In Situ Gas Heating and Real-Time 3-D Imaging Using a Transmission Electron Microscope

In situ atomic resolution imaging with gas injection and sample heating, and real-time three-dimensional structural and chemical characterizations are important features of the Hitachi H-9500 300 kV transmission electron microscope (TEM) (Hitachi High Technologies America, Inc., Pleasanton, CA). In situ gas environmental transmission electron microscopy (E-TEM) has been well recognized for its irreplaceable role in performing dynamic observation. The purpose of the E-TEM is to introduce gas into the specimen chamber of a TEM to study the gas–solid interactions between sample and gas. Since TEM works at high vacuum, gas is typically introduced into the microscope in two ways. The first method is to seal a sample in an environmental cell (E-cell) confined by electron transparent windows. The advantages of this window-type E-cell include simple design and low cost, but atomic resolution is difficult to achieve, and heating the sample is risky because the window materials may be damaged at elevated temperatures, causing gas to leak into the microscope column.

The second method is to install a differential pumping system on the microscope. Restricting apertures are installed above and below the sample area to reduce the influence of gas pressure in the specimen chamber on the other parts of the microscope. This design directly exposes the TEM sample to the electron beam, and high-resolution imaging and in situ heating are no longer problematic. However, because the restricting apertures below the sample area narrow down the collectable diffraction angle, recording high-angle electron diffractions and high-angle annular dark field (HAADF) images becomes impossible. In the H-9500 TEM, the differential pumping system was built above the sample chamber only; therefore the microscope serves as a high-performance tool for routine research and allows gas to be introduced into the specimen area when desired.

In nanomaterials studies, electron tomography is in high demand to examine the 3-D structure of nanomaterials and nanodevices. One widely used electron tomography technique requires the acquisition of tens to more than a hundred images from different directions of a TEM sample and uses computer software to process the image set and reconstruct the 3-D structure. Using the H-9500 TEM and a Hitachi 3-D sample holder, real-time structural and chemical characterizations can be done while working with the microscope without having to wait for postcomputer processing. The postreconstruction provides 3-D results without the artifacts caused by the missing sample tilting angle.

Gas injection heating–atomic resolution imaging

The LaB6 electron emitter H-9500 TEM is equipped with a differential pumping system above the sample area. Working with a Hitachi gas injection–heating sample holder,1 users can inject gas into the sample area inside the microscope and heat the sample to as high as 1500 °C. Atomic resolution imaging can be performed with a gas inlet and at elevated temperatures2 due to the mechanical stability design of the sample holder. High-resolution digital images or videos reflecting the dynamic structural changes in the samples can be recorded using a fast-speed charge-coupled device (CCD) camera. (All gas heating imaging data shown in this article were obtained from the National Application Center of Hitachi High Technologies [Hitachi NAKA Shi, Japan]; the project is led by Dr. Takeo Kamino.)

Figure 1 - Atomic resolution study of an SnO2 nanocrystal at 200 °C in a 0.02-Pa atmosphere.

Figure 1 shows an example in which an SnO2 nanocrystal was heated to 200 °C. The high-resolution image was taken at 200 °C in an atmosphere of 0.02 Pa. The high-resolution video and images were taken to demonstrate the structural change at the edge of the SnO2 crystal. The dark contrast spots in Figure 1 correspond to the projected atomic columns. An arrow indicates an atom located at a dislocation position on the surface.

The atomic resolution in situ gas heating E-TEM is extremely valuable in nanocatalyst research. Figure 2 shows Pt nanocatalysts at 800 °C in an H-9500 TEM; air was injected into the sample chamber. The dynamic changes in shape and size as well as coalescence among particles were observed. This instrumental development offers an economic platform for a broad range of applications in academia and industry for performing gas–solid interaction studies, and for gaining insight into the fundamental behavior of nanomaterials, nanocatalysts, and other materials at the atomic level.

Figure 2 - Atomic resolution study of Pt nanocatalysts at 800 °C in a 2 × 10–3 Pa atmosphere.

Real-time 3-D imaging

Electron tomography allows the characterization of 3-D structures with the aid of reconstruction computer software. The 3-D structural reconstruction relies on the TEM image data sets acquired from a limited sample tilting range (–70° to 70°, total ≠ 180°), and the missing angles cause the 3-D reconstruction quality to suffer because of the artifacts. In addition, the method is not real-time because of the required computer processing.

Figure 3 - Hitachi 3-D sample holder for real-time 3-D structural and chemical characterizations.

A new approach to the study of 3-D structures employs a patented 3-D sample holder to work with the H-9500 TEM. Figure 3 is a schematic of the 3-D holder. Under an electron beam, the sample can be tilted ±15° around the holder axis and rotated from 0 to 360° around the axis perpendicular to the holder axis. With this tilting–rotation combination, the sample can be observed and chemically mapped from essentially any direction to obtain structural and chemical distribution in the 3-D space. Novel electron tomography can be done with a full tilting angle range (0–180°, total = 180°) without missing any tilting angles; therefore the tomography reconstruction provides the most accurate results. Full-angle sample tilting can also work with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS) systems to analyze 3-D elemental distribution. Most importantly, because the technique is real-time, 3-D structures can be viewed while working with a TEM.

Figure 4 - High-resolution electron micrograph images and corresponding electron diffraction patterns taken from an Si sample. A Hitachi 3-D TEM sample holder was used. (Courtesy of Dr. Toshie Yaguchi of the National Application Center of Hitachi High Technologies.)

Figure 4 shows high-resolution images and the corresponding selected area electron diffraction patterns of an Si sample. Note that the two images were taken from two directions perpendicular to each other. This cannot be done using a conventional TEM sample holder.

Figure 5 is an electron micrograph for an Au nanoparticle inside an SiO2 shell. From this single two-dimensional image, it is difficult to tell whether the Au particle is in the center of the shell or on the surface. Full tilting in an electron microscope confirmed that the Au particle is inside. Obviously, this full-range tilting capability is extremely useful in the study of nanodevices and nanomaterials with complex morphologies, and biological safety of nanomaterials.

Figure 5 - Electron micrograph of an Au nanoparticle inside an SiO2 shell. (This is one of a series of images obtained from titling the sample from 0 to 360°. Courtesy of Dr. Konrad Jarausch, Hitachi High Technologies America, Inc.)

Conclusion

The state-of-the art H-9500 300-kV TEM provides in situ gas heating atomic resolution imaging and real-time 3-D structural and chemical characterizations. Gas can be introduced directly into the specimen chamber via a Hitachi gas heating sample holder. Samples can be heated up to 1500 °C to activate the gas–solid interaction that can be studied at an atomic resolution level. The research results should have a significant impact on nanoscience and nanotechnology, catalysts, fuel cells, environmental safety, gas sensors, geochemistry, and the toxicity study of nanomaterials.

Using the Hitachi 3-D sample holder, full-range tilting electron tomography can be achieved, and 3-D structural and chemical characterizations are in real-time without the need to wait for postcomputer processing.

References

  1. Kamino, T.; Yaguchi, T.; Konno, M.; Watabe, A.; Marukawa, T.; Mima, T.; Kuroda, K.; Saka, H.; Arai, S.; Makino, H.; Suzuki, Y.; Kishita, K. Development of a gas injection/specimen heating holder for use with a transmission electron microscope. J. Elec. Microsc. 2005, 54, 497–503.
  2. Zhang, X.F.; Kamino, T. Imaging gas–solid interactions in an atomic resolution environmental TEM. Microscopy Today  2006, 14, 16–18.

Dr. Zhang is Senior Product Development Manager, Hitachi High Technologies America, Inc., 5100 Franklin Dr., Pleasanton, CA 94588, U.S.A.; tel.: 925-218-2814; fax: 925-218-3230; e-mail: xiao.zhang@hitachi-hta.com.

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