Freddy Callens

Ghent University, Gand, Flemish, Belgium

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Publications (204)449.61 Total impact

  • P. Leblans · L. Struye · S. Elen · I. Mans · Henk Vrielinck · Freddy Callens
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    ABSTRACT: CsI:Eu2+ needle crystal layers were produced by Physical Vapour Deposition (PVD). The luminescence properties were studied before and after thermal anneal. It was shown by Electron Paramagnetic Resonance (EPR) that annealing produces Eu2+ monomer centres, probably starting from Eu clusters in the needle crystalline layers. These centres seem structurally very similar to the Eu2+ monomer centres in annealed CsBr:Eu2+ needle layers. They give rise to a narrow luminescence band peaking at about 450 nm (2.76 eV) at room temperature, both under X-ray and UV excitation. The luminescence intensity of as-deposited layers is low. Annealing enhances photo- and radioluminescence on average by a factor of 1.5-3. Surprisingly, further sensitization of radioluminescence with a factor of up to 7 is possible by exposing the annealed needle layers to X-rays or UV, while in contrast, UV excited luminescence is slightly reduced. X-ray enhancement thus leads to a structured scintillator with a conversion efficiency of about 42,000 photons / MeV upon X-ray excitation. A similar radioluminescence enhancement is observed for annealed CsI:Eu2+ fine powder, but not for large single crystals or conglomerated polycrystals. The measurements and observations are consistent with Eu2+ monomer centres stabilized by water molecules. These molecules are incorporated via annealing, and that this happens when the morphology is such that water can diffuse throughout the volume of the crystals. The presence of stabilized Eu2+ monomers is necessary to enhance radioluminescence by X-ray exposure. A mechanism is proposed for the phenomenon.
    No preview · Article · Sep 2015 · Journal of Luminescence
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    ABSTRACT: Doping the well-known metal-organic framework MIL-53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as-synthesised vanadium-doped MIL-53 are recorded at S-, X-, Q- and W-band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0-2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with (1) H, (27) Al and (51) V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Aug 2015 · ChemPhysChem
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    P. Leblans · L. Struye · S. Elen · I. Mans · H. Vrielinck · F. Callens

    Full-text · Dataset · May 2015
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    P. Leblans · L. Struye · S. Elen · I. Mans · H. Vrielinck · F. Callens

    Full-text · Dataset · May 2015
  • Jevgenij Kusakovskij · Freddy Callens · Henk Vrielinck
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    ABSTRACT: Solid-state sucrose is a well-known dosimetric system, which is capable of reliable dose estimates only at a considerable time after exposure. Immediately after irradiation at room temperature, its electron paramagnetic resonance (EPR) spectrum is dominated by contributions from unstable radicals, which are studied here using continuous-wave EPR and electron−nuclear double resonance (ENDOR) spectroscopy. Four hyperfine tensors of proton couplings were determined, associated with two radical species, and subsequently compared to density functional theory calculation results, which led to the identification of the species with lower abundance (U2) as a radical formed by a H abstraction from C4. The more abundant center (U1) has not been definitively identified yet, but we present compelling evidence that it should be a C6 centered radical. Comparison of the simulated EPR spectra with all available data to the experimental ones suggests that the EPR spectrum of X-irradiated sucrose immediately after irradiation can now be almost entirely understood.
    No preview · Article · May 2015 · The Journal of Physical Chemistry B
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    ABSTRACT: The electronic properties of manganese in crystalline germanium have been investigated by means of deep level transient spectroscopy (DLTS). Mn was diffused in the material by a thermal treatment at 700 °C. Next to the deep levels of nickel and copper, which are known contaminants in Ge treated at high temperature, three not previously reported levels were observed. These two hole and one electron traps, with apparent energy level positions at EV +0.136 eV, EV +0.342 eV and EC − 0.363 eV, were assigned to substitutional Mn. The analysis of the carrier capture cross-sections, the absence of field-assisted emission and the observation of the Mn2− electron paramagnetic resonance spectrum in n-type Ge:Mn at low temperature are all compatible with Mn introducing two acceptor and one donor levels in the band gap of Ge.
    No preview · Article · Apr 2015 · Journal of Physics D Applied Physics
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    ABSTRACT: A successful method for doping the metal-organic framework Al-MIL-53 with vanadium ions is reported. By using multi-frequency electron paramagnetic resonance and Q-band electron-nuclear double resonance spectroscopy, it was demonstrated that V=O units replace Al-OH in the as-synthesized metal-organic framework structure. More information can be found in the Full Paper by I. Nevjestić et al. (DOI:10.1002/cphc.201500522).
    No preview · Article · Jan 2015
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    ABSTRACT: Quenching experiments have been performed on both n- and p-type Ge in a dedicated furnace using infrared lamp heating. The capture and emission characteristics of the induced deep-level defects in the quenched samples were investigated by means of deep level transient spectroscopy. For all defect levels, a high impact of capture in the transition region (slow capture) was found. An empirical approach to analyse this effect is presented, which allows to extract reliable capture cross section parameters. The defect parameters thus obtained were compared with previously published data and it was found that some prominent quench-inginduced deep levels are related to metal impurities (Cu and Ni), while others may be vacancy-related.
    Full-text · Article · Nov 2014 · Semiconductor Science and Technology
  • J. Lauwaert · F. Moens · F. Callens
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    ABSTRACT: 1. Introduction. Mn doped germanium was studied almost 60 years ago by means of Hall effect [1] and photo conductivity [2] measurements. Based on the temperature dependence of the resistivity and carrier concentration, two levels, one at 0.37eV from the conduction band and one at 0.16eV from the valence band have been assigned to Mn impurities[1]. In agreement with the valence bond model and similar observations by counterdoping with donors for Fe, Co and Ni, these impurities have been assigned to double acceptors. Moreover, the presence of a double acceptor level was confirmed by the observation of the Mn2- state with Electron Paramagnetic Resonance (EPR). [3] However it was also noted in the early studies that the presence of contamination with acceptor impurities may cast doubt on the charge transition assignment. Since the availability of Deep Level Transient Spectroscopy (DLTS), which is a more reliable technique to study deep levels in semiconductors, the electronic properties of most of the 3d transition metals have already been studied with a higher energy resolution. Metal in-diffusion was successful to introduce Cu and Ni[4]. More recently the defect levels of Ti, Cr, Fe and Co have been identified with DLTS on metal-implanted Ge [5,6]. The present study reports on results obtained on Mn doped Ge. 2. Experimental conditions. The source material are single crystal germanium wafers, supplied by Umicore, with a shallow dopant concentration of 1014 cm-3 Ga and Sb for p- and n-type, respectively. To introduce Mn impurities, 99.995% pure Mn was thermally evaporated on the Ge surface. Afterwards the specimens were placed in a resistive furnace under Ar atmosphere for 10 minutes at 700°C. Finally, the residual Mn on the surface was removed by etching with a mixture of HNO3 and HF (3:1?). Schottky diodes for DLTS were prepared by evaporation of Au (n-type) or In (p-type) barriers immediately after etching. To identify the contamination introduced during the processing, Schottky diodes were also prepared on parts of the specimens outside the Mn covered areas. Schottky diodes on n-Ge were also prepared by annealing the n-Ge:Mn samples (after indiffusion at 700°C) to 400°C for 20 min in order to form a MnGe metallic alloy contact. 3. Results and discussion. Fig. 1 a shows the DLTS spectrum of n-Ge:Mn with a MnGe Schottky contact. A similar spectrum was recorded on the specimen with an Au-Schottky barrier. Conventional DLTS shows one electron trap (Mn-E1), while injection of minority carriers (Vp>0) reveals two additional levels (Mn-H1 and Mn-H2). Fig. 1 b shows the DLTS spectrum of p-type Ge, with presence of Mn-H1 and Mn-H2 only after Mn diffusion. Figure 1: DLTS spectrum of a) n-Ge/MnGe diode. b) p-type Ge with and without Mn-difussion. The electron trap Mn-E1 (E=0.36eV) and the hole trap Mn-H1 (E=0.14eV) correspond with the two levels reported before [1,2]. The Mn-H2 peak (E=0.34eV) has to our knowledge not been reported before in the context of transition metal impurities. Nonetheless, its appearance only in Mn diffused samples, its almost equal amplitude and concentration profile to Mn-H1 suggests that it is also related to substitutional Mn in Ge. This may indicate that the substitutional Mn defect can adopt 4 charge states in Ge, one of which certainly corresponds with 2-, as identified with EPR. 4. Conclusions. Three Mn related defect levels have been identified with DLTS. The striking similarity with previously published results for two of the observed levels suggest the assignment to substitutional Mn in germanium. The third level was not reported before but can most probably also be assigned to Mn. References [1]H. H. Woodbury and W.W. Tyler, Phys. Rev., 100, p. 659 (1955). [2]R. Newman, H.H. Woodbury and W.W. Tyler, Phys. Rev., 102, p. 613 (1956). [3]G.D. Watkins, Bull. Am. Phys. Soc., 2, 235 (1957). [4]P. Clauws and E. Simoen Mater. Sci. Semicond. Process. 9,546 (2006) [5]J. Lauwaert, J. Van Gheluwe, J. Vanhellemont, E. Simoen and P. Clauws, J. Appl. Phys. 105, 073707 (2009) [6]J. Lauwaert, J. Vanhellemont, E. Simoen, H. Vrielinck and P. Clauws, J. Appl. Phys. 111, 113713 (2012)
    No preview · Conference Paper · Oct 2014
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    ABSTRACT: The nitrogen split interstitial defect introduced by high-energy particle irradiation in $n$-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and $Q$-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the $g$-tensor anisotropy, which is not resolved at $X$ or $Q$ band. The good agreement of the principal values ${g}_{xx}=1.9966,{g}_{yy}=2.0016$, and ${g}_{zz}=2.0036$ with the theoretically predicted $g$ tensor confirm the $(\mathrm{N}\text{$-${}}\mathrm{N}){}_{\mathrm{N}}{}^{0}$ defect model. The hyperfine interactions of this defect have been studied by $Q$-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20\char21{}90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon interstitials is also the split interstitial configuration.
    Preview · Article · Aug 2014 · Physical Review B
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    ABSTRACT: We review our research of the past decade towards identification of radiation-induced radicals in solid state sugars and sugar phosphates. Detailed models of the radical structures are obtained by combining EPR and ENDOR experiments with DFT calculations of g and proton HF tensors, with agreement in their anisotropy serving as most important criterion. Symmetry-related and Schonland ambiguities, which may hamper such identification, are reviewed. Thermally induced transformations of initial radiation damage into more stable radicals can also be monitored in the EPR (and ENDOR) experiments and in principle provide information on stable radical formation mechanisms. Thermal annealing experiments reveal, however, that radical recombination and/or diamagnetic radiation damage is also quite important. Analysis strategies are illustrated with research on sucrose. Results on dipotassium glucose-1-phosphate and trehalose dihydrate, fructose and sorbose are also briefly discussed. Our study demonstrates that radiation damage is strongly regio-selective and that certain general principles govern the stable radical formation.
    Preview · Article · Jun 2014
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    ABSTRACT: Sucrose, the main component of table sugar, present in nearly every household and quite radiation sensitive, is considered as an interesting emergency dosemeter. Another application of radiation-induced radicals in sugars is the detection of irradiation in sugar-containing foodstuffs. The complexity of electron paramagnetic resonance (EPR) spectra of radicals in these materials, as a result of many hyperfine interactions and the multi-compositeness of the spectra of individual sugars, complicate dose assessment and the improvement of protocols for control and identification of irradiated sugar-containing foodstuffs using EPR. A thorough understanding of the EPR spectrum of individual irradiated sugars is desirable when one wants to reliably use them in a wide variety of dosimetric applications. Recently, the dominant room temperature stable radicals in irradiated sucrose have been thoroughly characterised using EPR, electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. These radicals were structurally identified by comparing their proton hyperfine and g-tensors with the results of Density Functional Theory calculations for test radical structures. In this paper, the authors use the spin Hamiltonian parameters determined in these studies to simulate powder EPR spectra at the standard X-band (9.5 GHz), commonly used in applications, and at higher frequencies, up to J-band (285 GHz), rendering spectra with higher resolution. A few pitfalls in the simulation process are highlighted. The results indicate that the major part of the dosimetric spectrum can be understood in terms of three dominant radicals, but as-yet unidentified radicals also contribute in a non-negligible way.
    No preview · Article · May 2014 · Radiation Protection Dosimetry
  • Gauthier Vanhaelewyn · Henk Vrielinck · Freddy Callens
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    ABSTRACT: In the past, decennia radiation-induced radicals were successfully identified by electron magnetic resonance (EMR) in several solid-state amino acids and sugars. The authors present a room temperature (RT) EMR study of the stable radicals produced by X-ray-irradiation in the amino acid l-threonine (CH3CH(OH)CH(NH3(+))COO(-)). Its chemical structure is similar to that of the well-known dosimetric material l-alanine (CH3CH(NH3(+))COO(-)), and radiation defects in l-threonine may straightforwardly be compared with the extensively studied l-alanine radicals. The hyperfine coupling tensors of three different radicals were determined at RT using electron nuclear double resonance. These results indicate that the two most abundant radicals share the same basic structure CH3(•)C(OH)CH(NH3(+))COO(-), obtained by H-abstraction, but are stabilised in slightly different conformations. The third radical is most probably obtained by deamination (CH3CH(OH)(•)CHCOO(-)), similar in structure to the stable alanine radical.
    No preview · Article · Apr 2014 · Radiation Protection Dosimetry
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    ABSTRACT: In the deep-level transient spectroscopy (DLTS) spectra of the 3d-transition metals cobalt and chromium in p-type germanium, evidence is obtained that hole emission from defect levels can occur by two parallel paths. Besides classical thermal emission, we observed a second, slower and temperature-independent emission. We show that this extra emission component allows determining unambiguously whether or not multiple DLTS peaks arise from the same defect. Despite similar characteristics, we demonstrate that the origin of the non-thermal emission is not tunnelling but photoionization related to black-body radiation from an insufficiently shielded part of the cryostat.
    Full-text · Article · Oct 2013 · Journal of Physics D Applied Physics
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    ABSTRACT: Ionizing radiation induces a composite, multi-line electron paramagnetic resonance (EPR) spectrum in sucrose, that is stable at room temperature and whose intensity is indicative of the radiation dose. Recently, the three radicals which dominate this spectrum were identified and their proton hyperfine tensors were accurately determined. Understanding the powder EPR spectrum of irradiated sucrose, however, also requires an accurate knowledge of the g tensors of these radicals. We extracted these tensors from angular dependent electron nuclear double resonance-induced EPR measurements at 110 K and 34 GHz. Powder spectrum simulations using this completed set of spin Hamiltonian parameters are in good agreement with experimentally recorded spectra in a wide temperature and frequency range. However, as-yet non-identified radicals also contribute to the EPR spectra of irradiated sucrose in a non-negligible way.
    No preview · Article · May 2013 · The Journal of Physical Chemistry B
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    ABSTRACT: Vacuum-deposited CsBr needle plates doped with Eu(2+) have been investigated with Q (similar to 34 GHz) and W (similar to 95 GHz) band electron paramagnetic resonance in a large temperature interval (4 K: room temperature). At low temperatures (< 35 K), two Eu-related centers were found with different symmetry (tetragonal and orthorhombic), while in earlier studies at room temperature only one center was found with tetragonal symmetry. Possible models for the three defects are investigated, taking into account their temperature behavior, their symmetry, and the atypical values of their zero-field splitting parameters.
    Full-text · Article · Apr 2013 · Physical review. B, Condensed matter
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    ABSTRACT: Radicals generated in trehalose single crystals by X radiation at room temperature were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR measurements, together with periodic density functional theory calculations. In the first days after irradiation, three radical species (I1, I2 and I3) were detected, two of which (I1 and I2) dominate the EPR spectrum and could be identified as H-abstracted species centered at C3' (I1) and C2 (I2), the latter with additional formation of a carbonyl group at C3. Annealing the sample at 40°C for 3 days or storing it in ambient conditions for three months resulted in another, more stable EPR spectrum. Two major species could be characterized in this stage (S1 and S2), only one of which was tentatively identified as an H-abstracted, C2-centered species (S1). Our findings disagree with a previous EPR study [Gräslund and Löfroth (23)] on several accounts. This work stresses the need for caution when interpreting composite EPR spectra and thermally induced spectral changes of radiation-induced species, even in these relatively simple carbohydrates. It also provides further evidence that the pathways for radiation damage critically depend on the specific conformation of a molecule and its environment, but also that carbonyl group formation is a common process in the radiation chemistry of sugars and related compounds.
    No preview · Article · Jan 2013 · Radiation Research
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    ABSTRACT: In this paper, we describe the post-functionalization of a V-containing Metal-organic framework with TiO(acac)2 to create a bimetallic oxidation catalyst. The catalytic performance of this V/Ti-MOF was examined for the oxidation of cyclohexene using molecular oxygen as oxidant in combination with cyclohexanecarboxaldehyde as co-oxidant. A significantly higher cyclohexene conversion was observed for the bimetallic catalyst compared to the non-functionalized material. Moreover, the catalyst could be recycled at least 3 times without loss of activity and stability. No detectable leaching of V or Ti was noted. Electron paramagnetic resonance measurements were performed to monitor the fraction of V-ions in the catalyst in the +IV valence state. A reduction of this fraction by ∼17% after oxidation catalysis is observed, in agreement with the generally accepted mechanism for this type of reaction.
    Full-text · Article · Dec 2012 · Catalysis Today
  • Hendrik De Cooman · Tim Desmet · Freddy Callens · Peter Dubruel
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    ABSTRACT: Electron paramagnetic resonance (EPR) measurements were performed on poly-ε-caprolactone (PCL) films at different stages of the postplasma-grafting process. PCL films prepared by solvent casting (SC) or electrospinning (ESP) yield very similar EPR spectra after Ar-plasma treatment and subsequent exposure to air, but the EPR signal is much stronger in the PCL-ESP films. The free radicals appear to be mainly, and possibly exclusively, oxygen centered. The radicals generated by UV irradiation in PCL-ESP films were studied in situ with EPR, using a UV-LED (λ = (285 ± 5) nm). Their EPR spectrum is distinctly different from the plasma-induced signal, indicative of carbon-centered radicals, and appears to be independent of the plasma pretreatment. UV-induced homolytic splitting of (hydro)peroxide bonds was not observed. Both the plasma- and UV-induced radicals decay at room temperature (RT), even in an inert atmosphere. This study demonstrates the potential of electrospun films and UV-LEDs for the study of plasma- and UV-generated free radicals with EPR in polyesters, and raises questions with respect to the validity of some generally accepted molecular mechanisms underpinning the postplasma grafting technique for polyesters. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012
    No preview · Article · Jun 2012 · Journal of Polymer Science Part A Polymer Chemistry
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    ABSTRACT: The structure of Eu2+ monomer centers in CsBr single crystals is investigated using electron paramagnetic resonance (EPR) spectroscopy. These centers are produced by heating the melt-grown crystals above 600 K in vacuum followed by a rapid quench to room temperature (RT) or 77 K. The angular dependence of their EPR spectrum demonstrates that these centers have cubic symmetry. At RT the EPR spectrum decays by aggregation of the Eu2+ ions. This strongly contrasts with the situation for CsBr:Eu needle image plates synthesized by physical vapor deposition, where the Eu2+-related EPR spectrum was observed to exhibit long-term stability at RT.
    Full-text · Article · Apr 2012 · Physical review. B, Condensed matter

Publication Stats

2k Citations
449.61 Total Impact Points

Institutions

  • 1985-2015
    • Ghent University
      • • Department of Solid-state Physics
      • • Center for Molecular Modeling
      • • Faculty of Sciences
      • • Department of Analytical Chemistry
      Gand, Flemish, Belgium
  • 2002
    • University of Antwerp
      • Department of Physics
      Antwerpen, Flemish, Belgium
  • 2001
    • Instytut Chemii i Techniki Jądrowej
      Warszawa, Masovian Voivodeship, Poland
  • 1999
    • Vrije Universiteit Brussel
      Bruxelles, Brussels Capital, Belgium