Surfaces of Complex Metallic Alloys and Their Adsorption Properties

Abstract

Quasicrystals are a novel phase of solids that were first reported in 1982. Qua sicrystals display aperiodic long-range order but lack translational symmetry. Furthermore, quasicrystals often exhibit classically forbidden rotational symmetries such as five-fold and ten-fold. Surfaces of icosahedral (i)–Ag–In–Yb quasicrystals and their approximants have been a subject of extensive research recently. Approximants are periodic metallic compounds with large unit cells that share similar chemical composition and atomic structure to their related quasicrystals. The basic building block of i–Ag–In–Yb quasicrystals and approximants are rhombic triacontahedral(RTH) clusters, which are formed by five successive atomic shells.

Surface studies carried out in this thesis are: the growth of pentacene (Pn) films on the two-fold and three-fold surfaces of i–Ag–In–Yb quasicrystals, the study of (110) surface of Au-Al-Gd 1/1 approximant and the (010) surface of Ga3Ni2 crys tal. To understand the surface structure, scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) tools were employed.

Pn molecules on the two-fold surface were found to be arranged in rows and aligned predominantly along the high-symmetry five-fold and two-fold axes of the surface. The row separation perpendicular to the five-fold symmetry axes is either short S = 0.79 ± 0.03 nm or long L = 1.24 ± 0.02 nm. The ratio between them is close to the golden mean (τ = 1.618..) and the row separations generate Fibonacci sequence segments, indicating that deposited Pn molecules have quasicrystaline order. The Fast Fourier-Transform (FFT) of the images produced by (STM) also reveals a two-fold quasicrystalline long-range order. Pn molecules are adsorbed on Yb sites because the distance between two sites is close to the length of Pn molecules. This is consistent with previous work on five-fold surface i–Ag–In–Yb where molecules were found to adsorb on Yb.

Similarly, Pn molecules deposited on the three-fold surface of i–Ag–In–Yb formed quasicrystalline monolayers on the surface. The FFT and the autocorrelation of high-resolution images show a three-fold pattern with maxima located at τ -scaling distances, confirming the quasicrystalline order of molecules. Rod and triangle motifs were the two types features observed in STM. Adsorbed molecules aligned along three-fold axes form the triangular motif. Rods shape were formed by molecules located at three-fold axes and rotated 30◦ with respect to the three-fold axes. These STM features can be understand if Pn molecules selectively adsorb on Yb. The behaviour of selective adsorption can be understood in terms of the geometry of the Yb sites in the surface substrate. The Yb–Yb separations are close to the C–C or H–H distances in a Pn molecule, indicating that Pn adsorbed on Yb sites

The surface study of the (110) surface of the Au-Al-Gd 1/1 approximant showed the (110) orientation is predominant, but different other orientations were also identified. The comparison of observed step heights and high resolution STM images with the bulk structure model reveal that terraces form on planes that break the icosahedron, the third shell of the RTH cluster, which is the building block of the system.

STM and LEED from the Ga3Ni2(010) surface reveal four different structures, depending on annealing temperature. These structures are (4×2), c(2×2), (2×1), and (1×1) observed at increasing annealing temperatures from 360 ◦C to 500 ◦C. In addition, the annealing time for transfer between different structures is identified.