emission mechanism of the ZnO-nanorods/InGaN LEDs, CSEM was used to observe the EL intensity distributions at different focal planes.

The electroluminescence mechanism of non-doping PhOLEDs based on CBP/Ir(ppy)3 investigated by delayed EL measurements

In this paper, the non-doped PhOLED based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl(CBP) and fac-tris(2-phenylpyridine) iridium(Ir(ppy)3) with the emitting layer of [CBP/Ir(ppy)3]n/CBP in which n is equal to 2,3,4 and 5, respectively, are prepared. The electroluminescence and the luminance-voltage-current density characteristics of devices are detected. The wavelength range of these devices is varied from 517 nm to 540 nm. And the delay EL measurements are used to elucidate the carrier recombination and light emission mechanism in non-doped PhOLEDs. In this technique, the devices are driven using a square pulse driving scheme, with a forward bias pulse width of 1 ms, which is sufficiently long for prompt EL to reach its steady-state intensity. The delay EL results show that changing the number of the non-doped EML leads to marked changes in the charge-trapping and host–host TTA patterns, which suggests that the carrier transport and recombination processes depends on the number of non-doped EML. When the number of non-doped EML is less than 3, only the trapped carrier recombination signal are detected in the delay EL measurement. For the devices with more non-doped EML than 3, both the trapped carrier recombination and host–host TTA signal are detected. All these results are discussed and give the evidence for the electroluminescence mechanism of prepared devices.

A Solid-State Thin-Film Incandescent Light-Emitting Device

A new type of solid-state thin-film white light-emitting device has been reviewed and discussed. Light is emitted due to thermal excitation of the nano-sized conductive paths formed after the dielectric breakdown of the amorphous metal oxide thin film deposited on a silicon wafer. The mechanism of light emission, optical characteristics, reliability, driving methods, and efficiency are discussed. The complete device is made of the IC-compatible materials and process.

(Ga,In)P nanowires grown without intentional catalyst

We have grown (Ga,In)P nanowires through the MOCVD method without a intentional catalyst. The organometallic precursor triethylgallium ((C2H5)3Ga), used as Ga source, is transported by the N2 gas carrier to the reactor chamber where reacts with the InP vapor pressure producing the nanowires. Two different reactor pressures (70 and 740Torr) were used leading to nanowires with different In contents. The nanowires are straight or wool-like and exhibit a twinned structure. They emit an intense orange to red color visible even to the naked eyes. Interface tunneling process at Ga1−xInxP/Ga1−yInyP interfaces (x≠y) is proposed to explain this efficient light emission mechanism.

Dynamics and acoustic energy dissipation in conical bubble collapse

We follow the dynamics and acoustics of conical bubble collapse (CBC) in a U-tube device, to understand its associated phenomena such as: light emission, turbulence, bubble cloud formation, strong rebound pressures, shock wave emission, and liquid–gas interface behaviour. High-speed video frames linked with the waveforms acquired by piezoelectric transducers and photomultipliers during the collapse are analysed. All of the data acquisition is synchronised to the same timeline. Acoustic energy dissipation is investigated in detail by analysing the piezoelectric waveforms using Fourier transforms and wavelets.The primary experimental results demonstrate that as the compression proceeds, (a) the liquid meniscus reaches the conical zone in cavitation conditions; (b) the liquid meniscus undergoes a geometric transformation (2D to 3D), it becomes a “bulb with a nozzle”, wherein the instabilities are dragged and confined inside it; (c) the nozzle is retained as part of the new meniscus that continues to push the gas pocket; and (d) both structures (bulb and gas pocket) are connected by a neck/nozzle and will eventually form a “slug”. Furthermore, during the collapse, the bulb eventually becomes a bubble cloud, and the bubble structures exhibit their own expansion–contraction rate. These phenomena are widely discussed.A detailed analysis of the acquired signals yields frequencies and scales, which are associated with the onset of shock waves and its propagation, as well as the frequency bands that occur when the energy has dissipated. Good agreement was found between the experimental measurements and two different models for CBC dynamics. From the analysis of the dynamics and acoustics, we consider that various light emission mechanisms are activated during the collapse of a conical bubble, these are: thermal, chemical, and electrical in nature.

Light emission from Ag(111) driven by inelastic tunneling in the field emission regime

We study the light emission from a Ag(111) surface when the bias voltage on a scanning tunneling microscope (STM) junction is ramped into the field emission regime. Above the vacuum level, scanning tunneling spectroscopy (STS) shows a series of well defined resonances associated with the image states of the surface, which are Stark shifted due to the electric field provided by the STM tip. We present photon–energy resolved measurements that unambiguously show that the mechanism for light emission is the radiative decay of surface localized plasmons excited by the electrons that tunnel inelastically into the Stark shifted image states. Our work illustrates the effect of the tip radius both in the STS spectrum and the light emission maps by repeating the experiment with different tips.

Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS₂ and Metal/WSe₂ Heterojunctions.

Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are free of dangling bonds and therefore make more "ideal" Schottky junctions than bulk semiconductors, which produce Fermi energy pinning and recombination centers at the interface with bulk metals, inhibiting charge transfer. Here, we observe a more than 10× enhancement in the indirect band gap photoluminescence of transition metal dichalcogenides (TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS2 and WSe2 and subsequently recombine radiatively with minority holes in the TMDC. Since the conduction band at the K-point is 0.5 eV higher than at the Σ-point, a lower Schottky barrier exists for the Σ-point band, making electron injection more favorable. Also, the Σ band consists of the sulfur pz orbital, which overlaps more significantly with the electron wave functions in the metal. This enhancement in the indirect emission only occurs for thick flakes of MoS2 and WSe2 (≥100 nm) and is completely absent in monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures, which is consistent with the hot electron injection model.

Mechanisms of enhanced light emission in GaN-based light-emitting diodes by V-shaped micropits

We have investigated the light emission mechanisms of V-shaped, hexagonal-micropit-arranged, InGaN/GaN light-emitting diodes (HMA-LEDs). By near-field scanning optical microscopy and the confocal scanning electroluminescence microscopy, we found that enhanced light output of HMA-LED was significantly contributed by not only improved light-extraction efficiency but also locally improved crystalline quality near the micropit. Etch pit density and cross-sectional transmission electron microscope image indicate that threading dislocation (TD) bending lead to decrease the TD density near the micropit. Furthermore, partially linearly polarized light from the inclined facets of the HMA-LEDs was observed.

Solid State Incandescent Light Emitting Device Made of WOx Embedded Zr-Doped HfO2 High-k Stack on Si

The phenomenon of white light emission from the WOx embedded Zr-doped HfO2 high-k dielectric MOS device has been investigated. The embedding of the WOx layer into the high-k stack increased the breakdown voltage due to the larger physical thickness. Light was emitted from many discrete nano dots due to thermal excitation of conductive paths formed from the dielectric breakdown process. The inclusion of the W component in the conductive path enhanced the emission of the red to IR portion of the light. The light intensity corresponds to the current density, which increases with the diving voltage. Judged from the light emission mechanism and optical properties, the light emission phenomenon is close to the blackbody emission. This kind of device can be applied to many products to take the advantage of its unique optical and thermal characteristics as well as the IC compatible fabrication process.

Light emission mechanism of mixed host organic light-emitting diodes

Light emission mechanism of organic light-emitting diodes with a mixed host emitting layer was studied using an exciplex type mixed host and an exciplex free mixed host. Monitoring of the current density and luminance of the two type mixed host devices revealed that the light emission process of the exciplex type mixed host was dominated by energy transfer, while the light emission of the exciplex free mixed host was controlled by charge trapping. Mixed host composition was also critical to the light emission mechanism, and the contribution of the energy transfer process was maximized at 50:50 mixed host composition. Therefore, it was possible to manage the light emission process of the mixed host devices by managing the mixed host composition.

Effect of Stepwise Doping on Lifetime and Efficiency of Blue and White Phosphorescent Organic Light Emitting Diodes.

We investigated a light emission mechanism of blue phosphorescent organic light emitting diodes (PHOLEDs), using a stepwise doping profile of 2, 8, and 14 wt.% within the emitting layer (EML). We fabricated several blue PHOLEDs with phosphorescent blue emitter iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate doped in N,N'-dicarbazolyl-3,5-benzene as a p-type host material. A blue PHOLED with the highest doping concentration as part of the EML close to an electron transporting layer showed a maximum luminous efficiency of 20.74 cd/A, and a maximum external quantum efficiency of 10.52%. This can be explained by effective electron injection through a highly doped EML side. Additionally, a white OLED based on the doping profile was fabricated with two thin red EMLs within a blue EML maintaining a thickness of 30 nm for the entire EML. Keywords: Blue Phosphorescent Organic Light Emitting Diodes, Stepwise Doping Structure, Charge Trapping Effect.

From Lighting to Photoprotection: Fundamentals and Applications of Rare Earth Materials

Light and rare earths (RE) have a long lived relationship that dates from the discoveries of these elements in the nineteenth century. Since then, the increasing comprehension of their spectroscopic properties conducted to a wide and alluring literature about light absorption and emission by this particular and fascinating group. For more than 50 years, RE-containing solid-state optical materials have undoubtfully been an important subject for the development of more efficient lighting, visualization, communication and health everyday applications. Therefore, this review introduces the spectroscopic properties of rare earth materials and their applications, with a brief discussion of the main mechanisms of light absorption and emission from a 4f elements inorganic physical chemistry perspective. Special attention is devoted to the use of these elements in systems concerning photoprotection and to the fundamentals of visible light generation for lighting and visualization.

Efficient electrochemiluminescence of a readily accessible boron difluoride formazanate dye.

Electrochemiluminescence (ECL) of a boron difluoride formazanate dye was investigated in the presence of tri-n-propylamine as a reductive co-reactant. The ECL mechanism was studied using ECL-voltage curves, spooling ECL, and accumulative ECL spectroscopy. The ECL occurs at 724 nm by three distinct, voltage-dependent mechanisms of light emission.