Low-Cost, Atmospheric-Pressure Scanning Transmission Electron Microscopy
Niels de Jonge,1 * Elisabeth A. Ring,1 Wilbur C. Bigelow,2 and Gabriel M. Veith3 1Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN 37232 2 Materials Science and technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 3 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109
*
niels.de.jonge@
vanderbilt.edu
Introduction Solid materials in subambient gaseous environments have
been imaged using in situ transmission electron microscopy (TEM), for example to study dynamic effects: carbon nanotube growth [1], nanoparticle changes during redox reactions [2], and phase transitions in nanoscale systems [3]. In these studies the vacuum level in the specimen region of the electron microscope was increased to pressures of up to 10 mbar using pump-limiting apertures that separated the specimen region from the rest of the high-vacuum electron column [4], but it has not been possible to achieve the higher pressures that are desirable for catalysis research [5]. TEM imaging at atmospheric pressure and at elevated temperature was achieved with 0.2-nm resolution by enclosing a gaseous environment several micrometers thick between ultra-thin, electron transparent silicon nitride windows [6]. Although Ångström-level resolution in situ TEM has been demonstrated with aberration-corrected systems [7], the key difficulty with TEM imaging is its dependence on phase contrast, which requires ultra-thin specimens, limiting the choice of experiments. Te parameter space changes radically when using
scanning transmission electron microscopy (STEM). We have recently introduced a simple and inexpensive system for in situ for STEM imaging through 360 micrometers of gas at atmospheric pressure [8].
Instrumentation In the present work, the flow cell was composed of two
silicon microchips supporting silicon nitride (SiN) windows placed directly in the vacuum chamber of the electron microscope (Figure 1). A 0.36-mm spacer created a gap between the chips. Plastic tubing mounted into the flow cell allowed gas to flow to and from the sample. Te entire flow cell and the tubing were sealed with epoxy. Nanoparticles were fixed on the entrance window, which was defined with respect to the electron beam direction. Images were obtained by scanning the focused electron beam over the sample and detecting elastically scattered electrons with an annular dark-field (ADF) detector. Te dimensions of the silicon microchips (Protochips,
Inc.) were 2.00 × 2.60 × 0.30 mm3, and each chip supported a 50 µm × 200 µm × 50 nm SiN window [9] (Figures 2A and 2B). Tese dimensions presented an optimum balance between the field of view and the strength to withstand the pressure difference between the interior of the flow cell and the vacuum of the electron microscope. Te sides of the silicon chips were diced vertically with a precision of ±10 µm with respect to the SiN window to aid in the precise alignment of the two microchips. A thin-film Au/TiO2 catalyst sample was prepared on the
SiN side of one of the Si chips before it was assembled into a flow cell [8]. Gold supported on TiO2 was selected as the catalyst because of the high contrast of gold in the STEM, the
Figure 1: Schematic of the flow system for atmospheric-pressure STEM. (A) A cross section of a sample compartment filled with gas at atmospheric pressure is enclosed between two silicon microchips supporting electron-transparent SiN windows. The microchips are separated by a spacer and sealed with epoxy. The flow cell is placed in the vacuum of the electron microscope. Images are obtained by scanning a focused electron beam over nanoparticles attached to the top window and detecting elastically scattered transmitted electrons. The dimensions and angles are not to scale. (B) Top view schematic of the flow cell. The gas flow path is from the input tubing, over the SiN window in the interior of the flow cell, and out the output tubing. Small pieces of tubing serve as the spacer. Figure modified from [8].
16 doi:10.1017/S1551929511000228
www.microscopy-today.com • 2011 May
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76