Michael Fronk

Optical and magneto-optical study of nickel and cobalt ferrite epitaxial thin films and submicron structures

Epitaxial films and ordered arrays of submicron structures of nickel and cobalt ferrites were deposited on Nb doped SrTiO3 by pulsed laser deposition. X-Ray diffraction and atomic force microscopy showed that the films have a good crystalline quality and smooth surfaces. A larger number of phonon bands was observed in the polarization dependent Raman spectra of the ferrite films than expected for the cubic spinel structures. This is explained by short range ordering of the Ni2+ (or Co2+) and Fe3+ cations at the octahedral sites inducing a lowering of the symmetry. The same behavior was also observed in the Raman spectra measured for the submicron structures, suggesting the same cation distribution as in the films. The diagonal components of the dielectric function for nickel and cobalt ferrites are determined from ellipsometry in the 0.73–5 eV photon energy range. The absorption edge was analyzed using a bandgap model and the energies for the indirect and direct optical transitions were calculated. It was...

Magneto-Optical Kerr Effect Spectroscopy—A Sensitive Tool for Investigating the Molecular Orientation in Organic Semiconductor Films

The detection and control of the molecular growth mode is a key prerequisite for fabricating opto-electronic devices. In this work we present the magneto-optical Kerr effect (MOKE) spectroscopy to be a highly sensitive method for the detection of the molecular orientation. On the example of metal free phthalocyanine (H(2)Pc) in thin films, it will be shown that also for diamagnetic molecules a strong magneto-optical response can be expected. The growth mode and thus the intensity of the MOKE signal of H2Pc is strongly influenced by a templating effect using ultrathin layers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). From the MOKE spectra in the energy range from 1.5 to 5.0 eV and the optical constants, the Voigt constant of thin organic films was determined. From the strong in-plane/out-of-plane anisotropy of the optical constants and the value of the Voigt constant the average molecular tilt angle of H2Pc molecules with respect to the substrate plane can be obtained.

Optical and magneto-optical properties of metal phthalocyanine and metal porphyrin thin films

The optical constants together with the magneto-optical Voigt constants of several phthalocyanine (Pc) and methoxy functionalized tetraphenylporphyrin (TMPP) thin films prepared on silicon substrates are presented. The materials investigated are MePc with Me = Fe, Co, Ni, Cu, Zn and MeTMPP with Me = Cu, Ni. We also compared our results to the metal-free H2Pc, H2TPP and H2TMPP. The experimental results will be supported by electronic structure calculations based on density functional theory (DFT) and interpreted using the perimeter model initially proposed by Platt. The model allows for qualitative understanding of the forbidden character of transitions in planar, aromatic molecules, and is able to qualify differences between Pc and TMPP type materials.

Electronic states and the influence of oxygen addition on the optical absorption behaviour of manganese phthalocyanine.

The effects of electronic states and air exposure on the spectroscopic properties of manganese phthalocyanine (MnPc) have been examined. The observed features of the Q-band in the absorption spectra can be explained by intrinsic electronic properties of MnPc, i.e., the formation of singly charged molecules by charge transfer excitations. However, the reaction of MnPc with atmospheric molecular oxygen leads to deviations in peak intensities but does not change the fundamental characteristics of the spectra. Nevertheless, the reaction with oxygen changes the spin state from S = 3/2 to S = 1/2. X-ray diffraction measurements also indicate a slow diffusion process of the oxygen into the MnPc crystal. We discuss both influences to explain the behaviour of MnPc in various spectroscopic methods (EELS, ellipsometry, PES). Furthermore, we support the experimental investigations by detailed ab-initio calculations of spectroscopic properties using methods of the density functional theory framework.

Tuning the magneto-optical response of TbPc2 single molecule magnets by the choice of the substrate

In this work, we investigated the magneto-optical response of thin films of TbPc2 on substrates which are relevant for (spin) organic field effect transistors (SiO2) or vertical spin valves (Co) in order to explore the possibility of implementing TbPc2 in magneto-electronic devices, the functionality of which includes optical reading. The optical and magneto-optical properties of TbPc2 thin films prepared by organic molecular beam deposition (OMBD) on silicon substrates covered with native oxide were investigated by variable angle spectroscopic ellipsometry (VASE) and magneto-optical Kerr effect (MOKE) spectroscopy at room temperature. The magneto-optical activity of the TbPc2 films can be significantly enhanced by one to two orders of magnitude upon changing the molecular orientation (from nearly standing molecules on SiO2/Si substrates to nearly lying molecules on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) templated SiO2/Si substrates) or by using metallic ferromagnetic substrates (Co).

Optical properties and electrical transport of thin films of terbium(III) bis(phthalocyanine) on cobalt

Summary The optical and electrical properties of terbium(III) bis(phthalocyanine) (TbPc2) films on cobalt substrates were studied using variable angle spectroscopic ellipsometry (VASE) and current sensing atomic force microscopy (cs-AFM). Thin films of TbPc2 with a thickness between 18 nm and 87 nm were prepared by organic molecular beam deposition onto a cobalt layer grown by electron beam evaporation. The molecular orientation of the molecules on the metallic film was estimated from the analysis of the spectroscopic ellipsometry data. A detailed analysis of the AFM topography shows that the TbPc2 films consist of islands which increase in size with the thickness of the organic film. Furthermore, the cs-AFM technique allows local variations of the organic film topography to be correlated with electrical transport properties. Local current mapping as well as local I–V spectroscopy shows that despite the granular structure of the films, the electrical transport is uniform through the organic films on the microscale. The AFM-based electrical measurements allow the local charge carrier mobility of the TbPc2 thin films to be quantified with nanoscale resolution.

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

The interface formation between manganese phthalocyanine (MnPc) and cobalt was investigated combining ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The transport band gap of the MnPc increases with the film thickness up to a value of (1.2 ± 0.3) eV while the optical band gap as determined from spectroscopic ellipsometry amounts to 0.5 eV. The gap values are smaller compared to other phthalocyanines due to metallic Mn 3d states close to the Fermi level. The transport band gap was found to open upon air exposure as a result of the disappearance of the occupied 3d electronic states.

Epitaxial growth and electronic properties of well ordered phthalocyanine heterojunctions MnPc/F16CoPc

We have prepared phthalocyanine heterojunctions out of MnPc and F16CoPc, which were studied by means of X-ray absorption spectroscopy. This heterojunction is characterized by a charge transfer at the interface, resulting in charged MnPc(δ +) and F16CoPc(δ -) species. Our data reveal that the molecules are well ordered and oriented parallel to the substrate surface. Furthermore, we demonstrate the filling of the Co 3d(z(2)) orbital due to the charge transfer, which supports the explanation of the density functional theory, that the charge transfer is local and affects the metal centers only.

Field-dependent magneto-optical Kerr effect spectroscopy applied to the magnetic component diagnosis of a rubrene/Ni system

Polar magneto-optical Kerr effect (MOKE) spectroscopy in the energy range from 1.75 eV to 5 eV at different magnetic field strength was applied to study Ni nanostructures formed on rubrene nanoislands. The magnetic hysteresis curves measured by MOKE change the shape depending on the photon energy and therefore deviate from those measured by superconducting quantum interference device (SQUID) magnetometry. Similar optical effects were previously observed in inorganic heterostructures. Our observations show that it correlates to the change in lineshape of the MOKE rotation and ellipticity spectra as a function of magnetic field strength. We show that this spectral dependence on magnetic field can be exploited to separate the contributions of two magnetic components to the magneto-optical spectra and hysteresis. The proposed model does not require the a priori knowledge of the (magneto-)optical constants of the heterostructure and its components.

Chemisorption of Exchange‐Coupled [Ni2L(dppba)]+ Complexes on Gold by Using Ambidentate 4‐(Diphenylphosphino)benzoate Co‐Ligands

A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2L(Cl)]ClO4 (1ClO4), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2L(dppba)](+), which can be isolated as an air-stable perchlorate [Ni2L(dppba)]ClO4 (2ClO4) or tetraphenylborate [Ni2L(dppba)]BPh4 (2BPh4) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II)2Au(I) complex [Ni(II)2L(dppba)Au(I)Cl]ClO4 (3ClO4). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II)2L(dppba)Au(I)Ph]BPh4 (4BPh4), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J = +15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H = -2 JS1S2). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative Au-P bonds in a monolayer.