Study On The Ordered Structures Of Organic Molecules On Single-crystalline Metal Surfaces

Based on the fundamental issues concerning organic semiconductors, in this thesis, I focused my attention on two aspects related to organic thin films. One is the growth behavior of monolayer organic semiconductors on single-crystalline metal surfaces. The other is the interface behavior between organic thin films and metals. Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) measurements and photoemission spectroscopy (PES) based on a series of typical model systems of organic semiconductors/metals interfaces were conductedThe growth of well-ordered organic nanostructures has attracted much attention in the past two decades. By choosing the highly symmetrical molecules such as perylene and tetracene, and the low-index substrate of Ru (1010) and Ag(110), the growth mechanism of monolayer organic semiconductors on metal substrates have been studied by LEED and STM. The results manifested that perylene and tetracene lied flat on metal surfaces. In the case of monolayer perylene or tetracene on Ag(110), molecules appeared in STM images not as monomers but as dimers, even trimers, suggesting the interactions between molecules are comparable to those between molecule and substrate and play a manifested role in the growth process. In the case of tetracene on more reactive surface, Ru (1010), no long range ordered structure was found, but STM images show that the long axis of tetracene is along either the [0001] or the [1210] azimuth, randomly, due to rather strong molecule-substrate interactions. All these revealed that the subtle balance between molecule-molecule interaction and molecule-substrate interaction results in the well-ordered monolayer organic film.Because the adsorbed layer and the substrate could not be investigated via STM simultaneously, it is needed to combine with theoretical calculations based on the first principle such as Gaussian and Dmol3 to determine the adsorption site of molecules. Based on calculations within Guassian98, it was found that perylene adsorbs on the short-bridge site of Ag (110) surface with separation of 3.4 A between the molecule and substrate, its long axis is 50°off the [110] azimuth. Based on calculations, the centre of the tetracene molecule is either on the short bridge site with the long molecular axis in the [0001] azimuth or on the hollow site with the long molecular axis in the [1210] azimuth. The separations between the molecule and substrate are 2.2 and 2.5 A respectively, indicating stronger interactions between molecule and substrate. Based on the calculations within Dmol3, the structure with alternating bright and dark is understood since that the separation between the molecule and substrate is different.Energy level alignment at organic/metal (electrode) interfaces, that is, how energy levels of organic semiconductors and electrodes are aligned at their interfaces, is one of the fundamental issues in organic based electronic devices. UPS on perylene/Ag(110) interface and tetracene/Ru(1010) interface were performed. Due to weaker interactions the valence bands of perylene are seldom affect by Ag substrate. However, because of stronger interaction, the HOMO of tetracene is fixed at 2.1 eV and the other valence bands shift is clear. The change of cut-off indicates that the decrease in work function upon the thickness can be attributed to the strong interface polarization.