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“Exploring possibilities of band gap measurement with off-axis EELS in TEM”. Korneychuk S, Partoens B, Guzzinati G, Ramaneti R, Derluyn J, Haenen K, Verbeeck J, Ultramicroscopy 189, 76 (2018). http://doi.org/10.1016/J.ULTRAMIC.2018.03.021
Abstract: A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inelastic electron scattering probabilities demonstrate the beneficial influence of the Bessel aperture in this setup even for high accelerating voltages. The importance of selecting the optimal experimental convergence and collection angles is highlighted. The effect of the created off-axis acquisition conditions on the selection of the transitions from valence to conduction bands is discussed in detail on a simplified isotropic two band model. This opens the opportunity for deliberately selecting certain transitions by carefully tuning the microscope parameters. The suggested approach is experimentally demonstrated and provides good signal to noise ratio and interpretable band gap signals on reference samples of diamond, GaN and AlN while offering spatial resolution in the nm range. (C) 2018 Elsevier B.V. All rights reserved.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Condensed Matter Theory (CMT)
Impact Factor: 2.843
Times cited: 7
DOI: 10.1016/J.ULTRAMIC.2018.03.021
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“Casting light on the darkening of colors in historical paintings”. da Pieve F, Hogan C, Lamoen D, Verbeeck J, Vanmeert F, Radepont M, Cotte M, Janssens K, Gonze X, Van Tendeloo G, Physical review letters 111, 208302 (2013). http://doi.org/10.1103/PhysRevLett.111.208302
Abstract: The degradation of colors in historical paintings affects our cultural heritage in both museums and archeological sites. Despite intensive experimental studies, the origin of darkening of one of the most ancient pigments known to humankind, vermilion (α-HgS), remains unexplained. Here, by combining many-body theoretical spectroscopy and high-resolution microscopic x-ray diffraction, we clarify the composition of the damaged paint work and demonstrate possible physicochemical processes, induced by illumination and exposure to humidity and air, that cause photoactivation of the original pigment and the degradation of the secondary minerals. The results suggest a new path for the darkening process which was never considered by previous studies and prompt a critical examination of their findings.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); AXES (Antwerp X-ray Analysis, Electrochemistry and Speciation)
Impact Factor: 8.462
Times cited: 30
DOI: 10.1103/PhysRevLett.111.208302
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“Degradation process of lead chromate in paintings by Vincent van Gogh studied by means of spectromicroscopic methods : 3 : synthesis, characterization, and detection of different crystal forms of the chrome yellow pigment”. Monico L, Janssens K, Miliani C, Brunetti BG, Vagnini M, Vanmeert F, Falkenberg G, Abakumov A, Lu Y, Tian H, Verbeeck J, Radepont M, Cotte M, Hendriks E, Geldof M, van der Loeff L, Salvant J, Menu M;, Analytical chemistry 85, 860 (2013). http://doi.org/10.1021/ac302158b
Abstract: The painter, Vincent van Gogh, and some of his contemporaries frequently made use of the pigment chrome yellow that is known to show a tendency toward darkening. This pigment may correspond to various chemical compounds such as PbCrO4 and PbCr1-xSxO4, that may each be present in various crystallographic forms with different tendencies toward degradation. Investigations by X-ray diffraction (XRD), mid-Fourier Transform infrared (FTIR), and Raman instruments (benchtop and portable) and synchrotron radiation-based micro-XRD and X-ray absorption near edge structure spectroscopy performed on oil-paint models, prepared with in-house synthesized PbCrO4 and PbCr1-xSxO4, permitted us to characterize the spectroscopic features of the various forms. On the basis of these results, an extended study has been carried out on historic paint tubes and on embedded paint microsamples taken from yellow-orange/pale yellow areas of 12 Van Gogh paintings, demonstrating that Van Gogh effectively made use of different chrome yellow types. This conclusion was also confirmed by in situ mid-FTIR investigations on Van Goghs Portrait of Gauguin (Van Gogh Museum, Amsterdam).
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); AXES (Antwerp X-ray Analysis, Electrochemistry and Speciation)
Impact Factor: 6.32
Times cited: 79
DOI: 10.1021/ac302158b
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“Degradation process of lead chromate in paintings by Vincent van Gogh studied by means of synchrotron X-ray spectromicroscopy and related methods : 1 : artificially aged model samples”. Monico L, van der Snickt G, Janssens K, de Nolf W, Miliani C, Verbeeck J, Tian H, Tan H, Dik J, Radepont M, Cotte M, Analytical chemistry 83, 1214 (2011). http://doi.org/10.1021/ac102424h
Abstract: On several paintings by artists of the end of the 19th century and the beginning of the 20th Century a darkening of the original yellow areas, painted with the chrome yellow pigment (PbCrO4, PbCrO4·xPbSO4, or PbCrO4·xPbO) is observed. The most famous of these are the various Sunflowers paintings Vincent van Gogh made during his career. In the first part of this work, we attempt to elucidate the degradation process of chrome yellow by studying artificially aged model samples. In view of the very thin (1−3 μm) alteration layers that are formed, high lateral resolution spectroscopic methods such as microscopic X-ray absorption near edge (μ-XANES), X-ray fluorescence spectrometry (μ-XRF), and electron energy loss spectrometry (EELS) were employed. Some of these use synchrotron radiation (SR). Additionally, microscopic SR X-ray diffraction (SR μ-XRD), μ-Raman, and mid-FTIR spectroscopy were employed to completely characterize the samples. The formation of Cr(III) compounds at the surface of the chrome yellow paint layers is particularly observed in one aged model sample taken from a historic paint tube (ca. 1914). About two-thirds of the chromium that is present at the surface has reduced from the hexavalent to the trivalent state. The EELS and μ-XANES spectra are consistent with the presence of Cr2O3·2H2O (viridian). Moreover, as demonstrated by μ-XANES, the presence of another Cr(III) compound, such as either Cr2(SO4)3·H2O or (CH3CO2)7Cr3(OH)2 [chromium(III) acetate hydroxide], is likely.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); AXES (Antwerp X-ray Analysis, Electrochemistry and Speciation)
Impact Factor: 6.32
Times cited: 113
DOI: 10.1021/ac102424h
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“Nanoscale investigation of the degradation mechanism of a historical chrome yellow paint by quantitative electron energy loss spectroscopy mapping of chromium species”. Tan H, Tian H, Verbeeck J, Janssens K, Van Tendeloo G, Angewandte Chemie: international edition in English 52, 11360 (2013). http://doi.org/10.1002/anie.201305753
Abstract: Getting the picture: The investigation of 100 year old chrome yellow paint by transmission electron microscopy and spectroscopy has led to the identification of four types of coreshell particles. This nanoscale investigation has allowed a mechanism to be proposed for the darkening of some bright yellow colors in Van Gogh's paintings (e.g. in Falling leaves (Les Alyscamps), 1888).
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); AXES (Antwerp X-ray Analysis, Electrochemistry and Speciation)
Impact Factor: 11.994
Times cited: 29
DOI: 10.1002/anie.201305753
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“Non-invasive and non-destructive examination of artistic pigments, paints, and paintings by means of X-Ray methods”. Janssens K, van der Snickt G, Vanmeert F, Legrand S, Nuyts G, Alfeld M, Monico L, Anaf W, de Nolf W, Vermeulen M, Verbeeck J, De Wael K, Topics in Current Chemistry 374, 81 (2016). http://doi.org/10.1007/S41061-016-0079-2
Abstract: Recent studies are concisely reviewed, in which X-ray beams of (sub)micrometre to millimetre dimensions have been used for non-destructive analysis and characterization of pigments, minute paint samples, and/or entire paintings from the seventeenth to the early twentieth century painters. The overview presented encompasses the use of laboratory and synchrotron radiation-based instrumentation and deals with the use of several variants of X-ray fluorescence (XRF) as a method of elemental analysis and imaging, as well as with the combined use of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Microscopic XRF is a variant of the method that is well suited to visualize the elemental distribution of key elements, mostly metals, present in paint multi-layers, on the length scale from 1 to 100 μm inside micro-samples taken from paintings. In the context of the characterization of artists pigments subjected to natural degradation, the use of methods limited to elemental analysis or imaging usually is not sufficient to elucidate the chemical transformations that have taken place. However, at synchrotron facilities, combinations of μ-XRF with related methods such as μ-XAS and μ-XRD have proven themselves to be very suitable for such studies. Their use is often combined with microscopic Fourier transform infra-red spectroscopy and/or Raman microscopy since these methods deliver complementary information of high molecular specificity at more or less the same length scale as the X-ray microprobe techniques. Since microscopic investigation of a relatively limited number of minute paint samples, taken from a given work of art, may not yield representative information about the entire artefact, several methods for macroscopic, non-invasive imaging have recently been developed. Those based on XRF scanning and full-field hyperspectral imaging appear very promising; some recent published results are discussed.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); AXES (Antwerp X-ray Analysis, Electrochemistry and Speciation)
Impact Factor: 4.033
Times cited: 50
DOI: 10.1007/S41061-016-0079-2
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“Deeper insights into the photoluminescence properties and (photo)chemical reactivity of cadmium red (CdS1-xSex) paints in renowned twentieth century paintings by state-of-the-art investigations at multiple length scales”. Monico L, Rosi F, Vivani R, Cartechini L, Janssens K, Gauquelin N, Chezganov D, Verbeeck J, Cotte M, D'Acapito F, Barni L, Grazia C, Buemi LP, Andral J-L, Miliani C, Romani A, The European Physical Journal Plus 137, 311 (2022). http://doi.org/10.1140/EPJP/S13360-022-02447-7
Abstract: Cadmium red is the name used for denoting a class of twentieth century artists' pigments described by the general formula CdS1-xSex. For their vibrant hues and excellent covering power, a number of renowned modern and contemporary painters, including Jackson Pollock, often used cadmium reds. As direct band gap semiconductors, CdS1-xSex compounds undergo direct radiative recombination (with emissions from the green to orange region) and radiative deactivation from intragap trapping states due to crystal defects, which give rise to two peculiar red-NIR emissions, known as deep level emissions (DLEs). The positions of the DLEs mainly depend on the Se content of CdS1-xSex; thus, photoluminescence and diffuse reflectance vis-NIR spectroscopy have been profitably used for the non-invasive identification of different cadmium red varieties in artworks over the last decade. Systematic knowledge is however currently lacking on what are the parameters related to intrinsic crystal defects of CdS1-xSex and environmental factors influencing the spectral properties of DLEs as well as on the overall (photo)chemical reactivity of cadmium reds in paint matrixes. Here, we present the application of a novel multi-length scale and multi-method approach to deepen insights into the photoluminescence properties and (photo)chemical reactivity of cadmium reds in oil paintings by combining both well established and new non-invasive/non-destructive analytical techniques, including macro-scale vis-NIR and vibrational spectroscopies and micro-/nano-scale advanced electron microscopy mapping and X-ray methods employing synchrotron radiation and conventional sources. Macro-scale vis-NIR spectroscopy data obtained from the in situ non-invasive analysis of nine masterpieces by Gerardo Dottori, Jackson Pollock and Nicolas de Stael allowed classifying the CdS1-xSex-paints in three groups, according to the relative intensity of the two DLE bands. These outcomes, combined with results from micro-/nano-scale electron microscopy mapping and X-ray analysis of a set of CdS1-xSex powders and artificially aged paint mock-ups, indicated that the relative intensity of DLEs is not affected by the morphology, microstructure and local atomic environment of the pigment particles but it is influenced by the presence of moisture. Furthermore, the extensive study of artificially aged oil paint mock-ups permitted us to provide first evidence of the tendency of cadmium reds toward photo-degradation and to establish that the conversion of CdS1-xSex to CdSO4 and/or oxalates is triggered by the oil binding medium and moisture level and depends on the Se content. Based on these findings, we could interpret the localized presence of CdSO4 and cadmium oxalate as alteration products of the original cadmium red paints in two paintings by Pollock.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Antwerp X-ray Imaging and Spectroscopy (AXIS)
Impact Factor: 3.4
Times cited: 3
DOI: 10.1140/EPJP/S13360-022-02447-7
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“0 and &pi, phase Josephson coupling through an insulating barrier with magnetic impurities”. Vávra O, Gaži S, Golubović, DS, Vávra I, Dérer J, Verbeeck J, Van Tendeloo G, Moshchalkov VV, Physical review : B : condensed matter and materials physics 74, 020502 (2006). http://doi.org/10.1103/PhysRevB.74.020502
Abstract: We have studied the temperature and field dependencies of the critical current I(C) in the Nb-Fe(0.1)Si(0.9)-Nb Josephson junction with a tunneling barrier formed by a paramagnetic insulator. We demonstrate that in these junctions coexistence of both the 0 and the pi states within one tunnel junction occurs, and leads to the appearance of a sharp cusp in the temperature dependence I(C)(T), similar to the I(C)(T) cusp found for the 0-pi transition in metallic pi junctions. This cusp is not related to the 0-pi temperature-induced transition itself, but is caused by the different temperature dependencies of the opposing 0 and pi supercurrents through the barrier.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.836
Times cited: 27
DOI: 10.1103/PhysRevB.74.020502
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“2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy”. Tan H, Turner S, Yücelen E, Verbeeck J, Van Tendeloo G, Physical review letters 107, 107602 (2011). http://doi.org/10.1103/PhysRevLett.107.107602
Abstract: Using a combination of high-angle annular dark-field scanning transmission electron microscopy and atomically resolved electron energy-loss spectroscopy in an aberration-corrected transmission electron microscope we demonstrate the possibility of 2D atom by atom valence mapping in the mixed valence compound Mn3O4. The Mn L2,3 energy-loss near-edge structures from Mn2+ and Mn3+ cation sites are similar to those of MnO and Mn2O3 references. Comparison with simulations shows that even though a local interpretation is valid here, intermixing of the inelastic signal plays a significant role. This type of experiment should be applicable to challenging topics in materials science, such as the investigation of charge ordering or single atom column oxidation states in, e.g., dislocations.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 8.462
Times cited: 115
DOI: 10.1103/PhysRevLett.107.107602
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“Aberration-corrected microscopy and spectroscopy analysis of pristine, nitrogen containing detonation nanodiamond”. Turner S, Shenderova O, da Pieve F, Lu Y-G, Yücelen E, Verbeeck J, Lamoen D, Van Tendeloo G, Physica status solidi : A : applications and materials science 210, 1976 (2013). http://doi.org/10.1002/pssa.201300315
Abstract: Aberration-corrected transmission electron microscopy, electron energy-loss spectroscopy, and density functional theory (DFT) calculations are used to solve several key questions about the surface structure, the particle morphology, and the distribution and nature of nitrogen impurities in detonation nanodiamond (DND) cleaned by a recently developed ozone treatment. All microscopy and spectroscopy measurements are performed at a lowered acceleration voltage (80/120kV), allowing prolonged and detailed experiments to be carried out while minimizing the risk of knock-on damage or surface graphitization of the nanodiamond. High-resolution TEM (HRTEM) demonstrates the stability of even the smallest nanodiamonds under electron illumination at low voltage and is used to image the surface structure of pristine DND. High resolution electron energy-loss spectroscopy (EELS) measurements on the fine structure of the carbon K-edge of nanodiamond demonstrate that the typical * pre-peak in fact consists of three sub-peaks that arise from the presence of, amongst others, minimal fullerene-like reconstructions at the nanoparticle surfaces and deviations from perfect sp(3) coordination at defects in the nanodiamonds. Spatially resolved EELS experiments evidence the presence of nitrogen within the core of DND particles. The nitrogen is present throughout the whole diamond core, and can be enriched at defect regions. By comparing the fine structure of the experimental nitrogen K-edge with calculated energy-loss near-edge structure (ELNES) spectra from DFT, the embedded nitrogen is most likely related to small amounts of single substitutional and/or A-center nitrogen, combined with larger nitrogen clusters.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 1.775
Times cited: 37
DOI: 10.1002/pssa.201300315
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“Accuracy and precision in model based EELS quantification”. Bertoni G, Verbeeck J, Ultramicroscopy 108, 782 (2008). http://doi.org/10.1016/j.ultramic.2008.01.004
Abstract: We present results on model based quantification of electron energy loss spectra (EELS), focusing on the factors that influence accuracy and precision in determining chemical concentrations. Several sources of systematical errors are investigated. The spectrometer entrance aperture determines the collection angle, and the effects of its position with respect to the transmitted beam are investigated, taking into account the diffraction by the crystal structure. The effect of the orientation of the sample is tested experimentally and theoretically on SrTiO3, and finally, a simulated experiment on c-BN at different thicknesses confirms the superior results of the model based method with respect to the conventional method. A test on a set of experimental reference compounds is presented, showing that remarkably good accuracy can be obtained. Recommendations are given to achieve high accuracy and precision in practice. (C) 2008 Elsevier B.V. All rights reserved.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.843
Times cited: 44
DOI: 10.1016/j.ultramic.2008.01.004
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“Analytical TEM study of CVD diamond growth on TiO2 sol-gel layers”. Lu Y-G, Verbeeck J, Turner S, Hardy A, Janssens SD, De Dobbelaere C, Wagner P, Van Bael MK, Van Tendeloo G, Diamond and related materials 23, 93 (2012). http://doi.org/10.1016/j.diamond.2012.01.022
Abstract: The early growth stages of chemical vapor deposition (CVD) diamond on a solgel TiO2 film with buried ultra dispersed diamond seeds (UDD) have been studied. In order to investigate the diamond growth mechanism and understand the role of the TiO2 layer in the growth process, high resolution transmission electron microscopy (HRTEM), energy-filtered TEM and electron energy loss spectroscopy (EELS) techniques were applied to cross sectional diamond film samples. We find evidence for the formation of TiC crystallites inside the TiO2 layer at different diamond growth stages. However, there is no evidence that diamond nucleation starts from these crystallites. Carbon diffusion into the TiO2 layer and the chemical bonding state of carbon (sp2/sp3) were both extensively investigated. We provide evidence that carbon diffuses through the TiO2 layer and that the diamond seeds partially convert to amorphous carbon during growth. This carbon diffusion and diamond to amorphous carbon conversion make the seed areas below the TiO2 layer grow and bend the TiO2 layer upwards to form the nucleation center of the diamond film. In some of the protuberances a core of diamond seed remains, covered by amorphous carbon. It is however unlikely that the remaining seeds are still active during the growth process.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.561
Times cited: 16
DOI: 10.1016/j.diamond.2012.01.022
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“Artificial construction of the layered Ruddlesden-Popper manganite La2Sr2Mn3O10 by reflection high energy electron diffraction monitored pulsed laser deposition”. Palgrave RG, Borisov P, Dyer MS, McMitchell SRC, Darling GR, Claridge JB, Batuk M, Tan H, Tian H, Verbeeck J, Hadermann J, Rosseinsky MJ;, Journal of the American Chemical Society 134, 7700 (2012). http://doi.org/10.1021/ja211138x
Abstract: Pulsed laser deposition has been used to artificially construct the n = 3 Ruddlesden Popper structure La2Sr2Mn3O10 in epitaxial thin film form by sequentially layering La1-xSrxMnO3 and SrO unit cells aided by in situ reflection high energy electron diffraction monitoring. The interval deposition technique was used to promote two-dimensional SrO growth. X-ray diffraction and cross-sectional transmission electron microscopy indicated that the trilayer structure had been formed. A site ordering was found to differ from that expected thermodynamically, with the smaller Sr2+ predominantly on the R site due to kinetic trapping of the deposited cation sequence. A dependence of the out-of-plane lattice parameter on growth pressure was interpreted as changing the oxygen content of the films. Magnetic and transport measurements on fully oxygenated films indicated a frustrated magnetic ground state characterized as a spin glass-like magnetic phase with the glass temperature T-g approximate to 34 K. The magnetic frustration has a clear in-plane (ab) magnetic anisotropy, which is maintained up to temperatures of 150 K. Density functional theory calculations suggest competing antiferromagnetic and ferromagnetic long-range orders, which are proposed as the origin of the low-temperature glassy state.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 13.858
Times cited: 27
DOI: 10.1021/ja211138x
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“Atomic layer epitaxy of Ruddlesden-Popper SrO(SrTiO3)n films by means of metalorganic aerosol deposition”. Jungbauer M, Huehn S, Egoavil R, Tan H, Verbeeck J, Van Tendeloo G, Moshnyaga V, Applied physics letters 105, 251603 (2014). http://doi.org/10.1063/1.4905055
Abstract: We report an atomic layer epitaxial growth of Ruddlesden-Popper (RP) thin films of SrO(SrTiO3)(n) (n = infinity, 2, 3, 4) by means of metalorganic aerosol deposition (MAD). The films are grown on SrTiO3(001) substrates by means of a sequential deposition of Sr-O/Ti-O-2 atomic monolayers, monitored in-situ by optical ellipsometry. X-ray diffraction and transmission electron microscopy (TEM) reveal the RP structure with n = 2-4 in accordance with the growth recipe. RP defects, observed by TEM in a good correlation with the in-situ ellipsometry, mainly result from the excess of SrO. Being maximal at the film/substrate interface, the SrO excess rapidly decreases and saturates after 5-6 repetitions of the SrO(SrTiO3)(4) block at the level of 2.4%. This identifies the SrTiO3 substrate surface as a source of RP defects under oxidizing conditions within MAD. Advantages and limitations of MAD as a solution-based and vacuum-free chemical deposition route were discussed in comparison with molecular beam epitaxy. (C) 2014 AIP Publishing LLC.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.411
Times cited: 32
DOI: 10.1063/1.4905055
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“Atomic resolution coordination mapping in Ca2FeCoO5 brownmillerite by spatially resolved electron energy-loss spectroscopy”. Turner S, Verbeeck J, Ramezanipour F, Greedan JE, Van Tendeloo G, Botton GA, Chemistry of materials 24, 1904 (2012). http://doi.org/10.1021/cm300640g
Abstract: Using a combination of high-angle annular dark field scanning transmission electron microscopy and atomically resolved electron energy-loss spectroscopy at high energy resolution in an aberration-corrected electron microscope, we demonstrate the capability of coordination mapping in complex oxides. Brownmillerite compound Ca2FeCoO5, consisting of repetitive octahedral and tetrahedral coordination layers with Fe and Co in a fixed 3+ valency, is selected to demonstrate the principle of atomic resolution coordination mapping. Analysis of the Co-L2,3 and the Fe-L2,3 edges shows small variations in the fine structure that can be specifically attributed to Co/Fe in tetrahedral or in octahedral coordination. Using internal reference spectra, we show that the coordination of the Fe and Co atoms in the compound can be mapped at atomic resolution.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 9.466
Times cited: 33
DOI: 10.1021/cm300640g
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“Atomic resolution mapping of phonon excitations in STEM-EELS experiments”. Egoavil R, Gauquelin N, Martinez GT, Van Aert S, Van Tendeloo G, Verbeeck J, Ultramicroscopy 147, 1 (2014). http://doi.org/10.1016/j.ultramic.2014.04.011
Abstract: Atomically resolved electron energy-loss spectroscopy experiments are commonplace in modern aberration-corrected transmission electron microscopes. Energy resolution has also been increasing steadily with the continuous improvement of electron monochromators. Electronic excitations however are known to be delocalized due to the long range interaction of the charged accelerated electrons with the electrons in a sample. This has made several scientists question the value of combined high spatial and energy resolution for mapping interband transitions and possibly phonon excitation in crystals. In this paper we demonstrate experimentally that atomic resolution information is indeed available at very low energy losses around 100 meV expressed as a modulation of the broadening of the zero loss peak. Careful data analysis allows us to get a glimpse of what are likely phonon excitations with both an energy loss and gain part. These experiments confirm recent theoretical predictions on the strong localization of phonon excitations as opposed to electronic excitations and show that a combination of atomic resolution and recent developments in increased energy resolution will offer great benefit for mapping phonon modes in real space.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.843
Times cited: 22
DOI: 10.1016/j.ultramic.2014.04.011
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“Atomic scale electron vortices for nanoresearch”. Verbeeck J, Schattschneider P, Lazar S, Stöger-Pollach M, Löffler S, Steiger-Thirsfeld A, Van Tendeloo G, Applied physics letters 99, 203109 (2011). http://doi.org/10.1063/1.3662012
Abstract: Electron vortex beams were only recently discovered and their potential as a probe for magnetism in materials was shown. Here we demonstrate a method to produce electron vortex beams with a diameter of less than 1.2 Å. This unique way to prepare free electrons to a state resembling atomic orbitals is fascinating from a fundamental physics point of view and opens the road for magnetic mapping with atomic resolution in an electron microscope.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.411
Times cited: 90
DOI: 10.1063/1.3662012
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“Atomic scale investigation of a PbTiO3/SrRuO3/DyScO3 heterostructure”. Egoavil R, Tan H, Verbeeck J, Bals S, Smith B, Kuiper B, Rijnders G, Koster G, Van Tendeloo G, Applied physics letters 102, 223106 (2013). http://doi.org/10.1063/1.4809597
Abstract: An epitaxial PbTiO3 thin film grown on self-organized crystalline SrRuO3 nanowires deposited on a DyScO3 substrate with ordered DyO and ScO2 chemical terminations is investigated by transmission electron microscopy. In this PbTiO3/SrRuO3/DyScO3 heterostructure, the SrRuO3 nanowires are assumed to grow on only one type of substrate termination. Here, we report on the structure, morphology, and chemical composition analysis of this heterostructure. Electron energy loss spectroscopy reveals the exact termination sequence in this complex structure. The energy loss near-edge structure of the Ti-L-2,L-3, Sc-L-2,L-3, and O K edges shows intrinsic interfacial electronic reconstruction. Furthermore, PbTiO3 domain walls are observed to start at the end of the nanowires resulting in atomic steps on the film surface. (C) 2013 AIP Publishing LLC.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.411
Times cited: 12
DOI: 10.1063/1.4809597
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“Boron-rich inclusions and boron distribution in HPHT polycrystalline superconducting diamond”. Lu Y-G, Turner S, Ekimov EA, Verbeeck J, Van Tendeloo G, Carbon 86, 156 (2015). http://doi.org/10.1016/j.carbon.2015.01.034
Abstract: Polycrystalline boron-doped superconducting diamond, synthesized at high pressure and high temperature (HPHT) via a reaction of a single piece of crystalline boron with monolithic graphite, has been investigated by analytical transmission electron microscopy. The local boron distribution and boron environment have been studied by a combination of (scanning) transmission electron microscopy ((S)TEM) and spatially resolved electron energy-loss spectroscopy (EELS). High resolution TEM imaging and EELS elemental mapping have established, for the first time, the presence of largely crystalline diamond-diamond grain boundaries within the material and have evidenced the presence of substitutional boron dopants within individual diamond grains. Confirmation of the presence of substitutional B dopants has been obtained through comparison of acquired boron K-edge EELS fine structures with known references. This confirmation is important to understand the origin of superconductivity in polycrystalline B-doped diamond. In addition to the substitutional boron doping, boron-rich inclusions and triple-points, both amorphous and crystalline, with chemical compositions close to boron carbide B4C, are evidenced. (C) 2015 Elsevier Ltd. All rights reserved.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 6.337
Times cited: 20
DOI: 10.1016/j.carbon.2015.01.034
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“Cation ordering and flexibility of the BO42- tetrahedra in incommensurately modulated CaEu2(BO4)4 (B = Mo, W) scheelites”. Abakumov AM, Morozov VA, Tsirlin AA, Verbeeck J, Hadermann J, Inorganic chemistry 53, 9407 (2014). http://doi.org/10.1021/ic5015412
Abstract: The factors mediating cation ordering in the scheelite-based molybdates and tungstates are discussed on the basis of the incommensurately modulated crystal structures of the CaEu2(BO4)(4) (B = Mo, W) red phosphors solved from high-resolution synchrotron powder X-ray diffraction data. Monoclinic CaEu2(WO4)(4) adopts a (3 + 1)-dimensionally modulated structure [superspace group I2/b(alpha beta 0)00, a = 5.238 73(1)A, b = 5.266 35(1) A, c = 11.463 19(9) A, gamma = 91.1511(2)degrees, q = 0.56153(6)a* + 0.7708(9)b*, R-F = 0.050, R-p = 0.069], whereas tetragonal CaEu2(MoO4)(4) is (3 + 2)-dimensionally modulated [superspace group I4(1)/ a(alpha beta 0)00(-beta alpha 0)00, a = 5.238 672(7) A, c = 11.548 43(2) A, q(1) = 035331(8)a* + 0.82068(9)b*, q(2) = -0.82068(9)a* + 0.55331(8)b*, R-F = 0.061, R-p = 0.082]. In both cases the modulation arises from the ordering of the Ca/Eu cations and the cation vacancies at the A-sublattice of the parent scheelite ABO(4) structure. The cation ordering is incomplete and better described with harmonic rather than with steplike occupational modulation functions. The structures respond to the variation of the effective charge and cation size at the A-position through the flexible geometry of the MoO42- and WO42- tetrahedra demonstrating an alternation of stretching the B-O bond lengths and bending the O-B-O bond angles. The tendency towards A-site cation ordering in scheelites is rationalized using the difference in ionic radii and concentration of the A-site vacancies as parameters and presented in the form of a structure map.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 4.857
Times cited: 48
DOI: 10.1021/ic5015412
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“Characterization of crystal defects in mixed tabular silver halide grains by conventional transmission electron microscopy and X-ray diffractometry”. Goessens C, Schryvers D, van Landuyt J, Amelinckx S, Verbeeck A, de Keyzer R, Journal of crystal growth 110, 930 (1991)
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 1.698
Times cited: 40
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“Circular dichroism in the electron microscope: progress and applications (invited)”. Schattschneider P, Ennen I, Stoger-Pollach M, Verbeeck J, Journal of applied physics 107, 09d311 (2010). http://doi.org/10.1063/1.3365517
Abstract: According to theory, x-ray magnetic circular dichroism in a synchrotron is equivalent to energy loss magnetic chiral dichroism (EMCD) in a transmission electron microscope (TEM). After a synopsis of the development of EMCD, the theoretical background is reviewed and recent results are presented, focusing on the study of magnetic nanoparticles for ferrofluids and Heusler alloys for spintronic devices. Simulated maps of the dichroic strength as a function of atom position in the crystal allow evaluating the influence of specimen thickness and sample tilt on the experimental EMCD signal. Finally, the possibility of direct observation of chiral electronic transitions with atomic resolution in a TEM is discussed.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.068
Times cited: 28
DOI: 10.1063/1.3365517
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“Computational aspects in quantitative EELS”. Verbeeck J, Van Aert S, Zhang L, Haiyan T, Schattschneider P, Rosenauer A, Microscopy and microanalysis 16, 240 (2010). http://doi.org/10.1017/S143192761005511X
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 1.891
DOI: 10.1017/S143192761005511X
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“Corrigendum: Structural phase transition at the percolation threshold in epitaxial (La0.7Ca0.3MnO3)1-x:(MgO)x nanocomposite films”. Moshnyaga V, Damaschke B, Shapoval O, Belenchuk A, Faupel J, Lebedev OI, Verbeeck J, Van Tendeloo G, Mücksch M, Tsurkan V, Tidecks R, Samwer K, Nature materials 4, 104 (2005)
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 39.737
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“Creating arrays of electron vortices”. Niermann T, Verbeeck J, Lehmann M, Ultramicroscopy 136, 165 (2014). http://doi.org/10.1016/j.ultramic.2013.10.002
Abstract: We demonstrate the production of an ordered array of electron vortices making use of an electron optical setup consisting of two electrostatic biprisms. The biprism filaments are oriented nearly orthogonal with respect to each other in a transmission electron microscope. Matching the position of the filaments, we can choose to form different topological features in the electron wave. We outline the working principle of the setup and demonstrate fist experimental results. This setup partially bridges the gap between angular momentum carried by electron spin, which is intrinsic and therefore present in any position of the wave, and angular momentum carried by the vortex character of the wave, which can be extrinsic depending on the axis around which it is measured. (C) 2013 Elsevier B.V. All rights reserved.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.843
Times cited: 9
DOI: 10.1016/j.ultramic.2013.10.002
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“Crystal Structure and Luminescent Properties of R2-xEux(MoO4)(3) (R = Gd, Sm) Red Phosphors”. Morozov VA, Raskina MV, Lazoryak BI, Meert KW, Korthout K, Smet PF, Poelman D, Gauquelin N, Verbeeck J, Abakumov AM, Hadermann J;, Chemistry of materials 26, 7124 (2014). http://doi.org/10.1021/cm503720s
Abstract: The R-2(MoO4)(3) (R = rare earth elements) molybdates doped with Eu3+ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R2-xEux(MoO4)(3) (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (alpha) and orthorhombic (beta') R2-xEux(MoO4)(3) (R = Gd, Sm; 0 <= x <= 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic beta'-phases could be synthesized only by quenching from high temperature to room temperature for Gd2-xEux(MoO4)(3) in the Eu3+-rich part (x > 1) and for all Sm2-xEux(MoO4)(3) solid solutions. The transformation from the alpha-phase to the beta'-phase results in a notable increase (similar to 24%) of the unit cell volume for all R2-xEux(MoO4)(3) (R = Sm, Gd) solid solutions. The luminescent properties of all R2-xEux(MoO4)(3) (R = Gd, Sm; 0 <= x <= 2) solid solutions were measured, and their optical properties were related to their structural properties. All R2-xEux(MoO4)(3) (R = Gd, Sm; 0 <= x <= 2) phosphors emit intense red light dominated by the D-5(0)-> F-7(2) transition at similar to 616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO8 to RO7 for the alpha- and beta'-modification, respectively. The Gd2-xEux(MoO4)(3) solid solutions are the most efficient emitters in the range of 0 < x < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm2-xEux(MoO4)(3) for higher Eu3+ concentrations (1.5 <= x <= 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet-visible-infrared regions of the EELS spectrum.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 9.466
Times cited: 24
DOI: 10.1021/cm503720s
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“Deconvolution of core electron energy loss spectra”. Verbeeck J, Bertoni G, Ultramicroscopy 109, 1343 (2009). http://doi.org/10.1016/j.ultramic.2009.06.010
Abstract: Different deconvolution methods for removing multiple scattering and instrumental broadening from core loss electron energy loss spectra are compared with special attention to the artefacts they introduce. The Gaussian modifier method, Wiener filter, maximum entropy, and model based methods are described. Their performance is compared on virtual spectra where the true single scattering distribution is known. A test on experimental spectra confirms the good performance of model based deconvolution in comparison to maximum entropy methods and shows the advantage of knowing the estimated error bars from a single spectrum acquisition.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.843
Times cited: 13
DOI: 10.1016/j.ultramic.2009.06.010
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“Determination of size, morphology, and nitrogen impurity location in treated detonation nanodiamond by transmission electron microscopy”. Turner S, Lebedev OI, Shenderova O, Vlasov II, Verbeeck J, Van Tendeloo G, Advanced functional materials 19, 2116 (2009). http://doi.org/10.1002/adfm.200801872
Abstract: Size, morphology, and nitrogen impurity location, all of which are all thought to be related to the luminescent properties of detonation nanodiamonds, are determined in several detonation nanodiamond samples using a combination of transmission electron microscopy techniques. Results obtained from annealed and cleaned detonation nanodiamond samples are compared to results from conventionally purified detonation nanodiamond. Detailed electron energy loss spectroscopy combined with model-based quantification provides direct evidence for the sp3 like embedding of nitrogen impurities into the diamond cores of all the studied nanodiamond samples. Simultaneously, the structure and morphology of the cleaned detonation nanodiamond particles are studied using high resolution transmission electron microscopy. The results show that the size and morphology of detonation nanodiamonds can be modified by temperature treatment and that by applying a special cleaning procedure after temperature treatment, nanodiamond particles with clean facets almost free from sp2 carbon can be prepared. These clean facets are clear evidence that nanodiamond cores are not necessarily in coexistence with a graphitic shell of non-diamond carbon.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 12.124
Times cited: 100
DOI: 10.1002/adfm.200801872
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“Direct space structure solution from precession electron diffraction data: resolving heavy and light scatterers in Pb13Mn9O25”. Hadermann J, Abakumov AM, Tsirlin AA, Filonenko VP, Gonnissen J, Tan H, Verbeeck J, Gemmi M, Antipov EV, Rosner H, Ultramicroscopy 110, 881 (2010). http://doi.org/10.1016/j.ultramic.2010.03.012
Abstract: The crystal structure of a novel compound Pb13Mn9O25 has been determined through a direct space structure solution with a Monte-Carlo-based global optimization using precession electron diffraction data (a=14.177(3) Å, c=3.9320(7) Å, SG P4/m, RF=0.239) and compositional information obtained from energy dispersive X-ray analysis and electron energy loss spectroscopy. This allowed to obtain a reliable structural model even despite the simultaneous presence of both heavy (Pb) and light (O) scattering elements and to validate the accuracy of the electron diffraction-based structure refinement. This provides an important benchmark for further studies of complex structural problems with electron diffraction techniques. Pb13Mn9O25 has an anion- and cation-deficient perovskite-based structure with the A-positions filled by the Pb atoms and 9/13 of the B positions filled by the Mn atoms in an ordered manner. MnO6 octahedra and MnO5 tetragonal pyramids form a network by sharing common corners. Tunnels are formed in the network due to an ordered arrangement of vacancies at the B-sublattice. These tunnels provide sufficient space for localization of the lone 6s2 electron pairs of the Pb2+ cations, suggested as the driving force for the structural difference between Pb13Mn9O25 and the manganites of alkali-earth elements with similar compositions.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.843
Times cited: 24
DOI: 10.1016/j.ultramic.2010.03.012
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“Direct structural and spectroscopic investigation of ultrathin films of tetragonal CuO: Six-fold coordinated copper”. Samal D, Tan H, Takamura Y, Siemons W, Verbeeck J, Van Tendeloo G, Arenholz E, Jenkins CA, Rijnders G, Koster G, Europhysics letters 105, 17003 (2014). http://doi.org/10.1209/0295-5075/105/17003
Abstract: Unlike other 3d transition metal monoxides (MnO, FeO, CoO, and NiO), CuO is found in a low-symmetry distorted monoclinic structure rather than the rocksalt structure. We report here of the growth of ultrathin CuO films on SrTiO3 substrates; scanning transmission electron microscopy was used to show the stabilization of a tetragonal rocksalt structure with an elongated c-axis such that c/a similar to 1.34 and the Cu-O-Cu bond angle similar to 180 degrees, pointing to metastable six-fold coordinated Cu. X-ray absorption spectroscopy demonstrates that the hole at the Cu site for the CuO is localized in 3d(x2-y2) orbital unlike the well-studied monoclinic CuO phase. The experimental confirmation of the tetragonal structure of CuO opens up new avenues to explore electronic and magnetic properties of six-fold coordinated Cu. Copyright (C) EPLA, 2014
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 1.957
Times cited: 15
DOI: 10.1209/0295-5075/105/17003
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