Weighing Supported Nanoparticles: Size-Selected Clusters as Mass Standards in Nanometrology

We present a new approach to quantify the mass and 3D shape of nanoparticles on supports, using size-selected nanoclusters as mass standards in scanning transmission electron microscope. Through quantitative image intensity analysis, we show that the integrated high angle annular dark field intensities of size-selected gold clusters soft-landed on graphite display a monotonic dependence on the cluster size as far as $\sim 6500$ atoms. We applied this mass standard to study gold nanoparticles prepared by thermal vapor deposition and by colloidal wet chemistry, and from which we deduced the shapes of these two types of nanoparticles as expected.

Figure 1

HAADF-STEM images and their intensity analysis. (a)–(c) Representative images of Au clusters with selected mass of 147, 2057, and 6525 atoms on graphite supports. The images were obtained at 900 kX magnification at a resolution of $1024\times 1024$ pixels and they were low-pass filtered. (d) The HAADF intensity integrated over clusters as a function of number ($N$) of Au atoms in the clusters. The error bars were taken from the standard deviation of distribution measured by STEM. The inset shows the logarithmic fit of the intensity versus $N$, $I=68862N^{0.92315}$. The residual of the fit $R=0.99733$.

Figure 2

HAADF-STEM images taken from Au nanoparticles deposited on a Cu TEM grid coated with amorphous carbon thin film. (a) The nanoparticles were prepared by wet chemistry method. (b) The nanoparticles were prepared by thermally evaporating Au onto the TEM grid held at room temperature under high vacuum condition. The corresponding size distribution is shown in (c) and (d), respectively.

Figure 3

Relationship between particle diameter and $N^{1/3}$ for Au nanoparticles prepared by three different methods. (a) Colloidal nanoparticles, (b) nanoparticles by thermal evaporation, and (c) size-selected clusters preformed in gas phase and soft landed on the surface. Spherical cluster approximation (the blue solid line) and hemispherical cluster approximation (the red dashed line) are shown to allow comparison of overall particle geometry. Schematics of growth mechanism of Au nanoparticles are also shown in the insets.