Publications

2021

• Synthesis and characterization of a Sb2Te3/Bi2Te3 p-n junction heterostructure via electrodeposition in nanoporous membranes
  
  • Rani Rashmi
  • Tusseau-Nenez Sandrine
  • Coulon Pierre-Eugene
  • Wade Travis L.
  • Konczykowski Marcin
  iScience, Elsevier, 2021, 24 (6), pp.102694. (10.1016/j.isci.2021.102694)
  DOI : 10.1016/j.isci.2021.102694
• Understanding first order Raman spectra of boron carbides across the whole stoichiometry range
  
  • Roma Guido
  • Gillet Kevin
  • Jay Antoine
  • Vast Nathalie
  • Gutierrez Gaelle
  Physical Review Materials, American Physical Society, 2021, 5, pp.063601. Boron carbide, a lightweight, high temperature material, has various applications as a structural material and as a neutron absorber. The large solubility range of carbon in boron, between $sim$ 9 % and 20 %, has been theoretically explained by some of us by the thermodynamical stability of three icosahedral phases at low temperature, with respective carbon atomic concentrations: 8.7 % (B$_{10.5}$C, named OPO$_1$), 13.0 % (B$_{6.7}$C, named OPO$_2$), whose theoretical Raman spectra are still unknown, and 20 % (B4C), from which the nature of some of the Raman peaks are still debated. We report theoretical and experimental results of the first order, non-resonant, Raman spectrum of boron carbide. Density functional perturbation theory enables us to obtain the Raman spectra of the OPO$_1$ and OPO$_2$ phases, which are perfectly ordered structures with however a complex crystalline motif of 414 atoms, due to charge compensation effects. Moreover, for the carbon-rich B$_4$C, with a simpler 15-atom unit cell, we study the influence of the low energy point defects and of their concentrations on the Raman spectrum, in connection with experiments, thus providing insights into the sensitivity of experimental spectra to sample preparation, experimental conditions and setup. In particular, this enables us to propose a new structure at 19.2 % atomic carbon concentration, B$_{4.2}$C, that, within the local density approximation of density functional theory (DFT-LDA), lies very close to the convex hull of boron carbide, on the carbon-rich side. This new phase, derived from what we name the "3+1" defect complex, helps in reconciling the experimentally observed Raman spectrum with the theory around 1000 cm-1. Finally, we predict the intensity variations induced by the experimental geometry and quantitavely assess the localisation of bulk and defect vibrational modes and their character, with an analysis of "chain" and "icosahedral" modes. (10.1103/PhysRevMaterials.5.063601)
  DOI : 10.1103/PhysRevMaterials.5.063601
• Electrons irradiation of III-V//Si solar cells for NIRT conditions
  
  • Medjoubi Karim
  • Lefèvre Jérémie
  • Vauche Laura
  • Veinberg-Vidal Elias
  • Jany Christophe
  • Rostaing Cedric
  • Amalbert Vincent
  • Chabuel Fabien
  • Boizot Bruno
  • Cariou Romain
  Solar Energy Materials and Solar Cells, Elsevier, 2021, 223, pp.110975. In this study, we evaluate the potential of III-V//Si solar cell technology for space applications. We present experimental results on wafer bonded 2-terminal III-V//Si solar cells degradation under 1 MeV electrons irradiation. Beginning-Of-Life (BOL) and End Of Life (EOL) electrical performances were measured under Normal Irradiance (100% AM0) and Room Temperature (300K) conditions (NIRT). No degradation of the wafer bonding interface was detected. The impact of electron irradiation on effective diffusion length in the silicon bottom cell was calculated using the Internal Quantum Efficiency model for different solar cells architectures. Improvement paths towards higher EOL III-V//Si solar cells are discussed. (10.1016/j.solmat.2021.110975)
  DOI : 10.1016/j.solmat.2021.110975
• Exploration of Hg-based cuprate superconductors by Raman spectroscopy under hydrostatic pressure
  
  • Auvray N.
  • Loret B.
  • Chibani S.
  • Grasset R.
  • Guarnelli Y.
  • Parisiades P.
  • Forget A.
  • Colson D.
  • Cazayous M.
  • Gallais Y.
  • Sacuto A.
  Physical Review B, American Physical Society, 2021, 103 (19). The superconducting phase of the HgBa2CuO4+δ (Hg-1201) and HgBa2Ca2Cu3O8+δ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of Tcwith pressure is slower in the Hg-1223 cuprate compared to Hg-1201 due to a charge carrier concentration imbalance accentuated by pressure in Hg-1223. We find that the energy variation under pressure of the apical oxygen mode from which the charge carriers are transferred to the CuO2 layer, is the same for both the Hg-1223 and Hg-1223 cuprates and it is controlled by the interlayer compressibility. At last, we show that the binding energy of the Cooper pairs related to the maximum amplitude of the d-wave superconducting gap at the antinodes does not follow Tcwith pressure. It decreases while Tcincreases. In the particular case of Hg-1201, the binding energy collapses from 10 to 2 KBTc as the pressure increases up to 10 GPa. These direct spectroscopic observations joined to the fact that the binding energy of the Cooper pairs at the antinodes does not follow Tceither with doping, raises the question of its link with the pseudogap energy scale which follows the same trend with doping. (10.1103/PhysRevB.103.195130)
  DOI : 10.1103/PhysRevB.103.195130
• An overview of the thermal erasure mechanisms of femtosecond laser induced nanogratings in silica glass
  
  • Wang Yitao
  • Cavillon Maxime
  • Ollier Nadège
  • Poumellec Bertrand
  • Lancry Matthieu
  Physica Status Solidi A (applications and materials science), Wiley, 2021, 218, pp.2100023. The Type II modifications induced by IR femtosecond (fs) laser are used in many optical devices due to their excellent thermal stability at high temperatures (typically> 800 °C). The characteristic feature of Type II modifications is the formation of nanogratings, which can easily be detected through birefringence measurements. However, the measured birefringence is an aggregate value of multiple contributions that include form birefringence, stress-induced birefringence due to permanent volume changes, and point defects. In this work, we investigate the thermal erasure kinetics for each one of these contributions in silica glass. Firstly, we irradiate silica glass samples with a fs-laser using different conditions (polarization, energy). Secondly, we perform accelerated aging experiments to evaluate the stability of the laser-induced modifications, including defects, densification, stress field and porous nanogratings. Finally, the aforementioned contributions to the thermal stability of the nanogratings are identified and discussed using spectroscopic techniques (Raman and Rayleigh scattering, UV-Vis absorption) and electron microscopy. Moreover, porous nanogratings erasure kinetic is simulated using the Rayleigh-Plesset (R-P) equation. This work provides a valuable framework in the realization of silica glass-based optical devices operating at high temperatures (>>800 °C) by 1) evidencing the effect of annealing on each erasure mechanism and 2) providing information on the optical response (mainly the birefringence) upon annealing. (10.1002/pssa.202100023)
  DOI : 10.1002/pssa.202100023
• An Overview of the Thermal Erasure Mechanisms of Femtosecond Laser‐Induced Nanogratings in Silica Glass
  
  • Ollier N.
  Physica Status Solidi A (applications and materials science), Wiley, 2021, 218. (10.1002/pssa.202100023)
  DOI : 10.1002/pssa.202100023
• A novel approach to fabricate bioinspired programmable composite materials : the 3D Printing way
  
  • Lantean Simone
  , 2021. This thesis work deals with 4D printing. That is to say the manufacturing of 3D printed objects whose shape and/or properties vary in a controlled way in time by the application of external stimuli. In particular, the use of magnetic fields is very promising because they can be easily applied, are not dangerous for health and allow the printed object to be remotely controlled.During our thesis work, we have incorporated magnetic charges (Fe3O4) in photocurable formulations in order to print magneto-active objects. For the 3D printing process, we used the digital light processing (DLP).In a first step, we modified the reactivity of the photo-curable formulations by varying the amount of reactive diluent (butyl acrylate, BA) in the acrylic resin (Ebecryl 8232). In particular, by varying the amount of BA reagent, we were able to control the stiffness of the polymers. We then showed that the maximum charge of magnetic particles that can be dispersed in the resin was 6% by weight, which allowed us to obtain printed parts with a resolution of about 400 µm. By varying the mechanical properties of the printed composites, we exploited different movements of the printed objects: rolling and translation for rigid matrices and folding and unfolding for softer polymers.To improve the remote control of printed materials, we have exploited the self-assembly process of magnetic particles to program their microstructure. Indeed, when magnetic charges are dispersed in a liquid medium and exposed to a uniform magnetic field, they spontaneously assemble into wire structures oriented along the field lines. By dispersing the magnetic particles in a photosensitive formulation, once the desired spatial arrangement was obtained, we irradiated the formulation to "freeze" the chains of magnetic particles in the host matrix.The self-assembly process as well as the rotation of the magnetic particle chains was studied by optical microscopy. In addition, a simplified theoretical model was proposed to describe both phenomena, and numerical simulations were performed to characterize the system. The self-assembly process and the rotation of the chains were also observed in situ and at the nanometer scale by scanning X-ray microscopy on the Hermes line of the SOLEIL synchrotron. This gave us detailed information on the evolution of the microstructure in a liquid film (in 2D) as a function of the applied magnetic field.To control the microstructure of a 3D printed object, our strategy consisted in reproducing the 2D control on the magnetic chains in each printed layer of the object. This was done by modifying a DLP printer to be able to apply magnetic fields of variable intensity and direction during the printing process. By optical microscopy, we demonstrated that the proposed DLP device is effective in programming the microstructure of the composites printed in 3D.Magnetic characterizations showed that samples incorporating oriented magnetic chains have an easy magnetic axis that coincides with the orientation of the microstructure. Thanks to this property, we were able to program the macroscopic magnetic behavior of the manufactured materials.Magneto-sensitive polymers containing oriented microstructures behave like magnetic compasses. Therefore, when a uniform magnetic field is applied, the objects are rotated to align their axis of easy magnetization along the field lines. Exploiting this phenomenon, we produced objects, which depending on their mechanical properties can undergo rotation or bending. In addition, we have printed gears composed of toothed wheels or magneto-active clamps that undergo a programmed rotation by the remote application of a magnetic field.
• Paramagnetic intrinsic point defects in alkali phosphate glasses: unraveling the $P_3$ center origin and local environment effects
  
  • Giacomazzi Luigi
  • Shcheblanov Nikita S.
  • Martin-Samos Layla
  • Povarnitsyn Mikhail E.
  • Kohara Shinji
  • Valant Matjaž
  • Richard Nicolas
  • Ollier N.
  Journal of Physical Chemistry C, American Chemical Society, 2021, 125, pp.8741 - 8751. In this work, we carry out a first-principles investigation of intrinsic paramagnetic point defects in P$_2$O$_5$ and in Na$_2$O−P$_2$O$_5$ glasses as a representative of alkali phosphate glasses. Glass models are generated by combining classical molecular dynamics and Monte Carlo simulations and validated by comparing their corresponding structure factors with the available X-ray and neutron scattering experiments. We use density functional theory to calculate the electron paramagnetic resonance parameters for a large set of paramagnetic oxygen-vacancy configurations. Our investigation, also by unveiling the effect of the local environment and disorder on the hyperfine tensor, enables us to propose a new model for the much debated P$_3$ center. In particular, we establish the occurrence of two variants, which we name P$_3^a$ and P$_3^b$ centers, that are instrumental to explaining the experimental shifts of the hyperfine splittings observed in alkali phosphate glasses as a function of the alkali content x in the phosphate glass. Our scenario predicts that for low to intermediate alkali contents (0 < x < 50%), a mixture of P$_1$ and P$_3^a$ centers should be generated under irradiation. For x > 50%, essentially only P$_3^a$ and P$_3^b$ centers would be generated, while P$_1$ will be absent. Therefore, our findings, by providing an improved mapping of P centers in phosphate glasses, pave the way for fine-controlling/tuning the optical absorption in a wide range of technological applications. (10.1021/acs.jpcc.0c11281)
  DOI : 10.1021/acs.jpcc.0c11281
• Optical Absorption of Excimer Laser‐Induced Dichlorine Monoxide in Silica Glass and Excitation of Singlet Oxygen Luminescence by Energy Transfer from Chlorine Molecules
  
  • Skuja Linards
  • Ollier N.
  • Kajihara Koichi
  • Bite Ivita
  • Leimane Madara
  • Smits Krisjanis
  • Silins Andrejs
  Physica Status Solidi A (applications and materials science), Wiley, 2021, 218, pp.210009. An optical absorption (OA) band of interstitial dichlorine monoxide moleculeswith peak at 4.7 eV and halfwidth 0.94 eV is identified in F$_2$ laser-irradiated((ħω $\equiv$7.9 eV) synthetic silica glass bearing both interstitial O$_2$and Cl$_2$molecules.Alongside with intrinsic defects, this OA band can contribute to solarization ofsilica glasses produced from SiCl$_4$. Although only the formation of ClClO isconfirmed by its Raman signature, its structural isomer ClOCl may alsocontribute to this induced OA band. Thermal destruction of this band between300°C and 400°C almost completely restores the preirradiation concentration ofinterstitial Cl$_2$. An additional weak OA band at 3.5 eV is tentatively assigned to ClO$_2$ molecules. The strongly forbidden 1272 nm infrared luminescence band ofexcited singlet O$_2$ molecules is observed at 3–3.5 eV excitation, demonstrating anenergy transfer process from photoexcited triplet Cl$_2$ to O$_2$. The energy transfermost likely occurs between Cl$_2$ and O$_2$ interstitial molecules located in neigh-boring nanosized interstitial voids in the structure of SiO$_2$ glass network. (10.1002/pssa.202100009)
  DOI : 10.1002/pssa.202100009
• The physical properties of the Hall current
  
  • Faisant F.
  • Creff M.
  • Wegrowe J.-E.
  Journal of Applied Physics, American Institute of Physics, 2021, 129, pp.144501. We study the stationary state of Hall devices composed of a load circuit connected to the lateral edges of a Hall bar. We follow the approach developed in a previous work [Creff et al., J. Appl. Phys. 128, 054501 (2020)] in which the stationary state of an ideal Hall bar is defined by the minimum power dissipation principle. The presence of both the lateral circuit and the magnetic field induces the injection of a current: the so-called Hall current. Analytical expressions for the longitudinal and transverse currents are derived. It is shown that the efficiency of the power injection into the lateral circuit is quadratic in the Hall angle and obeys to the maximum transfer theorem. For usual values of the Hall angle, the main contribution of this power injection provides from the longitudinal current flowing along the edges instead of the transverse current crossing the Hall bar. (10.1063/5.0044912)
  DOI : 10.1063/5.0044912
• A dominant positron capture and annihilation at vacancies in MAPbI$_3$ and CsMAFAPb(I$_x$Br$_{1-x}$)$_3$ layers on PEDOT-PPS/ITO/glass substrates
  
  • Aversa P.
  • Kim Minjin
  • Léger V.
  • Lee H.
  • Tondelier D.
  • Plantevin Olivier
  • Botsoa J.
  • Desgardin P.
  • Bourée Jean-Eric
  • Liszkay L.
  • Dickmann M.
  • Egger W.
  • Barthe M. F.
  • Geffroy Bernard
  • Corbel C.
  , 2021. Hybrid inorganic-organic halide perovskitesattract much attention for their application in optoelectronic devices. However, the performancesstrongly depend on the quality of the active layers and their capacity to withstand device operation without irreversible damage [1,2]. Light illumination is reported to induce ion migration in HOIPs [3]. Applying a bias in dark in CH3NH3PbI3(MAPbI3) based solar cells also results in ion migration [4]. Dark current measurements give evidence of temperature–dependent charge transport mechanisms in MAPbI3that are respectively related to electron/hole and ion transport [5]. This questions the existenceand/orgeneration of defects in HOIPs and their role in defect-assisted mechanisms of ion migration under bias and light illumination on photovoltaïc performance. This work focuses on vacancy-type defects. When in neutral or negatively charged states, such defects capture thermalized positronsin their open volume and give rise to annihilation fingerprints specific to the nature of the vacancy-type defects. Positrons have a most striking reproducible and stable behavior in MAPbI3 and CsMAFAPb(IxBr1-x)3layers spin coated on PEDOT:PPS/ITO/glass substrates in similar conditions by solution growth process. The annihilation characteristics, e-_e+ annihilating pair momentum distribution and positron lifetime spectra, are consistent with huge native vacancy concentration, ≥3*1018cm-3, that efficientlycapture thermalized positrons before their annihilation. An additional noticeable property is that the coverage with a PCBM electron transport layer has little effect on these native vacancies. The positron annihilation lifetime in the vacancies, 334(5) ps,has been also earlier observed in sintered MAPbI3 pellets [6]. The nature of the vacancies and their stability with ageingis discussed.
• Electron dynamics in layered materials
  
  • Dong Jingwei
  , 2021. Currently,layered materials attract great interest due to their electrical and optical properties. Such crystals display an electronic band structure that strongly depends on the sample thickness.The large tunability of the electronic screening and gap size can be very attracting for the creation of heterostructures whose properties can be designed on demand. We can say that the research field of low dimensional materials has been boosted by the discovery of graphene and quickly has been enlarged to other materials as transition metal dichalcogenides,black phosphorous and Indium selenide.Our work will focus on the excited state dynamics in these compounds,as well as on the evolution of the band structure upon surface doping.The manuscript is organized as follow:Chapter1 provides a general introduction of layered materials.In particular,we discuss the structural and electronic properties of some relevant compounds.Chapter2 describes the principles of ultrafast spectroscopic methods and shows many applications to the case of the layered materials.We mainly focus our attention on the electron dynamics in semiconducting crystals and charge density waves systems. The electron dynamics of layered materials have been investigated by means of time-and angle-resolved photoelectron spectroscopy (TrARPES),which is a powerful tool to directly map the electronic band structure and to follow the dynamics of the electrons photoinjected via an ultrafast laser source.Chapter3 discusses the experimental technique of choice and the setup where we have been performed in the reported measurements.we begin the discussion of our original data in Chapter4.The TrARPES measurements of layered black phosphorus(BP) monitor the electronic distribution in the conduction and valence band as a function of delay time from photoexcitation.The data show that,after thermalization,the photo-injected electrons do not lead to sizable band gap renormalization,neither do they generate an appreciable amount of carrier multiplication.On the other hand,a Stark broadening of the valence band is ascribed to the inhomogeneous screening of a local potential around charge defects.Chapter5 shows time resolved ARPES data on a BP surface that is doped in-situ by means of alkali metals evaporation. We monitor the collapse of the band-gap in the accumulation layer with unmatched accuracy and we observe that the buried states detected by the low energy photons of our probing pulse acquire a surprisingly high band velocity at large dopants concentration.Chapter6 deals with the modification of hot carrier dynamics upon increasing the surface doping of BP.In this case the reported analysis is still preliminary and needs to be backed by ab-initio calculations.Chapter7 contains our work on layered ɛ-InSe.As in the case of BP,we generate an accumulation layer of varying electronic density on the surface of such semiconductor.By spanning the doping level from the semiconducting to the metallic limit,we observe that quantum screening of Longitudinal Optical phonons is not as efficient as it would be in a strictly bidimensional system,indicating a remote coupling of confined states to polar phonons of the bulk.Furthermore,we show that a 3D Fröhlich interaction with Thomas-Fermi screening can be used to mimic the effects of such a remote coupling at the ɛ-InSe surface.In Chapter8,we study the layered 1T-TaS2.This material belongs to the Charge density waves (CDW) systems and has been extensively investigated by several research groups.In 1T-TaS2,the combination of structural distortion with high electronic correlations leads to a complex and fascinating phase diagram.We could reproduce controversial data that have been recently published in the literature and that identify a new instability in proximity of the metal to-insulator transition.Finally,chapter9 summarizes the conclusions of our work and briefly discusses the perspectives of some future directions of research.
• The geometrical meaning of spinors as a key to make sense of quantum mechanics
  
  • Coddens Gerrit
  , 2021. This paper aims at explaining that the key to understanding quantum mechanics (QM) is a perfect geometrical understanding of the spinor algebra that is used in its formulation. Spinors occur naturally in the representation theory of certain symmetry groups. The spinors that are relevant for QM are those of the homogeneous Lorentz group SO(3,1) in Minkowski space-time R 4 and its subgroup SO(3) of the rotations of three-dimensional Euclidean space R 3. In the three-dimensional rotation group the spinors occur within its representation SU(2). We will provide the reader with a perfect intuitive insight about what is going on behind the scenes of the spinor algebra. We will then use the understanding acquired to derive the free-space Dirac equation from scratch proving that it is a description of a statistical ensemble of spinning electrons in uniform motion, completely in the spirit of Ballentine's statistical interpretation of QM. This is a mathematically rigorous proof. Developing this further we allow for the presence of an electromagnetic field. We can consider the result as a reconstruction of QM based on the geometrical understanding of the spinor algebra. By discussing a number of problems in the interpretation of the conventional approach, we illustrate how this new approach leads to a better understanding of QM.
• Near‐IR Radiation‐Induced Attenuation of Aluminosilicate Optical Fibers
  
  • Alessi Antonino
  • Guttilla Angela
  • Agnello Simonpietro
  • Sabatier Camille
  • Robin Thierry
  • Barnini Alexandre
  • Di Francesca Diego
  • Li Vecchi Gaetano
  • Cannas Marco
  • Boukenter Aziz
  • Ouerdane Youcef
  • Girard Sylvain
  Physica Status Solidi A (applications and materials science), Wiley, 2021, pp.2000807. (10.1002/pssa.202000807)
  DOI : 10.1002/pssa.202000807
• Amorphization of a Proposed Sorbent of Strontium, Brushite, CaHPO4•2H2O, Studied by X-ray Diffraction and Raman Spectroscopy
  
  • de Noirfontaine Marie-Noëlle
  • Garcia-Caurel Enrique
  • Funes-Hernando Daniel
  • Courtial Mireille
  • Tusseau-Nenez Sandrine
  • Cavani Olivier
  • Jdaini Jihane
  • Cau-Dit-Coumes Céline
  • Dunstetter Frédéric
  • Gorse-Pomonti Dominique
  Journal of Nuclear Materials, Elsevier, 2021, 545, pp.152751. We present a systematic study of the transformation of brushite (dicalcium phosphate dihydrate, CaHPO4•2H2O) under irradiation of electrons of well-defined energy (2.5 MeV) and flux as a function of the irradiation dose. Contrarily to model hydroxides such as portlandite and brucite, which are very resistant to electron radiation damage, the studied brushite decomposes quite easily, even for very low irradiation doses. Irradiated brushite samples were characterized using X-ray diffraction (XRD) and Raman spectroscopy to get complementary information about changes in atomic structure and chemical composition respectively under irradiation. XRD showed that irradiation causes a very limited dilatation of unit cell of crystalline brushite, which becomes progressively amorphous with increasing radiation dose. Raman spectroscopy complemented XRD results and confirmed that the transformation of brushite to the amorphous phase was not abrupt, but rather progressive. Raman spectroscopy allowed for the identification of the amorphous phase as a calcium pyrophosphate. Both techniques showed that the amorphization of brushite was not fully complete at the maximum dose used, 5.5 GGy (4 C). Interestingly, monetite phase, (CaHPO4 dicalcium phosphate), was not detected at any step of the transformation as it is the case when brushite is thermally decomposed. This study reveals the high sensitivity to electron radiation of both hydrogen bonds and protonated phosphate units in brushite, thus facilitating the transformation into pyrophosphate. The damage of brushite by energetic electrons is to be carefully considered for applications related to the use of brushite as ion-exchanger in the decontamination of effluents polluted with strontium-90, an efficient beta ray emitter. (10.1016/j.jnucmat.2020.152751)
  DOI : 10.1016/j.jnucmat.2020.152751
• Radiation Effects in Glass
  
  • Ollier Nadège
  • Girard Sylvain
  • Peuget Sylvain
  , 2021 (1). (10.1002/9781118801017.ch3.13)
  DOI : 10.1002/9781118801017.ch3.13
• Ultrashort high energy electron bunches from tunable surface plasma waves driven with laser wavefront rotation
  
  • Marini S
  • Kleij P S
  • Pisani F
  • Amiranoff F
  • Grech M
  • Macchi A
  • Raynaud Michèle
  • Riconda C
  Physical Review E, American Physical Society (APS), 2021, 103. We propose to use ultrahigh intensity laser pulses with wave-front rotation (WFR) to produce short, ultraintense surface plasma waves (SPW) on grating targets for electron acceleration. Combining a smart grating design with optimal WFR conditions identified through simple analytical modeling and particle-in-cell simulation allows us to decrease the SPW duration (down to a few optical cycles) and increase its peak amplitude. In the relativistic regime, for Iλ 2 0 = 3.4 × 10 19 W/cm 2 μm 2 , such SPW are found to accelerate high charge (few 10 s of pC), high energy (up to 70 MeV), and ultrashort (few fs) electron bunches. (10.1103/physreve.103.l021201)
  DOI : 10.1103/physreve.103.l021201
• Stabilizing the hexagonal diamond metastable phase in silicon nanowires
  
  • Béjaud R
  • Hardouin Duparc O.
  Computational Materials Science, Elsevier, 2021, 188. At the nanoscale, the proportion of atoms at the surface of solids becomes significant, which may change the equilibrium of atomic edifices. In the case of silicon, experimental observations have evidenced indeed the presence of the hexagonal diamond (HD) metastable structure in as-grown nanowires of a few nm in diameter, as if that phase could become the stable one in such small objects. We present ab initiocalculations that demonstrate the existence of stable domain for the HD structure in silicon nanowires. Surfaces of HD Si are first studied without and with hydrogen, including possible relaxations, and compared to CD Si surfaces, with a globally favourable energy ratio for HD surfaces. The energies of several plausible HD and CD Si NWs of different thicknesses are then calculated and compared to estimate their relative phase stabilities, again in favour of HD NWs, without and with hydrogen at surfaces. Analytically extrapolating theab initioresults as functions of bulk, surface and edge energy contributions, the main result is that HD Si NWs are intrinsically stable with respect to CD Si NWs for effective diameters up to only about 15 nm, for pure Si NWs as well as for surface hydrogenated NWs. Thicker HD NWs can thus only be metastable. This may explain why Si HD NWs are so difficult to grow. The diameter size limit for germanium, silicon's big brother, is three times smaller, making HD Ge NWs much less likely, in agreement with recent experimental attempts to grow Ge NWs. (10.1016/j.commatsci.2020.110180)
  DOI : 10.1016/j.commatsci.2020.110180
• Investigation of new solar cell technology III-V//Si behavior under irradiations for space applications
  
  • Medjoubi Karim
  , 2021. This work focuses on the behavior in space environment of a new photovoltaic solar cell technology: the III-V//Si tandems (2- and 3-junction), obtained by direct bonding. These cells have been exposed to electron and proton irradiations and tested in two types of environment: a) normal irradiance, 1 sun, and 300K room temperature, NIRT condition (Earth orbits) and b) low irradiance, 0.03 sun, and 120K low temperature, LILT condition (deep space). In a preliminary stage, a comparative study was conducted on 2 solar simulators, respectively equipped with a flash lamp and LED lamps, in order to ensure the reliability and reproducibility of the measurements of these multi-junctions. For the flash simulator, a tandems characterization method for I-V under 1 sun that dispense the use of isotype reference cells has been adopted, based on EQE and flash spectrum measurements. For the LED simulator, mounted in-situ on the irradiation beam, a spectrum optimization was performed in order to approach the low irradiance reference, i.e. ~3% AM0. This comparative study also allowed to establish the validity of the extrapolation by calculating I-V measurements under 1 sun towards low irradiances.Then, the compatibility of this tandem III-V//Si technology with thermal cycling on the one hand and irradiances on the other hand has been demonstrated. The bonding interface maintains its mechanical and electrical integrity face to these constraints. The impact of the irradiations on the cell performances has revealed certain similarities at 300 K and 120 K: - a marked decrease in the short-circuit current (linked to the decrease in the diffusion length) - a smaller decrease in the open-circuit voltage (generation type defects). Due to the series connection of the sub-cells, the degradation of the limiting Si (low intrinsic resistance to irradiation) dominates the behavior of the multi-junction. It has been shown that the addition of an increasing number of cells on the Si results in an increased sensitivity to irradiation; indeed, the tandem configuration restricts the absorption band of the Si to the near infrared, the spectral part most affected by the decrease in diffusion length. The use of a model based on the IQE allowed the qualification of this diffusion length degradation of the Si in tandem, as well as the damage coefficient. Unlike electrons, 1 MeV proton irradiations are at the origin of a non-homogeneous degradation in Si; by EQE measurements coupled with simulation, we have correlated this non-homogeneous degradation in Si with the position of the corresponding Bragg peak.For the low-temperature study, a linear increase in efficiency was observed up to ~150K; and below this, anomalies of I V characteristics were detected; of "S-like shape" and "flat spot" type, these defects affect the FF and thus the efficiency. Reported in the literature, these effects are characteristic of LILT conditions, and are often related to changes in the metal/semiconductor interfaces. Although significant, the LILT end-of-life electrical performance degradation of III-V//Si has been shown to be more predictable than that of III-V/Ge LILT (statistical dispersion). We have also shown that a 300 K annealing after irradiation at 120 K leads to a marked healing of the short-circuit current; this underlines the importance of in-situ characterizations to quantify cell aging under operating conditions. The Displacement Damage Dose (DDD) approach was applied for 1 MeV electrons and protons in order to compare the rate of induced degradation. This approach allows to predict the degradation of these cells whatever the fluence, particles and energy, for a space mission at 300 K.
• Ultrashort high energy electron bunches from tunable surface plasma waves driven with laser wavefront rotation
  
  • Marini S.
  • Kleij P. S
  • Pisani F.
  • Amiranoff F.
  • Grech M.
  • Macchi A.
  • Raynaud M.
  • Riconda C.
  Physical Review E, American Physical Society (APS), 2021, 103 (2). We propose to use ultra-high intensity laser pulses with wavefront rotation (WFR) to produce short, ultra-intense surface plasma waves (SPW) on grating targets for electron acceleration. Combining a smart grating design with optimal WFR conditions identified through simple analytical modeling and particle-in-cell simulation allows to decrease the SPW duration (down to few optical cycles) and increase its peak amplitude. In the relativistic regime, for Iλ 2 0 = 3.4 × 10 19 W/cm 2 µm 2 , such SPW are found to accelerate high-charge (few 10's of pC), high-energy (up to 70 MeV) and ultra-short (few fs) electron bunches. (10.1103/PhysRevE.103.L021201)
  DOI : 10.1103/PhysRevE.103.L021201
• Exact theory of the Stern-Gerlach experiment - extended version
  
  • Coddens Gerrit
  , 2021. The Stern-Gerlach experiment is notoriously counter-intuitive. The official theory is that the spin of a fermion remains always aligned with the magnetic field. Its directions are thus quantized: It can only be spin-up or spin-down. But that theory is based on mathematical errors in the way it (mis)treats spinors and group theory. We present here a mathematically rigorous theory for a fermion in a magnetic field, which is no longer counter-intuitive. It is based on an understanding of spinors in SU(2) which is only Euclidean geometry. Contrary to what Pauli has been reading into the Stern-Gerlach experiment, the spin directions are not quantized. The new corrected paradigm, which solves all conceptual problems, is that the fermions precess around the magnetic-field just as Einstein and Ehrenfest had conjectured. Surprizingly this leads to only two energy states, which should be qualified as precession-up and precession-down rather than spin-up and spin down. Indeed, despite the presence of the many different possible angles $\theta$ between the spin axis ${\mathbf{s}}$ and the magnetic field ${\mathbf{B}}$, the fermions can only have two possible energies $m_{0}c^{2}\pm\mu B$. The values $\pm\mu B$ do thus not correspond to the continuum of values $-{\boldsymbol{\mu\cdot}}{\mathbf{B}}$ Einstein and Ehrenfest had conjectured. The energy term $V= -{\boldsymbol{\mu\cdot}}{\mathbf{B}}$ is a macroscopic quantity. It is a statistical average over a large ensemble of fermions distributed over the two microscopic states with energies $\pm\mu B$, and as such not valid for individual fermions. The two fermion states with energy $\pm\mu B$ are not potential-energy states. We also explain the mathematically rigorous meaning of the up and down spinors. They represent left-handed and right-handed reference frames, such that now everything is intuitively clear and understandable in simple geometrical terms. The paradigm shift does not affect the Pauli principle.
• Crucial Role of Conjugation in Monolayer-Protected Metal Clusters with Aromatic Ligands: Insights from the Archetypal Au$_{144}$L$_{60}$ Cluster Compounds
  
  • Sinha-Roy Rajarshi
  • López-Lozano Xóchitl
  • Whetten Robert
  • Weissker H.C.
  Journal of Physical Chemistry Letters, American Chemical Society, 2021, 12 (38), pp.9262-9268. Ligand-protected metal clusters are employed in a great many applications that include notably energy conversion and biomedical uses. The interaction between the ligands and the metallic cores, mediated by an often complex interface, profoundly influences the properties of small clusters, in particular. Nonetheless, the mechanisms of interaction remain far from fully understood. The Au144L60 class of cluster compounds has long played a central role in the study of monolayer-protected clusters, but total structure determination has been achieved only recently for a thiolated and an all-alkynyl cluster. Both ligands contain aromatic rings but differ in their ligation to the metal core: conjugation along a triple bond in the latter, saturation in the former. We demonstrate the paramount importance of the conjugation in the connection between aromatic ligand rings and metal cores for the electronic and optical properties and, by extension, the critical transport properties, providing a crucial element for the development of design-principle-based synthesis. (10.1021/acs.jpclett.1c02597)
  DOI : 10.1021/acs.jpclett.1c02597
• Ultrafast electron energy-dependent delocalization dynamics in germanium selenide
  
  • Chen Zhesheng
  • Xiong Heqi
  • Zhang Hao
  • Gao Chaofeng
  • Cheng Yingchun
  • Papalazarou Evangelos
  • Perfetti Luca
  • Marsi Marino
  • Rueff Jean-Pascal
  Communications Physics, Nature Research, 2021, 4, pp.138. Ultrafast scattering process of high-energy carriers plays a key role in the performance of electronics and optoelectronics, and have been studied in several semiconductors. Core-hole clock spectroscopy is a unique technique for providing ultrafast charge transfer information with sub-femtosecond timescale. Here we demonstrate that germanium selenide (GeSe) semiconductor exhibits electronic states-dependent charge delocalization time by resonant photo exciting the core electrons to different final states using hard-x-ray photoemission spectroscopy. Thanks to the experiment geometry and the different orbital polarizations in the conduction band, the delocalization time of electron in high energy electronic state probed from Se 1s is~470 as, which is three times longer than the delocalization time of electrons located in lower energy electronic state probed from Ge 1s. Our demonstration in GeSe offers an opportunity to precisely distinguish the energy-dependent dynamics in layered semiconductor, and will pave the way to design the ultrafast devices in the future. (10.1038/s42005-021-00635-y)
  DOI : 10.1038/s42005-021-00635-y
• Synthesis of boron carbide from its elements up to 13 GPa
  
  • Chakraborti Amrita
  • Guignot Nicolas
  • Vast Nathalie
  • Le Godec Yann
  Journal of Physics and Chemistry of Solids, Elsevier, 2021, 159, pp.110253. The synthesis of boron carbide from its elements (boron and carbon) has been studied under pressures up to 13 GPa and optimum parameters have been determined by varying the (P, T, reactants) conditions. Stoichiometric mixtures of amorphous boron and amorphous carbon have been subjected to a range of temperatures from 1673 K to 2273 K at the pressure values of 2 GPa and 5 GPa. The formation temperatures have been compared to those obtained from mixtures of β rhombohedral boron and graphite, and β rhombohedral boron and amorphous carbon at 2 GPa and 5 GPa. The formation temperature is thus shown to be affected by the pressure and the choice of the reactants. The carbon concentration of boron carbide is also shown to be affected by the pressure at which it is synthesised from elements, and we propose pressure as a means to control the carbon content. Temperature cycling has also been shown to reduce the formation temperature of boron carbide at 13 GPa. The formation of α rhombohedral boron as an intermediate phase is seen at 5 GPa before the formation of boron carbide. (10.1016/j.jpcs.2021.110253)
  DOI : 10.1016/j.jpcs.2021.110253
• H2 production under gamma irradiation of a calcium aluminate cement: an experimental study on both cement pastes and its stable hydrates
  
  • Acher Loren
  • de Noirfontaine Marie-Noëlle
  • Chartier David
  • Gorse -Pomonti Dominique
  • Courtial Mireille
  • Tusseau-Nenez Sandrine
  • Cavani Olivier
  • Haas Jérémy
  • Dannoux- Papin Adeline
  • Dunstetter Frédéric
  Radiation Physics and Chemistry, Elsevier, 2021, 189, pp.109689. The objective of this paper is to investigate the use of calcium aluminate cements as alternative cements within the context of nuclear waste stabilization by solidification. Using an external 60 Co source, the effect of γ-radiation on H2 gas production of one of the calcium aluminate cement-based materials (cement "Ciment Fondu") and its stable hydrates, was studied. The amount of H2 produced by these cement pastes is found to be much lower (up to five times less) than that of the Portland cement pastes containing the same amount of water, especially in the low range of water to cement ratios (W/C ≤ 0.4) where water is essentially engaged in the hydrates. The H2 production of the two major hydrates of Ciment Fondu, gibbsite AH3 and katoite hydrogarnet C3AH6, is very low compared with that of the main hydrates of other cements (Portland cement, Calcium Sulfo-Aluminate and Magnesium Phosphate cements). The type of water engaged in the hydrates, as hydroxyl groups and/or molecular water, influences significantly the H2 production. Thus, the nature of the hydrate is a key parameter to the aim of optimizing cement matrices with respect to the gas production under irradiation. XRD analysis shows that the crystal structures of gibbsite and katoite are preserved up to very high doses under electron irradiation (3 GGy). This makes calcium aluminate cements (CAC) potential good candidates for nuclear waste conditioning from the point of view of their stability under irradiation. (10.1016/j.radphyschem.2021.109689)
  DOI : 10.1016/j.radphyschem.2021.109689