Elsevier

Ultramicroscopy

Volume 108, Issue 7, June 2008, Pages 656-662
Ultramicroscopy

STEM nanodiffraction technique for structural analysis of CoPt nanoparticles

https://doi.org/10.1016/j.ultramic.2007.10.006Get rights and content

Abstract

Studying the structure of nanoparticles as a function of their size requires a correlation between the image and the diffraction pattern of single nanoparticles. Nanobeam diffraction technique is generally used but requires long and tedious TEM investigations, particularly when nanoparticles are randomly oriented on an amorphous substrate. We bring a new development to this structural study by controlling the nanoprobe of the Bright and Dark Field STEM (BF/DF STEM) modes of the TEM. The particularity of our experiment is to make the STEM nanoprobe parallel (probe size 1 nm and convergence angle <1 mrad) using a fine tuning of the focal lengths of the microscope illumination lenses. The accurate control of the beam position offered by this technique allowed us to obtain diffraction patterns of many single nanoparticles selected in the digital STEM image. By means of this technique, we demonstrate size effects on the order-disorder transition temperature in CoPt nanoparticles when their size is smaller than 3 nm.

Introduction

Nanoparticles can present, as a function of their size, structural transformations due to the competition between surface and volume energies [1], [2]. It is indeed well known that, in nanoparticles where the proportion of surface atoms is large, the equilibrium structures may differ from those predicted by the equilibrium phase diagram of the bulk material. Therefore, the knowledge of the atomic arrangement of nanoparticles is of fundamental importance to understand their physical and chemical properties [3], [4], [5].

In order to study the structural properties of nanoparticles, it is necessary to be able to synthesize nanoparticle assemblies with a well-controlled size, which is generally a difficult experimental task. As an example, synthesis of nanoparticles by physical routes on amorphous substrate leads to randomly oriented clusters with a broad size distribution, generally between 10% and 30% of the mean particle size. If we are interested in characterizing the structure of nanoparticles as a function of their size, it is then necessary to use techniques that combine both image and diffraction information at the nanometer scale. In addition, to obtain statistical results it is important to perform this structural analysis on a large number of particles.

The most widely used techniques are the high-resolution transmission electron microscopy (HRTEM) and the nanobeam diffraction (NBD) using an almost parallel beam of nanometric size combined with conventional bright field or dark field (BF/DF) imaging. These methods however cannot give statistical information because of the following limitations:

  • (I)

    On HRTEM images we directly observe the morphology and the structure of the particles, but each analysis requires zone-axis orientation conditions. Moreover, when lattice parameters are small, particularly in the case of metallic nanoparticles, the current point-to-point resolution of electron microscopes only permits one to observe the structure of particles oriented along the low indexes zone axes. Due to this limitation of the analysis possibilities and to the necessity to analyze many particles, the determination of the nanoparticles structures is a very tedious task using HRTEM, particularly when nanoparticles are randomly oriented.

  • (II)

    The NBD technique gives rise to diffraction patterns of single nanoparticles. For the particular case of chemically ordered and disordered structures, as for the CoPt alloy, any diffraction pattern of single nanoparticles contains useful information. Ordered structures are characterized by the presence of superstructure reflections in their diffraction patterns. For some zone-axis orientations, the absence of superstructure reflections is not necessarily the signature of a disordered structure. In that case, as for the HRTEM technique, nanoparticles have to be oriented along particular zone-axis in order to determine their structural state without any ambiguity. In addition, NBD has the drawback to give an indirect correlation between the morphology and the structure. This correlation requires the acquisition of an image of each analyzed particle and to switch from image to diffraction mode. This step is sometimes difficult on an assembly of almost identical nanoparticles.

The difficult step of the orientation of each nanoparticle may be overcome by developing epitaxial growth of nanoparticles on suitable crystalline substrates. However, the structure of the nanoparticles can strongly be influenced by the epitaxial relationship and the induced stress between the particles and the substrate.

In this paper, we present a method based on the use of the STEM mode imaging in a TEM with parallel illumination conditions, very close to the ones used in nanobeam diffraction experiments. We will show how the use of this technique has allowed us to demonstrate a size effect on the equilibrium structure of the CoPt nanoparticles prepared by pulsed laser deposition (PLD).

Section snippets

STEM nanodiffraction technique using parallel illumination conditions

The experiments have been performed on a JEM-2100F (TEM/STEM) field emission electron microscope operating at 200 kV. The microscope is equipped with a high-resolution objective lens (Cs=0.5 mm, point-to-point resolution at the Scherzer defocus=0.19 nm in TEM mode). Nanobeam diffraction patterns were obtained in the STEM mode condition, which was performed using the digital STEM system of the microscope. This technique is already available on the newly developed TITAN 80/300 FEI microscopes, under

CoPt nanoparticles preparation

The CoPt nanoparticle thin films on amorphous carbon were produced by pulsed laser deposition in an ultra-high vacuum chamber. The pressure in the chamber is better than 10−7 mbar. A typical target–substrate configuration is used to deposit separately metals by PLD using a KrF excimer laser at 248 nm with pulse duration of 25 ns at a repetition rate of 5 Hz. The laser energy can be chosen in the range 150–250 mJ depending on the ablation threshold of the selected target. The distance from targets to

Results on CoPt nanoparticles

The equiatomic bulk CoPt alloy presents a phase transition at 825 °C between a tetragonal ordered phase (L10) at low temperature (Fig. 2) and a disordered face centered cubic structure (fcc) at high temperature [10]. This phase transition can be described as a chemical ordering transition on the fcc lattice. The L10 ordered phase has a large magnetocrystalline anisotropy which is at the origin of important research efforts devoted to synthesize and to study CoPt nanoparticles with this

Conclusion

TEM illumination conditions have been modified to develop parallel nanobeam diffraction experiment in STEM mode. By using the facilities of digital STEM nowadays available on new generation TEM, it is possible to control the beam and to analyze the structure of nanoparticles by diffraction and to obtain their image at the same time without changing the lenses configuration of the microscope. This technique has been applied to the structural study of CoPt nanoparticles and has demonstrated a

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