On-Surface Synthesis of Two-DimensionalCovalent Organic Structures versus Halogen-Bonded Self-Assembly: Competing Formationof Organic NanoarchitecturesDavid Peyrot and Fabien Silly*
TITANS, CEA, IRAMIS, SPEC, CNRS, Universite Paris Saclay, CEA Saclay, F-91191 Gif sur Yvette, France
*S Supporting Information
ABSTRACT: The competition between the on-surfacesynthesis of covalent nanoarchitectures and the self-assembly of star-shaped 1,3,5-Tris(4-iodophenyl)benzenemolecules on Au(111) in vacuum is investigated usingscanning tunneling microscopy above room temperature.The molecules form covalent polygonal nanoachitectures atthe gold surface step edges and at the elbows of the goldreconstruction at low coverage. With coverage increasingtwo-dimensional halogen-bonded structures appear andgrow on the surface terraces. Two different halogen-bondednanoarchitectures are coexisting on the surface and hybrid covalent-halogen bonded structures are locally observed. Athigh coverage covalent nanoarchitectures are squeezed at the domain boundary of the halogen-bonded structures. Thecompetitive growth between the covalent and halogen-bonded nanoarchitectures leads to formation of a two-layer filmabove one monolayer deposition. For this coverage, the covalent nanoarchitectures are propelled on top of the halogen-bonded first layer. These observations open up new opportunities for decoupling covalent nanoarchitectures fromcatalytically active and metal surfaces in vacuum.
Engineering two-dimensional (2D) carbon-based nano-architectures has received tremendous attention due tothe unusual exceptional electronic properties of these
materials. Graphene is for example expected to revolutionizenumerous industrial technologies because of its high mobility.1
The zero band gap property of graphene is, however, adrawback for its integration in semiconductor-based devices.The calculations of Pedersen et al., however, predict thatcreating a hole-superlattice in a semimetallic graphene layerwould lead to a drastic modification of its electronicproperties.2 The so-prepared graphene layer would thusbecome a semiconductor with a significant and tunable energyband gap. This engineering at the atomic scale opens newopportunities for fabricating carbon layers with specificelectronic properties. Graphene can be patterned using top-down techniques like electron-beam lithography. However, thegeneration of large-scale highly ordered superlattice ofnanoholes separated by less than 10 nm still remains achallenging task to experimentally achieve.3,4 Alternativemethods have been developed. On-surface bottom-up synthesisof patterned graphene nanoarchitectures at atomic scale can forexample be achieved by taking advantage of the ability of some
specific organic building blocks to generate covalent carbon-structures via intermolecular interactions.5−11 Several reactionshave been identified to generate carbon nanoarchitectures viasurface-assisted intermolecular covalent C−C coupling. Thisincludes Glaser coupling,12 Bergman cyclization,13 cyclo-dehydrogenation,14,15 dehydrogenative coupling,16,17 and Ull-mann coupling.18−21 The current state-of the-art of surface-activated molecular covalent reactions is described in the reviewpapers of Dong et al. and Fan et al.22,23 Porous carbon-nanoarchitectures,24−28 graphene nanoribbons,29−31 have thusbeen successfully synthesized by surface-assisted reactions. Thedimensions of such structures are, however, quite limited andthe number of defects is usually important because the covalentbond formation is a nonreversible process preventing self-healing.Molecules with peripheral halogen atoms are promising
organic building blocks to engineer different types of two-dimensional porous carbon-nanoarchitectures. The molecular
halogen atoms (X) can stabilize highly ordered organicnanoarchitectures through the formation of intermolecularhalogen bonds.32−36 Halogen-bonded self-assembled nano-architectures have been successfully created using moleculeswith bromine37−40 and iodine substituents.41 In addition, thesebuilding blocks can also be used to engineer covalentnanoarchitectures taking advantage of on-surface polymer-ization. 2D covalent-nanoarchitectures have been fabricatedusing molecules with bromine atoms18,29,34,42−45 as well asmolecules with iodine atoms.34,46−48 However, the competingformation of halogen-bonded nanoarchitectures and covalentstructures on catalytically active metal surfaces has still to beelucidated to optimize the formation of one specific type oforganic structure. This is essential for example to trigger theformation of porous graphene-nanoarchitectures over theformation of self-assembled halogen-bonded films at roomtemperature.Tailoring the formation of porous 2D carbon-nanoarchitec-
ture is only one of the challenges in carbon-based nanostructureengineering. Intercalating an intermediate layers between themetal surface and the covalent carbon structures is another one.Walter et al. already succeeded in intercalating an intermediateoxygen layer between the metal surface and a graphene layerafter exposure to air49 and Li et al. suceeded in intercalating Siheteroatoms between graphene and Ru(0001) in vacuum.50
Eder et al. developed a drop-casting method to deposit 1,3,5-Tri(4-iodophenyl)benzene (Figure 1a) covalent aggregates on
top of a layer supposed to be made of chemisorbed iodineatoms and disordered molecules adsorbed on Au(111).46 Theyclaimed that the presence of the covalent aggregates in a secondlayer is unique to their solution approach and can not besuccessfully achieved in vacuum.47
In this paper, we investigate the self-assembly of 1,3,5-Tris(4-iodophenyl)benzene molecules and the formation of covalentnanoarchitectures on Au(111) in vacuum. Scanning tunnelingmicroscopy (STM) reveals that molecules form covalentstructures at the Au step edges at low concentration, whereasdifferent halogen-bonded networks appear at higher coverage.The competitive growth of the covalent and halogen-bondednanoarchitectures leads to formation of a two layer film, where
the covalent organic nanoarchitecture lays on top of thehalogen-bonded structures.
RESULTS AND DISCUSSIONThe chemical structure of the 1,3,5-Tris(4-iodophenyl)benzenemolecule is presented in Figure 1a. This 3-fold symmetrymolecule is a star-shaped molecule. The molecular skeleton isconsisting of a central benzene ring connected to threeperipheral 4′-iodophenyl groups. Two molecules are expectedto form on metal surface a covalent dimer (Figure 1b) throughsurface-assisted Ullmann coupling.
Initial Growth of the Organic Layer: 0.02−0.10Monolayer Coverage. The Figure 2a shows the Au(111)surface after low concentration deposition (less than 0.02monolayer (ML)) of 1,3,5-Tris(4-iodophenyl)benzene mole-cules. The STM image reveals that molecules preferentiallyadsorb at the gold step edges. The molecules form small zigzagchains on each side of the step edge. The center-to-centerdistance of adjacent molecules is 1.3 ± 0.1 nm. This indicatesintermolecular covalent bonding, Figure 1b. Bright spots can beobserved at the apex of molecular arms (highlighted by blackarrows in Figure 2a). It has been previously experimentallyobserved with STM that halogen atoms, such as bromine andiodine, appear brighter than molecular carbon atoms.41,51 Thesebright spots therefore reveal that molecular iodine atoms arenot systematically dissociated from molecular skeleton whenmolecules are adsorbed on the Au(111) surface at roomtemperature, in contradiction with previous reports.45,47,52
STM images of the surface step edges after 0.10 MLdeposition are presented in the Figure 2b,c. The STM imagesshow that small porous structures are growing from theAu(111) step edges to the surface terraces. The STM imagesalso reveal that bright spots are now adsorbed on the stepedges. This is particularly obvious in the high resolution STMimage in Figure 2c. Iodine adatoms created during thepolymerization of molecular building blocks appear topreferentially adsorb on the Au(111) step edges.Figure 2d shows that small covalent porous structures are
locally observed at the elbows of the gold herringbonereconstruction for 0.10 ML deposition. The formation ofcovalent structures appears however to be quite limited. Onlyone or two covalent cycles are usually observed in the elbows ofthe Au herringbone surface reconstruction.
Covalent Organic Polygonal Nanoarchitecture for0.1−0.2 Monolayer Coverage. Above 0.2 monolayerdeposition, a 2D porous nanoarchitecture is observed on thesurface. This structure is composed of organic pores (Figure 3)and zigzag chains (Figure 3h). The pores adopt a polygonalgeometry. Tetragonal, pentagonal, heptagonal, and octagonalcavities are mainly composing the organic structure, (Figure3c−g) . The measured center-to-center distance of the star-shaped molecular building block is 1.3 ± 0.1 nm. This revealsthat the molecules are covalently linked through thedehalogenation of the molecular peripheral iodine atoms. Theimage in Figure 3b is in addition showing that iodine adatomsare sometimes trapped inside the polygonal-cavities of thecovalent nanoarchitecture.
Appearance of Halogen-Bonded Nanoarchitecturesabove 0.2 Monolayer Coverage. The STM images in Figure4 show molecular self-assembly on the Au(111)−(22 × √3)terraces for 0.2 monolayer deposition. Molecules now formsmall well-organized domains trapped between the goldreconstruction lines, Figure 4a,b. Ordered bright features can
Figure 1. Scheme of 1,3,5-Tris(4-iodophenyl)benzene (C21H15I3)dimer building block. Carbon atoms are gray, iodine atoms purple,and hydrogen atoms white, respectively.
be observed inside the molecular domains. The size of thedomain is increasing with molecular concentration. Molecularcovalent structures can be observed at the edge of thesemolecular domains, Figure 4c. For one monolayer deposition(1 ML) the whole Au(111) surface is covered as shown inFigure 4d. Different organic networks are coexisting, different
bright patterns can be observed in the organic layer, Figure4c,d. The bright patterns can adopt a well-ordered sine-wave(Figure 5) or bow-tie structure (Figure 6) . The boundaries ofthe molecular network appear in comparison quite defective.
Sine-wave X-bonded Structure. Figure 5a shows a highresolution STM image of the sine-wave organic nano-architecture observed in Figure 4. The STM image revealsthat the molecules are self-assembled into an ordered porousstructure on the surface. Neighboring side-by-side moleculesare rotated by an angle of 180°, as it is highlighted by themolecular schemes superimposed to the STM image in Figure5a. The molecular iodine atoms appear brighter than molecularcarbon skeleton in the STM image. The bright sine-wavepattern results from the local arrangement of molecular iodineatoms. Each bright wave corresponds to an infinite iodinesynthon X∞. The STM images presented in Figure 5b,c areshowing that some molecular arms are locally dehalogenated.
Figure 2. Initial growth of the organic layer on Au(111)−(22 ×√(3): (a) 22 × 12 nm2, Vs = 0.9 V, It = 20 pA; (b) 45 × 38 nm2, Vs= 1.3 V, It = 205 pA; (c) 20 × 20 nm2, Vs = 1.3 V, It = 245 pA; (d)36 × 36 nm2, Vs = 1.3 V, It = 205 pA.
Figure 3. STM image of the 1,3,5-Tris(4-iodophenyl)benzene self-assembled covalent porous nanoarchitecture on Au(111), (a) 21 ×12 nm2, Vs = 0.9 V, It = 20 pA, (b) 75 × 60 nm2, (c) 3 × 3 nm2, (d)3 × 3 nm2, (e) 3 × 3 nm2, (f) 3 × 3 nm2, (g) 3 × 3 nm2, (h) 3 × 3nm2, Vs = 1.0 V, It = 25 pA.
This does not affect the structure of the organic nano-architecture. The missing iodine atoms are highlighted by awhite dotted circle superimposed to the STM images. The unitcell of the sine-wave nanoarchitecture is represented by a blackdashed line in Figure 5d. The network unit cell of this porousstructure is a rectangle with 2.1 ± 0.2 nm and 1.9 ± 0.2 nm unitcell constants. The unit cell is composed of two molecules. Themolecular architecture appears to be stabilized by halogen···halogen bonds between neighboring molecules. The anglebetween I−C groups of neighboring molecules is 180° and120°.Bow-Tie X-Bonded Structure. A second well-ordered
organic nanoarchitecture is also usually observed on theAu(111) surface. High resolution STM images of this structureare presented in Figure 6. This organic nanoarchitecture is alsoporous. Six molecular schemes have been superimposed to theSTM image in Figure 6b to visualize molecular assembly.Neighboring molecules are rotated by an angle of 180°. Themolecules are however not adopting a strict side-by-sidearrangement, as previously observed in the sine-wave structure.The molecular iodine atoms are forming 6-synthons (X6)adopting a bow-tie shape. The network unit cell of thisstructure is a parallelogram with 2.2 ± 0.2 nm and 1.7 ± 0.2 nmunit cell constants and an angle of 75 ± 3° between the axes.
The unit cell is composed of two molecules. The moleculararchitecture appears to be stabilized by halogen···halogen bondsbetween neighboring molecules. The angle between I−Cgroups of neighboring molecules is 60° and 120°.
Covalent-Dimer X-Bonded Structure. Another organicnanoarchitecture is locally observed on the Au(111) surface.An STM image of this structure is presented in Figure 7.Molecular iodine atoms are now paired and are aligned on thesurface. The high resolution STM image reveals that thebuilding block of this structure is the 1,3,5-Tris(4-iodophenyl)-benzene covalent-dimer, previously presented in Figure 1b. Themolecular dimers are aligned along their axis on the surface andthey are forming parallel chains. Neighboring dimers are bindedalong their axis through double X···X bonds. The anglebetween I−C groups is 60°. The model of this arrangement isrepresented in the Figure 7d. The network unit cell of this
Figure 4. STM images of the Au(111) terraces at increasingmolecular coverage, (a) 40 × 31 nm2, Vs = 2.2 V, It = 55 pA, (b) 50× 35 nm2, Vs = 2.2 V, It = 55 pA, (c) 26 × 12 nm2, Vs = 0.9 V, It =245 pA. (d) 36 × 36 nm2, Vs = 0.6 V, It = 245 pA.
Figure 5. High resolution STM images of the 1,3,5-Tris(4-iodophenyl)benzene sine-wave nanoarchitecture, (a) 10 × 8 nm2,Vs = 1.0 V, It = 23 pA, (b) 5 × 3 nm2, Vs = 1.0 V, It = 23 pA, (c) 3 ×5 nm2, Vs = 1.0 V, It = 23 pA, (d) model of the organicnanoarchitecture. Molecular skeleton is represented by an orangestar and the iodine atoms are represented by yellow balls. Thenetwork unit cell is represented by a dotted black rectangle.
structure is a parallelogram rectangle with 2.7 ± 0.2 nm and 1.7± 0.2 nm unit cell constants and an angle of 43 ± 3° betweenthe axes. The unit cell is composed of one dimer (twocovalently linked molecules). Local defects can be observed inthe organic layer. These structure defects correspond todehalogenated dimers (Figure 7b,c) and local bow-tie nano-architecture (see superimposed star molecule scheme in Figure
7a). This structure is a mixed nanoarchitecture, combiningcovalent and X···X halogen bonds. The angle between I−Cgroups of neighboring molecules is 120°.
Covalent-Chain Assembly. The STM image presented inFigure 8a reveals the existence of a fourth organic organizednanoarchitecture on the Au(111) surface. This structure iscomposed of covalent molecular zigzag chains arranged side byside. The zigzag chains result from the sequential covalentbinding of molecules alternatively rotated by an angle of 180°.The model of the zigzag chain arrangement is presented inFigure 8b. The network unit cell of this structure is a rectanglewith 2.2 ± 0.2 nm and 1.6 ± 0.1 nm unit cell constants. Theunit cell is composed of two covalently linked molecules. Thisstructure is, however, only locally observed on the Au(111)surface.
Figure 6. High resolution STM images of the 1,3,5-Tris(4-iodophenyl)benzene bow-tie nanoarchitecture, (a) 14 × 14 nm2,Vs = 1.3 V, It = 20 pA, (b) 5 × 6 nm2, Vs = 1.3 V, It = 20 pA. (c)Model of the organic nanoarchitecture. The network unit cell isrepresented by a dotted black parallelogram.
Figure 7. STM images of the mixed covalent/X-bonded 1,3,5-Tris(4-iodophenyl)benzene nanoarchitecture on Au(111), (a) 10 ×10 nm2, Vs = 1.3 V, It = 205 pA, (b) 7 × 7 nm2, Vs = 1.3 V, It = 205pA, (c) 7 × 7 nm2, Vs = 1.3 V, It = 205 pA. (d) Model of themolecular arrangement. The network unit cell is represented by adotted black parallelogram.
Double-Layer Organic Film above 1 MonolayerDeposition. Figure 9 shows the Au(111) surface after 1.2ML molecular deposition. The Au(111) surface is now fullycovered by an organic layer. The STM images show that thefirst organic layer on the Au(111) surface is essentiallycomposed of the sine-wave and bow-tie nanoarchitectures,whereas the dimer and chain-structures are only locallyobserved (Figure 9c). The second organic layer appears to beexclusively composed of the covalent polygonal nanoarchitec-ture that grows at low coverage (Figure 3). The line profile inFigure 9b shows the height difference between the halogen-bonded first layer and the supported covalent-nanoarchitec-tures.The self-assembly of 1,3,5-Tris(4-iodophenyl)benzene mol-
ecules on Au(111) surface in vacuum has been investigatedusing STM. STM images reveal that at low coverage themolecules preferentially adsorb on the Au(111) step edges. Themolecules form there covalent structures through iodinedehalogenation. Small covalent structures are also observed atthe herringbone elbows of the Au(111) reconstruction. Acoverage increase essentially leads to the growth of halogen-bonded nanoarchitectures on the surface; the formation ofcovalent nanoarchitecture is in comparison less favored. Incontrast with previous reports,45,47,53 our measurements showthat the iodine dehalogenation process is a limited process onAu(111) at room temperature since iodine atoms can beundoubtedly identified with STM on the molecular skeleton innumerous case on Au(111), Figure 2. Our STM images are in
fact showing that Au(111) step edges are preferential reactionsites for Ullmann coupling reaction. This effect was previouslyproposed by Saywell et al. for bromine molecules.42 They alsoobserved that Au(111) step edges and kinks act as “active sites”and catalytically induce the cleavage of the molecular halogenatoms. This explains why covalent structures preferentially startgrowing from Au(111) step edges, Figure 2a,b.The free iodine atoms produced during the Ullmann
coupling are preferentially adsorbing on the Au(111) stepedges. These atoms then form chains along the step edges, as itcan be observed in Figure 2b,c. The saturation of Au(111) stepedges with iodine atoms reduces the catalytic activity of theAu(111) surface. At high deposition the formation of covalentnanoarchitectures is highly diminished. The moleculespreferentially form halogen-bonded nanoarchitectures on theAu(111) surface. Molecules with missing iodine atoms are onlylocally observed in these structures, not only at roomtemperature but also at high temperature (see Figures S3 andS4 in Supporting Information). Increasing further the coverageleads to the competitive growth of the covalent structure and
Figure 8. (a) STM image of the covalent chain-nanoarchitecture onAu(111), 14 × 12 nm2, Vs = 1.9 V, It = 55 pA. (b) Model of thezigzag chain-nanoarchitecture. The network unit cell is representedby a dotted black square.
Figure 9. STM images of the Au(111) surface after 1.2 MLmolecular deposition, (a) 110 × 60 nm2, Vs = 2.1 V, It = 445 pA.(b) Line profile taken along the blue dotted line in (a); (c) 60 × 60nm2, Vs = 1.9 V, It = 445 pA.
halogen-bonded nanoarchitectures. STM is showing that thehalogen-bonded structures are preferentially covering thesurface and are pushing away the covalent nanoarchitectures.The covalent nanoarchitectures are then decorating the edgesof X-bonded structures. The covalent structures are finallysqueezed at the domain boundary of X-bonded structures for 1ML deposition, Figure 4c. This results in the formation of verydefective domain boundaries, Figure 4d. Two X-bondednanoarchitectures coexist. They are presented in the highresolution STM images in Figure 5 and Figure 6. However, itshould be noticed that hybrid X-bonded-covalent nano-architectures (Figure 7) and covalent chain nanoarchitectures(Figure 8) are only locally observed; their domain area is neverlarger than 200 nm2. Above one monolayer deposition, theAu(111) surface is fully covered by an organic layer, essentiallycomposed of the X-bonded nanoarchitectures. A second layer isobserved on top of the first X-bonded layer, Figure 9. STMimages reveal that the second layer is surprisingly composed ofpolygonal-covalent nanoarchitectures. As the Au(111) surface isnow covered by an organic layer and the step edges are alreadysaturated with iodine adatoms, Ullmann reaction cannot occuranymore for new molecules reaching the gold surface. Itappears therefore that the preferential adsorption of the X-bonded nanoarchitectures on the Au(111) surface is propellingthe initially formed covalent-structures on top of the X-bondednanoarchitectures above 1 ML deposition. Figure 3 obviouslyshows that the packing density of the covalent nanoarchitectureis largely lower than the one of the X-bonded nano-architectures, Figure 4. The growth of the most compactstructures is usually favored on surfaces because thesestructures not only favor intermolecular interactions but theyalso lower the surface free-energy of the sample.54 Thestructure density is 1 mol/1.99 nm2 for the sine-wave network,1 mol/1.81 nm2 for the bow-tie network, 1 mol/1.56 nm2 for
the hybrid dimer network, and 1 mol/1.76 nm2 for the hybridzigzag network. In comparison, the density of the covalenthexagonal network is the smallest with 1 mol/2.16 nm2, seeTable 1. As the covalent nanoarchitecture is the less dense, it ispushed away from the Au(111) surface by the growth of thedenser nanoarchitectures. In contradiction with Elder et al.statement, we prove that the presence of the covalentaggregates in a second layer is not unique to the solutionapproach and can also be obtained in vacuum46
The effect of surface temperature on Ullmann couplingreaction efficiency has been investigated. Molecules have beendeposited on a hot surface and molecules have also beendeposited on a surface at room temperature, followed by apostannealing. The temperature range investigated goes fromroom temperature to 500 °C. This was done for differentsurface coverages. STM images, however, reveal that temper-ature increase has no effect on the formation of covalentnanoarchitectures (see Supporting Information). X-bondedstructures are still observed at high temperature and thecompetitive growth of the different structures is not modified.Therefore, it appears that the formation of covalent-architectures is intrinsically connected to surface coverageand, therefore, the saturation of surface reaction sites (stepedges and elbows of the Au(111) herringbone reconstruction)more than temperature. It should be noticed that fortemperatures higher than 500 °C, molecules and iodineadatoms are both desorbing from the Au(111) surface.Bui et al. previously classified the different types of C−X···
X−C bonds depending on the angle between X−C groups.55
The type-I interaction is of van der Waals type. The type-IIinteraction is an attractive interaction between the nucleophilic(−) and electrophilic (+) areas of halogen atoms. The anglebetween the molecular X−C group axis is 90−120°. The sine-wave structure appears therefore to be stabilized by type-I and
Table 1. Structure, Bonding, and Packing Density of the 1,3,5-Tris(4-iodophenyl)benzene Nanoarchitecturesa
aThe carbon skeleton of th molecular building block is represented by an orange star, and the iodine atoms are represented by yellow balls.
type-II halogen bonds, whereas the bow-tie and dimerstructures appear to be stabilized by type-II halogen bondsonly, according to ref 55.
CONCLUSIONTo summarize, the on-surface synthesis of covalent nano-architectures and the self-assembly of star-shaped 1,3,5-Tris(4-iodophenyl)benzene molecules was investigated using scanningtunneling microscopy. STM shows that at low coverage,covalent polygonal nanostructures appear at the Au(111) stepedges and at the elbows of the Au(111) surface reconstruction.The iodine atoms generated by molecule dehalogenationdiffuse on the surface and are then adsorbed at the surfacestep edges. At high coverage two-dimensional halogen-bondednanoarchitectures are preferentially growing on the goldsurface. These structures are pushing away the covalentnanoarchitectures at their domain boundaries. Above onemonolayer deposition the whole Au(111) surface is coveredwith an organic layer and the covalent structures are propelledon top of the halogen-bonded organic layer. These observationsopen up new opportunities for decoupling covalent nano-architectures from catalytically active and metal surfaces invacuum. The electronic decoupling induced by the halogenbonded structures localized between the covalent structuresand the surface could be investigated, for example, using lowtemperature scanning tunneling spectroscopy.
EXPERIMENTAL SECTIONExperiments were performed in an ultrahigh vacuum (UHV) chamberat a pressure of 10−8 Pa. The Au(111) surface was sputtered with Ar+
ions and then annealed in UHV at 600 °C for 1 h. 1,3,5-Tris(4-iodophenyl)benzene molecules (90%, Aldrich), Figure 1a, wereevaporated at 180 °C and deposited on the gold surface. Cut Pt/Irtips were used to obtain constant current STM images at roomtemperature with a bias voltage applied to the sample. STM imageswere processed and analyzed using the homemade FabViewerapplication.56
ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.6b01938.
The distribution of covalent polygonal nanoarchitectureversus halogen-bonded arrangements on Au(111) fordifferent coverages, the height of the covalent andhalogen-bonded molecular structures in the first organiclayer, STM images of the halogen-bonded organicstructures at high temperatures. (PDF)
ACKNOWLEDGMENTSThe research leading to these results has received funding fromthe European Research Council under the European Union’sSeventh Framework Programme (FP7/2007-2013)/ERC grantagreement no. 259297. The authors thank F. Merlet forprecious technical support.
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