Showing papers on "Cathodoluminescence published in 2006"

Aligned AlN Nanorods with Multi-tipped Surfaces—Growth, Field-Emission, and Cathodoluminescence Properties

Abstract: Aluminum nitride, an important member of the group III nitrides with the highest bandgap of about 6.2 eV, has excellent thermal conductivity, good electrical resistance, low dielectric loss, high piezoelectric response, and ideal thermal expansion, matching that of silicon. The interest in field-emission (FE) applications of AlN materials has grown because they exhibit a negative electron affinity. Exhibiting a negative electron affinity means that electrons excited into the conduction band can be freely emitted into vacuum. In addition, high-current emission at a relatively low field is most attractive for FE applications. As a result, significant effort is being devoted to reducing the tip size and increasing the density of the emitting sites by using hierarchical nanostructures. Therefore, the synthesis of AlN nanostructures, such as nanowires, nanotubes, nanocones, nanotips, hierarchical comb-like structures, and nanobelts, with controlled high-aspect-ratio shapes and sizes is an important topic worthy of exploration. The promise that one-dimensional (1D) nanostructures may dramatically improve the desired properties for many applications has stimulated great enthusiasm. For example, FE properties of various AlN nanostructures have been investigated. The turn-on fields of various 1D aluminum nitride nanostructures have been measured, such as nanowires (8.8 Vlm), nanocones (12 Vlm), nanotips (3.1–4.7 V lm), and hierarchical comb-like structures (2.45–3.76 Vlm). On the other hand, reports on the luminescence properties of AlN nanostructures are scarce. The AlN nanocones have been observed to have an emission band centered at 481 nm, referred to as a deeplevel or trap-level state. In the present study, we report the growth of well-aligned AlN nanorods with hairy surfaces by a vapor–solid (VS) process. The well-aligned AlN nanorods with hairy surfaces reported here not only provide a new hierarchical nanostructure, but also serve as a promising candidate for FE emitters because of their low electron affinity and the geometry of the multiple-nanotip surfaces. Compared with previous reports on hierarchal growth of AlN nanostructures, in this communication we report a higher density of smaller nanotips (∼ 3–15 nm) that were radially grown on the surfaces of AlN nanorods. Each nanotip may serve as an ultrasmall emitter. In addition, growing well-aligned AlN nanorods on Si substrates is amenable to current technology for the fabrication of Sibased microelectronics devices. The subsequent characterization of their cathodoluminescence (CL) reveals that these hierarchical AlN nanostructures possess an intense emission peak, further suggesting potential applications in optoelectronic nanodevices. The structure of the as-grown products has been determined by X-ray diffraction (XRD). As shown in Figure 1, all of the diffraction peaks in the XRD pattern can be identified; they correspond to a hexagonal wurtzite-structured AlN crys-

Application of cathodoluminescence imaging to the study of sedimentary rocks

Abstract: Preface Acknowledgements 1. Introduction Part I. Principles and Instrumentation: 2. Cathodoluminescence and its causes 3. Instrumentation and techniques Part II. Applications: 4. Provenance interpretation 5. Cathodoluminescence characteristics of diagenetic minerals and fabrics in siliciclastic sedimentary rocks 6. Luminescence characteristics and diagenesis of carbonate sedimentary rocks 7. Miscellaneous applications of cathodoluminescence to sedimentary rocks References Index.

Anomalous blueshift in emission spectra of ZnO nanorods with sizes beyond quantum confinement regime

Abstract: Cathodoluminescence (CL) spectroscopy has been employed to study the electronic and optical properties of well-aligned ZnO nanorods with diameters ranging from 50to180nm. Single-nanorod CL studies reveal that the emission peak moves toward higher energy as the diameter of the ZnO nanorod decreases, despite that their sizes are far beyond the quantum confinement regime. Blueshift of several tens of meV in the CL peak of these nanorods has been observed. Moreover, this anomalous energy shift shows a linear relation with the inverse of the rod diameter. Possible existence of a surface resonance band is suggested and an empirical formula for this surface effect is proposed to explain the size dependence of the CL data.

Direct imaging of propagation and damping of near-resonance surface plasmon polaritons using cathodoluminescence spectroscopy

Abstract: Cathodoluminescence imaging spectroscopy is used to determine the propagation distance of surface plasmon polaritons near the surface plasmon resonance on both silver and gold films. Surface plasmon polaritons are generated by a focused (diameter of 5nm) electron beam spot in the metal and coupled out through a grating. By gradually varying the distance between the excitation spot and the grating the damping is probed. Propagation lengths as small as several hundred nanometers are probed, and an increase in propagation length is observed if the wavelength is increased above resonance. The measured data are compared with the calculated propagation lengths taking into account both absorption in the film and leakage radiation, and it is found that other loss mechanisms appear to be significant as well.

Luminescent properties of solution-grown ZnO nanorods

Abstract: The optical properties of solution-grown ZnO nanorods were investigated using photoluminescence and cathodoluminescence. The as-grown nanorods displayed a broad yellow-orange sub-band-gap luminescence and a small near-band-gap emission peak. The sub-band-gap luminescence can only be observed when exciting above band gap. Scanning cathodoluminescence experiments showed that the width of the sub-band-gap luminescence is not due to an ensemble effect. Upon reduction, the sub-band-gap luminescence disappeared and the near-band-gap emission increased. Compared to ZnO powders that are stoichiometric and oxygen deficient, we conclude that the yellow-orange sub-band-gap luminescence most likely arises from bulk defects that are associated with excess oxygen.

Application of Cathodoluminescence Imaging to the Study of Sedimentary Rocks: Introduction

Abstract: Preface Acknowledgements 1. Introduction Part I. Principles and Instrumentation: 2. Cathodoluminescence and its causes 3. Instrumentation and techniques Part II. Applications: 4. Provenance interpretation 5. Cathodoluminescence characteristics of diagenetic minerals and fabrics in siliciclastic sedimentary rocks 6. Luminescence characteristics and diagenesis of carbonate sedimentary rocks 7. Miscellaneous applications of cathodoluminescence to sedimentary rocks References Index.

Enhancement and patterning of ultraviolet emission in ZnO with an electron beam

Abstract: An intense enhancement of ultraviolet (UV) emission was observed in various kinds of ZnO samples that were prepared using a wet chemical method when they were under electron-beam irradiation. The UV emission can increase to more than two times its initial value, whereas the visible emission reduces to a negligible value. We suggest that this enhancement effect mainly results from electron-stimulated desorption of adsorbed water. Applying this effect, we have developed a simple technique of directly writing submicrometer UV emission patterns in ZnO with an electron beam without changing the material’s surface morphology.

Sol?gel synthesis and characterization of SiO2@CaWO4,SiO2@CaWO4:Eu3+/Tb3+ core?shell structured spherical particles

Abstract: Nanocrystalline CaWO4 and Eu3+ (Tb3+)-doped CaWO4 phosphor layers were coated on non-aggregated, monodisperse and spherical SiO2 particles by the Pechini sol?gel method, resulting in the formation of SiO2@CaWO4, SiO2@CaWO4:Eu3+/Tb3+ core?shell structured particles. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), photoluminescence (PL), low-voltage cathodoluminescence (CL), time-resolved PL spectra and lifetimes were used to characterize the core?shell structured materials. Both XRD and FT-IR indicate that CaWO4 layers have been successfully coated on the SiO2 particles, which can be further verified by the FESEM and TEM images. The PL and CL demonstrate that the SiO2@CaWO4 sample exhibits blue emission band with a maximum at 420?nm (lifetime = 12.8??s) originated from the WO42? groups, while SiO2@CaWO4:Eu3+ and SiO2@CaWO4:Tb3+ show additional red emission dominated by 614?nm (Eu3+:5D0?7F2 transition, lifetime = 1.04?ms) and green emission at 544?nm (Tb3+:5D4?7F5 transition, lifetime = 1.38?ms), respectively. The PL excitation, emission and time-resolved spectra suggest that there exists an energy transfer from WO42? to Eu3+ and Tb3+ in SiO2@CaWO4:Eu3+ and SiO2@CaWO4:Tb3+, respectively. The energy transfer from WO42? to Tb3+ in SiO2@CaWO4:Tb3+ is more efficient than that from WO42? to Eu3+ in SiO2@CaWO4:Eu3+.

Fabrication of ZnO microtubes with adjustable nanopores on the walls by the templating of butterfly wing scales

Abstract: ZnO microtubes with adjustable arrayed nanopores on the walls are prepared by using butterfly wing scales as natural biotemplates. Flat butterfly wing scales used as templates rolled into tubes during the calcinations; all the windows on the scales were reserved and formed porous walls. Furthermore the density of the pores on the walls is partially determined by experimental conditions, such as temperature and time of treatment. The nano/microstructures obtained have been investigated by means of x-ray diffraction and field emission scanning electron microscopy. The room temperature (T = 300 K) cathodoluminescence of these unique ZnO microtubes is also studied. The spectrum shows a sharp near band edge emission and a broad deep level emission, which are similar to those for other previously synthesized ZnO microtubes.

CdCl2 treatment, S diffusion, and recombination in polycrystalline CdTe

Abstract: Time-resolved photoluminescence measurements on glass∕SnO2∕CdTe and glass∕SnO2∕CdTe∕CdS structures indicate that the CdCl2 process, without any S present, significantly reduces recombination. However, S diffusion is required for lifetimes comparable to those observed in high-efficiency solar cells. Low-temperature photoluminescence, cathodoluminescence, and scanning electron images indicate how defect chemistry, grain-boundary passivation, and morphology are affected by S diffusion and the CdCl2 treatment.

Pulsed lateral epitaxial overgrowth of aluminum nitride on sapphire substrates

Abstract: The authors report on pulsed lateral epitaxial overgrowth of aluminum nitride films on basal plane sapphire substrates. This approach, at temperatures in excess of 1150°C, enhanced the adatom migration, thereby significantly increasing the lateral growth rates. This enabled a full coalescence in wing regions as wide as 4–10μm. Atomic force microscopy and cross-section transmission electron microscopy were used to establish the reduction of threading dislocations in the lateral growth. Cross-sectional monochromatic cathodoluminescence and photoluminescence measurements confirmed the improved optical properties of the laterally overgrown aluminum nitride films.

Single-crystalline AlZnO nanowires/nanotubes synthesized at low temperature

Abstract: Single-crystalline AlZnO nanomaterials were synthesized through a proposed alloy-evaporation deposition method at the low temperature of 550°C by thermal chemical vapor deposition. Transmission electron microscopy images show that AlZnO nanowires, or nanowire/nanotube junction structures, can be synthesized where the Al∕(Al+Zn) atomic ratio is determined to be about 2.5 and 12at.%, respectively, by electron energy loss spectrometry. Room-temperature cathodoluminescence measurements show that the AlZnO nanowires exhibit a strong ultraviolet emission, which shifts to a higher energy from 3.29to3.34eV due to Al incorporation.

Texture and electronic activity of grain boundaries in Cu(In,Ga)Se2 thin films

Abstract: The influence of different film textures on the electronic properties of polycrystalline Cu(In,Ga)Se2 absorbers is studied by measuring the laterally resolved optoelectronic properties of differently textured Cu(In,Ga)Se2 films with Kelvin probe force microscopy and cathodoluminescence. The grain boundaries in (112)- and (220/204)-textured films behave differently. The work-function profile measured with the Kelvin probe across a grain boundary in (112)-textured films shows a dip indicating positive charges at the grain boundaries. In panchromatic cathodoluminescence mappings in a transmission electron microscope, such grain boundaries appear dark, i.e. the strongly reduced luminescence indicates that the grain boundaries represent strong non-radiative recombination centers. In contrast, grain boundaries in (220/204)-textured films give rise to a dip or a step in the work function indicating slightly negative charge or neutrality. Cathodoluminescence is reduced at such grain boundaries, but less dramatically than in the (112)-textured case. However, when Na is present in the (220/204)-textured films, the grain boundaries are almost invisible in cathodoluminescence mappings. This strong passivating action of Na occurs only in the (220/204)-textured films, due to a particular grain-boundary population. In (112)-textured films and films without pronounced texture, this passivation effect is much less noticeable.

High spatial resolution picosecond cathodoluminescence of InGaN quantum wells

Abstract: The authors have studied InxGa1−xN∕GaN (x≈15%) quantum wells (QWs) using atomic force microscopy (AFM) and picosecond time resolved cathodoluminescence (pTRCL) measurements. They observed a contrast inversion between monochromatic CL maps corresponding to the high energy side (3.13eV) and the low energy side (3.07eV) of the QW luminescence peak. In perfect correlation with CL images, AFM images clearly show regions where the QW thickness almost decreases to zero. Pronounced spectral diffusion from high energy thinner regions to low energy thicker regions is observed in pTRCL, providing a possible explanation for the hindering of nonradiative recombination at dislocations.

Tunable photoluminescent and cathodoluminescent properties of ZnO and ZnO:Zn phosphors.

Abstract: ZnO and ZnO:Zn powder phosphors were prepared by the polyol-method followed by annealing in air and reducing gas, respectively. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectra (XPS), electron paramagnetic resonance (EPR), and photoluminescence (PL) and cathodoluminescence (CL) spectra, respectively. The results indicate that all samples are in agreement with the hexagonal structure of the ZnO phase and the particle sizes are in the range of 1−2 μm. The PL and CL spectra of ZnO powders annealed at 950 °C in air consist of a weak ultraviolet emission band (∼390 nm) and a broad emission band centered at about 527 nm, exhibiting yellow emission color to the naked eyes. When the sample was reduced at the temperatures from 500 to 1050 °C, the yellow emission decreased gradually and disappeared completely at 800 °C, whereas the ultraviolet emission band became the strongest. Above this temperature, the green emission (∼50...

Whispering-gallery-modelike-enhanced emission from ZnO nanodisk

Abstract: Hexagonal nanodisks of ZnO were fabricated by a solution process using ZnO nanoparticles and their cathodoluminescence characteristics were investigated. Monochromatic cathodoluminescence images showed that luminescence was spatially localized near the boundary of the nanodisk and spectral analysis in conjunction with the intensity profile consistently ascribed the spatial localization of luminescence to whispering-gallery-modelike-enhanced emission.

Straight and thin ZnO nanorods: hectogram-scale synthesis at low temperature and cathodoluminescence.

Abstract: A novel seed-assisted chemical reaction at 95 degrees C has been employed to synthesize uniform, straight, thin, and single-crystalline ZnO nanorods on a hectogram scale. The molar ratio of ZnO seed and zinc source plays a critical role in the preparation of thin ZnO nanorods. At a low molar ratio of ZnO seed and zinc source, javelin-like ZnO nanorods consisting of thin ZnO nanorods with a diameter of 100 nm and thick ZnO nanorods with a diameter of 200 nm have been obtained. In contrast, straight ZnO nanorods with a diameter of about 20 nm have been prepared. Dispersants such as poly(vinyl alcohol) act spatial obstructors to control the length of ZnO nanorods. The morphology, structure, and optical property of the ZnO nanostructures prepared under different conditions have been characterized by transmission electron microscopy, field emission scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and cathodoluminescence. The formation mechanisms for the synthesized nanostructures with different morphologies have been phenomenologically presented.

Multi-excitonic complexes in single InGaN quantum dots

Abstract: Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.

Prestrained effect on the emission properties of InGaN∕GaN quantum-well structures

Abstract: The authors demonstrate the spectral redshift of the quantum wells (QWs) designated for green emission into the orange range in a light-emitting diode by adding a violet-emitting QW at the bottom in metal-organic chemical vapor deposition An electroluminescence redshift of 53nm was obtained The cathodoluminescence spectra indicated that the long-wavelength QWs close to the violet one were strongly influenced by this added QW and mainly emitted the orange photons Those near the top were less affected This influence is supposed to originate from the prestrained effect in the barrier layer right above the violet QW Such a prestrained effect is expected to be more effective when the underlying QW is well shaped and the heterojunction strain is strong, like the case of the violet QW This effect is weak between the high-indium QWs, in which the formation of indium-rich clusters releases the strain

Growth and luminescence of elongated In2O3 micro- and nanostructures in thermally treated InN

Abstract: Indium oxide elongated micro- and nanostructures have been grown by thermal treatment of InN powder. Chains of nanopyramids connected by nanowires, forming a necklace-like structure, as well as cubes and arrow-like structures consisting of a long rod with a micron size pyramid on the top, grow at temperatures in the range 600–700°C in a catalyst free process. The structures have been characterized by scanning electron microscopy and cathodoluminescence.

Carrier recombination near threading dislocations in GaN epilayers by low voltage cathodoluminescence

Abstract: The authors present a low voltage cathodoluminescence (CL) study of as grown GaN and GaN:Si epilayers on sapphire. At 1kV they resolve individual threading dislocations on the sample surface at low temperature (5K), which appear as correlated dark spots. Analysis of CL intensity profiles across individual dislocation cores provides a direct measurement of the exciton and minority carrier diffusion lengths. Using this approach at 5K, an exciton diffusion length of 62±28nm was found for GaN:Si (∼3×1018cm−3) compared with 81±20nm for a nominally undoped n-type GaN (∼1×1016cm−3).

Charge contrast imaging of material growth and defects in environmental scanning electron miscroscopy—linking electron emission and cathodoluminescence

Abstract: An electron-based technique for the imaging of crystal defect distribution such as material growth histories in non- and poorly conductive materials has been identified in the variable pressure or environmental scanning electron microscope. Variations in lattice coherence at the meso-scale can be imaged in suitable materials. Termed charge contrast imaging (CCI), the technique provides images that correlate exactly with emitted light or cathodoluminescence in suitable materials. This correlation links cathodoluminescence and an electron emission. The specific operating conditions for observation of these images reflect a complex interaction between the electron beam, the positive ions generated by electron-gas interactions in the chamber, a biased detector, and the sample. The net result appears to be the suppression of all but very near surface electron emission from the sample, probably from of the order of a few nanometres. Consequently, CCI are also sensitive to very low levels of surface contaminants. Successful imaging of internal structures in a diverse range of materials indicate that the technique will become an important research tool.

Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots

Abstract: A systematic study of the impact of annealing on the electronic properties of single InAs∕GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within 8-band k∙p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position.

Low-temperature growth of uniform ZnO particles with controllable ellipsoidal morphologies and characteristic luminescence patterns.

Abstract: Uniform ellipsoidal ZnO particles have been synthesized in an aqueous solution in the presence of triethonalamine (TEA) mediated by sonication at the temperature below 80 degrees C. Scanning electron microscopy observations reveal that the ellipsoidal particles are highly uniform with a hexagonal cross-section. The morphologies of the ZnO particles can be systematically controlled from elongated rugby ball-like ellipsoidal to half-ellipsoidal by increasing the TEA concentration. Spatial resolved cathodoluminescence measurements at room temperature show that the ellipsoidal ZnO particles are intrinsically encoded with barcode-like ultraviolet luminescence patterns, which are of either a wide stripe or a narrow stripe perpendicular to the length at the core of the particles depending on the growth temperature. Moreover, the luminescence spectra of the ellipsoidal particles can be tuned by heat treatments at elevated temperatures, while maintaining the luminescence patterns. We believe that the well-defined uniform ellipsoidal ZnO particles embedded with unique luminescence characteristics hold great potential for use in bioengineering and photonics, such as biological labeling and optical probes.

Monodisperse spherical core-shell structured SiO2–CaTiO3:Pr3+ phosphors for field emission displays

Abstract: Nanocrystalline CaTiO3:Pr3+ phosphor layers were coated on nonaggregated, monodisperse, and spherical SiO2 particles by the sol-gel method, resulting in the formation of core-shell structured SiO2–CaTiO3:Pr3+ particles. X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence, cathodoluminescence spectra, as well as lifetimes were utilized to characterize the core-shell structured SiO2–CaTiO3:Pr3+ phosphor particles. The obtained core-shell structured phosphors consist of well dispersed submicron spherical particles with a narrow size distribution. The thickness of the CaTiO3:Pr3+ shell could be easily controlled by changing the number of deposition cycles (about 70nm for four deposition cycles). The core-shell SiO2–CaTiO3:Pr3+ particles show a strong red emission corresponding to D21–H43 (612nm) of Pr3+ under the excitation of ultraviolet (326nm) and low voltage electron beams (1–5kV). These particles may ...

Zinc oxide: hydrothermal growth of nano- and bulk crystals and their luminescent properties

Abstract: Hydrothermal growth, crystal chemistry and catodoluminescence of ZnO single crystals were studied. Zinc oxide was grown as bulk crystals so as crystalline films and powders with different form-factors (size and shape of the particles). The dependence of form-factor on the growth conditions was studied. Growth morphology and spectra of impulse cathodoluminescence (ICL) were analysed for the samples depending on growth conditions. Two emission bands were registered in ICL-spectra with band lengths ~385 and 600 nm. The shift of the band in visible part of spectrum (VIS-band) was found depending on KOH concentration in starting solutions.