Henryk Bednarski

New Conjugated Azomethines Containing Triphenylamine Core —Characterization and Properties

A novel polyazomethines bearing triphenylamine core and a proper model compound were synthesized and their spectroscopic (FTIR, 1 H NMR) and optical (UV-vis, photoluminescence) properties and also a molecular dynamic calculations were investigated. The polymers which were soluble in organic solvents (chloroform, dimethylacetamide, m-cresol) and formed transparent foils, emitted blue light and their photoluminescence band and intensity were solvent dependent. Additionally, the effects of blending the new polyazomethines with other polymers—PMMA, polyvinylophenol on UV-vis spectra were examined. Non-covalent ionic-type interactions between azomethines and methanesulfonic acid (MSA), m-cresol (MC) and p-chlorophenol (pClp) were also investigated. The structure formation of azomethines complexes are discussed on the basis of 1H NMR and FTIR. Photoluminescence and absorption properties of the azomethines compounds after doping with MSA, MC and pClp were tested.

Electronic structure of poly(azomethine) thin films.

Poly(1,4-phenylene-methylidynenitrilo-1,4-phenylenenitrilomethylidyne) (PPI) backbone approximated with poly(p-phenylene vinylene)like polymer composed of alternate phenylene and vinylenelike units is treated within pi electron approximation in terms of the chain composed of united atoms built up of virtual benzene and ethylene atoms. Electronic structure of the united atom is derived from interactions of benzene p and beta bands with V band of ethylene, taking into account that continuity of their pi systems results from overlap of vinylenelike highest occupied molecular orbital and lowest unoccupied molecular orbital orbitals with relevant components of benzene molecular orbitals having phase at parapositions. Electronic band structure has been derived within pi-electron approximation in a way resembling tight binding approximation usually applied to semiconductors. The proposed model is suitable to interpret UV-visible spectra of PPI with additional explaining vibronic progressions. Additionally, an expected location of lone pair related level is proposed.

Bound-magnetic-polaron molecule in diluted magnetic semiconductors

We formulate a complete microscopic theory of a coupled pair of bound magnetic polarons, the bound-magnetic-polaron molecule (BMPM) in a diluted magnetic semiconductor by taking into account both the proper two-body nature of the impurity-electron wavefunction and within the general spin-rotation-invariant approach to the electronic states. Also, the model takes into account both the Heisenberg and the antiferromagnetic kinetic-exchange interactions, as well as the ferromagnetic coupling within the common spin BMPM cloud. In this manner, we correct, unify and extend the weakly interacting BMP pair models of Wolff-Bhatt-Durst (2002 Phys. Rev. B 65 235205) and the model of nonoverlapping polarons considered by Angelescu and Bhatt (2002 Phys. Rev. B 65 75211). The resulting BMPM Hamiltonian is solved within the continuum-medium and the effective-mass approximations for the donor case and the thermodynamics is derived. In our approach the thermodynamic fluctuations of magnetization of the spins within BMPM are taken as Gaussian. It appears that the fluctuations can strongly stabilize the spin-triplet state, which may constitute a precursor effect of a ferromagnetic ordering in a many-impurity system.

Randomness of the magnetic ion distribution in dilute magnetic semiconductor epilayers grown by molecular beam epitaxy

Abstract We have applied our extended generalized pair approximation to dilute magnetic semiconductor epilayers grown by molecular beam epitaxy and in this paper we have compared our calculations of the heavy-hole exciton Zeeman splitting with those we measured on Zn1−xMnxSe and those measured by other authors on Cd1−xMnxTe epilayers. It appears that there can be found experimental results, including ours, which exhibit a systematic tendency to lie well below the corresponding theoretical curves based on random distribution. We can clearly attribute this reduced paramagnetic behavior to a non-random distribution of the magnetic ions and we present a way to determine the degree of non-random distribution.

Effect of thermodynamic fluctuations of magnetization on the bound magnetic polaron state in ferromagnetic semiconductors

We extend the theory of the bound magnetic polaron (BMP) in diluted paramagnetic semiconductors to the situation with a ferromagnetic phase transition. This is achieved by including the classical Gaussian fluctuations of magnetization from the quartic (non-Gaussian) term in the effective Ginzburg–Landau Hamiltonian for the spins. Within this approach, we find a ferromagnetically ordered state within the BMP in the temperature range well above the Curie temperature for the host magnetic semiconductor. Numerical results are compared directly with the recently available experimental data for the ferromagnetic semiconductor GdN. The agreement is excellent, given the simplicity of our model, and is because the polaron size ( nm) encompasses a relatively large but finite number () of quasiclassical spins coming from Gd3+ ions. The presence of BMP invalidates the notion of critical temperature and thus makes the incorporation of classical Gaussian fluctuations sufficient to realistically describe the situation.

Phenomenological parameters for a magneto-optical characterization of Cd1−xMnxTe with moderate magnetic ion concentration

Abstract The modified Brillouin function approach is improved in order to obtain a correct description of the low-temperature behavior of the Zeeman splittings in Cd 1− x Mn x Te. This work is based on computer simulations, within the extended generalized pair approximation, of the two relevant phenomenological parameters, i.e. the effective spin and the temperature shift for moderate magnetic ion concentration. A significant temperature dependence of the temperature shift is predicted.

Magnetic properties of (Cd1−x−yZnyMnx)3As2

Abstract We have studied the specific heat, magnetization and susceptibility of the tetragonal diluted magnetic semiconductor (DMS) ( Cd 1− x − y Zn y Mnx x ) 3 As 2 (CZMA) with various compositions in dc magnetic fields up to 17 T and in the temperatu range 1.3−300 K. The complicated crystal structure of CZMA has inspired us to generalize the pair approximation, widely used in analysis of the magnetic behaviour of II-VI DMSs, for an arbitrary structure. The resulting model, which may be called the generalized pair approximation (GPA), has been subsequently applied to interpret the results of our magnetic measurements performed on CZMA. Based on theoretical analyses concerning the Mn-Mn interaction in DMSs, which show that the dominant roles in this process are played by superexchange and the Bloembergen-Rowland exchange, we have obtained satisfactory agreement between our approach and experiment by using the analytical formulae with only two adjustable parameters, i.e. the first nearest-neighbour exchange constants for both mechanisms, which appear to be strongly dependent on Zn content.

P3HT:PCBM blend films phase diagram on the base of variable-temperature spectroscopic ellipsometry

In this work we present an in-depth study of the how the composition of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films influences their phase transitions using variable-temperature spectroscopic ellipsometry. We demonstrate that this non-destructive method is a very sensitive optical technique to investigate the phase transitions and to determine the glass transition temperatures and melting crystallization points of the P3HT:PCBM blend films. By analyzing the influence of the temperature T on the raw ellipsometric data, we have identified a high sensitivity of the ellipsometric angle Δ at a wavelength of 280 nm to temperature changes. Characteristic temperatures determined from the slope changes of the Δ(T) plot appeared to be very good guess values for the phase transition temperatures.

Optical properties of phenylene–thiophene-based polyazomethine thin films

Thin films of new aromatic polyazomethines, containing thiophene rings in their polymer chain structures, were obtained by the chemical vapor deposition method, via a polycondensation process from thiophene-2,5-dicarbaldehyde and one of four different aromatic diamines, as initial monomers. Spectroscopic ellipsometry, attenuated total reflectance mode of Fourier transform infrared spectroscopy, atomic force microscopy, and wide-angle X-ray diffraction techniques have been used to characterize these films. However, the optical transmission and reflectivity measurements, within the wide spectral UV-Vis-NIR range (0.5–6.2 eV), were the basic method of presented investigations. On the basis of optical measurements, the absorption coefficient spectra of these thin films were obtained and the absorption edge parameters, that is, the energy gap (E G) and Urbach energy (E U), were calculated in a way typical for amorphous semiconductors. These parameters allowed one to evaluate the length of conjugation and the extension of localized, defect states inside the energy gap, respectively, and to discuss the electronic transitions, being dependent on the chemical structure and conformation of polymer chains. Optical properties of these new phenylene–thiophene-based polyazomethine films were compared with their entirely phenylene counterparts. In general, the energy gaps of investigated films were clearly smaller (about 0.2 eV) than those of their phenylene counterparts, confirming that the presence of thiophene rings in polyazomethine structures improves conjugation.

Pressure dependence of magnetization in diluted magnetic semiconductors

Abstract We have introduced a scaling procedure of the pressure dependence of magnetization in diluted magnetic semiconductors (DMS) based on quite general considerations. This provides the direct relation between the magnetization at certain value of magnetic field, temperature and pressure with that at zero pressure but for lower magnetic field and temperature. It is also shown that this scaling procedure should be applicable to various DMS at zero pressure, as illustrated by appropriate experimental data for Cd1−xMnxTe and Zn1−xMnxTe.