3D CHARACTERIZATION OF NANOSTRUCTURES BY ELECTRON TOMOGRAPHY

M. S. MORENO; J.C. HERNANDEZ-GARRIDO; P.A. MIDGLEY

Buenos Aires

Congreso; 10th Inter-American Congress of Electron Microscopy 2009, CIASEM; 2009

The realization of materials at the nanometer scale have become a routine process in manyscientific disciplines. Manipulation of matter at the nanometer scale creates a new challenge formaterials characterization and also for modeling their properties. In particular, morphologicalcharacterization and control of nanoparticle (NP) morphology has become increasingly importantas many of their physical and chemical properties are highly size and shape-dependent and requiresaccurate characterization techniques. Metallic core-shell nanostructures have attracted attentionsince it is possible to enhance physical and chemical properties that can not be obtained in singlecomponentnanoparticles.Characterization of nanostructured objects by Transmission electron microscopy (TEM) techniqueshas become a conventional method because it provides the high spatial resolution required. Despiteof the powerful set of techniques routinely available in a transmission electron microscope, all ofthem provides a 2D characterization of the morphology in the form of images or thickness maps.Scanning-transmission electron microscopy (STEM)-based electron tomography is a technique ofgrowing importance for 3D crystallographic and metrological studies of different kinds ofnanostructures. HAADF-STEM tomography allows a very precise 3D description of the particlesize and morphology as well as metrological aspects at the nanoscale [1]. We used STEM-basedelectron tomography to study the metrology of Au NP´s whose morphology was revealed to be notideal and Au/Ag core-shell NP´s. The detailed and precise knowledge of their morphology andassociated metrological characterization is crucial for understanding its opto-electronic properties.Electron tomography was performed on an FEI Tecnai F20 electron microscope. The tilt-series wasacquired in HAADF-STEM mode using a Fischione ultra-high-tilt tomography holder in the tiltrange of -76º to +66º, with images recorded every 2º. Once the acquisition of the tilt series wascompleted, images were spatially aligned by a cross-correlation using Inspect3D software, and 3Dreconstructions were achieved using a simultaneous iterative reconstruction algorithm (SIRT) ofconsecutive 2D slices. Visualization and segmentation were performed using AMIRA 3.1.Figure 1 (upper) shows some HAADF-STEM images from the series at different tilt of a Au NPsynthesized. The evident changes in the projection with tilt indicate a highly irregular shape of theparticle. A truncated prismatic geometry could be infered from the images acquired at low tilt, butimportant changes are appreciated only at high tilt, for example becoming an hexagon at -76°. Thisimage series makes evident the need for a detailed 3D characterization of nanoobjects in order toestimate their properties and to discern morphology effects. Figure 1 (down) show one projectionof the 3D surface rendering. The obtained results show the real shape of the Au NP together withdetailed features about its facets and edges which configure its anisotropic morphology. Such a finedescription was achieved after considering the missing wedge problems arising from the slightlyreduced tilting range. This provides an example of the nanometrology information achieved bymeans of the 3D surface rendering where measurements of distances or angles between thedifferent facets have been done. The irregular geometry is more evident when these results arecompared with the regular morphology proposed. These results make evident the deviation fromany regular or ideal geometry like a truncated prism as it could be assumed from the analysis of asingle image acquired at 0°.Figure 2 shows a tomographic reconstruction of a cluster of Au-Ag core-shell nanoparticles. As anexample, we highlight the sub-volume near a single nanoparticle. The mean diameter of the Aucore is 23 nm and the Ag shell has a mean thickness of 2.3 nm.