Electron Diffraction Pattern Identification
In Figure 1 three NPs of distinctly
different known materials are compared (red: rutile TiO2; green: mercury; blue: zinc). Tis illustrates non-overlapping µD patterns. Tere is the possibility, when using spot patterns, that such non-overlapping spot patterns could be from the same material, but these diffraction patterns were taken of differently oriented particles. In our case the particle differences were also confirmed by energy-dispersive X-ray spectrometry (EDS). Ring patterns, which are usually produced from many differently oriented particles, eliminate this ambiguity. Indeed, when there are numerous particles, the discrete diffracted beams combine to form a continuous ring, as shown in Figure 3A, as compared to Figure 4 where fewer NPs are producing the µD patterns; therefore, individually diffracted beams are visible. Overlaying these colorized diffrac-
Figure 1: Microdiffraction (µD) patterns of a known material (TiO2, red) and two unknown nanoparticles (NPs) from a colored tattoo (blue: Zn; green: Hg) collected at the same kV and camera length (120 kV and 340 mm). EDS spectra of Zn and Hg are shown in the inserts on the top-left corner. Using the colorized image overlay method and rotating the images relative to one another, no matching patterns could be identified. The white circle was inscribed to help center the beam stop. The blue dots, representing two diffracted beams from the same diffraction ring, are equidistant from the central spot. When the beam stop is correctly positioned, then the two blue diffracted beams are equidistant from the white circle.
work successfully, one must ensure that both the standard and the unknown are in the eucentric position for both imaging and diffraction pattern acquisition. Tis ensures accurate camera length comparisons. Images of the µD patterns were saved as TIFF files for data comparison in Adobe Photoshop, where the diffraction patterns were colorized and superimposed using the layer palette.
Results and Discussion Method 1: Aſter collecting the diffraction patterns, they
were pseudo-colored in Adobe Photoshop, and these colorized patterns were overlaid for direct comparison. Tis simple technique requires only the superimposition of the µD pattern of the standard metal or non-metal with the experimental pattern of the analyzed sample of interest. In order to obtain an accurate and reproducible superimposition, the patterns must be collected under the identical conditions discussed above and within relatively close time frames. By overlaying the patterns, we obtained instant and distinct matches or mismatches. Diffraction patterns considered as mismatches were patterns that contained extraneous d-spacings that were not present in the standards.
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tion patterns in Photoshop allows for rapid screening of samples, by match- ing diffraction patterns to known standards or seeing definitive differ- ences. Figures 2C and 3C show matching diffraction patterns of an unknown and known material. Tis is an effective way to rapidly verify if the identity of the unknown matches the standard and may be applicable as an analytical tool for the quality control of production batches of nanomaterials and/or biological-related materials (see skin tattoos shown in
Figure 1). Using this method, we have successfully identified the metal properties of plasmid DNA-molded nanodiscs of gold (Figure 2) and nickel nanoparticles (Figure 3). We were able to vary conditions to fabricate size tunable gold and nickel, and ensure by this method, that we were maintaining the same material. Method 2: A µD pattern from a standard of ZnO powder
on a carbon-coated grid was collected and printed on an overhead transparency. Te transparency was then overlaid onto the standard µD pattern on the CCD monitor to check if any distortion of the µD pattern occurred when printing onto the transparency. To compare a µD pattern from an unknown to the standard, the transparency was overlaid on the unknown diffraction pattern directly on the CRT monitor of the AMT camera. Diffraction patterns considered as mismatches were patterns that contained extraneous d-spacings from those in the standards. To be considered a match to the standard, the µD pattern on the CRT had to match with at least five of the standard’s d-spacings. As the synthesis processing was refined, and µD confirmed the homogeneous nature of the desired NPs, this method
www.microscopy-today.com • 2011 September
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