In-situ and Multi-modal 4D-STEM of Core@Shell Nanoparticles Interdiffusion

At elevated temperatures, Core@Shell nanoparticles undergo changes in both their local structure and chemical composition, which subsequently alters their properties [1]. However, due to limitations in characterization methods, it can be difficult to obtain a comprehensive understanding of both chemical and structural information within the same sample, particularly under high-temperature conditions. To address this challenge, we integrated energy-dispersive X-ray spectroscopy (EDX) and four-dimensional scanning transmission electron microscopy (4D-STEM) to simultaneously record both the elemental and lattice information of an Au@Pd nano cube during an in-situ heating experiment. Our study provides insights into the elemental distribution and strain relaxation of the nanoparticle as it undergoes interdiffusion and alloying between Au and Pd. Figure 1a illustrates the experimental setup of the multi-modal characterization method using 4D-STEM. A focused electron beam scans the Au@Pd sample, generating different types of signals due to electron-sample interactions. The characteristic X-rays generated by inelastically scattering electrons are recorded by SuperX detectors, providing elemental maps in real space (Fig. 1b-c). The diffraction patterns (Fig. 1d), produced by elastically scattering electrons, are captured by the electron microscope pixel array detector (EMPAD) [2] and provide a wealth of lattice information with sub-pm precision [3]. To mitigate tilt and multi-scattering effects, the power cepstrum (Fig. 1e) is derived from the diffraction pattern and is utilized to extract lattice information [4]. Figure 2 displays the chemical composition and local strain of a Au@Pd nano cube as it undergoes an in-situ heating experiment. At room temperature, the core@shell structure of the particle is evident in the elemental map (Fig. 2a) and the strain map (Fig. 2e), because of the distinct lattice constants of Au (4.08 Å) and Pd (3.89 Å). Upon increasing the temperature, the nano cube retains its core@shell structure until 600°C, where the Au and Pd begin to diffuse (Fig. 2b), resulting in variations in local strains (Fig. 2f). The interdiffusion process continues, and the variance of local strains decreases as the temperature is further increased to 650°C (Fig. 2c, 2g). Eventually, at 750°C, the particle transforms into a fully mixed Au-Pd alloy (Fig. 2d), with a uniform lattice constant (Fig. 2h). Notably, during the heating process, the diffusion occurs only internally, and the shape of the particle remains a cube with (100) facets, rather than changing to a hollow core or spherical shape. Our results demonstrate the interdiffusion and alloying process of Au@Pd nanoparticles during heating and provide a potential method for synthesizing uniform Au-Pd alloy nano cubes for various applications [5]. Multi-modal characterization of Au@Pd nanoparticle by integrating EDS and 4D-STEM. (a) Schematic of the instrumentation. (b,c) EDX maps that provide chemical information. (d) Electron diffraction pattern that recorded through the EMPAD providing lattice information. (e) The power cepstrum, which is the FFT of the diffraction pattern in (d). Heating induced changes in chemical components and strain of the Au@Pd nanoparticle. At room temperature, the particle shows clear core@shell structure (a and e). The Au and Pd diffused at 600°C, and the local chemical components (b, c) and strains (f, g) were changing under heating. At 750°C, the particle became a fully mixed alloy with a uniform lattice constant (d, h).