The valence electronic structure of conducting pyrrole polymers: Evidence against a linear metallic chain picture

A UPS study of various conducting polypyrrole films is presented. Most of the valence band features can be explained by states derived from the orbitals of the pyrrole monomer and the associated anion molecules. In close vicinity of the Fermi energy, a density of states is observed which decreases linearly towards E_F. The corresponding states are introduced by oligomer formation. The π-electronic density at E_F is reduced by at least two orders of magnitude compared to ordinary sp-metals. The UPS spectra are consistent with short conjugation lengths and large amounts of disorder, but the corresponding defect states can not directly be observed.     

RKKY interaction in metallic overlayers of fractional dimensionality

Within an analytical approach we study the RKKY interaction mediated by the electron gas, which shows fractional spectral dimensionality. We derive formula for the RKKY exchange integral in a system of nonintegral dimensionality. Also the modifications of magnetic interaction and magnetic moments in metallic overlayers due to the surface/interface effects are considered. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999.     

Prospects of conducting polymers in molecular electronics

Molecular electronics (ME) is rapidly evolving from physics, chemistry, biology, electronics and information technology. This is because the present-day advanced silicon chip can store about 16 million bytes of information within an area less than 1 cm². Organic materials such as proteins, pigments and conducting polymers (CPs) have been considered as alternatives for carrying out the same functions that are presently being performed by semiconductor silicon. Among these, CPs have demanded the maximum attention. These ME materials differ from conventional polymers by having a delocalized electronic structure that can accommodate charge carriers such as electrons and holes. Besides, these conjugated electronic materials exhibit Peierl’s instabilities due to built-in highly anisotropic interactions. It has been proposed that electrical conduction in CPs occurs via non-linear (or topological) defects (solitons/polarons) generated either during polymerization or as a consequence of doping. Solitons and polarons have recently been shown to have implications in the technical development of ME devices.CPs such as polypyrroles, polythiophenes and polyanilines have been projected for applications for a wide range of ME devices. One of the main reasons for such a wide-spread interest is due to the reported observation that these interesting electronic materials exhibit full range of properties from insulator to superconductor depending upon chemical modification. CPs have been found to have applications as optical, electronic, drug-delivery, memory and biosensing devices. The major challenge confronting the material scientists including chemists and physicists is how do the properties of these electronic materials differ from those of conventional semiconductors. Another advantage lies in the fact that these materials possess specific advantages such as high packing density, possibility of controlling shape and electronic properties by chemical modification.Our group has been actively working towards the application of CPs to Schottky diodes, metal–insulator–semiconductor devices and biosensors for the past about 10 years. This paper is a review of some of the results obtained at our laboratory in the area of CP ME.     

One-electron Hamiltonian matrix elements in conducting polymers

The Wannier function method used in a stable lattice is extended to calculate one-electron Hamiltonian matrix elements in polyacetylene. In this technique only a2P _z orbital of the electrons is taken into account and the matrix elements are given analytically, to show the electronic contribution to the nonneighbor hopping.     

Interaction-created effective flat bands in conducting polymers

For a general class of conducting polymers with arbitrary large unit cell and different on-site Coulomb repulsion values on different type of sites, I demonstrate in exact terms the emergence possibility of an upper, interaction-created “effective” flat band. This last appears as a consequence of a kinetic energy quench accompanied by a strong interaction energy decrease, and leads to a non-saturated ferromagnetic state. This ordered state clearly differs from the known flat-band ferromagnetism. This is because it emerges in a system without bare flat bands, requires inhomogeneous on-site Coulomb repulsions values, and possesses non-zero lower interaction limits at the emergence of the ordered phase.     

Spin-wave impurity states in metallic ferromagnets

It is shown that spin waves in dilute ferromagnetic transition metal alloys can be described in terms of effective matrix-matrix and impurity-matrix exchange integrals. Such a parametrization is exact within the random phase approximation for long-wavelength spin waves. The effective impurity-matrix exchange integral is determined in the tight-binding approximation in terms of the impurity potential and local density of states. The present theory is applied qualitatively to NiFe alloys.     

Layer-heterogeneous magnetic states in metallic nanosystems

The formation of layer-heterogeneous periodic magnetic states in metallic systems is explained in terms of the dependence of the energy of itinerant electrons on the magnetic ordering of localized spins. The interaction of the local magnetic moments with conduction electrons with a certain spin projection is described by a set of spin-dependent δ-function potentials. A matrix method is developed permitting one to calculate the energy spectrum and the density of states of itinerant electrons in the presence of a layered magnetic heterogeneity. This method is used to explain oscillations of the interlayer exchange interaction in metallic magnetic superlattices and the stabilization of spin-density wave structures in transition metals and alloys.     

Evolution of avalanche conducting states in electrorheological liquids

Charge transport in electrorheological fluids is studied experimentally under strongly nonequilibrium conditions. By injecting an electrical current into a suspension of conducting nanoparticles we are able to initiate a process of self-organization which leads, in certain cases, to formation of a stable pattern which consists of continuous conducting chains of particles. The evolution of the dissipative state in such a system is a complex process. It starts as an avalanche process characterized by nucleation, growth, and thermal destruction of such dissipative elements as continuous conducting chains of particles as well as electroconvective vortices. A power-law distribution of avalanche sizes and durations, observed at this stage of the evolution, indicates that the system is in a self-organized critical state. A sharp transition into an avalanche-free state with a stable pattern of conducting chains is observed when the power dissipated in the fluid reaches its maximum. We propose a simple evolution model which obeys the maximum power condition and also shows a power-law distribution of the avalanche sizes.     

Laser induced forward transfer of conducting polymers

We report on laser printing of conducting polymers directly from the solid phase. Laser induced forward transfer is employed to deposit P3HT:PCBM films on glass/ITO/PEDOT:PSS substrates. P3HT:PCBM is widely used as the active material in organic solar cells. Polyaniline films, which are also printed by laser induced forward transfer, find many applications in the field of biotechnology. Laser printing parameters are optimized and results are presented. To apply solid-phase laser printing, P3HT:PCBM films are spun cast on quartz substrates, while aniline is in-situ polymerized on quartz substrates.