C. Seidel

The influence of molecular aggregation on the device properties of organic light emitting diodes

Two new materials for the light-emitting layer in organic light emitting devices, N′N′Bis (2,6-xylyl)perylene-3,4:9,10-bis(dicarboximide) (DPP-PTCDI), and N,N′-Dimethylperylene-3,4:9,10-bis(dicarboximide) (DMe-PTCDI) are compared by an spectroscopic and opto-electronic study. Measurements of the absorbance-spectra, the I–V-characteristic, photovoltage- and electroluminescence-spectra were done. Both materials could be identified as n-type materials. DPP-PTCDI is much more efficient as a light emitting material than DMe-PTCDI due to a hampered dimerization for the DPP-PTCDI-molecules. The influence of the size of the functional group on the aggregation and therewith on the quantum-efficiency for fluorescence and electroluminescence is discussed.

A combined STM, LEED and molecular modelling study of PTCDA grown on Ag(110)

Abstract Perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) was commensurably grown on Ag(110) by organic molecular-beam deposition (OMBD) under ultra-high vacuum conditions at 5 × 10−8 Pa. In the monolayer regime, an epitaxial superstructure exists in a single domain orientation, as investigated with low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Molecular modelling calculations result in the same commensurate structure and provide, in addition, the absolute position of the adsorbate relative to the silver surface. As a result, the flat-lying PTCDA molecules are orientated in the (001) direction of the Ag(110) unit cell, leaving small holes with three uncovered silver atoms between the molecules.

Determination of tip–sample interaction forces from measured dynamic force spectroscopy curves

Abstract The forces between a sharp tip and a sample are characteristic for different sample materials. A new method for quantifying the elastic tip–sample interaction forces from measured frequency vs. distance curves is presented. The dynamic force–spectroscopy curves investigated were obtained by dynamic force microscopy under ultrahigh vacuum (UHV) conditions for large vibration amplitudes with commercial levers/tips. The full non-linear force–distance relationship is deduced via a numerical algorithm, where the equation of motion describing the oscillation of the tip is solved explicitly. The elastic force distance dependence can be determined by fitting the results of a computer simulation to experimental frequency vs. distance data. The obtained force–distance curves can be compared quantitatively with theoretical models.

Ar plasma treated and Al metallised polycarbonate: a XPS, mass spectroscopy and SFM study

Abstract Ar plasma etched and Al metallised bisphenol A carbonate was analysed by mass spectroscopy, photoelectron spectroscopy (XPS), and scanning force microscopy (SFM). We mainly used a technical polymer (Makrolon 2808, Bayer) made by injection-moulding, as well as spin coated bisphenol A carbonate ( n =1) and polycarbonate (PC) ( n =115). The mass spectroscopy during the etching process shows the degradation of the PC in the form of carbon monoxide, carbon dioxide and methyl groups. The photoelectron spectroscopy shows in detail the surface modification after Ar plasma treatment and metallisation. The plasma induces a reduction of the carboxylic carbon (C 1s), a strong reduction of singly bonded oxygen (O 1s) and also a slight reduction of doubly bonded oxygen. After Al metallisation, a reaction of Al with the oxygen groups and an interaction with the aromatic system is documented. Ar plasma etching increases the chemical interaction of Al mainly with the aromatic carbon. The X-ray photoelectron spectroscopy of metallised PC under different initial conditions shows a strong influence of incorporated water in the PC bulk that cannot be seen by XPS on uncoated PC. The O 1s signal increases during metallisation and results in an oxidation of Al probably caused by the fact that the hydrophobic surfaces becomes hydrophillic. Temperature-dependent XPS was done on technical PC samples and on spin coated samples ( n =1, n =115) and supports the influence of the bulk state for the Al–PC interface. For n =1 carbonate, a diffusion of Al into the PC volume was observed. The SFM measurements showed a roughening effect on the nanometer scale even after short treatment times. Al can be seen as a weakly bound cluster on the virgin PC, and if a pre-etching is done, Al seems to grow as a good wetting film. The adhesion force of Al films on PC without any influence of the volume can be explained by the chemical bonding of Al to the carboxylic and aromatic systems. The adhesion can be increased by plasma pre-treatment. A breakdown of the adhesion on technical PC is probably induced by a reaction of Al with mobile intercalated gas, that is enriched near the surface after Al coating.

Real-time monitoring of phase transitions of vacuum deposited organic films by molecular beam deposition LEED

Abstract We describe a special low energy electron diffraction (LEED) system that allows us to record diffraction patterns during the adsorption process of vacuum-deposited organic thin films. With this instrument that combines Knudsen cells within a reverse view LEED instrument we can evaporate three different organic materials independently and simultaneously from each other. In contrast to conventional LEED, this molecular beam deposition LEED (MBD–LEED) allows us to study the kinetics of structural phase transitions from the sub-monolayer regime up to several layers without changing the position of the sample which is located on a temperature-controlled sample holder. The performance of the device is demonstrated by a growth study of perylene-3,4,9,10-tetracarboxylic-dianhydride on Ag(110). A phase transition was in situ observed after the preparation of a single domain oriented homogeneous monolayer (“brick stone”) structure to a condensed (“herring bone”) structure which is commensurate in one direction.

Oriented growth of DMe-PTCDI on Ag(110); an LEED, XPS, SFM and STM characterisation

Abstract Epitaxial monolayer and multilayer structures of DMe-PTCDI (of 2,9-dimethyl-antra [2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone) were prepared on Ag(110) by molecular beam epitaxy. This work presents structural and electron photoemission data for further investigations where highly laterally oriented multilayer films are important. Preparation conditions for five highly ordered monolayer and two multilayer structures are presented. LEED images taken during preparation show the transitions between the exposure and temperature-induced structures. In addition, the structures were examined by XPS, SFM and STM.

Aluminium deposition on SF6 plasma‐treated polycarbonate: an AFM, XPS and mass spectroscopy study

A systematic investigation was made of the chemical and morphological influences of SF 6 plasma on polycarbonate and the influence of plasma treatment on Al metallization. Mass and ion spectroscopy were used for characterization of the plasma and the etching process. X-ray photoelectron spectroscopy (XPS) measurements were applied for the chemical characterization, while atomic force microscopy (AFM) (static and dynamic mode) served to inspect the surface morphology. All analytical techniques were performed in an ultrahigh vacuum system, in order to prevent the polycarbonate sample from being exposed to ambient air after the plasma treatment. During the etching process we used mass difference spectra to demonstrate the removal of masses 19, 28 and 32 corresponding to HF, CO (N 2 ) and CF. Additionally, the inclusion of fluorine was also observed by this technique. The XPS spectra of polycarbonate surfaces show a significant inclusion of fluorine (C-F, C-F 2 ) and a reduction of the oxygen content after the plasma treatment. Aluminium metallization leads to the formation of an A<F interlayer; metallic growth of Al is only observed when the metallic layers become thicker than a few nanometres. The AFM investigations have shown that even a short plasma treatment causes changes in morphology (structures with an extension of 20-40 nm). After extended plasma exposure the surface becomes very rough, resulting in poor Al adhesion. On untreated polycarbonate, Al grows in the form of weakly bound clusters, which can only be imaged in the dynamic AFM mode. After plasma treatment, Al grows in the form of well-adhering flat layers without clustering.

The suppression of water-diffusion in polycarbonate through Ar- and He-plasma as a new model for the origin of improved adhesion of Al

Abstract The diffusion process from water into and out of the bulk of an injection-molded bisphenol-A-polycarbonate sample has been characterized by weighing. In the equilibrium state, the amount of water inside the polycarbonate depends, therefore, on the environmental humidity. Transferring the experiments into UHV showed that the amount of water could be determined by the time-integrated H 2 O-signal of a mass-spectrometer as depending on the storage time in vacuum before measurement. After Ar-plasma etching; samples showed a reduced H 2 O-signal, which is the consequence of a hydrophobic barrier on the sample’s surface created by the plasma. The thesis of a barrier was supported by XPS-measurements, which showed a decrease of the polar carboxylic group of the polycarbonate.

The effect of annealing on the photoluminescence of epitaxial DMe-PTCDI multilayers on Ag(110)

Abstract Epitaxial multilayer structures of DMe-PTCDI (2,9-dimethyl-antra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone) on Ag(110) were prepared by molecular beam epitaxy. Photoluminescence spectra obtained under ultra high vacuum (UHV) conditions of the multilayers show significant polarisation dependence. By combining this result with a real space model derived from low energy electron diffraction (LEED) measurements the molecular transition dipole moment was determined. During an annealing process the fluorescence of the multilayer spectrum changes dramatically. Possible reasons for this result are discussed and compared with tapping-mode atomic force microscope (AFM) and fluorescence microscopy measurements.