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Quelques articles remarquables | Highlights



On-surface synthesis of aligned functional nanoribbons monitored by scanning tunnelling microscopy and vibrational spectroscopy   (overview article)

Nataliya Kalashnyk, Kawtar Mouhat, Jihun Oh, Jaehoon Jung, Yangchun Xie, Eric Salomon, Thierry Angot, Frédéric Dumur, Didier Gigmes & Sylvain Clair

DOI:10.1038/ncomms14735
contact : sylvain.clair@im2np.fr

In the blooming field of on-surface synthesis, molecular building blocks are designed to self-assemble and covalently couple directly on a well-defined surface, thus allowing the exploration of unusual reaction pathways and the production of specific compounds in mild conditions. Here we report on the creation of functionalized organic nanoribbons on the Ag(110) surface. C-H bond activation and homo-coupling of the precursors is achieved upon thermal activation. The anisotropic substrate acts as an efficient template fostering the alignment of the nanoribbons, up to the full monolayer regime. The length of the nanoribbons can be sequentially increased by controlling the annealing temperature, from dimers to a maximum length of about 10 nm, limited by epitaxial stress. The different structures are characterized by room-temperature scanning tunnelling microscopy. Distinct signatures of the covalent coupling are measured with high-resolution electron energy loss spectroscopy, as supported by density functional theory calculations.

 

Hierarchical design and control of NaCe(WO4)2 crystals: structural and optical properties   (overview article)

N. Dirany, M. Arab, A. Moreau, J. Ch. Valmalette and J. R. Gavarri

DOI: 10.1039/C6CE01340F
contact : madjid.arab@univ-tln.fr

Double rare earth tungstates NaCe(WO4)2 have been successfully synthesized hydrothermally for the first time with cerium as the trivalent cation in the tetragonal single phase. The resulting hierarchical superstructures were structurally characterized by X-ray diffraction in conjunction with Rietveld refinements and Raman spectroscopy. Their shape and size were observed by scanning and transmission electron microscopy coupled to energy dispersive spectroscopy. It was found that morphology modulation could be realized by controlling the amount of EDTA ligand (0.3–0.35 g) and the pH values (6–8) of the system solution. Hierarchical microstructures resulted firstly from the coalescence of nanosheets and secondly from the self-assembly of nanoplatelets, nanorods and nanosheets, giving rise to sphere, flower and novel spindle morphologies, respectively. Structural determinations showed that the crystal structure belongs to the scheelite family ABWO4, with Na and Ce atoms occupying the same sites. Refinement calculations revealed that the microstructures showed different distortions of the polyhedral structure according to the W–O and Ce(Na)–O bonds and angles splitting. The formation mechanisms of the resulting hierarchical architectures are put forward based on a series of time-dependent experiments. Furthermore, the optical properties of the microstructures were strongly related to their morphologies, crystallite size and polyhedral distortion.

 

Fabrication of core-shell nanostructures via silicon on insulator dewetting and germanium condensation: towards a strain tuning method for SiGe-based heterostructures in a three-dimensional geometry  (overview article)

Meher Naffouti, Thomas David, Abdelmalek Benkouider, Luc Favre, Martiane Cabie, Antoine Ronda, Isabelle Berbezier and Marco Abbarchi

DOI: 10.1088/0957-4484/27/30/305602
contact : marco.abbarchi@im2np.fr

We report on a novel method for the implementation of core-shell SiGe-based nanocrystals combining silicon on insulator dewetting in a molecular beam epitaxy reactor with an ex situ Ge condensation process. With an in situ two-step process (annealing and Ge deposition) we produce two families of islands on the same sample: Si-rich, formed during the first step and, all around them, Ge-rich formed after Ge deposition. By increasing the amount of Ge deposited on the annealed samples from 0 to 18 monolayers, the islands' shape in the Si-rich zones can be tuned from elongated and flat to more symmetric and with a larger vertical aspect ratio. At the same time, the spatial extension of the Ge-rich zones is progressively increased as well as the Ge content in the islands. Further processing by ex situ rapid thermal oxidation results in the formation of a core-shell composition profile in both Si and Ge-rich zones with atomically sharp heterointerfaces. The Ge condensation induces a Ge enrichment of the islands' shell of up to 50% while keeping a pure Si core in the Si-rich zones and a ~25% SiGe alloy in the Ge-rich ones. The large lattice mismatch between core and shell, the absence of dislocations and the islands' monocrystalline nature render this novel class of nanostructures a promising device platform for strain-based band-gap engineering. Finally, this method can be used for the implementation of ultralarge scale meta-surfaces with dielectric Mie resonators for light manipulation at the nanoscale.

 

Templated Solid-State Dewetting of Thin Silicon Films  (overview article)

Meher Naffouti, Thomas David, Abdelmalek Benkouider, Luc Favre, Anne Delobbe, Antoine Ronda, Isabelle Berbezier, and Marco Abbarchi

DOI: 10.1002/smll.201670230
contact : marco.abbarchi@im2np.fr

Thin film dewetting can be efficiently exploited for the implementation of functionalized surfaces over very large scales. Although the formation of sub- micrometer sized crystals via solid-state dewetting represents a viable method for the fabrication of quantum dots and optical meta-surfaces, there are several limitations related to the intrinsic features of dewetting in a crystalline medium. Disordered spatial organization, size, and shape fluctuations are relevant issues not properly addressed so far. This study reports on the deterministic nucleation and precise positioning of Si- and SiGe-based nanocrystals by templated solid-state dewetting of thin silicon films. The dewetting dynamics is guided by pattern size and shape taking full control over number, size, shape, and relative position of the particles (islands dimensions and relative distances are in the hundreds nm range and fluctuate ≈11% for the volumes and ≈5% for the positioning)

 

EPR investigation of pure and Co-doped ZnO oriented nanocrystals  (overview article)

A. Savoyant, H. Alnoor, S. Bertaina, O. Nur and M. Willander

DOI: 10.1088/1361-6528/28/3/035705
contact : adrien.savoyant@im2np.fr

Pure and cobalt-doped zinc oxide aligned nanorods have been grown by the low-temperature (90 °C) aqueous chemical method on amorphous ZnO seed layer, deposited on a sapphire substrate. High crystallinity of these objects is demonstrated by the electron paramagnetic resonance investigation at liquid helium temperature. The successful incorporation of Co2+ ions in substitution of Zn2+ ones in the ZnO matrix has also been confirmed. A drastic reduction of intrinsic ZnO nanorods core defects is observed in the Co-doped samples, which enhances the structural quality of the NRs. The quantification of substitutional Co2+ ions in the ZnO matrix is achieved by comparison with a reference sample. The findings in this study indicate the potential of using the low-temperature aqueous chemical approach for synthesizing material for spintronics applications.

 

In situ observation of solidification patterns in diffusive conditions (overview article)

Silvere Akamatsu, Henri Nguyen-Thi

Acta Materialia Volume 108, 15 April 2016, Pages 325–346
DOI:10.1016/j.actamat.2016.01.024
contact : henri.nguyen-thi@im2np.fr

We present a review of recent in situ experimentation studies of solidification front patterns and microstructures in alloys. Front-tracking diagnostics and real-time observation methods using high-resolution optical or X-ray imaging devices currently apply to model transparent systems as well as metallic alloys in thin and bulk samples. On a theoretical basis that spans the physics of nonequilibrium pattern formation and materials science, in combination with time-resolved numerical simulations, conclusive results of both fundamental- and applied-science interest have been obtained on major problems relative to multiscale microstructure selection, morphological transitions, and crystallographic effects during single- and multi-phase solidification. We will mainly focus on the dynamics of cellular, dendritic, and eutectic growth patterns in diffusive-growth conditions, that is, in the absence of convection in the liquid. This can be achieved in (semi-)thin samples, or, for bulk solidification, in the reduced-gravity environment of orbiting facilities. A selection of emerging work on, e.g., faceted growth and adaptive control of solidification patterns will furthermore be reported. We conclude by pointing out open questions and new perspectives for future research.

 

Atom probe tomography of nanoscale electronic materials

D.J. Larson, T.J. Prosa, D.E. Perea, K. Inoue and D. Mangelinck

MRS Bulletin, Volume 41, Issue 01, pp 30-34, January 2016
DOI: 10.1557/mrs.2015.308
contact : dominique.mangelinck@im2np.fr

As the characteristic length scale of electronic devices shrinks, so does the required scale for measurement techniques to provide useful feedback during development and fabrication. The current capabilities of atom probe tomography (APT), such as detecting a low number of dopant atoms in nanoscale devices or studying diffusion effects in a nanowire (NW), make this technique important for metrology on the nanoscale. Here we review recent APT investigations applied to transistors (including regions such as gate oxide, channel, source, drain, contacts, etc.), heterogeneous dopant incorporation in NWs, and Pt-based nanoparticles.As the characteristic length scale of electronic devices shrinks, so does the required scale for measurement techniques to provide useful feedback during development and fabrication. The current capabilities of atom probe tomography (APT), such as detecting a low number of dopant atoms in nanoscale devices or studying diffusion effects in a nanowire (NW), make this technique important for metrology on the nanoscale. Here we review recent APT investigations applied to transistors (including regions such as gate oxide, channel, source, drain, contacts, etc.), heterogeneous dopant incorporation in NWs, and Pt-based nanoparticles.

 

Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting

Meher Naffouti, Thomas David, Abdelmalek Benkouider, Luc Favre, Antoine Ronda, Isabelle Berbezier, Sebastien Bidault, Nicolas Bonod and Marco Abbarchi

Nanoscale, 8, 2844, 2016
doi : 10.1039/c5nr07597a
contact : marco.abbarchi@im2np.fr

We report the fabrication of Si-based dielectric Mie resonators via a low cost process based on solid-state dewetting of ultra-thin amorphous Si on SiO2. We investigate the dewetting dynamics of a few nanometer sized layers annealed at high temperature to form submicrometric Si-particles. Morphological and structural characterization reveal the polycrystalline nature of the semiconductor matrix as well as rather irregular morphologies of the dewetted islands. Optical dark field imaging and spectroscopy measurements of the single islands reveal pronounced resonant scattering at visible frequencies. The linewidth of the low-order modes can be ∼20 nm in full width at half maximum, leading to a quality factor Q exceeding 25. These values reach the state-of-the-art ones obtained for monocrystalline Mie resonators. The simplicity of the dewetting process and its cost-effectiveness opens the route to exploiting it over large scales for applications in silicon-based photonics.

 

Continuous and Collective Grain Rotation in Nanoscale Thin Films during Silicidation

M.-I. Richard, J. Fouet, M. Texier, C. Mocuta, C. Guichet, and O. Thomas

Physical Review Letters, vol. 115, 266101 (2015)
doi : 10.1103/PhysRevLett.115.266101
contact : marie-ingrid.richard@im2np.fr

Texture evolution is an important issue in materials and nanosciences. Understanding it is fundamental for controlling the final orientation, which in fine controls the desired properties of nanodevices. Here, we reveal the formation of a peculiar texture during the silicidation of nanoscale Pd thin films. We demonstrate that the crystallographic relationship observed between the silicide and the Si(001) substrate, named gyroaxy, evolves continuously and collectively during silicidation. This continuous rotation of the nanosized grains over a wide angular range is proposed to be associated with a diffusional mechanism.

 

Inversion Domain Boundaries in GaN Wires Revealed by Coherent Bragg Imaging

Stephane Labat, Marie-Ingrid Richard, Maxime Dupraz, Marc Gailhanou, Guillaume Beutier, Marc Verdier, Francesca Mastropietro, Thomas W. Cornelius, Tobias U. Schu lli, Joe Eymery, and Olivier Thomas

ACS Nano, vol. 9, n° 9, p. 9210-9216, 2015
doi :10.1021/acsnano.5b03857
contact : stephane.labat@im2np.fr

Interfaces between polarity domains in nitride semiconductors, the so-called Inversion Domain Boundaries (IDB), have been widely described, both theoretically and experimentally, as perfect interfaces (without dislocations and vacancies). Although ideal planar IDBs are well documented, the understanding of their configurations and interactions inside crystals relies on perfect-interface assumptions. Here, we report on the microscopic configuration of IDBs inside n-doped gallium nitride wires revealed by coherent X-ray Bragg imaging. Complex IDB configurations are evidenced with 6 nm resolution and the absolute polarity of each domain is unambiguously identified. Picoscale displacements along and across the wire are directly extracted from several Bragg reflections using phase retrieval algorithms, revealing rigid relative displacements of the domains and the absence of microscopic strain away from the IDBs. More generally, this method offers an accurate inner view of the displacements and strain of interacting defects inside small crystals that may alter optoelectronic properties of semiconductor devices.

 

Stability of a screw dislocation in a (011) Copper nanowire.

Jean-Marc Roussel and Marc Gailhanou

Phys. Rev. Lett., 115, 075503, August 2015
doi:10.1103/PhysRevLett.115.075503
contact : jean-marc.roussel@im2np.fr

The stability of a screw dislocation in a free  copper nanowire is investigated using atomistic calculations. This study reveals a strong anisotropy of the Eshelby potential well (EPW) that traps the dislocation. Moreover the depth of the EPW is found to vanish when the radius of the nanowire decreases. It is demonstrated that this behavior is due to the dissociated state of the dislocation.

 

In situ bending of an Au nanowire monitored by micro Laue diffraction


Cedric Leclere,a* Thomas W. Cornelius,a Zhe Ren,a Anton Davydok,a
Jean-Sebastien Micha,b Odile Robach,b Gunther Richter,c Laurent Belliard d and Olivier Thomas a

J. Appl. Cryst. (2015). 48, 291–296
doi:10.1107/S1600576715001107
contact : cedric.leclere@im2np.fr

This article reports on the first successful combination of micro Laue (mLaue) diffraction with an atomic force microscope for in situ nanomechanical tests of individual nanostructures. In situ three-point bending on self-suspended gold nanowires was performed on the BM32 beamline at the ESRF using a specially designed atomic force microscope. During the bending process of the selfsuspended wire, the evolution of mLaue diffraction patterns was monitored,
allowing for extraction of the bending angle of the nanowire. This bending compares well with finite element analysis taking into account elastic constant bulk values and geometric nonlinearities. This novel experimental setup opens promising perspectives for studying mechanical properties at the nanoscale.

 

EPR characterization of Co-doped ZnO microwires and distant spin-pair coupling

A Savoyant, F Giovannelli, F Delorme and A Stepanov

Semiconductor Science and Technology, vol. 30, p. 075004, 2015
doi:10.1088/0268-1242/30/7/075004
contact : adrien.savoyant@im2np.fr

Cobalt-doped ZnO microwires have been grown by the optical furnace method with Co concentration in the range 0–5%. The high crystallinity of these objects is demonstrated by the use of electron paramagnetic resonance (EPR), in which Co2+ ions serve as magnetic probes. The spin state of these isolated magnetic impurities and their location in the host crystal are addressed by EPR, as well as the coupling constant between non-nearest Co2+–Co2+ neighbors. Eight of these distant spin pairs are detected, among which two are confirmed to be ferromagnetic. The problem of their localization in the wurtzite structure is discussed.

 

The first X-ray diffraction measurements on Mars

David Bish, David Blake, David Vaniman, Philippe Sarrazin, Thomas Bristow, Cherie Achilles, Przemyslaw Dera, Steve Chipera, Joy Crisp, R. T. Downs, Jack Farmer, Marc Gailhanou, Doug Ming John Michael Morookian, Richard Morris, Shaunna Morrison, Elizabeth Rampe, Allan Treiman and Albert Ye

IUCrJ (International Union of Crystallography Journal), vol. 1, part 6, p. 514-522, november 2014
contact : marc.gailhanou@im2np.fr

The Mars Science Laboratory landed in Gale crater on Mars in August 2012, and the Curiosity rover then began field studies on its drive toward Mount Sharp, a central peak made of ancient sediments. CheMin is one of ten instruments on or inside the rover, all designed to provide detailed information on the rocks, soils and atmosphere in this region. CheMin is a miniaturized X-ray diffraction/X-ray fluorescence (XRD/XRF) instrument that uses transmission geometry with an energy-discriminating CCD detector. CheMin uses onboard standards for XRD and XRF calibration, and beryl:quartz mixtures constitute the primary XRD standards. Four samples have been analysed by CheMin, namely a soil sample, two samples drilled from mudstones and a sample drilled from a sandstone. Rietveld and full-pattern analysis of the XRD data reveal a complex mineralogy, with contributions from parent igneous rocks, amorphous components and several minerals relating to aqueous alteration. In particular, the mudstone samples all contain one or more phyllosilicates consistent with alteration in liquid water. In addition to quantitative mineralogy, Rietveld refinements also provide unit-cell parameters for the major phases, which can be used to infer the chemical compositions of individual minerals and, by difference, the composition of the amorphous component.

 

Imaging the Optical Near Field in Plasmonic Nanostructures (focal point review)

Alexandre Merlen & François Lagugné-Labarthet

Applied Spectroscopy, vol. 68, n°12, 1307-1326, 2014
contact : alexandre.merlen@im2np.fr

Over the past five years, new developments in the field of plasmonics have emerged with the goal of finely tuning a variety of metallic nanostructures to enable a desired function. The use of plasmonics in spectroscopy is of course of great interest, due to large local enhancements in the optical near field confined in the vicinity of a metal nanostructure. For a given metal, such enhancements are dependent on the shape of the structure as well as the optical properties (wavelength, phase, polarization) of the impinging light, offering a
large degree of control over the optical and spatial localization of the plasmon resonance. In this focal point, we highlight recent work that aims at revealing the spatial position of the localized plasmon resonances using a variety of optical and non-optical methods.

 

Wafer Scale Formation of Monocrystalline Silicon-Based Mie Resonatorsvia Silicon-on-Insulator Dewetting

Marco Abbarchi, Meher Naffouti, Benjamin Vial, Abdelmalek Benkouider, Laurent Lermusiaux, Luc Favre, Antoine Ronda, Sebastien Bidault, Isabelle Berbezier, and Nicolas Bonod

ACS Nano, article ASAP (on line, November 2014)
DOI : 10.1021/nn505632b
contact : marco.abbarchi@im2np.fr

Subwavelength-sized dielectric Mie resonators have recently emerged as a promising photonic platform, as they combine the advantages of dielectric microstructures and metallic nanoparticles supporting surface plasmon polaritons. Here, we report the capabilities of a dewetting-based process, independent of the sample size, to fabricate Si-based resonators over large scales starting from commercial silicon-on-insulator (SOI) substrates. Spontaneous dewetting is shown to allow the production of monocrystalline Mie-resonators that feature two resonant modes in the visible spectrum, as observed in confocal scattering spectroscopy. Homogeneous scattering responses and improved spatial ordering of the Si-based resonators are observed when dewetting is assisted by electron beam lithography. Finally, exploiting different thermal agglomeration regimes, we highlight the versatility of this technique, which, when assisted by focused ion beam nanopatterning, produces monocrystalline nanocrystals with ad hoc size, position, and organization in complex multimers.

Rabi oscillations of pinned solitons in spin chains: A route to quantum computation and communication

S. Bertaina, C.-E. Dutoit, J. Van Tol, M. Dressel, B. Barbara, and A. Stepanov

Phys. Rev. B 90, 060404(R) – Published 20 August 2014
DOI: 10.1103/PhysRevB.90.060404
contact : sylvain.bertaina@im2np.fr

We provide evidence for the coherence and Rabi oscillations of spin solitons pinned by the local breaking of translational symmetry in isotropic Heisenberg chains (simple antiferromagnetic Néel or spin Peierls). We show that these correlated spin systems that are made of hundreds of coupled spins bear an overall spin  S =1/2 and can be manipulated as a single spin. This is clearly contrary to known spin qubits which are paramagnetic centers, highly diluted to prevent decoherence. These results offer an alternative approach for spin qubits, paving the way for the implementation of a different type of quantum computer.

cette publication a fait l'objet d'une présentation dans les "Actualités scientifiques" de l'Institut de Physique du CNRS : "Le soliton électronique : un bit quantique résilient"

 

Interplay of Optical, Morphological, and Electronic Effects of ZnO Optical Spacers in Highly Effi cient Polymer Solar Cells

S. Ben Dkhil, D. Duché, M. Gaceur, A. K. Thakur, F. Bencheikh Aboura, L. Escoubas, J.-J. Simon*, A. Guerrero, J. Bisquert, G. Garcia-Belmonte, Q. Bao, M. Fahlman, C. Videlot-Ackermann, O. Margeat, Jörg Ackermann*.

Advanced Energy Materials, 2014, 1400805
DOI: 10.1002/aenm.201400805
contact : jean-jacques.simon@im2np.fr

Optical spacers based on metal oxide layers have been intensively studied in poly(3-hexylthiophene) (P3HT) based polymer solar cells for optimizing light distribution inside the device, but to date, the potential of such a metal oxide spacer to improve the electronic performance of the polymer solar cells simultaneously has not yet be investigated. Here, a detailed study of performance improvement in high efficient polymer solar cells by insertion of solution-processed ZnO optical spacer using ethanolamine surface modification is reported. Insertion of the modifi ed ZnO optical spacer strongly improves the performance of polymer solar cells even in the absence of an increase in light absorption. The electric improvements of the device are related to improved electron extraction, reduced contact barrier, and reduced recombination at the cathode. Importantly, it is shown for the fi rst time that the morphology of optical spacer layer is a crucial parameter to obtain highly effi cient solar cells in normal device structures. By optimizing optical spacer effects, contact resistance, and morphology of ZnO optical spacers, poly[[4,8- bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6diyl] [3-fl uoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl-C71-butyric acid (PC 70 BM) bulk heterojunction solar cells with conversion effi ciency of 7.6% are obtained in normal device structures with all-solution-processed interlayers.

 

Real-time soft-error rate measurements : A review

J.L. Autran, D. Munteanu, P. Roche, G. Gasiot

Microelectronics Reliability, Volume 54, Issue 8, August 2014, Pages 1455–1476
DOI: 10.1016/j.microrel.2014.02.031
Contact : jean-luc.autran@im2np.fr

The real-time (or life testing) soft-error rate (SER) measurement is an experimental reliability technique to determine the soft error sensitivity of a given component, circuit or system from the monitoring of a population of devices subjected to natural radiation and operating under nominal conditions. This review gives a survey over recent real-time SER experiments, conducted in altitude and/or underground, and investigating modern CMOS logic technologies, down to the 40 nm technological node. The review also includes our different contributions conducted during the last decade on the ASTEP Platform (Altitude Single Event Effects Test European Platform) and at the LSM facility (Underground Laboratory of Modane) to characterize soft error mechanisms in advanced static (SRAM) memories. Finally, the review discusses the specific advantages and limitations of this approach as well as its comparison with accelerated tests using intense particle beams or sources.

 

 

Sylvain Clair,   Mathieu Abel and   Louis Porte  

Chem. Commun., 2014, 50, 9627-9635
DOI: 10.1039/C4CC02678K
contact : sylvain.clair@im2np.fr

The formation of atomically precise covalent networks directly on a surface is a promising approach to produce single layers of two-dimensional organic materials (2D polymers). In the emerging field of on-surface synthesis, there is an urgent need for finding a rationale to new reaction pathways taking place directly on the surface. In this feature article we review and put into perspective recent results on the surface polymerisation of boronic acid based systems under ultrahigh vacuum conditions studied by scanning tunnelling microscopy. By presenting several approaches to control the growth of covalent networks, we aim at providing a comprehensive overview of the complex mechanisms involved.

 

Inverted cones grating for flexible metafilter at optical and infrared frequencies

Jean-Baptiste Brückner, Vincent Brissonneau, Judikaël Le Rouzo, Abdelkerim Ferchichi, Cécile Gourgon,
Christophe Dubarry, Gérard Berginc, and Ludovic Escoubas

Applied Physics Letters 104, 081114 (2014)
doi: 10.1063/1.4866864
online : http://dx.doi.org/10.1063/1.4866864
Contact : judikael.lerouzo@im2np.fr

By combining the antireflective properties from gradual changes in the effective refractive index and cavity coupling from cone gratings and the efficient optical behavior of a tungsten film, a flexible filter showing very broad antireflective properties from the visible to short wavelength infrared region and, simultaneously, a mirror-like behavior in the mid-infrared wavelength region and long-infrared wavelength region has been conceived. Nanoimprint technology has permitted the replication of inverted cone patterns on a large scale on a flexible polymer, afterwards coated with a thin tungsten film. This optical metafilter is of great interest in the stealth domain where optical signature reduction from the optical to short wavelength infrared region is an important matter. As it also acts as selective thermal emitter offering a good solar-absorption/infrared-emissivity ratio,
interests are found as well for solar heating applications.

 

 

A novel ozone gas sensor based on one-dimensional (1D) α-Ag2WO4 nanostructures.
L. F. da Silva, A. C. Catto, W. Avansi Jr., L. S. Cavalcante, J. Andres, K. Aguir, V. R. Mastelaro and E. Longo,

Nanoscale, Accepted Manuscript First published online 29 Jan 2014
DOI: 10.1039/C3NR05837A, Communication 
Contact : khalifa.aguir@im2np.fr

This paper reports on a new ozone gas sensor based on α-Ag2WO4 nanorod-like structures. Electrical resistance measurements proved the efficiency of α-Ag2WO4 nanorods, which rendered a good sensitivity even for a low ozone concentration (80 ppb), a fast response and short recovery time at 300oC, demonstrating great potential for a variety of applications.

 

 

Technology downscaling worsening radiation effects in bulk: SOI to the rescue
Roche, Philippe; Autran, Jean-Luc; Gasiot, Gilles; Munteanu, Daniela

Electron Devices Meeting (IEDM), 2013 IEEE International, pp.31.1.1,31.1.4, 9-11 Dec. 2013
doi: 10.1109/IEDM.2013.6724728
Contact : jean-luc.autran@im2np.fr

Atmospheric radiation is today as important to IC reliability as intrinsic failure modes. In non-critical consumer applications (cell phone, printer, gaming), a relatively high soft error rate (SER) is often tolerable. In contrast, a similar failure rate would be deemed unacceptably high in an arena where system reliability, accessibility, and serviceability are of paramount importance (networking, server, avionic, space), particularly where human life or safety is at risk (medical, automotive, transportation). Increasing number of industry segments are impacted due to growing amount of memory and logic components per circuit. Concurrently, sub-45nm downscaling has a profound impact on SER of bulk CMOS technologies. The enhanced resilience of latest SOI technologies helps to leverage existing robust design solutions. In this paper, experimental radiation test results and simulations are reported for the first time in UTBB FDSOI 28nm and compared to Bulk, PDSOI and FinFET alternatives.

 

Unusual crystallization behavior in Ga-Sb phase change alloys
M. Putero, M.V. Coulet, T. Ouled-Khachroum, C. Muller, C. Baehtz, S. Raoux

APL Materials 1, 062101 (2013)
DOI : 10.1063/1.4833035
Contact : magali.putero@im2np.fr

Combined in situ X-ray scattering techniques using synchrotron radiation were applied to investigate the crystallization behavior of Sb-rich Ga-Sb alloys. Measurements of the sheet resistance during heating indicated a reduced crystallization temperature with increased Sb content, which was confirmed by in situ X-ray diffraction. The electrical contrast increased with increasing Sb content and the resistivities in both the amorphous and crystalline phases decreased. It was found that by tuning the composition between Ga:Sb = 9:91 (in at.%) and Ga:Sb = 45:55, the change in mass density upon crystallization changes from an increase in mass density which is typical for most phase change materials to a decrease in mass density. At the composition of Ga:Sb = 30:70, no mass density change is observed which should be very beneficial for phase change random access memory (PCRAM) applications where a change in mass density during cycling is assumed to cause void formation and PCRAM device failure.

in collaboration with : Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01314 Dresden, Germany and IBM/Macronix PCRAM Joint Project, IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, USA

Cette publication a fait l'objet d'une présentation dans Physics Today : "A phase-change alloy that crystallizes without shrinking" by Charles Day, December 2013 [Eliminating density changes that accompany changes in reflectivity and resistance would make a promising class of data-storage materials more effective]

 

Displacement field of a screw dislocation in a <011> Cu nanowire : an atomistic study
M. Gailhanou, J.M. Roussel

Physical Review B, 88, 224101, 2013
DOI : 10.1103/PhysRevB.88.224101
Contacts : marc.gaihanou@im2np.fr, jean-marc.roussel@im2np.fr

By performing atomistic calculations with a tight-binding potential, we study the displacement field induced by a screw dislocation lying along a free (011) Cu cylindrical nanowire. For this anisotropic orientation that is often encountered experimentally, we show that the displacement field u z along the nanowire can be seen as the superposition of three different fields: the screw dislocation field in an infinite medium, the warping displacement field caused by the so-called Eshelby twist, and an additional image field induced by the free surfaces. A Fourier series analysis of this latter image displacement and stress fields is given. For a circular cross section of the wire, this image field corresponds mainly to an additional warping displacement u z xy . The dissociation mechanism of the dislocation into partials and the surface stress effects being also captured in our simulations, the present study enables one to quantify the various contributions to the formation of the x-ray diffractograms.

 

 

Optical properties of Silver and Gold tetrahedral nanopyramid arrays prepared by nanosphere lithography
M. Tabatabaei, A. Sangar, N. Kazemi-Zanjani, P. Torchio, A. Merlen, and F. Lagugné-Labarthet

Journal of Physical Chemistry C, 117, 14778−14786, 2013
DOI : 10.1021/jp405125c
Contacts : alexandre.merlen@im2np.fr, philippe.torchio@im2np.fr

Tetrahedral nanopyramids made of silver and gold over ITO/glass surfaces are fabricated. Our protocol is based on nanosphere lithography (NSL) with the deposition of thicker metal layers. After removing the microspheres used in the NSL process, an array of metallic tetrahedral nanostructures of 350–400 nm height is formed. The reported procedure avoids the use of any stabilizing surfactant molecules that are generally necessary to segregate the individual particles onto surfaces. We focus here on the optical and the physical properties of these plasmonic surfaces using near-field spectroscopy in conjunction with finite difference time domain (FDTD) modeling of the electric field. Remarkably, FDTD shows that the localized surface plasmon resonance is confined in the plane formed by the edges of two facing pyramids that is parallel to the polarization of the impinging excitation laser. The variable gap between the edges of two adjacent pyramids shows a broader localized surface plasmon and a larger specific surface as opposed to the usual nanotriangle array. Localized enhancement of the electric field is experimentally investigated by coating the plasmonic surface with a thin film of photosensitive azopolymer onto the surface of the nanopyramids. Upon irradiation, the deformation of the surface topography is visualized by atomic force microscopy and suggests the potentiality of these 3D nanopyramids for near-field enhancement. This last feature is clearly confirmed by surface-enhanced Raman scattering measurement with 4-nitrothiophenol molecules deposited on the pyramid platforms. The potentiality of such 3D nanostructures in plasmonics and surface spectroscopy is thus clearly demonstrated.

in collaboration with the Department of Chemistry and Centre for Advanced Materials and Biomaterials, University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7, Canada".

 

Grafting a homogeneous transition metal catalyst onto a silicon AFM probe: a promising strategy for chemically constructive nanolithography
Dmitry A. Valyaev, Sylvain Clair, Lionel Patrone, Mathieu Abel, Louis Porte, Olivier Chuzel and Jean-Luc Parrain

Chemical Science, 2013, 4, 2815-2821
DOI: 10.1039/C3SC50979F
contact : sylvain.clair@im2np.fr

We report a novel approach to chemically selective lithography using an atomic force microscope (AFM) probe with immobilized homogeneous catalyst, potentially giving access to diverse nanoscale transformations of the surface-bound functional groups. This new concept was proven for the local epoxidation of an alkene-terminated self-assembled monolayer on silicon using H2O2 as an oxidant and a catalytic silicon AFM tip charged with manganese complexes with 1,3,7-triazacyclononane type ligands.

in collaboration with iSm2 UMR 7313, Aix Marseille Université, CNRS, Marseille, France

cette publication a fait l'objet d'une présentation dans les "Actualités scientifiques" de l'Institut de Chimie du CNRS

Spatiotemporal Dynamics of Oscillatory Cellular Patterns in Three-Dimensional Directional Solidification.
N.Bergeon, D.Tourret, L.Chen, J.M.Debierre, R.Guérin, A.Ramirez, B.Billia, A.Karma and R.Trivedi

Physical Review Letters Published 28 May 2013, 226102.
DOI : 10.1103/PhysRevLett.110.226102
contact : nathalie.bergeon@im2np.fr

We report results of directional solidification experiments conducted on board the International Space Station and quantitative phase-field modeling of those experiments. The experiments image for the first time in situ the spatially extended dynamics of three-dimensional cellular array patterns formed under microgravity conditions where fluid flow is suppressed. Experiments and phase-field simulations reveal
the existence of oscillatory breathing modes with time periods of several 10’s of minutes. Oscillating cells are usually noncoherent due to array disorder, with the exception of small areas where the array structure is regular and stable

in collaboration with Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, Massachusetts 02115, USA / et Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50010, USA

cette publication a fait l'objet d'une présentation dans les "Actualités scientifiques" de l'Institut de Physique du CNRS

 

Concentration and Strain Fields inside a Ag/Au Core–Shell Nanowire Studied by Coherent X-ray Diffraction
Sabine T. Haag, Marie-Ingrid Richar, Udo Welzel, Vincent Favre-Nicolin, Olivier Balmes, Gunther Richter, Eric J. Mittemeijer, Olivier Thomas

DOI: 10.1021/nl303206u

contact : marie-ingrid.richard@im2np.fr

Three-dimensional coherent diffraction patterns of an isolated, single-crystalline Ag/Au core–shell nanowire were recorded at different X-ray beam energies close to the Au LIII absorption edge. Two-dimensional slices of the three-dimensional diffraction pattern, with the diffraction vector oriented perpendicular to the wire axis, were investigated in detail. In reciprocal space, facet streaks with thickness fringes were clearly observed in the two-dimensional diffraction patterns, from which the shape and size of the corresponding cross sections of the nanowire could be revealed. Comparison with simulated diffraction patterns exhibited the coherency strain field in the nanowire. During in situ annealing at temperatures which would lead to significant intermixing by volume diffusion in bulk material, according to literature data, a core–shell morphology was preserved; that is, intermixing in the nanowire was pronouncedly decelerated compared to bulk diffusion.

This work has been carried out in collaboration with Max Planck Institute for Intelligent Systems, ESRF, CEA-UJF, Université Grenoble-Alpes, Institute for Materials Science, University of Stuttgart and Im2np (Aix-Marseille Université, CNRS)

 

Impact of electronic defects on the Raman spectra from electrodeposited Cu(In,Ga)Se2 solar cells: Application for non-destructive defect assessment
C. M. Ruiz, X. Fontané, A. Fairbrother, V. Izquierdo-Roca, C. Broussillou, S. Bodnar, A. Pérez-Rodríguez3 and V. Bermúdez

Appl. Phys. Lett. 102, 091106 (2013)
http://dx.doi.org/10.1063/1.4793418
contact : carmen-ruiz.herrero@im2np.fr

This work reports on the electrical and Raman scattering analysis of Cu(In,Ga)Se2 cells synthesised with different densities of Se and Cu related point defects. The analysis of the Raman spectra from the surface region of the absorbers shows a direct correlation between the spectral features of the main Raman peak and the density of Se vacancies detected by admittance spectroscopy, being sensitive to the presence of vacancy densities higher than 1015 cm−3. These results corroborate the potential of Raman scattering for the non-destructive detection of electronic defects with potential impact on the characteristics of the solar cells.

This work has been carried out in collaboration with IREC (Catalonia Institute for Energy Research), IN2UB (Departament d'Electrònica, Universitat de Barcelona), Nexcis (Rousset, France) and Im2np (Aix-Marseille Université, CNRS)

 

Anomalous coherent diffraction of core-shell nano-objects : A methodology for determination of composition and strain fields
S. T. Haag, M.-I. Richard, S. Labat, M. Gailhanou, U. Welzel, E. J. Mittemeijer, and O. Thomas

PHYSICAL REVIEW B 87, 035408 (2013)
DOI: 10.1103/PhysRevB.87.035408
http://link.aps.org/doi/10.1103/PhysRevB.87.035408
contact : marie-ingrid.richard@im2np.fr

A procedure to retrieve the three-dimensional strain and composition fields of single-crystalline nanoparticles using a combination of anomalous and coherent x-ray diffraction was developed. The applicability of the method was demonstrated using numerically generated data for Ag/Au core-shell nanowires forwhich the epitaxy-induced strain field was simulated by the use of the finite element method for infinitely long nanowires with either abrupt ("as-prepared") or diffuse ("as-annealed") interfaces between core and shell. First, the influence of the elemental distribution and the associated inhomogeneous strains on the x-ray diffraction patterns were described. Next, the diffraction patterns as determined for different x-ray energies were inverted by applying phase retrieval algorithms. Only if diffraction patterns at different energies are available and inverted can the concentrations and
displacements be obtained subject to specific assumptions.

 

Inelastic scattering in nanoscale devices: One-shot current-conserving lowest-order approximation
H. Mera, M. Lannoo, C. Li, N. Cavassilas, and M. Bescond

PHYSICAL REVIEW B 86, 161404(R) (2012)
DOI: 10.1103/PhysRevB.86.161404
http://link.aps.org/doi/10.1103/PhysRevB.86.161404
contact : marc.bescond@im2np.fr

We describe a lowest-order approximation (LOA) to the nonequilibrium Green's function in the presence of interactions, and generally address how one can build Φ-derivable one-shot approximations that satisfy the continuity equation. These approximations produce conserved electronic currents in one shot, requiring only one self-energy evaluation and, when applicable, they are as accurate as but much faster than the corresponding self-consistent approximation. This challenges the currently adopted view that heavy self-consistent calculations are necessary to get a satisfactory prediction of transport in nanoscale structures. We illustrate this with the case of electron-phonon scattering expressed within the self-consistent Born approximation (SCBA). In the LOA, the SCBA is further approximated by accounting only for one-phonon processes. LOA and SCBA are compared in one-dimensional wire where electrons interact with one optical phonon mode at room temperature. The LOA is found to provide a considerable reduction in computational time. Its limitations and extensions to include two-phonon processes are discussed.

 

Decoherence Window and Electron-Nuclear Cross Relaxation in the Molecular Magnet V15.
J. H. Shim, S. Bertaina, S. Gambarelli, T. Mitra, A. Müller, E. I. Baibekov, B. Z. Malkin, B. Tsukerblat, and B. Barbara

Physical Review Letters, 109, 5, 050401 (2012)
DOI:  10.1103/PhysRevLett.109.050401
http://link.aps.org/doi/10.1103/PhysRevLett.109.050401
contact : sylvain.bertaina@im2np.fr

Rabi oscillations in the V15 single molecule magnet embedded in the surfactant  (CH3)2[CH3(CH2)16CH2]2N+ have been studied at different microwave powers. An intense damping peak is observed when the Rabi frequency ΩR falls in the vicinity of the Larmor frequency of protons ωN. The experiments are interpreted by a model showing that the damping (or Rabi) time τR is directly associated with decoherence caused by electron-nuclear cross relaxation in the rotating reference frame. This decoherence induces energy dissipation in the range ωN-σe<ΩR<ωN, where σe is the mean superhyperfine field induced by protons at V15. Weaker decoherence without dissipation takes place outside this window. Specific estimations suggest that this rapid cross relaxation in a resonant microwave field, observed for the first time in V15, should also take place, e.g., in Fe8 and Mn12.

Bonding and structure of a reconstructed (001) surface of SrTiO3 from TEM
Guo-zhen Zhu, Guillaume Radtke & Gianluigi A. Botton

Nature, 10 October 2012
DOI:10.1038/nature11563
contact : guillaume.radtke@im2np.fr

The determination of the atomic structure and the retrieval of information about reconstruction and bonding of metal oxide surfaces is challenging owing to the highly defective structure and insulating properties of these surfaces. Transmission electron microscopy (TEM) offers extremely high spatial resolution (less than one ångström) and the ability to provide systematic information from both real and reciprocal space. However, very few TEM studies have been carried out on surfaces because the information from the bulk dominates the very weak signals originating from surfaces. Here we report an experimental approach to extract surface information effectively from a thickness series of electron energy-loss spectra containing different weights of surface signals, using a wedge-shaped sample. Using the (001) surface of the technologically important compound strontium titanate, SrTiO3 as a model system for validation, our method shows that surface spectra are sensitive to the atomic reconstruction and indicate bonding and crystal-field changes surrounding the surface Ti cations. Very good agreement can be achieved between the experimental surface spectra and crystal-field multiplet calculations based on the proposed atomic surface structure optimized by density functional calculations. The distorted TiO6−x units indicated by the proposed model can be viewed directly in our high-resolution scanning TEM images. We suggest that this approach be used as a general method to extract valuable spectroscopic information from surface atoms in parallel with high-resolution images in TEM.

 

Atom-probe-tomographic Studies on Silicon-Based Semiconductor Devices
Koji Inoue, Ajay Kumar Kambham, Dominique Mangelinck, Dan Lawrence, and David J. Larson

Microscopy Today (2012), 20:38-44 Cambridge University Press
DOI:10.1017/S1551929512000740
contact : dominique.mangelinck@im2np.fr

The development of laser-assisted atom probe tomography (APT) and specimen preparation techniques using a focused ion beam equipped with high-resolution scanning electron microscopy (SEM) has significantly advanced the characterization of semiconductor devices by APT. The capability of APT to map out elements in devices at the atomic scale with high sensitivity meets the characterization requirements of semiconductor devices such as the determination of elemental distributions for each device region.

Light- induced electron transfer and ATP synthesis in a carotene synthesizing insect
Jean-Christophe Valmalette, Aviv Dombrovsky, Pierre Brat, Christian Mertz, Maria Capovilla, Alain Robichon

Scientific Reports 2, Article number:579, 16 August 2012
http://www.nature.com/srep/2012/120816/srep00579/full/srep00579.html

DOI:10.1038/srep00579

contact : jean-christophe.valmalette@im2np.fr

The organic pigments known as carotenoids may be involved in the photosynthesis of ATP in aphids, a paper published in Scientific Reports this week suggests. The study presents what may be the first tentative evidence of ATP photosynthesis in an insect.Carotenoids occur naturally in the chloroplasts and chromoplasts of plants, algae and some bacteria and fungi, where they have a role in photosynthesis. Unlike other insects, the aphid genome includes genes that code for the proteins needed to synthesize carotenoids, which were probably acquired via lateral gene transfer from fungi. Aphids therefore seem equipped for synthesizing carotenoids, but the potential physiological functions of this system have remained uncertain.Depending on environmental context, aphids can be manipulated to exhibit different amounts of carotene, which is reflected in their colour. Alain Robichon and colleagues compared orange, green and white aphid strains and found evidence suggesting that carotenoids in orange and green aphids may potentially be involved in absorbing light and using the energy to reduce the co-enzyme NADP+, which can then power ATP synthesis in the mitochondria. The findings hint at an archaic photosynthetic system in aphids, although the mechanisms involved are still uncertain and whether this ability confers a fitness advantage remains to be seen.

 

Inhomogeneous Relaxation of a Molecular Layer on an Insulator due to Compressive Stress
F. Bocquet, L. Nony, S. C. B. Mannsfeld, V. Oison, R. Pawlak, L. Porte, and Ch. Loppacher
Phys. Rev. Lett. 108, 206103 (2012)

DOI:10.1103/PhysRevLett.108.206103
contact : laurent.nony@im2np.fr

We discuss the inhomogeneous stress relaxation of a monolayer of hexahydroxytriphenylene (HHTP) which adopts the rare line-on-line (LOL) coincidence on KCl(001) and forms moiré patterns. The fact that the hexagonal HHTP layer is uniaxially compressed along the LOL makes this system an ideal candidate to discuss the influence of inhomogeneous stress relaxation. Our work is a combination of noncontact atomic force microscopy experiments, density functional theory and potential energy calculations, and a thorough interpretation by means of the Frenkel-Kontorova model. We show that the assumption of a homogeneous molecular layer is not valid for this organic-inorganic heteroepitaxial system since the best calculated energy configuration correlates with the experimental data only if inhomogeneous relaxations of the layer are taken into account. (a) Autocorrelation image of a molecular domain of HHTP on KCl(001) showing moiré fringes and periodic bright spots. The orientation of the fringes with respect to the <10> direction is (100+/-3)°. (b) Simulated moiré pattern which reproduces both the experimental fringes and the bright spots, respectively. (c) Illustration of the inhomogeneous molecular relaxation: nine HHTP molecules are forced within the distance of ten minima of the electrostatic energy profile for a single molecule along the <2-1> direction of the substrate. The adsorption energy of the molecules with even number is significantly larger when the molecules are spaced according to the Frenkel Kontorova model (lower ones) compared to the equally spaced molecules (upper ones, homogeneous chain).

 

 

Un nouveau procédé de nanolithographie par auto-assemblage
Two-dimensional polymer as a mask for surface nanopatterning
Sylvain Clair, Oualid Ourdjini, Mathieu Abel, Louis Porte
Advanced Materials, 24, 9 (2012) 1252-1254, 2012

DOI : 10.1002/adma.201200063
contact : sylvain.clair@im2np.fr

In this work we propose an original approach to create surface-supported nanostructures with a sequential process which is inspired by traditional lithography techniques but which is based exclusively on self-assembly processes. In particular, we take advantage of the exceptional thermal stability of a two-dimensional polymer based on boronic acid that we have recently developed in our laboratory. We show that NaCl islands grown on a Ag(001) surface can be used as a positive mask to grow a 2D polymer exclusively in the NaCl-free parts of the surface. After thermal evaporation of the NaCl, the thermally stable polymer is used as a negative mask to grow Fe patches replicating approximately the former position and shape of the NaCl mask.
The novel strategy presented should be widely applicable to various systems to create a large variety of controlled nanostructures, in particular when combined with other, more traditional patterning techniques.

Figure: Schematics of the sequence used in the nanolithography process and illustration with scanning tunneling microscopy images showing the removal the mask consisting of NaCl islands.

 

Revue sur le silicène - Un nouveau candidat pour l'électronique
A review on silicene - New candidate for electronics
Abdelkader Kara, Hanna Enriquez, Ari P. Seitsonen, L.C. Lew Yan Voon, Sébastien Vizzini, Bernard Aufray, Hamid Oughaddou
Surface Science Reports 67 (2012) 1–18

DOI :10.1016/j.surfrep.2011.10.001 
contact : sebastien.vizzini@im2np.fr

Silicene – the silicon-based counterpart of graphene – has a two dimensional structure that is responsible for the variety of potentially useful chemical and physical properties. The existence of silicene has been achieved recently owing to experiments involving epitaxial growth of silicon as stripes on Ag(001), ribbons on Ag(110), and sheets on Ag(111). The nano-ribbons observed on Ag(110) were found – by both high definition experimental scanning tunneling microscopy images and density functional theory calculations – to consist of an arched honeycomb structure. Angle resolved photo-emission experiments on these silicene nano-ribbons on Ag(110), along the direction of the ribbons, showed a band structure which is analogous to the Dirac cones of graphene. Unlike silicon surfaces, which are highly reactive to oxygen, the silicene nano-ribbons were found to be resistant to oxygen reactivity

On the theoretical side, recent extensive efforts have been deployed to understand the properties of standalone silicene sheets and nano-ribbons using both tight-binding and density functional theory calculations. Unlike graphene it is demonstrated that silicene sheets are stable only if a small buckling (0.44 Å) is present. The electronic properties of silicene nano-ribbons and silicene sheets were found to resemble those of graphene.

Although this is a fairly new avenue, the already obtained outcome from these important first steps in understanding silicene showed promising features that could give a new future to silicon in the electronics industry, thus opening a promising route toward wide-range applications. In this review, we plan to introduce silicene by presenting the available experimental and theoretical studies performed to date, and suggest future directions to be explored to make the synthesis of silicene a viable one.

This work has been carried out in collaboration with CINaM, ISMO, IM2NP, Physikalisch-Chemisches Institut, Wright State University, and Central Florida University.

 

Structure Atomique Tridimensionnelle de Nano-clusters Métastables dans les Semi-conducteurs Dopés.
Three-Dimensional Atomic Structure of Metastable Nanoclusters in Doped Semiconductors
Martin Couillard, Guillaume Radtke, Andrew P. Knights, and Gianluigi A. Botton
Physical Review Letters 107, 186104 (2011)

DOI: 10.1103/PhysRevLett.107.186104
contact : guillaume.radtke@im2np.fr

Le dopage des semi-conducteurs est une étape fondamentale pour le contrôle de leurs propriétés électriques, magnétiques ou optiques. Cependant, l’incorporation d’impuretés dans des semi-conducteurs communs comme le silicium, visant en général l’obtention d’une solution solide homogène afin d’en exploiter correctement les propriétés, représente souvent un état instable de la matière et, de ce fait, reste difficile à contrôler. Par exemple, la formation de clusters n’impliquant que quelques dizaines d’atomes d’impuretés dans le massif du semi-conducteur hôte est suspectée dans de nombreux cas mais leur caractérisation reste extrêmement délicate : leur faible volume limite l’utilisation de techniques de diffraction (X ou électronique) tandis que les inhomogénéités structurales du matériau compliquent fortement l’interprétation des signaux de nombreuses techniques (EXAFS, annihilation de positrons, spectroscopie de rétrodiffusion de Rutherford, ...), moyennés sur des populations mixtes d’impuretés isolées et de clusters. Une description non ambiguë de ces nano-phases passe donc en général par une analyse tridimensionnelle dans l’espace direct. Dans ce travail, nous déterminons la structure atomique de clusters métastables de tailles sub-nanométriques de CeSi enterrés dans une matrice de Si cristallin en nous basant sur la microscopie électronique à balayage par transmission corrigée des aberrations. Plus particulièrement, l’analyse combinée d’images acquises dans deux directions cristallographiques nous a permis de déterminer l’ordre chimique local (zinc-blende ou B3) à ces nano-objets. Ce travail illustre les nouvelles possibilités offertes par la microscopie électronique dans la caractérisation quantitative de nano-objets et dans l’étude des processus hors équilibre en matière condensée.

Ce travail a été mené en collaboration avec M. Couillard, G.A. Botton et A.P. Knight à l’université McMaster à Hamilton (Ont.), Canada.

 

lysélénophène

Origin of the different transport properties of electron and hole polarons in an ambipolar polyselenophene-based conjugated polymer
Zhuoying Chen, Matthew Bird, Vincent Lemaur, Guillaume Radtke, Jérôme Cornil, Martin Heeney, Iain McCulloch, and Henning Sirringhaus
Phys. Rev. B 84, 115211 (2011)] Published Tue Sep 27, 2011
DOI:10.1103/PhysRevB.84.115211
contact : zhuoying.chen@im2np.fr

Understanding the mechanisms limiting ambipolar transport in conjugated polymer field-effect transistors (FETs) is of both fundamental and practical interest. Here, we present a systematic study comparing hole and electron charge transport in an ambipolar conjugated polymer, semicrystalline poly(3,3′′-di-n-decylterselenophene) (PSSS). Starting from a detailed analysis of the device characteristics and temperature/charge-density dependence of the mobility, we interpret the difference between hole and electron transport through both the Vissenberg-Matters and the mobility-edge model. To obtain microscopic insight into the quantum mechanical wave function of the charges at a molecular level, we combine charge modulation spectroscopy (CMS) measuring the charge-induced absorption signatures from positive and negative polarons in these ambipolar FETs with corresponding density functional theory (DFT) calculations. We observe a significantly higher switch-on voltage for electrons than for holes due to deep electron trap states, but also a higher activation energy of the mobility for mobile electrons. The CMS spectra reveal that the electrons that remain mobile and contribute to the FET current have a wave function that is more localized onto a single polymer chain than that of holes, which is extended over several polymer chains. We interpret this as evidence that the transport properties of the mobile electrons in PSSS are still affected by the presence of deep electron traps. The more localized electron state could be due to the mobile electrons interacting with shallow trap states in the vicinity of a chemical, potentially water-related, impurity that might precede the capture of the electron into a deeply trapped state.

selected by the editors of PRB as Editors' Suggestion.
Metacinnabar (beta-HgS) : a strong 3D topological insulator with highly anisotropic surface states
F. Virot, R. Hayn, M. Richter and J.Van den Brink
Physical Review Letters, vol. 106, 236806 (10 June 2011)
DOI :
10.1103/PhysRevLett.106.236806
contact Im2np : roland.hayn@im2np.fr

Researchers from the Im2np (CNRS and University of Marseille, France) and the IFW (Dresden, Germany) discovered by ab initio calculations that mercury-sulfide (HgS) is a topological insulator. These new materials act as both insulators and conductors, with their interior preventing the flow of electrical currents while their surfaces allow the movement of charge.

They are very promising for a new generation of electronic devices: much faster (similar to graphene), having low-power consumption and using not only the charge but also the spin of the electron for information processing (spintronics). In recent years, topological insulators have become one of the hottest topics in physics. About ten candidate topological insulator materials have been identified so far.

Mercury-sulfide (HgS) stands out among these because its surface spontaneously forms a pattern of parallel channels that transport charge. Within the channels electric current flows rapidly and unimpeded while charge can barely move from one channel to another. In this way the surface acts as a set of parallel electric wires.

Interestingly, mercury-sulfide crystals have been known since time immemorial, known as cinnabar to the Romans and as vermillion in the Middle Ages, these crystals have a wonderful orange-red hue. The topologically protected metallic surface states are expected in their grey-black phase, known as meta-cinnabar. Somewhat duller in color perhaps, but harboring an intriguing new quantum state of matter. 

In all topological insulators, the spin direction is forced to be perpendicular to the direction of motion of the electrons. Therefore, regular scattering becomes ineffective and cannot cause dissipation. Moreover, a pure spin current has to go hand in hand with a charge current. This is a promising effect for spintronics, an emerging information-processing technology to build efficient transistors and memory devices.

 

Magnetic Couplings in CsV2O5: A New Picture
A. Saul and G. Radtke
Physical Review Letters, vol. 106, 177203 (2011)
DOI: 10.1103/PhysRevLett.106.177203
contact Im2np : guillaume.radtke@im2np.fr

The study of low-dimensional spin-1/2 quantum systems has been a very prolific field of condensed matter physics during the past decades. The family of vanadates, in particular, has provided a rich variety of compounds with different behaviors and topologies such as depleted quasi-two-dimensional square lattice, two-leg ladders, one-dimensional chains, or isolated dimers. Their magnetic structure and properties are primarily determined by the magnitude and the sign of the different effective exchange couplings arising between magnetic ions and therefore on the very details of their atomic and electronic structures. In this framework, the sole consideration of  the geometry of a compound, usually based on a simple analysis of the distances separating the magnetic centers,  is often incomplete and even misleading.  A detailed experimental investigation of (VO)2P2O7 by inelastic neutron scattering demonstrated, for example, that the magnetic properties of this compound, originally considered as an excellent realization of a two-leg spin ladder, were in fact those of an alternating Heisenberg antiferromagnetic chain running perpendicularly to the supposed ladder direction. The determination of exchange couplings is therefore crucial to unravel the complex magnetic behavior of certain compounds but also to guide the design of new systems.

In this work, we demonstrate, based on first-principles calculations and on an extensive use of the broken symmetry formalism, that the magnetic structure of CsV2O5 is made of strongly dimerized alternating 1D chains oriented along the c axis. Moreover, we show that the largest exchange coupling along the chain arises unexpectedly between the structural dimers, via the V4+-O-V5+-O-V4+ superexchange pathway or, in other words, that the structural dimers are not the magnetic dimers. This work should motivate further experimental investigations on this compound while emphasizing the importance of nonmagnetic bridging units in the design and the understanding of new systems.

This work has been done in collaboration with A. Saúl (CINaM, Marseille).

 

Single Layer of Polymeric Fe-Phthalocyanine: An Organometallic Sheet on Metal and Thin Insulating Film
Mathieu Abel, Sylvain Clair, Oualid Ourdjini, Mireille Mossoyan, and Louis Porte
J. Am. Chem. Soc., 133 (5), pp 1203–1205, 2011
DOI : 10.1021/ja108628r
contact Im2np : sylvain.clair@im2np.fr, mathieu.abel@im2np.fr

Supramolecular chemistry on a surface has produced a large variety of atomically controlled systems, but practical applications are seriously restricted by the use of weakly cohesive non-covalent bonds and by the confinement to a metal surface. Here we report on the formation of a well-ordered organometallic sheet consisting of two-dimensional polymeric phthalocyanine. Remarkably, the growth demonstrated on a metal surface can be extended onto a thin insulating film. We thus expect the intrinsic properties to be preserved, and the system should be easily transferable to real devices.

STM image of the 2D Fe-phthalocyanine polymer (poly-FePc) formed on a thin insulating NaCl island deposited on Ag(100) by successive deposition of TCNB and Fe atoms in a 2:1 ratio.

 

Interplay between Structural, Electronic, and Magnetic Degrees of Freedom in Sr3Cr2O8
Radtke, G; Saul, A; Dabkowska, HA; Luke, GM; Botton, GA
PHYSICAL REVIEW LETTERS 105 (3): Art No. 036401 2010
DOI 10.1103/PhysRevLett.105.036401
contact Im2np : guillaume.radtke@im2np.fr

The dimerization of localized spin ½, i.e. the formation of a spin singlet (S=0) magnetic ground state at low temperature, separated from the first excited triplet (S=1) states by a spin gap has been observed in a number of inorganic materials. A particular interest has been devoted to this type of material (for instance TlCuCl3 or BaCuSi2O6) since, in the limit where the interactions between these dimers are weak, the physics of these compounds maps onto the physics of Bose-Einstein condensation.

Sr3Cr2O8 appears as a spin ½ dimer system as Cr adopts the unusual 5+ oxidation state (corresponding to a 3d1 electronic configuration) and as its crystallography favors the formation of dimers built from two (CrO4)3- structural units. In this work we showed, based on a ab initio structural optimization accounting for the strong electron correlation arising within the Cr 3d shell, that the tetrahedrally coordinated Cr5+ ions are responsible for a structural transition (associated to a collective Jahn-Teller distortion) from the high-temperature hexagonal phase to a low-temperature monoclinic structure (at around 275K). The electronic structure, probed in electron energy loss spectroscopy, as well as the magnetic properties are in excellent agreement with our theoretical predictions, confirming the image of a system built from weakly interacting dimers in a singlet ground state. 

This work has been done in collaboration with A. Saúl (CINaM, Marseille), H. A. Dabkowska, G. M. Luke, and G. A. Botton (McMaster University, Canada).

Polarization Switching without Domain Formation at the Intrinsic Coercive Field in Ultrathin Ferroelectric PbTiO3
Highland MJ (Highland, Matthew J.), Fister TT (Fister, Timothy T.), Richard MI (Richard, Marie-Ingrid), Fong DD (Fong, Dillon D.), Fuoss PH (Fuoss, Paul H.), Thompson C (Thompson, Carol), Eastman JA (Eastman, Jeffrey A.), Streiffer SK (Streiffer, Stephen K.), Stephenson GB (Stephenson, G. Brian)
PHYSICAL REVIEW LETTERS    Volume: 105    Issue: 16  Article Number: 167601    Published: OCT 12 2010  
DOI: 10.1103/PhysRevLett.105.167601
contact Im2np : marie-ingrid.richard@im2np.fr

Polarization switching in ferroelectrics has been thought to occur only through the nucleation and growth of new domains. Here we use in situ synchrotron x-ray scattering to monitor switching controlled by applied chemical potential. In sufficiently thin PbTiO3 films, nucleation is suppressed and switching occurs by a continuous mechanism, i.e., by uniform decrease and inversion of the polarization without domain formation. The observed lattice parameter shows that the electric field in the film during switching reaches the theoretical intrinsic coercive field.

coll. : Argonne National Laboratory, Argonne, Illinois 60439, USA | Université Paul Cézanne Aix-Marseille, IM2NP, UMR CNRS 6242, Faculté des Sciences de St Jérôme, 13397 Marseille, France | Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, USA

 

 

Mesoscopic Arrays from Supramolecular Self-Assembly
Clair, S (Clair, Sylvain); Abel, M (Abel, Mathieu); Porte, L (Porte, Louis)
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 49 (44): 8237-8239 2010
DOI 10.1002/anie.201003335
contact Im2np : sylvain.clair@im2np.fr

Well-ordered superarrays of exceptionally large period (22 nm) and involving a substantial number of hexahydroxytriphenylene molecules (20×20 superlattice) form on a Ag(111) surface. The superstructure (see picture; triangular pattern) is an intrinsic property of the system and, remarkably, it does not depend on global surface coverage (lower right: partial structure).

 

Fast Growth Synthesis of GaAs Nanowires with Exceptional Length
Ramdani, MR; Gil, E; Leroux, C; Andre, Y; Trassoudaine, A; Castelluci, D; Bideux, L; Monier, G; Robert-Goumet, C; Kupka, R
NANO LETTERS 10 (5):1836-1841 2010
DOI 10.1021/nl100557d
contact Im2np : christine.leroux@im2np.fr

We report the first synthesis of GaAs nanowires (NWs) by Au-assisted vapor iquid solid (VLS) growth in the novel hydride vapor phase epitaxy (HVPE) environment. Forty micrometer long rodlike (111) monocrystalline GaAs nanowires exhibiting a cubic zinc blende structure were grown in 15 min with a mean density of 106 cm2. The synthesis of such long figures in such a short duration could be explained by the growth physics of near-equilibrium HVPE. VLSHVPE is mainly based on solidification after direct and continuous feeding of the arsenious and GaCl growth precursors through the AuGa liquid catalyst. Fast solidification (170 μm/h) is then assisted by the high decomposition frequency of GaCl. This predominant feeding through the liquid solid interface with no mass and kinetic hindrance favors axial rather than radial growth, leading to twin-free nanowires with a constant cylinder shape over unusual length. The achievement of GaAs NWs several tens of micrometers long showing a high surface to volume ratio may open the field of III V wires, as already addressed with ultralong Si nanowires.

coll. : Clermont Université, Université Blaise Pascal, LASMEA, BP 10448, F-63000 Clermont-Ferrand, France | CNRS, UMR 6602, LASMEA, F-63177 Aubière, France | Université du Sud Toulon-Var, IM2NP, UMR CNRS 6242, BP 20132, 83957 La Garde Cedex, France