Nanostructures and Thin Films for Multifunctional Applications

The capabilities and advantages of electrochemical impedance spectroscopy (EIS) as a useful and non-destructive technique are discussed. EIS provides the time dependent quantitative information about the electrode processes. The description of EIS is given in comprehensive way beginning from the theoretical basics of EIS and data interpretation in the frames of various equivalent electric circuits. The practical applications of EIS are described for the following thin film types: (i) cathodic metals/alloys films deposition; (ii) anodization of metals and characterization of oxide films and its growth by EIS including information provided by Mott-Schottky plots; (iii) underpotential deposition of metals; (iv) characterization of organic films onto metals; (v) application in development of biosensors and biofuel cells. The original data of EIS on cathodic electrodeposition of Co and Co-W are provided and reduction mechanisms involving adsorbed intermediates are discussed. The advantages of EIS in the oxide films characterization and their electrochemical properties are shown. EIS can be successfully applied for the characterization of biosensing surfaces and/or in evaluation of bioanalytical signals generated by biosensors. The glucose oxidase (GOx) based biosensor could be successfully analyzed by merged scanning electrochemical microscopy (SECM) and EIS techniques. Such combining study by SECM and EIS could be very attractive in order to evaluate the biofuel cell efficiency and in the modeling of biosensor action, because it is unavailable to obtain by other convenient electrochemical methods.

The results of theoretical and experimental investigations of physical phenomena that accompany the formation of oxide and hydroxide nano-metric pellicles on metal surfaces by applying pulsed electrical discharge machining (PEDM) are presented. The chemical composition of the processed surface determined by Energy Dispersive X-ray analysis (EDX) attests the presence of oxygen that reaches the abnormal ammounts (up to 60 % at.) for all investigated alloys. The surface phase analysis using X-ray Photoelectron Spectroscopy (XPS) allows one to affirm that the oxygen in film forms three basic structures: –O²⁻ (oxides), –OH⁻ (hydroxides) and structures of C–O and O–C=O types. Experimental investigations have shown that the surface active resistance of these pellicles increases by about 10⁷ times, the potential of corrosion increases to positive values and the speed of corrosion decreases in the chemically active media. Oxide pellicle formation occurs on flat, round and combined interior and exterior piece surfaces made of metal materials. It can be applied in oxide pellicle formation on piece surfaces aiming at providing anticorrosive protection; in surface passivation of construction pieces used in the chemical industry; in manufacturing active resistances of high values (10⁶ Ω) and small dimensions (1 × 1 × 0.01 mm) used in microelectronics; in the production of elements with electronic emission surfaces.

The paper presents the results of experimental research on the physics of natural graphite film formation, the establishment of chemical composition and functional properties of the graphite films, formed on metal surfaces, as a result of the action of plasma in the air environment, at a normal pressure, under the pulsed electrical discharge (PED) conditions. The continuity of the formed film, the thickness of the layer, the diffusion depth, the micro-hardness of the surface layer, the potential of corrosion, and the wear resistance are investigated. It is shown that the use of electrode tool made of pyrolytic graphite improves the mechanical properties at the microscopic level, such as tensile strength. The behaviour of films in an exceptional manner with reference to solubility in different environments is highlighted and the hypothesis that they could contain space structures such as fullerenes and carbon nanotubes are emphasized. Testing studies, using three processing regimes, have proved that the maximum breaking force of samples made from graphite-coated steel-3 increases essentially as compared to the untreated sample. It is demonstrated that the graphite films made by PED method have similar characteristics to those of fullerene or carbon nanotubes.

ZnS_xSe_1-x thin films with various x values in the range from 0 to 1 were prepared by vacuum evaporated technique on glass substrates using powders of ZnS and ZnSe. The structure, surface morphology and composition of thin films have been investigated by XRD, SEM and EDAX techniques. The composition analysis and the diffraction patterns revealed that ZnS_xSe_1-x thin films are nearly stoichiometric and have a cubic zinc blende type structure, with a strong orientation of the crystallite along the crystalline plane (111). The temperature dependence of electrical conductivity measured in a temperature range of 300–500 K demonstrated that the films are of n-type conductivity. The thermal activation energy was evaluated from electrical measurements, and the transparency of films in the wavelength range of 380–1100 nm has been demonstrated from oprical measurements. The lifetime of nonequilibrium charge carriers was determined from the relaxation curves of photoconductivity and photoluminescence, and the energy levels of recombination and trapping centers have been determined from the spectral dependence of photoluminescence and thermoluminesce.

This review gives a brief description of four types of photovoltaic devices based on A₂B₆ thin film semiconductors. Among A₂B₆ semiconductors CdTe is widely researched due to its near ideal band gap energy of 1.45 eV for the achievement of the theoretical maximum photovoltaic conversion efficiency of 31 %. The first part of the paper highlights the history of CdS/CdTe thin-film photovoltaic devices. It also describes the basic physics and design of CdS/CdTe and ZnSe/CdTe thin film heterojunction photovoltaic devices prepared by close spaced sublimation method at Moldova State University. The second part of this paper discusses the physical properties of novel TiO₂ oxide as a partner in heterojunction with CdSe and CdTe thin films. This section also describes the physics, design, and fabrication process of thin film photovoltaic devices based on TiO₂/CdSe and TiO₂/CdTe structures.

This chapter presents a theoretical basis of the anisotropic magnetoelectric (ME) effect in tri-layers of metglas and piezoelectric (PE) single crystals. The properties of various common PE and magnetostrictive substances are discussed, and arguments for the choice of the most appropriate materials are made. A linear description of the ME effects in terms of electric, magnetic and elastic material fields and material constants is presented. An averaging quasi-static method is used to illustrate the relation between the material constants, their anisotropy and the transversal direct ME voltage and charge coefficients. Subsequently, the aforementioned model is employed in the calculation of the maximum expected direct ME voltage coefficient for a series of tri-layered Metglas/Piezocrystal/Metglas composites as a function of the PE crystal orientation. The ME effects are shown to be strongly dependent on the crystal orientation, which supports the possibility of inducing large ME voltage coefficients in composites comprising lead-free PE single crystals such as LiNbO₃, LiTaO₃, α-GaPO₄, α-quartz, langatate and langasite through the optimization of the crystal orientation.

This chapter describes an experimental technique, developed experimental setup and respective experimental study of the dynamic properties of direct magnetolelectric (ME) effect measured in metglas-piezocrystal laminates. We have prepared a variety of different magnetoelectric laminates by bonding magnetostrictive metglas foils onto single-crystalline substrates of LiNbO₃ (LNO), GaPO₄ (GPO) and PMN-PT. The measurements have been performed as a function of the crystal cut, magnitude and orientation of the magnetic bias field and the frequency of the modulation field. Despite much weaker PE coefficients of LNO and GPO, direct ME effects have been found to have comparative magnitudes in the samples based on them and on PMN-PT. Greatly enhanced ME coefficients in certain resonance modes are explored and their relations to the material properties of the crystals and the geometry of the composites are investigated. We demonstrate that control of the PE crystal’s orientation can be successfully used in order to obtain almost any desired quasi-static and resonant anisotropic ME properties for some given application. Such unique features as chemical stability, linear piezoelectricity, thermal robustness open up a real perspective to use lead-free LNO and α-GPO based ME tri-layers, e.g., as vector magnetic field sensors working in a wide range of temperatures.

This chapter presents a review on studies of fundamental properties of narrow-bandgap solid solutions Hg_1–x Mn _x Te and Hg_1–x–y Cd _x Mn _y Te. The structure and parameters of energy bands, electrical, photoelectrical and photoluminescence properties of bulk monocrystals and epitaxial layers are also reviewed. The impact of manganese on the restructuring of energy spectrum of free carriers and localized states are discussed. The basic properties of semimagnetic materials Hg _1–x Mn _x Te and Hg _1–x–y Cd _x Mn _y Te are compared with the properties of more intensively studied Hg _1–x Cd _x Te solid solutions. It is shown that semimagnetic solid solutions Hg _1–x–y Cd _x Mn _y Te and Hg _1–x Mn _x Te are prospective alternatives to Hg _1–x Cd _x Te materials for the creation of infrared detector structures.

The present work reports on the influence of a Cobalt sublayer on a conventional exchange bias CoO_x/Cu₄₁Ni₅₉ interface. For superconducting spintronics the ability to exchange bias diluted ferromagnetic alloys is an essential building block, as they have advantages for the application in superconductor-ferromagnet spin-valve heterostructures. The magnetic properties are investigated by SQUID magnetometry and two separate strongly exchange biased signals are observed. The obtained results are compared with predictions of the domain state and spin-glass model of exchange bias.

Reducing the recombination rate of photo-generated electron-hole pairs in the surface of pristine TiO₂ materials can be done, among others, via local spatial charge separation in the photocatalytically active surface region. A local electric field acting for this purpose can be ensured, for instance, in hetero-junction regions formed at the interface between a TiO₂ film and a WO₃ ultra-thin layer. The option for the WO₃–TiO₂ semiconductor pair is related to the suitable fit of their band structures. To model the interface, a more in-depth knowledge of local atomic environment is required. Here, we discuss the local atomic ordering and the related effects in the interface region of TiO₂/WO₃ and WO₃/TiO₂ structures grown on Si(100) substrates. Materials characterization was done by using XRD, XPS and XAS techniques. We demonstrate that tungsten atoms enter as W⁶⁺ or W⁴⁺ cations into the rutile-type TiO₂ lattice, by substituting the Ti⁴⁺ cations. While W⁶⁺-Ti⁴⁺ substitution leaves the surrounding rutile matrix unchanged, the W⁴⁺-Ti⁴⁺ substitution induces a local rutile-to-anatase transition. The current results are relevant in designing new applications structures with enhanced photocatalytic performances.

The semiconductors like GaS, GaSe and InSe crystallize in lamellar structures with crystal lattice composed of B–A–A–B type packings bound by Van der Waals forces. Materials composed of Ga, In, Cd and Zn chalcogenide crystallites with sizes of 10–30 nm are obtained by heat treatment of GaS, GaSe, GaTe and InSe single crystalline plates in Zn and Cd vapor at temperatures from 653 to 853 K. Phase transformations of GaSe and GaTe compounds with formation of Ga ₂ Se ₃, CdGa ₂ Se ₄ and Ga ₂ Te ₃ compounds occur, as well as the formation of Ga-Cd and Cd-Te clusters on the surface, during the process of producing CdTe-GaTe and CdSe-GaSe nanocomposites.

GaS is an n-type semiconductor with indirect bandgap at 293 K equal to 2.5 eV. The average energy of phonons active in the formation of the edge of indirect absorption band is equal to 44.5 meV. The energy of transverse optical phonon of 23.9 meV was determined from the absorption spectra in the vibrations region of monophononic and crystal lattice. The phonons with average energy equal to 17 meV participate in the formation of the edge of indirect bandgap of GaSe crystals in absorption spectra. Direct bandgap width at 293 K is equal to 2.020 eV and 1.320 eV in GaSe and InSe, respectively. GaTe is a p-type semiconductor with direct bandgap width at 293 K equal to 1.6 eV. The edge of direct absorption in GaSe, GaTe and InSe lamellar crystals is formed by excitons with binding energy of 22 meV, 17 meV and 16 meV, respectively. Heterojonctions consisting of Cd and Zn chalcogenides and lamellar A^IIIB^VI semiconductors that are photosensitive in UV-VIS and NIR regions of spectrum were obtained by heat treatment of GaS, GaSe, GaTe and InSe single crystals in Cd and Zn vapor.

The photoluminescence (PL) spectra of GaS, GaSe, GaTe and InSe semiconductors used as the basis materials to obtain nanocomposite by heat treatment in Zn and Cd vapor were studied. The PL spectra of ZnS–GaS, CdSe–GaSe, CdSe–InSe, ZnSe–InSe composites consist of wide bands covering a wide range of wavelengths in the antistokes region for CdSe, ZnSe and GaS crystallites from composites. The antistokes branches of spectra are interpreted as the shift of PL bands to high energies for nanosized crystallites.

The nanoreliefs could be defined as surface asperities having height characteristics lower than one micrometer, in order to be expressed in nanometers. There are various applications that need nanoreliefs and this aspect determined the investigation of the possibilities offered by distinct manufacturing processes of obtaining nanoreliefs. A research concerning the manufacturing methods based on the material removal from workpiece was developed. The analysis revealed that there are machining methods by traditional cutting and nonconventional machining methods that use thermal or electrochemical phenomena in order to remove the material from workpiece. Some essential aspects of such machining methods were highlighted. The factors able to affect the asperities heights were showed. Experimental researches facilitated the identification of mathematical empirical models able to offer supplementary information concerning the influence exerted by some of the process input factors on the surface roughness parameter Ra. Surfaces profiles or images were obtained by means of instruments for investigation of the machined surfaces.

This chapter provides a review of methods for the production of metal nanotubes and their applications. The importance of nanotemplated growth of nanowires and nanotubes for nanofabrication, and the advantages of nanotubes over nanowires are revealed. Technological approaches for producing various templates, as well as advantages and drawbacks of specific templates, such as ion-track membranes, porous alumina templates, and porous semiconductor templates for nanofabrication are discussed, especially with respect to their suitability for the production of metal nanotubes. Technological methods applied for deposition of metal nanotubes with a focus on electrodepostion and electroless deposition are overviewed for each type of porous templates, and their mechanisms and peculiarities are evidenced. The prospects of application of nanomaterials based on porous nanotemplates in electronics, energy sector, optics, photonics, computers and communications, magnetism and biomedical sciences are explored.

In this chapter we present a review of the phonon thermal conductivity of segmented nanowires focusing on the theoretical results for Si and Si/Ge structures with the constant and periodically modulated cross-sections. We describe the use of the face-centered cubic cell and Born-von Karman models of the lattice vibrations for calculating the phonon energy spectra in the segmented nanowires. Modification of the phonon spectrum in such nanostructures results in strong reduction of the phonon thermal conductivity and suppression of heat transfer due to a trapping of phonon modes in nanowire segments. Possible practical applications of segmented nanowires in thermoelectric energy generation are also discussed.

In this chapter we present a review the terahertz devices based on carbon nanomaterials such as graphene and carbon nanotubes. First, the graphene and carbon nanotube physical properties at high frequencies are described. Then, antennas at terahertz frequencies based on graphene, the generation and detection of terahertz electromagnetic waves are analyzed and different devices and circuits are discussed.

The chapter presents some elements related to Abrasive Flow Machining (AFM), specifically state of the art, elements of technological system, and construction of equipment as well as experimental results. The technological system comprises machine, tooling, working media, carrier, abrasive grains. This equipment could be the base for further researches in AFM field, comprising two opposed cylinders, which assures extrusion with semisolid abrasive media back and forth through the work piece or through passages formed by the work piece and a fixture. By repeatedly extruding the media from one cylinder to the other, an abrasive action is produced as the media enter a restrictive passage and travel through or across the work piece. In this way, the work piece can be nanofinished under Ra = 0.1 μm.