Publications

2018

• Comments on “Nanoscale Investigation of Carrier Lifetime on the Cross Section of Epitaxial Silicon Solar Cells Using Kelvin Probe Force Microscopy”
  
  • Garrillo Pablo
  • Narchi Paul
  • Cabarrocas Pere Roca I
  • Grevin Benjamin
  • Borowik Lukasz
  IEEE Journal of Photovoltaics, IEEE, 2018, 8 (2), pp.661-663. (10.1109/JPHOTOV.2018.2793760)
  DOI : 10.1109/JPHOTOV.2018.2793760
• Feedback of a one year small electric vehicle use in the Paris suburb
  
  • Bourdin Vincent
  • Badosa Jordi
  • Migan-Dubois Anne
  • Calderón Obaldía Fausto
  • Bonnassieux Yvan
  , 2018.
• Kinetic theory of two-temperature polyatomic plasmas
  
  • Orlac’h Jean-Maxime
  • Giovangigli Vincent
  • Novikova Tatiana
  • Roca I Cabarrocas Pere
  Physica A: Statistical Mechanics and its Applications, Elsevier, 2018, 494, pp.503-546. (10.1016/j.physa.2017.11.151)
  DOI : 10.1016/j.physa.2017.11.151
• Tight-Binding modeling of CsPbI3 in several perovskite phases
  
  • Boyer-Richard S.
  • Pedesseau Laurent
  • Marronnier Arthur
  • Roma Guido
  • Traoré Boubacar
  • Katan Claudine
  • Bonnassieux Yvan
  • Jancu Jean-Marc
  • Seshadri Ram
  • Stoumpos Constantinos C
  • Kanatzidis Mercouri G
  • Even Jacky
  , 2018. Among perovskite solar cells, fully inorganic perovskite solar cells can compete as a stable and efficient alternative to hybrid cousins, with the recent report of a 13.4% efficient CsPbI3 perovskite quantum dot solar cell. Using synchrotron X-ray powder diffraction (SXRD), the detailed experimental structures of the three perovskite-phases of CsPbI3 (gamma at 325K, beta at 510K and alpha at 645K) have been recently reported. Based on these experimental results, we investigate the electronic properties of all three phases using a recently developed symmetry-based tight-binding (TB) model [1] as well as DFT calculations including both spin-orbit coupling (SOC) and many-body (GW approximation) effects.In fact, TB models afford an atomic-scale description to computing various properties, including distorted structures, at a significantly reduced computational cost compared to first-principles approaches. It allows tackling more difficult issues in terms of size, with complex heterostructures, nanostructures or composite materials, as well as properties with a great diversity of physical phenomena.The original empirical sp3 tight-binding (TB) model has been built for the reference Pm3m cubic phase of halide perovskite structures of general formula ABX3 and the TB parameters have been calibrated using available experimental and theoretical data for MAPbI3 (MA=CH3NH3+) [1]. To investigate less symmetric structure, such as the beta and gamma phases of CsPbI3, we show that this model can be extended by means of a simple d-2 Harrisson law, which allows tackling bond length variation. Given that very few experimental results are available on these phases of CsPbI3, we used the initial parameterization performed on MAPbI3. In order to gauge the quality and performances of this TB model, we further performed DFT calculations including both self-consistent GW corrections and relativistic effects (SOC). The band structure obtained with our TB model agrees nicely with results obtained from first-principles calculations. The model is then further exploited to inspect effect related to anharmonicity and results support the hypothesis of dynamical Rashba effects in CsPbI3. We may expect that such a model will be as relevant to the future of perovskite device modeling, as it has proved efficient for conventional semiconductors. (10.29363/nanoge.abxpvperopto.2018.046)
  DOI : 10.29363/nanoge.abxpvperopto.2018.046
• Evidence of halide ion migration in CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$ based perovskite solar cells and its effect on current-voltage hysteresis
  
  • Lee Heejae
  • Gaiaschi Sofia
  • Chapon Patrick
  • Marronnier Arthur
  • Tondelier Denis
  • Bonnassieux Yvan
  • Bourée Jean-Eric
  • Geffroy Bernard
  , 2018. Hybride perovskite solar cells (PSCs) have rapidly emerged as a promising candidate for the next generation photovoltaics with power conversion efficiencies (PCEs) attaining 22%. Low temperature solution processing, low cost raw material and relative insensitivity to intrinsic point defects are some of the attractive qualities of this emerging class of devices. But one of the major obstacles for the commercialization of PSCs lies in the long-term stability of the perovskite films subjected to different environmental conditions such as temperature, humidity and illumination. In this work, we focused on experimental evidence of halide ion migration in CH$_3$ NH$_3$ PbI$_{3-x}$ Cl$_x$ based solar cells and its effect on current-voltage hysteresis for which various mechanisms have been proposed. The inverted planar structure adopted for the PSCs was: glass/ITO/PEDOT:PSS/perovskite/PCBM/Ag. The perovskite thin films were deposited by 1-step spin-casting process and the organic PEDOT:PSS (hole-transporting layer) and PCBM (electron-transporting layer) layers were deposited by spin-coating process. Firstly the PCE under 1 sun equivalent illumination reached 12.7% for the best cell of a series of 10 samples with an active area of 0.28 cm$^2$. The J-V hysteresis effect was small (less than 2.5%) between the reverse and the forward direction, consistent with the results reported in the literature. Secondly, using glow discharge optical emission spectrometry (GD-OES), a spectrochemical technique allowing direct determination of major and trace elements, we have shown that halide ions (I-and Cl-) migrate inside the perovskite films under an applied bias in both directions, the time of migration being typically 2 min. Furthermore no migration of lead and nitrogen ions was observed in the same time scale. Thirdly we observed the hysteresis of current-voltage characteristics under dark conditions (thus without any photo-generated carriers) versus voltage scanning rate and temperature. The activation energy value of 0.253 eV derived from the Nernst-Einstein relation above 264 K, for which the perovskite phase is tetragonal, indicates that the conduction is dominated by the ions (instead of electrons for conventional semiconductors) and furthermore confirms that the conduction is ascribed to the migration of anion vacancies, which is well known in the perovskite-type halides such as CsPbCl$_3$ or CsPbBr$_3$. These experiments prove that there is a direct link between halide ion migrations in CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$ based perovskite thin films and current-voltage hysteresis.
• Effect of Defect Production on Photoluminescence Properties in He ion Implanted Methylammonium Lead Tri-Iodide Perovskite Layers
  
  • Aversa Pierfrancesco
  • Lee Heejae
  • Kim Minjin
  • Plantevin Olivier
  • Cavani Olivier
  • Ollier Nadège
  • Geffroy Bernard
  • Corbel Catherine
  , 2018.
• Factors influencing readthrough therapy for frequent cystic fibrosis premature termination codons
  
  • Pranke Iwona
  • Bidou Laure
  • Martin Natacha
  • Blanchet Sandra
  • Hatton Aurélie
  • Karri Sabrina
  • Cornu David
  • Costes Bruno
  • Chevalier Benoit
  • Tondelier Danielle
  • Girodon Emmanuelle
  • Coupet Matthieu
  • Edelman Aleksander
  • Fanen Pascale
  • Namy Olivier
  • Sermet-Gaudelus Isabelle
  • Hinzpeter Alexandre
  ERJ Open Research, European Respiratory Society, 2018, 4 (1), pp.00080-2017. Premature termination codons (PTCs) are generally associated with severe forms of genetic diseases. Readthrough of in-frame PTCs using small molecules is a promising therapeutic approach. Nonetheless, the outcome of preclinical studies has been low and variable. Treatment efficacy depends on: 1) the level of drug-induced readthrough, 2) the amount of target transcripts, and 3) the activity of the recoded protein. The aim of the present study was to identify, in the cystic fibrosis transmembrane conductance regulator (CFTR) model, recoded channels from readthrough therapy that may be enhanced using CFTR modulators. First, drug-induced readthrough of 15 PTCs was measured using a dual reporter system under basal conditions and in response to gentamicin and negamycin. Secondly, exon skipping associated with these PTCs was evaluated with a minigene system. Finally, incorporated amino acids were identified by mass spectrometry and the function of the predicted recoded CFTR channels corresponding to these 15 PTCs was measured. Nonfunctional channels were subjected to CFTR-directed ivacaftor-lumacaftor treatments. The results demonstrated that CFTR modulators increased activity of recoded channels, which could also be confirmed in cells derived from a patient. In conclusion, this work will provide a framework to adapt treatments to the patient's genotype by identifying the most efficient molecule for each PTC and the recoded channels needing co-therapies to rescue channel function. (10.1183/23120541.00080-2017)
  DOI : 10.1183/23120541.00080-2017
• Symmetric decomposition of experimental depolarizing Mueller matrices in the degenerate case
  
  • Vizet Jérémy
  • Ossikovski Razvigor
  Applied optics, Optical Society of America, 2018, 57 (5), pp.1159. (10.1364/AO.57.001159)
  DOI : 10.1364/AO.57.001159
• Electrical and optical degradation study of methylammonium-based perovskite materials under ambient conditions
  
  • Marronnier Arthur
  • Lee Heeryung
  • Lee Heejae
  • Kim Minjin
  • Eypert Céline
  • Gaston Jean-Paul
  • Roma Guido
  • Tondelier Denis
  • Geffroy Bernard
  • Bonnassieux Yvan
  Solar Energy Materials and Solar Cells, Elsevier, 2018, 178, pp.179 - 185. Hybrid perovskites have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells which combine the advantages of organic and inorganic semiconductors. One of the main obstacles to their commercialization is their poor stability under light, humidity, oxygen, and high temperatures. In this work, we compare the optical and the electrical light-induced degradation of CH 3 NH 3 PbI 3 (“MAPI”)-based solar cells using real-time ellipsometry measurements, electrical measurements and X-Ray Diffraction (XRD) techniques. We evidence that while the electrical degradation takes place on a short time scale (2–3 days of exposure to ambient light conditions in a nitrogen atmosphere), no optical degradation is observed before 10 days when the dissociation reaction of methylammonium lead iodide starts acting. We find a very good agreement between XRD and ellipsometry measurements; both show the appearance of PbI 2 after 1 week of exposure. We also confirm that the main mechanism at play is a light-induced degradation affecting the edges of the stack and the interfaces between the perovskite and the neighbouring layers. Last, a very good match is obtained on the optical constants of MAPI between our ellipsometry measurements and density functional theory calculations we performed, and we confirm the behavior of MAPI as an inorganic semiconductor. (10.1016/j.solmat.2018.01.020)
  DOI : 10.1016/j.solmat.2018.01.020
• Optical properties and performance of pyramidal texture silicon heterojunction solar cells: Key role of vertex angles
  
  • Mrazkova Zuzana
  • Sobkowicz Igor Paul
  • Foldyna Martin
  • Postava Kamil
  • Florea Ileana
  • Pištora Jaromír
  • Roca I Cabarrocas Pere
  Progress in Photovoltaics, Wiley, 2018, 26 (6), pp.369 - 376. (10.1002/pip.2994)
  DOI : 10.1002/pip.2994
• Analysis of Current-Voltage Hysteresis and Ageing Characteristics for CH3NH3PbI3-xClxBased Perovskite Thin Film Solar Cells
  
  • Lee Heejae
  , 2018. Organic-inorganic lead halide perovskites are very promising materials for the next generation of solar cells with intrinsic advantages such as a low-cost material due to the availability of source materials and low-temperature solution processing as well as a high power conversion efficiency of the sunlight. However, perovskite solar cells are still unstable and show deleterious current-voltage hysteresis effects. Inthis thesis, analyses of CH3NH3PbI3-xClx based perovskite thin films and solar cells are presented. The electrical transport characteristics and the ageing processes are investigated using different approaches.The synthesis of the halide perovskite materials is optimized in a first step by controlling the deposition conditions such as annealing temperature (80°C) and spinning rate (6000 rpm) in the one step-spin-casted process. CH3NH3PbI3-xClx based perovskite solar cells are then fabricated in the inverted planar structure and characterized optically and electrically in a second step.Direct experimental evidence of the motion of the halide ions under an applied voltage has been observed using glow discharge optical emission spectroscopy (GDOES). Ionic diffusion length of 140 nm and ratio of mobile iodide ions of 65 % have been deduced. It is shown that the current-voltage hysteresis in the dark is strongly affected by the halide migration which causes a substantial screening of the applied electric field. Thus we have found a shift of voltage at zero current (< 0.25 V) and a leakage current (< 0.1 mA/cm2) in the dark versus measurement condition. Through the current-voltage curves as a function of temperature we have identified the freezing temperature of the mobile iodides at 260K. Using the Nernst-Einstein equation we have deduced a value of 0.253 eV for the activation energy of the mobile ions.Finally, the ageing process of the solar cell has been investigated with optical and electrical measurements. We deduced that the ageing process appear at first at the perovskite grain surface and boundaries. The electrical characteristics are degraded through a deterioration of the silver top-electrode due to the diffusion of iodides toward the silver as shown by GDOES analysis.
• Modélisation multi-échelle de l'insertion du 3H et du 36Cl dans les graphites UNGG
  
  • Lechner Christoph
  , 2018. Au cours des prochaines années, neuf centrales nucléaires de type UNGG (Uranium Naturel Graphite Gaz) devront être démantelées en France. Ces centrales utilisent le graphite comme modérateur et réflecteur de neutrons. Pendant leur exploitation, celui-ci est activé. Leur démantèlement conduira à 23000 tonnes de déchets de graphite irradiés à gérer. Ce travail focalise sur deux radionucléides contenus dans ces déchets : le 36Cl et le 3H. Le 36Cl a l'une des demi-vies les plus longues (301 000 ans). Par contre, le 3H a une demi-vie plus courte (12 ans), mais contribue beaucoup à l'activité initiale des déchets. Différentes données expérimentales suggèrent que le 36Cl et le 3H sont piégés à différents endroits du graphite, comme les boucles de dislocation, les surfaces ou les joints de grains. Le seul mécanisme de migration des radionucléides est le relâchement. Pour cette raison, il est important de comprendre quels sont les pièges et les différentes conditions du relâchement.Le graphite UNGG a une structure complexe, hétérogène et multi-échelle qui diffère du monocristal idéal du graphite. Cependant, pour comprendre les données macroscopiques, les études théoriques à l'échelle nanoscopique et microscopique sont des outils importants, même si elles reposent sur des modèles plus simples. Dans cette thèse, une approche multi-échelle a été utilisée afin d’étudier les interactions des radionucléides avec le graphite ainsi que les mécanismes de diffusion et de piégeage à l'échelle du nm-μm.Les interactions du 3H et du 36Cl avec différents défauts du graphite ont été étudiées dans le cadre de la théorie fonctionnelle de la densité (DFT). L'hydrogène forme une liaison covalente avec le graphite massique ainsi qu'avec ses surfaces (001), (100) et (110). Plusieurs reconstructions de surface ont été explorées. Les résultats montrent que les hypothèses existantes sur le piégeage de l'hydrogène doivent être affinées. Le comportement du Cl est plus complexe. Sa chimisorption est observée sur les surfaces (100) et (110). Cependant, sur la surface (001), le Cl interagit par transfert de charge. Le Cl2 n'interagit que par interactions de van der Waals avec celle-ci. Le Cl2 se dissocie dans le graphite massique.Les diffusions du H et du Cl dans le graphite irradié ont été étudiées en effectuant des simulations de dynamique moléculaire. Les résultats ab initio ont été utilisés pour développer des potentiels de type « bond order » afin de modéliser l'interaction des radionucléides avec la matrice de graphite, qui possède des contributions à court et à long portée. Pour le Cl, un nouveau potentiel a été paramétré qui reproduit toutes les données obtenues au niveau DFT. Pour les interactions 3H-graphite, les potentiels AIREBO/M, pour les interactions C-H, et LCBOP, pour les interactions C-C, ont été utilisés.Pour évaluer l'influence de la structure complexe du graphite UNGG sur le comportement des radionucléides, plusieurs modèles atomiques ont été utilisés pour rendre compte de cette diversité, tels que les surfaces, les joints de grains et les nanopores.Pour le Cl, des simulations d'irradiation ont été réalisées pour une gamme d’énergie allant de 1 à 10 keV et une gamme de température de 200 à 500ºC. Les dépendances à la température et à la direction d'irradiation ont été étudiées. D’une façon générale, les dommages causés par l'irradiation perpendiculaire aux surfaces augmentent avec la température. L'irradiation à des angles d’incidence <90º aux surfaces peut causer plus ou moins de dommages par rapport à l'irradiation perpendiculaire selon le type de surface.Les diffusions du H et du Cl montrent que tous les bords de cristallites avec des liaisons pendantes sont des pièges. Pour le Cl, la diffusion dans le graphite nanoporeux a révélé deux emplacements préférés: les bords des cristallites où le Cl forme une liaison covalente et les coins des microfissures où le Cl interagit par transfert de charge.
• III-V/Si tandem solar cells : an inverted metamorphic approach using low temperature PECVD of c-Si(Ge)
  
  • Hamon Gwenaëlle
  , 2018. Combining Silicon with III-V materials represents a promising pathway to overcome the ≈29% efficiency limit of a single c-Si solar cell. While the standard approach is to grow III-V materials on Si, this work deals with an innovative way of fabricating tandem solar cells. We use an inverted metamorphic approach in which crystalline silicon or SiGe is directly grown on III-V materials by PECVD. The low temperature of this process (<200 °C) reduces the usual thermal expansion problems, and growing the group IV material on the III-V prevents polarity issues.The realization of the final tandem solar cell made of SiGe/AlGaAs requires the development and optimization of various building blocks. First, we develop the epitaxy at 175°C of Si(Ge) on (100) Si substrates in an industrial standard RF-PECVD reactor. We prove the promising electrical performances of such grown Si(Ge) by realizing PIN heterojunction solar cells with 1.5µm epitaxial absorber leading to a Voc up to 0.57 V. We show that the incorporation of Ge in the layer increases the Jsc from 15.4 up to 16.6 A/cm2 (SiGe28%).Meanwhile, we develop the growth of AlGaAs solar cells by MOVPE and its process flow. We reach an efficiency of 17.6 % for a single Al0.22GaAs solar cell. We then develop the tunnel junction (TJ), essential part of a tandem solar cell with 2-terminal integration. We develop the growth of n-doped GaAs with DIPTe precursor to fabricate TJs with peak tunneling currents up to 3000 A/cm2, reaching state-of-the art TJs.Then, the hetero-epitaxy of Si on GaAs by PECVD is studied. c-Si exhibits excellent structural properties, and the first stages of the growth are investigated by X-ray diffraction with synchrotron beam. We find an unexpected behavior: the grown Si is fully relaxed, but tetragonal. While the GaAs lattice parameter is higher than silicon one, we find a higher out-of-plane Si parameter (a⏊) than in-plane (a//), contradicting the common rules of hetero-epitaxy. We find a strong correlation between this tetragonal behavior and the presence of hydrogen in the Si layer. We furthermore show that hydrogen also plays a strong role in GaAs: the doping level of GaAs is decreased by one order of magnitude when exposed to a H2 plasma, due to the formation of complexes between H and the dopants (C, Te, Si). This behavior can be recovered after annealing at 350°C.Finally, the last step of device fabrication is studied: the bonding. We successfully bonded an inverted AlGaAs cell, removed it from its substrate, and processed a full 2” wafer. We succeeded in growing our first tandem solar cells by growing thick layers (>1 µm) of Si on an inverted AlGaAs solar cells followed by a TJ. The bonding and process of this final device is then performed, leading, as a next step, to the electrical measurement of the very first tandem solar cell grown by inverted metamorphic growth of Si on III-V.
• Deposition of hydrogenated silicon clusters for efficient epitaxial growth
  
  • Le Ha-Linh Thi
  • Jardali Fatme
  • Vach Holger
  Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2018. Epitaxial silicon thin films grown from the deposition of plasma-born hydrogenated silicon nanoparticles using plasma enhanced chemical vapor deposition techniques have widely been investigated due to their potential applications in photovoltaic and nanoelectronic device technologies. However, the optimal experimental conditions and the underlying growth mechanisms leading to high-speed epitaxial growth of thin silicon films from hydrogenated silicon nanoparticles remain far from being understood. In the present work, extensive molecular dynamics simulations were performed to study the epitaxial growth of silicon thin films resulting from the deposition of plasma-born hydrogenated silicon clusters at low substrate temperatures under realistic reactor conditions. Strong evidence is presented that a temporary phase transition of the substrate area around the cluster impact site to the liquid state is necessary for the epitaxial growth to take place. We predict further that a non-normal incidence angle for the cluster impact significantly facilitates the epitaxial growth of thin crystalline silicon films. (10.1039/c8cp00764k)
  DOI : 10.1039/c8cp00764k
• Novel three-dimensional carbon nanotube networks as high performance thermal interface materials
  
  • Kong Qinyu
  • Bodelot Laurence
  • Lebental Bérengère
  • Lim Yu Dian
  • Shiau Li Lynn
  • Gusarov Boris
  • Tan Chong Wei
  • Liang Kun
  • Lu Congxiang
  • Tan Chuan Seng
  • Coquet Philippe
  • Tay Beng Kang
  Carbon, Elsevier, 2018, 132, pp.359-369. Vertically aligned carbon nanotube (VACNT) arrays are considered as promising thermal interface materials (TIMs) due to their superior out-of-plane thermal conductivities. However the air gaps between adjacent CNTs within the CNT array hinder the in-plane heat transfer, thus significantly degrading the thermal performance of VACNT-based TIMs. To improve the in-plane thermal conduction of VACNT arrays, we propose a novel three dimensional CNT (3D CNT) network structure, where VACNTs are cross-linked by randomly-oriented secondary CNTs. Three different catalyst preparation methods for the secondary CNT growth are compared in terms of their ability to produce a dense network of secondary CNTs. The 3D CNT network grown using the chemical impregnation method shows a denser network structure, and thus is chosen for further thermal characterization. The temperature fields of the corresponding 3D CNT network under different heating powers are recorded using a 15 Œm-resolution infrared thermal imaging system. The in-plane thermal conductivity is then derived from these fields using numerical fitting with a 3D heat diffusion model. We find that the in-plane thermal conductivity of the 3D CNT network is 5.40 ± 0.92 W/mK, at least 30 times higher than the thermal conductivity of the primary VACNT array used to grow the 3D CNT network. (10.1016/j.carbon.2018.02.052)
  DOI : 10.1016/j.carbon.2018.02.052
• First-Principles Investigation of the Coupling-Induced Dissociation of Methane and its Transformation to Ethane and Ethylene
  
  • Varghese Jithin John
  • Saravanan Bharathi
  • Vach Holger
  • Peslherbe Gilles H.
  • Mushrif Samir H
  Chemical Physics Letters, Elsevier, 2018. Quantum chemical computations predict that compression of the methane dimer to an inter-nuclear separation lower than 2 Å facilitates a concerted coupling and dissociation of C-H bonds of the molecules to form ethane/ethylene. In this bimolecular, concerted mechanism, ethane formation is accompanied by production of H radicals from each methane moiety that may further abstract hydrogen atoms to lead to ethylene formation. Alternatively, transformation to ethane and ethylene proceeds via stepwise molecular hydrogen elimination, with the first eliminated hydrogen molecule originating from one of the methane moieties, accompanied by an intermolecular hydrogen transfer, and the second originating from both methyl groups.
• Conservation of the piezoelectric response of PVDF films under irradiation
  
  • Melilli G.
  • Lairez D.
  • Gorse D.
  • Garcia-Caurel E.
  • Peinado A.
  • Cavani O.
  • Boizot B.
  • Clochard M.-C.
  Radiation Physics and Chemistry, Elsevier, 2018, 142, pp.54-59. (10.1016/j.radphyschem.2017.03.035)
  DOI : 10.1016/j.radphyschem.2017.03.035
• Te doping of GaAs and GaInP using diisopropyl telluride (DIPTe) for tunnel junction applications
  
  • Hamon G.
  • Paillet N.
  • Alvarez J
  • Larrue A.
  • Decobert J.
  Journal of Crystal Growth, Elsevier, 2018, 498, pp.301-306. In this work, we have investigated the growth of highly n-doped gallium arsenide (GaAs) and gallium indium phosphide (GaInP) with tellurium (Te) by metal organic vapor phase epitaxy (MOVPE) using diisopropyl telluride (DIPTe), aiming at fabricating high performances tunnel junctions. A parametric study is performed in order to optimize the n++-type doping. Concentrations above 2.7 × 1019 cm−3 were achieved in both GaAs and GaInP layers. Using these Te-doped layers, we fabricated both n on p (n/p) and p on n (p/n) tunnel junctions. The p/n tunnel junction required additional annealing steps during growth, due to memory effect and surfactant properties of Te. We characterized GaAs/GaAs, GaAs/AlGaAs and AlGaAs/GaInP tunnel junctions with peak tunneling current densities as high as 250, 3000 and 1500 A/cm2 respectively. These tunnel junction performances are suitable for multijunction solar cells operating under high concentration. © 2018 (10.1016/j.jcrysgro.2018.07.003)
  DOI : 10.1016/j.jcrysgro.2018.07.003
• La quête des températures ultrabasses
  
  • Suchet Daniel
  , 2018. Des vendeurs de glace à l'industrie cryogénique, le froid est souvent perçu comme un produit. Pour le physicien, le froid, réduction de l'agitation aléatoire des particules, apparaît plutôt comme une force productrice, capable de faire émerger des structures complexes et de métamorphoser les propriétés des objets. Les basses températures ont été l'enjeu d'une course effrénée dont la première phase a pris fin en 1908 avec la liquéfaction de l'ensemble des gaz connus. Loin de clore l'aventure, cette étape a fait surgir des nouveaux états de la matière qui ne peuvent être interprétés que par la mécanique quantique. Pour étudier la supraconductivité, la superfluidité ou la condensation de Bose-Einstein, les expériences actuelles visent des températures dites ultrabasses, à quelques milliardièmes de degré au-dessus du zéro absolu. Cette présentation propose une mise en perspective historique de la recherche des très basses températures et une introduction aux expériences actuelles d'atomes froids.
• A comment on ‘‘The interaction of X 2 (X = F, Cl, and Br) with active sites of graphite” [Xu et al., Chem. Phys. Lett., 418, 413 (2006)]
  
  • Lechner Christoph
  • Baranek Philippe
  • Vach Holger
  Chemical Physics Letters, Elsevier, 2018, 698. A comment on "The interaction of X2 (X = F, Cl, and Br) with active sites of graphite" [Xu et al., Chem. Abstract In their article, Xu et al. [1] present the adsorption energies for the chemisorption of the three halogens F 2, Cl 2, and Br 2 on the active sites of graphite. The three investigated systems are the three most stable surfaces, (001), (100), and (110); the latter two are also called zigzag and armchair surface, respectively. Due to some inconsistencies in their article, we re-evaluated the results of Xu et al. in order to investigate the impact on the adsorption energies of the halogens. For the (001) surface, our results agree with Xu et al. However, for the other two surfaces, we find major differences. Contrary to Xu et al., we find that the halogens adsorb the strongest on the zigzag surface. The second strongest adsorption is found on the armchair surface for the symmetric configurations, the third strongest for the asymmetric configurations. Several reasons are given which explain this discrepancy. The most striking source of error in the work of Xu et al. is due to the fact that they did not choose the correct spin multiplicities for the model systems which means that they performed the calculations in excited states. This leads to errors between 50-600% for the zigzag surface and 3-42% for the armchair surface. (10.1016/j.cplett.2018.02.067)
  DOI : 10.1016/j.cplett.2018.02.067
• Conservation of the piezoelectric response of PVDF films under irradiation
  
  • Melilli G.
  • Lairez D.
  • Gorse D.
  • Garcia-Caurel E.
  • Peinado A.
  • Cavani O.
  • Boizot B.
  • Clochard M.-C.
  Radiation Physics and Chemistry, Elsevier, 2018, 142, pp.54-59. As opposed to piezo-ceramics (i.e PZT), flexibility and robustness characterize piezoelectric polymers. The main advantage of a piezoelectric polymer, such as Poly (vinylidene fluoride) (PVDF), is an electric power generation under large reversible elastic deformation. Starting from polarized PVDF, we have shown that, despite the fact that irradiation is known to structurally modify the PVDF by introducing defects (radicals, chain scission and crosslinks), the electro-active properties were not affected. At doses lower than 100 kGy, a comparison between swift heavy-ion (SHI) and e-beam irradiations is presented. A homemade device was realized to measure the output voltage as a function of the bending deformation for irradiated and non-irradiated PVDF film. DSC and FT-IR techniques give new insights on which crystalline part or structural change contributes to the conservation of the output voltage. Results suggest that despite the material after irradiation is composed of smaller crystallites, the β-phase content remains stable around 36%, which explains the remarkable preservation of the piezoelectric response in irradiated polarized PVDF films. (10.1016/j.radphyschem.2017.03.035)
  DOI : 10.1016/j.radphyschem.2017.03.035
• La quête des températures ultrabasses
  
  • Suchet Daniel
  , 2018. Des vendeurs de glace à l'industrie cryogénique, le froid est souvent perçu comme un produit. Pour le physicien, le froid, réduction de l'agitation aléatoire des particules, apparaît plutôt comme une force productrice, capable de faire émerger des structures complexes et de métamorphoser les propriétés des objets. Les basses températures ont été l'enjeu d'une course effrénée dont la première phase a pris fin en 1908 avec la liquéfaction de l'ensemble des gaz connus. Loin de clore l'aventure, cette étape a fait surgir des nouveaux états de la matière qui ne peuvent être interprétés que par la mécanique quantique. Pour étudier la supraconductivité, la superfluidité ou la condensation de Bose-Einstein, les expériences actuelles visent des températures dites ultrabasses, à quelques milliardièmes de degré au-dessus du zéro absolu. Cette présentation propose une mise en perspective historique de la recherche des très basses températures et une introduction aux expériences actuelles d'atomes froids.
• Raman investigation of air-stable silicene nanosheets on an inert graphite surface
  
  • Castrucci Paola
  • Fabbri Filippo
  • Delise Tiziano
  • Scarselli Manuela
  • Salvato Matteo
  • Pascale Sara
  • Francini Roberto
  • Berbezier Isabelle
  • Lechner Christoph
  • Jardali Fatme
  • Vach Holger
  • de Crescenzi Maurizio
  Nano Research, Springer, 2018. (10.1007/s12274-018-2097-6)
  DOI : 10.1007/s12274-018-2097-6
• Investigation of a plasma--target interaction through electric field characterization examining surface and volume charge contributions: modeling and experiment
  
  • Viegas Pedro
  • Slikboer Elmar
  • Obrusník Adam
  • Bonaventura Zdenek
  • Sobota Ana
  • Garcia-Caurel Enric
  • Guaitella Olivier
  • Bourdon Anne
  Plasma Sources Science and Technology, IOP Publishing, 2018, 27. Numerical simulations and experiments are performed to better understand the interaction between a pulsed helium plasma jet and a dielectric target. The focus of this work lies on the volume and surface charge influence on the electric field distribution. Experimentally, the electric field due to surface charges is measured inside an electro-optic target under exposure of a plasma jet, using the optical technique called Mueller polarimetry. For the first time, the time-resolved spatial distributions of both the axial and radial components of electric field inside the target are obtained simultaneously. A 2D fluid model is used in a complementary way to the experiments in order to study separately the contribution of volume charges and surface charges to the spatio-temporal evolutions of the electric field during the plasma--surface interaction. The experimental investigation shows that the average axial and radial components of electric field inside the dielectric target, only due to surface charges, are lower than generally reported for electric field values in the plasma plume. Thanks to the phenomenological comparison with experiments, simulations show that during the plasma--surface interaction two effects sequentially determine the electric field inside the target: firstly, a relatively high electric field is observed due to the proximity of the ionization front; afterwards, in longer timescales, lower electric fields are induced due to the contribution of both leftover volume charges close to the target and surface charges deposited on its surface. The experimental technique provides a unique way to examine this second phase of the plasma--surface interaction. (10.1088/1361-6595/aadcc0)
  DOI : 10.1088/1361-6595/aadcc0
• Imaging axial and radial electric field components in dielectric targets under plasma exposure
  
  • Slikboer Elmar
  • Sobota Ana
  • Guaitella Olivier
  • Garcia-Caurel Enric
  Journal of Physics D: Applied Physics, IOP Publishing, 2018, 51 (11), pp.115203. Mueller polarimetry is used to investigate the behavior of an electro optic target (BSO crystal) under exposure of guided ionization waves produced by an atmospheric pressure plasma jet. For the first time, this optical technique is time resolved to obtain the complete Mueller matrix of the sample right before and after the impact of the discharges. By analyzing the induced birefringence, the spatial profiles and local values are obtained of both the electric field and temperature in the sample. Electric fields are generated due to deposited surface charges and a temperature profile is present, due to the heat transferred by the plasma jet. The study of electric field dynamics and local temperature increase at the target, due to the plasma jet is important for biomedical applications, as well as surface functionalization. This work shows how Mueller polarimetry can be used as a novel diagnostic to simultaneously acquire the spatial distribution and local values of both the electric field and temperature, by coupling the external source of anisotropy to the measured induced birefringence via the symmetry point group of the examined material. (10.1088/1361-6463/aaad99)
  DOI : 10.1088/1361-6463/aaad99