Letter pubs.acs.org/JPCL
Structural and Electronic Properties of the TTF/ZnO(10−10) Interface: Insights From Modeling Sébastien Nénon, Raphael̈ Méreau, Seyhan Salman, and Frédéric Castet* Université de Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, 351 Cours de la Libération, 33405 Talence, France
Tanguy Van Regemorter, Sergiu Clima,† David Beljonne, and Jérôme Cornil Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium S Supporting Information *
ABSTRACT: The structural and electronic properties of a tetrathiafulvalene (TTF) monolayer adsorbed onto the ZnO(10−10) surface are investigated by using two different quantum-chemical approaches, namely, density functional theory and the self-consistent charge density functional-parametrized tight binding method. The two approaches yield strong hybridization of the highest occupied molecular orbital (HOMO) level of the TTF molecules with band states of ZnO in the most stable interfacial geometric configuration, which results in the pinning of the corresponding orbital in the hybrid system and a significant charge transfer across the interface. As a consequence, the work function of ZnO is significantly reduced. We discuss these results in the context of the design of new hybrid opto-electronic devices, where the deposition of organic layers onto inorganic surfaces allows modulating the barrier height for charge injection. SECTION: Electron Transport, Optical and Electronic Devices, Hard Matter
T
inorganic substrates. This crucial information cannot be easily accessed at the experimental level typically due to a lack of resolution in the experimental techniques, a poor control of the actual nature of the interface, or the ensemble averaging procedure associated with many measurements. Such interfacial properties have been widely investigated theoretically over the years for metal/organic semiconductor interfaces,8 whereas much less attention has been dedicated to hybrid inorganic/ organic semiconductor systems. In this contribution, we focus on the adsorption of a tetrathiafulvalene (TTF) molecule on the nonpolar zinc oxide (ZnO) (10−10) surface. ZnO is a wide band gap semiconductor (3.40 eV9) frequently used in DSSCs, 10 solar cells, 11 organic light-emitting diodes (OLEDs),7,12,13 and gas-sensing applications (for example, in desulfurization processes14). We have chosen TTF molecules as adsorbates because of their strong electron-donating character, which is expected to yield a pronounced electronic reorganization at the interface with ZnO. The adsorption of a single TTF molecule onto the ZnO(10−10) surface has been studied here by means of two radically different quantum-chemical methods, both using periodic boundary conditions (PBCs): a density functional theory (DFT) method with the PBE functional and a DZP basis set, as
he structural and electronic properties of interfaces between organic and metal-oxide semiconductors strongly impact the performance of hybrid opto-electronic devices such as dye-sensitized solar cells (DSSCs),1 hybrid solar cells incorporating inorganic nanoparticles or nanorods,2 or organic light-emitting diodes (OLEDs) involving inorganic buffer layers.3 The possibility to tune the effective work function of metal-oxide layers or objects through a surface functionalization via self-assembled monolayers (SAMs), or the noncovalent deposition of organic molecules, has motivated many recent works. For instance, the deposition of dipolar molecules chemically bound to the surface by using phosphonic or carboxylic acid units as anchoring groups has been shown to provide a wide range of tunability in the effective surface work function of indium−tin oxide electrodes.4−6 The work function of ZnO can also be efficiently controlled through surface modification using SAMs based on dipolar molecules, resulting in the enhancement of the charge injection efficiencies in hybrid organic−inorganic LEDs.7 As a matter of fact, the degree of electronic coupling and, in particular, the amount of charge transfer across the inorganic− organic heterojunction is highly dependent on both the morphology of the interface and the relative alignment of the electronic energy levels of the organic and inorganic parts. From a theoretical point of view, it is of prime interest to investigate at the atomistic level the structural properties of hybrid interfaces and the nature of the electronic processes accompanying the deposition of organic molecules on top of © 2011 American Chemical Society
Received: October 27, 2011 Accepted: December 6, 2011 Published: December 6, 2011 58
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Figure 1. Geometric structures of the three stable TTF/ZnO complexes optimized at the DFT (top) and SCC-DFTB (bottom) levels.
implemented in the SIESTA package,15,16 and the selfconsistent charge density functional-parametrized tight binding method (SCC-DFTB)17 available in the DFTB+ code.18 (See the Computational Section for details.) The PBE functional is known to describe ground-state properties properly but tends to underestimate the electronic gaps.19 Whereas hybrid functionals better describe the electronic properties of oxides20 and start to be available in PBC methods, they are computationally very expensive and were not considered here in view of the size of the systems under study. The SCC-DFTB approach has been shown to provide results of DFT quality for the ground-state structural and energetic properties of organic molecules and solid-state systems (including bulk ZnO, ZnO surfaces and systems in which small species such as H, CO2, and NH3 are adsorbed on ZnO surfaces21) with the computational cost of common semiempirical Hartree−Fock methods. Therefore, the motivation of this work is two-fold: (i) to describe the electronic processes occurring at organic− inorganic interfaces and (ii) to assess the accuracy of the simplified SCC-DFTB computational scheme against standard DFT calculations. Geometry optimizations of the bare ZnO(10−10) surface were first carried out at both the DFT and SCC-DFTB levels on the four top layers of the slab. (The four bottom layers were frozen to simulate bulk properties.) The calculated surface reconstruction, that is, the atomic displacements in the first layers at the interface are consistent with previous experimental and theoretical findings.22 At the DFT/PBE (SCC-DFTB) level, the Zn ions in the top layer are displaced downward by 0.21 (0.33) Å, and the oxygen atoms are displaced upward by 0.07 (0.03) Å; there is no significant atomic shift below the second layer (Figure S1, Supporting Information). We have next performed similar calculations at the SCCDTFB level on the TTF/ZnO(10−10) interface and identified three different stable geometric configurations (Figure 1). Geometry optimizations using the DFT approach calculations were further achieved on the basis of these structures. In structure 1, the TTF molecule features a significant distortion compared with its planar, gas-phase, equilibrium configuration. Such a geometric reorganization is associated with hybridization
between the molecular orbitals of TTF and the band states of ZnO. The degree of distortion of the TTF molecule can be quantified by the dihedral angles defined by the central C−C bond and the sulfur−carbon bonds in the rings. (Only one angle is presented because of the symmetry of the distortion for the two sulfur atoms of a given ring.) At the DFT level, these dihedral angles amount to 170 and 155°, whereas the SCCDFTB calculations yield slightly smaller values (162 and 149°). At the DFT level, the distances between the S atoms of the TTF and the closest Zn atoms of the surface are different for the two rings but are similar for the two S of a given ring (2.65 and 2.81 Å), whereas they are found to be similar and shorter (2.40 and 2.48 Å) using SCC-DFTB. In configurations 2 and 3, the TTF molecule remains essentially planar, and the Zn−S distances are similar (∼2.6 Å and 2.39 Å at the DFT and SCCDFTB levels, respectively). These calculated distances are slightly larger yet comparable to bond lengths measured by X-ray diffraction in Zn(II) complexes with sulfur ligands (in the range of 2.25 to 2.30 Å).23 To evaluate the relative stability of the three complexes, we have calculated the adsorption energy EADS of the TTF molecule on the ZnO surface using eq 1
EADS = E TTF/ZnO − E TTF − E ZnO
(1)
where ETTF/ZnO is the total energy of the TTF/ZnO complex and ETTF and EZnO are the total energy of the isolated TTF and ZnO(10−10) surface, respectively. (All energies in eq 1 refer to optimized structures, so that EADS accounts for both electronic interactions and structural deformations.) The EADS values collected in Table 1 show the same evolution between the two Table 1. Adsorption Energies (EADS, kJ/mol) of the Three TTF/ZnO Complexes, As Calculated at the DFT and SCCDFTB Levelsa DFT SCC-DFTB
structure 1
structure 2
structure 3
−188.3 −232.7
−150.7 (37.6) −194.5 (38.2)
−137.3 (51.0) −172.5 (60.2)
a
Energy differences (ΔEADS) with respect to structure 1 are given between parentheses.
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(reference structure) and S3 of the Supporting Information (structures 1 to 3). The relative positions of the energy levels of the isolated components in the reference system widely differ when using the DFT/PBE versus SCC-DFTB approaches. Although slightly overestimated, the electronic gap of ZnO is nicely reproduced when using SCC-DFTB (calculated at 3.74 eV vs 3.4 eV from two-photon photoemission experiments9), whereas it is dramatically underestimated at the DFT level (0.55 eV), as already reported in previous works.24 Both the DFT and SCCDFTB methods predict a HOMO−LUMO gap of 2.0 eV for the TTF molecule, which is lower than the experimental optical gap (2.8 eV) measured by UV−visible absorption in cyclohexane.25 In structure 1, both the HOMO and LUMO levels of the TTF molecule experience a large shift to lower energies with respect to the corresponding MOs of the isolated molecule, whereas the energy of the valence and conduction band edges of the oxide surface is hardly affected. It is worth stressing that the shift of the LUMO in structure 1, compared with the reference structure, cannot be clearly observed at the DFT level due to the deformation of the TTF molecule upon adsorption. As a consequence of the strong coupling with the ZnO surface, the TTF molecule loses its planarity, which induces an increase in the HOMO−LUMO gap. In addition, the molecular LUMO level gets broadened, which complicates the assignment of a given energy to the level. (The highest peak in the DOS originating from the LUMO of the molecule has been chosen in Table 2.) Despite the very different alignment of the frontier electronic levels of the isolated units at the two levels of theory, we find that the HOMO level of TTF is pinned at the valence band edge of the ZnO due to the significant hybridization between their orbitals. (In other words, the energy difference between the HOMO of TTF and the VBE of the metal oxide is found to be very similar at the two levels of theory despite the huge differences observed in the energy levels for the two isolated components.) Such a pinning is not observed for the LUMO due to the weaker electronic coupling between the orbitals of the two components. In structures 2 and 3, a similar shift toward lower energy is observed for both the HOMO and LUMO levels of TTF, although the shifts are smaller than those in structure 1 due to the weaker orbital interactions. In that case, the frontier MOs of TTF remain mainly localized over the molecule, and no pinning effect is observed. The displacement of the highest occupied frontier electronic levels upon adsorption of TTF on ZnO can be directly visualized from the computed DOS (Figure 2). When comparing the DOS of structures 2 and 3 with that of the reference system, the band corresponding to the TTF HOMO, which lies within the ZnO band gap, is shifted toward lower energies. As shown in Figure S4 of the Supporting Information, the atom-projected density of states (PDOS) confirm that this band indeed corresponds to the HOMO of the TTF molecule because its main contributions arise from the π orbitals of the sulfur and carbon atoms. In the case of structure 1, the HOMO of TTF merges with the valence band of ZnO, as confirmed by the PDOS analysis. (See the Supporting Information.) The amount of charge transfer between TTF and ZnO has further been analyzed by calculating the total net charge on the TTF molecule (qTTF) using the Mulliken approximation (and additionally the Hirshfeld scheme within DFT). As shown in Table 3, both approaches predict very similar trends: qTTF is positive in the three complexes, which indicates that TTF acts
levels of calculation. In particular, both theoretical approaches yield a larger binding energy for configuration 1 compared with structure 2 (ΔEADS ≈ 40 kJ/mol) and 3 (ΔEADS ≈ 50−60 kJ/mol). When comparing the density of states (DOS) of the various TTF/ZnO interfaces, it is important to keep in mind that the zero in energy is system dependent in periodic calculations so that the absolute energy scale of the DOS of different systems cannot be directly compared. To overcome this problem, we have aligned the energies with respect to a reference level. For the DFT calculations, the energies are aligned with the planeaveraged electrostatic potential calculated outside the lower part of the ZnO slab, which remains unchanged regardless of the orientation and the distance of the TTF molecule on the upper part of the slab. A shift of the same amplitude is therefore performed to align the DOS. For the DFTB method, the electrostatic potential cannot be extracted, but the alignment is directly performed on the DOS by shifting the low-lying energy levels to the same values because they are not affected by the presence of the molecule on the surface. Of course, the need to align the energies according to a reference level impedes direct comparison between the absolute energies of the levels of the isolated ZnO and TTF moieties. Accordingly, we have compared the DOS of structures 1 to 3 with the energies obtained from a single-point calculation on a structure in which the TTF molecule in its ground-state geometry optimized in gas phase is positioned parallel to the ZnO surface (optimized without TTF) at a distance of 7.5 Å to minimize the interactions between the molecule and the oxide surface. (Test calculations showed essentially similar results for a distance of 10 Å.) The DOS calculated for the four structures are presented in Figure 2, whereas the energy values of the frontier electronic levels calculated at the SCC-DFTB and DFT levels are gathered in Table 2 and visualized in Figures S2
Figure 2. DOS of the three TTF/ZnO complexes and of the reference structure, calculated at the DFT (top) and SCC-DFTB (bottom) levels. 60
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Table 2. Energy of the Frontier MOs of the Three TTF/ZnO Complexes and Energy Shifts (in electronvolts) with Respect to the Reference Structure, As Obtained at The DFT and DFTB Levels SCC-DFTB structure 1
structure 2
structure 3
reference
ZnO
VBEa
CBE
VBE
CBE
VBE
CBE
VBE
CBE
MO energy shift vs ref gap TTF
−5.78 0.01
−2.02 0.01
−5.76 0.02
−2.01 0.02
−5.77 0.01
−2.02 0.01
−5.78 0.00
−2.03 0.00
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
−5.73 −1.27
−3.61 −1.15
−5.00 −0.54
−3.12 −0.65
−4.67 −0.21
−2.78 −0.31
−4.46 0.00
−2.46 0.00
MO energy shift vs ref gap
3.75
3.76
2.12
3.75
1.88
3.74
1.89
2.00
DFT structure 1
structure 3
reference
VBE
CBE
VBE
CBE
VBE
CBE
VBE
CBE
MO energy shift vs ref gap TTF
−4.81 0.08
−4.21 0.08
−4.82 0.07
−4.22 0.07
−4.82 0.07
−4.16 0.13
−4.89 0.00
−4.29 0.00
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
−5.05 −0.82
−2.11 0.14
−4.52 −0.29
−2.47 −0. 22
−4.34 −0.11
−2.36 −0.11
−4.23 0.00
−2.25 0.00
MO energy shift vs ref gap a
structure 2
ZnO
0.60
0.60
2.94
0.66
2.05
1.98
0.60
1.98
VBE and CBE stand for valence band edge and conduction band edge, respectively.
Table 3. Total Net Charge on the TTF Molecule in the Three TTF/ZnO Complexes (qTTF, in |e|), As Calculated at the DFT/PBE (using DMol326 Implemented within Materials Studio) and SCC-DFTB Levels Using the Mulliken Approximationa DFT (Mulliken) DFT (Hirshfeld) SCC-DFTB (Mulliken)
structure 1
structure 2
structure 3
0.447 0.476 0.297
0.397 0.396 0.247
0.397 0.369 0.111
a
For comparison, the Hirshfeld charges obtained from the DFT calculations are also reported.
as an electron-donor in all geometric configurations, thus rationalizing the lowering of the frontier electronic levels of the TTF molecule in the complexes following the appearance of a partial positive charge on the TTF core. The absence of energy shift for the VBE and CBE is most likely explained by the fact that the negative charge transferred to the ZnO surface is delocalized over a large number of atoms in the crystalline network, thus limiting its impact on the electronic structure. The largest electron transfer is found in structure 1, where the orbital couplings between TTF and ZnO are the largest, and decreases in the order 1 > 2 > 3. Finally, the evolution of the plane-averaged electrostatic potential along the direction perpendicular to the interface (z axis) has been calculated at the DFT/PBE level for the three complexes (Figure 3) and compared with the electrostatic potential of the bare ZnO surface. As expected from the electron-donating character of TTF, the adsorption of a TTF molecule on the surface leads to a lowering of the electrostatic potential whatever the structural configuration. In structures 2 and 3, the electrostatic potential is reduced by ∼0.30 eV, whereas in complex 1 the lowering amounts to ∼0.45 eV. Because the band edges of ZnO are hardly affected by the presence of TTF, this electrostatic shift is essentially reflecting
Figure 3. Evolution of the plane-averaged electrostatic potential along the direction perpendicular to the surface (z axis) for the three TTF/ ZnO complexes and for the bare ZnO(10−10) surface, as calculated at the DFT level. The top of the surface, on which TTF is deposited, sits on the right side of the plot.
the reduction of the work function of ZnO. This work function lowering is comparable in magnitude to, yet smaller than, that obtained upon surface modification using carboxylic acid-based self-assembled dipolar molecules, with dipoles pointing away from the ZnO surface.7 In summary, although they yield rather different one-electron energy diagrams for the isolated partners, periodic DFT and SCC-DFTB calculations provide a fully consistent picture of the electronic structure for occupied levels at TTF/ZnO(10−10) interfaces. More specifically, the two methods yield a strong hybridization of the HOMO level of TTF with band states of ZnO in the most stable interfacial geometric configuration, which results in the pinning of the corresponding orbital in the hybrid system. Therefore, the metal oxide network dictates the energy of the HOMO level of the molecule in the inorganic−organic hybrid, irrespective of its relative position for the isolated TTF molecule. In contrast, the alignment predicted by the two 61
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(3) Bolink, H. J.; Coronado, E.; Repetto, D.; Sessolo, M. Air Stable Hybrid Organic-Inorganic Light Emitting Diodes Using ZnO as the Cathode. App. Phys. Lett. 2007, 91, 223501−1−3. (4) Khodabakhsh, S.; Poplavskyy, D.; Heutz, S.; Nelson, J.; Bradley, D. D. C.; Murata, H.; Jones, T. S. Using Self-Assembling Dipole Molecules to Improve Hole Injection in Conjugated Polymers. Adv. Funct. Mater. 2004, 14, 95−100. (5) Paniagua, S.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. Phosphonic Acid Modification of Indium-Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle Studies. J. Phys. Chem. C 2008, 112, 7809−7817. (6) Hotchkiss, P. J.; Li, H.; Paramonov, P. B.; Paniagua, S. A.; Jones, S. C.; Armstrong, N. R.; Brédas, J.-L.; Marder, S. R. Modification of the Surface Properties of Indium Tin Oxide with Benzylphosphonic Acids: A Joint Experimental and Theoretical Study. Adv. Mater. 2009, 21, 4496−4501. (7) Park, J. S.; Lee, B. R.; Lee, J. M.; Kim, J.-S.; Kim, S. O.; Song, M. H. Efficient Hybrid Organic-Inorganic Light Emitting Diodes with Self-Assembled Dipole Molecule Deposited Metal Oxides. Appl. Phys. Lett. 2010, 96, 243306−1−3. (8) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/ Organic Interfaces. Adv. Mater. 1999, 11, 605−625. (9) Tisdale, W. A.; Muntwiler, M.; Norris, D. J.; Aydil, E. S.; Zhu, X.-Y. Electron Dynamics at the ZnO (10−10) Surface. J. Phys. Chem. C 2008, 112, 14682−14692. (10) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 244. (11) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Efficient Hybrid Solar Cells from Zinc Oxide Nanoparticles and a Conjugated Polymer. Adv. Mater. 2004, 16, 1009−1013. (12) Bao, J.; Zimmler, M. A.; Capasso, F.; Wang, X.; Ren, Z. F. Broadband ZnO Single-Nanowire Light-Emitting Diode. Nano Lett. 2006, 6, 1719−1722. (13) Wadeasa, A.; Tzamalis, G.; Sehati, P.; Nur, O.; Fahlman, M.; Willander, M.; Berggren, M.; Crispin, X. Solution Processed ZnO Nanowires/Polyfluorene Heterojunctions for Large Area Lightening. Chem. Phys. Lett. 2010, 490, 200−204. (14) Samokhvalov, A.; Tatarchuk, B. J. Characterization of Active Sites, Determination of Mechanisms of H2S, COS and CS2 Sorption and Regeneration of ZnO Low-Temperature Sorbents: Past, Current and Perspectives. Phys. Chem. Chem. Phys. 2011, 13, 3197−3209. (15) Ordejón, P.; Artacho, E.; Soler, J. M. Self-Consistent Order-N Density-Functional Calculations for Very Large Systems. Phys. Rev. B 1996, 53, R10441−R10444. (16) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA Method for ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (17) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Self-Consistent-Charge DensityFunctional Tight-Binding Method for Simulations of Complex Materials Properties. Phys. Rev. B 1998, 58, 7260−7268. (18) Aradi, B.; Hourahine, B.; Frauenheim, Th. DFTB+, a Sparse Matrix-Based Implementation of the DFTB Method. J. Phys. Chem. A 2007, 111, 5678−5684. (19) Sham, L. J.; Schlüter, M. Density-Functional Theory of the Band Gap. Phys Rev B 1985, 32, 3883−3889. (20) Wróbel, J.; Kurzydłowski, J. K.; Hummer, K.; Kresse, G.; Piechota, J. Calculations of ZnO Properties Using the Heyd-ScuseriaErnzerhof Screened Hybrid Density Functional. Phys. Rev. B 2009, 80, 155124−1−8. (21) Moreira, N.; Dolgonos, H.; Bàlint, G.; da Rosa, A.; Frauenheim, A. L. T. Toward an Accurate Density-Functional Tight-Binding Description of Zinc-Containing Compounds. J. Chem. Theory Comput. 2009, 5, 605−614.
modeling schemes differs for the lowest unoccupied electronic levels, as a result of weaker electronic couplings between the organic and the inorganic components. The computed electronic structure of all TTF/ZnO configurations points to a significant charge transfer from the TTF molecule to the ZnO surface, resulting in the appearance of a significant interfacial dipole. As such, the deposition of TTF molecules on the ZnO(10−10) surface reduces the work function of ZnO and might help modulating the barrier for charge injection in hybrid optoelectronic devices. From a technical point of view, these calculations have also shown the overall good performance of the SCC-DFTB approach compared with the computationally more expensive DFT PBE/DZP method.
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COMPUTATIONAL SECTION Periodic DFT calculations have been performed using the SIESTA 3.0 program package16 with the GGA-PBE functional initially developed by Perdew, Burke, and Ernzerhof27 and the polarized double-ζ (DZP) basis set. SCC-DFTB17 calculations were performed using the DFTB+ code.18 The same unit cell parameters and Monkhorst-Pack k-point meshes have been used in DFT and SCC-DFTB calculations. The unit cell size has been chosen as the best compromise between the computational cost and the possibility to match one single TTF molecule on the surface, with as limited as possible interactions with its own images. The lattice parameters are a = 15.9879 Å, b = 16.570 Å, and c = 49.5667 Å with all angles equal to 90°. The slab includes eight layers, and a vacuum gap of 40 Å is added to avoid interactions between two slabs along the direction perpendicular to the interface (z axis). A (2 × 2 × 1) k-point mesh was used in geometry optimizations, and singlepoint electronic structure calculations were performed using a (5 × 5 × 3) grid. The plane-averaged electrostatic potential has been calculated using the technique presented in ref 28.
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ASSOCIATED CONTENT
S Supporting Information *
Geometry of the ZnO(10−10) surface; alignment of the electronic energy levels in the isolated systems; electronic energy levels in the TTF/ZnO complexes; and atom-projected density of states of the TTF/ZnO complexes. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Present Address † IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. ACKNOWLEDGMENTS This work was funded by the European Commission Seventh Framework Program (FP7/2007-2013) under grant agreement number 228424 (Project MINOTOR). The work in Mons was further supported by the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 6/27) and the Belgian National Fund for Scientific Research. J.C. and D.B. are senior FNRS research fellows.
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