(233f) Development of Quantitative STEM Technique for Jem-2100F to Measure Atom Numbers in Metallic Nanocatalysts

Catalysis is used in virtually all industrially important reactions, from oil-refining to manufacturing of clothing. Catalysis depends intimately on surface chemistry. Therefore, it is vital to understand the 3-dimensional structure of catalytic materials, i.e. the supported nanoparticles (NPs), as it will clearly affect their catalytic activity and selectivity. Transmission electron microscopy (TEM) is widely used to provide direct atomic structure information and chemistry of nano-catalyst materials. In 1970, Crewe et al. first reported visibility of single uranium (Z=92) atoms using high-angle annular dark-field (HAADF) imaging, also referred to as atomic number contrast or Z-contrast  in a dedicated scanning transmission electron microscope (STEM)[1].  Z-contrast is formed from the high-angle-scattered electrons by a sample, and the high-angle scattering of 100 mrad or larger leads a purely incoherent interpretation of the image [2.3]. Quantitative measurements of scattering cross-section of individual atoms and small clusters have been made before in measuring Re₆ clusters on a thin carbon grid with an error of ± 2 Re atoms [3].  However, this method was developed for a dedicated VG-STEM over a decade ago; unfortunately, VG-STEM has not been manufactured for more than 16 years. In the past decade, the rapid development of aberration-correctors have provided STEM capable of sub-Angstrom resolution with atomic detection sensitivity where atoms in a cluster can be directly counted [4,5]. However, aberration corrected STEMs are unique and costly instruments that are not readily available to most laboratories. Here, we present our development of the quantitative STEM (QSTEM) technique on a modern JEM-2100F S/TEM with a field emission gun (high probe current), which is a relatively inexpensive, popular and powerful TEM/STEMs, and can be found in many university facilities and national laboratories.

In the technological aspect, we faced to many challenges, and the post-sample column is more complicated for JEM 2100F.  There are 3 intermediate lenses plus 1 projective lens, whereas the old VG-STEM has none of them. The actual scattering electrons in JEM 2100F post sample column propagate more complicated trajectories. As there are no standard configurations of operations to meet the needs, we used free-lens control to customize the lens configurations.

The critical step for this technique is the measurement of the HAADF detector efficiency, which is the response of the detector as a function of acceptance angle (from the inner angle to the outer angle), under the same condition of the real scattering electron trajectories (post the sample). The free lens control was used to customize the lens configurations such that the electron beam scanned across the entire HAADF detector in order to quantify its efficiency. The low beam current is necessary to avoid saturating the detector. For the calibration of angle scan, the diffraction pattern from Au(100) thin film under STEM mode (STEM-EDP) was obtained with the BF detector. The inner and outermost (half) angles of the HAADF detector were measured as 100.3 mrad and 252.1 mrad, respectively. The detector efficiency was based on an averaged radial intensity as a function of scattering angles.

Au nanoparticles (NPs) were synthesized on an ultra-thin C-grid or other supports (e.g. gamma-alumina) using a variety of methods, including physical evaporation, chemical deposition-precipitation, and copolymer micellar encapsulations. For physical deposition of Au NPs, a UHV e-Beam evaporation system (made by Pascal) is employed, with a base pressure of 2E-9 torr.  The evaporating rate can be well controlled as low as 0.01 nm/s, and monitored with quartz-crystal microbalance sensors at both positions of the source and sample. A TEM grid holder was specially designed to hold multiple TEM grids, capable to heat with IR lamps assembled originally in the UHV chamber. The temperature used for creating Au NPs was between 250 ^oC and 300 ^oC to create crystalline and three-dimensional nanoparticles on the TEM grids. The Au NPs created by this method are a few nm in size.

Here we present our test on the QSTEM technique with Au NPs supported on ultrathin carbon TEM grids.  The HAADF images of Au NPs were taken under same camera length of 8cm (same as that taken the HAADF image). The electron beam current was measured with the GIFCCD camera using the gross integral count. The average size of the Au NP is 0.9 ± 0.2 nm with average of 27 ± 25 Au atoms. The large standard deviations of  the average atom number for Au NPs indicates a diversity of 3-dimensional shapes of Au NPs even though they have a relatively monodispere size distribution, e.g., the difference in the number of atoms between spherical and hemi-spherical nanoparticle is a factor of two. Once the diameter of NPs can be determined accurately, then different shapes of NPs, convoluted with the probe size can be modeled.  This modeling plus the information of the number of atoms of an NP can be used to uniquely determine the 3-dimensional structure of the supported metal NP. This technique can be transferred to the other modern TEMs with STEM attachments, including the advanced aberration corrected STEMs to extend its applications of quantitative measurements of nanocatalysts.

We gratefully acknowledge DOE-BES funding (DE-FG02-03ER15476).  We thank the user facilities at the NFCF at the University of Pittsburgh.

References:

[1] A. V. Crewe, J. Wall, and J. Langmore, Science 168 (3937) (1970) 1338.

[2] M. M. J. Treacy and S. B. Rice, Journal of Microscopy 156 (2) (1989) 211.

[3] A. Singhal, J. C. Yang, and J. M. Gibson, Ultramicroscopy 67 (1-4) (1997) 191.

[4] Z. Y. Li et al., Nature 451 (7174) (2008) 46.

[5] S. Van Aert et al., Nature 470 (7334) (2011) 374.