Investigations On The Adsorption Geometry And Electronic Structures Of Organic Semiconductors On Metal Surfaces

Organic materials offer an increasing alternative to inorganic semiconductor electronics due to low-cost deposition methods, flexible substrates, and simple packaging. The study of the adsorption of organic molecules on metal surfaces is an important paradigm for exploring the self-assembly of organic semiconductors from a fundamental point of view. It yields precious information at organic-inorganic interfaces and in particular on the adsorption properties of such molecules on metal electrodes which have been applied in organic devices.Thin ordered films of organic molecules on metallic surfaces have attracted much attention for the investigations of fundamental physical phenomena, such as charge transport, charge injection, and light emission, which essentially depend on both the electronic properties of the ordered structures and the morphologies of this thin film. The orientation and structure of the first monolayer are particularly important in determining the structural characteristics of a thin organic film. A thorough comprehension of the interaction between organic molecules and the substrate is a prerequisite for the possibility to design optimized organic devices with predetermined characteristics and better device performance.In this thesis, first, I present the investigations on the growth behavior of perylene on the Cu(100) substrate by using scanning tunneling microscopy (STM) and density functional theory (DFT). As revealed by STM images, perylene molecules prefer to adopt lying configuration with their molecular plane parallel to the substrate, and two symmetrically equivalent ordered domains were observed. DFT calculations show that perylene molecule prefers to adsorb on the top site of substrate Cu atoms with its long molecular axis aligning along the [011] or [011] azimuth of the substrate which is the most stable adsorption geometry according to its highest binding energy, Consequently two adsorption superstructures of c(8×4) and c(8×6), each containing two perylene molecules per unit cell, are proposed based on our STM images. The growth mechanism for ordered perylene domains on Cu(100) can be attributed to the balance between weak adsorbate-adsorbate interaction and comparable adsorbate-substrate interaction.Then the investigations on the electronic, structure and adsorption geometry of Iron-Phthalocyanine (FePc) adsorbed on Cu(110) by using ultraviolet photoelectron spectroscopy (UPS) and DFT calculations are presented. The emission features α,β,γ and δ originating from the FePc molecules in UPS spectra are located at 3.42,5.04,7.36, and 10.28 eV below Fermi level. The feature a is mostly deriving from Fe 3d orbital with some contributions from C 2p orbital. A considerable charge transfer from the Cu substrate to the Fe 3d orbital occurs upon the adsorption of FePc molecules. The angle-resolved UPS (ARUPS) measurements indicate that FePc molecules adopt lying-down configurations with their molecular plane nearly parallel to the Cu(110) substrate at monolayer stage. In combination with the DFT calculations, the adsorption structure is determined to be that FePc molecule adsorbs on the top site of Cu(110) with an angle of 45° between the lobes of FePc and the [110] azimuth of the substrate. These results should promote to deep understanding the nature at the organic/metal interfaces and growth behavior of organic semiconductors on metal surfaces.