Pierre Milbeo

Jean Martinez

Muriel Amblard

Monique Calmès

Baptiste Legrand

octane-2-carboxylic acid and derivatives as chiral constrained bridged scaffolds for foldamers and chiral catalysts

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1-Aminobicyclo[2.2.2]octane-2-carboxylic acid and derivatives as chiral constrained bridged scaffolds for foldamers and chiral catalysts

PIERRE MILBEO, JEAN MARTINEZ, MURIEL AMBLARD*, MONIQUE CALMES*, BAPTISTE LEGRAND* ligands whose combined with Cu(OAc) 2 led to efficient catalysts for the asymmetric Henry reaction of nitromethane with various aldehydes.

INTRODUCTION

β-amino acids are of great interest in various fields of organic and bioorganic chemistry such as medicinal, supramolecular chemistry and asymmetric catalysis. [START_REF] Cabrele | Peptides Containing β-Amino Acid Patterns: Challenges and Successes in Medicinal Chemistry[END_REF] They have been frequently

found in numerous bioactive natural products from various microorganisms and are essential monomers in natural antitumor agents such as taxol and bleomycin. [START_REF] Wani | Plant antitumor agents. VI. Isolation and structure of taxol, a novel antileukemic and antitumor agent from Taxus brevifolia[END_REF][START_REF] Umezawa | New Antibiotics, Bleomycin A and B[END_REF] Compared to the common α-amino acids, they possess one extra backbone carbon atom, which improved their stability and, β-peptides are inert to proteases. Consequently, they have been widely incorporated into peptides to generate peptidomimetics exhibiting significantly improved pharmacokinetic profiles.

Furthermore, the early works from Seebach, Gellman, and co-workers have shown that β-amino acid oligomers can fold into original architectures reminiscent of the protein secondary structures, and gave rise to the field of foldamers. [START_REF] Seebach | β-Peptides: A Surprise at Every Turn[END_REF][START_REF] Gellman | A Manifesto[END_REF][START_REF] Seebach | The World of Beta-and Gamma-Peptides Comprised of Homologated Proteinogenic Amino Acids and Other Components[END_REF] In this context, great efforts have been made to enlarge the β-amino acid family by developing conformationally constrained cyclic βamino acids. [START_REF] Ortuño | Enantioselective Synthesis of Conformationally Constrained β-Amino Acids[END_REF] To date, a broad range of single ring systems of different sizes have been synthesized and successfully used as precursors of various bioactive compounds such as antibiotics, enzyme inhibitors, antitumoral compounds. [START_REF] Fülöp | Alicyclic β-Amino Acids: Useful Synthons in Drug Research[END_REF][START_REF] Kuhl | Alicyclic β-Amino Acids in Medicinal Chemistry[END_REF] They have also been involved in asymmetric synthesis as efficient organo-catalysts both in their free form and their derivatives. [START_REF] Davies | Evaluating β-Amino Acids as Enantioselective Organocatalysts of the Hajos-Parrish-Eder-Sauer-Wiechert Reaction[END_REF][START_REF] D'elia | Short α/β-Peptides as Catalysts for Intra-and Intermolecular Aldol Reactions[END_REF] Their incorporation into oligomers significantly enlarged the diversity of foldamer structures, and allowed the development of innovative foldamers as therapeutic candidates [START_REF] Pasco | Foldamers in Medicinal Chemistry[END_REF] as well as catalysts of complex chemical reactions. [START_REF] Legrand | Catalytic Foldamers: When the Structure Guides the Function[END_REF] Over the past ten years, we focused on highly constrained amino acids built around a bicyclo[2.2.2]octane structure. Bicyclic molecules are less flexible and bulkier than single-ring compounds, and such a strengthened conformational restriction has been successfully used in the optimization of relevant molecules. Some bioactive natural and synthetic products containing a bicyclo[2.2.2]octane skeleton were reported, [START_REF] Seebacher | Synthesis of New Esters and Oximes with 4-Aminobicyclo[2.2.2]Octane Structure and Evaluation of Their Antitrypanosomal and Antiplasmodial Activities[END_REF][START_REF] Baran | Synthesis of Bicyclo[2.2.2]Octane-2,3,5,6,7,8 Hexols (Bishomoinositols) as Glycosidase Inhibitors[END_REF] while derivatives of this scaffold were used as templates for organic and asymmetric synthesis. [START_REF] Manner | Asymmetric Baker's Yeast Reductions of Bridgehead-Substituted Bicyclo[2.2.2]Octane-2,6-Dione Derivatives Followed by Conversion into Catalytically Active BODOLs for the Diethylzinc Addition to Benzaldehyde[END_REF][START_REF] Jayamurugan | Design and Synthesis of Aviram-Ratner-Type Dyads and Rectification Studies in Langmuir-Blodgett (LB) Films[END_REF] In the β-amino acid series, bulky bicyclic side chains significantly reduce the accessible range of backbone torsion angles, but also strongly influence the spatial orientation of adjacent chemical groups and building blocks. It is noteworthy that bicyclic carbo-bridged scaffolds have been less studied than bicyclic-fused and hetero-bridged systems. Among them, the synthesis of bicyclo[2.2.1]heptane derivatives was the most reported along with few examples of the synthesis of 3-aminobicyclo[2.2.2]octane-2carboxylic acids and derivatives. [22][START_REF] Park | Synthesis and Structural Analysis of 6-Aminobicyclo[2.2.1]heptane-2carboxylic Acid as a onformationally Constrained γ-Turn Mimic[END_REF][START_REF] Grygorenko | Bicyclic β-Amino Acids[END_REF] In the following parts, we discuss our efforts towards the synthesis of the enantiopure (R)-and (S)-1-aminobicyclo[2.2.2]octane-2-carboxylic acid (ABOC), [START_REF] Songis | Asymmetric Diels-Alder Cycloaddition of 1-Aminocyclohexadiene to Chiral Acrylate: Synthesis of Enantiopure Bridgehead-Aminobicyclo[2.2.2]Octane-2-Carboxylic Acid Derivatives[END_REF] a highly constrained bicyclic β-amino acid bearing a bridgehead amino group as well as its diamine derivative, the 1,2-diaminobicyclo[2.2.2]octane (DABO). [START_REF] Milbeo | Synthesis of Enantiopure 1,2-Diaminobicyclo[2.2.2Octane Derivatives, C1-Symmetric Chiral 1,2-Diamines with a Rigid Bicyclic Backbone[END_REF] Our first motivation was to investigate the propensity of such building blocks to accommodate secondary structures in hybrid/mixed oligoamides [START_REF] André | A Rigid Bicyclic β-Amino Acid as Turn Inducer[END_REF]3,[START_REF] Legrand | Unprecedented Chain-Length-Dependent Conformational Conversion Between 11/9 and 18/16 Helix in α/β-Hybrid Peptides[END_REF][START_REF] Simon | 12/10-Helix in Mixed β-Peptides Alternating Bicyclic and Acyclic β-Amino Acids: Probing the Relationship between Bicyclic Side Chain and Helix Stability[END_REF] and in oligoureas. 2,[START_REF] André | Mixed Oligoureas Based on Constrained Bicyclic and Acyclic β-Amino Acids Derivatives: On the Significance of the Subunit Configuration for Folding[END_REF][START_REF] Milbeo | A Bicyclic Unit Reversal to Stabilize the 12/14-Helix in Mixed Homochiral Oligoureas[END_REF] Then, we assessed ABOC and DABO derivatives as effective enantioselective catalysts. [START_REF] Milbeo | C1-Symmetric 1,2-Diaminobicyclo[2.2.2Octane Ligands in Copper-Catalyzed Asymmetric Henry Reaction: Catalyst Development and DFT Studies[END_REF][START_REF] Milbeo | N-Pyrrolidine-Based α/β-Peptides Incorporating ABOC, a Constrained Bicyclic β-Amino Acid, for Asymmetric Aldol Reaction Catalysis[END_REF]

SYNTHESIS OF ENANTIOPURE ABOC AND DABO

The route towards the original bicyclic β-amino acid ABOC involved an asymmetric Diels-Alder cycloaddition between the aminodiene 2 and the acrylate 1 equipped with the pantolactone-based chiral auxiliary (Scheme 1). [START_REF] Songis | Asymmetric Diels-Alder Cycloaddition of 1-Aminocyclohexadiene to Chiral Acrylate: Synthesis of Enantiopure Bridgehead-Aminobicyclo[2.2.2]Octane-2-Carboxylic Acid Derivatives[END_REF][START_REF] Gouverneur | Control of the Exo and Endo Pathways of the Diels-Alder Reaction by Antibody Catalysis[END_REF] The Diels-Alder cycloaddition between the dienophile (S)-1 and the diene 2 under microwave irradiation in solvent-free conditions resulted in a mixture of the expected cycloadducts in good yield (91%). Despite the moderate endo/exo selectivity (67%), an excellent facial selectivity (90%) allowed to isolate the (S)-1-aminobicyclo[2.2.2]octane-2carboxylic acid (S)-3 [(S)-ABOC] after separation by column chromatography, cleavage of the chiral auxiliary and concomitant hydrogenation/hydrogenolysis. The (R)-ABOC (R)-3 enantiomer was obtained using the same procedure starting from the dienophile (R)-1.

Scheme 1: Synthesis of H-(S)-ABOC-OH 3.

We then explored the pathway towards the bridged bicyclic diamino derivative of ABOC, i.e.

(R)-and (S)-DABO 17 (Scheme 2). The high interest in chiral diamines and their derivatives lies on their occurrence in biologically active compounds as well as their importance in stereoselective catalytic processes in a broad variety of organic reactions. 33

Scheme 2: Synthesis of enantiopure (R)/(S)-DABO 17.

A straightforward strategy to synthesize the diamine DABO proceeded through the rearrangement of the carboxylic acid moiety of ABOC to an amine. Hofmann experimental conditions were explored, starting from the saturated rac-6 or unsaturated "mix"-7 derivatives of Z-ABOC-OH obtained using non-asymmetric Diels-Alder cycloaddition between the diene 2 and neat methyl acrylate 4 under microwave activation at 150°C (Scheme 2A.). [START_REF] Milbeo | Synthesis of Enantiopure 1,2-Diaminobicyclo[2.2.2Octane Derivatives, C1-Symmetric Chiral 1,2-Diamines with a Rigid Bicyclic Backbone[END_REF] The cycloadduct "mix"-5 was isolated in good yield but as a rather complex diastereomeric mixture due to both poor endo/exo and facial selectivity. Hydrogenation of the double bond in the presence of Wilkinson's catalyst followed by saponification of the methyl ester yielded the racemic Nbenzyloxycarbonyl protected ABOC [rac-Z-ABOC-OH, rac-6)] whereas direct saponification yielded "mix"-7. The Hofmann rearrangement of the bicyclo[2.2.2]oct-5-ene amide derivative 9 carried out under mild conditions [START_REF] Waki | A Facile Synthesis of N2-Protected L-2,3-Diaminopropanoic Acid[END_REF] (Scheme 2B.) led mainly to the expected 1,2-diamine 12. By contrast, an unexpected divergent outcome investigated by DFT [START_REF] Milbeo | Synthesis of Enantiopure 1,2-Diaminobicyclo[2.2.2Octane Derivatives, C1-Symmetric Chiral 1,2-Diamines with a Rigid Bicyclic Backbone[END_REF] was observed under the same experimental conditions when using the saturated amide 8, affording mainly the corresponding cyclic urea 10 via an intramolecular trapping of the isocyanate intermediate by the carbamate nitrogen. After screening a few chiral alcohols to resolve the rather complex stereoisomeric mixture of unsaturated "mix"-7, pyrrolidinone 14 was identified as the most efficient auxiliary (Scheme 2C.). Thereby, the two pairs of diastereoisomers were obtained in good yield. Then, Hofmann rearrangement of each pair of the corresponding amide derivatives 9, followed by Nprotection of the free amine by a trifluoroacetamide group (Tfac) to ease the purification, saponification and hydrogenation/hydrogenolysis in AcOH yielded the enantiopure (R)-and (S)-DABO 17 as a diacetate salt.

HELICAL PEPTIDE/PEPTIDOMIMETIC FOLDAMERS

We first studied the constrained ABOC building block as a folding element in peptide sequences (Figure 1 and2). The carboxylic acid and amino groups of the ABOC residue were locked in a synclinal position that could potentially favour a turn within peptides. Reverse turns that are key structural motifs in biologically active peptides and proteins, have been often

substituted by dipeptide mimics and to a lesser extent by β-amino acids. [START_REF] Park | Synthesis and Structural Analysis of 6-Aminobicyclo[2.2.1]heptane-2carboxylic Acid as a onformationally Constrained γ-Turn Mimic[END_REF][START_REF] Robinson | β-Hairpin Peptidomimetics: Design, Structures and Biological Activities[END_REF][START_REF] Trabocchi | Synthesis of a Bicyclic δ-Amino Acid as a Constrained Gly-Asn Dipeptide Isostere[END_REF][START_REF] Godina | Mimics of Peptide Turn Backbone and Side-Chain Geometry by a General Approach for Modifying Azabicyclo[5.3.0]Alkanone Amino Acids[END_REF] Therefore, we have evaluated the ability of the ABOC motif to induce a turn in peptide sequences. For that purpose, following the Boc standard peptide strategy in solution, we have synthesized two model peptides, incorporating the ABOC motif in central position and varying the capping groups, i.e.

Z-Ala-(S)-ABOC-Phe-NHiPr 18 and iPrCO-Ala-(S)-ABOC-Phe-OCH 3 19. [START_REF] André | A Rigid Bicyclic β-Amino Acid as Turn Inducer[END_REF] Their structures were studied in solution by circular dichroism (CD), Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies, and in the solid-state by X-ray diffraction (XRD). These techniques were combined with molecular dynamics (MD) and DFT calculations.

We showed that ABOC was able to induce a reverse turn both in organic solvents and in the solid-state. The crystal structure of 18 showed C 9 and C 11 hydrogen bonds (Figure 1A). Thereby, these promising results have encouraged us to extend our investigations in the field of peptide foldamers. We focused on the use of ABOC motif in heterogeneous backbone systems that have significantly enlarged the diversity of attainable molecular architectures. [START_REF] Horne | Foldamers with Heterogeneous Backbones[END_REF] In this context, we synthesized hybrid oligomers of various lengths by combining ABOC and proteinogenic αamino acids (α-AAs), first with a 1:1 alternation of α-AA/(S)-ABOC residues, and then by decreasing the ABOC content, with 2:1 and 3:1 alternations (Figure 1B and 1C). 3,[START_REF] Legrand | Unprecedented Chain-Length-Dependent Conformational Conversion Between 11/9 and 18/16 Helix in α/β-Hybrid Peptides[END_REF] Although the 1:1 α-AA/(S)-ABOC crystal structure showed a 9/11-helix consistently with our previous model tripeptide, we detected a chain-length-dependent conformational polymorphism between the 9/11-and a wider 18/16-helix in solution (Figure 1B). [START_REF] Legrand | Unprecedented Chain-Length-Dependent Conformational Conversion Between 11/9 and 18/16 Helix in α/β-Hybrid Peptides[END_REF] The tetramer 20 adopted a 9/11-helix like in the solid-state, while the hexamer 21 structure was in equilibrium between the 9/11-and the 18/16-helices, and finally only typical NOE correlations of the 18/16-helix could be detected for the octamer 22. We hypothesized that the constraints imposed by the crystal lattice and the packing interactions might favour the 9/11-fold in the solid-state. Interestingly, the interconversion between the two helices in solution involved a unique rotation of about 90° of the α-AA ϕ angle, while the other backbone dihedral angle values remained nearly unchanged.

Conformational polymorphism between a 11-and a 14/15-helix was previously reported by Gellmann and co-workers for oligomers alternating α-AAs and trans-2-aminocyclopentane carboxylic acids (ACPCs). [START_REF] Hayen | Two Helical Conformations from a Single Foldamer Backbone: "Split Personality" in Short Alpha/Beta-Peptides[END_REF] On the other side, the 2:1 α-AA/(S)-ABOC sequences 23-25 adopted a single original 12/14/14-helix in solution stabilized by hydrogen bonds with opposite directionality, forming intramolecular 12-and 14-membered pseudocycles (Figure 1C). 3 The geometry of hexamer 24 was particularly attractive since the orientation of α-AA side chains matched rather well with those in the canonical α-helix. Finally, attempts to further dilute the ABOC residue among α-amino acids failed to give rise to stable architectures, e.g. short 1:3 α-AA/(S)-ABOC oligomers did not fold (unpublished results).

In parallel, we investigated ABOC-based β-peptides. In particular, we focused on (S)-ABOC homo-oligomers and oligomers with a 1:1 alternation of β 3 -homoamino acids (β 3 -hAAs) and (S)-ABOCs, i.e. β 3 -hAA/(S)-ABOC mixed oligomers (Figure 2). [START_REF] Simon | 12/10-Helix in Mixed β-Peptides Alternating Bicyclic and Acyclic β-Amino Acids: Probing the Relationship between Bicyclic Side Chain and Helix Stability[END_REF] β-peptide helices could adopt various shapes including the 10-, the 10/12-, the 12-and the 14-helices, depending on the βamino acid substitution patterns. [START_REF] Cheng | β-Peptides: From Structure to Function[END_REF] Among them, the 14-helix composed of acyclic β 3 -hAAs was extensively studied for therapeutic purposes including protein-protein interaction inhibitors and drug delivery compounds. [START_REF] Kritzer | β-Peptides as Inhibitors of Protein-Protein Interactions[END_REF][START_REF] Harker | Cell-Permeable β-Peptide Inhibitors of P53/HDM2 Complexation[END_REF] Besides, various homo-oligomers composed of cyclic motifs have been successfully synthesized and reported in the literature, affording an important contribution to the field of foldamers. [START_REF] Cheng | β-Peptides: From Structure to Function[END_REF] In our case, despite great efforts to obtain homo-ABOC oligomers and the successful syntheses of various homo-oligomers composed of cyclic motifs reported in the literature, we failed to elongate the poly-ABOC chain beyond two units. Indeed, the dimer 26 crystal structure showed a flat ribbon structure devoid of hydrogen bonds, both monomers sharing similar torsion angle values, i.e. ϕ = 59 ± 2°, θ = 45 ± 2° and ψ = -111 ± 13°

(unpublished results, Figure 2A). Both the rigidity and the steric hindrance of ABOC prevented the required approach of the third residue for the formation of a subsequent amide bond. In addition, the flat 'N-shape' adopted by consecutive ABOC residues in compound 26 might be incompatible with helical structures. These results prompted us to study mixed oligomers alternating β 3 -hAAs and (S)-ABOCs in a 1:1 pattern. [START_REF] Simon | 12/10-Helix in Mixed β-Peptides Alternating Bicyclic and Acyclic β-Amino Acids: Probing the Relationship between Bicyclic Side Chain and Helix Stability[END_REF] Incorporation of commercially available acyclic substituted β-homologated proteinogenic amino acids bearing a broad range of functional groups would allow the development of versatile oligomers for applications. We successfully synthesized 1:1 β 3 -hAA/ABOC oligomers 27 and 28 of various lengths. Hexamer 28 adopted a stable 12/10-Helix conformation in various organic solvents (Figure 2B). The mixed hydrogen bonds forming C 10 and C 12 pseudo rings were in opposite directions along the helix axis, resulting in a smaller macrodipole generally inducing a weaker stability in polar solvent compared to unidirectional helices. However, the real plus of the β 3 -hAA/ABOC helices was that their stability was not solvent-dependent while other building block combinations underwent partial or total loss of structure in polar solvents. [START_REF] Sharma | Robust Mixed 10/12 Helices Promoted by "Alternating Chirality" in a New Family of C-Linked Carbo-Beta-Peptides[END_REF][START_REF] Martinek | Effects of the Alternating Backbone Configuration on the Secondary Structure and Self-Assembly of Beta-Peptides[END_REF] We showed by DFT calculations that the strength of the C 10 hydrogen bonds was significantly higher than in acyclic β 3 -hAA/β 2 -hAA model oligomers. The NH and CO vectors were locked in optimal directions to establish strong C 10 hydrogen bonds.

Moreover, the amino group at the ABOC bridgehead involved in the C 10 was protected from the solvent.

HELICAL OLIGOUREA FOLDAMERS

In the meantime, we studied the effect of incorporation of bicyclo[2.2.2]octane rings in oligoureas. 2,[START_REF] André | Mixed Oligoureas Based on Constrained Bicyclic and Acyclic β-Amino Acids Derivatives: On the Significance of the Subunit Configuration for Folding[END_REF][START_REF] Milbeo | A Bicyclic Unit Reversal to Stabilize the 12/14-Helix in Mixed Homochiral Oligoureas[END_REF] Guichard and co-workers have shown earlier that oligoureas derived from acyclic β 3 -hAAs, i.e. composed of (S)-2-amino alkyl carbamoyl (AAC) residues, adopt typical 12/14-helices stabilized by bifurcated hydrogen bonds (Figure 3B and 3D). [START_REF] Fischer | The Canonical Helix of Urea Oligomers at Atomic Resolution: Insights into Folding-Induced Axial Organization[END_REF] In this context, the perspective to develop highly stable helices incorporating a few motifs combining both constrained bicyclic motifs and bifurcated hydrogen bonds stabilization brought by urea linkages, have stimulated these researches. Moreover, the longer urea link could significantly reduce the steric hindrance between bicycles and might allow the synthesis of homo-bicyclic amino carbamoyl oligomers, i.e. BAC oligoureas.

Thus, we have first successfully synthesized short (S)-BAC oligomers 29, 30 and 31 by stepwise assembly using the Boc standard strategy and the succinimidyl carbamate derivative of ABOC [Boc-(S)-BAC-OSu] (Figure 3A). 2 (S)-BAC oligomers adopted robust right-handed 12/14-helices both in solution and in the solid-state. Topological parameters were closed to those of the oligourea canonical helix with 2.5 residues per turn, a pitch of 5.1 Å, although the ϕ and θ 2 torsion angles values were swapped, i.e. ϕ = 63 ± 6°, θ 1 = 54 ± 9°, and θ 2 = -109 ± 21° in the crystal structures (Figure 3C, left panel and 3D). It was noteworthy that only two BAC residues already initiated the 12/14-helix conformation and no cis-trans urea bond isomerization was detected in solution by NMR. Moreover, CD spectra of hexamers showed that the homo-(S)-BAC oligourea 31 helix displayed a greater stability than the acyclic oligourea 32 helix (Figure 3E). Indeed, both compounds exhibited similar signatures with strong positive maxima around 204 nm but 31 showed a higher intensity. 3E). Moreover, the CD signal of the (S)-BAC/(S)-AAC sequences did not increase with the oligomer's length as typically observed for folded oligomers. [START_REF] André | Mixed Oligoureas Based on Constrained Bicyclic and Acyclic β-Amino Acids Derivatives: On the Significance of the Subunit Configuration for Folding[END_REF] NMR studies confirmed that homochiral oligomers were unfolded, while the heterochiral sequences adopted left-handed 12/14-helices in solution. The crystal structure of the (R)-BAC/(S)-AAC octamer 37 helix was obtained (Figure 3C, right panel). Nevertheless, the helix axis was significantly bent by the presence of solvent molecules (H 2 O and CH 3 OH) intercalated into a hydrogen bond network, acting as bridges between urea amide protons and carbonyl groups (Figure 3D). A close inspection of the (R)-BAC/(S)-AAC structures revealed that the AAC side chains were projected in inadequate intermediate positions generating steric clashes with the adjacent backbone carbonyls. The steric hindrance in the heterochiral helix was partially offset by the solvent molecule's insertion. In homochiral oligoureas, the AAC side chains would be in the axial orientations and would prevent the oligourea helix folding. Starting from these results, we anticipated that a simple reversal of the bicycle swapping the bridgehead position, and the 2-position within the final oligoureas, should lead to stable helices with AAC side chains in optimal lateral orientation. [START_REF] Milbeo | A Bicyclic Unit Reversal to Stabilize the 12/14-Helix in Mixed Homochiral Oligoureas[END_REF] Indeed, the reverse bicyclic amino carbamoyl unit (rBAC) shared similar torsion angle values with the acyclic AAC residues in the canonical oligourea helix, and the (S)-rBAC/(S)-AAC oligoureas would ideally fold with AAC substituents in lateral positions. repeating the coupling step until completion of the reaction. The CD spectra of (S)-AAC/(S)-rBAC oligomers hexamer 44 displayed the typical oligourea 12/14-helix signature (Figure 3E).

NMR and crystal structures showed that the (S)-AAC/(S)-rBAC oligomers adopted the typical right-handed 12/14-helix both in solution and in the solid-state, which was well-superimposed with the canonical oligourea helix composed of acyclic β 3 -hAA derivatives (Figure 4A). AAC side chains were ideally oriented in lateral position, perpendicularly to the helix axis and this latter was perfectly straight. Interestingly, both the homo-(S)-BAC and (S)-AAC/(S)-rBAC oligomers of same length shared comparable CD signal intensities, indicating a similar helix stability despite two times less bicyclic constrained motifs in the mixed oligourea (Figure 3E).

ASYMMETRIC CATALYZED REACTIONS

Asymmetric catalysis relies essentially on the ability of a chiral catalyst to interact with reaction substrates, and to exclusively stabilize the transition state towards the formation of a single product stereoisomer product. For that purpose, geometric constraints are key features of chiral catalysts to tune their steric properties and to control occupation of the three-dimensional chemical space. These properties are found in living systems where complex asymmetric biochemical reactions are achieved by enzymes with well-defined three-dimensional structures in thoroughly organized active sites.

In this context, the bicyclo[2.2.2]octane structure was applied to the design of novel peptide catalysts for asymmetric synthesis. Short peptides have received increasing attention in asymmetric organo-catalysis of diverse reactions as they possess a high degree of structural diversity and complexity, and are rather simple to synthesize. [START_REF] Metrano | Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms[END_REF] Over the past 20 years, many highly efficient peptide catalysts have been developed to promote the asymmetric aldol reaction, some of them exhibiting very high enantiomeric excesses and low catalyst loadings. [START_REF] Yamashita | Catalytic Enantioselective Aldol Reactions[END_REF] Nevertheless, only a few examples involve -amino acids. Thus, this reaction is particularly attractive to evaluate the impact of the constrained ABOC β-amino acids in catalysis. Therefore, a variety of α,β-peptides 46-60 containing proline and ABOC have been synthesized in solution using typical Boc strategies. They were tested for their catalytic properties in the asymmetric aldol reaction between p-nitrobenzaldehyde and acetone (Figure 5A). Dipeptides 46-49 were less effective than tripeptides and for these latter, the rate and stereoselectivity of the reaction was highly dependent on both the absolute configuration of the ABOC and on the nature of the Cterminal amino acid. As a general result, the heterochiral sequences gave better results than homochiral sequences, and the tripeptide 59, i. The best peptide catalyst 59 was used in the aldol reaction between diverse benzaldehyde and acetone under the optimised experimental conditions providing high yields and good enantioselectivities (up to 88%) in the presence of only 5 mol% catalyst. Given the important efficiency gap between heterochiral and homochiral catalysts, we studied the impact of the ABOC absolute configuration on the conformations of tripeptide 59 and 60 conformations. The crystal structure of heterochiral peptide 59 showed a turn structure with a salt-bridge between the amine function and the carboxylic acid both involved in the catalytic mechanism (Figure 5B.).

The close proximity between the two reactive functions was consistent with effective aldol reactions with high degrees of stereoselectivity. Unfortunately, we could not obtain suitable monocrystals of peptide 60, therefore we compared the structures of 59 and 60 using DFT calculations (Figure 5C). Both tripeptides adopted turn structures driven by the ABOC residue, most of the structures exhibited a salt-bridge between the secondary amine and the carboxylic acid. Additionally, a hydrogen bond between the ABOC HN and the carboxylate group was observed in structures of 59 (Figure 5B, left panel), while conformations of the homochiral peptide 60 were further stabilized by a C 9 pseudoring, according to the iPrCO-Ala-(S)-ABOC-Phe-OCH 3 model tripeptide crystal structure previously described (Figure 5B, right panel). [START_REF] André | A Rigid Bicyclic β-Amino Acid as Turn Inducer[END_REF] Like previous reports, [START_REF] D'elia | Short α/β-Peptides as Catalysts for Intra-and Intermolecular Aldol Reactions[END_REF][START_REF] Revell | Investigating Sequence Space: How Important Is the Spatial Arrangement of Functional Groups in the Asymmetric Aldol Reaction Catalyst H-Pro-Pro-Asp-NH2?[END_REF] we noticed the stabilization of a turn maintaining both the amine and carboxylic acid functions in direct vicinity that could correlate to efficient catalytic activities.

Nevertheless, the difference of rate and stereoselectivity between heterochiral and homochiral peptides was difficult to rationalize. DFT calculations showed an enhanced flexibility for the heterochiral compound 59 compared to its homochiral analogue 60, suggesting that a greater flexibility could guarantee a high adaptability, while 60 might be trapped in suboptimal conformations. This study, although not improving precedent reported results, provided valuable information about the relationship between conformation and efficiency of these catalysts that could be extremely useful in the development of future catalytic systems.

Capitalising on these results, we pursued by designing ligands based on the DABO residue to catalyse another well-documented reaction, the copper-catalysed asymmetric Henry reaction. Chiral 1,2-diamines are precursors of privileged chiral ligands such as salen-type ligands and bidentate ligands that play a key role in transition metal-based catalytic systems.

Usually based on constrained cyclic diamine scaffolds, these ligands often possess C 2symmetrical structures where their symmetry axis reduces the number of possible transition states and reaction pathways, for a catalysed reaction. [START_REF] White | A New Catalyst for the Asymmetric Henry Reaction: Synthesis of β-Nitroethanols in High Enantiomeric Excess[END_REF][START_REF] Kannan | Effect of Ligand N,N-Substituents on the Reactivity of Chiral Copper(II) Salalen, Salan, and Salalan Complexes toward Asymmetric Nitroaldol Reactions[END_REF] C 1 -symmetric scaffolds are less commonly used, although they have proven to be effective in asymmetric transformations. [START_REF] Arai | Practical Asymmetric Henry Reaction Catalyzed by a Chiral Diamine-Cu(OAc)2 Complex[END_REF] The copper-catalysed asymmetric Henry reaction is achieved with very high enantiomeric excesses typically using C 2 -symmetric ligands such as bisoxazolines. [START_REF] Murugavel | Copper(II)-Catalyzed Nitroaldol (Henry) Reactions: Recent Developments[END_REF] However, very few examples of non-symmetric ligands have been reported. In this context, the evaluation of DABO as chiral ligands was particularly interesting. Indeed, DABO provided such a C 1 -symmetric structure and was used as a precursor to synthesize bidentate and tetradentate ligands using classical procedures for reductive amination of diverse aldehydes. These ligands were utilised in the copper-catalysed asymmetric Henry reaction between p-nitrobenzaldehyde and nitromethane (Figure 6). Ligands with diverse aromatic substituents were tested to probe the effect of bulkiness and electronic properties. The ligand 66 with 1-naphtyl substituents on the amine groups provided the best yields and enantioselectivity. A broad range of Henry's substrates were tested under these experimental conditions, and moderate to very good yields as well as good enantioselectivities were obtained using both aromatic and aliphatic aldehydes (Figure 7A).

In an attempt to rationalize asymmetric induction in this reaction, theoretical studies of the transition structures were carried out (Figure 7B). The transition state of the reaction was considered to be composed of the ligand-copper complex with one acetate group, the 4pnitrobenzaldehyde and the nitronate anion. Free energies and energy barriers were obtained for a few transition-state structures, and calculations were optimized to obtain the most favoured calculated transition state. It exhibited an approach of the nitronate from the Re face of the aldehyde leading to a product with S configuration as observed experimentally. It is worth noting that the C 1 -symmetry of the ligand allows to hinder a large area in the surrounding of the copper atom making a single quadrant available for the substrates to react, in this favoured transition state structure. compromises must be found between molecular rigidity allowing stable folds and adaptability to accommodate local steric hindrance. The bicyclic motifs were prone to induce strong steric hindrance due to higher rigidity and crowding compared to single ring systems, which could limit their incorporation. Thus, strong clashes prevented the elongation of homo-ABOC oligoamides while the use of longer urea bonds as linkages pushed aside the consecutive bicycles and allowed the conception of highly stable oligourea helices. DABO derivative, i.e. rBAC, have

shown to be fully compatible with the oligourea canonical helix and could be incorporated in high contents. Since a broad range of applications were developed for this foldamer family, [START_REF] Douat | A Cell-Penetrating Foldamer with a Bioreducible Linkage for Intracellular Delivery of DNA[END_REF][START_REF] Guichard | Structural Basis for α-Helix Mimicry and Inhibition of Protein-Protein Interactions with Oligourea Foldamers[END_REF] rBAC could be highly valuable to tune the oligourea properties by increasing their global and local stability, and by limiting the flexibility of proximal side chains. In parallel, the singular frameworks of ABOC and DABO residues were successfully used in asymmetric synthesis to design efficient chiral catalysts, i.e. short α,β-peptide catalysts and chiral diamine ligands, to catalyse asymmetric adol and Henry reactions, respectively. The broad range of tested compounds have allowed the emergence of general rules to optimize both class of catalysts. In the future, the screening of new chiral catalysts for various reactions will be a huge task since multiple possibilities arose from both ABOC and DABO, varying the nature of the substituents and the achievable reactions. Besides, multiple prospects were opened for foldamers in health and in biomaterial areas. In particular, overlaps between the side chains of the naturally occurring α-helix and the 12/14/14-helix made this latter attractive as α-helix mimics. Likewise, the stable 10/12-helix could be readily faced for membrane recognition to develop antimicrobial compounds and vectors for the delivery of biologically active agents.

Figure 1 :

 1 Figure 1: Hybrid oligoamide structures composed of (S)-ABOC and α-AA.

Figure 2 :

 2 Figure 2: β-peptide structures composed of (S)-ABOCs and β 3 -hAAs. Comparison of the main

Figure 3 :

 3 Figure 3: Formula of succinimidyl carbamate derivatives. Canonical acyclic [(S)-AAC] 8 oligourea 33 (ccdc750017) 45 and BAC-containing oligoureas: poly-(S)-BAC tetramer 30 and (S)-

Figure 4 :

 4 Figure 4: Acyclic [(S)-AAC] 8 oligourea 33 and (S)-AAC/(S)-rBAC octamer 45 crystal

  e. H-Pro-(R)-ABOC-Asp-OCH 3 , was the most effective. The absence of a third chiral centre (peptides 50-53) or of a carboxylic acid function (peptide 58), as well as the presence of a second carboxylic acid function on the C-terminal amino acid (peptides 54-55), led to a lower enantioselectivity in both homo-and heterochiral sequences.All reactions were carried out at 25°C using 0.05 mmol of p-nitrobenzaldehyde and 20 mol% of catalyst in 0.9 ml of acetone:DMF (1:2). Conversion and enantioselectivity of aldol products were determined by analytical chiral HPLC analysis on a chiralpak AS-H column, detection at 270 nm. The absolute configuration of the major enantiomer was assigned by comparison with literature data.a The four examples involving dipeptide catalyst yielded the aldol product of absolute configuration (R). b Reaction carried out at the optimal temperature of -20°C.

Figure 5 :

 5 Figure 5: ABOC containing peptides as organocatalyst in the asymmetric aldol reaction and

  If the use of salen-type tetradentate ligands 61 and 62 resulted in a very slow reaction with modest enantioselectivity, bidentate ligands 63-70 substantially accelerated the reaction and provide up to 80 % enantiomeric excesses.Yields are isolated yields after column chromatography. Enantioselectivity of nitro-aldol products were determined by analytical chiral HPLC analysis of the crude on a chiralpak OD-H column, detection at 214 nm. a Reaction performed in toluene at room temperature, b reaction performed in isopropanol at 0 °C, c incomplete conversion.

Figure 6 .

 6 Figure 6. Evaluation of diamine ligands (L*) derived from DABO in copper-catalysed

Figure 7 :

 7 Figure 7: Scope of the reaction using the most efficient ligand 66 and rational for the

6 .

 6 SUMMARY AND OUTLOOKTopological restraints encoded in 1-aminobicyclo[2.2.2]octane-2-carboxylic acid were successfully used as driving forces to guide the folding of helical architectures when mixed with naturally-occurring α-AAs and commonly used β 3 -hAAs in peptide foldamers. The bulky bicyclo[2.2.2]octane induced a high steric pressure, which locked the ABOC backbone torsion angle values and affected the spatial arrangement of adjacent and remote residues. It provided robust helical shapes for only short sequences with limited effects of external factors such as temperature and solvent, thus ensuring the stability of the oligomers structure, while the α-and β 3 -hAA brought versatile side chains. The 9/11-, 18/16-, 12/14/14-and 10/12-helices exhibited various topological parameters (pitch, residue per turn, diameters, etc.) and thus rather different side chains positioning patterns. On the other side, all these helices share common features, which are intrinsically related to the chemical nature and the resulting preorganization of the ABOC monomer. In the whole structures, the (S)-ABOC residue exhibited backbone torsion angle values around ϕ = 80°, θ = 50° and ψ = -90°, the NH and CO vectors being directed up and down, and exclusively promoted helices with mixed hydrogen bond networks. Nonetheless,

  chemistry. He then carried out his Ph.D studies under the supervision of Dr. M. Calmès at the University of Montpellier (IBMM) and obtained his degree in 2017. After a first postdoctoral experience at Trinity College Dublin, Pierre joined the INSA Rouen-Normandie where he is currently doing postdoctoral research. His research interests include synthetic methodology, asymmetric synthesis and biomolecules. Jean Martinez received both his Ph.D and Dr. Sci from the University of Montpellier, France, under the direction of Prof. F. Winternitz. He spent 3 years as a postdoctoral fellow with Dr. E. Bricas, University of Paris Sud, and then with Prof. M. Bodanszky, Case Western Reserve University, Cleveland, Ohio, USA. He joined the University of Montpellier as a full professor in both organic and medicinal chemistry. He founded the Institute of Biomolecules Max Mousseron in 2007, acting as the director until 2014. His current research interests focus on peptide chemistry, stereoselective syntheses of biomolecules, design of biomaterials, green chemistry, biology and pharmacology of neuropeptides. He is currently Emeritus Professor at the University of Montpellier. Muriel Amblard received her Ph.D in Organic Chemistry at the University of Montpellier in France under the supervision of Prof. J. Martinez in 1991. Then, she joined the group of Dr. P. Anderson at Merck Sharp & Dohm in Pennsylvania as a postdoctoral fellow. In 1996, she became a CNRS senior researcher and leads her research at the Institute of Biomolecules Max Mousseron in the field of peptide science. Her research interests are mainly at the interface of chemistry, biology and structural analysis for developing bioactive compounds and chemical tools that can mimic biological systems. Monique Calmès was graduated as a chemical engineer in 1981 from the "Ecole Supérieure de Chimie de Montpellier" (France). In 1985, she received her Ph.D from the University of Montpellier (France) under the supervision of Prof. R. Jacquier in the field of new methodologies suitable for peptide synthesis. She joined in 1987, the group of Prof. L. Gosez at the University of Louvain la Neuve (Belgium) as a postdoctoral fellow. In 1988, she obtained a permanent position at the CNRS in Montpellier and holding a position of CNRS senior researcher at the Institute of Biomolecules Max Mousseron until she retired in 2019. Her research interests concerned the constrained amino acids, foldamers and asymmetric synthesis. Baptiste Legrand was born in Chartres, France, in 1982. He received a master degree in biochemistry and structural biology from the University of Orléans in 2006. He obtained his Ph.D from the University of Rennes 1 in 2009 under the supervision of Dr. A. Bondon. After a postdoctoral at the LCPM, ENSIC, University of Lorraine at Nancy, he joined the University of Montpellier (IBMM) in 2012 where he is currently research engineer. His research is devoted to the development of bioactive oligomers mainly focusing on the structural aspect.

ACKNOWLEDGMENTS

The authors are grateful to all the colleagues who contribute to this work, in particular to Drs. Marie Christine Averlant-Petit, Claude Didierjean, Emmanuel Aubert, Emmanuel Wenger, University of Lorraine, and Aurélien Lebrun, University of Montpellier. We thank all financial institutions and the analytical facilities of Universities of Montpellier and Lorraine who contributed to this work. Sylvie Gémeno is warmly thanked for drawing the tree of the graphical abstract.

ORCID

Pierre Milbeo: 0000-0002-6199-6636 Jean Martinez: 0000-0002-9267-4621 Muriel Amblard: 0000-0001-8922-1755 Monique Calmès: 0000-0001-7690-0206 Baptiste Legrand: 0000-0001-8931-7757 Present Addresses †Present address: Laboratoire COBRA, UMR CNRS 6014, 1 rue Tesnière -Bât. IRCOF (no 40) -76821 Mont Saint-Aignan Cedex

AUTHOR INFORMATION

Corresponding Author

*E-mail: muriel.amblard@umontpellier.fr, monique.calmes@gmail.com, baptiste.legrand@umontpellier.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources

CNRS, MESR, ANR (ANR-08-BLAN-0066-01, ANR-10-LABX-05-01), LabEx CheMISyst, FFCSA and WTU China.

Notes

The authors declare no competing financial interest Biographies Pierre Milbeo was born in Casablanca, Morocco, in 1991. He graduated from ENSC-Lille and the University of Lille (2014) with a master degree in chemical engineering and organic


Søren Roesgaard

Etienne Talbot

Constantinos Hatzoglou

John Lundsgaard Hansen

Brian Julsgaard
email: brianj@phys.au.dk

Structural characterization of tin nanocrystals embedded in silicon by atomic probe tomography

Keywords: Sn-nanocrystals, atom probe tomography, photoluminescence

Tin nanocrystals embedded in silicon are studied by atom probe tomography and by photoluminescence spectroscopy in the 0.76-1.07 eV region of emission energies. The nanocrystals have been fabricated by molecular beam epitaxy followed by a post-growth annealing step at various temperatures. One particular sample, annealed at a temperature of 725 °C, shows a distinctly higher optical activity. It is found, however, that the distinct behavior cannot be explained by variations in the nanocrystal composition or in the properties of Sn atoms dissolved in the surrounding Si matrix, which can be investigated by atom probe tomography.

For quite some time, there has been a strong desire to integrate optical functionality in standard complementary metal-oxide-semiconductor (CMOS) devices based on silicon (Si). In Si-photonics the aim is to combine the electrical functionality of Si with the optical functionality offered by direct-bandgap materials. The merging of these functionalities has often been approached by combining the Si with group-III-V-based systems, which however tend to be chemically incompatible with Si [1]. Another approach is to obtain the optical activity from a monolithic implementation of group-IV alloys [2]. This is possible due to the direct-bandgap nature of tin (Sn) in its diamond-structured and semiconducting α-Sn phase. Lately, germanium-tin (GeSn) alloys have been used in the demonstration of light-emitting diodes [3] and lasing [4][5][6] at temperatures up to 180 K. Furthermore, a multistack GeSiSn/GeSn has shown similar lasing activity [7]. Such thin films are currently of great interest; however, there might be interesting possibilities in turning to embedded nanocrystals since the control of the nanocrystal diameter could constitute another handle for tuning the bandgap energy, and in turn, allow for tunable lasers [8]. Light emission from Si with embedded Sn nanocrystals has previously been presented, where the Sn nanocrystals were formed upon post-annealing of a solid solution of Sn in Si grown by molecular beam epitaxy (MBE) [9]. In that work, a significant increase in optical activity was found within a narrow annealing temperature window around 725 °C, although no structural difference between Sn nanocrystals formed at different annealing temperatures could be detected by electron microscopy. Recently, it was found, surprisingly, that the sample with the increased light emission contained nanocrystals in the metallic β-Sn phase [10], which is in contradiction to previous findings stating that Sn-nanocrystals smaller than ∼8nm should be α-Sn [11]. The exact physical mechanism behind the light emission is still not understood, which calls for further investigation.

In the present manuscript we use laser-assisted three-dimensional atom probe tomography (APT) to investigate if there are any differences in the composition of the Sn nanocrystals or the surrounding matrix that could be correlated to the optical properties of the nanocrystals. The APT method, which provides three-dimensional mapping of atoms in a material, has previously been used to understand nanoscale clustering and measure accurate composition of nano-objects in semiconductor or oxide materials. For example, the composition of rare-earth silicate nanoclusters was determined by APT allowing to identify the phase-separation mechanism of rare-earth doped silicon oxide and to explain the optical properties [12,13]. By quantifying the Si-nanocluster characteristics (size distribution, composition, etc) in silica using APT, Roussel et alhave demonstrated the growth mechanism of Si-nanoclusters to depend on the supersaturation [14].

The samples investigated were grown by MBE. A composite layer of 30 nm thickness containing Si, Sn, and carbon (C) was evaporated onto a Si substrate, resulting in concentrations of 1.6% Sn and 0.04% C within the Si matrix, determined by secondary-ion mass spectrometry. The C was added in order to reduce the strain and suppress dislocations [15]. Post-growth annealing was carried out in a N 2 atmosphere at 650, 725, and 800 °C for 20 minin order to form the nanocrystals. Additional details are given in [9]. The resulting Sn-nanocrystals were investigated by scanning transmission electron microscopy (STEM) and the resulting images are shown in figure 1. The MBE-grown SiSnC layer together with the nanocrystals, formed upon post-annealing at temperatures 600 °C [16], are clearly visible. The images also indicate a columnar structure of the grown layers for all temperatures including the as-grown sample.

The samples were characterized optically by photoluminescence (PL) spectroscopy, using excitation laser pulses of 100 fs duration, 400 nm wavelength, and 8×10 -6 J cm -2 pump fluence. The samples were mounted in a cryostat and kept at a temperature of 16 K. The resulting PL spectra for the three samples annealed at different temperatures are shown in figure 2. The early-time integrated PL spectra, seen in figure 2(a), show a narrow peak centered around ≈1.035 eV, which is assigned to the second replica of the transverse optical phonon-assisted recombination in Si [17,18]. The broader luminescence features observed at lower energies has previously been ascribed to the Sn-nanocrystals [9] and are also seen in the late-time integrated PL spectra shown in figure 2(b). In this panel, no peak related to Si is observed, demonstrating that the lifetime of the Sn feature is significantly longer than the lifetime of the Si feature. Even though a part of the Sn-related PL extends below the detection limit of 0.76 eV, it is clearly seen that the sample annealed at 725 °C is the most luminescent. This is further elaborated in figure 2(c), which indicates the overall strength of the Sn-related spectral peak.

The Sn nanocrystals have been analyzed by a laser assisted, wide-angle, tomographic atom probe (LaWaTAP) instrument from CAMECA using ultra-violet laser pulses with 343 nm wavelength, 350 fs pulse duration, and 100 kHz repetition frequency and with the samples kept at a temperature of 80 K. A detailed description of this technique can be found in Lefebvre-Ulrikson, et al [START_REF]Atom Probe Tomography[END_REF]. The tip-shaped specimens, required for APT experiments, were prepared using the standard lift-out method and annular milling procedure [START_REF] Larson | [END_REF] in a dual-beam SEM-FIB ZEISS NVision 40 instrument. To prevent the layer of interest from damages, Ga implantation, and amorphization during the sample processing, a 500-nm-thick layer of Pt was deposited on the top of the sample prior to the milling procedure and a low-accelerating voltage (2 kV) was used in the final processing stage. APT is a very local probe, and hence there is a significant risk that any given sample tip contains Figure 2. In all panels, samples annealed at temperatures of 650, 725, and 800 °C are color coded as blue, green, and red, respectively. (a) Time-integrated PL spectra for early times ( t 0 64 ns). (b) Time-integrated PL spectra for late times (0t600 μs). (c) Integrated spectral area below each late-time spectra for emission energies in the range 0.76 eVE0.90 eV. The three specific annealing temperatures investigated here are marked by colored circles (see [9] for further information).

no nanocrystals, even though it has been prepared from a sample known to contain nanocrystals. For this reason, we have prepared a total of six APT samples, named as S650-1, S725-1, S725-2, S725-3, S800-1, and S800-2, where the first number in each sample name refers to the annealing temperature and the last number to different attempts. Samples S725-1, S725-2, and S800-1 contained no nanocrystals.

A typical mass spectrum obtained during APT experiments of the prepared samples is presented in figure 3. The presence of Si and Sn, constituting the samples, is clearly seen. Silicon is identified in the charged states Si 2+ and Si 1+ , whereas tin is detected only as Sn 2+ . All isotopes of each chemical element (3 for Si and 10 for Sn) are detected in good agreement with their respective relative natural abundances (inset of figure 3). The presence of carbon is indicated by the very weak peaks (M/n=12 and 40) related to C 1+ and SiC 1+ complexes. Some oxygen is detected in the form of SiO ions uniformly across the samples, most likely originating from the growth process or surface oxidation of specimens prepared for APT.

A 3D reconstruction volume related to the APT analysis of the SiSnC layer annealed at 800 °C (S800-2), encapsulated in the Si buffer, is shown in figure 4(a). The regions I correspond to the Si buffer and region II to the SiSnC layer. It is clearly seen in region II that Sn clusters are detected. Figure 4(b) represents the concentration profile for Si and Sn corresponding to the volume in figure 4(a). The Si and Sn content were found to be similar for different samples, and the concentration profile shows that Sn atoms are only located in the SiSnC layers (no diffusion through the Si buffer) with a measured concentration ofX Sn =(1.8±0.2)%, in good agreement with the 1.6% measured previously by second-ion-mass spectrometry [9].

Considering figure 4(a), the SiSnC layer appears rather homogeneous at first glance in the region around the Sn nanocrystals. However, density fluctuations can be difficult to see in a plot containing such a relatively large number of Sn atoms, and for this reason, the Sn distribution will be analyzed more carefully in the following. To this end, define NN1 as the detected nearest-neighbor distance for a given Sn atom as determined directly from the raw APT data, NN2 as the detected distance to the next-nearest neighbor, and so on. These distances will naturally be small in dense regions of Sn and large in more dilute regions. Figure 5 shows histograms of the average distance to the three nearest neighbors, NN1 NN2 NN3 3. This combination was chosen, on the one hand, to increase the sensitivity to dense regions. It is more likely for this combination than for NN1 alone to take on a low value in dense regions, since it only requires two atoms to statistically form a low value of NN1 in a dilute Sn region. On the other hand, the detection efficiency of the APT procedure is below unity (around 60%), which is the reason for not using the four nearest neighbors that might otherwise appear as an obvious choice given the bonding structure of the Si crystal. The detected location of Sn atoms has a finite measurement precision, and hence the distribution of distances between neighbors will be slightly broadened. In order to avoid edge effects in the histograms of figure 5, care was taken to exclude Sn atoms that were closer than 1 nm from the APT sample surface and from the interface between the SiSnC and Si buffer regions. The histograms show in general two features: firstly, if Sn nanocrystals are present, a small bump appears for mean distances around 0.25 nm characteristic of the bonding length between Sn atoms. This is most pronounced for the sample S800-2, which contains a rather large number of nanocrystals as already shown in figure 4(a). This bump is also quite clear in S650-1 and barely visible in S725-3. Secondly, a broad feature, typically centered around 0.8-1.0 nm, is present for all samples and reflects the properties of the Sn atoms dissolved in the surrounding SiSnC layer. The appearance of this broad feature seems to stand out with a more complex structure for the two samples S650-1 and S800-1; however, this can be ascribed to a low APT sample volume and therefore poor statistics. Evidently, in some samples the mean distance is smaller than in others. Since the average concentration, X Sn , of Sn atoms is the same for all samples, the variations in the mean neighbor distance corresponds to local variations in the degree to which Sn atoms have segregated towards each other. We wish to remind that the solubility of Sn in Si is < 0.1%, meaning that the solid solution of Sn in the SiSnC layer constitute a meta-stable configuration out of thermodynamic equilibrium. Since the mean distance between Sn atoms appears to be slightly smaller for the S800-1 and S800-2 samples as compared to the S650-1 sample, it is tempting to suggest that Sn atoms have segregated slightly more towards each other for the higher annealing temperature. However, similar differences can be seen between the three samples, S725-1, S725-2, and S725-3, which are all annealed at the same temperature. We thus have to conclude that within the investigated annealing temperature window, there are no detectable variations in the mean nearest-neighbor distance of the dissolved Sn atoms that can be related to temperature effects. The observed difference in the mean neighbor distance for different samples points to spatial variations across the SiSnC layer, which we therefore wish to investigate further.

The vertical dashed lines in figure 5 define the boundary of three chosen regions, (i), (ii), and (iii), of mean distances, which all correspond to quite dense parts of the histograms. The atoms, for which the mean neighbor distance falls within either of these three regions, have been included in the 3D reconstruction plot with different colors in figure 6 for the three nanocrystal-containing samples. In this way, only the Sn nanocrystals (primarily black and magenta) and the most dense part of the Sn solid solution (primarily green) appear. The plots indicate that the solid solution of Sn is located in a columnar structure, which is consistent with the contrast already seen in the STEM images in figure 1. This columnar structure becomes much more evident upon viewing the 3D plots in figure 6 from various angles, and for this reason, animations of the 3D plots are presented for all samples in the Supporting Information. The columns are aligned along the z-axis, which correspond to the growth direction. Their typical lateral extent is around 5 nm in all investigated samples, and from figure 1 it is evident that this columnar structure also exists in the as-grown sample. In other words, it must have emerged during the growth process and be independent of the post-annealing temperatures used in this investigation. It should be remarked that the columnar contrast in the STEM images in figure 1 in principle could have had their origin in the STEM specimen milling step. However, the APT has now evidenced that it is really the Sn atoms which are distributed into this columnar variation. No extended defects, like dislocations, stacking faults, or grain boundaries, have been observed from the grown layers using electron microscopy.

Turning towards the Sn nanocrystals, the composition can be measured from APT experiments. Prior to this step, Sn nanocrystals are evidenced and identified using a cluster identification algorithm. As with other techniques working at the atomic scale, APT is subjected to limitations (physical and instrumental). In the present case, a bias exists in the determination of the chemical composition of Sn nanocrystals. Indeed, trajectory aberrations occur for the evaporated atoms from the matrix in the proximity of nanoparticles. In our case, this induces an artificial presence of Si atoms within the Sn nanocrystals. The raw concentration measured on the Sn-nanocrystals from APT analysis is around X Sn ∼30-40 at% in all the samples. However, knowing the APT artefact, we can not directly conclude that Sn nanocrytals are not pure. To determine the real composition, we have applied a correction model [21] based on field evaporation simulations [22]. This model was already applied with success on similar materials [13,23]. The corrected composition from chemical biases is X Sn =100 at%, i.e.Sn-nanocrystals are pure in Sn with a radius of about 1 nm for all the samples. In other words, the Snnanocrystal composition and mean size are not affected by the variations in annealing temperature studied here. Considering the overall results of the APT analysis in connection to the observed PL, it must be concluded that there is no correlation between the particularly strong PL intensity, observed for the annealing temperature of 725 °C, and the properties offered by the APT analysis, i.e. nanocrystal composition and the level of Sn segregation in the surrounding Si matrix. In fact, based on APT the three investigated samples appear rather similar, which is consistent with previous studies showing that the nanocrystal size distribution does not depend on the annealing temperature within the investigated temperature range [9]. Hence, it seems that the physical mechanism behind the light emission must be sought on the atomic scale from e.g.point defects or complexes of few atoms, possibly located on the nanocrystal surface or within the SiSnC layer. With the present investigation in mind, such atomic complexes would contain Sn and possibly also carbon or oxygen. One could speculate that only one or a few particular types of complexes would lead to light emission and that these just happen to form upon annealing at 725 °C for 20 min. At higher temperatures, these could then evolve further to larger types of complexes that do not give rise to light emission. Light emission has been predicted previously from Sn nanocrystals in the semiconducting α-Sn phase [24]. The recent investigation of the sample annealed at 725 °C, showing it to consist of Sn in the metallic β-Sn phase, adds to the picture that the light emission is not related to the recombination of confined electrons and holes within the Sn nanocrystals.

Figure 1 .

 1 Figure 1. STEM images of the as-grown SiSnC layer and the Sn-nanocrystal-containing layers. The bright contrast is due to Sn atoms, seen either as a continuous band corresponding to the original SiSnC layer or as bright spots from individual Sn nanocrystals.

Figure 3 .

 3 Figure 3. APT mass spectrum obtained from the sample S800-2. The inset shows a magnification of the Sn 2+ peaks in the 55-64 M/n region.

Figure 4 .

 4 Figure 4. (a) APT reconstruction of the SiSnC layer of volume 52×52×90 nm 3 for the sample S800-2. (b) Si and Sn concentration profile (in at %) along the 3D sample tip.

Figure 5 .

 5 Figure 5. The distribution of the mean distance to the three nearest neighboring Sn atoms for the different APT samples. The graphs are shifted vertically for clarity. The vertical dotted lines separate three ranges in nm as: (i) [0; 0.2], (ii) [0.2; 0.4], and (iii) [0.4; 0.7], which are used for different coloring in figure 6.

Figure 6 .

 6 Figure 6. Map of atoms belonging to different regions of the mean distance to the three nearest neighbors. The range of distances, which correspond to the notations (i)-(iii) defined in figure 5, have been plotted with colors: (i) black, (ii) magenta, and (iii) green. The three panels correspond to different APT samples: (a) S650-1, (b) S725-3, and (c) S800-2. Animations of these 3D density fluctuations are available in the Supporting Information.

Acknowledgments

This work was supported by the Villum Foundation.