Quantitative Structure Analysis of Nano-sized Materials by Transmission Electron Microscopy

Abstract

AbstractNanostructured materials from almost all classes of materials are of great interest because the reduced dimensionality may drastically change the physical properties. In general, these properties are a function of size, shape, arrangement and chemical composition of the nano-sized materials. Transmission electron microscopy (TEM) is a powerful tool to get a detailed insight into the material characteristics. To correlate microstructure as well as microchemistry and materials properties the various TEM techniques for imaging, diffraction and spectroscopy have to be combined. The potential applicability of quantitative TEM will be demonstrated for different nano-sized objects, particularly for semiconductor islands, nanowires, quantum dots and for soft magnetic materials. The classical diffraction contrast method of conventional TEM is applied to analyse the size, shape and arrangement of nano-sized structures, where a quantitative analysis often requires image simulations of diffraction contrast for theoretical structure models. An alternative and powerful method is the three-dimensional reconstruction of the shape from two-dimensional phase mapping by means of electron holography. This reqires the exact calculation of the mean inner potential of the specimen. Quantitative high-resolution transmission electron microscopy (qHRTEM) has to be applied to analyse structure and chemical composition on an atomic scale of magnitude. Particularly the application of aberration-corrected HRTEM offers new possibilities for quantitative structure analysis due to a contrast transfer by means of negative spherical aberration imaging (NCSI) and the resulting strong suppression of image delocalisation effects. An example for quantitative composition analysis will be demonstrated for ternary semiconductor quantum structures by means of a combined analysis of dark-field imaging and qHRTEM. The results will be compared with analytical TEM data (energy-dispersive X-ray spectroscopy (EDXS), electron energy-loss spectroscopy (EELS), and energy-filtered TEM (EFTEM)). The retrieval of chemical information with atomic resolution will be illustrated for III-V semiconductor nanostructures using STEM (scanning TEM) Z-contrast imaging. The correlation of structure and magnetic properties of soft magnetic materials will be demonstrated by combined application of Lorentz microscopy and electron holography. The potential applicability of the different quantitative TEM methods will be shown for following systems:(i) (Si,Ge) islands(ii) ZnTe and (Zn,Mn)Te nanowires(iii) Ga(As,Sb) quantum dots (QDs) on GaAs substrate(iv) nc softmagnetic FeCo alloysThe possibilities and limitations of the various methods applied will be critically evaluated.