3D characterization and metrology of nanostructures by electron tomography

J.C. HERNANDEZ; A.B. HUNGRÍA; J.A. PEREZ-OMIL; M.S. MORENO; E.A. CORONADO; G. CEMPURA; A. KRUK; P.A. MIDGLEY

Albuquerque, New Mexico, USA

Congreso; Microscopy and Microanalysis 2008; 2008

Microscopy Society of America

With the design of materials at the nanometer scale starting to become a routine process in many scientific disciplines such as molecular nanotechnology, nano-electronics and heterogeneous catalysis, atomic-scale structure and compositional characterization can give great insight into the control and optimization of the functional and structural properties of materials. In particular, morphological control of nanoparticles has become increasingly important as many of their physical and chemical properties are highly shape-dependent and requires an accurate characterization technique. Conventional electron microscopy whether in TEM or STEM mode yield two-dimensional images which are typically only projections of the true three-dimensional structure. Interpretation and measurement will always be prone to projection artifacts that may lead to erroneous results. A full 3D analysis can be achieved using electron tomography, especially in STEM mode which is very well suited to materials science applications [1].   Here we present some recent results which illustrate the application of STEM based tomography for crystallographic and metrological studies of different kinds of nanostructures. HAADF-STEM tilt series were acquired on a Tecnai F20 microscope, usually in the tilt range of -74º to +74º, using a Fischione ultrahigh-tilt tomography holder, with images recorded every 2º. The images in the tilt series were aligned using the Inspect3D software and reconstructions carried out using the SIRT algorithm. Fig. 1 shows a reconstruction of a Au nanoparticle synthesized by chemical seed- mediated growth method. In particular, such noble metal nanoparticles exhibit a plasmon resonance which can be tuned with nanoparticle size, shape and dielectric environment, to such an extent that a very small change in their geometry leads to dramatic changes in their optical and electronic properties. STEM tomography allows a very precise 3D description of the particle shape and associated metrological characterization in order to compare with its opto-electronic properties. Fig. 2(a) shows a reconstruction of a Ce-Zr oxide catalyst nanocrystal. HAADF-STEM tomography reveals that these faceted crystallites have a shape close to that of an octahedron, in agreement with the observed increase of {111} facets in HREM images [2]. Such results could be key in understanding the excellent redox behavior observed in these catalysts. A third example is taken from a double-sided extraction replica of a low-carbon alloyed steel. Here, the main interest was to visualise the 3D shape of Ti-C-N precipitates in steels. Fig. 2(b) shows a reconstruction of a precipitate that as a result of the preparation process, possesses an empty inner core with a morphology close to an octahedron. The octahedral particle shrouded by this enveloping precipitate was lost in the extraction.