Adam Kiersnowski

Microstructure Evolution and Device Performance in Solution-Processed Polymeric Field-Effect Transistors: The Key Role of the First Monolayer

Probing the role of the first monolayer in the evolution of the film polymer microstructure is essential for the fundamental understanding of the charge carrier transport in polymeric field-effect transistors (FETs). The monolayer and its subsequent microstructure of a conjugated polymer [poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT] film were fabricated via solution deposition by tuning the dip-coating speed and were then studied as accumulation and transporting layers in FETs. Investigation of the microstructure of the layers prepared at different coating velocities revealed that the monolayer serves as an important base for further development of the film. Significant improvement of the charge carrier transport occurs only at a critical multilayer network density that establishes the required percolation pathways for the charge carriers. Finally, at a low dip-coating speed, the polymer chains are uniaxially oriented, yielding pronounced structural anisotropy and high charge carrier mobilities of 1.3 cm(2) V(-1) s(-1) in the alignment direction.

Graphene nanoribbon blends with P3HT for organic electronics.

In organic field-effect transistors (OFETs) the electrical characteristics of polymeric semiconducting materials suffer from the presence of structural/morphological defects and grain boundaries as well as amorphous domains within the film, hindering an efficient transport of charges. To improve the percolation of charges we blend a regioregular poly(3-hexylthiophene) (P3HT) with newly designed N = 18 armchair graphene nanoribbons (GNRs). The latter, prepared by a bottom-up solution synthesis, are expected to form solid aggregates which cannot be easily interfaced with metallic electrodes, limiting charge injection at metal-semiconductor interfaces, and are characterized by a finite size, thus by grain boundaries, which negatively affect the charge transport within the film. Both P3HT and GNRs are soluble/dispersible in organic solvents, enabling the use of a single step co-deposition process. The resulting OFETs show a three-fold increase in the charge carrier mobilities in blend films, when compared to pure P3HT devices. This behavior can be ascribed to GNRs, and aggregates thereof, facilitating the transport of the charges within the conduction channel by connecting the domains of the semiconductor film. The electronic characteristics of the devices such as the Ion/Ioff ratio are not affected by the addition of GNRs at different loads. Studies of the electrical characteristics under illumination for potential use of our blend films as organic phototransistors (OPTs) reveal a tunable photoresponse. Therefore, our strategy offers a new method towards the enhancement of the performance of OFETs, and holds potential for technological applications in (opto)electronics.

Exfoliation of montmorillonite in protein solutions

In the study we demonstrate a method to obtain stable, exfoliated montmorillonite-protein complexes by adsorption of the proteins extracted from hen-egg albumen. Analysis of the process by means of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the complexes are formed by sequential adsorption of ovotransferrin, ovalbumins, ovomucoid and lysozyme on the surface of the silicate. Structural studies performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the adsorption of ovotransferrin and albumins is accompanied by disintegration of clay stacks into discrete platelets. Further analysis by dynamic light scattering (DLS) revealed that at protein to silicate weight ratios exceeding 20, the synergistic adsorption of albumen components leads to reaggregation of silicate platelets into disordered, microgel-like particles. By means of DLS it was found that exfoliation predominantly leads to formation of particles with average hydrodynamic radii (R(h)) of 0.19 μm while their aggregation causes formation of particles having R(h) in of approx. 0.5 μm and larger.

Bulk Heterojunction Nanomorphology of Fluorenyl Hexa-peri-hexabenzocoronene–Fullerene Blend Films

In this study, the nanomorphology of fluorenyl hexa-peri-hexabenzocoronene:[6,6]-phenyl C61-butyric acid methyl ester (FHBC:PC61BM) absorber layers of organic solar cells was investigated. Different electron microscopical techniques, atomic force microscopy, and grazing incidence wide-angle X-ray scattering were applied for a comprehensive nanomorphology analysis. The development of the nanomorphology upon sample annealing and the associated change of the device performance were investigated. It was shown that the annealing process enhances the phase separation and therefore the bulk heterojunction structure. Due to π-π stacking, the FHBC molecules assemble into columnar stacks, which are already present before annealing. While the nonannealed sample consists of a mixture of homogeneously distributed PC61BM molecules and FHBC stacks with a preferential in-plane stack orientation, crystalline FHBC precipitates occur in the annealed samples. These crystals, which consist of hexagonal arranged FHBC stacks, grow with increased annealing time. They are distributed homogeneously over the whole volume of the absorber layer as revealed by electron tomography. The FHBC stacks, whether in the two phase mixture or in the pure crystalline precipitates, exhibit an edge-on orientation, according to results from grazing incidence wide-angle X-ray scattering (GIWAXS), dark-field transmission electron microscopy (DF TEM) imaging and selective area electron diffraction (SAED). The best solar cell efficiencies were obtained after 20 or 40 s sample annealing. These annealing times induce an optimized degree of phase separation between donor and acceptor material.

Killing of Gram-Negative Bacteria with Normal Human Serum and Normal Bovine Serum: Use of Lysozyme and Complement Proteins in the Death of Salmonella Strains O48

Serum is an environment in which bacterial cells should not exist. The serum complement system provides innate defense against microbial infections. It consists of at least 35 proteins, mostly in pre-activated enzymatic forms. The activation of complement is achieved through three major pathways: the classical, alternative, and lectin. Lysozyme, widely present in body fluids, catalyzes the hydrolysis of β 1,4 linkage between N-acetyloglucosamine and N-acetylmuramic acid in the bacterial cell wall and cooperates with the complement system in the bactericidal action of serum. In this study, ten strains of serotype O48 Salmonella, mainly associated with warm-blooded vertebrates and clinically important causing diarrhea in infants and children, were tested. The results demonstrated that the most efficient killing of Salmonella O48 occurred when all the components of normal bovine serum (NBS) and normal human serum (NHS) cooperated. To prove the role of lysozyme in the bactericidal activity of bovine and human serum, the method of serum adsorption onto bentonite (montmorillonite, MMT) was used. In order to investigate structural transitions accompanying the adsorption of serum components, we applied X-ray diffraction methods. The results of this investigation suggested that apart from lysozyme, other proteins (as, e.g., C3 protein or IgG immunoglobulin) were adsorbed on MMT particles. It was also shown that Ca2+ cations can be adsorbed on bentonite. This may explain the different sensitivities of the serovars belonging to the same O48 Salmonella serotype to NBS and NHS devoid of lysozyme.

Improved charge carrier transport in ultrathin poly(3-hexylthiophene) films via solution aggregation

Field-effect transistors based on poly(3-hexylthiophene) (P3HT) bulk films exhibit maximum charge carrier mobilities of up to 0.1 cm2 V−1 s−1. However, reducing the thickness of the polymer film beyond 10 nm results in a significant deterioration of the charge transporting properties. In our work, we demonstrate a strategy towards ultrathin (i.e. thinner than 10 nm) polymer layers with charge carrier mobilities identical to bulk films. The improvement in conduction is related to aggregation of P3HT in solution allowing the formation of fibrils in the ultrathin films. Changing the molar mass of P3HT as well as varying the solvent type, aging time, and spin coating parameters resulted in layers with different thicknesses and fibrillar microstructures. The crystal packing and microstructure of the P3HT films, studied by atomic force microscopy and X-ray diffraction, were correlated with the transistor performance. It has been found that P3HT nanofibrils serve in the ultrathin films as pathways for charge carriers. Films of 8 nm thickness revealing a high density and a sufficient length of nanofibrils, along with pronounced internal crystallinity and long π-stacking coherence length, yield a mobility of 0.1 cm2 V−1 s−1. In this way, we demonstrated that controlling the microstructure of the active film in the ultrathin regime does not have to be at the expense of charge carrier mobility.

New insights into the multilevel structure and phase transitions of synthetic organoclays

The lamellar structure of unmodified as well as octadecylamine-exchanged fluoromica was revisited using scanning and transmission electron microscopy as well as small-angle X-ray scattering (SAXS). The fluoromica particles were found to form piled, 10 nm-thick stacks of polygonal platelets having planar dimensions in a submicron range. Cross-sectional micrographs of the stacks indicated that they are made of nearly uniformly spaced platelets. Nonetheless, smaller peaks were identified, both in the Fourier transforms of the TEM patterns and in the X-ray diffractograms. They were preceding the main maximum corresponding to the spacing between subsequent platelets (the basal spacing, dI). Conventional analysis of peak positions would suggest that the silicate contains fractions having basal spacing of dI and 2 × dI. Direct comparison of SAXS and TEM patterns indicated that additional peaks in the diffractograms originate exclusively from discontinuities or defects located at the boundaries of stacks and not from the presence of any additional fractions of the fluoromica. Solid-state NMR investigations supported by SAXS and molecular dynamics simulations were utilized to probe the structure and physical transitions that the surfactant molecules intercalated in the galleries of the silicates undergo are based on the γ-gauche effect. During the first heating cycle, a disordering of the octadecylamine molecules is observed followed by a de-interdigitation and formation of a gauche-dominated liquid-like phase at a temperature around 80 °C. These changes cannot be reversed by simply cooling of the hybrids. However, cooling the system back to 25 °C causes the reoccurrence of trans conformers and decreases the mobility of methylene groups, which is accompanied by a change in heat capacity similar to that observed during a glass transition. The latter changes can be observed upon repeating heating/cooling cycles.

One-pot synthesis of PMMA/montmorillonite nanocomposites

Abstract This article describes simple preparation methods of poly(methyl methacrylate) (PMMA)/synthetic montmorillonite nanocomposites by single-step insitu polymerizations. Compatibility between PMMA and the silicate was ensured by an addition of (3-acrylamidepropyl) trimethylammonium chloride (AAPTMA). The work also compares how different synthetic routes, namely emulsion and solution polymerization, affect the structure as well as thermal and mechanical properties of obtained nanocomposites. The results of structural investigations clearly show, that both the techniques lead to intercalated nanocomposites, but emulsion polymerization allows more effective deflocculating and intercalating of the clay with acrylic copolymers. The addition of small amounts of layered silicates causes an increase in thermal stability and stiffness of the materials. It is demonstrated that at 5 wt. % of the filler, the temperature of 10 % weight loss was shifted up by nearly 50 K in comparison to the neat PMMA. In the same sample, the Young’s modulus of the material was found to be increased by 26 %.

Adsorption, aggregation, and desorption of proteins on smectite particles.

We report on adsorption of lysozyme (LYS), ovalbumin (OVA), or ovotransferrin (OVT) on particles of a synthetic smectite (synthetic layered aluminosilicate). In our approach we used atomic force microscopy (AFM) and quartz crystal microbalance (QCM) to study the protein-smectite systems in water solutions at pH ranging from 4 to 9. The AFM provided insights into the adhesion forces of protein molecules to the smectite particles, while the QCM measurements yielded information about the amounts of the adsorbed proteins, changes in their structure, and conditions of desorption. The binding of the proteins to the smectite surface was driven mainly by electrostatic interactions, and hence properties of the adsorbed layers were controlled by pH. At high pH values a change in orientation of the adsorbed LYS molecules and a collapse or desorption of OVA layer were observed. Lowering pH to the value ≤ 4 caused LYS to desorb and swelling the adsorbed OVA. The stability of OVT-smectite complexes was found the lowest. OVT revealed a tendency to desorb from the smectite surface at all investigated pH. The minimum desorption rate was observed at pH close to the isoelectric point of the protein, which suggests that nonspecific interactions between OVT and smectite particles significantly contribute to the stability of these complexes.

Microstructure-Dependent Charge Carrier Transport of Poly(3-hexylthiophene) Ultrathin Films with Different Thicknesses

Since the interfacial order of conjugated polymers plays an essential role for the performance of field-effect transistors, comprehensive understanding on the charge carrier transport in ultrathin semiconducting films below thicknesses of 10 nm is required for the development of transparent and flexible organic electronics. In this study, ultrathin films based on poly(3-hexylthiophene) as conjugated polymer model system with a thickness range from single monolayer up to several multilayers are investigated in terms of microstructure evolution and electrical properties of different molecular weights. Interestingly, a characteristic leap in field-effect mobility is observed for films with thickness greater than four layers. This threshold mobility regarding film thickness is attributed to the transition from 2D to 3D charge carrier transport along with an increased size of the P3HT aggregates in the upper layers of the film. These results disclose key aspects on the role of the film interlayer on the charge carrier transport through conjugated polymers in transistors.