ORCID Profile
0000-0001-5752-2852
Current Organisations
Faculty of Science and Medicine, University of Fribourg
,
Empa
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Publisher: American Chemical Society (ACS)
Date: 14-08-2003
DOI: 10.1021/IC034110T
Abstract: Spectroscopic, structural, and magnetic data are presented for Fe(C(3)H(4)N(2))(6)(NO(3))(2), which facilitate a precise definition of the electronic and molecular structure of the [Fe(Im)(6)](2+) cation. The structure was determined at 120(1) K by X-ray diffraction methods. The salt crystallizes in the trigonal space group R3 with unit-cell parameters a = 12.4380(14) A, c = 14.5511(18) A, and Z = 3. All the imidazole ligands are equivalent with an Fe-N bond distance of 2.204(1) A. Variable-temperature inelastic neutron scattering (INS) measurements identify a cold magnetic transition at 19.4(2) cm(-1) and a hot transition at 75.7(6) cm(-1). The data are interpreted using a ligand field Hamiltonian acting in the weak-field (5)D basis, from which the diagonal trigonal field splitting of the (5)T(2g) (O(h)) term is estimated as approximately 485 cm(-1), with the (5)A(g) (S(6)) component lower lying. High-field multifrequency (HFMF) EPR data and measurements of the magnetic susceptibility are also reported and can be satisfactorily modeled using the energies and wave functions derived from analysis of the INS data. The electronic and molecular structures are related through angular overlap model calculations, treating the imidazole ligand as a weak pi-donor.
Publisher: Springer Science and Business Media LLC
Date: 2013
DOI: 10.1557/OPL.2013.842
Abstract: We report on the maskless integration of micron-sized GaAs crystals on patterned Si substrates by metal organic vapor phase epitaxy. In order to adapt the mismatch between the lattice parameter and thermal expansion coefficient of GaAs and Si, 2 μm tall Ge crystals were first grown as virtual substrate by low energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs structures grown on top of the Ge crystals at the transition towards full pyramids with energetically stable {111} facets. A substantial release of strain is shown in GaAs crystals with a height of 2 μm and lateral sizes up to 15×15 μm 2 by both X-ray diffraction and photoluminescence.
Publisher: IEEE
Date: 10-2012
Publisher: American Chemical Society (ACS)
Date: 10-05-2005
DOI: 10.1021/IC050134J
Abstract: A new family of tetranuclear Ni complexes [Ni(4)(ROH)(4)L(4)] (H(2)L = salicylidene-2-ethanolamine R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni(4)O(4)] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni(II) centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximately -0.93 cm(-)(1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.
Publisher: American Chemical Society (ACS)
Date: 29-10-2003
DOI: 10.1021/JA0367086
Abstract: A new family of tetranuclear Mn complexes [Mn4X4L4] (H2L = salicylidene-2-ethanolamine X = Cl (1) or Br (2)) and [Mn4Cl4(L')4] (H2L' = 4-tert-butyl-salicylidene-2-ethanolamine, (3)) has been synthesized and studied. Complexes 1-3 possess a square-shaped core with ferromagnetic exchange interactions between the four Mn(III) centers resulting in an S = 8 spin ground state. Magnetochemical studies and high-frequency EPR spectroscopy reveal an axial magnetoanisotropy with D values in the range -0.10 to -0.20 cm(-1) for complexes 2 and 3 and for differently solvated forms of 1. As a result, these species possess an anisotropy-induced energy barrier to magnetization reversal and display slow relaxation of the magnetization, which is observed as hysteresis for 1 and 3 and frequency-dependent peaks in out-of-phase AC susceptibility measurements for 3. The effective energy barrier was determined to be 7.7 and 7.9 K for 1 and 3, respectively, and evidence for quantum tunneling of the magnetization was observed. Detailed magnetochemical studies, including measurements at ultralow temperatures, have revealed that complexes 1 and 2 possess solvation-dependent antiferromagnetic intermolecular interactions. Complex 3 displays ferromagnetic intermolecular interactions and approaches a ferromagnetic phase transition with a critical temperature of approximately 1 K, which is coincident with the onset of slow relaxation of the magnetization due to the molecular anisotropy barrier to magnetization reversal. It was found that the intermolecular interactions have a significant effect on the manifestation of slow relaxation of the magnetization, and thereby, these complexes represent a new family of "exchange-biased single-molecule magnets", where the exchange bias is controlled by chemical and structural modifications.
Publisher: AIP Publishing
Date: 13-01-2014
DOI: 10.1063/1.4861864
Abstract: We report on the mask-less integration of GaAs crystals several microns in size on patterned Si substrates by metal organic vapor phase epitaxy. The lattice parameter mismatch is bridged by first growing 2-μm-tall intermediate Ge mesas on 8-μm-tall Si pillars by low-energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs crystals towards full pyramids exhibiting energetically stable {111} facets with decreasing Si pillar size. The release of the strain induced by the mismatch of thermal expansion coefficients in the GaAs crystals has been studied by X-ray diffraction and photoluminescence measurements. The strain release mechanism is discussed within the framework of linear elasticity theory by Finite Element Method simulations, based on realistic geometries extracted from scanning electron microscopy images.
Publisher: American Chemical Society (ACS)
Date: 10-07-2004
DOI: 10.1021/IC049600F
Abstract: Nine members of a new family of polynuclear ferric complexes have been synthesized and characterized. The reaction of Fe(O(2)CMe)(2) with polydentate Schiff base proligands (H(2)L) derived from salicylidene-2-ethanolamine, followed in some cases by reaction with carboxylic acids, has afforded new complexes of general formulas [Fe(2)(pic)(2)(L)(2)] (where pic(-) is the anion of 2-picolinic acid), [Fe(3)(O(2)CMe)(3)(L)(3)], [Fe(4)(OR)(2)(O(2)CMe)(2)(L)(4)], and [Fe(5)O(OH)(O(2)CR)(4)(L)(4)]. The tri-, tetra-, and pentanuclear complexes all possess unusual structures and novel core topologies. Mössbauer spectroscopy confirms the presence of high-spin ferric centers in the tri- and pentanuclear complexes. Variable-temperature magnetic measurements suggest spin ground states of S = 0, 1/2, 0, and 5/2 for the bi-, tri-, tetra-, and pentanuclear complexes, respectively. Fits of the magnetic susceptibility data have provided the magnitude of the exclusively antiferromagnetic exchange interactions. In addition, an easy-axis-type magnetic anisotropy has been observed for the pentanuclear complexes, with D values of approximately -0.4 cm(-)(1) determined from modeling the low-temperature magnetization data. A low-temperature micro-SQUID study of one of the pentanuclear complexes reveals magnetization hysteresis at nonzero field. This is attributed to an anisotropy-induced energy barrier to magnetization reversal that is of molecular origin. Finally, an inelastic neutron scattering study of one of the trinuclear complexes has revealed that the magnetic behavior arises from two distinct species.
Publisher: American Chemical Society (ACS)
Date: 05-04-2005
DOI: 10.1021/IC048411U
Abstract: The syntheses, structures, and magnetochemical characterization of two novel mixed-chelate undeca- and dodecanuclear ferric complexes are reported. Preformed tri- and pentanuclear ferric complexes that possess tridentate Schiff base (L2- and (L'2-) and acetate ligands were reacted with 1,1,1-tris(hydroxymethyl)ethane (H3thme) to afford [Fe11O3(OH)(O2CMe)8(thme)2(L)6] (1) and [Fe12O4(O2CMe)8(thme)2(NH2(CH2)2O)2(L')6] (2), respectively, following structural agglomeration and rearrangement associated with ligand substitution. The incorporation of more than one type of ligand that can both chelate and bridge the Fe centers gives rise to the complicated molecular structures displayed by 1 and 2. As a result of the tripodal conformation of thme3-, the cores of both molecules incorporate several face-shared defect {Fe3O4}+ cuboidal subunits. Variable-temperature dc and ac magnetic susceptibility studies, together with low-temperature magnetization measurements, are consistent with S = 5/2 and S = 0 ground-state spins for 1 and 2, respectively, and suggest that excited states with higher spin values lie relatively close in energy to the ground state for both species. Low-temperature micro-SQUID measurements on oriented single crystals of 1 confirm the easy-axis type magnetic anisotropy suggested by conventional SQUID magnetometry. However magnetization hysteresis is not observed down to 0.04 K, which is ascribed to rapid quantum tunneling of the magnetization associated with transverse interactions.
Publisher: IEEE
Date: 06-2012
Publisher: IOP Publishing
Date: 14-03-2014
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C8CP06034G
Abstract: Micro transmittance spectroscopy of ultrathin crystalline organic salt semiconductors reveals strong low-energy and weak high-energy bands polarized along the crystallographic b - and c -axis. While the excitonic coupling pattern explains the occurrence of polarized bands, the observed intensities are opposite of expectations based on chromophore alignment within the crystal.
Publisher: Elsevier BV
Date: 04-2014
Publisher: The Electrochemical Society
Date: 12-08-2014
Abstract: We show that geometric shielding of the reactive flux in chemical vapor deposition by tall neighboring structures obtained by deep substrate patterning, along with short surface diffusion lengths, can provide nearly space filling arrays of high-quality epitaxial crystals despite large mismatches of lattice parameters and thermal expansion coefficients. The density of extended defects is strongly reduced by the method, and wafer bowing and crack formation largely inhibited. The concept is shown to be valid for SiGe/Si heterostructures ranging from pure Si to pure Ge both on Si(001) and Si(111) substrates. Here, dislocations are efficiently eliminated from three-dimensional faceted crystals with high-aspect ratios on top of micron-sized Si pillars. The application to 3C-SiC/Si(001) ridges, characterized by a lattice mismatch of nearly 20%, provides significantly lower stacking fault densities compared with layers grown on planar substrates.
Publisher: American Chemical Society (ACS)
Date: 23-11-2004
DOI: 10.1021/JA046095C
Abstract: Variable-temperature spectroscopic and crystallographic studies on the chromium(II) Tutton's salts, (MI)2Cr(X2O)6(SO4)2, where MI = ND4+, Rb+, or Cs+, and X = H or D, are reported. Inelastic neutron scattering (INS) and multifrequency EPR experiments facilitate a rigorous definition of the ground-state electronic structure from 1.5 up to 296 K, which is unprecedented for a high-spin d4 complex. Modeling of the INS data using a conventional S = 2 spin Hamiltonian reveals a dramatic variation in the axial and rhombic zero-field-splitting parameters. For the ammonium salt, D and E are -2.454(3) and 0.087(3) cm(-1) at 10 K and -2.29(2) and 0.16(3) cm(-1) at 250 K, respectively. A temperature variation in the stereochemistry of the [Cr(D2O)6]2+ complex is also identified, with an apparent coalescence of the long and medium Cr-O bond lengths at temperatures above 150 K. The corresponding changes for the rubidium and cesium salts are notable, though less pronounced. The experimental quantities are interpreted using a 5Ee Jahn-Teller Hamiltonian, perturbed by anisotropic strain. It is shown that good agreement can be obtained only by employing a model in which the anisotropic strain is itself temperature dependent. A new theoretical approach for calculating variable-temperature EPR spectra of high-spin d4 complexes, developed within the 5Ee coupling model, is described. Differences between spin-Hamiltonian parameters determined by INS and EPR are consistent with those of the different time scales of the two techniques.
Location: Switzerland
Location: Switzerland
No related grants have been discovered for Antonia Neels.