Lab-in-Gap TEM
Figure 2 : Photo of the Hitachi 300 kV CFEG HF-3300 gas environmental TEM/ STEM/SEM. The arrow indicates the secondary electron (SE) detector above the specimen position. SE and STEM images can be acquired simultaneously.
a high elevation angle to the specimen chamber located in the pole piece gap. T e accelerated ion beam irradiates the specimen, resulting in structural change and/or byproducts, which are imaged and recorded by a TEM camera. At IBM, the specially designed H-9000 allows the pressure in the specimen chamber to vary from 2 × 10 −10 to 5 × 10 −5 Torr. Since the time it started to generate data from 1993, numerous papers have been pulished, and this IBM TEM group has become one of the pioneers and leaders in the fi eld of in situ gas and liquid environmental TEM.
Differential pumping . The accommodation of gas in the TEM specimen chamber is not possible for conventional TEM systems. Differential vacuum pumping is needed to decrease the gas pressure stage by stage along the TEM column [ 3 ]. This way, the crucial high vacuum can be maintained, especially in the electron gun area, while the specimen chamber is filled with gas. The current-generation Hitachi 300 kV LaB 6 emission H-9500 gas environmental TEM (ETEM) system is designed based on this technology. While the H-9500 serves as an atomic-resolution and affordable gas ETEM platform, there is a demand for a gas ETEM system with multiple imaging modes, a large pole piece gap, and strong capability for spectroscopy analysis. Hitachi therefore designed the cold field emission gun (CFEG) HF-3300 gas ETEM platform, which combines TEM, scanning transmission electron microscopy (STEM), and secondary electron (SE) imaging modes on one column. Three- or four-stage differential pumping between the specimen chamber and electron gun maintains a sufficient vacuum in the gun area when the specimen chamber is filled with gas. A picture of this microscope is shown in Figure 2 . SE/STEM imaging . In Figure 2 an arrow indicates the SE detector located above the specimen position. T is high- effi ciency SE detector can catch SE signals emitted from the upper surface of the specimen, allowing the simulta- neous collection of STEM and SE images or movies [ 4 ]. Displayed side by side, the STEM image reveals the bulk structure of the specimen while the SE image shows the surface morphology. Pt nanoparticles . Figure 3 shows the side-by-side SE and
Figure 3: Simultaneously recorded SE and BF-STEM images at 300 kV accelerating voltage in the Hitachi HF-3300 ETEM. The catalysis sample under study contains Pt nanoparticles on a carbon support. The SE image reveals a pore formation on carbon surface during heating at 200°C in a 4 Pa air environment, whereas the BF-STEM image does not show such a structural change on sample surface.
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bright-fi eld (BF)-STEM images for Pt nanocatalysts dispersed on a carbon support. Pt particles are in white contrast in the SE image and dark contrast in the BF-STEM image. When the sample was heated to 200°C in the presence of air, pores were formed on the carbon surface as shown in the SE image, whereas the pores cannot be seen in the STEM image [ 4 ]. Such simultaneous and live SE/STEM imaging at elevated temper- ature with a gas environment can be benefi cial to the study of heterogeneous structures, especially when the surface structure and/or surface/object interface plays an important role like in heterogeneous catalysis systems. Functional devices . As aforementioned, the ideal Lab-In-Gap electron microscope not only needs a gas or liquid environment, but also miniaturized functional devices. T e easiest way to add a functional device to the specimen area in a TEM system is to build such a device in the tip of the TEM specimen holder. T e fi rst Hitachi in situ heating TEM holder was reported in 1993 [ 5 ]. T e built-in heating device can heat the specimen to as high as 1500°C, while sample driſt is low enough to allow for atomic- resolution imaging at temperatures above 1000°C [ 5 – 7 ].
In 2006 a Lab-In-Gap type of Hitachi TEM holder with two built-in heaters and
www.microscopy-today.com • 2016 January
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