Quantum dots (QDs) such CdSe nanocrystals attract much attention owing to their promising applications in light emitting devices and biomedical tags. They are usually coated by the wider band materials such CdS and ZnS in order to improve the photoluminescence quantum yield together with the chemical protection. However, it is not clarified the relationship between optical properties and core/shell particle structure, and we should optimize the coating treatment condition and the core/shell structure (shape and thickness), throughout many experiments. It is important to clarify the relationship between optical properties and particle structure. Direct evaluation about the optical properties of the individual particles is very effective. As we all know, TEM is a powerful tool for the shape structure evaluation of nanoparticle. In addition, even the optical properties from UV to NIR of individual single nanoparticles can be evaluated by the HR-EELS using the monochromator TEM. In this study, we have investigated the particle structure and optical property using advanced TEM technique. We have synthesized CdSe, CdSe/CdS and CdSe/CdS/ZnS multi-shell particles. The optical properties of individual single nanoparticles have been evaluated by HR-EELS using the monochromator TEM in addition to the usual particle structure evaluation by HR-STEM and EDX. All nanoparticles were synthesized in an organic solvent with some ligands such as amine, oleic acids and trioctylphoshine. The synthesized and refined core CdSe particles were re-heated together with CdS raw material in an organic solvent, and we have obtained the CdSe/CdS core/shell particles. The CdSe/CdS/ZnS core/shell/shell particles have been also obtained in the similar treatments. We have evaluated the particle size and shape by STEM. Their size and shape was homogeneous. CdSe core particles were spherical and ca. 5 nm. CdSe/CdS and CdSe/CdS/ZnS particles were cubic shape, and ca. 10 nm and 15 nm. In STEM-EDX analysis, we have been able to confirm the core/shell and core/shell/shell structure. We could obtain some spectral structures attributed to interband transitions in the CdSe core and CdS shell by EELS analysis. However, the spectral structure about ZnS outer shell could be hardly confirmed. This would be attributed the low crystallinity of ZnS. Through the detail investigation of spectra, dielectric property of multi-shell nanoparticle is clarified and relation with the photo luminescent property will be discussed. [Acknowledgement] The research was partially supported by JSPS KAKENHI Grant Number 26286026, Grant-in-Aid for Scientific Research (B). In addition, this STEM observation was partly supported by Advanced Characterization Platform of the Nanotechnology Platform Japan sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
A high accuracy measurement of the lattice parameter can be obtained by x-ray diffraction. However, these methods have low spatial resolution and are limited by sample thickness. Therefore, for heterolayers much better information about local distortions near the interface can be obtained by electron microscopy using convergent-beam illumination. Large-angle convergent-beam electron diffraction (LACBED) patterns and convergent-beam imaging (CBIM) were applied to study of the lattice distortion across the interface of the GaAs epilayer grown on [001] GaAs substrate and the GaAs layer grown at 200°C (called the low-temperature (LT) layer). These LT GaAs layers are grown by molecular beam epitaxy from As oversaturation and are known to be As rich. Particle-induced x-ray studies reveal up to 1.5 % extra As. This excess As leads to the expansion of the lattice parameter up to 0.15% when measured by x-ray diffraction along the (004) reflection. From this study, it was expected that a cubic expansion of the lattice parameter would be equal in all three directions. However, TEM studies do not reveal the formation of dislocations at the interface, which would be expected taking into account the difference in the lattice parameter across the interface . Convergence beam illumination methods were applied to study in detail the strain, lattice parameter variation, and crystallographic distortion across the interface, since these methods have spatial resolution several orders of magnitude higher than x-ray diffraction. A CBED pattern taken in the substrate and in the layer with an incidence close to the [530] direction (exact orientation [0.869 0.495 0]) reveals the lattice parameter change, since the position of the cross of the 1113 and 1113 lines changes when an electron beam is placed in the substrate and the layer, respectively (Fig. la,b).
Selected-area electron diffraction and convergent-beam electron diffraction (CBED) techniques are used to reveal the Ba doping effect on the crystal structure of (${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{)}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ [SBN(x)] in the temperature range from 293 to 693 K. Ba doping of ${\mathrm{Sr}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ (SN) causes weakening of the incommensurate modulation and deviation from the C-centered lattice. On the other hand, symmetries of fundamental reflections in CBED patterns of SBN(x) are seen to be the same as those of SN, showing a small structural deviation from Cmc${2}_{1}$. Crystal symmetries above and below the temperature of a relaxorlike dielectric anomaly in SBN ($x=0.32$) at 465 K remain unchanged. CBED experiments with a nanometer-sized electron probe do not show any indication of the presence of a nanodomain around this temperature. However, the presence of a few-micron-sized polar-inversion domains is found at around 573 K in SBN ($x=0.32$). The origin of the reported relaxorlike dielectric anomaly in SBN ($x=0.32$) is discussed on the basis of the experimental results of CBED.
The behavior of excitons excited in anatase crystalline materials affects the photocatalytic performance of the materials. Thus, investigating the properties of such excitons is essential for understanding the origin of their influence on photocatalytic performance. Here, angle-resolved electron energy-loss spectroscopy was applied to evaluate the size of the spatial spread of excitons in anatase TiO2. Three kinds of excitons (I–III), which were reported by polarized light absorption experiments, were experimentally identified in the electron energy-loss spectroscopy (EELS) spectra. Exciton size was evaluated from absorption intensity, which was derived by the Kramers–Kronig analysis of the EELS spectra, depending on the momentum transfer (q). The sizes of excitons I, II, and III were evaluated to be 8, 5, and 6 nm, respectively. The larger size of exciton I than that of III was with the same tendency as in the theoretical results. Exciton II, which was evaluated as a delocalized one, was evaluated to have a finite size in this experiment. The largest size of exciton I, approximately 8 nm, is the same order of the exciton diffusion lengths of the material. Therefore, exciton I should significantly influence the photocatalytic activity of anatase.
An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Abstract In the field of biological and materials sciences, the importance of energy filter transmission electron microscope(EF-TEM) is increasing. Because it is a powerful instrument for contrast enhancement and obtaining elemental mapping images. We have developed a 200kV EF-TEM equipped with a fieldemission gun and in-column spectrometer. The new EF-TEM JEM-2010FEF inherits the performance in high resolution imaging and analysis from field emission TEM. The outer view is shown in Fig.l. Figure 2 shows the lens configuration of JEM-2010FEF. An in-column Q-type spectrometer is introduced within the imaging lens system. It was designed to have image distortion less than 1% and dispersion power 1.2p.m/eV for 200keV electrons. There is no need of compensating procedure of distortion. Imaging lens system consists of two objective lenses, three intermediate lenses and three projector lenses. The 8-stage imaging lens system enables wide range of imaging modes equal to conventional TEMs; energy spectroscopic image of magnification from ×200 to × 1,500,000, energy spectroscopic diffraction of camera length from 200mm to 2,000mm.
