Darío M González
email: dario.gonzalez@pucv.cl

Jonathan Cisterna

Iván Brito

Thierry Roisnel

Jean-René Hamon

Carolina Manzur

Prof David Carrillo

Binuclear Schiff-Base Zinc(II) complexes: Synthesis, crystal structures and reactivity toward ring opening polymerization of rac-Lactide

Keywords: Binuclear Zinc complexes, Schiff-Base ligands: X-ray diffraction, Ring opening polymerization, Polylactide

Upon crystallization from DMF/diethyl ether or CH2Cl2/pyridine of tetracoordinated Zn(II) complexes containing either 4-fluorophenyl or 4-anisyl substituted O,N,O-tridentate Schiff base ligands and pyridine as auxiliary ligand, three new doubly phenoxide-bridged dimeric Zn(II) complexes were isolated and their catalytic properties for the ring opening polymerization (ROP) of rac-lactide were explored. The single-crystal XRD analysis allowed to corroborate the molecular structures of the binuclear complexes, showing that the pentacoordinated Zn(II) metal ion adopts a slightly distorted square pyramidal geometry. The basal plane Is formed by the O,N,O-donor set of the tridentate Schiff base ligand and a phenoxo bridging oxygen, while a pyridine or a DMF

molecule is located at the apex of the pyramid. The binuclear complexes were employed as catalysts in the ROP of rac-lactide for polylactide (PLA) synthesis. Polymerization conditions such as reaction time, catalyst concentration and use of benzyl alcohol as cocatalyst were studied, as well as their influence on the conversion rate, average molecular weight and polydispersity of the obtained PLA. The catalyst substituted with fluorine was more reactive, but with a lower control on the molecular weight compared to the methoxy-substituted catalyst. On the other hand, the presence of pyridine as an auxiliary ligand generates an improvement in the polydispersity of PLA.

Introduction

The development of biodegradable polymers is currently a subject of great scientific interest, as it points to the possibility of replacing some applications of traditional petrochemical plastic materials. In this context, in the last decade, polylactide (PLA) is one of the most promising and its research has grown significantly since in addition to its biodegradable characteristics, it is also biocompatible and of bio-sourced origin [1][2][3][4][5]. Another important characteristic of PLA is that it can achieve physical properties and mechanical behaviors similar to that of polyolefins when synthesized by ring-opening polymerization (ROP) from a lactide (LA) monomer unit using a suitable catalytic system [6][7][8][9]. The catalysts used for PLA synthesis have evolved over time: the first catalysts were metals like tin alkoxides [9][10][11], and have since reached increasingly complex species like some organometallic compounds, and, more recently, coordination complexes [12].

There are currently a wide variety of metal complexes that have been studied as ROP catalysts, with general success in the use of Al as a metal center [13][14][15][16][17][18][19], some alkaline [20][21][22]and alkaline earth metals [23][24][25],and also some transition metals like Ti or Zr [26]. However, Zinc-based complexes are among the most outstanding, mainly because of good polymerization control via ROP, which translates into good catalytic activity, high molecular weight, low polydispersity, and even stereochemical control of synthesized PLA [27][28][29][30][31][32][33].In addition, Zn has low toxicity compared to other metals.

With respect to the design of a coordination complex for ROP catalysis, the choice of central metal atom is undoubtedly a highly determining factor; that said, ligands also have an important role in the catalytic behavior of the complex, influencing reactivity and even some structural characteristics of the synthesized polymers [23,34]. Furthermore, the development of numerous synthesis routes has allowed a wide variety of coordination compounds to exist today, e.g., ligands with different structures, symmetries, chelating heteroatoms, and substituent groups. Because of this, the range of complexes used in ROP catalysis has grown enormously, though without consensus on the real influence of metal complex structure in ROP. Rather, it is generally only possible to approximate catalyst functions as a set of structural variables, taken from different polymerization experimental conditions, optimized for the use of each catalyst.

To this end, this paper proposes the use of Schiff-base ligands derived from β-diketones. The latter are characterized as versatile molecules and very good coordinators, with the ability to stabilize a wide variety of metals in different oxidation states. In addition, their simple synthesis routes are of good performance and so are easily functionalized [35][36][37][38][39]. Considering the good results found with Zinc in ROP reactions, the objective is to study Schiff-base ligands and their respective complexes with different substituent groups, such as fluorine and methoxyl, and to subsequently evaluate their catalytic activity in rac-LA polymerization in terms of catalytic performance, molecular weights and polydispersities in obtained PLA.

Experimental

General experimental methods

Reactions were performed under dry dinitrogen atmosphere using standard Schlenk lines. Solvents were dried and distilled according to standard procedures [START_REF] Armarego | Purification of Organic Chemicals[END_REF]. 1-(4-methoxyphenyl)-1,3butanedione [START_REF] Popic | [END_REF], 1-(4-fluorophenyl)-1,3-butanedione [42], and the Schiff base proligand 4-MeO- (1) [38], were synthesized according to literature procedures. All other chemicals were purchased from commercial sources and used without further purification. 1 H NMR spectra were recorded at 298 K with a Bruker 300 FT-NMR spectrometer.

C6H4-C(O)CH=C(CH3)N(H)-C6H4-2-OH

Chemical shifts (δ) are reported in parts per million (ppm) and referenced to the residual deuterated solvent peaks (CDCl3: δ=7.26 ppm; (CD3)2SO: δ=2.50 ppm). Coupling constants (J) are reported in Hertz (Hz), and integrations are reported as number of protons. High resolution electrospray ionization mass spectra (ESI-MS) were obtained at the Centre Regional de Mesures Physiques de l'Ouest (CRMPO, Université de Rennes 1, France) with a Thermo Fisher Scientific Q-Exactive mass spectrometer. Elemental analyses were conducted on a Thermo-Finnigan Flash EA 1112 CHNS/O analyzer of the CRMPO Microanalytical Service. Melting points were measured in evacuated capillaries on a Kofler Bristoline melting point apparatus and were not corrected. GPC measurements were performed on a Shimadzu DGU-20A3R with a Shimadzu RID-10A RI detector and a combination two-column Shimadzu Shim-pack GPC-800DP and GPC-80MD.The molecular weight and polydispersity index were calculated according to polystyrene standards.

Synthesis of Schiff base proligand 2 and complexes

Synthesis of O,N,O-tridentate Schiff base proligand 2: (4-F-C6H4)-C(O)CH=C(CH3)N(H)-C6H4-2-OH

A three necked round bottom flask equipped with a Dean-Stark apparatus was loaded with a magnetic stir bar, 1000 mg (5,6 mmol) of 1-(4-fluorophenyl)-butane-1,3-dione, 672 mg (6.2 mmol) of 2-aminophenol, and 50 mL of toluene. The reaction mixture was refluxed for 5 h.

After cooling to room temperature, 10 mL of diethyl ether were added and the solution was stored at -30 °C causing the precipitation of microcrystalline yellow solid. This material was collected by filtration, washed with 3 x 2 mL portion of diethyl ether/pentane mixture (1:1), and dried under vacuum, providing 1338 mg (86% yield). M.p. 118-120°C. 1 A Schlenk tube with a magnetic stir bar was loaded with Schiff base proligand 1 or 2 (0.71 mmol), potassium tert-butoxide (2.1 mmol), and 3 mL of THF. The reaction mixture was stirred for 15 min, with a dark red precipitate observed. Then, 2.1 mmol of pyridine was slowly added, and after 15 min of stirring, a solution of Zn(NO3)2x6H2O (1.1 mmol) in 1.5 mL of THF was added. The reaction mixture was vigorously stirred for 4 h at room temperature. The solvent was then evaporated to dryness under reduced pressure. The solid residue was dissolved in dichloromethane and filtered to remove excess salts. The solution was evaporated to dryness, yielding a solids sample of 3 or 4, respectively.

Binuclear complexes obtained by recrystallization: Compounds 5,6, and 7.

Complex 5 was obtained by slow evaporation of a saturated solution of compound 4 (100 mg) in CH2Cl2/pyridine (10:1) as orange monocrystals. This methodology requires a filtration procedure with celite to obtain an adequate and completely translucent solution, causing product losses and generating overall yields of 30%. For complexes 6 and 7, 100 mg of compounds 3 and 4, respectively, were dissolved in the minimum amount of DMF and subjected to slow vapor diffusion of diethyl ether for up to 7 days, yielding dark orange monocrystals of complexes 6 and 7. The overall yield reached 35%, through increasable up to 40% via recovery and concentration of the mother liquor and repeated recrystallization processes. All compounds were characterized via single-crystal XRD, corroborating their binuclear structures.

X-ray crystal structure determination

Some suitable crystals of compounds 5-7 were measured and their diffraction data were collected at 295(2) K on a Bruker D8 QUEST and D8 Venture diffractometer equipped with a bidimensional CMOS Photon100 detector, using graphite monochromated Cu-K(λ = 1.54178 Å) and Mo-K(λ = 0.71073 Å) radiation, respectively. The diffraction frames were integrated using the APEX3 package [START_REF]Bruker APEX3[END_REF] and were corrected for absorptions with SADABS. The structures of 5 -7 were solved by intrinsic phasing [START_REF] Sheldrick | [END_REF] using the OLEX 2 program [45]. All the structures were then refined with full-matrix least-square methods based on F 2 (SHELXL-2014) [START_REF] Sheldrick | [END_REF]. For the three compounds, nonhydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were included in their calculated positions, assigned fixed isotropic thermal parameters and constrained to ride on their parent atoms. A summary of the details about crystal data, collection parameters and refinement are documented in Table 1, and additional crystallographic details are in the CIF files.

ORTEP views were drawn using OLEX2 software [45].

rac-Lactide polymerization

In a typical experiment for polymerization in melt, a round bottom flask was charged with a stirring bar, 1.5 g (10.4 mmol) of rac-LA, 22 mg (0.013 mmol, 0.125% mol) of catalyst 5, and 55 mg (0.5 mmol) of benzylalcohol. The reaction mixture was melted at 130 °C with an oil bath and stirred for 4 h. The flask was taken off, cooled to room temperature, and opened in air. An aliquot was analyzed by 1H NMR spectroscopy for determining the conversion. 0.5 mL of acidic methanol (1.0 M, HCl/MeOH solution) was added in the flask, and the polymer was precipitated with excess methanol. Then, the supernatant solution was removed with a pipet and the polymer was dried under vacuum to constant weight. The ratios of rac-lactide, catalyst, benzylalcohol, and reaction times employed for the various catalytic tests are given in section 3.3 (Table 3).

Results and discussions

Synthesis of ligands and complexes

The synthetic route to the known 1 [38] and new 2 ligand precursors corresponds to a Claisen condensation between the appropriate 4-substituted acetophenone and ethyl acetate leading to the formation of the corresponding betadiketone, and subsequent condensation with 2-aminophenol to generate the desired O,N,O tridentate Schiff base proligands (Scheme 1). This route provided both compounds in high yields, around 85%. Subsequent synthesis of mononuclear complexes 3 and 4 consists in abstracting two acidic protons of the ligands with an excess of potassium tert-butoxide.

Pyridine is then incorporated as an auxiliary ligand -in slight excess to stabilize tetracoordinated complexes-before incorporating the Zn(II) salt (Zn(NO3)2x6H2O) (Sheme 1). The observed color changes indicated reaction progress: the yellow and orange colors of ligands 1 and 2, respectively, turned intense red upon reaction with t-BuOK, and finally the Zn(ii) complexes 3 and 4 shaded to a color very similar to that of their respective ligands. Complexes 3 and 4 are isolated as powdered solids in 66% and 52% yields, respectively. They are soluble in dimethylformamide (DMF), dimethylsulfoxide (DMSO) and pyridine but are poorly soluble in more common organic solvents such as dichloromethane, acetonitrile and acetone.

Scheme 1. Synthesis of compounds 1-7

Recrystallization of 4 from a CH2Cl2/pyridine mixture leads to the isolation of the doubly phenoxide-bridged dimeric specie 5 with the pyridine ligand occupying the fifth coordination site at the Zn(II) metal center (Scheme 1). Similarly, recrystallization of 3 and 4 from vapor diffusion of diethyl ether into respective DMF solutions affords isostructural binuclear complexes 6 and 7, respectively, though with a DMF molecule instead of pyridine as the fifth Zn(II) coordination ligand (Scheme 1). This ability of Zn(II) to form binuclear complexes has been reported in previous works [23,31,32,46,47]. Only those three binuclear complexes 5-7 were tested as catalysts in the ROP of rac-Lactide depicted in this paper (see Section 3.3).

Compound characterization

Mononuclear complexes 3 and 4 were characterized by satisfactory elemental analysis, 1 H NMR spectroscopy and mass spectrometry as detailed in the experimental section, clearly informing that they correspond to the proposed structures. Fig. 1 compares the 1 H NMR spectra of these complexes to those of their respective ligands. Spectra of tridentate ligands 1 and 2 recorded in deuterated chloroform show all proton peaks. Both compounds have fairly similar spectra, with slight differences in the chemical shifts, due to phenyl ring substitution with MeO-for 1 and F-for 2. The acceptor character of the fluorine substituent causes an downfield shift of the proton signals (d and e, Fig. 1). Ligand 2 has greater aromatic proton signal multiplicity: the triplet observed at 6.86 ppm for compound 1 (g, Fig. 1), appears again for ligand 2 at 6.89 ppm. The signal here is a triplet of doublet, confirming F nucleus coupling with phenyl ring proton nuclei. In spectra of both 1 and 2,an acidic N-H proton is observed above 12 ppm, likely due to its intramolecular hydrogen bond with the carbonyl oxygen.

Fig. 1 shows the 1 H NMR spectra of Zn(II) complexes 3 and 4.Unlike their respective ligands, these were insoluble in deuterated chloroform, so measurements were made in lower concentrations of deuterated DMSO solutions. Here, all protons were also assigned, largely similar, though with aromatic proton displacement differences. Three proton signals corresponding to coordinated pyridine were observed in both compounds (j, k, and l, Fig. 1). Finally, the absence of acidic N-H protons in both complexes confirms ligand coordination to the metal center through the amino nitrogen atom. The 1 H NMR signals did not present large differences in chemical displacement, with the only exception in the methyl proton peaks (b, Fig. 1).From complexes to corresponding ligands, these protons are downfield shifted, passing from 1.67 and 1.77 ppm for 1 and 2, to 2.32 and 2.33 ppm for 3 and 4, respectively. Those chemical shift differences bear testimony to the coordination of the ligands to the Zn(II) metal ion and formation of rigid structures.

Fig. 1 1 H NMR spectra of 1 and 2 obtained in CDCl3; and of 3 and 4 recorded in DMSO-d6.

X-ray crystallography

The molecular structure of 5 is displayed in Fig. 2 while those of 6 and 7 are presented in Fig. 3.

Selected bond lengths and angles of the first Zn(II) coordination sphere of 5-7 are given in Thermal ellipsoids are drawn with 30% of probability (i = 𝑋 ̅ , 𝑌 ̅ , 𝑍 ̅ ).

All the bond distances and angles (Table 2) are in agreement with those of previously reported zinc(II) complexes with similar features [49,50] and the formation of five-and six-membered metallacycles shows distortions similar to that observed for known structurally characterized Schiff base complexes [37,38,51]. Nevertheless, the metrical parameters measured in the two fused heterometallacycles are indicative of a delocalization over the entire system.

It is also important to highlight that the distances between apical ligand (Py for compound 5 and, DMF for compound 6 and 7), present a bond distance slightly larger than other similar compounds [49,50] . This feature could be responsible for their catalytic response in ring opening polymerization of rac-lactide, according to its catalytic mechanism [8,25]. Between the three complexes, there are not major differences in their structural parameters (Table 2). However, the bond distances in the apical ligand in each compound, allow to infer about the catalytic response between compounds 5 -7, due to DMF as ligand, is a weak ligand than Py. This could explain the greater reactivity that compound 7 presented under mild polymerization conditions, but at the same time, a lower control in the polydispersity of PLA. These results are discussed in more detail in section 3.4.

(3) a X ̅ , 1-Y, Z ̅ ; b X ̅ , 2-Y, 1-Z; c 1-X, 1-Y, 1-Z

Reactivity studies of complexes 5-7 in ROP of rac-lactide

One of the main objectives of this work was to study the catalytic activity of these three complexes 5-7 in ring opening polymerization to obtain PLA. Table 3 shows the polymerization tests performed with the three synthesized binuclear complexes. In short, bulk polymerizations in a melted reaction mixture of monomer rac-LA at 130°C for 4-18 hours and different concentration ratios of monomer and catalyst were studied. Additionally, benzyl alcohol has been incorporated as a cocatalyst in some tests, since there are previous reports indicating that different alcohols in low concentration improve the catalytic activity of some complexes [29]. Table 3 also shows the influence of these polymerization variables on the degree of conversion and the molecular weight of the PLA obtained. Fig. 4 shows an example of how 1 H NMR characterization allows calculating the degree of conversion through clear differences in monomer and polymer proton signals. Such signals can be individually integrated, giving rise to the composition of the reaction mixture.

Furthermore, 1 H NMR approximates PLA molecular weight through PLA polymer chain terminal group proton signals (e and g, Fig. 4). Considering the contribution of their areas in relation to the rest of the protons in the spectrum, the number of repetitive monomeric units was calculated, reporting approximate chain length averages (Mn). Synthesized PLA samples were also characterized by GPC, which, in addition to the average molecular weight, gives chain size distribution by calculating the polydispersity index (Table 3). 1 H NMR; c Average molecular weight and polydispersity index determined by GPC calibrated versus polystyrene standards and corrected by a factor of 0.58 according to literature recommendations [52].

Table 3 shows some general trends of the catalytic behavior of complexes 5-7. After 4 hours of reaction, no complex is able to generate the highest possible degree of conversion; after 18 hours, however, or with BnOH as co-catalyst, all conversions reach high values (>90%). Of the three catalysts studied, complex 7 achieves 78% conversion in 4 hours of reaction, making it the fastest catalyst under these conditions (Entries 1, 5, and 9).Reaction times of 18 hours provide conversions of around 98% for all catalysts, similar to results obtained with benzyl alcohol (Entries 2, 6, and 10). On the other hand, reducing catalyst concentration by half ([rac-LA]:[Zn]=800:1) negatively affects conversion rates, independent of co-catalyst (entries 11 and 12). This is quite expected, due to the lower concentration of active centers to initiate polymerization.If the same concentration of catalyst is allowed to react for a longer time, however, conversions can be equally high as in cases where the concentration of catalyst is higher (entries 4, 8, and 15). Decreasing the catalyst concentration may affect PLA molecular weight. Indeed, initial catalyst concentrations generally generated relatively low molecular weights, ranging from 1.7 to 4.4 kg mol -1 (400:1). Thus a decrease in active polymerization sites should generate longer PLA chains if polymerization is stable over time.

To discuss the influence of catalysts and reaction conditions on the molecular weight of PLA, it is necessary to consider some differences of the two methods by which the average molecular weight (Mn) value has been calculated. Although GPC is one of the most widely used, and considered the most suitable method for the determination of Mn of polymers, the 1 H NMR method also provides a good approximation of this value. Table 3 shows how the 1 H NMR method approximates more closely to GPC results when molecular weight values are lower. In such cases terminal group proton contribution is greater, accounting for the majority of integrated signals. In addition, methodology results are similar for low polydispersity samples because 1 H NMR calculations assume equal chain size. In some cases, the methods differ by less than 10% (entries 11,12,14); in others, more considerably (30%, entry 8). Nevertheless, increasing or decreasing trends for PLA molecular weight are of the same respect to the polymerization conditions variation. Thus both methodologies provide information for catalyst behavior and reaction condition effects on Mn.

The results presented in Table 3 show that Mn increases when the concentration of catalyst decreases. Catalyst 6 reaches 13.3 kg mol -1 with only a slight increase in polydispersity. This is remarkable since other Mn increases are accompanied by considerable increases in polydispersity.

Next, as time and conversion rate, PLA chain length also tends to increase, though with different Mn and polydispersity effects for each complex. For example, decreased concentrations of catalyst 5 increase chain length by up to 200% (entry 4), without seriously affecting polydispersity. This implies heterogeneous catalyst reactivity: over an extended period of 18 hours, not all active centers are equally effective, therefore generating chains of different length. On the other hand, and unique to catalyst 5, Mn can be slightly increased by increasing time without decreasing concentration, while mostly maintaining polydispersity (entry 3). In this case, a higher catalyst concentration favors more stable polymerization. The latter behavior could be attributed to the presence of pyridine as an auxiliary ligand in catalyst 5. Finally, although BnOH as a co-catalyst allows better conversions, the effect on Mn is negative. In the only case where there is a considerable increase in Mn, polydispersity is highly increased. In general, it is advisable to reduce concentration of the three catalysts and extend reaction times.

The three catalysts behave similarly to changes in reaction conditions, e.g., BnOH presence or different concentrations. That said, differences may be attributable to structural modifications, like the auxiliary ligand (DMF vs. pyridine) or the nature of the substituents (F vs. MeO). For example, catalysts 5 and 7, which have fluoride in their structure, generate better conversions at shorter reaction times, but lower PLA molecular weights, implying that the acceptor effect of fluoride favors more reactive, less-controlled polymerization. Less reactive catalyst 6 bearing the MeOsubstituent, achieves higher molecular weights with the advantage of maintaining polydispersity.

Finally, the effect of the auxiliary ligand under identical conditions for complexes 5 and 7 (entries 3 and 15) shows very little difference in conversion percentage; that said,at higher Mnvalues, the DMF catalyst presents an important increase in polydispersity, which does not occur in the case of pyridine (catalyst 5).

Conclusions

Zn(II) binuclear complexes 5, 6, and 7 synthesized via dimerization of their fully characterized mononuclear precursors 3 and 4 during recrystallization were found to be stable compounds under environmental conditions, capable of being used as catalysts in the rac-LA polymerization reactions. They are similar in regards to ROP, with high sensitivity toward the presence of BnOH as a co-catalyst or higher conversion rates over longer periods of time. On the other hand, PLA with different characteristics can be synthesized by controlling polymerization conditions (time and catalyst concentration) in the absence of BnOH. Here, the catalytic behavior is attributed to structural differences. Complexes with a fluoride substituent were slightly more reactive but had less control over average molecular weight than the catalyst bearing the donor methoxy group. The inclusion of pyridine instead of DMF as an auxiliary fifth coordination ligand generates better control of PLA polydispersity. Finally, the MeO-substituted catalyst generated the highest Mn PLA when used in low concentrations, though with a reaction time of 18 h and a conversion rate slightly lower than other catalysts under the same polymerization conditions. Binuclear (II) zinc complexes were obtained from dimerization of their mononuclear precursors with the aim of studying the catalytic activity of these compounds in the ring-opening polymerization of rac-lactide, considering the influence of its structural features on the conversion percentage, molecular weight average and polydispersity of the obtained PLA.

Table 2 .

 2 compounds 5, 6 and 7 crystallize in the orthorhombic, triclinic and monoclinic crystal systems in the centrosymmetric space groups Pbca, P1 ̅ and P21/c, respectively. The X-ray crystal structures of 5-7 indicate that the three complexes are doubly phenoxide-bridged dimeric species in which each monomeric unit is composed of a pentacoordinated Zn(II) metal center through a dianionic O,N,O-tridentate Schiff base ligand, a bridging oxygen atom and either a pyridine (5) or a DMF (6 and 7) molecule. On the other hand, the three dimeric complexes generate the whole molecular structure related by an inversion center (1 ̅ ), in the middle of the molecule due to their high molecular symmetry. Moreover, all the compounds present a slightly distorted square pyramidal geometry (τ5 =0.07, 0.06 and 0.65, respectively)[48] .

Fig. 2

 2 Fig. 2 ORTEP plot of compound 5. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn with 30% probability.

Fig. 3

 3 Fig. 3 ORTEP plots of compounds 6 (above) and 7 (below). Hydrogen atoms are omitted for clarity.

Fig. 4

 4 Fig. 4 Example of 1 H NMR spectrum of PLA with 87% conversion, obtained with complex 7 as catalyst and BnOH as cocatalyst (Table 3, Entry 12).

  Dec.). Anal. Calcd for C21H17FN2O2Zn (413.76 g mol -1 ): C, 60.96; H, 4.14; N, 6.77. Found: C, 59.90; H, 4.11; N, 6.55. ESI-MS (m/z) calcd for C21H17N2O2F 64 Zn: 412.056, found: 412.0559 (0 ppm). 1 H NMR (300 MHz, DMSO-d6): 2.32(s, 3

	Compound 3: yellow solid; 200 mg (66% yield). 180°C (Dec.). Anal. Calcd for C22H20N2O3Zn
	(425.79 g mol -1 ): C, 62.06; H, 4.73; N, 6.58. Found: C, 60.95; H, 4.68; N, 6.18. ESI-MS (m/z) calcd
	for C22H20N2O3 64 Zn: 424.07599, found: 424.0760 (0 ppm). 1 H NMR (300 MHz, DMSO-d6): 2.32
	(s, 3 H, CH3), 3.82 (s, 3 H, OCH3), 5.70 (s, 1 H, CH=C), 6.53 (t, J=7.3 Hz 1 H, C6H4-2), 6.86 (t, J=
	7.3 Hz, 1 H, C6H4-2), 7.01 (d, J=8.7 Hz, 2 H, 4-C6H4O), 7.07 (d, J=8.0 Hz, 1 H, C6H4-2), 7.18 (d,
	J=7.7 Hz 1 H, C6H4-2), 7.40 (m, 2 H, C5H5N), 7.80 (m, 1 H, C5H5N), 7.93 (d, J=8.7 Hz, 2 H, C6H4-
	2), and 8.58 (m, 2 H, C5H5N).
	Compound 4: orange solid; 153 mg (52% yield). 200°C (H, CH3), 5.72 (s, 1 H, CH=C), 6.53 (m, 1 H, C6H4-2), 6.88 (m, 1 H, C6H4-2), 7.08 (m, 1 H, C6H4-
	2),7.16 (m, 1 H, C6H4-2) 7.27 (m, 2 H,(4-C6H4F), 7.42 (m, 2 H, C5H5N), 7.82 (m, 1 H, C5H5N ),
	8.00 (m, 2 H, 4-C6H4F), 8.57 (m, 2 H, C5H5N).

Table 1 .

 1 Crystal data parameters for compounds 5-7.

	Compound	5	6	7
	Empirical Formula	C42H34F2N4O4Zn2	C40H44N4O8Zn2	C38H38F2N4O6Zn2
	Formula mass, g mol -1	827.47	839.53	815.46
	Collection T, K	295(2)	295(2)	295(2)
	crystal system	Orthorhombic	Triclinic	Monoclinic
	space group	Pbca	P1 ̅	P21/c
	a (Å)	16.771(3)	8.3006(3)	11.2365(6)
	b (Å)	9.8456(17)	10.19115(4)	8.3087(4)
	c (Å)	22.075(4)	11.3579(4)	19.4733(10)
	α(°)	90	81.273(2)	90
	 (°)	90	89.629(2)	93.571(2)
	γ (°)	90	70.055(2)	90
	V (Å 3 )	3645(10)	954.68(6)	1814.51(16)

Table 2 .

 2 Selected Bond distance (Å) and angles (º) of the Zn(II) coordination core for compounds 5-7.

	Compound	5	6	7
		Bond distances
	Zn(1)-O(1)	1.963(2)	2.004(12) b	1.999(2)
	Zn(1)-O(2)	2.090(2) / 2.030(2) a	2.010(11) / 2.124(11) b	2.027(2) / 2.123(14) c
	Zn(1)-N(1)	2.060(2)	2.024(13) b	2.013(19) c
	Zn(1)-N(2)	2.095(2)	---	---
	Zn(1)-O(3)	---	2.005(12)	2.045(2)
	Zn(1)-Zn(1)'	3.250(2)	3.121(3)	3.117(7)
			Bond angles
	O(1)-Zn(1)-L	105.07(10)	95.15(5)	97.05(6)
	N(1)-Zn(1)-L	100.33(10)	113.27(6)	117.26(7)
	O(2)-Zn(1)-L	107.06(10)	92.00(2)	89.93(6)
	N(1)-Zn(1)-O(2)	78.36(9)	80.08(5)	79.86(6)
	O(1)-Zn(1)-N(1)	94.52(9)	92.49(5)	92.42(6)
	O(1)-Zn(1)-O(2)	147.84(10)	171.28(5)	171.39(5)
	Zn(1)-O(2)-Zn(1)'	104.13(9)	98.00(5)	97.78(8)
	L: 5=N(2); 6 and 7=O

Table 3 .

 3 ROP of rac-lactides catalyzed by complexes 5, 6, and 7.

	Entry	Complex	[rac-LA ]:[Zn]:[BnOH]	Time (h)	Conv. a (%)	Mn b (kg mol -1 )	Mn c 1 ) (kg mol -	Mw/Mn c
	1	5	400:1:0	4	69	2.2	1.7	1.30
	2	5	400:1:20	4	97	2.3	1.7	1.33
	3	5	400:1:0	18	97	3.5	4.4	1.38
	4	5	800:1:0	18	95	4.9	6.0	1.55
	5	6	400:1:0	4	68	2.0	1.5	1.17
	6	6	400:1:20	4	98	4.1	5.8	1.60
	7	6	400:1:0	18	98	4.0	4.1	1.76
	8	6	800:1:0	18	92	8.5	13.3	1.34
	9	7	400:1:0	4	78	2.5	1.8	1.28
	10	7	400:1:20	4	97	2.5	1.9	1.34
	11	7	800:1:0	4	67	3.1	3.0	1.30
	12	7	800:1:20	4	87	3.6	4.0	1.52
	13	7	400:1:0	18	97	4.8	5.9	1.90
	14	7	400:1:20	18	98	2.6	2.4	1.54
	15	7	800:1:0	18	95	3.6	4.9	1.59
	16	7	800:1:20	18	98	3.6	4.4	1.60
	a Conversion percentage determined by 1 H NMR; b Average molecular weight determined by

Acknowledgements

The authors thank F. Lambert (CRMPO, Rennes) for helpful assistance with MS measurements. Financial support from FONDECYT Postdoctoral project 3160673 (D.M.G.), FONDEQUIP (EQM130154), the Vicerrectoría de Investigación y Estudios Avanzados, Pontificia Universidad Católica de Valparaíso, Chile (D.M.G. and C.M.) , the CNRS and the Université de Rennes 1 is gratefully acknowledged. I.B. and J.C. thank FONDEQUIP (EQM 130021) and financial support from Universidad de Antofagasta for the access to Cambridge Structural Database (CSD), and J.C. thanks de Universidad de Antofagasta for the Postdoctoral Fellowship.

Appendix A: supplementary information.

CCDC 1862423 (for 5), 1863233 (for 6) and 1862424 (for 7) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.as.uk/data_request/cif


A Takacs
email: atakacs@laas.fr

H Aubert
email: haubert@laas.fr

Keywords: Energy harvesting, rectennas, satellite applications, microwave circuits, autonomous wireless sensors. I

This paper addresses the microwave energy harvesting on board of geostationary satellites for health satellite monitoring. To prove the feasibility of such a concept, we investigated the electromagnetic environment existing on antenna panels. Based on established cartographic maps, three designs of rectennas are proposed. Measured DC powers ranging from 0.256 mW to 1.28 mW can be harvested for electric field levels ranging from 91 V/m to 121 V/m and by using very simple and compact designs. The harvesting structures consist of only one Schottky diode per rectenna and present a total surface of 2.4 cm 2 . They are suitable for powering the new generation of ultralow power transceivers, thus enabling autonomous wireless power networks for satellite health monitoring.

power autonomous wireless sensors used for monitoring the structural health of the satellite. Moreover, the electromagnetic power radiated by microwave antennas is almost constant and consequently the DC power regulatory circuits should be minimal for harvesting systems.

The electromagnetic environment of the antenna panels is analyzed in Section II. Rectenna (RECTifier antENNA) topologies [START_REF] Popovic | Cut the Cord Low-power far-field wireless powering[END_REF] and design methodology developed for proving the concept of microwave power harvesting for satellite health monitoring are discussed in Section III. The main goal of this paper is to demonstrate that microwave energy harvesting based on specific rectenna designs is an ideal solution for powering autonomous sensors for satellite health monitoring.

II. ELECTROMAGNETIC ENVIRONMENT

Broadcasting satellite antennas operating in C, X, Ku and K band radiate electromagnetic power between 50W and 100W with linearly or circularly polarized electromagnetic field. Recently, Thales Alenia Space© performed an analysis of the electromagnetic energy illuminating antenna panels. Intensive electromagnetic simulations using GRASP software from TICRA [2] were performed by taking into account the case of a Spacebus class C satellite. Figs. 1 to 4 show typical electromagnetic field levels (E-field in V/m, peak values) on panels located on the lateral side or on the Earth side of a broadcasting geostationary satellite. These levels were obtained in C, Ku and K frequency bands. We observed that large areas of antenna panels are covered with field levels higher than 4 V/m. This electromagnetic field available at the surface of antenna panels can be advantageously harvested by using rectennas [START_REF] Brown | The history of power transmission by radio waves[END_REF] for supplying power to autonomous sensors dedicated to satellite health monitoring. As shown in Fig. 3 a maximum E-field level of 180 V/m (peak value) can be reached at 17.7 GHz.

Microwave Power Harvesting for Satellite Health Monitoring

Alexandru Takacs, Member, IEEE, Herve Aubert, Senior member, IEEE, Stephane Fredon, Laurent Despoisse, Henri Blondeaux As shown previously (Fig. 1 to Fig. 4) satellite panels can be illuminated with electromagnetic energy at various frequencies. Moreover the incident waves illuminating satellite panels arrives various directions and have various polarizations (dependently on satellite antennas types and their mounting position). Thus the antenna of the rectenna has to be engineered to meet these criteria (operating frequency, polarization and radiation pattern shape/tilt angle) in order to maximize the amount of the RF power injected at the input of the (rectenna) rectifier.

III. MICROWAVE RECTENNAS: DESIGN AND MEASUREMENT RESULTS

Rectennas are intensively used for wireless power transfer or energy harvesting applications. Many designs and topologies have been proposed for various applications, such as GSM [START_REF] Ladan | Highly Efficient Compact Rectenna for Wireless Energy Harvesting Application[END_REF], GPS [START_REF] Zhu | Electrically Small GPS L1 Rectennas[END_REF], C-band (ISM 2.45 GHz) [START_REF] Harouni | A Dual Circularly Polarized 2.45-GHz Rectenna for Wireless Power Transmission[END_REF]- [START_REF] Suh | A high-efficiency dual-frequency rectenna for 2.45-and 5.8-GHz wireless power transmission[END_REF], 5.8 GHz [START_REF] Suh | A high-efficiency dual-frequency rectenna for 2.45-and 5.8-GHz wireless power transmission[END_REF] and X-band [START_REF] Epp | A compact dual polarized 8.51-GHz rectenna for high-voltage (50 V) actuator applications[END_REF], [START_REF] Hagerty | Recycling ambient microwave energy with broad-band rectenna arrays[END_REF] applications. Dual-band [START_REF] Sun | Design of a High-Efficiency 2.45-GHz Rectenna for Low-Input-Power Energy Harvesting[END_REF] or broadband [START_REF] Hagerty | Recycling ambient microwave energy with broad-band rectenna arrays[END_REF] designs have been studied while applications beyond 10 GHz have been addressed in the past mainly for wireless power transfer [START_REF] Strassner | Microwave Power Transmission: Historical Milestones and system Components[END_REF] application [START_REF] Brown | The history of power transmission by radio waves[END_REF], [START_REF] Yoo | Theoretical and experimental development of 10 and 35 GHz rectennas[END_REF]- [START_REF] Ren | 35 GHz rectifying antenna for wireless power transmission[END_REF] but not for power harvesting because of the lack of powerful environmental electromagnetic sources for terrestrial applications. Several research works have been reported on the rectenna design for energy harvesting at frequencies higher than 10 GHz [START_REF] Shinohara | Development of 24 GHz rectennas for Fixed Wireless Access[END_REF]- [START_REF] Collado | 24 GHz Substrate Integrated Waveguide (SIW) Rectenna for Energy Harvesting and Wireless Power Transmission[END_REF]. A reconfigurable rectenna device that is capable of adapting itself to the incident power level is described in [START_REF] Marian | Strategy for Microwave Energy Harvesting From Ambient Field or a Feeding Source[END_REF]. In order to evaluate the feasibility of such microwave energy harvesting, several rectennas were designed and the measured DC power was compared with typical power requirements for wireless transceivers. Note that ultra-low power receivers are now available [21] and their power consumption is around 2 mW only (voltage supply of 1.2V-1.8V with current below 2 mA).

A. Rectenna topology

In order to keep the proposed designs compatible with the specific technical requirements for space applications, several design guidelines were adopted: (i) the rectenna topology has to be as simple as possible; (ii) all the components in the rectenna (dielectric substrate, diode, capacitors, etc) must be qualified for space applications, ready to be qualified or at least with performances compatible with space requirements; (iii) the operating frequencies must be chosen by taking into account the electromagnetic environment existing on board of satellite antenna panels (typical E-field distributions were presented in Section II) and, (iv) a non-optimal DC loading for rectenna could be chosen, in this a worst case scenario the input impedance/resistance of the sensors and/or the transceiver to be powered may be far from the optimal DC load that maximizes the DC power generated by the rectenna. Keeping in mind these design requirements, the rectenna was designed to power a temperature sensor (load) with a typical resistance in the range of 9-10 kΩ. The rectenna was fabricated on Rogers 6002 substrate (relative permittivity: 2.94, loss tangent: 0.0012, thickness: 508 µm, metallization thickness: 35 µm) [22]. Two GaAs flip chip Schottky diodes were selected: M/A Com MA4E-1317 diode [23] and Aeroflex/Metelics MZBD-9161 diode [24]. These diodes were chosen based on their RF power handling, thermal capabilities and high operating frequency. An important issue in rectenna design is the antenna that should: (i) match the polarization of the incident electromagnetic field, (ii) have a gain as high as possible in order to increase rectenna conversion efficiency, (iii) present a high large half-power beamwidth to efficiently operate for various directions of the incident field and, (iv) be easily matched to the rectifier at the operating frequency. Reproducing in Laboratory the electromagnetic environment existing on satellite panels is not an easy task. As presented in Section III.C, by using the setup S2 an incident wave illuminating the rectenna under test with a linearly polarized E-field, with various power densities (E-field levels) and at various frequencies can be easily generated in Laboratory. Consequently we selected (for our rectennas designs) linearly polarized antennas with at least 50° half-power beam-width and a maximum gain of at least 4 dBi.

The proposed rectenna designs are based on the topology shown in Fig. 5. The antenna collects the surrounding electromagnetic power density and transfers it at the input of the rectifier (Schottky diode) through a matching circuit. The matching circuit allows the conjugate impedance matching and maximizes the RF power level flowing from the antenna to the rectifier. Moreover, the matching circuit should stop the reflected power at the operating frequency and harmonics to be radiated by the antenna. The rectifier converts the RF power to a DC power. The DC power is injected into the load through a low-pass filter that blocks any undesirable RF signal at operating frequency or harmonics. In order to facilitate the testing and debugging of such a rectenna, a 50Ω test point (end-launch K connector) was added between the antenna and the matching circuit. Although it may introduce undesirable losses and impedance mismatch, this 50Ω test point was very useful in the early stages of the design. Based on the topology sketched in Fig. 5, three rectennas were designed, fabricated and measured. These rectennas, named R1 (R1a and R1b), R2 (R2a and R2b) and R3 (R3a and R3b), are shown in Fig. 6. The rectennas R1a (using the MA4E-1317 diode) and R1b (using the MZBD-9161 diode) were the first developed demonstrator. Only the rectifier and its 50Ω input port are shown in Fig. 6. They used only a single stub matching circuit and a lumped RC low-pass filter. The 10kΩ load impedance is integrated in the filter structure. The results obtained with these simple topologies were recently reported by the authors [START_REF] Shinohara | Development of 24 GHz rectennas for Fixed Wireless Access[END_REF]. The main drawback of the R1rectenna is its low efficiency at 17.7 GHz and consequently a high gain (horn) antenna is needed for harvesting a DC power higher than 2 mW. Two new rectenna designs named R2 and R3 with improved harvesting performances were developed and are now presented. antenna and operating at 2.45 GHz, was proposed in [START_REF] Sun | Design of a High-Efficiency 2.45-GHz Rectenna for Low-Input-Power Energy Harvesting[END_REF]. The antenna was excited by a coplanar stripline. No matching circuit was necessary, and the maximum power transfer (between antenna and diode/rectifier input) and the suppression of the undesirable reflected power at the second harmonic were achieved by properly controlling the input impedance of the antenna. A capacitance of 1.5 pF was used for shunting the RF signal at the operating frequency and its harmonics. In order to maximize the performances of this design, it is crucial to properly control the diode mounting position.

B. Rectennas design and simulation methodology

One of the major challenges in the effective design of rectennas in the microwave frequency range (Ku band and higher) is the accurate non-linear modeling of the diode. The most relevant parameters of the MA4E-1317 diode were derived from an available datasheet [23] (total capacitance: 0.045 pF, series resistance: Rs=4 Ω) and from the extrapolation of the I-V measured characteristic (derived ideality factor: n≈1.2 and saturation current: Is≈0.1 pA). For the MZBD-9161 diode, the Spice equivalent model proposed in [24] was used but with a much lower breakdown voltage (3.5 V instead of the 10 V given in the datasheet). A lower breakdown voltage was needed to accurately model the observed nonlinear power behavior of the diode. The simulation model for the MZBD-9161 diode given in [24] uses two anti-parallel diodes (with different parameters) in order to simulate the single Schottky diode used in the rectenna. Fig. 7 shows the complete simulation model in AWR software [25] for the rectenna R2 (without the patch antenna) using closedform models for the distributed components (transmission lines, stubs and via hole). The design procedure for the design of the rectenna R3 was quite different. First the DDL antenna was designed and simulated using FEKO [27] for operation at the desired frequency (17.7 GHz). The antenna radiation pattern, gain and radiation efficiency were calculated and the distribution of the electric currents on the metallic strips were analyzed. The design requires a low load resistance for working efficiently. A load of 510Ω was used. For simulation purposes, the diode was replaced by a voltage port while the capacitance (1.5 pF) and the load (510 Ω) were modeled as port loads. Fig. 8 shows a top view of the rectenna R3. The dimensions of the R3 design are: l x =20mm, l y =14mm, l d =5.5mm, l r =15mm, l s =1.4mm, l ax =6.3mm and l ay =8.3mm. The coplanar strip line is characterized by a strip width of w=0.8 mm and a gap size of g=0.4 mm. A metallic plate (dotted line contour in the Fig. 8) was printed below the radiating element (l gx =10mm and l y =14mm). R3 design is very compact with a total surface of 2.4 cm 2 .

C. Experimental setup

Two experimental setup configurations, named S1 and S2, were used. In the S1 setup shown in Fig. 9 a continuous millimeter-wave signal generated from an Anritsu MG3694B generator was directly injected at the 50Ω test point of the manufactured prototypes and the resulting DC voltage was measured at the input port of the load impedance with a Keythley 2000 multimeter. (e.g. R1a, R1b, R2a) of the rectennas when equipped with a 50Ω test point (coaxial K connector). In the S2 setup shown in Fig. 10, a microwave signal generated from an Anritsu MG3694B generator was injected at the input of a horn antenna (VT220HA20-SK from Vector Telecom [START_REF]VT220HA20+SK[END_REF]) which illuminated the rectenna under test with a linear polarized Efield. This setup is appropriate for testing the rectennas (R2b, R3) or rectifiers equipped with a 50Ω test point/coaxial K connector (e.g. R1a, R1b, R2a) connected with an external antenna.

D. Simulation and experimental results

The layouts of rectennas were accommodated with the tolerances required by a fabrication in a laboratory equipped for general-purpose printed circuit boards (PCB). A solid silver-filled epoxy system EPO-TEK®H20E was used for soldering the diodes by a low temperature process (110°C). The simulation results obtained by using AWR simulation model for R2a are represented with a continuous (blue) line (diamond marker). Fig. 11 and Fig. 12 show a comparison between the measured performances (by using the setup S1) of the rectifiers R1a and R2a in terms of the harvested DC voltage (Fig. 11) and DC power (Fig. 12) at the input of the load impedance (10 kΩ). A DC voltage up to 6.4 V and a DC power up to 4mW can be harvested with the R2a design when a RF power around 15 dBm is injected at the input of the rectifier (Schottky diode). A maximum worst-case efficiency of 15.4% was achieved for a RF input power of 13 dBm (experimental results). This worst case efficiency takes into account the losses and the impedance mismatch introduced by the 50Ω K-connector and by the male-to-male K-adapter. This low efficiency is mainly due to the non-optimal load impedance (10 kΩ) chosen as reference.

Fig. 12. Measured DC output power (setup S1) for R1a (dotted red line, square marker) and R2a (dotted violet line, 'x' marker design). The simulation results obtained by using AWR simulation model for R2a are represented with a continuous (blue) line (diamond marker).

Precursory of the R1a/R1b structures, reported in [START_REF] Takacs | Energy harvesting for powering wireless sensor networks on-board geostationary broadcasting satellites[END_REF]- [START_REF] Takacs | Design and implementation of a rectenna for satellite application[END_REF] were tested in CNES (French Space Agency) Laboratory by applying a standard thermal cycling procedure for space applications (30 thermal cycles performed between -65°C and +125°C). Despite of the soldering-free home-made fixation system the prototypes still remained operational after the thermal cycling without a major impact on the RF performances. The output DC voltage as a function of the operating frequency obtained for a RF input power of 13 dBm is displayed in the Fig. 13. The maximum voltage of 5.4 V was obtained at 17.7 GHz, which is the central operating frequency of the single stub matching circuit. Fig. 14 shows the measured DC voltage harvested by the R2b-rectenna design when illuminated with the electric field of a horn antenna (gain: 18.6 dBi at 17.7 GHz). A microwave power ranging from 10 to 24 dBm was injected at the input of the horn antenna. The distance between the radiating aperture of the horn antenna and the rectenna R2b is 15 cm. The input port (coaxial K connector) of the transmitting horn antenna is located at 25 cm above the rectenna R2b under test. A maximum DC voltage of 1.6 V was measured and a low harvested DC power of 0.256 mW was obtained. In order to measure the RF power collected by the patch antenna, a calibrated Anritsu ML2437A powermeter was used. The measured RF power received with the patch antenna alone (gain: 5dBi at 17.7GHz) was found to be 4.5dBm: when this RF power was directly injected at the input port of the rectifier (using the 50Ω test point of R2a in the setup S1), the harvested DC voltage was found to be 1.63 V. Consequently, the measurement results obtained using setup S2 (Fig. 14) are coherent with the results depicted in Fig. 11. To obtain a harvested DC power higher than 2mW, an antenna with a gain of at least 12dBi should be used instead of the patch antenna. Alternatively a DC combining circuit could be proposed to combine the DC power harvested from many rectennas [START_REF] Popovic | Cut the Cord Low-power far-field wireless powering[END_REF].

The power P received at a given frequency by an antenna of gain G at a location where an electric field level E exists can be approximated by [START_REF] Balanis | Antenna theory analysis and design[END_REF]: [START_REF] Popovic | Cut the Cord Low-power far-field wireless powering[END_REF] where denotes the free-space wavelength. Consequently, the RF power at the output of the receiving antenna (input of the 50Ω test point) can be estimated for a given incident E-field level (table 1). Eq. ( 1) can be also written as:

(2) When specifying the required RF power at the output of a calibrated antenna (with a known gain), the needed E-field level can then be derived from Eq.(2). Three typical antenna gains were considered: 18.6 dBi (e.g. horn antenna), 11 dBi (for comparison purposes) and 5 dBi (e.g. patch antenna). To design rectenna R3, the variation of the input impedance of DDL antenna as a function of the diode mounting position was analyzed. The simulated input impedance Z in of the antenna viewed at the distance l r -l d along the coplanar stripline is depicted in Fig. 15 as a function of l d (see Fig. 7). The imaginary part (reactance) was found to be positive. As the estimated input impedance of the MZBD-9161 diode is 8j*80 Ω the diode should be located approximately at l d =5.5 mm to ensure the conjugate impedance matching condition. Moreover since the (simulated) impedance Z in at l d =5.5 mm is 24.5+j*130 Ω at 35.4 GHz (i.e., at the first harmonics) and

        4 120 2 2 G E P RF input power (dBm) DC_Voltage (V) G P E     120 2   204+j*203 Ω at 53.1 GHz (i.
e., at the second harmonics), the reflected power radiated back by the antenna of the rectenna (R3 design) is expected to be small at the first two higherorder harmonics. The simulated radiation pattern displayed in Fig. 16 shows that the maximum gain of the DDL antenna is approximately 6 dBi. The measured DC voltage (setup S2, power injected at the input of a horn antenna: 23 dBm, distance between the horn aperture and the R3a rectenna: d=15 cm) as a function of the frequency is depicted in Fig. 17.

A maximum DC voltage of 0.81 V (DC power of 1.28 mW) was recorded at a frequency of 18.8 GHz while a DC voltage of only 0.36 V (DC power of 0.25 mW) is measured at the targeted operating frequency f=17.7 GHz. This can be explained by: (i) the inaccuracy of the adopted design methodology (in the electromagnetic model the antenna is loaded by a voltage electromagnetic port instead of the actual loading of the diode); (ii) manufacturing tolerances (all the rectennas were fabricated in an university laboratory not equipped for high-frequency PCB manufacturing); (iii) error in the positioning of the diode during its mounting on the PCB. However the rectenna R3a is very compact (2.8 cm 2 ) and performs very well in a very wide frequency range (from 15 to 20 GHz). The measured RF power (using a calibrated Anritsu ML2437A powermeter) at the output of manufactured patch antenna is 2dBm and is consistent with the E-field level of 91 V/m at 17.7 GHz estimated on the rectenna surface. In order to calculate the efficiency  (in %) of this rectenna the following worst case definition is used [START_REF] Popovic | Low-Power Far-Field Wireless Powering for Wireless Sensors[END_REF], [START_REF] Falkenstein | Low-power wireless power delivery[END_REF]:

(3) where P DC is the measured harvested DC power, S is the incident power density and A G denotes the area of the radiating surface. The power density (expressed in µW/cm 2 ) can be computed as function of E-field effective value E on the antenna surface as follows:

(4)

In the case of the R3 design an efficiency of 42% is derived for A G =1.4 cm 2 and E=91 V/m (we consider here that the geometrical surface of antenna is the surface of the metallic plate printed below the DDL radiating element, that is 10mmx14mm). Taking into account the geometrical surface of the patch antenna which is approximately 1 cm 2 for the rectenna R2b we obtain a worst-case efficiency of 11.4 % (A G =1 cm 2 and E=91 V/m) according to eq. 3. This low efficiency can be explained by: (i) the non-optimal loading of the MA4E-1317 diode by a 10 kΩ load and (ii) the MA4E-1317 diode (forward voltage in the range of 0.7 V) serially connected with the patch antenna requires a high RF input power in order to be properly self-biased and to operate efficiently. This high RF input power cannot be delivered by the patch antenna under the test condition (setup S2, incident E-field : E=91 V/m).

Several rectennas operating at frequencies beyond 10 GHz were reported in the literature [START_REF] Yoo | Theoretical and experimental development of 10 and 35 GHz rectennas[END_REF]- [START_REF] Collado | 24 GHz Substrate Integrated Waveguide (SIW) Rectenna for Energy Harvesting and Wireless Power Transmission[END_REF]. Table 2 summarizes the performances (efficiency: eff in % as function of the frequency: f in GHz and size in mm 2 ) of these rectennas compared with our work. We note that only the references [START_REF] Yoo | Theoretical and experimental development of 10 and 35 GHz rectennas[END_REF]- [START_REF] Ren | 35 GHz rectifying antenna for wireless power transmission[END_REF] present measured values (with a setup and We note that for lower frequencies (e.g. 2.45 GHz or below) higher efficiencies (up to 80%) can be obtained.

IV. CONCLUSION

We present the cartography of the electromagnetic environment available on antenna panels of broadcasting geostationary satellites. Large areas of those panels are illuminated with an electromagnetic field ranging from 4 V/m to 127 V/m. This electromagnetic energy can be harvested to supply power to wireless sensor networks for satellite health monitoring. In order to prove the feasibility of the concept three rectennas, named R1, R2 and R3, were proposed. The rectennas were developed in a worst case scenario, namely when the rectifier/Schottky diode is not optimally loaded by the DC load impedance. Moreover, constraints concerning the space applications were taken into account in the rectennas design. Measured DC powers ranging from 0.256 mW (R2 design for an incident E-field level around 121 V/m) to 1.28 mW (R3 design for an incident E-field around 91 V/m) were harvested. If a higher antenna gain is used in the R2 design, a DC power higher than 2 mW, (i.e. the typical power consumption of the new generation of ultra-low power transceivers) may be harvested. The very compact rectenna (design R3 with a total surface of 2.8 cm 2 ) can harvest a DC power around 2 mW if more electromagnetic energy is available (e.g. if the E-field level reaches 127 V/m). Moreover, the well-known stability of the microwave links will reduce the complexity of any DC power management, storage, or regulatory circuits. expert on RF satellite tests in Compact Antenna Test Range: antennas, payload and compatibility performances.

Fig. 1 .

 1 Fig. 1. E-field distribution (peak value) on a lateral panel at 3.5 GHz (C-band). Radiated power: 90 W. The x-and y-coordinates are in millimeter.

Fig. 3 .

 3 Fig. 3. E-field distribution (peak values) on Earth side of satellite at 17.7 GHz. Radiated power: 70W. The x-and y-coordinates are in millimeter.

Fig. 4 .

 4 Fig. 4. E-field distribution (peak values) on a lateral side of satellite at 21.4 GHz. Radiated power: 85W and. The x-and ycoordinates are in millimeter.

Fig. 5 .

 5 Fig. 5. Rectenna topology: the vertical dotted line indicates the position (reference plane) of the 50Ω test point (end-launch K connector).

Fig. 6 .

 6 Fig. 6. Photos of the developed rectenna designs: R1 (R1a and R1b), R2 (R2a and R2b) and R3 (R3a and R3b)The rectenna R2 uses the MA4E-1317 diode, a double stub matching circuit and a distributed low-pass filter containing two radial stubs. The radial stubs were designed for rejecting the operating frequency (17.7 GHz) and the second-order harmonic (35.4 GHz). The rectenna R2a was composed of a rectifier and a patch antenna having a gain around 4 dBi. The rectifier and the antenna are connected using a K-adapter. The

Fig. 7 .

 7 Fig. 7. Simulation model (AWR) of the rectenna R2a (without patch antenna) and its dimensions.The harmonic balance simulation engine was used for performing non-linear simulations. A generic non-physical

Fig. 8 .

 8 Fig. 8. Simulation model (Feko) of the R3-rectenna design (see the text for the dimensions)

Fig. 9 .

 9 Fig. 9. Setup S1.The setup (S1) is appropriate for testing the rectifier part

Fig. 10 .

 10 Fig. 10. Setup S2. An automatic acquisition routine was implemented in Labview software from National Instruments [29] in order to speed-up the acquisition process. The harvested DC voltage was measured with the microwave source OFF (DC noise: V_off) and with microwave source ON (DC voltage: V_on). The DC voltage of interest (Vout) was then derived from the difference Vout=V_on -V_off. Digital filters were used for minimizing the noise level. The measurement was performed twice and the average value of Vout was registered as the final result.

Fig. 11 .

 11 Fig. 11. Measured DC output voltages (setup S1) for R1a (dotted red line, square marker) and R2a (dotted violet line, 'x' marker design).

Fig. 13 .

 13 Fig.[START_REF] Strassner | Microwave Power Transmission: Historical Milestones and system Components[END_REF]. Measured DC output voltage (setup S1) for R2a design as a function of the frequency for an RF power of 13 dBm at the input of 50Ω test point.

Fig. 14 .

 14 Fig. 14. Measured DC output voltage (setup S2) for the R2brectenna design as a function of the RF power injected at the input port of a transmitting (horn) antenna (operating frequency: 17.7 GHz).

Fig. 15 .

 15 Fig. 15. Simulated input impedance Z in of the antenna (R3 design) viewed at the distance l r -l d along the coplanar stripline as a function of l d .

Fig. 16 .

 16 Fig. 16. Simulated radiation pattern (gain) of rectenna R3a: a) 3D radiation pattern and b) E-plane polar cut: main (E_phi) and cross (E_theta) components of the total gain (values in dB, angles in degrees) are represented with continuous (red) and dotted (green) lines, respectively.

  experimental conditions well detailed) for the overall efficiency of the rectennas.

Table 1 :

 1 Estimated RF power at the input of the 50Ω test point, measured DC voltage extracted from Fig.11and the computed DC power for R2a design (f=17.7GHz)

				R2a design (MA4E-1317)
	E (V/m)	G (dBi)	RF power (dBm)	DC voltage (V)	DC power (mW)
	107	18.6	17	6.48	4.2
	67	18.6	13	5.54	3.07
	48	18.6	10	3.67	1.37
	34	18.6	8	2.78	0.77
	27	18.6	5	1.75	0.31
	15	18.6	0	0.58	0.033
	127	11	10.9	4.27	1.82
	91	11	8	2.78	0.77
	72	11	6	2.07	0.43
	51	11	3	1.21	0.015
	36	11	0	0.58	0.033
	127	5	4.9	1.75	0.31
	121	5	4.5	1.6	0.28
	114.5	5	4	1.47	0.21
	102	5	3	1.21	0.015
	91	5	2	0.94	0.088
	72	5	0	0.58	0.033

Table 2 :

 2 Comparison of the rectennas performances for reported design (beyond of 10 GHz).

	eff (%)	f (GHz)	Diode	Size (mm 2 )		Note
					[14]: a waveguide array simulation
					technique [34] was used to measure
					the overall rectenna efficiency; the
	39	35	DMK6606 (Alpha)	>24	reported efficiency defined as the ratio of DC power to the input power
					into diode was measured for 60 mW
					of input power into the diode and for
					an optimum load of 400Ω
					[15]:	the	reported	efficiency
	35	35	M/A Com MA4E-1317	>110	(received RF power/output DC power) was measured using a free space setup for an incident power density of 30 mW/cm 2 and for a load
					resistance of 50Ω
					[16]: the efficiency was measured by
					illuminating the rectenna with a
					signal delivered by a waveguide
			M/A Com		expander positioned in the close
	54.6	24	MADS-001317-	NR *	vicinity of the rectenna under test. The reported value was obtained for
			1320AG		130 mW of RF input power
					(supposed to be at the input of the
					rectifier) and an optimal load of 400
					Ω
					[17]: the reported efficiency (for 5
	82	24	M/A Com MA4E-	NR *	mW of RF power at the input of the rectifier) concerns only the
			1317		microwave rectifier loaded by an optimal resistive load (value NR * )
					[19]: only the efficiency of the
			M/A Com		rectifier part is reported; value
	17	25.7	MA4E-	328.5	obtained for a RF power of 8 dBm at
			2502L		the input of the diode and an optimal
					load of 1500 Ω
					This paper: the rectenna efficiency
	42	18.8	Metelics MZBD-9161	280	(worst case definition: eq. 3) was measured by using the setup S2, for an incident E-field of 91 V/m (incident power density around of 2.2 mW/cm 2 ) and for a load of 510 Ω

* NR : Not Reported

ACKNOWLEDGMENTS

The authors acknowledge Tonio Idda (former CNRS-LAAS engineer) for his valuable support during the early stage of the project, Samuel Charlot from CNRS-LAAS for diode mounting, the support of the European Commission and of the National Funding Agency (Region Midi-Pyrénées) from France through the MNT-ERA.NET project MEMIS (MEMS based Millimeter wave Imaging System) for developing the simulation model for GaAs Schottky diodes successfully used in the design of our rectennas. The electronic laboratory of University of Toulouse (Paul Sabatier) is acknowledged for PCB manufacturing.

This work was supported by French Space Agency (CNES) under research grant R&T n° 115052 and R&T2012 RS12/MT-0002-012.


A Takacs
email: atakacs@laas.fr

S Charlot
email: charlot@laas.fr

P Calmon

D Dragomirescu

HDRM VHF/UHF Kapton supported antenna for cubesat applications

Keywords: dual-band, VHF/UHF antenna, cubesat, flexible planar antenna

This paper addresses an original concept for very low mass planar flexible Kapton supported antenna for cubesat application. In order to illustrate the concept a dual-band VHF/UHF planar cross dipole antenna is presented. This circularly polarized antenna is compatible with the integration requirements for a cubesat mission and demonstrates very good performances in terms of input matching and radiation pattern.

I. INTRODUCTION

The interest for Cubesat (CS) miniaturized satellite for space research program rising exponentially in last years. Their small size, ranging from 0.5 U to 6U (1 U is a volume of 10 cm x 10 cm x 11 cm) and low weight (typically less than 10 kg for a 6U CS) involve very challenging design requirements mainly for DC power units (including battery and solar cells) and RF units (including transceivers and antennas). Antennas are used for TTC (Telemetry, Tracking and Control) functions or (scientific) data communications. The TTC function requires a very reliable low bit rate radio link established between the CS and the ground station (GS). This radio link should be effective even in a worst case scenario (e.g. when the DC power reserve of the CS is very low or when the satellite orientation with the respect of GS is not well controlled). Thus the VHF or/and UHF frequency bands are privileged because of the 'low' propagation losses between the GS and CS (as compared with higher frequency bands). But VHF and even UHF antennas are large and bulky for CS and Hold-Down and Release Mechanism (HDRM) should be used in order to fit the antenna in the allowed CS volume during launching and then deploying them during the flight mission. Typically wire antennas (e.g. dipole, monopole, turnstile, helical, quadrifilar helix) are used for such TTC functions in VHF/UHF bands [START_REF] Imb Riale | Space Antenna Handbook[END_REF]. For the first time, we present here a novel concept of self-deployable planar VHF/UHF antennas based of the use of a flexible kapton technology. The design of the proposed kapton supported antennas is presented in Section II while the simulation results are presented in Section III.

II. KAPTON SUPPORTED CROSS DIPOLES ANTENNA

Kapton is a polyimide based material combining very good mechanical, thermal, chemical and electrical properties [2]. Kapton is already used for space various space instruments embarqued on spacecraft and satellites. The electrical properties of the Kapton (dielectric constant: 3.4, loss tangent: 0.0018) recommended Kapton as a good candidate for flexible antennas [START_REF] Yang | Flexib le Substrate Technology for Millimeter Wave Wireless Power Transmission[END_REF]- [START_REF] Yang | Flexible Substrate Technology For Millimeter Wave Applications[END_REF]. Kapton HN film is available with thickness of: 25µm, 50µm, 75µm and 125µm and can be used as support for very thin planar and flexible antennas. The Kapton mass is only 3.55mg/cm2 (thickness 25µm, no metallization) and 5.1mg/cm2 (thickness 25 µm with 0.8 µm of gold metallization). A VHF Planar Cross Dipole Antenna (PDCA) operating at 144 MHz (assuring the TTC function) was designed and simulated in order to illustrate the proposed concept (VHF kapton antenna for CS applications). This is a 'worst case' design scenario because at such low frequencies the antenna is typically large and bulky. The proposed PCDA is composed by two half-wavelength planar dipoles disposed in cross configuration as shown in Fig. 1 a) and fed in phase quadrature. A review of crossed dipole antennas is presented in [START_REF] Ta | Crossed Dipole Antennas: A review[END_REF]. Elliptical holes are performed into the dipoles in order to reduce both the mass and the aerodynamic footprint of the antenna. Before the deployment the antenna should be folded as illustrated in Fig. 1 b). A HDRM based on split spool device/thermal knife is intended to be used. We note that no spring based mechanism is required for PCDA as for the 'classical' VHF wire antennas. The tensile stress of the flexible kapton film (that can be controlled and tuned by an appropriate technological process [START_REF] Yang | Flexible Substrate Technology For Millimeter Wave Applications[END_REF]) is enough to deploy the antenna once the hold-down mechanism released. The main dimensions of the PDCA are reported in the Fig. 2. The length 'L' should be in the range of the quarter wavelength and basically fix the operating frequencies. The width 'w' should be accomodated with the mechanical design of the CS (< 10 cm for a 3U CS). This dimension can be larger with the benefit of the increased frequency bandwith of the PCDA but with the expense of a more complicated folding procedure and HDRM. Because the electrical currents flows mainly on the exterior contour of the PCDA a hole can be introduced in order to reduce the weight and the aerodinamic footprint of the antenna. For simplicity reasons an elliptical hole was used. The kapton substrate is keep only under the metallized region (blue in Fig. 2).

Feed ports

Fig. 2. Top view of the half of each constituve dipole of the PCDA with its main dimmensions III. RESULT Full-wave electromagnetic simulations were performed by using Feko software (Method of Moment). The body of the CS was modelled as a lossy metallic cuboid (Al based) of 10 cm x 10 cm x 34 cm and was taken into account in the simulation. First, a parametric analysis was performed in order to quantify the impact of the main dimensions of the PCDA: length (L) and width (w). The length was keep constant L=45.5 cm (L1=41 cm) while w was varied from 2mm to 84mm. No hole was considered for this analysis. The simulated reflection coefficient is represented in Fig. 3 while the maximum gain is depicted in Fig. 4. As shown in Fig. 3 the PCDA exhibits two resonant frequencies (targeted 144 MHz and 432 MHz) corresponding to λ/2 and 3λ/4 operating mode. The resonant frequencies are increased while w decreased and the frequency bandwidth is increased while w increases. We note that, as expected, the PCDA exhibits a dual band behavior with λ/2 (VHF frequency) and 3λ/4 (UHF frequency) radiation pattern. Thus a single PCDA can be used for uplink (e.g. 144 MHz) and downlink (e.g. VHF) path. As illustrated in Fig. 4 the 'VHF' gain (at 144 MHz) is not significantly impacted by the width parameter 'w' while the UHF gain (at 432 MHz) increased while w decreased. A parametric analysis concerning the dimensions of the elliptical hole was also conducted. It was found that the holes dimensions do not impact significantly the VHF resonant frequency and impact moderately the UHF frequency. Finally a PCDA (L=45.5 cm, L1=41cm, w=84 mm) with an elliptical hole (minor radius: 2.5 cm and a major radius 20cm) was selected. The mass of this PCDA design is approximately 3.9g (kapton thickness 25 µm with 0.8 µm of gold metallization). For a no-hole PCDA design the mass is expected to be twice higher. The manufacturing process developed in our laboratory [START_REF] Yang | Flexible Substrate Technology For Millimeter Wave Applications[END_REF] is intended to be used. This process allows metallization quality and manufacturing resolutions far beyond the requirements of VHF/UHF applications but, as a matter of principle, other low-cost process based on inkjet printing can be used. The simulation results (Fig. 5) show very good results in terms of circular polarization (CP) at 144 MHz (the diference between co and cross components of the gain is greater than 10 dB in an angular bandwith of 104°). Good CP behaviour is obtained also at 434 MHz but in a lower angular bandwith because of a typical 3λ/4 radiation shape. IV. CONCLUSION A dual-band Kapton supported planar crossed dipole antenna for cubesat applications was proposed. This very low weight (mass<4g) antenna is easy to integrate into a cubesat as VHF/UHF antenna. The design methodology was presented with an emphasis of the design guidelines. The electromagnetic simulations predict very good performances in terms of input matching and radiation pattern. The manufacturing of such antenna is under run and experimental results will be presented at conference.

Fig. 1 .

 1 Fig. 1. Proposed kapton supported PCDA : a) deployed on the 3U CS, and b) folded before deployment (CS body is not represented)

Fig. 3 .

 3 Fig. 3. Reflection coefficient (simu lation results, FEKO) for various width 'w' while the length L is keep constant at L=45.5 cm.

Fig. 4 .

 4 Fig. 4. Simu lated maximu m gain as function of the frequency and of the width of the PCDA.

Fig. 5 .

 5 Fig. 5. Co (LHC, continuous blue line) and cross (RHC, dotted red line) components of the gain obtained in XoZ (phi=0°) and YoZ (phi=90°) cuts. Simulations results obtained at 144 MHz by using FEKO (MoM) software


Tetyana Torchynska

Brahim El Filali

Larysa Khomenkova

Xavier Portier

Fabrice Gourbilleau

Phase transformation and light emission in Er-doped Si-rich HfO 2 films prepared by magnetron sputtering

The impact of phase transformation on the emission properties of Er-doped Si-rich HfO 2 films obtained by RF magnetron sputtering has been investigated by means of the scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and photoluminescence techniques. It has been observed that thermal treatment of the films at 950 and 1100 °C governs a phase separation process. The formation of HfO 2 nanocrystals of the tetragonal phase together with the Si-quantum dots (QDs) occurs at 950 °C. Upon annealing at 1100 °C, the tetragonal SiO 2 and monoclinic HfO 2 nanocrystals appear. The appearance of bright emission in the visible-near-infrared spectral range, related to the optical transitions in the 4f intrashell energy levels of Er ions, has been detected. The investigation of the annealing effect on the luminescent properties has revealed that the enhancement of Er 3+ emission occurs due to an effective energy transfer from Si-QDs toward the Er ions. The oxidation of Si-QDs at high temperature annealing (1100 °C) leads to a reduction in the intensity of the Er ion related emission. Since hafnia-based materials have high density and are very sensitive to high-energy excitation, the results offer multifunctional applications of doped hafnia films, such as the luminescent materials for traditional phosphors.

I. INTRODUCTION

The great interest shown on hafnia-based materials can be attributed to their applications as alternative dielectrics to SiO 2 in the Si-based CMOS technology 1,2 or as silica-hafnia-based waveguides. 3,4 Meanwhile, HfO 2 is characterized by a wide bandgap (≈5.8 eV), high transparency in the ultraviolet-visible spectral ranges, and a high refractive index (2.0-2.1 at 1.95 eV), making this material a good and attractive candidate for future photonic and optoelectronic applications. [5][6][7] Additionally, the HfO 2 -based materials are characterized by a low phonon cut-off energy (∼about 780 cm -1 ) that reduces the probability of nonradiative phonon assisted relaxation and is attractive for doping these materials with different elements. The photoluminescence (PL) in pure HfO 2 was detected in near ultraviolet (4.0-4.2 eV) and visible (2.5-3.5 eV) spectral ranges. 8,9 The ultraviolet emission was attributed to the self-trapped exciton, but it is almost decayable at room temperature. On the other hand, the visible emission was assigned to the recombination via the different types of oxygen vacancies with trapped electrons. 8,9 Some papers were related to the emission study in the HfO 2 films doped with the rare-earth elements, such as Nd, Er, Pr, etc. 7,[10][11][12][13][14] Among different rare-earth trivalent elements, the Er 3+ ion is one of the most popular due to its radiative transitions in the green ( 4 S 3/2 → 4 I 15/2 ) and infrared ( 4 I 13/2 → 4 I 15/2 ) spectral ranges being extensively used as an eye-safe source in the atmosphere, laser radar, and medical and surgical applications ( 4 I 11/2 → 4 I 13/2 ). 15,16 However, the number of papers related to the spectroscopic investigation of HfO 2 ;Er based materials is few.

Binary silica-hafnia systems are of great interest as well because they allow thermal variation of the optical and structural properties, separating the SiO 2 and HfO 2 phases, and exert to study an influence of the nearest coordination shells on the emission of embedded rare-earth ions. 17 It is worth noting that Si-rich HfO 2 materials are considered mainly for microelectronic applications and that only a few emission studies on Er-doped SiO 2 -HfO 2 materials have been presented till date. 10,12 However, the mechanism of the Er 3+ ion excitation has not been reported. Moreover, the phenomenon of any formation of the Si nanocrystals or Si quantum dots (QDs) in hafnia has also not been observed.

The fact that the first coordination shell around the Er 3+ ions is composed of five to six oxygen atoms at a distance ≈2.32-2.35 Å was discovered early. However, in regard to the other coordination shells, the situation is not clear yet. It was shown that in a binary system, the Er 3+ ions were mainly dispersed in an amorphous HfO 2 environment. 17 But the question of the interaction of rare-earth ions with host defects and the dependence of their emission on the crystal phase of the surrounding matrix has not been clarified yet.

In the present paper, we focus on the study of the phase Note: This paper is part of the Special Topic Collection on Complex Oxides. a) Electronic mail: ttorch@esfm.ipn.mx transformation in the Si-rich hafnia oxide system versus annealing temperatures and its impact on the emission via the Er 3+ ion inner electronic shells.

II. EXPERIMENTAL SETUP

Er-doped Si-rich HfO 2 films were grown by RF magnetron sputtering. The deposition was performed on the 2-in. B-doped (100) oriented Si wafers with a resistivity of about 15 Ω cm. All substrates were submitted to standard RCA cleaning [dipped in a diluted hydrofluoric solution (10%) followed by cleaning in deionized water and drying in pure N 2 flow] prior to their transfer in a load-lock vacuum chamber of the deposition setup. A pure HfO 2 target (99.9%) topped with calibrated Si and Er 2 O 3 pellets was used. The Si and Er 2 O 3 pellets covered about 12% of the target surface each. The deposition was performed in a pure Ar plasma with an argon flow f Ar = 3 sccm at a constant plasma pressure [P = 0.039 mbar (3.9 Pa)]. The RF power density and the substrate-cathode distance were 0.74 W/cm 2 and 57 mm, respectively. The temperature of the substrate was kept at 500 °C and the deposition time was 200 min. All the films were found to be homogeneous and uniform. The average thickness was about 800 nm.

After the deposition, each substrate was cut into a set of small pieces (usually 1 × 1 cm 2 ) to study the effect of the annealing treatment. It was performed in a conventional horizontal furnace at different temperatures in a continuous nitrogen flow [48 sccm with a pressure of about 1.4 mbar (140 Pa)]. Henceforth, the samples annealed at T A = 950 and 1100 °C during t A = 15 min will be discussed in detail.

Several experimental techniques were used to analyze the properties of the films. The PL emission was excited with a 325 nm line of a He-Cd laser and a 476 or 488 nm line of an Ar laser. PL spectra were recorded using several setups: (i) a Jobin Yvon TRIAX180 monochromator linked with a fast Hamamatsu photomultiplier and a Stanford Research Systems (SRS) lock-in amplifier (SP830 DPS) (for a 400-1000 nm spectral range) and (ii) a Jobin Yvon 1 m single grating monochromator coupled to a Northcoast Germanium detector cooled with liquid nitrogen and an SRS lock-in amplifier (SP830 DPS) (for an 800-1700 nm spectral range). The referenced chopping frequency was 20 Hz.

To study the surface morphology of the films, as well as to get information on their chemical composition, a scanning electronic microscope (SEM) Quanta 3D FEG-FEI with an additional detector Apollo X10 mark EDAX for energy dispersive x-ray spectroscopy (EDS) was used. X-ray diffraction (XRD) results were obtained using the equipment of Model X'PERT MRD with a Pixel detector, a three axis goniometry, and a parallel collimator with an angular resolution of 0.0001°. X-ray beam was achieved from the Cu source (K α1 line, λ = 1.5406 Å).

To detect the film chemical composition, the x-ray photoelectron spectroscopy (XPS) was used that was realized in the Thermo Scientific™ K-Alpha™ XPS spectrometer. X-rays were obtained from the Al anode (K a radiation with an energy of 1486.7 eV) operated at 15 kV (90 W) at a pressure of 1.33 × 10 -7 Pa during the data collection. The 400 μm spot of the x-ray beam was set in two pass energy modes of 160 and 40 eV. To analyze the XPS spectra, the THERMO AVANTAGE V5.938 software was applied. All experiments were carried out at room temperature.

III. RESULTS AND DISCUSSION

A. EDS and SEM study

The surface morphology of as-grown and annealed samples was studied by the SEM method. As one can see from I).

The annealing treatment results in the evolution of the shape of EDS spectra. It is at a significant range of E = 0.35-0.55 keV, i.e., the intensity of the O Kα signal (Fig. 2, curves 2 and 3) increases with annealing temperature enlargement (Table II). This can be caused by the adsorption of water-like entities from the atmosphere on the surface of as-grown films and oxygen penetration into the film depth during annealing. Besides, an appearance of nitrogen in the films after annealing (Table II) can also modify these spectra. The N Kα signal is observed usually at E = 0.398-0.402 keV and can be hardly distinguished due to its overlapping with the Hf Nα signal [Fig. 2(a)]. The enrichment of the films in oxygen and nitrogen can be explained taking into account the results of Ref. 18. It was reported that annealing at 1000 °C in a nitrogen atmosphere of HfSiON thin films causes the transport of O and N elements into the film depth. 18 Similar processes can occur in the studied films as well. It is noteworthy that the intense Si Kα signal can be a superposition of the signal from the Si phase inside the film (if any) and that from the underlying substrate.

B. XRD study

XRD patterns recorded with the symmetric geometry are shown in Fig. 3. It is seen that the XRD pattern of the as-grown film has two broadbands centered at about 32.16°a nd 55.11°[Fig. 3(a)]. These two XRD bands can be attributed to the amorphous phase of the chemical composition HfSiO x . Actually, these peaks were detected in the amorphous phase and then in the crystalline hafnium silicate of the tetragonal phase (ICSD Ref. code 00-036-0088) after annealing. 2,4,7 Additionally, the XRD pattern presents a set of small intensity XRD peaks, i.e., at 2Θ = 30.09°(a), 38.57°(b), 44.84°(c), 53.12°(d), 56.01°(e), 59.64°(f ), 65.72°(g), 76.39°(h), and 78.96°(k) [Fig. 3(a)]. These peaks can originate from other phases that appeared due to the deposition of the films at 500 °C. It is possible that the crystallization process occurs at such temperature.

The Thus, annealing at T A = 1100 °C stimulates the transformation of the tetragonal HfSiO 4 and HfO 2 crystal phases, detected in the as-grown samples, into the monoclinic HfO 2 phase. At the same time, the XRD signal from the Si phase decreases, which can be caused by the formation of the tetragonal SiO 2 phase.

The coherent domain sizes, d, were estimated for all investigated films from the half width of the most intensive peaks using Scherrer's formula. [START_REF] Cullity | Elements of X-ray Diffractions[END_REF] It was found that in some as-grown films d = 13.7 Å, demonstrating the trend to increase up to d = 29.4 Å (T A = 950 °C) and 82.5 Å (T A = 1100 °C) in annealed films. appearance of orange-red emission (2.03-2.13 eV) upon the annealing treatment. This PL band was found to be similar to that observed in the Si-rich SiO 2 materials 21,22 and was explained by the exciton emission in Si-quantum dots. [START_REF] Khomenkova | [END_REF] Note that XRD results presented above for similar Si-rich HfO 2 films confirmed the existence of Si inclusions in the tetragonal HfO 2 matrix. Thus, the PL band at 2.03 eV in studied films can be assigned to the exciton emission in Si-QDs.

The PL peak centered at 2.40 eV, which has been detected after annealing at 950 °C [Fig. 4(a)] and has not appeared after annealing at 1100 °C [Fig. 4(b)], can be assigned to the recombination via the different types of oxygen vacancies with trapped electrons in the films. 8,9 The additional oxidation of the films annealed at 1100 °C, that was confirmed by the EDS study (Fig. 2), reduces the amount of oxygen vacancies. As a result, the intensity of the PL band peaked at 2.40 eV decreases as well [Fig. 4(b)].

The formation of Si-QDs in the films mediates the excitation of rare-earth ions. It is known that the absorption cross section of rare-earth ions for 4f-4f transitions is about 10 -19 -10 -21 cm -2 , and these ions require a high power laser for their direct excitation. [23][24][25][26][27][28] The more promised transition is the 4f-5d ones with a larger absorption cross section (about 10 -18 cm -2 ). However, the excitation belongs to the ultraviolet spectral range, restricting its use for many applications.

It was reported that the host-mediated excitation of 4f-4f transitions can be very effective due to energy transfer. This approach was widely used for the rare-earth doped silica with embedded Si nanoclusters or QDs. [23][24][25][26][27] The Si-QD incorporation allowed increasing the absorption cross section of Er 3+ ions from 10 -21 cm -2 (Ref. 28) up to 10 -16 cm -2 . 26,27 Simultaneously, the bright emission from Er 3+ ions was achieved due to the effective energy transfer from Si-QDs into Er ions. For this purpose, the visible broadband excitation was used offering the safe applications of these materials. Thus, on the basis of the analogy with Si-QDs embedded in the SiO 2 host, one can suppose that an incorporation of Si-QDs in the hafnia oxide matrix can allow one to achieve an effective Er 3+ ion excitation in the visible spectral range.

Thus, two PL bands centered at 2.78 and 3.18 eV [Fig. 4(a)], as well as the IR emission (Fig. 5) detected in the film annealed at 950 °C, have been attributed to the well-known optical transitions via the 4f intrashell energy levels in the Er 3+ ions: 4 [29][30][31] Furthermore, the bright PL emission from the films annealed at 950 °C can be explained by the excitation energy transfer from the Si-QD, or native defects (V O ), toward the Er 3+ ions, and/or by the reabsorption of Si-QD or native defect emissions by the Er 3+ ion and its excitation via the up-conversion mechanism. 32 The PL spectrum of the film annealed at T A = 1100 °C has changed dramatically [Fig. 4(b)]. The PL intensities of all PL bands related to the optical transitions via the 4f intrashell energy levels in the Er ions have increased and the new PL bands appear. Note that the PL band (at 2.03 eV), related to the exciton emission in Si-QDs, and the PL band (at 2.40 eV) owing to the recombination via oxygen vacancies have been not detected in the PL spectrum. This correlates with the XRD results presented above, which show that the Si inclusions have not been detected in the films after the high temperature treatment (1100 °C).

PL peak positions of new PL bands and their legends are summarized in Table III. At high temperature, the process of phase transformation is realized efficiently, and two phases, the tetragonal SiO 2 and the monoclinic HfO 2 , have been revealed in the XRD study. Earlier it was shown that in binary silica-hafnia systems, the Er 3+ ions were located mainly in the hafnia oxide. 17 Thus, we can conclude that Er 3+ ions in the Si-HfO 2 :Er film after annealing at 1100 °C locate in the monoclinic HfO 2 phase mainly. Nevertheless, the different full widths at the half maximum of detected PL bands [Fig. 4(b)] testify that the Er ions ( partially) exist in the second tetragonal SiO 2 phase as well.

D. XPS study

To confirm that the Si-QDs exist in the films after annealing at 950 °C and disappear after the temperature treatment at 1100 °C, the high resolution x-ray photoelectron spectra for the Si2p line were measured (Fig. 6). Two peaks centered at 98.48 and 101.48 eV have been detected in the XPS spectrum of as-grown films (Fig. 6, curve 1). These two peaks were assigned to x-ray photoelectron excitation from the Si2p energy levels of Si atoms located in the monocrystalline Si (98.48 eV) and in the amorphous HfSiO x phase (101.48 eV). [START_REF]XPS database[END_REF] Annealing at 950 °C triggers a partial decrease in the intensity of the Si2p peak related to the monocrystalline Si (98.48 eV) and the shift to 102.78 eV of the second Si2p peak connected with the Si atoms located in the Si suboxide. The latter process is attributed to the Si atoms in the tetragonal SiO x crystal lattice. Annealing at 1100 °C leads to a total disappearence of the Si2p peak related to the monocrystalline Si (98.48 eV) and a significant increase of the Si2p peak at 103.68 eV owing to the Si atoms in the tetragonal SiO 2 crystal lattice. [START_REF]XPS database[END_REF] Thus, at high annealing temperature, the Si atoms contribute to the formation of the tetragonal SiO 2 phase. One can conclude that the disappearence of Si-QD in the films at high annealing temperature (1100 °C) is the reason for the reduction in the PL intensity in comparison with that registered for the films annealed at 950 °C.

IV. SUMMARY AND CONCLUSIONS

In the present study, the impact of phase transformation on the emission properties of the Si-rich HfO 2 :Er films obtained by RF magnetron sputtering was investigated by means of the SEM, EDS, XRD, XPS, and PL techniques. It was observed that the thermal treatment governs a phase separation process and the formation of (i) the small nanocrystals (≈2.9 nm) of tetragonal HfO 2 phase together with the inclusion of Si-QDs at 950 °C annealing and (ii) the larger nanocrystals (≈8.2 nm) of the tetragonal silica and monoclinic hafnia phases at 1100 °C annealing. Bright emission in the visible-near-infrared spectral range occurred. The investigation of the effect of annealing treatment on luminescent properties revealed that the enhancement of Er 3+ emission occurs due to an effective energy transfer from Si-QDs, or naive defects, toward Er ions. The oxidation of Si-QDs at high temperature annealing (1100 °C) leads to a reduction of intensity in the Er ion related emission. Since hafnia-based materials have high density and are very sensitive to highenergy excitation, our results offer multifunctional applications of doped hafnia films, such as luminescent materials for traditional phosphors.

Fig. 1 ,

 1 FIG. 1. SEM images of Si-HfO 2 :Er films: as-grown (a) and after annealing at 950 °C (b) and 1100 °C (c).

  FIG. 2. EDS spectra recorded for the as-grown (1) and annealed at 950 °C (2) and 1100 °C (3) films. The energy ranges are 0-0.8 keV (a) and 1.0-2.4 keV (b).

  2Θ = 44.84°(c) can be assigned to the tetragonal HfSiO 4 crystal lattice (ICSD Ref. code 00-036-0088), whereas the XRD peaks at 2Θ = 38.57°(b), 53.12°(d), 65.72°(g), and 78.96°(k) were identified as the monoclinic Er 2 Si 2 O 7 crystal structure (ICSD Ref. code 01-072-0650). The thermal treatment of the films at T A = 950 °C led to changes in the XRD pattern. The signals from the tetragonal HfSiO 4 and monoclinic Er 2 Si 2 O 7 phases disappeared, while the XRD peaks owing to the tetragonal HfO 2 phase centered at 2Θ = 30.09°(1), 35.09°(2), 49.55°(3), and 59.64°(5) increased significantly [Fig. 3(b)]. The XRD peak at 2Θ = 56.01°(4) related to the cubic Si crystal lattice is still possible to detect in the XRD pattern [Fig. 3(b)]. Thermal treatment of the films at T A = 1100 °C results in the further evolution of XRD patterns. The signals from the tetragonal HfO 2 phase have been not detected anymore. Instead the two sets of XRD peaks with significant intensities have appeared [Fig. 3(c)]. The first set of XRD peaks detected at 2Θ = 28.38°(1), 34.55°(3a), 35.58°(3b), 41.17°( 4), 50.38°(6), and 55.57°(7) has been assigned to the monoclinic HfO 2 phase (ICSD Ref. code 00-034-0104). The second set of XRD peaks detected at 2Θ = 30.24°(2), 45.67°( 5), 60.52°(8), and 62.97°(9) has been attributed to the tetragonal SiO 2 crystal structure (ICSD Ref. code 00-045-1374).

C

  FIG. 4. Visible PL spectra of Si-HfO 2 :Er films: as-grown (1a,1b) and annealed at 950 °C (2a) and 1100 °C (2b).

FIG. 3 .

 3 FIG. 3. XRD patterns of Si-HfO 2 :Er films: as-grown (a) and annealed at 950 °C (b) and 1100 °C (c).

  F 3/2.5/2 -4 I 15/2 at 455 nm (2.72 eV), 4 G 11/2 -4 I 15/2 at 400 nm (3.10 eV), and 4 I 13/2 -4 I 15/2 at 1.525-1.545 μm (0.803-0.813 eV).

FIG. 5 .

 5 FIG. 5. Infrared PL spectra of the films annealed at 1100 °C (1) and 950 °C (2). Excitation light wavelength is 476 nm.

TABLE I .

 I Atomic-electron binding energies (in keV) for the elements of interest (data extracted from the Table of Isotopes, CD-ROM Edition, Version 1.0, March 1996). The bold values correspond to arrays in Fig.2.

				Element
	Atomic shell	7 N	8 O	14 Si	68 Er	72 Hf
	K	0.4016	0.5320	1.8389	57.4855	65.3508
	L 1	0.0244	0.0285	0.1487	9.7513	11.2707
	L 2	0.0092	0.0071	0.0995	9.2643	10.7394
	L 3	0.0092	0.0071	0.0989	8.3579	9.5607
	M 1			0.0076	2.2065	2.6009
	M 2			0.0030	2.0058	2.3654
	M 3			0.0030	1.8118	2.1076
	M 4				1.4533	1.7164
	M 5				1.4093	1.6617
	N 1				0.4491	0.5381
	N 2				0.3662	0.4370
	N 3				0.3200	0.3804
	N 4				0.1767	0.2238
	N 5				0.1676	0.2137
	TABLE II. Chemical film composition.
		As-deposited	T A = 950	°C	T A = 1100	°C
	Samples element		(%)	(%)		(%)
	O K		49.5	60.3		66.2
	Er M		9.7	7.3		6.2
	Hf M		26.7	20.1		17.2
	Si K		12.6	9.5		8.1
	N K		1.5	2.8		2.3

TABLE III .

 III Detected PL peaks and their legends.

	Peak energy	Peak wavelength
	(eV)	(nm)	Transition	Reference
	1.698	730	4 F 11/2 -4 I 15/2	30
	1.746	710	4 F 9/2 -4 I 15/2	30
	2.194	565	4 S 5/2 -4 I 15/2	30
	2.254	550	4 S 3/2 -4 I 15/2	30
	2.339	530	2 H 11/2 -4 I 15/2	30
	2.530	490	4 F 7/2 -4 I 15/2	31
	2.720	455	4 F 5/2 -4 I 15/2	31
	2.850	435	4 F 3/2 -4 I 15/2	30
	2.952	420	2 H 9/2 -4 I 15/2	30
	3.178	390	4 G 11/2 -4 I 15/2	30

ACKNOWLEDGMENTS

This work was partly supported by the National Academy of Sciences of Ukraine (Project No. III-4-16), the Ministry of Education and Science (Project ID 89452), the Normandie Region and the European Regional Development Fund (ERDF) (Project LUMIERE), as well as by the CONACYT Mexico (Project No. 258224) and SIP-IPN Mexico (Project No. 20180495).