Radiative Recombination Of Electrons Research Articles

Manipulation of Shallow-Trap States in Halide Double Perovskite Enables Real-Time Radiation Dosimetry.

Storage phosphors displaying defect emissions are indispensable in technologically advanced radiation dosimeters. The current dosimeter is limited to the passive detection mode, where ionizing radiation-induced deep-trap defects must be activated by external stimulation such as light or heat. Herein, we designed a new type of shallow-trap storage phosphor by controlling the dopant amounts of Ag+ and Bi3+ in the host lattice of Cs2NaInCl6. A distinct phenomenon of X-ray-induced emission (XIE) is observed for the first time in an intrinsically nonemissive perovskite. The intensity of XIE exhibits a quantitative relationship with the accumulated dose, enabling a real-time radiation dosimeter. Thermoluminescence and in situ X-ray photoelectron spectroscopy verify that the emission originates from the radiative recombination of electrons and holes associated with X-ray-induced traps. Theoretical calculations reveal the evolution process of Cl-Cl dimers serving as hole trap states. Analysis of temperature-dependent radioluminescence spectra provides evidence that the intrinsic electron-phonon interaction in 0.005 Ag+@ Cs2NaInCl6 is significantly reduced under X-ray irradiation. Moreover, 0.025 Bi3+@ Cs2NaInCl6 shows an elevated sensitivity to the accumulated dose with a broad response range from 0.08 to 45.05 Gy. This work discloses defect manipulation in halide double perovskites, giving rise to distinct shallow-trap storage phosphors that bridge traditional deep-trap storage phosphors and scintillators and enabling a brand-new type of material for real-time radiation dosimetry.

Electrically Driven Plasmons in Metal-Insulator-Semiconductor Tunnel Junctions: The Role of Silicon Amorphization.

We investigate electrically driven plasmon (EDP) emission in metal-insulator-semiconductor tunnel junctions. We find that amorphization of the silicon crystal at a narrow region near the junction due to the applied voltage plays a critical role in determining the nature of the emission. Furthermore, we suggest that the change in the properties of the insulating layer above a threshold voltage determines the EDP spatial properties, from being spatially uniform when the device is subjected to low voltages, to a spotty pattern peaking at high voltages. We emphasize the role of the high-energy emission as an unambiguous tool for distinguishing between EDP and radiative recombination of electrons and holes in the semiconductor.

Localized Excitonic Electroluminescence from Carbon Nanodots

Localized excitons are expected to achieve high-performance electroluminescence and have been widely investigated in GaN-based light-emitting diodes (LEDs). Although carbon nanodot (CD) based LEDs have been achieved with the radiative recombination of electrons and holes, localized excitonic electroluminescence has been not reported before. In this Letter, localized excitonic electroluminescent devices have been fabricated using fluorescent CDs as an active layer. The CDs show strong localized excitonic yellow emission with a fluorescence quantum yield of 76% and Stokes shift of 2.1 eV. The CD-based LEDs present a sub-bandgap turn-on voltage of 2.4 V and a maximum luminance of 60.2 cd m-2, which is the lowest driving voltage among the CD-based electroluminescent devices. Localized centers trap carriers effectively, resulting in sub-bandgap light emission. The current results manifest that localized excitons may furnish a promising approach to boost the development of CD-based LEDs.

FUV observations of variations in thermospheric composition and topside ionospheric density during the November 2004 magnetic superstorm

We revisited the November 2004 superstorm by analyzing TIMED/GUVI data. The 135.6 nm limb radiances at 520-km are mainly due to the O+ and electron radiative recombination and represent the daytime ionosphere density at the altitude. The 135.6 nm radiances clearly showed a signature of ionospheric equatorial arcs and their variations during the November 2004 magnetic superstorm. When an intense eastward Interplanetary Electric Field (IEF) occurred, the dayside equatorial arcs were enhanced and their latitude separation increased. The enhanced equatorial arcs were hemispherically symmetric or asymmetric in the region with non-depleted O/N2 or hemispherically asymmetric O/N2 depletion, respectively. When O/N2 depletion reached the magnetic equator, there was no observable enhancement in the equatorial arcs regardless the IEF conditions, indicating O/N2 condition significantly modulated the variations in storm-time equatorial arcs. GUVI observations also showed that a westward IEF and/or disturbance dynamo electric field could also suppress the dayside equatorial arcs.

Ionospheric and Thermospheric Contributions in TIMED/GUVI O 135.6 nm Radiances

AbstractWe developed an empirically determined, data‐driven method to extract the ionospheric and thermospheric contributions in the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (GUVI) 135.6 nm radiances on dayside to improve the O/N2 product. The technique assumes a reference dayside ratio of 130.4 and 135.6 nm radiances versus solar zenith angle (SZA) established from geomagnetically quiet observations and a fixed night side 130.4–135.6 nm radiance ratio of 1.2. The method effectively separates the ionospheric and thermospheric contributions in the observed 135.6 nm radiance. This not only improves the estimation of the thermospheric O/N2 ratio, (an important space weather parameter), by removing the ionospheric contribution in the GUVI 135.6 nm radiance data, but also provides the ionospheric signature in the 135.6 nm radiance. Comparison between GUVI ionospheric 135.6 nm radiances due to O+ and electron radiative recombination with coincident GPS TEC data shows a similar morphology of equatorial arcs, such as the arc latitude separation and nonmigrating tidal signature, confirming that the method is reliable. When there are no equatorial arcs, the ionospheric contribution is low, also consistent with low TEC data. This method can be applied to DMSP/SSUSI and other far ultraviolet data with simultaneous disk 130.4 and 135.6 nm measurements.

(Invited) The Electroluminescence of Graphene

Electroluminescence is the phenomenon by which a material emits light in response to the passage of an electric current. In solids, it is the prerogative of semiconductors and related organic materials, and it results from the radiative recombination of electrons and holes. Today, electroluminescent devices represent the majority of lighting and display devices and are a building block of the global information and telecommunication network.We report on two observations signaling the electroluminescence of high-mobility graphene field-effect transistors (FET). The semi-metallic nature of graphene a priori forbids electroluminescence: Indeed, in semiconductors, the bandgap energetically insulates the electrons from the holes. Such protection doesn’t exist in graphene which is gapless. Nonetheless, electroluminescence is possible, because (i) of the remarkable inefficiency of the non-radiative carrier relaxation in graphene, and (ii) thanks to an original carrier injection mechanism specific to 2D semimetals: the Zener-Klein (ZK) tunnel conductance. [1]Our first observation consists in the thermometry of graphene’s electrons with applied bias, see figure 1a. Using electronic noise thermometry, we observe that, after an initial warm up with the deposited Joule power, the electron gas cools down, starting at a doping-dependent threshold. The turning point is concomitant with the onset of electron-hole injection by Zener-Klein (ZK) tunneling. The cooling mechanism involves the radiative emission, due to the electron-hole recombination, in electromagnetic near-field modes of the hBN-graphene heterostructure. It is reminiscent of far-field electroluminescent cooling that can be observed in regular LEDs [2] except that it’s much more efficient (~10 mW vs. ~10 pW, i.e., 9 orders of magnitude larger). [3]Our second observation consists in the detection of the middle-wave infrared radiation (MIR) emitted by a high-mobility graphene FET operating in ambient conditions, see figure 1b. We observe a sizeable optical emission in the range 80–250 meV, starting at the ZK tunneling threshold bias. The spectral analysis of the emitted light indicates that it originates from the scattering of the hBN-graphene heterostructure near-fields, as observed in reference [4] by surface near-field optical microscopy. Bibliography [1] W. Yang et al, A graphene Zener-Klein transistor cooled by a hyperbolic substrate. Nat. Nanotechnol. 2018, Vol. 47, 13.[2] P. Santhanam et al, Room temperature thermo-electric pumping in mid-infrared light-emitting diodes. Appl. Phys. Lett. 2013, Vol. 103, 18, p. 183513.[3] E. Baudin et al, Hyperbolic Phonon Polariton Electroluminescence as an Electronic Cooling Pathway. Adv. Funct.. Mater. 2020, Vol. 30, 8, p. 1904783.[4] S. Dai et al, « Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial ». Nat. Nanotech. 2015, Vol. 10, 8, p. 682. Figure 1

Performance analysis of anomalous photocatalytic activity of Cr-doped TiO2 nanoparticles [Cr(x)TiO2(1−x)

We report the synthesis and characterisation of pristine and chromium (Cr) metal ion-doped titanium dioxide nanoparticles [Cr(x)TiO2(1−x)] to study the anomalous effect of Cr doping on the photocatalytic property of TiO2. The presence of dopants generates more number of recombination pairs and increases surface coverage sites which decreases photocatalytic activity. We study the structural morphology of the synthesised Cr(x)TiO2(1−x) samples using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy analysis. The effect of Cr3+ ions on the optical properties of TiO2 has been studied using various imaging and spectroscopic techniques. Further, the effect of doping of Cr on the photocatalytic activity of TiO2 has been analysed in detail. The concentration of Cr in TiO2 has been chosen as 0, 1, 5 and 10% by weight. It has been observed that the pristine TiO2 exhibits better photocatalytic activity as compared to Cr-doped TiO2 irrespective of the Cr concentration. This can be attributed to the fact that due to Cr doping in TiO2, the number of available adsorption sites for malachite green reduces which degrades its photocatalytic activity. It is also confirmed by photoluminescence (PL) and time-resolved photoluminescence spectroscopy. PL intensity increases, and lifetime decreases with increase in doping concentration. Radiative recombination of electron and hole pairs of Cr3+ in TiO2 degrades its photocatalytic activity. The degradation efficiency is found to be 96% in the case of pristine TiO2 which reduces to 12% when doped with x = 10% concentration of chromium. Therefore, it is observed that in comparison with Cr-doped TiO2, pristine TiO2 exhibits an improved photocatalytic activity which shows the anomalous effect of Cr doping on the photocatalytic property of TiO2.

Unconventional Thermally Activated Indirect to Direct Radiative Recombination of Electrons and Holes in Tin Disulfide Two-Dimensional van der Waals Material

Tin disulfide (SnS2) is a two-dimensional semiconducting van der Waals material with an indirect band gap. We measured the mobility and recombination dynamics of charge carriers as a function of te...

Electron radiative recombination with a hydrogen-like ion

We survey the results of a long-term study of the process of radiative recombination (RR). A rigorous theory of non-relativistic electron RR with a hydrogen-like ion is used to calculate the total cross section of the process, the effective radiation, the recombination rate coefficient, and the emission coefficient in a plasma with a Maxwellian electron distribution. The exact results are compared with the numerous known asymptotic and interpolation formulas. We propose interpolation formulas which ensure a uniform approximation of all mentioned quantities in a wide range of plasma temperatures.

Optical and Structural Properties of Fluorine Doped SnO2 on Si (100) for Photovoltaic Application

Photovoltaic material plays a vital role in the production of energy storage devices, more specifically in solar cell fabrications. In this work, ITO:F-doped materials were coated over the silicon substrate through spray pyrolysis technique. X-ray diffraction studies were conducted for porous silicon (PSi) coated with ITO:F structures formed at different current densities. This pore formation is evident from the broad peak at 69.9°, revealing an amorphous-like nature but at the same location where the single crystalline peak also is observed. These pores are explicitly shown in the SEM images in which very fine surface fragments are observed. At 20 mA/cm2, well-defined porous patterns that were uniformly distributed over the surface were observed. The microstructures observed via atomic force microscopy for these PSi coated with ITO:F structures are randomly aligned and almost evenly distributed over the entire surface of these nanorods, which are approximately 40 nm. Radiative recombination of electrons from a level in the conduction band or its subband to a level at an energy difference of greater than 1.7 eV in the valance band or its subband will emit visible light.

Polarization anisotropy of electroluminescence in indium antimonide

Mid-infrared electroluminescence spectra under impact ionization conditions in strong electric fields are studied experimentally in monocrystalline n-InSb. The emission band observed in electroluminescence spectra is associated with a radiative recombination of electrons and holes between the conduction and valence bands. Spectral measurements of the electroluminescence intensity for different light polarizations relative to the direction of the applied electric field reveal the polarization anisotropy of the electroluminescence intensity in InSb, which is caused by the anisotropy of the distribution function of nonequilibrium electrons as well as by the anisotropy of the angular dependence of the interband optical matrix element in momentum space.

Photocurrent and photoluminescence characteristics of AlGaAs/GaAs double-heterostructures with a pair of two-dimensional electron and hole channels

AlGaAs/GaAs/AlGaAs double-heterostructures having two-dimensional electron and hole channels at the respective interfaces are studied by measuring their photocurrent and photoluminescence characteristics. Under the weak photoexcitation, it is found that photo-generated electrons and holes are driven by a built-in electric field between the two channels and flow out mostly as a photocurrent to the respective electrodes, making the photoluminescence negligibly small. When the excitation reaches a certain level, some of photo-generated electrons and holes accumulate in each channel and weaken the built-in field, leading to an exponential increase in photoluminescence or the radiative recombination of electrons and holes. When the excitation gets strong, photo-generated carriers are lost mostly in the form of photoluminescence, resulting in the saturation of photocurrent. A theoretical model to explain these findings is presented. A possibility of using this type of study to clarify operating mechanisms of super-junction devices is suggested.

The quenching of silver nanoparticles photoluminescence by graphene oxide: spectroscopic and morphological investigations

Graphene derivatives attracted tremendous attention world-wide owing to their promising properties and potential applications and interest in studying the photoluminescence quenching in nanomaterials has constantly been on the increase in recent years. In this sense, the current study provides an investigation of this optical phenomenon in graphene oxide (GO) -silver nanoparticles (AgNPs) composite (Ag-NPs/GO). The structural and morphological characterizations showed remarkable interactions between the two nanomaterials which has an important effect on their optical properties. In this regard, our observations revealed the quenching photoluminescence of silver nanoparticles by GO sheets. Indeed, the excited electrons of silver were attracted by the charged sheets of the GO causing the radiative recombination of electron and hole and/or a lower electron–hole recombination rate resulting in PL quenching. Considering the efficient fluorescence quenching of GO on AgNPs, we believe that the photoinduced electron transfer from AgNPs to GO should play an important role in quenching related applications.

Efficient light-emitting diodes based on nanocrystalline perovskite in a dielectric polymer matrix.

Electroluminescence in light-emitting devices relies on the encounter and radiative recombination of electrons and holes in the emissive layer. In organometal halide perovskite light-emitting diodes, poor film formation creates electrical shunting paths, where injected charge carriers bypass the perovskite emitter, leading to a loss in electroluminescence yield. Here, we report a solution-processing method to block electrical shunts and thereby enhance electroluminescence quantum efficiency in perovskite devices. In this method, a blend of perovskite and a polyimide precursor dielectric (PIP) is solution-deposited to form perovskite nanocrystals in a thin-film matrix of PIP. The PIP forms a pinhole-free charge-blocking layer, while still allowing the embedded perovskite crystals to form electrical contact with the electron- and hole-injection layers. This modified structure reduces nonradiative current losses and improves quantum efficiency by 2 orders of magnitude, giving an external quantum efficiency of 1.2%. This simple technique provides an alternative route to circumvent film formation problems in perovskite optoelectronics and offers the possibility of flexible and high-performance light-emitting displays.

Large-Scale Synthesis of Highly Pure Novel Cadmium Semi-Spheres and Their Anomalous Optical Properties

ABSTRACTHighly pure and large-scale novel cadmium (Cd) semi-spheres were prepared via Chemical Vapor Deposi-tion (CVD). The growth process is described with multi-steps of Vapor-Liquid-Solid (VLS) and decompositionof CdS nanostructure in Ar(90%)+H 2 (10%) mixture gas stream at definite temperature and selected zone.In contrast to previous Cd nanostructures, our individual Cd semi-spheres show strong room-temperature pho-toluminescence (PL) at around 591 nm, which is detected for the first time. Its long lived ( > ∼1.5 ns) electronicstate by time-resolved PL supports the origin of strong emission, which is attributed to radiative recombina-tion of electrons in the s , p conduction band near the Fermi surface and the holes at the d -bands or localmicrostructures generated by UV-visible laser excitation. It is a plasmonic band as that in Au, Ag, Cu tuned bysizes. Strong surface enhanced Raman scattering (SERS) of pyridine species have been detected from this Cdsemi-sphere, as strong as the silver metals does for SERS agents. The above optical responses has strongrelation to their growth process and as formed microstructures in one sphere. It reflects that such Cd spheresmay be a much cheaper candidates for applications in LEDs and other optoelectronic devices, for example, theplasmonic enhanced solar cells.KEYWORDS: Chemical Vapor Deposition, Cadmium, Metals, Photoluminescence, Silver.

Analytical ETL/EML Layer Investigation of Blue OLEDs

Organic light emitting diodes (OLEDs) based display technologies are getting more and more important for smartphones, laptops and televisions. OLED devices consist of thin organic layer stacks between two electrodes. Located around an emission layer, where the radiative recombination of electrons and holes takes place, are layers for the transport and blocking of charge carriers. Thickness, composition and structure of the layers influence color, efficiency and lifetime of OLED devices. Characterization for process qualification and failure analysis of the OLED stacks is pivotal to improve these properties. The challenge in this material is to prepare the relevant area, obtain contrast to resolve the layers and not damage the layers even under high beam current analytical experimental conditions. In this paper the complete workflow from preparation of a sample from a working mobile phone to examination of critical organic LED layers is presented.

New Ternary Europium Aluminate Luminescent Nanoribbons for Advanced Photonics

AbstractDeveloping novel one‐dimensional (1D) luminescent nanostructures (e.g., nanowires and nanoribbons) is highly desired for enabling progress in nanophotonics and other emerging optical technologies. Previous studies on 1D luminescent nanostructures were mostly focused on elemental and binary semiconductor materials, the light emission of which originates from the radiative recombination of electrons and holes via either intrinsic states or extrinsic defect states. Herein, three kinds of ternary europium aluminate nanoribbons are reported that have localized Eu2+ luminescent centers and exhibit new compositions, new crystal lattice structures, and new luminescence properties and mechanisms. These three europium aluminate nanoribbons are: blue luminescent EuAl6O10 with a new composition and a new tetragonal lattice structure, green luminescent EuAl2O4 with a monoclinic lattice structure, and orange luminescent EuAl2O4 with a new hexagonal lattice structure and extremely large band width and Stokes shift of emission. These materials have promising applications as nanometer‐scale light generators and waveguides in nanophotonics and as light converting phosphors in warm white light‐emitting diodes.

MONTE CARLO CALCULATION FOR CATHODOLUMINESCENCE OF AlGaAs/GaAs NANOSTRUCTURE

The cathodoluminescence technique (CL) performed in the Scanning Electron Microscope (SEM) offers opto-electronic characterization of some nanostructures and it is a powerful tool for studying the compositional variation or band structure of three-dimensional microscale or nanoscale structures. The major problem of the CL technique is the difficulty of high spatial resolution of the low-dimensional structure. In the present paper we propose a simple Monte Carlo calculation model to describe the interaction of electron beam with Al x Ga 1-x As - GaAs nanostructure. This model takes into account the confinement phenomenon in the quantum well by an easy method. The influence of different parameters such as the thickness of barriers, Al mole fraction (x), and the diffusion length are studied. The carrier excess generated during the collision of the incident electron with the atoms of the material (random walk) is calculated taking into account the confinement phenomenon within the quantum well. The radiative recombination of electron–hole pairs is collected as a light (CL signal).

Investigation of Optical Effects in Silicon Quantum Dots by Using an Empirical Pseudopotential Method

A computer simulation using a pseudopotential approach has been carried out to investigate the band gap as a function of the size and the shape of small silicon (Si) dots having 3 to 44 atoms per dot with and without surface passivation. We used an empirical pseudo-potential Hamiltonian, a plane-wave basis expansion and a basic tetrahedral structure with undistorted local bonding configurations. In our simulation, the structures of the quantum dots were relaxed and optimized before and after passivation. We found that the gap increased more for an oxygenated surface than a hydrogenated one. Thus, both quantum confinement and surface passivation determined the optical and the electronic properties of Si quantum dots. Visible luminescence was probably due to radiative recombination of electrons and holes in the quantum-confined nanostructures. The effect of passivation of the surface dangling bonds by hydrogen and oxygen atoms and the role of surface states on the gap energy was also examined. We investigated the entire energy spectrum starting from the very low-lying ground state to the very high-lying excited states. The results for the sizes of the gap, the density of states, the oscillator strength and the absorption coefficient as functions of the size are presented. The importance of the confinement and the role of surface passivation on the optical effects are also discussed.

Parity nonconservation in the radiative recombination of electrons with heavy hydrogen-like ions

The parity nonconservation effect on the radiative recombination of electrons with heavy hydrogen-like ions is studied. Calculations are performed for the recombination into the 21S0 state of helium-like thorium and gadolinium, where, due to the near-degeneracy of the opposite-parity 21S0 and 23P0 states, the effect is strongly enhanced. Two scenarios for possible experiments are studied. In the first scenario, the electron beam is assumed to be fully polarized while the H-like ions are unpolarized, and the polarization of the emitted photons is not detected. In the second scenario, the linearly polarized photons are detected in an experiment with unpolarized electrons and ions. Corresponding calculations for the recombination into the 23P0 state are presented as well.