Physics and chemistry at surfaces : exploring molecular architectures and their properties

Abstract

In this thesis we combine surface chemistry and surface physics to architecture molecular layers in a bottom-up approach. The formation of self-assembled molecular structures at surfaces on the basis of dipole-dipole interactions, H-bonding, metal coordination and covalent bonding is studied. The molecules of different structure and with specific functional groups are investigated on selected substrates, namely Au(111), Ag(111), Cu(111), Cu(100) or Bi reconstructed Cu(100). A number of model cases for controlling on-surface architectures and their properties has been found and is reported about in this thesis: 1) Architecture control of a coordination polymer, comprised from chiral and flexible molecular building blocks, by tuning of the intermolecular bonding motif; 2) Chirality transfer in a 1D coordination polymer formed from chiral molecules; 3) Dimensionality (0D, 1D, 2D) control via selection of the transition metal adatom, which modifies a ligand and participates in an on-surface coordination complex; 4) Demonstration of a 2D molecular ‘sponge’, created on the basis of a borylene derived covalent link with angular flexibility; 5) Investigation of confined 2D electron states in quantum boxes of different size and shape; 6) Self -sorting of bi-molecular system in a 2D array by the coulomb interaction of the surface dipole which depends on band-alignment, charge transfer and the screening in the substrate. Specifically, we show that chiral and flexible [7]helicene molecules with cyano-groups, covalently attached in symmetric positions, give rise to a 1D arrangement. The intrinsically chiral species imprints its chirality onto the weakly H-bonded assembly, which occurs if molecules are deposited on samples held at low (~90 K) temperatures. This imprint vanishes under the influence of stronger metal-coordination bonds formed after providing metal coordination centers to the H-bonded assembly. The flexibility of the helicene as well as the competition between intermolecular and molecule-surface interactions allow the coordinated chains to assemble in structures with the mirror symmetry apparently being reduced. The next important issue, addressed in this thesis, is the on-surface modification of the ligand as an approach to control the dimensionality of the resulting on-surface polymer. We present a novel metal-specific reaction of amino- /imino- functionalized perylene derivatives. This precursor is modified upon addition of Co, Fe or Ni at room-temperature into an endo-ligand. In contrast, the presence of Cu adatoms in conjunction with thermal activation leads to the formation of an exo-ligand. Thus the type of metal ligand defines whether a 1D or 2D coordinated polymer can be formed. We show that borylene-functionalized molecules react upon thermal activation with trimesic acid in a novel on-surface reaction. Moreover, the covalent connection, formed in this reaction, exhibits a high degree of flexibility and allows for the formation of the differently sized pores. The resulting molecular ‘sponge’, created this way, serves as a template confining the surface state electrons. We investigate the effect of size and shape of the pores on this quantum phenomenon. Furthermore, we present a new way of creating highly-ordered bimolecular self-sorted chessboard arrays. The bi-component mixture of Mn-phthalocyanine (MnPc) and Cu-phthalocyanine (CuPc) on Bi/Cu(100) self-assembles without participation of any chemical bonding or molecular functionalization but only on the basis of the lateral 2D Coulomb interactions. We resolve charge-transfer channels of two types, directing the supramolecular self-assembly: one oriented perpendicular to the substrate surface, the other oriented in-plane. These investigations are performed in ultra-high vacuum conditions (UHV) with the use of variable temperature Scanning Tunneling Microscopy / Spectroscopy (STM/STS), X-Ray Photoelectron Spectroscopy (XPS) and synchrotron-based Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The experimental results are supported by Density Functional Theory (DFT) calculations, performed by research partners.