L. V. Govor

Self‐Organized Networks Based on Conjugated Polymers

Univ Oldenburg, Fac Phys, Dept Energy & Semicond REs, D-26111 Oldenburg, Germany. Belarussian State Univ, Dept Phys, Minsk 220050, Byelarus. Res Inst Phys & Chem Problems, Minsk 220050, Byelarus. Limburgs Univ Ctr, B-3590 Diepenbeek, Belgium.Parisi, J, Univ Oldenburg, Fac Phys, Dept Energy & Semicond REs, D-26111 Oldenburg, Germany.

Self-organized formation of low-dimensional network structures starting from a nitrocellulose solution

Low-dimensional solid state structures in the form of a hexagonal network are particularly interesting for unveiling new physical properties of condensed matter and, moreover, due to their properties as photonic crystals. Besides the so far well-established structuring with advanced lithographic methods, it is also possible to take advantage of self-organization processes for the gain of those low-dimensional structures. In the following paper, we introduce an experimental method that is capable of producing highly regular polymer network patterns on the basis of different cell types. The diameter of a hexagonal cell amounts to 1.5–2.0 μm. Our method is based on the wetting of a drop of the polymer solution (nitrocellulose in amyl acetate) on the surface of distilled water cooled down to a temperature of 3 to 5°C and the influence of the water vapor on the created polymer thin film. Following the self-organized process of precipitating water vapor drops on the polymer layer, pulling the latter to the water drop, and subsequently evaporating the solvent, we end up with a structuring of the polymer thin film to a hexagonal network. Depending on the time elapsed after the water vapor has begun to affect the polymer layer, one obtains different forms of net structures. The size of the hexagonal cells results from the extension of the water vapor drop. We propose a structuring model capable of explaining the morphology of the individual cells inside the network obtained in the experiment.

Nanoparticle ring formation in evaporating micron-size droplets

Self-assembly process of CoPt3 particles into a ring pattern (ring diameter ranging from 0.6 to 1.5 μm, particle diameter 6 nm) results from phase separation in the thin film of a binary mixture, giving rise to a bilayer structure and subsequent decomposition of the top layer into droplets. Evaporation of the droplet leads to a shrinking of its contact line, and the particles located at the contact line follow its motion and self-assemble along the line.

Detailed investigation of the conducting channel in poly(3-hexylthiophene) field effect transistors

In this study, the conducting channel in poly(3-hexylthiophene) (P3HT) organic field effect transistors (OFETs) was investigated. The effect of varying the P3HT layer thickness on the OFET parameters was studied. The threshold voltage and the field effect mobility were determined from both the linear and saturation regime of the OFET output characteristics for all film thicknesses and the results are compared and discussed. A gated four probe technique was used to investigate the formation and evolution of the conducting channel by monitoring changes in potential at different points in the channel during measurement. It was found that the device performance of the OFETs was significantly influenced by the thickness of the P3HT layer. Bulk currents were found to dominate device performance for thicker P3HT layers.

Formation of close-packed nanoparticle chains.

The self-assembly of CoPt(3) particles (diameter 6 nm) into low-dimensional ordered arrays results from phase separation in a hexane solution containing nanoparticles, hexadecylamine, and water. The evaporation of hexane from the thin film of solution initiates the formation of a water layer on the solid substrate. Subsequent dewetting of this water layer leads to the formation of water droplets. The nanoparticles follow the motion of the contact line of the dewetting water layer and thus assemble into ordered arrays at the periphery of the water droplets.

Preparation of Self-Assembled Carbon Network Structures with Magnetic Nanoparticles

Nanosized particles of different metals in a polymer matrix have attracted considerable interest in various research fields of chemistry, because of their physical and chemical catalytic properties and their application potential in nanoelectronics. In this paper, we present an experimental method for the preparation of self-assembled honeycomb carbon network patterns with Co and Ni particles. Starting from a 2% carboxylated nitrocellulose solution in amyl acetate submerged in cooled distilled water, we already observe the above self-organized uniform-size net structure at the top of the water surface. The submergence of the carboxylated nitrocellulose network in 0.25-M water solution of cobalt acetate Co(CH 3 COO) 2 ) (or nickel acetate Ni(CH 3 COO) 2 for 1 h leads to the ion-exchange introduction in inorganic cations Co 2+ (or Ni 2+ ) to a polymer matrix. In order to obtain samples with a large content of Co (or Ni) cations, we have carried out the sedimentation of the ion-connected Co (or Ni) with the oxalic acid H 2 C 2 O 4 and, next, repeated the sorption of carboxylated nitrocellulose with the cations Co 2+ (or NI 2+ ). For the fabrication of the Co (or Ni) nanoparticles, the carboxylated nitrocellulose networks, received after a first, second, and third sorption cycle, were heated under vacuum conditions (10 -5 mbar) at T≥500°C for 2 h. The process of implementing the Co and Ni nanoparticles in the carbon network was systematically characterized with the help of measuring the specific resistivity in the temperature range from 4.2 to 295 K.

Electrical field dependence of hopping conduction in self-organized carbon networks

The influence of the electrical field on the variable range hopping process of porous carbon networks is examined in the range of validity of the law ln σ(T)∝T−1/2, where σ and T mean electrical conductivity and temperature, respectively. We show that the field dependence of the samples investigated in the vicinity of the metal–insulator transition clearly distinguishes four characteristic regions. At low values of the applied electrical field, we have ohmic conductivity. Upon increasing the electrical field E, the electrical conductivity σ rises, first following the law ln σ(E)∝En, where n changes from 1.4 to 2.6 with increasing distance from the metal–insulator transition on the insulating side. Then, at higher electrical field, the conductivity turns to the relation ln σ(E)∝E1.0. The temperature dependence of the hopping length of the charge carriers, determined within the above field regime, develops as l(T)∝T−0.9. At temperatures where the ohmic behavior in the Coulomb gap occurs and obeys the law ln...

Self-assembled patterns from evaporating layered fluids

We studied the formation of tree-like patterns of polymer aggregates and rings of nanoparticles during evaporation from a fluid film. We utilize phase separation between two immiscible fluids to generate a double-layer film which dries up in a sequential manner. Both fluid layers may contain a solute, polymer aggregates or nanoparticles. During evaporation of the top layer, instabilities may occur and direct a self-assembly process of the solute which may be further affected by an instability of the bottom layer at a later stage. We present two cases where, after evaporation of the top fluid layer, the solute was adsorbed on the surface of the bottom fluid layer. In comparison to dewetting of a single fluid layer on a solid substrate, the advantage of our double-layer approach lies in the deposition of the solute on the surface of the bottom fluid layer. The relatively high mobility of the solute on such a fluid surface favors the formation of ordered patterns, driven by an instability of the bottom layer.

Treelike branched structures formed in dewetting thin films of a binary solution

The authors present evidence for treelike patterns which developed during solvent evaporation from a phase separated bilayer resulting from a binary polymer solution spin coated onto a solid substrate. Initially, a bilayer structure containing a poly(isobutyl methacrylate) (BMA) solution layer on top of a nitrocellulose (NC) solution layer. During subsequent solvent evaporation, the top BMA solution layer becomes unstable and transforms into short ridges. Finally, solvent evaporation from the NC solution layer connects the BMA ridges to treelike patterns.

Coulomb gap and variable-range hopping in self-organized carbon networks

Carbon networks fabricated by means of a self-organized process, which is in the focus of our interest, represent disordered porous systems. The degree of disorder and, accordingly, the values of their electric conductivity extending from insulator to metal behavior change via heat treatment under vacuum conditions at process temperatures in the range from 600 to 1000 °C. Upon varying the ambient temperature from 4.2 to 295 K, four transport mechanisms can be observed. For carbon nets whose conductivity is far beyond the metal–insulator transition (MIT), the specific resistivity ρ depends on the temperature T as ρ(T)∝T−b exp([T0/T ]1/p). In the low-temperature range, a Coulomb gap in the density of states located near the Fermi energy level occurs, which means that the characteristic value of the exponent is p=2. At high temperatures, the pre-exponential part ρ(T)∝T−b dominates. In the intermediate temperature range, we disclose Mott’s hopping law with p=3. However, the specific resistivity of the carbon ...