Device relevant organic films and interfaces: A surface science approach (original) (raw)
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Organic Electronics, 2011
The energy-level alignment of vacuum-sublimed thin films of a series of differently alkyl-substituted sexithiophenes (6T) (a,x-didecylsexithiophene (DD6T), a,x-diethylsexithiophene (DE6T) and 3 0 ,4 0 -dimethyl-a,x-dihexylsexithiophene (Me-DH6T)) on Ag(1 1 1) substrates was investigated by means of ultraviolet photoelectron spectroscopy (UPS). The orientation dependent ionization energy of the 6T derivatives allowed determining the thin film growth mode of the molecules, which varied with the length and substitution-position of the alkyl chains on the conjugated 6T backbone. DE6T and Me-DH6T are lying almost flat in the monolayer and in multilayers, whereas DD6T undergoes an orientational transition from lying to standing from the second molecular layer onwards.
Thin Solid Films, 2006
The electronic and structural properties of sexiphenyl and sexithiophene films grown under ultra-high vacuum conditions on a variety of wellcontrolled substrate surfaces have been examined. We show that, in contradiction with the general notion, the ionisation potential of the organic films is not a material constant. Considering the variable ionisation potential, the validity of the Schottky-Mott relationship, which expresses a dependence of the band alignment on the substrate work function, is analysed. We also briefly discuss the relevance of the built-in potential behaviour in organic films for the band-bending concept. (J. Ivanco).
Advanced Functional Materials, 2001
Ultraviolet photoelectron spectroscopy was used to determine the energy level alignment at the interfaces formed by either the deposition of para-sexiphenyl (6P) onto samarium thin films, or by growing samarium on 6P films. We find that for both cases (6P on Sm, and Sm on 6P) the interaction at the interface is weak (physisorption), and the interfacial dipole is smaller than 0.2 eV. This weak interaction enabled us to demonstrate that small morphological changes in the substrate Sm film lead to a reversal in the direction of the interfacial dipole. For native silicon oxide substrates the appearance of a photoemission signal more than 1 eV above the Fermi level is observed, and explained by the energy level alignment process for wide bandgap organic semiconductors. Additionally, we demonstrate that large lateral vacuum level shifts (ca. 1 eV) can be locally realized on an inhomogeneous sample.
Oriented organic semiconductor thin films
Synthetic Metals, 2003
In this part of our investigations, we mainly use atomic force microscopy to study the growth of para-sexiphenyl (PSP) films on mica. It is shown that self-organization of PSP molecules occurs during the deposition controlled by the substrate temperature and deposition time. In addition, X-ray diffraction (XRD) measurements were performed using synchrotron radiation. They confirmed the very high crystalline quality of the grown films. #
Optical Materials, 1999
The deposition of thin ®lms of 3,3 HH ,4 HHH ,3 HHHHH -tetrahexyl-2,2 H :5 H ,2 HH :5 HH ,2 HHH :5 HHH ,2 HHHH :5 HHHH ,2 HHHHH -sexithiophene (4H6T) has been performed in ultra-high vacuum conditions by using organic molecular beam deposition (OMBD). Once temperature and pressure are optimized, the substrate choice is the key factor in determining the properties of the deposited ®lms. From the structural analysis of samples deposited on dierent inorganic and organic substrates as well as from the comparison with crystalline polymorphs of 4H6T, useful information for the control of both structural order and molecular orientation in the ®lms are inferred. The results of structural investigations by X-ray diraction, compared with optical analyses (electronic absorption and photoluminescence), are indicative of either intermolecular or sub-strate±4H6T interactions. Ó
Synthetic Metals, 1998
Highly ordered and oriented deposition of substituted sexithiophene molecules is obtained on single crystal organic substrates. The study of X-ray diffraction and of the optical absorption of the films measured under different angles of incidence and with different directions of light polarization permits a complete structural model to be proposed. In addition, some interesting conclusions about the average molecular orientation with respect to the substrate are drawn.
Revealing the buried interface: para-sexiphenyl thin films grown on TiO2(110)
Physical Chemistry Chemical Physics, 2010
The thickness dependent optical and electronic structure of para-sexiphenyl thin films grown on TiO 2 (110) at around 400 K reveals that the substrate is first wet by one monolayer of molecules lying with their long axis parallel to the [001] direction of the substrate, while the molecules in subsequent layers are almost standing upright. Whilst ultraviolet photoemission spectroscopy (UPS) is sensitive to the molecules in the outermost layer, reflection difference spectroscopy (RDS) shows that the molecules at the buried interface do not dewet and maintain the orientation of the original wetting monolayer.