Materials Science Forum Vol. 518

Abstract: A brief review of fascinating properties of fullerene is presented on the basis of the concept of effectively non-paired electrons. A versatile chemistry of fullerenes follows from the regioselectivity of their atoms and the uniqueness of donor-acceptor abilities. Computational synthesis of the fullerene derivatives is discussed. Applications of the basic concepts to medicinal applications of fullerenes as well as their magnetic properties and ability to form technomimetic species are considered.

Abstract: Nanostructured films are considered as a characteristic (distinctive) type of consolidated nanostructured materials (NMs). Their benefits as compared to other types of NMs are described in detail. Some new interesting results related to mechanical and physical properties of nanostructured films based on high-melting point compounds (nitrides, borides and carbides), metals, and oxides are discussed. Data on film hardness, type of deformation, effect of additional magnetic field at deposition of films, properties of twins, conductivity, coercivity, and the Hall coefficient are reported and commented.

Abstract: InxGa1-xAs1-yNy/Al0.33Ga0.67As multiquantum wells grown by plasma-assisted molecular beam epitaxy are studied by resonant inelastic light scattering. Sharp vibration modes have been observed at 323, 402, 454 and 501 cm-1. Their intensities resonate at the barrier bandgap reduced by the presence of N. Their resonance energies reveal the influence of the N concentration on the barrier gap at the multiquantum well interfaces. These peaks are interpreted in terms of local vibrations involving the pairing of N atoms, which seems to occur mostly at the quantum well interfaces due to preferential bonding of N to Al.

Abstract: Carbon nanohorns and multiwalled carbon nanotubes have been synthesized by DC arcdischarge carried out at room pressure in air and Ar-enriched environment, by a specially designed experimental device. The resulting nanostructured material, characterized by electron microscopy and X-ray diffraction, shows different structures according to the condensation channels through which the sublimated carbon atoms are re-condensed in the solid state. Multi-Walled Carbon Nano- Tubes are mainly found in the hard crust formed at the cathode, while nano-horned particles can be recovered from a cylindrical collector surrounding the discharge. Further material, rag-like shaped and with an amorphous structure, can be collected in the reaction area. When the discharge occurs under Ar atmosphere, a larger quantity of this latter phase is synthesized. This suggests that the atmospheric oxygen could play an active role by burning the most reactive among the synthesized phases, like amorphous carbon contributing so to an “in situ” purification of the raw material.

Abstract: An experimental and theoretical comparative analysis of the output characteristics of λ ≈ 9m GaAs/Al0.45Ga0.55As quantum cascade lasers based on single and double phonon resonance depopulation mechanisms were presented. The layer structures were grown with solid source molecular beam epitaxy and consist of 48 or 36 active stages embedded in a symmetrical plasmon enhanced waveguide. From the wafers, ridge waveguide lasers were fabricated by optical lithography and dry etching. The theoretical model is based on a fully non-equilibrium Schrödinger- Poisson self-consistent analysis of the coupled scattering rate and single-temperature energy balance equations, taking all relevant electron-LO phonon, electron-electron and electron-ionised impurity scattering processes into account. Single phonon resonance devices exhibit clear current saturation, simultaneously with a decrease of the optical power. In the moderate doping regime, a quasi-linear dependence of both the threshold and saturation current densities on injector doping, were measured, in a very good agreement with theoretical predictions. Double phonon resonance lasers exhibit ‘saturation’ mechanism evident from their decrease in optical power, but without pronounced current saturation. Previously reported saturation of the ‘maximal’ current under higher injector doping in single phonon resonance lasers, is also observed in the double phonon resonance structure for injector sheet doping above 8x1011cm-2.

Abstract: We have analyzed the spin-filtering effects of the electron current in asymmetric ZnSe/Zn1-xMnxSe multilayer structures, under the influence of both an external magnetic field and a bias voltage. In this type of semiconductor systems, conduction band electrons interact with 3d electrons of the magnetic Mn2+ ions. Because of this sp-d exchange interaction, an external magnetic field modulates the effective potential profile seen by spin-up and spin-down electrons, giving rise to a large Zeeman effect. It is found that the degree of spin polarization changes significantly when the electrical bias is switched from forward to reverse, thus the proposed structure displays obvious behavior of spin-filter diode. This originates from the effective “lifting” of the potential for spin-up electrons, which tunnel through actual potential barriers. Structural parameters optimization, with the goal of maximizing the spin-filtering coefficient, was performed by using simulated annealing algorithm. The described effect may be important for designing new tunable spin-based multifunctional semiconductor devices.

Abstract: Thin films and interfaces of crystalline organic dyes with semiconducting properties attracted a lot of attention in the last decade due to their numerous applications in electronics and optoelectronics. One of the most studied molecules is 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA); an archetypal organic material that can grow into multilayer films. Despite the great interest and intensive investigations, its optical properties are still not completely understood. The interpretations range from the Wannier-Mott exciton model to models of excitons of small radii. In the present work, we apply the Frenkel exciton model in order to describe the optical behavior of the solid phase of PTCDA, influenced by the transfer of excitations between different molecules. We are able to model the anisotropy of dielectric tensor, lineshape of the complex index of refraction, exciton dispersion and the large Stokes shift between absorption and photoluminescence, results of electron-energy loss spectroscopy, and photoluminescence transition energies and decay times. In addition, we made an extension of the model towards ultrathin PTCDA films.

Abstract: The Green’s functions technique suitable for broken symmetry structure analysis was developed. With the help of this new technique the phonon subsystem was analysed in ultrathin films and in cylindrical nanotubes with finite height. The most interesting results of mentioned analyses are spatial dependence of thermodynamical characteristics, existence of phonon gap and extremely low specific heat and thermal conductivity at low temperatures. This promises wide application of films and finite nanotubes in technology. The same technique was applied to investigate electron subsystems in rectangular nanostructures of all dimensions as well as in simple and full nanotubes. The most interesting conclusion of these analyses is the presence of autoreduction effect being the consequence of nonisomorphic transition configuration – momentum space. This effect represents a qualitative difference between nano and macroscopic structures. The skin effect is present in all types of nanostructures except nano-parallelepiped where antiskin effect takes place. The latter is quite understandable, since in nano-parallelepiped nodes are on boundaries.

Abstract: Exciton states in type-II InP/InGaP and GaSb/GaAs self-assembled quantum dots and quantum-dot superlattices subject to a normal magnetic field are calculated. Strain is explicitly taken into account in single particle models of the electronic structure, while an exact diagonalization approach is adopted to compute the exciton states. Strain reverts type II band alignment in InP quantum dots to type I, therefore no transitions between the lowest energy states of different angular momenta are observed. On the other hand, strain increases the barrier for the electron in the conduction band of GaSb/GaAs quantum dots, therefore the exciton, being composed of electron and hole states of various angular momenta, may have a finite angular momentum in the ground state. Consequently, the oscillator strength in the InP single quantum dot and quantum-dot superlattice increases with the magnetic field, while the angular momentum transitions between the bright and the dark exciton states in the GaSb system bring about decay of the oscillator strength when the magnetic field exceeds a certain value.

Abstract: Neutral beam etching is proposed as a candidate for reducing plasma-process-induced damage in nanoscale devices. In this paper, neutralization of ion beams due to both gas phase collisions and ion surface interactions based on a PIC (Particle in Cell) simulation of realistic Capacitively Coupled Plasma is presented. It was found that a satisfactory degree of neutralization might be achieved by a combined effect of charge transfer and surface collisions.