On the imaging of Pt atoms in zeolite frameworks

We compare the relative merits of high-resolution bright-field imaging in the TEM and high-angle annular-dark-field imaging (Z-contrast) in the STEM, for the detection and measurement of small ( <1 nm) noble-metal clusters in zeolites. Pt in K-zeolite L is used as an example system. It is confirmed that high-resolution bright-field imaging is better suited for resolving the zeolite framework. However, even with contrast enhancements gained from image-processing techniques, such as Fourier-filtering, bright-field images are ineffective for detecting clusters containing fewer than ∼ 20 Pt atoms in supports thicker than ∼ 10 nm. This is attributed mainly to ambiguous phase contrast speckle patterns associated with beam-damaged regions of the zeolite framework. Z-contrast images obtained with a STEM high-angle annular detector using a ∼ 0.2 nm probe are shown to be capable of detecting single Pt atoms against a ∼ 20 nm thick zeolite support. However, the precision with which atomic-sized clusters can be located relative to the unit cell is limited by the beam-damage-induced distortion of the zeolite framework.