domain: phys chim chim.mate

text:
Ranjith Kumar

Neena Prasad

Fabien Veillon

Wilfrid Prellier
email: wilfrid.prellier@ensicaen.fr

Raman spectroscopic and magnetic properties of Europium doped Nickel oxide nanoparticles prepared by microwave-assisted hydrothermal method

Keywords: Nickel oxide, Europium, defects, ferromagnetism, Raman spectroscopy

Pure and 5 mol% Eu doped nickel oxide (NiO) nanoparticles were synthesized by microwave-assisted hydrothermal method. X-ray diffraction and Rietveld refinement analysis confirmed the phase and purity of prepared nanoparticles. Scanning electron microscopy analysis revealed that the undoped nanoparticles are porous flake-like structures while the doped samples were aggregated. X-ray photoelectron spectroscopy analysis disclosed that Eu doping increases hole concentration, Ni 2+ vacancies, and defective oxygen concentration in the nanoparticles. The magnetic analysis shows the presence of weak ferromagnetism in lower fields of the undoped nanoparticles. On doping with Eu, the ferromagnetic behavior increases due to increased oxygen defects and uncompensated surface spins. Raman spectroscopy investigation shows the absence of 2M vibrational mode and reduced antiferromagnetic coupling in the prepared samples. It also substantiated the presence of defects in the prepared samples. Eu doping produced local vibrations that overlap with SO mode of NiO and shifting of 1M band due to its higher mass. Eu dopant can modify the magnetic property of NiO nanoparticles that can be used in spintronic applications.

Introduction

Nanoparticles are extensively studied since they exhibit highly attractive and unusual properties, due to the more significant surface electrons and surface and interface effects than the bulk materials [START_REF] Winkler | Surface anisotropy effects in NiO nanoparticles[END_REF]. Nickel oxide (NiO) is an extensively studied magnetic material since it manifests room temperature antiferromagnetic behaviour with high Neel temperature (TN = 523 K) in bulk form. But, when the particle size is reduced to the nanoscale regime, NiO shows unique and exciting properties like high charge mobility, high sensing, good photocatalytic activity, etc. [START_REF] Delgado | Redox and catalytic properties of promoted NiO catalysts for the oxidative dehydrogenation of ethane[END_REF][START_REF] Khalil | Nickel oxide nanocrystals as a lithium-ion battery anode: structure-performance relationship[END_REF][START_REF] Duan | Sub-3 nm NiO nanoparticles: Controlled synthesis, and photocatalytic activity[END_REF][START_REF] Liu | Facile synthesis and hierarchical assembly of flowerlike NiO structures with enhanced dielectric and microwave absorption properties[END_REF]. The remarkable change in properties is due to the quantum size effect, surface effect, shape, and volume effects [START_REF] Kodama | Finite size effects in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Makhlouf | Magnetic anomalies in NiO nanoparticles[END_REF]. When doped with other metals, like Mn, Fe, Cu, Ce, etc., NiO nanoparticles show drastic changes in the magnetic and optical properties [START_REF] Emamdoust | Surface and electrochemical properties of flower-like Cu-NiO compounds[END_REF][START_REF] Patel | Effect of Mn doping concentration on structural, vibrational and magnetic properties of NiO nanoparticles[END_REF][START_REF] Ponnusamy | Structural, optical and magnetic properties of undoped NiO and Fe-doped NiO nanoparticles synthesized by wet-chemical process[END_REF][START_REF] Boukhari | Synthesis, characterization, optical and magnetic properties of pure and Mn, Fe and Zn doped NiO nanoparticles[END_REF]. Also, the synthesis route can cause considerable changes in the electronic structure of NiO nanoparticles [START_REF] Biju | Electronic structure of nanostructured nickel oxide using Ni 2p XPS analysis[END_REF][START_REF] Yousaf | Tuning the structural, optical and electrical properties of NiO nanoparticles prepared by wet chemical route[END_REF]. Hence, NiO nanoparticles of various size, shape, and different dopants have been synthesized by different methods, and its potential application in numerous fields have been studied [START_REF] Emamdoust | Surface and electrochemical properties of flower-like Cu-NiO compounds[END_REF][START_REF] George | Synthesis and characterisation of nickel oxide nanofibre webs with alcohol sensing characteristics[END_REF][START_REF] Li | Preparation of nano-NiO particles and evaluation of their catalytic activity in pyrolyzing biomass components[END_REF][START_REF] Meher | Microwave-mediated synthesis for improved morphology and pseudocapacitance performance of nickel oxide[END_REF][START_REF] Vijayakumar | Supercapacitor studies on NiO nanoflakes synthesized through a microwave route[END_REF][START_REF] Zhang | Effect of nickel vacancies on the room-temperature NO2 sensing properties of mesoporous NiO nanosheets[END_REF]. Since the rare earth elements possess partially filled 4f orbitals, these elements can be utilized as dopants to modulate NiO nanoparticles [START_REF] Bünzli | New opportunities for lanthanide luminescence[END_REF][START_REF] Chauhan | Synthesis and Characterization of Neodymium Doped Nickel Oxide Nanoparticles[END_REF][START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF]. The structural, magnetic, optical, and dielectric behaviour of rare earth (Y, Pr, Sm, Gd, and Er) doped NiO nanoparticles were extensively studied by Al Boukhari et al. [START_REF] Boukhari | Synthesis, characterization, optical and magnetic properties of pure and Mn, Fe and Zn doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF][START_REF] Boukhari | Improving the dielectric behavior of NiO nanoparticles by Samarium doping for electromagnetic applications[END_REF]. Their investigation shows that rare earth doping increases the bandgap, dielectric constant, electrical conductivities, and magnetization in NiO nanoparticles. However, the effect of such dopants on the vibrational properties of the NiO nanoparticles was not reported. Naseem Siddique et al. have synthesized Ce 3+ doped NiO nanoparticles by sol-gel method and explored the optical and magnetic properties [START_REF] Siddique | Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures[END_REF]. They have reported that Ce 3+ doping produces defects that shift and broaden the 1LO and 2LO vibration modes and suppresses the 2M magnon mode. Chauhan et al. have investigated the effect of Nd 3+ dopant on the optical properties of NiO nanoparticles prepared by the sol-gel method and reported the possible application for optical glucose sensors [START_REF] Chauhan | Synthesis and Characterization of Neodymium Doped Nickel Oxide Nanoparticles[END_REF].

In all the reports mentioned above, weak ferromagnetism has been observed in the NiO nanoparticles when doped with rare earth elements. The origin of such weak ferromagnetism has been attributed to the presence of Ni vacancy, and photoluminescence results substantiated it. Among the rare earth metals, Europium (Eu) is known to be the most reactive one. It can exist as Eu 3+ or Eu 2+ in compounds. Eu 2+ is formed in reduced conditions, and Eu 3+ is known to exist as a more stable state in most compounds [START_REF] Dai | Mechanism of the reduction and energy transfer between Eu 2+ and Eu 3+ in Eu-doped CaAl 2 Si 2 O 8 materials prepared in air[END_REF]. Goumri-Said et al.

have investigated, theoretically and experimentally, the effect of Eu doping on the optical properties and crystal structure of NiO thin films [START_REF] Goumri-Said | Europium incorporation dynamics within NiO films deposited by sol-gel spin coating: experimental and theoretical studies[END_REF]. It was found that the incorporation of Eu produced changes in the polarization of NiO, which affects the magnetic moment.

Microwave-assisted hydrothermal method is known for the rapid synthesis rate, increased chemical kinetics, higher yield, etc. [START_REF] Meher | Microwave-mediated synthesis for improved morphology and pseudocapacitance performance of nickel oxide[END_REF][START_REF] Komarneni | Microwave-hydrothermal synthesis of ceramic powders[END_REF]. We have successfully synthesized pure and 5 mol% Eu-doped NiO nanoparticles by microwave assisted hydrothermal method in the present work. We have concentrated on investigating the effect of Eu doping on the vibrational and magnetic properties of NiO nanoparticles. The crystal structure, chemical state of the constituent elements, and morphology of the prepared nanoparticles were also examined.

Experimental details

Synthesis of the nanoparticles

Commercially available nickel acetate tetrahydrate (Ni(CH3COO)2.4H2O) (Alfa Aesar, 98%), Sodium hydroxide (NaOH) (Alfa Aesar, 98%), and Europium acetate (Eu(CH3COO)3) (Sigma Aldrich, 99.9%) were used without further purification as precursors for the synthesis of the nanoparticles. Distilled water prepared in the laboratory was used throughout the experiment. For the synthesis of nickel oxide (NiO) nanoparticles, Ni(CH3COO)2.4H2O and NaOH, in 1:2 molar ratio, were dissolved in 35 ml and 30 ml of distilled water, respectively.

Then, the NaOH solution was added dropwise to Ni(CH3COO)2 solution under constant stirring. The obtained suspension was transferred to a 90 ml microwave reactor vessel and then placed on the turntable inside the microwave digestive system (CEM MARS 6) for uniform heating. At a heating rate of 5 °/min, the suspension was heated to 180 °C for 30 min and then allowed to cool naturally. The obtained product was washed with distilled water and ethanol several times by centrifugation to remove all unreacted ions from the products and dried in air at 180 °C for 3 hrs. It was then manually ground into a fine powder using agate mortar, secured using platinum crucible, and annealed in a muffle furnace at 400 °C for 3 hrs.

The heating rate was kept constant at 5 °/min, and cooling was done naturally.

For the synthesis of Eu doped NiO nanoparticles, 5 mol% of Eu(CH3COO)3 was used as Eu source, and a similar procedure was adopted. The obtained final products were NiO and Ni0.95E0.5O, respectively, and used for further characterizations.

Characterization

The phase and crystallinity of the synthesized nanoparticles were analyzed by the X-ray diffraction (XRD) technique using the PanAlytical X'pert Pro X-ray diffractometer having Cu Kα (λ = 1.5418 A ̊) source. The XRD patterns in the 2θ range of 20 -90° were obtained at a step size of 0.025°/sec. Chemical oxidation states of the nanoparticles were investigated by X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD) with monochromatic Al Kα radiation (1486.8 eV). Before recording the spectra, the binding energies were calibrated with reference to adventitious carbon 1s peak at 284.6 ± 0.1 eV. The morphology and composition of the nanoparticles were examined by high-resolution field emission scanning electron microscopy (FE-SEM, Carl ZEISS-Ultra55) equipped with energy-dispersive X-ray spectroscopy (EDS). Magnetization measurements were conducted using a superconducting quantum interference device (SQUID, Quantum design) magnetometer between a magnetic field of -60 to +60 kOe. Micro Raman spectra were obtained using LabRAM HR800 Raman spectrometer equipped with diode-pumped solid-state laser (Quantum MPC 6000) working at 532 nm wavelength. The laser was operated at a power of 50 mW, and the spectra were obtained in the wavenumber range of 250 -1600 cm -1 . The instrument was calibrated concerning Si-Si vibration mode at 520 ± 0.25 cm -1 . All measurements were performed at room temperature.

Results and discussion

Structure and morphological analysis

Figure 1 shows the refined XRD patterns of the synthesized nanoparticles. The diffraction pattern of the undoped (NiO) and doped (Ni0.95E0.5O) samples show five distinct peaks. These peaks correspond to (111), (200), (220), (311), and (222) orientations of the FCC crystal structure that belongs to the Fm-3m space group [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Siddique | Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures[END_REF][START_REF] Farbod | Size dependence of optical and magnetic properties of nickel oxide nanoparticles fabricated by electric arc discharge method[END_REF]. The crystallinity and purity of the NiO nanoparticles can be confirmed from the high-intensity narrow peaks and the absence of impurity peaks. In the case of Ni0.95E0.5O nanoparticles, no characteristic peaks of Europium compounds can be found, suggesting that the Eu ions are successfully integrated into NiO lattice [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF]. Rietveld analysis, done using the MAUD program [START_REF] Lutterotti | Total pattern fitting for the combined size-strain-stress-texture determination in thin film diffraction[END_REF], corroborates these observations. From the diffraction patterns, we can perceive that the Ni0.95E0.5O nanoparticles have broader peaks and relatively lower intensity than the pure NiO nanoparticle. This variation in intensity and peak width indicates that the crystallinity of the Ni0.95E0.5O nanoparticles has decreased compared to the undoped samples, which complies with the results reported in the literature [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF][START_REF] Boukhari | Improving the dielectric behavior of NiO nanoparticles by Samarium doping for electromagnetic applications[END_REF].

Likewise, it can be noticed that the relative intensity of the peaks corresponding to (311) and (222) orientations have decreased, which signifies that doping with Eu atoms disrupts the growth of NiO crystals along with those directions. While examining the peak positions, it was observed that the peaks corresponding to (111) and (200) orientations of the Ni0.95E0.5O nanoparticles have been shifted by 0.06° and 0.03°, respectively, towards the lower angle, compared to the pure NiO nanoparticles. Since the atomic radius of Eu 3+ (0.0947 nm) is more significant than Ni 2+ (0.069 nm), the lattice will undergo distortion on Eu substitution. The distortion in the lattice will cause tensile stress in the crystals, which causes the peak to shift towards the lower angle. Also, the peak shift towards the lower angle indicates that the lattice parameter has increased [START_REF] Siddique | Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures[END_REF]. A standard LaB6 sample was used to determine the instrumental line broadening of each peak. The average crystallite size (D) of both the samples was calculated from the three high-intensity peaks, i.e., (111), (200), and (220), using the Scherrer equation given by [START_REF] Mote | Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles[END_REF],

D = (1)
where, the shape factor K = 0.9, λ is the wavelength of the X-ray, β is the full width half maximum (FWHM) at the Bragg angle θ. The lattice parameter (a) was estimated using the expression 2 [START_REF] Goumri-Said | Europium incorporation dynamics within NiO films deposited by sol-gel spin coating: experimental and theoretical studies[END_REF] for the cubic crystal system, =

where, d is the interplanar spacing, and h k l are the Miller indices. The lattice strain (ε) was estimated using equation 3 [START_REF] Mote | Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles[END_REF],

ε = (3)
The estimated average crystallite size for the NiO and Ni0.95E0.5O samples were 13.45 and 6.51 nm, respectively. The lattice strain was found to be 0.0066 and 0.0138 for the NiO and Ni0.95E0.5O nanoparticles, respectively. The calculated lattice parameter was 4.179 A ̊ and When the size of the nanoparticle is reduced, the fraction of surface atoms increases, which in turn increases the uncompensated surface spins [START_REF] Popkov | Size effects in the formation of an uncompensated ferromagnetic moment in NiO nanoparticles[END_REF].

Figure 2 shows the high magnification FE-SEM micrographs and the corresponding EDS spectra of the pure NiO and Ni0.95E0.5O nanostructures. The NiO nanoparticles (Figure 2(a))

were found to be perforated flakes of different shapes and various sizes with an average thickness of about 15 nm. Since the particles can be distinguished individually with clearly visible boundaries, their sizes can be determined from image analysis [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF]. The size of the flakes varies from 30 to 230 nm, with the more significant number of particles being in the 90 -150 nm range. The average size of the particle was found to be 120 nm (Figure 2(e)). The size of the pores varies from 3 to 12 nm based on the size of the flakes. The larger size can be due to the aggregation of the crystallites during synthesis. This flake-like morphology observed in NiO nanoparticles synthesized by microwave-assisted route is due to oriented attachment [START_REF] Vijayakumar | Supercapacitor studies on NiO nanoflakes synthesized through a microwave route[END_REF][START_REF] Manikandan | Comparative investigation of NiO nano-and microstructures for structural, optical and magnetic properties[END_REF]. Similar flake-like mesoporous NiO nanoparticles were prepared by Zhang et al. by hydrothermal method using nickel chloride as a precursor, and its potential application as NO2 gas sensor has been explored [START_REF] Zhang | Effect of nickel vacancies on the room-temperature NO2 sensing properties of mesoporous NiO nanosheets[END_REF]. The relatively bigger size and porous structure compared to those reported in the literature can be attributed to longer synthesis time. The nucleation rate and growth of the NiO nanoflakes are augmented by microwaves during the synthesis. When exposed to microwave for a longer duration, the neighbouring nanoparticles aggregate and grow along a specific direction and form nanoflakes [START_REF] Vijayakumar | Supercapacitor studies on NiO nanoflakes synthesized through a microwave route[END_REF]. Thus, the porous structures might be due to the incomplete aggregation of the nanoparticles and perhaps the evaporation of water molecules during annealing in air. When doped with Eu, we can observe considerable changes in the morphology of the nanoparticles. The flake size appears to be reduced, and the particles are agglomerated, as seen in Figure 2(c). This nonuniform shape and highly agglomerated morphology changes from almost uniform, and less agglomerated particles might minimise the surface energy [START_REF] Manikandan | Comparative investigation of NiO nano-and microstructures for structural, optical and magnetic properties[END_REF]. Such accumulation of nanoparticles was also observed when NiO nanoparticles were doped with rare earth elements like Pr, Sm, Gd, Er, etc. [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF][START_REF] Boukhari | Improving the dielectric behavior of NiO nanoparticles by Samarium doping for electromagnetic applications[END_REF]. These observations indicate that doping causes aggregation of the nanoparticles and increases the interaction between them. This accumulation might be prevented by using appropriate surfactants. Synthesis of NiO nanoparticles by wet chemical routes is characterised by a large number of Ni 2+ vacancies and a higher surface to volume ratio [START_REF] Maniammal | X-ray diffraction line profile analysis of nanostructured nickel oxide: Shape factor and convolution of crystallite size and microstrain contributions[END_REF]. The presence of defects greatly affects the electrical, magnetic, and properties of the oxide nanostructures [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF][START_REF] Boukhari | Improving the dielectric behavior of NiO nanoparticles by Samarium doping for electromagnetic applications[END_REF][START_REF] Siddique | Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures[END_REF].

Compositional analysis

EDS was performed over 30 μm × 30 μm to estimate the atomic percentage of constituent elements present in the samples. The EDS spectra (Figure 2 The prepared nanoparticles were characterised by XPS, which gives the elemental composition and their oxidation states at a depth of a few atomic layers' thicknesses from the surface. Figures 3 and4 show the obtained core level XPS spectra of constituent elements present in the pure NiO and Ni0.95E0.5O samples. Peak fittings were carried out using CasaXPS software with Shirley's background. The peaks were deconvoluted using the Gaussian-Lorentzian function. The details of the position, FWHM, and area of the peaks are summarised in Table 2.

Due to the complexity of the 2p spectrum, it is challenging to analyse the chemical state of nickel oxide and nickel hydroxide [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn[END_REF]. In the present work, the oxidation states have been assigned from the binding energies of Ni 2p3/2 spectra, which is generally used as the fingerprint of NiO [START_REF] Khawaja | XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide[END_REF]. On deconvolution, the acquired core level spectra of Ni 2p3/2 yields four peaks, characteristic central peak with a shoulder at ~ 1.5 eV and two satellite peaks at a higher binding energy level [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn[END_REF][START_REF] Khawaja | XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide[END_REF]. These peaks that are intrinsic for nickel oxide can be observed in both NiO and Ni0.95E0.5O samples (Figures 3(a) and 4(a)). The central peak at 853.8 eV is due to the local screening effect, which gives rise to 3d 9 ligand band hole at the core level site induced by the X-ray photons. The shoulder peak at 855.5 eV is due to the surface and non-local screening effects [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn[END_REF][START_REF] Khawaja | XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide[END_REF]. The 2p core hole formation and the corresponding charge transfer process that causes screening of the core hole is accompanied by d-d transitions, which produces the shake-up peaks at 860.8 and 864.1 eV [START_REF] Zou | The electronic structures of NiO nanoparticles coated with stearates[END_REF]. The deconvolution of the spectrum of Ni0.95E0.5O sample yields peaks located at 854.0, 855.7, 861.1, and 865.1 eV. Thus, when doped with Eu, these peaks are shifted to a higher binding energy side. Since the doping of Eu atoms distorts the lattice, it creates an asymmetry in the electron cloud over the neighbouring atoms. This changes the bond length and hence increases the binding energy [START_REF] Allen | A study of a number of mixed transition metal oxide spinels using X-ray photoelectron spectroscopy[END_REF]. We can also observe that the area of all the peaks has decreased by almost half after doping. The ratio of the area of the shoulder peak with respect to the main peak has decreased when doped with Eu atoms. This decrease might be due to the decrease in the Ni 3d -O 2p hybridization in the nanoparticles, which in turn decreases the covalency and electron-phonon coupling [START_REF] Biju | Electronic structure of nanostructured nickel oxide using Ni 2p XPS analysis[END_REF]. The FWHM of the shake-up peaks has increased when doped with Eu, indicating that the concentration of the holes has increased.

The number of surface atoms and the Ni 2+ vacancies in the Ni0.95E0.5O nanoparticles increases due to particle size reduction. Since each Ni 2+ vacancy gives rise to two holes, the concentration of holes increases when doped with Eu atoms.

Deconvolution of O 1s spectrum yields three peaks located at 528.7, 530.4, and 531.7 eV for the pure NiO sample (Figure 3(b)). These peaks correspond to the lattice oxide, defective oxygen, and adsorbed water, respectively [START_REF] Khawaja | XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide[END_REF][START_REF] Payne | Use of oxygen/nickel ratios in the XPS characterisation of oxide phases on nickel metal and nickel alloy surfaces[END_REF]. Whereas, in the case of Ni0.95E0.5O sample, the deconvoluted O 1s spectrum yields two peaks located at 528.8 and 530.6 eV (Figure 4(b)). The shift towards higher binding energy might be due to the higher electronegativity of Ni, which increases the Eu-O bond ionicity [START_REF] Mercier | XPS study of Eu (III) coordination compounds: Core levels binding energies in solid mixed-oxo-compounds EumXxOy[END_REF]. It can be observed that doping causes broadening and an increase in the intensity of the defective oxygen peak. Also, the peak area of the defective oxygen has doubled after doping. Thus, when doped with Eu, the defective oxygen (the neighbouring oxygen atoms of Ni vacancy) concentration increases. that give rise to unusual magnetic properties [START_REF] Zhang | Preparation and magnetic properties of polycrystalline Eu2O3 microwires[END_REF]. Thus, we can presume that the enhancement in oxygen vacancy concentration in the Ni0.95E0.5O nanoparticles might be from the contribution of Eu atoms present in the nanoparticles.

Magnetic analysis

Defective structures, doping, and finite-size effects cause the magnetic moment in most of the antiferromagnetic nanoparticles. NiO nanoparticles exhibit anomalous magnetic properties due to their finite size effect, where the uncompensated surface spins cause changes in the magnetic order [START_REF] Kodama | Finite size effects in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Makhlouf | Magnetic anomalies in NiO nanoparticles[END_REF][START_REF] Berkowitz | Anomalous properties of magnetic nanoparticles[END_REF]. Figure 5 shows the hysteresis loop of the synthesized pure NiO and Ni0.95E0.5O nanoparticles. The M -H curve of pure NiO (figure 5a) shows an almost linear response with a low degree of magnetisation and low hysteresis loss, which is the typical behaviour of NiO nanoparticles when the size is reduced. Since no impurities were observed form XRD and EDS analysis, the presence of weak ferromagnetism in the pure NiO nanoparticles at the low field region can be attributed to vacancies in the prepared nanoparticles [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF]. The remnant magnetization (Mr) and coercivity (Hc) were 113.5×10 -5 emu/g and 0.195 kOe, respectively. The relatively higher coercivity is due to exchange coupling between uncompensated surface spins and antiferromagnetic core [START_REF] Makhlouf | Magnetic anomalies in NiO nanoparticles[END_REF].

On doping with Eu, the hysteresis response of the sample shows drastic changes (figure 5b).

The linearity has impaired, and an increase in Mr (113.1×10 -3 emu/g) by two orders of magnitude and Hc (0.198 kOe) by a small value can be observed. Likewise, the hysteresis loss is high as compared to the undoped NiO nanoparticles. When the nanoparticles are in close contact, the exchange interaction is increased. As observed from FE-SEM micrographs, the particles are agglomerated when doped with Eu. Thus, when doped with Eu atoms, linear behaviour is impaired, and the rate of magnetization has increased significantly. Similar results were obtained by Al Boukhari et al. when NiO nanoparticles were doped with rare earth elements like Y, Pr, Sm, Gd, and Er [START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF]. Their analysis shows that the ferromagnetic behaviour of the rare earth doped NiO nanoparticles depends on the type of dopant used.

Depending on the dopant element, the vacancies in the NiO nanoparticles varies. Also, the reduction in the size of the particles that result in a higher number of exchange bonds being broken on the particle surface can affect the magnetization [START_REF] Moura | Tuning the surface anisotropy in Fe-doped NiO nanoparticles[END_REF]. nanoparticle was synthesized by the wet-chemical route [START_REF] Ponnusamy | Structural, optical and magnetic properties of undoped NiO and Fe-doped NiO nanoparticles synthesized by wet-chemical process[END_REF]. Their results suggest that the linear behaviour was due to the contribution of uncompensated surface spins to the magnetization. Also, the free electrons from the higher valent dopant can contribute to the exchange interactions. The change in magnetic properties might be due to Eu 3+ bonding with O, which gives rise to a more significant number of oxygen vacancies [START_REF] Zhang | Preparation and magnetic properties of polycrystalline Eu2O3 microwires[END_REF]. As observed from XPS analysis, the oxygen vacancy concentration has increased when doped with Eu. The incorporation of Eu 3+ ions in NiO induces ferromagnetism by polarizing the adjoining vacancy atoms [START_REF] Siddique | Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures[END_REF]. Goumri-Said et al. have reported that when Eu is incorporated into the NiO matrix, it undergoes spin polarization, and hence the magnetization is increased.

Because of the presence of the Eu impurity, the magnetic moment of the Ni atoms changes such that the Ni atoms closer to the Eu atom have higher magnetic moments, and this, in turn, induces magnetic moment in oxygen atom [START_REF] Goumri-Said | Europium incorporation dynamics within NiO films deposited by sol-gel spin coating: experimental and theoretical studies[END_REF]. Thus, the magnetic property of the NiO nanoparticles can be altered not only by varying the size but also by doping with different rare earth elements.

Raman analysis

Raman spectroscopy is an effective non-destructive material characterization technique that commonly demonstrates the structural arrangements, crystallographic orientations, structural defects, doping effects, lattice disorder, etc. [START_REF] Prasad | Phase-dependent structural, optical, phonon and UV sensing properties of ZnS nanoparticles[END_REF][START_REF] Prasad | A Raman spectral probe on polar w-ZnS nanostructures and surface optical phonon modes in nanowires[END_REF][START_REF] Rani | Structural, optical and magnetic properties of NiO nanopowders[END_REF]. Moreover, Raman spectroscopy can be successfully utilized to probe the doping effect and local vibrational modes, electro-phonon coupling, spin-phonon coupling, etc. [START_REF] Aytan | Spin-phonon coupling in antiferromagnetic nickel oxide[END_REF][START_REF] Prasad | Raman spectral probe on increased local vibrational modes and phonon lifetimes in Ho3+-doped Bi2O3 micro-rods[END_REF][START_REF] Sarkar | Ultrafast Carrier Dynamics of Undoped and Ho3+-Doped α-Bismuth Oxide Microrods[END_REF]. Figure 6 shows the recorded room temperature Raman spectra of the prepared pure NiO and Ni0.95E0.5O nanostructures within the range from 250 to 1300 cm -1 . The spectra are deconvoluted using multiple Lorentzian fits. The Raman band below 600 cm -1 is generally owing to first-order phonon scattering, and the peaks above 600 cm -1 are due to second-order scattering [START_REF] George | Synthesis and characterisation of nickel oxide nanofibre webs with alcohol sensing characteristics[END_REF]. The deconvolution of the undoped NiO nanoparticles spectrum gives eight peaks (Figure 6a), which are the characteristic Raman mode vibration of face-centered cubic NiO nanostructures. In detail, the Raman bands observed at 368, 445, 537, 666, 827, 937, 1048, and 1140 cm -1 are corresponding to one phonon (1P) transverse optical (TO) modes, surface optical (SO) modes, 1P longitudinal optical (LO) mode, two-phonon (2P) TO (730), 2P LO+TO, 2P SO, the vibrational origin of 2LO and one-magnon (1M) bands respectively [START_REF] Patel | Effect of Mn doping concentration on structural, vibrational and magnetic properties of NiO nanoparticles[END_REF]. The obtained Raman active mode is in good accordance with the reported characteristic phonon mode of vibrations of intrinsic NiO nanostructures [START_REF] Duan | Size effects on properties of NiO nanoparticles grown in alkalisalts[END_REF][START_REF] Ravikumar | Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Zhao | Monolayer graphene/NiO nanosheets with two-dimension structure for supercapacitors[END_REF]. Hence, from the Raman spectral analysis, the phase purity of the prepared NiO nanoflakes can be confirmed.

The appearance of 1P SO mode suggests that the prepared nanoparticles are imperfect [START_REF] Duan | Size effects on properties of NiO nanoparticles grown in alkalisalts[END_REF].

Moreover, the appearance of 1P LO and TO are usually originated from the parity breaking defects in NiO, which arises due to the nickel vacancy concentration [START_REF] Wang | Synthesis of NiO nanorods by a novel simple precursor thermal decomposition approach[END_REF]. The presence of defects and surface-induced disorder and the imperfectness of the prepared nanoflakes can also be the reason for the appearance of 1LO [START_REF] Patel | Effect of Mn doping concentration on structural, vibrational and magnetic properties of NiO nanoparticles[END_REF]. Oftentimes, NiO exhibits a 2M band that supports the existence of the antiferromagnetic state of pure NiO nanostructures at room temperature due to the dominant superexchange interaction mechanism between the nextnearest-neighbouring Ni ions through O in the Ni 2+ -O 2--Ni 2+ linear atomic chain [START_REF] Ravikumar | Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Gandhi | Size confined magnetic phase in NiO nanoparticles[END_REF].

Ulamne et al. reported the reduction of 2M mode and complete vanishing of 2M with a decrease in particle size and for crystallite size below 100 nm, respectively [START_REF] Babu | An investigation of flower shaped NiO nanostructures by microwave and hydrothermal route[END_REF][START_REF] Mironova-Ulmane | Magnon and Phonon Excitations in Nanosized NiO[END_REF]. Finitesize effect, smaller crystallites, and disorder-induced defects can reduce spin correlation length that can be a reason for the reduction and vanishing of 2M modes in NiO nanostructures [START_REF] Duan | Size effects on properties of NiO nanoparticles grown in alkalisalts[END_REF][START_REF] Ravikumar | Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Mironova-Ulmane | Magnon and Phonon Excitations in Nanosized NiO[END_REF].

Moreover, two types of magnons associated with the spin up and spin down of two electrons can be observed in the case of bulk antiferromagnetic NiO [START_REF] George | Synthesis and characterisation of nickel oxide nanofibre webs with alcohol sensing characteristics[END_REF]. 2M is significant to probe the magnetic domain structure since it is well separated from phonon contribution and the elastic scattering line [START_REF] Mironova-Ulmane | Polarisation dependent Raman study of single-crystal nickel oxide[END_REF]. The transition from antiferromagnetic to ferromagnetic nature NiO leads to the disappearance of 2M peaks since the magnon has a strong dependence on the magnetic nature of NiO [START_REF] George | Synthesis and characterisation of nickel oxide nanofibre webs with alcohol sensing characteristics[END_REF]. Interestingly, in the present investigation, there was no sign of the appearance of the 2M band was observed in the Raman spectra of undoped NiO nanoparticles. The absence of the 2M band indicates the substantial antiferromagnetic coupling reduction with an average crystallite size of 13.45 nm and particle size ~120 nm [START_REF] Babu | An investigation of flower shaped NiO nanostructures by microwave and hydrothermal route[END_REF]. Hence, we can conclude that the absence of magnon peak in the Eu-doped NiO nanoparticles might be due to the presence of uncompensated surface spins, smaller crystallite size, disorder-induced defects that result in the reduction of spin correlation between the next nearest neighbour Ni 2+ ions in the crystal lattice and hence, the reduction in antiferromagnetic coupling strength [START_REF] Ravikumar | Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Mironova-Ulmane | Magnon and Phonon Excitations in Nanosized NiO[END_REF][START_REF] Mccluskey | Local vibrational modes of impurities in semiconductors[END_REF][START_REF] Sunny | Raman Spectral Probe on Size Dependent Surface Optical Phonon Modes and Mangnon Properties of NiO Nanoparticle[END_REF].

The deconvolution of Ni0.95E0.5O spectrum gives seven distinct peaks around 361, 537, 678, 841, 946, 1040 and 1138 cm -1 (Figure 5(b)). From the peak positions, it can be observed that the 1P TO band is shifted from 368 to 361 cm -1 when doped with Eu. This slight shift can be associated with the vacancies and structural defects of the nanostructure [START_REF] Patel | Effect of Mn doping concentration on structural, vibrational and magnetic properties of NiO nanoparticles[END_REF][START_REF] Ascencio | Study of NiO nanoparticles, structural and magnetic characteristics[END_REF]. Since doping with Eu increases the number of oxygen vacancies as noticed from XRD and XPS analysis, this observation is in good accordance with the results. Also, we can observe a small shift in the 2LO mode when doped with Eu, which can be due to a reduction in crystallite size.

Furthermore, the 1P modes of the Ni0.95E0.5O nanostructures were found to be broadened, and the Raman band from 820 to 1150 cm -1 found to have asymmetry in the lower wavenumber region as compared with the undoped NiO nanostructures. This change can be due to the reduction of phonon lifetime in the nanocrystalline regime [START_REF] Rani | Structural, optical and magnetic properties of NiO nanopowders[END_REF]. Usually, a band centered around 450-550 cm -1 that were quite rarely perceived in the Raman spectrum for Eu-doping into metal oxides [START_REF] Kolesnikov | Photoluminescence properties of Eu 3+ ions in yttrium oxide nanoparticles: defect vs. normal sites[END_REF]. This band is generally attributed to local vibrational mode (LVM), which is purely a dopant induced vibrational mode. When Nickel or oxygen lattice sites are substituted with Eu-dopant, dopant related LVM might appear in the Raman spectra of the Eu doped NiO nanoflakes along with the characteristic modes of pure NiO [START_REF] Udayabhaskar | Role of micro-strain and defects on band-gap, fluorescence in near white light emitting Sr doped ZnO nanorods[END_REF]. According to the harmonic approximation, the frequency of LVM can be written as [START_REF] Mccluskey | Local vibrational modes of impurities in semiconductors[END_REF],

= ( " " ) /% …………….. (4)
The above expression can write as in terms of wavenumber as, In the present studies, it is interesting that the SO mode of NiO nanostructures found to disappear with Eu-doping, and a remarkable increase in intensity for 1TO and 1LO modes were seen for the Ni0.95E0.5O samples along with 1LO peak broadening. Since the LVM of Eu is around 450 -550 cm -1 , it can influence the SO phonon mode around 445 cm -1 . Hence disappearance of SO phonon mode and 1LO peak broadening and enhancement in intensity may be due to the overlapping of SO modes and LVM [START_REF] Prasad | Raman spectral probe on increased local vibrational modes and phonon lifetimes in Ho3+-doped Bi2O3 micro-rods[END_REF]. The contribution of surface modes in NiO broadens 1LO mode and also the 2LO band [START_REF] Mironova-Ulmane | Magnon and Phonon Excitations in Nanosized NiO[END_REF]. Since the 1LO and 2LO

were observed in the Raman spectra, the intensity ratio of 1LO to 2LO (I2LO/I1LO) can be found to estimate the coupling strength [START_REF] Duan | Size effects on properties of NiO nanoparticles grown in alkalisalts[END_REF][START_REF] Sarkar | Ultrafast Carrier Dynamics of Undoped and Ho3+-Doped α-Bismuth Oxide Microrods[END_REF][START_REF] Ascencio | Study of NiO nanoparticles, structural and magnetic characteristics[END_REF]. The (I2LO/I1LO) ratio was found to be 0.86 and 0.43 for NiO and Ni0.95E0.5O, respectively. The ease off in the coupling strength of Ni0.95E0.5O samples in comparison with pure NiO can be due to the reduction in phonon lifetime that arises due to the defects on the surface [START_REF] Prasad | Raman spectral probe on increased local vibrational modes and phonon lifetimes in Ho3+-doped Bi2O3 micro-rods[END_REF][START_REF] Sarkar | Ultrafast Carrier Dynamics of Undoped and Ho3+-Doped α-Bismuth Oxide Microrods[END_REF]. This observation is in accordance with the results obtained from XPS analysis. Besides, with Eu doping, the peak position of the 1M shifts towards lower wavenumber was observed, which may be due to the substitution of larger mass and higher atomic number of Eu compared to the host matrix [START_REF] Gandhi | Size confined magnetic phase in NiO nanoparticles[END_REF].

This energy shift in the magnon peak can be due to the local symmetry lowering at Ni 2+ sites resulted from the chemical substitution and vacancies [START_REF] Kolesnikov | Photoluminescence properties of Eu 3+ ions in yttrium oxide nanoparticles: defect vs. normal sites[END_REF]. These outcomes are in good agreement with the obtained XRD and XPS, which provides the evidence for the chemical substitution of Eu into the NiO lattice.

Conclusion

Pure

4 .

 4 184 A ̊, for the NiO and Ni0.95E0.5O nanoparticles. With the uncertainty of ±0.004 A ̊, the obtained values are in good agreement with reported literature[START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF][START_REF] Boukhari | Improving the dielectric behavior of NiO nanoparticles by Samarium doping for electromagnetic applications[END_REF]. It can be observed that Eu doping has decreased the lattice parameter value and reduced the crystallite size by 50% as compared to the pure NiO nanoparticles. Similar results were reported when NiO nanoparticles were doped with other rare earth elements[START_REF] Boukhari | Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural and electrical investigations of pure and rare earth (Er and Pr)-doped NiO nanoparticles[END_REF][START_REF] Boukhari | Structural analysis and dielectric investigations of pure and rare earth elements (Y and Gd) doped NiO nanoparticles[END_REF][START_REF] Boukhari | Improving the dielectric behavior of NiO nanoparticles by Samarium doping for electromagnetic applications[END_REF][START_REF] Siddique | Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures[END_REF]. The decrease in size increases the surface area to volume ratio, which causes remarkable changes in the properties of the material[START_REF] Kodama | Finite size effects in antiferromagnetic NiO nanoparticles[END_REF][START_REF] Makhlouf | Magnetic anomalies in NiO nanoparticles[END_REF][START_REF] Farbod | Size dependence of optical and magnetic properties of nickel oxide nanoparticles fabricated by electric arc discharge method[END_REF][START_REF] Duan | Size effects on properties of NiO nanoparticles grown in alkalisalts[END_REF]. Rinaldi-Montes et al. have investigated the effect of NiO nanoparticles size on the TN by neutron diffraction experiment[START_REF] Rinaldi-Montes | Size effects on the Néel temperature of antiferromagnetic NiO nanoparticles[END_REF]. Their analysis shows that the TN of NiO nanoparticles decreases when the size is reduced. Popkov et al. reported that the decrease in the size of the NiO nanoparticles increases the ferromagnetic contribution.

  (b) and (d)) confirms the presence of Ni and O elements in the prepared samples and the existence of the Eu element along with Ni and O in the doped sample. The atomic % of constituent elements present in the samples are shown in the table given in the inset of each EDS spectra. The atomic percentage ratio of [Eu] to [Ni] was estimated to be 4.56 in the Ni0.95E0.5O sample, which is very close to the amount of dopant concentration (5 mol%) used during synthesis. Also, the absence of other elements confirms the phase purity of the prepared samples.

  Such broken exchange bonds give rise to many uncompensated spins and hence increases the exchange coupling. Similar results were obtained by Ponnusamy et al. and Moura, K. O., et al. when Fe-doped NiO

  -Ni and Eu-O separately contribute to the effective mass; hence the Eu dopant can occupy either Ni or O sites and produce two different LVM wavenumbers. The harmonic approximation method gives two peak positions at 485.51 cm -1 and 548.08 cm -1 correspondings to Eu-Ni and Eu-O, respectively. The dopant related vibrational mode due toEu-Ni effective mass contribution positioned around 485 cm -1 is in between TO and LO mode. Typically, the frequency of the SO phonon modes exhibits dispersion in-between long wavelength LO and TO mode. Interestingly, in the present investigation, the SO mode (445 cm -1 ) is found to disappear with Eu doping. This disappearance of SO mode with Eu doping can be due to the overlap of SO mode of NiO and LVM due to Eu-Ni effective mass contribution since both of these vibrational modes may appear in between the TO and LO mode[START_REF] Prasad | Raman spectral probe on increased local vibrational modes and phonon lifetimes in Ho3+-doped Bi2O3 micro-rods[END_REF].

  NiO and Eu-doped NiO (Ni0.95E0.5O) nanoparticles were synthesized by microwaveassisted hydrothermal method. XRD, Rietveld refinement and EDS analysis confirmed the phase, purity, and the successful substitution of Eu in NiO host material. FE-SEM analysis disclosed porous flake-like morphology in NiO and agglomerated structures in the case of Ni0.95E0.5O samples. XPS investigation revealed the presence of holes and oxygen vacancies in the prepared nanoparticles. Doping with Eu decreases the Ni 3d -O 2p hybridization and increases the concentration of defects in the nanoparticles. Magnetic measurements showed the influence of Eu-doping on the magnetic properties of the nanoparticles. NiO nanoparticles exhibit weak ferromagnetism at low fields due to exchange coupling between uncompensated surface spins and antiferromagnetic core. On doping with Eu, the coercivity and the rate of magnetization were found to increase, which might be due to the change in the number of uncompensated surface spins, reduction in particle size, shape, defects in the prepared nanoparticles, and the effect of spin polarization of the Eu atom. Raman spectroscopy analysis confirmed the presence of defects in the prepared samples. The absence of the 2M band shows antiferromagnetic coupling reduction in the samples. Eu doping produced local vibrations that overlap with the SO mode of NiO and shifting of the 1M band due to its higher mass.

Figure 1 .

 1 Figure 1. Refined x-ray diffraction pattern of (a) pure NiO and (b) Eu-doped NiO nanoparticles.

Figure 2 .

 2 Figure 2. SEM image and the corresponding EDS of (a) undoped and (b) doped NiO nanoparticles (c) and (d). Particle size distribution of undoped NiO nanoparticles.

Figure 3 .

 3 Figure 3. Core level XPS spectra of (a) Ni 2p3/2 and (b) O 1s of undoped NiO nanoparticles.

Figure 4 .

 4 Figure 4. Core level XPS spectra of (a) Ni 2p3/2, (b) O 1s and (c) Eu 3d of doped NiO nanoparticles.

Figure 5 .

 5 Figure 5. Hysteresis loops of (a) undoped, and (b) Eu doped NiO nanoparticles at room temperature.

Figure 6 .

 6 Figure 6. Room temperature Raman spectra of (a) undoped and (b) doped NiO nanoparticles.

Table 1 .

 1 Rietveld refined parameters of the prepared nanoparticles

	Sample	Rwp	Rexp	Lattice parameter (A ̊)	Crystallite size
		(%)	(%)			(nm)
				Calculated	Refined
	NiO	2.34	1.27	4.17	4.17	13.45
	Ni0.95E0.5O	2.19	1.42	4.18	4.18	6.51

Table 2 .

 2 Summary of XPS results of the prepared nanoparticles

	Sample	Element	Peak energy	Peak width	Peak Area
			(eV)	FWHM (eV)	(eV×cps)
			853.8	1.1	4613.7
		Ni 2p3/2	855.5	2.9	10159.4
			860.8	3.8	8006.0
	NiO		864.1	5.1	2366.2
			528.7	0.9	4277.7
		O 1s	530.4	1.1	947.5
			531.7	2.6	2856.3
			854.0	1.1	2245.0
		Ni 2p3/2	855.7	2.9	5769.6
			861.1	3.9	4394.3
	Ni0.95E0.5O		865.1	5.4	1419.7
		O 1s	528.8	0.9	2438.5
			530.6	1.7	1958.6
		Eu 3d	1133.6	4.5	1233.8
			1163.6	6.7	825.3

Acknowledgement

Ranjith Kumar P would like to thank the Indo-French Laboratory of Solid-State Chemistry and the CNRS for the financial support. Also, Prof. Bikramjit Basu for his support and A. Hoez, S. Gascoin and F. Veillon are acknowledged for their help during synthesis and measurements. The authors acknowledge the Micro and Nano Characterization Facility (MNCF), Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science (IISc), Bengaluru, India, for providing the characterization facilities.


Claudine Sculier

MD Delphine Taussig
email: delphine.taussig@aphp.fr

Olivier David

PhD Jerry Blustajn

PhD Leila Ayoubian

MD Julie Bonheur

MD, PhD Christine Bulteau

MD, PhD Mathilde Chipaux

Nathalie Dorison

MD Emmanuel Raffo

MD ; Olivier David

MD, PhD Sarah Ferrand- Sorbets

MD Georg Dorfmüller

MD Martine Fohlen

Focal polymicrogyria in children: Contribution of invasive explorations and epileptogenicity mapping in the surgical decision

Keywords: polymicrogyria, epilepsy, surgery, children, invasive recordings, epileptogenicity mapping

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

PMG is a malformation of cortical development, with an excessive number of abnormally small and partly fused cortical gyri with shallow sulci and abnormal cortical lamination [START_REF] Severino | Definitions and classification malformations of cortical development: practical guidelines Brain[END_REF]. PMG can be associated with other abnormalities such as corpus callosum dysgenesis, schizencephaly, periventricular and subcortical heterotopia, focal cortical dysplasia, or hemimegalencephaly [START_REF] Barkovich | A developmental and genetic classification for malformations of cortical development[END_REF]. The incidence of epilepsy is reported in up to 65-87% of cases. For most patients with PMG-related epilepsy, the onset occurs in childhood, and frequently becomes refractory to medical treatment [START_REF] Leventer | Clinical and imaging heterogeneity of polymicrogyria: a study of 328 patients[END_REF]. Patients can also present a global development delay or motor deficit, depending on the extent and localization of the lesion.

When focal, the localization is mainly perisylvian: in the largest imaging study, it is the region of maximal radiologic severity in 65% of patients [START_REF] Leventer | Clinical and imaging heterogeneity of polymicrogyria: a study of 328 patients[END_REF].

Epileptogenicity of the PMG zone appears to be heterogeneous and complex. The epileptogenic zone (EZ) most frequently involves only a part of the PMG cortex sometimes with additional remote cortical areas [START_REF] Maillard | Epilepsy surgery for polymicrogyria: a challenge to be undertaken[END_REF]. It may also fully overlap or include only remote cortical areas, particularly mesial temporal structures or associated cortical malformations [START_REF] Chassoux | Intralesional recordings and epileptogenic zone in focal polymicrogyria[END_REF], [START_REF] Ramantani | Intracranial evaluation of the epileptogenic zone in regional infrasylvian polymicrogyria[END_REF], [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF]. A well-conducted invasive intracranial study then appears warranted in most cases to delineate the EZ, while the neuroimaging studies are less reliable. Surgical treatment has not been a frequent option so far due to the extent of the malformation and the presence of functional cortex within the lesion [START_REF] Araujo | Language and motor fMRI activation in polymicrogyric cortex[END_REF], [START_REF] Chang | Predictors of seizure freedom after surgery for malformations of cortical development[END_REF]. However, recent studies, including a retrospective multicentric study of 58 patients, showed a good outcome after a complete disconnection/resection or tailored resection in well-chosen patients, with 50 to 72% achieving seizure freedom [START_REF] Chassoux | Intralesional recordings and epileptogenic zone in focal polymicrogyria[END_REF][START_REF] Chang | Predictors of seizure freedom after surgery for malformations of cortical development[END_REF], [START_REF] Ramantani | Intracranial evaluation of the epileptogenic zone in regional infrasylvian polymicrogyria[END_REF], [START_REF] Wang | Surgical management of medically refractory epilepsy in patients with polymicrogyria[END_REF], [START_REF] Cossu | Surgical treatment of polymicrogyria-related epilepsy[END_REF], [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF], [START_REF] Jalloh | The role of surgery in refractory epilepsy secondary to polymicrogyria in the pediatric population[END_REF]. All those studies included adults or a mixed population of children and adults. A single article was exclusively pediatric [START_REF] Jalloh | The role of surgery in refractory epilepsy secondary to polymicrogyria in the pediatric population[END_REF]. The surgical treatment is likely to have a better prognosis with shorter epilepsy duration [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF]. We could then assume that those patients affected by PMG-related epilepsy should be considered for surgery during childhood as early as possible.

In this paper, we present a series of pediatric patients with focal PMG-related epilepsy referred for surgical considerations. The aim of this study was to assess the contribution of invasive EEG (iEEG) and epileptogenicity mappings (EM) in the evaluation of the EZ and the concordance with PMG in order to guide the feasibility of surgery. We also focused on surgical outcome and follow-up.

Patients and methods:

Patients

From our database, we retrospectively reviewed all the patients with focal PMG-related refractory epilepsy referred for potential resective surgery between 2012 and 2019. Over the same period, a total number of 271 patients underwent a focal surgery for drug-resistant epilepsy of various etiologies. All patients were referred by several neuropediatric departments in France and abroad but evaluated in a single centre (Rothschild Foundation) and operated on by two senior neurosurgeons (GD, SFS).

Medically refractory epilepsy was defined as the failure of adequate trials of two antiepileptic drugs appropriately selected and dosed for the patient's seizure type [START_REF] Kwan | Definition of drug resistant epilepsy: consensus proposal by th ad hoc task force of the ILAE commission on therapeutic strategies[END_REF].

Inclusion criteria for our study were [START_REF] Severino | Definitions and classification malformations of cortical development: practical guidelines Brain[END_REF] Based on the results of this work-up, our multidisciplinary team discussed indications and modalities of intracranial exploration and subsequent surgery.

IEEG was used to further delineate the EZ and/or identify a critical area when presumably embedded within the polymicrogyric cortex, either with SEEG or with SDE using few depth electrodes depending on the location of the suspected EZ [START_REF] Taussig | Invasive evaluation in children (SEEG vs subdural grids)[END_REF].

A tailored surgery was guided by the results of the pre-surgical investigation, electroclinical data and structural MRI, aimed at EZ resection. Subtotal resection was intentionally undertaken if the EZ overlapped with functionally eloquent cortex. A callosotomy was offered if necessary as palliative surgery.

Seizure outcome was assessed by the Engel's score [START_REF] Engel | Outcome with respect to epileptic seizures[END_REF]. Postoperative neurological deficits and surgical complications were identified for each patient. Patients were followed for three months in our institution, with routine EEGs, MRIs and antiepileptic drug adjustments according to seizure outcomes. They then received yearly check-ups at our outpatient clinic.

Data analysis

We retrospectively analyzed the patients' charts and contacted them to get the latest outcome.

We reviewed all the video and EEG (scalp and invasive) data blindly to the neurophysiologist who wrote the report/completely blind to the neurophysiologist's report. Similarly, MRIs were blindly reviewed by a neuroradiologist experienced in epilepsy surgery. IEEG-recorded seizures were also analyzed quantitatively using post-hoc Epileptogenicity Mapping (EM) [START_REF] David | Imaging the seizure onset zone with stereoelectroencephalography[END_REF]. EM of the explored volume aims at evaluating the propensity of the implanted regions to generate significant HFOs at seizure onset (ictal HFO). The frequency band of interest was visually tailored for every subject by selecting the highest band with significant change of early ictal power in high gamma band, defined within the maximum range of [50 -170] Hz.

After co-registering of electrodes with subject's MRI and local interpolation of iEEG spectral power on a 3 mm isotropic grid, the output of the EM processing [START_REF] David | Imaging the seizure onset zone with stereoelectroencephalography[END_REF], [START_REF] Ayoubian | Epileptogenicity Mapping: A Quantitative Approach to Identify the Seizure Onset[END_REF] is an epileptogenicity map that corresponds to the statistical image of the difference of logpower of ictal HFO between seizure onset and baseline (two-sample t-test). The threshold p-value was set at 0.05, family-wise error corrected for multiple comparisons. Baseline was defined as a period of 20 s devoid of artifacts or epileptic activity selected in a range between 10 and 60 s before the onset. Duration of onset period for EM analysis was set to 4 s.

Results

From 2012 to 2019, eight consecutive patients were identified, five males and three females.

Seizure description (Table 1)

The age range at seizure onset was 3 months to 10 years (mean 40.3 months).

Seizure frequency was highly variable, from barely one per month to 10 per day. Patients presented one to three different types of seizures. All patients had focal seizures, two of them presenting two different semiologies (patients 1 and 5). The second type was asymmetric spasms which were recorded in two patients (patients 1 and 6). A third patient had a history of spasms in infancy (patient 5). The third type was atypical absences with eyelid myoclonias which were recorded in patient 6. Three patients had focal seizures with evolution to bilateral tonic-clonic seizures (patients 3, 4 and 7). All patients had refractory epilepsy, resistant to at least three antiepileptic drugs.

Neurological examination (Table 1)

Slight to moderate hemiparesis was observed in six patients (patients 1, 2, 3, 5, 6, and 7).

All were, however, independently walking. Three patients (patients 5, 6, and 7) were able to use their hand while patients 1, 2 and 3 were limited to palmar prehension. No deficit was observed in case 8 but she had a mild amyotrophy of the left hand.

Two patients (patients 3 and 8) had normal cognitive abilities with learning difficulties in patient 3. Two patients (patients 5 and 6) had a mild cognitive delay and four had a moderate delay (cases 1, 2, 4 and 7). Three patients had an autistic spectrum disorder (patients 1, 4 and 5).

Images (Tables 1 and2) MRI revealed unilateral multilobar PMG, with perisylvian involvement in five patients. It was located within the left hemisphere in four patients. In three of them, PMG involved the perisylvian region (cases 3, 5 and 6). In case 3, the PMG was more extended, also involving the central region (Figure 1, left image). One was restricted to the central region (case 2, Figure 2, left image). Four had right-hemispheric PMG: fronto-insular (case 1), perisylvian-precentral (case 4), fronto-parietal (case 7), and perisylvian-temporal (case 8).

Associated malformations included ventricular heterotopia (case 1), schizencephaly (cases 2 and 5), short corpus callosum (case 4) and focal atrophy (cases 4, 6, and 8). Six cases had no underlying etiology found. One presented an epidermal nevus syndrome with associated occipital lobe hypertrophy (case 7), and one had an unknown disease with progressive hemispheric atrophy associated with congenital PMG (case 8). 18 F-FDG-PET was performed in four of our patients: the metabolic abnormalities were more extended than the PMG in three patients (patients 3, 4, and 5) and distinct from the PMG in the fourth (patient 8).

Functional MRI (fMRI) (Table 3) showed that motricity was located within the PMG zone in patients 2 and 3 and not in patient 6. In patients 2 and 6 there was also a contralateral representation. In the three patients with left-hemispheric PMG on whom an MRI was performed, speech lateralization was on the right (patients 3, 5 and 6).

Scalp EEG (Table 1)

Scalp video-EEG monitoring lasted two to five days. Interictal and ictal discharge localizations are summarized in Table 1. The ictal onset zone assessed by scalp EEG was concordant with the PMG (patients 1, 2, and 4) or overlapped with the PMG lesion (patients 3, 6, and 7). It was different in one patient (patient 8) and not informative in patient 5.

Invasive explorations (Table 2)

The mean age during exploration was 10.8 years (range 7 to 15). The mean duration of epilepsy before invasive exploration was 6.7 years (range 4-12.5).

Ten iEEG were performed in eight patients, two of them having been explored twice.

Five patients were explored with SEEG only, one patient with SDE only (case 7) and two patients first underwent SEEG and then SDE (cases 2 and 3).

The implantation scheme and interictal and ictal abnormalities are summarized in Table 2. The interictal epileptic abnormalities were always multifocal and extended beyond the borders of the lesion, without clear limits. The EZ strictly corresponded to the lesion defined by the MRI in one case (case 2). The EZ was smaller than the lesion in two patients (patients 1 and 7). The EZ only partially involved the lesion and also included adjacent apparently normal cortex in two cases (cases 5 and 6). The EZ was different than the PMG cortex in case 8.

Three SEEG and one SDE were inconclusive either because no seizure occurred (SEEG in case 2 and SEEG and SDE in case 3) or because only unusual seizures could be recorded (SEEG in case 4). Case 2 was explored using SDE the following year and seizures were recorded.

In three cases 18 F-FDG-PET abnormalities were more extended than the EZ (patients 2, 3, and 4) and in one it was coherent with the EZ (patient 8).

Results of EM are available for seven patients and presented in Table 2. In patient 4, the EZ defined by EM is more limited than in visual analysis, but the study was limited to unusual seizures. In patients 1 and 5, the results are identical with both methods and in patients 2, 6, 7 and 8, EM showed an EZ larger than what was visually assessed

(Supplementary Figure).
Motor electrical stimulations (Tables 2 and3) were performed in all patients and additional electrical stimulations in four only. When stimulating within the polymicrogyric cortex, two patients had a motor response (patients 2 and 3), none had any speech response, and three had primary somatosensory and insular responses (patients 3, 6, and 8). In patients 3 and 8, seizures could be evoked by stimulating within the EZ (one within and one outside PMG).

Surgery (Table 4)

Five patients had surgery. Four of them had a tailored resection (patients 1, 2, 3, and 7) and patient 6 had a total callosotomy. Patient 1 had a total PMG resection in nonfunctional cortex (right fronto-insular disconnection, including motor opercular area). In patient 2, the EZ was concordant with the PMG and a total resection was advised. The motor cortex controlling the right arm had to be sacrificed, but the seizures were very disabling with falls and the child already had a moderate deficit. This decision was discussed with the patient and his parents to carefully weigh the benefits and risks. He therefore underwent a left central resection, sparing the motor cortex controlling the leg, although involved in the PMG. Unfortunately, on the post-operative MRI, the deepest part of the lesion was left and a second surgery is presently being considered (Figure 2, right image). Patient 7 underwent a right frontal disconnection, sparing the precentral gyrus. Patient 3 had a left operculo-insular resection (Figure 1, right image). This decision was based on interictal abnormalities which were clearly focal and on the evocation of regular reported seizures from electrical stimulation in the inferior part of the precentral gyrus (Table 2). This zone was checked to be nonfunctional using electrical stimulations.

Patient 6 had at least two lobes implicated in the EZ, with motor cortex largely involved, but with a very discrete hemiparesia. Aggravating the deficit was not an option, so a callosotomy was performed in order to decrease the seizures with falls.

Three patients (62.5%) were deemed ineligible for surgery in our series. Patient 5 had four lobes implicated in the EZ, as the seizures could have different starting points. A left hemispherotomy was then proposed but refused by the family, because of the expected postoperative deficit of the hand. Patient 8 had a focal EZ located in the motor cortex without any motor deficit. Patient 4 did not undergo surgery because no regular seizures were recorded. This last situation was unexpected as the family had reported frequent seizures.

We summarized the results of invasive investigations and surgery in a flow chart (Figure 3).

The sole complication that occurred was in Patient 2. He presented vascular injury during surgery with a subsequent motor deficit of the leg which worsened and then recovered within four months following surgery with an ability to walk unaided. He kept an anticipated motor deficit of the hand.

Histological results were consistent with polymicrogyria in two cases (patients 1 and 7); for patients 2 and 3 the samples were too small to properly analyze cortical architecture and showed rare dysplastic neurons (Table 4).

Genetic tests were done in four patients (2, 3, 7, and 8). They were normal in three; only one patient (patient 7) carried PIK3CA pathogenic variants (p.Glu545Lys).

Outcome (Table 4)

For the five surgical patients, the post-surgical follow-up is 32.6 months (range 7-98).

Two patients (patients 1 and 3) achieved seizure freedom and improved regarding behavior and development. Patient 1 demonstrated better social interactions and improved language skills, and patient 3 improved academically. Patient 2 remained seizure-free for three months, but then the seizures reappeared with time and became stronger with a relapse of seizures with sudden falling. One year post-operatively, the clinical and EEG conditions were comparable to the presurgical situation. No change was noticed regarding his behavior and autonomy. Patient 7 showed no improvement after surgery. In patient 6, following the callosotomy, the seizures with absences and falls completely ceased; nevertheless, daily focal seizures occurred that were much less disabling. He showed dramatic improvement regarding motor autonomy and quality of life.

Discussion

In this article, we present eight children with focal PMG-related refractory epilepsy who underwent invasive exploration and, when appropriate, surgery. Our paper presents a small series; however, it is the largest pediatric group of focal PMG cases explored with iEEG. Indeed, Cossu et al. [START_REF] Cossu | Surgical treatment of polymicrogyria-related epilepsy[END_REF] had only five children explored with SEEG, Maillard et al. [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF] two, and Jalloh et al. five children explored with SDE [START_REF] Jalloh | The role of surgery in refractory epilepsy secondary to polymicrogyria in the pediatric population[END_REF]. Moreover, we provide data about eloquent areas and their complex relationship with PMG. Lastly, it is the first article with a systematic use of EM in this pathology.

1.

Major contribution of invasive explorations in the identification of the EZ.

Surgery can be performed without preceding iEEG in two particular situations, as reported in previous studies. The first situation concerns patients with extended or hemispheric PMG and a pre-existing motor deficit. These cases can be treated successfully with a hemispherotomy [START_REF] Caraballo | Congenital hemiparesis, unilateral polymicrogyria and epilepsy with or without status epilepticus during sleep: a study of 66 patients with long-term follow-up[END_REF], [START_REF] Fohlen | Parasagittal hemispherotomy in hemispheric polymicrogyria with electrical status epilepticus during slow sleep: Indications, results and follow-up[END_REF]. The second situation where iEEG can be bypassed concerns rare cases of small-sized unilobar PMG with fully concordant electroclinical findings. These patients are candidates for a direct resection of the lesion, with or without using intraoperative corticography [START_REF] Cossu | Surgical treatment of polymicrogyria-related epilepsy[END_REF], [START_REF] Jalloh | The role of surgery in refractory epilepsy secondary to polymicrogyria in the pediatric population[END_REF], [START_REF] Maillard | Epilepsy surgery for polymicrogyria: a challenge to be undertaken[END_REF]. Apart from these situations, invasive explorations appear warranted and this was the case for all of our patients probably due to the fact that we are a tertiary centre. In our population with focal epilepsies, a direct surgery could never be considered.

Both SEEG and SDE procedures are used in this indication. The advantages of these techniques are known to be complementary and choosing one of them can be challenging [START_REF] Taussig | Invasive evaluation in children (SEEG vs subdural grids)[END_REF]. SDE offers the best coverage of the cortical convexity, which is particularly interesting in extended unilateral lesions like PMG to determine the borders of the EZ or its relationships with a functional area, in particular the speech area. This functional cortical mapping is useful in PMG with central or perisylvian localization. PMG indeed often involves motor cortex with preserved functionality but the anatomy of gyri appears malformed and hard to recognize in many cases. fMRI can help to understand the reorganization of functional cortical areas.

Bilateral motor representation is also possible [START_REF] Staudt | Searching for motor functions in dysgenic cortex: a clinical transcranial magnetic stimulation and functional magnetic resonance imaging study[END_REF]. The sampling error related to SEEG can lead to overlooking eloquent cortices and makes SDE a better option in our experience. On the other hand, SEEG allows the exploration of deeper cortical structures often involved in the PMG and is preferred to help define three-dimensional epileptic networks. This is useful in PMG when the involvement of a remote located area is suspected. The strategy for SEEG should follow the general rules, i.e. implanting the suspected EZ, the neighboring regions and the adjacent potential eloquent areas as well as the alternative hypothesis with some electrodes within the lesion [START_REF] Taussig | Invasive evaluation in children (SEEG vs subdural grids)[END_REF] . However, as PMG can be extensive, its complete exploration is not an option. Electroclinical data should play a particular role in targeting the electrodes in the parts of the PMG potentially involved in the seizures. For instance, in patient 3 we only explored a small part of the PMG.

Our study supports the heterogeneous concordance between MRI-defined PMG and the EZ as already pointed out in previous studies, ranging from 0 to 50% [START_REF] Chassoux | Intralesional recordings and epileptogenic zone in focal polymicrogyria[END_REF], [START_REF] Wang | Surgical management of medically refractory epilepsy in patients with polymicrogyria[END_REF], [START_REF] Ramantani | Intracranial evaluation of the epileptogenic zone in regional infrasylvian polymicrogyria[END_REF], [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF].

The EZ was fully concordant with the PMG lesion in only one case (12.5%), and was more extended than the PMG lesion with additional MRI-normal cortex in three cases (37.5%), comparable with 16% and 60% respectively in Maillard's series. The EZ was smaller than the PMG zone in two cases (25%) compared to 14% in Maillard's publication. Case 8 presented a very focal EZ apparently involving the motor cortex that appeared normal on MRI. The PMG was therefore not implicated in the seizure onset. Five (10%) similar cases were reported in Maillard's study. This lack of concordance was not predictable in any of our patients following only non-invasive explorations.

In PMG 18 F-FDG-PET seems to be of little help : the metabolic abnormalities showed the EZ in only 25% of our patients in whom it was performed and were so extended in the others that it was not even possible to properly explore all the involved zones. In the largest published study, PET was concordant with the EZ in only 29% of patients [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF].

Relationship between focal PMG and eloquent cortex

Coexistence of eloquent cortex and focal PMG is reported in the literature, as assessed by fMRI or electrical stimulations during invasive recording [START_REF] Araujo | Language and motor fMRI activation in polymicrogyric cortex[END_REF], [START_REF] Ramantani | Intracranial evaluation of the epileptogenic zone in regional infrasylvian polymicrogyria[END_REF], [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF]. It was for instance the case in 34% of Maillard's series. Six of our patients had a mild to moderate hemiparesis.

All together fMRI and electrical stimulations localized the motor area within the polymicrogyric zone in two patients. Regarding language evaluation, fMRI performed in cooperating patients with left-sided PMG showed a right-sided hemispheric specialization in all of them, and no speech deficits were evoked with electrical stimulation in the left hemisphere. Additionally, in three patients other functional responses were evoked with electrical stimulations. One should, however, emphasize that electrical stimulations in children, in particular with developmental delays, are properly feasible for motor function only [START_REF] Taussig | Stereoelectroencephalography (SEEG) in 65 children: an effective and safe diagnostic method for pre-surgical diagnosis, independent of age[END_REF]. When children have an important speech delay and we cannot perform either fMRI or electrical stimulations, we hypothesize a right lateralization of speech in case of lefthemispheric epilepsy. Indeed, in the event of early unilateral injury, some authors found evidence of equipotentiality of the left and right hemispheres in the development of verbal and nonverbal abilities [START_REF] Muter | A longitudinal study early intellectual development in hemiplegic children[END_REF]. Our study, although comprising only a few patients, provides clues in favor of the previously reported presence of motor functions within the PMG whereas the language functions in all our left-sided PMG patients were shifted to the right hemisphere, illustrating brain plasticity in this specific etiology.

2.

Focal PMG and surgery PMG, although being the most common brain malformation, represents only a small percentage of the patients operated on for epilepsy (in pediatric as well as in adult series). Of the 271 patients who underwent a focal surgery for drug-resistant epilepsy of various etiologies between 2012 and 2019 in our institution, only 3% were PMG patients. In a much larger series involving histopathological studies that spanned 25 years and included 2623 children operated on for all types of epilepsy, PMG was found in only 2.1% of the patients [START_REF] Blumcke | Histopathological findings in brain tissue obtained during epilepsy surgery[END_REF].So the surgical rate is very low in PMG compared with other cortical malformations and acquired pathologies. In our study, only half of the patients were eligible for focal resection, which is low compared to our overall population with various etiologies: all together, we offer surgery in 78% of patients after SEEG [START_REF] Taussig | Stereoelectroencephalography (SEEG) in 65 children: an effective and safe diagnostic method for pre-surgical diagnosis, independent of age[END_REF]. Reasons not to operate on the patients of the present series were anatomico-electrophysiological discordances and the frequent involvement of functional eloquent area. Only two patients had clinical seizures compatible with a perisylvian onset while four had seizures suggestive of a central origin, which made them less accessible to surgery and most of them had widespread interictal and ictal scalp EEG. In the literature, fewer patients with focal PMG (28 to 41%) were excluded from focal surgery after invasive explorations [START_REF] Chassoux | Intralesional recordings and epileptogenic zone in focal polymicrogyria[END_REF], [START_REF] Cossu | Surgical treatment of polymicrogyria-related epilepsy[END_REF], [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF]. This can be related to a mainly adult population (80% in Maillard's paper) and a predominance of unilobar PMG while our patients all had multilobar PMG.

Of our five surgical cases, only two (40%) achieved seizure freedom (Engel I) which is a poor outcome compared to our global results in children explored with SEEG (67% Engel 1, [START_REF] Taussig | Stereoelectroencephalography (SEEG) in 65 children: an effective and safe diagnostic method for pre-surgical diagnosis, independent of age[END_REF])

or iEEG in general (68%, [START_REF] Taussig | Invasive explorations in children younger than 3years[END_REF]). This is also low compared to the published articles, where

Engel class 1 patients range from 50 to 75% [START_REF] Chassoux | Intralesional recordings and epileptogenic zone in focal polymicrogyria[END_REF], [START_REF] Ramantani | Intracranial evaluation of the epileptogenic zone in regional infrasylvian polymicrogyria[END_REF], [START_REF] Wang | Surgical management of medically refractory epilepsy in patients with polymicrogyria[END_REF], [START_REF] Cossu | Surgical treatment of polymicrogyria-related epilepsy[END_REF], [START_REF] Maillard | Stereoelectroencephalography and surgical outcome in polymicrogyria-related epilepsy: a multicentric study[END_REF], [START_REF] Jalloh | The role of surgery in refractory epilepsy secondary to polymicrogyria in the pediatric population[END_REF]. Authors reported outcomes considering partial versus total resection of PMG. According to the literature and our personal experience, the important issue is the total removal of the EZ, and not of the radiological abnormality. In our patients, the EZ was probably not sufficiently well-defined, which could explain surgical failures. Moreover, in one of our patients with a poor outcome, the planned surgery was unfortunately not completed. EM gives clues to the insufficiency of the visual analysis, as both patients who failed surgery had discordant results between quantitative analysis and visual analysis of the EZ. This suggests that this method should be systematically used in those difficult to analyze explorations. The other explanation is the reluctance of our team to create a post-operative deficit in order to cure the patient of his epilepsy. In the above series with successful surgical outcomes, 25 to 64% of patients had a new neurological deficit after surgery.

Conclusion

Performing epilepsy surgery in pediatric patients with PMG-associated focal epilepsy is complex because of a double challenge: the first one is the difficulty in this specific etiology to adequately place the intracranial electrodes and the second is the frequent involvement of eloquent areas which complicates surgical decisions and feasibility. iEEG shows unpredictable concordance between the EZ and PMG, and frequent overlapping of

  age under 18 years, (2) diagnosis of PMG on magnetic resonance imaging (MRI), (3) refractory focal epilepsy and (4) evaluation by invasive procedure, either stereoelectroencephalography (SEEG) or subdural electrodes exploration (SDE). A comprehensive medical history of epilepsy was obtained for each patient. They had pre-exploratory as well as pre-and post-surgical clinical examinations done by a pediatric neurologist. Cognitive abilities were determined based on the clinical examination. They were classified under normal, mild, moderate or severe intellectual deficiency, according to the DSM V criteria. Every patient underwent a long-duration scalp video-EEG, the recording of all types of seizures being a requirement before iEEG. Information on localization of epileptic discharges and interictal abnormalities was collected. Characteristics of the PMG and other associated malformations were diagnosed by a neuroradiologist on structural brain MRI (3D T1 T2 Flair weighted sequences) in all cases and 18 F-FDG-PET images were analyzed and merged to MRI. If necessary and feasible, functional MRI (fMRI) for language lateralization and motricity was performed.

Table 1 :

 1 eloquent cortex, PMG and EZ exists. iEEG can lead to surgical abstention or insufficient resection because of functional risk. EM is of great help in those complex cases and should be systematically used to reach a surgical decision. Larger pediatric studies on selection of surgical candidates and invasive explorations are required. Children with polymicrogyria-related epilepsy who are candidates for surgery: clinical features, MRI and 1 8 F-FDG-PET Abbreviations: iEEG: intracranial electroencephalography F: female, M: male, R: right L: left, Y: year, PMG: polymicrogyria, ASD: autistic spectrum disorders, NA: not available

	Age at iEEG (y) Sex	Duration of	Neurological examination at the time of iEEG	Cognitive	MRI-PMG 18 F-FDG-PET
			epilepsy (y)		disabilities
	1 7	F	6.5	Moderate L hemiparesis with palmar prehension	Moderate	R Fronto-	NA
				ASD		insular
	2 15	M	9.5	Moderate R hemiparesis with palmar prehension.	Moderate	L Central	Frontal, central and
						(fronto-	temporal
						parietal)	hypometabolism
	3 9	M	4.5	Slight R arm paresis with palmar prehension.	No	L	Extended L
						Perisylvian	hypermetabolism,
						(fronto-	Frontal, central and
						parieto-	parietal
						temporal)
						and central
	4 7	M	6.5	No motor deficit	Moderate	R	R frontal, insular, central
				ASD		Perisylvian-	and anterior temporal
						insular-	hypometabolism
						precentral
	5 13	M	12.5	Slight R hemiparesis, with palmar digital prehension	Mild	L	NA
				ASD		Perisylvian-
						insular
	6 14	M	4	Slight R hemiparesis with palmar digital prehension Mild	L	NA
						Perisylvian-
						insular
	7 12.4	F	12	Moderate L hemiparesis with palmar digital	Moderate	R Fronto-	NA
				prehension		parietal
	8 9	F	4	No motor deficit. Slight L hand amyotrophy	No	R	hypometabolism of the R
						Perisylvian-	paracentral lobule
						temporal

Table 2 :

 2 Overview of invasive EEG in children with polymicrogyria-related epilepsy: schemas, invasive EEG findings and surgical resections

	P	Implantation
	a
	ti
	e
	n
	t

Abbreviations: NA: not done or not interpretable (lack of cooperation), PMG: polymicrogyria, R: right, fMRI: functional MRI.

Table 4 :

 4 surgery and follow-up

		Surgery	Histopathology Post	Engel score	Follow-up	Schooling
				operative		duration
				neurologic		(months)
				al exam
	1	Right	polymicrogyria No deficit	I	98	institution
		Fronto-		behaviour
		insular		al
		disconnectio		improvem
		n		ent
	2	Left Central	dysmorphic	Expected	IV	23	institution
		resection	neurons	right arm
				motor
				deficit R
				hemiparesi
				s following
				ischemia.
	3	Left	dysmorphic	No deficit I	7	Regular
		Operculo-	neurons				school
		insular
		resection
	6	Callosotomy		No deficit III (no	16	Special
					seizures		school
					with falls)
	7	Right Frontal	polymicrogyria Moderate	IV	19	Institution
		disconnectio		aggravatio
		n		n of L arm
				and hand

Acknowledgements:

We thank the physicians who referred the patients to epilepsy surgery.

We are indebted to our EEG technologists Mrs M. Benghezal and Mrs I. Cheramy.

We thank Mrs. Aisha Yu, native English speaker, for careful revisions of the language. Disclosures: none of the authors has any conflict of interest to disclose O David has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 616268 F-TRACT, the European Union's Horizon 2020 Framework Programme for Research and Innovation under Spcific Grant Agreement No. 785907 and No. 945539 (Human Brain Project SGA2 & SGA3)

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