Simon Desbief

Semi-metallic polymers

Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly(3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

Nanoscale investigation of the electrical properties in semiconductor polymer–carbon nanotube hybrid materials

The morphology and electrical properties of hybrids of a semiconducting polymer (namely poly(3-hexylthiophene) P3HT) and carbon nanotubes are investigated at the nanoscale with a combination of Scanning Probe Microscopy techniques, i.e., Conductive Atomic Force Microscopy (C-AFM) and time-resolved Current Sensing Force Spectroscopy Atomic Force Microscopy (CSFS-AFM, or PeakForce TUNA™). This allows us to probe the electrical properties of the 15 nm wide P3HT nanofibers as well as the interface between the polymer and single carbon nanotubes. This is achieved by applying controlled, low forces on the tip during imaging, which allows a direct comparison between the morphology and the electrical properties at the nanometre scale.

Superhydrophobic Aluminum Surfaces by Deposition of Micelles of Fluorinated Block Copolymers

Superhydrophobic surfaces are generated by chemisorption on aluminum substrates of fluorinated block copolymers synthesized by reversible addition-fragmentation chain transfer in supercritical carbon dioxide. In an appropriate solvent, those block copolymers can form micelles with a fluorinated corona, which are grafted on the aluminum substrate thanks to the presence of carboxylic acid groups in the corona. Water contact angle and drop impact analysis were used to characterize the wettability of the films at the macroscale, and atomic force microscopy measurements provided morphological information at the micro- and nanoscale. The simple solvent casting of the polymer solution on a hydroxylated aluminum surface results in a coating with multiscale roughness, which is fully superhydrophobic over areas up to 4 cm(2).

Synthesis and Characterization of Nanocomposites Based on Functional Regioregular Poly(3-hexylthiophene) and Multiwall Carbon Nanotubes.

New functionalized poly(3-hexylthiophene)s (P3HT) have been designed and synthesized with the aim of increasing the dispersion of carbon nanotubes (CNT) in solutions and in thin films of semiconducting polymers. Dispersion in solution has been assessed by sedimentation tests while the thin film morphology has been analyzed by TEM and AFM. Both the physisorption of P3HT chains (via pyrene end-groups) or their chemical grafting (onto amine functions generated on the CNT surface) lead to a much better dispersion in solution and in the solid. In thin films, P3HT fibrils are observed to arrange perpendicular to the CNT surface, which can be understood on the basis of molecular modeling simulations. Finally, the effect of dispersing those P3HT/CNT nanocomposites in bulk-heterojunction P3HT-based photovoltaic devices has been evaluated.

Synthesis and characterisation of π-conjugated polymer/silica hybrids containing regioregular ionic polythiophenes

Highly regioregular poly{3-[n-(1-methylimidazolium-3-yl)alkyl]thiophene-2,5-diyl bromide} (n = 4, 6, 8) (10–12) has been prepared by reaction of poly[3-(n-bromoalkyl)thiophene-2,5-diyl] (n = 4, 6, 8) (7–9) with 1-methylimidazole. Acid-catalyzed hydrolysis and polycondensation of tetraethoxysilane (TEOS) were carried out in the presence of these polymers. The strong ionic interactions between the imidazolium moieties of the polythiophene and the charged silanol groups enabled the homogeneous dispersion of the polythiophene within silica. The resulting composite materials were characterised by SEM, TEM and tapping mode atomic force microscopy (TM-AFM). The optical and thermal properties were investigated by UV and TGA measurements.

PTFE Surface Etching in the Post-discharge of a Scanning RF Plasma Torch: Evidence of Ejected Fluorinated Species

The texturization of poly(tetrafluoroethylene) (PTFE) surfaces is achieved at atmospheric pressure by using the post-discharge of a radio-frequency plasma torch supplied in helium and oxygen gases. The surface properties are characterized by contact angle measurement, X-ray photoelectron spectroscopy and atomic force microscopy. We show that the plasma treatment increases the surface hydrophobicity (with water contact angles increasing from 115 to 1558) only by modifying the PTFE surface morphology and not the stoichiometry. Measurements of sample mass losses correlated to the ejection of CF2 fragments from the PTFE surface evidenced an etching mechanism at atmospheric pressure.

Synthesis of poly[(4,4′-(dihexyl)dithieno(3,2-b;2′,3′-d)silole)] and copolymerization with 3-hexylthiophene: new semiconducting materials with extended optical absorption

The synthesis of poly[(4,4′-(dihexyl)dithieno(3,2-b;2′,3′-d)silole)] has been carried out through Kumada catalyst transfer polycondensation and its copolymerization with 3-hexylthiophene (3HT) leads to well-defined diblock copolymers in terms of molecular characteristics. These copolymers show a wider absorption window than P3HT, which is of potential interest for photovoltaic applications.

Confined wrinkling: impact on pattern morphology and periodicity

Wrinkling instabilities are observed in compressed multilayers made of materials with contrasted properties. Here, we study the influence of confinement, i.e. using foundations with dimensions smaller than 50 nm, on the wrinkled patterns. We show that confinement generates a change in morphology from labyrinths to a dotted pattern with hexagonal symmetry. This transition occurs when the amplitude of the wrinkles becomes comparable to the thickness of the upper membrane, i.e. A ≃ h. For very thin foundations (h < 100 nm), a drastic deviation from the usual evolution of the wavelength with the foundation thickness is observed. This new regime was explained by considering the van der Waals interactions between the metal layer and the substrate. The VDW energy term was involved in a new scaling law model that agrees with the experimental data.

Single and binary self-assembled monolayers of phenyl- and pentafluorophenyl-based silane species, and their phase separation with octadecyltrichlorosilane.

In this paper, we first present the study of the formation of phenyltrichlorosilane film and self-assembled monolayers of phenylalkyltrichlorosilane (PATCl), pentafluoro-phenylalkyltrichlorosilane (PFATCl), and a mixture of the two, on silicon covered by its native oxide. These monolayers are shown to grow in two steps with characteristic time constants. The first step is characterized by a similar time constant of growth for all the studied trichlorosilane molecules and attributed to chemisorption. The second step corresponds to the arrangement between molecules, accelerated by the presence of the short alkyl chain (3-4 carbon atoms), and by mixing phenyl and pentafluoro-phenyl terminal moieties, which is accounted for by hydrogen bonding CH···FC and/or attractive quadrupolar interactions within a face-to-face phenyl/pentafluoro-phenyl alternating stack arrangement. Such results should allow improvement of intermolecular stacking within conjugated molecular domains, which is particularly important for molecular electronic devices. In the second part, we studied how PATCl, PFATCl, and their mixture phase separate with octadecyltrichlorosilane (OTS) molecules in various ratios. The way to improve phase separation was studied modifying aromatic ring to ring as well as aromatic-aliphatic interactions. OTS island size and coverage are shown to be smaller with the aromatic phase that involves stronger ring to ring interactions, i.e., attractive interactions between the phenyl species by mixing phenyl and pentafluoro-phenyl rings. The best phase separation is obtained with PFATCl as the aromatic molecule. If nanoislands of aromatic molecules could not be observed in these experiments, we show that they are attainable by mixing OTS and aromatic small organotriethoxysilanes whose grafting kinetics is slower. These results pave the way to the control improvement of the composition and nanostructuration of SAMs, essential for their further use within molecular devices.

Etching Processes of Polytetrafluoroethylene Surfaces Exposed to He and He–O2 Atmospheric Post-discharges

A comparative study of polytetrafluoroethylene (PTFE) surfaces treated by the post-discharge of He and He-O(2) plasmas at atmospheric pressure is presented. The characterization of treated PTFE surfaces and the species involved in the surface modification are related. In pure He plasmas, no significant change of the surface has been observed by X-ray photoelectron spectroscopy (XPS), dynamic water contact angles (dWCA) and atomic force microscopy (AFM), in spite of important mass losses recorded. According to these observations, a layer-by-layer physical etching without any preferential orientation is proposed, where the highly energetic helium metastables are the main species responsible for the scission of -(CF(2))(n)- chains. In He-O(2) plasmas, as the density of helium metastables decreases as a function of the oxygen flow rate, the treatment leads to fewer species ejected from the PTFE surfaces (in agreement with mass loss measurements and the detection of fluorinated species onto aluminum foil). However, the dWCA and AFM measurements show an increase in the hydrophobicity and the roughness of the surface. The observed alveolar structures are assumed to be caused by an anisotropic etching where the oxygen atoms etch mainly the amorphous phase.