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Spectro-Microscopy Characterization of Organic Semiconductor

Author: Ateeq-ur-Rehman
Tutor: BaoShiNing
School: Zhejiang University
Course: Condensed Matter Physics
Keywords: layer Organic semiconductors FePc substrate surface photoemission molecular function structure deposition properties current semiconductor result between deposited lattice molecule tunneling inter
CLC: O472
Type: PhD thesis
Year: 2013
Downloads: 12
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Abstract

This thesis reports on the ultra-high vacuum studies on the surface and interface electronic structure, electronic properties, and growth behavior of iron phthalocyanine (FePc) molecules on the single crystal surfaces of conventional semiconductor and coinage metals. A cluster of surface analysis instruments including valence band and core level photoelectron spectroscopy (PES:VB-UPS, CL-XPS) and scanning tunneling microscopy (STM) have been used to address these subjects. Moreover, the interpretation of the experimental data has been supported by density functional theory modeling (DFT) to gain a comprehensive picture.PES data of the FePc-Si(110) interface disclosed interesting electronic properties. A larger charge transfer (per molecule) from benzene carbons (Cbenz,3.06×10-21C) as compared to the pyrrole carbons (Cpyr,1.47×10-21C) to the electrode is observed. The width of the resulting dipole is estimated circa1.87A. In addition to this the surface charge densities due to the benzene (σbenz) and pyrrole carbons (σpyr), respectively, are found to be5.27×10-39C2/nm2and0.81×10-39C2/nm2. Given the PES evidence we conclude that molecule and substrate have donor and acceptor character, respectively. A clear difference between the work functions of the FePc multilayer (4.23eV), monolayer (4.12eV), and the Si(110) sample surface (4.31eV) exhibits that the vacuum level shift caused by the FePc adsorption is of magnitude Vb=0.2eV. Also, the calculated barrier heights for electron (Obe,-0.29eV) and hole injection (Φbh,1.82eV) across the interface are not equal indicating a correlation between the overlayer and the substrate.When FePc deposited on the Cu(100) surface and investigated by STM and DFT-based calculations, it is found that at sub-monolayer (sub-ML) coverage molecules tend to adsorb dispersedly with their molecular planes parallel to the crystallographic directions of the substrate. Another interesting observation for this coverage is that the molecular axes of FePc are aligned along [037](as well as[037]) azimuth. After annealing the ML films to433K for10hours, two types of ordered structures; namely the quasi quadratic and oblique structure, respectively, with the lattice vectors separated by angle (θ) of (95±3)°and (90±3)°are observed. In both cases, the dimensions and orientations of the overlayer lattice indicate a commensurate structural relationship between the superstructure and the crystalline substrate. The packing densities for quasi quadratic and oblique lattices are0.444molecules/nm2and0.535molecules/nm2, respectively. With slightly increasing the coverage, a single distorted molecule on the face of the first ML is observed. However, at1.3ML coverage molecular clusters are formed particularly near the step edges. Our DFT findings for quasi quadratic structure indicate that molecules prefer to adsorb on the top site in a flat-lying geometry with a separation of2.70A between the molecule and the substrate. The molecular axis of FePc is along [037] azimuth; same as that of an isolated FePc molecule on the Cu (100) surface, indicating that the inter-molecular interaction is weak as compared to the overlayer-substrate interaction. In the case of oblique superstructure, molecules prefer to adsorb on the hollow site in a flat-lying geometry with a separation of3.30A between the adsorbate and the substrate. In this case, the neighboring molecules within the unit cell rotate their molecular axes with respect to the [037] azimuth by an angle of6.7°. This is the result of the increased inter-molecular interaction which then leads to increase the molecular packing density.A comparative study on the electronic and structural properties of FePc on Cu(100) and Cu(l10) surfaces is solely carried out by means of DFT calculations. In the case of FePc on Cu(100), a swear molecular rotation is observed. Further to this, an upward bend in the molecular plane ranging from7°to10°is observed; giving almost a buckled shape to the molecule. However, in the case of FePc on Cu(110), neither a swear bend nor a sizable rotation is observed. This comparative study, which also includes charge transfer, dipole formation, work function changes and molecular orbitals shift, show that FePc relatively strongly interacts with Cu(100) surface as compared to Cu(110) surface. Using CL-PES, we have been able to study the growth modes of FePc on Ag(110) surface. The C1s and Fe2p lines exhibited rigid shifts towards lower binding energies following the deposition of organic films, each by a different magnitude. A greater change and a larger shift in the Fe2p photoemission as compared to the Cls core level reveal that the adsorbate interacts with the substrate mainly via the Fe atom, located at the centre of the molecule. An increase/decrease in the intensity of C1s/Ag3d photoemission is found exponentially linked to the density of the adsorbing molecules. Finally, the so called growth/decay curve indicates that FePc thin films initially develops following the FM growth mode and then transform to the SK mode resulting in3D island aggregation.

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