Atmospheric-Pressure STEM
of the experiment). Gas flow was verified by observing the volume of water displaced in an inverted graduated cylinder containing water, and flow rates were estimated to be about 0.4 cc/hour (>200 gas exchanges/hour). Te integrity of the flow cell was confirmed because we maintained vacuum in the electron microscope in this configuration.
Results Figure 4A shows a STEM image of gold nanoparticles
adjacent to a 0.36-mm thick layer of CO/O2/He gas mixture at atmospheric pressure.
Image noise was reduced using
a convolution filter with a kernel of (1, 1, 1; 1, 5, 1; 1, 1, 1) (Image J soſtware, NIH). Te image shows several different sizes of gold islets. Te background signal varied over the image, which possibly can be explained by a combination of thickness variation in the TiO2 layer and the formation of carbon contamination during imaging. Two of the smallest nanoparticles are indicated with the arrows 1 and 2. Line scans over these nanoparticles are shown in Figures 4B and 4C. Tey exhibit a FWHM of the peak above the background level of 0.8 nm and 1.0 nm, respectively. Te spatial resolution was 0.4 nm [8].
Discussion Te electron probe of the STEM enters the flow cell through
the top SiN window. Te interaction between the window and the electron beam leads to a small amount of broadening of the electron probe only [8]. Te beam is elastically scattered by the specimen located immediately below the SiN window, and this forms the contrast for the ADF detector. Te scattered electrons then interact with the gas molecules of the 0.36-mm thick layer of 1-percent CO, 5-percent O2, He at atmospheric pressure. Te effect of these interactions can be calculated from the elastic scattering cross section and mean free path length [11]. Te mean free path length in, for example, He2 gas at atmospheric pressure is 4 mm [8]. Because the mean free path length is much larger than the thickness of the gas column, the efficiency of detection will not change noticeably from that of imaging in a vacuum. Inelastic scattering can be neglected because it does not affect the angles of the electrons scattered into the ADF detector but merely causes a small reduction in the energy of some of the electrons compared to the total beam energy. Higher pressures than ambient pressure can probably be achieved with the present system, and, if needed, smaller or thicker SiN windows could be used. Atomic resolution should be achievable when using aberration-corrected STEM. Advanced microchip technology can be used to provide heating and cooling rates of 106 degrees Celsius per second [12].
Conclusions Our results show that subnanometer resolution can be
achieved through a 0.36-mm thick layer of gas at atmospheric pressure, using a simple and inexpensive system that is compatible with current electron microscopes. Te relatively large space between the SiN windows, and the ability to control the pressure of the sample, opens the possibility for a wide variety of experiments, involving clusters of nanoparticles, micrometer-sized samples, electrodes, mechanical probes, actuators, sensors, and even light guides, all without the need for complex, expensive equipment.
Acknowledgements We thank L.F. Allard, P.S. Herrell, and D.C. Joy for help and discussions. Te silicon chips were designed in collaboration
2011 May •
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Figure 4: Atmospheric STEM image of gold nanoparticles in a 360-m thick 1-percent CO/ 5-percent O2/ He gas-filled enclosure. (A) Image showing gold islets on the top SiN window recorded at a magnification M = 600,000, a pixel size of 0.17 nm, a pixel-dwell time of 8 seconds. For improved visibility of the nanoparticles, a convolution filter was applied and the signal intensity was color-coded. (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). From [8].
with Protochips Inc. (NC). Tis 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 (GMV, NJ), and by Vanderbilt University Medical Center (EAR, NJ).
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