Takashi Sekiguchi

Zn vacancy induced green luminescence on non-polar surfaces in ZnO nanostructures

Although generally ascribed to the presence of defects, an ultimate assignment of the different contributions to the emission spectrum in terms of surface states and deep levels in ZnO nanostructures is still lacking. In this work we unambiguously give first evidence that zinc vacancies at the (1010) nonpolar surfaces are responsible for the green luminescence of ZnO nanostructures. The result is obtained by performing an exhaustive comparison between spatially resolved cathodoluminescence spectroscopy and imaging and ab initio simulations. Our findings are crucial to control undesired recombinations in nanostructured devices.

Facile synthesis of vertically aligned hexagonal boron nitride nanosheets hybridized with graphitic domains

Motivated by the recent quest for producing novel two dimensional nanomaterials, we developed a facile synthetic method for growing boron nitride–carbon (BN–C) phase-separated composite nanosheet coatings on silicon/silicon dioxide (Si/SiO2) substrates. The coatings were composed of compact partially vertically aligned nanosheets with a nanoscale roughness. The majority of the obtained BN–C nanosheets were less than 5 nm in thickness, mostly consisting of 2–15 atomic layers. Electron energy loss spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed the natural sp2 hybridization of the product, and cathodoluminescence spectroscopy measurements showed strong luminescence emission in the ultraviolet region at room temperature. Ultraviolet-visible spectroscopy demonstrated that the composite structure of alternating BN and C domains has different optical band gap features compared to pure h-BN nanosheets and graphenes, making it a promising material for further fundamental physical studies and potential applications in optoelectronics. Moreover, due to the rough morphology and nanoscale features of the BN–C coatings, they exhibited excellent water repellency (superhydrophobicity).

Low-energy cathodoluminescence microscopy for the characterization of nanostructures

Abstract Spatially and spectrally resolved low-energy cathodoluminescence (CL) microscopy was applied to the characterization of nanostructures. CL has the advantage of revealing not only the presence of luminescence centers but also their spatial distribution. The use of electrons as an excitation source allows a direct comparison with other electron-beam techniques. Thus, CL is a powerful method to correlate luminescence with the sample structure and to clarify the origin of the luminescence. However, caution is needed in the quantitative analysis of CL measurements. In this review, the advantages of cathodoluminescence for qualitative analysis and disadvantages for quantitative analysis are presented on the example of nanostructures.

Moisture-induced degradation and its mechanism of (Sr,Ca)AlSiN3:Eu2+, a red-color-converter for solid state lighting

(Sr,Ca)AlSiN3:Eu2+ (SCASN) is a very promising red phosphor used as a down-conversion luminescent material in solid state lighting. In this study, the moisture-induced degradation of SCASN was comprehensively investigated by treating it under severe conditions with high-pressure water steam. The degradation initiated at 150 °C, and the luminescence of SCASN was quenched quickly, with the powder sample being bleached after the treatment. Both the microstructure and phase changed obviously with oxidation, and the host turned finally into NH3, (Sr,Ca)Al2Si2O8 and Ca(OH)2. Using a variety of spectroscopic, surface and microstructure analytical techniques, the degradation mechanism was clarified and proposed to occur via the oxidant-gas penetration mechanism through the moisture-enhanced oxidation of both the SCASN host and divalent europium. The activation energy for the moisture-induced degradation was about 66.32 kJ mol−1.

Sharing of secondary electrons by in-lens and out-lens detector in low-voltage scanning electron microscope equipped with immersion lens.

To understand secondary electron (SE) image formation with in-lens and out-lens detector in low-voltage scanning electron microscopy (LV-SEM), we have evaluated SE signals of an in-lens and an out-lens detector in LV-SEM. From the energy distribution spectra of SEs with various boosting voltages of the immersion lens system, we revealed that the electrostatic field of the immersion lens mainly collects electrons with energy lower than 40eV, acting as a low-pass filter. This effect is also observed as a contrast change in LV-SEM images taken by in-lens and out-lens detectors.

Tantalum oxide nanomesh as self-standing one nanometre thick electrolyte

Tantalum oxide (TaO3) nanosheets coated on the surface of a LiCoO2 cathode decrease its interfacial resistance in a solid-state battery by two orders of magnitude. Since the interfacial resistance is rate-determining in the solid-state system, the interfacial structure of the nanosheet is anticipated to pave the way for realising high-performance solid-state lithium batteries. The reduction in the interfacial resistance also strongly suggests that the TaO3 nanosheet is a self-standing solid electrolyte layer with an ultimate thinness of 1 nm. It has a wide band gap and a mesh structure with openings that are almost the same in size as the lithium ion, which prevents electronic conduction and allows the penetration of lithium ions, respectively.

Reduced thermal degradation of the red-emitting Sr2Si5N8:Eu2+ phosphor via thermal treatment in nitrogen

The red phosphor of Sr2Si5N8:Eu2+ was synthesized by a solid state reaction. The as-synthesized phosphor powders were post-treated in a N2 atmosphere. The prepared samples were analyzed by XRD, FE-SEM, TG-DTA, FT-IR, zeta potential, cathodoluminescence (CL), photoluminescence (PL), quantum efficiencies (QEs), and temperature-dependent PL and QE techniques. After the thermal treatment in N2, it was found that the N2-treatment caused a negligible influence on the phase purity and particle morphology; the surface of the phosphor particle became more hydrophilic; the isoelectric point (IEP) of the suspension containing phosphor powder shifted to a higher pH value; the edge area (formed surface layer) of the phosphor particle had lower CL intensity than the inner part but it inhibited the surface damage caused by e-beam irradiation; more significantly, the formed surface layer plays a passivating role in preventing the Eu2+ activator from being oxidized, consequently, effectively reducing thermal degradation that deteriorates the PL intensity of the Sr2Si5N8:Eu2+ phosphor.

Suppression of concentration quenching of Er-related luminescence in Er-doped GaN

Erbium-doped GaN with different doping concentrations were grown by ammonia-source molecular beam epitaxy. The intra-4f-shell transitions related green luminescence were observed by both photoluminescence (PL) and cathodoluminescence (CL) measurements. It was found that concentration quenching of Er-related luminescence was observed in PL measurements while not in CL measurements. The different excitation and relaxation processes are suggested as the cause of the concentration quenching characteristics between PL and CL. The strong Er-related CL intensity in highly doped GaN demonstrates that high energy excitation is a promising approach to suppress the concentration quenching in Er-doped GaN.

Secondary electron emission from freely supported nanowires

We present secondary electron (SE) emission results from freely supported carbon/silicon nitride (Si3N4) hybrid nanowires using scanning electron microscopy. We found that, contrary to bulk materials, the SE emission from insulating or electrically isolated metallic nanowires is strongly suppressed by the penetrating beam. A mechanism of the SE suppression by the positive specimen charging is proposed, which is based on a total emission yield calculation using the Monte Carlo technique. This finding provides an important basis for studying low-energy electron emission from nanostructures under a penetrating electron beam.

Local analysis of Eu2+ emission in CaAlSiN3

Abstract We have investigated the local luminescence properties of Eu-doped CaAlSiN3 by using low-energy electron beam (e-beam) techniques. The particles yield broad emission centered at 655 nm with a shoulder at higher wavelength under light excitation, and a broad band around 643 nm with a tail at 540 nm under e-beam excitation. Using cathodoluminescence (CL) in a scanning electron microscope (SEM), we have observed small and large particles, which, although with different compositions, exhibit Eu2+-related emissions at 645 and 635 nm, respectively. Local CL measurements reveal that the Eu2+ emission may actually consist of several bands. In addition to the red broad band, regularly spaced sharp peaks have been occasionally observed. These luminescence variations may originate from a variation in the composition inside CaAlSiN3.