Advances in Science and Technology Vol. 77

Abstract: In recent years, the interest in the application of organic materials in electronic devices (light emitting diodes, field effect transistors, solar cells), has shown a rapid increase. Polymer memory devices (PDMs) is a very recent addition to the organic electronics. The polymer memory devices can be fabricated by depositing a blend (an admixture of organic polymer, small organic molecules and metal or semiconductor nanoparticles) between two metal electrodes. We demonstrate the memory effect in the device with simple structure based on blend of polymer with different materials like ionic compound (NaCl), ferroelectrical nano-particles (BaTiO3) and small organic molecules In 2007 Paul has proposed a model to explain memory effect a switching between two distinctive conductivity states when voltage is applied based on electrical dipole formation in the polymer matrix. Here, we investigate if our memory devices based on different types of materials are fitted with the proposed model.

Abstract: In the last years oxide materials for electronics show significant progress. However, many details regarding technology control of the properties have to be solved. For electronics, thin films and heterestructures are important taking advantage of integration and synergetic concepts leading to new types of devices and functionalities. It is notable that, while fabrication of new devices and materials showing new phenomena are booming, the growth principles and concepts are somehow developing slowly within this general trend. This is because in many cases, growth of materials is very personalized. Understanding of the bi-directional relationship between the general and particular principles deserves attention. The immediate benefit is that knowledge on growth for one material can be transferred to another one. In our work we have analyzed such relationships for some oxide multicomponent perovskites. Materials used in our examples are Bi-Sr-Ca-Cu-O and YBa₂Cu₃O₇, (Ca, Sr)CuO₂, (Ca, Ba)CuO₂ and Bi₄Ti₃O₁₂. Presented thin films or heterostructures are with c-axis and non-c-axis orientations and based on these examples we discuss some of the growth principles.

Abstract: Magnetoelectric effect in multiferroic materials is widely studied for its fundamental interest and practical applications. The magnetoelectric effect observed for single phase materials like Cr₂O₃, BiFeO₃, Pb(Fe_0.5Nb_0.5)O₃ is usually small. A much larger effect can be obtained in composites consisting of magnetostrictive and piezoelectric phases. This paper investigates the magnetostrictive and magnetoelectric properties of nickel ferrite Ni_0.3Zn_0.62Cu_0.08Fe₂O₄ - relaxor Pb(Fe_0.5Nb_0.5)O₃ bulk composites. The magnetic properties of composites shows a dependence typical of such composite materials, i.e. it consists of a dominating signal from ferrimagnetic phase (ferrite) and a weak signal from paramagnetic (antiferromagnetic) phase (relaxors). Magnetoelectric effect at room temperature was investigated as a function of static magnetic field (300-7200 Oe) and frequency (10 Hz-10 kHz) of sinusoidal modulation magnetic field. The magnetoelectric effect increase slightly before reaching a maximum at H_DC = 750 Oe and then decrease. The magnetoelectric coefficient increases continuously as frequency is raised, although this increase is less pronounced in the 1-10 kHz range.

Abstract: Bismuth layer-structured compounds in the Bi-Ti-Fe-O system known as Aurivillius phases are single phase multiferroics. It was stated that substitution of some rare earth elements for bismuth ions in such structure can modified its magnetic properties. Powders of Bi7Fe3Ti3O21 and Bi6.3Sm0.7Fe3Ti3O21 were prepared by co-precipitation – calcination method and then were sintered to dense polycrystalline materials. Low field DC susceptibility was measured in the zero field cooled (ZFC) and field cooled (FC) modes at 10÷350 K. For selected temperatures magnetisation curves and hysteresis loops were also measured. The FC and ZFC curves of both samples diverge at temperatures below 250 K indicating a spin glass-like behaviour. The compound with samarium exhibits magnetic hysteresis already at room temperature with the coercive field increasing to 870 Oe at 10 K. The low temperature hysteresis loops of the samarium containing compound are shift with respect to zero field which can be attributed to a magneto-electrical coupling of the samarium sublattice "exchange biased" by the iron one, which orders anti-ferromagnetically at a higher temperature than the samarium sublattice.

Abstract: Reflection coefficient of electromagnetic waves from TbMnO3 surface with sinusoidal structure and permittivity are examined. Model of material with spin, elastic and electromagnetic interactions used. Resonance kind of reflection coefficient and of permittivity was shown. Found spectrum and reasoned existing of band-gaps in coefficient of reflection.

Abstract: Complementary Sierpinski Triangular Resonators (C-STR) in coplanar waveguide (CPW) configuration have been studied for microwave applications. The peculiarity of the coupled triangular structures to exhibit multiple resonances is proposed for dual and three-band purposes, to obtain efficient metamaterial-inspired unit cells for high performance microwave tunable oscillators.

Abstract: Metamaterials are composite artificial materials composed of sub-wavelength resonant building blocks designed with state-of-the-art configurations, which exhibit novel and unique electromagnetic (EM) properties. The building blocks are usually made of metallic material, where surface plasmon polaritons can be excited at the interface, leading to many interesting and promising phenomena and applications. However, some drawbacks are accompanied such as intrinsic loss, narrow working frequency, and tunability limitation. We have designed a class of metamaterials composed of building blocks made of ferrite materials, it is accordingly termed magnetic metamaterials (MM). It is demonstrated that with the MM we can construct a magnetically tunable negative index material with the impedance matched to the air. The excitation of the magnetic surface plasmon (MSP) resonance can also be observed in the MM. Due to the MSP resonance and the time reversal symmetry breaking nature of the MM, the unidirectional waveguiding of the EM wave is demonstrated.

Abstract: Novel concepts of nonlinear-optical (NLO) photonic metamaterial are proposed. They concern photonic materials that support backward electromagnetic or vibration waves (BWs) and provide coherent nonlinear-optical energy exchange between ordinary and BWs as applied to three- and four-wave mixing processes. Three different classes of materials which support BWs are considered: plasmonic negative-index (NI) metamaterials (NIMs), metamaterials with specially engineered spatial dispersion of the nanoscopic building blocks, such as standing carbon nanotubes, and crystals that support optical phonons with negative group velocity. The possibility to exploit ordinary crystals instead of plasmonic NLO metamaterials that are very challenging to engineer is proposed. It is shown that extraordinary nonlinear optical frequency-conversion propagation processes attributed to NIMs can be mimicked in the proposed metamaterials. It is also shown that the detrimental effects of strong losses can be mitigated in the short-pulse regimes, which exhibit exotic properties when ordinary and BWs are involved in the NLO coupling. Comparative review of unparallel properties of coherent energy exchange between ordinary and backward electromagnetic waves in NIMs and between ordinary electromagnetic waves coupled through backward vibration waves is given. Unique photonic devices are proposed.

Abstract: The paper deals with a measurement method of the input impedance in the radiating section of a coplanar waveguide (CPW) Composite Right / Left-Handed (CRLH) antenna. In this respect were used the technical facilities offered by an on-wafer measurement equipment contacting the antenna feeding line. With appropriate formulas derived from the theory of the transmission lines, the resistive Rs and the reactive Xs parts of the CRLH radiating structure impedance Zs = Rs + jXs were found. The results for three antenna samples were: Zs1 (Ω) = 48.5326 - j×16.9639, Zs2 (Ω) = 48.6053 - j×16.8143 and Zs3 (Ω) = 48.1786 - j×16.3707. As it is normal, the reactive parts of the measured impedances are negative due to the CRLH line construction with series interdigital capacitors inducing left hand behavior.

Abstract: We invented a novel method to fabricate graphene transistors on oxidized silicon wafers without the need to transfer graphene layers. By means of catalytic chemical vapor deposition (CCVD) the in-situ grown monolayer graphene field-effect transistors (MoLGFETs) and bilayer graphene transistors (BiLGFETs) are realized directly on oxidized silicon substrate, whereby the number of stacked graphene layers is determined by the selected CCVD process parameters. In-situ grown MoLGFETs exhibit the expected Dirac point together with the typical low on/off-current ratios between 16 (hole conduction) and 8 (electron conduction), respectively. In contrast, our BiLGFETs possess unipolar p-type device characteristics with an extremely high on/off-current ratio up to 1E7 exceeding previously reported values by several orders of magnitude. We explain the improved device characteristics by a combination of effects, in particular graphene-substrate interactions, hydrogen doping and Schottky-barrier effects at the source/drain contacts as well. Besides the excellent device characteristics, the complete CCVD fabrication process is silicon CMOS compatible. This will allow the usage of BiLGFETs for digital applications in a hybrid silicon CMOS environment.