Thomas Gredig

Angular Dependence of Vortex Annihilation Fields in Asymmetric Co Dots

Shape asymmetries in nominally circular nanomagnets provide a potential means for vortex chirality control. However, in realistic arrays their effects are challenging to probe since asymmetric magnetization reversal processes are often averaged to include distributions over all angles. Here we investigate how shape asymmetry influences the vortex reversal in arrays of sub-micron edge-cut Co dots. We find that the vortices can be manipulated to annihilate at particular sites under different field orientations and cycle sequences. The vortex annihilation field and degree of chirality control depend sensitively on the angular position of the applied field relative to the flat edge of the dots. For small angles, the major loop annihilation field is significantly larger than that found from the half loop and the vortex chirality can be well controlled. At intermediate angles the chirality control is lost and an interesting crossover in the annihilation field is found: the half loop actually extrudes outside of the major loop, exhibiting a larger vortex annihilation field. At large angles the annihilation fields along major and half loops become degenerate.

Origin of the anomalous X-ray diffraction in phthalocyanine films

The impact of the submolecular electron density on the X-ray diffraction profile of a layer-stacked thin film is studied experimentally and compared with numerical simulations based on the molecular structure and angular arrangement. Important structural information is contained in the X-ray diffraction profile of highly anisotropic molecular thin films, such as phthalocyanines. The results show that the intensity distribution of the diffraction peaks belonging to the same series of lattice planes provides important structural information including the molecular tilt angle and the center electron density of the molecule.

Height-Height Correlation Function to Determine Grain Size in Iron Phthalocyanine Thin Films

Quasi one-dimensional iron chains are formed in thermally evaporated iron phthalocyanine (FeC32N8H16) thin films on silicon substrates. The chain length is modified by the deposition temperature during growth. Atomic force microscopy images show spherical grains at low deposition temperatures that become highly elongated at high deposition temperatures due to diffusion. The grain distributions are quantitatively characterized with a watershed-based segmentation algorithm and a height-height correlation function. The grain size distributions are found to be characteristically distinct for the α-phase and β-phase samples. The average effective grain size from the distribution is proportional to the correlation length found from the height-height correlation function and grows exponentially with deposition temperature. The long-range roughness and Hurst parameter increase only slightly with the deposition temperature.

Mobility saturation in tapered edge bottom contact copper phthalocyanine thin film transistors

Copper phthalocyanine (CuPc) thin film transistors were fabricated using a tapered edge bottom contact device geometry, and mobility saturation was observed for devices with CuPc thicknesses of 12 monolayers (MLs) and greater. The mobility saturation is attributed to a significantly decreased contact resistance resulting from a bilayer resist lift-off method, as compared with a single layer resist lift-off method. Threshold voltages are also found to saturate above 12 ML CuPc thicknesses.

Molecular tilting and columnar stacking of Fe phthalocyanine thin films on Au(111)

Scanning tunneling microscopy and x-ray absorption spectroscopic results at the Fe K edge of Fe phthalocyanine (FePc) thin films grown on Au substrates, together with theoretical calculations, allow us to refine the structure of the film. In particular, we show that the columnar stacking of the FePc molecules is different from that found in bulk α and β phases. Moreover, the molecules do not lay parallel to the surface of the substrate. These structural findings are relevant to understand magnetism of FePc films.

Tunable finite-sized chains to control magnetic relaxation

The magnetic dynamics of low-dimensional iron ion chains have been studied with regards to the tunable finite-sized chain length using iron phthalocyanine thin films. The deposition temperature varies the diffusion length during thin film growth by limiting the average crystal size in the range from 40 to 110 nm. Using a method common for single chain magnets, the magnetic relaxation time for each chain length is determined from temporal remanence data and fit to a stretched exponential form in the temperature range below 5 K, the onset for magnetic hysteresis. A temperature-independent master curve is generated by scaling the remanence by its relaxation time to fit the energy barrier for spin reversal, and the single spin relaxation time. The energy barrier of 95 K is found to be independent of the chain length. In contrast, the single spin relaxation time increases with longer chains from under 1 ps to 800 ps. We show that thin films provide the nano-architecture to control magnetic relaxation and a testbed to study finite-size effects in low-dimensional magnetic systems.

Morphology-defined interaction of copper phthalocyanine with O2/H2O

Abstract. Copper phthalocyanine (CuPc) is an important hole transport layer for organic photovoltaics (OPVs), but interaction with ambient gas/vapor may lead to changes in its electronic properties and limit OPV device lifetimes. CuPc films of thickness 25 and 100 nm were grown by thermal sublimation at 25°C, 150°C, and 250°C in order to vary morphology. We measured electrical resistance and film mass in situ during exposure to controlled pulses of O2 and H2O vapor. CuPc films deposited at 250°C showed a factor of 5 higher uptake of O2 as detected by a quartz crystal microbalance (QCM), possibly due to the formation of β-CuPc at T>200°C which allows higher O2 mobility between stacked molecules. While weight-based measurements stabilize after ∼10  min of gas exposure, resistance response stabilizes over times >1  h, suggesting that mass change occurs by rapid adsorption at active surface sites whereas resistive response is dominated by slow diffusion of adsorbates into the bulk film. The 25 nm films exhibit higher resistive response than 100 nm films after an hour of O2/H2O exposure due to fast analyte diffusion down to the film/electrode interface. We found evidence of decoupling of CuPc from the gold-coated QCM crystal due to preferential adsorption of O2/H2O molecules on gold.

Magnetic anisotropy in Fe phthalocyanine film deposited on Si(110) substrate: Standing configuration

In this contribution we report on the structural and magnetic properties of an Fe phthalocyanine (FePc) thin film deposited on a silicon substrate. The planar FePc molecules order spontaneously in a standing configuration, i.e., with the molecular plane perpendicular to the substrate. The x-ray linear polarized absorption and x-ray magnetic circular dichroism experiments at the Fe-L2,3 edges at T = 6 K were performed, concluding that at this temperature the film displays magnetic anisotropy with the main easy axis perpendicular to the substrate. This result is explained in terms of the FePc single molecule anisotropy which has its larger moment in the molecule plane. Thus, the standing configuration in polycrystalline thin films favors statistically that, at the macroscopic array level, the magnetic easy anisotropy axis is normal to the substrate.

Effect of film morphology on oxygen and water interaction with copper phthalocyanine

Copper phthalocyanine (CuPc) films of thickness 25 nm and 100 nm were grown by thermal sublimation at 25°C, 150°C, and 250°C in order to vary morphology. Using a source-measure unit and a quartz crystal microbalance (QCM), we measured changes in electrical resistance and film mass in situ during exposure to controlled pulses of O2 and H2O vapor. Mass loading by O2 was enhanced by a factor of 5 in films deposited at 250°C, possibly due to the ~200°C CuPc α→β transition which allows higher O2 mobility between stacked molecules. While gas/vapor sorption occurred over timescales of < 10 minutes, resistance change occurred over timescales < 1 hour, suggesting that mass change occurs by rapid adsorption at active surface sites, whereas resistive response is dominated by slow diffusion of adsorbates into the film bulk. Resistive response generally increases with film deposition temperature due to increased porosity associated with larger crystalline domains. The 25 nm thick films exhibit higher resistive response than 100 nm thick films after an hour of O2/H2O exposure due to the smaller analyte diffusion length required for reaching the film/electrode interface. We found evidence of decoupling of CuPc from the gold-coated QCM crystal due to preferential adsorption of O2/H2O molecules on gold, which is consistent with findings of other studies.