Microstructural investigation into calcium phosphate biomaterials by spatially resolved cathodoluminescence

From cathodoluminescence (CL) investigations of synthetic and natural calcium phosphates it can be concluded that the CL of pure synthetic apatite is mainly characterized by intrinsic luminescence, whereas the luminescence of naturally occurring apatites is frequently activated by trace elements. CL revealed internal structures within plasma‐sprayed hydroxyapatite coatings which were not discernible by SEM‐BSE imaging. However, cathodoluminescence microscopy alone can presently not be used in every case to characterize synthetic calcium phosphate biomaterials because of the dominant intrinsic blue CL emission. In the future, optimum results will likely be achieved by using a combination of CL microscopy and spectroscopy with other spatially resolved analytical methods such as SEM‐BSE, SEM‐CL or micro‐Raman spectroscopy. In the present study, different types of tetracalcium phosphate dental cements could be distinguished due to varying CL colours and CL spectra that are caused by a different content of impurity Mn. These results emphasize the advantages of spectral CL measurements for spatially resolved detection of trace elements in solids.     

Imaging the hidden modes of ultrathin plasmonic strip antennas by cathodoluminescence

We perform spectrally resolved cathodoluminescence (CL) imaging nanoscopy using a 30 keV electron beam to identify the resonant modes of an ultrathin (20 nm), laterally tapered plasmonic Ag nanostrip antenna. We resolve with deep-subwavelength resolution four antenna resonances (resonance orders m = 2-5) that are ascribed to surface plasmon polariton standing waves that are confined on the strip. We map the local density of states on the strip surface and show that it has contributions from symmetric and antisymmetric surface plasmon polariton modes, each with a very different mode index. This work illustrates the power of CL experiments that can visualize hidden modes that for symmetry reasons have been elusive in optical light scattering experiments.     

Spatially and angularly resolved cathodoluminescence study of single ZnO nanorods

Single ZnO nanorods were studied with cathodoluminescence at high spatial and angular resolution. A newly developed luminescence detector consisting a fiber probe controlled by a nano-manipulator is attached to a scanning electron microscope to carry out the cathodoluminescence measurements. Excitonic emission from the sidewalls and redshifted near band edge emission guided along the nanorod axis are observed as the fiber probe axis is aligned to be perpendicular and parallel to the nanorod axis, respectively, demonstrating the angular resolving power of the experimental setup and waveguiding behavior of the nanorods. High spatial resolution cathodoluminescence measurement shows that the near band edge emission can propagate parallel and perpendicular to the nanorod axis and an increased propagation distance results in more redshift of the guided luminescence. In addition, the high spatial resolution and temperature dependent cathodoluminescence measurements demonstrate the important role of free exciton-longitudinal optical phonon interaction in the waveguiding behavior and the propagation of the near band edge emission in ZnO nanorods.     

Validation of luminescent source reconstruction using single-view spectrally resolved bioluminescence images

We characterize the capabilities and limitations of the Living Image Software 3D Analysis package (Xenogen, Alameda, California) in the reconstruction of calibrated light sources. Sources shallower than the mean free path of light propagation suffered reconstruction inaccuracy. For sources deeper than the mean free path, the average error in depth and intensity reconstruction was less than 4% and 12%, respectively, for homogeneous tissue. The reconstruction of luminescent beads implanted within an optically heterogeneous mouse abdomen proved less accurate. The ability to distinguish multiple sources decreased with increasing source depth. A number of factors influence the accuracy of light source reconstruction.     

Strong morphological dependence of luminescence efficiency and emission wavelength in hexagonal GaN crystallites directly imaged by scanning cathodoluminescence microscopy

The microstructure and morphology of hexagonal GaN crystallites grown on c-axis sapphire substrates by low pressure chemical vapor deposition is correlated with the luminescence efficiency and emission wavelength. Microscopic variation of local band gap monitored by the luminescence wavelength on the a- and c-planes of hexagonal GaN-crystallites are directly mapped by means of low temperature scanning cathodoluminescence (CL) and CL wavelength imaging (CLWI). Beside minor fluctuations from crystallite to crystallite, the a-planes show pronounced red shift of emission energy of more than 132 meV with respect to the luminescence from the c-planes. The c-plane itself shows additional inhomogeneity on a micron scale. Strongly red shifted luminescence (λ>400 nm) originates from the very center region correlated with a high dislocation density found in TEM. The CL intensity shows a reticulated structure over the c-plane visualizing the local dislocation network.     

Direct observation of plasmonic modes in au nanowires using high-resolution cathodoluminescence spectroscopy

We use cathodoluminescence imaging spectroscopy to excite and investigate plasmonic eigenmodes of Au nanowires with lengths of 500-1200 nm and approximately 100 nm width. We observe emission patterns along the Au nanowire axis that are symmetric and strongly wavelength dependent. Different patterns correspond to different resonant modes of the nanowire. From the observed patterns, we derive the spatial and spectral properties of the wire eigenmodes and determine the dispersion relation for plasmonic Au nanowire modes.     

Experimental study of the phase-shift miscalibration error in phase-shifting interferometry: use of a spectrally resolved white-light interferometer

The white-light interferogram in a spectrally resolved white-light interferometer is decomposed in its constituent spectral components by a spectrometer and displayed along its chromaticity axis. A piezoelectric transducer phase shifter in such an interferometer can give a desired phase shift of pi/2 only at one wavelength. The phase shift varies continuously at all other wavelengths along the chromaticity axis. This situation is ideal for an experimental study of the phase error due to the phase-shift error in the phase-shifting technique, as it will be shown in this paper.     

Spectrally and spatially resolved cathodoluminescence of nanodiamonds: local variations of the NV(0) emission properties

Here we report the spectrally and spatially resolved cathodoluminescence of diamond nanoparticles using focused fast electron beams in a transmission electron microscope. We demonstrate the possibility of quickly detecting various individual colour centres of different kinds on wide areas (several micrometres square) contained in nanoparticles separated by subwavelength distances. Among them, nanoparticles containing one or more neutral nitrogen-vacancy (NV(0)) intensity maxima have been seen, attributable to individual emitters. Thanks to a spatial resolution which is solely limited by charge carrier diffusion in the case of a fast electron (80 keV) setup, the spectra of two individual NV(0) emitters separated by 80 nm inside a nanoparticle have been spatially discerned. A shift of the zero phonon line (ZPL) between the two emitters, which we attribute to internal stress, is shown to arise even within the same nanoparticle. Detailed emission spectra (ZPL, phonon lines and Huang-Rhys factor, directly linked to the relaxation energy of the colour centre) in 51 individual NV(0) centres have been measured in 39 particles. The ZPL and Huang-Rhys factor are found to be measurably dispersed, while the phonon energies keep constant.     

Study of critical buckling loads and modes of cross-ply laminated annular plates

Buckling of composite annular plates under uniform internal and external radial edge loads have been investigated using energy method. Trefftez rule is used in the stability equations. The symmetric buckling of symmetric cross-ply laminates is considered. In this paper, buckling behavior for the three laminates (90/0)_2s, (90/0₂/90)_s and (90₂/0₂)_s are studied. Influence of some parameters such as thickness, stacking sequence, type of supports and the ratio of hole to sheet radius on buckling loads and modes are investigated. The results of the energy method are compared with the results of numerical method. Based on the results, in the plates with clamped boundary conditions the symmetric buckling assumption is not accurate, contrary to other boundary conditions.     

Analysis of spectrally resolved thermoluminescence of LiF:Mg,Cu,P detectors by the surface fitting method using algorithm for unrestricted peak positions

Recently developed surface fitting technique is a natural replacement for the widely used glow curve deconvolution (GCD) technique. Surface fitting can be applied for advanced spectrally resolved thermoluminescence (TL) measurements. It combines both kinetic and emission-band analysis. Owing to greater number of parameters and data points the algorithm is more time-consuming than usual GCD. However, it offers greater reliability in determination of trap parameters. This is especially important for spectrally resolved measurements that are usually performed at low-light level conditions. This paper demonstrates an application of the surface fitting method to the analysis of TL-3D data from LiF:Mg,Cu,P detectors. The spectra were analysed using two different variants of surface fitting--for restricted and unrestricted peak positions.     

Spatial distribution of impurities in ZnO nanotubes characterized by cathodoluminescence

Low-energy cathodoluminescence (CL) imaging and spectroscopy technique was employed to study the impurity distribution in individual ZnO hexagonal nanotubes fabricated by metalorganic chemical vapor deposition on the sapphire (0001) substrate. The CL spectra at 10 K show that acceptor and donor impurities are incorporated in the ZnO nanotubes. CL monochromatic images indicate that the concentration of donor is higher at the bottom part and the distribution of acceptors is more inhomogeneous at the surface of the nanotubes. The non-uniform defects and impurities distributions are explained by unstable growth conditions and contamination from the environment. These results indicate that the low-energy CL is a very powerful method to investigate the inhomogeneity of luminescence properties in the individual nanostructures.     

Recovery of spectral reflectances of objects being imaged by multispectral cameras

Acquisition of spectral information of objects being imaged through the use of sensor responses is important to reproduce color images under various illuminations. In the past several models have been proposed to recover the spectral reflectances from sensor responses. The accuracy of the spectral reflectances recovered by five different models is compared by using multispectral cameras. It is shown that the Wiener estimation that uses the noise variance estimated as proposed in IEEE Trans. Image Process.15, 1848 (2006) recovers the spectral reflectances more accurately than the others when the test samples are different from learning samples.     

Observations of contact damage in MgO and LiF crystals by cathodoluminescence

Cathodoluminescence (CL) studies in a scanning electron microscope of the static and dynamic contact damage in MgO and LiF crystals are described. The main luminescence for both MgO and LiF was found to be associated with the plastically deformed zone at and around the contact site, although there were differences of details in the CL behaviour of the two materials. It was also found that in MgO the intensity of luminescence from screw dislocations was markedly higher than that from the edge dislocations for all possible orientations of the specimen. It is proposed that this simple and rapid technique can be used for assessing the mechanical state of a surface.     

Precursors of the zeolite ZSM-5 imaged by Cryo-TEM and analyzed by SAXS

The microstructural evolution in the nucleation stage of a synthesis reaction of ZSM-5 zeolite was studied, with particular emphasis on the role of the organic cation, TPA⁺ (tetrapropylammonium). Direct observation of the microstructure was achieved by cryo-transmission electron microscopy (cryo-TEM). The quantitative evaluation of the structural units was obtained by small-angle X-ray scattering (SAXS) measurements. In the presence of the organic cation TPA⁺, we found globular structural units 5 nm in diameter that aggregate to elongated bodies 44 nm long. The globular structural units were found in reaction mixtures with or without the organic template TPA-OH, as long as the pH of the initial solution was kept above 11.6. Without TPA⁺, aggregation to cylindrical particles was not observed after 2 h of heating, and no ZSM-5 crystallinity was attained at the end of the reaction (after 8 d). These results suggest that the zeolite ZSM-5 building blocks are globular structural units, 5 nm in diameter, containing silica, alumina, organic cation, and water, which fuse together to produce elongated aggregates that may compose the final unit cell. It is concluded that the globular structural unit is a cluster of tetrapods similar to those found in the final ZSM-5 crystals.     

In situ measurement of silicon oxidation kinetics by monitoring spectrally emitted radiation

When a highly polished silicon wafer is thermally oxidized, its spectral emittance fluctuates systematically, as the protective silica film grows thicker. If the spectral intensity of the emitted radiation at a wavelength where silica is transparent is monitored, the film thickness can be obtained.