We report on recent doping experiments of cubic GaN epilayers by Ge and investigate in detail the optical properties by photoluminescence spectroscopy. Plasma-assisted molecular beam epitaxy was used to deposit Ge-doped cubic GaN layers with nominal thicknesses of 600 nm on 3C-SiC(001)/Si(001) substrates. The Ge doping level could be varied by around six orders of magnitude by changing the Ge effusion cell temperature. A maximum free carrier concentration of 3.7×1020 cm-3 was measured in the GaN layers via Hall-effect at room temperature. Low temperature photoluminescence (PL) showed a clear shift of the donor-acceptor emission to higher energies with increasing Ge-doping. Above a Ge concentration of ∼ 2x1018cm-3 the near band edge lines merge to one broad band. From temperature dependent measurements of the observed excitonic and donor-acceptor transitions a donor-energy of ∼ 36 meV could be estimated for Ge.

This work explores responsive hydrophilic polymers for convergent functions of climate control with architectural material systems. In buildings, the transition across exterior and interior space occurs through the envelope, which is an enclosure system that mediates heat, light, air and moisture transfer functions. Conventional building envelopes are typically constructed to form a barrier that insulates and hermetically separates outdoor and indoor conditions. The dynamic environmental responses of superporous intelligent hydrogels are shown to be beneficial at the interior layer of a double-skin glazing system for building envelope applications. If the hydrogels are integral to the building envelope system, then various environmental functions (such as natural daylighting, heat transfer, airflow and moisture control) can be achieved through integrated actuators to result in improved building energy performance.

The composite embodiments emulate bio-analytical functions when embedded microbore-tube water channels serve as actuators for swelling and deswelling kinetics respectively. Each prototype is conceived in response to hot-arid climate contexts. The prototype presented here is a lightweight ventilation cooling and daylighting system. Initial prototypes are inserted into an environmental test-bed that is consequently divided into two chambers to represent an outdoor and indoor condition. The input chamber includes controllable heat and light elements that affect the dynamics of the hydrogel system. The output chamber on the opposite side of the prototype division includes temperature, humidity and photo sensors that are connected to an Arduino board for data collection. Dependent upon the environmental conditions of chamber two, a control program actuates small hydro-pump to saturate the gels with water.

The initial results provide correlations between mechanical (elasticity) and thermal (conductivity) properties. Current work in progress includes documentation of average rates for sorption-desorption kinetics and correlations between saturation loading and visible transmittance. The physical test data will also be integrated into building-scale energy performance simulations and hygrothermal transfer numerical analysis for building envelope compositions. The embedded material logic of the hydrogel is exploited in an architectural configuration for a convergence of prior building mechanical system and building envelope functions. The current work demonstrates a highly promising application of soft-skin membranes for much needed reductions in energy consumption within the building sector.

We present an evaluation carried out on a large-scale teacher training activity, the festival “Science on Stage” (SonS) 2013. SonS is a European initiative offering teachers the opportunity to share ideas and teaching practices both on the national and international level. We evaluated the project’s effectiveness in pursuing clear objectives, in clearly defining the target group, and in internationally disseminating teaching projects. Our study documents the transfer of knowledge from the festival to the classroom, aims to identify the prerequisites thereof and, finally, allows formulating key factors for successful future large-scale science communication activities.

Practical use of Resistive Random Access Memory (ReRAM) depends on thorough understanding of the resistive switching (RS) mechanism in polycrystalline metal oxide films. Based on experimental and theoretical results of NiO based ReRAM, we have proposed a grain surface tiling model, in which grain surfaces (i.e. grain boundaries) are composed by insulating and conductive micro surface structures. This paper reports the adequacy of our model to the NiO based ReRAM and universality of surface electronic properties in metal oxides of NiO, CoO and MgO. Experimental results of RS operating modes suggest that the resistance changes in the grain boundaries, supporting our model. First-principles calculation results suggest that our model can be adopted to other metal oxide materials and the RS from a low resistance to a high resistance can be caused at 1000 K, which agrees with previous experimental reports.

Hybrid perovskite solar cells (PSCs) under normal operation will reach a temperature above ∼ 60 °C, across the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI3). Whether the structural phase transition could result in dramatic changes of ionic, electrical and optical properties that may further impact the PSC performances should be studied. Herein, we report a structural phase transition temperature of MAPbI3 thin film at ∼ 55 °C, but a striking contrast occurred at ∼ 45 °C in the ionic and electrical properties of MAPbI3 due to a change of the ion activation energy from 0.7 eV to 0.5 eV. The optical properties exhibited no sharp transition except for the steady increase of the bandgap with temperature. It was also observed that the activation energy for ionic migration steadily increased with increased grain sizes, and reduction of the grain boundary density reduced the ionic migration.

Spherical carbon particles (CPs) with controllable size and surface functionalities were prepared via hydrothermal treatment using sucrose as a precursor and then post modification methods. The CPs were prepared in two steps; firstly, dewatering of sucrose molecules at relatively low temperatures and secondly, carbonization at high temperatures. The micro/nano-sized CPs were functionalized by 2-Acrylamido-2-methylpropane sulfonic acid (AMPS), (3-Acrylamidopropyl)-trimethylammonium chloride (APTMACI), and N-isopropylacrylamide (NIPAM) in order to have the corresponding polymers on the surface of CPs. The monomers were polymerized via atom transfer radical polymerization (ATRP) by means of grafted initiator onto the CPs surface. Additionally, copolymer of these monomers was synthesized to introduce additional properties. The stability of the functionalized CPs in different solutions, and pH, temperature responsive behaviors of the newly prepared CP-polymers composite were evaluated.

We have studied the structural and morphological properties on the pyrochlore (Er2-x Srx)Ru2O6 system, for x = 0.0, 0.02, 0.05, 0.07, 0.10, and 0.15. Polycrystalline samples were prepared by solid-state reaction (SR) and sol-gel acrylamide polymerization (SGAP). Thermogravimetric Analysis (TGA) was used to follow the thermal transformations such as reagents decomposition, phase transformation, chemical stability, and volatilization of organic material of samples. The reagents and synthesized products by the different methods of synthesis were characterized using powder X-ray diffraction (XRD). All samples crystallize Er2Ru2O6 PDF (72-7620) in the cubic unit cell with Fd$\bar{3}$m (No. 227) space group and form a solid solution up to x = 0.15. Scanning electron microscopy (SEM) shows considerable variations and similitudes in sizes, very few phases and shapes of polycrystals can be observed. Polycrystalline samples prepared by solid-state reaction (SR) present a grain size varies between 77 nm to 250 nm.

TMNCN (where TM = Mn2+, Fe2+, Co2+ or Ni2+) have been recently proposed as electrochemically active materials for Na-ion insertion that operate via conversion reaction. Their electrochemical performance for Na-ion batteries is presented here with an emphasis on long-term cycling. With a very low voltage for Na insertion of ∼0.1V vs Na+/Na for MnNCN, the overpotential observed in batteries of MnNCN plays a very important role in their performance, evidencing big differences in the electrochemical performance between materials produced with different nano- and micrometer particle sizes evidenced by SEM images. A more suitable voltage for the conversion reaction accompanied by less overpotential is shown by FeNCN, CoNCN and NiNCN. Despite the lower reversible capacity achieved by FeNCN (450 mAh/g) in comparison with CoNCN and NiNCN in the first cycle; the smallest first-cycle irreversible capacity (220 mAh/g) and the lower voltage plateau (0.3 V vs Na+/Na) make FeNCN a good candidate as an anode material for sodium ion batteries. The voltages of conversion reaction are correlated with the calculated enthalpies of formation suggesting that thermodynamics dominates the observed electrochemical conversion reaction.

Cubic boron nitride (cBN) is a synthetic wide band gap material that has attracted attention due to its high thermal conductivity, optical transparency and optical emission. In this work, defects in cBN have been investigated using experimental and theoretical X-ray absorption near edge structure (XANES). Vacancy and O substitutional defects were considered, with O substituted at the N site (ON) to be the most energetically favorable. All defects produce unique signatures in either the B or N K-edges and can thus be identified using XANES. The calculations coupled with electron-irradiation / annealing experiments strongly suggest that ON is the dominant defect in irradiated cBN and remains after annealing. This defect is a likely source of optical emission in cBN.

We have investigated organic light emitting diode (OLED) backside contacting for the enhancement of luminance uniformity as a superior alternative to gridlines. In this approach, the low-conductivity OLED anode is supported by a high-conductivity auxiliary electrode and vertically contacted through via holes. Electrical simulations of large-area OLEDs have predicted that this method allows comparable luminance uniformity while sacrificing significantly less active area compared to the common gridline approach.

The method for fabricating backside contacts is comprised of five steps: (1) Thin-film encapsulation of the OLED, (2) Patterning of the OLED surface with lithography (resist mask defining via hole positions), (3) Via hole formation to the bottom anode by a plasma etching process, (4) Organic residues removal and sidewall insulation. (5) Contacting of the anode with a high-conductivity auxiliary electrode.

Backside-contacted OLEDs processed by organic vapor phase deposition show high luminance uniformity. Scanning electron microscopy pictures and electrical breakthrough measurements confirm efficient sidewall insulation.

Bismuth displays puzzling superconducting properties. In its crystalline equilibrium phase, it does not seem to superconduct at accessible low temperatures. However, in the amorphous phase it displays superconductivity at ∼ 6 K. Under pressure bismuth has been found to superconduct at Tcs that go from 3.9 K to 8.5 K depending on the phase obtained. So the question is: what electronic or vibrational changes occur that explains this radical transformation in the conducting behavior of this material? In a recent publication we argue that changes in the density of electronic and vibrational states may account for the behavior observed in the amorphous phase with respect to the crystal. We have now undertaken an ab initio computational study of the effects of pressure alone maintaining the original crystalline structure and compressing our supercell computationally. From the results obtained we infer that if the crystal structure remains the same (except for the contraction), no superconductivity will appear.

Optical transmission spectra are collected in the ferromagnetic state of GdN at various temperatures. The temperature dependent extinction coefficient extracted from the transmission measurements show that the conduction and valence band states are spin polarized. The results confirm the LSDA+U estimates of the energy gap associated with the majority/minority-spins and also the magnitude of the spin splitting.

Growing interest in the use of CO2 as a feedstock for fuel generation has led to increased interest in solar CO2 electrolysis for renewable fuel generation which has a variety of applications ranging from providing renewable sources for energy-dense carbon fuels, to curbing high-density emissions from power plants, industries and automobiles. The challenges of integrated solar-to-carbon fuel converters, where the photovoltaic (PV) material is immersed in the electrolyte, are well-known: the need for unique PV cell designs; material incompatibility; corrosion; and optical losses. In this paper, a PV-electrolysis system is presented, where a flow-cell electrolyzer is power-matched to a high-performance solar PV module array which has two system design advantages: 1) use of standard PV cells external to the electrolyzer, which allows de-coupling the design, fabrication and operation of the PV system from that of the electrolyzer; and 2) enabling optimization of the PV configuration to maximize power coupling efficiency to the specific electrolyzer Tafel curve, with or without the use of electronic power-conditioning devices. The implemented system resulted in a peak SFE of 6.5%, a competitive solar-to-fuel efficiency (SFE) figure to those reported in literature.

We present the optical characterization of quantum dot cascade laser (QDCL) structures using photoluminescence. In this work, two InAs/AlAs quantum dot structures are investigated with different GaAs QW thicknesses. Low temperature photoluminescence measurements show peaks at 1.03 eV, 1.28 eV, and 1.51 eV for QDCL1, and 1.07 eV, 1.27 eV, and 1.47 eV for sample QDCL2, corresponding to ee-hh transition in both QD and QW. A three dimensional QD-QW model in Nextnano is developed to simulate the energy states, wave functions, and transition rates between conduction and valence bands using a typical QD size obtained from atomic force microscope measurements. The simulation results agree well with the experimental data. This allows us not only to understand the optical characteristics of the QD-QW structure, but also to optimize the QDCL structure design.

Carbon nanotube thin film transistors (TFTs) with characteristics resembling those of TFTs constructed on amorphous silicon, low-temperature polycrystalline silicon and metal oxides were fabricated on (6,5) single chirality single-walled carbon nanotube (SWCNT) thin film deposited from electronically pure semiconducting (6,5) single chirality single-walled carbon nanotube (SWCNT) ink. This ink was extracted in industrial scale from raw SWCNTs produced using high pressure carbon monoxide conversion, and deposited on pretreated substrates to form uniform and consistent (6,5) HiPCO SWCNT thin film using solution process. The (6,5) HiPCO SWCNT thin films were characterized as pure semiconductor without metallic impurities showing classic nonlinear current-bias curves in Schottky-type diodes. Both N-type and P-type (6,5) HiPCO SWCNT TFTs were fabricated with femto Ampere off-current and ION/IOFF ratio of 108 by depositing SiNx and HfO2 dielectrics on the top of (6,5) HiPCO SWCNT thin films, respectively. The (6,5) HiPCO SWCNT inverter with voltage gain of 52 was also demonstrated by wire-bonding one P-type HiPCO SWCNT TFT to one N-type HiPCO SWCNT TFT.

Due to their high actinide content MOX residues require immobilization within a robust host matrix. Although it is possible to immobilize actinides in vitreous wasteforms; ceramic phases, such as brannerite (UTi2O6), are attractive due to their high waste loading capacity and relative insolubility. Brannerites Gd0.1U0.9Ti2O6, Ce0.1U0.9Ti2O6 and Gd0.1U0.81Ce0.09Ti2O6 were prepared using an oxide route. Charge compensation of trivalent cations was expected to occur via the oxidation of U (IV) to higher valence states (U (V) or U (VI)). Gd was added to act as a neutron absorber in the final Pu bearing wasteform and Ce was used as a structural surrogate for Pu. X-ray absorption spectroscopy showed that Ce (IV) was reduced to Ce (III) in all cases. X-ray powder diffraction of synthesized specimens found that the final phase assemblage was strongly affected by processing atmosphere (air or argon). Prototypical brannerite was formed in all compositions, secondary phases observed were found to vary according to processing atmosphere and stoichiometry. Microstructural analysis (SEM) of the sintered samples confirmed the results of the X-ray powder diffraction.

The microstructure of Ni-Mg-Al mixed oxides obtained by thermal decomposition of hydrotalcite-like compounds synthesized by a co-precipitation method has been studied by using X-ray diffraction (XRD) and atomic resolution transmission electron microscopy (TEM). XRD patterns revealed the formation of NixMg1-xO (x=0÷1), α-Al2O3 and traces of MgAl2O4 and NiAl2O4 phases. The peaks profile analysis indicated a small grain size, microdeformations and partial overlapping of peaks due to phases with different, but similar interplanar spacings. The microdeformations point out the presence of dislocations and the peaks shift associated with the presence of excess vacancies. The use of atomic resolution TEM made it possible to identify the phases, directly observe dislocations and demonstrate the vacancies excess. Atomic resolution TEM is achieved by applying an Exit Wave Reconstruction procedure with 40 low dose images taken at different defocus. The current results suggest that vacancies of metals are predominant in MgO (NiO) crystals and that vacancies of Oxygen are predominant in Al2O3 crystals.

This paper discusses a pulse electroplating method for developing tin (Sn)-decorated gas diffusion electrodes (GDEs) for the electrochemical conversion of carbon dioxide (CO2) to formate. The pulse-plated Sn electrodes achieved current densities up to 388 mA/cm2, more than two-fold greater than conventionally prepared electrodes (150 mA/cm2), both at a formate selectivity of 80%. Optical and microscopic analyses indicate improvements in deposition parameters could further enhance performance by reducing the catalyst particle size.

Interaction of GaAs with sulfur can be immensely beneficial in reducing the deleterious effect of surface states on recombination attributes. Bonding of sulfur on GaAs is also important for developing novel molecular devices and sensors, where a molecular channel can be connected to GaAs surface via thiol functional group. However, the primary challenge lies in increasing the stability and effectiveness of the sulfur passivated GaAs. We have investigated the effect of single and double step surface passivation of n-GaAs(100) by using the sulfide and fluoride ions. Our single-step passivation involved the use of sulfide and fluoride ions individually. However, the two kinds of double-step passivations were performed by treating the n-GaAs surface. In the first approach GaAs surface was firstly treated with sulfide ions and secondly with fluoride ions, respectively. In the second double step approach GaAs surface was first treated with fluoride ions followed by sulfide ions, respectively. Sulfidation was conducted using the nonaqueous solution of sodium sulfide salt. Whereas the passivation steps with fluoride ion was performed with the aqueous solution of ammonium fluoride. Both sulfidation and fluoridation steps were performed either by dipping the GaAs sample in the desired ionic solution or electrochemically. Photoluminescence was conducted to characterize the relative changes in surface recombination velocity due to the single and double step surface passivation. Photoluminescence study showed that the double-step chemical treatment where GaAs was first treated with fluoride ions followed by the sulfide ions yielded the highest improvement. The time vs. photoluminescence study showed that this double-step passivation exhibited lower degradation rate as compared to widely discussed sulfide ion passivated GaAs surface. We also conducted surface elemental analysis using Rutherford Back Scattering to decipher the near surface chemical changes due to the four passivation methodologies we adopted. The double-step passivations affected the shallower region near GaAs surface as compared to the single step passivations.

Formation of the carbon nanotube (CNT) sock, which is an assemblage of nanotubes in a thin cylindrical shape, is a prerequisite for continuous production of thread and sheet using the floating catalyst growth method. Although several studies have considered sock formation mechanisms, the dynamics of the sock behavior during the synthesis process are not well understood. In this work, a computational technique is utilized to explore the multiphysics environment within the nanotube reactor affecting the sock formation and structure. Specifically the flow field, temperature profile, catalyst nucleation, and residence time are investigated and their influence on the sock formation and properties are studied. We demonstrate that it is critical to control the multiphysics synthesis environment in order to form a stable sock. Sock production rate was studied experimentally and found to be linearly dependent on the amount of effective catalyst (iron in the sock) inside the reactor. To achieve a high sock production rate, the proportion of effective iron has to be high when increasing the total amount of catalyst in the reactor. Based on the analysis, we suggest that using small size catalyst and growing longer CNTs by increasing temperature, increasing residence times etc. will increase the CNT production rate and improve the properties of CNT thread/sheet produced from the sock.