Light Emitters Research Articles

Influence of dynamic morphological modifications of atom probe specimens on the intensity of their photoluminescence spectra

The photoluminescence intensity of a light emitter embedded in an atom probe needle-shaped specimen varies with the morphological evolution of the latter during field evaporation. Light absorption and emission patterns within such an evolving system were calculated considering the increase in the reflectivity induced by the high electrostatic field present at the apex surface. A good agreement is obtained between the experimental and calculated photoluminescence intensity as a function of the evaporation progress. These methods could be applied to more general situations in which the properties of nanoscale objects are modulated by surface chemistry or morphology changes.

Halide Perovskites Films for Ionizing Radiation Detection: An Overview of Novel Solid-State Devices

Halide perovskites are a novel class of semiconductors that have attracted great interest in recent decades due to their peculiar properties of interest for optoelectronics. In fact, their use ranges from the field of sensors and light emitters to ionizing radiation detectors. Since 2015, ionizing radiation detectors exploiting perovskite films as active media have been developed. Recently, it has also been demonstrated that such devices can be suitable for medical and diagnostic applications. This review collects most of the recent and innovative publications regarding solid-state devices for the detection of X-rays, neutrons, and protons based on perovskite thin and thick films in order to show that this type of material can be used to design a new generation of devices and sensors. Thin and thick films of halide perovskites are indeed excellent candidates for low-cost and large-area device applications, where the film morphology allows the implementation on flexible devices, which is a cutting-edge topic in the sensor sector.

Multi‐spectral flare designs using both red‐ and green‐light emitters: Progress towards wavelength selectable signals

AbstractPyrotechnic flares are used extensively, with each color relating to specific information such as landing zone, enemy location, medical evacuation, or submarine location. While all current military flares are restricted to a single illumination color, progress towards a wavelength selectable signal and/or multispectral signal is of interest. In this effort, we use additive manufacturing, via direct ink write approach, to print three unique configurations with green/red light emitter compositions (GLE/RLE). Varying the infill design provides the opportunity to fire either of the base colors individually (red‐ and/or green‐light illuminant) or to fire them simultaneously to achieve a multispectral result. In this work, architecture is shown to play a critical role, and when the GLE composition is approx. 70 wt% of the overall, then a reasonable multi‐spectral result is achieved. While early, this work demonstrates that there is significant promise to optimize wavelength selectable signals via architecture achievable with additive manufacturing.

Radiative pumping of exciton-polaritons in 2D hybrid perovskites

In addition to their attractive technological applications in photovoltaics and light emitters, the perovskite family of semiconductors has recently emerged as an excellent excitonic material for fundamental studies. Specifically, the 2D hybrid organic-inorganic perovskite (HOIP) offers the added advantage of room temperature investigations owing to their large exciton binding energy. In this work, we strongly couple excitons in 2D HOIP crystals to planar microcavity photons sustaining exciton-polaritons under ambient conditions resulting in a Rabi splitting of 290 meV. Dark excitons directly pump the polariton branch along its dispersion in resonance with the Stokes shifted emission state (radiative pumping), creating a high density of polaritons at higher in-plane momentum (k||). We further probe the nonlinear polariton dispersion dynamics at varying input laser fluence, which indicates efficient polariton-polariton scattering and decay to k|| = 0 from higher k||. The observation of Stokes shift-assisted energy exchange of dark states with lower polaritons coupled with evidence of efficient polariton-polariton scattering makes 2D HOIPs an attractive platform to study exciton-polariton many-body physics and Bose-Einstein like condensation (BEC) at room temperature.

Localization-enhanced moiré exciton in twisted transition metal dichalcogenide heterotrilayer superlattices

The stacking of twisted two-dimensional (2D) layered materials has led to the creation of moiré superlattices, which have become a new platform for the study of quantum optics. The strong coupling of moiré superlattices can result in flat minibands that boost electronic interactions and generate interesting strongly correlated states, including unconventional superconductivity, Mott insulating states, and moiré excitons. However, the impact of adjusting and localizing moiré excitons in Van der Waals heterostructures has yet to be explored experimentally. Here, we present experimental evidence of the localization-enhanced moiré excitons in the twisted WSe2/WS2/WSe2 heterotrilayer with type-II band alignments. At low temperatures, we observed multiple excitons splitting in the twisted WSe2/WS2/WSe2 heterotrilayer, which is manifested as multiple sharp emission lines, in stark contrast to the moiré excitonic behavior of the twisted WSe2/WS2 heterobilayer (which has a linewidth 4 times wider). This is due to the enhancement of the two moiré potentials in the twisted heterotrilayer, enabling highly localized moiré excitons at the interface. The confinement effect of moiré potential on moiré excitons is further demonstrated by changes in temperature, laser power, and valley polarization. Our findings offer a new approach for localizing moiré excitons in twist-angle heterostructures, which has the potential for the development of coherent quantum light emitters.

SIMULATION AND ANALYSIS OF ELECTRO-OPTICAL CHARACTERISTICS OF ORGANIC COMPOUNDS IN ORGANIC LIGHT-EMITTING DIODES (OLEDs)

Organic light emitters (OLEDs) work according to the principles of electroluminescence. These OLEDs are commercially available and can be used in smartphone and television displays due to their low power consumption, flexibility and higher brightness than inorganic de-vices. The copolymer based on 3,4-ethylene dioxythiophene (EDOT) and poly(N-vinylcarbazole) (PVK) was synthesized using previously published procedures. The copolymer was synthesized by an oxidative copolymerization reaction, while the DFT/B3LYP/6-31G(d,p) density function theory method was used to perform quantum calculations. This paper presents the simulation results by SILVACO-TCAD simulation software of the PVK-PEDOT organic matrix with calcium as cathode and ITO as anode. The simulation is based on the distribution of the Langevin recombination model including the proposed structure, and the electrical and optical characteristics, such as current versus voltage, luminescence power, and current versus electric field for different thicknesses, and charge carrier densities of the emitting layer, as well as the I-V characteristics for different temperature values. The model presented here will be useful in the future for optimization of better electrical parameters.

Fabrication and chemical modification of carbon nanodots/monolayer hexagonal boron nitride/substrate heterostructures and their terahertz optoelectronic properties

Carbon nanodots (CNDs) can be potentially applied in optoelectronic devices such as full-color display and LED. In this study we fabricate, for the first time, CNDs/monolayer hexagonal boron nitride (ML-hBN)/sapphire heterostructures, where i) ML-hBN is compatible with both CNDs and sapphire due to the presence of the van der Waals (vdW) forces between ML-hBN and these materials and ii) the CNDs can be chemically modified by attaching chemical bonds and/or functional groups on the surface/edges of the carbon-cores of CNDs. The basic physical and chemical properties of CNDs and the heterostructures are characterized. We further apply terahertz time-domain spectroscopy to measure the optical conductivity of CNDs/ML-hBN vdW heterostructures and to determine optically the key physical parameters of these heterostructures. The temperature dependence of these parameters is examined. We find that the agglomeration of dried CNDs on sapphire substrate can be largely suppressed by the presence of ML-hBN and the electronic and optoelectronic properties of the heterostructure can be controlled and modulated by chemical modification on CNDs. The results obtained from this study can be helpful for the fabrication, integration and modulation of CND based vdW heterostructures for practical device applications.

Over Two-Fold Photoluminescence Enhancement from Single-Walled Carbon Nanotubes Induced by Oxygen Doping.

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) is a promising way to improve their photoluminescent (PL) brightness and thus make them applicable as a base material for infrared light emitters. We report as high as over two-fold enhancement of the SWCNT PL brightness by using oxygen doping via the UV photodissociation of hypochlorite ions. By analyzing the temporal evolution of the PL and Raman spectra of SWCNTs in the course of the doping process, we conclude that the enhancement of SWCNTs PL brightness depends on the homogeneity of induced quantum defects distribution over the SWCNT surface.

Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41° and 1.76 106 cm−1, respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.

Advances in scintillator screen technology for neutron imaging

Modern digital imaging detectors have been implemented with scintillator screens as the key component. They capture neutrons and emit light, which is registered by suitable converts into the digital signal, used for analyses.The two established detector systems are either based on highly-efficient sensitive cameras or on amorphous silicon flat panels, which are in direct contact with the scintillator screen.Because the neutron beam intensity is quite low (e.g. compared to synchrotron light), it is essential to have scintillator screens with highest light emission per neutron. This property defines the efficiency of the detection process and the needed acquisition time for a valid image.On the other hand, highest spatial resolution is demanded in order to see smallest feature of a structure under investigation.In order to distinguish small contrast variation in the image data, a possible highest signal-to-noise-ratio is required. These conditions cannot however be satisfied in only one unique device at the same time.Although the principle neutron absorbers for the neutron detection and the most powerful light emitters have been known for many years, there is still potential for further improvements and optimization. Even exotic neutron absorbers and new scintillation materials are under investigation for the improvement of the scintillator screens, used in the neutron imaging detectors.Our report will summarize the current status of the development and describe the performance of common, commercialized materials. On the other hand, we will give the outlook for further approaches and first results of running developments.

Strategies to Extend the Lifetime of Perovskite Downconversion Films for Display Applications.

Metal halide perovskite nanocrystals (PeNCs) have outstanding luminescent properties that are suitable for displays that have high color purity and high absorption coefficient; so they are evaluated for application as light emitters for organic light-emitting diodes, light-converters for downconversion displays, and future near-eye augmented reality/virtual reality displays. However, PeNCs are chemically vulnerable to heat, light, and moisture, and these weaknesses must be overcome before devices that use PeNCs can be commercialized. This review examines strategies to overcome the low stability of PeNCs and thereby permit the fabrication of stable downconversion films, and summarizes downconversion-type display applications and future prospects. First, methods to increase the chemical stability of PeNCs are examined. Second, methods to encapsulate PeNC downconversion films to increase their lifetime are reviewed. Third, methods to increase the long-term compatibility of resin with PeNCs, and finally, how to secure stability using fillers added to the resin are summarized. Fourth, the method to manufacture downconversion films and the procedure to evaluate their reliability for commercialization is then described. Finally, the prospects of a downconversion system that exploits the properties of PeNCs and can be employed to fabricate fine pixels for high-resolution displays and for near-eye augmented reality/virtual reality devices are explored.

Ultrabright Light Emission Properties of All-Inorganic and Hybrid Organic–Inorganic Copper(I) Halides

All-inorganic and hybrid copper(I) halides have recently emerged as candidate optical materials due to their extremely high light emission efficiencies, nontoxic and earth-abundant elemental compositions, and low-cost solution processability. Originally inspired and motivated by the research on the halide perovskites family, the development of copper(I) halide light emitters has been following its unique path. In this perspective, we discuss the distinct low-dimensional crystal structures of all-inorganic halides, which enable strong charge localization and formation of room-temperature self-trapped excitonic (STE) states, giving rise to largely Stokes-shifted light emission with near-unity quantum yields. Due to their unique electronic structures, all-inorganic copper halides are predominantly blue emitters with unusually weak tunability of their optical properties (e.g., halide substitution has a minimal impact on their photoluminescence properties). These shortcomings paved the way for the exploration of more robust and diverse families of hybrid organic–inorganic copper(I) halides, which are discussed next. Our discussions of crystal and electronic structures and optical properties of all-inorganic and hybrid Cu(I) halides are complemented by the reported uses of these materials in optoelectronic applications. Finally, we identify remaining fundamental questions, knowledge gaps, and practical challenges and outline several paths forward for addressing these issues, including through new materials discovery and in-depth studies of photophysics of Cu(I) halides and derivative materials.

Extended-SWIR GeSn LEDs with Reduced Footprint and Efficient Operation Power

Complementary metal oxide semiconductor-compatible short- and midwave infrared emitters are highly coveted for the monolithic integration of silicon-based photonic and electronic integrated circuits to serve a myriad of applications in sensing and communication. In this regard, the group IV germanium–tin (GeSn) material epitaxially grown on silicon (Si) emerges as a promising platform to implement tunable infrared light emitters. Indeed, upon increasing the Sn content, the bandgap of GeSn narrows and becomes direct, making this material system suitable for developing an efficient silicon-compatible emitter. With this perspective, microbridge PIN GeSn LEDs with a small footprint of 1520 μm2 are demonstrated, and their operation performance is investigated. The spectral analysis of the electroluminescence emission exhibits a peak at 2.31 μm, and it red-shifts slightly as the driving current increases. It is found that the microbridge LED operates at a dissipated power as low as 10.8 W at room temperature and just 3 W at 80 K. This demonstrated low operation power is comparable to that reported for LEDs having a significantly larger footprint reaching 106 μm2. The efficient thermal dissipation of the current design helped reduce the heat-induced optical losses, thus enhancing light emission. Further performance improvements are envisioned through thermal and optical simulations of the microbridge design. These simulations indicate that the use of GeSn-on-insulator substrate for developing a similar microbridge device is expected to improve the optical confinement toward the realization of electrically driven GeSn lasers.

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S0-S1 transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (BCPL) up to 66.5 M-1 cm-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (gabs) and luminescence dissymmetry (glum) factor.

Zero-Dimensional Hybrid Cuprous Halide of [BAPMA]Cu2Br5 as a Highly Efficient Light Emitter and an X-Ray Scintillator.

Lead halide perovskites have been explored as a new kind of promising X-ray with wide applications in radiation-associated fields, but low light yield and serious toxicity extremely restrict further applications. To address these issues, we herein demonstrated one new zero-dimensional (0D) organic-inorganic hybrid cuprous halide of [BAPMA]Cu2Br5 (BAPMA = N,N-Bis(3-aminopropyl) methylamine) containing discrete [Cu4Br10]6- tetramers as excellent lead-free scintillators. Upon UV light excitation, [BAPMA]Cu2Br5 displays highly efficient broadband yellowish-green light emission with one dominant peak at 526 nm, a large Stokes shift of 244 nm, and a high photoluminescent quantum yield of 53.40%. Significantly, this broadband light emission can also be excited by higher-energy X-ray as radioluminescence with a high scintillation light yield of 43,744 photons/MeV. The detection limit of 0.074 μGyair/s is also far less than the required value for regular medical diagnostics of 5.5 μGyair/s. The solution-assembled hybrid structure facilely enables the [BAPMA]Cu2Br5-based scintillation screen to display high-performance X-ray imaging with a spatial resolution of 15.79 lp/mm showcasing potential application in X-ray radiography. In brief, combined merits of low toxicity and cost, negligible self-absorption, a low detection limit, considerable light yield, and spatial resolution highlight the excellent scintillation performance of 0D hybrid cuprous halide.

Synthesis and investigation of the physicochemical properties of polymorphic 3C–SiC

Silicon carbide (SiC) nanopowders were synthesized under Ar atmosphere via the carbonization reaction method using silicon powders and expanded graphite as initial materials, and ferric nitrate as catalyst precursor. The effects of heat treatment temperature and the C/Si molar ratio on SiC products were studied. The microstructure, phase composition and chemical state of the products were analyzed by XRD, SEM, XPS, TEM and Raman. The photoluminescence (PL) properties of SiC nanopowders were tested. The results showed that the content of the 3C–SiC product is higher after heat treatment at 1400 °C for 3 h with a molar ratio of C/Si of 1:1. 3C–SiC exists in the form of particles and wires. The formation process of 3C–SiC can be explained by a two-step method, nucleation under catalysis and growth under Vapor-Solid (VS) mechanism. Two ultraviolet light emission peaks are observed at about 318 and 380 nm. The main PL spectra of 3C–SiC nanopowders exhibit larger blueshifts. The blueshift is attributed to the stacking fault, oxygen defects, morphology and quantum confinement effect. It has potential application prospects in ultraviolet detectors, optoelectronic devices and light emitters.

INFIERI 2021: Hands-on Lab – LiDAR

LiDAR is an acronym of “Light Detection and Ranging”, which is to measure the distance to an object (“Ranging”) by detecting the light being reflected (“Light Detection”). The "hands-on" lab is made of two sections: (1) overviewing the technology, the application, and the key devices of LiDAR, and (2) a laboratory where we operate a LiDAR setup, do calibration, and look into signals in the electronics. In the section (1), we learn the “basics” of the LiDAR. In the section (2), we learn “real stuff” by using a LiDAR setup. We use a simple desktop LiDAR setup developed by Hamamatsu photonics K.K. (HPK), adapted for the hands-on lab in collaboration with the author. The LiDAR setup, which is using “cutting-edge” solid-state devices, is a direct time-of-flight LiDAR equipped with a 905 nm laser diode and a 16-ch MPPC photon counting image sensor. The light emitter is a pulsed solid-state laser (PLD) of 905 nm infra-red light. The light receiver (a photon counting image sensor (PCI)) is a linear-array of 16 channel MPPC (a solid-state photomultiplier) with a readout ASIC. The setup is also equipped with a visible-light camera, with which we correlate the visual image with the feature of the LiDAR setup. In the LiDAR laboratory, we do “aiming” (i.e., calibrate) by detecting the infra-red laser light, and correlating visual imaging and LiDAR “ranging”. We get insight into the LiDAR setup by monitoring time-of-flight (TOF) and energy (pulse height with time-over-threshold technology (TOT)) signals in the electronics with an oscilloscope. Through the process, we learn the real stuff of LiDAR and the issues associated.

Self-Organized Germanium Quantum Dots/Si3N4 Enabling Monolithic Integration of Top Si3N4-Waveguided Microdisk Light Emitters and p-i-n Photodetectors for On-Chip Sensing

Using a coordinated combination of lithographic patterning and self-assembled growth, Ge spherical quantum dots (QDs) were controllably generated within host layers of Si3N4 as active medium for Si photonics. A significant fabrication advantage of our approach is the high-temperature thermal stability of Ge QDs that are formed by thermal oxidation of poly-SiGe lithographically patterned structures at 800 °C–900 °C, offering flexibility in the waveguide (WG)-material choices, co- design, and integration of Ge photonic devices. Our Ge QDs enable monolithic integration of microdisk light emitters and p-i-n photodetectors (PDs) with top-Si3N4 WG-coupled structures using standard Si processing. Low dark current of 0.3 mA/cm2 at 300 K and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.2\,\mu \text{A}$ </tex-math></inline-formula> /cm2 at 77 K in combination with 3-dB frequency of 12 GHz for Ge-QD PDs and low threshold power of 0.6 kW/cm2 for optically pumped Ge QD/SiN microdisks light emission evidence the high degree of crystallinity of our Ge QDs being an effective building block for 3-D SiN photonic integrated circuits.

On-the-Fly Nonadiabatic Dynamics Simulations of Single-Walled Carbon Nanotubes with Covalent Defects

Single-walled carbon nanotubes (SWCNTs) with covalent surface defects have been explored recently due to their promise for use in single-photon telecommunication emission and in spintronic applications. The all-atom dynamic evolution of electrostatically bound excitons (the primary electronic excitations) in these systems has only been loosely explored from a theoretical perspective due to the size limitations of these large systems (>500 atoms). In this work, we present computational modeling of nonradiative relaxation in a variety of SWCNT chiralities with single-defect functionalizations. Our excited-state dynamics modeling uses a trajectory surface hopping algorithm accounting for excitonic effects with a configuration interaction approach. We find a strong chirality and defect-composition dependence on the population relaxation (varying over 50-500 fs) between the primary nanotube band gap excitation E11 and the defect-associated, single-photon-emitting E11* state. These simulations give direct insight into the relaxation between the band-edge states and the localized excitonic state, in competition with dynamic trapping/detrapping processes observed in experiment. Engineering fast population decay into the quasi-two-level subsystem with weak coupling to higher-energy states increases the effectiveness and controllability of these quantum light emitters.

Er3+-doped Y4Al2O9 nanophosphors for advance display applications: Synthesis, crystal chemistry and down conversion photoluminescent investigation

A gel-combustion approach was positively used to synthesize a series of cool greenish light emitter Y4-xAl2O9:xEr3+ (x = 1–5 mol %) nanophosphors and their structural, vibrational, morphological and luminescent characteristics have been examined for display applications. Studies of X-ray diffraction and structural refinement show that synthesized nanophosphors are made up of a single phase with a monoclinical crystal structure having P21/c space group. Transmission electron microscope (TEM) image manifests occurrence of non-uniform agglomeration of particles in the dominion of nano region. Emission spectra entail strongest band located at 549 nm due to 4S3/2 → 4I15/2 transition of trivalent erbium ion, producing greenish light. Concentration quenching (CQ) was attributed to exchange interactions (quadruple-quadruple), with an optimum concentration at 3 mol %. The lifetime of 549 nm emission decreases with higher Er3+ addition. The CIE chromaticity coordinates of Y4-xAl2O9:xEr3+ (x = 1–5 mol %) nanosamples were found in the greenish area. These obtained results of Y4-xAl2O9:xEr3+ (x = 1–5 mol %) nanophosphors are supportive for the usage of prepared nanophosphors as green component in display fabrication and other lighting foundations.