HRTEM and EELS Studies of GdSi 2 Nanostructures Grown by Self-Assembly
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HRTEM and EELS Studies of GdSi2 Nanostructures Grown by Self-Assembly Jiaming Zhang1, Gangfeng Ye1, R. Loloee2, Martin A. Crimp1, and Jun Nogami3 1 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, 48824-1226 USA 2 Department of Physics and Astronomy, Michigan State University, East Lansing, MI, 48824-1226 USA 3 Department of Materials Science and Engineering, University of Toronto, 184 College St, Toronto, ON, Canada M5S 3E4 ABSTRACT There has been considerable interest in the properties of rare earth metal nanostructures grown epitaxially on Si(001) surfaces. The questions arises as to what extent the nanometer scale lateral dimensions (width and thickness) affect the material properties. We compare thin film Gd silicide samples with Gd nanostructures grown on Si(001) using high resolution transmission electron microscopy (HRTEM). The nanostructures have the same orthorhombic crystal structure as the thicker silicide films. Electron energy loss spectroscopy measurements from metallic Gd and the thin film silicides are also shown as references for similar measurements on the nanostructures.
INTRODUCTION The epitaxial growth of rare earth (RE) silicides on Si(001) has been investigated since it was found that nanowires or islands can self-assemble by depositing a small amount of RE metal on the Si(001) surface with subsequent high temperature annealing [1]. These silicide nanowires and islands have potential for applications in nanometer scale electronic devices. In addition, the heteroepitaxial interfaces between RE silicides and Si have special characteristics and should give some basis for understanding the growth of epitaxial thin films in lattice mismatched systems in general. Chen et at. reported that the formation of the self-assembled Er disilicide nanowires on Si(001) is due to the anisotropic mismatch between the hexagonal AlB2 type silicide crystal structure and the Si(001) substrate [2]. This mismatch can also explain the morphology of silicide nanowires and islands that form when other RE metals such as Ho, Y and Gd are grown on Si(001) [3-5]. Previous HRTEM studies of Dy silicide nanostructures showed that they can form either coherent or semicoherent interfaces with the substrate [6]. Because of the physical constraints of the interfaces and surface of nanostructures, material behavior may deviate significantly from that of the bulk. For nanostructures being considered for electronic applications, it is critical to understand if the properties differ between the bulk and nanophase. Electron-energy-loss spectroscopy (EELS) combined with TEM is a powerful technique to provide a wealth of information including electronic structure at the nanometer scale. Specifically, the M45 edges of RE metals in the electron-energy-loss spectrum, known as white lines, are dominated by the transition of 3d electrons to unoccupied 4f states (from the dipole selection rule M5 : 3d5/2→4f7/2, M4 : 3d3/2→4f5/2) [7]. Consequently, the intensity of t
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