A look inside epitaxial cobalt-on-fluorite nanoparticles with three-dimensional reciprocal space mapping using GIXD, RHEED and GISAXS

S M Suturin,A M Korovin,N S Sokolov,V V Fedorov,S G Konnikov,G A Valkovskiy,M Tabuchi

doi:10.1107/s0021889813008777

Abstract

In this work epitaxial growth of cobalt on CaF2(111), (110) and (001) surfaces has been extensively studied. It has been shown by atomic force microscopy that at selected growth conditions stand-alone faceted Co nanoparticles are formed on a fluorite surface. Grazing-incidence X-ray diffraction (GIXD) and reflection high-energy electron diffraction (RHEED) studies have revealed that the particles crystallize in the face-centered cubic lattice structure otherwise non-achievable in bulk cobalt under normal conditions. The particles were found to inherit lattice orientation from the underlying CaF2 layer. Three-dimensional reciprocal space mapping carried out using X-ray and electron diffraction has revealed that there exist long bright 〈111〉 streaks passing through the cobalt Bragg reflections. These streaks are attributed to stacking faults formed in the crystal lattice of larger islands upon coalescence of independently nucleated smaller islands. Distinguished from the stacking fault streaks, crystal truncation rods perpendicular to the {111} and {001} particle facets have been observed. Finally, grazing-incidence small-angle X-ray scattering (GISAXS) has been applied to decouple the shape-related scattering from that induced by the crystal lattice defects. Particle faceting has been verified by modeling the GISAXS patterns. The work demonstrates the importance of three-dimensional reciprocal space mapping in the study of epitaxial nanoparticles.

Highlights

In recent years, intense research efforts have been devoted to the study of heterostructures with magnetically ordered ferroand antiferromagnetic layers
The present study focuses on the structural properties of well shaped high-crystalline-quality Co nanoparticles epitaxically grown on (111), (110) and (001) CaF2 surfaces by an improved double-stage technique
The results obtained during investigation of cobalt nanoparticle growth on three differently oriented CaF2 surfaces may be summarized as follows

Summary

Introduction

Intense research efforts have been devoted to the study of heterostructures with magnetically ordered ferroand antiferromagnetic layers. Depositing Co on different CaF2 faces is the way to look at virtually the same object from different viewing angles as the oriented particles turn different sides to the viewer This is especially helpful for microscopy and grazing-incidence diffraction studies where the view from the back hemisphere is shadowed by the substrate. The CaF2(111) layer is grown on an Si(111) substrate following the recipe known to result in uniform films with regular surface morphology (Sokolov et al, 2013). At this temperature the orientation of the fluorite layer is the same as that of the Si substrate (Pasquali et al, 2001) This surface has typical roughness below 1.5 nm and consists of nanometre-sized square pyramidal huts with {111} slopes (Fig. 1b). The rest of the fluorite was deposited at 673 K to produce a microridged surface with ridges of nanometre size having {111} slopes (Fig. 1c)

Cobalt nanoparticles: atomic force microscopy

Cobalt nanoparticles: electron diffraction

Cobalt nanoparticles: grazing-incidence X-ray diffraction

Cobalt nanoparticles

Findings

Concluding remarks