FWHM Research Articles

High-efficiency top-emitting organic light-emitting diodes based on metal/ITO composite electrodes

High-performance top-emitting organic light-emitting diodes (TEOLEDs) integrating the metal/ITO composite anodes were developed. The results revealed that covering the surface of Al anode with ITO layer could effectively improve the charge injection efficiency and balance the hole–electron charge. The effect of the thickness of ITO on the performance of TEOLEDs with Al/ITO anodes was further studied. The TEOLEDs with Al/ITO (5 nm) anode showed optimized performance, with the current efficiency (CE) and external quantum efficiency (EQE) enhanced by 40.0% and 34.5% compared to that of the OELD with pure Al anode, and increased by 93.1% and 33.5% compared to that of bottom-emitting OLEDs. In addition, the full width at half maximum (FWHM) was narrowed to 36 nm due to the micro-cavity effect of the top-emitting structure and the turn-on voltage decreased to as low as 2.3 V owing to the efficient charge injection and well-matched energy level. In addition, TEOLEDs using Ag/ITO as anode exhibited a slow roll-off of CE and EQE and a narrower FWHM of 30 nm, greatly improving the color purity. The strategy is simple and can significantly improve the efficiency of the TEOLEDs, which promotes the applications of OLEDs in the fields of ultra-high-definition displays and micro-displays.

Multi-wavelength confocal displacement sensing using a highly dispersive flat-field concave grating.

A multi-wavelength confocal displacement sensor based on a flat-field concave grating (FFCG) was proposed and designed; the large dispersion and small volume of the FFCG make it an ideal candidate for replacing the complex dispersive lens group. The designed displacement sensor was calibrated by displacement meter, and the characteristics were measured. Consequently, for the proposed displacement sensor, the displacement range of 6.8mm was measured with the R-square linearity evaluation coefficient of 0.998, and the sensitivity preceded 17.1nm/mm. The resolution of the displacement sensor was characterized by 70µm, as well as a full width at half maximum (FWHM) fluctuating around 1.63nm, indicating high precision and accuracy in displacement measurement. Moreover, the stability and reliability of the sensor were verified within 20min, with no significant wavelength shifts, and gentle power fluctuations of 557.73 counts at 520nm and 563.67 counts at 545.05nm, respectively.

Laser-Induced Plasmas of Plutonium Dioxide in a Double-Walled Cell.

Plutonium research has been stifled by the significant number of administrative controls and safety procedures, space and instrumentation limitations in radiological gloveboxes, and the potential for personnel and equipment contamination. To address the limited number of spectroscopic studies in Pu-bearing compounds in the current scientific literature, this work presents the use of double-walled cells (DWCs) in "clean" buildings/laboratories as an alternative to research in radiological gloveboxes. This study reports the first laser-induced breakdown spectroscopy (LIBS) experiments of a PuO2 pellet contained within a DWC, where the formation of elemental (atomic and ionic) species as well as the evolution from elemental to molecular products (PuxOy) was measured. Raman spectroscopy was also used to characterize the surface of the ablated pellet and the particulates deposited on the window of the inner cell. The full width half-maximum of the T2g band enabled us to obtain an estimate of the temperature at the pellet surface after the ablation pulse and the particulates based on the crystal lattice disorder. Particulates deposited on the window of the DWC during laser ablation were characterized using scanning electron microscopy, where molten irregular particulates and spheroids were observed. This exciting research conducted in a DWC describes our initial attempts to incorporate LIBS in the arsenal of spectroscopic tools for nuclear forensics applications.

Phase Engineering Reinforced Energy Transfer for High-Performance Blue Perovskite Light-Emitting Diodes.

Layered metal-halide perovskites, a category of self-assembled quantum wells, are of paramount importance in emerging photonic sources, such as lasers and light-emitting diodes (LEDs). Despite high trap density in two-dimensional (2D)perovskites, efficient non-radiative energy funneling from wide- to narrow-bandgap components, sustained by the Förster resonance energy transfer (FRET) mechanism, contributes to efficient luminescence by light or electrical injection. Herein, it is demonstrated that bandgap extension of layered perovskites to the blue-emitting regime will cause sluggish and inefficient FRET, stemming from the tiny spectral overlap between different phases. Motivated by the importance of blue LEDs and inefficient energy transfer in materials with phase polydispersity, wide-bandgap quasi-2D perovskites with narrow phase distribution, improved crystallinity, and the pure crystal orientation perpendicular to the charge transport layer are developed. Based on this emitter, high-performance blue perovskite LEDs with improved electroluminescence (EL) external quantum efficiency (EQE) of 7.9% at 478nm, a narrow full width at half-maximum (FWHM) of 22nm and a more stable EL spectra are achieved. These results provide an important insight into spectrally stable and efficient blue emitters and EL devices based on perovskites.

Effect of growth temperature on the microstructure and properties of epitaxial MoS2 monolayers grown by metalorganic chemical vapor deposition

Metalorganic chemical vapor deposition (MOCVD) is a promising technique for wafer-scale synthesis of MoS2 monolayers for 2D field-effect transistors (2D-FETs) and related devices. Epitaxial growth of MoS2 on sapphire provides films that are crystallographically well-oriented but typically contain low-angle grain boundaries (e.g., mirror twins), voids, and other defects depending on growth conditions and substrate characteristics. In this study, we investigate microstructure, optical properties, and field-effect characteristics of wafer-scale MoS2 monolayers grown by MOCVD on c-plane sapphire over a narrow window of growth temperatures (900–1000 °C). The density of low-angle grain boundaries in the MoS2 monolayer was found to decrease dramatically from 50% areal coverage for films grown at 900 °C to 5% at 1000 °C. This decrease in low-angle grain boundary density is correlated with an increase in the room-temperature photoluminescence intensity of A excitons and a decrease in the full-width-half maximum (FWHM) of the Raman A1g peak, which are typically indicative of a general reduction in defects in MoS2. However, the best transport properties (e.g., mean field-effect mobility mFE = 17.3 cm2/V s) were obtained in MoS2 monolayers grown at an intermediate temperature of 950 °C. It was found that as the growth temperature increased, small regions bound by high-angle boundaries begin to appear within the monolayer and increase in areal coverage, from ∼2% at 900 °C to ∼5% at 950 °C to ∼10% at 1000 °C. The growth temperature of 950 °C, therefore, provides an intermediate condition where the combined effects of low-angle and high-angle boundaries are minimized. The results of this study provide guidance on MOCVD growth and characterization that can be used to further optimize the performance of MoS2 2D-FETs.

Predicting the Type of Narrow Bipolar Pulse: A Machine Learning Approach

Narrow Bipolar Pulses (NBP) depicts the electric field (E-Field) changes due to a Compact Intra-cloud Discharge (CID) and are of two types namely Positive NBP (PNBP) and Negative NBP (NNBP). In this study, 437 NBPs were statistically investigated using a dataset collected in Sri Lanka in 2013, 2015, 2016 and 2017. Seven independent variables (Pulse Duration (PD), Rise Time (RT), Slow Front Duration (SFD), Zero Crossing Time (ZCT), Full Width at Half Maximum (FWHM) and Ratio between the Initial and Overshoot Peak Amplitudes (RIOPA)) and one dependent variable (Type of NBP) were analyzed. Two machine learning classification models, the Random Forest (RF) model and the Binary Logistic Regression (BLR) model, were used to predict the dependent variable based on the independent variables. Two models were compared in terms accuracy (ACC), sensitivity (SE), specificity (SP), AUC (Area Under the Curve)-ROC (Receiver Operator Characteristic) curve and kappa statistic. RF model scored the highest in terms of ACC (0.91), specificity (0.94), Kappa statistics (0.82) and AUC (0.98). In conclusion, RF model had the best performance in predicting the type of NBP hence can be used as a suitable automated method to classify the type of NBP.

Two-step growth of β-Ga2O3 on c-plane sapphire using MOCVD for solar-blind photodetector

In this work, a two-step metal organic chemical vapor deposition (MOCVD) method was applied for growing β-Ga2O3 film on c-plane sapphire. Optimized buffer layer growth temperature (T B) was found at 700 °C and the β-Ga2O3 film with full width at half maximum (FWHM) of 0.66° was achieved. A metal−semiconductor−metal (MSM) solar-blind photodetector (PD) was fabricated based on the β-Ga2O3 film. Ultrahigh responsivity of 1422 A/W @ 254 nm and photo-to-dark current ratio (PDCR) of 106 at 10 V bias were obtained. The detectivity of 2.5 × 1015 Jones proved that the photodetector has outstanding performance in detecting weak signals. Moreover, the photodetector exhibited superior wavelength selectivity with rejection ratio (R 250 nm/R 400 nm) of 105. These results indicate that the two-step method is a promising approach for preparation of high-quality β-Ga2O3 films for high-performance solar-blind photodetectors.

A very thin MRPC developed for TOF-PET

PET (Positron Emission Tomography) is a medical molecular imaging modality employed in cancer diagnosis and monitoring. Multi-gap Resistive Plate Chambers (MRPCs) are utilized in Time of Flight (TOF)-PET due to their superior time resolution. Our research group has successfully developed a 4-chamber 32-gap MRPC prototype with 0.128 mm gap thickness. The measured time resolution for cosmic rays is 20 ps, while the time resolution for the time difference between 0.511 MeV photon pairs is 193 ps Full Width at Half Maximum (FWHM). The 23.6 mm thickness of the MRPC prototype will affect the time resolution. To investigate the influence of the number and total thickness of gas gaps on the time resolution, Geant4 software is used to simulate the interaction between particles (such as cosmic rays and photons) and MRPC detectors. Most of the gamma photons interact with the resistive plates in MRPCs. It is ejected mostly a Compton electron into one gas gap, causing the gas ionization. In this paper, two 8-gap MRPC prototypes with 0.128 mm gap thickness and 4.2 mm total thickness are fabricated and tested. The time resolution of 39 ps for cosmic rays and 166 ps FWHM for 0.511 MeV photons is obtained using the fast front-end amplifier and waveform acquisition system. So, a real time PET image can be obtained with a pair of MRPCs, and the detection efficiency for gamma can be improved by increasing the number of thin MRPCs.

Multiple ultra-narrow band-stop filters based on MIM plasmonic waveguide with nanoring cavities

In this work, multiple ultra-narrow band-stop filters based on metal–insulator–metal (MIM) plasmonic waveguide filters with high efficiency are designed and analyzed numerically. The relationship between incident radiation and transmission spectra is investigated between 0.45 μm and 1.5 μm in the electromagnetic spectrum by finite-difference time-domain (FDTD) method. The designed structures have a bus waveguide coupled with nanoring cavity resonators of different sizes. Minimum transmission is 1.3% at 622 nm. The full width at half maximum (FWHM) is 8.64 nm and the quality factor is obtained as 72.33 in this wavelength. The highest quality factor is 185.48 and the lowest FWHM is 4.2 nm at 779 nm. The designed waveguide-based filters can be used for integrated optical devices from visible to near-infrared regimes.

Resonant-type Luneburg lens for broadband low-frequency focusing

In this paper, a novel structural Luneburg lens with local resonators is proposed. This lens allows the realization of subwavelength focusing in low-frequency range. The lens is achieved by graded refractive index from the lens's centre to the outer surface. Numerical simulations are conducted to obtain data on wave propagation waveform, maximum displacement amplitude, and full width at half-maximum (FWHM) of the lens's focal region. The results show that a broadband frequency range can be achieved for subwavelength focusing. This provides a straightforward and adaptable method for designing the structural Luneburg lens for numerous applications.

Multi-zone taylor expansion method for broadband achromatic polarization-insensitive metalens design

Broadband achromatic polarization-insensitive metalens with large numerical aperture (NA), large diameter and high efficiency are in demand. Existing methods can dramatically improve one of the above performances at the expense of the others. In this work, the multi-zone Taylor expansion method (MZTEM) is proposed to reduce these trade-offs between different key figures of merit. In this method, a metalens is devided into odd ring and even ring regions, then the odd (even) ring metalens focus peak is shifted forward with tailored functions of even (odd) ring region, and finally the arrangement of the meta-units is optimized to get higher efficiency. Based on the MZTEM, an achromatic polarization-insensitive lithium niobate on insulator (LNOI) metalens is designed working in the visible from wavelength λ = 430 nm to 750 nm, which exhibits a focusing efficiency of 35%–63% and NA of 0.255. Besides, the full width at half maxima (FWHM) variation is less than 0.14 μm and the focusing efficiency variation is less than 2% within 0°–10° incidence angle.

Tunable emission of Ce3+ in (K, Rb)3GdSi2O7 phosphors toward broadband cyan luminescence

AbstractIn an effort to obtain a novel cyan‐emitting phosphor, the [K1‐xRbx]3GdSi2O7: yCe3+ materials (x = 0, 0.1, 0.2, 0.3, 0.4, y = 0, 0.005, 0.01, 0.02, 0.03, 0.04) are synthesized via a solid‐state reaction pathway, and their crystal structure and luminescence behaviors are studied systematically. The chemical substitution of Rb+ in the K+ sites can enlarge the K3GdSi2O7 host lattice and help to the blue‐shifting of emission band of Ce3+ ions due to the decreased crystal field splitting effect. It is also confirmed that the Ce3+ ions tend to enter [Gd1O6] and [Gd2O6] polyhedrons simultaneously. Consequently, the [K0.7Rb0.3]3GdSi2O7: Ce3+ phosphor yields a broadband cyan emission centered at 492 nm with the full width at half maximum (FWHM) of ∼115 nm upon 365 nm excitation. The optimal concentration of Ce3+ dopant is determined to be 0.01, and the concentration quenching effect can be attributed to the dipole−dipole interactions. The white light emitting diode (WLED) device fabricated by employing the discovered [K0.7Rb0.3]3GdSi2O7: 0.01Ce3+ phosphor displays a high color rendering index (Ra = 91.7) and a low correlated color temperature (CCT = 3749 K). This work may promote the development of cyan phosphors for near ultraviolet‐converted WLEDs with high performance.

Growth and Characterization of Yb: CALYGLO Crystal for Ultrashort Pulse Laser Applications

Yb:CALYGLO crystals with a dopant concentration of 5 at.% were successfully grown using the Czochralski method. The crystal samples were extensively studied to analyze their structure, room temperature and low temperature spectra, and laser properties. The highest absorption cross-section at 977 nm was calculated to be 1.83 × 10−20 cm2 for σ polarization and 5.32 × 10−20 cm2 for π polarization. Similarly, the emission cross-section was determined to be 1.38 × 10−20 cm2 at 980 nm for σ polarization and 2.28 × 10−20 cm2 at 981 nm for π polarization, with a full width at half maximum (FWHM) of 50.3 nm and 89.5 nm, respectively. The fluorescence lifetime of the 5 at.% Yb:CALYGLO crystal at 2F5/2 was measured to be 1.10 ms. Additionally, gain cross-sections were calculated for different β values. In the continuous laser experiment, the crystal demonstrated a laser output of 20.15 W at 1057 nm, with a slope efficiency of 53.3%. These experimental findings indicate that the lattice of Y3+ in the crystal is partially replaced by Lu3+ and Gd3+, resulting in a broader spectrum. Consequently, this crystal shows promising potential as a gain medium for ultrashort pulse laser crystals.

Structural and electrocaloric properties of Yttrium modified Bi0.35Na0.35Sr0.3TiO3 composite

This communication presents the effect of Yttrium and Strontium doping on structural and electrocaloric characteristics of Bismuth Sodium Titanate. Bi0.25Na0.35Sr0.3Y0.1TiO3 electroceramics was synthesized by traditional solid-state mixed-oxide technique. The crystallographic phase formation was confirmed using the X-ray diffraction technique. Rietveld refinement was carried out to extract the unit cell parameters. Average particle size was estimated from the full-width half maxima of the strong XRD peak using Debye-Scherrer’s equation. The temperature dependence of the maximum polarization obtained from P-E loops, the electrocaloric ability of the BNSYT is estimated by calculating the change in the entropy at a field of 6 kV/cm using the standard thermodynamics relation.

Improving the Resolution of Correlation Imaging via the Fluctuation Characteristics

The resolution is an important factor in evaluating image quality. In general, the resolution of correlation imaging is taken to the full width at half maximum (FWHM) of the point spread function (PSF) produced by the second-order correlation function. In this paper, we show that the resolution of correlation imaging can be improved by the fluctuation characteristic of the second-order correlation function. It is demonstrated both experimentally and theoretically that the resolution of the system can be drastically improved. We also prove that the FWHM of the PSF can be narrowed by 2n by extracting the n-order fluctuation information of the second-order correlation function.

Highly uniform microspheres of broadband near-infrared-emitting Ba(Hf1-xCrx)O3 perovskite phosphor

Chromium activated BaHfO3 (BHOC) perovskite phosphor with varying amount of Cr3+ ion (1–10 %) was prepared by hydrothermal synthesis and was characterized systematically using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), positron annhilation lifetime spectroscopy (PALS) and time resolved photoluminescence (TRPL). FESEM show the formation of highly uniform microsphere of BHOC with very smooth surface texture. XPS showed the stabilization of chromium in +3 oxidation states, formation of oxygen vacancies and improved crystallinity in 6 % doped in BHOC sample. Under 500 nm excitation, BHOC perovskite demonstrated a very broad near infrared (NIR) emission band (700–1050 nm) centered at 815 nm with a full width at half-maximum (FWHM) of ∼150 nm and a wide absorption band, suggesting its potential application as NIR phosphor. The fraction of positrons getting trapped in the defects and vacancy clusters decreased with increase in Cr3+ doping and showed minima at 6.0 % doping and this sample also showed highest NIR light emission.

Intense O + E + S-band emission from Pr3+-doped ZnF2-based glasses

Pr3+-doped ZnF2-based glasses were prepared by using a melt-quenching method in dry N2 atmosphere. Under the excitation of a 588 nm light emitting diode (LED), ultrabroadband emissions ranging from 1245 to 1640 nm were obtained from the Pr3+-doped ZnF2-based glasses, which originate from the transitions 1D2→1G4 (producing E + S-band emission) and 1G4→3H5 (producing O-band emission) of Pr3+. The shape of the emission spectra could be tailored by varying the concentration of Pr3+. Emission spectra with the maximum full width at half maximum (FWHM) of 215 nm (1289 nm-1504 nm, covering the O + E + S-band) was obtained in the ZnF2-based glass at a doping concentration of 5000 ppm. The effects of the phonon energy of the matrix on O + E + S-band emission were also investigated. Our results showed that Pr3+-doped ZnF2-based glasses with low phonon energy might be used for constructing O + E + S-band lasers and optical amplifiers.

Technical NEMA NU2-2018 Performance Assessment of Time-of-Flight-Integrated Digital PET-CT System.

Aim  The objective of this study includes the NEMA (National Electrical Manufacturer Association) NU2-2018 performance evaluation of the uMIvista PET-CT (positron emission tomography-computed tomography) system. Methods  The latest NEMA NU2-2018 guidelines have been followed for the evaluation of performance parameters of this PET-CT scanner: axial, tangential, and radial spatial resolution, sensitivity, counting losses, scatter, randomness, random and counting loss correction, image quality, time and energy resolution, image uniformity, and image registration alignment post installation of country first uMIvista PET-CT. Results  The measured NEMA sensitivity of the uMIvista PET scanner was 12.053 cps/kBq. The spatial resolutions of the PET were measured as tangential, radial, and transaxial spatial resolutions at 10 mm, with 3.01 mm, 2.95 mm, and 2.93 mm, respectively; at 100 mm, with 3.17 mm, 3.42 mm, and 3.05 mm, respectively; and at 200 mm, with 3.65 mm, 4.54 mm, and 3.17 mm, respectively, at full-width half-maximum (FWHM); while at full-width tenths-maximum (FWTM), the values at 10 mm were 5.79 mm, 5.57 mm, and 5.69 mm, respectively, and at 100 mm were 5.59 mm, 5.96 mm, and 5.91 mm, respectively. The measured time-of-flight (TOF) timing resolution was 302.294 ps and the measured energy resolution was 11.76% with FWHM and FWTM. Conclusion  The NEMA NU2-2018 performances of this TOF-integrated digital PET-CT system are extremely good in all parameters.

Polarization-independent wideband meta-material absorber based on resistor-loaded hexagonal ring resonators

A polarization-independent meta-material-based absorber with wideband response is presented in this study. The proposed absorber’s unit cell consists of a resistance-loaded metallic hexagonal ring and four rectangular metal strips placed inside the hexagonal ring patterned on the upper surface of a dielectric substrate. The absorber exhibits wideband absorption with absorptivity more than 90% in the frequency range 7.14 to 13.68 GHz. The Full-Width Half Maxima (FWHM) of the absorber is 8.8 GHz (5.2–14 GHz). The four-fold symmetrical structure of the absorber confirms polarization-insensitive behaviour. The absorber performance is also validated with fabrication and measurement. There is a fair degree of agreement between simulated and measured results. The novelty of the proposed absorber is having wideband absorption with high packing density due to its hexagonal shape. The metamaterial absorber finds its use in defence-related applications such as radar surveillance, stealth technology, terrestrial communications, and satellite communication.

Design and optimization of dielectric metalens with quasi-periodic arrays

Metalenses have attracted a great deal of research interests due to the property of light controllability. How to enhance the focusing effect of metalens has always been a key problem that many researchers have been working on. In this paper, we propose a novel sub-dimensional resonance-based optimization method for dielectric metalens with golden angle spiral arrays. The waveguiding effect is first applied to the initial design of the quasi-periodic metalens through the defined equivalent period. Then the genetic algorithm is improved for adjusting the sub-dimensional structure of the initial metalens. The optimization results show that the peak intensity of central hot spot increases by 32 % while keeping the full width at half maximum (FWHM) almost unchanged. At the same time, the field intensity of side lobes around the hot spot decreases, which means that a potential way for high-contrast imaging is provided.