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

Abstract

We present a new approach to quantify the mass and 3D shape of nanoparticles on supports, using sizeselected 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 $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. DOI: 10.1103/PhysRevLett.101.246103 PACS numbers: 81.07.Àb, 61.46.Bc, 61.46.Df, 68.37.Ma Since the properties of nanoclusters and nanoparticles depend critically on their size [1,2], the measurement of their mass is crucial. For example, knowledge of the mass, i.e., the number of atoms in the clusters, when coupled with the measured projected area of the nanoparticles, would enable us to deduce the 3D shape of the nanoparticles. This is of particular importance in areas such as environmental health [3] and catalysis [4,5], because it regulates the effective surface area of the (typically supported) particles. However, atom counting is a challenging task for nanoparticles on surfaces, in terms of accuracy, efficiency, and mass range. Here we show that soft-landed size-selected atomic clusters can be used as mass standards which enable surface mass spectrometry by scanning transmission electron microscopy (STEM). For Au clusters on carbon surfaces, we find that the measured relationship between mass and the high angle annular dark field (HAADF) intensity in STEM displays a monotonic dependence on cluster size as far as $6; 500 atoms. This extends the applicable mass range for STEM-based mass spectrometry by about 3 orders of magnitude [6,7]. The method is demonstrated by the 3D shape determination of colloidal, evaporated and size-selected Au nanoparticles.There are a number of ways that the masses of nanoparticles on surfaces can be measured. Cantilever-based techniques have recently been demonstrated for mass detection at the zeptogram level (10 À21 g) [8,9]. Electronmicroscopy-based techniques have a long history [6,7,[10][11][12][13][14][15][16][17][18]. In Zeitler and Bahr's early work of 1962, the mass of nanoparticles has been determined down to 10 À18 g using STEM with an accuracy of 10% [17]. The advantage of this approach is that the mass information obtained can be directly correlated to the structure and properties of the particle studied. The concept of atomic-level mass determination by STEM, as developed by Howie, is a quantitative application of the HAADF-STEM imaging method [18,19]. This requires an accurate knowledge of the electron scattering cross section ( ) as a function of the number of atoms (N) and the number of electrons in the focused electron beam. Ab initio calculation of high angle electron scattering cross section of nanoparticles is a challenging...

Compositional changes of Pd-Au bimetallic nanoclusters upon hydrogenation

“…Since it is improbable that hydrogenation flattens the nanoclusters, a lower integrated HAADF intensity must correspond to a higher palladium concentration. In general wetting for comparable nanoclusters has not been observed before, 12,22 a change in shape is unlikely to occur and therefore not a source of deviation.…”

“…Indeed, a linear relation for size selected gold nanoclusters has been observed before in such diagrams. 12,22 The integrated HAADF intensity of the nanoclusters as a function of the volume in Fig. 3͑A͒ shows that a wide range of integrated HAADF values are present for each diameter.…”

Catalysis with Supported Size-selected Pt Clusters

“…For other metals with high atomic number Z, i.e. Au, Pd [336,337,238,237], the dependency of the STEM image intensity on Z, so called Z-contrast imaging, in the ADF mode was used in order to discriminate between different sized clusters. The feasibility of this approach is shown, however the work suffers from poor size selection (±5%), the use of considerably large clusters and neglecting the influence of other parameters on the Z-contrast [290].…”

Coalescence dynamics of size-selected gold clusters studied by time-resolved transmission electron microscopy

“…Figure 1(b), shows three discrete peaks at 2.73 ± 0.13 nm, 3.66 ± 0.12 nm and 4.26 ± 0.41 nm, respectively. This size distribution is different from evaporated Au nanoparticles, which had a single broad distribution peak [6]. The Au-Au nearest neighbour distance is 0.288 nm [7] and the theoretical diameter of a spherical Au 923 cluster (full-shell geometry) is 3.45 nm, which is close to the value of 3.36 ± 0.16 nm measured by an aberrationcorrected scanning transmission electron microscope (AC-STEM) [8].…”