08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Epitaxy and scanning tunneling microscopy image contrast of copper-phthalocyanine on graphite and MoS2(1994) Ludwig, Christoph; Strohmaier, Rainer; Petersen, Jörg; Gompf, Bruno; Eisenmenger, WolfgangMonolayers of copper–phthalocyanine (Cu–Pc) on highly oriented pyrolytic graphite (HOPG) and MoS2 prepared by organic molecular beam epitaxy have been investigated by scanning tunneling microscopy. On both substrates there exist well defined preparation conditions leading to ordered two-dimensional arrays of flat lying molecules. On HOPG they form a close-packed structure with a nearly quadratic unit cell, whereas on MoS2 we found two phases, one close-packed and one rowlike phase. This rowlike phase can be explained by a long range interaction due to an adsorbate induced superstructure of the substrate, which also can be seen in the scanning tunneling microscopy images. In images with submolecular resolution, the molecules appear different on the two substrates. On MoS2 they look like a four-leaved clover, on graphite they show a more detailed inner structure.Item Open Access Dynamics of poling PVDF between 25°C and 120°C(1991) Eberle, Gernot; Eisenmenger, WolfgangPVDF films with high β-content are poled in an electric field of 60 MV/m at temperatures between 25°C and 120°C. At 25°C the alignment of dipoles takes place in a central poling zone during several hours of poling. When the temperature is increased to 120°C the poling time necessary to align the dipoles in this narrow zone is reduced to several seconds. In addition, at temperatures higher than 90°C and increased poling times the central poling zone first increases but later decreases in its dielectric displacement. Simultaneously in a 10 μm regime adjoining the positive electrode a secondary displacement zone starts to grow. The strong reduction of this peak under short circuit conditions indicates hetero-charge accumulation in front of an anode.Item Open Access Field induced gas emission of polymer films(1992) Selle, Michael; Eberle, Gernot; Gompf, Bruno; Eisenmenger, WolfgangPoling polyvinylidene fluoride under vacuum conditions causes strong gas emission, which is analyzed with a quadrupole mass spectrometer. The dominant peak in the mass spectrum is HF, which is produced by electrochemical reactions at the sample surface. After the electric field is switched off, the gas emission unexpectedly increases to a value higher than under the field. This can be explained by ionic charge detrappingand charge migrationItem Open Access Thermal motion of one-dimensional domain walls in monolayers of a polar polymer observed by Video-STM(1993) Ludwig, Christoph; Eberle, Gernot; Gompf, Bruno; Petersen, Jörg; Eisenmenger, WolfgangScanning tunneling microscopy (STM) has been used to investigate monolayers of the ferroelectric copolymer polyvinylidenefluoride/trifluoroethylene P(VDF/TrFE) showing images of ordered polymer monolayers. By scanning with video frame rate, direct observation of the motion of onedimensional domain walls was also possible for the first time. The images clearly show domain walls normal to the polymer chains. From measurements of the temperature dependence of the domain wall velocities the activation energy for the thermally generated kink motion was estimated. These results are compared with theoretical models describing domain wall motion in ferroelectric PVDF.Item Open Access High frequency phonon transmission through amorphous films(1990) Mebert, Joachim; Maile, Bernd; Eisenmenger, WolfgangWe now report on phonon spectroscopy measurements in the frequency range from 300 GHz - 1 THz on thin amorphous films of several materials (i.e. e-beam evaporated SiO2., α-Si and α-Ge). Some films were prepared in a residual gas atmosphere as H20 and N2.Item Open Access Polarization distribution in PVDF obtained by poling under constant current condition(1990) Neumann, Guido; Bihler, Eckardt; Eberle, Gernot; Eisenmenger, WolfgangSince the resistivity of PVDE films increases when the films are poled, the voltage across the samples must be increased during the poling process in order to keep the current constant. Under these conditions, the formation of a polarization zone in the center of the film depth is observed. The thickness of the polarization-free zone close to the film surfaces is found to decrease with larger current densities. The results are consistent with a model assuming charge trapping in deep traps associated with the orientation of polar crystallites.Item Open Access Phonon physics : acoustics at terahertz frequencies(1992) Eisenmenger, WolfgangIt is the natural aim of phonon physics at terahertz frequencies to extend the possible range of phonon spectroscopic investigations to the upper limit of 10THz. There are some approaches in this direction. Especially semiconductor physics and microelectronics see the importance of phonon aspects. With respect to the transitions of photons in high energetic phonons in semiconductors as Ga/As very interesting results have been obtained.Item Open Access STM investigations of PTCDA and PTCDI on graphite and MoS2 : a systematic study of epitaxy and STM image contrast(1994) Ludwig, Christoph; Gompf, Bruno; Petersen, Jörg; Strohmaier, Rainer; Eisenmenger, WolfgangMonolayers of the organic molecules perylene-3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and diimide (PTCDI) on graphite and MoS₂ have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations.Item Open Access Piezo and pyroelectricity in electrets : caused by charges, dipoles, or both?(1992) Sessler, Gerhard M.; Das-Gupta, Dilip K.; deReggi, Aimé S.; Eisenmenger, Wolfgang; Furukawa, Takeo; Giacometti, José A.; Gerhard-Multhaupt, ReimundIn a homogeneous medium space charges cannot cause piezo or pyroeiectricity. If stressed, there will be affine geometrical changes throughout the medium, and therefore the induction charges on the electrodes will remain the same, and piezoelectricity cannot be generated. However, if dipoles are present in such a homogeneous dielectric, then piezoelectricity is possible. In an inhomogeneous medium such as a semicrystalline polymer, charges and dipoles can cause piezoelectricity. If you have, for instance, a semicrystalline polymer then the crystalline and the amorphous parts are, upon compression, subjected to different degrees of deformation; and for this reason, a piezoelectric effect can be generated.Item Open Access Fibre-optic probe hydrophone for ultrasonic and shock-wave measurements in water(1993) Staudenraus, Joachim; Eisenmenger, WolfgangAimed at lithotripter acoustic output measurements, a new fibre-optic probe hydrophone overcomes most of the problems involved with the use of piezoelectric hydrophone technology in non-linear ultrasonic and shock-wave fields. The fibre-optic principle allows for extremely wide bandwidth (larger than 1 GHz) and superior electromagnetic shielding. Contrary to hitherto existing hydrophones a high cavitation threshold at the water-silica interface provides undistorted detection of strong rarefractional pulse pressures. Considering pure compression there is good agreement between maximum pulse pressure derived from fibre-optic hydrophone theory and the corresponding amplitudes obtained from acoustically calibrated PVDF membrane and needle hydrophones.
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