S. Elagoz

Electric field dependence of the excitonic properties in graded double quantum wells

A quasi-bound state approximation is used to obtain the electric field dependence of the eigenvalues, eigenfunctions, electron-hole overlap integrals and excitonic binding energies for coupled, graded double quantum wells (GDQWs). In GDQWs the intra- and interband transitions have a much stronger electric field dependence than those in the square quantum well structures. Also, unlike the single or double square wells the operation wavelength of GDQW devices can be tuned by changing not only the magnitude but also the polarity of the applied voltage. Thus GDQWs have potential for applications in optoelectronic devices such as tunable electro-absorption modulators and infrared photodetectors.

The orbit centre dependence of the energy levels in a single quantum well under external tilted magnetic and electric fields

The analytical solutions of the Schrodinger equation for a square well system subjected to an externally applied electric field in the growth direction and an externally applied tilted magnetic field are obtained and the results are discussed. The dependence of the energy spectrum of the system on the external electric fields as a function of the orbit centre is also discussed.

InGaAs/InAlAs SLs via MOCVD for QCL applications

T objective of the present study is to analyze the microstructure and mechanical properties of a TWIP steel at different temperatures. For this purpose, tensile tests were performed in a Fe-22Mn-0.65C TWIP (Twinning-induced plasticity) steel in a temperature range between 25 °C and 400 °C. The microstructure after deformation was characterized via optical microscopy. It was observed that the microstructure consists of mainly deformation twins at low temperatures, whereas dislocation bands are the predominating feature at high temperatures. The yield stress, ultimate tensile strength, total elongation, strain hardening index and the area reduction were measured at different temperatures. The analysis of mechanical data suggests a transition of deformation mechanism from twinning at low temperature to dislocation slip at high temperatures. The work hardening rate and area reduction variations with temperature are discussed and correlated to the decrease of twinning contribution to the deformation mechanism. The role of other process, such as dynamic strain aging and precipitation hardening, are discussed. A thermodynamic-based description for the dependence of Yield stress with temperature was developed, suggesting two acting work hardening mechanisms. This is consistent with the computed activation energy for each mechanism. The stacking fault energy (SFE) was computed by means of Olson and Cohen model, at different temperatures finding that at temperatures higher than 325°C, dislocation glide was the predominant deformation mechanism, which is in accordance with experimental results. Twinning-volume fraction (TVF) in samples tested at different temperatures were computed, finding a decrease in TVF as testing temperatures increases, which in agreement with previous experimental features.T measurement of the magnetic saturation in reference to the pure Co is utilized for quality control in cemented carbides. This measurement is an estimation of binder phase components. WC-Co cemented carbides, in which Co is chosen as a binder, are relatively tough and fatigue-resistant composite materials used widely for cutting tools and rock drilling inserts. However, a substitution for Co as a binder is in urgent demand due to its health threat and fluctuating price. This work aims to investigate the correlation between the COM value and binder phase components for a new binder Ni85Fe15 (at. %) through first principles calculations. The magnetic behavior of Ni/WC interface and the binder segregation are also studied. The equation for calculating the COM value of WC-Ni85Fe15 cemented carbides is constructed. The COM value is decreased by W and C compositions dissolved into the binder phase. We further compare theoretically predicted COM values with experimental measurements for several cemented carbides. And theoretical results agree well with experimental values. The interface investigation shows that spin polarized Ni atoms around the Ni(111)/WC(0001) interface possess lower magnetic moments than bulk Ni atoms. The segregation near the impurity W in the binder phase indicates that the W prefers Fe instead of Ni. Factors that would affect the magnetic behavior of WC-Ni85Fe15 alloys are analyzed.CRISTIAN PANTILIMON1, GEORGE COMAN1, CATALIN GRADINARU1, CLAUDIA TARCEA1, SORIN CIUCA2, MIRELA SOHACIU1*, ANDREI BERBECARU1*, ECATERINA MATEI1, ANDRA PREDESCU1, CRISTIAN PREDESCU1 1University Politehnica of Bucharest, Faculty of Materials Science and Engineering, Department of Metallic Materials Processing and Eco-Metallurgy, 313 Splaiul Independentei, 060042, Bucharest, Romania 1University Politehnica of Bucharest, Faculty of Materials Science and Engineering, Department of Metallic Materials Sciences and Physical Metallurgy, 313 Splaiul Independentei, 060042, Bucharest, RomaniaStatement of the Problem: Memristive systems based on two-dimensional (2D) crystals such as graphene, graphene oxide, molybdenum disulphide, etc., 1-5, are considered as a new type of electronic elements with extremely low energy consumption and with ultra-high scalability for processing and storage of information. The unique electronic and optical properties of 2D crystals demonstrate the enormous potential for creating ultra-high density nano-and bioelectronics for innovative imaging systems. The purpose of this study is to develop memristors with a floating photogate so-called photomemristors2,3 based on graphene and nanocrystals.I superlattices (SLs) are very attractive and suitable for QCL applications due to the availability of lattice matching on InP substrate and large conduction band offset. When InxGa1-xAs and InyAl1-yAs compounds are lattice matched to InP substrate, this allows fabricating QCL devices with an emission wavelength at λ>4μm. Similarly, to go larger wavelengths, the same materials can be used by utilizing a technique known as strain-balancing to overcome the difficulties arise from lattice mismatch. Precise thickness control, alloy composition control and repeatability of the SLs are the most critical issues to be dealt with in growth studies to obtain the desired device structures. The thinnest layer thickness is a few monolayers and the device performance is quite sensitive to interface roughness. Molecular beam epitaxy (MBE) is the generally preferred growth technique due to the requirement of having very thin layers with sharp interfaces. However, QCL also includes thick layers such as claddings for which MOCVD suits the best. For these reasons, it is worth efforts to find a way to grow the whole structure via MOCVD. Using special growth conditions and smaller mass flow controllers (MFCs) it is possible to precisely control the gas flow quantity dilution and injection of metalorganic sources. Transmission Electron Microscope (TEM), Scanning Tunneling Electron Microscope (STEM) and similar techniques are widely used to determine the exact thickness of epitaxially grown SLs. However, these techniques are destructive, relatively expensive, time consuming and require an elevated level of expertise for the sample preparations as well as the sample measurement. The high-resolution x-ray diffraction is a non-destructive, economic, quick and robust technique than electron microscopes and depending on the scan type it is quite sensitive to thickness change, alloy composition and interface quality and, as we demonstrate, it can be used to find the thicknesses for very thin layers.Z nitride (Zn3N2) is a material with an antibixbyite structure in its crystal form and a band gap energy of 1.23 eV. It is deposited by radio frequency magnetron sputtering and molecular beam epitaxy (MBE) at low temperatures (T < 500 K) using reactive N2 plasma and tends to form polycrystalline films. Despite its low temperature growth, it presents high mobilities (100 cm2/V s, in sputtering samples, and 350 cm2/V s, in MBE samples) and low resistivities (10-2-10-3 Ω.cm). Those are attractive features for applications in flexible electronics for which common substrates do not often tolerate high temperature growth. An intrinsic property of the material is its metastability in ambient conditions. The as-grown material has a black appearance but, through the reaction with the water molecules in air, it oxidizes completely to produce a translucent whitish film of ZnO. As a result of the transformation, the material becomes electrically insulating. Through our extensive work on the material characteristics, a good correlation between the transformation span and the storage conditions was found. Thus, at a constant temperature, the lifetime of the nitride layer reduces as the relative humidity increases. The irreversible characteristic of the nitride degradation makes our devices suitable for potential applications in industry. In particular, the thickness of the Zn3N2 layer can be tuned to adapt the device lifetime to the degradation time of a perishable product in transit during long-distance transportation or long-time storage. These products suffer sudden changes on the ambient conditions that could spoil them or diminish their quality. The device is fabricated on polyethylene substrates and can be read out either optically or electronically. In order to further develop the technology, we investigated material passivation using a ZnO layer on top of the nitride. The results indicate that the cap layer improves the stability of the electrical characteristics, enabling the fabrication of thin film transistors, which deliver good output characteristics and field effect mobilities close to those achieved in amorphous Si technology.M a new family of low-dimensional materials, have received a lot of interest due to their unique physical, chemical, and mechanical properties [1]. MXenes have already shown a great potential in storage applications due to their impressive capacitive performance [2]. Here, we study the electronic and transport properties of Ti3C2 MXene using density-functional theory (DFT) in combination with the nonequilibrium Green’s function formalism [3, 4]. Fluorinated, oxidized and hydroxylated surfaces are considered. We found that the surface termination has a considerable impact on the electronic transport [3]. For example, the fluorinated sample shows the largest transmission, whereas surface oxidation results in considerable reduction of the electronic transmission. Such enhanced transmission originates from the extended electronic states and smaller variations of the electrostatic potential profile. We also study the effect of lithium and sodium ion adsorption on the electronic transport properties of the MXene [4]. Optical properties of MXene are also affected bysurface functionalization [5]. For example, in the visible range of the spectrum, the oxidized sample shows larger absorption, whereas surface fluorination results in weaker absorption as compared to pristine MXene. Recently, MXene nanosheets have also emerged as ultrathin and high-flux sieving membranes [6]. In addition to ultrafast water flux, both hydration radius and charge dependent transport of ions have been observed. MXenes are also shown to be highly resistive to biofouling [7]. Here we present the results of our DFT calculations to explore the possible mechanisms for the charge-selective ionic transport through Ti3C2X2 (X=O, OH or F) Mxene [8, 9]. We show that the charge selectivity originates from the charged nature of the MXene layers: the system shows dynamic response to the intercalating ions, even in their hydrated states, by changing the interlayer spacing. We also address the stability of MXene membranes and discuss the possibilities of enhancing their stability by molecular and nanoparticle intercalations. We present the results of our atomistic scale calculations for structural, electronic water sieving properties of hydrophobic graphene and hydrophilic MXene monolayers (see Fig. 1)..C nanotubes are materials of great scientific and technological interest that has called attention of many scientists, since the discovery of them by Ijima in 1991[1]; This great interest due to their unique physical and chemical properties such as chemical corrosion resistance, thermal stability, high thermal and electrical conductivity, low density and high mechanical strength. The present work shows an experimental fluidized bed catalytic chemical vapor deposition reactor (see Figure), as the best technology to produce carbon nanotubes, using a clean process that allows the simultaneous production of hydrogen, based on catalytic decomposition of ethanol, which is a renewable carbon source, and a catalyst based on nickel. Operational parameters as initial catalyst amount, fluidization velocity, temperature and residence time, was analyzed and optimized for the experimental design, making the respective experiments to obtain a kinetic model of the reactor. As results was found that the granulometric distribution size of the catalyst have an impact in the yield of the reaction, being increased as the size down; the nature and behavior of the catalyst inside the reactor affect the requirements to keep the fluidization regimen, increasing the inlet flow of gases when agglomeration of catalyst is created with the increase of the temperature inside the reactor; the time of reaction, to obtain the same yield is decreased while the gas concentration of the ethanol is increased in the inlet gases; the outer diameter of the CNTs strongly depends on the reaction temperature; an increase of the reaction temperature leads to an increase in H2 production, this associated with the thermal decomposition of the ethanol and the CNT production; finally the TEM micrographs show that the nanotubes were multi-walled for the range of conditions studied. The production of carbonaceous materials (mainly carbon nanotubes) was between 1 g/g catalyst and 14 g/g catalyst. Materials, catalysts, and reaction products was characterized using analytical techniques such as XRD, TGA, SEM, TEM .H friction stir welding (HFSW) is an advanced solid-state welding process that can produce sound joint in between dissimilar materials possess different thermo-physical properties such as aluminium and steel. The crucial problem in joining of aluminium to steel is the low solubility of aluminium in iron leads to form brittle intermetallic phase layer. The existence of intermetallic phase layer is desirable for suitable joint quality; however, excessive growth of the phase layer deteriorate the joint strength. Conventional friction stir welding successfully joins aluminium to steel with controlling the growth of phase layer thickness, however, excessive tool wear and forming defects are the significant limitations of this process. These drawbacks may be overcome in HFSW process by introducing an additional heat source to pre-heat the harder material like steel and at the same time placing the tool in an optimal location for sufficient material flow around the tool. In the published literature, little efforts are available to understand the influence of joining conditions on growth of the phase layer thickness in hybrid friction stir welding of aluminium to steel. In the present study, an attempt is undertaken to estimate the growth of the intermetallic phase layer in laser-assisted hybrid friction stir butt welding of 2.5 mm thick AA5052 alloy to 1.4 mm thick ultrahigh strength steel DP590 through numerical analysis. A 3-dimensional conduction heat transfer based model using finite element method is developed to simulate the HFSW process. The estimated thermal cycles and peak temperatures at the joint interface from the numerical model are used to estimate the phase layer thickness. Further, the impact of the distance between the tool and laser on the growth of layer thickness is studied.M Science and Optics are intrinsically related from the earlier study on radiation-matter interaction. Over the last years, photonics has rapidly evolved towards more compact and sophisticated devices from the visible-near infrared (VIS-NIR) towards Mid and Far Infrared (MIR and THz). The challenge is the realization of integrated structures as a powerful technology for “packing” sources, detectors, electronics and optics into single and low costs platforms. In particular MIR and THz spectral region are very attractive for scientific and applicative reasons:this spectral zone is the “so-called” fingerprint region in which many substances exhibitvery strong characteristic absorptions: simple molecules (CO2, H2O, H2S, etc.), complex molecules (dioxins, explosives, organic fluids, etc.). Key light sources for mid infrared sensing and spectroscopy are coming from Material Science Research:Interbandand Quantum Cascade Lasers.For spectroscopic and metrology it is very important to have stable sources with narrow linewidth. In this view, a new class of materials (nonlinear crystals) and devices enable frequency conversion,in order to realize optical referencesusing VIS-NIR sources or Laser Frequency Combs. A new Physics and new classes of devices are coming from research in crystalline Whispering Gallery Mode Resonators (WGMRs). These devices enable nonlinear generation of optical frequency combs, recently exploited in the Telecom region (making use of micro-resonators where light is coupled in and out by opticalfibers). Such WGMRs are also providing outstanding performance in laser stabilization, even in the mid infrared-MIR spectral range. They can also be used for direct sensing in gaseous or liquid compounds, with innovative applications in the field of medicine, human health and study of capillarity phenomena and viscous-elastic properties of fluids. Here, we report our recent researchactivity on crystalline and liquid WGMRs, used as powerful tool for nonlinear optics, bio-chemical sensing and mid-IR laser frequency stabilization,passive and active optical cavity-assisted surface-plasmon-resonance sensors as well ason nonlinear crystals for generation of metrological mid-IR coherent light. These results open the way to new classes of compact MIR sources with a number of applications in Space missions, Metrology, Chemistryand Fundamental Physics.