[Show abstract][Hide abstract] ABSTRACT: The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer-polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semi-crystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈ 140°C) at temperatures of 90°C, 100°C and 110°C for time periods from 2 min up to 250 minutes. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.
[Show abstract][Hide abstract] ABSTRACT: Glass forming systems are characterized by a stability against crystallization upon heating and by the easiness with which their liquid phase can be tranformed into a solid lacking of long-range order upon cooling (glass forming ability). Here, we report the thickness dependence of the thermal phase transition temperatures of poly(L-lactide acid) thin films supported onto solid substrates. The determination of the glass transition, cold crystallization and melting temperatures down to a thickness of 6 nm, permitted us to build up parameters describing glass stability and glass forming ability. We observed a strong influence of the film thickness on the latter, while the former is not affected by 1D confinement. Further experiments permitted us to highlight key structural morphology features giving insights to our ellipsometric results via a physical picture based on the changes in the free volume content in proximity of the supporting interfaces.
[Show abstract][Hide abstract] ABSTRACT: The origins of specific polymorphic phases within thin films are still not well understood. The polymorphism of the molecule dioctyl-terthiophene is investigated during the presence of a silicon-oxide surface during the crystallisation process. It is found that a monolayer of molecules forms two-dimensional crystals on the surface. In the case of thicker films crystalline islands are formed, A comparison of the three polymorphic phases observed within thin films and the thermodynamically more stable single crystal phases reveals distinct differences which can be related to an adaption of the molecular packing with the flat surface of the substrate.
Chemical Physics Letters 04/2015; 22. DOI:10.1016/j.cplett.2015.04.027 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Substrate-induced phases (SIPs) are polymorphic phases which are found in thin films of a material and are different from the single crystal or "bulk" structure of a material. In this work, we investigate the presence of a SIP in the family of benzothieno[3,2-b]benzothiophene (BTBT) organic semiconductors and the effect of aging and solvent vapor annealing on the film structure. Through extensive X-ray structural investigations of spin coated films, we find a SIP with a significantly different structure to that found in single crystals of the same material forms; the SIP has a herringbone motif while single crystals display layered π-π stacking. Over time, the structure of the film is found to slowly convert to the single crystal structure. Solvent vapor annealing initiates the same structural evolution process but at a greatly increased rate, and near complete conversion can be achieved in a short period of time. As properties such as charge transport capability are determined by the molecular structure, this work highlights the importance of understanding and controlling the structure of organic semiconductor films and presents a simple method to control the film structure by solvent vapor annealing.
[Show abstract][Hide abstract] ABSTRACT: The shape transition in the neutron-rich Os isotopes is studied by investigating the neutron-rich 196Os nucleus through in-beam γ-ray spectroscopy using a two-proton transfer reaction from a 198Pt target to a 82Se beam. The beam-like recoils were detected and identified with the large-acceptance magnetic spectrometer PRISMA, and the coincident γ rays were measured with the advanced gamma tracking array (AGATA) demonstrator. The de-excitation of the low-lying levels of the yrast-band of 196Os were identified for the first time. The results are compared with state-of-the-art beyond-mean-field calculations, performed for the even-even 188−198Os isotopes. The new results suggest a smooth transition in the Os isotopes from a more axial rotational behavior towards predominately vibrational nuclei through triaxial configurations. An almost perfect γ-unstable/triaxial rotor yrast band is predicted for 196Os which is in agreement with the experimentally measured excited states.
Physical Review C 08/2014; 90(021301(R)). DOI:10.1103/PhysRevC.90.021301 · 3.73 Impact Factor
[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 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 04/2014; 9(4):e92999. DOI:10.1371/journal.pone.0092999 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, we present an exploratory experiment performed at REX-ISOLDE to investigate neutron-rich Sr, Y and Zr nuclei around mass A = 100 by cluster transfer reactions of radioactive Rb-98/Sr-98 beams on a Li-7 target. The goal of the experiment is to test the potential of cluster-transfer mechanism to study the low-lying structure of neutron-rich nuclei, performing, as a first case, a gamma-spectroscopy study of the structure in deformed nuclei beyond N = 60 populated, so far, only via p decay and spontaneous fission experiments.
33rd Mazurian Lakes Conference on Physics; 02/2014
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1140/epje/i2013-13003-6 · 1.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A cold neutron induced fission experiment recently took place at the Institute Laue-Langevin (ILL) in Grenoble. The neutron beam was provided by the nuclear reactor facility at ILL and the detector setup that was used for the gamma-spectroscopy of the fission products consisted mainly of the detectors of the EXOGAM array , thereby the name of the campaign is EXILL. The main purpose of our measurement was to investigate the nuclei in the region with N = 50 close to Ni-78 as well as the nuclei close to the N = 82 shell closure. In this paper, the motivation of the experiment is described as well as the experimental setup and the status of the ongoing data analysis.
The European Physical Journal Conferences 01/2013; 62:01006. DOI:10.1051/epjconf/20136201006
[Show abstract][Hide abstract] 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; 405. DOI:10.1016/j.jcis.2012.12.019 · 3.37 Impact Factor