ALBERT

Description: We have investigated the thickness-dependent transport properties of La 1/3 Sr 2/3 FeO 3 thin films grown on SrTiO 3 (001) and (111) substrates. At a thickness of ∼40 nm, both films show a clear transition in resistivity associated with the characteristic charge disproportionation at approximately 190 K. The transition temperature of the charge disproportionation is nearly unchanged with decreasing film thickness down to a certain thickness of ∼13 nm for both orientations, while the change in resistivity gradually decreases. Below this thickness, the transition becomes unclear, strongly suggesting the suppression of the charge disproportionation at the critical thickness of ∼13 nm. Furthermore, there is no significant difference in the thickness dependence of La 1/3 Sr 2/3 FeO 3 thin films between the (001) and (111) orientations. The negligible crystallographic-orientation dependence may reflect the isotropic nature for the domain of charge disproportionation states in La 1/3 Sr 2/3 FeO 3 .

Description: Titanium oxide (TiO x ) has attracted a lot of attention as an active material for resistive random access memory (RRAM), due to its versatility and variety of possible crystal phases. Although existing RRAM materials have demonstrated impressive characteristics, like ultra-fast switching and high cycling endurance, this technology still encounters challenges like low yields, large variability of switching characteristics, and ultimately device failure. Electroforming has been often considered responsible for introducing irreversible damage to devices, with high switching voltages contributing to device degradation. In this paper, we have employed Al doping for tuning the resistive switching characteristics of titanium oxide RRAM. The resistive switching threshold voltages of undoped and Al-doped TiO x thin films were first assessed by conductive atomic force microscopy. The thin films were then transferred in RRAM devices and tested with voltage pulse sweeping, demonstrating that the Al-doped devices could on average form at lower potentials compared to the undoped ones and could support both analog and binary switching at potentials as low as 0.9 V. This work demonstrates a potential pathway for implementing low-power RRAM systems.

Description: In this work, electron induced modifications on the bulk etch rate, structural and optical parameters of CR-39 polymer were studied using gravimetric, FTIR (Fourier Transform Infrared) and UV–vis (Ultraviolet–Visible) techniques, respectively. CR-39 samples were irradiated with 10 MeV electron beam for different durations to have the absorbed doses of 1, 10, 550, 5500, 16 500, and 55 000 kGy. From the FTIR analysis, the peak intensities at different bands were found to be changing with electron dose. A few peaks were observed to shift at high electron doses. From the UV-vis analysis, the optical band gaps for both direct and indirect transitions were found to be decreasing with the increase in electron dose whereas the opacity, number of carbon atoms in conjugation length, and the number of carbon atoms per cluster were found to be increasing. The bulk etch rate was observed to be increasing with the electron dose. The primary objective of this investigation was to study the response of CR-39 to high electron doses and to determine a suitable pre-irradiation condition. The results indicated that, the CR-39 pre-irradiated with electrons can have better sensitivity and thus can be potentially applied for neutron dosimetry.

Description: Low temperature thermal to electrical energy converters have the potential to provide a route for recovering waste energy. In this paper, we propose a new configuration of a thermal harvester that uses a naturally driven thermal oscillator free of mechanical motion and operates between a hot heat source and a cold heat sink. The system exploits a heat induced liquid-vapour transition of a working fluid as a primary driver for a pyroelectric generator. The two-phase instability of a fluid in a closed looped capillary channel of an oscillating heat pipe (OHP) creates pressure differences which lead to local high frequency temperature oscillations in the range of 0.1–5 K. Such temperature changes are suitable for pyroelectric thermal to electrical energy conversion, where the pyroelectric generator is attached to the adiabatic wall of the OHP, thereby absorbing thermal energy from the passing fluid. This new pyroelectric-oscillating heat pipe (POHP) assembly of a low temperature generator continuously operates across a spatial heat source temperature of 55 °C and a heat sink temperature of 25 °C, and enables waste heat recovery and thermal energy harvesting from small temperature gradients at low temperatures. Our electrical measurements with lead zirconate titanate (PZT) show an open circuit voltage of 0.4 V (AC) and with lead magnesium niobate–lead titanate (PMN-PT) an open circuit voltage of 0.8 V (AC) at a frequency of 0.45 Hz, with an energy density of 95 pJ cm −3 for PMN-PT. Our novel POHP device therefore has the capability to convert small quantities of thermal energy into more desirable electricity in the nW to mW range and provides an alternative to currently used batteries or centralised energy generation.

Description: Electrode materials selection guidelines for oxide-based memory devices are constructed from the combined knowledge of observed device operation characteristics, ab-initio calculations, and nano-material characterization. It is demonstrated that changing the top electrode material from Ge to Cr to Ta in the Ta 2 O 5 -based memory devices resulted in a reduction of the operation voltages and current. Energy Dispersed X-ray (EDX) Spectrometer analysis clearly shows that the different top electrode materials scavenge oxygen ions from the Ta 2 O 5 memory layer at various degrees, leading to different oxygen vacancy concentrations within the Ta 2 O 5 , thus the observed trends in the device performance. Replacing the Pt bottom electrode material with CMOS compatible materials (Ru and Ir) further reduces the power consumption and can be attributed to the modification of the Schottky barrier height and oxygen vacancy concentration at the electrode/oxide interface. Both trends in the device performance and EDX results are corroborated by the ab-initio calculations which reveal that the electrode material tunes the oxygen vacancy concentration via the oxygen chemical potential and defect formation energy. This experimental-theoretical approach strongly suggests that the proper selection of CMOS compatible electrode materials will create the critical oxygen vacancy concentration to attain low power memory performance.

Description: Thus far, despite many investigations have been carried out for photo-triggered drug delivery systems, most of them suffer from an intrinsic drawback of without real-time monitoring mechanism. Incident intensity of light is a feasible parameter to monitor the drug release profiles. However, it is difficult to measure the incident laser power irradiated onto the photo-triggered carriers in drug delivery systems during in vivo therapy. We design an online measurement method based on the fluorescence intensity ratio (FIR) technique through upconversion nanoparticles. FIR value varies with temperature of sample due to the thermal effect induced by the incident laser, which validates the laser power measurement. Effects of rare earth doping concentration, as well as experimental conditions including laser spots and wavelengths on the measurement behavior were also investigated.

Description: Several candidate phenomenological expressions are studied for self-rippling energy that drives ripple formation of free single-layer graphene sheets. One phenomenological expression is admitted, while all others are rejected because they cannot admit stable periodic ripple mode. The admitted phenomenological expression contains two terms: one quadratic term which acts like a compressive force and has a destabilizing effect, and another fourth-order term which acts like a nonlinear elastic foundation and has a stabilizing effect. The two associated coefficients depend on specific mechanism of self-rippling and can be determined based on observed wavelength and amplitude of ripple mode. Based on the admitted expression, the effect of an applied force on ripple formation is studied. The present model predicts that the rippling can be controlled or even suppressed with an applied tensile force or collapsed into narrow wrinkles (of deformed wavelengths down to around 2 nm) under an applied compressive force, and the estimated minimum tensile strain to suppress rippling is in remarkable agreement with some known data. Our results show that self-rippling energy dominates ripple formation of sufficiently long free graphene ribbons, although it cannot drive self-rippling of sufficiently short free graphene ribbons. Consequently, a critical length is estimated so that self-rippling occurs only when the length of free single-layer graphene ribbons is much longer than the critical length. The estimated critical length is reasonably consistent with the known fact that self-rippling cannot occur in shorter free graphene sheets (say, of length below 20 nm).

Description: A single electron dynamic memory is designed based on the non-equilibrium dynamics of charge states in electrostatically defined metallic quantum dots. Using the orthodox theory for computing the transfer rates and a master equation, we model the dynamical response of devices consisting of a charge sensor coupled to either a single and or a double quantum dot subjected to a pulsed gate voltage. We show that transition rates between charge states in metallic quantum dots are characterized by an asymmetry that can be controlled by the gate voltage. This effect is more pronounced when the switching between charge states corresponds to a Markovian process involving electron transport through a chain of several quantum dots. By simulating the dynamics of electron transport we demonstrate that the quantum box operates as a finite-state machine that can be addressed by choosing suitable shapes and switching rates of the gate pulses. We further show that writing times in the ns range and retention memory times six orders of magnitude longer, in the ms range, can be achieved on the double quantum dot system using experimentally feasible parameters, thereby demonstrating that the device can operate as a dynamic single electron memory.

Description: Chemical vapor deposition methods were developed, using stoichiometric reactions of specialty Ge 3 H 8 and SnD 4 hydrides, to fabricate Ge 1- y Sn y photodiodes with very high Sn concentrations in the 12%–16% range. A unique aspect of this approach is the compatible reactivity of the compounds at ultra-low temperatures, allowing efficient control and systematic tuning of the alloy composition beyond the direct gap threshold. This crucial property allows the formation of thick supersaturated layers with device-quality material properties. Diodes with composition up to 14% Sn were initially produced on Ge-buffered Si(100) featuring previously optimized n -Ge/ i -Ge 1- y Sn y / p -Ge 1- z Sn z type structures with a single defected interface. The devices exhibited sizable electroluminescence and good rectifying behavior as evidenced by the low dark currents in the I-V measurements. The formation of working diodes with higher Sn content up to 16% Sn was implemented by using more advanced n -Ge 1- x Sn x / i -Ge 1- y Sn y / p -Ge 1- z Sn z architectures incorporating Ge 1- x Sn x intermediate layers ( x ∼ 12% Sn) that served to mitigate the lattice mismatch with the Ge platform. This yielded fully coherent diode interfaces devoid of strain relaxation defects. The electrical measurements in this case revealed a sharp increase in reverse-bias dark currents by almost two orders of magnitude, in spite of the comparable crystallinity of the active layers. This observation is attributed to the enhancement of band-to-band tunneling when all the diode layers consist of direct gap materials and thus has implications for the design of light emitting diodes and lasers operating at desirable mid-IR wavelengths. Possible ways to engineer these diode characteristics and improve carrier confinement involve the incorporation of new barrier materials, in particular, ternary Ge 1- x - y Si x Sn y alloys. The possibility of achieving type-I structures using binary and ternary alloy combinations is discussed in detail, taking into account the latest experimental and theoretical work on band offsets involving such materials.

Description: Temperature-dependent characteristics of GeSn light-emitting diodes with Sn composition up to 9.2% have been systematically studied. Such diodes were based on Ge/GeSn/Ge double heterostructures (DHS) that were grown directly on a Si substrate via a chemical vapor deposition system. Both photoluminescence and electroluminescence spectra have been characterized at temperatures from 300 to 77 K. Based on our theoretical calculation, all GeSn alloys in this study are indirect bandgap materials. However, due to the small energy separation between direct and indirect bandgap, and the fact that radiative recombination rate greater than non-radiative, the emissions are mainly from the direct Γ-valley to valence band transitions. The electroluminescence emissions under current injection levels from 102 to 357 A/cm 2 were investigated at 300 K. The monotonic increase of the integrated electroluminescence intensity was observed for each sample. Moreover, the electronic band structures of the DHS were discussed. Despite the indirect GeSn bandgap owing to the compressive strain, type-I band alignment was achieved with the barrier heights ranging from 11 to 47 meV.

Description: Due to their high strength and advantageous high-temperature properties, tungsten-based alloys are being considered as plasma-facing candidate materials in fusion devices. Under neutron irradiation, rhenium, which is produced by nuclear transmutation, has been found to precipitate in elongated precipitates forming thermodynamic intermetallic phases at concentrations well below the solubility limit. Recent measurements have shown that Re precipitation can lead to substantial hardening, which may have a detrimental effect on the fracture toughness of W alloys. This puzzle of sub-solubility precipitation points to the role played by irradiation induced defects, specifically mixed solute-W interstitials. Here, using first-principles calculations based on density functional theory, we study the energetics of mixed interstitial defects in W-Re, W-V, and W-Ti alloys, as well as the heat of mixing for each substitutional solute. We find that mixed interstitials in all systems are strongly attracted to each other with binding energies of −2.4 to − 3.2   eV and form interstitial pairs that are aligned along parallel first-neighbor 〈 111 〉 strings. Low barriers for defect translation and rotation enable defect agglomeration and alignment even at moderate temperatures. We propose that these elongated agglomerates of mixed-interstitials may act as precursors for the formation of needle-shaped intermetallic precipitates. This interstitial-based mechanism is not limited to radiation induced segregation and precipitation in W–Re alloys but is also applicable to other body-centered cubic alloys.

Description: This paper presents the first design and experimental demonstration of an ultrahigh frequency complete phononic crystal (PnC) bandgap aluminum nitride (AlN)/air structure operating in the GHz range. A complete phononic bandgap of this design is used to efficiently and simultaneously confine elastic vibrations in a resonator. The PnC structure is fabricated by etching a square array of air holes in an AlN slab. The fabricated PnC resonator resonates at 1.117 GHz, which corresponds to an out-of-plane mode. The measured bandgap and resonance frequencies are in very good agreement with the eigen-frequency and frequency-domain finite element analyses. As a result, a quality factor/volume of 7.6 × 10 17 /m 3 for the confined resonance mode was obtained that is the largest value reported for this type of PnC resonator to date. These results are an important step forward in achieving possible applications of PnCs for RF communication and signal processing with smaller dimensions.

Description: In this work, titanium dioxide nanotubes were prepared by using titanium foils via electrochemical anodization in ethylene glycol solutions containing different amounts of water and fluoride in the ranges of 1%–3% and 0.15%–0.5%, respectively, to determine their effects on morphology, optical, and crystalline structure properties. Annealing processes were performed on all samples in the range between 273 and 723 K. Morphology and structure properties of the samples were studied by scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy. Titanium dioxide (TiO 2 ) nanotubes, through anodization method, are strongly influenced by conditions, like fluoride concentration and applied voltages. Tube lengths between 2 and 7  μ m were obtained, exhibiting different diameters and wall thicknesses. When alternating voltage was applied, the outer surface of the nanotubes exhibited evenly spaced ring-shaped regions, while smooth tubes were observed when constant voltage was applied. Reflection peaks, corresponding to Brookite, Anatase, and Rutile, of TiO 2 phases, were observed from the XRD pattern. These phases were corroborated via μXRD measurements, and the Ti 3 O 5 phase was also observed in detail. Absorption coefficient (α), optical band gap (Eg), and extinction coefficient (ε) of TiO 2 nanotubes were calculated by transmittance spectra in the UV–Vis range. Strong absorption was noted in the UV region from reflectance and absorbance measurements. A correlation between synthesis parameters and physical properties is presented.

Description: Similar to electromagnetism, described by the Maxwell equations, the physics of magnetoelectric (ME) phenomena deals with the fundamental problem of the relationship between electric and magnetic fields. Despite a formal resemblance between the two notions, they concern effects of different natures. In general, ME-coupling effects manifest in numerous macroscopic phenomena in solids with space and time symmetry breakings. Recently, it was shown that the near fields in the proximity of a small ferrite particle with magnetic-dipolar-mode (MDM) oscillations have the space and time symmetry breakings and the topological properties of these fields are different from the topological properties of the free-space electromagnetic fields. Such MDM-originated fields—called magnetoelectric (ME) fields—carry both spin and orbital angular momenta. They are characterized by power-flow vortices and non-zero helicity. In this paper, we report on observation of the topological ME effects in far-field microwave radiation based on a small microwave antenna with a MDM ferrite resonator. We show that the microwave far-field radiation can be manifested with a torsion structure where an angle between the electric and magnetic field vectors varies. We discuss the question on observation of the regions of localized ME energy in far-field microwave radiation.

Description: The epitaxial integration of functional oxides with wide band gap semiconductors offers the possibility of new material systems for electronics and energy conversion applications. We use first principles to consider an epitaxial interface between the correlated metal oxide SrRuO 3 and the wide band gap semiconductor TiO 2 , and assess energy level alignment, interfacial chemistry, and interfacial dipole formation. Due to the ferromagnetic, half-metallic character of SrRuO 3 , according to which only one spin is present at the Fermi level, we demonstrate the existence of a spin dependent band alignment across the interface. For two different terminations of SrRuO 3 , the interface is found to be rectifying with a Schottky barrier of ≈1.3–1.6 eV, in good agreement with experiment. In the minority spin, SrRuO 3 exhibits a Schottky barrier alignment with TiO 2 and our calculated Schottky barrier height is in excellent agreement with previous experimental measurements. For majority spin carriers, we find that SrRuO 3 recovers its exchange splitting gap and bulk-like properties within a few monolayers of the interface. These results demonstrate a possible approach to achieve spin-dependent transport across a heteroepitaxial interface between a functional oxide material and a conventional wide band gap semiconductor.

Description: The aim of this work is to analyze in more depth a model of particle deposition by characterizing different parameters such as profile density, bonds and perimeter, and substrate coverage, all being involved in the description of deposits as bulk. Thus, this study is an extension of a previous work on non-equilibrium interface-growth systems where two different interface-growth models, called Standard Adherence Rule Model and Potential Adherence Rule Model, were characterized. In this work, bulk characterization is implemented for the complete range of Peclet numbers. The zones of density profile (Near-Wall, Plateau, and Active-Growth) are studied by proposing an adjustment for each of them and determining the full-setting density profile depending on the Peclet number. The density profiles are compared with other one- and two-stage models. Furthermore, an algorithm is proposed to calculate the number of bonds of the particles and the perimeter that a substrate forms over time. Finally, to analyze the coating, its temporal behavior is adjusted to an exponential function by comparing the results with those found for Random Sequential Adsorption models which describe systems like colloidal particles on solid substrates, adsorption of proteins at mineral surfaces, or oxidation of one-dimensional polymer chains.

Description: The recently discovered hexagonal ABC -type hyperferroelectrics, in which the polarization persists in the presence of the depolarization filed, exhibit a variety of intriguing and potentially useful properties [Garrity et al ., Phys. Rev. Lett. 112 , 127601 (2014)]. For the existing prototype of LiBeSb, we present detailed first-principles calculations concerning the lattice dynamics, electronic structure, and optical properties. An unstable longitudinal optic mode in the high-symmetry structure and a large polarization of 0.5 C/m 2 in the polar phase are reported, including the remarkable dependence of Born effective charges on structural distortion. Using the HSE06 hybrid functional, we predict that LiBeSb has a small band-gap of 1.5 eV and shows dominant asymmetric covalent bonding character. Importantly, we find that there are remarkable absorptions in the whole visible spectrum. These features, combined with the enhanced carrier mobility, make LiBeSb as well as the whole family of hexagonal ABC -type hyperferroelectrics as promising candidates for ferroelectric photovoltaic materials with large bulk photovoltaic effect in the visible spectrum.

Description: We combine classical nucleation theory with superharmonic focusing to predict necessary pressures to induce nucleation in acoustic droplet vaporization. We show that linear acoustics is a valid approximation to leading order when particle displacements in the sound field are small relative to the radius of the droplet. This is done by perturbation analysis of an axisymmetric compressible inviscid flow about a droplet with small surface perturbations relative to the mean radius subjected to an incoming ultrasonic wave. The necessary nucleation pressure threshold inside the droplet is calculated to be −9.33 ± 0.30 MPa for typical experimental parameters by employing results from classical homogeneous nucleation theory . As a result, we are able to predict if a given incident pressure waveform will induce nucleation.

Description: Resonant dipolar relaxation in poly( ε -caprolactone) (PCL) is reported using thermally stimulated discharge current spectroscopy. PCL is a bio-medically known shape memory polymer having a well defined γ , β , α , and α ′ relaxations, respectively, centered around 125 K, 170 K, 220 K, and 270 K as seen by the measurements. By employing a new protocol variable poling temperature at constant freezing temperature, resonant dipolar relaxation in PCL could be induced, especially in the vicinity of α relaxation. Such a protocol is useful in de-convoluting the features in a more meaningful fashion. By an analysis of activation process, we could show a clear contrast enhancement of the dynamics of the participating dipoles by means of a minimum in the activation energies situated around the glass transition region. The relevant parameters of interest such as activation energies and relaxation times are estimated and discussed.

Description: We report the growth of thin films of the mixed valence compound YbAl 3 on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction, and aberration-corrected scanning transmission electron microscopy, we establish that the films are ordered in the bulk as well as at the surface. Our films show a coherence temperature of 37 K, comparable to that reported for bulk single crystals. Photoelectron spectroscopy reveals contributions from both f 13 and f 12 final states establishing that YbAl 3 is a mixed valence compound and shows the presence of a Kondo Resonance peak near the Fermi-level.

Description: An improved method for characterizing the magnetic anisotropy of films with cubic symmetry is described and is applied to an yttrium iron garnet (111) film. Analysis of the ferromagnetic resonance (FMR) spectra performed both in-plane and out-of-plane from 0.7 to 8 GHz yielded the magnetic anisotropy constants as well as the saturation magnetization. The field at which FMR is observed turns out to be quite sensitive to anisotropy constants (by more than a factor ten) in the low frequency (

Description: In this study, fullerene like carbon (FL-C) is introduced in hydrogenated amorphous carbon (a-C:H) film by employing a direct current plasma enhanced chemical vapor deposition. The film has a low friction and wear, such as 0.011 and 2.3 × 10 −9 mm 3 /N m in the N 2 , and 0.014 and 8.4 × 10 −8 mm 3 /N m in the humid air, and high hardness and elasticity (25.8 GPa and 83.1%), to make further engineering applications in practice. It has several nanometers ordered domains consisting of less frequently cross-linked graphitic sheet stacks. We provide new evidences for understanding the reported Raman fit model involving four vibrational frequencies from five, six, and seven C-atom rings of FL-C structures, and discuss the structure evolution before or after friction according to the change in the 1200 cm −1 Raman band intensity caused by five- and seven-carbon rings. Friction inevitably facilitates the transformation of carbon into FL-C nanostructures, namely, the ultra low friction comes from both such structures within the carbon film and the sliding induced at friction interface.

Description: It was shown that drilling of multi-layered target placed in the air by tightly focused femtosecond laser radiation with high fluence (up to 1000 J/cm 2 ) can be monitored online using plasma-induced X-ray emission and second harmonic of incident laser radiation. The technique based on X-rays registration is appeared to be more flexible than the method based on detection of second harmonic since its accuracy depends crucially on the target type. We demonstrated that the X-ray signal clearly indicates the transition from one layer to another during the microdrilling of targets consisting of 2–4 layers of titanium foil when a laser beam is focused beneath the target surface at a depth comparable to the layer thickness. The diagnostics of microchannel production in the chicken eggshell was performed for the first time. It was found that the presence of albumen beneath the shell accounts for longtime generation of X-ray pulses.

Description: This work reports on the effects of a deep high-dose hydrogen ion implant on damage accumulation, defect retention, and silver diffusion in silver implanted ZnO crystals. Single-crystal ZnO samples were implanted with Ag ions in a region ∼150 nm within the surface, and some of these samples were additionally implanted with hydrogen ions to a dose of 2 × 10 16  cm −2 , close to the depth ∼250 nm. Rutherford backscattering/ion channeling measurements show that crystal damage caused by Ag ion implantation and the amount of defects retained in the near surface region following post-implantation annealing were found to diminish in the case with the H implantation. On the other hand, the additional H ion implantation resulted in a reduction of substitutional Ag atoms upon post-implantation annealing. Furthermore, the presence of H also modified the diffusion properties of Ag atoms in ZnO. We discuss these findings in the context of the effects of nano-cavities on formation and annihilation of point defects as well as on impurity diffusion and trapping in ZnO crystals.

Description: The formation and evolution of spin wave band gaps in the transmission spectrum of a magnonic crystal have been studied. A time and space resolved magneto inductive probing system has been used to map the spin wave propagation and evolution in a geometrically structured yttrium iron garnet film. Experiments have been carried out using (1) a chemically etched magnonic crystal supporting the propagation of magnetostatic surface spin waves, (2) a short microwave pulsed excitation of the spin waves, and (3) direct spin wave detection using a movable magneto inductive probe connected to a synchronized fast oscilloscope. The results show that the periodic structure not only modifies the spectra of the transmitted spin waves but also influences the distribution of the spin wave energy inside the magnonic crystal as a function of the position and the transmitted frequency. These results comprise an experimental confirmation of Bloch′s theorem in a spin wave system and demonstrate good agreement with theoretical observations in analogue phononic and photonic systems. Theoretical prediction of the structured transmission spectra is achieved using a simple model based on microwave transmission lines theory. Here, a spin wave system illustrates in detail the evolution of a much more general physical concept: the band gap.

Description: The 8-band k · p parameters which include the direct band coupling between the conduction and the valence bands are derived and used to model optical intersubband transitions in Ge quantum well heterostructure material grown on Si substrates. Whilst for Si rich quantum wells the coupling between the conduction bands and valence bands is not important for accurate modelling, the present work demonstrates that the inclusion of such coupling is essential to accurately determine intersubband transitions between hole states in Ge and Ge-rich Si 1− x Ge x quantum wells. This is due to the direct bandgap being far smaller in energy in Ge compared to Si. Compositional bowing parameters for a range of the key modelling input parameters required for Ge/SiGe heterostructures, including the Kane matrix elements, the effective mass of the Γ 2 ′ conduction band, and the Dresselhaus parameters for both 6- and 8-band k · p modelling, have been determined. These have been used to understand valence band intersubband transitions in a range of Ge quantum well intersubband photodetector devices in the mid-infrared wavelength range.

Description: Rare-earth orthoniobates constitute a class of materials that has been exploited due to their interesting physical properties depending on the lanthanide element. Besides paramagnetism, ferroelasticity, and negative compressibility, these materials are known by their interesting optical properties and mixed types of conduction processes (protonic, ionic, and electronic). In this work, two types of SmNbO 4 samples were studied: polycrystalline samples, prepared by a sol-gel route using the Pechini method, and single crystalline fibres grown by the Laser Floating Zone technique. These samples were structurally characterized based on powder and single-crystal X-ray diffraction studies. A metastable tetragonal phase, stabilized by grain size, was identified in the synthesized powders. After a sintering process of such powders, a single monoclinic phase was obtained. Complementarily, scanning electron microscopy and Raman spectroscopy analyses were performed to these samples. Photoluminescence and photoluminescence excitation spectroscopic studies allowed identifying more than one optically active centre of the trivalent samarium ion in the analysed material. Impedance spectroscopy measurements have shown a large variation of the ac conductivity as a function of temperature, assigned to a protonic conduction and to native ionic conduction mechanisms.

Description: A series of polycrystalline YMn 1−x Ru x O 3 (0 ≤ x ≤ 0.2) samples were prepared by traditional solid-state reaction method. Effects of doping on the microstructures and magnetic properties were investigated. Microstructural results reveal that samples crystallize in a single hexagonal structure with P 6 3 cm group for x ≤ 0.1. Lattice parameters a, c, and unit-cell volume of YMn 1−x Ru x O 3 are found to increase with doping content, which are ascribed to the larger radius of Ru 3+ than that of Mn 3+ . Weak ferromagnetism is found and is enhanced with increasing doping concentration x. The bond angles of Mn-O3-Mn and Mn-O4-Mn are changed in the opposite trends, which break the Mn-Mn exchange interaction, thus the antiferromagnetic ordering. The MnO 5 bipyramid is found to be relieved and the trimerization of Mn ions is weakened. These structural modifications result in the increase of weak ferromagnetism ordering in samples.

Description: To increase the thermoelectric efficiency and reduce the thermal fatigue upon cyclic heat loading, alloying of amorphous NbO 2 with all 3d and 5d transition metals has systematically been investigated using density functional theory. It was found that Ta fulfills the key design criteria, namely, enhancement of the Seebeck coefficient and positive Cauchy pressure (ductility gauge). These quantum mechanical predictions were validated by assessing the thermoelectric and elastic properties on combinatorial thin films, which is a high-throughput approach. The maximum power factor is 2813  μ W m −1  K −2 for the Ta/Nb ratio of 0.25, which is a hundredfold increment compared to pure NbO 2 and exceeds many oxide thermoelectrics. Based on the elasticity measurements, the consistency between theory and experiment for the Cauchy pressure was attained within 2%. On the basis of the electronic structure analysis, these configurations can be perceived as metallic, which is consistent with low electrical resistivity and ductile behavior. Furthermore, a pronounced quantum confinement effect occurs, which is identified as the physical origin for the Seebeck coefficient enhancement.

Description: We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ 8C conduction band edge.

Description: Quite recently, the magnon Hall effect of spin excitations has been observed experimentally on the kagome and pyrochlore lattices. The thermal Hall conductivity κ xy changes sign as a function of magnetic field or temperature on the kagome lattice, and κ xy changes sign upon reversing the sign of the magnetic field on the pyrochlore lattice. Motivated by these recent exciting experimental observations, we theoretically propose a simple realization of the magnon Hall effect in a two-band model on the honeycomb lattice. The magnon Hall effect of spin excitations arises in the usual way via the breaking of inversion symmetry of the lattice, however, by a next-nearest-neighbour Dzyaloshinsky-Moriya interaction. We find that κ xy has a fixed sign for all parameter regimes considered. These results are in contrast to the Lieb, kagome, and pyrochlore lattices. We further show that the low-temperature dependence on the magnon Hall conductivity follows a T 2 law, as opposed to the kagome and pyrochlore lattices. These results suggest an experimental procedure to measure thermal Hall conductivity within a class of 2D honeycomb quantum magnets and ultracold atoms trapped in a honeycomb optical lattice.

Description: We perform molecular dynamics simulations to investigate the static and dynamic fragmentation of metallic liquid sheets of tin induced by random surface fluctuations. The static regime is analyzed by simulating sheets of different thicknesses, and the dynamic fragmentation is ensured by applying along the longitudinal direction of a sheet an instantaneous expansion velocity per initial unit length (expansion rate) with values ranging from 1 × 10 9 to 3 × 10 10  s −1 . The simulations show that the static/dynamic fragmentation becomes possible when the fluctuations of the upper and lower surfaces of the sheets can either overlap or make the local volume density of the system go down below a critical value. These two mechanisms cause locally in the sheet the random nucleation of pores of void, on a timescale that exponentially increases with the sheet thickness. Afterwards, the pores develop following distinct stages of growth, coalescence, and percolation, and later in time aggregates of liquid metal are formed. The simulations also show that the fragmentation of static sheets is characterized by relatively mono-dispersed surface and volume distributions of the pores and aggregates, respectively, whereas in extreme conditions of dynamic fragmentation (expansion rate typically in the range of 1 × 10 10  s −1 ), the distributions are rather poly-dispersed and obey a power law decay with surface (volume). A model derived from the simulations suggests that both dynamic and static regimes of fragmentation are similar for expansion rates below typically 1 × 10 7  s −1 .

Description: The objective of this paper is to demonstrate that the addition of properly tuned nonlinearities to a nonlinear system can increase the range over which a specific resonance responds linearly. Specifically, we seek to enforce two important properties of linear systems, namely, the force-displacement proportionality and the invariance of resonance frequencies. Numerical simulations and experiments are used to validate the theoretical findings.

Description: HoCrO 3 , Ho 0.67 Gd 0.33 CrO 3 , and GdCrO 3 bulk powder samples were prepared by citrate route. The phase purity and the structural properties of the samples were examined by x-ray diffraction and Raman spectroscopic measurements. The dc magnetization data revealed that the Cr 3+ ordering temperatures (Néel temperature) for the HoCrO 3 , Ho 0.67 Gd 0.33 CrO 3 , and GdCrO 3 samples are 140 K, 148 K, and 167 K, respectively, while the ac magnetization data revealed that the rare-earth (Ho) ordering occurs at ∼8 K for HoCrO 3 and Ho 0.67 Gd 0.33 CrO 3 samples. Temperature-induced magnetization reversal and spin reorientation were observed in GdCrO 3 bulk sample, which depends on applied magnetic field and disappears at ∼1500 Oe and 500 Oe, respectively. By fitting the dc magnetic data with Curie-Weiss law, the effective magnetic moments were calculated to be 11.66 μ B , 10.23 μ B , and 9.90 μ B for the HoCrO 3 , Ho 0.67 Gd 0.33 CrO 3 , and GdCrO 3 samples, respectively. The isothermal magnetization data showed that the magnetic behavior changed from canted antiferromagnetic in low temperature region (below Néel temperature) to paramagnetic at high temperature. It was found that Gd substitution considerably improves the magnetocaloric effect of HoCrO 3 . Pure GdCrO 3 bulk sample showed giant magnetocaloric entropy change (31.6 J/kg K at temperature ∼5 K and at ∼70 kOe), which is higher than that for polycrystalline RMnO 3 , RCrO 3 , and RFeO 3 bulk powder samples. This renders GdCrO 3 useful for potential applications in low-temperature magnetic refrigeration.

Description: Electric field-induced changes in the domain wall motion of (1−x)Bi(Mg 0.5 Ti 0.5 )O 3 –xPbTiO 3 (BMT-xPT) near the morphotropic phase boundary (MPB) where x = 0.37 (BMT-37PT) and x = 0.38 (BMT-38PT), are studied by means of synchrotron x-ray diffraction. Through Rietveld analysis and profile fitting, a mixture of coexisting monoclinic ( Cm ) and tetragonal ( P 4 mm ) phases is identified at room temperature. Extrinsic contributions to the property coefficients are evident from electric-field-induced domain wall motion in both the tetragonal and monoclinic phases, as well as through the interphase boundary motion between the two phases. Domain wall motion in the tetragonal and monoclinic phases for BMT-37PT is larger than that of BMT-38PT, possibly due to this composition's closer proximity to the MPB. Increased interphase boundary motion was also observed in BMT-37PT. Lattice strain, which is a function of both intrinsic piezoelectric strain and elastic interactions of the grains (the latter originating from domain wall and interphase boundary motion), is similar for the respective tetragonal and monoclinic phases.

Description: We fabricated homogeneous double-layer amorphous Si-doped indium oxide (ISO) thin-film transistors (TFTs) with an insulating ISO cap layer on top of a semiconducting ISO bottom channel layer. The homogeneously stacked ISO TFT exhibited high mobility (19.6 cm 2 /V s) and normally-off characteristics after annealing in air. It exhibited normally-off characteristics because the ISO insulator suppressed oxygen desorption, which suppressed the formation of oxygen vacancies (V O ) in the semiconducting ISO. Furthermore, we investigated the recovery of the double-layer ISO TFT, after a large negative shift in turn-on voltage caused by hydrogen annealing, by treating it with annealing in ozone. The recovery in turn-on voltage indicates that the dense V O in the semiconducting ISO can be partially filled through the insulator ISO. Controlling molecule penetration in the homogeneous double layer is useful for adjusting the properties of TFTs in advanced oxide electronics.

Description: A compact and integrated optical gas sensor on a silicon-on-insulator platform based on surface plasmon interference for hydrogen detection is theoretically introduced in this paper. The basic sensor element consists of a thin layer of palladium (Pd) embedded in a silicon waveguide. Two decoupled surface plasmon polariton waves propagate simultaneously on either side of the Pd layer, which combine and interfere at the end of the Pd layer. The interference mode can be either constructive or destructive, which is highly sensitive to volumetric hydrogen concentration. The proposed sensor is of great potential as a basic building block for lab-on-chip-scale devices owing to its high integration and compactness.

Description: A study is performed on a photonic-state mixing-pattern in an insulator-metal-insulator cylindrical silver nanoshell and its rich variations induced by changes in the geometry and dielectric media of the system, representing the combined influences of plasmon coupling strength and cavity effects. This study is performed in terms of the photonic local density of states (LDOS) calculated using the Green tensor method, in order to elucidate those combined effects. The energy profiles of LDOS inside the dielectric core are shown to exhibit consistently growing number of redshifted photonic states due to an enhanced plasmon coupling induced state mixing arising from decreased shell thickness, increased cavity size effect, and larger symmetry breaking effect induced by increased permittivity difference between the core and the background media. Further, an increase in cavity size leads to increased additional peaks that spread out toward the lower energy regime. A systematic analysis of those variations for a silver nanoshell with a fixed inner radius in vacuum background reveals a certain pattern of those growing number of redshifted states with an analytic expression for the corresponding energy downshifts, signifying a photonic state mixing scheme beyond the commonly adopted plasmon hybridization scheme. Finally, a remarkable correlation is demonstrated between the LDOS energy profiles outside the shell and the corresponding scattering efficiencies.

Description: The wave property of phonons is employed to explore the thermal transport across a finite periodic array of nano-scatterers such as circular and triangular holes. As thermal phonons are generated in all directions, we study their transmission through a single array for both normal and oblique incidences, using a linear dispersionless time-dependent acoustic frame in a two-dimensional system. Roughness effects can be directly considered within the computations without relying on approximate analytical formulae. Analysis by spatio-temporal Fourier transform allows us to observe the diffraction effects and the conversion of polarization. Frequency-dependent energy transmission coefficients are computed for symmetric and asymmetric objects that are both subject to reciprocity. We demonstrate that the phononic array acts as an efficient thermal barrier by applying the theory of thermal boundary (Kapitza) resistances to arrays of smooth scattering holes in silicon for an exemplifying periodicity of 10 nm in the 5–100 K temperature range. It is observed that the associated thermal conductance has the same temperature dependence as that without phononic filtering.

Description: Interplay between structural disorder and magnetic interaction is investigated here for a multiferroic candidate material, CdCr 2 Se 4 . Ferromagnetic order in CdCr 2 Se 4 sets in below T C  ∼ 130 K as a result of competition between the direct Cr-Cr spin coupling and the near neighbour Cr-Se-Cr exchange interactions. Hence, a small change in the crystal structure is expected to drastically affect its magnetic order. In this report, local lattice distortions within the overall cubic symmetry were brought about by replacing a small percentage of Se by isovalent S . Detailed crystal structure study using EXAFS and Raman Spectroscopy reflects the presence of local distortions within the overall cubic symmetry. Contrary to the expectation, magnetic properties of the substituted compositions do not show any drastic changes. Though, a signature of spin-phonon coupling is present across the magnetic ordering temperature. No structural phase transition occurs within the investigated temperature range of 80–300 K.

Description: In this paper, we studied the localized surface plasmon polariton (SPP) resonance in hole arrays in transparent conducting aluminum-doped zinc oxide (AZO). CMOS-compatible fabrication process was demonstrated for the AZO devices. The localized SPP resonance was observed and confirmed by electromagnetic simulations. Using a standing wave model, the observed SPP was dominated by the standing-wave resonance along (1,1) direction in square lattices. This research lays the groundwork for a fabrication technique that can contribute to the core technology of future integrated photonics through its extension into tunable conductive materials.

Description: We studied the optical absorption and luminescence of agate (SiO 2 ), topaz (Al 2 [SiO 4 ](F,OH) 2 ), beryl (Be 3 Al 2 Si 6 O 18 ), and prehnite (Ca 2 Al(AlSi 3 O 10 )(OH) 2 ) doped with different concentrations of transition metal ions and exposed to fast neutron irradiation. The exchange interaction between the impurity ions and the defects arising under neutron irradiation causes additional absorption as well as bands' broadening in the crystals. These experimental results allow us to suggest the method for obtaining new radiation-defect induced jewellery colors of minerals due to neutron irradiation.

Description: We analyzed carefully the experimental kinetics of the low-temperature diffusion-controlled F, H center recombination in a series of irradiated alkali halides and extracted the migration energies and pre-exponential parameters for the hole H centers. The migration energy for the complementary electronic F centers in NaCl was obtained from the colloid formation kinetics observed above room temperature. The obtained parameters were compared with data available from the literature.

Description: The effect of low-temperature uniaxial deformation on the self-trapping-limited mean free path of excitons in a KI–Tl crystal was revealed from x-ray luminescence spectra. The analysis of the dependence of the intensity ratio of the Tl-center emission (2.85 eV) and the luminescence of self-trapped excitons (π-component; 3.3 eV) on the extent of low-temperature deformation showed that in the KI–Tl crystal (3 × 10 −3 mol. %) the self-trapping-limited mean free path of excitons is comparable with the distance between Tl atoms (20–27) a under a deformation ε = 2%. As the compression increases to ε ≥ 2%–5%, the mean free path drops to (27-5.35) a . The results of modeling based on the continuum approximation showed that with increasing temperature and the degree of low-temperature deformation the height of the potential barrier for the exciton self-trapping drops, which is consistent with the reduction of the mean free path of excitons in the KI–Tl crystal.

Description: Free volume and pore size distribution size in functional micro and macro-micro-modified Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 ceramics are characterized by positron annihilation lifetime spectroscopy in comparison with Hg-porosimetry and scanning electron microscopy technique. Positron annihilation results are interpreted in terms of model implication positron trapping and ortho-positronium decaying. It is shown that free volume of positron traps are the same type for macro and micro modified Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 ceramics. Classic Tao-Eldrup model in spherical approximation is used to calculation of the size of nanopores smaller than 2 nm using the ortho-positronium lifetime.

Description: In order to predict optical properties of insulating materials under intensive laser excitation, we generalized methods of quantum electrodynamics, allowing us to simulate excitation of electrons and holes, interacting with each other and acoustic phonons. The prototypical model considers a two-band dielectric material characterized by the dispersion relations for electron and hole states. We developed a universal description of excited electrons, holes and acoustic phonons within joint quantum kinetics formalism. Illustrative solutions for the quasiparticle birth-annihilation operators, applicable at short laser pulses at 0 K, are obtained by the transition from the macroscopic description to the quantum field formalism.

Description: Monoclinic antiferromagnetic NiWO 4 was studied by far-infrared (30–600 cm −1 ) absorption spectroscopy in the temperature range of 5–300 K using the synchrotron radiation from SOLEIL source. Two isomorphous CoWO 4 and ZnWO 4 tungstates were investigated for comparison. The phonon contributions in the far-infrared range of tungstates were interpreted using the first-principles spin-polarized linear combination of atomic orbital calculations. No contributions from magnetic excitations were found in NiWO 4 and CoWO 4 below their Neel temperatures down to 5 K.

Description: The creation spectrum of stable F centres (being part of F-H pairs of Frenkel defects) by synchrotron radiation of 7–40 eV has been measured for highly pure NaCl single crystals at 12 K using a highly sensitive luminescent method. It is shown that the efficiency of F centre creation in a closely packed NaCl is low at the decay of anion or cation excitons (7.8–8.4 and 33.4 eV, respectively) or at the recombination of relaxed conduction electrons and valence holes. Only the recombination of nonrelaxed (hot) electrons with holes provides the energy exceeding threshold value E FD , which is sufficient for the creation of Frenkel defects at low temperature.

Description: An unambiguous attribution of the absorption spectra to definite paramagnetic centres identified by the EPR techniques in the most cases is problematic. This problem may be solved by applying of a direct measurement techniques—the EPR detected via the magnetic circular dichroism, or briefly MCD–EPR. The present survey reports on the advantages and disadvantages applying the MCD–EPR techniques to simple and complex paramagnetic centres in crystals as well as glasses and glass-ceramics.

Description: Luminescence properties of SiO 2 in different structural states are compared. Similar comparison is made for GeO 2 . Rutile and α-quartz structures as well as glassy state of these materials are considered. Main results are that for α-quartz crystals the luminescence of self-trapped exciton is the general phenomenon that is absent in the crystal with rutile structure. In rutile structured SiO 2 (stishovite) and GeO 2 (argutite) the main luminescence is due to a host material defect existing in as-received (as-grown) samples. The defect luminescence possesses specific two bands, one of which has a slow decay (for SiO 2 in the blue and for GeO 2 , in green range) and another, a fast ultraviolet (UV) band (4.75 eV in SiO 2 and at 3 eV in GeO 2 ). In silica and germania glasses, the luminescence of self-trapped exciton coexists with defect luminescence. The latter also contains two bands: one in the visible range and another in the UV range. The defect luminescence of glasses was studied in details during last 60–70 years and is ascribed to oxygen deficient defects. Analogous defect luminescence in the corresponding pure nonirradiated crystals with α-quartz structure is absent. Only irradiation of a α-quartz crystal by energetic electron beam, γ-rays and neutrons provides defect luminescence analogous to glasses and crystals with rutile structure. Therefore, in glassy state the structure containing tetrahedron motifs is responsible for existence of self-trapped excitons and defects in octahedral motifs are responsible for oxygen deficient defects.

Description: The ground state properties of cubic scandium trifluoride (ScF 3 ) perovskite were studied using first-principles calculations. The electronic structure of ScF 3 was determined by linear combination of atomic orbital (LCAO) and plane wave projector augmented-wave (PAW) methods using modified hybrid exchange-correlation functionals within the density functional theory (DFT). The comprehensive comparison of the results obtained by two methods is presented. Both methods allowed us to reproduce the lattice constant found experimentally in ScF 3 at low temperatures and to predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1 s x-ray absorption spectra.

Description: Photoluminescence and excitation spectra of microcrystalline and nanocrystalline nickel tungstate (NiWO 4 ) were measured using UV-VUV synchrotron radiation source. The origin of the bands is interpreted using comparative analysis with isostructural ZnWO 4 tungstate and based on the results of recent first-principles band structure calculations. The influence of the local atomic structure relaxation and of Ni 2+ intra-ion d–d transitions on the photoluminescence band intensity are discussed.

Description: In this paper a novel method for synthesis of LaInO 3 :Er 3+ is reported and upconversion luminescence properties of the synthesized material at different temperatures (9–300 K) are studied. The samples were prepared by co-precipitation and subsequent heat treatment of lanthanum, indium and erbium hydroxides. It is shown that the excitation at 980 nm leads to a strong green upconversion luminescence in the material. At the concentrations above 0.1 mol. % of Er 3+ the energy transfer upconversion mechanism of the luminescence becomes evident. Further increase of Er 3+ content in the material leads to higher red-to-green upconversion luminescence intensity ratio. The mechanisms responsible for the observed variation are discussed.

Description: Possible proximity effects in gases of cold, multiply charged atoms are discussed. Here we deal with rarefied gases with densities n d of multiply charged ( Z ≫ 1) atoms at low temperatures in the well-known Thomas-Fermi (TF) approximation, which can be used to evaluate the statistical properties of single atoms. In order to retain the advantages of the TF formalism, which is successful for symmetric problems, the external boundary conditions accounting for the finiteness of the density of atoms (donors), n d ≠ 0, are also symmetrized (using a spherical Wigner-Seitz cell) and formulated in a standard way that conserves the total charge within the cell. The model shows that at zero temperature in a rarefied gas of multiply charged atoms there is an effective long-range interaction E proxi ( n d ), the sign of which depends on the properties of the outer shells of individual atoms. The long-range character of the interaction E proxi is evaluated by comparing it with the properties of the well-known London dispersive attraction E Lond ( n d ) 〈 0, which is regarded as a long-range interaction in gases. For the noble gases argon, krypton, and xenon E proxi 〉0 and for the alkali and alkaline-earth elements E proxi 〈 0. At finite temperatures, TF statistics manifests a new, anomalously large proximity effect, which reflects the tendency of electrons localized at Coulomb centers to escape into the continuum spectrum. The properties of thermal decay are interesting in themselves as they determine the important phenomenon of dissociation of neutral complexes into charged fragments. This phenomenon appears consistently in the TF theory through the temperature dependence of the different versions of E proxi . The anomaly in the thermal proximity effect shows up in the following way: for T ≠ 0 there is no equilibrium solution of TS statistics for single multiply charged atoms in a vacuum when the effect is present. Instability is suppressed in a Wigner-Seitz model under the assumption that there are no electron fluxes through the outer boundary R 3 ∝ n −1 d of a Wigner-Seitz cell. E proxi corresponds to the definition of the correlation energy in a gas of interacting particles. This review is written so as to enable comparison of the results of the TF formalism with the standard assumptions of the correlation theory for classical plasmas. The classic example from work on weak solutions (including charged solutions)—the use of semi-impermeable membranes for studies of osmotic pressure—is highly appropriate for problems involving E proxi . Here we are speaking of one or more sharp boundaries formed by the ionic component of a many-particle problem. These may be a metal-vacuum boundary in a standard Casimir cell in a study of the vacuum properties in the 2 l gap between conducting media of different kinds or different layered systems (quantum wells) in semiconductors, etc. As the mobile part of the equilibrium near a sharp boundary, electrons can (should) escape beyond the confines of the ion core into a gap 2 l with a probability that depends, among other factors, on the properties of E proxi for the electron cloud inside the conducting walls of the Casimir cell (quantum well). The analog of the Casimir sandwich in semiconductors is the widely used multilayer heterostructures referred to as quantum wells of width 2 l with sides made of suitable doped materials, which ensure statistical equilibrium exchange of electrons between the layers of the multilayer structure. The thermal component of the proximity effects in semiconducting quantum wells provides an idea of many features of the dissociation process in doped semiconductors. In particular, a positive E proxi 〉 0 (relative to the bottom of the conduction band) indicates that TF donors with a finite density n d ≠ 0 form a degenerate, semiconducting state in the semiconductor. At zero temperature, there is a finite density of free carriers which increases with a power-law dependence on T .

Description: Structural, dielectric, and ferroelectric properties, and electrocaloric effects of pure and Gd doped ( Na 0.5 Bi 0.5 ) 0.94 Ba 0.06 TiO 3   ceramics prepared by the conventional solid-solid method have been carried out. The X-ray diffraction analysis confirms a pure perovskite structure with the coexistence of tetragonal and rhombohedra structures in both powders. The thermal and frequency dependences of the dielectric constants of both ceramics revealed relaxor behavior. The two compounds exhibited two phase transitions: ferroelectric/antiferroelectric (FE/AFE) transition followed by an antiferroelectric/paraelectric (AFE/PE) transition at higher temperatures. Remarkably, we noticed that the small amount of Gd doping (2%) highly enhanced the dielectric properties of the parent compound by about 71%. The phase diagram was as well influenced by the Gd doping, where the FE/AFE transition temperature rose from 90 in the parent compound to 115 °C in the doped one whereas the AFE/PE transition temperature was decreased from 320 to 270 °C, respectively. The direct electrocaloric measurements performed on both compounds showed that the ferroelectric/antiferroelectric phase transition was accompanied by a significant electrocaloric effect. The Gd 3+ doping improved the electrocaloric properties of the parent compound, where a remarkable temperature variation of 1.4 K was obtained in the doped ceramic. The results of the direct electrocaloric measurements will be compared and discussed with those derived from the indirect method.

Description: The free vibration analysis of a multiple rotating nanobeams' system applying the nonlocal Eringen elasticity theory is presented. Multiple nanobeams' systems are of great importance in nano-optomechanical applications. At nanoscale, the nonlocal effects become non-negligible. According to the nonlocal Euler-Bernoulli beam theory, the governing partial differential equations are derived by incorporating the nonlocal scale effects. Assuming a structure of n parallel nanobeams, the vibration of the system is described by a coupled set of n partial differential equations. The method involves a change of variables to uncouple the equations and the differential transform method as an efficient mathematical technique to solve the nonlocal governing differential equations. Then a number of parametric studies are conducted to assess the effect of the nonlocal scaling parameter, rotational speed, boundary conditions, hub radius, and the stiffness coefficients of the elastic interlayer media on the vibration behavior of the coupled rotating multiple-carbon-nanotube-beam system. It is revealed that the bending vibration of the system is significantly influenced by the rotational speed, elastic mediums, and the nonlocal scaling parameters. This model is validated by comparing the results with those available in the literature. The natural frequencies are in a reasonably good agreement with the reported results.

Description: Polycrystalline lanthanum lead zirconate titanate (PLZT) thin films were deposited on Pt/TiO 2 /SiO 2 /Si substrates to study the effects of the thickness and grain size on their structural and piezoresponse properties at nanoscale. Thinner PLZT films show a slight (100)-orientation tendency that tends to random orientation for the thicker film, while microstrain and crystallite size increases almost linearly with increasing thickness. Piezoresponse force microscopy and autocorrelation function technique were used to demonstrate the existence of local self-polarization effect and to study the thickness dependence of correlation length. The obtained results ruled out the bulk mechanisms and suggest that Schottky barriers near the film-substrate are likely responsible for a build-in electric field in the films. Larger correlation length evidence that this build-in field increases the number of coexisting polarization directions in larger grains leading to an alignment of macrodomains in thinner films.

Description: Ge-chalcogenide films show various photo-induced changes, and silver photo-diffusion is one of them which attracts lots of interest. In this paper, we report how silver and Ge-chalcogenide layers in Ge 33 S 67 /Ag/Si substrate stacks change under light exposure in the depth by measuring time-resolved neutron reflectivity. It was found from the measurement that Ag ions diffuse all over the matrix Ge 33 S 67 layer once Ag dissolves into the layer. We also found that the surface was macroscopically deformed by the extended light exposure. Its structural origin was investigated by a scanning electron microscopy.

Description: The motion of antiferromagnetic interfacial spins is investigated through the temperature evolution of training effect in a Co/CoO film with in-plane biaxial anisotropy. Significant differences in the training effect and its temperature dependence are observed in the magnetic easy axis and hard axis (HA) and ascribed to the different motion modes of antiferromagnetic interfacial spins, the collective spin cluster rotation (CSR) and the single spin reversal (SSR), caused by different magnetization reversal modes of ferromagnetic layer. These motion modes of antiferromagnetic spins are successfully separated using a combination of an exponential function and a classic n −1/2 function. A larger CSR to SSR ratio and a shorter lifetime of CSR found in the HA indicates that the domain rotation in the ferromagnetic layer tends to activate and accelerate a CSR mode in the antiferromagnetic spins.

Description: In an attempt to reduce the reliance on fossil fuels, associated with severe environmental effects, the current research is focused on the identification of the thermoelectric potential of n -type Pb 1− x Ti x Te alloys, with x values of up to 3%. A solubility limit of 0.5 at. % Ti in PbTe was identified, while beyond this composition, a precipitation of a TiTe 2 phase was occurred. An impressive maximal dimensionless thermoelectric figure of merit ZT of ∼1.2 was obtained upon 0.1% Ti doping at 500 °C, indicating a ∼9% efficiency enhancement compared to an undoped PbTe. It is shown that generating a functionally graded material based on undoped PbTe as a low temperature segment and a 0.1% Ti doped PbTe as a high temperature segment has a potential to enhance the efficiency by ∼14% compared to the undoped sample.

Description: SrTiO 3 is not only of enduring interest due to its unique dielectric, structural, and lattice dynamical properties, but is also the archetypal perovskite oxide semiconductor and a foundational material in oxide heterostructures and electronics. This has naturally focused attention on growth, stoichiometry, and defects in SrTiO 3 , one exciting recent development being such precisely stoichiometric defect-managed thin films that electron mobilities have finally exceeded bulk crystals. This has been achieved only by molecular beam epitaxy, however (and to a somewhat lesser extent pulsed laser deposition (PLD)), and numerous open questions remain. Here, we present a study of the stoichiometry, defects, and structure in SrTiO 3 synthesized by a different method, high pressure oxygen sputtering, relating the results to electronic transport. We find that this form of sputter deposition is also capable of homoepitaxy of precisely stoichiometric SrTiO 3 , but only provided that substrate and target preparation, temperature, pressure, and deposition rate are carefully controlled. Even under these conditions, oxygen-vacancy-doped heteroepitaxial SrTiO 3 films are found to have carrier density, mobility, and conductivity significantly lower than bulk. While surface depletion plays a role, it is argued from particle-induced X-ray emission (PIXE) measurements of trace impurities in commercial sputtering targets that this is also due to deep acceptors such as Fe at 100's of parts-per-million levels. Comparisons of PIXE from SrTiO 3 crystals and polycrystalline targets are shown to be of general interest, with clear implications for sputter and PLD deposition of this important material.

Description: We investigated chemical sputtering of silicon films by H y + ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H 2 inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H 2 inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3–0.4) ± 0.1 Si atom per bombarding H y + ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions S F (S F (%) = [SiH 4 ]/[H 2 ]*100) ranging from S F  = 0% to 20%. We experimentally observed that the SiH y + flux is not proportional to S F but decreasing from S F  = 3.4% to 20%. This counterintuitive SiH y + flux trend was partly explained by an increasing chemical sputtering rate with decreasing S F and partly by the reaction between H 3 + and SiH 4 that forms SiH 3 + .

Description: Nanostructured zirconium dioxide (zirconia) films are very promising for catalysis and biotechnological applications: a precise control of the interfacial properties of the material at different length scales and, in particular, at the nanoscale, is therefore necessary. Here, we present the characterization of cluster-assembled zirconia films produced by supersonic cluster beam deposition possessing cubic structure at room temperature and controlled nanoscale morphology. We characterized the effect of thermal annealing in reducing and oxidizing conditions on the crystalline structure, grain dimensions, and topography. We highlight the mechanisms of film growth and phase transitions, which determine the observed interfacial morphological properties and their resilience against thermal treatments.

Description: We present a new approach to perform beam steering in reflecting type apertures such as reflectarray antennas. The proposed technique exploits macro-scale mechanical movements of parts of the structure to achieve two-dimensional microwave beam steering without using any solid-state devices or phase shifters integrated within the aperture of the antenna. The principles of operation of this microwave beam steering technique are demonstrated in an aperture occupied by ground-plane-backed, sub-wavelength capacitive patches with identical dimensions. We demonstrate that by tilting the ground plane underneath the entire patch array layer, a phase shift gradient can be created over the aperture of the reflectarray that determines the direction of the radiated beam. Changing the direction and slope of this phase shift gradient on the aperture allows for performing beam steering in two dimensions using only one control parameter (i.e., tilt vector of the ground plane). A proof-of-concept prototype of the structure operating at X-band is designed, fabricated, and experimentally characterized. Experiments demonstrate that small mechanical movements of the ground plane (in the order of 0.05 λ 0 ) can be used to steer the beam direction in the ±10° in two dimensions. It is also demonstrated that this beam scanning range can be greatly enhanced to ±30° by applying this concept to the same structure when its ground plane is segmented.

Description: The effective electric charge of a nanoparticle in an ionic magnetic colloidal system (an ionic ferrofluid) is determined by using the impedance spectroscopy technique. The electric response of the samples to a harmonic external electric field excitation is described by means of the Poisson-Nernst-Planck model. The model proposed for the theoretical interpretation of the impedance spectroscopy data considers that the magnetic particles are electrically charged with H + and have in their vicinity Cl − counterions, resulting in an effective charge Q eff . In the presence of an harmonic, in time, external field (frequency bigger than 10 4 Hz ) particles are assumed to be at rest, due to inertial reason. In this framework, the response of the cell is due to the H + and Cl − present in the solution. From the spectra of the real and imaginary components of the electric impedance of the cell, by means of a best fit procedure to our model, we derive the effective electric charge of the magnetic particles and the bulk density of ions. From an independent measurement of the ζ -potential of the suspension, it is possible to calculate the hydrodynamic radius of the particle, in good agreement with that independently measured.

Description: Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge 1−x C x (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge 0.998 C 0.002 shows a bandgap reduction supporting these results. Growth of Ge 0.998 C 0.002 using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III–V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.

Description: In this work, spin transport in corrugated armchair graphene nanoribbons (AGNRs) is studied. We survey combined effects of spin-orbit interaction and surface roughness, employing the non-equilibrium Green's function formalism and multi-orbitals tight-binding model. Rough substrate surfaces have been statistically generated and the hopping parameters are modulated based on the bending and distance of corrugated carbon atoms. The effects of surface roughness parameters, such as roughness amplitude and correlation length, on spin transport in AGNRs are studied. The increase of surface roughness amplitude results in the coupling of σ and π bands in neighboring atoms, leading to larger spin flipping rate and therefore reduction of the spin-polarization, whereas a longer correlation length makes AGNR surface smoother and increases spin-polarization. Moreover, spin diffusion length of carriers is extracted and its dependency on the roughness parameters is investigated. In agreement with experimental data, the spin diffusion length for various substrate ranges between 2 and 340 μ m. Our results indicate the importance of surface roughness on spin-transport in graphene.

Description: This article proposes an array of nonlinear piezomagnetoelastic energy harvesters (NPEHs) for scavenging electrical energy from broadband vibrations with low amplitudes ( 〈 2 m/s 2 ). The array consists of monostable NPEHs combined to generate useful power output (∼100  μ W) over wide bandwidth. The nonlinearity in each of the NPEHs is induced by the magnetic interaction between an embedded magnet in the tip mass of cantilever and a fixed magnet clamped to the rigid platform. The dynamic responses of two NPEHs, one with attractive configuration and the other with repulsive configuration, are combined to achieve a bandwidth of 3.3 Hz at a power level of 100  μ W. A parametric study is carried out to obtain the gap distances between the magnets to achieve wide bandwidth. Experiments are performed to validate the proposed idea, the theoretical predictions, and to demonstrate the advantage of array of NPEHs over the array of linear piezoelectric energy harvesters (LPEHs). The experiments have clearly shown the advantage of NPEH array over its linear counterpart under both harmonic and random excitations. Approximately, 100% increase in the operation bandwidth is achieved by the NPEH array at harmonic excitation level of 2 m/s 2 . The NPEH array exhibits up to 80% improvement in the accumulated energy under random excitation when compared with the LPEH array. Furthermore, the performance of NPEH array with series and parallel connections between the individual harvesters using standard AC/DC interface circuits is also investigated and compared with its linear counterpart.

Description: The majority of intuition on phonon transport has been derived from studies of homogenous crystalline solids, where the atomic composition and structure are periodic. For this specific class of materials, the solutions to the equations of motions for the atoms (in the harmonic limit) result in plane wave modulated velocity fields for the normal modes of vibration. However, it has been known for several decades that whenever a system lacks periodicity, either compositional or structural, the normal modes of vibration can still be determined (in the harmonic limit), but the solutions take on different characteristics and many modes may not be plane wave modulated. Previous work has classified the types of vibrations into three primary categories, namely, propagons, diffusions, and locons. One can use the participation ratio to distinguish locons, from propagons and diffusons, which measures the extent to which a mode is localized. However, distinguishing between propagons and diffusons has remained a challenge, since both are spatially delocalized. Here, we present a new method that quantifies the extent to which a mode's character corresponds to a propagating mode, e.g., exhibits plane wave modulation. This then allows for clear and quantitative distinctions between propagons and diffusons. By resolving this issue quantitatively, one can now automate the classification of modes for any arbitrary material or structure, subject to a single constraint that the atoms must vibrate stably around their respective equilibrium sites. Several example test cases are studied including crystalline silicon and germanium, crystalline silicon with different defect concentrations, as well as amorphous silicon, germanium, and silica.

Description: Hexagonal and cubic GaN—integrated on on-axis Si(100) substrate by metalorganic chemical vapor deposition via selective epitaxy and hexagonal-to-cubic-phase transition, respectively—are studied by temperature- and injection-intensity-dependent cathodoluminescence to explore the origins of their respective luminescence centers. In hexagonal (cubic) GaN integrated on Si, we identify at room temperature the near band edge luminescence at 3.43 eV (3.22 eV), and a defect peak at 2.21 eV (2.72 eV). At low temperature, we report additional hexagonal (cubic) GaN bound exciton transition at 3.49 eV (3.28 eV), and a donor-to-acceptor transition at 3.31 eV (3.18 eV and 2.95 eV). In cubic GaN, two defect-related acceptor energies are identified as 110 and 360 meV. For hexagonal (cubic) GaN (using Debye Temperature ( β ) of 600 K), Varshni coefficients of α = 7.37 ± 0.13 × 10 − 4   ( 6.83 ± 0.22 × 10 − 4 ) eV / K and E 0 = 3.51 ± 0.01   ( 3.31 ± 0.01 )  eV are extracted. Hexagonal and cubic GaN integrated on CMOS compatible on-axis Si(100) are shown to be promising materials for next generation devices.

Description: In this paper, we report a systematic study that shows how the numerous processing parameters associated with ion implantation (II) and pulsed laser annealing (PLA) can be manipulated to control the quantity and quality of graphene (G), few-layer graphene (FLG), and other carbon nanostructures selectively synthesized in crystalline SiC (c-SiC). Controlled implantations of Si − plus C − and Au + ions in c-SiC showed that both the thickness of the amorphous layer formed by ion damage and the doping effect of the implanted Au enhance the formation of G and FLG during PLA. The relative contributions of the amorphous and doping effects were studied separately, and thermal simulation calculations were used to estimate surface temperatures and to help understand the phase changes occurring during PLA. In addition to the amorphous layer thickness and catalytic doping effects, other enhancement effects were found to depend on other ion species, the annealing environment, PLA fluence and number of pulses, and even laser frequency. Optimum II and PLA conditions are identified and possible mechanisms for selective synthesis of G, FLG, and carbon nanostructures are discussed.

Description: Results characterising the performance of thin (2  μ m i-layer) Al 0.52 In 0.48 P p + -i-n + mesa photodiodes for X-ray photon counting spectroscopy are reported at room temperature. Two 200  μ m diameter and two 400  μ m diameter Al 0.52 In 0.48 P p + -i-n + mesa photodiodes were studied. Dark current results as a function of applied reverse bias are shown; dark current densities

Description: A simple method for the preparation of bulk quantities of magnetic carbon materials, which contain uniformly dispersed transition metals ( M  = Fe, Co, Ni, and Cu) as the magnetic components, is presented. By using highly chlorinated metal phthalocyanine as the building block and potassium as the coupling reagent, phthalocyanine-based carbon materials (PBCMs) containing transition metals were obtained. Our experiments demonstrate the structure of these PBCMs consists of transition metals embedded in graphitic carbon that includes a square planar M N 4 magnetic core and the Fe and Co-PBCM possess spontaneous magnetization at room temperature. In addition, carbon-coated transition metal particles were obtained by the Wurtz-type reaction with excess amount of potassium coupling agent. The large transition metal surface area and magnetization of these M -PBCMs are useful for spintronic and catalytic applications.

Description: We report on a comparative study of transfer doping of hydrogenated single crystal diamond surface by insulators featured by high electron affinity, such as Nb 2 O 5 , WO 3 , V 2 O 5 , and MoO 3 . The low electron affinity Al 2 O 3 was also investigated for comparison. Hole transport properties were evaluated in the passivated hydrogenated diamond films by Hall effect measurements, and were compared to un-passivated diamond films (air-induced doping). A drastic improvement was observed in passivated samples in terms of conductivity, stability with time, and resistance to high temperatures. The efficiency of the investigated insulators, as electron accepting materials in hydrogenated diamond surface, is consistent with their electronic structure. These surface acceptor materials generate a higher hole sheet concentration, up to 6.5 × 10 13  cm −2 , and a lower sheet resistance, down to 2.6 kΩ/sq, in comparison to the atmosphere-induced values of about 1 × 10 13  cm −2 and 10 kΩ/sq, respectively. On the other hand, hole mobilities were reduced by using high electron affinity insulator dopants. Hole mobility as a function of hole concentration in a hydrogenated diamond layer was also investigated, showing a well-defined monotonically decreasing trend.

Description: The density of trap states (DOS) in organic p-type transistors based on the small-molecule 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), the polymer poly(triarylamine) and blends thereof are investigated. The DOS in these devices are measured as a function of semiconductor composition and operating temperature. We show that increasing operating temperature causes a broadening of the DOS below 250 K. Characteristic trap depths of ∼15 meV are measured at 100 K, increasing to between 20 and 50 meV at room-temperature, dependent on the semiconductor composition. Semiconductor films with high concentrations of diF-TES ADT exhibit both a greater density of trap states as well as broader DOS distributions when measured at room-temperature. These results shed light on the underlying charge transport mechanisms in organic blend semiconductors and the apparent freezing-out of hole conduction through the polymer and mixed polymer/small molecule phases at temperatures below 225 K.

Description: We investigated the effect of Sn doping on the optical, electrical, and magneto transport properties of epitaxial α-Ga 2 O 3 thin films grown by mist-Chemical Vapour Deposition. Sn introduces a shallow donor level at ∼0.1 eV and has a high solubility allowing doping up to 10 20  cm −3 . The lowest obtained resistivity of the films is 2.0 × 10 −1  Ω cm. The Sn doped films with a direct band gap of 5.1 eV remain transparent in the visible and UV range. The electrical conduction mechanism and magneto-transport have been investigated for carrier concentrations below and above the insulator-metal transition. The magnetic properties of the neutral Sn donor and the conduction electrons have been studied by electron spin resonance spectroscopy. A spin S = 1/2 state and C 3V point symmetry of the neutral Sn donor is found to be in good agreement with the model of a simple Sn Ga center.

Description: An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al 2 O 3 layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatory to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficient for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.

Description: An experimental technique capable of determining the dynamic tensile (spall) strength of metals in the liquid state is described. Relying on this technique, spall data on samples of tin, lead, and zinc pre-heated to 20 K above their melting points were obtained. It is found that the spall strength of the metals is low, 40–100 MPa, but not zero and is, seemingly, affected by material purity and by the rate of tensile deformation preceding sample spallation.

Description: We study whether a direct measurement of the absolute temperature of a Magnetic Tunnel Junction (MTJ) can be performed using the high frequency electrical noise that it delivers under a finite voltage bias. Our method includes quasi-static hysteresis loop measurements of the MTJ, together with the field-dependence of its spin wave noise spectra. We rely on an analytical modeling of the spectra by assuming independent fluctuations of the different sub-systems of the tunnel junction that are described as macrospin fluctuators. We illustrate our method on perpendicularly magnetized MgO-based MTJs patterned in 50 × 100 nm 2 nanopillars. We apply hard axis (in-plane) fields to let the magnetic thermal fluctuations yield finite conductance fluctuations of the MTJ. Instead of the free layer fluctuations that are observed to be affected by both spin-torque and temperature, we use the magnetization fluctuations of the sole reference layers. Their much stronger anisotropy and their much heavier damping render them essentially immune to spin-torque. We illustrate our method by determining current-induced heating of the perpendicularly magnetized tunnel junction at voltages similar to those used in spin-torque memory applications. The absolute temperature can be deduced with a precision of ±60 K, and we can exclude any substantial heating at the spin-torque switching voltage.

Description: We investigated the relationship between the fine structure of spin-coated conductive polymer poly(3,4-ethylenedioxythiphene):poly(styrene sulfonate) (PEDOT:PSS) films and the photovoltaic performance of PEDOT:PSS crystalline-Si (PEDOT:PSS/c-Si) heterojunction solar cells. Real-time spectroscopic ellipsometry revealed that there were two different time constants for the formation of the PEDOT:PSS network. Upon removal of the polar solvent, the PEDOT:PSS film became optically anisotropic, indicating a conformational change in the PEDOT and PSS chain. Polarized Fourier transform infrared attenuated total reflection absorption spectroscopy and Raman spectroscopy measurements also indicated that thermal annealing promoted an in-plane π-conjugated C α  = C β configuration attributed to a thiophene ring in PEDOT and an out-of-plane configuration of -SO 3 groups in the PSS chain with increasing composition ratio of oxidized ( benzoid ) to neutral ( quinoid ) PEDOT, I qui /I ben . The highest power conversion efficiency for the spin-coated PEDOT:PSS/c-Si heterojunction solar cells was 13.3% for I qui /I ben  = 9–10 without employing any light harvesting methods.

Description: In an attempt to reduce our reliance on fossil fuels, associated with severe environmental effects, the current research is focused on the identification of the thermoelectric potential of p -type (GeTe) 1− x (Bi 2 Te 3 ) x alloys, with x values of up to 20%. Higher solubility limit of Bi 2 Te 3 in GeTe, than previously reported, was identified around ∼9%, extending the doping potential of GeTe by the Bi 2 Te 3 donor dopant, for an effective compensation of the high inherent hole concentration of GeTe toward thermoelectrically optimal values. Around the solubility limit of 9%, an electronic optimization resulted in an impressive maximal thermoelectric figure of merit, ZT , of ∼1.55 at ∼410 °C, which is one of the highest ever reported for any p -type GeTe-rich alloys. Beyond the solubility limit, a Fermi Level Pinning effect of stabilizing the Seebeck coefficient was observed in the x  = 12%–17% range, leading to stabilization of the maximal ZT s over an extended temperature range; an effect that was associated with the potential of the governed highly symmetric Ge 8 Bi 2 Te 11 and Ge 4 Bi 2 Te 7 phases to create high valence band degeneracy with several bands and multiple hole pockets on the Fermi surface. At this compositional range, co-doping with additional dopants, creating shallow impurity levels (in contrast to the deep lying level created by Bi 2 Te 3 ), was suggested for further electronic optimization of the thermoelectric properties.

Description: We report the controllable nanosized local thinning of multi-layer (2 L and 3 L)-thickness MoS 2 films down to the monolayer (1 L) thickness using the simple method of annealing in a dry oxygen atmosphere. The annealing temperature was optimized in the range of 240 °C to 270 °C for 1.5 h, and 1 L thick nanosized pits were developed on the uniform film of the 2 L and 3 L MoS 2 grown using the chemical vapor deposition method. We characterized the formation of the 1 L nanosized pits using nanoscale confocal photoluminescence (PL) and Raman spectroscopy. We observed that the PL intensity increased and the Raman frequency shifted, representative of the characteristics of 1 L MoS 2 films. A subsequent hydrogen treatment process was useful for removing the oxygen-induced doping effect resulting from the annealing.

Description: In this work, phosphate-borate based glasses with molar composition 20.7P 2 O 5 –17.2Nb 2 O 5 –13.8WO 3 –34.5A 2 O–13.8B 2 O 3 , where A = Li, Na, and K, were prepared by the melt quenching technique. The as-prepared glasses were heat-treated in air at 800 °C for 4 h, which led to the formation of glass-ceramics. These high chemical and thermal stability glasses are good candidates for several applications such as fast ionic conductors, semiconductors, photonic materials, electrolytes, hermetic seals, rare-earth ion host solid lasers, and biomedical materials. The present work endorses the analysis of the electrical conductivity of the as-grown samples, and also the electrical, dielectric, and structural changes established by the heat-treatment process. The structure of the samples was analyzed using X-Ray powder Diffraction (XRD), Raman spectroscopy, and density measurements. Both XRD and Raman analysis confirmed crystals formation through the heat-treatment process. The electrical ac and dc conductivities, σ ac and σ dc , respectively, and impedance spectroscopy measurements as function of the temperature, varying from 200 to 380 K, were investigated for the as-grown and heat-treated samples. The impedance spectroscopy was measured in the frequency range of 100 Hz–1 MHz.

Description: The relationship between growth temperature and the formation of periodic interfacial misfit (IMF) dislocations via the anion exchange process in InSb/GaAs heteroepitaxy was systematically investigated. The microstructural and electrical properties of the epitaxial layer were characterized using atomic force microscope, high-resolution x-ray diffraction, transmission electron microscopy, and Hall resistance measurement. The formation of interfacial misfit (IMF) dislocation arrays depended on growth temperature. A uniformly distributed IMF array was found in a sample grown at 310 °C, which also exhibited the lowest threading dislocation density. The analysis suggested that an incomplete As-for-Sb anion exchange process impeded the formation of IMF on sample grown above 310 °C. At growth temperature below 310 °C, island coalescence led to the formation of 60° dislocations and the disruption of periodic IMF array. All samples showed higher electron mobility at 300 K than at 77 K.

Description: We report that the light-matter interaction in metallic nano-hole array structures possess a subwavelength hole radius and periodicity. The transmission coefficient for nano-hole array structures was measured for different angles of incidence of light. Each measured transmission spectrum had several peaks due to surface plasmon polaritons. A theory of the transmission coefficient was developed based on the quantum density matrix method. It was found that the location of the surface plasmon polariton and the heights of the spectral peaks were dependent on the angle of incidence of light. Good agreement was observed between the experimental and theoretical results. This property of these structures has opened up new possibilities for sensing applications.

Description: Micromagnetic simulations were performed to study the effect of grain shape and defect layer in Nd-Fe-B magnets. It was found that the coercivity can be increased by a factor of ∼2 by changing the grain shape from the triangular prism to the spheroid. Both the anisotropy field contribution and the shape contribution to the coercivity, and thus also the final coercivity, were found to decrease in the order: spheroid 〉 circular prism 〉 hexagonal prism 〉 square prism 〉 triangular prism. Sputtered columnar grains and hot-deformed platelet grains with a constant volume were also considered. Results show that the coercivity initially increases with the aspect ratio and then nearly saturates above the ratio of ∼4. Simulations of multigrain ensembles showed that depending on the grain shape, compared to the case of single grain, a further decrease of ∼10%–45% in the coercivity is induced by magnetostatic coupling.

Description: We present an indirect method of estimating the strength of a shock wave, allowing on line monitoring of its reproducibility in each laser shot. This method is based on a shot-to-shot measurement of the X-ray emission from the ablated plasma by a high resolution, spatially resolved focusing spectrometer. An optical pump laser with energy of 1.0 J and pulse duration of ∼660 ps was used to irradiate solid targets or foils with various thicknesses containing Oxygen, Aluminum, Iron, and Tantalum. The high sensitivity and resolving power of the X-ray spectrometer allowed spectra to be obtained on each laser shot and to control fluctuations of the spectral intensity emitted by different plasmas with an accuracy of ∼2%, implying an accuracy in the derived electron plasma temperature of 5%–10% in pump–probe high energy density science experiments. At nano- and sub-nanosecond duration of laser pulse with relatively low laser intensities and ratio Z/A ∼ 0.5, the electron temperature follows T e  ∼ I las 2/3 . Thus, measurements of the electron plasma temperature allow indirect estimation of the laser flux on the target and control its shot-to-shot fluctuation. Knowing the laser flux intensity and its fluctuation gives us the possibility of monitoring shot-to-shot reproducibility of shock wave strength generation with high accuracy.

Description: Using the first-principles nonequilibrium Green's function method, we study effects of Cu and Ni@Cu used as the Cu-based gate electrode on the charge transport of graphene in the field effect transistors (FET). We find that the transmission of graphene decreases with both Cu and Ni@Cu absorbed in the scatter region. Especially, noticeable transmission gaps are present around the Femi level. The transmission gaps are still effective, and considerable cut-off regions are found under the non-equilibrium environment. The Ni@Cu depresses the transmission of graphene more seriously than the Cu and enlarges the transmission gap in armchair direction. The effects on the charge transport are attributed to the redistribution of electronic states of graphene. Both Cu and Ni@Cu induce the localization of states, so as to block the electronic transport. The Ni@Cu transforms the interaction between graphene and gate electrode from the physisorption to the chemisorption, and then induces more localized states, so that the transmission decreases further. Our results suggest that besides being used to impose gate voltage, the Cu-based gate electrode itself will have a considerable effect on the charge transport of graphene and induces noticeable transmission gap in the FET.

Description: The effects on the local structure due to self-irradiation damage of Ga stabilized δ -Pu stored at cryogenic temperatures have been examined using extended x-ray absorption fine structure (EXAFS) experiments. Extensive damage, seen as a loss of local order, was evident after 72 days of storage below 15 K. The effect was observed from both the Pu and the Ga sites, although less pronounced around Ga. Isochronal annealing was performed on this sample to study the annealing processes that occur between cryogenic and room temperature storage conditions, where damage is mostly reversed. Damage fractions at various points along the annealing curve have been determined using an amplitude-ratio method, a standard EXAFS fitting, and a spherical crystallite model, and provide information complementary to the previous electrical resistivity- and susceptibility-based isochronal annealing studies. The use of a spherical crystallite model accounts for the changes in EXAFS spectra using just two parameters, namely, the crystalline fraction and the particle radius. Together, these results are discussed in terms of changes to the local structure around Ga and Pu throughout the annealing process and highlight the unusual role of Ga in the behavior of the lowest temperature anneals.

Description: We have carried out first-principles spin polarized calculations to obtain comprehensive information regarding the structural, magnetic, and electronic properties of the Mn-doped GaSb compound with dopant concentrations: x  = 0.062, 0.083, 0.125, 0.25, and 0.50. The plane-wave pseudopotential method was used in order to calculate total energies and electronic structures. It was found that the Mn Ga substitution is the most stable configuration with a formation energy of ∼1.60 eV/Mn-atom. The calculated density of states shows that the half-metallic ferromagnetism is energetically stable for all dopant concentrations with a total magnetization of about 4.0  μ B /Mn-atom. The results indicate that the magnetic ground state originates from the strong hybridization between Mn- d and Sb- p states, which agree with previous studies on Mn-doped wide gap semiconductors. This study gives new clues to the fabrication of diluted magnetic semiconductors.

Description: We investigate the bow of free standing (0001) oriented hydride vapor phase epitaxy grown GaN substrates and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the substrates. The origin of the stress gradient and the curvature is attributed to the correlated inclination of edge threading dislocation (TD) lines away from the [0001] direction. A model is proposed and a relation is derived for bulk GaN substrate curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substrates as determined by high resolution x-ray diffraction. The results show a close correlation between the experimentally determined parameters and those predicted from theoretical model.

Description: This work presents a study of the structural characterization of Cu 2 ZnSnSe 4 (CZTSe) thin films by X-ray diffraction (XRD) and microdiffraction measurements. Samples were deposited varying both mass (M X ) and substrate temperature (T S ) at which the Cu and ZnSe composites were evaporated. CZTSe samples were deposited by co-evaporation method in three stages. From XRD measurements, it was possible to establish, with increased Ts, the presence of binary phases associated with the quaternary composite during the material's growth process. A stannite-type structure in Cu 2 ZnSnSe 4 thin films and sizes of the crystallites varying between 30 and 40 nm were obtained. X-ray microdiffraction was used to investigate interface orientations and strain distributions when deposition parameters were varied. It was found that around the main peak, 2ϴ = 27.1°, the Cu 1.8 Se and ZnSe binary phases predominate, which are formed during the subsequent material selenization stage. A Raman spectroscopy study revealed Raman shifts associated with the binary composites observed via XRD.

Description: Graphite cathodes are widely used due to their good emission properties, especially their long lifetime. Some previous papers have researched their lifetime under certain conditions and uncovered some important phenomena. This paper is dedicated to research the lifetime of the graphite cathode under higher power. In the lifetime test, the voltage and current amplitudes are about 970 kV and 9.7 kA, respectively. The repetition rate is 20 Hz. An X-band relativistic backward wave oscillator is used to generate high power microwave by utilizing the electron beam energy. The experimental results demonstrate that the emission property of the graphite cathode remains quite stable during 10 5 pulses, despite some slight deteriorations regarding the beam and microwave parameters. The macroscopic morphology change of the cathode blade due to material evaporation is observed by a laser microscope. The mass loss of the graphite cathode is about 60  μ g/C. Meanwhile, the observation by a scanning electron microscope uncovers that the original numerous flaky micro-structures are totally replaced by a relatively smooth surface at the mid region of the cathode blade and a large number of new micro-protrusions at the blade edges during the lifetime test.

Description: The effects of annealing process on the electrical properties of n + -Si/n-SiC and p + -Si/n-SiC junctions fabricated by using surface-activated bonding are investigated. It is found by measuring the current-voltage ( I-V ) characteristics of n + -Si/n-SiC junctions that the reverse-bias current and the ideality factor decreased to 2.0 × 10 −5  mA/cm 2 and 1.10, respectively, after the junctions annealing at 700 °C. The flat band voltages of n + -Si/n-SiC and p + -Si/n-SiC junctions obtained from capacitance-voltage ( C-V ) measurements decreased with increasing annealing temperature. Furthermore, their flat band voltages are very close to each other irrespective of the annealing temperature change, which suggests that the Fermi level is still pinned at the bonding interface even for the junctions annealing at high temperature and the interface state density causing Fermi level pinning varies with the junctions annealing. The reverse characteristics of n + -Si/n-SiC junctions are in good agreement with the calculations based on thermionic field emission. In addition, the calculated donor concentration of 4H-SiC epi-layers and flat band voltage is consistent with the values obtained from C-V measurements.

Description: Ab initio , electronic energy bands of MoS 2 single layer are reported within the local density functional approximation. The inclusion of spin orbit coupling reveals the presence of two excitons A and B. We also discuss the change of physical properties of MoS 2 from multilayer and bulk counterparts. The nature of the band gap changes from indirect to direct when the thickness is reduced to a single monolayer. The imaginary and real dielectric functions are investigated. Refractive index and birefringence are also reported. The results suggest that MoS 2 is suitable for potential applications in optoelectronic and photovoltaic devices. The ab initio study is essential to propose the crucial parameters for the analytical model used for A-B exciton properties of the monolayer MoS 2 . From a theoretical point of view, we consider how the exciton behavior evolves under environmental dielectrics.

Description: This study reports a method to reuse GaAs substrates with a batch process for thin film light emitting diode (TF-LED) production. The method is based on an epitaxial lift-off technique. With the developed reclaim process, it is possible to get an epi-ready GaAs surface without additional time-consuming and expensive grinding/polishing processes. The reclaim and regrowth process was investigated with a one layer epitaxial test structure. The GaAs surface was characterized by an atomic force microscope directly after the reclaim process. The crystal structure of the regrown In 0.5 (Ga 0.45 Al 0.55 ) 0.5 P (Q 55 ) layer was investigated by high resolution x-ray diffraction and scanning transmission electron microscopy. In addition, a complete TF-LED grown on reclaimed GaAs substrates was electro-optically characterized on wafer level. The crystal structure of the epitaxial layers and the performance of the TF-LED grown on reclaimed substrates are not influenced by the developed reclaim process. This process would result in reducing costs for LEDs and reducing much arsenic waste for the benefit of a green semiconductor production.

Description: Cu(In,Ga)Se 2 (CIGS) solar cells with superstrate-type structure of soda-lime glass (SLG)/epoxy/Al/ZnO:Al (AZO)/ZnO/CdS/CIGS/back n-type transparent conductive oxide (TCO) electrode/Al are fabricated by lift-off process. AZO or In 2 O 3 :Sn (ITO) is used as the back n-type TCO electrode. Ohmic-like contact between p-type CIGS and n-type D-TCO (damage-TCO), namely, D-AZO or D-ITO, is formed through the trap-assisted recombination. The D-TCO, meaning TCO with high sputtering damage on the CIGS surface, is prepared under the optimization of its deposition condition, namely, the power density of 2.4 W/cm 2 for D-AZO or 3.3 W/cm 2 for D-ITO, for high defect density on the CIGS surface to promote the trap-assisted recombination. Ultimately, the superstrate-type CIGS solar cell with a bi-layer of D-AZO/AZO as back n-type TCO electrode with conversion efficiency (η) of 9.2% is achieved, which is 70% of η of the substrate-type CIGS solar cell before lift-off process. The bi-layer of D-AZO/AZO is utilized owing to high resistivity of D-AZO (about 0.1 Ω cm). On the other hand, the superstrate-type CIGS solar cell with D-ITO as the back n-type TCO electrode with η of 10.4% is attained, which is 93.7% of η of the substrate-type CIGS solar cell, where the resistivity of the D-ITO layer is low at about 5.0 × 10 −3 Ω cm.