High Spatiotemporal Resolution STEM Imaging at High Temperature

Abstract

Following the remarkable progress of aberration-corrected scanning transmission electron microscopy (STEM), atomic and electronic structures in materials and devices have been well studied. However, the experimental observations are mostly limited to static rather than dynamic in materials. For further understanding of materials properties, it is inevitable to perform in-situ atomic-resolution observation under controlled conditions such as gas/liquid atmosphere and low/high temperature. Since a STEM image is formed by a raster fashion, the temporal resolution is limited to a few seconds [1]. To significantly improve the temporal resolution in STEM, we developed a high-speed scanning probe system with a frame rate of 25 frames per second (fps) for 512×512 pixels, corresponding to 40 ms per frame [2]. In this study, we will present recent progress and applications using high spatiotemporal resolution STEM imaging at high temperature [3, 4]. Platinum nanoparticles (Pt NPs) on rutile TiO2 (110) surface is one of the most studied systems as a heterogeneous catalyst. Higher temperatures are required to promote the oxidation of carbon monoxide, but catalytic degradation is also accelerated. It is therefore essential to directly observe the atomic dynamics of Pt NPs at high temperature. Figure 1(a) shows a single frame of atomic-resolution annular dark-field (ADF) STEM image obtained from Pt NPs deposited on the atomically flat TiO2 (110) surface [4], where the acquisition time is 83 ns/pixel at 54.8 pA, and a total of 672 frames were acquired. Figure 1(b) and (c) show the unit-cell averaged ADF-STEM image and the structure model of TiO2 viewed along the [1-10] direction. As indicated by the white rectangle, an isolated Pt-trimer can be seen on the TiO2 (110) surface. To track the locations of Pt atoms, we used a maximum intensity map, where for each pixel the maximum intensity from all frames is selected. When a Pt atom stays at a certain position in a single frame, the pixel value could be the highest in the whole frames, and one can find the distribution of Pt positions over the time in the maximum intensity map. We find that many bright pixels in the maximum intensity map, suggesting that the Pt-trimer moved dynamically in the white rectangle region of Fig. 1(a). In addition to the tracking of the Pt-trimer, we performed 3D reconstruction of Pt-trimer frame-by-frame, using a geometric constraint. With the aid of density functional theoretical calculations, it is revealed that the lifetime of the meta-stable Pt-trimer configuration is considerably short as 80 ms, but our high-speed scanning system can capture such meta-stable structures of the Pt-trimer. We will discuss the details of the 3D reconstruction algorithm, the dynamics of the Pt-trimer, and their energetic stability. (a) Atomic-resolution ADF-STEM image (single frame) of Pt NPs on TiO2 (110) surface, and a Pt-trimer is in the white rectangle. (b) Unit-cell averaged ADF-STEM image of TiO2 viewed along the [1-10] orientation. (c) The structure model of TiO2.