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Cathodoluminescence System


in which the plasmon excitation occurs in a similar manner.


As proof of principle, CL tomography was performed on a plasmonic crescent consisting of a polystyrene core and a gold shell (see Figures 4 a and 4 b) [ 14 ]. Such crescents have interesting magneto- electric optical properties and serve as building blocks for metamaterials. Rather than tilting a single crescent, seven crescents with similar dimensions but different orienta- tions were imaged for simplicity. In an alternative method, a virtual tilt series was created from a single measurement using the symmetry of the crescent system. Figure 4c shows the result of this reconstruction method where the crescent shows a rich 3D CL structure where the core and base are dominant for blue light. Toward the infrared, the top of the crescents become more dominant because of the strong plasmonic tip mode that exists there. Materials science . Materials like rare-earth-doped phosphors and III/V semiconductors play an important role in optoelectronic devices, including scintillators, light-emitting- diodes, diode lasers, and solar cells. As mentioned, nanostructuring is employed increasingly for optimi- zation of the optical properties in such materials. CL can be used to study these materials (both in bulk and in nanostructured materials) and to determine their light-emitting properties at the nanoscale. Figure 5 shows CL spectra for a variety of dielectric and semiconductor materials, illustrating the wide scope of materials that can be studied. By combining the spectral sensitivity with the high spatial resolution of the technique, subtle changes in the output spectrum, defect luminescence, angular profi le, and emission polarization [ 10 ] can be analyzed on a deep-subwavelength scale. For active CL color impurities, such as rare-earth ions, the detection limit can be as low as 10 14 atoms/cm 3 if the host material is non-absorbing and if there are no competing CL emission lines [ 3 ]. This is well below the ppm level and the detection limit of common SEM-based X-ray techniques, making CL spectroscopy a highly valuable technique for visual- izing trace element variations. Geology . The SPARC system is also useful in the field of geology, more specifically in petrology. Petrology is the branch of geoscience that is concerned with the analysis of the origin, distribution, texture, and composition of rocks. Microanalysis of rocks plays a key role in establishing the


2016 May • www.microscopy-today.com


Figure 4 : (a), Schematic of a crescent consisting of a polystyrene (PS) core and an Au shell. (b) Bright-fi eld TEM image of an individual crescent. (c) Reconstructed CL signal at different wavelengths (color coding from blue to red). The orientation of the crescent in the fi gure is the same as in (a) (image by Ashwin Atre [ 15 ]).


Figure 5 : Normalized CL spectra excited for a variety of dielectric and semiconductor materials. The SiO 2 :Er 3+ , GaAs, thermal SiO 2 on silicon, YAG:Ce 3+ , and GaN emission was measured on bulk/planar materials. The other spectra were measured on nanomaterials: InP (nanowire grown with the vapor-liquid-solid method), CdS (colloidal platelet), InGaN (quantum well embedded in GaN), and ZnO (nanoparticle powder).


aforementioned properties. The SEM provides an ideal platform for such studies because it supports diff erent types of imaging modes that can be applied and combined at small length scales (< 1 µm).


CL has been widely used in geology for decades and is


oſt en applied together with energy-dispersive or wavelength- dispersive X-ray spectroscopy (EDS, WDS) [ 15 ], as well as secondary electron (SE), backscattering electron (BSE), and electron backscattering diffraction (EBSD) imaging. These imaging modes provide information on elemental composition, topology, density, and crystal structure, respec- tively. CL microscopy complements these techniques because it is sensitive to elemental composition, to the phases present (crystal structure), and to the presence of defects and trace elements in the material. Such characteristics strongly depend


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