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STEM with Atomic Resolution


Figure 2: Windowed gas cell chip with heating capability. (left) Cutaway schematic of the six-hole gas cell heating chip with gold contacts to the heater. (right) Bright-field image of one of the holes in the SiC ceramic and the SiNx


membrane window.


of HAADF-STEM imaging encountered in the differentially pumped ETEM. In addition, most powder samples can be eas- ily loaded on this gas cell holder. For example, the powders are first dispersed in solvent, and the suspension is deposited directly onto one of the windows (the upper one or the lower one, depending on whether TEM mode or STEM mode is to be used). Te powder samples are thus situated between the two windows, sealed in a gaseous environment. Meanwhile, MEMS-based technology enables the integra- windows into Si chips with heating capabili-


tion of the SiNx


ties. As shown in the schematic of the Protochips Atmosphere gas cell system (Figure 2), both chips are etched in the center region and support a thin (<50 nm) amorphous SiNx


window


μm holes patterned in a thin low-conductivity SiC ceramic (see cutaway schematic in Figure 2), which serves as a heating unit. Gold electrode contact pads are fabricated on the two chips to allow current to pass from electrical leads coming through the specimen holder. With this design the heating membrane can achieve heating and cooling rates on the order of 106


over the open imaging area. In one style of the window chip, the amorphous SiNx


°C/s,


allowing more rapid thermal response and stabilization than with standard heat- ing holders. It has been demonstrated that atomic resolution can be realized even at 1000 °C and 1 atm pressure in an aberra- tion-corrected STEM/TEM instrument [7].


Results CO-induced Pt nanoparticle surface


reconstruction Tis example is an obser- vation of structural rearrangement on the surface of a Pt nanoparticle induced by adsorption of CO at saturation coverage and elevated temperature [8]. It is well known that the adsorption of reactive gaseous spe- cies can cause the reconstruction of metal surfaces, and it is also known that different configurations of surface atoms have differ- ent catalytic reactivities [9]. Tus, charac- terizing metallic surface structures under gaseous reaction conditions at atomic scale is critical for understanding reactivity.


18 window is supported over an array of 9 Te specimen consists of ∼10 nm diameter Pt nanopar-


ticles in the shape of truncated octahedrons supported on carbon. As illustrated in Figure 3a, the atomic-scale HAADF- STEM image, taken at 150 °C in 500 Torr of N2


, shows an edge


view of flat {100} atomic planes at the surface of the as-pre- pared Pt nanoparticle (beam along the <110 > zone axis). Tis {100} surface configuration was stable under N2


atmosphere


at 150 °C, and no morphological changes were identified dur- ing 30 minutes of in situ observation. At the end of that initial observation, and while the 150 °C temperature was main- tained, the N2


was pumped out, and 500 Torr of reactive gas (5


vol% CO in 95% Ar) was introduced into the gas cell. It is nota- ble that the partial pressure of CO reached 25 Torr, ensuring a saturation CO coverage on the Pt nanoparticle surfaces (satu- ration requires ∼10 Torr) [10]. As a result, Figure 3b shows an in situ HAADF-STEM image of the same Pt nanoparticle aſter exposure to the CO environment for 10 minutes. For a clear comparison of structural differences induced by CO exposure, layers 1 and 3 are marked as a reference in both the initial and reconstructed states. Upon CO adsorption, a new atomic plane of Pt atoms (marked as Layer 0) emerged on top of the original


Figure 3: In situ STEM measurement of CO-induced Pt nanoparticle surface reconstruction. (a) HAADF- STEM image of a Pt nanoparticle under a N2


environment. Scale bar = 2 nm. (b) Same particle under a


CO environment, showing a surface reconstruction on the {100} surface. (c) Wulff constructions of a Pt nanoparticle, based on DFT-calculated surface-free energies of clean surfaces and CO-saturated surfaces.


www.microscopy-today.com • 2019 May


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