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And nanorods and the crystal structures of different

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and nanorods, and the crystal structures of different parts of a sample. In SAD, the condenserlens is defocussed to produce parallel illumination at the specimen and a selected area aperture
Chapter 5:Tools to Characterize Nanomaterials159is used to limit the diffracting volume. SAD patterns are often used to determine the Bravaislattices and lattice parameters of crystalline materials by the same procedure used in XRD.Although TEM has no inherent ability to distinguish atomic species, electron scattering isexceedingly sensitive to the target element and various spectroscopy techniques are developedfor the analysis of chemical composition.TEM has also been used to determine the melting points of nanocrystals. Heatingof a nanocrystal by the electron beam cancause melting, which can be identified by thedisappearance of crystalline diffraction. TEMhas also been used to measure the mechanicaland electrical properties of individual nanowiresand nanotubes. Thus, one can develop structure–property correlations in nanomaterials. Figure 5.5shows nano-quasicrystalline particles in a Zr alloy.High-resolution TEM (HRTEM) is the ultimatetool for imaging atomic-scale defects. In favourablecases, it shows a two-dimensional projection of thecrystal with defects and other features. Of course,this only makes sense if the two-dimensionalprojection is down some low-index direction, whenatoms are exactly on top of each other. Consider avery thin slice of a crystal that has been tilted suchthat a low-index direction is exactly perpendicularto the electron beam. All lattice planes nearlyparallel to the electron beam will be close enoughto the Bragg position and will diffract the primarybeam. The diffraction pattern can be consideredas the Fourier transform of the periodic electronpotential in crystals. In the objective lens, alldiffracted beams and the primary beam are broughttogether again; their interference provides a back-transformation and leads to an enlarged image ofthe periodic potential. This image is magnified bythe following electron-optical system and is finallyseen on the screen at typical magnifications of 106(Fig. 5.6) or higher.5.5ATOMIC FORCE MICROSCOPY (AFM)AFM is a high-resolution imaging technique thatcan resolve features as small as an atomic lattice inthe real space. It is also commonly referred to asFig. 5.5Nano-quasicrystalline particlesin Zr-Cu-Al-O alloy. (Source:BS Murty, IITMadras).50 nmFig. 5.6High-resolution TEM image ofamorphous nano-pockets in ultrafine-grainedAl. (Source:BS Murty, IIT Madras).2 nmAmorphousFCC-Al
160Nanoscience and Nanotechnologyscanning probe microscope (SPM). It allows researchers to both observe as well as manipulatemolecular- and atomic-level features. The AFM is being used to solve processing and materialsproblems in a wide range of technologies affecting the electronics, telecommunications,biological, chemical, automotive, aerospace and energy industries. Almost every materialranging from polymers to ceramics to composites are being investigated using AFM. AFM

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Term
Fall
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Tags
Materials Science, Nanotechnology, Quantum dot, nanomaterials, Carbon nanotube

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