M Sferrazza

Université Libre de Bruxelles, Bruxelles, Brussels Capital Region, Belgium

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Publications (95)197.46 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A detailed structural study of the bulk and thin film phases observed for two potential high-performance organic semiconductors has been carried out. The molecules are based on [1]benzothieno[3,2-b]benzothiophene (BTBT) as conjugated core and octyl side groups which are anchored either symmetrically at both sides of the BTBT core (C8-BTBT- C8) or non-symmetrically at one side only (C8-BTBT). Thin films of different thickness (8 nm to 85 nm) have been prepared by spin-coating for both systems and analyzed by combining specular and grazing incidence X-ray diffraction. In the case of C8-BTBT- C8, the known crystal structure obtained from single crystal investigations is observed within all thin films, down to a film thickness of 9 nm. In the case of C8-BTBT, the crystal structure of the bulk phase has been determined from X-ray powder diffraction data with a consistent matching of experimental and calculated X-ray diffraction patterns (Rwp = 5.8%). The packing arrangement of C8-BTBT is similar to that of C8-BTBT- C8, that is, consisting in a lamellar structure with molecules arranged in a "herringbone" fashion, yet with lamellae composed of two head-to-head (or tail-to-tail as the structure is periodic) superimposed molecules instead of only one molecule for C8-BTBT-C8. As for C8-BTBT- C8, we demonstrate that the same phase is observed in bulk and thin films for C8-BTBT whatever the film thickness investigated.
    ACS Applied Materials & Interfaces 08/2014; · 5.01 Impact Factor
  • Caroline Housmans, Michele Sferrazza, Simone Napolitano
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    ABSTRACT: Recent experimental evidence showed a strong correlation between the behavior of polymers under confinement and the presence of a layer irreversibly adsorbed onto the supporting substrate, hinting at the possibility to tailor the properties of ultrathin films by controlling the adsorption kinetics. At the state of the art, however, the study of physisorption of polymer melts is mainly limited to theory and simulations. To overcome this gap, we present the results of an extensive investigation of the kinetics of irreversible adsorption of entangled melts of polystyrene onto silicon oxide. We show that the process of chain pinning proceeds via a first order reaction mechanism, which slows down at large surface coverage, and the adsorbed amount scales with the predictions of reflected random walk. We propose an analytical form of the time evolution of the thickness of the adsorbed layer with two well-defined regimes: linear at short times and logarithmic at longer times, separated by a temperature independent crossover thickness and a molecular weight independent crossover time, in line with simulations and theory.
    Macromolecules 05/2014; 47(10):3390–3393. · 5.93 Impact Factor
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    ABSTRACT: Probing molecules using perdeuteration (i.e deuteration in which all hydrogen atoms are replaced by deuterium) is extremely useful in a wide range of biophysical techniques. In the case of lipids, the synthesis of the biologically relevant unsaturated perdeuterated lipids is challenging and not usually pursued. In this work, perdeuterated phospholipids and sterols from the yeast Pichia pastoris grown in deuterated medium are extracted and analyzed as derivatives by gas chromatography and mass spectrometry respectively. When yeast cells are grown in a deuterated environment, the phospholipid homeostasis is maintained but the fatty acid unsaturation level is modified while the ergosterol synthesis is not affected by the deuterated culture medium. Our results confirm that the production of well defined natural unsaturated perdeuterated lipids is possible and gives also new insights about the process of desaturase enzymes.
    PLoS ONE 01/2014; 9(4):e92999. · 3.73 Impact Factor
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    ABSTRACT: Polymorphism of α,ω-dioctylterthiophene (DOTT) thin films (less than 100 nm-thick) deposited onto standard silicon substrates is investigated by combining specular and in-plane grazing incidence X-ray diffraction measurements. Two crystal phases are revealed: the bulk phase (b) and the surface-induced phase (s). Previous results showed that s-phase was the most favored close to the substrate surface while b-phase is predominant at the DOTT-air interface. We show here that the relative proportion of these polymorphs is related to sample preparation. Indeed, the preferential crystallization of one phase or the other can be tuned as a function of substrate temperature during film deposition, s-phase being mainly present at high substrate temperature while b-phase is most observed at low substrate temperature. Particularly noteworthy is that thin films exhibiting only b-phase or only s-phase can be obtained according to sample preparation. Furthermore, when samples are annealed above the melting temperature (smectic G phase), the polymorph observed after cooling back to room temperature depends upon the cooling rate, s-phase being predominant at high cooling rate while it is rather b-phase for slow cooling. This behavior is independent of the crystalline phase that was present at room temperature before annealing.
    Journal of Crystal Growth 01/2014; 386:128–134. · 1.55 Impact Factor
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    ABSTRACT: The concentration profile of deuterated myoglobin (Mb) adsorbed onto polystyrene substrates displaying poly(ethylene glycol) (PEG) brushes is characterized by neutron reflectometry (NR). The method allows to directly distinguish between primary adsorption at the grafting surface, ternary adsorption within the brush, and secondary adsorption at the brush outer edge. It complements depth-insensitive standard techniques, such as ellipsometry, radioactive labeling and quartz crystal microbalance. The study explores the effect of the PEG polymerization degree, N, and the grafting density, σ, on Mb adsorption. In the studied systems there is no indication of secondary or ternary adsorption but there is evidence of primary adsorption involving a dense inner layer at the polystyrene surface. For sparsely grafted brushes the primary adsorption involves an additional dilute outer protein layer on top of the inner layer. The amount of protein adsorbed in the inner layer is independent of N but varies with σ, while for the outer layer it is correlated to the amount of grafted PEG and thus sensitive to both N and σ. The use of deuterated proteins enhances the sensitivity of NR and enables monitoring exchange between deuterated and hydrogenated species.
    Langmuir 10/2013; · 4.38 Impact Factor
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    ABSTRACT: The order in molecular monolayers is a crucial aspect for their technological application. However, the preparation of defined monolayers by spin-coating is a challenge, since the involved processes are far from thermodynamic equilibrium. In the work reported herein, the dynamic formation of dioctyl-benzothienobenzothiophene monolayers is explored as a function of temperature by using X-ray scattering techniques and atomic force microscopy. Starting with a disordered monolayer after the spin-coating process, post-deposition self-reassembly at room temperature transforms the initially amorphous layer into a well-ordered bilayer structure with a molecular herringbone packing, whereas at elevated temperature the formation of crystalline islands occurs. At the temperature of the liquid-crystalline crystal-smectic transition, rewetting of the surface follows resulting in a complete homogeneous monolayer. By subsequent controlled cooling to room temperature, cooling-rate-dependent kinetics is observed; at rapid cooling, a stable monolayer is preserved at room temperature, whereas slow cooling causes bilayer structures. Increasing the understanding and control of monolayer formation is of high relevance for achieving ordered functional monolayers with defined two-dimensional packing, for future applications in the field of organic electronics.
    ChemPhysChem 05/2013; · 3.35 Impact Factor
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    ABSTRACT: The effect of terminally anchored chains on the structure of lipid bilayers adsorbed at the solid/water interface was characterized by neutron reflectivity. In the studied system, the inner leaflet, closer to the substrate, consisted of head-deuterated 1,2-distearoyl-sn-glycero-3-phosphorylcholine (DSPC) and the outer leaflet comprised a mixture of DSPC and polyethylene glycol (PEG) functionalized 1,2-distearoyl-sn-glycero-3-phosphoethanolamine. The DSPC headgroups were deuterated to enhance sensitivity and demarcate the bilayer/water interface. The effect on the inner and outer headgroup layers was characterized by w (1/2) , the width at half-height of the scattering length density profile. The inner headgroup layer was essentially unperturbed while w (1/2) of the outer layer increased significantly. This suggests that the anchored PEG chains give rise to headgroup protrusions rather than to blister-like membrane deformations.
    The European Physical Journal E 01/2013; 36(1):9818. · 2.18 Impact Factor
  • Nicolas Brouette, Michele Sferrazza
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    ABSTRACT: The Langmuir-Schaefer technique allows horizontal transfer of monolayer films of amphiphilic molecules from an air-water interface to a hydrophobic solid substrate. Spectroscopic ellipsometry has been used to estimate the transfer ratio of Polyethylene Glycol (PEG) polymer brushes prepared by this Langmuir-Schaefer method. We measured the transfer ratio for various degrees of polymerization and surface densities at the air-water interface. We do not observe a significant change of transfer ratio with the degree of polymerization. We also show that the transfer ratio is lower from an air-water monolayer film in the liquid phase than from a monolayer in the solid phase. Control over the transfer ratio is a crucial step in many experiments since the efficacy of the repression of proteins depends strongly on the assumption of the characteristics of the brushes.
    Journal of Colloid and Interface Science 12/2012; · 3.17 Impact Factor
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    ABSTRACT: The structure of adsorbed globular protein layers on hydrophobic surfaces is elucidated in detail by combining the use of a fully deuterated protein, myoglobin, and the neutron reflectivity technique. The hydrophobic surfaces consist of grafted self-assembled monolayer of octadecyltrichlorosilane (OTS) and polystyrene (PS) layer on silicon substrates. Different protein concentrations ranging from 1mg/ml to 0.01mg/ml are used. On the OTS surface and for low protein concentration, the adsorbed protein layer consists of a dense layer of thickness around 13Å indicating that proteins are denaturated when adsorbed on the hydrophobic interface - myoglobin being a globular protein with an average diameter of about 40Å. At high protein concentration, an additional layer is observed on the top of this first denaturated layer. The thickness of this layer corresponds roughly to the dimensions of the myoglobin suggesting that additional proteins in their bulk conformation are adsorbed on the top. In the case of PS, the protein is significantly less flattened at the interface, PS being a less hydrophobic surface.
    Journal of Colloid and Interface Science 09/2012; · 3.17 Impact Factor
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    ABSTRACT: We demonstrate using neutron reflectometry that the internal interfaces in a trilayer system of two identical thick polystyrene layers sandwiching a much thinner (deuterated) poly(methyl methacrylate) layer 15 nm thick (viscosity matched with the polystyrene layers) increase in roughness at the same rate. When the lower polystyrene layer is replaced with a layer of the same polymer of much greater molecular mass, two different growths of the interfaces are observed. From the growth of the interface for this asymmetric case in the solid regime using the theoretical prediction of the spinodal instability including slippage at the interface, a value of the Hamaker constant of the system has been extracted in agreement with the calculated value. For the symmetric case the rise time of the instability is much faster.
    Physical Review E 09/2012; 86(3-1):032801. · 2.31 Impact Factor
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    ABSTRACT: Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.
    Langmuir 05/2012; 28(22):8530-6. · 4.38 Impact Factor
  • Wilfried Béziel, Michele Sferrazza
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    ABSTRACT: We have investigated the dewetting behaviour of a thin polystyrene film on top of cross-linked network of the same polymer. By changing the cross-linking density, the dynamic of the dewetting is modified. The behaviour of the contact angle and dewetting velocity can be related to the interfacial width between the polymers. We observe moreover that while the lost of entropy of the network connected to the penetration of the free chains will favour the dewetting; the possible presence of “connectors” between the layers will tend to stabilize the wetting. By modifying the entropy of the mesh by swelling the cross-linked system with a reservoir of the same polymer, the mesh is saturated and a stable interface without interdiffusion between the layers is obtained: in this case a constant contact angle and slippage length are obtained. Networks with higher cross-linking density are more difficult in general to swell – the mesh size is smaller - and the complete saturation is not reached.
    Macromolecular Symposia 05/2012; 315(1).
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    ABSTRACT: We report on the observation of a new isomeric state in $^{68}$Ni. We suggest that the newly observed state at 168(1) keV above the first 2$^+$ state is a $\pi(2p-2h)$ 0$^{+}$ state across the major Z=28 shell gap. Comparison with theoretical calculations indicates a pure proton intruder configuration and the deduced low-lying structure of this key nucleus suggests a possible shape coexistence scenario involving a highly deformed state.
    Physical Review C 02/2012; 85(3). · 3.72 Impact Factor
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    ABSTRACT: At the early stages of the division process in Escherichia coli, the protein FtsZ forms a septal ring at the midcell. This Z-ring causes membrane constriction during bacterial division. The Z-ring associates to the lipid membrane through several membrane proteins, ZipA among them. Here, a simplified FtsZ-ZipA model was reconstituted onto Langmuir monolayers based in E. coli polar lipid extract. Brewster angle and atomic force microscopy have revealed membrane FtsZ-polymerization upon GTP hydrolysis. The compression viscoelasticity of these monolayers has been also investigated. The presence of protein induced softening and fluidization with respect to the bare lipid membrane. An active mechanism, based on the internal forces stressed by FtsZ filaments and transduced to the lipid membrane by ZipA, was suggested to underlie the observed behavior.
    Langmuir 02/2012; 28(10):4744-53. · 4.38 Impact Factor
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    ABSTRACT: The structure of dense poly(N-isopropylacrylamide) (PNIPAM) brushes and oligo(ethylene glycol) (OEG) monolayers has been probed using neutron reflectometry and ellipsometry. The PNIPAM brush is swollen below the Lower Critical Solution Temperature (LCST) of 32 ∘C and is collapsed at 37 ∘C. Neutron reflectivity shows that below the LCST, the brush is described by a two-layer model: an inner dense layer and a hydrated outer layer. Above the LCST the collapsed brush forms a homogenous layer. With a fully deuterated myoglobin protein to increase the neutron scattering length density contrast, the reflectivity data show no detectable primary adsorption on the grafted OEG surface. A bound on the ternary adsorption onto PNIPAM chains forming dense brushes below and above the LCST is obtained.
    The European Physical Journal Special Topics 01/2012; 213(1):343-353. · 1.80 Impact Factor
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    ABSTRACT: Cited By (since 1996): 1, Export Date: 15 December 2012, Source: Scopus
    RSC Advances 01/2012; 2(10):4404-4414. · 2.56 Impact Factor
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    ABSTRACT: The formation of solvent-cast, poly(methyl methacrylate) (PMMA) thin films from dilute bromobenzene solutions was studied using an ellipsometry technique. Bromobenzene has a relatively high refractive index (compared to PMMA), which provides contrast in ellipsometry, allowing the concentration to be determined. The solvent also has a relatively low evaporation rate, which makes the film formation slow enough to capture via the technique. The formation of the glassy film is thus studied in situ, and information on solvent and void concentration in the thin film during the film formation process is obtained. There is evidence that nanovoids (representing intramolecular space) develop in the film when solvent evaporates. Copyright © 2011 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 11/2011; 43(11). · 1.22 Impact Factor
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    ABSTRACT: The protein resistance of poly(N-isopropylacrylamide) brushes grafted from silicon wafers was investigated as a function of the chain molecular weight, grafting density, and temperature. Above the lower critical solution temperature (LCST) of 32 °C, the collapse of the water-swollen chains, determined by ellipsometry, depends on the grafting density and molecular weight. Ellipsometry, radio assay, and fluorescence imaging demonstrated that, below the lower critical solution temperature, the brushes repel protein as effectively as oligoethylene oxide-terminated monolayers. Above 32 °C, very low levels of protein adsorb on densely grafted brushes, and the amounts of adsorbed protein increase with decreasing brush-grafting-densities. Brushes that do not exhibit a collapse transition also bind protein, even though the chains remain extended above the LCST. These findings suggest possible mechanisms underlying protein interactions with end-grafted poly(N-isopropyl acrylamide) brushes.
    Langmuir 06/2011; 27(14):8810-8. · 4.38 Impact Factor
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    Polymer Journal - POLYM J. 01/2011; 43(2):214-217.
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    ABSTRACT: An experimental investigation of the surface modification of polytetrafluoroethylene (PTFE) by an Ar and Ar/O2 plasma created with an atmospheric-pressure radio frequency (r.f.) torch is presented here. The surfaces were analyzed by atomic force microscopy (AFM), XPS and water contact angle (WCA) to get an insight of the surface morphology and chemistry. An increase of roughness is observed with the Ar/O2 plasma treatment. The WCA analysis shows that these surfaces are more hydrophobic than pristine PTFE; a contact angle of 135° was measured. When a PTFE surface is treated by Ar plasma, no roughening or significant change of the surface morphology and chemistry of PTFE was observed. The effects of the Ar and O2 fluxes on the PTFE surface treatment were analyzed, as well as the effect of the power and treatment time. The plasma phase was also analyzed by optical emission spectroscopy, and some correlations with the treatment efficiency of the plasma are made. The chemistry on the surface is finally discussed and the competition between etching and re-deposition chemical reactions on the surface is proposed as a possible explanation of the results. Copyright © 2010 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 04/2010; 42(6‐7):1014 - 1018. · 1.22 Impact Factor

Publication Stats

704 Citations
197.46 Total Impact Points

Institutions

  • 2005–2014
    • Université Libre de Bruxelles
      • • Department of Physics
      • • Faculty of Sciences
      • • Analytical and Interface Chemistry Unit (CHANI)
      Bruxelles, Brussels Capital Region, Belgium
  • 2013
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 2012
    • Graz University of Technology
      • Institut für Festkörperphysik
      Graz, Styria, Austria
  • 1994–2007
    • Purdue University
      • Department of Chemistry
      West Lafayette, IN, United States
  • 2001–2006
    • University of Surrey
      • Department of Physics
      Guildford, ENG, United Kingdom
  • 1996–2003
    • University of Cambridge
      • Department of Physics: Cavendish Laboratory
      Cambridge, ENG, United Kingdom
  • 2002
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1993–1996
    • INFN - Istituto Nazionale di Fisica Nucleare
      Frascati, Latium, Italy
  • 1993–1995
    • North Carolina State University
      Raleigh, North Carolina, United States