Showing papers on "Cathodoluminescence published in 2021"

Optical Excitations with Electron Beams: Challenges and Opportunities

Abstract: Free electron beams such as those employed in electron microscopes have evolved into powerful tools to investigate photonic nanostructures with an unrivaled combination of spatial and spectral prec...

Optical coherence transfer mediated by free electrons

Abstract: We theoretically investigate the quantum-coherence properties of the cathodoluminescence (CL) emission produced by a temporally modulated electron beam. Specifically, we consider the quantum-optical correlations of CL produced by electrons that are previously shaped by a laser field. Our main prediction is the presence of phase correlations between the emitted CL field and the electron-modulating laser, even though the emission intensity and spectral profile are independent of the electron state. In addition, the coherence of the CL field extends to harmonics of the laser frequency. Since electron beams can be focused to below 1 A, their ability to transfer optical coherence could enable the ultra-precise excitation, manipulation, and spectrally resolved probing of nanoscale quantum systems.

Spontaneous and stimulated electron-photon interactions in nanoscale plasmonic near fields.

Abstract: The interplay between free electrons, light, and matter offers unique prospects for space, time, and energy resolved optical material characterization, structured light generation, and quantum information processing. Here, we study the nanoscale features of spontaneous and stimulated electron–photon interactions mediated by localized surface plasmon resonances at the tips of a gold nanostar using electron energy-loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL), and photon-induced near-field electron microscopy (PINEM). Supported by numerical electromagnetic boundary-element method (BEM) calculations, we show that the different coupling mechanisms probed by EELS, CL, and PINEM feature the same spatial dependence on the electric field distribution of the tip modes. However, the electron–photon interaction strength is found to vary with the incident electron velocity, as determined by the spatial Fourier transform of the electric near-field component parallel to the electron trajectory. For the tightly confined plasmonic tip resonances, our calculations suggest an optimum coupling velocity at electron energies as low as a few keV. Our results are discussed in the context of more complex geometries supporting multiple modes with spatial and spectral overlap. We provide fundamental insights into spontaneous and stimulated electron-light-matter interactions with key implications for research on (quantum) coherent optical phenomena at the nanoscale.

Modulation of Cathodoluminescence Emission by Interference with External Light.

Abstract: Spontaneous processes triggered in a sample by free electrons, such as cathodoluminescence, are commonly regarded and detected as stochastic events. Here, we supplement this picture by showing through first-principles theory that light and free-electron pulses can interfere when interacting with a nanostructure, giving rise to a modulation in the spectral distribution of the cathodoluminescence light emission that is strongly dependent on the electron wave function. Specifically, for a temporally focused electron, cathodoluminescence can be canceled upon illumination with a spectrally modulated dimmed laser that is phase-locked relative to the electron density profile. We illustrate this idea with realistic simulations under attainable conditions in currently available ultrafast electron microscopes. We further argue that the interference between excitations produced by light and free electrons enables the manipulation of the ultrafast materials response by combining the spectral and temporal selectivity of the light with the atomic resolution of electron beams.

Band gap measurements of monolayer h-BN and insights into carbon-related point defects

Abstract: Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is (6.8 ± 0.2) eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of (0.7 ± 0.2) eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level.

Optoelectronics and Nanophotonics of Vapor–Liquid–Solid Grown GaSe van der Waals Nanoribbons

Abstract: 2D/layered semiconductors are of interest for fundamental studies and for applications in optoelectronics and photonics. Work to date focused on extended crystals, produced by exfoliation or growth and investigated by diffraction-limited spectroscopy. Processes such as vapor-liquid-solid (VLS) growth carry potential for mass-producing nanostructured van der Waals semiconductors with exceptionally high crystal quality and optoelectronic/photonic properties at least on par with those of extended flakes. Here, we demonstrate the synthesis, structure, morphology, and optoelectronics/photonics of GaSe van der Waals nanoribbons obtained by Au- and Ag-catalyzed VLS growth. Although all GaSe ribbons are high-quality basal-plane oriented single crystals, those grown at lower temperatures stand out with their remarkably uniform morphology and low edge roughness. Photoluminescence spectroscopy shows intense, narrow light emission at the GaSe bandgap energy. Nanophotonic experiments demonstrate traveling waveguide modes at visible/near-infrared energies and illustrate approaches for locally exciting and probing such photonic modes by cathodoluminescence in transmission electron microscopy.

Tunable Optical Properties of Thin Films Controlled by the Interface Twist Angle.

Abstract: Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodo...

Cyan Broad-Band Emission Phosphor with Scandium Silicon Multiple-Ring Structure for White Light-Emitting Diodes and Field Emission Displays.

Abstract: Silicate phosphor KBaScSi2O7:Ce3+ (KBSS:Ce3+), as a novel material, has been prepared by the solid-state method in this study. The crystal structure, luminescent properties, thermal stability, and cathodoluminescence (CL) properties of this material were analyzed systematically. It concludes that the phosphor can emit cyan light with emission peak at 509 nm under n-UV light excitation (300-400 nm). By coating KBSS:Ce3+ with a red-emitting CaAlSiN3:Eu2+ on a n-UV (365 nm) light-emitting diode (LED) chip, the intense warm white light with high RA (83.4) and low CCT (3652) can be produced under a 350 mA forward bias current.In addition, the CL performance shows that KBSS:0.10Ce3+ has high saturation current and voltage and good color stability under low voltage conditions. All these results indicate that KBSS:Ce3+ phosphor will be very promising in LED and field emission display applications.

Origin of Red Emission in β‐Ga 2 O 3 Analyzed by Cathodoluminescence and Photoluminescence Spectroscopy

Abstract: The spectroscopic techniques of cathodoluminescence (CL) and photoluminescence (PL) are used to study the origin of red emission in β-Ga 2O 3 grown using the edge-defined film-fed grown (EFG) method and hydride vapor phase epitaxy. Room-temperature CL shows red emission peaks from samples doped with Fe, Sn, and Si and from unintentionally doped (UID) samples. Narrow emission lines around 690 nm are seen strongly in the Fe and UID samples. Temperature-dependent PL analysis of the two prominent red emission lines reveals properties similar to the R lines in sapphire for all samples but with different levels of emission intensities. These lines are attributed to Cr 3+ ionic transitions rather than Fe 3+, as reported previously. The most likely origin of the unintentional Cr incorporation is the source material used in the EFG method.

Nanoscale modification of WS$_2$ trion emission by its local electromagnetic environment

Abstract: Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties, including the generation of single photon emitters. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications, due to the large gap in spatial resolution between optical spectroscopy and nanometer resolved techniques, such as transmission electron microscopy or scanning tunneling microscopy. Here, we bridge this gap by obtaining nanometer resolved optical properties using electron spectroscopy, specifically electron energy loss spectroscopy (EELS) for absorption and cathodoluminescence (CL) for emission, which were directly correlated to chemical and structural information. In an h-BN/WS$_2$/h-BN heterostructure, we observe local modulation of the trion (X$^{-}$) emission due to tens of nanometer wide dielectric patches, while the exciton, X$_A$, does not follow the same modulation. Trion emission also increases in regions where charge accumulation occurs, close to the carbon film supporting the heterostructures. Finally, localized exciton emission (L) detection is not correlated to strain variations above 1 $\%$, suggesting point defects might be involved in their formations.

Luminescence, structure and insight on the inversion degree from normal to inverse spinel in a ZnAl(2−x)Fex3+O4 system

Abstract: ZnAl2O4 doped with various concentration (x) of Fe3+ were prepared using the Pechini synthesis. In the first set of samples, Fe3+ substituted Al3+ to create a system of the form ZnAl(2−x)Fex3+O4 (x = 0–0.08). To study the effect of charge compensation, Fe3+ substituted Zn2+ to create Zn(1−x)Fex3+Al2O4 (x = 0.05) system. The structure and particle morphology of the phosphors were studied using X-ray diffractometer (XRD) and field emission scanning electron spectroscopy, respectively. From the XRD data, the oxygen parameter and the inversion degrees were estimated. The elemental composition and electronic states of the phosphors were analyzed using X-ray photoelectron spectroscopy (XPS). XPS results showed that part of the Fe3+ were reduced to Fe2+ in the doped samples. Both photoluminescence and cathodoluminescence properties of the phosphors were also studied. Luminescence excitations using a xenon lamp and X-ray showed two broad emission bands located around 470 and 730 nm, and were ascribed to Fe ions occupying the tetrahedral (tet) and the octahedral (oct) sites in ZnAl2O4, respectively. A change in the lifetime of these two emission bands upon Fe doping further confirmed the existence of Fe in the tet and oct sites in the ZnAl(2−x)Fex3+O4 matrix.

A novel high efficiency and ultra-stable red emitting europium doped pyrophosphate phosphor for multifunctional applications

Abstract: The development of high quantum efficiency and stable red emitting phosphors with high color purity is an urgent need for multifunctional optical applications. In this work, we propose a nonlinear optical material (NLOM)-inspired methodology to synthesize a novel red emission pyrophosphate phosphor Rb2Ba3(P2O7)2:Eu3+. XRD Rietveld refinement and energy dispersive X-ray spectroscopy were then exploited to obtain information on the phase purity, crystal structure and chemical composition. Moreover, the optical bandgap of different Eu3+ doped Rb2Ba3(P2O7)2 phosphors was analyzed by diffuse reflectance spectroscopy and further confirmed by first principles calculation. Furthermore, the Eu3+ site occupation preference, the characteristic luminescence properties and luminescence kinetics as a function of Eu3+ doping concentration were investigated by employing the cryogenic spectra at 4 K, room temperature emission spectra and decay curves. The luminescence analysis indicated that Rb2Ba3(P2O7)2:Eu3+ could emit red emission light with high quantum efficiency (IQE ∼ 77.04%), high color purity (96.4%) and excellent thermal stability (87%@ 140 °C) under ultraviolet light excitation. In addition, the excellent stability of Rb2Ba3(P2O7)2:Eu3+ against external pressure, cathode-ray irradiation, water and acid–base was further proved by pressure-driven (from 0 to 20 GPa) luminescence analysis, cathodoluminescence analysis and emission photography after infiltrating in water, acid, and alkali. The current work indicates that Rb2Ba3(P2O7)2:Eu3+ exhibits unprecedented excellent luminescence properties compared to other recently discovered phosphate red phosphors, and can serve as a potential red phosphor candidate in multifunctional optical applications, like LEDs, FEDs, artistic appreciation or some applications under extreme conditions (high temperature, strong pressure, and infiltrating in water, acid, and alkali).

Cathodoluminescence of Ultrathin Twisted Ge1–xSnxS van der Waals Nanoribbon Waveguides

Abstract: Ultrathin van der Waals semiconductors have shown extraordinary optoelectronic and photonic properties. Propagating photonic modes make layered crystal waveguides attractive for photonic circuitry and for studying hybrid light-matter states. Accessing guided modes by conventional optics is challenging due to the limited spatial resolution and poor out-of-plane far-field coupling. Scanning near-field optical microscopy can overcome these issues and can characterize waveguide modes down to a resolution of tens of nanometers, albeit for planar samples or nanostructures with moderate height variations. Electron microscopy provides atomic-scale localization also for more complex geometries, and recent advances have extended the accessible excitations from interband transitions to phonons. Here, bottom-up synthesized layered semiconductor (Ge1-x Snx S) nanoribbons with an axial twist and deep subwavelength thickness are demonstrated as a platform for realizing waveguide modes, and cathodoluminescence spectroscopy is introduced as a tool to characterize them. Combined experiments and simulations show the excitation of guided modes by the electron beam and their efficient detection via photons emitted in the ribbon plane, which enables the measurement of key properties such as the evanescent field into the vacuum cladding with nanometer resolution. The results identify van der Waals waveguides operating in the infrared and highlight an electron-microscopy-based approach for probing complex-shaped nanophotonic structures.

Formation of wide-blocky calcite veins by extreme growth competition

Abstract: Liassic limestones on the coast of Somerset in the UK contain dense arrays of calcite microveins with a common, but poorly understood microstructure, characterized by laterally wide crystals that form bridges across the vein. We investigated the mechanisms of formation and evolution of these ‘wide-blocky’ vein microstructures using a combination of high-resolution analytical methods, including virtual petrography, optical cathodoluminescence and scanning electron microscopy techniques (e.g. energy-dispersive X-ray spectrometry, back-scattered electron imaging, cathodoluminescence and electron back-scattered diffraction), laboratory experiments and multiphase field modelling. Our results indicate that the studied veins formed in open, fluid-filled fractures, each in a single opening and sealing episode. As shown by the optical and electron back-scattered diffraction images, the vein crystals grew epitaxially on grains of the wall rock and we hypothesize that their growth rates differed depending on whether the crystals were on a wall rock grain substrate that fractured intergranularly (slow growth rates) or transgranularly (rapid growth rates). Our multiphase field models support this hypothesis, showing that wide, blocky crystals only form where there are significant differences in the growth rate and are dependent on the type of seed grain. These results provide strong evidence for extreme growth competition, a process that we propose controls vein-filling in many micritic carbonate reservoirs, as well as demonstrate that the characteristics of the fracture wall can affect the filling processes in syntaxial veins. Supplementary material: The description and images of the studied thin sections are available at https://doi.org/10.6084/m9.figshare.c.5172371. High-resolution optical microscopy mosaics (under plane-polarized- and crossed polarized light) of the thin section collection in PetroScan file format are available on request from the authors.

Effect of Ca2+ - Si4+ on Y3Al5O12:Ce ceramic phosphors for white laser-diodes lighting

Abstract: Y2.985-xCe0.015CaxAl5-xSixO12 (YAG:1.5%Ce+xCS) ceramic phosphors (CPs) were synthesized by a vacuum sintering method. The addition of CaCO3 and SiO2 significantly reduces the sintering temperature of CPs. The effects of Ca2+ - Si4+ on luminescent properties and microstructures of YAG:Ce CPs are discussed. The Ca2+ - Si4+ cannot be completely dissolved in the YAG:Ce CPs when x reaches to 0.2. Combining the results of XRD, photoluminescence, and cathodoluminescence spectra proved the existence of the Ca2Al2SiO7. The performances of the titled CPs in high-power white laser diodes lighting are characterized.

Site control of quantum emitters in gallium nitride by polarity

Abstract: Gallium nitride (GaN) is a promising platform for integrated nanophotonic circuitry due to highly versatile growth protocols for the material. With the discovery of quantum emitters hosted by its lattice, potential applications of GaN have expanded to quantum-based technologies, despite the fact that the atomic structures of the emitters are unknown. Thus, we investigate the nature of quantum emitters grown in various samples of differing growth orientations—namely, Ga-polar, N-polar, and a combination of the two in an alternating periodic pattern. We showcase the unique growth technique used to fabricate these samples and characterize the emitters that form as a result. Through measurements of photoluminescence, cathodoluminescence, and Raman spectroscopy, we observe consistent formation of quantum emitters within Ga-polar regions of the grown GaN, attributed to overall defectivity caused by the specific growth procedure used to synthesize Ga-polar GaN. Our findings shed light onto the origins of the quantum emitters and are used to demonstrate site-selective formation of the emitters in GaN.

Engineering nitrogen- and hydrogen-related defects in ZnO nanowires using thermal annealing

Abstract: The chemical bath deposition (CBD) of ZnO nanowires (NWs) is of high interest, but their formation occurs in a growth medium containing many impurities including carbon, nitrogen, and hydrogen, rendering the accurate determination of predominant crystal defects as highly debated. In addition to the typical interstitial hydrogen in bond-centered sites (HBC) and zinc vacancy-hydrogen (VZn−nH) complexes, we reveal that nitrogen-related defects play a significant role on the physical properties of unintentionally doped ZnO NWs. We show by density functional theory that the VZn−NO−H defect complex acts as a deep acceptor with a relatively low formation energy and exhibits a prominent Raman line at 3078cm–1 along with a red-orange emission energy of ∼1.82 eV in cathodoluminescence spectroscopy. The nature and concentration of the nitrogen- and hydrogen-related defects are found to be tunable using thermal annealing under oxygen atmosphere, but a rather complex, fine evolution including successive formation and dissociation processes is highlighted as a function of annealing temperature. ZnO NWs annealed at the moderate temperature of 300 °C specifically exhibit one of the smallest free charge carrier densities of 5.6×1017cm–3 along with a high mobility of ∼60 cm2/Vs following the analysis of longitudinal optical phonon-plasmon coupling. These findings report a comprehensive diagram showing the complex interplay of each nitrogen- and hydrogen-related defect during thermal annealing and its dependence on annealing temperature. They further reveal that the engineering of the nitrogen- and hydrogen-related defects as the major source of crystal defects in ZnO NWs grown by CBD is capital to precisely control their electronic structure properties governing their electrical and optical properties in any devices.

LPE growth of Tb3Al5O12:Ce single crystalline film converters for WLED application

Abstract: The possibility of development of an efficient phosphor converters (PC) for white LED (WLED) based on single crystalline films (SCF) of Ce3+ doped Tb3Al5O12 garnet (TbAG:Ce), grown using liquid-phase epitaxy method onto Y3Al5O12 (YAG) substrates, is evidenced for the first time in this work. The detail investigation of the structural properties of the TbAG:Ce SCF/YAG epitaxial structures was performed. High-resolution scanning transmission electron microscopy and composition analysis revealed an interface with high structural quality including the formation of a transition layer with a 5–7 nm thickness between TAG:Ce SCF and YAG substrate. The transition layer consisted of solid solutions between Tb3Al5O12:Ce and Y3Al5O12 garnets gradually changing from undoped YAG in substrate towards TbAG:Ce in the film and allowed to reduce the mismatch-stress. The absorption, cathodoluminescence, photoluminescence properties of TbAG:Ce SCFs were investigated as well. Furthermore, the dependence of photoconversion properties on the film thicknesses was studied to construct the prototype of efficient warm white LEDs. The change in the crystal thickness enabled tuning of the white light tons from cold white/daylight to neutral white. These results can be useful for the development of a novel generation of phosphor converters for white LEDs.

High-performance deterministic in situ electron-beam lithography enabled by cathodoluminescence spectroscopy

Abstract: The application of solid-state quantum emitters in real-world quantum information technologies requires precise nanofabrication platforms with high process yield. Self-assembled semiconductor quantum dots with excellent emission properties have proven to be among the best candidates to meet the needs of a number of novel quantum photonic devices. However, their spatial and spectral positions vary statistically on a scale that is far too large for their system integration via fixed lithography and inflexible processing schemes. We solve this severe problem by introducing a flexible and deterministic manufacturing scheme based on precise and convenient cathodoluminescence spectroscopy followed by high-resolution electron-beam lithography. The basics and application examples of this advanced in situ electron-beam lithography are described in this article. Although we focus here on quantum dots as photon emitters, this nanotechnology concept is very well suited for the fabrication of a variety of quantum nanophotonic devices based on quantum emitters that exhibit suitably strong cathodoluminescence signals.

Purcell effect of nitrogen-vacancy centers in nanodiamond coupled to propagating and localized surface plasmons revealed by photon-correlation cathodoluminescence

Abstract: We measured the second-order correlation function of the cathodoluminescence intensity and investigated the Purcell effect by comparing the lifetimes of quantum emitters with and without metal structure. The increase in the electromagnetic local density of state due to the coupling of a quantum emitter with a plasmonic structure causes a shortening of the emitter lifetime. Since the plasmon-enhanced electric field is confined well below the wavelength of light, the quantum emitter lifetime is changed in the nanoscale range. In this study, we combined cathodoluminescence in scanning (transmission) electron microscopy with Hanbury Brown--Twiss interferometry to measure the Purcell effect with nanometer and nanosecond resolutions. We used nitrogen-vacancy centers contained in nanodiamonds as the emitters and compared their lifetime in different environments: on a thin $\mathrm{Si}{\mathrm{O}}_{2}$ membrane, on a thick flat silver film, and embedded in a silver film. The lifetime reductions of nitrogen-vacancy centers were observed in the samples with silver. We evaluated the lifetime by analytical calculation and numerical simulations and revealed the Purcell effects of emitters coupled to the propagating and localized surface plasmons.

A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content

Abstract: With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11-22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x ≈ 0.57-0.85) and dopant concentration (3 1018-1 1019 cm-3) in various series of Al x Ga1-x N layers grown on (0001) and (11-22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0-5.4 eV as well as various deep impurity transition peaks in the range 2.7-4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.