pubs.acs.org/NanoLett
Atmospheric Pressure Scanning Transmission Electron Microscopy Niels de Jonge,*,†,‡ Wilbur C. Bigelow,§ and Gabriel M. Veith‡ †
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee 37232, and § Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 ‡
ABSTRACT Scanning transmission electron microscope (STEM) images of gold nanoparticles at atmospheric pressure have been recorded through a 0.36 mm thick mixture of CO, O2, and He. This was accomplished using a reaction cell consisting of two electrontransparent silicon nitride membranes. Gold nanoparticles of a full width at half-maximum diameter of 1.0 nm were visible above the background noise, and the achieved edge resolution was 0.4 nm in accordance with calculations of the beam broadening. KEYWORDS In situ electron microscopy, scanning transmission electron microscopy, gold nanoparticles, catalytic reaction, reaction cell
I
n situ transmission electron microscopy (TEM) has been used to image solid materials in subambient gaseous environments, e.g., to study the growth of carbon nanotubes,1 dynamic changes of nanoparticles occurring during redox reactions,2 and phase transitions in nanoscale systems.3 Typically in these studies the vacuum level in the specimen region of the electron microscope is increased to pressures of up to 10 mbar by the use of pump-limiting apertures separating the specimen region from the rest of the electron column, which is maintained at a high vacuum. These existing in situ microscopy apparatuses involve complex and expensive equipment and cannot achieve higher pressures, which are desirable for catalysis research.4 TEM imaging with 0.2 nm resolution at atmospheric pressure and at elevated temperature was achieved by enclosing a gaseous environment of several micrometers thickness between ultrathin, electron-transparent silicon nitride windows.5 However, the components of this system were also of high complexity, and the space for the sample was only a few micrometers thick, which limits the choice of specimen and experiments. While angstrom-level resolution in situ TEM has been demonstrated with aberration-corrected systems,6 the key difficulty with TEM imaging is its dependence on phase contrast which requires ultrathin specimens. The parameter space changes radically when using scanning transmission electron microscopy (STEM) providing subangstrom resolution.7 Here, we describe a simple and inexpensive system for in situ STEM imaging through 360 µm of gas at atmospheric pressure. The key components were two silicon microchips supporting silicon nitride (SiN) windows, which formed a flow cell that was placed directly in the vacuum chamber of
the electron microscope; see Figure 1. A gap between the chips was provided by means of a 0.36 mm thick spacer. Gas was fed to and from the samples through plastic tubing mounted into the flow cell. The entire flow cell and the tubing were sealed with epoxy. Nanoparticles were fixed on the entrance window 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. The two silicon microchips (manufactured by Protochips, Inc.) had dimensions of 2.00 × 2.60 × 0.30 mm, and each supported a 50 nm thick SiN window of 50 × 200 µm.8 This size, thickness, and rectangular shape presented an optimum balance between field of view and strength to withstand the pressure difference occurring when the flow cell was placed in the vacuum of the electron microscope. Others have published that 30 nm thick SiN windows of 0.1 × 0.1 mm, thinner than those used by us, reliably withstand a pressure difference over 1 atm.9 The sides of the silicon chips were diced vertically and with a precision of (10 µm with respect to the SiN window, such that alignment of two microchips could be accomplished with high precision. Two of the windows with their SiN sides facing were mounted to form a flow cell as follows. A silicon chip was placed with the SiN side facing up on the pole of a mounting device of local design (Figure 2a). Pieces of plastic tubing (Peek tubing, Upchurch Scientific) with an inner diameter of 50 µm and an outer diameter of 0.36 mm were placed on the window using tweezers (Figure 2b) to serve as spacers. The second window was then placed with the SiN side facing down on top of this stack, the top pole of the mounting tool was lowered on the stack, and pressing at their sides with tweezers aligned the silicon chips. On account of the diced edges of the chips, alignment of the two
* Corresponding author,
[email protected]. Received for review: 12/23/2009 Published on Web: 02/10/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl904254g | Nano Lett. 2010, 10, 1028–1031
FIGURE 1. Schematic of the flow system for atmospheric pressure scanning transmission electron microscopy (STEM). 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. Gas entry and exit are not shown in this drawing. 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.
FIGURE 2. Specimen holder for atmospheric pressure STEM. (a) Picture of the assembled flow cell kept in place between the metal poles of a loading device. The plastic tubing is glued in place using epoxy. (b) Schematic cross section of the flow cell. The gas flow path is through and from tubing over the SiN window in the interior of the flow cell. Pieces of tubing serve as the spacer. (c) Picture of the specimen rod (Hitachi style) consisting of two shafts. Shaft 1 leads the tubing to the exterior of the microscope. Shaft 2 contains the tip positioned in the center of the microscope. The flow cell is placed in the tip and its tubing is fed through the hollow interior of shaft 2. Epoxy provides a vacuum seal around the tubing inside the shaft. Shaft 2 fits into shaft 1 via the small O-ring. The larger O-ring provides a vacuum seal in the microscope. (d) Close-up of the tip. The flow cell is fixed in place with a clamp inside a cartridge. Tubing is fed through the hollow interior of the shaft.
silicon nitride windows occurred during this last step. The chips were then fixed in place by vacuum epoxy (Torr Seal, Varian), but leaving the left side in Figure 2a open. After 2 h of drying, the tubing for gas flow was inserted into the flow cell and fixed in place by epoxy. The stack was dried overnight. The flow cell with tubing attached to it was then placed in the specimen rod of local design; see Figure 2c. The specimen rod consisted of two separable shafts, where shaft #2 fit into shaft #1 and a vacuum seal was made with the smaller O-ring in Figure 2c. The vacuum seal between the rod and the microscope was formed by the larger O-ring. The tubing fit through the hollow interior of the shafts. A © 2010 American Chemical Society
vacuum seal was made with epoxy at the outer end of shaft #2 (left side of Figure 2c) also sealing the tubing in place. The flow cell was then placed into a cartridge of matching dimensions in place in the tip of the specimen rod and held by a spring clamp and screw. The purpose of the cartridge was to make it possible to reshape the tip region if needed, without having to change the whole shaft. For the present experiment a catalyst sample was prepared on the “interior” side of one of the Si chips before it was assembled into a flow cell. Gold supported on TiO2 was selected as our catalyst due to the contrast of gold in STEM, the room temperature activity of Au/TiO2 toward the oxidation of CO, and the exothermic nature of CO oxidation, 1029
DOI: 10.1021/nl904254g | Nano Lett. 2010, 10, 1028-–1031
which would provide heat promoting gold migration which could be studied in the electron microscope.10 A ∼2 nm thick TiO2 film was deposited via dc magnetron sputtering of a TiO2 target (99.9%, Kurt J. Lesker) using 99.99995% pure argon at an applied power of 18 W and a deposition pressure of 16.2 mTorr. Gold atoms were deposited on the TiO2 film via rf magnetron sputtering of a 99.99% pure gold target (Refining Systems, Nevada) at an applied power of 10 W and a pressure of 14.8 mTorr generated from the highpurity argon. Deposition rates were estimated using a quartz crystal balance. The deposited gold atoms nucleated and formed gold clusters on the TiO2 surface. For the purpose of testing the flow cell, we used a catalyst with low activity compared to other Au/TiO2 catalysts,11 thus avoiding gold particle migration and coarsening. The activity was measured to be 0.003 mol of COreacted (mol of Au)-1 s-1. Electron microscopy was performed with a Hitachi HD2000 STEM operated in the high-resolution mode at 200 kV with an approximate probe current of 0.1 nA. An imaging session started with the adjustment of the vertical position of the stage by focusing on the top window of the flow cell using the secondary electron detector. The flow cell was then imaged in transmission mode with the annular dark-field (ADF) detector. The brightness and contrast settings were adjusted for optimal visibility of the gold nanoparticles. Images 1280 × 960 pixels in size were recorded with an imaging time of 10 s. Images were first recorded with the tubing open and thus with the interior of the flow cell at atmospheric pressure. This experiment verified that the windows did not rupture at atmospheric pressure and under exposure of the electron beam. The tubing was then connected using valves and fittings from Upchurch Scientific to a premade mixture of 1% CO and 5% O2 in He stored in an Al cylinder to prevent the formation of Fe(CO)5 (Air Products). Gas was allowed to flow through the cell at a pressure slightly higher than ambient conditions (743 Torr, for the location of the experiment). Gas flow was verified by observing the volume of water displaced in an inverted graduated cylinder containing water; flow rates were estimated to be about 0.4 cm3/h (>200 gas exchanges/hour). The integrity of the flow cell was confirmed since we maintained vacuum in the electron microscope in this configuration. Figure 3a shows a STEM image obtained from a layer of gold nanoparticles on top of a 0.36 mm thick layer of CO/ O2/He gas mixture at atmospheric pressure. The image is a selected area of 680 × 680 pixels. A convolution filter with a kernel of (1, 1, 1; 1, 5, 1; 1, 1, 1) was applied to reduce the image noise (Image J software). The STEM image shows gold islets of several different sizes. The background signal varied over the image, which can possibly be explained by a combination of thickness variation of the TiO2 layer and the formation of carbon contamination during imaging. To determine the spatial resolution achieved, we recorded line scans over six of the smallest nanoparticles. Two of these are indicated by the arrows in Figure 3a, and their line scans © 2010 American Chemical Society
FIGURE 3. Atmospheric STEM imaging of gold nanoparticles in 1% CO/5% O2/He gas. (a) Image showing gold islets on the top SiN window recorded at a magnification M ) 600000, a pixel size of 0.17 nm, and a pixel-dwell time of 8 µs. For improved visibility of the nanoparticles, a convolution filter was applied and the signal intensity was color-coded. The thickness of the gas column was 360 µm. (b) Line-scan signal versus horizontal position, over the nanoparticle indicated with arrow #1 in (a) The background level was set to zero. (c) Line-scan signal versus horizontal position, over the nanoparticle indicated with arrow #2 in (a).
are shown in panels b and c of Figure 3. The background level was determined from the average of the 20 pixels at the left side of the peak and then set to zero. The full width at half-maximum (fwhm) of the peak above the background level in Figure 3b is 0.8 nm, and in Figure 3c is 1.0 nm, giving an indication of the sizes of the nanoparticles. The signalto-noise level for the detection of these nanoparticles was 8. As a measure of the resolution of the STEM imaging, we used the 25-75% rising edge width x25-75, realizing that the probe size was much smaller than the nanoparticles. For the peak in Figure 3b the average of the left and the right side of the peak was x25-75 ) 0.5 nm. For Figure 3c the value was 0.4 nm. The average for six measured nanoparticles was x25-75 ) 0.4 ( 0.1 nm. The principle of imaging through a thick layer of gas can be understood as follows. The electron probe of the STEM enters the flow cell through the top SiN window and is only minimally broadened by interactions of the electron beam with the window. Elastic scattering by the specimen located immediately below the SiN window forms the contrast for the ADF detector. The scattered electrons then travel through the 0.36 mm thick layer of 1%CO, 5% O2, He at atmospheric pressure and so also interact with the gas molecules. The effect of these interactions can be calculated from the equation12
N T ) 1 - exp - , N0 l
( )
l)
W σFNA
(1)
in which N is the total number of elastically scattered electrons, N0 the number of incident electrons, T the thickness of the material, l the mean-free-path length for elastic scattering, F the mass density, W the atomic weight, NA Avogadro’s number, and σ the total cross section for elastic scattering given by12 1030
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σ)
Z2R2λ2(1 + E/E0)2 2
(2)
E0 ) m0c2
(3)
πaH
Our results thus show that subnanometer resolution can be achieved through a layer of gas 0.36 mm thick at atmospheric pressure. The large space provided between the SiN windows opens the possibility for a wide variety of experiments, involving clusters of nanoparticles, micrometersized samples, electrodes, mechanical probes, actuators, sensors, and even light guides. Higher resolution could be achieved by using thinner SiN windows (windows thinner than 30 nm would likely have to be constructed of a smaller size to prevent rupture). Higher pressures than ambient pressure can probably be achieved with the present system, but if needed, smaller or thicker SiN windows can be used. Advanced microchip technology can be used to provide heating and cooling rates of 106 °C s-1.13 The same system could also be used for TEM imaging in the scatter contrast mode, but then the specimen should be mounted on the interior side of the exit window.
where
λ)
hc
√2EE0 + E2
R ) aHZ-1/3 E ) Ue with electron accelerating voltage U (in V), atomic number Z, aH the Bohr radius, m0 the rest mass of the electron, c the speed of light, h Planck’s constant, and e the electron charge. Scattering in, for example, He2 gas at atmospheric pressure gives l ) 4 mm. Because l . T the efficiency of detection will not change noticeably compared with the case of vacuum below the specimen. Note that inelastic scattering can be neglected because it does not affect the angles of the electrons but merely causes a reduction of the energy of some of the electrons by a small amount compared to the total beam energy. A cold field emission STEM has a typical probe size containing 50% of the current of d50 ) 0.2 nm. For a Gaussian beam profile x25-75 = d50/2 ) 0.1 nm. The electron probe is broadened by interaction of the electron beam with the entrance SiN window. The beam broadening through a material expressed as the width of an intensity distribution across a sharp edge, where the intensity goes from 25% to 75% of the total intensity, x25-75 is given by12
x25-75 ) 1.05 × 103
F W
( )
1/2
Z(1 + E/E0) 3/2 T E(1 + E/2E0)
Acknowledgment. We thank L. F. Allard and D. C. Joy for discussions. The silicon chips were designed in collaboration with Protochips Inc. (NC). This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. Financial support by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy, and by Vanderbilt University Medical Center (NJ). REFERENCES AND NOTES (1)
(2) (3)
(4)
(4) (5)
The chemical composition of the SiN window approximately equals that of Si3N4, and thus, F ) 3.2 × 106 g/m3, W ) 3/7 × 28 + 4/7 × 14 ) 20.0 g/mol, and Z ) (3/7 × 142 + 4/7 × 72)1/2 ) 10.6. The 50 nm thick window then gives x25-75 ) 0.3 nm. The influence of the TiO2 layer is negligible compared to the much thicker Si3N4. The contributions of the broadening and the probe size in vacuum have be added in quadrature leading to a total x25-75 ) 0.3 nm. Scanning a beam over an infinitely sharp edge would thus lead to a profile with x25-75 ) 0.3 nm, which corresponds to the measured value within the error margin. Since additional broadening due to the rounded edge of the nanoparticle has also to be considered, we conclude that the STEM imaging has indeed occurred with a probe sharper than x25-75 ) 0.4 nm.
© 2010 American Chemical Society
(6) (7)
(8) (9) (10) (11)
(12) (13)
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DOI: 10.1021/nl904254g | Nano Lett. 2010, 10, 1028-–1031
