Cathodoluminescence Emission Intensity Research Articles

Cathodoluminescence emission and electron energy loss absorption from a 2D transition metal dichalcogenide in van der Waals heterostructures

Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSe2 flakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO2/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiO2 make this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.

Photoluminescence, cathodoluminescence degradation and surface analysis of Gd2O3:Bi pulsed laser deposition thin films

Gd2-xO3:Bix = 0.003 thin films were successfully deposited on Si (100) substrates in vacuum and an oxygen atmosphere at different substrate temperatures using the pulsed laser deposition technique. The microstructure, surface topography, chemical composition analysis and luminescent properties of the samples were studied. The influence of prolonged electron beam exposure on the surface state, chemical and cathodoluminescence (CL) stability of the samples was investigated. The background atmosphere and substrate temperature were found to significantly affect the microstructure and photoluminescence (PL). According to scanning electron microscopy cross-sections, the thicknesses for the thin films were relatively constant around 100 nm. The CL emission intensity degradations in a vacuum and an oxygen atmosphere were checked synchronously with the Auger peak-to-peak heights using the same electron beam for both measurements. The effect of the electron bombardment on the surface state of the samples was studied by using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). All major elements (gadolinium and oxygen) were located, with additional carbon and chlorine that were removed during the early stages of electron bombardment. Generally, the CL intensity of the thin films with a blue-green emission was stable under electron bombardment, indicating that this phosphor may be appropriate for field emission displays applications.

Pressure effect on cathodoluminescence emission intensity recorded in metamorphosed detrital zircons of the Sanbagawa schists

AbstractThe cathodoluminescence (CL) emission and spectra analyses were conducted on detrital zircons in the Cretaceous Sanbagawa schists to describe changes in CL properties of zircons associated with metamorphism. In addition, high‐pressure (HP) and high‐temperature (HT) experiments were conducted on the non‐metamorphosed Miocene Ashizuri igneous zircons, and the CL emission intensities obtained from them were compared with those of the Sanbagawa zircons. As a result, as compared to low‐grade Sanbagawa zircons, the emission intensities of the high‐grade Sanbagawa zircons reduced. The emission intensities produced from the HP‐treated Ashizuri zircons, on the other hand, were twice as high as those generated from untreated zircons, but less than half of those generated from HT‐treated zircons. Combining our results with previous studies on the temperature influence on zircon CL properties, it indicates that increasing metamorphic pressure decreases the increase in the CL emission intensity of metamorphosed detrital zircons resulting from the increasing temperature. The pressure effect on the emission intensity of zircon during metamorphism might be larger than the temperature effect.

Correlation between deep-level defects and functional properties of β-(SnxGa1-x)2O3 on Si photodetectors

Heterogeneous integration of β-(SnxGa1−x)2O3 (TGO) UV-C photodetectors on silicon substrates by molecular beam epitaxy is demonstrated. Multimodal electron microscopy and spectroscopy techniques reveal a direct correlation between structural, compositional, and optical properties of TGO and the functional properties of the photodetectors. Wavelength dispersive x-ray spectroscopy results accurately determine Sn concentrations (x) in the region of 0.020, and room temperature cathodoluminescence (CL) hyperspectral imaging shows changes in the CL emission intensity in TGO compared with a Ga2O3 sample with no Sn. Alloying Ga2O3 with Sn is shown to quench the red emission and enhance the blue emission. The increase in blue emission corresponds to the rise in VGa-related deep acceptors responsible for the high gain observed in the TGO detectors. A Ga2O3 nucleation layer is shown to improve the TGO surface quality and give better device properties compared to TGO grown directly onto the Si substrate, including a higher specific detectivity on the order of 1012 Jones.

Luminescence properties and energy transfer mechanism of Eu3+ and Tm3+ Co-doped AlN thin films

Eu3+, Tm3+ co-doped AlN thin films have been prepared through ion implantation for the first time. The implanted samples were annealed at 1180 °C in N2 to recover implantation damages and activate the rare-earth ions. Luminescence properties were investigated by photoluminescence and cathodoluminescence spectra. Under 325 nm laser excitation, for both Eu3+-doped AlN and Eu3+, Tm3+ co-doped AlN, the sharp characteristic emission lines corresponding to intra-4fn-shell transitions of Eu3+ were observed, but no any emission peak associated to Tm3+ ion was observed. According to the cathodoluminescence spectra, both Eu3+ and Tm3+ ions can be excited efficiently under high-energy electron beam. For Eu3+, Tm3+ co-doped AlN, the cathodoluminescence emission intensity of Eu3+ and Tm3+ ions varies with the dose ratio of these two ions. A possible energy transfer mechanism between Tm3+ and Eu3+ is proposed. Through changing the dose ratio of Eu3+ respect to Tm3+ ions, the chromaticity coordinates and color temperatures could be effectively regulated.

Color tunable cathodoluminescence properties of RE2WO6:Ln3+ (RE, Ln = Er3+ and Tm3+) phosphor and its microscopic imaging

Highly crystalline RE2-xWO6:xLn3+ (RE, Ln = Er3+ and Tm3+, x = 0.02 to 0.50 mol.) phosphors were successfully synthesized by the solid-state reaction method. The cathodoluminescence (CL) properties of the RE2-xWO6:xLn3+ phosphors were studied in detail by changing the Ln3+ (Er, Tm) doping concentration, accelerating voltage and tuning the electron beam current. The CL spectra of the undoped Er2WO6 phosphor showed bright red emission due to the 4F9/2→4I15/2 (Er3+) transition, while Tm3+ doped Er2WO6 display the strong green emission due to 4S3/2→4I15/2 (Er3+) transitions. The Er3+ doped Tm2WO6 phosphor showed strong white light emission due to the combination of several bands in the CL spectra. The CL emission intensity of the phosphors linearly enhanced when the electron beam current was increased from 1.00 nA to 7.0 nA and no saturation emission was noted in this range. The Commission Internationale del’Eclairage chromaticity coordinates were measured for the RE2WO6:Ln3+ phosphors.

Strong red emission with excellent thermal stability in double-perovskite type Ba2GdSbO6:Eu3+ phosphors for potential field-emission displays

Abstract The novel strong red-emitting in double-perovskite type Ba2GdSbO6:Eu3+ phosphors was successfully demonstrated by the simple high-temperature solid-state reaction at 1400 °C. The crystal structure was investigated by the X-ray diffraction analysis and further studied via employing the General Structure Analysis System. The chemical purity, surface crystalline function, elemental mapping, energy dispersive X-ray spectroscopy and photoluminescence behaviors were analyzed in detail. The decay curve of the optimized sample was measured with the long lifetime value of 3.51 ms and its microenvironment among doping ions was further investigated by the Judd-Oflet theory. Furthermore, the thermal stability was evaluated, and its emission intensity still remained 79.27% at 483 K. Eventually, the cathodoluminescence (CL) emission spectra exhibited strong emission intensity under low accelerating voltages, suggesting that the red-emitting Ba2GdSbO6:Eu3+ phosphors with excellent thermal stability and strong CL emission intensity could be considered as a promising red-emitting material for field-emission displays.

Controlled synthesis of nonpolar GaInN/GaN multiple-quantum-shells on GaN nanowires by metal-organic chemical vapour deposition

Nonpolar GaInN/GaN multiple-quantum shells (MQSs) on nanowires (NWs) were investigated for high-efficiency light-emitting diodes (LEDs). The growth conditions of NWs were systematically optimized via a continuous growth mode in metal-organic chemical vapour deposition (MOCVD). The In incorporation rate on the m-planes decreased as the growth temperature elaborated, whereas the crystalline quality is improved. The cathodoluminescence (CL) results revealed that longer growth time of the GaInN well can induce additional In-rich droplets and degrade the emission properties of MQSs. The CL emission intensity and the peak wavelength increased as the number of MQS pairs increased from one to three pairs, which was attributed to the increased In incorporation as the diameter enhanced. The linearly enhanced CL emission intensity with barrier thickness was ascribed to the increase of the electron-hole states from the GaN barrier to the wells, resulting in a larger recombination probability. The scanning transmission electron microscopy (STEM) results demonstrated that a thicker barrier shell can suppress the formation of In-rich droplets. Overall, the feasibility of obtaining high-quality m-plane coaxial GaInN/GaN MQSs structures are promising for NW-based white and micro LEDs.

Zoned quartz phenocrysts in supercooled melt inclusions in granulites from continental collision orogens

AbstractFelsite inclusions (FIs) in granulite garnets from continental collision orogens are supercooled felsic melt inclusions. Not only do these represent non‐equilibrium textures, such as dendritic and spherulitic crystals of quartz and other minerals, but the porphyritic texture is that of hypabyssal and volcanic rocks. This paper presents representative zoned quartz phenocryst‐bearing FIs in various granulites from the Highland Complex in central Sri Lanka, the Lützow‐Holm Complex in East Antarctica, the Grenville Province in southeastern Canada, and the Kerala Khondalite Belt in southern India. Quartz phenocrysts show simple and definite cathodoluminescence (CL) zoning with euhedral bright cores and dark rims and overgrowths. Euhedral quartz never develops in subsolidus metamorphic rocks except for vein quartz, and is characteristic of growth in a melt phase. Groundmass quartz surrounding quartz phenocrysts within the FIs is always dark in CL, whereas matrix quartz outside garnet and single‐grain inclusion quartz in garnet are usually brighter than the cores of quartz phenocrysts. The close relationship between violet CL emission intensity and Ti content of quartz indicates different growth temperatures in harmony with the inferred crystallization sequence. The preservation of CL growth zoning in quartz phenocrysts and fine non‐equilibrium textures imply that the cooling rates of some granulites are 1 to 2 orders of magnitude faster than so far presumed.

Ethylene glycol-assisted ultrafast synthesis and luminescent properties of novel multifunctional EuSr2F7 and TbSr2F7 nanostructures for WLEDs, displays and anti-counterfeiting

In this paper, novel EuSr2F7 and TbSr2F7 nanostructures were successfully synthesized via an ultrafast wet-chemical route at room temperature. The elemental compositions, morphology, luminescent behaviors, thermal stability, decay curves and quantum yields of the as-prepared samples were investigated. The fabricated white light-emitting diodes (WLEDs) exhibited excellent color rendering index and correlated color temperature values. The security inks and soft polydimethylsiloxane films were prepared for potential anti-counterfeiting and flexible display applications. Eventually, under low accelerating voltage and filament current, the as-prepared samples showed strong cathodoluminescence emission intensity. The above results suggest that the novel EuSr2F7 and TbSr2F7 nanostructures with outstanding luminescent performances could be promising for field-emission displays, WLEDs, flexible display films, and anti-counterfeiting applications.

Photoluminescence, cathodoluminescence and thermal stability of Sm3+ -activated Sr3 La(VO4 )3 red-emitting phosphors.

A series of Sm3+ -activated Sr3 La(VO4 )3 phosphors were synthesized by a facile sol-gel method. X-ray diffraction patterns and photoluminescence (PL)/cathodoluminescence (CL) spectra as well as PL decay curves were employed to characterize the obtained samples. Upon 402 nm light excitation, the characteristic emissions of Sm3+ ions corresponding to 4 G5/2 →6 HJ transitions were observed in all the as-prepared products. The PL emission intensity was increased with increase in Sm3+ ion concentration, while concentration quenching occurred when the doping concentration was over 4 mol%. The non-radiative energy transfer mechanism for concentration quenching of Sm3+ ions was dominated by dipole-dipole interaction and the critical distance was around 21.59 Å. Furthermore, temperature-dependent PL emission spectra revealed that the obtained phosphors possessed good thermal stability with an activation energy of 0.19 eV. In addition, the CL spectra of the samples were almost the same as the PL spectra, and the CL emission intensity showed a tendency to increase with increase in accelerating voltage and filament current. These results suggest that the Sm3+ -activated Sr3 La(VO4 )3 phosphors with good color coordinates, high color purity and superior thermal stability may be a potential candidate for applications in white light-emitting diodes and field-emission displays as red-emitting phosphors.

Efficiency of Cathodoluminescence Emission by Nitrogen-Vacancy Color Centers in Nanodiamonds.

Correlated electron microscopy and cathodoluminescence (CL) imaging using functionalized nanoparticles is a promising nanoscale probe of biological structure and function. Nanodiamonds (NDs) that contain CL-emitting color centers are particularly well suited for such applications. The intensity of CL emission from NDs is determined by a combination of factors, including particle size, density of color centers, efficiency of energy deposition by electrons passing through the particle, and conversion efficiency from deposited energy to CL emission. This paper reports experiments and numerical simulations that investigate the relative importance of each of these factors in determining CL emission intensity from NDs containing nitrogen-vacancy (NV) color centers. In particular, it is found that CL can be detected from NV-doped NDs with dimensions as small as ≈40 nm, although CL emission decreases significantly for smaller NDs.

Upconversion emission and cathodoluminescence of Er3+-doped NaYbF4 nanoparticles for low-temperature thermometry and field emission displays

The Er3+-doped NaYbF4 nanoparticles were fabricated by a hydrothermal method. The green and red emissions located at around 525, 542 and 657 nm corresponding to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ions, respectively, were observed when pumped at 980 nm light. Furthermore, with the help of the fluorescence intensity ratio technique, the thermometric properties of as-prepared products from the thermally coupled 2H11/2 and 4S3/2 levels of Er3+ ions were studied by analyzing temperature-dependent upconversion (UC) emission spectra. The maximum sensitivity for the Er3+-doped NaYbF4 nanoparticles was found to be around 0.0043 K− 1 with a temperature range of 93–293 K. In addition, the cathodoluminescence (CL) spectrum of the synthesized nanoparticles was nearly the same as the UC emission spectrum and the CL emission intensity did not exhibit saturation with the increase of accelerating voltage and filament current.

Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors

Tb3+-doped BiOCl phosphors were synthesized by a facile solid-state reaction method. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and cathodoluminescence (CL) spectra, and PL thermal stability. Under 377 nm near-ultraviolet light excitation, all the samples showed the characteristic green emissions of Tb3+ ions owing to the 5D4 → 7FJ (J = 6, 5, 4, 3) transitions. The dependence of PL emission intensity on the Tb3+ doping concentration was investigated and the optimal doping concentration was found to be 9 mol%. With the help of theoretical calculation, the critical distance was determined to be about 10.58 Å and the concentration quenching was dominated by multipole-multipole interaction. Furthermore, thermal stability of these phosphors was investigated by measuring the temperature-dependent PL spectra. In addition, these Tb3+-doped BiOCl phosphors also exhibited strong CL green emissions, and the CL emission intensity did not show saturation with the increase of accelerating voltage and filament current. These results indicated that the Tb3+-doped BiOCl phosphors may have potential applications in white light-emitting diodes (LEDs) and field emission displays (FEDs) as green-emitting phosphors.

Diagenesis of echinoderm skeletons: Constraints on paleoseawater Mg/Ca reconstructions

One of the most profound environmental changes thought to be reflected in chemical composition of numerous geological archives is Mg/Ca ratio of the seawater, which has varied dramatically throughout the Phanerozoic. Echinoderms that today typically form high magnesium calcite skeletons are increasingly being utilized as a proxy for interpreting secular changes in seawater chemistry. However, accurate characterization of the diagenetic changes of their metastable high magnesium calcite skeletons is a prerequisite for assessing their original, major-element geochemical composition. Here we expand the existing models of diagenesis of echinoderm skeleton by integration of various analytical methods that up to now rarely have been used to assess the diagenetic changes of fossil echinoderms. We validated the preservation of a suite of differently preserved echinoderm ossicles, mostly crinoids, ranging in age from the Cambrian through Recent. In 13 of 99 fossil echinoderm ossicles we found well-preserved porous microstructure (stereom), non-luminescent behaviour or blotchy dark color in cathodoluminescence, and distinct nanostructural features (layered and nanocomposite structure). Moreover, in representatives of such preserved samples, distribution of sulphates associated with organic matter is identical to those in Recent echinoderms. Only such ossicles, despite of local micrometer-scale diagenetic changes, were herein considered well-preserved, retaining their original major-element skeletal composition. By contrast, majority of samples show transformation to the stable low magnesium calcite that leads to obliteration of the primary geochemical and micro/nanostructural features and is accompanied with increase in cathodoluminescence emission intensity. Using only well-preserved fossil echinoderm samples, we found purely random variation in Mg/Ca in echinoderm skeletons through the observed time series; any periodicities in echinoderm skeletal Mg/Ca ratio which might be related to the secular transitions in calcite and aragonite seas were not confirmed. These findings suggest that, in contrast to some groups of organisms with relatively weak control over their biomineralization (such as algae, sponges, and bryozoans), in which polymorph mineralogies consistently changed according to the seawater type, application of fossil echinoderms, in particular crinoids, to seawater Mg/Ca reconstructions is unreliable. These data emphasize a key-role of physiological factors (the so-called vital effects) in echinoderm biomineralization.

Low voltage cathode-luminescent properties of Zn co-doped Y2O3:Eu red phosphor

Zn co-doped and undoped Y2O3-Eu red phosphor was prepared by citric acid gel method and it was investigated to know its cathodoluminesecent properties at low voltage excitation (<2kVs). In this connection the cathodoluminescence (CL) spectra of Zn co-doped and undoped phosphors were taken and they showed various line emissions along with a sharp red emission at 611nm, irrespective of Zn concentration, due to 5D0–7F2 transitions from Eu luminescent center. It was found that the incorporation of Zn could significantly enhance the CL emission intensity of Eu at about 33% when compared with Zn undoped phosphor. It was also found the Zn co-doping could not alter the crystal structure whereas it could change the morphology of the particles. The promising results revealed the possible application of this phosphor materials as red phosphor for field emission displays [FEDs].

Upconversion emission and cathodoluminescence of Yb3+ ions activated BiOCl:Ho3+ phosphors

Upconversion (UC) emission and cathodoluminescence (CL) properties of Ho3+/Yb3+-codoped BiOCl phosphors were reported. Under 980nm light excitation, all the samples exhibited strong green and red UC emissions corresponding to the (5F4,5S2)→5I8 and 5F5→5I8 transitions of Ho3+ ions, respectively. Furthermore, the UC emission intensity as a function of Yb3+ ion concentration was investigated to determine an optimum doping concentration and the UC mechanism was found to be a two-photon process. Additionally, excellent CL properties were observed in Ho3+/Yb3+-codoped BiOCl phosphors and the CL emission intensity increased with increasing the accelerating voltage and filament current.

Upconversion emission, cathodoluminescence and temperature sensing behaviors of Yb3+ ions sensitized NaY(WO4)2:Er3+ phosphors

A series of Yb3+ ions sensitized NaY(WO4)2:Er3+ phosphors were synthesized through a solid-sate reaction method. The X-ray diffraction (XRD), upconversion (UC) emission and cathodoluminescence (CL) measurments were applied to characterize the as-prepared samples. Under the excitation of 980nm light, bright green UC emissions corresponding to (2H11/2,4S3/2)→4I15/2 transitions of Er3+ ions were observed and the UC emission intensities showed an upward trend with increasing the Yb3+ ion concentration, achieving its optimum value at 25mol%. Furthermore, the temperature sensing behavior based on the thermally coupled levels (2H11/2,4S3/2) of Er3+ ions was analyzed by a fluorescence intensity ratio technique. It was found that the obtained samples can be operated in a wide temperature range of 133–773K with a maximum sensitivity of approximately 0.0112K−1 at 515K. Ultimately, strong CL properties were observed in NaY(WO4)2:0.01Er3+/0.25Yb3+ phosphors and the CL emission intensity increased gradually with the increment of accelerating voltage and filament current.

Observations of exciton–surface plasmon polariton coupling and exciton–phonon coupling in InGaN/GaN quantum wells covered with Au, Ag, and Al films

The coupling of excitons to surface plasmon polaritons (SPPs) and longitudinal optical (LO) phonons in Au-, Ag-, and Al-coated InxGa1−xN/GaN multiple and single quantum wells (SQWs) was studied with time-resolved cathodoluminescence (CL) and CL wavelength imaging techniques. Excitons were generated in the metal-coated SQWs by injecting a pulsed high-energy electron beam through the thin metal films, which is found to be an ideal method of excitation for plasmonic quantum heterostructures and nanostructures which are opaque to laser/light excitation. The Purcell enhancement factor (Fp) at low temperatures was obtained by the direct measurement of changes in the carrier lifetime caused by the SQW exciton–SPP coupling. The deposition of thin films of Al, Ag, and Au on an InGaN/GaN QW enabled a comparison of exciton–SPP coupling for energy ranges in which the surface plasmon energy is greater than, approximately equal to, and less than the QW excitonic transition energy. We investigated the temperature dependence of the Huang–Rhys factors for exciton-to-LO phonon coupling for the metal-covered and bare samples. CL imaging and spectroscopy with variable excitation densities are used to examine the spatial correlations between CL emission intensity, carrier lifetime, QW excitonic emission energy, and the Huang–Rhys factor, all of which are strongly influenced by local fluctuations in the In composition and formation of InN-rich centers.

STEM‐CL investigations on the influence of stacking faults on the optical emission of cubic GaN epilayers and cubic GaN/AlN multi‐quantum wells

AbstractWe report the influence of {111} stacking faults on the cathodoluminescence (CL) emission characteristics of cubic GaN (c‐GaN) films and cubic GaN/AlN multi‐quantum wells. Transmission electron microscopy (TEM) measurements indicate that stacking faults (SFs) on the {111} planes are the predominant crystallographic defects in epitaxial films, which were grown on 3C‐SiC/Si (001) substrates by plasma‐assisted molecular beam epitaxy. The correlation of the SFs and the luminescence output is evidenced with a CL setup integrated in a scanning TEM (STEM). By comparing the STEM images and the simultaneously measured CL signals it is demonstrated that SFs in these films lead to a reduced CL emission intensity. Furthermore, the CL emission intensity is shown to increase with increasing film thickness and decreasing SF density. This correlation can be connected to the reduction of the full width at half maximum of X‐ray diffraction rocking curves with increasing film thickness of c‐GaN films. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)