1. 
 Bhowmick, Somnath, et al.
(författare)

Xray absorption spectra : Graphene, hBN, and their alloy
 2013

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 87:15, s. 155108

Tidskriftsartikel (refereegranskat)abstract
 Using firstprinciples density functional theory calculations, in conjunction with the MahanNozieresde Dominicis theory, we calculate the xray absorption spectra of the alloys of graphene and monolayer hexagonal boron nitride on a Ni (111) substrate. The chemical neighborhood of the constituent atoms (B, C, and N) inside the alloy differs from that of the parent phases. In a systematic way, we capture the change in the Kedge spectral shape, depending on the chemical neighborhood of B, C, and N. Our work also reiterates the importance of the dynamical corehole screening for a proper description of the xray absorption process in sp(2)bonded layered materials.


2. 
 Bidermane, Ieva, et al.
(författare)

Experimental and theoretical study of electronic structure of lutetium biphthalocyanine
 2013

Ingår i: Journal of Chemical Physics.  American Institute of Physics (AIP).  00219606. ; 138:23, s. 234701

Tidskriftsartikel (refereegranskat)abstract
 Using Near Edge XRay Absorption Fine Structure (NEXAFS) Spectroscopy, the thickness dependent formation of Lutetium Phthalocyanine (LuPc2) films on a stepped passivated Si(100)2x1 reconstructed surface was studied. Density functional theory (DFT) calculations were employed to gain detailed insights into the electronic structure. Photoelectron spectroscopy measurements have not revealed any noticeable interaction of LuPc2 with the Hpassivated Si surface. The presented study can be considered to give a comprehensive description of the LuPc2 molecular electronic structure. The DFT calculations reveal the interaction of the two molecular rings with each other and with the metallic center forming new kinds of orbitals in between the phthalocyanine rings, which allows to better understand the experimentally obtained NEXAFS results.


3. 
 Kocevski, Vancho, et al.
(författare)

Transition between direct and indirect band gap in silicon nanocrystals
 2013

Ingår i: Physical Review B. Condensed Matter and Materials Physics.  10980121. ; 87:24, s. 245401

Tidskriftsartikel (refereegranskat)abstract
 Using groundstate density functional theory we study the transition from indirect to direct band gap in hydrogenterminated silicon nanocrystals (NCs) as a function of decreasing diameter. The studied range, from 1.0 to 4.6 nm diameter of nanocrystals, with spherical and Wulffshape NCs, covers the transition from nanoto bulk regime. A change in the symmetry of the lowest unoccupied state as a function of decreasing NC diameter is observed, gradually increasing the oscillator strength of transitions from the highest occupied to the lowest unoccupied state. Real space and Fourier space characteristics of highest occupied and lowest unoccupied states are explored in detail and linked to a smooth transition from nanoto bulk regime.


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7. 
 Lidbaum, H, et al.
(författare)

EMCD in the TEM  Optimization of signal acquisition and data evaluation
 2008

Konferensbidrag (refereegranskat)abstract
 The method of electron magnetic circular dichroism (EMCD) was recently proposed anddemonstrated by Peter Schattschneider et al. [I]. The EMCD signal consists in differences of L3 andL2 edge intensities of ferromagnetic materials at specific diffraction vectors. EMCD is thereforeelement specific. Furthermore. it was shown recently that sum rules apply to the EMCD methodwhich is an essential progress to obtain quantitative magnetic information from this method [23].Though. the theoretical derivations of sum rule suppose that the obtained spin to orbital magneticmoments do not depend on the choice of scattering vector. This is a simplification which must beverified both by simulations and experiments to make EMCD a quantitative method for the study ofmagnetic moments.In the experiment. the dichroic signal. i.e. the change ofLJ and L2 edges at different diffractionvectors is small. i.e. of the order of 515% of the total signal intensity. Therefore. the acquisitionconditions must be optimized to both approach a di ffraction geometry where sum rules can beapplied and to maximize the signal to noise ratio.In this work. we optimize both. the signal and the signal/noise ratio. The experiments were carriedout on a FEI Tecnai F30 equipped with a Gatan GIF2002 energy filter. Instead of recording kvectordependent single spectra as in the original work [I], we acquire energy filtered diffraction patternsin the energy interval around the transition metal Ledge. This allows for extraction of the EMCDsignal at kvectors where the signal contains quantitative magnetic information. All measurementsof2 dimensional kspace maps of the EMCD signal are compared with simulations of the EM CDsignal. We find very good agreement between theoretical predictions and experimental values forboth. the kspace evolution and quantitative agreement the EMCD signal. The data evaluationincludes a careful normalization procedure. a statistical optimization of the signal to noise ratio aswell as the consideration of the entire edge intensity. Finally. we demonstrate the quantitativeprecision of the proposed method at the example ofa slightly textured iron layer (figures I and 2).References[I] P Schattschneider et aJ.. Nature 44 I (2006) 486.[2] J. Rusz, O. Eriksson, P. Novak, P. M. Oppeneer. Phys. Rev. B 76 (2007) 060408.[3] L. Calmels et aJ.. Phys. Rev. B 76 (2007) 060409.


8. 
 Lidbaum, Hans, et al.
(författare)

Experimental conditions and data evaluation forquantitative EMCD measurements in the TEM
 2008

Ingår i: European Microscopy Conference.

Konferensbidrag (refereegranskat)abstract
 The recently demonstrated technique electron energyloss spectroscopy (EMCD) [I)opens new routes for characterization of magnetic materials using transmission electronmicroscopy. The technique enables quantitative measurements of orbital to spinmagnetic moments with element specificity, according to the recently derived sum rules[2). Electron energyloss spectra is obtained at well defined scattering geometries, seefigure I.The principle of the technique having been demonstrated, further progress isrequired to obtain reliable quantitative information about the magnetic properties of thesample. By using energy filtered diffraction patterns, the distribution of the EM CDsignal in reciprocal space is obtained. The ava,lability of these data sets from a fullreciprocal plane allows for the optimisation of the data treatment. We study the theinfluence of experimental geometries on the EMCD signal and optimise data analysis ofthe probed reciprocal plane. This is essential to obtain correct and reliable magneticinformation. Especially normalization, signal to noise optimization and consideration ofthe entire edge intensities are important. The data cubes consisting of the reciprocalplane and energyloss were acquired using a FEI Tecnai F30ST microscope equippedwith a Gatan GIF2002 spectrometer. In figure 2, two spectra that were extracted at theP+ and P positions are shown. The experimental results are compared with calculationsof the EMCD signal for a thin Fe film, showing very good agreement.I. P. Schattschneider, S. Rubino, C. H~bert, J. Rusz, J. Kune~, P Novak, E. Carlino,M. Fabrizioli, G. Panaccione and G. Rossi, Nature 441 (2006), p. 486488.2. J. Rusz, O. Eriksson, P. Novak and P.M. Oppeneer, Phys. Rev. B 76 (2007),060408(R).


9. 
 Lidbaum, Hans, et al.
(författare)

Quantitative magnetic information from reciprocal space maps in transmission electron microscopy
 2009

Ingår i: Physical Review Letters.  00319007. ; 102:3, s. 037201

Tidskriftsartikel (refereegranskat)abstract
 One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08±0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative elementspecific technique for magnetic studies, using a widely available instrument with superior spatial resolution.


10. 
 Lidbaum, Hans, et al.
(författare)

Reciprocal and real space maps for EMCD experiments
 2010

Ingår i: Ultramicroscopy.  03043991. ; 110:11, s. 13801389

Tidskriftsartikel (refereegranskat)abstract
 Electron magnetic chiral dichroism (EMCD) is an emerging tool for quantitative measurements of magnetic properties using the transmission electron microscope (TEM), with the possibility of nanometer resolution. The geometrical conditions, data treatment and electron gun settings are found to influence the EMCD signal. In this article, particular care is taken to obtain a reliable quantitative measurement of the ratio of orbital to spin magnetic moment using energy filtered diffraction patterns. For this purpose, we describe a method for data treatment, normalization and selection of mirror axis. The experimental results are supported by theoretical simulations based on dynamical diffraction and density functional theory. Special settings of the electron gun, so called telefocus mode, enable a higher intensity of the electron beam, as well as a reduction of the influence from artifacts on the signal. Using these settings, we demonstrate the principle of acquiring real space maps of the EMCD signal. This enables advanced characterization of magnetic materials with superior spatial resolution.

