STEM electron tomography of gold nanostructures

J.C. HERNANDEZ; M.S. MORENO; E.A. CORONADO; P.A. MIDGLEY

Aachen, Alemania

Congreso; 14th European Microscopy Congress, EUREM 2008; 2008

The electronic and optical properties of many nanoparticles can depend critically ontheir size and morphology [1]. As such, the morphological control of noble metalnanoparticles, technologically important in many fields such as catalysis, optics andsurface enhanced Raman spectroscopy, has become of great interest. Metallic core-shellnanostructures have attracted attention since it is possible to enhance physical andchemical properties that cannot be obtained in single-component nanoparticles. Theirplasmonic properties can be tuned with nanoparticle geometry (size and shape) anddielectric environment to such extent that a very small change in their geometry leads todramatic changes in their optical properties [2]. Knowledge of the 3D morphology aswell as the precise thickness of the shell is needed in order to understand the unusualphysico-chemical properties of these core-shell nanostructures. Here we apply STEMbasedelectron tomography, a technique well suited to materials science applications [3],to achieve a full 3D analysis of gold and gold-silver core-shell nanostructures.Tomographic reconstruction allows 3D visualization of the nanoparticle shape andenables measured opto-electronic properties to be related to the particle morphology. Inparticular, we would like to understand variation in the surface plasmon resonance as afunction of core-shell dimension and composition [4].HAADF-STEM tilt series were acquired on a FEI Tecnai F20 microscope, using aFischione ultrahigh-tilt tomography holder. The images in the tilt series were alignedand reconstructed using Inspect3D software. Our first example is taken from a goldnanoparticle synthesized by chemical seed-mediated growth. Figure 1 shows atomographic reconstruction, using the simultaneous iterative reconstruction technique(SIRT), from a preliminary STEM-HAADF tilt series recorded in 10° steps. Even withsuch limited data, an approximate decahedral morphology is apparent from thetomographic reconstruction. The five dominant facets, delineated by the dashed lines inthe figure, are likely to be {111} planes. However, for accurate metrology (surface area,volume, angles between facets, etc), finer tilt steps are needed and/or the use of novelreconstruction algorithms such as discrete tomography [5]. Figure 2 shows a SIRTtomographic reconstruction of a cluster of Au-Ag core-shell nanoparticles. HAADFSTEMimages were recorded every 2° over +/-74°. Here we highlight the sub-volumenear the uppermost nanoparticle as an example of nanoscale quantitative analysis: themean diameter of the Au core is 23 nm and the Ag shell has a mean thickness of 2.3 nm.