Size Effect in Gold Nanoparticles Investigated by Electron Holography and STEM

Abstract

Gold clusters are extremely interesting nanosystems with a high catalytic activity, exploited for sensing applications and to promote the growth of nanostructures [1,2]. All these potential applications motivated numerous structural studies on Au nano-clusters. The mean electrostatic potential (MIP) is a fundamental quantity and its value is crucial for accurate evaluation and simulation of experimental data from TEM imaging and electron diffraction. Recently, a dependence of the MIP on particle size has been reported, measured by electron holography (EH): the increase of the MIP over the bulk value for particle sizes lower than 5 nm has been shown, suggesting a correlation with the catalytic behaviour of gold [3,4]. Despite these very promising results, a similar effect was observed in amorphous carbon films as the result of a thickness independent phase shift [5]. EH is capable of providing a quantitative determination of this surface phase shift since it depends on the projected sample thickness. Unfortunately, gold particles are reported to change their contact angle with the substrate reducing their dimension, as a result of a size-dependent change in the particle-support interaction [3]. To overcome these limitations, and to determine unambiguously information on surface phase effects in gold clusters, a combination of High Angle Annular Dark Field STEM (HAADF-STEM) and electron holography has been used, exploiting the local sample thickness dependence of the HAADF-STEM signal. HAADF intensity depends also on the imaging conditions, on the atomic number and of the density of the observed material. By keeping fixed all these parameters, it is possible to directly correlate image intensity to sample thickness variations.